rt Hydrologic and Human Aspects of the 1976-77 Drought GEOLOGICAL SURVEY PROFESSIONAL PAPER 1130Hydrologic and Human Aspects of the 1976—77 Drought By HOWARD F. MATTHAI GEOLOGICAL SURVEY PROFESSIONAL PAPER 1 13 0 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 16237 loin.UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress number 79-600188 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock number 024-001-03243-1CONTENTS Conversion factors........................v Abstract..................................1 Introduction..............................1 Historical setting........................3 Drought and its ramifications.............5 Impacts on people........................14 Public water supplies................14 Rural water supplies.................15 Water for irrigation.................16 Water for hydroelectric power ... 16 Water for forests....................16 Water quality........................17 Water for fisheries..................18 Water for recreation.................18 Water for navigation.................18 Adaptation to drought....................19 Short-term planning..................19 Changes in water use..............19 Changes in irrigation practices . 19 The role of forecasting .... 20 Weather.....................20 Runoff......................20 Ground-water use............20 Base flow projections .... 20 Long-term planning................21 Legal aspects.....................22 Chronology of the 1976-77 drought . . 23 Great Lakes—WRC Region 04 . . .24 Precipitation and runoff .... 24 Ground-water conditions . . . .26 Water quality...................26 Activities resulting from the drought..................26 Upper Mississippi—WRC Region 07 and Souris-Red-Rainy—WRC Region 09.......................27 Previous droughts...............28 Precipitation and runoff .... 29 Ground-water conditions .... 31 Water quality...................32 Activities resulting from the drought..................32 Missouri Basin—WRC Region 10 (upper part)....................33 Precipitation and runoff .... 34 Ground-water conditions .... 35 Water quality...................36 Activities resulting from the drought..................36 Missouri Basin—WRC Region 10 (lower part)....................36 Previous droughts...............37 Precipitation and runoff .... 37 Ground-water conditions .... 38 Water quality...................38 Activities from the drought . .39 Arkansas-White-Red—WRC Region 11.......................39 Precipitation and runoff .... 39 Ground-water conditions .... 41 Water quality...................41 Activities resulting from the drought..................41 Upper Colorado—WRC Region 14 .41 Precipitation and runoff .... 43 Ground-water conditions .... 46 Water quality...................46 Activities resulting from the drought..................46 The Great Basin—WRC Region 16 .47 Previous droughts...............47 Precipitation and runoff .... 49 Ground-water conditions .... 50 Water quality...................51 Forests.........................51 Activities resulting from the drought...................53 Pacific Northwest—WRC Region 17.......................53 Previous droughts...............53 Precipitation and runoff .... 54 Ground-water conditions .... 58 Water quality...................59 Forests.........................59 The fishery.....................59 Activities resulting from the drought...................60 California—WRC Region 18 .... 61 Precipitation and runoff .... 62 Ground-water conditions .... 66 Water quality...................67 Land subsidence.................69 inIV CONTENTS Hydroelectric power generation...................71 Forests.........................71 Activities resulting from the drought..................71 Hawaii—WRC Region 20................72 Previous droughts...............72 Precipitation and runoff .... 73 Ground-water conditions .... 73 Water quality...................74 Activities resulting from the drought..................74 The eleven other WRC Regions . . 74 Precipitation and runoff .... 74 Ground-water conditions .... 75 Water quality....................75 Activities resulting from the drought...................75 Epilogue............................76 Summary.................................77 A look to the future....................77 Selected references.....................83 ILLUSTRATIONS FIGURE 1. Map of the United States showing regions designated by Water Resources Council............................................2 2. Graphs of low-flow frequency curves for six streams in areas affected by the drought......................................7 3. Maps of United States showing number of months of deficient streamflow...................................................8 4. Graph of water level decline caused by reduced natural recharge during the drought.................................10 5. Graph of fluctuations, of water level, 1960-77, in well 16/15-34N4 near Cantua Creek, 35 miles southwest of Fresno, Calif.....H 6. Maps of United States showing values of the Palmer index on four selected dates in 1976 12 7. Maps of United States showing values of the Palmer index on four selected dates in 1977 13 8. Photograph of stunted ears of corn caused by drought in Midwest . 17 9. Map of Great Lakes—WRC Region 04............................25 10. Graph of monthly discharge of Fox River at Rapide Croche Dam, near Wrightstown, Wis.......................................27 11. Map of Upper Mississippi—WRC Region 07 and Souris-Red- Rainy—WRC Region 09.........................................28 12. Photograph of parched soil in Iowa in 1976 ................ 30 13. Map of Missouri Basin—WRC Region 10 (upper part)...........34 14. Map of Missouri Basin—WRC Region 10 (lower part)...........37 15. Map of Arkansas-White-Red—WRC Region 11.....................40 16. Map of Upper Colorado—WRC Region 14.........................42 17. Photographs of snow cover in Colorado Rockies a. April 1976 ............................................ 44 b. April 1977 ............................................ 45 18. Map of the Great Basin—WRC Region 16........................48 19. Map of Pavant Valley, Utah showing change of water levels from March 1977 to March 1978 ......................32 20. Map of Pacific Northwest—WRC Region 17......................54 21. Graph of water content of Columbia River basin snowpack as a percentage of the April 1 average.....................55CONTENTS V 22. Photographs of snow cover in the Cascades, Washington a. April 1976 .............................................. 56 b. April 1977 .............................................. 57 23. Map of California—WRC Region 18.................................61 24. Graph of water content of snowpack in California as a percentage of the April 1 average.............................63 25. Graph of monthly mean discharges for four selected drought years, North Fork American River at North Fork Dam, Calif. ... 64 26. Graph showing trends in streamflow at selected sites in California, October 1975 to January 1978 ..................... 65 27. Photographs showing depleted conditions of water in storage in two reservoirs a. Pardee Reservoir near Valley Springs, Calif., March 26, 1977 . 67 b. Shasta Lake near Redding, Calif., September 5, 1977 .... 67 28. Map of California showing ground-water level changes 1975-77 . . 68 29. Map of Sacramento-San Joaquin Delta showing annual maximum intrusion of salinity for selected years............70 30. Map of Hawaii—WRC Region 20.................................72 CONVERSION FACTORS Inch-pound acre acre-ft (acre-foot) board-foot bushel bushels per acre Multiply by 4.047 x 10-3 0.4047 1.233 x 10~3 2.360 x 10“3 35.24 0.0871 ft /s (cubic foot per second) 28.32 x 10 ft (foot) 0.3048 gal/d (gallons per day) 3.785 x 10 bgd (billion gallons per day) 3.785 in. (inch) 25.40 mi (mile) 1.609 2 mi (square mile) 2.590 Metric 2 km (square kilometer) hm (square hectometer) 3 hm (cubic hectometer) 3 m (cubic meter) L (liter) 3 2 m /hm (cubic meter per square hectometer) 3 m /s (cubic meter per second) m (meter) 3 m /d (cubic meters per day) 3 hm /d (cubic hectometers per day) mm (millimeter) km (kilometer) km (square kilometer)HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT By Howard F. Matthai ABSTRACT The drought of 1976-77 was the most severe one in at least 50 years in many parts of the United States. Record low amounts of rainfall, snowfall, and runoff, and increased withdrawals of ground water were prevalent. The use of carry-over storage in reservoirs during 1976 maintained streamflow at near normal levels, but some reservoirs went dry or dropped below the outlet works in 1977. Carry-over storage in the fall of 1977 was very low. Ground-water levels were at or near record low levels in many aquifers, hundreds of wells went dry, and thousands of wells were drilled. Yet no wide-spread deterioration of ground-water quality was reported. Water-quality problems arose in some streams and lakes, but most were localized and of short duration. Water rationing became a way of life in numerous areas, and water was hauled in many rural areas and to a few towns. Water use was affected by legal agreements or decisions, some of which were modified for the duration of the drought, and by the inability of water managers to efficiently manage surface and ground waters as one resource under existing law. There are still many drought related problems to solve and many challenges to be met before the next drought occurs. The advancement of techniques in many fields of endeavor in recent years plus ongoing, planned, and proposed research on drought and the risks involved are promising thrusts that should make it easier to cope with the next drought. INTRODUCTION A drought is primarily a natural event, but the consequences can be significantly altered by humans. The United States and other parts of the world have been afflicted intermittently by droughts from time immemorial; and one of them, a severe one, occurred in many parts of the United States in 1976 and 1977. By the late winter of 1977-78, enough rain and snow had fallen over much of the drought areas that the drought was considered by many people to be over. The wet period, however, may be only a brief interruption in an extended dry period. Historical records show that a wet period or year has occurred in the midst of a number of dry periods or years and, conversely, that dry years have occurred among groups of wet ones. This report was prepared to document the drought of 1976-77 in the United States as a hydrologic event, how it affected humans, and how they reacted to it. Whether this report is a final report or an interim one depends mainly upon the weather in the future. Many people have rather short memories in relation to droughts in the past, some have vivid recollections of experiences in their younger days, and a few have such active imaginations that their stories about past events have little relation to reality. To put the 1976-77 drought into context, several brief descriptions of previous droughts are included herein. The descriptions plus the references cited should give the reader some perspective on the latest drought. Because there is a wide range in the factors related to climate, hydrology, topography, water use, and legal and economic conditions over the United States, some of the complex interrelations of these factors as they affect the drought or help to describe it are presented in the section, "Drought and its Ramifications." The impacts of the drought on people are presented in a general way because many of them were similar. There was also a similarity in the actions taken by people in response to the drought. Legal actions are quite prevalent if not almost customary in today's civilization. Drought-related activities provided circum- 12 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT Regions with major drought effects Regions with limited drought effects Regions with localized drought effects No drought Water Resources Regions Figure 1. Map of United States showing regions designated by Water Resources Council. stances that affected the well being of individuals or the responsibilities of public entities; therefore, legal actions were instituted that sought redress for losses related to the drought, and hearings were held to discuss interpretations of existing regulations and possible modifications of them. Some specific examples are given in the section, "Legal Aspects." The Water Resources Council has divided the United States into 21 regions (fig. 1) which are major river basins or groups of river basins. Of the 21 regions, 18 were affected by the drought to some degree. The drought had minor effects in regions 05, 06, 12, 13, 15, and 21, and only localized effects in regions 02 and 03. The chronology of the drought and its effects are described in detail for 10 WRC regions, by region, and in general for the other 11 WRC regions later in this report. The regions affected by the drought are identified in figure 1. How people and agencies adapted to the drought and the beginnings of the recovery from the drought are also described. The report concludes with a section, "A Look to the Future." The drought of 1976-77 caught many people in a state of complacency with respect to water supply and water use, but the problems encountered and the plans made to overcome them provided a valuable learning experience. Several plans have been proposed and others are in preparation to create means by which people can cope with a drought in the future in an orderly and less costly manner. Actions taken in response to such a crisis, whether locally or nationally, are not very efficient and often too late. The spring of 1978 is too early to know how many plans will be adopted and implemented, and another drought period will be needed before the success of plans can be evaluated. The information in this report was obtained from numerous Federal, State, county, and municipal agencies, public utilities, the news media, and private individuals. The offices of the U.S. Geological Survey, Water Resources Division, provided data and information.HISTORICAL SETTING 3 HISTORICAL SETTING The 1976-77 drought in the United States affected a larger part of the country more severely than other droughts in the 20th century. Yet archaeologic and tree-ring evidence indicate that, meterologically and hydrologically at least, droughts in the past have been more severe and have lasted longer than the 1976-77 drought. In southern California, tree rings for the past 560 years show dry periods ranging from 6 to 40 years (Troxell, 1957) and periods that are predominantly wet last, on the average, 12.5 years, and those that are predominantly dry last about 14.5 years (Thomas and others, 1963e). An extended drought occurred in the Southwest from about 1276 to 1300. It is considered to be one of the prime reasons leading to the abandonment of the cliff dwellings and other community sites of several Indian groups in New Mexico and Arizona. The more notable droughts in the 20th century are: 1910 drought in the Great Plains 1924-34 drought in California 1930-40 drought in Oklahoma and the Midwest (the Dust Bowl) 1942-56 drought in the Southwest 1952-56 drought in the Midcontinent 1961-67 drought in the Northeastern States In 1910, the Dakotas, Nebraska, Kansas, and Oklahoma were hit by a drought. Precipitation in the Great Plains area was 71 percent of normal (Hoyt, 1938), but the area affected was large. Hoyt ranked the drought of 1910 as the third worst in semiarid States and as the seventh worst in humid States among the 15 worst droughts between 1880 and 1936. The period 1924-34 in California was predominantly dry. This drought acted as a catalyst in the development of the State Water Plan and the Central Valley Project. The drought started abruptly in 1924, which was the driest year on record in the State until 1977. Runoff values for 1924 or for all or parts of the period 1924-34 were used as annual or multiyear criteria for the design of projects that required carry-over storage to provide firm low flows. The "Dust Bowl" that resulted from the drought of 1930-40 in western Oklahoma, particularly in the Panhandle, and in eastern Colorado, and surrounding States is probably the drought that many people think of when a drought is mentioned. Now that the old-timers who lived through those days are not so numerous, the drought of 1976-77 may take its place as a conversation piece. Precipitation in Oklahoma was below normal in 7 of the 11 years, but only 3 of them had less than 85 percent of normal. The cumulative departure from the average for the 11 years was -19.79 in. which is almost the amount of rain that might be expected in 8 months, on the average. Nearly half the Great Plains area had a desert climate in 1934 that produced arid and semiarid conditions in places that are normally subhumid. Seven weather observers in Oklahoma reported no rainfall during a calendar month in the summer of 1934, and rainfall was less than 0.25 in. at about 25 locations in Kansas. The deficient rainfall during the growing season from April through October was bad enough, but the area was the hottest in more than 40 years. High temperature records were set for June or July or for the April-August period. Monthly temperatures ranged from 2.5° to 4.5°F above normal, and new daily high temperatures occurred at many places. Winds were frequent and strong enough to cause wind damage to structures. During the previous decade, rainfall had been above average and sufficient to encourage farmers to plow up and plant land that was marginal without an ample water supply. With the cover of grass removed, the combination of deficient rainfall, the hot summer, and the wind easily converted the area into the "Dust Bowl." The parched soil was blown into drifts that buried what crops managed to sprout and made life miserable for the residents. About 50 million acres was affected at the height of the drought. It was this drought that caused an exodus from the farms in the stricken area and that was the impetus for soil conservation programs and better farming methods. An interesting perspective on the "Dust Bowl" is that though it was certainly dusty, there have been more recent periods of drought in Oklahoma that meterologically and hydrologically were more severe. Precipitation in 1956 in Oklahoma, New Mexico, and Kansas was 35 percent of normal, and the average flow of the Washita River near Durwood,4 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT 95 mi south of Oklahoma City, Okla., for the 15-year period 1958-72 was only 70 percent of that for the 11-year period 1930-40. The improved farming practices and other factors reduced the impact on the public so that the more recent droughts received less notoriety, but losses still ran into billions of dollars. The drought of 1942-56 in the southwestern United States is documented in a series of Geological Survey Professional Papers, Nos. 372A through 372G, by Gatewood or Thomas and other co-authors. The effects of this drought in an arid area fluctuated during the period at many localities, but it was severe enough that 282 counties in three states, mainly in Texas, plus all of Arizona and New Mexico and most of Utah were declared disaster areas. Nace and Pluhowski (1965) reported that the rare occurrence of a succession of drought-producing meteorologic events during 1952-56 caused critical water deficiencies in much of the southern half of the nation. The accumulated precipitation deficiencies during the 5-year drought period, expressed in percentage of the average precipitation for 1 year, ranged from 25 to 225 percent. Low-flow frequency data for eastern Kansas indicate that the drought had a recurrence interval of more than 50 years. Statistical studies of long-term precipitation records for the southern Great Plains indicate that drought of equivalent severity has a recurrence interval of about 140 years in parts of the area. Ground-water levels declined steadily in much of the Midcontinent, and levels were reduced by tens of feet in some places. The decline in water supplies caused considerable financial loss and many personal hardships. The 1961-67 drought in the Northeast was the longest and most severe in the history of the region, and it affected human activities mainly by its impact on water resources related to agriculture and water supplies (Barksdale and others, 1966). The Water Resources Council (1966) estimated that in the Northeast a drought of this duration occurs, on the average, once in about 160 years. Though drought conditions eased at different times in different parts of the Northeast, the drought was not over until the spring of 1967. At least four of the six definitions of droughts suggested by the World Meteorological Organization (see p. 5 ) are applicable to each of the droughts briefly described above. Meteorologically, absolute amounts of precipitation were deficient for specific durations. Climatologically, precipitation, expressed as percentages of normal values, was significantly less than 100 percent. Atmospheric conditions, namely wind, relative humidity, and temperature, were contributing factors in causing the Dust Bowl. Agriculturally, soil moisture was reduced or depleted and the growth of vegetation and crops was hindered. Hydrologically, stream flows and ground-water levels were low and storage in lakes and reservoirs was reduced. In some areas, water-management practices had not progressed enough to provide integrated water-supply systems and surface or subsurface storage to equalize the water supply seasonally or from year to year. The reader must remember that construction equipment used to build surface structures, well-drilling equipment and methods, and pumps were not as advanced 40 or 50 years ago as they are today. Hence, water management's ability to deliver water or develop ground-water supplies during the earlier droughts should not be judged solely by today's standards. Also, farm practices have improved, and hardier corn, wheat, and other crops have been developed since the 1930's; therefore, better production can be achieved now under drought conditions. All these developments and improvements make it easier to adapt to a drought now. Therefore, the perceived severity of a drought also depends on how well adaptations work. In the context of a meteorological drought and using the seasonal (July 1 to June 30) precipitation at San Francisco, Calif., as an example, there have been four periods prior to 1976 when precipitation was below average for several years in succession. These periods are tabulated below along with the average deficiency per season and the total deficiency, as well as similar data for the period 1974-77. Period Number Deficiency of (inches years per year) Total deficiency (inches) 1897-1904 7 3.74 1927-1934 7 5.18 1946-1950 4 5.04 1958-1966 8 5.25 1974-1977 3 9.83 26.20 36.25 20.14 42.03 29.49DROUGHT AND ITS RAMIFICATIONS 5 The average deficiency for 1974-77 is close to twice the rates of the previous droughts listed above, and the total deficiency of 29.49 in. is 81 percent of that from 1927-34 and 70 percent of that from 1958-66; yet it developed in just 3 years rather than in 7 or 8 years. DROUGHT AND ITS RAMIFICATIONS Numerous definitions of drought have been proposed and used by prominent individuals and organizations, yet only the very generalized definitions have much of a relation to the drought of 1976-77. A precise definition is not practical because a drought is the result of many complex factors acting on and interacting with the environment. Among the natural factors are the climate of an area; the antecedent conditions as exemplified by the amounts of soil moisture, rain, and snow; the distribution of rain and snow in time and space; water-table levels during the drought; water quality; and soil types. Human factors include the degree of development of water storage and distribution systems; the number, locations, and depths of wells; the patterns of water use and per capita consumption; the legal aspects relating to property rights, project operating rules, water-quality standards, and service contracts; economic considerations; and many more. Therefore, a definition of a drought must be tailored to the conditions in an area at a given time. Even so, the result is a general definition. A drought may be defined as a condition where and when the water supply is deficient enough and for a long enough time to damage the growth of vegetation, industrial production, or domestic activities (J. S. Crag-wall, Jr., written commun., 1977). Nace and Pluhowski (1965) mention the concept that a drought occurs wherever there is less water than the amount to which people have become accustomed. [Emphasis by writer.] The World Meteorological Organization has defined six types of drought as follows (Subrah-manyam, 1967): 1. Meteorologic drought—defined only in terms of precipitation deficiencies in absolute amounts, for specific durations. 2. Climatological drought—defined in terms of precipitation deficiencies, not in spe- cific quantities but as a ratio to mean or normal values. 3. Atmospheric drought—definitions involve not only precipitation, but possibly temperature, humidity, or wind speed. 4. Agricultural drought—definitions involve principally the soil moisture and plant behavior, perhaps for a specific crop. 5. Hydrologic drought—defined in terms of reduction of streamflows, reduction in lake or reservoir storage, and lowering of ground-water levels. 6. Water-management drought—this classification is included to characterize water deficiencies that may exist because of the failure of water-management practices or facilities such as integrated water-supply systems and surface or subsurface storage to bridge over normal or abnormal dry periods and equalize the water supply through the year. Each type is rather restrictive and by definition describes only one part of a complex whole. Also, the kinds of drought enumerated by the World Meteorological Organization may occur sequentially, or overlap, or be combined in several ways. For example: A small amount of precipitation, a meteorological drought, over a long enough period will become a climatological drought as the precipitation will be a small percentage of normal. In turn, streamflow and storage in reservoirs and underground will be reduced to create a hydrologic drought which brings on problems of distribution and use which is a water-management drought. In humid regions where rainfall normally occurs frequently during the growing season, a drought effect on nonirrigated agriculture can be achieved after only a few days of hot, dry weather. J. C. Hoyt (1938, p. 2) stated: In general, however, in humid and semi-arid states there are no serious drought effects unless the annual precipitation is as low as 85 percent of the mean; that is, unless there is an annual deficiency of 15 percent or more. Hoyt also mentioned that the above criterion has shortcomings because it does not consider the effects of temperature and the6 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT distribution of precipitation nor that some areas can withstand large variations in precipitation whereas other areas cannot. Other hydrologists have found that the criterion of a 15 percent annual deficiency is meaningless if applied to desert areas where the mean annual precipitation does not exceed 5 inches because precipitation in many years is less than 85 percent of the mean and a drought implies a large departure from the norm. In arid or semiarid areas, water deficiencies are chronic. Irrigation is necessary for most agriculture, and metropolitan areas—Los Angeles, San Diego, and San Francisco for example—also depend upon imported water for domestic, municipal, commercial, and industrial supplies. Therefore, drought conditions can be experienced where and when the demand exceeds the supply. Hydrologists use analyses of low-flow frequencies to "define" a hydrological drought on an annual basis. If the actual low flow of a natural stream for a selected number of days has a small probability of occurrence then one can conclude that a drought, in a hydrologic sense, is in progress. Both the number of days and the level of probability considered to be an uncommon event are arbitrary. Low-flow frequency curves have been determined from long-term records for six streams representing different areas affected by the drought in 1976-77. Frequency curves for selected numbers of consecutive days have been plotted in figure 2. Also, the discharges for 1976 and 1977 and the minimums of record are plotted. In figure 2a, the probability that the minimum average discharge for 30 consecutive days in a given year will be less than 26 ft3/s is 0.10 (Point X). Because the recurrence interval, which is the average number of years between events, is the reciprocal of the probability, the flow will be less than 26 ft 3/s at intervals averaging 10 years. In 1977, the 30-day minimum discharge was 13 ft ^s which has a probability of 0.005 or a recurrence interval of 200 years. Because frequency curves are most reliable in the vicinity of the mean, a probability of 0.5, probabilities determined near the extremes should be considered only as reasonable estimates, not exact probabilities. The low-flow frequency curves and the flows in 1976 and 1977 at the six locations are not directly comparable. Differences may be caused by chance, by different basin characteristics, by different temperatures and precipitation at various times, or by combinations of these factors. However, several interesting conclusions can be made. 1. The North Fork American River at North Fork Dam, 31 mi northeast of Sacramento, Calif., and the Beaver River near Beaver, 48 mi northeast of Cedar City, Utah had record minimum flows in 1977 at all numbers of consecutive days selected. Probabilities were roughly 0.01 to 0.005, or the recurrence intervals are in the 100 to 200 year range. 2. Flows of the Sturgeon River near Sidnaw, 90 mi northwest of Escanaba, Mich., for periods up to 183 days had probabilities equivalent to recurrence intervals greater than 200 years. The probabilities are from long extrapolations of the frequency curves and should be considered only as estimates. That the very low flows of the Sturgeon River are extremely rare events can be substantiated by the combination of two natural events. The drought in 1976 reduced the summer and fall flows to a new record low just slightly more than half the previous low flow of record. Then the extremely cold winter struck the area, and flows remained low until the latter half of March 1977. Thus the lowest 183-day period started on July 23, 1976 and ended on January 21, 1977. 3. Flows of the other three streams in other years were significantly less than in 1976 or 1977 though none of them had probabilities less than 0.012, about an 80-year recurrence interval. 4. Record low flow for the entire year was the damaging feature related to the drought. Records for low annual flows were set at four of the six streams, and the recurrence intervals are in the 100- to 170-year range. Whipple (1966) derived relations between the duration of a drought, in years, versus the probability of occurrence for five streams from Illinois to New Mexico and for three streams in Massachusetts. The relation curves are nearly alike for durations of 4 years or longer. On the basis of this small sample, the probability of a drought lasting 4 years isDISCHARGE,IN CUBIC FEET PER SECOND DISCHARGE,IN CUBIC FEET PER SECOND DISCHARGE,IN CUBIC FEET PER SECOND DROUGHT AND ITS RAMIFICATIONS 7 Figure 2. Low-flow frequency curves for six streams in areas affected by the drought8 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT about 0.02 and that for one lasting 6 years is about 0.01. The probability of a 2-year drought is in the range from 0.03 to 0.05. A factor related to the probabilities of low flows is the duration of low runoff. The number of months that streamflow was deficient is shown in figures 3a, 3b, and 3c during 12-, 18-, and 23-month periods starting in February 1976. Monthly streamflow is deficient when it is within the lowest 25 percent of record for the month. See figure 10 as an example. A month or two of deficient streamflow, or even 3 or 4 months in some areas, will not be a very serious hydrologic drought, particularly if the months are not consecutive. The number of months of deficient stream-flow does not indicate how many of them were consecutive nor when they occurred; but a hydrologic drought is evident because when 8 to 11 months out of 12 or 16 to 22 of 23 months are deficient, many of the months are consecutive and occur during the growing Figure 3. Maps of the United States showingDROUGHT AND ITS RAMIFICATIONS 9 LIXPLANATION Months 16-18 EXPLANATION Months number of months of deficient streamflow.10 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT season. These conditions are depicted in a general way by the patterns in figure 3. Another way to recognize a drought is to compare ground-water levels and rates of decline with historic records. When precipitation is below normal, natural recharge is less, and ground-water levels may drop. This response is usually slow; that is, 2 or more years may pass before the effect of reduced recharge is evident in a given well. The major problem in trying to quantify this effect is that the water level in the vast majority of wells is affected by pumping in the vicinity, and any response to a change in recharge is masked by the fluctuations caused by the pumping. One observation well that registers the water-table level in the Snake River basin about 130 mi east of Boise, Idaho is not affected by pumping or return flows from irrigation. The hydrograph for this well is plotted in figure 4, and the decline of the water surface because of reduced natural recharge is clearly shown. The normal rise of 1.3 to 1.7 ft usually between October and May was only 0.45 ft in the winter of 1976-77. And the annual decline, which had ranged between 1.0 and 2.2 ft, was 2.9 ft in 1977. If ground-water use increases to offset deficiencies in the surface-water supply, the rate of decline accelerates, and shallower wells may go dry. The magnitude of the effect of increased pumping is illustrated in figure 5 for a well in the Central Valley of California. The seasonal fluctuations from pumping are Figure 4. Water level decline caused by reduced recharge during the drought. Well is in the Snake River basin 130 miles east of Boise, Idaho. superimposed on a rising trend that started in 1968 when water was first imported to the area and pumping was reduced. The water level prior to the importation reached an all time low of 618 ft below land surface in 1967. Water-level records on this well have been obtained since 1960 when the level was 495 ft below land surface. The decline of 123 ft was caused by the overdraft during the 7 years, 1961-67. In the ensuing 9 years the water level rose 266 ft, but in only 8 months in 1977 the increased pumping lowered the water level 165 ft which is 62 percent of the rise in just 7 percent of the time or 1.3 times the decline in the 7 years prior to the importation of water. Still another indication of drought conditions is a deterioration of water quality. Both surface water and ground water are subject to changes in quality when new stresses are applied to a water system during a drought, and in most cases the change is for the worse. Reduced freshwater flow into an estuary usually allows the saltwater to encroach farther than normal into the estuary or even into the tidal reaches of the tributary streams. The Sacramento-San Joaquin Delta in California was one of the areas that had problems because of saltwater encroachment. In some coastal areas, freshwater is injected into wells near the ocean to establish a hydraulic gradient that will prevent saltwater intrusion into the aquifer. During a drought, there may not be enough freshwater available to maintain the freshwater-saltwater interface at an acceptable position, and salinity problems might develop. Lower streamflows during a drought usually mean higher water temperatures with adverse effects on fish and greater aquatic growths that cause increased eutrophication of water bodies. Special releases and spills at dams on the Columbia River were made in 1977 to help preserve the fishery resource during the migration period. The National Oceanic and Atmospheric Administration, Environmental Data Service, uses a drought index developed by W. C. Palmer (1965) to classify drought severity. Briefly, the method is directed at a quantitative assessment of periods of prolonged meteorological anomalies by considering antecedent precipitation, the precipitation during a given period, and the duration and magnitude of the abnormal moisture deficiency. The difference280 DROUGHT AND ITS RAMIFICATIONS 11 ±33d ni '30Vdans onvi MO!3a a3±VA/\ Ol Hld3Q cd o o a CO CD s- CO (D £ -C -4-* 3 O CO CO 0 LO CO £ (D CD fc* 0 cd 3 4-J c cd O t- cd (D C £ CO 1 LO CD £ fcs- tr- l o CO o* 0 > 0 fc- 0 4-» cd £ «+-H o c o +-> cd 3 +-> 0 3 r-H Ph “ +1 0 m 1 rn 1 1 i— Arroyo Seco near Pasadena, Calif + 100 0 rn— 1 1—i 1 r~r~ North Fork American River at -1 N. The maximum accumulated deficiency for the 26 \ month period, October 1975 to November 1977, o z D -100 - \ North Fork Dam, Calif The maximum accumulated deficiency for the 27 month -2 \ is equivalent to the normal runoff for 1.6 years. \ Runoff was below normal for 24 of the 26 months ” is -200 \ period, October 1975 to December 1977, is equiv-N alent to the normal runoff for 1.7 years. Runoff -3 \ CC QJ O u. -300 - \ was below normal for 26 consecutive months -4 \ ° < cc < -400 - -5 / V UJ O cc (ft -500 -6 c \ D o 1- z cc < -600 \ -7 r \ 2 \ a. D ss O *- -700 \ -8 E \ uj z 1 < -800 -9 3 -900 \ -10 -11 Q. \ . O \ / - t— / - I 1 1 1 1 1 l—l— O o < -1000 -1100 i i 1 1 1 1 1 1 ft d c O OJ CD c d c Q> CD C o c 0) CD 5 ^ c d c 0) c d c 0) CD O Q -> 1975 1976 O “» 3 1977 O -> O Q -> 1975 3 1976 Q " 3 1977 Q “> Figure 26. Trends in streamflow at selected sites in California, October 1975 to January 1978. cumulated departure from normal runoff, in thousands of acre-feet, by months. A downward slope of the line for any period indicates deficient flow in that period. The 26- or 27- month period was based upon the month, November or December 1977, when the maximum deficiency occurred. This is the lowest point on each graph.66 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT The largest accumulated deficiencies were equivalent to the normal runoff for periods ranging from 1.1 to 2.0 years. The shortest period is for the Salmon River at Somes Bar, a tributary of the Klamath River on the north coast, and reflects a smaller effect in 1976. This is compatible with the nondrought conditions in the Pacific Northwest in 1976. The longest period is for the Napa River near St. Helena, a northern tributary to San Francisco Bay, where the Palmer index indicated the most severe drought conditions in the State. See figures 6 and 7. Except for the Salmon River where the accumulated deficiency in 1976 was 24 percent of that for the 2-year period, the accumulated deficiencies at the other five sites were almost equally divided between the two years. This relation does not hold for regulated streams where releases from storage were used to augment the flow, and the graphs in figure 26 should not be interpreted to indicate that the graph must rise to zero deficiency before the basin is back to normal. The very poor snowpack in 1977 was bad enough by itself, but its damaging effects were compounded because it was the seond year in a row with a snowpack having a water content much below normal. This sequence was the reason that the water withdrawn from reservoirs in 1975 and 1976 to meet agricultural and other demands was not replaced in 1976 or 1977; therefore, reservoir levels reached all-time lows in the fall of 1977. The April-July 1976 runoff into major reservoirs between the American and San Joaquin Rivers was either the lowest of record or nearly so. Storage in six reservoirs in the Central Valley was depleted to dead storage levels by September 30, 1976. The total capacity of 79 reservoirs in the Central Valley is 27.0 million acre-ft, but on September 30, 1976 there was only 9.8 million acre-ft in storage or 58 percent of average. Comparable figures for 74 other reservoirs in California are 7.4 million acre-ft total capacity, 3.8 million in storage or 84 percent of average. The depleted condition of water in storage in two reservoirs is illustrated in figure 27. Figure 27a shows the intake tower at Pardee Reservoir near Valley Springs, on the Moke-lumne River southeast of Sacramento, when it was out of water on March 26, 1977. The contents was 47,300 acre-ft, only 23 percent of capacity, and the water level was 112.3 ft below the spillway elevation which is just below the walkway to the tower. The old U.S. Highway 99 bridge across the Sacramento River is shown in figure 27b. The bridge was submerged during the filling of Shasta Lake three decades ago, and it reappeared for the first time since then in 1977. The picture was taken on September 5, 1977 when the contents was 572,900 acre-ft or 13 percent of capacity and when the water surface was 229 ft below the top of the gates on the spillway of Shasta Dam. Several good storms brought precipitation to northern California in September and November 1977; but even so, on December 1, 1977 the Central Valley reservoirs contained only 5.8 million acre-ft which is just 22 percent of the total capacity and 38 percent of the average for that date. Storage in the other major reservoirs in the State dropped to 1.8 million acre-ft which is 24 percent of the total capacity and 43 percent of average for December 1. Ground-water Conditions Ground-water levels in a large part of California were lower in the spring of 1976 than they were in the spring of 1975, and, in some places, they were even lower than those in the fall of 1975. A few wells located where water levels remained nearly the same had lower yields in 1976 than in 1975, and a few wells less than 50 feet deep went dry. Declines in water levels from the spring of 1975 to the spring of 1976 were mainly in the 1- to 6-foot range; but wells in Yolo County west of Sacramento averaged about 7 ft lower, the Madera, Tulare, and lower Kaweah and Tule River areas of the San Joaquin Valley reported declines of 7 to 8 feet, and there were declines of 23 ft in the Shafter-Wasco area northwest of Bakersfield, and 25 ft near Chowchilla northwest of Madera. The average decline in Santa Clara County at the south end of San Francisco Bay was 15 ft, but wells in the southern part of the county dropped as much as 30 ft (California Department of Water Resources, 1976). Water levels declined an additional 15 ft by August 1976 in the northern Sacramento Valley, but they recovered about 6 ft when rain fell in the late summer and pumping wasCHRONOLOGY OF THE 1976-77 DROUGHT 67 reduced. All time record low levels were reached in the lower Sacramento Valley where wells are the major source of water. In Yolo County, declines were in the 10- to 20-ft range. Outside the Sacramento Valley, water levels in northern California were near normal. The depletion of ground-water storage in the San Joaquin Valley during 1976 has been estimated as between 3 and 3.5 million acre-ft. This is about two to three times the normal annual extraction of 1.2 million acre-ft. The additional pumping in California during 1977 increased the overdraft to 6.5 million acre-ft and caused an average lowering of ground-water levels of 13 to 20 ft. One consequence of the heavier pumping was that land subsidence resumed in the San Joaquin Valley after a 2- to 3-year cessation. Figure 28 shows the ground-water level changes from 1975 to 1977. Because diversions of Sacramento River water were restricted in the summer of 1977, irrigators drilled new wells; and some of these were close enough to the river that they probably drew water from the river. The relative amounts drawn from the river and from the ground-water body have not been determined (California Department of Water Resources, 1978). The drought affected runoff in the Owens Valley area in the eastern Sierras where the intake to the Owens Valley aqueduct of the city of Los Angeles is located. Los Angeles sought permission to about double its pumping from the Owens Valley ground-water basin, but pumping was limited by a court injunction until the effectiveness of the water rationing program was demonstrated. This was done, and increased pumping from deep wells started on August 1, 1977 and provided an additional 10,000 acre-ft per month for 8 months. The decreased use of surface water was partly offset by increased use of ground water. The cooperative way that water managers operated ground-water basins throughout southern California was beneficial to all of southern California as well as the entire State. However, the managers are now concerned over significantly lower water levels, many of them being at all-time lows. The 2-year overdraft is estimated as 500,000 acre-ft (E. L. Griffith, 1978). The number of reports received by the State of wells put into operation was 8,300 in 1975, 11,200 in 1976, and 20,000 in 1977. Because all wells drilled are not reported, the total for 1977 has been estimated as 28,000 wells. Water Quality The reduced inflow into the Sacramento-San Joaquin River Delta allowed the saltwaterHYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT Figure 28. Ground-water level changes, 1975-77, in feet. Adapted from California Department of Water Resources (1977c).CHRONOLOGY OF THE 1976-77 DROUGHT 69 of the San Francisco Bay system to encroach farther upstream than usual. The point of maximum encroachment is defined as the point farthest upstream where the chloride concentration reaches 1,000 mg/L (milligrams per liter). Water of this salinity, when used for irrigation over a considerable period of time, will severely inhibit the yield of most crops. Comparative chloride concentrations are 19,000 mg/L for sea water and near 7 mg/L for Sacramento River water. Figure 29 is a map of the Delta showing the maximum intrusion of salinity for selected years, including 1976 and 1977. The encroachment shown for 1931 and 1939, two dry years, occurred prior to the completion of Shasta Dam in 1944. Since then, fresh water has been released from Shasta Lake to repel the salinity. In 1952 and 1958, heavy flood runoff occurred, and 1966 was a dry year, 75 percent of average runoff, before the completion of Oroville Dam in 1968. Runoff in 1975 was 113 percent of average. Runoff into the Delta dropped to 60 percent of average in 1976 and to 28 percent of average in 1977. The salinity intrusion advanced about 7 mi in 1976 from the position in 1975 and about 4 mi more in 1977. To halt the saltwater encroachment in parts of the Delta, temporary earthfill barriers were erected in 1977 across several channels in the Delta. A pumping plant was built to provide higher quality water to the Contra Costa Canal, and several diversions were changed on an interim basis to points upstream to tap better quality water for use within the Delta. Operations of the upstream reservoirs of the State Water Project and the Federal Central Valley Project are coordinated to provide water in the Delta for local consumptive use, for exports by the State and Federal projects, and to maintain water-quality standards set by the State Water Resources Control Board. Twice during 1977, once on February 8 and again on June 2, the water-quality standards were modified so that less water was required for Delta outflow, thus conserving the short supplies upstream so that they would be available later in the season to protect the water quality in the Delta. Seawater intrusion raised the chloride concentrations in wells near the mouth of the Pajaro River near Watsonville, Calif., about 70 mi south of San Francisco. The lack of recharge and the increased pumping during the drought caused water levels to decline an average of 10 ft between November 1975 and November 1977. Under normal conditions, the water levels rise during the winter months to elevations above sea level; but throughout the drought, the water levels in most wells remained below sea level. This adverse gradient was not sufficient to cause the intrusion to advance any farther inland than approximately 1 mi which was also observed in 1974, but the increases in chloride concentrations were dramatic. In a well perforated in the 100- to 200-ft interval, the chloride concentration rose from 114 mg/L in November 1975 to 571 mg/L in November 1977. The maximum increase in a well tapping the aquifer at the 300- to 600-ft level was 100 mg/L to a high of 229 mg/L. The use in gardens, etc., of "grey water" from bathing, dishwashing, and laundering was approved in some localities and banned in others as detrimental to public health. The State Water Quality Control Board imposed restrictions in April 1977 on several communities discharging sewage effluent into the Russian River because most of the fresh water released from storage near the headwaters was diverted and the waste discharged into the river exceeded the controlled flow in the downstream reaches. In southern California, reclaimed water has been used for irrigation and for recharge to aquifers, but the use of less fresh water increased the salinity of effluents enough to make it less desirable or even unusuable for recycling. High water temperatures associated with low flows in September 1977 caused a fish kill in the Trinity River below Clair En^le Lake. The water temperature rose almost 9^ to 70°F which is warm for trout. Land Subsidence Subsidence is a problem in the San Joaquin and Santa Clara Valleys. It is directly related to hydraulic stresses induced by ground-water pumping and the resulting compaction of water-bearing deposits. After three decades of pumping overdraft and water-level declines, the widespread subsidence in the San Joaquin Valley was halted or significantly reduced after 1968 when imported canal water replaced ground-water pumpage. The drought of70 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT 122° 00' 121°45' 121°30' 121°1 5' Figure 29. Map of Sacramento-San Joaquin Delta showing annual maximum intrusion of salinity for selected years. Adapted from Bureau of Reclamation.CHRONOLOGY OF THE 1976-77 DROUGHT 71 1976-77 reversed the trend because the increased use of ground water to meet the demands for water initiated another period of pumping overdraft and the resumption of subsidence (Lofgren, 1977). Subsidence causes significant changes in storage characteristics of an aquifer system during the first cycle of water-level decline. Pore water is squeezed from the water-bearing deposits, and permanent compaction occurs in the fine-grained beds. Therefore, the pore space available for storage is reduced, and pumping rates that occurred in 1977 will lower the water levels much faster, possibly 10 to 20 times faster, than the same rate of pumping would have caused during the first cycle that ended in the late 1960's. Hydroelectric Power Generation Hydroelectric power generated in northern and central California is an integrated operation of 105 hydroelectric plants. Virtually every plant was adversely affected by the drought of 1976-77. Only a few plants were actually shut down for lack of water, but at other plants the hours of operation were reduced and the capability of the generators at variable head plants decreased because of low heads. The significant result was that hydroelectric generation in 1977 was 10 billion kilowatt-hours; whereas, the average output is 24 billion kilowatt-hours. The deficit of 14 billion kilowatt-hours was directly related to the drought, and the replacement of most of the deficit by purchases from other electric utilities and additional use of steam generating plants increased production costs to one utility by $326 million (E. F. Kaprielian, written commun., 1978). The major hydroelectric development in the southern Central Valley is in the upper San Joaquin River basin. Hydroelectric power generation by this project averages 3.5 billion kilowatt-hours; but in the 1976 water year, output was only 2.5 billion kilowatt-hours and in 1977 it dropped to 1.1 billion kilowatt-hours. The snow and rain that fell on the watersheds in 1976 was not enough to produce the 2.5 billion kilowatt-hours; therefore, heavy demands were made on water in storage which was not replaced during the winter of 1976-77 when precipitation was very low. The system still met peak demands for short periods only because special operating schedules were developed by careful planning. The cost to replace the 2-year deficit of 3.4 billion kilowatt-hours was $85 million. Hydroelectric power generation at several small plants on streams without storage is usually reduced when the snowmelt runoff ceases. In both 1976 and 1977 power generation had to be curtailed earlier than usual. Forests The drought conditions caused the fire season to start early. The larger fires of the 13,300 fires that occurred in California in 1976 were in June and July. Forest fires usually burn about 25 percent of the amount burned annually by July 15; but 85 percent of the 194.000 acres burned in 1976 had been burned by then. In September 1977, the California Department of Forestry estimated damage to timber and watersheds by recent fires at $250 million. About 1,850 fires occurred in California in August 1977, burning in excess of 410.000 acres, and the 11,900 fires in 1977 burned 449,000 acres. The loss of trees because of the two-year drought was tremendous. About six million trees that could be used for commercial timber died from drought-induced afflictions. This loss has been estimated to be between 2.5 and 3.8 billion board feet of marketable lumber. The fire that started in the hills near Santa Barbara, Calif, spread into areas with expensive homes and caused millions of dollars of damage. Activities Resulting from the Drought The Director of the California Department of Water Resources (Robie, 1978) reported that more than 100 communities in California adopted some form of mandatory rationing during 1977. Allowances were as low as 50 gal/d per person. Statewide urban consumption was 20 percent less than that in 1976, but communities on the Monterey Peninsula and in the Marin Municipal Water District reduced water use by 49 and 53 percent, respectively, during the first 9 months of 1977 compared to the same period in 1976. The reduction in water use in 1977 was slightly more than 400.000 acre-ft which exceeds the projected72 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT annual yield of the Auburn Dam project, a major water development in the Sierras east of Sacramento. Water exchanges were arranged by the Department of Water Resources to distribute the available water supplies more equitably. The Metropolitan Water District of southern California reduced its demands on the State Water Project by 400,000 acre-ft and used additional water from the Colorado River. The water that would have gone to southern California was delivered to agricultural users in the San Joaquin Valley and to urban users in the San Francisco Bay area. This exchange made it possible for farmers served by the State Water Project to receive 91 percent of their 1977 entitlement instead of about the 40 percent they would have received without the exchange. The Metropolitan Water District normally supplies about 6 percent of Los Angeles' needs but supplied about 25 percent in 1977. The District offered an incentive rebate to member agencies of $20 for each acre-foot of water conserved in 1977 below 90 percent of their 1976 use of surface supplies. None of the 27 agencies failed to meet its new allotment. Actually, an additional saving of 180,000 acre-ft was accomplished at a cost to the district of over $3 million. Farmers pumped ground water into the California Aqueduct where its quality was compatible with the canal water. The pumping took place during periods of low water use by the farmers, and the water was stored in the canal until it was needed. The Department of Water Resources, the Department of Fish and Game, and the owners of several duck clubs in the Suisun Marsh, between San Francisco Bay and the Delta, shared the cost of bringing higher quality water into a local wildlife management area. Soil salinity was reduced; therefore, the production of waterfowl food was improved. The State Water Resources Control Board directed large water suppliers to maintain reserves in upstream storage facilities to protect against another dry year in 1978. Forty-seven of California's 58 counties and 4 counties in Oregon were declared disaster areas, and anyone who suffered losses because of the drought was eligible for financial assistance. Also, Governor Edmund G. Brown, Jr. created the Governor's Drought Emergency Task Force on March 4, 1977. The task force was responsible for the direction and coordination of State efforts to combat the drought and for the education of citizens regarding the nature and extent of the drought. The activities mentioned herein are only a few of the many activities that occurred in California, but they are types that illustrate that when a crisis develops, agencies and individuals can cooperate to alleviate adverse conditions to everyone's mutual benefit. Hawaii—WRC Region 20 There are eight islands in the State of Hawaii, but information in this report is related to the five major islands, Kauai, Oahu, Molokai, Maui, and Hawaii (fig. 30). The topography of the islands causes an extreme range in precipitation from about 13.5 in. annually near sea level on Maui to 486 in. at Mount Waialeale in the center of Kauai, one of the world's wettest spots. With this wide a range, the departure from average at some locations can be more than the total annual precipitation at other sites. Also, the average precipitation is markedly different over short distances as the greatest distance from the coast to the center of an island is only 42 mi on the island of Hawaii. Under these conditions, the existence of a drought is difficult to determine. Previous Droughts A drought started to develop in the winter of 1951-52 when only one soaking rain occur- Figure 30. Hawaii—WRC Region 20.CHRONOLOGY OF THE 1976-77 DROUGHT 73 red. Rains carried by the trade winds provided adequate water between February and June 1952 except on Molokai where precipitation was less than half of average. The late summer was dry and so was the early winter. Pastures on Molokai and Maui were brown, and truck crops wilted. Some wells ran dry or became brackish, and drinking water was being trucked from higher elevations to the Kona area, on the west coast of Hawaii. January 1953 was very dry with Maui receiving only 10 percent of normal rainfall. Ground water was pumped into the elaborate ditch system on Maui to provide flows near a quarter of its capacity. Thunderstorms in mid-February brought some relief, but the drought continued throughout the summer of 1953. At Waipahu, Oahu, near Pearl Harbor west of Honolulu, monthly rainfall was well below normal for 18 of the 21 months from January 1952 through September 1953. This record indicates the lack of the usual number of or substantial Kona storms which are the widespread storms that usually occur during the winter. The deficient rainfall from the trade winds during the rest of the year is indicated by the record at Luakaha in the mountains back of Honolulu where rainfall was below normal in 12 of the same 21 months and in 6 of the last 7 months. Rains in October and November 1953 ended the drought. Previous record low rainfall had been in 1921 on Kauai, in 1933 on Oahu, and in 1917 on Maui and Hawaii. What was called a dry spell by some and a drought by others occurred in the summer of 1971. Maui was particularly hit hard, and less severe conditions existed on Hawaii, Molokai, Oahu, and Kauai. The major cause was the absence of normal rainfall from the trade winds that bring moisture to the higher elevations on the windward sides of the islands. There are no ground-water sources on East Maui to supplement a shortage in the streams; therefore, when the number of rainy days and the amount of rainfall were reduced significantly, drought conditions existed. Irrigation and domestic uses were sharply curtailed on Maui and Hawaii, water was trucked to families on Molokai and to irrigate macadamia nuts on Hawaii, temporary pumps were installed to raise water from a low elevation reservoir into a higher level ditch system on Maui, and planting of sugar cane was delayed until sig- nificant rains occurred. Rains ended the drought in September 1971. The May through August 1971 precipitation at Waikamoi Dam on East Maui was the lowest since records began in 1911 and was 30 percent of normal as compared to 40 percent in 1953. The 4-month rainfalls at other Maui sites were in the range of 19 to 52 percent of normal. Precipitation and Runoff Precipitation for 1975 at the four index stations on the four larger islands ranged from 75 to 107 percent of normal. The below normal trend started about March 1975 with the bulk of the deficiency occurring during the summer and fall. Even the 107 percent of normal at Honolulu was caused primarily by almost four times the normal rainfall in November whereas 10 of 12 months in 1975 were below normal. Rainfall was below normal during 11 of the 12 months in 1976 at Honolulu, Oahu and Kahului, Maui. Only February was above normal. Precipitation was deficient for 9 of the 12 months at Hilo, Hawaii and for 8 of them at Lihue, Kauai. At Honolulu, only 15 percent of the normal rainfall occurred during the period September through December 1976, and December with only 0.06 in. was the driest December in the 100-year period of record. Below normal precipitation continued into March 1977, then above normal precipitation in April on all the islands and in May and June on Kauai and Oahu relieved the drought conditions to some extent. But the pattern of below normal rainfall was reestablished in July and continued through December. Runoff during the last 8 months of 1975 was below normal on the islands of Hawaii and Kauai and was normal for most of that time on Maui and Oahu. Deficient runoff conditions continued into 1976 and became serious enough by August 1976 to be considered a drought which persisted to March 1977. Runoff increased in April and in May on Kauai and Oahu in response to the above normal precipitation, and was again generally deficient from July through December 1977. Ground-water Conditions Water levels in five observation wells declined to record low levels. Three of the mini-74 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT mum levels were reached in March 1977 on Maui and two in late August or early September 1977 on Oahu. Both basal water tables and a high elevation aquifer are represented in this sample. The record on the latter started in 1936. Though water levels were very low, no significant reductions in water yield occurred. There was no significant increase in the number of wells drilled in 1976 or 1977. Water Quality The chemical quality of surface waters did not change significantly during the drought. Dissolved-solids concentration increased slightly, but sediment concentrations and discharges decreased. The sediment discharge during the 1977 water year of Waikele Stream into Pearl Harbor was one-ninth that of the previous year. Though flows were less, stream water temperatures were near seasonal normals. Activities Resulting from the Drought The below normal runoff produced the most serious problems in the areas that have no development of ground-water resources such as East Maui. The areas that rely on surface-water supplies and limited water storage facilities were hard pressed to meet the demand. The Kona area on the island of Hawaii was declared in a state of drought emergency in January 1977. Water was hauled to families in the area, lawn watering and car washing were prohibited, macadamia nut growers were asked to reduce the amount of water used for irrigation, and cattle ranchers had less water for their livestock. Similar constraints were imposed on most of Maui where residents were asked to reduce their water consumption by 30 percent, and the truck farmers were limited to two irrigations per week. The least effect was on Kauai. No state of emergency was declared, but sugar cane growers hoped for rain to supplement the diminished flows in the irrigation systems. On Oahu, requests were made to cut water use by 10 percent for residential use, by 15 percent for commercial and agricultural uses, and by 50 percent on parks and golf courses. Though no drought emergency was declared in 1977, the Honolulu Board of Water Supply proposed rules and regulations to initiate manda- tory water conservation measures in the summer of 1978, if necessary. Tlie Eleven Other WRC Regions Seven of the other eleven WRC regions stretch from Maine to California, along the Atlantic and Gulf coasts, and along the Mexican border. Two are interior regions, the Ohio and the Tennessee regions, and the other two regions are Alaska and Puerto Rico. There was no or virtually no drought in three regions; New England, Region 01; Lower Mississippi, Region 08; and Alaska, Region 19. In six regions, Ohio, Tennessee, Texas-Gulf, Rio Grande, Lower Colorado, and Caribbean, respectively Regions 05, 06, 12, 13, 15, and 21, the effects of the drought were relatively minor—either in localized areas or for short durations similar to other dry years. The Mid-Atlantic, Region 02 and the South Atlantic-Gulf, Region 03, had more serious drought problems in small parts of the regions. Brief descriptions of the conditions in Regions 02 and 03 and a few highlights from the other Regions are presented in this section. Previous droughts have been described by many writers, among them Barksdale and others (1966), Benson and Gardner (1974), Gatewood and others (1964), Hoyt (1936, 1938), Nace and Pluhowski (1965), Thomas and others (1963a-d), and Water Resources Council (1966). Palmer and Denny (1971) have compiled a bibliography of other drought reports. Precipitation and Runoff Four months of deficient rainfall from April to July 1977 was the primary cause of drought conditions in South Carolina. Runoff during the previous winter was near normal, but the dry period started about 2 months earlier than usual, thus causing adverse impacts on agriculture. Rain in early August prevented a severe drought from developing. Deficient rainfall in Georgia and the longer than usual intervals between rains, some as long as 8 weeks, caused farmers to delay their planting of crops because moisture is needed to activate the chemicals in pesticides and weed control agents. Monthly rainfall in April and May in southwest Georgia usually exceeds 4 in.; but rainfall in April 1977 was only 1.29 in., and during the first 19 days of May it was only 0.33 in.CHRONOLOGY OF THE 1976-77 DROUGHT 75 Streamflows in Maryland and Delaware were below normal during the summer of 1977 and generally were the lowest since 1970, but they were well above the minimums of record, many of which occurred during the summer of 1966. The exception was the Choptank River near Greensboro, Md., 65 mi east of Washington, D.C. where new monthly minimum flows occurred in May, June, and July 1977, and the flows were in the deficient range for 7 months from February to August 1977. There was much publicity and concern in the Washington, D.C. area when the flow of the Potomac River near there dropped to 40 percent of normal during May and June 1977. However, enough rain fell in the basin during the summer to maintain the flow at more than three times the diversion rate for municipal supplies. The drought in Virginia was the most severe in the north, central and eastern parts of the State in 1977. The September runoff of the Rapidan River near Culpeper, about 65 mi southwest of Washington, D.C., was the sixth lowest flow for September since records began in 1930. The lowest September flow occurred in 1954 when it was a fourth of that in 1977. In North Carolina the drought was not continuous, but the eastern half of the State was affected in both 1976 and 1977. Streamflow was below normal from March through September 1976 and was near the minimum of the last 50 years. Deficient streamflow occurred again from April through mid-August 1977. Flows of some streams in South Carolina in July 1977 receded to the level where the 7-day average flow reached recurrence intervals of 2 to 6 years. Runoff and precipitation in New Mexico are not very high in normal times, but the below normal precipitation and runoff reduced the storage in major reservoirs that provide water for irrigation to less than 50 percent of average in December 1977. Ground-water Conditions In several States, some wells went dry or did not yield enough water, but the main problems were related to the wells rather than the ground-water supply. Wells were not deep enough, or not in a very good location, or had not been developed correctly after they were drilled. Water levels in North Carolina were generally below normal from June through September 1976 and about the same period in 1977. However, no serious ground-water deficiencies developed. The use of ground water for irrigation in Georgia is a "whole new use" of that resource according to the State Geologist. Some shallow wells in southern Georgia went dry when overpumped. Therefore, the Governor ordered a survey of ground-water consumption as an early step toward the development of a coordinated water and land use policy. There was also concern that the large number of new wells drilled and the increased withdrawals might overtax the ground-water resource. Water Quality The severe winter of 1976-77 in the eastern United States coupled with below normal runoff caused an increase in the ice cover on streams and lakes—both in thickness and areal extent. Fish kills occurred in some ice covered streams and stock ponds. These factors also affected the flow of the lower Mississippi River and the position of the freshwater-saltwater interface which is where the chloride concentration reaches 5,000 mg/L. At Baton Rouge, La., the discharge dropped to 175,000 ft Vs in October 1976, but this was 100,000 ft Vs more than the minimum flow of record in 1939. On October 6, 1976, the leading edge of saltwater had intruded 57 mi up the Mississippi River to Myrtle Grove, but increased flows pushed the leading edge downstream 24.5 mi by October 20 and 46 mi by November 12. With flows under 200,000 ftfys starting on November 20, 1976, the leading edge moved back upstream 14 mi by mid-December and reached a point 47 mi above Head of Passes by mid-August 1977. It receded to river mile 11 again by September 28, 1977. The penetration of saltwater as far as Myrtle Grove has a mean recurrence interval between 5 and 10 years. Communities along the lower reach of the Mississippi River that use river water for their supply had to import freshwater to dilute their supply to acceptable levels. Activities Resulting from the Drought Crop damage in Virginia was high, and municipal water supplies were dangerously76 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT low. Restrictions on water use were in effect during the late summer of 1977 mainly in northern Virginia. In the summer of 1977, the drought severely stressed the water-storage facilities of a few municipalities in eastern North Carolina where the facilities were known to be marginal, before the drought. Restrictions on water use were imposed and the water shortage at Chapel Hill in north-central North Carolina was severe. The city of Raleigh completed a 3.4 billion gallon auxiliary reservoir late in 1976 which filled during the winter and prevented a recurrence of the water shortages of 1976. Orange County which includes Chapel Hill formed a water and sewer authority to plan long-term solutions to their water supply problems. In the meantime, they planned on auxiliary supplies from new wells and possibly a pipeline from the Haw River. The North Carolina Department of Natural Resources and Community Development evaluated the adequacy of water supplies statewide. Farmers in Georgia who raised soybeans, peanuts, and other crops became interested in irrigation for the first time in more than 20 years because rainfall was deficient and there were intervals between rains extending to 8 weeks. The farmers who suffered financial setbacks in 1977 because of the drought planned significant capital outlays for irrigation systems to prevent a similar setback in the future. Pecan, corn, peanut, and soybean production was affected by the drought more than the tobacco and cotton crops. Hay was cut early in 1977, but none later in the season because the pastures burned up for lack of rain. Army worms infested corn in Georgia and Alabama. Some water for irrigation was obtained by draining farm fish ponds, but this impaired the raising of fish for food. One farmer attended so many fish fries that he lost his appetite for fish! The drought in Puerto Rico started in March 1977 and ended in October 1977, but it was not much different than the drought in 1974. Water rationing began on April 14 on a voluntary basis, became mandatory on June 17, ended in some areas in August and the rest in October. Rationing was accomplished in a different manner; namely, by the public utility shutting off all supplies between 9 p.m. and 6 a.m. except to vital service units. The local people adjusted to drought conditions since they have had to in the past, but the tourists had problems. Cattlemen's costs rose when they had to buy feed for their cattle because the pastures dried up. The Virgin Island's main water supply is desalinized water; therefore, the impact of the drought was felt mostly in the rural areas. Pastures dried out, and cattlemen had to buy feed for their stock. Farmers in the Middle Rio Grande Valley in New Mexico received water from the Colorado River basin through the San Juan-Chama Project, thus averting serious effects from the deficient precipitation. Several municipalities instituted mandatory water conservation practices and limited water use. Many counties in most of the States were declared disaster areas. Epilogue The drought of 1976-77 may be over or it may be only interrupted. Precipitation between September 1977 and January 1978 was above normal in most of the western United States. A few ski resorts had operational problems caused by snow depths up to 150 percent of normal for January, and very stormy conditions closed mountain highways and airports. However, the wet period that began in September 1977 ended about mid-January 1978 along most of the west coast when the high pressure system moved northward and just off the coast forcing the storm paths to bypass the area to the north. The more northerly storm path brought cold arctic air into the Midwest causing blizzards and very cold weather—a partial repeat of the severe winter of 1976-77. Since January 1978 precipitation has been generally above normal, and drought conditions have almost disappeared. The fall and winter rains of 1977-78 were welcome events. The persistence and, at times, the intensity of the precipitation were sufficient to increase streamflows into the normal or above normal regimes. The runoff replenished nearly depleted supplies in reservoirs. For example: Storage in 10 of the larger reservoirs in California increased by 3.3 million acre-ft in January 1978. The water content of the heavy snowpack was enough to make releases of recently stored water neces-SUMMARY 77 sary to preserve enough space for flood control if needed later in 1978 at a few sites in California, Oregon, and Washington. Ground-water levels, particularly in California, were still below average; and more than a few months' time is needed for ground-water levels to respond to normal or above normal precipitation and runoff. Water rationing has been abandoned in most locations, but many water rates are higher than they were before the drought. Looking back on the drought, its impacts, and the adaptations made, a very brief summary of an important human aspect in relation to the drought is contained in a statement that someone made that did not characterize the drought, per se, but expressed a view of the overall conditions very well. The statement is "The year 1977 can be remembered as the year of cooperation and compromise." SUMMARY Preparation of a summary of the drought of 1976-77 is analogous to picking up mercury with one's fingers. There were so many aspects that changed with time and location, that an adequate summary would be overlong; therefore, this summary will include only the more important aspects of the drought. The brunt of the drought was felt in both or parts of 1976 and 1977 over large parts of the United States; however, drought effects began to develop in the Midwest in 1974. The record low amounts of precipitation in many localities and the longer than usual intervals between rains seriously affected non-irrigated crops, yet in a number of areas enough rain fell at the right time during the growing season to produce respectable crop yields. The record low snowpacks, particularly in the Sierras in California, and in the Cascades in the Pacific Northwest and the Rocky Mountains caused record low runoff in many western streams. The use of carry-over storage in reservoirs cushioned the impact in 1976, but surface-water supplies were insufficient to meet all demands in 1977. The very cold winter of 1976-77 in most of the eastern part of the country compounded the problems. All these factors made the drought of 1976-77 the most severe one in at least 50 years in many parts of the country. Ground-water level declines greater than those in earlier years occurred in many area as ground water was used to supplement the dwindling surface-water supplies. Wells went dry, yields diminished, and thousands of new wells were drilled. Despite the added stresses on the aquifers, very few serious water-quality problems arose. The increased withdrawals of ground water in the San Joaquin Valley of California brought on a renewal of land subsidence. Most water-quality problems were localized and of relatively short duration. Some were anticipated and other were not. Though water in storage was in short supply, there was enough flexibility in most water development systems that additional releases from reservoirs were made to dilute or flush stream reaches to reduce or eliminate any degradation in water quality that became serious. Water rationing was common in all areas except in the Pacific Northwest where only a few towns introduced rationing. Trucking of water to users and trucking of cattle to water occurred in many States. Legal constraints on water use were modified on temporary bases to provide more water where and when it was needed. Emergency funds were made available to truck cattle or for pumps, pipes, and equipment for emergency water supplies. The public was educated by various agencies and the media on the seriousness of the drought and on how to conserve water. The water systems constructed since the "Dust Bowl Days" of the 1930's, the development of better machinery, better strains of corn, wheat and other crops, better management of irrigation water, and better farming practices contributed to reducing the adverse impacts upon people during the drought of 1976-77 as compared to earlier droughts. The cooperation of agencies and individuals at all levels of government and in the public sector to alleviate drought related problems was encouraging in spite of some unsucessful efforts. The outlook for coping with a drought in the future is an optimistic one. A LOOK TO THE FUTURE The writer does not claim any abilities to predict droughts in the future, though the National Oceanic and Atmospheric Administration has reported (Upper Mississippi River78 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT Basin Commission, 1977) that there is a tendency toward a higher risk of drought in the 1990's than in the 1980's, but a drought may come in the 1980's. A number of facts, ideas, plans, programs, and intentions for the future have been accumulated, discussed, and proposed because of the drought of 1976-77, and some of these are presented below. Man usually learns something from his experiences, though the transfer of his new knowledge to a workable solution or the mitigation of a problem is easier said than done. Benjamin Franklin once said, "Some people are weatherwise but most are otherwise." Since his time, more and more people have become weatherwise or at least concerned about the weather because the weather does affect the quality of their lives and their economic well being. Probably the most generally recognized fact that was reemphasized by the drought is that water is both a limited and a renewable resource. This apparent dichotomy is mainly a matter of degree, but it is also related to changes in other factors in both time and space. Water may be limited by the short supply as evidenced by the small amounts of storage left in many reservoirs by the fall of 1977. Water in a few reservoirs was near the normal amount, and water in aquifers was still ample for the current needs, but it was limited because the means to deliver the water did not exist, or the cost to pump water from greater depths was excessive, or as in California enough baseload power needed to pump ground water or water in canals, was not available during the seasons of high demand. The rains and snow that came late in 1977 did renew the supply in reservoirs and brought some ground-water levels up. However, the aquifers where subsidence occurred will never be able to contain as much water as they did before the subsidence because the compaction of the materials in the aquifer has eliminated a significant amount of the pore space. In several areas the withdrawal of ground water exceeds the recharge; therefore, the water replenished just prolongs the time until other serious problems may develop. Any plans for the future will not be very sound unless they are founded on a good data base and competent analyses and interpretations of the information. The quantity and quality of surface and ground waters as they occur from day to day must be determined in sufficient detail so that probabilities of occurrence of rare events and the risk involved may be estimated with more reliability. Also, because water data are collected only at selected sites, regional relations need improvement so that the data base can be extrapolated to any point of interest more reliably than at present. This is particularly true of low streamflows. Discharge measurements of low flows are made by the Geological Survey at gaging stations and numerous miscellaneous sites where a project is proposed to improve regional low-flow relations. There are many areas of the United States where little is known about ground-water conditions or where only reconnaissance level studies have been made. The growing population in arid and semi-arid areas has increased the demands for water in parts of the nation where development was not foreseen a few years ago and consequently increased the need for more water data. Another important need for future planning is a better determination of water use. This is especially true in time of drought, and it is also important in long-term planning for future droughts. Efforts along this line have been made in the past (MacKichan, 1951; Murray and Reeves, 1977), and a new program has been started by the Geological Survey in cooperation with the States to obtain current data on water use. The National Research Council has proposed a study on planning, preparedness, and management in relation to droughts, and the role of water use data is a prominent part of the study. The quantification of water use will be the basis for analyses of trends and changing patterns in water use that in turn will improve plans for development of water resources. Johnston (1978) presented another viewpoint on water use when he stated: If per capita water usage in an area can be permanently cut by one-third through conservation and reclamation, then developers would have a powerful argument to justify more growth. More development, however, would cause other problems such as traffic congestion, smog, less open space and increased demands for tax-financed services such as schools, police and fireA LOOK TO THE FUTURE 79 protection and welfare. And if Westerners learn to get by on less water by eliminating waste, the next time a drought comes the margin by which water consumption can be reduced without imposing real hardships may be dramatically lessened. If the water saved by conservation is put to use by others, the next drought could mean hardships that the reservoirs, the new wells and the water reclamation equipment will not be able to overcome. Also, he could have pointed out that solutions to the problems he mentions would require more water. Agencies at all levels of government found that they could cooperate during the drought to alleviate or solve drought induced problems within their mission and the legal and other constraints imposed upon them. In the instances when cooperation could not be accomplished, some reevaluation of policies, priorities, operating agreements, and legal aspects seems in order so that reaction to a drought crisis can be replaced by planned actions before a drought occurs. For example: Irrigators in Wisconsin claimed that though emergency drought relief programs were in effect, the delays in providing relief reduced the usefulness of the assistance. Also, they were concerned that a 30-day state-of-emergency period was too short a time considering the duration of the drought in 1976. The promised extension was for another 30 days, but it came after an interval of a month. Also, the U.S. General Accounting Office (1977) questioned whether existing water resource plans and programs adequately meet the competing demands for water uses. Companies and municipalities supplying domestic water raised their rates to maintain their level of income while water sales were down because of rationing during the drought. At the end of the drought, part of the raise was commonly rescinded, but the increased rates may be the initial step toward the end of "cheap" water. One benefit of the water shortage is that people who looked upon water as an inexpensive resource, a "freebie" as one person expressed it, and one readily obtained are now concerned about the adequacy of the water supply and its worth in today's market. Usually the consumer who used more water paid a lower rate, but proponents of water conservation argue that the more water used, the higher the rate should be. A small step toward water conservation has occurred in Wisconsin where legislation has been introduced to amend the plumbing code to restrict flow rates to 3 gal/min regardless of pressure. The water situation has been compared to the circumstances relating to the change in the cost of energy in 1973. More changes in the pricing policies for water will probably occur in the future. The recycling of water used by industry and by a few households seems to be here to stay. The degree to which recycling is developed depends, among other things, upon the attitude of the public toward the uses of recycled water, the elimination of health hazards, the costs of recycling equipment, the cost of fresh water, and the availability of fresh water. Currently, the prevailing view in relation to recycling in homes is that treatment at the source will be less expensive and safer than if it is done by nonprofessionals in the home. There are those who contend that people who move into desert areas where the water supply is insufficient for their needs should not receive water from systems subsidized by the nation's taxpayers. One alternative for these people is to rely on desalinated water which is relatively expensive, but many people in the United States and in other countries do so now. The drought refocused attention on desalinization of seawater; however, its cost is about three to nine times the cost of present supplies. Furthermore massive growth in desalinization would require major expenditures of energy—another commodity increasingly in short supply. A drought is a catalyst that makes the "have-nots" look toward the "haves" for supplies of additional water. The distinction between the two groups is usually relative and often nebulous, but nevertheless interest in interbasin transfers of water is renewed. Whether or not the United States can afford to build very large storage and distribution systems is a question to be settled in the future probably under different social and economic conditions than exist in 1978. The costs of even intrabasin transfers or exchanges of water are high, and many complex factors must be considered and integrated into a plan80 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT before a decision is made to proceed with or reject a project. One approach to better management of the water resource is the conjunctive use of surface and ground water. Basically this technique involves the recharge of ground-water aquifers using surplus surface water in wet periods and the withdrawal of ground water to supplement surface supplies during dry periods. The natural recharge is augmented by artificial means which include injection wells where water is pumped into the aquifers, spreading basins that allow increased percolation into the aquifers, and dams that store flood waters so that the water can be released over an extended period of time to increase percolation through the streambed. The elimination of evaporation when water is stored underground is a big advantage over surface storage, but the energy needed to recover the water is becoming a more important factor in overall planning. Where the surface-water resources are highly developed to increase the basin yield in normal years, the flexibility of management and the amount of water in reserve are restricted during a drought, particularly a multiyear drought. The legal complications of conjunctive use often prevent its implementation. Decisions, frequently by the courts, must be made regarding ownership of the water when underground, how the older surface-water rights relate to the surface water that has become ground water, and other questions that are peculiar to a given area. Times have changed; but someone claimed that up to not so long ago, more people had been killed in disputes over water or water rights than had been killed in lover's quarrels! The United States Department of the Interior (1951) issued a report on the drought in the southwestern United States as of October 1951. The first recommendation made was that ... all state and local agencies be urged to initiate immediately the necessary action to establish effective controls over ground-water development and withdrawals in order to insure a stable agricultural economy in those areas that must depend entirely or in part on ground water as a source of irrigation water supply. Unfortunately, little action, immediate or otherwise, followed this recommendation made 27 years ago. The drought of 1976-77 should provide new and stronger stimulus to implement this suggestion. Another promising approach to water problems is recent enabling legislation in a few states that allows the formation of conservancy districts. These have the potential of controlling the use of water to provide more equitable distribution during dry periods and droughts. Because conservancy districts are relatively new and the law in all states is not the same, the effects of actions by the districts during the drought of 1976-77 were mixed—some successes, some defeats. The dual problems of water as a user of energy and energy as a user of water will become more important in the near future. The transportation of water from a source to a point of use requires a considerable amount of energy. Power is needed to pump water from wells, and those pumps lifting water several hundred feet use large amounts of power. The huge pumps that lift water into the Delta-Mendota Canal and the California Aqueduct and over the Tehachapi Mountains in California need large blocks of the less expensive baseload or off-peak power which is not as plentiful as it was. When water is used to generate hydroelectric power that is used to lift the same water into a canal, a loss of energy results, but the process is essential to move the water to where it is needed. The development of new, but not necessarily different, sources of energy requires water whether the development is of coal in the northern Great Plains States, oil shale in Colorado and Utah, or nuclear plants or stream generating power plants wherever they may be. Water is used to make slurries to transport coal by pipeline, in the oil-shale extraction process, for rehabilitation and replanting of strip-mined areas, and for cooling in nuclear and steam plants. The volumes of water required are large enough to cause conflicts with existing uses and rights, and resolutions of these conflicts will be needed in the future. The reduction of major or irreversible damage caused by a drought will require many different measures. The main objectives are to increase the water supply or reduce the use of it. A wide array of measures have been used, but not all are feasible or suitable inA LOOK TO THE FUTURE 81 every case. Besides tailoring the measures to a specific area to alleviate drought conditions, the selection of the measures must be based also on the quantity and quality of the water and the importance of the supply locally, regionally and nationally. Different agencies have different responsibilities relating to water most of which are complimentary, but some are conflicting. Also, different interests among the public such as the commercial, agricultural, environmental, development, and sportsmen's groups support or obstruct various measures according to the impact of the proposed measures upon the activities of the groups. To ease the immediate impacts of the drought of 1976-77, agencies and groups met and made compromises which worked. If cooperation occurs in the future, the outlook for diminshed water-related problems will be brighter. The drought also highlighted the fact that an adequate water supply is no longer solely the concern of arid regions. Humid areas in the Pacific Northwest, the Southeast, and the Midwest had serious problems; therefore, changes in the status quo may be expected that will affect the entire nation. There have been a few attempts to plan and manage water on an integrated basis for an entire river basin. To do so involves land use planning also because of the close link between land and water. More of this type of an approach is needed in the future. This concept and others being tried may change the perceptions of the governments and the public about water resources. Water is limited as described previously, and it is not something that is part of a person's birthright. To prepare properly for the future using any of the activities mentioned above, research and development are necessary. Though the data base has increased enormously in recent years and new techniques and instruments have been useful, there are still many unanswered questions. The inability to obtain the types of data required when and where they are needed is another serious problem. To overcome the shortcomings, viable research projects—both basic and applied—are needed. Many aspects of water quality are not clearly understood, and some long-standing concepts have been challenged. A few examples: How can desalinization be improved, can reductions of oxygen in water be pre- vented or alleviated, what changes in the water resource are caused by urban runoff, is the chlorination of water really harmful to health? Research is needed before these and similar problems can be solved. Better management plans to make the best use of the available water supply, better conservation practices such as trickle irrigation and no-till or shallow plowing of fields, and better instruments and equipment will depend to a large extent on the research efforts devoted to them. More research is needed to develop and refine remote sensing to locate recharge areas and springs and to evaluate snowpacks and other sources of water. Satellite imagery has proven to be a highly useful way to obtain synoptic information, and aerial photography is a powerful tool when detailed studies are made. Research and development are in progress in the fields of weather forecasting and weather modification. The National Oceanic and Atmospheric Administration anticipates that during the next 10 years they will have the capability to project month-by-month and season-to-season weather developments with much more skill than they do now. A National Academy of Science panel of scientists foresees serious climate changes beginning sometime in the next century. If the world increases its dependency on coal for energy, the panel expects more air pollution and a greenhouse effect that will warm the atmosphere, melt ice caps, raise the level of the oceans and flood coastal areas, and alter the patterns of atmospheric circulation. However, there are other scientists who do not believe this will happen. Both governmental agencies and private organizations are researching the physics related to weather modification, and they are conducting field tests and applying their state-of-the-art knowledge in a number of areas primarily in the 17 western States. The proponents claim successful operations in most of the areas. The legal problems relating to the results or even the anticipated results have not been resolved. See page 23. The Geological Survey is involved in research related to climate as it affects land and water resources (Smith, 1978). Investigations have been divided into five categories: (1) Present climate-related processes and indices that provide baseline data for climatic inter-82 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT pretation; (2) geologically short-term changes in climate: (3) geologically longer term climate changes; (4) the areal distribution of past climates; and (5) dating and correlation methods. A few of the projects in the first category involve measurements of streamflow, ground-water levels, glaciers, erosion rates, channel geometry, evaporation, lakes, and animal and plant communities; rainfall-runoff modeling, water chemistry, weathering, and soil formation. The intent of the research is to provide better bases for resource management and for estimating climate-related risks and hazards such as a drought. Several studies have been made in which the water supplies at some future time have been estimated. The Water Resources Council reported (Nation's Business, 1977) that only the South Atlantic region, the Ohio River basin, and New England would be expected to have adequate water supplies by the year 2000 unless additional reservoirs are built and other steps taken to help nature provide the water that will be needed. The water resources available to metropolitan areas are limited, and several routinely supply more water than they could during a drought (U.S. General Accounting Office, 1977). By the year 2000, 85 percent of the population may be living in metropolitan areas, an increase of about 10 percent. The drought in the Northeastern States during the 1960's affected several metropolitan areas; yet since then no major water supply projects have been built to serve the three most critical areas; Washingon, D.C., New York City, and the East Massachusetts-Rhode Island metropolitan areas. Water projects take many years to plan, authorize, design, and build; and because metropolitan areas must obtain water from outside the area, any project will infringe upon nearby communities and rural areas thereby raising numerous legal, political, and environmental problems. Though most of the nation's population is served with safe drinking water by central water-supply systems, about 31 million people must rely on their own domestic supply, mostly from wells. There are also 6 million persons, primarily in remote areas and with low incomes, who have no running water in their dwellings. There are Federal programs to help improve water supply facilities in rural areas, but many small communities are faced with a water supply that is not sufficient in quantity or quality or both to maintain or improve the viability of the community. The Comptroller General of the United States (1977) reported on the drought in California during 1976-77 and concluded that the State water plan shows that dependable water supplies will not provide for State needs through the year 2000, even if planned Federal, State, and local surface and ground-water projects are completed and if waste water is reclaimed and re-used. The State expects the deficit to be made up through the continued overdraft of ground water. If the overdraft in the eastern San Joaquin Valley is not rectified within the next 20 to 25 years by restricting ground-water use and importing surface water, more than 1 million acres of California's more productive agricultural land could be lost. This situation and others related to the drought imply that a re-examination of the State water plan in conjunction with the Federal government is desirable. Many water and power projects in the Western States that had been deferred for various reasons are being reevaluated in relation to how they might alleviate drought conditions in the future and projected shortages of water and power. The above examples briefly indicate some of the complexities associated with water resources and water supply. Part of the picture is not very encouraging. Though individuals, groups, and agencies are working and planning improvements to make more water of better quality available, apathy is apt to dominate the public's views during wet periods. Bond issues for local water developments have been defeated, and tax dollars for some projects acceptable to all parties involved have not been provided. Nace and Pluhowski (1965) stated, "Man is an optimist, and during periods of ample water supply, he tends to forget past adversities and acts as though the adversities cannot recur." It is difficult to think about and plan for drought conditions when it is raining. The bright side of the picture shows the rapid development of techniques and abilities during the last 10 or 15 years. Large computers can now be used to help in more sophisticated analyses of larger areas so that the interactions of surface and ground waters and changes in quality can be estimated withSELECTED REFERENCES 83 more reliability than was possible just a few years ago. The data base is expanding and more scientists in fields directly related to water are working with professionals in the fields of planning, economics, legislation, etc., to develop better integrated solutions to water problems. The importance of water quality has been recognized as the controlling factor in many cases, and legislation and funds have been provided to improve water quality. These activities indicate that man has entered a renaissance with respect to water resources. SELECTED REFERENCES Barksdale, H. C., O'Bryan, Deric, and Schneider, W. J., 1966, Effect of drought on water resources in the northeast: U.S. Geological Survey Hydrologic Atlas 243. Bensen, M. A., and Gardner, R. A., 1974, The 1971 drought in south Florida and its effect on the hydrologic system: U.S. Geological Survey Water Resources Investigations 12-74, 45 p. Buchanan, T. J., and Gilbert, B. K., 1977, The drought, in Water spectrum, summer 1977: U.S. Department of the Army, p. 6-12. California Department of Water Resources, 1976, The California drought—1976: California Department of Water Resources, 99 P- _____ 1977a, Water Conditions in California: California Cooperative Snow Surveys Bulletins. _____ 1977b, The California drought—1977, an update: California Department of Water Resources, 150 p. _____ 1977c, The continuing California drought: California Department of Water Resources, 138 p. _____ 1978, The Sacramento Valley water use survey, 1977: California Department of Water Resources. Columbia River Water Management Group, 1978, Columbia River water management report for water year 1977, p. 45-55. Committee on Fishery Operations, 1977, Special drought year operation for downstream fish migrants: Columbia River Water Management Group, 70 p. Comptroller General of United States, 1977, California drought of 1976 and 1977—Extent, damage, and government response, 92 P- Environmental Data Service, 1976, Climatological data, national summary, 1975: National Oceanic and Atmospherip Administration, v. 26, no. 13, 124 p. Foehner, O. H., 1977, Weather modification—a major resource tool: in Proceedings Western Snow Conference, Albuquerque, New Mexico, April 18-21, 1977, p. 1-7. Freeman, O. W., 1929, Evidence of prolonged droughts on the Columbia Plateau prior to white settlement: Monthly Weather Review, v. 57, June 1929, p. 250-251. Gatewood, J. S., Wilson, Alfonso, Thomas, H. E., and Kister, L. R., 1964, General effects of drought on water resources of the southwest: U.S. Geological Survey Professional Paper 372-B, 53 p. Griffith, E. L., 1978, Southern California's drought response program: Journal American Water Works Association, v. 70, no. 2, February 1978, p. 74-78. Hoyt, J. C., 1936, Droughts of 1930-34: U.S. Geological Survey Water-Supply Paper 680, 103 p. ______ 1938, Drought of 1936 with discussion on the significance of drought in relation to climate: U.S. Geological Survey Water-Supply Paper 820, 60 p. Johnston, David, 1978, Is the drought really over?: Sundancer Magazine, February 1978: Hughes Air West, v. 7, no. 2, p. 107. Lappala, E. G., 1978, Quantitative hydrogeology of the upper Republican Natural Resources District, southwest Nebraska: U.S. Geological Survey Water Resources Investigations 78-38, 200 p. Lofgren, B. E., 1977, Changes in aquifer-system properties with ground-water depletion: in Proceedings, 11th California Biennial Groundwater Conference, September 15-16, 1977, Fresno, California, p. 48-68. Mackichan, K. A., 1951, Estimated use of water in the United States—1950: U.S. Geological Survey Circular 115, 13 p. Murray, C. R., and Reeves, E. B., 1977, Estimated use of water in the United States in 1975: U.S. Geological Survey Circular 765, 39 p. Nace, R. L., and Pluhowski, E. J., 1965, Drought of the 1950's with special reference to the Midcontinent: U.S. Geological Survey Water-Supply Paper 1804, 88 p.84 HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT Nation's Business, 1977, Water: The next resource crisis?, September 1977, p. 50-55. Palmer, W. C., 1965, Meteorological drought: U.S. Weather Bureau Research Paper no. 45, 58 p. Palmer, W. C., and Denny, L. M., 1971, Drought bibliography: Environmental Data Service, National Oceanic and Atmospheric Administration, Technical Memorandum EDS 20, 233 p. Robie, R. B., 1978, California's program for dealing with the drought: Journal Ameri- can Works Association, v. 70, no. 2, February 1978, p. 64-68. Sellers, W. D., 1960, Precipitation trends in Arizona and western New Mexico in Proceedings, Western Snow Conference, Santa Fe, New Mexico, p. 81-94. Shelton, M. L., 1977, The 1976 and 1977 drought in California: Extent and severity: Weatherwise, v. 30, no. 4, August 1977, p. 139. Smith, G. I., 1978, Climate variation and its effects on our land and water: U.S. Geological Survey Circular 776-B, 52 p. Soil Conservation Service, 1977a, Water supply outlook for Idaho ______ 1977b, Water supply outlook for Nevada ______ 1977c, Water supply outlook for Oregon ______ 1977d, Water supply outlook for Washington ______ 1977e, Water supply outlook for western United States State of Washington, 1977, Governor's ad hoc executive water emergency committee: Biweekly reports. Subrahmanyam, V. P., 1967, Incidence and spread of continental drought: World Me- terological Organization, International Hydrological Decade, Reports on WMO/IHD Projects, no. 2, Geneva, Switzerland. Thomas, H. E., and others, 1963a, Effects of drought in central and south Texas: U.S. Geological Survey Professional Paper 372-C, 31 p. _____ 1963b, Effects of drought in the Rio Grande basin: U.S. Geological Survey Professional Paper 372-D, 58 p. _____ 1963c, Effects of drought in basins of interior drainage: U.S. Geological Survey Professional Paper 372-E, 50 p. ______ 1963d, Effects of drought in the Colorado River basin: U.S. Geological Survey Professional Paper 372-F, 51 p. _____ 1963e, Effects of drought along Pacific Coast in California: U.S. Geological Sur- vey Professional Paper 372-G, 25 p. _____ 1963f, General summary of effects of the drought in the Southwest: U.S. Geological Survey Professional Paper 372-H, 22 p. Troxell, H. C., 1957, Water resources of Southern California with special reference to the drought of 1944-51: U.S. Geological Survey Water-Supply Paper 1366, 139 p. Upper Mississippi River Basin Commission, 1977, Drought, dilemma, decisions: Proceeding of Symposium on drought in mid-America, 162 p. U.S. Department of Interior, 1951, Drought in southwestern United States as of October 1951, 65 p. U.S. Environmental Protection Agency, 1976, Quality criteria for water, Washington, U.S. Government Printing Office, 256 p. U.S. General Accounting Office, 1977, Water resources planning, management, and development: What are the nation's water supply problems and issues, July 28, 1977. Water Resources Council, 1966, Drought in northeastern United States, a third appraisal. Wells, Wade, 1978, Fires producing "waterproof soils": Sediment Management Newsletter no. 3, Spring 1978, California Institute of Technology and Scripps Institute of Oceanography, p. 3. Whipple, W., Jr., 1966, Regional drought frequency analysis: American Society Civil Engineers, Journal Irrigation and Drainage Division, v. 92, no. IR2, June 1966, p. 11-31.v/- / / 3 / / UAT3 Rubidium-Strontium Geochronology and Plate-Tectonic Evolution of the Southern Part of the Arabian Shield GEOLOGICAL SURVEY PROFESSIONAL PAPER 1131 Prepared in cooperation with the Saudi Arabian Ministry of Petroleum and Mineral Resources, Directorate General of Mineral ResourcesRubidium-Strontium Geochronology and Plate-Tectonic Evolution of the Southern Part of the Arabian Shield / By ROBERT J. FLECK, WILLIAM R. GREENWOOD, DONALD G. HADLEY, R. ERNEST ANDERSON, and DWIGHT L. SCHMIDT GEOLOGICAL SURVEY PROFESSIONAL PAPER 1131 Prepared in cooperation with the Saudi Arabian Ministry of Petroleum and Mineral Resources, Directorate General of Mineral Resources A study of the age of rocks and the geologic history of southwestern Saudi Arabia UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data United States. Geological Survey. Rubidium-strontium geochronology and plate-tectonic evolution of the southern part of the Arabian Shield. (Geological Survey professional paper ; 1131) Bibliography: p. 1. Ruhidium-strontium dating. 2. Plate tectonics. 3. Geology—Arabia. I. Fleck, Robert J. II. Title. III. Series United States. Geological Survey. Professional paper ; 1131. QE508.U5 1979 555.3 79-13596 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402CONTENTS Page Abstract_________________________________________________ 1 Introduction_____________________________________________ 1 Acknowledgments__________________________________________ 1 General geologic setting_________________________________ 3 Volcanic and sedimentary rocks______________________ 3 Basaltic assemblage ____________________________ 3 Andesitic assemblage ___________________________ 5 Plutonic rocks _____________________________________ 5 Dioritic batholiths_____________________________ 5 Granodiorite gneiss_____________________________ 6 Late-orogenic or postorogenic plutons___________ 7 Layered gabbro and associated rocks----- 7 Granodiorite to granite intrusions______ 7 Najd fault system___________________________________ 8 Previous geochronologic studies__________________________ 8 Analytical techniques ___________________________________ 8 Results of rubidium-strontium analyses___________________ 9 Basaltic assemblage________________________________ 15 Page Results of rubidium-strontium analyses—Continued Andesitic assemblage ______________________________ 16 Undisturbed isotopic systems------------------ 16 Disturbed isotopic systems-------------------- 17 Foliated diorite to trondhjemite batholiths----- 19 Granodiorite gneiss domes _________________________ 22 Late-orogenic or postorogenic granodiorite to granite plutons______________________________ 24 Tindahah batholith ___________________________ 24 Bani Thuwr pluton_____________________________ 24 Wadi Shuwas __________________________________ 25 Jabal Shada and Jabal Ibrahim_________________ 25 Jabal Qal and Jabal ’Aya---------------------- 25 Wadi Halal ___________________________________ 26 Wadi al Miyah and Wadi Musayrah--------------- 26 Other late-orogenic or postorogenic plutons _ 28 Discussion of the results------------------------------ 29 Conclusions ___________________________________________ 36 References cited--------------------------------------- 36 ILLUSTRATIONS Page Figure 1. Map of the western part of the Arabian Peninsula showing the area studied and the exposed part of the Arabian Shield________________________________________________________________________ 2 2. Chart showing a generalized chronology of igneous and sedimentary units of the southern part of the Arabian Shield and of tectonic events affecting them------------------------------------- 3 3. Geologic map of the southern part of the Arabian Shield in the Kingdom of Saudi Arabia------------ 4 4. Index map of the location of analyzed samples from the Arabian Shield and of quadrangles---------- 10 5-25. Total-rock rubidium-strontium isochron diagrams: 5. Basalt of Wadi al Faqh______________________________________________________________________ 15 6. Arphan Formation of Hadley and meta-andesite of the Junaynah quadrangle--------------------- 16 7. Khadrah Formation metavolcanic rocks, rhyolite of the Bi’r Juqjuq quadrangle, and Arphan Formation of Hadley______________________________________________________________________ 16 8. Metavolcanic rocks of Wadi bin Dwaynah______________________________________________________ 17 9. Metavolcanic rocks of Hishat al Hawi and Wadi Shuklalah------------------------------------- 17 10. Rhyolite of the Murdama Group___________________________________________________________ 18 11. Juqjuq Formation of Hadley-------------------------------------------------------------- 18 12. Quartz diorites of Wadi Khadrah and Wadi ash Shaqah ash Shamiyah, Al Lith area, and of Biljurshi and Wadi Qanunah, Biljurshi area________________________________________ 19 13. Quartz diorites of An Nimas, Wadi Tarj, and Al Mushirah and the trondhjemite of Wadi Asmak, An Nimas batholith____________________________________________________________ 20 14. Quartz diorites of Wadi Makhdhul and Simlal and the diorite and trondhjemite of the Wadi Tarib batholith, Wadi Malahah quadrangle---------------------------------------- 21 15. Quartz diorites of Jabal Umm al Hashiyah and the southwestern Al Qarah quadrangle------- 22 16. Quartz diorite of Malahah dome and granodiorite of Wadi Malahah_____________________________ 22 17. Granodiorite gneiss of Jabal Mina_______________________________________________________ 23 18. Granodiorite gneiss of Wadi Bagarah and granitic gneiss of Jiddah Airport--------------- 23 IIIIV CONTENTS Figures 5-25. 26. 27, 28. 29. 30. Total-rock rubidium-strontium isochron diagrams—Continued 19. Granitic gneisses of Harisi dome and Wadi Bishah ___________________________________________ 20. Tindahah batholith and the Bani Thuwr pluton, quartz monzonites of Wadi Shuwas, and granites of Jabal Shada and Jabal Ibrahim _____________________________________________ 21. Quartz monzonites of Jabal Qal and Jabal ’Aya_______________________________________________ 22. Granodiorite of Wadi Halal__________________________________________________________________ 23. Granodiorites of Wadi al Miyah and Wadi Musayrah and an aplite dike of the southwestern A1 Qarah quadrangle____________________________________________________________________ 24. Hornblende diorite of Hadley________________________________________________________________ 25. Quartz monzonite and gneissic granodiorite of the Bi’r Juqjuq quadrangle and the quartz monzonite of Jabal Tarban______________________________________________________________ Diagrams showing variation of ^Sr/^Sr and of Rb/Sr with age_________________________________________ Sketch maps showing geographic distribution of ages: 27. Dioritic complexes and metavolcanic rocks older than 680 million years---------------------- 28. Volcanic rocks and late-orogenic or postorogenic plutonic rocks younger than 680 million years -------------------------------------------------------------------------------------- Diagram of a plate-tectonic model of the origin and deformation of the Arabian Shield--------------- Map showing a comparison of continental-collision-related structures of Asia with orogenic structures of the Arabian Shield_________________________________________________________________________ Page 23 25 26 26 27 28 28 30 32 33 34 36 TABLES Page Table 1. Analytical results for rocks of the Arabian Shield---------------------------------------------------- 11 2. Summary of Rb-Sr ages and isochron data_______________________________________________________________ 14 CONVERSION FACTORS Metric unit Inch-Pound equivalent Length millimeter (mm) — 0.03037 inch (in) meter (m) 3.28 feet (ft) kilometer (km) = .62 mile (mi) Area square meter (m2) 10.76 square feet (ft2) square kilometer (km2) z= .386 square mile (mi2) hectare (ha) = 2.47 acres Volume cubic centimeter (cm3) — 0.061 cubic inch (in3) liter (L) zz 61.03 cubic inches cubic meter (m3) — 35.31 cubic feet (ft3) cubic meter — .00081 acre-foot (acre-ft) cubic hectometer (hm3) = 810.7 acre-feet liter zz 2.113 pints (pt) liter zz 1.06 quarts (qt) liter zz .26 gallon (gal) cubic meter zz .00026 million gallons (Mgal or 10« gal) cubic meter = 6.290 barrels (bbl) (1 bbl = 42 gal) Weight gram (g) 0.035 ounce, avoirdupois (oz avdp) gram zz .0022 pound, avoirdupois (lb avdp) metric tons (t) zz 1.102 tons, short (2,000 lb) metric tons = 0.9842 ton, long (2,240 lb) Specific combinations kilogram per square 0.96 atmosphere (atm) centimeter (kg/cm2) kilogram per square — .98 bar (0.9869 atm) centimeter cubic meter per second = 35.3 cubic feet per second (ft3/s) (m3/s) Metric unit Inch-Pound equivalent Specific combinations—Continued liter per second (L/s) _ .0353 cubic foot per second cubic meter per second per square kilometer [ (m3/s)/km2] 91.47 cubic feet per second per square mile [ (ft3/s)/mi2] meter per day (m/d) = 3.28 feet per day (hydraulic conductivity) (ft/d) meter per kilometer 5.28 feet per mile (ft/mi) (m/km) kilometer per hour .9113 foot per second (ft/s) (km/h) meter per second (m/s) = 3.28 feet per second meter squared per day (m3/d) = 10.764 feet squared per day (ft2/d) (transmissivity) cubic meter per second (m3/s) = 22.826 million gallons per day (Mgal/d) cubic meter per minute 264.2 gallons per minute (gal/min) liter per second (L/s) - 15.85 gallons per minute liter per second per meter [(L/s)/mJ zz 4.83 gallons per minute per foot [(gal/min)/ft] kilometer per hour — .62 mile per hour (mi/h) (km/h) meter per second (m/s) = 2.237 miles per hour gram per cubic 62.43 pounds per cubic foot (lb/ft3) centimeter (g/cm3) gram per square — 2.048 pounds per square foot (lb/ft2) centimeter (g/cm2) gram per square zz .0142 pound per square inch (lb/in2) centimeter Temperature degree Celsius (°C) = 1.8 degrees Fahrenheit (°F) degrees Celsius = [ (1.8x °C)-f-32] degrees Fahrenheit (temperature)RUBIDIUM-STRONTIUM GEOCHRONOLOGY AND PLATE-TECTONIC EVOLUTION OF THE SOUTHERN PART OF THE ARABIAN SHIELD By Robert J. Fleck, William R. Greenwood, Donald G. Hadley, R. Ernest Anderson, and Dwight L. Schmidt ABSTRACT Rubidium-strontium studies of Precambrian volcanic and plutonic rocks of the Arabian Shield document an early development of the Arabian craton between 900 and 680 m.y. (million years) ago. Geologic studies indicate an island-arc environment characterized by andesitic (dioritic) magmas, volcaniclastic sedimentation, rapid deposition, and contemporaneous deformation along north- or northwest-trending axes. Magmatic trends show consistent variation in both composition and geographic location as a function of age. The oldest units belong to an assemblage of basaltic strata exposed in western Saudi Arabia that yield an age of 1165±110 m.y. The oldest andesitic strata studied yield an age of 912±76 m.y. The earliest plutonic units are diorite to trondhjemite batholiths that range from 800 to 900 m.y. in age and occur along the western and southern parts of Saudi Arabia. Younger plutonic units, 680 to 750 m.y. in age, range from quartz diorite to granodiorite and become more abundant in the central and northeastern parts of the Arabian Shield. Initial s,Sr/MSr ratios for both dioritic groups range from 0.7023 to 0.7030 and average 0.7027. The absence of sialic detritus in sedimentary units and the evidence for an island-arc environment suggest the early development of the Arabian craton at a convergent plate margin between plates of oceanic lithosphere. Active subduction apparently extended from at least 900 m.y. to about 680 m.y. Subsequent to this subduction-related magmatism and tec-tonism, called the Hijaz tectonic cycle, the Arabian craton was sutured to the late Precambrian African plate in a collisional event. This period of orogeny, represented in Arabia and eastern Africa by the Mozambiquian or Pan-African event, extended from some time before 650 m.y. to at least 540 m.y. and perhaps 520 m.y. B.P. Although the tectonic processes of subduction and continental collision during the 900+ to 500-m.y. period require similar directions of plate convergence, the differences in magmatic and tectonic styles of Hijaz orogenesis from those of the Pan-African and the temporal break between them in much of the southern part of the Arabian Shield support division into at least two events. As defined by the ages of major plutonic units, the axis of magmatic and tectonic activity migrated eastward or northeastward during the Hijaz cycle, the predominantly dioritic plutonic rocks becoming younger and more siliceous to the east. Granodiorite to granite plutonism of the Pan-African event, however, shows no geographic bias, being distributed throughout the Arabian Shield. Although the Hijaz diorites and Pan-African granitic rocks exhibit strong contrasts in composition and age differences as great as 250 m.y. in the westernmost parts of the area, the two groups are less distinct compositionally and nearly the same age in the eastern part. INTRODUCTION Geologic and geochronologic studies of the southern part of the Arabian Shield present a complex but surprisingly clear view of late Precambrian island-arc development and crustal evolution. The geology of the Arabian Shield between lat 21°30' N. and the Saudi Arabian-Yemen border (fig. 1) is discussed by Schmidt and others (1973) and by Greenwood and others (1973, 1976, 1977) ; geologic mapping was compiled by Greenwood and others (1974). Geochronologic studies of the shield include those by Fleck and others (1972, 1973, and 1976). Results of geologic, geochemical, and preliminary geochronologic studies have been interpreted as indicating formation of the Arabian Shield in an intraoceanic island arc by andesitic volcanism, associated plutonism, and cannibalistic sedimentation (Greenwood and others, 1976, 1977). This report presents Rb-Sr results that document a detailed chronology for the development of the shield area. These results were obtained by the U.S. Geological Survey as part of the program of geologic investigations of the Directorate General of Mineral Resources, Ministry of Petroleum and Mineral Resources, Kingdom of Saudi Arabia. ACKNOWLEDGMENTS We thank A. Berry, J. Saburomaru, and W. Doering for laboratory assistance with the rubidium-strontium analyses reported here. We also thank T. H. Kiilsgaard, W. C. Prinz, R. G. Coleman, J. C. Ratte, and H. R. Cornwall for assistance of various kinds. M. A. Lanphere and F. C. W. Dodge made many useful suggestions. 12 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD AMMAN JERUSALEM Al 'Aqabah KUWAIT, / Al 4. / KUWAIT AN NAFUD Tabuk Khaybar Jabal ; al Wask: Al Hufuf AR RIYAD Harad Ad Dawadimi Mahd adh Dhqhqb MAKKAH; Ranyah Turabah Bishah; Biljurshi Port Sudan' An Nimas: AR RUB' AL KHALI Al Qunfudhah' Hamdqh: • •Khqrnis Abha Wushayt Zahran# Area of study (tig. 3)X YEMEN Massawa Asmara# Shabwah 100 200 KILOMETERS A| Hudaydah 200 MILES Figure 1.—Western part of the Arabian Peninsula showing the area studied and the exposed part of the Arabian Shield (shaded). Hachures are on Paleozoic side of contact between Paleozoic units and units of the shield. Only-representative major faults of the Najd fault system are shown.GENERAL GEOLOGIC SETTING 3 GENERAL GEOLOGIC SETTING The Arabian Shield represents a terrane of slightly to intensely deformed stratified units and undeformed to partly remobilized plutonic units of late Precambrian (Proterozoic) age. The shield is exposed over an area of 610,000 km2 in the western part of the Arabian Peninsula (fig. 1). On the north and east the shield is overlain by a veneer of nearly flat-lying Phanerozoic sedimentary rocks. On the west the Red Sea rift, containing “oceanic” or “basaltic” crust of Tertiary and Quaternary age, separates the Arabian Shield from its previously continuous counterpart, the Nubian (or African) Shield in Egypt, Sudan, and Ethiopia. A summary of the major sedimentary and igneous units of the southern part of the shield and a generalization of the type of tectonic environment are shown in figure 2. The distribution of these units in the area studied is shown on the geologic map (fig. 3), which is modified from Greenwood and others (1974). VOLCANIC AND SEDIMENTARY ROCKS BASALTIC ASSEMBLAGE The oldest rocks exposed in the southern part of the Arabian Shield constitute an assemblage of clastic and volcaniclastic marine sedimentary rocks, basalt and basaltic andesite flows, flow breccias, tuffs, and, in lesser amounts, chert and impure limestone (marble). Sedimentary units exhibit graded bedding, slump structures, intraformational breccias, Figure 2.—Generalized chronology of igneous and sedimentary units of the southern part of the Arabian Shield and of tectonic events affecting them. Periods indicated by shading were of decreased igneous activity or deposition and are shown as time transgressive because of a general southwest to northeast decrease in age of the early shield-forming units.4 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD Igneous and sedimentary rocks and alluvium Granodiorite to granite circular plutons, ring dike complexes, sheets, and irregular bodies—Diapirically to passively intruded Gneissic quartz diorite to quartz monzonite domes and complex diapirs Diorite to trondhjemite-Subordinate gabbro and serpentinized ultramafic rocks Andesitic assemblage-Metamorphosed andesitic to dacitic flows, breccia, and tuff, associated volcaniclastic rocks, and subordinate carbonaceous rocks and marble; includes units assigned to the Jiddah, Ablah, Halaban, and Murdama Groups Basaltic assemblage-Metamorphosed basaltic flows, breccia, and tuff, graywacke, chert, and minor marble; includes units assigned to the Baish and Bahah Groups PHANEROZOIC CAMBRIAN AND PRECAMBRIAN PROTEROZOIC Unconformity Figure 3.—Geologic map of the southern part of the Arabian Shield in the Kingdom of Saudi Arabia. Modified from Greenwood and others (1974).GENERAL GEOLOGIC SETTING 5 and similar features characteristic of turbidites. Algal structures have been identified in some carbonaceous units (T. Kiilsgaard, oral commun., 1977). Neither potassium feldspar clasts nor terrigenous lithic fragments have been reported from these strata. Contacts between this “basaltic assemblage,” as we shall refer to these units, and any other rock unit within the area studied are either tectonic or indicate this unit to be the older. No evidence has been found within the southern part of the Arabian Shield to suggest deposition of the basaltic assemblage on sialic crust, and no older rock units are known. The basaltic assemblage includes units assigned by various workers to the Baish and Bahah Groups. Bakor and others (1976) identify a mafic-ultramafic complex at Jabal al Wask in the northern part of the shield (fig. 1) as “back-arc ophiolite.” Although serpentinite occurs within the southern part of the shield, no relation to rocks of the basaltic assemblage has been demonstrated. The sedimentary aspects of the “ophiolite” described by Bakor and others are similar to those described here, but none of the volcanic rocks studied in the southern shield could be classified as spilitic. Any relation between this “ophiolite” and the basaltic assemblage, however, remains to be demonstrated. ANDESITIC ASSEMBLAGE The stratified units of the southern part of the Arabian Shield not part of the basaltic assemblage are classified here as the “andesitic assemblage.” This generalization of previously published stratigraphic nomenclature is necessary for several reasons. Correlation of units of an extremely thick, laterally variable, highly deformed, and slightly to highly metamorphosed sequence of lithologically ren-etitious strata over 610,000 km2 must be described as difficult at best. This remains true when the study area is restricted to the southern one-third of the shield area. Available age determinations are inadequate for regional correlation of strata at a forma-tional level and probably questionable at the group level. Alteration, sedimentary reworking, and varying degrees of metamorphism further complicate the geochronologic interpretation and, as will be shown, invalidate many of the age determinations made on fine-grained volcanic or volcaniclastic units. The andesitic assemblage consists predominantly of calc-alkalic andesite, basaltic andesite, dacite, associated volcaniclastic rocks, and subordinate carbonaceous rocks and marble. The volcanic rocks include flows, breccias, and large amounts of tuff, including welded ash-flow tuff. Locally, younger units of this assem- blage rest unconformably on older units of the assemblage, on older plutonic rocks, or on the basaltic assemblage. In some localities, the younger units contain dioritic to granitic detritus; in other areas, the rocks cannot be subdivided easily on this basis. The relation between the oldest units of the andesitic assemblage and the basaltic assemblage is uncertain because all contacts between these two groups of strata are tectonic. Because the two assemblages are chemically and lithologically distinct, never occur in the same tectonic block, and contain units whose characteristics require significantly different environments of deposition, we conclude that these assemblages accumulated at different times or in areas originally remote one from another and were juxtaposed subsequently by faulting along north-south shear zones. Although the nature of this displacement is inadequately known, it appears to have occurred during deposition of the andesitic assemblage and therefore must be consistent with the island-arc origin of that sequence. PLUTONIC ROCKS DIORITIC BATHOLITHS The basaltic assemblage and the older units of the andesitic assemblage are complexly deformed, metamorphosed, and intruded by diorite, quartz diorite, and trondhjemite batholithic complexes. The intrusive sequence within these complexes generally follows a differentiation trend, beginning with hornblende diorite and gabbro followed by quartz diorite and trondhjemite. In places, however, the sequence was repeated, the trondhjemite being intruded by younger quartz diorite or hornblende diorite. Chemical variations within the batholiths include ranges in Si02 from 55 to 73 percent, K20 from less than 0.4 to more than 1.7 percent, and CaO from less than 2 to more than 8.5 percent (Greenwood and Brown, 1973). Normative orthoclase commonly makes up less than 10 percent, especially in the western part of the area. As are the older strata, the diorite batholiths are cut by zones of intense shearing that segment the bodies into narrow north-trending slices, at many places separated by similarly sheared blocks of stratified units. Unlike the two older stratified assemblages, which never occur in the same tectonic slices, the batholiths retain a general coherence and, though cut by the same shear zones, exhibit no significant lateral displacement. This relation suggests that batholithic intrusion was probably concurrent with much of the deformation but that the major translation required to juxtapose the older6 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD units of the ande»itic assemblage with the basaltic terrane occurred earlier. Diorite-trondhjemite complexes (fig. 3) in the southwesternmost part of the shield have been assigned to three large batholiths—Biljurshi, An Nimas, and Wadi Tarib—that exhibit an en echelon pattern from northwest to southeast. The northwest-ernmost complex, the Biljurshi batholith (Greenwood, 1975a), includes dioritic bodies between A1 Lith and the major north-trending shear zone at approximately long 41°45' E. South and east of the Biljurshi batholith, dioritic units are assigned to the An Nimas batholith (Greenwood, 1979a; Anderson, 1977), which extends north from the town of An Nimas between long 41°45' and long 42°30' E. Dioritic intrusive rocks south and east of the An Nimas batholith are assigned to the Wadi Tarib batholith, which occupies a broad area in the southeastern part of the Arabian Shield (Greenwood, 1979c, d). The Wadi Tarib batholith includes both dioritic and granodioritic units mapped previously as Khamis Mushayt Gneiss (Coleman, 1973a). Northeast of the en echelon batholiths, exposures of diorite are generally less extensive, as younger andesitic units cover and younger granodiorite to granite bodies intrude the older terrane. East of the town of Bishah, discontinuous exposures suggest that a large dioritic batholith may once have occupied a large area but this has been reduced by emplacement of a younger granitic body, the granodiorite of Wadi Musayrah. Subsequent to emplacement of the early dioritic batholiths, younger units of the andesitic assemblage, including conglomerates containing diorite and trondhjemite cobbles, were deposited on both older volcanic assemblages and on erosionally exposed plutons of the batholiths themselves. The younger units of the andesitic assemblage indicate a continuation of andesitic to dacitic volcanism and deposition of andesitic volcaniclastic, fine-grained terrigenous, and carbonate sediments, but the plu-tonic detritus in the abundant breccias and intrafor-mational conglomerates indicates local uplift and erosion of the early dioritic bodies. Because the younger andesitic strata were deposited on both older andesitic units and the basaltic assemblage, juxtaposition of these two older sequences must have been largely complete before that time. Because these younger andesitic units are also displaced by the north-south shear zones and range in metamorphic grade up to almandine-amphibolite facies (including kyanite and sillimanite in some localities), deformation and metamorphism clearly must have continued for a significant period of time. In some areas, the younger units of the andesitic assemblage are intruded by a second, or younger, generation of diorite batholiths that may in turn be overlain by even younger graywacke, andesite, volcanic tuff, breccia, and agglomerate (fig. 2). Plutonic detritus increases upward in the assemblage but is clearly controlled by local exposures of older diorites. As lateral variations of detrital components are common within these strata, the presence or absence of dioritic detritus at a given locality is unreliable as a means of correlation. GRANODIORITE GNEISS The first major shift in plutonic rock type occurred in the western part of the Arabian Shield after the major dioritic complexes were emplaced in that area. Compositionally variable gneiss domes, ranging from biotite-bearing trondhjemite to biotite-mus-covite granite, were emplaced with both internal and host-rock foliations concordant with the intrusive margins. Greenwood (1979a) describes the granodiorite orthogneiss of Jabal Mina as “... a nested to interconnected series of phacoidal and lensoidal bodies separated by thin septae of metamorphic layered rocks. The bodies appear to be regionally conformable with the structure of the metamorphic rocks but have slightly to moderately discordant contacts when viewed on an outcrop scale.” Gneiss domes of this general description, though not abundant, are common in the southwestern part of the Arabian Shield. The rocks range from quartz diorite to quartz monzonite and generally contain both biotite and muscovite but no hornblende. The fabric of these rocks is strongly planar, defined primarily by oriented micaceous minerals and stretched inclusions. This foliation is commonly antiformal or domical, paralleling that of the deformed metamorphic rocks at the margins. Contacts between the gneiss domes and adjacent stratified units vary in definition, some being determinable to within a millimeter, as where the intruded strata are amphibolite-grade metavolcanic rocks, and others being almost completely gradational from sillimanite-bear-ing metapelites into biotite-muscovite granodiorite gneiss. Locally, garnet is an accessory mineral. The metamorphic grade of stratified units adjacent to these gneiss domes is generally high, and kyanite and sillimanite are common where ALO, contents are adequate for their formation. Grade of metamorphism decreases away from the domes to that of an average greenschist facies, the minimum metamorphic grade of all older strata in this area of theGENERAL GEOLOGIC SETTING 7 Arabian Shield. This metamorphic gradient adjacent to the domes indicates a clear genetic relationship between the gneiss domes and high-pressure, high-temperature metamorphism, but designation of either as cause or effect is generally uncertain. The gneiss domes are generally elongate, doubly plunging antiforms whose axes and longest dimensions are concordant with the generally north-south trend of schistosity, axes of folds, and faults in the strata intruded. Although the gneiss domes may be bounded by the north-trending shear zones, few are actually truncated by the zones. Minor fractures offset the domes, but in general, most field evidence indicates that emplacement of the domes was simultaneous with movement on the north-south shear zones and with the most intense deformation of both the basaltic and andesitic assemblages. These characteristics have led some workers to refer to the domes as “syntectonic” granites (see, for example, Coleman, 1973a, b). The emplacement of the gneiss domes marks a significant point in the geologic evolution of the Arabian Shield. Strata younger than the domes may be included within the andesitic assemblage but are generally only broadly folded, except near or in large northwest-trending wrench-fault zones of the Najd fault system (Brown and Jackson, 1960; Delfour, 1970). Metamorphism of post-gneiss-dome units does not exceed the greenschist-facies except in contact metamorphic aureoles around younger, late-orogenic or postorogenic granitic plutons. These youngest units of the andesitic assemblage commonly contain large amounts of plutonic detritus but are included with the assemblage because of the dominance of andesitic to dacitic volcanic and vol-caniclastic detritus. The more variable distribution of flow rocks in the post-gneiss-dome strata compared to the older units suggests that volcanic centers were more widely spaced than before, giving rise to broad areas of largely detrital units bounded by areas with interbedded flows and clastic strata with fewer areas of predominantly volcanic units. LATE-OROGENIC OR POSTOROGENIC PLUTONS Volumetrically, the units discussed represent the primary constructional phase of the southern part of the Arabian Shield. These units were emplaced or deposited during a period of active orogenesis, and most are deformed and metamorphosed, exhibiting isoclinal folding, cataclasis, and local migmatization. Early plutonic rocks are less obviously deformed than are stratified units, but they are generally foliated and commonly highly sheared. Late-orogenic or postorogenic units, however, show little evidence of deformation or metamorphism, although those bodies adjacent to such younger structures as faults of the Najd fault system may exhibit local effects. Two major groups of plutonic rock units occur as late-orogenic or postorogenic bodies: (1) layered gabbro and associated rocks and (2) granodiorite to granite intrusions. LAYERED GABBRO AND ASSOCIATED ROCKS Gabbroic masses occur as mafic phases of the dio-rite batholiths, as small masses of metagabbro intruding metavolcanic units, and as largely unmetamorphosed, undeformed, subcircular, layered bodies. The first two of these groups are associated with the active orogenic andesite and diorite magmatism of the island arc and probably represent the plutonic equivalents or more mafic differentiates of island-arc basalts. The layered gabbros are younger than the other groups, being intruded after the gneiss domes but before the last thermal events affecting the Arabian Shield. The layering of these gabbros dips inward from the margins, where it is commonly steep, and flattens toward the center (Coleman and others, 1972, 1973). Compositionally, the layering is rhythmic, controlled largely by variations in amount of plagioclase, and represents crystallization and settling from a subalkaline tholeiitic magma. GRANODIORITE TO GRANITE INTRUSIONS Late-orogenic or postorogenic granodiorite to granite plutons that yield K-Ar ages between 520 and 620 m.y. (Fleck and others, 1976) intrude the pre-gneiss-dome units and either intrude the youngest andesitic strata or are their plutonic equivalents. These plutons occur throughout the Arabian Shield, representing the last major shield-forming event, which has been equated with the Pan-African event or orogeny of the African continent (Fleck and others, 1976). The intrusions commonly deflect and invade the north-trending shear zones of the shield, at only a few places showing small displacements produced by late movement. The plutons are affected, however, by left-lateral offsets, some of which may be large, along northwest-trending fractures of the Najd fault system and by small, generally right-lateral displacements having an east-west trend that may be conjugate to that of the Najd (Fleck and others, 1976). In composition, the plutons range from granodiorite to granite and follow a calc-alkaline differentiation trend (Greenwood and Brown, 1973).8 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD Although the granodiorite to granite bodies occur as batholiths in some areas, especially in the eastern part of the Arabian Shield, the more common occurrence is as subcircular ring structures or arcuate, lensoidal cone-sheets emplaced between similarly shaped plates of host rock. Many have steeply dipping primary foliations that parallel their margins. Some of these bodies are interpreted as subvolcanic intrusions, whereas cleavage in others suggests dia-piric emplacement. With the exception of volumetri-cally insignificant mafic and siliceous dikes that may cut these plutons, the granodiorite to granite bodies represent the youngest plutonic units of the Arabian Shield. NAJD FAULT SYSTEM Strata younger than the late-orogenic or post-orogenie plutons are confined to the northern part of the shield and are generally related to down-faulted areas of the Najd fault system (Hadley, 1974; Delfour, 1970). Although volcanic units occur within these strata, the units are dominantly clastic sedimentary rocks derived by erosion from adjacent fault blocks. The Najd fault system trends northwest-southeast across the entire exposure of the Arabian Shield (fig. 1). Brown (1972) suggests a composite left-lateral offset of as much as 240 km along the system. Only the easternmost part of the area studied by us (fig. 3) is within the Najd fault system. PREVIOUS GEOCHRONOLOGIC STUDIES Most of the previously reported age determinations from the Arabian Shield are either K-Ar analyses or Rb-Sr model ages on minerals. The Rb-Sr ages were calculated from assumed initial 87Sr/86Sr ratios. These results provide a reasonably accurate view of the most recent thermal or tectonic event but generally “telescope” older ages toward that event as mineral ages are reset. In this paper the term “apparent age” is used to describe those ages that are analytically correct but are not necessarily representative of the time of formation or “true age” of the unit studied. “Indicated age” is used in a similar sense by some authors. Many workers—Brown and others (1963a, b), Bramkamp and others (1963a, b), and Jackson and others (1963)—report both K-Ar and Rb-Sr apparent ages on 1:500,000 scale maps of the shield. Other age determinations are reported by Coleman and others (1972), Brown (1970, 1972), and Lenz and others (1973). Fleck and others (1976) studied rocks from the southern part of the shield in order to investigate the effectiveness of the K-Ar technique in age studies of this area. Because of the obvious inadequacies of K-Ar studies in the older units, studies reported here were begun using Rb-Sr whole-rock isochrons. Fleck and others (1976) conclude that a pervasive orogenic event with at least two pulses or thermal maxima affected the Arabian Shield between 510 and about 610 m.y. (B.P.) and correlate this event with the Pan-African (Mozambiquian) orogeny or event of eastern Africa. Potassium-argon ages of amphiboles showed that the diorite-granodiorite (here called diorite to trondjhemite) batholiths and the tonalite to granite (here called granodiorite) gneiss domes were at least 765 m.y. old. Preliminary results of the Rb-Sr studies (Fleck and others, 1973) suggested ages of 958 ±22 m.y. and 759 ±36 m.y. (A^=1.39x 10-11 yr-1), respectively, for one of the diorite batholiths and one gneiss dome. Preliminary interpretations of results presented in this report are cited by Greenwood and others (1976, 1977), who suggest a general chronology of orogenesis and plate-tectonic evolution of the Arabian Shield from before 960 m.y. to 550 m.y. The present report, presenting Rb-Sr whole-rock isochrons for the maj or rock groups, modifies that chronology and extends the plate-tectonic model to explain the late-stage orogenesis of the Pan-African event. ANALYTICAL TECHNIQUES Most rock samples collected for Rb-Sr analyses were heavier than 10 kg, although locations and access occasionally obviated the collecting of samples of this size. Weathered material was removed. In all cases the volume of material powdered for analysis was greater than that of a cube whose edges were at least 10 times the maximum dimension of any contained mineral grain. In no case, however, was the weight of material used less than 1 kg. In foliated rocks, these dimensions were arbitrarily increased such that the maximum width of mineral bands was used instead of grain size to determine sample size. Most crushed samples averaged 3 kg or more. This material was crushed so that the entire amount would pass 150-mesh Tyler sieves. This sieve size was changed to 200 mesh in the latter part of the study. After sieving, the powder was mixed thoroughly and an aliquant of approximately 30 g was taken for analyses. Rubidium and strontium concentrations used in this study were measured either by mass-spectro-metric isotope dilution or by X-ray fluorescenceANALYTICAL TECHNIQUES 9 analysis calibrated by standards measured by isotope dilution. For isotope-dilution analyses, standard silicate-digestion (HF + HC104) and ion-exchange techniques were followed. Because of the use of an S7Rb spike, rubidium and strontium were analyzed in separate splits to permit correction of 87Sr/86Sr ratios for even very low levels of rubidium. X-ray fluorescence analyses were made using the technique of Norrish and Chappell (1967), modified slightly to correct for short-term instrumental drift. Uncertainties in isotope-dilution analyses were generally less than 1 percent, as determined by replicate measurements. The principal uncertainty appears to have been sample inhomogeneity, as data for both blank runs and spike calibrations indicate errors of less than 0.5 percent (one standard deviation). Standard deviations of isotope-ratio measurements were generally less than 0.0002, with standard errors of the mean of less than 0.0001. Uncertainties in concentrations determined by X-ray fluorescence were calculated as the standard deviation of the calibration curve determined on standards analyzed with the unknowns, using three to six standards with each set of unknowns. In most cases, at least one unknown from each data set was analyzed by isotope dilution and included in the calibration curve. Standard deviations varied from less than 1 ppm (part per million) to about 6 ppm for rubidium (unknowns from 1 to 200 ppm) and from about 2 to 15 ppm for strontium (unknowns from 10 to 1,200 ppm). Except those for concentrations below 20 ppm, these values generally represented uncertainties between 1 and 4 percent (one standard deviation). Isochron data were calculated using the least-squares regression technique of York (1969) with provision for correlated errors. No correlation of errors was used, however, because none of the 87Sr/86Sr values exceeded 1.0. Studies of two-error regression techniques by Brooks and others (1972) found no significant correlation of errors in samples with 87Sr/86Sr below 1.0; their recommendation to use a correlation coefficient of zero in these cases has been followed here. Uncertainties in age and strontium initial-ratio reported in this study represent values calculated by the York (1969) regression technique using estimates of analytical errors and do not reflect the actual fit of data points to, or degree of scatter about, the best-fit line. As shown by York (1969), to incorporate a measure'of the actual degree of scatter of the data points about the best line, the uncertainties reported here are multiplied by the index of fit, (SUMS/(n-2))14, given here simply as “Index.” Because Index values are less than 1.0 in all but three of the isochrons in this study, uncertainties reflecting this degree of fit are much lower than those reported. As discussed by Brooks and others (1972), this Index may be used as an indicator of geologic error on the basis of its comparison of the observed dispersion of the data with that calculated from assigned experimental errors. They suggest a cutoff between analytical and geologic error at an Index level of 1.58 \ With the exception of those for two rock units, Index values obtained in this study are consistently below this value, even when geologic error is implied by other evidence. Because the estimated experimental errors in the concentrations of rubidum and strontium are based hot on replicate isotope-dilution analyses of similar samples but on regressions of standards for X-ray fluorescence and on only limited numbers of strictly similar duplicates, we use the Index only as a comparative device, not as a quantitative measure of geologic error. As used in this paper, “isochron” refers only to the type of diagram or the best-fit line for a given set of points and does not imply any conclusion regarding presence or absence of geologic error. The decay constant of 87Rb used here is Xg = 1.42x 10-11 yr-1 (Neumann and Huster, 1976). A rubidium abundance ratio 85Rb/87Rb = 2.59265 and atomic weight of Rb = 85.46776 (Catanzaro and others, 1969) were used. Strontium isotope-ratios were normalized to the Nier (1938) 8GSr/88Sr value of 0.1194. Measurements of 87Sr/86Sr in National Bureau of Standards SrCO., 987 during the period of analyses (n = 69) yield a weighted mean of 0.71024± 0.00002 (standard error of the mean). RESULTS OF RUBIDIUM-STRONTIUM ANALYSES Locations of samples used in total-rock Rb-Sr studies reported here are shown in figure 4. Locality numbers shown correspond to those in table 1, which presents the analytical results. Samples represent all of the major geologic units shown in figure 2 with the exception of the quartz-porphyry and gabbro bodies. Rb-Sr ages obtained from these data (table 2) are discussed in the context of the generalized geologic units (fig. 2). In all cases these ages are calculated from whole-rock Rb-Sr analyses. Those 1 Following Brooks and others (1972), most authors, including us, have used (SUMS/(n — 2) ) % as the equivalent of the mean square of weighted deviates (MSWD) of McIntyre and others (1966). As pointed out by Roddick (1978), however, the MSWD is actually equivalent to SUMS/ (n —2) and not its square root. Values reported here as Index should be squared to be compared directly with MSWD and the cutoff value of 2.5 recommended for that quantity by Brooks and others (1972).10 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD 22° 20° 30 MINUTE QUADRANGLES A. Bi'r Juqjuq B. Wadi Sadiyah C. Wadi al Miyah D. Jabal Yafikh E. Al Lith F. Jabal 'Afaf G. Jabal Ibrahim H. Al 'Aqiq I. Junaynah J. Al Qarah K. Jabal Shada L. Biljurshi M. Wadi Tarj N. Wadi Harjab P. Wadi Yiba Q. An Nimas R. Khadrah S. Wadi Hali T. Jabal 'Aya U. Khaybar V. KhamisMushayt W. Wadi Malahah X. Wadi Wassat Y. Wadi 'Atf Z. Mayza Figure 4.—Location of analyzed samples from the Arabian Shield (unpatterned areas) and of quadrangles. Cross-ruled areas represent areas of Paleozoic strata. Tertiary and Quaternary units are not shown.RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 11 Table 1.—Analytical results for rocks of the Arabian Shield LNo!ity Sample No' North lat East long Rb (ppm) Sr (ppm) ^Rb/^Sr ^Sr/^Sr Rock type 1 — . 71-8-9A 18°18.6' 42-52.1' 345.7 253.5 3.958 0.73905 Granodiorite-quartz monzonite 2... . 71-8-9B 18°20.5' 42-53.4' 150.9 349.9 1.249 .71487 Do. 3 . 71-8-9C . . 18°23.5' 42-55.3' 156.1 283.6 1.594 .71788 Do. 4... 71-8-9D ... 18°26.7' 42-40.1' 107.6 163.1 1.911 .72153 Garnet-bearing granite 71-8-9E 18°26.7' 42-40.1' 112.8 71.17 4.603 .74846 Do. 71-8-9F 18°26.7' 42-40.1' 113.7 50.05 6.608 .76607 Do. 5— -71-8-11K 18°40.7' 42-39.5' 294.6 235.4 3.631 .73467 Quartz monzonite 6 -71-8-13H 19°55.0' 41-37.0' 39.9 377 .3058 .70652 Quartz diorite 7 71-8-131 19°51.0' 41-36.0' 17.6 358 .1417 .70442 Do. 8— .71-8-13J _ . 19°50.0' 41-34.5' 58.5 345 .4838 .70877 Do. 9— .71-8-14A 19°50.5' 41-19.0' 163.0 85.70 5.528 .75365 Biotite, muscovite granite 10__. ..71-8-14B 20°25.5' 41-08.5' 131.9 222.4 1.718 .71930 Do. 11—_ .71-8-15A 19°48.0' 41-52.5' 123.2 209.3 1.705 .71888 Quartz monzonite 12 .71-8-15C 19°44.5' 41-52.5' 24.68 493.5 .1447 .70485 Do. 13 .71-8-15D 20°07.0' 41-55.0' 31.71 218.3 .4131 .70709 Perthitic quartz monzonite 14—_ .71-8-15E 20°07.5' 41-54.0' 49.22 52.94 2.6955 .72859 Do. 15— .71-8-18M 20°48.6' 43-48.9' 57.00 121.0 1.364 .71711 Rhyolite tuff 71-8-18N 20°48.6' 43°48.9' 60.95 51.02 3.466 .73383 Do. 71-8-18P 20°48.6' 43°48.9' 52.05 75.19 2.006 .72244 Do. 16— .724-28J .. . 20°08.2' 42-51.0' 20.8 245 .2450 .70528 Andesite tuff 724-28K __ _ 20°08.2' 42-51.0' 23.1 187 .3584 .70656 Do. 724-28L 20°08.2' 42-51.0' 3.7 208 .0518 .70312 Do. 17—. 724-28N 20°03.6' 42-54.3' 54.2 692 .2266 .70486 Quartz diorite 18 — 724-28P 20°02.7' 42°54.2' 41.5 680 .1766 .70439 Do. 19— 724-29H . 19°46.5' 42-55.4' 60.2 534 .3262 .70587 Do. 724-291 19°46.5' 42-55.4' 92.1 545 .4887 .70760 Do. 20... .725-6L .. 18°52.0' 42-19.0' 202 101 5.830 .75419 Quartz monzonite 21 — 725-6M __ 18°43.7’ 42-09.1' 150 116 3.745 .73651 Do. 725-6N 18°43.7' 42-09.1' 160 28.4 16.50 .84947 Granite 22... .725-6P 18°48.2' 42-07.1' 131 396 .9573 .71188 Granodiorite 23 — .725-6Q 18°44.0' 42-02.0' 56.1 388 .4178 .70773 Granodiorite gneiss 24— 725-7C 18°57.9’ 42-02.7' 34.1 167 .5903 .70960 Do. 25— 725-7D 18°58.5' 42-03.4' 33.2 370 .2600 .70600 Do. 26— .725-7E 18°49.0' 42-01.2' 53.7 385 .4036 .70783 Do. 27— 725-7F 18°45.6' 42-01.6' 21.6 366 .1723 .70548 Do. 28— .725-8N 18°56.0' 41-59.0' 35.0 415 .2438 .70583 Do. 29... 742-15A 21°31.3' 39-16.0' 66.6 214 .9017 .71251 Do. 742-15B __ 21°31.3' 39-16.0' 62.3 239 .7532 .71081 Do. 30... 742-15C 21°31.1' 39-16.0' 68.6 226 .8802 .71219 Do. 31— 742-15D ... 21°30.7' 39-16.3' 66.9 214 .9035 .71237 Do. 32... .742-19R __ 17°40.8' 43-45.1' 12.5 114 .3169 .70615 Metadacite 742-19S 17°40.8' 43-45.1' 12.2 136 .2609 .70618 Do. 742-19T . . 17°40.8' 43-45.1' 22.1 150 .4282 .70761 Do. 33... 742-19U 17°41.2' 43-45.0’ 26.7 212 .3648 .70710 Do. 742-19V . _ 17°41.2' 43-45.0' 22.9 180 .3683 .70717 Do. 742-19W 17°41.2' 43-45.0' 28.7 138 .6013 .70908 Do. 34... .742-20A 17°36.2' 43-49.1' 38.6 570 .1958 .70489 Quartz diorite 742-2OB 17°36.2' 43-49.1' 94.2 157 1.736 .71905 Granodiorite 35— 742-20C ._ 17°36.3' 43-49.0' 51.9 304 .4936 .70719 Do. 742-20D 17°36.3' 43-49.0' 40.7 602 .1955 .70472 Quartz diorite 742-20E 17°36.3' 43-49.0' 45.6 396 .3332 .70586 Granodiorite 742-20F 17°36.3' 43-49.0' 55.9 379 .4261 .70678 Do. 742-20G 17°36.3' 43-49.0' 94.6 207 1.324 .71493 Do. 36— 742-201 .. 18°23' 44-04' 25.5 526 .1403 .70427 Quartz diorite 742-20J _ _ 18°23' 44-04' 35.9 562 .1844 .70467 Do. 742-2OK 18°23' 44-04' 26.2 704 .1075 .70394 Do. 742-20L _ .. 18°23' 44-04' 16.4 626 .0760 .70378 Do. 37... .742-21F ._ 18°22' 44-00' 44.1 530 .2405 .70521 Do. 742-21G 18°22' 44-00' 31.5 615 .1482 .70412 Do. 742-21H 18°22' 44-00' 37.2 767 .1401 .70402 Do. 38... 742-21A 18°05' 44-05' 29.5 496 .1715 .70455 Do. 742-2IB 18-05' 44-05' 18.4 537 .0989 .70387 Do. 40— .742-22A .. 19°01' 42-13' 60.2 62.5 2.797 .73433 Granodiorite gneiss 742-22B 19-01' 42-13' 59.5 62.9 2.746 .73318 Do. 742-22C . . 19-01' 42-13' 63.6 96.8 1.902 .72477 Do. 742-22 D 19°01' 42-13' 57.3 61.9 2.686 .73335 Do. 742-22E 19-01' 42°13' 69.8 92.0 2.200 .72724 Do. 41 — 742-22F 19-00' 42“13' 64.4 132 1.410 .71638 Do.12 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD Table 1.—Analytical results for rocks of the Arabian Shield—Continued L°No!ity Sample No- North lat East long Rb (ppm) Sr (ppm) 87Rb/80Sr 87Sr/8CSr Rock type 42 742-23F 19°03.0' 42-12.0' 46.6 329 0.4097 0.70765 Quartz diorite 742-23G 19°03.0' 42-12.0' 36.2 422 .2479 .70559 Do. 742-23H 19°03.0' 42-12.0' 1.1 488 .0063 .70282 Do. 742-231 19°07' 42-09' 35.4 365 .2807 .70616 Do. 43 742-24C 19°15' 42-09' 54.8 170 .9352 .71142 Meta-andesite or dacite 742-24D 19°15' 42°09' 14.0 228 .1779 .70477 Do. 742-24E . _ 19°15' 42-09' 34.1 209 .4721 .70739 Do. 742-24F .. . 19-15' 42-09' 36.8 345 .3085 .70662 Do. 742-24G 19°15' 42°09' 12.9 662 .0565 .70320 Do. 742-24H 19°15' 42-09' 8.5 409 .0601 .70358 Do. 742-241 19-15' 42°09' 22.2 255 .2521 .70575 Do. 44... .742-24J . ___ 19-08' 42°58' 21.4 167 .3704 .70623 Meta-andesite 742-24K 19-08' 42°58’ 50.5 58.7 2.494 .72925 Do. 742-24L _ _ 19-08’ 42“58' 50.5 52.0 2.818 .73438 Do. 742-24M . 19°08' •42-58’ 22.0 109 .5837 .70839 Do. 45— 742-25J ._ 20-40.7' 42-42.9' 113 142 2.312 .72418 Quartz monzonite 46... 742-25K 20-40.1' 42°43.2' 118 165 2.066 .72176 Do. 742-25L 20-40.1’ 42°43.2' 121 141 2.481 .72492 Do. 47... 742-25M 20-38.4' 42-37.2' 1.5 43.4 .1024 .70463 Meta-andesite 742-25N 20-38.4' 42-37.2' 2.3 53.5 .1222 .70497 Do. 742-25P 20-38.4' 42°37.2' 1.0 46.1 .0607 .70483 Do. 742-25Q 20-38.4' 42-37.2' 6.2 41.0 .4347 .70806 Do. 742-25R 20-38.4' 42-37.2' 1.1 46.7 .0695 .70445 Do. 742-25S ... 20-38.4' 42-37.2' 20.8 270 .2224 .70508 Do. 742-25T 20-38.4' 42-37.2' 11.7 327 .1033 .70439 Do. 742-25U ... 20-38.4' 42-37.2' 8.8 232 .1090 .70435 Do. 48— 742-26A 20-16.7' 42-57.2' 69.7 401 .5030 .70769 Granodiorite 742-26B 20-16.7' 42-57.2' 97.6 299 .9439 .71159 Do. 49... 742-26C . 20-17.9' 43-01.1' 115 270 1.235 .71435 Do. 742-26D 20-17.9' 43-01.1' 65.0 311 .6048 .70866 Do. 742-26E 20-17.9' 43-01.1' 137 264 1.505 .71665 Do. 50— 742-26F 20-17.2’ 43-05.0' 75.2 340 .6404 .70903 Do. 742-26G 20-17.2' 43-05.0' 107 262 1.182 .71347 Do. 51... 742-26M _ _ 20-08.9' 43-08.5' 25.4 794 .0925 .70359 Quartz diorite 742-26N 20-08.9' 43-08.5' 24.2 886 .0790 .70380 Do. 742-26P 20-08.9' 43-08.5' 82.2 316 .7537 .70945 Aplite 742-26Q 20-08.9' 43-08.5' 19.1 764 .0723 .70347 Quartz diorite 742-26R 20-08.9' 43-08.5' 74.8 304 .7119 .70908 Aplite 52.- 742-27D 21-04.0' 43-53.6’ 81 312 .7500 .70997 Rhyolite ash-flow 742-27E 21-04.0' 43-53.6' 82 352 .6743 .70911 Do. 742-27F 21-04.0' 43°53.9' 92 208 1.2814 .71572 Do. 53— 742-271 21-25.2' 43°43.5' 14 650 .0648 .70397 Pyroxene andesite 742-27J 21-25.2' 43°43.5' 39 390 .2899 .70596 Do. 742-2 7K 21-25.2' 43-43.5' 40.8 620 .1904 .70508 Basaltic andesite 54... 742-27L 21-23.7' 43-37.9' 87.0 82.1 3.073 .73137 Rhyolite ash-flow 742-27M 21-23.7' 43“37.9' 90.6 95.2 2.758 .72862 Do. 55... 742-2 7N 21-23.5’ 43-37.6' 36.6 698 .1516 .70449 Basaltic andesite 742-27P 21-23.5' 43°37.6' 13.0 358 .1050 .70412 Volcanic breccia 742-27Q 21-23.5' 43°37.6' 13.0 358 .1200 .70414 Basaltic andesite 742-27R 21-23.5' 43-37.6' 32.2 465 .2000 .70558 Volcanic breccia 742-27S 21-23.5' 43-37.6' 42.4 619 .1979 .70556 Do. 56— 742-28A 21-14.2' 43°56.9' 90.4 334 .7823 .71019 Granodiorite 742-28B 21-14.2’ 43°56.9' 89.7 338 .7667 .71024 Do. 742-28C 21-14.2' 43-56.9' 93.7 350 .7752 .71003 Do. 57... .742-28D 21-16.8' 43-54.9’ 107 296 1.044 .71222 Do. 742-28E 21-16.8’ 43°54.9' 94.2 346 .7865 .71007 Do. 58—. 742-28F 21-16.5' 43-59.4' 6.8 773 .0254 .70352 Hornblende diorite 742-28G 21-16.5’ 43°59.4' 15.7 726 .0625 .70380 Do. 742-28H 21-16.5' 43-59.4' 11.7 662 .0511 .70370 Do. 59— 742-281 21-28.1' 43°58.6' 147 312 1.364 .71619 Quartz monzonite 742-28J 21-28.1' 43°58.6' 157 332 1.371 .71553 Do. 742-28K 21-28.1' 43°58.6' 148 306 1.400 .71540 Do. 60—_ 742-28L 21-29.6' 43°52.5' 110 338 .9427 .71157 Granodiorite 742-28M 21-29.6' 43°52.5' 112 322 1.045 .71213 Do. 61— . 742-28N . 21-19.5' 43-50.6' 108 195 1.603 .71764 Quartz monzonite 742-28P 21-19.5' 43-50.6' 110 177 1.808 .71949 Do.RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 13 Table 1.—Analytical results for rocks of the Arabian Shield—Continued L°No'ity Sample No. North lat East long Rb (ppm) Sr (ppm) 87Rb/8GSr 87Sr/8CSr Rock type 62... 743-3B 20°30.9' 40-08.5' 28.6 500 0.1656 0.70469 Quartz diorite 743-3C . 20°30.9' 40-08.5' 26.4 512 .1490 .70448 Do. 743-3D 20°30.9' 40-08.5' 18.4 609 .0872 .70344 Do. 63... 743-3E 20°29.5' 40-20.6' 16.3 578 .0818 .70384 Do. 743-3F 20°29.5' 40-20.6' 10.1 492 .0593 .70337 Do. 743-3G 20°29.5' 40-20.6' 13.7 558 .0708 .70346 Do. 64... 743-3H 20°22.6' 40-15.9' 4.82 548 .0255 .70335 Metabasalt 743-31 . . 20°22.6' 40-15.9' 0.42 416 .0030 .70292 Do. 743-3J 20°22.6' 40-15.9' 12.9 347 .1077 .70468 Do. 65... .743-4G . . 20°20.6' 40-56.2' 35.2 516 .1973 .70471 Quartz diorite 743-4H 20°20.6' 40-56.2' 14.9 620 .0694 .70395 Do. 743-41 20o20.6' 40-56.2' 15.4 591 .0755 .70427 Do. 743-4J _ . 20-20.6' 40-56.2' 11.7 512 .0656 .70382 Do. 66— 743-4K 20-19.0' 40-52.3' 22.8 584 .1128 .70487 Do. 743-4L _ .. 20-19.0' 40-52.3' 44.5 458 .2812 .70623 Do. 743-4M 20-19.0' 40-52.3' 38.2 402 .2754 .70678 Do. 743-4N 20-19.0' 40-52.3' 43.3 433 .2893 .70683 Do. 67... 743-6A _ . 19-46.8' 42-17.4' 4.87 510 .0276 .70322 Trondhiemite 68... 743-6B 19-42.3' 42-18.4' 18.8 408 .1329 .70448 Do. 69... 743-6C 19-44.0' 42-19.4' 28.0 593 .1366 .70443 Quartz diorite 70 743-6D . 19-43.7' 42-19.8' 22.8 539 .1225 .70442 Do. 71 — .743-6E . 19-43.3' 42-21.5' 39.9 448 .2578 .70571 Do. 743-6F 19-43.3' 42-21.5' 11.8 701 .0486 .70333 Do. 72... 743-6G 19-43.0' 42-21.8' 17.3 744 .0674 .70348 Do. 73... 7411-9A 17-31.0' 43-03.5' 179 131 3.953 .74638 Granodiorite 74... .7411-9B 17-30.8' 43-04.1' 62.8 914 .1986 .70492 Quartz monzonite 75... 7411-9C 17-30.9' 43-04.0' 121 286 1.219 .71404 Do. 76— J-653 19-36.0' 41-58.5' 18.5 411 .1290 .70420 Quartz diorite 77— J-659 . 19-33.0' 41-57.6’ 18.1 400 .1296 .70417 Do. 78— .J-667 . _ _. 19-31.0' 41-59.3' 19.3 439 .1258 .70407 Do. J-669 _ 19-31.0' 41-59.3' 6.6 589 .0316 .70312 Do. 79— J-606 . . 19-54.4' 41-34.2' 16.3 370 .1277 .70450 Do. 80... -J-625 ... __ 19-51.3' 41-34.6' 11.5 381 .0874 .70401 Do. 81... J-632 19-52.6' 41-34.8' 19.0 394 .1395 .70464 Do. 82... .725-9A .. . 19-29.0' 41-37.0' 26.2 376 .2014 .70552 Do. 83 — .725-9B ___ 19-23.6' 41-35.3' 23.6 407 .1686 .70490 Do. 85— 742-26H 20-20.1' 43-10.0' 21.7 703 .0891 .70387 Do. 742-261 20-20.1' 43-10.0' 21.4 798 .0776 .70327 Do. 86... 742-26J 20-19.8' 43-08.5' 13.1 948 .0401 .70329 Do. 87— .742-26K ... 20-05.6' 43-04.0' 52.1 567 .2655 .70547 Do. 88... 7411-7A 18-21.7' 43-59.5' 33.9 867 .1131 .70394 Granodiorite 89— .7411-7B 18-24.3' 43-45.6' 18.2 1214 .0433 .70314 Quartz diorite 7411-7C 18-24.3' 43°45.6' 54.4 858 .1834 .70465 Granodiorite 7411-7D 18-24.3' 43°45.6' 39.8 440 .2622 .70539 Quartz diorite 90... 7411-7E 18-23.3' 43°46.8' 23.0 1019 .0653 .70351 Do. 7411-7F 18-23.3' 43-46.8' 33.4 821 .1177 .70395 Granodiorite 7411-7G . 18-23.3' 43-46.8' 26.9 862 .0901 .70375 Do. 7411-7H 18-23.3' 43°46.8' 19.7 979 .0581 .70347 Quartz diorite 91... .7411-8A .. 18-21.9' 43-42.3' 72.7 518 .4062 .70684 Granodiorite 7411-8B 18-21.9' 43-42.3' 74.3 508 .4236 .70701 Do. 92... .7411-8C 18-28.3' 43-44.8' 43.3 723 .1732 .70457 Do. 7411-8D 18-28.3' 43°44.8' 24.4 509 .1388 .70451 Quartz diorite 93—. .7411-8H 18-08.2' 43-38.5' 3.02 636 .0138 .70232 Do. 7411-81 18-08.2' 43-38.5' 2.72 1294 .0061 .70242 Do. 94... .7411-8J . 18-07.0' 43-37.8' 2.8 1048 .0077 .70256 Trondhjemite14 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD Table 2.—Summary of Rb-Sr ages and isochron data [See table 1 for listing: of localities] Rock unit Locality No. Fig. No. Number of points Mean Rb/Sr Apparent Age (m.y.) (87Sr/8GSr)o (SUMfin>V4 64 0.016 Basaltic assemblage: Basalt of Wadi al Faqh----------- Andesite assemblage: Rhyolite of the Murdama Group_____ Meta-andesite, Junaynah quadrangle_____________________ Meta volcanic rocks of Wadi Shuklalah______________________ Meta volcanic rocks of Wadi bin Dwaynah 1 ______________________ Khadrah Formation metavolcanic rocks _________________________ Metavolcanic rocks of Hishat al Hawi __________________________ Arphan Formation of Hadley (1976) ________________________ Arphan Formation of Hadley (1976) ________________________ Juqjuq Formation of Hadley (1976) ________________________ Rhyolite of the Bi’r Juqjuq quadrangle_______________________ Diorite to trondhjemite batholiths, Al Lith and Biljurshi areas: Quartz diorite of Biljurshi (set A) _______________________ Quartz diorite of Biljurshi (set B) _______________________ Quartz diorite of Wadi Khadrah____ Quartz diorite of Wadi ash Shaqah ash Shamiyah____________ Quartz diorite of Wadi Qanunah 2 _ Diorite to trondhjemite batholiths, An Nimas batholith: Quartz diorite of An Nimas_______ Trondhjemite of Wadi Asmak_______ Quartz diorite of Wadi Tarj______ Quartz diorite of Al Mushirah____ Diorite, trondhjemite, and granodiorite batholiths, central and eastern areas: Quartz diorite of Jabal Umm al Hashiyah ______________________ Quartz diorite of the Al Qarah quadrangle_____________________ Hornblende diorite of Hadley (1976) _______________________ Quartz diorite of Wadi Makhdhul _ Quartz diorite of the Malahah Dome___________________________ Granodiorite of Wadi Malahah_____ Quartz diorite of Simlal_________ Granodiorite gneiss-domes Granodiorite gneiss of Jabal Mina __________________________ Granitic gneiss of Wadi Bishah___ Granodiorite gneiss of Wadi Bagarah_______________________ Granitic gneiss of Jiddah Airport _ Granitic gneiss of Harisi Dome___ Late-orogenic, or postorogenic, granodiorite to granite plutons : Tindahah batholith of Coleman (1973a) _____________________ Bani Thuwr pluton of Coleman (1973b)3 _____________________ Granite of Jabal Shada3__________ Granite of Jabal Ibrahim 3_______ Quartz monzonite of Wadi Shuwas________________________ 15 10 3 .786 16 6 3 .076 32,33 9 6 .135 43 8 4 .070 44 7 4 .569 47 9 4 .062 52 7 3 .312 55 6 2 .047 53 11 3 .063 54 7 2 1.005 6-8 12 3 .107 79-81 12 3 .041 62-63 12 6 .035 65-66 12 8 .059 82-83 12 2 .064 42 13 4 .082 67, 68 13 2 .028 69, 71, 72 13 4 .044 76-78 13 4 .036 17-19 15 4 .105 51, 87 15 4 .044 58 24 3 .016 37 14 3 .061 89 16 2 .053 36, 88-92 16 13 .057 38 14 2 .047 40 17 5 .851 4 19 3 1.506 23-28 18 6 .120 29-31 18 4 .297 73-74 19 2 .716 1-3 20 3 .782 5 20 1 (1.251) 9 20 1 (1.902) 10 20 1 (.593) 11-14 20 4 .428 1,165+110 0.7029+0.0001 0.28 568± 29 .7061+ .0007 .47 -3 00 Or 1+ 96 .7025 ± .0003 .02 593+ 53 .7040+ .0003 .15 912+ 76 .7024+ .0002 .40 746+ 16 .7021+ .0002 .38 + 1 o 43 •7039±. 0001 1.67 761 + 23 .7018 ± .0003 .15 775 (T) .7028 (T) — 620+ 95 .7034+ .0003 .01 612 (T) .7045 (T) 890+ 67 .7026+ .0003 .05 848±282 .7030+ .0005 .01 895±173 .7025+ .0003 .81 853+ 72 .7030+ .0002 1.61 (846) (.7028) — 837+ 50 .7027+ .0002 .48 838(T) .7029(T) 818+ 95 .7028+ .0002 .42 747+178 .7028+ .0003 .22 723+107 .7025+ .0005 .10 724± 93 .7027± .0002 .42 522+429 ,7033± .0003 .07 843+273 .7023+ .0006 .01 720(T) .7027(T) 684+ 43 .7029+ .0001 .14 815(T) .7027 (T) — 746+114 .7043+ .0036 .40 664+ 9 .7035+ .0007 1.01 763+ 53 .7033 ± .0003 .90 763+159 .7026+ .0019 .25 773(T) .7027 (T) 626+ 17 .7037+ .0004 .16 (608) (.7032) (640) (.7032) (657) (.7032) — 636± 21 ,7035± .0003 .43RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 15 Table 2.—Summary of Rb-Sr ages and isochron data—Continued Rock unit Locality No. Fig. No. Number of points Mean Rb/Sr Apparent Age (m.y.) («Sr/»>Sr) o (SUM(SIn0>;24E 24iq^76 EXPLANATION / O Granitic gneiss of Jiddah — / / Airport (Iocs. 29-31) S' / O Granodiorite gneiss of ^^0.7033 ± 0.0003 Wadi Bagarah (Iocs. ^0.7026 ± 0.0019 I I I L 23-28) i i i l i by us for whole-rock Rb-Sr analysis (Iocs. 29-31). Results from these samples yield a whole-rock isochron age of 763 ±159 m.y., an r4 of 0.7026 ±0.0019, and Index of 0.25. These data are inconsistent with an age of 1,050 m.y., at all levels of confidence up to 92.9 percent. They do not invalidate ages reported for dioritic rocks closer to Mecca but suggest a greater similarity to two-mica granitic rocks such as those of Wadi Bagarah or Jabal Mina. The granite gneiss of Harisi Dome in the southwestern part of the Wadi ’Atf quadrangle (Iocs. 73, 74) is represented by only two samples and the uncertainties associated with the “isochron” obtained cannot be adequately evaluated (fig. 19). These rocks, like the other gneiss domes, are biotite-muscovite gneisses with foliation concordant with both the intrusive margins and the schistosity in the adjacent andesitic assemblage rocks. The Harisi dome, like the Wadi Bagarah and Jabal Mina gneiss domes, intrudes and deforms rocks assigned to the Ablah Group (Anderson, 1978a). The apparent age (773 m.y.) and r, (0.7027) obtained from the two samples are consistent with results from the other two intrusives. Rubidium-strontium studies of the garnet-bearing granitic gneiss of Wadi Bishah (loc. 4) indicate an apparent age of 664 ±9 m.y., significantly less than the 761-m.y. average of the other four domes. The Index value of 1.01 is higher but cannot be interpreted as indicating disturbance of the Rb-Sr system (fig. 19). The rt of 0.7035±0.0007 is in agreement Figure 19.—Total-rock Rb-Sr isochron diagram of the granitic gneisses of Harisi dome and Wadi Bishah. See figure 17 for explanation of isochron data.24 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD with values obtained for the other domes. The three sample sites within the body were separated by at least 100 m but represent the same general location. Because the body is strongly deformed and contains a metamorphic mineral assemblage, the possibility of metamorphic homogenization of strontium cannot be entirely disregarded. Amphibolites of the andesite unit that were intruded by the gneiss dome have high strontium concentrations and low 87Sr/8eSr ratios. Because of the relatively low strontium (95 ppm, average) in the gneiss, exchange between the gneiss and amphibolites during metamorphism could reset the 87Sr/86Sr ratio to the low value of 0.7035, even though the body has a Rb/Sr ratio of 1.5. Although we consider this possibility unlikely and regard the 664-m.y. age as a reasonable value for the age of intrusion, the data do not permit a more definitive conclusion. Regardless of whether the apparent age represents the time of intrusion or of metamorphism, major deformation occurred in the southern part of the Arabian Shield at or subsequent to 664 ±9 m.y. ago. LATE-OROGENIC OR POSTOROGENIC GRANODIORITE TO GRANITE PLUTONS Fleck and others (1976), reporting K-Ar and 40Ar/39Ar ages for late-orogenic or postorogenic (so-called “Pan-African”) granodiorite to granite plu-tons (fig. 3), conclude that most of the plutons were emplaced between about 610 m.y. and about 560 m.y., as determined by concordant mineral ages. Recalculating these data to the decay constants of Beckinsale and Gale (1969) and potassium abundance values of Garner and others (1975) changes this range to about 620 m.y. to 570 m.y. Rb-Sr results for 14 different granodiorite-granite plutons or intrusive complexes are shown in table 2. Total-rock isochron ages obtained for 8 bodies range from 587 to 643 m.y., but the youngest of these, from Wadi al Miyah (Iocs. 45, 46), has a very large uncertainty (one standard deviation is 17 percent) and high r,. The remaining isochron ages cluster between 620 m.y. and 643 m.y., averaging 629 m.y. with a mean rt of 0.7032. Model ages obtained for five other bodies by assuming this initial ratio (0.7032) range from 608 m.y. to 657 m.y., with a mean of 632 m.y., nearly identical with the mean value of isochron ages. The Rb-Sr ages indicate that the oldest K-Ar ages within the granodiorite to granite bodies are between 15 and 45 m.y. too young. Because many of these bodies contain no amphiboles and K-Ar ages reported are for micas, some of the differentials might be explained by slow cooling. As discussed by Fleck and others (1976, p. 20), however, a later thermal pulse or phase between about 550 and 520 m.y. (recalculated values) is required by the irregular regional pattern of discordance in K-Ar ages. Studies using 40Ar/39Ar ages indicate that even the oldest apparent ages have been affected to some degree. As shown by the Rb-Sr ages, the timing of the last major intrusive phase of shield formation was earlier than that defined by K-Ar ages, commencing about 645 m.y. and continuing for at least 45 m.y., and probably longer. TINDAHAH BATHOLITH The Tindahah batholith of Coleman (1973a), in the eastern Khamis Mushayt quadrangle, intrudes deformed units of the andesite assemblage and gneissic to migmatitic bodies of quartz diorite to granodiorite, called the Khamis Mushayt Gneiss by Coleman (1973a), which represent the western part of the Wadi Tarib batholith. The Tindahah batholith, which ranges in composition from granodiorite to quartz monzonite, exhibits little or no evidence of deformation and forms an elongate body whose largest dimension parallels the fold axes and foliation in adjacent units. In contrast to many other late-orogenic or postorogenic bodies, the margins of the Tindahah batholith are irregular, invading adjacent stratified units both across and along structural trends. Rubidium-strontium total-rock data for the Tindahah batholith, shown in figure 20, form an excellent isochron, with an age of 626 ±17 m.y., rt of 0.7037±0.0004, and Index of 0.16. Potassium-argon ages reported by Fleck and others (1976) for this body range from 567 to 579 m.y. when recalculated using the new constants. Although the differences between K-Ar ages are not significant at the 95-percent level of confidence, they are clearly discordant with the Rb-Sr age and indicate either slow cooling or subsequent reheating. As discussed by Fleck and others (1976), however, discordant 10Ar/39Ar age-spectra and the irregular regional pattern of K-Ar ages indicate argon loss due to a thermal event subsequent to initial cooling. BANI THUVVR PLUTON The Bani Thuwr pluton of Coleman (1973b) is a complex of coalescing ring structures that intrudes the same rock units as the Tindahah batholith approximately 20 km northwest of that body. It is a coarse-grained quartz monzonite containing large phenocrysts of pink microcline in some facies. Potassium-argon ages reported by Fleck and othersRESULTS OF RUBIDIUM-STRONTIUM ANALYSES 25 Figure 20.—Total-rock Rb-Sr isochron diagram of the Tindahah batholith and the Bani Thuwr pluton, quartz monzonites of Wadi Shuwas, and the granites of Jabal Shada and Jabal Ibrahim. Data for all isochrons are shown in table 2. Sample locations are shown in figure 4 and listed in table 1. (1976) range from 529 m.y. to 590 m.y., with obvious discordance. Because only a single sample was suitable for Rb-Sr study, no isochron could be obtained. As shown in figure 20, the single data point falls somewhat below the isochron for the Tindahah batholith but is not significantly different at the 95-percent level of confidence. A model age of 608 m.y. is obtained by assuming an r{ of 0.7032, but its uncertainty could be as much as 30 m.y., because of analytical errors or an incorrect assumption of r WADI SHUWAS Coalescing arcuate lenses or sheets of quartz mon-zonite, granodiorite, and quartz-bearing syenite (Greenwood, 1975a, 1975b) invade a dominantly diorite to quartz diorite terrane in the southeastern part of the Aqiq quadrangle and northeastern part of the Biljurshi quadrangle along Wadi Shuwas (Iocs. 11-14). Data from all of these units, called the quartz monzonites of Wadi Shuwas, form an isochron (fig. 20) with an age of 636±21 m.y., r, of 0.7035 ±0.0003, and Index of 0.43, supporting the postulate of cogenesis. Potassium-argon ages reported by Fleck and others (1976) are discordant, yielding apparent ages from 618 m.y. to 545 m.y. and having hornblende values greater than those of coexisting biotite in all but one sample (71-8-15D, loc. 13). This single hornblende concentrate was examined by 40Ar/39Ar incremental-heating (Fleck and others, 1976), evolving a discordant age-spectrum with minimum age of 588 m.y. We conclude that the discordance cannot be due to slow cooling but rather is the result of argon loss subsequent to the 636-m.y. time of intrusion. The age of the event causing argon loss is poorly defined but is presumed to be less than 545 m.y. old. JABAL SHADA AND JABAL IBRAHIM Two small subcircular quartz monzonite to granite plutons occur in quadrangles adjacent to the Wadi Shuwas bodies at Jabal Shada (loc. 9) and Jabal Ibrahim (loc. 10). The single analyses reported (table 1) for each pluton represent the only samples from these locations, and sampling was made originally for K-Ar studies (Fleck and others, 1976). Recalculated to new constants, the K-Ar ages are from Jabal Shada, biotite 602 m.y. and muscovite 598 m.y. and for biotite from Jabal Ibrahim, 610 m.y. Rubidium-strontium model ages, shown in table 2, were calculated for these bodies assuming the mean r.t of total-rock isochrons of the granodiorite to granite plutons (0.7032). Compared with the isochron for the nearby Wadi Shuwas bodies (r, = 0.7035), however, the value for the Jabal Shada sample fits precisely and the Jabal Ibrahim data point is only slightly above (fig. 20). We conclude that the granite of Jabal Shada may have been oogenetic with the quartz monzonites of Wadi Shuwas (at 636 m.y.) and that the Jabal Ibrahim body may have been emplaced slightly earlier, between 640 and 657 m.y. ago. JABAL QAL AND JABAL ’AYA Quartz monzonite to granite plutons in the western part of the Jabal ’Aya quadrangle (Iocs. 21, 22) at Jabal Qal and Jabal Barkouk intrude metasedi-26 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD mentary and metavolcanic strata of both the basaltic and andesitic assemblages. Major north-south shear zones are truncated by the intrusives, and foliation in the metamorphic units adjacent to the bodies is deflected from the regional trend. The plutons are nearly circular bodies with arcuate screens of mafic rocks subparallel to the margins. They are undeformed and clearly postdate all major deformation within the area. As shown in figure 21 and table 2, sample values from these units form a linear array on the isochron diagram, yielding an age of 620 ± 18 m.y., r4 of 0.7084 ±0.0005, and Index of 0.03. Data from a sample of a small closely related pluton at Jabal ’Aya (loc. 20) 15 km east of the Jabal Qal body also fit the isochron, suggesting that the Jabal ’Aya intrusive was coeval with Jabal Qal. Adding the data for Jabal ’Aya would reduce the isochron age slightly, yielding 614 ±13 m.y., an rt of 0.7035 ±0.0004, and an Index of 0.34. The 614±13-m.y. value represents a minimum age for the last movements on major north-trending shear zones in this area, which extend for distances as great as 300 km (see, for example, Greenwood and others, 1974). WADI HALAL The granodiorite of Wadi Halal intrudes a sequence of deformed ash-flow tuffs of the andesitic sequence in the Mayza quadrangle (Anderson, 1978b). The body is essentially circular but is itself cut by a younger reddish granitic phase. Foliations in the andesitic unit are deflected around the intrusion, Figure 21.—Total-rock Rb-Sr isochron diagram of the quartz monzonites of Jabal Qal and Jabal ’Aya. See figure 20 for explanation of isochron data. which has metamorphosed some of these strata to the amphibolite facies. The metavolcanic rocks of Wadi Shuklalah (Iocs. 32, 33) are from this metamorphosed sequence, and as stated, yield a whole-rock isochron age of 593 ±53 m.y. Values for samples of the granodiorite are plotted in figure 22, where the array defines a good isochron showing an age of 643 ±20 m.y., r, of 0.7028 ±0.0002, and Index of 0.52. Samples 742-20A and 742-20B were collected approximately 0.5 km east of the other samples in what Anderson maps as a separate “border” phase of the body. Both samples appear to be displaced slightly above the isochron, indicating a slightly higher r4 (0.7031) but an identical (2-point isochron) age of 644 m.y. The younger “red-granite” phase that intrudes the granodiorite may well be responsible for homogenization of strontium isotopes in the metavolcanic rocks at about 593 m.y. That phase was not collected as part of the current study, however. Because much of the granodiorite of Wadi Halal is foliated, it is hypothesized that emplacement may have been diapiric, as the magma of the younger, granitic phase drove a nearly circular “piston” of metavolcanic rocks and granodiorite upward. This hypothesis is strengthened by the presence of subcircular fractures that are generally concentric with arcuate sheets of red granite. WADI AL MIYAH AND WADI MUSAYRAH Granodiorite units in the area north and east of the town of Bishah (fig. 3) show a vague, possibly primary, foliation but are structurally, chemically, Figure 22.—Total-rock Rb-Sr isochron diagram of the granodiorite of Wadi Halal. See figure 20 for explanation of isochron data.RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 27 and isotopically distinct from the highly deformed (gneissic) granodiorite and quartz diorite bodies such as those of the southwestern part of the A1 Qarah quadrangle (Iocs. 51, 87) that give ages of 724 m.y. Samples of the granodiorite of Wadi Mu-sayrah (Iocs. 48-50, fig. 4) define a precise isochron (fig. 23) of 623±18 m.y. with an rjof 0.7033 ±0.0003 and Index of 0.42. The average Rb/Sr ratio of the samples is significantly greater than those of the older batholiths (table 2), while Sr concentrations are less than half (table 1). The granodiorite of Wadi al Miyah (Iocs. 45, 46) intrudes metavolcanic strata of the andesite unit (loc. 47), but its relation to dioritic units in the region is not known. Feldspars in samples of the Wadi al Miyah body are somewhat altered, but it is not clear whether this alteration accompanied deformation or was significantly later. A statistically acceptable isochron may be drawn through the three data points, giving an apparent age of 587 ±77 m.y. (fig. 23), but the large uncertainty reflects both the dispersion of the data and the inadequate spread in Rb/Sr ratio. Only one point from the Wadi al Miyah set, 742-25L, diverges significantly from the Wadi Musayrah isochron. That point has the highest Rb/Sr ratio and falls below the isochron, a characteristic pattern of geologic error in Rb-Sr systems (see, for example, Brooks and others, 1972). The data are consistent with a 623-m.y. age for both bodies, although stron- Figure 23.—Total-rock Rb-Sr isochron diagram of the grano-diorites of Wadi al Miyah and Wadi Musayrah and an aplite dike of the southwestern Al Qarah quadrangle. See figure 20 for explanation of isochron data. tium concentrations in the Wadi al Miyah samples are much lower (table 1). On its southern and eastern margins, the granodiorite of Wadi Musayrah intrudes a broad area of quartz diorite discussed previously (Iocs. 17-19, 51, 87; fig. 15) that gives an age of 723 m.y. At one of these localities (loc. 51) an aplite dike was sampled that cuts across foliation and compositional banding in the gneissic quartz diorite but is tightly folded coaxially with isoclinal folds in the banding of the gneiss. As the dike is less tightly folded than is the banding in the quartz diorite, its emplacement clearly postdated initial deformation. The folding, however, does indicate significant deformation subsequent to intrusion, probably at temperatures sufficiently elevated for plastic flow. Rb-Sr data for two samples of the dike are shown in figure 23. The points fall slightly below the isochron of samples of the granodiorite of Wadi Musayrah located 15 to 20 km to the north, but a line through the two points is clearly parallel to the isochron. The apparent age of the two-point “isochron” is 621 m.y., essentially identical with the Wadi Musayrah value of 623 ±18 m.y. The rf of the dike, 0.7028, is equal to the lowest value obtained from any of the late-orogenic or post-orogenic bodies but is exactly the value to be expected if the aplite were derived by partial melting (mobilization) of the 723-m.y.-old gneissic host rock at that location (southwestern Al Qarah quadrangle) 623 m.y. ago. The weak foliation of the granodiorites of Wadi Musayrah and Wadi al Miyah and the obvious deformation of the aplite dike of the same age show that deformation and, undoubtedly, metamorphism continued in the Bishah area after 620 m.y. ago. Because this deformation involved folding and the development of foliation along the major north-south trend at a time when all similar deformation in the area of Jabal Qal (loc. 21, 22) had ceased, deformation in the Bishah area must have occurred, or at least continued, after that to the west. Although the granodiorites described are among the youngest intrusive bodies in the shield, a still younger “red granite” represents the youngest intrusive unit within this region. Potassium-argon studies by Fleck and others (1976) indicate concordant biotite and hornblende ages of about 605 m.y. from these red granites, although some biotite and plagioclase ages are as young as 536 m.y and indicate subsequent loss of radiogenic argon. The red granites intrude the granodiorite of Wadi al Miyah, apparently disturbing the Sr isotopic systems to some degree. Because of the low strontium"Sr / "Sr 28 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC concentrations and high Rb/Sr ratios of the Wadi al Miyah granodiorite, small disturbances of the system could be apparent even though they may occur within 30-50 m.y. after intrusion. OTHER LATE-OROGENIC OR POSTOROGENIC PLUTONS Late-orogenic or postorogenic intrusions in the northeastern part of the region studied (fig. 3) show the same range in age as those in the southern and western parts, but in the northeast the bodies are generally deformed by the Najd fault system (see, for example, Greenwood and Brown, 1973; Hadley, 1976). The system is represented not only by northwest-trending, apparently left-lateral strike-slip faults but also by penetrative deformation of units adjacent to the major shear zones, forming strong planar and linear fabrics. These structures, however, have distinctly northwesterly trends, whereas the generally older regional trends are more nearly north-south or slightly northeasterly. Effects of deformation after 600 m.y. ago are well displayed in the Bi’r Juqjuq quadrangle (Iocs. 52-61), studied by Hadley (1976). Although the resetting of Rb-Sr ages of units of the andesitic assemblage can probably be related to emplacement of the granodiorite to granite plutons, the age of hornblende diorite in the eastern part of the quadrangle (loc. 58) cannot. As seen on the isochron diagram for samples of this unit, figure 24, the three samples, all with Rb/Sr ratios less than 0.03, yield an apparent age of 522 ±429 m.y., r( of 0.7033 ±0.0003, and Index of 0.07. The samples all show petrographic evidence of greenschist-facies metamorphism, and the unit is intruded by a granodiorite to quartz monzonite complex that occurs within much Figure 24.—Total-rock Rb-Sr isochron diagram of the hornblende diorite of Hadley (1976). See figure 20 for explanation of isochron data. EVOLUTION, SOUTHERN ARABIAN SHIELD of the eastern one-third of the Bi’r Juqjuq quadrangle. Of the Rb-Sr results from samples from four different locations (Iocs. 56, 57, 59, 61) within this complex intrusion, shown in the isochron diagram of figure 25, only the two from the Jabal Tarban area (loc. 61) have sufficient variation in Rb/Sr ratio to give an independent indication of age for a single locality. That value is 635 m.y., with an intercept of 0.7031, close to the average rt (0.7032) of isochrons from rocks of this group. The remaining granodiorite or quartz monzonite samples show significant dispersion that may be the result of metamorphism but also may indicate differences in age and (or) rt within the intrusive complex. Model ages generated by using the weighted mean 87Sr/86Sr and 87Rb/86Sr values for each area and and assuming an r{ of 0.7032 yield apparent ages of 620 m.y. for the gneissic granodiorite (loc. 56, 57) and 636 m.y. for quartz monzonite to the northeast (loc. 59). If all samples are presumed to be coeval, they yield an isochron age of 651 ±25 m.y., r4 of 0.7029 ±0.0004, and Index of 0.92. These values indicate a significantly greater age than the age of the hornblende diorite (522 m.y.) intruded by the complex. In addition to showing evidence of greenschist-facies metamorphism, the hornblende diorite body is cut by en echelon fractures of the Najd fault system, is the host for a closely spaced system of mafic dikes that parallel the trend of the Najd system, and contains a pervasive foliation with a similar northwest trend. We suggest that the apparent age of the diorite may represent the time of homogenization of strontium as a result of deformation within the Najd fault system, although the uncertainty in the age is ex- Figure 25.—Total-rock Rb-Sr isochron diagram of the quartz monzonite and gneissic granodiorite of the Bi’r Juqjuq quadrangle and the quartz monzonite of Jabal Tarban. See figure 20 for explanation of isochron data.RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 29 ceptionally large because of the small range of the Rb/Sr ratio. By this interpretation, the 522-m.y. value would reflect the closing of strontium isotopic systems at the end of major deformation and green-schist metamorphism along the Najd fault system and is quite similar to that suggested by Fleck and others (1976) for that event. DISCUSSION OF THE RESULTS Rubidium-strontium ages reported here represent all major rock units of the southern part of the Arabian Shield with the exception of the layered gabbros and several exposures of serpentinite. The layered bodies are considered to be largely late-orogenic or postorogenic but older than most of the granodiorite to granite plutons (Coleman and others, 1972). In the Khaybar quadrangle (Coleman, 1973b), hornblende from the Jabal Shayi gabbro (Coleman and others, 1973) yields an 10Ar/30Ar age of 626±8 m.y. (corrected to new decay constants) and is intruded by the Bani Thuwr pluton (608 m.y., table 2). The gabbro postdates the granitic gneiss of Wadi Bishah (loc. 4), which yields an age of 664±9 m.y. (table 2). These data and those for other gabbros reported by Fleck and others (1976) are consistent with an age range of 620-660 m.y. for the layered gabbros. With the exception of the serpentinite, then, the Rb-Sr data reported here provide a detailed chronology for the evolution of the shield. The Arabian Shield is not Archean continental crust remobilized during one or more periods of intracratonic events. There is at this time no evidence to support the presence of any sialic crust before 1,000 m.y. ago. The oldest plutonic rocks studied aie about 900 m.y. old. Units of the basaltic assemblage represent the oldest rocks in the southern part of the shield and the constituent rock types indicate marine sedimentation and volcanism remote or isolated from a continental landmass. Whether part of the andesitic assemblage might be as old as the 1,165-m.y. age obtained from the basaltic assemblage cannot be resolved at this time. The oldest rocks of the andesitic assemblage studied yield an age of 912 ±76 m.y., almost precisely the age of the oldest plutonic rocks studied. Although none of the major rock units has been so adequately sampled as to represent the entire shield, the similar age ranges, initial 87Sr/86Sr ratios, chemical compositions and Rb/Sr ratios of flow units of the andesitic assemblage and the dio-ritic batholiths are strong evidence of a common genesis. The greatest time spans not represented by either andesitic or dioritic units during the period from 920 to 720 m.y. are 37 m.y. (between 890 and 853 m.y.) and 30 m.y. (between 815 and 785 m.y.). Considering the problem of representative sampling of an area the size of the Arabian Shield, as well as the uncertainties in the ages themselves, however, these “gaps” cannot be assigned much significance at present. The model of Proterozoic evolution of the Arabian Shield presented by us previously (Greenwood and others, 1976) is consistent with data presented here, although the earlier results have been modified to reflect the newly recommended decay constant for 87Rb as well as the analyses of additional samples. Although the tectonic episodes delineated by Greenwood and others (1976), which represent specific deformational periods in the areas where they were defined, may eventually be recognized more widely, we cannot extend these “events” shieldwide. At given periods of time, deformation, uplift, and erosion occurred in some areas while others were the sites of rapid accumulation of volcanic, volcaniclas-tic, and even carbonate strata. This situation was reversed later, providing evidence of repeated local cycles of deposition, deformation, intrusion, and erosion. Folded schistosities in the oldest strata demonstrate the repetitive nature of deformation, although folds from later events may be coaxial with those developed earlier. In short, multiple tectonic and magmatic phases are detectable within specific areas during the period between 920 and 680 m.y. ago, but some of these episodes were both temporally and areally restricted and did not affect broad areas of the Arabian Shield. This period of andesitic volcanism, diorite intrusion, island-arc sedimentation, and deformation with generally north-south structural-trends is temporally defined by the ages of the diorite batholiths. We favor treatment of this orogenesis as a single geologic event, the Hijaz tectonic cycle or orogeny (Brown, 1972; Greenwood and others, 1973, 1976), which represents the major period of development of the Arabian subcontinent and formation of continental crust within, an “oceanic” geologic environment. Whereas plutonism and island-arc volcanism and sedimentation of the Hijaz tectonic cycle spanned a period of about 200 m.y., the culminating plutonic phase of orognesis was comparatively brief, covering approximately 40 m.y. between about 650 and 610 m.y. ago. Magmas of this late-orogenic phase were much more evolved than those in earlier phases; plutons average over 70 percent Si02,30 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD whereas diorite bodies, including trondhjemite, average only 64 percent (Greenwood and Brown, 1973). As shown by Greenwood and Brown (1973), the differentiation trends of the two groups diverge significantly, suggesting derivation of the magmas from different source materials. Rubidium-strontium studies not only support the interpretation of differences in the sources of Hijaz and Pan-African magmas but also provide data that reflect the evolution of sialic crust in Saudi Arabia. From correlation diagrams of (87Sr/86Sr)0 and (Rb/Sr) with age (fig. 26), it is evident that both ratios are universally low in the early magmas. Even the metavolcanic rocks of Wadi bin Dwaynah (912 m.y.), which contain 74 percent Si02, have low Rb/Sr ratios and probably represent trondhjemitic magma. Between 770 and 780 m.y., however, magmas enriched in rubidium (Rb/Sr ratios greater than 0.3) appeared for the first time. Magmas in which Rb/Sr ratios were below 0.2 were still common, but the average value was distinctly greater 2.5i-1----1---1----1---1---1—ctt A Volcanic rocks 2.0 O Diorite-trondhjemite 1 O □ Granodiorite gneiss 1 1 CO 1.5 O Diorite-gronodiorite 1 d n nr ID O Granodiorite-granite 1 l 1 o 1 A - □ □ O 0 A o o 0.5 0.2 a/? 1 rs i a ^ADn R> i O 1 1 0 900 800 700 600 B AGE, IN MILLION YEARS Figure 26.—Variation of (A) initial 87Sr/“Sr ratio and (B) Rb/Sr with age. Vertical dashed lines are drawn at two periods of major change in strontium isotope composition and Rb/Sr ratio of magmas. Strontium-evolution line (solid line) in A represents single-stage evolution from a meteorite initial s,Sr/wSr ratio of 0.699 to the average modem island-arc value of 0.7037 (Faure and Powell, 1972; Dickinson, 1970). Dashed evolution line is least-squares fit of data points shown here and listed in table 2 having Rb/Sr ratio less than 0.2, yielding s,Sr/“Sr values of 0.7036 for the present and 0.6989 at 4,500 m.y. than that of the more primitive diorite-trondhjemite suite. Although many of the plutons having Rb/Sr ratios greater than 0.2 occur as gneiss domes, volcanic units are also represented in this group. We suggest that the change in magma type reflects an evolution of the source from one dominated by oceanic lithosphere and perhaps mantle to one including previously differentiated (that is, sialic) crust. Because this sialic component is juvenile, however, no increase in initial 87Sr/86Sr ratios is observed. As shown by Kistler and Peterman (1973), in places where magma was derived at least in part from ancient sialic crust 87Sr/86Sr ratios are elevated above those of magma derived solely from mantle or oceanic lithosphere. Initial 87Sr/8800 m.y. • Dioritic complexes ■ Volcanic rocks Rb-Sr ages 680-800 m.y. O Dioritic complexes □ Volcanic rocks 75 KILOMETERS 50 MILES Figure 27.—Distribution of dioritic complexes and metavolcanic rocks older than 680 m.y. primary volcanic material dominated by the products of presumably subaerial eruptions, including ash-flow tuff. Even though the older units of the andesitic assemblage were deposited in an area remote or isolated from eroding continental masses, by 540 m.y. ago the Arabian Shield had become part of the African plate, and the granodiorite to granite suite represents the last major magmatic event. We suggest that this event, known as the Pan-African event or orgeny (Kennedy, 1964; Holmes, 1965; Clifford, 1967, 1968, 1970) and expressed not only in Saudi Arabia but throughout much of the eastern part of the African continent, represents a phase of continental collision or suturing of Arabia to the Proterozoic African plate. Subduction would have ceased as the continental margin of AfricaDISCUSSION OF THE RESULTS 33 Figure 28.—Distribution of volcanic rocks and late-orogenic or postorogenic plutonic rocks less than 680 m.y. old. entered the trench, making this event the culminating phase of a prolonged period of plate convergence. Our model suggests that prior to the collisional phase, consumption of a margin of oceanic crust on the east side of the Proterozoic African plate by subduction along an eastward-dipping oceanic-oceanic plate-boundary (island-arc-type subduction zone) generated new sialic crust along the Arabian island arc (fig. 29). As subduction proceeded, the distance between the Proterozoic African Continent and the Arabian island arc decreased, and progressively larger amounts of terrigenous detritus were added to andesitic volcanic and sedimentary sequences in the arc (see fig. 2). Because the areas of active volcanism, plutonism, and sedimentation mi-34 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD WEST EAST 850 Proterozoic African Continent si- Continental crust1**-: 780 m.y. Volcanic arc i I ! Transitional crust Volcanic arc S.L.f 700 m. 620 Molasse S.L. Figure 29.—Plate-tectonic model of the origin and deformation of the Arabian Shield.DISCUSSION OF THE RESULTS 35 grated to the eastern part of the Arabian island arc before collision, however, exposure of the older parts of the arc may have provided both a source of juvenile plutonic detritus and a barrier limiting amounts of African continental detritus. Because subduction would have continued until the time of actual collision of the African continent with the island arc and because the axis of subduction-related volcanism and plutonism (that is, andesite and dio-rite) was migrating eastward, the hiatus between island-arc volcanism and intrusion of the Hijaz cycle and the onset of collision-related (Pan-African) intrusion and tectonism would decrease from west to east. In the easternmost parts of the Arabian arc, subduction-related tectonism and plutonism would have progressed directly into those related to collision with no hiatus at all. Deformation of 640-630-m.y.-old granodiorite in the Bi’r Juqjuq and Mayza quadrangles and the presence of 684-m.y.-old gneissic quartz diorite in Wadi Malahah may be evidence of this transition. As collision began, deformation, metamorphism, and granodiorite to granite plutonism became shieldwide. The collisional phase (the Pan-African event) probably began prior to 650 m.y. ago, subsequent to emplacement of the major diorite batholiths. Location of a suture or sutures between the Proterozoic African plate and the island arc or arcs is currently uncertain, although the report of an ophiolite body at Jabal al Wask (Bakor and others, 1976) raises the possibility that part of such a structure may lie within the Arabian Peninsula. Study of both diorite and granodiorite to granite intrusive bodies in northeastern Sudan (Neary and others, 1976), however, revealed the presence of intrusive rock types characteristic of the Arabian Shield. Darracott (1972) reports a gravity “signature” of the edge of the Pan-African orogen in East Africa, but this may well represent the limit of deformation within the African plate and could be well west of an actual suture. We would locate the westernmost parts of the Proterozoic island arc west of all occurrences of 900- to 700-m.y.-old diorite but within the area affected by the Pan-African event. The presence of subsidiary or even imbricate zones of subduction within the Arabian Peninsula cannot be discounted, but such zones are inadequately documented at present. PossibleTocations along the major north-trending shear zones, such as that at about long 42°30' E., or the major contact between the basaltic and older andesitic assemblage (fig. 3) might be suggested as areas for investigation. In addition to explaining the early evolution of the present Arabian craton, this model accounts for intraplate deformations subsequent to subduction. Tapponierand Molnar (1976) presented an indentation-deformation model for continental-collision tectonics that has application to the Arabian Peninsula. Similar, though less rigorously developed, collision models have been presented by Sengor (1976), Dewey and Burke (1973), and Dewey and others (1973). During the Pan-African event, the Arabian Peninsula was subjected to simultaneous plutonism, folding, graben formation, and transcurrent faulting with large displacement (Fleck and others, 1976; Delfour, 1970; Brown, 1972; Al-Shanti and Mitchell, 1976). Folding about nearly north-southtrending axes, such as that of 621-m.y.-old aplite dikes in the Al Qarah Quadrangle (loc. 51), must be accommodated by the same tectonic model invoked to explain the northwest-trending Najd fault system. We suggest that a continental-collision model such as that proposed for the Himalayan deformation by Tapponnier and Molnar (1976) provides a unifying explanation of Arabian intraplate tectonics. As shown by those workers, left-lateral strike-slip faults of large displacement, such as the Altyn Tagh fault north of the Himalayas, would be predicted by analogy to indentation of a semiinfinite rigid-plastic medium (Asia) by a rigid in-denter (India). The angular relations of the north-west-trending Najd fault system to the nearly north-south trend of compressional structures in Saudi Arabia are nearly identical to those between the Altyn Tagh fault and the Himalayan fold system (fig. 30). These collisionlike intraplate tectonics combined with the strong evidence of pre-Pan-African isolation from continental areas, the chemi-ical evidence cited by Greenwood and Brown (1973) and Greenwood and others (1976) for an eastdipping subduction zone, and the extended period of eastward-migrating, subduction-related magma-tism suggest that the Pan-African event represents the culmination of orogenesis in Arabia during which time the juvenile Arabian subcontinent or island arc was sutured to the Proterozoic African Continent. This last tectonic episode probably extended from before 650 m.y. to at least 540 m.y. ago, and the cooling of rocks now exposed was completed after about 520 m.y. ago.36 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD Figure 30.—A comparison of continental-collision-related structures of Asia (after Tapponnier and Molnar, 1976) with oro-genic structures of the Arabian Shield. Large arrows indicate implied direction of subduction prior to collision. CONCLUSIONS Basaltic volcanism with island arc affinities rather than mid-ocean-rift affinities occurred prior to 1,100 m.y. ago in the area presently incorporated into the Arabian Shield. Island-arc volcanism, sedimentation, and plutonism began prior to 900 m.y. ago; magmas were dioritic to trondhjemitic, probably derived by partial melting of oceanic lithosphere or, less probably, upper mantle. After about 775 m.y. ago, more alkali-rich melts appeared with quartz diorite-granodiorite replacing diorite-trondhjemite. The youngest dioritic units occur in the eastern parts of the shield, emplaced between 680 and 720 m.y. ago when the north- or northwest-trending zone of magmatism migrated to its most easterly position. After this period of subduction-related orogenesis, called the Hijaz tectonic cycle, the newly formed arc was sutured to the Proterozoic African plate during a collisional phase recognized in Arabia and eastern Africa as the Pan-African orogeny. Plutonism related to this event began between 660 and 640 m.y. ago and continued after the major deformational period was concluded. The youngest, possibly post-orogenic, plutonic rocks of the shield were probably formed between about 610 and 570 m.y. ago, but low-grade metarrorphism and wrench-fault tec-tonism continued until about 520 m.y. ago. REFERENCES CITED Al-Shanti, A. M. S., and Mitchell, A. H. G., 1976, Late Pre-cambrian subduction and collision in the A1 Amar-Idsas region, Arabian Shield, Kingdom of Saudi Arabia: Tec-tonophysics, v. 30, p. T41-T47. Anderson, R. 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York, D., 1969, Least-squares fitting of a straight line with correlated errors: Earth and Planetary Science Letters, v. 5, p. 320-324. ☆ U.S. GOVERNMENT PRINTING OFFICE: 1980 0-341-614/19*It) Z Oh | A~f / Recent Vegetation Changes Along the Colorado River Between Glen Canyon Dam and Lake Mead, Arizona GEOLOGICAL SURVEY PROFESSIONAL PAPER 1132 PEPAlrfaFNT j ■ 0 1980 LJBSA.'iY UNIVERSITY Cf CALIFORNIA s. dep°s1tor‘ FEB 15 1980Recent Vegetation Changes Along the Colorado River Between Glen Canyon Dam and Lake Mead, Arizona By RAYMOND M. TURNER and MARTIN M. KARPISCAK GEOLOGICAL SURVEY PROFESSIONAL PAPER 1132 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Turner, Raymond M. Recent vegetation changes along the Colorado River between Glen Canyon Dam and Lake Mead, Arizona. (Geological Survey professional paper ; 1132) Bibliography: p. 22-24. 1. Botany—Arizona—Ecology. 2. Botany-Colorado Valley—Ecology. 3. Vegetation dynamics-Arizona. 4. Vegetation dynamics-Colorado Valley. I. Karpiscak, Martin M., joint author. II. Title. III. Series: United States. Geological Survey. Professional Paper ; 1132. QK147.T87 581.5 79-25928 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03269-4CONTENTS Page List of common plant names and scientific equivalents ______IV Abstract____________________________________________________ 1 Introduction _______________________________________________ 1 Acknowledgments ____________________________________________ 3 Changes in Colorado River streamflow regime ________________ 3 Floods _________________________________________________ 3 Daily stage_____________________________________________ 6 Annual discharge________________________________________ 7 Monthly discharge________________________________________7 Changes in channel and alluvial deposits in the Colorado River below Glen Canyon Dam ______________________________ 8 A history of photography on the Colorado River_____________ 10 Vegetation________________________________________________ 12 Distribution of major plant species_________________________13 Bermuda grass___________________________________________14 Russian olive___________________________________________14 Saltcedar_______________________________________________14 Elm ____________________________________________________15 Camelthorn _____________________________________________15 Catclaw ________________________________________________15 Western honey mesquite _________________________________15 Fremont cottonwood _____________________________________16 Arrowweed_______________________________________________16 Longleaf brickellia ____________________________________16 Page Distribution of major plant species—Continued Rabbitbrush ___________________________________________16 Desert broom___________________________________________16 Waterweed______________________________________________17 Seep willow ___________________________________________17 Emory seep willow _____________________________________17 Apache plume __________________________________________17 Netleaf hackberry _____________________________________17 Redbud ________________________________________________17 Cattail________________________________________________17 Reed___________________________________________________18 Spiny aster____________________________________________18 Sandbar willow_________________________________________18 Goodding willow ______________________________________ 18 Desert isocoma_______________________________________ 18 Photographic documentation of changes _____________________18 Summary of changes ________________________________________19 Zone of postdam fluvial sediments----------------------19 Zone of predam fluvial sediments ______________________19 Zone of predam flood terraces, eolian deposits, and stabilized talus slopes______________________________21 General conclusion ____________________________________21 References cited __________________________________________22 ILLUSTRATIONS Page Plate 1. Map showing Colorado River from Glen Canyon Dam to Lake Mead.____________________________________________In pocket 2. Maps showing distribution of selected plant species along the Colorado River. [Includes figs. 7-15.]-----In pocket 3. Maps showing distribution of selected plant species along the Colorado River. [Includes figs. 16-25.] ---In pocket Figure 1. Graph showing yearly range between minimum daily and maximum discharge and yearly range between minimum and maximum stage of Colorado River at Lees Ferry and near Grand Canyon--------------------------- 4 2. Graph showing daily variation in river stage during two selected years, Colorado River at Lees Ferry---------- 6 3. Graph showing total annual streamflow (by calendar year) of Colorado River as recorded at Lees Ferry and near Grand Canyon ---------------------------------------------------------------------------------------- 7 4. Graph showing mean monthly discharge as a percentage of total annual discharge, Colorado River, Lees Ferry---- 8 5. Graph showing regression analyses of annual (water year) discharge versus sediment yield______________________ 9 6. Graph showing accumulated volume of degradation below Glen Canyon Dam for various periods between 1956 and 1975 ____________________________________________________________________________________________ 10 7-25. Maps showing distribution of selected plant species along the Colorado River from Glen Canyon Dam to Lake Mead: 7. Bermuda grass and elm _____________________________________________________________________________Plate 2 8. Russian olive and desert broom_____________________________________________________________________Plate 2 9. Saltcedar _________________________________________________________________________________________Plate 2 10. Camelthorn and Apache plume_______________________________________________________________________Plate 2 11. Catclaw___________________________________________________________________________________________Plate 2 12. Western honey mesquite__________________________________________________________________________ Plate 2 13. Fremont cottonwood _______________________________________________________________________________Plate 2 14. Arrowweed ________________________________________________________________________________________Plate 2 15. Longleaf brickellia_______________________________________________________________________________Plate 2 16. Rabbitbrush and waterweed ________________________________________________________________________Plate 3 IIIIV CONTENTS Figure 17. Seep willow ____________________________________________________________________________________Plate 3 18. Emory seep willow_______________________________________________________________________________Plate 3 19. Netleaf hackberry __________________________________________________________________________.__Plate 3 20. Redbud and desert isocoma______________________________________________________________________Plate 3 21. Cattail ______________________________________________________________________________________Plate 3 22. Reed _________________________________________________________________________________________Plate 3 23. Spiny aster __________________________________________________________________________________Plate 3 24. Sandbar willow _______________________________________________________________________________Plate 3 25. Goodding willow________________________________________________________________________________Plate 3 26-73. Matched photographs showing changes in vegetation ___________________________________________________26-125 TABLES Table Page 1. Mean, standard deviation, and coefficient of variation of yearly maximum stage for the Colorado River at Lees Ferry and near Grand Canyon_____________________________________________________________________________ 5 2. Mean, standard deviation, and coefficient of variation of yearly minimum stage for the Colorado River at Lees Ferry and near Grand Canyon__________________________________________________________________________ 5 3. Camera station descriptions, including dates, location, altitude, and photograph credits------------------------- 20 LIST OF COMMON PLANT NAMES USED AND SCIENTIFIC EQUIVALENTS agave Agave spp. Apache plume Fallugia paradoxa (D.Don) Endl. arrowweed Pluchea sericea (Nutt.) Coville barrel cactus Ferocactus acanthodes (Lemaire) Britt. & Rose beavertail cactus Opuntia basilaris Engelm. & Bigel. bebbia Bebbia juncea (Benth.) Greene Bermuda grass Cynodon dactylon (L.) Pers. brittle bush Encelia farinosa Gray camelthorn Alhagi camelorum Fisch. carrizo (see reed) Phragmites communis Trin. catclaw Acacia greggii Gray cattail Typha spp. coldenia Coldenia spp. creosote bush Larrea tridentata (DC.) Coville desert broom Baccharis sarothroides Gray desert isocoma Haplopappus acradenius (Greene) Blake desert plume Stanleya pinnata (Pursh) Britton desert trumpet Eriogonum inflatum Torr. & Frem. dicoria Dicoria spp. dogbane Apocynum spp. dropseed Sporobolus spp. dyssodia Dyssodia spp. elm Ulmus minor Mill. Emory seep willow Baccharis emoryi Gray evening primrose Oenothera pallida Lindl. four-wing saltbush Atriplex canescens (Pursh) Nutt. Fremont cottonwood Populus fremontii Wats. globemallow Sphaeralcea spp. Goodding willow Salix gooddingii Ball great bulrush Scirpus validus Vahl. hackberry Celtis reticulata Torr. horsetail Equisetum spp. horseweed Conyza canadensis Cronquist jointfir Ephedra torreyana Wats. longleaf brickellia Brickellia longifolia Wats. mistletoe Phoradendron californicum Nutt. monkey flower Mimulus cardinalis Dougl. Mormon tea Ephedra trifurca Torr. netleaf hackberry Celtis reticulata Torr. ocotillo Fouquieria splendens Engelm. peppergrass Lepidium montanum Nutt. poison ivy Rhus radicans L. rabbitbrush Chysothamnus spp. range ratany Krameria parvifolia Benth. red brome Bromus rubens L. redbud Cercis occidentalis Torr. red willow Salix laevigata Bebb. reed (see carrizo) Phragmites communis Trin. Russian olive Elaeagnus angustifolia L. Russian thistle Salsola kali L. var. tenuifolia Tausch saltcedar Tamarix chinensis Lour. sandbar willow Salix exigua Nutt. sand verbena Abronia elliptica A. Nels. scouring rush Equisetum hiemale L. seepweed Suaeda torreyana Wats. seep willow Baccharis glutinosa Pers. shrub liveoak Quercus turbinella Greene slender poreleaf Porophyllum gracile Benth. smooth horsetail Equisetum laevigatum A. Braun spiny aster Aster spinosus Benth. watercress Rorippa nasturtium-aquaticum (L. Schinz & Thell waterweed Baccharis sergiloides Gray western honey mesquite Prosopis glandulosa var. torreyana (Ben son) Johnston white bursage Ambrosia dumosa (A. Gray) Payne willow Salix spp. wolfberry Lycium spp. wire lettuce Stephanomeria pauciflora (Torr.) A Nels.RECENT VEGETATION CHANGES ALONG THE COLORADO RIVER BETWEEN GLEN CANYON DAM AND LAKE MEAD, ARIZONA By Raymond M. Turner and Martin M. Karbiscak ABSTRACT Vegetation changes in the canyon of the Colorado River between Glen Canyon Dam and Lake Mead were studied by comparing photographs taken prior to the completion of the Glen Canyon Dam in 1963 with those taken afterwards at the same sites. The old photographs, taken by J. K. Hillers, T. H. O’Sullivan, William Bell, F. A. Nims, R. B. Stanton, N. W. Carkhuff, N. H. Darton, L. R. Freeman, E. C. LaRue, and others, document conditions as they were between 1872 and 1963. In general, the older pictures show an absence of riparian plants along the banks of the river. The new photographs of each pair were taken in 1972 through 1976. The most obvious vegetation change revealed by the photograph comparison is the increased density of many species. Exotic species, such as saltcedar and camelthorn, and native riparian plants, such as sandbar willow, ar-rowweed, desert broom, and cattail, now form a new riparian community along much of the channel of the Colorado River between Glen Canyon Dam and the Grand Wash Cliffs. The matched photographs also reveal that changes have occurred in the amount of sand and silt deposited along the banks. The photographs show that in some areas erosion has been significant since the time of the earlier photograph while at other locations sediment has accumulated on river bars and terraces. Detailed maps are presented showing distribution of 25 plant species. Some of these, such as Russian olive and elm, were unknown along the Grand Canyon reach of the Colorado River before 1976. Relevant data are presented to show changes in the hydrologic regime since completion of Glen Canyon Dam. Flooding, as expressed by annual maximum stage, has decreased in amplitude, and its season of occurrence has changed from spring (May-June) to a longer period from April through October. Dam construction has had a moderating influence on several other hydrologic variables. Compared to the predam era, discharge through the year now varies within narrow limits, changing little from month to month or season to season; annual maximum discharges are now strikingly uniform, and sediment load has materially decreased. Increases have occurred in some characteristics, however, such as daily variation in river stage and median discharge. The interaction of decreased flooding, decreased sediment load, and increased riparian plant coverage makes the future of existing river fans, bars, and terraces uncertain. The establishment of a new ecological equilibrium at the bottom of the Grand Canyon may require many decades. INTRODUCTION When viewed as a water conveyance system, the Colorado River is unspectacular. Its long-term average annual discharge is 16,600 hm3 (Stockton and Jacoby, 1976), only about one-thirtieth the flow of the Mississippi—less than the flow of such well-known rivers as the Snake, the Missouri, and the Potomac, and less even than such little-known rivers as the Atch-afalaya (Louisiana), the Skagit (Washington), or the Apalachicola (Florida). Yet, if measured in terms of its impact on regional and national requirements for recreation, energy, and water, it is, for its size, of disproportionate importance. Many of our national parks and monuments lie within its scenic basin and the Colorado River is the erosional force that shaped the Grand Canyon, one of the great scenic wonders of the world and the main attraction in one of the more heavily visited National Parks. The Colorado River serves as a major power source for the region’s cities and industries, and as a major water source for its domestic and agricultural users. When measured in terms of the impact it has upon the daily lives of the many human occupants of the arid southwestern United States and northwestern Mexico, the Colorado River’s presence must be accorded far more importance than would be ascribed to it on the basis of discharge alone. Until recently man was not a major factor affecting the vegetational and fluvial features along the Colorado River within the Grand Canyon. But during the last few decades the river has become one of the most-used rivers in America—both in terms of recreational use and in terms of water consumption. To provide a perspective for viewing the changes along the river, some examples are cited. During the first 86 years of river travel through the Grand Canyon (.1869-1955), only 185 persons traversed the canyon by boat (Wallace, 1972); in the 1970’s roughly 15,000 persons travel through the canyon by boat each year (Larson, 1974). Coupled with the heavy recreational use has been an increasingly heavy use of the river’s water. By the construction of the four units of the Colorado River Storage Project (Flaming Gorge Dam [1962], Navajo Dam [1962], Curecanti Unit dams [1965-66], and Glen Canyon Dam [1963]) in the Upper Colorado River Basin, man has been able to control the flow in the river, curbing the spring floods and distributing throughout the year the water normally carried during these annual events of high flow. The modifications in flow re- 12 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA gime have disrupted the equilibrium which formerly existed within the Grand Canyon, the reach of the river of primary concern in this report. But even before major dams appeared in the Upper Colorado River Basin, the Lower Colorado River was impounded by a series of dams beginning with Hoover Dam (1935). By far the greatest diversions from the river occur from these impoundments lying below the Grand Canyon. Before the first diversions were made, approximately 16,600 hm3 (Stockton and Jacoby, 1976) of water reached the Sea of Cortez (Gulf of California) annually. Today, less than 1 percent of its virgin flow ever reaches the mouth of the Colorado River. Glen Canyon Dam probably has had greater impact on the riparian habitat within the Grand Canyon than the combined effects of all other river system modifications in the Upper Basin. It is difficult to conceive of a change in regional climate of sufficient magnitude to reduce average annual maximum flows from 2,486 m3/s to 803m3/s, to increase the median discharge from 210 m3/s, to increase the average diurnal fluctuation in stage from a few centimeters to several meters, to reduce the average annual water temperature from a range of 0.2° to 28°C during the predam period (1949-1962) to a range of 5.5° to 18°C during the postdam period (1962-1976), and to simultaneously reduce the median sediment concentration from 1,500 to 7 parts per million (ppm). Yet the foregoing changes have all been recorded at Lees Ferry, 26 kilometers downstream from Glen Canyon Dam. Each of these changes, and others, has had an effect upon the riparian ecosystem since the completion of Glen Canyon Dam, and inevitably, adjustments in the biota and the physical setting have occurred. Although establishment of a new ecological balance requires many years in response to the new fluvial regime, sufficient time has elapsed since the dam’s completion in 1963 to reveal many vegetational and riverine shifts. The study was undertaken to determine the nature of these transformations and to provide a basis for predicting future trends. Conditions existing prior to the regulation of flow in the Colorado River were established by examining photographs and hydrologic records made between 1872 and 1963. Postdam conditions were documented by referring to recent hydrologic records, by photographically matching scenes shown in the predam pictures, and by recording in detail the distribution of riparian plant species. In the following material, frequent reference will be made to localities along the Colorado River. In keeping with common practice, "mileage” designations downstream from Lees Ferry will follow those established by the U.S. Geological Survey in 1923 (Birdseye and Burchard, 1924). Lees Ferry, at the stream gaging station, is taken as kilometer 0. All distances are based upon this datum. Distances upstream are taken from Glen Canyon National Recreation Area map (National Park Service, no date). The metric system is given preference in this report; accordingly, distances are given in kilometers. Convention and long usage has firmly established proper names along the river such as Seventyfive Mile Rapid and Two Hundred and Ninemile Canyon. Where these place names appear they are used alone without metric equivalents. For the few localities lying upstream from kilometer 0, distance will be shown as kilometers above Lees Ferry. The long-established convention of referring to river-banks as left or right when viewed downstream is followed here. The altitude given in the captions refers to altitude at river level, regardless of the camera’s position. Before examining the changes, certain features of the Colorado River and the canyon through which it flows should be reviewed. The reach of the Colorado that we examine in this report lies between Glen Canyon Dam, 26 km above Lees Ferry, Ariz., and Pearce Ferry, Ariz., at kilometer 450 below Lees Ferry (pi. 1). This segment of the river is only a small reach of the 2,700-km-long river, yet because it traverses the Grand Canyon, it is the best known and most famous portion. Strictly speaking, the Grand Canyon extends from the mouth of the Paria River near Lees Ferry downstream to the Grand Wash Cliffs; the reach above the Paria is part of Glen Canyon. The Grand Canyon is 21 km across at its widest point, with a depth of as much as 1,800 meters. Various divisions of the Grand Canyon have long been recognized: These include such reaches as Marble Canyon, Conquistador Aisle, and Upper, Middle, and Lower Granite Gorges. In its course through the Grand Canyon, the Colorado River moves through a narrow valley and is confined by steep, high walls of mostly hard and resistant rock. There is no flood plain along much of the Colorado’s course through the Grand Canyon, and the absence of a flood plain broad enough to reduce the force of annual floods produced a predam valley devoid of the dense riparian community typical of other streams of the region. If one ignores the alternating pools and rapids, the river profile is smooth and nearly straight (Leopold, 1969). The river gradient is under dominant control of the tributary fans. The river’s tendency to move laterally is greatly reduced compared to rivers in unconfined channels (Leopold, 1969), and the river’s poorly understood proclivity for vertical entrenchment rather than lateral movement has served to maintain through millions of years the narrow gorge that today is viewed annually by nearly 3 million people. Although no river width and depth data exist for theCHANGES IN COLORADO RIVER STREAMFLOW REGIME 3 entire reach described in this report, data are available for selected shorter segments (Leopold, 1969). The available depth measurements made in 1963 represent predam conditions with a flow of 1,375 nF/s. Through the first 223.7 km below Lees Ferry, maximum river depth was 33.5 m (kilometer 183.9). Roughly 20 percent of the depth measurements, taken at 0.16-km intervals, equaled or exceeded 15.5 m, and 50 percent equaled or exceeded 11.0 m. Width measurements have been taken at 0.16-km intervals from 1965 aerial photographs for the reach between kilometer 45.1 and kilometer 177.0 (Leopold, 1969). Although only generalized width values were given, these provide a broad picture of river conditions. For example, of the approximately 800 measurements, fewer than 5 percent were less than 61.0 m, 50 percent of the observations equaled or exceeded 97.5 m, and 20 percent equaled or exceeded 125.0 m. In general terms this is the river examined here. In following chapters specific characteristics of the river will be discussed with particular emphasis on the impact of Glen Canyon Dam. ACKNOWLEDGMENTS Completion of this study required the assistance of many people. We owe a major debt of gratitude to the staff of the Museum of Northern Arizona, who provided transportation and support of many kinds during the course of the study. Support by the Museum included financial support for the junior author during the early months of his work on the study. Dr. Steven Carothers’ knowledge of the Grand Canyon ecosystem and his skill as a boatman were substantial contributions during several trips through the canyon. Others from the Museum to whom we are especially indebted include Dr. A. M. Phillips III, Dr. B. G. Phillips, Mr. George Ruffner, Mr. S. H. Aitchison, and Mr. D. S. Tomko. To the Phillips’ we owe a special debt of gratitude for their assistance in augmenting our plant distribution data. We also wish to express our appreciation for the skill and assistance of Mr. R. A. Heinz and Ms. Carroll Bennett in obtaining some of the photographic matches. Librarians at the New York Public Library, Manuscript and Archives Division; the U.S. Geological Survey Photographic Library, Denver; and Grand Canyon National Park were important contributors aiding us in the search for old photographs. The National Park Service has been a major collaborator, providing assistance in many ways, including the opportunity to accompany a Park Service crew on a boat trip through the canyon, as well as providing boat transportation between Lees Ferry and Glen Canyon Dam. To Messrs. M. A. Turner, M. S. Pierce, and Thomas Workman we owe thanks for assistance while in the canyon. Dr. P.S. Martin was an early observer of postdam vegetation changes, and his shared insight and knowledge of the area is gratefully acknowledged. Finally, the patience of our companions on each of six photograph-matching trips, as they waited while we tried to find the exact camera position of some early-day photographer, is acknowledged, with appreciation CHANGES IN COLORADO RIVER STREAMFLOW REGIME Construction of a dam across a river produces many changes in the hydrologic regime of the river system both above and below the structure. The changes immediately upstream from the dam, such as water impoundment and silt accumulation are often the most striking. The more subtle downstream hydrologic modifications include smoothing the flow duration curve, lowering maximum stages, and increasing base flow (Leopold and others, 1964). The downstream alterations in the discharge regime may directly affect riparian biotic communities. In the present chapter, pertinent streamflow records for the periods before and after construction of Glen Canyon Dam are presented as a basis for interpreting the vegetation changes that will be noted in later sections. In the chapter following this, we have also examined channel changes and some of the causes for the altered channel geometry. FLOODS Before the construction of dams along the Colorado River, flooding was commonplace. One of the better known floods occurred in November 1905, when the Colorado River left its old channel via a manmade canal and flowed into the Salton Sink, thus forming the Salton Sea (Sykes, 1937). The river was not returned to its original channel until February 1907 (Grunsky, 1907; LaRue, 1916). With completion of a series of dams along the lower Colorado River, a recurrence of this event is unlikely. Similarly, floods through the Grand Canyon have been curtailed by thg construction of Glen Canyon Dam. A river characteristic that is closely associated with flooding is annual maximum stage (fig. LA). Stage records for the Colorado River at Lees Ferry and near Grand Canyon, Ariz., have been used to illustrate general changes in maximum stage throughout the reach of the Colorado River examined in this report. (The stream gage "near Grand Canyon” is 0.4 km upstream from Bright Angel Creek and 7.5 km northeast of Grand Canyon, Ariz.) Because of variations in channel and valley geometry, the values provide only a relative measure of the height to which banks might be inundated by flood waters.4 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA 10 — EXPLANATION ? Lees Ferry CC 8 — Near Grand Canyon 1895 1920 1930 1940 1950 WATER YEAR 1960 1970 1980 Figure 1.—Yearly range between minimum daily and maximum discharge (A) and yearly range between minimum and maximum stage (B) of Colorado River at Lees Ferry and near Grand Canyon. Excluding 1965, an anomalous year, the yearly maximum stage (fig. L4; table 1) has varied little at Lees Ferry since Glen Canyon Dam was completed (mean = 3.48 m; std. dev. = 0.19 m). (The high discharge in 1965 resulted from the release of water through a diversion tunnel and hollow jets as a meansCHANGES IN COLORADO RIVER STREAMFLOW REGIME 5 Table 1.—Mean, standard deviation, and coefficient of variation of yearly maximum stage for the Colorado River at Lees Ferry and near Grand Canyon [Data, based on water years, are values for the total period of record, the predam period, and the postdam period] Period of record (1921-76) Predam (1921-62) Postdam (1963-76) (1963-76, excl. 1965)' Lees Ferry Number of years 56 42 14 13 Mean (meters) 4.67 5.04 3.56 3.48 Standard deviation (meters) 1.06 .96 .35 .19 Coefficient of variation .23 .19 .10 .06 Near Grand Canyon (1923-76) (1923-62) (1963-76) (1963-76, excl. 1965)' Number of years 54 40 14 13 Mean (meters) . 6.35 6.89 4.79 4.69 Standard deviation (meters) 1.57 1.40 .85 .79 Coefficient of variation .25 .20 .18 .17 'The year 1965 was anomalous. See text. of fulfilling downstream commitments before all the generators were in operation. Since 1965, with all generators operating, flow has been sufficient to meet downstream requirements (A.O. Dewey, U.S. Bureau of Reclamation, oral commun., 1975).) A similar but less marked stability is apparent in the record from near Grand Canyon (mean = 4.69 m; std. dev. = 0.79 m). Annual minimum stage (fig. L4; table 2) appears little changed at Lees Ferry (mean = 1.47 m; std. dev. = 0.15 m) although the record from near Grand Canyon (mean = 0.70 m; std. dev. = 0.45 m) shows a slight increase in these minimal values.1 (This increase is the result of a December 1966 flood on Bright Angel Creek which deposited new bouldery debris in the Colorado River channel, altering the control for the Colorado River gaging station (Cooley and others, 1977).) At both gaging stations the range between annual maxima and annual minima has been narrowed, especially because of reduced maxima. Thus, the effect of flowing water upon shore-zone plants is now characteristically confined to a rather narrow band at these stations and presumably elsewhere in the canyon. The maximum stages in figure 1A clearly show the reduction in the streamflow amplitude after completion of Glen Canyon Dam. The stage, which had reached 11.43 m near Grand Canyon in 1921, has not exceeded 6.07 m since 1963. Proportional changes in the maximum stage have also been recorded at Lees Ferry. Means based on the predam and postdam records are shown in table 1. The mean maximum stage at Lees Ferry for the postdam period is 3.48 m, excluding the anomalous values for 1965; the mean for the period prior to the dam is 5.04 m. The mean value for the 'These values have been obtained, in part, from unpublished data, including recording charts. When no stage was recorded, estimates were made from available data. Table 2.—Mean, standard deviation, and coefficient of variation of yearly minimum stage for the Colorado River at Lees Ferry and near Grand Canyon [Data, based on water years, are values for the total period of record, the predam period, and the postdam period] Period of record (1922-76) Predam (1922-62) Postdam (1963-76) (1963-76, excl. 1965)' Lees Ferry Number of years 55 41 14 13 Mean (meters) 1.68 1.76 1.46 1.47 Standard deviation (meters) .21 .17 .15 .15 Coefficient of variation .12 .09 .10 .10 Near Grand Canyon (1924-76) (1924-62) (1963-76) (1963-76, excl. 1965)' Number of years 53 39 14 13 Mean (meters) .52 .46 .67 .70 Standard deviation (meters) .31 .23 .45 .45 Coefficient of variation .61 .50 .68 .65 'The year 1965 was anomalous. See text. postdam period is slightly greater than the smallest maximum value (3.37 m), recorded in 1934, for any predam year (fig. 1A). Mean values for the Colorado River near Grand Canyon are also given in table 1 and show that as a result of dam construction, mean stage has fallen more than at Lees Ferry. This difference largely results from differences in valley and channel configuration at the two sites. The coefficients of variation (CV) of yearly maximum stages (table 1) during these two periods emphasize further the postdam stability of streamflow. This statistic is a measure of the magnitude of the standard deviation relative to its mean. At Lees Ferry, for the 42-year period before 1963, the CV is 0.19; for the 13-year postdam period, it is 0.06 (table 1). Near Grand Canyon the CV’s for the predam and postdam periods were 0.20 and 0.17, respectively. The decrease in CV is far less near Grand Canyon than at Lees Ferry, a fact largely attributable to flow from the Little Colorado River which enters the Colorado River between the two gaging stations. Flow in the Little Colorado is erratic, there being no large dams along the river, and the unregulated streamflow during flood stage is great enough to affect the flow of the Colorado mainstem. Prior to 1963, maximum discharges (fig. IB) were almost always greater at Lees Ferry than farther downstream near Grand Canyon. This downstream decrease in maximum discharge probably occurred because of channel storage along the 140.8-km channel from Lees Ferry to the gage near Grand Canyon. In almost all cases, the peaks at Lees Ferry and near Grand Canyon occurred during the same runoff event. Since completion of Glen Canyon Dam, annual peak flows at these stations have not been temporally correlated, and annual peak flows near Grand Canyon have usually been greater than those at Lees Ferry. Before 1963, annual peak flows near Grand Canyon exceeded6 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA those at Lees Ferry by more than 10 percent during only two years out of 40 (1923 and 1955). In contrast, during the relatively short period 1963-76, peak flows at Grand Canyon have exceeded those at Lees Ferry by more than 10 percent in 1969, 1971, and 1973 (fig. IB). The major reason for the greater peak discharges each time was the flow contributed by the Little Colorado River. In this connection LaRue (1925, p. 116) noted that the large increase in flow at the gage near Grand Canyon in 1923 was produced by a flood on the Little Colorado River. The reduced flooding along the Colorado River has provided stability in a habitat that was previously highly unstable. Sites along the riverbanks below Glen Canyon Dam are likely to experience approximately the same maximum water depth each year. Before 1963, most newly established plants were subject to possible inundation and uprooting by floodwaters that reached levels considerably higher than those of the postdam period. The dam effectively diminishes the ability of the river to maintain a bank that is periodically stripped of its vegetation. DAILY STAGE Figure 2 illustrates daily variations in river stage at Lees Ferry for two randomly selected years: one (fig. 2A) prior to the construction of Glen Canyon Dam and the other (fig. 2B) after dam construction. In the predam period, depicted by water year 1939, daily variation in stage was usually only a fraction of a decimeter except for periods of flooding when changes of several decimeters might occur. (Freeman (1930, p. 363) mentioned that when the U.S. Geological Survey party was on the river at Lava Falls in September 1923, the river rose 4.27 m during one night.) In the postdam period, as depicted by water year 1973, daily variations of 4 (/) 0c 1X1 h LU ^ 3 Z LU o < c/) 2 CC LU > tr 1 Figure 2.—Daily variation in river stage during two selected years, Colorado River at Lees Ferry: (A) water year 1939, before Glen Canyon Dam was built; (B) water year 1973, after Glen Canyon Dam was completed. Bars connect the daily maximum and daily minimum stage.CHANGES IN COLORADO RIVER STREAMFLOW REGIME 7 more than 1.5 m became typical, and daily variations of less than 0.3 m are almost nonexistent. In 1973, the abnormal releases during March and April resulted from a court decision preventing water of Lake Powell from rising to the base of Rainbow Bridge (Mann, 1976). The predam water year, 1939, shows no difference between stages on any periodic basis. However, the post dam water year, 1973, shows a significant 7-day periodic drop in stage on most Sundays with some striking drops seen on holidays such as Christmas, New Year’s, Memorial Day, and Labor Day, as a consequence of decreased power demands on Glen Canyon Dam generators. One additional note that should be made is that 1939 was a year with below normal runoff at Lees Ferry (less than 11,714 hm3) and with a relatively small maximum discharge (less than 1,416 nrVs). ANNUAL DISCHARGE Within the Grand Canyon vast differences in annual streamflow occurred prior to the construction of Glen Canyon Dam (fig. 3). Discharge, as recorded at the gage near Grand Canyon located 141 km below Lees Ferry, ranged from a low of 5,200 hm3/calendar year in 1934 to 24,500 hnrVcalendar year in 1929. Since 1962, however, annual flow has ranged from a low of 2,000 hm3/ calendar year, when Lake Powell was filling, to 14,500 hnrVcalendar year in 1965. Postdam streamflow generally falls within the range of 9,900-12,300 hm3/ calendar year. The greater flow at the Grand Canyon compared with that at Lees Ferry, averaging approximately 493 hm3 annually, results from the contribution of the Little Colorado River (Thomas and others, 1960). MONTHLY DISCHARGE Prior to construction of Glen Canyon Dam, the maximum monthly mean discharges generally occurred during the month of June as a result of spring snowmelt in the high mountains at the headwaters of the Colorado River. During postdam years the maximum monthly means have occurred in May and are primarily the result of power and irrigation demands. During the predam period the maximum monthly mean discharge, 4,300 hm3, was more than ten times greater than the lowest monthly mean discharge. During the postdam period, the ratio of maximum to minimum is only 1.8 : 1. The seasonal variability was strongly unimodal during the predam period with a maximum during May and June and a minimum during December, January, and February (fig. 4). Much of the old pattern is now lost. The timing of the annual peak is little changed, Figure 3.—Annual streamflow (by calendar year) of Colorado River at Lees Ferry and near Grand Canyon.8 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 4.—Monthly mean discharge as a percentage of total annual discharge, Colorado River, Lees Ferry: open bars, period before Glen Canyon Dam (calendar year 1901 through calendar year 1962); solid bars, period after Glen Canyon Dam (April 1963 through March 1977). but the relative magnitude of the monthly values now shows little variation. CHANGES IN CHANNEL AND ALLUVIAL DEPOSITS IN THE COLORADO RIVER BELOW GLEN CANYON DAM Except through the Marble Gorges where the canyon is narrow and its walls descend steeply to the river, there are few reaches of the Colorado through the Grand Canyon in which recent alluvium is not a conspicuous feature of the fluvial environment. In some reaches, such as those between Glen Canyon Dam and Lees Ferry, from Lava Canyon to Unkar Creek, and the segment above and below Granite Park, the canyon floor is fairly broad and alluvial flats are well developed (figs. 27, 30, 33, 47, and 64).2 In most other places, however, deposits of fine alluvium are discontinuous, commonly appearing at the tributary mouths, in eddy zones upstream from rockfalls and boulder deposits, or as scattered flood deposits on lower talus slopes. The characteristics of alluvial deposits are determined by the interplay among channel features and recent discharges of water and sediment. The extent of deposits and their textural composition are controlled by the nature of recent streamflow events. The predam sediment deposits along the Colorado River probably underwent seasonal variations due to the strongly seasonal character of streamflow events. Because the postdam hydrologic regime shows little seasonal var- iability, the deposits have had to adjust to the new fluvial environment. Marked changes in riparian plant cover would also affect these deposits through the stabilizing influence of the plants. The amount of silt carried by rivers has long been known to depend largely on the characteristics of the precipitation producing the runoff. These precipitation characteristics include intensity, duration, frequency, distribution, and season of occurrence (Daines, 1949; Thomas and others, 1960). Nevertheless, there is a close relationship between discharge alone and the amount of suspended sediment recorded at Grand Canyon (Daines, 1949; Thomas and others, 1960; Kister, 1964). Interestingly, the relationship between these two streamflow variables shifted markedly in the early 1940’s. Figure 5 shows that prior to 1943 a given annual discharge in cubic hectometers (hm3) at Lees Ferry was related to sediment yield in megagrams (Mg) by a regression line slope of 16.46. From 1943 to 1963, the slope of the regression line was 5.74. During the postdam period the near absence of sediment at any discharge has resulted in a regression line with a slope of approximately 0. Similar shifts in the sediment-discharge relationship for the station near Grand Canyon occurred at the same times. The 1943 shifts in the sediment-discharge relationships have been discussed by Daines (1949), Thomas and others (1960), and Kister (1964) and may be the result of changes in measurement techniques or of the regional drought in arid watersheds. That Glen Canyon Dam has a dominant effect on the entire reach to Lake Mead is shown by comparing the predam and postdam sediment yield at the station near Grand Canyon with that at Lees Ferry (fig. 5). In spite of contributions from the Paria and the Little Colorado Rivers below Lees Ferry, sediment yield near Grand Canyon is greatly reduced in the postdam period and is little more than the Lees Ferry values. In a predam analysis of monthly sediment data, Thomas and others (1960) observed a consistent seasonal trend in sediment load. A spring period, usually May and June, was characterized by high sediment load which correlated with high discharge values. These high values coincided with the period of snowmelt in the river’s headwaters. A secondary maximum in runoff and sediment load coincided with the period of summer rains and usually came in August or September. During this late summer and fall period the concentrations of sediment were usually higher than during the spring period. With the closure of the Colorado River by Glen Canyon Dam, sediment carried by the Colorado River is being trapped in Lake Powell at an estimated annual rate of 128xlOfi m3 (Gessel, 1963). Releases from the 2Figures 26-73 are matched photographs found at end of report.CHANGES IN CHANNEL AND ALLUVIAL DEPOSITS 9 DISCHARGE, IN CUBIC HECTOMETERS Figure 5.—Regression analyses of annual (water year) discharge versus sediment yield as recorded at Lees Ferry and near Grand Canyon. dam are therefore of clear water and the river’s capacity to transport sediment has sharply increased. Because closure has lowered the peak stages, the river’s competence to rework tributary debris has di- minished and the nature of transported sediment has changed. The changes in fluvial regime have brought about channel changes, many of which were anticipated before dam construction began.10 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Twenty channel cross sections were surveyed by the Bureau of Reclamation in the reach of river between Glen Canyon damsite and Lees Ferry during 1956, the year dam construction started. These measurements provide base-line data for later observations. The same twenty sections were resurveyed in 1959, 1965, and 1975 (Pemberton, 1976). Erosion occurred at an accelerated rate immediately below the damsite during the construction period from 1956 to 1959 (fig. 6). The degradation that first occurred just below the dam progressed downstream during the next measurement period between 1959 and 1965. By 1975, within the 26-km reach between the dam and the mouth of the Paria River at Lees Ferry, resurveys showed that about 9.87 x 10s m:i of bottom sediment had been removed from the channel. As this sediment has been removed coarser material has been exposed, resulting in considerable channel armouring by gravel. Approximately 10 gravel-cobble control bars occur through the 26-km reach above the Paria, effectively stabilizing the channel. Comparison of 1975 profiles with those from 1965 indicate that the channel has been quite stable during the past decade with only 12xl04 m:i of sediment being scoured from the river bottom during this period. As another indication of stability, Pemberton (1976) noted that some sandbanks were being held in check by increasing plant cover. In a study of postdam sediment transport through the Grand Canyon, Laursen and others (1976) found considerable evidence of bank erosion between Glen Canyon Dam and Lees Ferry. They found that most talus slopes and beaches either had vertical slump faces or they were protected by exposed rock. The beaches that remain, they found, were in the lee of obstructions or other bank configurations that produce lee eddys. They noted: "On the few sizable beaches left the 'campsite’ sign has had to be moved back several times as the beachline retreated.” They concluded that DISTANCE BELOW GLEN CANYON DAM, IN METERS Figure 6.—Accumulated volume of degradation below Glen Canyon Dam for various periods between 1956 and 1975 (modified from Pemberton, 1976). since completion of Glen Canyon Dam, channel degradation had progressed to the vicinity of Lees Ferry and that continuing degradation could be expected downstream through the Grand Canyon. Extrapolating from the known rate of degradation above Lees Ferry, they concluded that somewhat more than 200 years will pass before most of the beaches vanish from the reach below Lees Ferry. In view of the anticipated changes noted in the foregoing, we have looked in the photographs presented in a later section for evidence of geomorphic changes related to dam construction. A HISTORY OF PHOTOGRAPHY ON THE COLORADO RIVER A brief chronological history of photography on the Colorado River will be presented here. Several centuries of exploration preceded the development of the photographic process; it was not until 1871, at the time of Powell’s second trip and the Wheeler Survey, that photographs were taken along the Colorado River within the Grand Canyon. There are several sources giving the history of earlier explorations. Notable among these is Dellenbaugh (1903). The first trip by boat through the Grand Canyon was made by Major J. W. Powell and his men. Powell left Green River, Wyoming, on May 24, 1869, with a crew of nine. Although photography had been developed to a crude but dependable stage by this date, no photographer accompanied this first historic trip. By the time the expedition reached the mouth of the Virgin River on August 30, 1869, four men had quit the group. Powell and two others left the canyon at the Virgin River, leaving four to continue toward the Gulf of California (Smithsonian Institution, 1875; Powell, 1895; Kolb, 1914). Because of the numerous problems encountered on the first trip, including the loss of much equipment, the results were not as desired and Powell later arranged to make a second trip. The second trip was made in two parts. Powell and a group of 10 men left Green River, Wyoming, on May 22, 1871, and arrived at the mouth of the Paria River on October 23, 1871 (Dellenbaugh, 1908; Thompson, 1939; Bartlett, 1962). After a lapse of several months, Powell’s second passage down the Colorado River was resumed at Lees Ferry on August 17, 1872, and ended on September 7, 1872, at Kanab Canyon (Dellenbaugh, 1903; Kolb, 1914; Thompson, 1939; Bartlett, 1962; Fowler, 1972). Among the members of this second expedition, as it left Green River, was a professional photographer, E. O. Beaman, who made approximately 350 photographs during the next few months, both on and off the river (Darrah, 1948). Beaman did not continue past LeesHISTORY OF PHOTOGRAPHY 11 Ferry, and his duties were taken over for a brief period by Clem Powell, who was later replaced by James Fen-nemore, a professional photographer (Thompson, 1939). Fennemore photographed the river from Fremont Creek (Dirty Devil River) down to Lees Ferry. Because of ill health, however, he was forced to quit after having taken some 70 photos (not all on the river). His replacement was J. K. Hillers, who, although initially hired as a boatman, was quick to grasp the photographer’s art with the aid of Fennemore. Hillers later became chief photographer for the U.S. Geological Survey. He took some 3,000 photographs between 1872 and 1878 on Powell’s expeditions in the Colorado River region (Darrah, 1948). Hillers had a keen eye for composition, and many of his photographs are regarded as masterpieces. While Powell and his party were making their second trip down the Colorado River, the Wheeler expedition started out on September 16, 1871, from Camp Mohave to go up the Colorado some 322 km to Diamond Creek. On October 19, 1871, after 33 days of very difficult travel, they arrived at their destination (Wheeler, 1872; Dellenbaugh, 1903; Horan, 1966). T. H. O’Sullivan, who took his photographic apprenticeship with M. B. Brady and became the first photographer of the U.S. Geological Survey, was a member of the Wheeler expedition (Horan, 1966). He took some 300 photographs while with Wheeler, a few of which were taken in the Grand Canyon (Wheeler, 1889; Horan, 1966). The following year (1872) William Bell, an English physician, took O’Sullivan’s place as photographer, accompanying the Wheeler Survey into the canyon (Wheeler, 1874; Horan, 1966; Watkins, 1969) both along Kanab Creek and at Lees Ferry. Bell was apparently the first Grand Canyon photographer to experiment with dry-plate photography but with little success (Watkins, 1969). In 1873, O’Sullivan again joined the Wheeler Survey as photographer. He visited and photographed the river at Lees Ferry during that year. By the time the Powell and Wheeler Surveys had ended, approximately 3,500 photographs had been taken of the Grand Canyon region. That so many photographs were taken during this early period of Colorado River exploration is all the more surprising when one realizes that the most advanced photographic process at the time was the wet-plate technique which required that the photographer make his own negative by applying wet chemicals to a glass plate just prior to its use. He then had to develop the negative without delay following exposure. The operation required a portable darkroom with many chemicals, jars, bottles, and glass plates. This fragile, bulky cargo, including a large box camera, accompanied the first photographers of the river and often must have been an unappreciated extra burden. After about 1874 this method was replaced by other processes requiring less bulky equipment. In 1889, two decades after Powell’s first historic trip, a railroad survey for the proposed Denver, Colorado Canyon, and Pacific Railway was organized by F. M. Brown, with R. B. Stanton as chief engineer and F. A. Nims as photographer. The expedition started out on May 22, 1889, from Green River, Utah, and ended about 51.5 km below Lees Ferry. By the time the party had reached this point, three men, including Brown, had drowned—needlessly, it seems, for Brown had rejected recommendations to take life jackets for the men. The surviving members of the party climbed out of the canyon near Vaseys Paradise (Stanton, 1965). A second expedition, with Stanton now in full charge and with all hands supplied with specially designed life jackets, resumed the survey on December 10, 1889. The new boats were hauled overland from Green River, Utah, to the mouth of Crescent Creek in Glen Canyon (Stanton, 1965; Smith, 1967). The party entered Marble Canyon below Lees Ferry on December 28, 1889 (Stanton, 1965). Four days later on January 1, 1890, Nims was seriously injured in an accident. He was lifted and carried out of the canyon, and the expedition continued without him. At this time Stanton decided to assume Nims’ duties as photographer, never before having taken a photograph. He was to take some 2,200 photographs during the entire trip but did not know until after the first 1,200 were taken whether any of the photographs were properly exposed. The cameras on this trip used roll film and were far easier to employ than those used earlier on the Powell and Wheeler expeditions. The photographs taken by Nims and Stanton are a particularly rich source of data because the camera was used—as an adjunct to the surveyor’s transit—to show an almost unbroken panoramic view of the river from its headwaters to its mouth (Stanton, 1965). Several years elapsed before other photographers entered the canyon. G. W. James and his companion, Nathan Galloway, started from Lees Ferry in about 1898 and travelled up the river through Glen Canyon, then down river past Lees Ferry a few kilometers to Soap Creek Rapids (James, 1907). James took many photographs of the river. H. G. Peabody made numerous photographs of the canyon around 1900 using dry-plate negatives (Watkins, 1969). In 1901-02 photographs were taken along the lower Colorado River from Gregg’s Ferry (Walapai damsite) to Yuma on two trips led by J. B. Lippincott (Lippincott, 1903; Lippin-cott and Ahern, 1903). F. S. Dellenbaugh also took pic-12 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA tures of the Colorado River in 1907 (Dellenbaugh, 1908). Many others including F. H. Maude, A. F. Mes-singer, and Oliver Lippincott were to capture the Colorado and its canyons on film (James, 1907). Other photographs were taken by a group including C. S. Russell, E. R. Monett, and Albert Loper who started downriver from Green River, Utah, on September 20, 1908. Four days later below the junction of the Green and Colorado Rivers, Loper’s boat was damaged along with his camera and plates thus ending the photographic coverage for that trip. Loper remained behind for boat repairs at Hite while Russell and Monett proceeded to Lees Ferry to wait. They got tired of waiting for Loper and continued on to Needles. When Loper finally arrived at the Paria and found that the others had continued on without him, he left the river (James, 1910; 1914). In 1909 a trip was organized by J. F. Stone and Nathan Galloway to go down the Colorado River for the specific purpose of taking photographs. They were accompanied by a photographer, R. A. Coggswell, and by S. S. Deubendorff and C. C. Sharp (Kolb, 1914). The party left Green River, Wyo., on September 12, 1909, and arrived at Needles on November 19, 1909 (James, 1910; Kolb, 1914; Stone, 1932). The first motion pictures of the canyon were made by the Kolb brothers, E. L. and E. C., using a Pathe camera. They began their trip at Green River, Wyo., on September 8, 1911, and arrived at Needles on January 18, 1912 (Kolb, 1914). The U.S. Geological Survey made studies of potential damsites along the Colorado River in the early 1920’s (LaRue, 1925). In the fall of 1921, Cataract Canyon was surveyed by W. R. Chenoweth, E. C. LaRue, Sidney Paige, Frank Stoudt, E. L. Kolb, and, as photographer, E. C. Kolb (Freeman, 1923, p. 360). L. R. Freeman and several others went up the Colorado River from Lees Ferry to Halls Crossing and then down again with LaRue and others to Lees Ferry in 1922, surveying Glen Canyon for a damsite and also taking photographs (Freeman, 1930, p. 76-77). Another U.S. Geological Survey team left from Lees Ferry on August 1, 1923, arriving at Needles on October 19, 1923, some 734 km downstream (LaRue, 1925, p. 126). Lt. Col. C. H. Birdseye, Chief Topographic Engineer of the Geological Survey, was the leader. Some other members of the expedition were R. W. Burchard, R. C. Moore, E. C. LaRue, and L. R. Freeman (Freeman, 1937). E. C. LaRue, L. R. Freeman, and E. C. Kolb were responsible for taking still and moving pictures on the trip (Freeman, 1930). After the studies by the U.S. Geological Survey were completed in 1923, the exploratory phase of the Colorado River travel ended. Detailed maps of the river and its canyon from Lees Ferry to the Virgin River were published in 1924. Trips by boat through the Grand Canyon continued, but until large inflatable boats became available in the 1950’s the trip was too dangerous to appeal to most people. In 1968 a U.S. Geological Survey expedition was orgnized by E. M. Shoemaker with H. G. Stephens serving as photographer. The party followed the route of the second Powell expediton and successfully secured new comparison photographs of 95 percent of the Powell photographs that are preserved in the National Archives. Ten photographs from this 1968 expedition have been published with their earlier Powell expedition counterparts, but these are all from the reach upstream from Lake Powell (Shoemaker and Stephens, 1975). The National Park Service, in 1974, began an ecological survey of the reach of the Colorado River we study here. Using photographs assembled for the present study, a number of camera stations were found and duplicate views recorded on film (Karpiscak, 1976). VEGETATION The difference in altitude from the plateaus flanking the Grand Canyon to the Colorado River at the canyon bottom locally exceeds 1,520 m. This great range fosters a varied vegetation pattern within the vertical 1.5 km above the river. Dense coniferous forests at the rim may overlook open desertscrub far below on the valley floor, and several vegetation zones occupy the intervening slopes. Much less variety in vegetation occurs on the arid slopes along the floor of the canyon through the 474.7 km reach from Glen Canyon Dam to Pearce Ferry. At the lower end of this reach, the vegetation is predominantly an extension of Mojave Desertscrub; at the upper end, Great Basin Desertscrub (Brown and Lowe, 1974a; b). Through this reach there is a fall of 571.5 m, an associated rise in temperature, and an apparent increase in desert plant biomass. The gradual increase in temperature is correlated with changes in species distributions along the river valley; none of the floral changes affects the basic open shrubby appearance of the arid-slope communities. The Colorado River, in its passage through the arid environment along the bottom of the canyon, creates a moist riparian habitat along its banks. Sand and silt deposits, gravel bars, rock piles, and cliff faces all provide more or less suitable substrates for plants at the water’s edge. These varied habitats contribute greatly to the variety of plant life along the river. Several early travelers through the Grand Canyon made brief notes of vegetation. Powell (1875) noted the high waterline marked by scattered hackberry trees.DISTRIBUTION OF MAJOR PLANT SPECIES 13 Stanton made few references to vegetation, although he did record the presence of mesquite groves from Point Hansbrough to a point 22.5 km below the mouth of the Little Colorado (Stanton, 1965). Several botanical observations were made along the Colorado River at locations accessible from the canyon rims and below the Grand Wash Cliffs. The reach of the Colorado River from the mouth of the Bill Williams River to a point 96.5 km upstream was traversed by J. M. Bigelow, a member of the Whipple expedition of 1853-54. The most common species along the river noted were cottonwood, mesquite, and willow (Bigelow, 1856). J. S. Newberry and a Mr. Mollhousen, who were members of the Lt. J. C. Ives expedition of 1857-58, made some plant collections along the lower sections of the Colorado River including the mouth of Diamond Creek. They commented on the common presence of arrowweed along the banks of the Colorado for the first 805 km above the mouth of the river. In addition, mesquite and catclaw were found to be common below Black Canyon (Ives, 1861). Cannon (1906) noted that a species of Baccharis was one of the primary plants growing along the river at the foot of Bright Angel Trail. Aside from these brief observations, there was no general description available of the vegetation in the canyon until the detailed floristic study of Clover and Jotter (1944), who were with the party led by N. D. Nevilles. Their route followed the course of the Green River from Green River, Utah, to the river’s confluence with the Colorado River and then along the Colorado River to Hoover Dam. The trip by boat through the canyon was made in 1938. Additional observations and collections were made in 1939 at a few localities that could be reached by foot or vehicle. Although their purpose was mainly to collect and identify plants and record plant distributions, they gave brief descriptions of the plant habitats and dominants of these sites. They described five habitats found along the canyon bottom: a margin of moist sand next to the river; above that, dry sandy shores; rubble and boulder areas; talus slopes; and areas about springs and waterfalls. One of their general observations of the vegetation is noteworthy: "Owing to constantly changing conditions of the talus by landslides, and of the river’s edge in consequence of periodic floods, there is little climax vegetation in the Canyon of the Colorado. However, vegetation may remain undisturbed for years, chiefly at springs and on stabilized portions of the lower talus” (Clover and Jotter, 1944, p. 620). Several lists of plant species have been published for the Grand Canyon or for adjoining areas (McDougall, 1947; Deaver and Haskell, 1955; Gaines, 1960; Phillips and Phillips, 1974; and Phillips, 1975). In a description of postdam riparian conditions in the Grand Canyon, three zones were recognized by Dolan and others (1974). According to their scheme, the lowest zone is at the river’s edge and consists of postdam fluvial sediments. The dominant plants here are arrowweed, saltcedar, coyote willow, and Bermuda grass. The second zone is of predam fluvial sediments, reworked by eolian processes, and may lie as much as 5.5 m above the present highwater. The common species here are arrowweed, red brome, camelthorn, and Russian thistle. The third zone, of predam flood terraces and eolian deposits, forms the highest beach zone described and may lie as high as 9 m above present high-water. The important species in this area are Apache plume, catclaw, western honey mesquite, and desert broom. DISTRIBUTION Of MAJOR PLANT SPECIES In September 1976, we made virtually continuous observations of the occurrence of the dominant plant species growing along the valley of the Colorado River from Lees Ferry to Diamond Creek. These data are based largely upon observations made from the river while aboard boats. A few collections were made during the same trip. Low plants such as Bermuda grass were difficult to sight from the raft and are probably underrepresented in our data. These detailed sightings were supplemented by more casual observations between Lees Ferry and Glen Canyon Dam during the same period and from Lees Ferry to Lake Mead in 1972. In addition, unpublished records of sightings and collections by scientists from the Museum of Northern Arizona and from P. S. Martin, University of Arizona, (written commun., 1970 and 1971) have added significantly to the record. Sightings and collections based on the 1938 trip of Clover and Jotter have also been included as have Jotter’s 1939 records from localities accessible by foot (Clover and Jotter, 1944). Data from all these sources are presented in a series of distribution maps (pis. 2, 3, figs. 7-25) showing the occurrence of 24 species at 1-mile intervals along the river beginning at Glen Canyon Dam. The use of miles instead of kilometers for plotting distributions was mainly for convenience. Because no map with distances marked off in kilometers was available for field use, we used as a base for recording distributions a map with river distances in miles (Belknap, 1969). When referring to the maps, the differences in data reliability should be kept in mind: Casual observations were made for the Glen Canyon Dam-Lees Ferry reach and the Diamond Creek-Pearce Ferry reach, and virtually continuous observations were made for the Lees Ferry-Diamond Creek reach. The distribution records are based on occurrences14 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA near the floor of the main canyon; thus, plants growing nearby in tributary canyons have been intentionally excluded. Observations were confined to the area extending from the present water level to the band of vegetation immediately above the predam flood line. The habitats in which most of the plants occur are postdam fluvial sediments, predam fluvial sediments, and predominantly fine-grained stabilized talus slopes. The communities in which these species occur are narrow with sharply defined boundaries. The communities on the stabilized talus slopes are apparently limited on the upper side, now as in the past, by unstable conditions or by too little soil. The lower edge of this community was generally sharply defined by the scouring action of major predam floods. Because large floods of predam magnitude no longer occur, this boundary may become less sharp with time. The written descriptions of distributions that follow indicate which species are apparently undergoing broad changes in distribution and which are merely becoming more abundant within their preexisting range. The set of maps will be useful to future observers for detecting trends in plant range variations. Five of the 25 plant species for which we present distribution data are introduced to this area. Those five are discussed first. BERMUDA GRASS A native of Eurasia, Bermuda grass is almost ubiquitous in warmer parts of the world and was growing along irrigation ditches in Tucson, Arizona, as early as 1891 (specimen, University of Arizona Herbarium). Clover and Jotter (1944) cite occurrences of the species at a few places on or near the Colorado River in 1938, and the grass was probably well established at that time. They found it as far upstream as Bright Angel Creek (kilometer 143.2) but not in the main canyon (fig. 50). Our records (pi. 2, fig. 7) for the Colorado River valley show its first occurrence 144.8 km farther downstream (kilometer 228). Clover and Jotter (1944) also noted that it was apparently a recent colonist rapidly becoming established along the shores of Lake Mead. This plant probably occurs throughout the reach of river we studied, although our records do not show this. RUSSIAN OLIVE Russian olive is a common naturalized species in Arizona and adjacent states. It is found along the Rio Grande as far south as El Paso but is better adapted to more northerly environments along that river above Elephant Butte Dam (Campbell and Dick-Peddie, 1964). Christensen (1963) reports the plant established in nature by 1925 in Nevada, by 1942 in Arizona, by 1948 in Utah, and by 1954 in Colorado. Its ability to spread rapidly into a new area is shown by Harlan and Dennis (1976) who noted that the species was planted in Canyon de Chelly National Monument, Arizona, in 1964; by 1974, it was one of the dominant trees of the canyon bottoms. The tree was first noted in the Grand Canyon at the mouth of Kanab Creek in 1973 (R. R. Johnson, U.S. National Park Service, written commun., 1978), and it is known from the Paria River near Lees Ferry (M. G. Simons, written commun., 1978). Four of the recent sightings of this tree along the Colorado River have been within 16 km of Lees Ferry (pi. 2, fig. 8). Two collections by the Museum of Northern Arizona at kilometers 231 and 232 (shown as one locality in figure 8) are just below the mouth of Kanab Creek. The tree may have entered the valley of the Colorado River via the Paria River and Kanab Creek and will likely spread from the present localities near these tributaries. SALTCEDAR Saltcedar was probably brought to Arizona before 1900 and was found in the wild state along the Salt River in 1901 (Robinson, 1965). It was reported along the Colorado River near the mouth of the San Juan River during the period between 1933 and 1938 (Woodbury and Russell, 1945). Tidestrom (1925) lists Tamarix gallica, a species often confused with T. chinensis,3 as an escaped plant along the Virgin River near St. Thomas, Nevada. Thus, by the late 1920’s and 1930’s, saltcedar was probably a common plant throughout the Colorado River drainage basin, although as Christensen (1962) noted, the fastest rate of invasion may have occurred during the 20-year period from 1935 to 1955. Clover and Jotter (1944) noted that saltcedar was found in 1938 in the entire length of the area we later mapped, except for "a considerable stretch in Marble Canyon.” In 1936 it was noted from Nankoweap Creek to the base of Tanner Trail (Patraw, 1936) and at the mouth of Bright Angel Creek (Dodge, 1936). McDougall (1947) did not mention it in his checklist of Grand Canyon plants. Robinson (1965) mapped the occurrence of saltcedar in the western states and showed it along the Colorado River both above and below our area. On his map it was conspicuously absent from the entire Grand Canyon. The absence of this species in all but one of the early photographs (fig. 36A) indicates that it occurred during the predam period merely as widespread isolated plants. The taxonomic status of the introduced and naturalized saltcedar is unsettled. If several species of this difficult group occur in Arizona, as claimed by Baum (1967), then it is possible that our observations along the Colorado River include more than one species. Until the confusion over the identity of the introduced saltcedar is cleared, we prefer to regard all the saltcedars recorded along the Colorado River through the Grand Canyon as one species, Tamarix chinensis Lour.DISTRIBUTION OF MAJOR PLANT SPECIES 15 We believe that its nearly continuous presence today (pi. 2, fig. 9) postdates Glen Canyon Dam. This plant reproduces vigorously from seed that remains viable for only a few weeks (Horton and others, 1960). The seedlings require high levels of soil moisture for a prolonged period before establishment occurs. The daily flooding of river bars provides a reliable source of water during the critical period of seed production (approximately April to October). The plants are prolific seed producers; the soil surface may receive as many as 17 viable seeds per square centimeter per season (Warren and Turner, 1975). Saltcedars grow densely along the Colorado River today because of the uniform, dependable moisture supply on the river bars, because of the stable habitat, and the abundant seed source that is available through most of the warm season. ELM In 1976, A. M. Phillips III, Museum of Northern Arizona, (oral commun., 1976) found elm at several places near Glen Canyon Dam (pi. 2, fig. 7). It presumably reached the Colorado River from plantings almost directly above at the Page, Ariz., golf course. We know of no other wildland occurrences of the plant. A related species, Siberian elm (Ulmus pumila L.), has been rapidly increasing in lowland areas of Utah since its establishment in about 1935 (Christensen, 1964). CAMELTHORN The spiny shrub camelthorn was introduced from Asia. The plant entered California via shipments of alfalfa seed prior to the 1930’s (Robbins and others, 1941) and has been known in Arizona since the 1930’s when it was collected (specimens at the University of Arizona Herbarium) along the Little Colorado River west of Leupp in 1934 and on the Gila River at Gillespie Dam in 1937. Camelthorn was first reported along the Colorado River in 1970 (P. S. Martin, University of Arizona, written commun., 1970) at kilometer 269.5 and at Cardenas Creek, 114.2 km below Lees Ferry, where it had only recently become established. We found it (1976) 1 km below the mouth of the Little Colorado River, and A. M. Phillips III, Museum of Northern Arizona, collected it just below the mouth of the Little Colorado River (written commun., 1978). It occurs at many sites downstream from there, but we do not know of its occurrence above this locality (pi. 2, fig. 10). The shrub probably reached the Colorado River valley via the Little Colorado drainage. It spreads rapidly by means of underground rootstocks and thus is not dependent upon special surface-moisture conditions for seedling establishment. This may explain why it commonly occurs above the zone of daily inundation. CATC LAW The shrub catclaw occurs in warm, arid areas of Arizona and adjacent states (Little, 1976) at altitudes up to 1,830 m. The plant was first seen at kilometer 63.4 on the left bank (pi. 2, fig. 11). Downstream from this locality it is an almost constant component of the predam flood-line community which commonly occurs on stable talus and predam alluvial deposits. Predam scouring action of floodwaters produced a sharply defined lower limit to this community (figs. 43A, 44A, 45A, 47A, 64A, and others). Today, many catclaw seedlings occur below the old community boundary and foretell future conditions. For example, in an area below the old flood line on the debris fan at the mouth of Horn Creek, we counted 22 catclaw seedlings growing in an area of 100 m2. This is equivalent to 4.5 m2 per plant, a value not unlike that for dense catclaw thickets. These data suggest, assuming that no plants die and that the plants reach a size typical of nearby areas, enough seedlings have already become established here to produce a dense thicket .comparable to the predam flood-line community. WESTERN HONEY MESQUITE The western honey mesquite, a shrub or small tree, is widespread in southwestern North America where it and the other varieties of mesquite are among a number of shrubs and trees that have increased in prominence within their ranges in the last 100 years, especially in grassland areas bordering the desert. It is first encountered on the right bank at kilometer 63.4 across the river and slightly upstream from the first occurrence of catclaw. Mesquite was noted by Powell (1875) at approximately the same location, indicating that its range in the Grand Canyon has not expanded during the past century. Stanton also recorded having seen mesquite but farther downstream, between the mouth of the Little Colorado River (kilometer 98.8) and kilometer 120.7 (Stanton, 1965). Western honey mesquite is part of the predam floodline community growing on stabilized talus and on dunes, occuring almost continuously from its first appearance until kilometer 120.7, after which its presence becomes discontinuous and sporadic for the next 148 km (pi. 2, fig. 12). It reappears near National Canyon (kilometer 267.9) and from that point downstream shares dominance in the flood-line community with catclaw. Scattered young individuals of western honey mesquite can be seen growing with saltcedar, sandbar willow, and other members of the postdam riparian strip vegetation. Few of these new entrants have yet grown to tree size, but indications are that this species will become a minor but significant member of the16 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA riparian community. It is also expected to occupy the niche between the river and the predam flood line that was devoid of woody plants during predam times. Infestation of western honey mesquites by mistletoe begins abruptly at kilometer 282.9 approximately 25.7 km downstream from the parasite’s first occurrence on catclaw (P. S. Martin, University of Arizona, written commun., 1971). FREMONT COTTONWOOD Fremont cottonwood is found along the entire Colorado River mainstem from the river’s mouth to its headwaters (Little, 1971), at least to an altitude of 1,515 m (Benson and Darrow, 1954). Along the lower Colorado River it was a common occupant of silt flats (MacDougal, 1904; Sykes, 1937), but its position there has now been taken over by saltcedar. Except for figure 60A, we found no evidence of Fremont cottonwood occurring along the banks of the Colorado River through the Grand Canyon until after the completion of Glen Canyon Dam. The seedlings typically become established in the open on newly deposited moist sediment. Through most of the Grand Canyon these deposits were among the more unstable habitats, and the trees were probably repeatedly uprooted by floods. The tree was noted by Clover and Jotter (1944) within the Grand Canyon at two locations but only within tributary canyons. P. S. Martin, University of Arizona (written commun., 1971) noted several occurrences of the tree that seemed to predate Glen Canyon Dam. It is still seen only infrequently along the river (pi. 2, fig. 13). Apparently its establishment is less successful on the postdam sediments than that of the other riparian species, such as saltcedar and sandbar willow, and once the sediment is occupied by the other species, Fremont cottonwood seedlings are effectively excluded. Fremont cottonwood is a preferred food of the beaver, and most trees observed showed signs of having been cut. Perhaps the plant’s limited occurrence is related to the heavy use it receives from these animals. ARROWWEED Arrowweed is often seen in pure stands in alluvial soils along streams in Arizona and adjacent states up to an altitude of about 915 m (Vines, 1960). The riparian habitat with which it is usually associated is also ideal for saltcedar, and the area formerly occupied by arrowweed has decreased in some regions (Turner, 1974) since the spread of saltcedar along many watercourses in southwestern North America. This willowlike shrub was recorded in 1938 at several locations on the Green River by Clover and Jotter (1944). It is first seen within our area at Lees Ferry (pi. 2, fig. 14). It is common along the Lower Colorado River near Yuma (McDonald and Hughes, 1968). Apparently unable to withstand the scouring action of floodwaters, this plant occurred only sporadically through the Grand Canyon before Glen Canyon Dam was built. It sprouts readily from roots and most stems in a thicket may be connected by a common root system (Gary, 1963). Its ability to reproduce vegetatively enables the plant to rapidly colonize open alluvial deposits. Arrowweed has become one of the more prominent members of the riparian community throughout the reach of the Grand Canyon we studied. LONGLEAF BRICKELLIA This shrub occurs in northern Arizona, Utah, and California at altitudes from 535 m to 1,830 m or higher. It is commonly seen as a subdominant of the arid communities on the slopes above the flood-line community. It is notable in the postdam ecosystem (pi. 2, fig. 15) as one of the species to first dominate the coarse debris fans and talus slopes lying below the predam high flood line (fig. 54 B). Whether it will retain this dominant role as these habitats approach an equilibrium with the present flood-free regime is unknown. RABBITBRUSH This species barely enters the study area, occurring along the Colorado River only upstream from Lees Ferry. There are several sites at which it forms dense thickets in the 1970’s within that short reach; other areas that were dominated by this plant in the late 19th century now support other species, mainly saltcedar (pi. 3, fig. 16; figs. 26, 27, 28, 30, 32, and 33). DESERT BROOM Desert broom is a common shrub in Arizona and adjacent states at altitudes below 1,675 m. This ruderal species is a common early occupant of disturbed areas such as roadsides, streambanks, and abandoned fields and is uncommon in more stable habitats. Clover and Jotter (1944) recorded it from only two localities— Lava Falls (kilometer 288.3) and Two Hundred and Fivemile Canyon (kilometer 330.5). P. S. Martin, University of Arizona (written commun., 1971), noted its abundant occurrence at kilometer 265.5 and its common occurrence at Lava Falls and downstream from there. Our records show it at a few localities upstream from kilometer 265.5 (pi. 2, fig. 8), but it is nowhere abundant until that section of the river is reached. Where found, desert broom may be seen next to theDISTRIBUTION OF MAJOR PLANT SPECIES 17 river or in the formerly bare strip above the beach but below the predam flood level. It was present in the predam flood-line community, but these older plants are now dying in many places. Perhaps its early establishment on the formerly flooded canyon slopes represents an early stage in a series of stages leading to a permanent, more complex, community. WATERWEED Waterweed and desert broom are morphologically and ecologically similar. The two are difficult to distinguish in the vegetative condition, but during the fall when both are in flower they may be distinguished by floral differences. The two occur together near the river and as ruderal species on the canyon sides below the predam high flood line. Waterweed is not known upstream from kilometer 46.7; it is found at Stanton Cave (kilometer 51.5) and sporadically downstream from there; and it is missing, according to our records, between kilometer 281.6 and kilometer 493.3 (pi. 2, fig. 16). It is much less abundant than desert broom throughout the region of their range overlap. SEEP WILLOW Seep willow is a common willow-like plant along water courses in Arizona and adjacent states at altitudes below about 1,525 m. It occurs near permanent or semipermanent bodies of water where the water table is near the surface. This tall shrub was the dominant species on recent alluvium throughout much of its range until the introduction of saltcedar. Judging from the early photographs of the Colorado River through the Grand Canyon, this plant was not commonly seen before the construction of Glen Canyon Dam. Today seep willow occurs through the Grand Canyon at short intervals especially at Buck Farm Canyon (kilometer 66) and downstream from there (pi. 3, fig. 17). It is found at Lees Ferry but apparently is not a common plant above this locality. Gaines (1960) collected it on the Colorado River in predam Glen Canyon during 1957 at a locality 8.8 km below Klondike Bar, San Juan County, Utah. Clover and Jotter (1944) recorded it at Lees Ferry in 1939 but not on the river above that location. Today the plant is generally subordinate to saltcedar and sandbar willow in the narrow riparian community at the edge of the river. As the riparian habitat reaches stability in the postdam period, seep willow will probably remain a conspicuous but minor element along the banks of the Colorado River. EMORY SEEP WILLOW The shrub emory seep willow is strikingly similar, ecologically and morphologically, to seep willow and is difficult to distinguish from it except during late summer and fall at which time both species are in flower and differences in the position of inflorescences may be used to distinguish the two, even at a distance. Emory seep willow occupies the same habitat as seep willow but according to our records is slightly less common than the latter (pi. 3, fig. 18). The shrub was common near Green River, Utah, in 1939 (Clover and Jotter, 1944), and it has been recorded 12.9 km above Lees Ferry. APACHE PLUME Apache plume is an evergreen shrub found from western Texas to southeastern California and south into Mexico. It is normally found at altitudes from 1,065 m to 2,285 m. It reaches its lower elevational limit in the Grand Canyon and distinctly marks the old flood line from Glen Canyon downstream to about kilometer 93.3; it is rare below that locality (pi. 2, fig. 10). Its position of dominance in the old flood-line community has not changed during the postdam period (fig. 35). NETLEAF HACKBERRY The netleaf hackberry, a widespread deciduous tree, is usually found in valleys from Oklahoma and Colorado to northern Mexico. In the Grand Canyon it is characteristic of steep slopes at or above the old flood line and occurs discontinuously over the full length of the canyon (pi. 3, fig. 19). It has recently become established below the predam flood line at kilometer 12.6 (P. S. Martin, University of Arizona, written commun., 1971) and can be expected to appear in this newly available habitat at other localities. REDBUD The redbud is a small tree inhabiting mesic situations such as alcoves on north-facing slopes, seeps, and the old predam flood line. It is commonly seen from Glen Canyon Dam to the vicinity of 'Nankoweap (kilometer 85.3). It has recently become established below the old flood line at Vaseys Paradise (kilometer 51.5) (P. S. Martin, University of Arizona, written commun., 1971). Redbud is more abundant than our map (pi. 3, fig. 20) indicates; it is commonly seen from the river in habitats above the predam flood line for which we do not show data. CATTAIL Two similar species of cattail, Typha latifolia and T. domingensis, occur in the Grand Canyon; in the field close examination is required to separate them. Our sightings were made mostly from a raft on the river,18 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA and we could not see the plants on the shore clearly enough for identification. The two are ecologically similar and are not separated in our records (pi. 3, fig. 21). These plants are practically limited to aquatic situations with quiet water; the rhizome requires complete and permanent immersion (Ridley, 1930). These plants are important food for beavers and muskrats, the seeds are eaten by some waterfowl, and the dense cover produced by these plants provides shelter and nesting cover for marsh birds, waterfowl, and songbirds. Stands of these plants were probably rare or absent in the predam period but are common today (figs. 37B, 65B, and 68B) and contribute importantly to the new postdam ecosystem along the Colorado River. REED The large grass called reed is a cosmopolitan species found in most of the temperate and tropical parts of the world. It characteristically occurs in marshes, at seeps, and along rivers and streams in our region. Clover and Jotter (1944) observed the grass in 1938 at three locations in the Grand Canyon: at the mouth of Bright Angel Creek, near Deer Creek Falls, and downstream from Upset Rapids at kilometer 244.6. The plant was probably well established in the main canyon at seeps above the predam flood level and within tributary canyons. It now occurs as a common member of the new riparian community (pi. 3, fig. 22). SPINY ASTER The spiny aster is a green, broom-like herbaceous perennial with poorly developed leaves and sparse thorns. It is a widespread plant in moist habitats up to altitudes of about 1,200 m from Texas to California and south to Costa Rica. In 1938-39 Clover and Jotter (1944) found it at Lees Ferry and downstream from there at several places. It is present along the Colorado River from Glen Canyon Dam to the Sea of Cortez (Gulf of California) in moist alluvial deposits where conditions are stable enough to permit establishment. Spiny aster spreads by rhizomes; it quickly colonizes open areas and can be expected to become common throughout the Grand Canyon (pi. 3, fig. 23). SANDBAR WILLOW Sandbar willow is a shrub that attains heights of 2 to 4 m (rarely 5 m). It is one of the more abundant shrubs along the Colorado River today (pi. 3, fig. 24) and is found throughout the Colorado River basin and along most of its tributaries to an altitude of about 2,100 m. In 1970 Martin found it spreading very rapidly along the river below the flood line (P. S. Martin, University of Arizona, written commun., 1970). The shrub is restricted to substrate near river level except at seeps, such as those at Vaseys Paradise, Deer Creek Falls, and Lava Falls. In 1938, Clover and Jotter (1944) reported it at localities from Lees Ferry to kilometer 308.9. Sandbar willow spreads rapidly from roots and is an important beach stabilizer (R. R. Johnson, U.S. National Park Service, written commun., 1978). This plant and saltcedar are the dominant species of the riparian community. Because of its short stature, this shrub may be partially replaced as equilibrium is reached in those areas where it grows with taller plants such as saltcedar (Campbell and Dick-Peddie, 1964). In recent years, its rapid establishment on bare, unstable mud bars below saltcedar has been observed (A. M. Phillips III, Museum of Northern Arizona, written commun., 1978). GOODDING WILLOW The only large willow growing along the Colorado River below Glen Canyon Dam is Goodding willow (pi. 3, fig. 25). Clover and Jotter (1944, p. 602) observed that willows in the lower part of the Grand Canyon "became so well established in some locations as to attain a height of thirty or forty feet.” The few places that tree willows appear in predam photographs are from the lower part of the Grand Canyon (figs. 62A, 63A, and 66A). DESERT ISOCOMA The species desert isocoma is normally less than 1 m tall and is typically found as scattered individuals in the riparian community. It was found at Lees Ferry in 1938-39 (Clover and Jotter, 1944) but has not been reported upstream from this locality. Our records show it (pi. 3, fig. 20) at scattered locations along the Colorado River to the vicinity of Rampart Cave (kilometer 442); it probably occurs downstream from there. Desert isocoma is nowhere important in the habitats below the predam flood line, but in old alluvial deposits above this level it may be the dominant plant. PHOTOGRAPHIC DOCUMENTATION OF CHANGES The use of photographs for recording changes in landscapes has many advantages over other methods because the camera records in great detail many features that would otherwise be overlooked. The camera faithfully records such subtle features as highwater stains on streamside outcrops, the intricate details of hexagonal soil cracks on alluvial silt beds, vertical banks formed by recent floods, the presence or absenceSUMMARY OF CHANGES 19 of plants on flood plains, and numerous details that would easily be overlooked or could be described well only by making painstaking drawings, measurements, and verbal descriptions. Photographs provide an unbiased and unusually complete record of conditions existing at a specific time (Malde, 1973). And when a new photograph is exactly matched against an old, any difference between the two can be readily discerned and taken as evidence for change. In this section are 48 sets of long-interval, time-lapse photographs. In most instances, the photographs are paired, oblique, terrestrial views—one taken in 1963 or earlier, the other dating from 1972 through 1976. The exceptions are (1) the single aerial photograph series in which the oldest view was taken in September 1952, repeated in May 1965(about 2 years after the dam was completed) and again in June 1973, and (2) in one instance where three matched photographs in one set were used instead of two. The photographs are arranged, for the most part, in an orderly sequence beginning with the station farthest upstream. The photographs in this report represent approximately one-third of those acquired during the study. The unpublished photographs may prove useful to future students of the Grand Canyon. Negatives for all the recent photographs have been retained in Tucson, Ariz., by the U.S. Geological Survey, Water Resources Division, or by the Museum of Northern Arizona, Flagstaff, Ariz. Most of the predam photographs are from the files of the U.S. Geological Survey Library, the New York Public Library, or from the National Park Service Library at Grand Canyon. The source of all photographs used in this publication is given, with appropriate credit, in table 3. The location is shown on plate 1 of all sites at which published matching photographs were taken. SUMMARY OF CHANGES In the preceding section we have attempted to document changes occurring along the Colorado River from the period prior to 1963, the year water impoundment in Lake Powell began, and the early 1970’s when our field work was accomplished. Changes can be considered within three separate communities: the community of postdam fluvial sediments lying nearest the river, the zone of predam fluvial sediments found next above, and, the highest of the three zones, communities of predam flood terraces, eolian deposits, and stabilized talus slopes. ZONE OF POSTDAM FLUVIAL SEDIMENTS Because all predam photographs show the near absence of plants in this situation, we believe that in the short period of 13 years the zone of postdam fluvial deposits has been transformed from a barren skirt on both sides of the river to a dynamic double strip of vegetation. Here conditions for plant establishment and growth are excellent, and under the new hydro-logic regime many plants now grow densely. Foremost among these are saltcedar and sandbar willow. In addition, there occur desert broom, Bermuda grass, carrizo, seep willow, Emory seep willow, and cottonwood. Cattails grow on submerged deposits, as do horsetail and great bulrush. The photographs show that this community accounts for most of the striking changes observed. It is doubtful that equilibrium has been reached, and the community is probably still undergoing change, both in compostion and density. From our observations of the rapid spread of camelthorn since 1970 and the recent appearance of Russian olive and elm along the river, we feel that these species, at least, will continue to expand in importance within this zone causing some changes in riparian community composition. Moreover, many of the new plants have not reached full size; as they do, increased community coverage will result. The increase in plant biomass in this zone has doubtless had effects on the fauna of this habitat. Recent work (Carothers and Johnson, 1975; Carothers, Aitchi-son, and Johnson, 1976; Ruffner and Carothers, 1975; Tomko, 1975) has shown profound responses to the new habitat by insects, reptiles, small and large mammals, and birds. Beavers are becoming common as a ready source of food has developed. They will obviously influence the composition and density of the zone of postdam fluvial sediments because of their preferential harvesting of cottonwoods and willows. Numerous large cottonwoods as well as willows were observed to have been downed by beaver, but no beaver sign was observed on saltcedar. The increasing population of beaver would appear to be a factor favoring plants other than cottonwoods and certain willows within the postdam fluvial zone. ZONE OF PREDAM FLUVIAL SEDIMENTS Prior to construction of Glen Canyon Dam, this predam zone and the previous zone were indistinguishable. Both were under control of frequent floods, and few plants became established. In situations where the floor of the canyon is broad, the scouring action of floods is reduced. If the fluvial deposits are thick and ground water is shallow, the zone of predam fluvial deposits supports plants like arrowweed and desert broom. Thick predam eolian deposits with even greater depth to ground water support dropseed (fig. 4OB ). Free now from flooding, this zone possesses the stability for new community development but lacks the water to20 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Table 3.—Camera station descriptions, including dates, location, altitude, and photograph credits Figure Location Direction Altitude Original photography Repeat photography Remarks No. [km (mi)/ ri verba nk of view at river Date level Photographer Credit Collection designation Date Location Negative number left, right, (m) negative center] 26 + 24.1 __957 1889 --NYPL' USGS2 757 Stanton, 1932, facing p. 25. (15.0)/L #237 1975 27 + 20.6 do --956 1889 do --NYPL K. B. Stanton 11 June USGS 754 (12.8VL #239 1975 28 + 11.3 Downstream _ _ _954 1889 do- ------ --NYPL R. B. Stanton 11 June USGS 752 (7.0)/R #244 1975 29 + 6.4 Upstream -_953 1889 _ do --NYPL K. B. Stanton 11 June USGS 750 (4.0)/L #245 1975 30 + 4.8 --953 1872 —USGSD3 Wheeler photo album #68 USGS 756 (3.0)/L 1975 31 + 0.5 Downstream _ _ --951 1923 —USGSD Topo. Div. #6 22 August 1972 USGS 710 (0.3)/L 32 0 __949 1873 —USGSD Wheeler photo album #70 22 August 1972 USGS 706 Bartlett, 1972, facing p. 361; (0)/L Horan, 1966, p. 270. 33 o Downstream -_949 USGSD Wheeler photo album #47 27 June 1972 USGS 671 (0)/R ca. 1873 34 1.0 947 24 Sept. 1952 USGSA4 (0.6) Photo #20-54 16 June USGS 003 WRD 14 May USGS2 Roll #1 1973 6-16-73 1965 Photo #1 35 6.9 Upstream -_942 21 Oct. NPS5 NPS #2354 21 August 1972 USGS 704 (4.3)/C 1952 36 12.6 Downstream -939 19 June do NPS NPS #2297 21 August USGS 705 (7.8)/L 1952 1972 37 17.7 __933 2 August 1923 USGSD 22 August 1972 USGS 672 (11.0)/L #338 38 34.6 Across --907 6 August do -USGSD E. C. LaRue 23 August USGS 714 (21.5)/R 1923 #353 1972 39 39.4 893 6 August 1923 -USGSD 16 Sept. 1976 USGS 797 Karpiscak, 1976, p. 8. (24.5)/L #355 40 45.5 884 20 August 1872 J. K. Hillers USGSD J. K. Hillers 23 August 1972 USGS 673 Darrah, 1947, facing p. 9; Darrah, 1951, Illus. #12; (28.3)/L #445 Stegner, 1954, following p. 238; LaRue, 1916, plate 9A; Dellenbaugh, 1903, p. 321; James, 1910, facing p. 248; Rusho, 1969, p. 3. 41 51.3 875 8 August 1923 USGSD Topo. Div. #27 MNA6 2 Karpiscak, 1976, p. 10. (31.9)/C 1974 42 53.3 Downstream -.873 (August?) 1923 _.USGSD Grand Canyon #173 MNA 3 Karpiscak, 1976, p. 12. (33.D/L 1974 43 74.8 .-858 11 August 1923 E. C. LaRue ___ _.USGSD MNA 5 (46.5)/R #390 1974 44 84.6 --847 (August?) 1923 E C. Kolb USGSD Grand Canyon #45 MNA 9 Karpiscak, 1976, p. 14. (52.6)/R 1974 45 98.8 Upstream --826 1872 J. K. Hillers ..USGSD J. K. Hillers 23 August USGS 685 James, 1910, (61.4)/C #885 1972 facing p. 159. 46 98.8 -_826 13 July 1963 J. Blaisdell . _ _NPS NPS #4288 2 Sept. 1973 USGS 730 (61.41/L 47 105.4 Downstream _ _ -_817 1872 USGSD 26 July 1974 MNA 46 (65.5)/R #858 48 123.1 -_780 (August?) 1923 USGSD Topo. Div. #36 23 August 1972 USGS 715 (76.5)/L 49 126.3 --770 1872 J. K. Hillers - -- USGSD MNA 11 (78.5)/R #449 1974 LaRue, 1916, plate 2A; Dellenbaugh, 1903, p. 219; James, 1907, p. III. 50 140.6 -_741 NPS NPS #2349 (87.4)/L 1952 23 August USGS 716 19 June J. blaisdell & NPS NPS #4251 1972 1963 A. Wolfe 51 143.1 Upstream -.732 February D. T. MacDougal . ARHS7 Al-41 20 Sept. USGS 801 (88.9)/L 1903 1976 52 174.7 Across -666 1901 N. W. Carkhuff .USGSD N. W. Carkhuff 23 August USGS 688 (108.6)/L #A62 1972 53 185.8 --645 February 1890 __NYPL R. B. Stanton 22 Sept. USGS 802 (115.5)/L #542 1976 54 200.8 Downstream _ _ -623 6 Sept. 1923 .USGSD 22 Sept. 1976 USGS 691 (124.8)/R #514 55 202.7 Upstream -619 1872 J. K. Hillers .USGSD J. K. Hillers 31 July MNA 54 Darrah, 1951, Illus. 11; (126)/L #879B 1974 Dellenbaugh, 1908, p. 218. 56 219.1 Across -588 10 Sept. E. C. LaRue -.USGSD E. C. LaRue 24 August USGS 717A (136.2)/L 1923 #546 1972 57 231.2 Upstream --572 10 Sept. _- -do_ _ _ —USGSD E. C. LaRue 24 August USGS 718 (143.7)/L 1923 #553 1972 58 287.4 Downstream _ -511 18 Sept. do _-USGSD E. C. LaRue 3 July USGS 697 (178.6)/L 1923 #601 1972 59 288.5 Across .. --511 19 June William Belknap, Jr. NPS C-57 26 Sept. USGS 803 (179.3)/R 1950 1976 60 313.6 Downstream _ 471 25 Sept. 1923 —USGSD 25 August 1972 USGS 720 (194.9)/L #628 61 328.9 --454 27 Sept. 1923 -USGSD 3 August 1974 MNA 60 (204.4)/R #633 62 336.0 442 28 Sept. 1923 --USGSD MNA 30 (208.8)/L #643 (left half! 1972 63 336.0 _442 28 Sept. 1923 --USGSD MNA 30 Karpiscak, (208.8 )/L #646 (right half) 1974 1976, p. 28. 64 336.6 .439 28 Sept. 1923 -.USGSD MNA 21 (209.2)/L #643 1974 65 350.3 Downstream 424 30 Sept. do .--USGSD E. C. LaRue 4 July USGS 703 (217.7)/L 1923 #653 1972 66 358.1 413 1 Oct. -USGSD 25 August 1972 USGS 722 (222.5)/L 1923 #664 67 360.7 408 -USGSD USGS 804 (224.2)/L 1923 #665 1976 See footnotes at end of table.SUMMARY OF CHANGES 21 Table 3.—Camera station descriptions, including dates, location, altitude, and photograph credits—Continued Figure Location Direction Altitude Original photography Repeat photography Remarks No. [km (mi)/ riverbank left, right, center] of view at river level (m) Date Photographer Credit .Collection designation Date Location negative Negative number 68 363.0 (225.6)/L Upstream __ 408 1902 N. H. Darton USGSD N. H. Darton #911 25 August 1972 USGS 723B 69 363.2 (225.7)/L - do 407 22 Sept. 1922 E. C. LaRue USGSD E. C. LaRue #675 29 Sept. 1976 USGS 805 LaRue, 1925, plate 45-A; Karpiscak, 1976, p. 34. 70 368.0 (228.71/R Downstream . 395 7 Oct. 1923 do USGSD E. C. LaRue #690 26 August 1972 USGS 724 71 406.0 (252.3VL Upstream 322" 13 Oct. 1923 do USGSD E. C. LaRue #747 26 August 1972 USGS 725 72 437.6 (272.0)/R Downstream 282" 15 Oct. 1923 do _ USGSD E. C. LaRue #759 6 August 1974 MNA 72 73 441.3 (274.3VL do 280" 15 Oct. 1923 do USGSD E. C. LaRue #761 6 August 1974 MNA 73 'New York Public Library, New York, N.Y. 2U.S. Geological Survey, Project Office, Tucson, Ariz. 3U.S Geological Survey, Photographic Library, Denver, Colo. 4U.S. Geological Survey, Arizona District Office, Tucson, Ariz. 'National Park Service, Grand Canyon, Ariz. ^Museum of Northern Arizona, Flagstaff, Ariz. 7Arizona Historical Society, Tucson, Ariz. "River level prior to filling of Lake Mead. rapidly produce dense plant growth. Changes here are slow, and although underway by the time of our study, few signs of change were detected photographically. The new community, judging from recent plant establishments, will comprise species commonly found upslope. These include catclaw, Apache plume, western honey mesquite, dropseed, brittle bush, and rabbitbrush. Camelthorn, an exotic species, is also rapidly becoming an important member of the community. Man’s influence on the vegetational changes along the Colorado is not limited to his control of river flow. He has become an important element for change by his presence on the river. The annual passage of approximately 15,000 people through the Grand Canyon facilitates the downstream movement of plant disseminules from one beach to another. Species that inhabit the zone of predam fluvial sediments are likely to be affected. The zone below may be similarly affected, but most of the migrant disseminules there are probably carried and deposited by water. ZONE OF PREDAM FLOOD TERRACES, EOLIAN DEPOSITS, AND STABILIZED TALUS SLOPES Predam terraces, eolian deposits, and stable talus slopes were situated above the zone of annual inundation and in many places even above the zone reached by extreme flooding. This habitat was marked by general stability and by little competition from plants along its lower margin. As a consequence, the plants composing this community grew to large size and were mostly closely spaced. The effect of dam construction on this community will probably be a decrease in its density, especially on level flood terraces that no longer are periodically inundated. The change is conspicuous at several sites above Lees Ferry and is illustrated in figures 33Aand B. In this same zone on stabilized talus slopes, there are signs in some areas of a decline in the size and number of the woody plants comprised in this community. This is not a widespread phenomenon and was recorded in only one photograph pair (fig. 34 A and B) of the several herein, showing the dense fringe of predam flood-line vegetation. The dominant plants here are rabbitbrush, Apache plume, western honey mesquite, catclaw, and canyon hackberry. These species provide the seed source for the plants newly occupying the predam fluvial deposits on the zone next below. On talus slopes clearly above the influence of predam flooding, the vegetation is remarkably stable. The photographs in this series do not support earlier conclusions (Clover and Jotter, 1944) that landslides made the talus slopes so unstable that a climax vegetation did not develop. From field observations and from the photographs, we judge that most talus is stable as is the vegetation it supports. Unstable talus is seen in figure 48 and a recent rock fall in figure 57B. In the many other views of talus, unstable surfaces are not evident. Man is directly influencing the stability of this zone above annual inundation by his destruction of plants at camping areas and elsewhere. Trampling of vegetation and trail cutting are especially evident at prime attractions such as the ruins at Nankoweap or at Deer Creek Falls in the area above the old high water line community. These heavily disturbed areas are inhabited predominantly by ephemeral species which thrive on disturbed ground, and many complete their life cycle before the river runners start their season. GENERAL CONCLUSION Any serious attempt to predict future changes must await passage of additional time. The vegetation has probably not reached an equilibrium with the new environment, and the final stage cannot be foreseen. The problem of assessing equilibrium lies only partly in the lack of data for determining rate of vegetation change. It lies partly in our inability to predict changes22 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA in river flow, for even though relatively stable, stream discharge still has the potential for causing great change. The flow in the Colorado River through the Grand Canyon is determined by power-generation needs, by water-supply needs, and by water availability. The last variable in this complex relationship is dependent on regional weather and is unpredictable. The other variables are under man’s control yet their effects are hardly more predictable and there is still a large element of uncertainty. As an example, diversion of the dependable allocated water supply relies upon completion of long range programs such as the Central Arizona Project. Completion of this program is behind schedule, and as a result Colorado River flow may be affected. As was noted by the Bureau of Reclamation (1976), "the dependable water supply [of the Colorado River] has been allocated but some of the facilities for its use have not yet been constructed. Since 1962, the resulting temporary excess of supply over demand has been stored in new facilities [such as Lake Powell]. In the near future, storage facilities will be filled to their operating limits requiring the frequent release of excess water. This condition will then continue until the allocated water supply is fully used or a period of deficient water supply occurs.” Thus, for the foreseeable future, flow through the Grand Canyon was expected to be greater than during the earlier postdam period, although this prediction was tempered by the acknowledged effect that regional precipitation deficiency would have on any surplus. The spring of 1977 provided a timely example of the unpredictable effect of drought on the flow of water through the canyon. Instead of the predicted excess water, the river below Glen Canyon Dam actually carried flows far below average. The discharge rate was so low that in one instance river parties were stranded in the canyon and a special release of water from Glen Canyon Dam was required to flush them out. Should release of excess water become necessary then effects on the riparian vegetation are likely to occur. To the extent that the greater flows inundate established vegetation for longer periods or to greater depths than before, there may be a change in the zones of plants within the newly established riparian strips. If, as occurred during April 1973 (fig. 1), increased flow from Lake Powell is accomplished by maintaining near constant daily flow, some plant species may be eliminated from positions they now occupy because of inability to withstand inundation for long periods. If release of excess water is accomplished by increasing the daily maximum values while maintaining the old minima, then plants now established would probably remain intact and new plants would become estab- lished at higher positions on the banks. This release pattern would presumably increase the breadth of the riparian strips, unless, of course, the increased daily maxima result in scouring and uprooting of plants. The exact form of the future plant communities along the Colorado River will in the final analysis depend on the interplay between the whims of man and the vagaries of weather and is therefore probably unpredictable. Yet it is apparent that the future relatively stable riparian vegetation will be dense and will probably include the species that are there now plus new introductions that will occasionally reach the valley of the Colorado River. REFERENCES CITED Aitchison, S. W., 1976, Campsite usage and impact, in Carothers, S. W., and Aitchison, S. W., eds., An ecological survey of the riparian zone of the Colorado River between Lees Ferry and the Grand Wash Cliffs, Arizona: Final research rept. prepared for and sponsored by Natl. Park Service (Contract No. CX821500007), Colorado River Research Ser. Contr. 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M, 1936, Check-list of plants of Grand Canyon National Park: Grand Canyon Nat. History Assoc. Bull. no. 6, 75 p. Pemberton, E. L., 1976, Channel changes in the Colorado River below Glen Canyon Dam: Third Federal Interagency Sedimentation Conf. Proc., p. 5-61 to 5-73. Pewe, T. L., 1969, Colorado River Guidebook—Lees Ferry to Phantom Ranch: Phoenix, Lebeau Printing Company, 78 p. Phillips, A. M., Ill, 1975, Flora of the Rampart Cave area, Lower Grand Canyon, Arizona: Arizona Acad. Sci. Jour., v. 10, no. 3, p. 148- 159. Phillips, B. G., and Phillips, A. M., Ill, 1974, Spring wildflowers of the Inner Gorge, Grand Canyon, Arizona: Plateau, v. 46, no. 4, p. 149- 156. Powell, J. W., 1875, Exploration of the Colorado River of the West and its tributaries—explored in 1869, 1870, 1871, and 1872: Washington, U.S. Govt. Printing Office, 291 p. ------ 1895, Canyons of the Colorado: Meadville, Flood and Vincent, The Chautauqua-Century Press, 397 p. ------1961, The exploration of the Colorado River and its canyons: New York, Dover Pubs., Inc., 400 p. Ridley, H. N., 1930, The dispersal of plants throughout the world: Ashford, Kent, L. Reeve and Co., Ltd., 744 p. Robbins, W. W., Bellue, M. K., and Ball, W. S., 1941, Weeds of California: Sacramento, Calif. State Dept. Agriculture, 491 p. Robinson, T. W., 1965, Introduction, spread and areal extent of saltcedar (Tamarix) in the Western States: U.S. Geol. Survey Prof. Paper 491-A, 12 p. Ruffner, G. A., and Carothers, S. W., 1975, Recent notes on the distributions of some mammals of the Grand Canyon region: Plateau, v. 47, no. 4, p. 154-160. Rusho, W. L., and Crampton, C. G., 1975, Desert river crossing— Historic Lee’s Ferry on the Colorado River: Salt Lake City, Peregrine Smith, Inc., 126 p. Shoemaker, E. M., and Stephens, H. G., 1975, First photographs of the canyon lands, in Canyonlands country—A guidebook of the Four Corners Geological Society, J. E. Fassett (ed.), 8th Field Conf., Sept. 22-25, 1975: p. 111-122. Simmons, G. C., and Gaskill, D. L., 1969, River runners’ guide to the canyons of the Green and Colorado Rivers, with emphasis on geologic features—Marble Gorge and Grand Canyon: Flagstaff, Northland Press, 132 p. Smith, D. L., ed., 1967, The photographer and the river 1889-1890—The Colorado canon diary of Franklin A. Nims with the Brown-Stanton railroad survey expedition: Santa Fe, Stagecoach Press, 75 p. Smithsonian Institution, 1875, Exploration of the Colorado River of the West and its tributaries: Washington, U.S. Govt. Printing Office, 285 p. Stanton, R. B., 1965, Down the Colorado: Edited by D. L. Smith, Norman, Oklahoma Univ. Press, 237 p. Stockton, C. W., and Jacoby, G. C., Jr. 1976, Long-term surface-water supply and streamflow trends in the Upper Colorado River Basin: Lake Powell Research Proj. Bull. no. 18, Los Angeles, California Univ., Institute of Geophysics and Planetary Physics, 70 p. Stone, J. F., 1932, Canyon country: New York, G.P. Putnam’s Sons, 442 p. Sykes, Godfrey, 1937, The Colorado delta: Am. Geog. Soc. Spec. Pub. no. 19, 193 p. Thomas, H. E., Gould, H. R., and Langbein, W. B., 1960, Life of the reservoir, in Comprehensive survey of sedimentation in Lake Mead, 1948-49: U.S. Geol. Survey Prof. Paper 295, p. 231-244. Thompson, A. H., 1939, Diary of Almon Harris Thompson: Utah Hist. Quart., v. 7, p. 1-140. Tidestrom, Ivar, 1925, Flora of Utah and Nevada: Contr. U.S. Natl. Herbarium, v. 25, p. 1-665. Tomko, D. S, 1975, The reptiles and amphibians of the Grand Canyon: Plateau, v. 47, no. 4, p. 161-166. Turner, R. M., 1974, Quantitative and historical evidence of vegetation changes along the upper Gila River, Arizona: U.S. Geol. Survey Prof. Paper 655-H, 20 p. U.S. Bureau of Reclamation, 1976, River flows between Davis Dam and Yuma, Arizona—A forecast of conditions and impacts for the period 1977 to 1986: U.S. Bur. Reclamation Rept., Colorado River front work and levee system, Arizona-California-Nevada, 20 p. U.S. Geological Survey, Surface Water Supply of the United States—Part 9, Colorado River Basin: published annually through 1960. ------Water Resources Data for Arizona—Part 1, Surface Water Records: published annually from 1961. Vines, R. A., 1960, Trees, shrubs, and woody vines of the Southwest: Austin, Texas Univ. Press, 1104 p. Wallace, Robert, 1972, The Grand Canyon: New York, The American Wilderness—Time-Life Books, 184 p. Warren, D. K., and Turner, R. M., 1975, Saltcedar (Tamarix chinen-sis) seed production, seedling establishment, and response to inundation: Arizona Acad. Sci. Jour., v. 10, no. 3, p. 135-144. Watkins, T. H., ed., 1969, The Grand Colorado—The story of a river and its canyons: Palo Alto, American West Publishing Company, 310 p. Wheeler, G. M., 1872, Preliminary report of explorations in Nevada and Arizona 1871: Washington, U.S. Govt. Printing Office, 96 p. ------1874, Progress report upon geographical and geological explorations and surveys west of the one hundredth meridian in 1872: Washington, U.S. Govt. Printing Office, 56 p. ------1889, Report upon United States geographical surveys west of the one hundredth meridian, Vol. 1—Geographical report: Washington, U.S. Govt. Printing Office, 780 p. Woodbury, A.M., and Russell, H. N., Jr., 1945, Birds of the Navajo Country: Utah Univ. Biol. Ser. Bull., v. 9, no. 1, p. 1-160.FIGURES 26-7326 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 26A.-—(1889). F. A. Nims, photographer with the Stanton-Brown railroad survey expeditions, took this photograph looking upstream from a point 1.6 kilometers below the present site of Glen Canyon Dam (24.4 kilometers above Lees Ferry). Woody vegetation on the far bank appears as a dense thicket separated from the water at this river stage by a barren sand bar. The thicket appears to be rabbitbrush. The high sandy terrace supporting the dense growth was a common feature above Lees Ferry but uncommon below. Only in years of extremely high floods would there be overbank flow sufficient to scour out these plants. The sharp lower boundary results from the scouring action of floodwaters. At the base of the distant cliffs above the river in center background are clumps of what appear to be netleaf hackberry. (Altitude 957 meters.)FIGURES 26-73 27 Figure 26B.—(1975). Glen Canyon Dam lies just out of view around the bend of the river. Construction activity related to the dam has considerably altered the steep slopes of the left bank. The netleaf hackberry seen in the previous view has been covered by rubble piles below tunnels drilled into the cliffs above. The netleaf hackberry has not reoccupied the niche at the cliff bases, although saltcedar relatively quickly became established along the base of the rubble piles. The foreground has also been altered by rubble from above, although many of the rocks carry over from the older view. The thicket on the opposite bank has enlarged, and its composition has changed—saltcedar dominates today and has spread downward over the barren skirt as far as fluctuations in river level will permit. The photograph was taken at 1330 hours, a time of peak-load demand, and the river is probably at near maximum stage. Large saltcedars dominate on the left bank also, forming a narrow almost continuous streamside band along the base of the steep slopes below the cliffs. Maximum lowering in mean streambed elevation (1.83 meters) occurred during the 1956-59 period at a point near the far bend in the river. Subsequently the channel has been stabilized by gravel and cobble-size armor, eliminating continued degradation (Pemberton, 1976).28 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 21A.—(1889). The Colorado River is deeply entrenched in the Upper Triassic(?) and Jurassic Navajo Sandstone through the lower section of Glen Canyon between Glen Canyon Dam and Lees Ferry, 20.6 kilometers above Lees Ferry. The view is upstream. The camera station is located near the level of maximum river stage. A single netleaf hackberry (arrow) is at the same level a few hundred feet upstream from the camera. The coarse material of the foreground supports little more than a few grasses and forbs. On the far bank is a thicket of what is probably rabbitbrush. (Altitude 956 meters.)FIGURES 26-73 29 Figure 27B.—(1975). Although not an exact match, the new camera position is probably within a few feet of the old. Two towers near the brink of the canyon wall support lines that carry power generated at Glen Canyon Dam. Saltcedar has overgrown both banks and in the vicinity of the camera station occupies what is probably the predam highwater level. Several nearby saltcedars at this level have died. Longleaf brickellia is common on the steep slopes of coarse rubble in the foreground. The sandy bar on the opposite bank is covered by plants to lower levels now than in the earlier view. The deposit is partly submerged by the river, which is at higher stage than before, making difficult any estimate of erosion since the earlier photograph.30 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 28A.—(1889). This downstream view was taken below the crest of an actively eroding alluvial deposit, 11.3 kilometers above Lees Ferry. Scattered seedlings have become established on the unstable surface marked by horizontal banding. This photograph was taken at a time of low river stage. At the top of the bank can be seen a dense cover of what is probably rabbitbrush. The dense community has been undercut by the river. (Altitude 954 meters.)FIGURES 26-73 31 Figure 28S.—(1975). The old camera position cannot be exactly located so dense is the vegetation now growing on the alluvial deposit. Saltcedar, arrowweed, and sandbar willow occur from the river’s edge up the steep bank, and on to the flat surface above. Rabbitbrush still occurs on top of the terrace where it is mixed with the other riparian species. The dense growth of vegetation providing stability to the steep bank is undoubtedly new since completion of Glen Canyon Dam.32 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA ‘f Figure 29A.—(1889). Members of the Stanton-Brown railroad survey team are seen in this upstream view, 6.4 kilometers above Lees Ferry. The vegetation on the foreground terrace is Great Basin Desertscrub (Brown and Lowe, 1974a, b). The dense vegetation crossing the picture in the right midground occupies the arroyo leading from Water Holes Canyon. The river is at low stage exposing a sandy bar near the far shore. (Altitude 953 meters.)FIGURES 26-73 33 Figure 29S.—(1975). This photograph is not an exact match of the earlier one but is probably off less than 8 meters. The foreground vegetation is much the same as in the 1889 view and comprises jointfir, fourwing saltbush, beavertail cactus, coldenia, and various grasses. The tallest plants along the midground arroyo are shrub liveoak and netleaf hackberry. Shrub liveoak is rare at river level downstream from this site. The netleaf hackberry is common along the canyon for about 80.5 kilometers below Lees Ferry. It is rare below that station although it can be seen near the river 318.6 kilometers below Lees Ferry. The sandy bar of the preceding view persists to the present and is shallowly inundated at this river stage. Saltcedar now occurs as a narrow band on both sides of the river.34 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 30A.—(1872). This upstream view was taken by William Bell, photographer with the Wheeler Survey. The dense riparian vegetation seen in the background on both banks is typical of beach deposits along this section of the river, 4.8 kilometers above Lees Ferry. At the river stage shown in this picture, the riparian community and the river are separated by bare beach.FIGURES 26-73 35 Figure 3OB.—(1975). The old camera location is overgrown with bushes and lies about 3.0 meters upslope to the right. Judging from the position of the water relative to the rocks at the right, the river stage is roughly the same in the two views, and unlike most of the photograph pairs taken along the river, the river level can therefore be used as a datum for judging changes. Saltcedar and sandbar willow (both visible in the right foreground) grow along the section of the beach that was formerly bare. The large trees at the upper edge of the riparian zone are saltcedars. Cattails grow in a few areas at the river’s edge.36 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 31A.—(1923). The building in left foreground is a boathouse. The cluster of buildings across the river on the right is Lees Ferry. The U.S. Geological Survey river-stage recorder is housed in the cylindric structure standing near the river at the left. This gage marks 'Mile 0” in the system used for assigning mileages along the reach of the Colorado River from Lees Ferry to Lake Mead. The Upper Triassic Shinarump Member of the Chinle Formation near the gage and on the opposite side of the Colorado River dips below the level of the river at an inclination of 15° to the east. The plants in the dense thicket near the river cannot be positively identified but included are probably arrowweed, rabbitbrush, and sandbar willow. Saltcedar may be established here, although no record exists of its presence at Lees Ferry until 1938 (Clover and Jotter, 1944). At any rate, no plants on the near shore have reached heights typical of mature saltcedar. (Altitude 951 meters.)FIGURES 26-73 37 Figure 3 LB.—(1972). Lees Ferry has grown. The large inflated rafts (center) are evidence of the town’s main industry as a launching facility for boat and raft trips. Saltcedar is the main plant along the near shore, and because of its height, much of the river is screened from view. The cable from which measurements of streamflow are made by the U.S. Geological Survey has been moved from the abandoned tower in the foreground and is now located upstream from the camera station.38 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 32A.—(1873). The mouth of the Paria River is seen on the left in this northwesterly view looking across the Colorado River and up the valley of the Paria. The main ferry crossing, still little used by the date of this photograph, lies about 2 kilometers upstream from the Paria. This photograph was taken by T. H. O’Sullivan, photographer with the Wheeler Survey (Bartlett, 1962). In 1872, J. D. Lee, his seventeenth wife, Emma, and their children built a house (arrow) and began farming on the broad, flat floor of the Paria River valley, becoming the first permanent residents of the area (Rusho and Crampton, 1975). In 1776 the Spanish padres Dominguez and Escalante camped at the base of the cliff of the Upper Triassic Shinarump Member of the Chinle Formation on the right (Rusho and Crampton, 1975). The large trees lining the Paria are probably willows (A) and Fremont cottonwoods (B). The dense shrub community (C) extending up the Paria on the right is probably mainly rabbitbrush (see fig. 33A). A bare sandy bank slopes from the thicket (of sandbar willow?) to the Colorado River. (Altitude 949 meters.)FIGURES 26-73 39 Figure 326.—(1972). Almost a century after the preceding view was taken, many of the rocks on the steep slope below the Echo Monocline are still in place. The Paria has shifted its course, accounting for some of the vegetation changes in that valley. The changes along the Colorado River are marked. The thicket of riparian plants has expanded across the bare shore toward the river. Nearest the water’s edge is a low community comprising horsetail and a species of bulrush. Plants at this level are probably inundated daily. Shoreward from this low community is a dense forest of saltcedar and a few sandbar willows. This community is out of reach of the diurnal ebb of the river’s fluctuating stage.40 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 33A.—(1873). In this classic view looking down the Colorado River from a vantage point 0.5 kilometers below Lees Ferry, the Vermillion Cliffs define the distant skyline at right. The low cliffs in the mid-distance are the Chocolate Cliffs, comprising the Upper Triassic Shinarump Member of the Chinle Formation above and the Triassic Moenkopi Formation below. A band of riparian species (probably rabbitbrush) occupies the right bank across the foreground. Evidence of overbank flooding is seen in the piles of debris scattered in the dense thicket. Flow from the mouth of the Paria River, just out of view on the right, crosses at midground along the edge of the fan. This fan, formed by the Paria River, forces the Colorado River toward the left bank. The near shore (left foreground) and the fan are mostly devoid of plants. For scale, note the figure at the bottom, just right of center. (Altitude 949 meters.)FIGURES 26-73 41 Figure 33B.—(1972). In this view, taken almost a century after the first, the band of dense riparian vegetation has moved to lower ground, encroaching upon what was formerly a bare shore. The terrace, which was occupied by tall dense vegetation a century earlier, now supports an open growth of arrowweed, Russian thistle, and seepweed. The dense riparian vegetation of today is mainly saltcedar. The fan now supports a continuous broad band of riparian species along its margin. The Paria River has changed its course and now enters the Colorado River below the fan. The dugway road (arrow), seen here crossing the Triassic Moenkopi Formation, was in use for 30 years beginning in 1898 (Rusho and Crampton, 1975).42 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONAFIGURES 26-73 43 Figure 34.—Three aerial photographs, spanning a 21-year period, record conditions at the mouth of the Paria River near Lees Ferry. The photographs were taken on September 24, 1952; May 14, 1965; and June 16, 1973. Many features shown in figures 31 through 33 can be seen in this series of aerial views. The Echo Monocline forms the cliffs to the right of the Paria River which flows toward the Colorado from the upper right of the photographs. The approximate location of the 1873 channel of the Paria (see fig. 334) is marked with the letter A. The dugway road that was in use until the ferry was abandoned in 1929 can be seen running from point B toward the west above the left bank of the river. The U.S. Geological Survey stream gaging station is located at point B. This is also near the location from which figure 33 was taken. Lees Ferry, seen at upper left in these photographs, is the embarkation point for boats and rafts that float through the Grand Canyon. In the 1973 photograph, 37 of these craft can be seen tied up either at Lees Ferry or across the river from the town. Two rafts have just departed and appear as the light-colored, elliptical objects in the river toward the right in the 1973 photograph. Lees Ferry has grown in 21 years as has the number of persons embarking here for trips through the Grand Canyon. In 1952, about 50 persons made the trip; in 1965, 547 persons; and in 1973, 15,219 persons (Aitchison, 1976). The configuration of the debris fan at the mouth of the Paria River changes little from 1952 to 1973, although it appears different as the river level varies. At the time of the September 24, 1952, photograph, the river stage on the gage at B was roughly e 2.7 meters; on May 14, 1965, the value was 3.4 meters; and on July 16, 1973, the level was between 1.8 and 2.7 meters. By 1973, the fringe of saltcedar has become pronounced along some sections of the shoreline. The fan and the plants it supports is shown from another perspective in figure 32 which was taken from point marked C in the 1952 aerial view. Most of the fan surface is probably below the level of maximum flooding, yet some plants have become established there, apparently surviving all but the most severe floods.44 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 34.—Continued.FIGURES 26-73 45 Figure 34.—Continued.VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA 46 Figure 35A.—(1952). This upstream view of the river is from the Navajo Bridge on U.S. Highway 89, 6.9 kilometers below Lees Ferry. The bridge was completed in 1928, after which the ferry at Lees Ferry was abandoned. The vertical cliffs expose rocks of the Lower Permian and Toroweap Formation overlying Kaibab Limestone. The interrupted dense thicket is immediately above the infrequently flooded high water level and is composed of netleaf hackberry and Apache plume. Bare beaches of sand appear at this river stage. (Altitude 942 meters.)FIGURES 26-73 47 Figure 35B.—(1972). The river is at higher stage here than in the 1952 view, and many of the former sandy areas are covered with water. The new dense riparian community is mainly saltcedar, sandbar willow, and Apache plume with infrequent stands of cattails.48 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 36A. (1952). Overlooking Badger Creek rapids from a point above the left bank just north of Jackass Creek, 12.6 kilometers below Lees Ferry. Slopewash blankets the Hermit Shale Formation at river level here. The cliffs visible above consist of the Lower Permian Coconino Sandstone and overlying Toroweap Formations. At the time of this photograph (June 19), the discharge for the day at Lees Ferry was 2,799 m's (US. Geol. Survey, issued annually). Note the large shrub surrounded by water just off the right bank. This shrub persists in the 1972 photograph. (Altitude 939 meters.)FIGURES 26-73 49 Figure 36B.—(1972). The 24-hour discharge at Lees Ferry on the day of this photograph (August 21) was 529.6 m:'/s, approximately one-fifth the volume for the date of the previous photograph (U.S. Geol. Survey, issued annually). Note the raft passing the Badger Creek rapids and several other rafts near the right bank below the rapids. The water through the rapids is obviously more turbulent at this low stage than at the high stage in the 1952 view. The large shrub of the early view is visible and is saltcedar. This plant was present along the river at many places through the Grand Canyon as early as 1938 (Clover and Jotter, 1944), but it probably grew only at scattered localities at or slightly above the contour of maximum river stage. In this view, saltcedar occurs as an interrupted band along both banks of the river.50 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 37A.—(1923). This downstream view shows the stretch of quiet water above Soap Creek rapids, 17.7 kilometers below Lees Ferry. The alluvium near the river is devoid of plants. A few outcrops of the Lower Permian Hermit Shale on the slope above the river are exposed through the covering of debris that has fallen from above. (Altitude 933 meters.)FIGURES 26-73 51 Figure 37B.—(1972). The large saltcedar shading this spot had a stem diameter (at ground level) of nearly 46 centimeters in 1976. This tree may have become established before the dam during a time of sustained high water. The river no longer reaches this level. Cattails grow in the protected embayment in the foreground. Because the new camera position is too far right for an exact match, judgments concerning erosion of the foreground alluvium cannot be made.52 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 38A.—(1923). The U.S. Geological Survey team camped on the right bank, August 6, 34.6 kilometers below Lees Ferry. A portion of the Pennsylvanian and Lower Permian Supai Group is here exposed at the upper right. The stick in the foreground is a mast for a radio antenna. The conspicuous plants on the sandy knoll behind the four men are probably wire lettuce and spiny aster. On the opposite bank, a discontinuous line of shrubs, probably Apache plume, marks the level of maximum river stage. (Altitude 907 meters.)FIGURES 26-73 53 Figure 38B.—(1972). Saltcedar, growing to heights of 6 meters, is the dominant plant on the site. Longleaf brickellia (right foreground), wire lettuce, and spiny aster are also present.54 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 39A.—(1923). E. C. LaRue, a member of the 1923 U.S. Geological Survey team, took this upstream view at the lower end of Tanner Wash rapids, 39.4 kilometers below Lees Ferry. These rapids have been difficult to navigate in the past: Powell posted and lined here in 1869; two members of the Brown-Stanton party drowned here in 1889; Bert Loper’s boat capsized here in 1939, and he was never seen again (Hamblin and Rigby, 1969; Pewe, 1969). A large pile of driftwood has accumulated on the silt deposit in front of the boats. Pre-Glen Canyon high-water surges probably overflowed the uppermost boulders. Roughly 2 kilometers upstream from this station recent river silts 6-7.5 meters above river level remain as evidence of the height of predam flows in this reach of the canyon (Hamblin and Rigby, 1968). No vegetation can be seen along this section of the river. (Altitude 893 meters.)FIGURES 26-73 55 Figure .39/1.—(1974). Most of the boulders seen in 1923 are still in place on the debris fan at the mouth of Tanner Wash. The large pile of driftwood seen in the earlier view is gone as is the deposit of fine alluvium beneath it. Saltcedar is growing near the river, and longleaf brickellia is the common shrub at higher levels on the beach. The camera for this photograph is located slightly too far left for an exact match.56 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 40A.—(1872). J. W. Powell’s boat, the Emma Dean, with Powell’s armchair strapped to the deck, is seen in this upstream view at 45.5 kilometers below Lees Ferry. The photograph was taken from a small fan at the mouth of a minor side canyon. The narrow canyon and vertical walls of the Mississippian Redwall Limestone promote high velocities and attendant scouring during times of large streamflow volume. This section of Marble Canyon appears to support no vascular plants. (Altitude 884 meters.)FIGURES 26-73 57 Figure 40B.—(1972). The tree on the right is saltcedar. The dominant small shrub on the fan is longleaf brickellia. Shrubs occur at scattered localities on the opposite side of the river. Since Powell’s trip, there has developed in this short reach of the canyon an open terrestrial biotic community.58 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 41A.—(1923). L.R. Freeman, a member of the 1923 U.S. Geological Survey team, took this picture of E.C. LaRue photographing Vaseys Paradise, 51.5 kilometers below Lees Ferry. This is one of the better known springs along the Colorado River and was named by Powell for Dr. G. W. Vasey, a botanist (Powell, 1961). Stanton Cave, where members of the Brown-Stanton expedition cached their equipment when they abandoned their first trip in 1889, lies only a few hundred meters upstream. Stanton Cave and the cave system from which springs flow at Vaseys Paradise are evidence of the susceptibility of Mississippian Redwall Limestone to solution. The moist area in the scene is generally northeast facing and supports a dense growth of redbud and poison ivy. A bare skirt just above the river marks the strip scoured periodically by heavy flow. (Altitude 875 meters.)FIGURES 26-73 59 Figure 41B.—(1974). Many of the redbuds appear dead in this March 17 photograph; however, since this photograph was taken many plants have been periodically observed to have progressively recovered (A.M. Phillips III, oral commun., 1978). Many herbaceous plants now grow within the old scour zone, including scouring rush, watercress, and monkey flower. These plants were reported at Vaseys Paradise by Clover and Jotter (1944) in 1938 and were probably among those plants seen in the 1923 photograph. Saltcedar (right foreground) is now established on the gravel bar near the camera station.60 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 424.—(1923). This photograph provides a downstream view of the Grand Canyon from within Redwall Cavern, 53.3 kilometers below Lees Ferry. The cavern, formed by solution of the Mississippian Redwall Limestone (Hamblin and Rigby, 1968) is one of the major attractions of the Grand Canyon. Prior to the contraction of Glen Canyon Dam, the river entered the cavern during high flows producing conditions too unstable for plant establishment. Powell camped here during a rainstorm and noted that the floor would be inundated during periods of peak flow by a "raging flood” (Powell, 1875). Waves during an earlier period of high water have produced marks in the sand that are high above the river level in this 1923 view. Erosion of the alluvium is apparent from the vertical bank near the edge of the water. The large blocks on the far side of the cavern and in the foreground have fallen from the roof. The plant in the foreground is dogbane. (Altitude 873 meters.)FIGURES 26-73 61 Figure 42B.—(1974). Comparison of this photograph with the earlier one reveals several changes. The impact of wave action is no longer apparent away from the river, but wind ripples are evident in the sand of the foreground. Using the collapse blocks as references, it is apparent that aggradation, perhaps from wind, has occurred on the higher parts of the beach and erosion has reduced the sand deposit toward the base of the sloping beach. The sand bar across the river to the right appears new. The only plants visible are scattered saltcedars. The lack of more vegetation probably results from the low light intensity within the cavern and trampling by the thousands of visitors that stop here each year.62 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 43A.—(1923). This view was taken from the top of a talus slope on the right bank a short distance above Triple Alcoves, 74.8 kilometers below Lees Ferry. The vertical outcrops with conspicuous bedding at right foreground are the Middle Cambrian Muav Limestone. The Mississippian Redwall Limestone forms the vertical cliffs above the Muav. The flood-line vegetation is especially well developed on the talus slopes on the right bank. A few logs of driftwood can be seen along the upper part of the boulder-strewn beach; elsewhere on the beach, there are no signs of plants. (Altitude 858 meters.)FIGURES 26-73 63 Figure 43S.—(1974). Driftwood is still visible in the same location as before, and many boulders are the same in both views. Much of the sand deposit is still relatively unstable and devoid of plants. In this March view the flood-line community, composed mainly of western honey mesquite in this section of Upper Marble Canyon, is leafless and cannot be easily compared with its counterpart in the August 1923 photograph. Western honey mesquite is first seen in the canyon only 11.5 kilometers upriver from here. Some of the plants (jointfir) on the talus slope at the right seem to be the same as those in the earlier view. The vegetation at the river’s edge is predominantly saltcedar and willow, with sparse seep willow, arrowweed, and cattail. Other species found on the beach are peppergrass, desert plume, Russian thistle, red brome, globemallow, brittlebush, dyssodia, longleaf brickellia, and desert trumpet.64 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 444.—(1923). Ancient Indian structures, which were possibly used to store grain, can be seen in a cave within the Mississippian Redwall Limestone in this downstream view near the mouth of Nankoweap Canyon. The photographic station is roughly 259 meters above the Colorado River and 84.6 kilometers below Lees Ferry. A dense stand of catclaw, western honey mesquite, and netleaf hackberry occurs above the level attained by the river during flood stage (Altitude 847 meters.)FIGURES 26-73 65 Figure 44B.—(1974). This matching photograph was taken in March before many of the shrubs and trees in the valley were in full leaf. The preceding photograph was taken in August when foliage was fully developed. Because the decrease in density of the plants at A might be the result of differences in seasonal development, additional photographs, taken by John Richardson, Southern Illinois University, in July 1978, were obtained. These photographs (not shown) were taken at a time of maximum leaf development and reveal the same decline in the predam flood-line community as in this March view. The dense thicket on the terrace above the boats (at B) was thinned by a wildfire that burned through the thicket in May 1970 (P. S. Martin, written commun., 1971). During the 51-year interval between the two photographs, there has been extensive development of the riparian belt near the margins of the river. Saltcedar, arrowweed, sandbar willow, and Emory seep willow are the dominant plants with occasional growths of cattail, smooth horsetail, and great bulrush. Trails have become prominent features of the slope leading to the Indian ruins.66 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 45A.—(1872). Two of Powell’s boats are shown tied up at a small island at the mouth of the Little Colorado River 98.8 kilometers below Lees Ferry. The mouth of the Little Colorado is just out of view on the right. This upstream view of the Colorado River valley shows a well established zone of dense vegetation above the level of maximum river stage and at the approximate base of the Middle Cambrian Bright Angel Shale. Several light-colored deposits of sand appear on both banks; these accumulations are deposited during spring high flows. (Altitude 826 meters.)FIGURES 26-73 Figure 45B.—(1972). Two boats representative of styles commonly in use today appear in this view. Saltcedar now occupies the habitat nearest the river. The zone of dense vegetation above the old high water line is little changed in 100 years and is mainly catclaw. The river stage is several feet higher in the 1972 view than at the time of Powell’s visit. A silt deposit now covers formerly bare surfaces on the rocky promontory in the Lower and Middle Cambrian Tapeats Sandstone at right.68 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 46A.—(1963). The mouth of the Little Colorado River as seen from Cape Solitude, 1,167 meters above river level. This upstream view of the Colorado River was taken after completion of Glen Canyon Dam and during a period of low flow when Lake Powell was filling. From this vantage point, the interrupted line of dense vegetation marking the flood level is visible, especially on the left bank. A few shrubs (circle) have become established below the upper fringe of plants. The Little Colorado River enters from the right, and its waters appear turbid in this July view. A dense stand of riparian vegetation lines its left bank. (Altitude 826 meters.)FIGURES 26-73 69 Figure 46S.—(1973). In the 10-year period between the dates of the two photographs the dense stand of saltcedar remains approximately the same along the lowest reach of the Little Colorado River. Vegetation changes along the Colorado River mainstem show three trends, depending upon location: Changes are slight or nonexistent within the dense stand of catclaw marking the former high water line; changes are conspicuous near the edge of the water where a dense growth of saltcedar has recently appeared; the few shrubs that had become established below the upper fringe by 1963 have increased in size but not in number.Figure 47A.—(1872). This photograph was taken by J. K. Hillers from near the mouth of Lava Canyon 105.4 kilometers below Lees Ferry. The vegetation on the opposite bank grows on a fan at the mouth of Palisades Creek. The dominant plants appear to be western honey mesquite and catclaw. Note the ripple marks on the block at the right. (Altitude 817 meters.) VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONAFIGURES 26-73 71 Figure 47S.—(1974). The foreground rocks show little change in the 102 years since the first photograph of this pair was taken. On the opposite shore, the bare sandy beach of the previous century is now covered by sandbar willow and saltcedar with minor inclusions of seep willow and arrowweed. The zone with western honey mesquite and catclaw is little changed and, although not discernible in this view, is separated from the riparian vegetation bordering the river by an open zone of mainly ephemeral plants.Figure 48/1.—(1923). The U.S. Geological Survey team is shown packing up for portage at the head of Hance Rapids, 123.1 kilometers below Lees Ferry. The pack animals were able to reach the river from the South Rim by descending the Hance Trail. The base of the vertical exposure of Precambrian Shinumo Quartzite (upper left) is well defined by differential weathering of the underlying Hakatai Shale. A thicket of desert shrubs can be seen on the left above the zone scoured by floods. (Altitude 780 meters.)FIGURES 26-73 73 Figure 48B.—(1972). The depth of the sand deposit has increased and saltcedar grows densely along the edge of the river. The sand dune is new on the sloping area at left midground. The thicket of desert shrubs at left, comprising western honey mesquite, catclaw, and four-wing saltbush, is rearranged but may have changed little in biomass. The Hance Trail is no longer maintained and is not usable by pack animals. The remains of a campfire and the many human tracks attest to the heavy use the area receives as a campground by parties floating down the river. In 1974 this campsite was given a subjective human impact rating of 20.0 on a scale from 10.0 (no impact) to a maximum impact value of 22.6 (Aitchison, 1976). In spite of the heavy use, saltcedar has become established in abundance.74 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 49A.—(1872). This downstream view, taken by J. K. Hillers on Powell’s second expedition, shows the head of the rapids named Sockdolager by Powell in 1869. In boxing parlance this name means a heavy or finishing blow. The rapids are 126.3 kilometers below Lees Ferry and are the first in the Upper Granite Gorge, which begins about 2.4 kilometers upstream from here. Because the lower Precambrian rocks through the Upper Granite Gorge are resistant to erosion, steep walls and a narrow V-shaped canyon are produced. The great length of the rapids and the thunder produced by the sound of the waves reverberating from the walls as the water tumbles through the gorge, gives the rapids a frightening aspect. There is little substrate for terrestrial plants near the river or above on the cliff faces. (Altitude 770 meters.)FIGURES 26-73 75 Figure 49/1.—(1974). Except for one clump of saltcedar, leafless in the March view, little has changed in 102 years. Because of the narrow canyon and the near vertical walls, fluctuations in water level are exaggerated through this reach. The reaches of the river with the least vegetation change are the "Inner Gorges” represented by this view.76 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 50A.—(1952). This photograph is the first in a set of three that spans 20 years. The photograph is taken from the Kaibab Trail just above the Kaibab Suspension Bridge, 140.6 kilometers below Lees Ferry, and shows the mouth of Bright Angel Creek. The debris fan at the mouth of Bright Angel Canyon is devoid of plants except on higher ground where western honey mesquite is the dominant large plant. The large trees around the base of the hill near midground are Fremont cottonwoods. Phantom Ranch is out of view upstream on Bright Angel Creek. The area near the mouth of Bright Angel Creek is the most heavily visited location in the bottom of the Grand Canyon and is served by two trails from the south rim and one from the north. (Altitude 741 meters.)FIGURES 26-73 77 .-d£.-4r<3 Figure 50B.—(1963). On the date of this photograph, Glen Canyon Dam had been completed and the water was being impounded in the reservoir area above it. No plants occur at the river’s edge. Most of the boulders on the beach are the same in this and the earlier photograph.78 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 50C.—(1972). In the 9 years since the previous photograph was taken many new features are apparent. In December 1966, record flooding occurred in the Bright Angel watershed and the creek left the old channel, flowing across the debris fan toward the camera point. Boulders in the path of the flood were removed from the fan (or covered?) and finer material was deposited in their place. In the absence of subsequent flooding along the Colorado River, this area has remained free of boulders. The bridge across the Colorado River on the left was built in 1967 and is used as a foot bridge. The bridge also supports a pipeline carrying water from the upper reaches of Bright Angel Creek to the south rim. The cottonwoods at midview have declined as have the mesquites on the right (between the bare fan and the trail above). Plants now occupying the debris fan near the edge of the water include sandbar willow, saltcedar, and several Fremont cottonwood saplings. The fan was visited in May 1978. Beavers had cut the sapling cottonwoods, but the trees had produced multiple basal spouts.FIGURES 26-73 7980 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 51A.—(1903). Bright Angel Trail reaches the Colorado River at the mouth of Pipe Creek (kilometer 143.1), the point from which this upstream view was taken. The floor of the canyon is devoid of plants. In the vicinity of the camera station, the only apparent features near the river are sand, cobbles, and consolidated rocks. (Altitude 732 meters.)FIGURES 26-73 81 Figure 51B.—(1976). The sand deposits seen in the earlier view are largely gone and a few plants have become established along the river banks. The route of a trail cut through the Precambrian Vishnu Schist in 1936 (Hughes, 1967) can be seen above to the right. A large rock block, present in the earlier picture near the river at right, has fallen. It may have been displaced by falling rubble at the time of trail construction. Although not seen in this view, high water levels of predam time are expressed as sand and coarse gravel plastered against the canyon walls 12 to 18 meters above the river (Hamblin and Rigby, 1969).VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 52A.—(1901). The Colorado River flows from right to left in this view up Shinumo Creek, 174.7 kilometers below Lees Ferry. The photograph was taken from the opposite bank and shows horizontal water stains on cliffs to the left of the canyon mouth. The deposit of sand and boulders at the mouth of Shinumo Creek is not occupied by plants. (Altitude 666 meters.)FIGURES 26-73 83 Figure 52B.—(1972). The river is at higher stage than in the previous view, but several changes are apparent: the sand deposit at left is gone, and saltcedar now flanks the mouth of Shinumo Creek and also occupies the camera location. The pile of rounded boulders has changed little since 1901.84 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 53A.—(1890). The three boats of the Stanton Survey party are tied up at a small sandy beach 185.8 kilometers below Lees Ferry. This upstream view shows the well-bedded Lower and Middle Cambrian Tapeats Sandstone at river level again, having been at varying elevations above the river since roughly the mouth of the Little Colorado River (fig. 45). Sand deposits near the camera station and upstream on the opposite bank have been left by high flows. No plants are established below the upper edge of these deposits. (Altitude 645 meters.)FIGURES 26-73 85 Figure 53S.—(1976). The river is at higher stage now than in the earlier view but loss of much of the higher sand deposits is evident. A few plants of dicoria occur on the sand. Saltcedar is perhaps 3-4 meters tall where it grows in scattered pockets near the edge of the river.86 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 54A.—(1923). This downstream view shows the debris fan at the mouth of Fossil Canyon (out of view on left), 200.8 kilometers below Lees Ferry. The Middle Cambrian Bright Angel Shale is exposed near river level through this reach of the canyon, although it is mostly covered by slope wash. A line of scattered shrubs marks the high water line above the barren beach of rocks and patches of sand. (Altitude 623 meters.)FIGURES 26-73 87 ips \ ;3& I--' ^ Figure 54B.—(1976). The line of shrubs seen in the previous view is composed of catclaw and appears to be little changed. Both views were taken in September, a month when the shrubs are in full leaf. The camera is too far back for an exact match. The catclaw right of center has grown in size. The herbaceous plants of the foreground slope are approximately as dense now as in 1923. This camera station is on a well-travelled burro trail. Saltcedar grows densely along sandy stretches on the opposite bank and long-leaf brickellia grows in an open stand on the rocky areas. The sand deposits on the opposite bank are less thick than in 1923.88 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 55A.—(1872). A. H. Thompson, who was in charge of topographical work on Powell’s second expedition (Dellenbaugh, 1908; Bartlett, 1962), was photographed looking upstream from the left bank a short distance below Fossil Canyon at the head of the Middle Granite Gorge, 202.7 kilometers below Lees Ferry. The Lower and Middle Cambrian Tapeats Sandstone forms a shallow inner gorge here. There is no observable vegetation on the pile of boulders in the foreground. (Altitude 619 meters.)FIGURES 26-73 89 Figure 55B.—(1974). Although the photograph is not an exact match, one can easily see that the configuration of the pile of boulders has changed little, if at all. After 102 years, such plants as dropseed, seep willow, and saltcedar have become established among the large rocks. The light colored sand deposits appear smaller now than before.90 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 56A.—(1923). The water in Deer Creek plunges more than 30 meters through a slot cut in the Lower and Middle Cambrian Tapeats Sandstone just before entering the Colorado River, 219.1 kilometers below Lees Ferry. Angular blocks from the cliff face above lie on the banks of the river. The Colorado River flows from right to left in this picture. (Altitude 588 meters.)FIGURES 26-73 91 Figure 56B.—(1972). Few raft parties pass here without stopping: between 1963, when Glen Canyon Dam was completed, and August 1972, when this photograph was taken, an estimated 50,000 people had visited this site. Saltcedar, sandbar willow, and seep willow grow densely at the mouth of Deer Creek in spite of the heavy human impact. The large blocks and boulders have not changed noticeably but changes involving some of the smaller rocks along the beach are evident. The large sand deposit on the left in the old photograph is gone.92 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 57A.—(1923). Kanab Creek (left foreground) enters the Colorado River 231.2 kilometers below Lees Ferry and is seen here from a camera station approximately 100 meters above the river. Powell ended his second trip here in 1872, leaving the Grand Canyon via Kanab Canyon. The Middle Cambrian Muav Limestone is exposed at the entrance to Kanab Canyon, with Mississippian Redwall Limestone and Devonian Temple Butte Limestone forming the vertical walls above. The development of a well-defined row of shrubs above the position of maximum flood stage is the most conspicuous feature of the vegetation in this view. (Altitude 572 meters.)FIGURES 26-73 93 Figure 57B.—(1972). The row of shrubs marking the predam high flood stage is catclaw and has persisted with little change through almost half a century. The new riparian community comprises several species, including saltcedar, seep willow, waterweed, Emory seep willow, cattail, and desert isocoma. A rock avalanche at upper right is new since 1923.94 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 58A.—(1923). A bare skirt, several times wider than the length of the U. S. Geological Survey boat at left, is seen in this view of the right bank a short distance above Lava Falls, 287.4 kilometers below Lees Ferry. A dense shrub community occurs above the reach of flood water. (Altitude 511 meters.)FIGURES 26-73 95 Figure 58B.—(1972). Slight thinning may have occurred along the upper fringe of the dense plant community present in 1923. (The 1923 and 1972 photographs were taken in September and July, respectively, and the degree of foliation should be approximately the same in both.) The community is dominated by western honey mesquite and catclaw. The open desert community on the upper parts of the talus slopes beneath the cliffs is composed mainly of creosote bush which reaches its range limit 15 kilometers upstream from this point (P. S. Martin, written commun., 1971). The riparian community is dominated by saltcedar with lesser amounts of arrowweed and desert broom. The latter species became an important member of this community beginning about 24 kilometers upstream from this station. Sandbar willow and cattail grow along the lower edge of the riparian community in the strip subjected to diurnal flooding. Cambrian strata, mostly covered by slope wash, lie just above river level. Upper Cenozoic basalt flows overlie the sedimentary rocks. Layers of river gravels, laid down during periods between successive flows, are exposed on the cliff face.96 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 59A.—(1950). Most boatmen agree that Lava Falls Rapids, which lies 288.5 kilometers below Lees Ferry, represent the greatest hazard to navigation on the Colorado River. This remarkable photograph was taken of the rapids from the right bank looking toward the mouth of Prospect Canyon. Flow in the river was approximately 1,473 m3/s. The great turbulence created when the water encounters erosional debris from Prospect Canyon is clearly seen in this photograph. The large dark shrubs in this view were still present in 1976 (fig. 59B) and are catclaw. Some of the shrubs on the opposite bank are desert broom. (Altitude 511 meters.)FIGURES 26-73 97 Figure 59B.—(1976). This photograph was taken from a slightly different position and at a time when the Colorado River was carrying only 102 m3/s. There has been a notable biomass increase in the riparian community during the 26 years since the original photograph was taken. The erect, coarse grass at the mouth of Prospect Creek is carrizo. Other plants growing near the river include arrowweed, desert broom, cattail, saltcedar, and horseweed. On the dissected fan at the mouth of the canyon is creosote bush, catclaw, ocotillo, and barrel cactus. The last seems to have declined in number since 1950.98 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 6QA.—(1923). This photograph was taken from the left bank 313.6 kilometers below Lees Ferry. A Quaternary basalt flow is seen at river level on the right. The canyon of the Colorado River is broad enough here so that the scouring action of flood waters is dampened and some perennial plants became established in the alluvium near the river. A flood, 6 days earlier, overtook the U.S. Geological Survey crew at Lava Falls, 25.1 kilometers upstream from here and the river stage increased by 6.7 meters (Freeman, 1930). The signs of recent wave action high on the beach in this photograph probably stem from the same flood. Several plants such as the tree (Fremont cottonwood or willow) at right midground, desert broom (left foreground), and what is probably saltbush (near the men), withstood these floodwaters. (Altitude 471 meters.)FIGURES 26-73 99 Figure 60B.—(1972). At the time of this photograph a Goodding willow was growing at the same location as the tree in the previous view. The present tree is smaller than the one in the earlier photograph. Either the original tree was broken and then resprouted or there is now a different tree growing in the same location. Desert broom is the dominant woody plant of the foreground beach. The grasses, which have increased notably over the beach area, will serve to stabilize the sandy soil of the beach. The new camera location closely matches the old, although the lack of stable foreground features precludes exactly matching the two views. An increase in sand deposition on the beach is obvious. The giant grass, carrizo, grows at the edge of the river with saltcedar.100 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 6L4.—(1923). This upstream view was taken from a point just above Spring Canyon, 328.9 kilometers below Lees Ferry. The Middle Cambrian Bright Angel Shale, mostly covered by slope wash, is at river level. A well-developed community occurs above the flood line on both sides of the river. At the present river stage, a large bar, unoccupied by plants, is exposed. (Altitude 454 meters.) MirFIGURES 26-73 101 Figure 6 IB.—(1974). The camera station is too far forward and too far to the right for an exact match. The plants that now densely occupy the higher ground of the old bar include saltcedar, seep willow, desert broom, catclaw, arrowweed, and some large western honey mesquites. The present exposed bar is probably low enough to be inundated by the regular daily high flows. Because of the slightly darker tones of western honey mesquite and catclaw foliage compared to the color of the riparian plants, the contact between the old highwater community and the new riparian community is apparent. Across the river where the talus slopes have a northerly aspect, the highwater community is mostly western honey mesquite and catclaw. This elongate community appears slightly more open in 1974 (August) than in 1923 (September). The same highwater community below the camera station has a southerly aspect, and four species share dominance: western honey mesquite, wolfberry, creosote bush, and catclaw. On the slope in the foreground is Mohave Desertscrub vegetation comprising ocotillo, brittlebush, creosote bush, Morman tea, and barrel cactus.102 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 62A.—(1923). The mouth of Two Hundred and Ninemile Canyon is seen across the river from this camera station in a small cove at Granite Park, 336 kilometers below Lees Ferry. The first cliffs above the river to the right of the canyon mouth are remnants of a basalt flow and are preserved here, as elsewhere along this reach of the Grand Canyon on the inside of a meander bend. The willow at left is one of those seen from a distant hill in figure 644. Once these plants have developed a large enough root system, they withstand considerable buffeting by floods in the broad valley at Granite Park. A well-developed flood-line community can be seen across the river at the mouth of Two Hundred and Ninemile Canyon. (Altitude 442 meters.)FIGURES 26-73 103 Figure 62B.—(1974). Because the old camera station is now overgrown by saltcedar and desert broom, this photograph was taken from a slightly different location than in 1923. The tree at left is Goodding willow and is the same plant as in the earlier view. Bermuda grass now grows on the beach near the tree. The flood-line community at the mouth of Two Hundred and Ninemile Canyon comprises western honey mesquite and catclaw. Although not evident in the photograph, close field inspection revealed that both species, trimmed through grazing by burros, are infested by mistletoe.104 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 63A.—(1923). E. C. LaRue used a panoramic camera that took pictures through a wide angle of view. This photograph was taken with that camera and is a continuation to the right of figure 62A.Temple Butte Limestone forms much of the conspicuous butte on the right; thicker bedded limestones form cliffs, thinner bedded limestones form the shallow ledges. The large shrubs near the U.S. Geological Survey boats appear to be willows and the smaller shrubs behind them to the right, arrowweeds. The vertical banks below the flood-line community at right midground and on the beach near the boats suggest that erosional processes are active. (Altitude 442 meters.)FIGURES 26-73 105 Figure 63S.—(1974). The clump of large shrubs seen near the boats in the previous view are gone and have been replaced by a few small Goodding willows which are obscured by the more abundant saltcedar. The band of arrowweed has expanded and now occupies, with desert broom, the sandy beach between the saltcedar fringe along the river and the scarp below the first terrace. On the terrace, the dark shrubs of the old flood-line community are mainly western honey mesquite with some catclaw.106 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 64A.—(1923). This upstream view was taken from a point 168 meters above the Colorado River and shows Granite Park, 336 kilometers below Lees Ferry. A flood-line community is strongly developed along this section of the canyon. Some trees have become established at the edge of the river, a habitat that is here more stable than usual because of the great width of the valley floor. (Altitude 442 meters.)FIGURES 26-73 107 Figure 64B.—(1974). This view, taken in March, shows a greatly diminished flood-line community, but this reduction in plant biomass is more apparent than real—the shrubs comprising the community are leafless at this season, whereas in the 1923 photograph, taken in September, the plants were in full leaf. The streamside plants that were present in 1923 and which persist to the present are willows. Unlike the condition in 1923, the beach at Granite Park now supports a dense growth of plants. The dominant species on the beach are willow, arrowweed, saltcedar, camelthorn, red brome, and Bermuda grass. All but the first two species have been introduced to this continent from other parts of the world. Feral burros, another introduced species, are found only on the right side of the river. On areas of poorly stabilized sand, grow sand verbena, evening primrose, and dropseed. The vegetation of the bajada is typical Mohave Desertscrub with creosote bush, white bursage, brittlebush, ocotillo, range ratany, and various cacti. Sand deposits on the large formerly barren island are becoming stabilized by such plants as dropseed, sand verbena, evening primrose, bebbia, globemallow, longleaf brickellia, and slender poreleaf. The arm of the river to the right of the island in the photograph now carries little flow and may be an incipient marsh. Along its margin grow such marsh species as cattail, earrizo, and horsetail in addition to saltcedar, seep willow, willow, and arrowweed.108 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 65A.—(1923). This downstream view was taken at kilometer 350.3 from a station just below the mouth of Two Hundred and Seventeenmile Canyon and only a short distance into the Lower Granite Gorge. Except where sediment has entered the canyon from tributaries, there are few sand or gravel shores within the Lower Granite Gorge, so steeply have the Precambrian rocks eroded to river level. Even where substrate exists for riparian communities, as on the foreground debris fan, the narrowly defined valley tends to increase the depth of flood waters making these habitats too unstable for plant establishment. The flood-line community is also missing on the steep slopes above the river. (Altitude 424 meters.)FIGURES 26-73 109 Figure 65B.—(1972). Seasonal flow maxima are greatly reduced under the present flow regime and formerly inhospitable debris fans now support a varied plant life. Among the species on the foreground alluvium are arrowweed, cattail, carrizo, desert broom, sandbar willow, and saltcedar. The conspicuous shrub below the sloping flat boulder at upper left is catclaw; the plant has changed little in nearly half a century.110 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 66A.—(1923). Members of Powell’s second river expedition found quartz crystals on an ant hill near the pyramid-shaped peak at center and the peak became known as Diamond Peak (Dellenbaugh, 1908). The valley is broad through this section of the Lower Granite Gorge, and alluvial deposits are common. There is a single plant at the left on the far side of the gravel bar in this downstream view taken 358.1 kilometers below Lees Ferry. The plant’s identity is uncertain, but it appears bent over, perhaps from the same flood of a few days earlier that overtook the U.S. Geological Survey team at Lava Falls. The flood-line community is conspicuous in this photograph taken in early October. In the foreground, a large willow can be seen. (Altitude 413 meters.)FIGURES 26-73 111 Figure 66B.—(1972). The camera station is now in a dense stand of arrowweed. Many saltcedar plants grow around the edge of the gravel bar where a half century earlier only one plant was visible. In addition, two western honey mesquites, desert broom, Goodding willow, and desert isocoma grow near the shore. The flood-line community, mainly of catclaw with few western honey mesquites, is still present but seems diminished in this August view. Close inspection of this community shows that there are numerous burro trails and the basal portions of flood-line community shrubs are stripped of foilage where in reach of the animals.112 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 67A.—(1923). View down the V-shaped canyon cut through Precambrian metamorphic rocks. The stratified formation capping the Precambrian rocks is the Lower and Middle Cambrian Tapeats Sandstone. The far skyline is defined by a formation of the Pennsylvanian and Permian Supai Group. The sand deposit in the foreground and the one across the river are both at the mouths of minor tributary canyons 360.7 kilometers below Lees Ferry. A flood line is apparent from stains on the rocks of the opposite shore and no plants are visible below that level. (Altitude 408 meters.)FIGURES 26-73 113 Figure 67B.—(1976). The boulder with the man standing on it in the previous photograph is now hidden by saltcedars, which form a discontinuous strip along both shores. The depth of sand around the base of the boulders is less now than before. The sand deposit is now partially stabilized by dropseed (the coarse grass), red brome, and Russian thistle. Signs of burros are abundant.114 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 68A.—(1902). This photograph was taken from the small promontory of rocks seen beyond the fan in the previous pair of pictures and shows in close view the same large tree near the river as seen in the previous views. For the tree to have reached this size would probably require 10 or more years of growth. Thus, the tree became established in about 1892 or earlier and persisted until at least 1923 (fig. 69A). (Altitude 408 meters.)FIGURES 26-73 115 Figure 68B.—(1972). Saltcedar, arrowweed, and seep willow grow thickly at the site occupied earlier by the single tree. Slightly to the right of that position can be seen a clump of cattails.116 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 69A.—(1923). Diamond Creek enters the Colorado River 363.2 kilometers below Lees Ferry and is the first place in that distance where vehicles can reach the river. The mouth of Diamond Creek was visited by Ives and his party in 1858 (Ives, 1861) and was the terminus of the Wheeler Expedition in 1871 (Wheeler, 1872). The Diamond Creek Hotel was in operation here for sightseers from 1884 to 1889 before scenic areas farther east were developed (Barnes, 1960; Simons and Gaskill, 1969). In this upstream view, the only plant at the river’s edge is a large tree visible on the opposite bank in a protected area below a bar. (See also figure 68A.) Large plants on the fan at the mouth of Diamond Creek (right midground) are probably mesquites. On the uppermost sandy terrace of the fan a dense growth of small shrubs is evident. (Altitude 407 meters.)FIGURES 26-73 117 Figure 69B. (1976). A dense community of saltcedar, arrowweed, and seep willow has become established along the shore on both sides of the river. The fan at right is more heavily overgrown by plants than before, except for the formerly stabilized high terrace which now seems to be covered by a sand deposit. The large tree on the opposite bank has not persisted to the present. The Colorado River no longer floods the large bare area with boats and people on it; as a result, fine material, deposited by flooding on Diamond Creek, now covers the gravels that were present earlier. Bermuda grass grows in moist soil near the shelters. The lower reach of Diamond Creek (foreground) is lined by a riparian community dominated by saltcedar.118 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 70A.—(1923). The U.S. Geological Survey team has moored its boats near a large rock at a sand bar on the right bank, 368.0 kilometers below Lees Ferry. Although no flood-line community is found on the steep walls of the Lower Granite Gorge, the flood line is visible because of stains on the rocks of the talus slope. No plants are found below that line, and above it are seen widely scattered desert shrubs. (Altitude 395 meters.)FIGURES 26-73 119 Figure 70S.—(1972). The new flood line now lies below the large rock and several plants of brittlebush grow on the slope between the new and the old flood lines. The bar in the foreground is unusual in that no plants have become established there. The Inner Gorge is narrow here and during times of high flow the bar is flooded, making for an unstable surface for plant establishment.120 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 71A.—(1923.) This upstream view of the Colorado River was taken from a hill above the mouth of Maxson or Reference Point Canyon, 406.0 kilometers below Lees Ferry. At the time of the photograph, barren silt accumulations deposited during floods could be seen high on the steep walls of the Inner Gorge. For explanation of the dashed line, see the next figure. (Altitude 322 meters.)FIGURES 26-73 121 Figure 71B.—(1972). Silt had accumulated to a depth of over 50 meters in this reach of the Lower Granite Gorge by 1948, approximately 13 years after the completion of Hoover Dam (Pampel, 1960). The old camera station is now within the reservoir area of Hoover Dam and is buried beneath Lake Mead silt. The new station has been moved upslope. Saltcedar is the dominant plant of the foreground. The dashed line in the previous figure marks the approximate location of the surface of the silt deposit seen in this view. The vertical silt bank extends approximately 5.5 meters above the present water surface and has developed since 1963 when the water level in Lake Mead was lowered sharply at the time impoundment of water in Lake Powell began. A secondary riparian community has developed at the base of the bank. At the time of the photograph, the lake level was approximately 352.5 meters above mean sea level (U.S. Geological Survey, issued annually). By the following May, the lake level reached 361.8 meters above mean sea level (U.S. Geological Survey, issued annually), at which time the silt bank was probably inundated resulting in the death of many of the saltcedars.122 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 72 A.—(1923). The Lower Marble Canyon ends at about kilometer 414.3 and the steep walls of the Inner Gorge give way to less steep talus slopes. In this view, looking down the canyon of the Colorado River 437.6 kilometers below Lees Ferry, the flood-line community is well developed above the conspicuous highwater line. The Middle Cambrian Bright Angel Shale is at river level. Typical Mohave Desertscrub species of the foreground include ocotillo, white bursage, and agave. The floodline community probably includes western honey mesquite and catclaw. (Altitude 282 meters.)FIGURES 26-73 123 Figure 72S.—(1974). The old camera station lies beneath Lake Mead silt several meters below and to the right of the new position. Before Hoover Dam was built the thalweg altitude was roughly 275 meters; now the thalweg altitude exceeds 350.5 meters, representing an accumulation of 76.2 meters of sediment (Pampel, 1960). Saltcedar is the only plant seen on the deeply cracked silt deposit. Many of the plants are dead in this August view, presumably from submergence during earlier months when this site was covered by water to a depth of 2.7 to 3.7 meters. Some taller plants survived. Seedlings growing from the deep cracks in the foreground became established after the water receded. This scene illustrates the dynamic nature of the saltcedar community on Lake Mead silt.124 VEGETATION CHANGES ALONG COLORADO RIVER, ARIZONA Figure 73A.—(1923). What appear to be catclaw and western honey mesquite form a dense community lining the channel of Cave Canyon which enters the Grand Canyon from the left bank 441.3 kilometers below Lees Ferry. This community merges with the flood-line community of the Colorado River valley. There is apparently a moist area in a travertine deposit on the bank opposite the mouth of Cave Canyon which supports a low growth of plants that extends to the edge of the river. The slopes near the camera station are dominated by creosote bush and white bursage. (Altitude 280 meters. Estimated from Pampel, 1960; Belknap, 1969.)FIGURES 26-73 125 Figure 73B.—(1974). The original camera station is now covered by water and silt and cannot be reoccupied. By 1948-49, silt had filled the valley at this location to depths as great as 73.2 meters (Pampel, 1960). The upper surface of the silt deposit is exposed near the opposite bank and is densely overgrown with saltcedar. Plants of the saltcedar thicket are leafless and dead at the time of this August photograph; death was probably from inundation during 1973. That year the lake reached the highest levels since 1963 when diversion began for filling Glen Canyon Dam. GPO 689-143UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 1132 PLATE 1 SCALE 1:500 000 10 0 10 20 30 40 MILES I—II—ii—ii—ii—ii— --1 i — i I 10 0 10 20 30 40 50 60 KILOMETERS l—l M |—| |—| |—| | I I-- -i i .. ' | MAP SHOWING COLORADO RIVER FROM GLEN CANYON DAM TO LAKE MEADUNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 1132 PLATE 3 Poverty Flat lOTH NDARD Poverty Flat NORTH TtIndard milion Cliffs' -Big Sefingso - STANDARD ) Cedar Ridge oCedar Norin0 »TANC Grand Canyon PARALLEL NORTH NORTH. NORTH. ~T»«t L. EXPLANATION Rabbitbrush Waterweed Springs^ 'STANOARI FIGURE 16. — Map showing distribution of rabbitbrush and waterweed along the Colorado River from Glen Canyon Dam to Lake Mead. ■SJandarc 'sTANOARd PARAL FIGURE 18.-Map showing distribution of Emory seep willow along Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 1 7. - Map showing distribution of seep willow along the Colorado River from Q[en Canyon Dam to Lake Mead PARALLEL \. AN DARI NORTH Poverty Flat NORTH "Jacob i_‘ Lake Clifts 6- STANDARD PARAI LE L MTTrumbull tyOR jORl North,-, Grand Canyon shallow PARALLEL NORTH EXPLANATION • Redbud A Desert isocoma 'STANOARt j^ANDARI ‘STAN DARI FIGURE 19. - Map showing distribution of netleaf hackberry along Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 21.- Map showing distribution of cattail along Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 20. - Map showing distribution of redbud and desert isocoma along Colorado River from Glen Canyon Dam to Lake Mead. Poverty Flat north north Marble Canyon —Big Springsp : .STANDARD STANOARD Kangaroo The Dome 5*87' ALLEI \iort|i ' OR 11 Grand Canyon /Coer nino1 PARALLEL PARALLEL RTH ^STANOARd RTH PARAL ☆ Interior Geological Survey, Reston, Va. Map showing distribution of sandbar willow along Colorado River from Glen Canyon Dam to Lake Mead. W79209 FIGURE 22. - Map showing distribution of reed along Colorado River from Glen Canyon Dam to Lake Mead. Map showing distribution of spiny aster along Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 24. FIGURE 2 3 ^Littlefield V /I Ml Bangs >3!0' \ ) oV / ) « 4 Mud Mtn\ 5/96 • ) - -JJL \ r Sr , ? : \..... i r- ’■ "para LLEL \* /■ \ V r c*n y ■ess » ) ^ - ■ 5707 .Lost Sprin, \ T' s> <; ^ \ ^ ih i 1 1 " 7 ( i S7*°Y J * / fss. * STANDARD $ j * / o ( S . 5 i ui O Mt eo" 0 •1 I Logan \ \x #MI Emma ) ) i|°m H 'JEL > \ y 17 1 \ \ *0 > Z O J 6 7TH Gold p • Iron M • H O' i 96 \ .... —(e.d m \ Spring Wash \ if ‘ / C 'v. - i -B > \ Hills FIGURE 25. - Map showing distribution of Goodding willow along Colorado River from Glen Canyon Dam to Lake Mead. 20 SCALE 1:1 000 000 20 ___40 60 80 MILES 3 20 0 I—I I—I I—II—I I—I h- 20 40 60 80 100 120 KILOMETERS NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAPS SHOWING DISTRIBUTION OF SELECTED PLANT SPECIES ALONG THE COLORADO RIVER Base from U.S. Geological Survey State Map 1:1,000,000, 1974UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 1132 PLATE 2 FIGURE 7. — Map showing distribution of Bermuda grass and elm along Colorado River from Glen Canyon Dam to Lake Mead Flat north" LLEL lOTH Marble Cebyof., 0111101? Cliff*' -W*S«tin«*< iTANDARD - 1ALI-C' 'STanT Grand Canyon Moqui Tpsayan EXPLANATION Russian olive Desert broom 'STANOARC ■r*U-Ei FIGURE 8. Map showing distribution of Russian olive and desert broom along Colorado River from Glen Canyon Dam to Lake Mad. The Gap Shadow*?; • *** ^ JA t <>*' Moccasin PARAjLL^L V NORTH IOTH ISTANOARD Ip^RoV Nf O R * H ■ *Tanc Grand Canyoni PARALLEL Camp1 Map showing distribution of saltcedar along Colorado River from Glen Canyon to Lake Mead. FIGURE 9 Moccasin NORTH PARAll NDARD ff .siiat ujh 7 PefiW’._______ Grand Canyon EXPLANATION louble Too. Camelthorn Apache plume '“‘ton mA 'lC>- «„ l-'“ 1 fl r ^ H \ 5 o Ltr X /* \f~- Ft- 7_ V o oGjay M I MMf td N ' /r / l-Si atm FL Willows. I Camo I FIGURE 10. - Map showing distribution of camelthorn and Apache plume along Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 11.— Map showing distribution of catclaw along Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 12. - Map showing distribution of western honey mesquite along Colorado River from Glen Canyon Dam to Lake Mead. —) ^sColorodo HE. Poverty Flat NORTH LLEL \*. PARAll Cliffs' STANDARD IALLEL "ETaNC PF*n» Grand Canyon Xameron PARAL stan5aRd" NORTH. 'STAN PARC FIGURE 13. - Map showing distribution of Fremont cottonwood along Colorado River from Glen Canyon Dam to Lake Mead. Poverty Flat Andard ittlefield «* Swung* o STANDARD Findlay ^ORIH North0 •TANC Grand Canyon Too. 'STANOARC FIGURE 14. — Map showing distribution of arrowweed along Colorado River from Glen Canyon Dam to Lake Mead. Poverty Flat PARAjLLEL ANDARD Cliffs' Boyssg 559 .Mount Sinyala Grand Canyon 1 NORTH. fsTANDARd ir Interior — Geological Survey, Reston, Va. — 1979 — W79209 Map showing distribution oflongleaf brickellia along the Colorado River from Glen Canyon Dam to Lake Mead. FIGURE 15 Base from U.S. Geological Survey State Map 1:1,000,000, 1974 SCALE 1:1 000 000 20______________________0____________________20___________________40____________________60_____________ 80 MILES i~~i i—i i—i i—i i—i i ....... > i I i 20 0 20 40 60 80 100 120 Kl LOMETERS FTH 'H i-i i-i i- -I---- l------- H l- I -----| NATIONAL GEODETIC VERTICAL DATUM OF 1929 MAPS SHOWING DISTRIBUTION OF SELECTED PLANT SPECIES ALONG THE COLORADO RIVER*°4Y} M Clastic Dikes of Heart Mountain Fault Breccia, Northwestern Wyoming, and Their Significance GEOLOGICAL SURVEY PROFESSIONAL PAPER 1133Clastic Dikes of Heart Mountain Fault Breccia, Northwestern Wyoming, and Their Significance By WILLIAM G. PIERCE GEOLOGICAL SURVEY PROFESSIONAL PAPER 1133 UNITED STATES GOVERNMENT PRINTING OFFICE, . WASHINGTON : 1979UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Pierce, William Gamewell, 1904- Clastic dikes of Heart Mountain fault breccia, northwestern Wyoming, and their significance. (Geological Survey professional paper ; 1133) Includes bibliographical references. Supt.ofDocs.no.: 119.16:1133 1. Dikes (Geology)-Wyoming-Heart Mountain. 2. Breccia—Wyoming— Heart Mountain. I. Title. II. Series: United States. Geological Survey. Professional paper ; 1133. QE611.5U6P53 551.8’8 79-607991 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03252-0CONTENTS Page Abstract________________________________________________________ 1 Introduction --------------------------------------------------- 1 General features of the Heart Mountain fault-------------------- 3 Clastic dikes of fault breccia__________________________________ 3 General features of clastic dikes in upper-plate carbonate rocks__________________________________________ 4 General features of clastic dikes in volcanic rocks--------- 4 Description of dikes at specific sites ------------------------- 5 Area 1, south of Silver Gate, Mont. ------------------------ 5 Calcibreccia dikes in Paleozoic rocks of the upper plate_______________________________________ 5 Calcibreccia dikes in the Wapiti Formation______________ 6 Area 2, southeast of Silver Gate, Mont----------------------10 Page Description of dikes at specific sites—Continued Area 3, north of Republic Mountain ____________________13 Area 4, south of Pilot Creek___________________________13 Area 5, north of Jim Smith Peak________________________15 Area 6, near White Mountain____________________________17 Area 7, west of Trout Creek____________________________18 Other sites of clastic dikes___________________________20 Breccia and brecciation produced by the Heart Mountain fault 20 Mechanism of dike injection________________________________21 Geologic significance of the clastic dikes ________________23 Summary ___________________________________________________24 Acknowledgments ___________________________________________24 References cited __________________________________________24 ILLUSTRATIONS Page Figure 1. Map showing location of four phases of Heart Mountain fault and sites of clastic dikes of fault breccia ------------- 1 2. Photograph of calcibreccia dikes in the Three Forks Formation, area 1----------------------------------------------- 4 3. Cross section in area 1 showing calcibreccia dikes in blocks of the upper plate and in the Wapiti Formation --------5 4-8. Photographs from area 1 showing: 4. Calcibreccia dike with three components ____________________________________________________________________ 6 5. Specimen of calcibreccia dike containing carbonized wood____________________________________________________ 7 6. Calcibreccia dike containing carbonized wood _______________________________________________________________ 8 7. Irregular borders between calcibreccia dikes and enclosing dikelike body of the Wapiti Formation------------ 9 8. Calcibreccia dike containing xenoliths of Precambrian rocks ------------------------------------------------ 9 9. Cross section in area 1 showing movement of fault breccia and basal part of the Wapiti Formation into dikes and dikelike bodies_______________________________________________________________________________________________10 10. Cross section in area 2 showing relation of calcibreccia to Heart Mountain fault, surface of tectonic denudation, and the Wapiti Formation__________________________________________________________________________________________11 11-22. Photographs showing: 11. Polished specimen of plume of Wapiti Formation extending into calcibreccia dike, area 2_____________________11 12. Upper end of calcibreccia dike, area 2 ---------------------------------------------------------------------12 13. Antiform of calcibreccia, area 2 ___________________________________________________________________________12 14. Calcibreccia dike intermixed on its borders with the Wapiti Formation, area 3 ---------------------------- 13 15. Heart Mountain fault breccia overlain by the Wapiti Formation, area 4 --------------------------------------14 16. Polished specimen of upper third of Heart Mountain fault breccia, area 4____________________________________14 17. Mound of Heart Mountain fault breccia, area 4------------------------------------------------------------- 14 18. Polished specimen of calcibreccia from mound, area 4________________________________________________________15 19. Polished specimen of calcibreccia dike in lower part of the Wapiti Formation, area 4 -----------------------15 20. Flat-lying calcibreccia dike, area 4------------------------------------------------------------------------15 21. Polished specimen of calcibreccia interlayered with basal part of the Wapiti Formation, area 5--------------16 22. Irregular layer of calcibreccia in basal part of the Wapiti Formation, area 5 ______________________________16 23. Cross section in area 7 showing relation of a dike of the Wapiti Formation containing stringers of calcibreccia to wallrock of Madison Limestone and Wapiti Formation ______________________________________________________________18 24. Photograph of dike of volcanic breccia of the Wapiti Formation containing calcibreccia in contact with fault block of Madison Limestone, area 7______________________________________________________________________________________19 25. Photographs showing polished specimens of calcibreccia from dike of volcanic rock of the Wapiti Formation, area 7 -19 26. Cross section in area 3 showing degrees of deformation in brecciated limestone mass above Heart Mountain fault------22 27. Photograph showing polished specimen of brecciated limestone, area 3------------------------------------------------22 illIV CONTENTS TABLE Table 1. Location of and brief data on other calcibreccia dikes Page . 20CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING, AND THEIR SIGNIFICANCE By William G. Pierce ABSTRACT Structural features in northwestern Wyoming indicate that the Heart Mountain fault movement was an extremely rapid, cataclysmic event that created a large volume of carbonate fault breccia derived entirely from the lower part of the upper plate. After fault movement had ceased, much of the carbonate fault breccia, here called calcibreccia, lay loose on the resulting surface of tectonic denudation. Before this unconsolidated calcibreccia could be removed by erosion, it was buried beneath a cover of Tertiary volcanic rocks: the Wapiti Formation, composed of volcanic breccia, poorly sorted volcanic breccia mudflows, and lava flows, and clearly shown in many places by interlensing and intermixing of the calcibreccia with basal volcanic rocks. As the weight of volcanic overburden increased, the unstable water-saturated calcibreccia became mobile and semifluid and was injected upward as dikes into the overlying volcanic rocks and to a lesser extent into rocks of the upper plate. In some places the lowermost part of the volcanic overburden appears to have flowed with the calcibreccia to form dikelike bodies of mixed volcanic rock and calcibreccia. One calcibreccia dike even contains carbonized wood, presumably incorporated into unconsolidated calcibreccia on the surface of tectonic denudation and covered by volcanic rocks before moving upward with the dike. Angular xenoliths of Precambrian rocks, enclosed in another calcibreccia dike and in an adjoining dikelike mass of volcanic rock as well, are believed to have been torn from the walls of a vent and incorporated into the basal part of the Wapiti Formation overlying the clastic carbonate rock on the fault surface. Subsequently, some of these xenoliths were incorporated into the calcibreccia during the process of dike intrusion. Throughout the Heart Mountain fault area, the basal part of the upper-plate blocks or masses are brecciated, irrespective of the size of the blocks, more intensely at the base and in places extending upward for several tens of meters. North of Republic Mountain a small 25-m-high upper-plate mass, brecciated to some degree throughout, apparently moved some distance along the Heart Mountain fault as brecciated rock. Calcibreccia dikes intrude upward from the underlying 2 m of fault breccia into the lower part of the mass and also from its top into the overlying volcanic rocks; an earthquake-related mechanism most likely accounts for the observed features of this deformed body. Calcibreccia dikes are more common within the bedding-plane phase of the Heart Mountain fault but also occur in its transgressive and former land-surface phases. Evidence that the Wapiti Formation almost immediately buried loose, unconsolidated fault breccia that was the source of the dike rock strongly suggests a rapid volcanic deposition over the area in which clastic dikes occur, which is at least 75 km long. Clastic dikes were injected into both the upper-plate and the volcanic rocks at about the same time, after movement on the Heart Mountain fault had ceased, and therefore do not indicate a fluid-flotation mechanism for the Heart Mountain fault. The difference between contacts of the clastic dikes with both indurated and unconsolidated country rock is useful in field mapping at localities where it is difficult to distinguish between volcanic rocks of the Cathedral Cliffs and Lamar River Formations, and the Wapiti Formation. Thus, calcibreccia dikes in the Cathedral Cliffs and Lamar River Formations show a sharp contact because the country rock solidified prior to fault movement, whereas calcibreccia dikes in the Wapiti Formation in many instances show a transitional or semifluid contact because the country rock was still unconsolidated or semifluid at the time of dike injection. INTRODUCTION General features of the Heart Mountain fault have been described by Dake (1918), Hewett (1920), Bucher (1933, 1940), Pierce (1941, 1957,1960), Voight (1974), and Prostka (1978); detailed mapping of the fault and of the areal extent of the upper plate have been published (see fig. 1 for references). In this report attention will be focused upon clastic dikes of the Heart Mountain fault breccia, which furnish striking and unusual evidence on the brief lapse of time involved in the Heart Mountain fault emplacement process and provide some clues on the probable mechanism of both fault movement and dike injection. Four major points will be developed in this report. (1) The calcibreccia dikes intruded two quite different kinds of country rock—Paleozoic carbonate and Tertiary volcanic rocks—but apparently were injected at essentially the same time and show a common mechanism, namely, lithostatic pressure due to burial by a rapidly accumulating cover of volcanic rocks, the Wapiti Formation. (2) The dikes are an aid in distinguishing fault-emplaced from nonfault-emplaced volcanic rocks. (3) Their wide distribution indicates that at least the lower part of the Wapiti Formation was rapidly deposited over a large area. (4) The dikes confirm an earlier conclusion (Pierce, 1968, 1973b) that the Heart Mountain fault movement was a cataclysmic event and cast further doubt on some of the mechanisms proposed for this fault movement. In the course of their preparation of a geologic map of the Pilot Peak quadrangle, Wyoming, Pierce, Nelson, and Prostka (1973) differed as to whether the Heart Mountain fault movement had occurred during or after deposition of the Lamar River Formation, a name pro- lCLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING 45' 30' 109° 15' 45 00' Heart Mountain fault block - Faults within block not shown Trace of Heart Mountain fault - Hachures on upper plate or preserved surface; fault-breccia dikes also on hachured side Heart Mountain break-away fault - Squares on depositional side. Dotted where removed by erosion High-angle fault - U, upthrown side; D, downthrown side Site of fault-breccia dike - Number refers to areas 1-7 described in text Letters A-H 44 30' Additional site of fault-breccia dike refer to table 1 10 I 15 20 25 KILOMETERS _l_____J 0 10 15 MILES Figure 1.—Map showing location of four phases of Heart Mountain fault and sites where clastic dikes of fault breccia are found. For geologic setting and detailed location of calcibreccia dikes in northwestern part of area, see Pierce, Nelson, and Prostka (1973). GQ numbers refer to Geologic Quadrangle Maps and I number to Miscellaneous Investigations Series Map published by U.S. Geological Survey at a scale of 1 : 62,500. posed by Smedes and Prostka (1972) for the lowest volcanic rocks of northeastern Yellowstone National Park including all rocks of the Cathedral Cliffs Formation (Pierce, 1963a). These differences remained unresolved at the time of publication, and so areas with alternate interpretation were designated accordingly on the map. Subsequent studies of the Heart Mountain fault, fault breccia, and clastic dikes (for example, areas 1-3 and 4-5, fig. 1, which Prostka (1978, p. 428) interpreted as sites of fault-emplaced volcanic rocks) have shown that the volcanic rocks are depositional over the Heart Mountain fault surface of tectonic denudation.CLASTIC DIKES OF FAULT BRECCIA 3 On their map, Pierce, Nelson, and Prostka designated the volcanic rocks in areas 1-3 (fig. 1) as the Lamar River and Cathedral Cliffs Formations, undivided; this assignment is unsatisfactory because the Cathedral Cliffs Formation is part of the upper plate and thus is older than the Heart Mountain fault, and the Lamar River Formation in its type area (Smedes and Prostka, 1972) in the Abiathar Peak quadrangle (Prostka and others, 1975) also predates the faulting. In view of the difficulty in distinguishing between the Lamar River and Wapiti Formations east of the Heart Mountain breakaway fault, it is suggested that the Lamar River Formation be discontinued in the area of the Heart Mountain fault and that the Cathedral Cliffs Formation be used for the pre-Wapiti volcanic rocks. GENERAL FEATURES OF THE HEART MOUNTAIN FAULT The Heart Mountain fault is in northwestern Wyoming, between the northeast corner of Yellowstone National Park and the west flank of the Bighorn Basin. Near the close of the early Eocene Epoch, rocks in a 1,300-km2 area between the breakaway and transgressive phases of the Heart Mountain fault (fig. 1) became detached along a bedding plane and moved southeast. During movement, the upper plate broke into more than 50 blocks, ranging from a few tens of meters to 8 km across, scattered over an area of 3,400 km2 that stretched 105 km in a northwest-southeast direction. Horizontal movement of the most southeasterly blocks was 50 km; to the northwest, the extent of movement diminished progressively toward the breakaway fault. The overall slope of the fault, which can be determined fairly closely, was less than 2°. The upper plate of the Heart Mountain fault is composed of about 500 m of predominantly carbonate rocks of the Bighorn Dolomite (Ordovician), the Jefferson and Three Forks Formations (Devonian), and the Madison Group (Mississippian); and of from 0 to 600 m of volcanic rocks of the Cathedral Cliffs and Lamar River Formations (Eocene). The detachment horizon is underlain by 360 m of Cambrian shale and limestone of the Snowy Range, Pilgrim, and Gros Ventre Formations, and by the Flathead Sandstone, which rests un-conformably on Precambrian granitic rocks. The distribution of the Crandall Conglomerate, an unusual stream-channel deposit (Pierce, 1973a), indicates a preliminary southeastward movement in the northeastern part of the Heart Mountain fault mass that opened a deep rift about 1 km wide through the upper plate. Erosion then cut a channel as deep as 100 m into the underlying Cambrian shale, in which 150 m or more of this coarse conglomerate was subsequently deposited. This preliminary movement was followed in rapid succession by the Reef Creek detachment fault (Pierce, 1963b) and then by a main movement of the entire upper plate that left the lower part of the conglomerate in place but carried the upper part, along with rocks of the upper plate, roughly 24 km southeastward. CLASTIC DIKES OF FAULT BRECCIA The clastic dikes of carbonate fault breccia were first described by Pierce (1968) as limestone dikes at 6 general sites; since then, the number of sites has been increased to 15. These dikes are composed of finely crushed dolomite and limestone in a matrix of carbonate flour, firmly cemented by calcium carbonate to form a resistant rock that breaks with conchoidal fracture. For brevity the term "calcibreccia dike” will be used here in place of the longer "clastic calcium and magnesium carbonate breccia dike”; the term "calcibreccia” will be used at times to refer to the Heart Mountain fault breccia either as tectonic breccia or as dike rock. The source of carbonate fault breccia or calcibreccia was the clastic rock formed by movement of the upper plate of the Heart Mountain fault. The Heart Mountain fault breccia is a cataclastic rock (cataclasite) composed of angular carbonate clasts in a microbreccia of smaller fragments without primary cohesion. The fault breccia is not mylonite, even in a broad sense, nor does it have fluxion structure; it is not only found on the fault surface beneath the large blocks of Paleozoic rock composing the upper plate, but also is irregularly distributed on the surface of tectonic denudation between these blocks. A few calcibreccia dikes are seen in Paleozoic rocks that form remnants of the upper plate, but most are found in the volcanic rocks deposited on the surface of tectonic denudation. The dikes are believed to have intruded both upper-plate and volcanic rocks at essentially the same time, soon after deposition of the first postfault volcanic rocks. The associated volcanic rocks in which clastic dikes occur, originally informally known as the early basic breccia of Hague (1899), were subsequently named the Wapiti Formation by Nelson and Pierce (1968). In the Pilot Peak quadrangle map (Pierce and others, 1973), the volcanic rocks containing clastic dikes were called the Lamar River and Cathedral Cliffs Formations, undivided. However, inasmuch as throughout a much larger area of the Heart Mountain fault the volcanic rocks that were deposited on the surface of tectonic denudation are unquestionably included in the Wapiti Formation, that designation will be followed here, es-4 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING pecially because the surface of tectonic denudation permits a precise time correlation between units. Features of the Heart Mountain fault breccia clearly indicate that it is the source material for the calcibrec-cia dikes in both the upper-plate blocks of Paleozoic rocks and in the volcanic rocks. The lower parts of many blocks of the upper plate are intensely brec-ciated, and so the dike rock that they enclose was evidently derived from fault breccia. However, most of the volcanic rocks containing calcibreccia dikes are not from the upper plate, nor were they fault emplaced. They are postfaulting in age, as shown by: (1) the absence of clasts of volcanic rocks in the fault breccia, in contrast to the abundance of carbonate fault breccia; (2) the semifluid mixing of calcibreccia with volcanic rock (figs. 21, 22); and (3) their depositional relation against and over Heart Mountain fault masses of all sizes (figs. 3, 26). Consequently, the only possible source for the calcibreccia in the dikes in volcanic rocks also was the Heart Mountain fault breccia. Some geologic features indicating that the dikes in both the upper-plate and volcanic rocks were in fact derived from Heart Mountain fault breccia are: (1) the fault breccia and dike rock are lithologically similar except for minor additions of wallrock to the dikes, (2) the dikes are most abundant in the basal part of the rocks resting on the Heart Mountain fault, (3) many dikes extend downward to the Heart Mountain fault but none are below it, and (4) the dikes pinch out upward and exhibit an upward-branching trend. GENERAL FEATURES OF CLASTIC DIKES IN UPPER-PLATE CARBONATE ROCKS Calcibreccia dikes in blocks of Paleozoic rock in the upper plate of the Heart Mountain fault have been observed at four sites (areas 1, 3, and 6 and locality E, fig. 1). These dikes were not formed by the movement of country rock along dike walls, because the adjoining rock is not brecciated and the dikes show abrupt changes in trend through the country rock. An example of this irregular trending is a 2- to 5-cm-wide calcibreccia dike in thin-bedded shale of the Three Forks Formation in area 1 (fig. 2). Although numerous faults have been observed and mapped in Paleozoic rocks of the upper plate, none have been found that contain calcibreccia. The calcibreccia dikes observed in upper-plate Paleozoic rocks die out upward and can be traced downward to the Heart Mountain fault or, in most instances, to concealed areas within 1 m or so of the fault. Therefore, these dikes, which are composed of fault breccia, were evidently injected upward along fractures in the upper plate. Figure 2.—Calcibreccia dike (cb) in steeply inclined thin-bedded shale of the Three Forks Formation in upper plate of the Heart Mountain fault in area 1, south of Silver Gate, Mont, (see fig. 3 for location). Dike is as much as 5 cm thick and pinches and swells as it alternately follows and cuts across bedding. End of hammer handle marks edge of dike. GENERAL FEATURES OF CLASTIC DIKES IN VOLCANIC ROCKS Calcibreccia dikes in volcanic rocks have been observed at 15 sites (areas 1-7 and localities A-H, fig. 1), nearly four times the number for those in the upper plate. This unequal distribution may be more apparent than real because (1) more of the Heart Mountain fault area is covered by the Wapiti Formation rather than by upper-plate rocks, and (2) calcibreccia dikes are more likely to be observed in the volcanic rocks because of the greater contrast between dikes and country rock. The volcanic rocks in which calcibreccia dikes occur almost exclusively are in the lowermost part of the early or middle Eocene Wapiti Formation, which is composed of volcanic breccia and of poorly sorted andesitic volcaniclastio rocks and lava flows; the calcibreccia dikes are more commonly found in the volcanic breccia. Much of the trachyandesitic material in the breccia consists of volcanic detritus, either pyroclastic rock or fragments of preexisting solid volcanic rocks ofDESCRIPTION OF DIKES AT SPECIFIC SITES 5 the Wapiti Formation. The matrix of the breccia is fine grained and is composed dominantly of silt- to coarse sand-size detritus. Although some calcibreccia dikes in the Wapiti Formation consist entirely of carbonate fault breccia, most contain scattered fragments of volcanic rock. A few dikes have a central core of calcibreccia with borders of volcanic dike rock (fig.6), or with borders of mixed calcibreccia and volcanic rock (fig. 8). One calcibreccia dike contains carbonized wood (fig. 5) and another, fragments of Precambrian granitic rock (fig. 8). Several dikes consist of volcanic breccia mixed with a minor to moderate amount of carbonate breccia; this breccia is referred to as a mixed breccia. The dikes usually range in thickness from a few centimeters to 0.5 m; commonly they pinch and swell and are discontinuous, but some have a fairly uniform thickness over short distances. The calcibreccia also occurs as irregular bodies, lenses, and pods in the volcanic rocks (figs. 17, 21, and 22), and is irregularly distributed over or against small blocks of the upper plate (fig. 10); at one site (area 2, fig. 1), calcibreccia has flowed into a small antiform (fig. 13). DESCRIPTION OF DIKES AT SPECIFIC SITES Seven sites of dike occurrence to be discussed are numbered on the map (areas 1-7, fig. 1) : areas 1 to 3 are in the Silver Gate-Cooke City, Mont., area; areas 4 and 5 are 12 km and 14 km to the southeast, respectively; area 6 is near White Mountain, 32 km farther southeast; and area 7 is west of Trout Creek, 28 km still farther to the southeast. Eight additional sites of calcibreccia dikes are lettered (Iocs. A-H, fig. 1), and data on them are summarized in table 1. Clastic dikes of fault breccia have not been described in detail in the geologic literature, so far as this author is aware, although fault gouge injected into cracks in the upper-plate rocks of the Muddy Mountain thrust, Nevada, have been noted (Stanley and Morse, 1974; Brock and Engelder, 1977). Their occurrence, however, in both upper-plate and volcanic rocks is most unusual if not unique. The exceptional circumstances associated with the dikes of Heart Mountain fault breccia and the collateral geologic information that they provide make it desirable to give a detailed account of a number of dikes, rather than merely a generalized description. AREA 1, SOUTH OF SILVER GATE, MONTANA The calcibreccia dikes in area 1, south of Silver Gate, Mont., are the most easily accessible and can be reached on foot by a 200-m climb up the steep mountain slope due south of the town. As indicated by figure 3, a variety of calcibreccia dikes can be seen there, both in upper-plate Paleozoic rocks and in the Eocene Wapiti Formation composed of volcanic rocks. CALCIBRECCIA DIKES IN PALEOZOIC ROCKS OF THE UPPER PLATE On the far right of figure 3 is the previously mentioned calcibreccia dike in the Three Forks Formation, shown in figure 2. Left of center in figure 3 is a much smaller, highly deformed upper-plate block with calcibreccia dikes. There, a very small Heart Mountain 0 50 METERS Approximate scale Figure 3.—Diagrammatic cross section of relations in area 1, south of Silver Gate, Mont., between calcibreccia dikes (cb) in Paleozoic rocks of the upper plate (Pz) and in volcanic rocks of the Wapiti Formation (Tw). Dots indicate flow banding adjacent to dikes and intermixing of calcibreccia with volcanic rock. HMf, Heart Mountain fault; td, surface of tectonic denudation; Ob, basal bed of Bighorn Dolomite; €gc, Grove Creek Limestone Member of the Snowy Range Formation. Faults in Paleozoic rocks do not extend into the Wapiti Formation. Not to scale.6 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING fault block of the Three Forks Formation, about 6 m across and 5 m high, contains several small irregular dikes of calcibreccia; one, about 1.5 m long, is in the west side of the block. The Wapiti Formation overlies this fault block, extends down its sides, and rests on the Heart Mountain fault surface of tectonic denudation adjacent to the block. In the east side of the block is one dike composed mainly of carbonate rock and some scattered fragments of shale of the Three Forks Formation, but the presence of volcanic rock fragments indicates that this dike was injected after the adjoining volcanic rocks were deposited. Evidence does not suggest that the dike material moved down, because that would be very unlikely, considering that the dike is composed almost entirely of fault breccia from beneath the block and from the adjoining surface of tectonic denudation. Rather, it appears that the calcibreccia in the dike moved up, and as it did so, a lateral flow occurred, both of underlying calcibreccia and of calcibreccia mixed with some volcanic rock from the adjoining area. Such general lateral flow of calcibreccia toward the dikes, as shown in figure 9, seems implicit as a means of acquiring the required volume of dike-filling material. Layering is well developed in the calcibreccia parallel to the dike walls and is due to the flow of material through the dike. Although debris conceals a 1.5-m interval between the bottom of the dike and the Heart Mountain fault, the dike can be traced diagonally upward for 1.5 m to the edge of the fault block, where it terminates at the contact between the Three Forks Formation and a 1.5-m-wide dikelike body (not shown in fig. 3) that consists of mixed volcanic and carbonate breccia containing stringers and pod-shaped bodies of calcibreccia. Above and to the southwest, a carbonate dike in volcanic rocks of the Wapiti Formation penetrates this dikelike body for about 0.5 m before pinching out 1 m from the end of the calcibreccia dike intruding shale of the Three Forks Formation. CALCIBRECCIA DIKES IN THE WAPITI FORMATION The dikes in the Wapiti Formation indicated in figure 3 will be discussed beginning with_lhe one slightly to the right of the center of the figure. This one, shown in figure 4, is a lensing dike of calcibreccia and volcanic breccia about 25 m long, intruded into the Wapiti Formation. Rubble conceals a 7-m interval between the lowermost exposed part of the dike and the Heart Mountain fault. The dike strikes N. 30° E., has an arcuate shape, and is near vertical. A short distance from the bottom of the exposed portion of the dike is a detached block of Madison Limestone in the upper plate of the Heart Mountain fault (see figs. 3, 4). The Wapiti Formation, which covers the limestone block, Figure 4.—Calcibreccia dike in area 1, south of Silver Gate, Mont., containing three components: calcibreccia (cb), volcanic breccia of the Wapiti Formation (vb), and mixed calcibreccia and volcanic breccia (mb) of the Wapiti Formation (Tw). The Madison Limestone (Mm) is part of detached block of upper plate lying on Heart Mountain fault 6 m below view, "x” indicates a 15-cm limestone clast. contains a few limestone fragments as much as 30 m across that were presumably picked up and incorporated into the Wapiti as it was deposited over the block. The dike shown in figure 4 is made up of three components: calcibreccia, volcanic rocks of the Wapiti Formation, and a mixture of these two materials. In the central part of the dike is an irregular lenslike body of fine-grained calcibreccia containing scattered angular clasts of volcanic rock from less than 0.1 to 50 cm or more in diameter. The bulk of the fragments, however, are 1 to 2 cm across. Where the volcanic fragments are 1 to 2 cm in size, they constitute no more than 10 percent of the calcibreccia; where they are smaller, they appear to form a lesser proportion of the calcibreccia. Within the calcibreccia, in the centralDESCRIPTION OF DIKES AT SPECIFIC SITES 7 part of the dike, is an irregular mass of breccia about 2 m long composed of about equal proportions of carbonate and volcanic rock fragments as much as 30 cm across. This breccia body is elongated roughly parallel to the walls of the dike. At one place in the lower part of the calcibreccia is a limestone clast 15 cm across (fig. 4). Volcanic breccia, much coarser than the calcibreccia, forms a zone 0.3 to 1 m wide on either side of the calcibreccia core and extends upward about 15 m above the core before pinching out. The volcanic breccia is intensely sheared and exhibits more or less vertically oriented, striated shear surfaces parallel to the dike walls. A specimen of the dike taken at the boundary between the calcibreccia and volcanic breccia, showing a very narrow transitional contact 1 mm or so thick between the two rock types, indicates that the volcanic breccia was unconsolidated to some degree at the time the calcibreccia was injected. Within the specimen, elongate grains and fragments of volcanic rock in both the calcibreccia and volcanic breccia show a preferential lengthwise orientation parallel to the dike walls. About 25 m above and to the southeast of this dike, a similar, less prominent, thinner body of calcibreccia occurs within a moderately distinct dike of volcanic breccia. This dike, indicated on figure 3 by an arrow to the area of figures 5 through 7, is about 120 m long, trends N. 55° E., and contains fragments of volcanic rock. Calcibreccia within the dike of volcanic breccia pinches and swells, is discontinuous, and is locally absent, ranging from less than a few centimeters to 30 cm in width. At one place in the dike are two thin (2-5 cm) tabular bodies of calcibreccia 0.5 m apart. In the summer of 1973, while I was showing a group of visitors through this area, Charles Frye noticed a few small fragments of what appeared to be carbonized wood in one calcibreccia body in this dike; on further examination later in the summer, I identified definite particles of carbonized wood, some as much as 4 cm long (fig. 5), in calcibreccia in other parts of the dike. The wood occurs as numerous small pieces in a matrix of very fine grained carbonate breccia. Although fossil wood has been found in many places in the Wapiti Formation, the wood in this dike evidently was not secondarily derived from the volcanic rocks because it is entirely enclosed within calcibreccia and is free of associations to volcanic rock in the dike. Furthermore, the wood particles seem to have been incorporated into the calcibreccia after that breccia was formed because the wood is much too fragile to have withstood the deformation that produced the fault breccia. It is surprising that the wood has withstood the deformation accompanying its injection into the dike as well as it has, inasmuch as some of the wood, represented by very small specks in the calcibreccia, has thoroughly disin- Figure 5—Specimen of calcibreccia dike in area 1, south of Silver Gate, Mont., containing fragments of carbonized wood (W) and a few fragments of volcanic rock (V) in a matrix of carbonate breccia (cb). Photograph by Lowell Kohnitz. tegrated. The surface of tectonic denudation, created by movement along the Heart Mountain fault, was exposed for perhaps only a few hours or days after the fault movement ceased before being buried by volcanic rocks of the Wapiti Formation (Pierce, 1968). During this extremely brief interval, the carbonized wood became a constituent of the Heart Mountain fault breccia presumably by aerial or aqueous transport from a short distance away. The conditions that permitted inclusion of this wood into the Heart Mountain fault breccia probably were extremely rare because carbonized wood has been found only in this one dike. However, similar fossil wood, albeit much silicified and presumably from the same source as that in the calcibreccia, has also been noted in volcanic breccia of the dike near its northeast end. The body of calcibreccia with the carbonized wood occurs within the seemingly flow banded volcanic breccia of the Wapiti Formation that constitutes the main mass of the dike (fig. 6). The bodies of volcanic breccia in the dike that borders the calcibreccia are about 10 cm wide, slightly darker than the adjoining wallrock, and contain a small amount of fine calcareous breccia. The core of calcibreccia at the southwest end of this dike lies about 25 m above the Heart Mountain fault, and the enclosing breccia of the Wapiti Formation pinches out a few meters higher up in the dike. At the northeast end of the dike, the discontinuous calcibreccia core can be traced down to within 3 m of the fault, below which it is concealed by talus. The contact between calcibreccia and volcanic breccia in the dike (fig. 7) is extremely irregular rather than planar, and the fragments in the calcibreccia do not show a preferred orientation parallel to the contact. Very thin stringers and protuberances of volcanic breccia into the calcibreccia indicate that the volcanic rock was in a some-8 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING Figure 6.—Calcibreccia dike (cb) in area 1, south of Silver Gate, Mont., containing carbonized wood. The Wapiti Formation (Tw) seems to be flow banded (Twd) on both sides of calcibreccia dike. what mobile state, that is, incompletely consolidated at the time of dike injection, and also that the fault movement which formed the calcibreccia preceded deposition of the Wapiti Formation. One calcibreccia dike from a few to 45 cm wide (left, fig. 3), well exposed at the bottom and on the west side of a steep ravine, pinches and swells and is irregularly distributed within another, larger 1- to 2-m-wide dike of volcanic breccia of the Wapiti Formation that is sheared and has fairly well defined shear borders with the enclosing volcanic rock (fig. 8); the contact between the calcibreccia and intrusive volcanic breccia is gradational. The calcibreccia is predominantly carbonate rock but contains abundant fragments of volcanic rock. At the outer border of the calcibreccia is a 2-cm-wide zone of fine-grained rock composed of volcanic breccia intermixed with a small amount of calcibreccia. The contact of the mixed breccia with the calcibreccia and the outer contact with the volcanic breccia are both gradational through a thin zone. In addition to this irregular but more or less continuous body of calcibreccia (fig. 8), other discontinuous stringers less than 1 cm to a few centimeters wide occur within the dike. The composite dike of volcanic breccia and calcibreccia is bordered by vertically sheared dikelike bodies of rock of the Wapiti Formation. The dikelike body on the south side (left, fig. 8) is about 3.5 m wide, with the outer half less sheared and its outer border less well defined than its inner border. The dikelike body of the Wapiti on the north side of the calcibreccia dike (right, fig. 8) is mostly covered, but the part that can be observed is similar to its counterpart on the south side of the dike. An unusual feature of the dike shown in figure 8 is the occurrence of fragments of Precambrian granitic rock in the rocks that form the dike and in the dikelike Figure 7.—Caption on facing page.DESCRIPTION OF DIKES AT SPECIFIC SITES 9 Figure 7.—Polished specimens of calcibreccia dike (cb) and adjoining dikelike body of the Wapiti Formation (Twd) in area 1, south of Silver Gate, Mont. Very thin stringers (not shown here) and protuberances of Wapiti Formation into calcibreccia indicate that country rock was incompletely consolidated at time calcibreccia dike of Heart Mountain fault breccia was injected into it. Photographs by Lowell Kohnitz. bodies of sheared Wapiti volcanic breccia that border it. The xenoliths in the dike are angular and range from 1 or 2 cm across in the calcibreccia to 1 m across in Figure 8.—Calcibreccia dike (cb) in area 1, south of Silver Gate, Mont., within and part of a dike of the Wapiti Formation (Twd). Calcibreccia dike and enclosing volcanic breccia both contain xenoliths of Precambrian rock (pG). The bordering, slightly less dikelike bodies of the Wapiti Formation (Twdl) also contain xenoliths of Precambrian rocks. None of the dikes extend beneath Heart Mountain fault (3 m below view shown). the adjacent dikelike bodies of Wapiti breccia. The xenoliths occur in the lowest exposed parts of the dike and adjacent breccia, about 3 m above the Heart Mountain fault, and are scattered upward for 15 m in the dike and in the Wapiti that borders it. The Precambrian fragments in the vertically sheared Wapiti breccia occur close to the dike. Inasmuch as the calcibreccia originally was irregularly distributed between the base of the Wapiti Formation and the surface of tectonic denudation, the calcibreccia must have flowed laterally and upward to form the dike, as illustrated in figure 9. Vertical stria-tions and shears in the rocks of the Wapiti Formation that enclose the calcibreccia, as well as the relation10 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING 0 1 METER Approximate scale Figure 9.—Diagrammatic cross section of calcibreccia dike (cb) on left side of figure 3 in area 1, south of Silver Gate, Mont., showing its relation to the Heart Mountain fault carbonate breccia (fb), dike (Twd), dikelike (Twdl) bodies of Wapiti Formation, and bordering rocks of the Wapiti Formation (Tw). pG, angular xenoliths of Precambrian granitic rocks; Ob, basal bed of Bighorn Dolomite; Ggc, Grove Creek Limestone Member of Snowy Range Formation; td, Heart Mountain fault surface of tectonic denudation. Arrows indicate vent and incorporated into the basal part of the volcanic rocks immediately after the Heart Mountain fault movement. AREA 2, SOUTHEAST OF SILVER GATE, MONT. In area 2, southeast of Silver Gate, Mont., most of the calcibreccia dikes and related features are exposed on the east side of Falls Creek, and these features are described first. Here, the upper part of the Grove Creek Limestone Member of the Snowy Range Formation is well exposed on the east side of a steep V-shaped valley and is overlain by 2 to 3 m of the basal bed of the Bighorn Dolomite. The top of this bed is the surface of tectonic denudation produced by movement on the Heart Mountain fault. Resting on this surface is (1) a small block of fault-transported Madison Limestone about 3 m high and (2) as much as 2/3 m of carbonate fault breccia and mixed carbonate and volcanic breccia that are buried by the Wapiti Formation. The volcanic direction of flow. and mixing of the volcanic rock and calcibreccia, indicate that the Wapiti must have flowed with the carbonate fault breccia. The angularity of the granitic clasts, their wide range in size, and their lack of association with any sedimentary deposits seem to rule out any derivation by stream transport. A search was made for clasts of Precambrian rock in the basal part of the flat-lying Wapiti Formation but without success; however, about 100 m to the southwest, near the location of figure 4 shown on figure 3, crystalline clasts were found less than 1 m above the base but not higher up in the sequence. The basal volcanic rocks overlying a 25-m-high mass of deformed Devonian rocks in area 3, north of Republic Mountain, also contain clasts of Precambrian rock. So far as is known, the Wapiti Formation never rests directly on Precambrian crystalline rocks, and Precambrian clasts have never been observed in the Heart Mountain fault breccia that is overlain by upper-plate rocks. Therefore, these clasts are interpreted to be xenoliths torn from the walls of aDESCRIPTION OF DIKES AT SPECIFIC SITES 11 cover contains numerous irregular to dikelike bodies of calcibreccia. These features are diagrammatically shown to the center and left in figure 10. The fault contact between the block of Madison Limestone and the underlying basal bed of the Bighorn Dolomite is well exposed, and no carbonate fault breccia or volcanic breccia occur there. This block, as well as numerous other blocks of the Madison Limestone resting on the fault surface, are considered to have toppled off the trailing edge of a larger mass and come to rest on the surface of tectonic denudation before the overlying volcanic rocks were deposited (Pierce, 1957). Also in area 2, a yellow-weathering calcibreccia commonly containing a small amount of volcanic rock is irregularly distributed between the basal bed of the Bighorn Dolomite and the overlying Wapiti Formation; a thin zone of mixed carbonate and volcanic breccia at the base of the volcanic cover overlies the layer of calcibreccia. The contact between the Wapiti Formation and the calcibreccia dike shown in figure 11 suggests that the volcanic rocks were not fully consolidated at the time of contact with the calcibreccia. Apparently, a considerable amount of fault breccia lay loose on the fault surface in this area as a result of the cataclysmic fault movement because a layer of calci -breccia 1 to 2 cm thick is plastered irregularly onto the sides and top of the block of Madison Limestone, as shown in figure 10. The contact between the Madison and the calcibreccia is sharp and distinct, that between the calcibreccia and mixed breccia is less distinct, and that between the mixed breccia and the Wapiti Formation is least distinct. Figure 11.—Polished specimen of calcibreccia dike in area 2, on east side of Falls Creek. Curving plume of calcibreccia (cb) extending into the Wapiti Formation (Tw) suggests semifluid mixing. Photograph by Lowell Kohnitz. Numerous irregular pods and dikelike masses as much as 1 m across, commonly tapering into short dikelike forms composed of light-tan- to yellowish-weathering calcibreccia, occur in the basal 1 to 2 m of the Wapiti Formation in area 2. The central parts of 0 10 METERS Approximate scale Figure 10.—Diagrammatic cross section in area 2, southeast of Silver Gate, Mont., illustrating relation of calcibreccia to Heart Mountain fault (HMf), surface of tectonic denudation (td), and the Wapiti Formation (Tw). Mm, Heart Mountain fault block of Madison Limestone; cb, fault breccia (calcibreccia) on surface of tectonic denudation and in dikes and irregular bodies in the Wapiti Formation; mb, mixture of calcibreccia and volcanic breccia; Ob, basal bed of Bighorn Dolomite; -Ggc, Grove Creek Limestone Member of the Snowy Range Formation. Not to scale.12 CLASTIC DIKES OF HEART MOUN TAIN FAULT BRECCIA, NOR THWES TERN WYOMING these bodies usually consist almost entirely of calci-breccia and a few volcanic fragments, although some pods contain numerous small lenses and inclusions of volcanic rock as much as a few centimeters across. The outer parts of the calcibreccia bodies generally contain more volcanic rock than the inner parts, and the adjoining wallrock contains some calcibreccia. The contacts between some calcibreccia masses and the enclosing Wapiti Formation are sharp and distinct, but other contacts are gradational. Not uncommonly, a single calcibreccia mass may display both kinds of contacts. A few meters higher up in the Wapiti but usually within the lower 10 m of the volcanic rocks, the calcibreccia occurs in dikelike bodies a few meters long and as much as 0.3 m across. These bodies commonly contain some volcanic rock derived from the enclosing Wapiti Formation, and the adjoining rocks also contain some calcibreccia (fig. 12). An exceptionally well exposed body of calcibreccia in a cliff 15 m west of Falls Creek in area 2 (fig. 13), in the same mass of the Wapiti Formation as dikes on the east side of the creek (fig. 10), forms a unique feature about 2 m high, slightly resembling an anticlinal fold—not a true fold, but apparently related more to flowage than folding. For lack of a better term, this feature is here referred to as an antiform. The base of this body is exposed to within 1 m of the Heart Mountain fault and presumably extends downward to the fault; 2 m to the west (right, fig. 13), the Wapiti Figure 12.—Upper end of calcibreccia dike (cb) 5 m above base of Wapiti Formation (Tw) and Heart Mountain fault, area 2. Calcibreccia contains a minor amount of volcanic rock derived from enclosing country rock, and adjoining country rock contains some carbonate breccia. The mixing of calcibreccia and volcanic breccia and the contacts between them indicate that the Wapiti Formation was emplaced in a mobile or semifluid state onto carbonate fault breccia, which is now incorporated into it as dikes and irregular masses. Figure 13.—View southwest along trend of an antiform, or calcibreccia mass resembling an anticlinal fold, at base of the Wapiti Formation (Tw) in area 2, 15 m west of Falls Creek. Shearing surfaces (s) in inner part of the calcibreccia (cb) also have an anti -clinal form. Irregular masses of calcibreccia border flanks of anti-form, and lozenge-shaped mass of calcibreccia lies in a shear plane along right flank of the antiform. To right, the Wapiti Formation rests on Heart Mountain fault surface of tectonic denudation (td) and on basal bed of Bighorn Dolomite (Ob). The Wapiti Formation near and adjoining calcibreccia is calcareous, a feature suggesting that some calcibreccia became mixed with it during deposition on the fault surface. Formation can be seen to rest on the fault surface. The absence there of carbonate fault breccia is not surprising, however, inasmuch as the fault breccia was swept off the fault surface into the antiform. The calcibreccia on the flanks of the antiform is composed of angular carbonate fragments, less than 1 to a few millimeters across, bonded in a carbonate matrix containing scattered angular fragments of volcanic rock 2 to 8 cm across. Polished specimens of the calcibreccia do not show any distinct or layered orientation of fragments, although a few thin, wavy layers a few centimeters long of contrasting material suggest flowage. The volcanic breccia of the Wapiti Formation in which the antiform occurs is highly sheared and fractured on the flanks of and above the antiform; these features become less distinct and disappear farther out on the flanks. The Wapiti Formation above this body is inaccessible, but the lines of shear that can be observed from a distance are inclined at about 45° parallel to the inner shearing surfaces (fig. 13) and apparently die out upward in an inverted "V”. The fact that the steeper side of the antiform lies on the southeast suggest a movement from northwest to southeast; the fault surface, on which the Wapiti Formation was deposited (that is, the surface of tectonic denudation), sloped to the southeast in the general direction of movement along the Heart Mountain fault.DESCRIPTION OF DIKES AT SPECIFIC SITES 13 AREA 3, NORTH OF REPUBLIC MOUNTAIN In area 3, on the north side of Republic Mountain, several calcibreccia dikes occur in the eastern part of a highly brecciated and fractured limestone mass that probably belongs to the Jefferson Formation (fig. 26). One dike, 8 m long and as much as 15 cm wide, extending to within about 1.5 m of the Heart Mountain fault, consists essentially of carbonate rock with some specks of asphalt; no volcanic rock was observed. A few meters east of this dike, an irregular but somewhat linear mass as much as 1 m across of calcibreccia of the same composition has sharp contact with the enclosing limestone, which is contorted, deformed, and brecciated. The fact that this deformation does not extend to any of the calcibreccia dikes or to their contacts with the limestone is convincing evidence that injection of the dikes occurred after the enclosing rock was deformed. Overlying this brecciated limestone block are volcanic rocks of the Wapiti Formation, the lower 1 m or so of which contains a few scattered angular xenoliths of Precambrian rock. Along the contact and within the basal few meters, thin irregular streaks and discontinuous layers of calcibreccia occur; several short, thin calcibreccia dikes extend several meters upward from the contact into the Wapiti Formation (fig. 14). One small dike, less than 1 mm wide in its lower part but increasing upward in width to several centimeters, Figure 14.—Polished specimen of calcibreccia dike in area 3, north of Republic Mountain. Mixture (mb) of calcibreccia (cb) and volcanic breccia of the Wapiti Formation (Tw) suggests that the Wapiti Formation was unconsolidated at the time the calcibreccia was intruded. Photograph by Lowell Kohnitz. seems to indicate that the lower part of the dike may have been squeezed together after the calcibreccia was injected. Such an event may explain the fact that many calcibrecci dikes cannot be traced to their points of origin within the underlying Heart Mountain fault breccia. AREA 4, SOUTH OF PILOT CREEK Area 4 is about 1 km south of Pilot Creek, near the foot of a waterfall of a north-flowing stream that heads on the west side of Jim Smith Peak. The Grove Creek Limestone and the basal bed of the Bighorn Dolomite, which has the Heart Mountain fault surface of tectonic denudation at its top, are well exposed on the east side of the valley from the waterfall to a point 150 m north. The 2-m-thick basal bed of the Bighorn Dolomite is overlain either by carbonate fault breccia or by volcanic rocks of the Wapiti Formation. The volcanic rocks contain several dikes and irregular bodies of calcibreccia. At a site 300 m to the north, an upper-plate block of Madison Limestone 30 m long and 10 m high rests on 30 to 35 cm of fault breccia. A layer of calcibreccia 0.7 m thick (figs. 15, 16), exposed 100 m north of the waterfall, rests on the basal bed of the Bighorn Dolomite and is overlain in turn by the Wapiti Formation. No volcanic rock was observed at these sites in the fault breccia, which consists almost entirely of carbonate rock and a few very fine fragments of chert. A murky-brown submicroscopic matrix in the uppermost part of a polished specimen of the breccia (fig. 16) gives that part a darker color. Also in area 4, 15 m north of the basal bed of the Bighorn Dolomite shown in figure 15, a mound of Heart Mountain fault breccia almost 2 m high rests on the basal bed of the Bighorn Dolomite and is adjoined and was overlain by the Wapiti Formation (fig. 17). A polished specimen that was collected near the top of this mound of breccia (fig. 18) is composed entirely of carbonate rock displaying a well-defined wavy to crinkled layering. A specimen from the lower part of the mound is similar, except that high-angle offsets of 1 to 2 mm in the layering are more numerous than in the upper part. It is suggested that the crinkled, wavy layering with high-angle offsets may reflect strong earth tremors at the time the fault breccia was unconsolidated. The Wapiti Formation, which overlies the Heart Mountain fault in area 4, contains several calcibreccia dikes, some near the fault and others higher up, even as much as 20 to 35 m above the fault. One dike lower down is distinctly layered parallel to the dike walls, and nonspherical fragments in the dike are similarly alined (fig. 19). The contact between the dike and the enclosing volcanic rock is irregularly convolute. Many14 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING •^w^T Si ’WWs’*' _Jh»' ,. r Sb*%k. *■- 25 mm ______I Figure 16.—Polished specimen of upper third of Heart Mountain fault breccia, showing its angular character. Top of specimen is at contact with the overlying Wapiti Formation. Photograph by Lowell Kohnitz. Figure 15.—Heart Mountain fault breccia (calcibreccia) on fault in area 4, south of Pilot Creek; Ob, basal bed of the Bighorn Dolomite; fb, fault breccia. The Wapiti Formation (Tw) is cut by small dikes of carbonate fault breccia (cb). Head of hammer marks base of fault breccia. of the dikes in this area are curving in plan and inclined from vertical to 45° or less, others are inclined at a very low angle, and a few are nearly horizontal. One that is 1 m thick and inclined at 35° has a fluted top and is 10 m above the Heart Mountain fault; it consists mostly of carbonate rock, probably dolomite, because it effervesces slowly on acid treatment and contains, in addition, some volcanic material. Another calcibreccia dike (fig. 20) is nearly horizontal and pinches and swells irregularly. About 15 m further east and 35 m above the Heart Mountain fault is an irregular dikelike mass of calcibreccia 0.5 to 0.7 m thick and 7 m across, composed mostly of carbonate rock; its east border is steeply inclined to vertical, and the west margin is inclined Figure 17.—Mound of Heart Mountain fault breccia (fb) in area 4, south of Pilot Creek. The mound, nearly 2 m high, is composed entirely of carbonate breccia and rests on the basal bed of the Bighorn Dolomite (Ob).DESCRIPTION OF DIKES AI SPECIFIC SI I ES 15 Figure 18.—Polished specimen of calcibreccia from near top of 2-m-high mound in figure 17. Note unusual crinkled, wavy layering. Photograph by Lowell Kohnitz. about 20°. As revealed in a polished specimen, a 1-cm-thick layer of calcibreccia adjoining the volcanic country rock is finer grained than the rest of the calcibreccia in the dikelike mass but is not layered and shows no distinct orientation of fragments. The bloblike mass, which crops out 15 m or more east of the other calcibreccia bodies, could not be traced downward; it exhibits a sharp but highly irregular contact with the volcanic wallrock, which is darker than the rock enclosing the other calcibreccia bodies. AREA 5, NORTH OF JIM SMITH PEAK Area 5 is about 0.5 km east of area 4 and extends southeast for 1.5 km along the Heart Mountain fault to Jim Smith Creek, which crosses the fault northeast of Jim Smith Peak. In the northwestern part of this site, two large upper-plate blocks of Madison Limestone, each about 80 m long and 20 to 35 m high, as well as three smaller upper-plate blocks, one of Madison Limestone and the other two of the Three Forks and Jeffer- Figure 19.—Polished specimen of calcibreccia dike (cb) in lower part of the Wapiti Formation (Tw) in area 4, south of Pilot Creek, showing irregularly convolute contact with wallrock and layering parallel to dike wall. Photograph by Lowell Kohnitz. Figure 20.—Flat-lying calcibreccia dike (cb) in area 4, south of Pilot Creek. Dike pinches and swells irregularly and is about 20 m above base of the enclosing Wapiti Formation (Tw), which rests on the Heart Mountain fault surface of tectonic denudation. son Formations, rest on the basal bed of the Bighorn Dolomite, which is of less than its normal thickness here. Underlying the large block of Madison Limestone farthest to the northwest is 5 to 8 cm of fault breccia; no volcanic rock fragments occur in the fault breccia or in the adjoining rocks. About 35 m southeast of this large block, a smaller block of Madison Limestone, about 5 m high and 8 m across, crops out in the middle of a steep16 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING ravine. Adhering to the top of the small block is an irregular layer of mixed calcibreccia and volcanic breccia 0.5 to 4 cm thick; resting on the breccia layer are volcanic rocks of the Wapiti Formation, the lowermost several centimeters of which contains lenses of mixed calcibreccia and volcanic breccia (fig. 21). The wavy to cuspate contact between the calcibreccia lenses and the volcanic rocks indicates that both rock types were unconsolidated at the time of intermixing. Between these upper-plate limestone blocks just described and Jim Smith Creek, which heads just east of Jim Smith Peak, the Heart Mountain fault is well exposed along steep ravines. Along this segment of the fault the Wapiti Formation rests on the basal bed of the Bighorn Dolomite, which ranges in thickness from less than 1 m near Jim Smith Creek to 2 m in the area to the west. At the bottom of a ravine 50 m west of the creek, 1 m of the Heart Mountain fault breccia rests on this basal bed. This carbonate fault breccia, which has considerable volcanic rock mixed with it, including abundant angular and subangular volcanic fragments from 0.5 to 15 cm across, is overlain by Wapiti rocks showing no evidence of tectonic brecciation. At this same site, a small calcibreccia dike 15 cm thick extends upward from the fault breccia into the Wapiti Formation, and several other calcibreccia dikes of similar size occur nearby in the lowermost part of the volcanic rocks. In some places the base of the volcanic rocks has some carbonate rock mixed with it, and in others it does not. Where carbonate rock occurs, it is either intimately mixed with the volcanic rocks or forms irregular stringers or layers, such as the one shown in figure 22. Prostka (1978, p. 430-431) has suggested that in the area north of Jim Smith Peak (areas 4 and 5 of this report), rocks from the top part of Heart Mountain fault blocks have spilled forward over the leading edge of the blocks, owing to their momentum as the fault blocks were coming to rest; these features he called "low-angle ramp structures or spillovers.” He cited as evidence steep dips to the southeast —in the direction of sliding—on the leading edges of volcanic rocks; low-angle ramp structures were found to occur only on the leading edges of volcanic fault masses, whereas the trailing edges and sides of blocks are steep. He concluded "*** that the ramp structures were not caused by simple collapse and slumping of steep slopes, but that the momentum of the moving volcanic masses was necessary to produce the spillovers.” He believed that these volcanic rocks are fault emplaced, but to me the relation between the Heart Mountain fault breccia and the volcanic rocks resting on the Heart Mountain fault (figs. 15-17, 21, 22), as well as the occurrence of calcibreccia dikes, indicates that the volcanic rocks were deposited on the fault surface. Figure 21.—Polished specimen of calcibreccia (cb) interlayered with basal part of the Wapiti Formation (Tw) in area 5, north of Jim Smith Peak. Inasmuch as calcibreccia was formed concurrently with the Heart Mountain fault movement, the volcanic rocks with which it is interlayered must be postfaulting. Wavy to cuspate interlayering indicates that the volcanic rock was unconsolidated at the time the calcibreccia was mixed with it. Photograph by Lowell Kohnitz. Figure 22.—Irregular layer 2 to 5 cm thick of carbonate breccia or calcibreccia (cb) 1.5 m above base of the Wapiti Formation (Tw) in area 5, west of Jim Smith Creek. Volcanic breccia also has carbonate breccia intimately mixed with it. Observations mentioned above in discussing areas 4 and 5 negate the spillover concept and indicate that most of the volcanic rocks are depositionally emplaced in these areas. Several small fault blocks of Paleozoic carbonate rock north of Jim Smith Peak, shown on the geologic map of the Pilot Peak quadrangle (Pierce and others, 1973), rest on the Heart Mountain fault. Volcanic rocks overlie these blocks and extend downward to the fault surface, as shown in figures 3 and 10. Four exposures of fault breccia—one overlain by volcanic rock (fig. 15) and three overlain by Paleozoic rocks— consist entirely of carbonate fault breccia that containsDESCRIPTION OF DIKES AT SPECIFIC SITES 17 no volcanic material. These occurrences of carbonate fault breccia without volcanic material are incompatible with Prostka’s spillover concept and with his conclusion that all of this volcanic rock is fault emplaced as part of the Heart Mountain fault movement. AREA 6, NEAR WHITE MOUNTAIN In area 6, near White Mountain, calcibreccia dikes occur in upper-plate rocks on the southwest side of the mountain and at sites 1,900 and 2,100 m to the east-northeast(Nelson and others, 1972, fig. 4). At the site 1,900 m away, some fragments and disconnected pieces of igneous dike rock occur in a broken and faulted upper-plate block of the Bighorn Dolomite. One of the calcibreccia dikes at this locality passes through one of the igneous dike fragments, a feature indicating that the igneous rock is older than the clastic dike of Heart Mountain fault breccia and evidence compatible with the conclusion of Nelson, Pierce, Parsons, and Brophy (1972) that some of the igneous dikes at White Mountain are older than the Heart Mountain fault movement. Above and to the northeast of the block containing calcibreccia dikes in the Bighorn Dolomite, a well-exposed high-angle fault brings the Jefferson Formation into contact with the Bighorn Dolomite; however, no fault breccia occurs along this fault, nor has fault breccia similar to that in the calcibreccia dikes been found in the numerous other high-angle faults mapped at White Mountain. Furthermore, although calcibreccia dikes occur along fractures in the upper plate, the trends of many clastic carbonate dikes at White Mountain are too irregular for the enclosed carbonate material to have been fragmented by movement of one side of the dike relative to the other (Nelson and others, 1972). At the site 200 m farther northeast, two clasts of dark-gray shale less than 1 m apart are enclosed in about the middle of a calcibreccia dike in dolomitic limestone, about 2 m thick and dipping 25° west: one is 15 cm across and well rounded, the other about 13 cm long, 3 cm across, and well tapered at both ends. Although both clasts resemble and presumably were derived from the Three Forks Formation, it is not known whether they came from the fault breccia or from the wallrock. The dike itself pinches out about 7 m above the clasts. The calcibreccia dikes on the southwest side of White Mountain are unusual in that they intrude a block of Paleozoic rock of the upper plate that is metamorphosed and strongly folded in contrast to other Heart Mountain fault blocks. This metamorphism ends abruptly downward at the Heart Mountain fault. Rounded fragments of carbonate rock in the fault brec- cia show that the metamorphism occurred before fault movement. Another unusual feature of these calcibreccia dikes is that they contain abundant fragments of igneous rock. These dikes can be traced down to a thick layer of fault breccia at the base of the upper plate that is similarly rich in igneous fragments derived from igneous dikes injected into the upper plate before it was emplaced by the Heart Mountain fault movement (Nelson and others, 1972). However, the absence of such fragments in the shale-containing dike 2,100 m east-northeast and in the closest body of fault breccia 600 m northeast suggests that the source of the calcibreccia in each dike was fault breccia from within a very limited area. If, as seems likely, the carbonate dikes containing igneous fragments were derived from a small body of fault breccia at the base of the upper-plate block that contains them, then these dikes must have have been injected after the upper-plate block was emplaced. AREA 7, WEST OF TROUT CREEK Area 7 lies on the west side of Trout Creek, at the south tip of a narrow strip of upper-plate Madison Limestone that is bordered and partly overlapped on the west by volcanic rocks of the Wapiti Formation, which were deposited on the Heart Mountain fault surface (the surface of tectonic denudation). The volcanic rocks in area 7 are for the most part flow breccia but include some lighter, hornblende-bearing polybreccia similar to the Cathedral Cliffs Formation. The vertical contact between the volcanic rocks and the upper-plate block of Madison Limestone is well exposed and accessible in the lower and upper parts of a steep face 100 m high but inaccessible in the intermediate part. At the lower exposure, lenses and stringers of calcibreccia occur in a dike about 3 m wide, somewhat variable and irregular in width, composed of volcanic rocks of the Wapiti Formation (fig. 23), similar to the dikes of volcanic rock containing calcibreccia described in the section "Area 1, South of Silver Gate, Montana.” (See figs. 4, 6, and 8.) The contact between this dike of volcanic rock and the Madison Limestone is sharp but irregular to wavy, and the adjoining limestone is not sheared or brecciated. On the other hand, the contact between volcanic breccia of the dike and the Wapiti Formation is indistinct because their compositions are much the same, the only differences being the vague, more or less horizontal flow lines in the Wapiti Formation, the presence of carbonate breccia in the dike, and the steep to vertical shears in the volcanic breccia of the dike (fig. 24). The calcibreccia stringers in the dike are vertically alined and occur in a zone about 1 m wide adjacent to the limestone. Vertical lineations and'striations in the18 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING Figure 23.—Cross section in area 7, on west side of Trout Creek, showing relations of a dike of the Wapiti Formation (wd) enclosing irregular stringers of carbonate fault breccia (cb) to country rocks of the Wapiti Formation (Tw) and an upper-plate block of Madison Limestone (Mm). Scattered dots in dike of Wapiti Formation indicate some intermixed carbonate rock. Stippled area at bottom of left diagram indicates talus cover. Heart Mountain fault (HMf) and surface of tectonic denudation (td), which are obsured by talus, lie about 20 m below lowermost exposure. dike of volcanic rock are also most pronounced in this zone. The part of this zone that could be observed and sampled lies in the lowermost 4 m of outcrop, above which it becomes inaccessible and apparently contains no calcibreccia because none could be seen from a distance. Most calcibreccia stringers are irregularly layered parallel to the dike walls (fig. 25A) and are composed almost entirely of carbonate breccia, even near the contact with the enclosing volcanic breccia. One irregular calcibreccia pod (upper left of enlarged diagram, fig. 23), however, exhibits irregular wavy bands, and the irregular, nonparallel banding is horizontal to inclined rather than vertical (fig. 25B). The presence of carbonate rock in the volcanic matrix ad-DESCRIPTION OF DIKES AT SPECIFIC SITES 19 Figure 24.—Dike of volcanic breccia of the Wapiti Formation containing calcibreccia (cb) in contact with edge of a Heart Mountain fault block of Madison Limestone in area 7, west of Trout Creek. Note sharp, irregular to wavy contact between the Wapiti Formation (Tw) and the Madison Limestone (Mm), and vertical shear (s) between irregular pod of calcibreccia to left (on which hammer rests) and calcibreccia stringer on right. For relation to general area, see figure 23. Photograph by Allan Krill. joining these stringers indicates that both the volcanic matrix and the calcibreccia were unconsolidated at the time of injection. The upper, accessible part of the contact between the Wapiti Formation and the Madison Limestone is about 100 m above the lower part. Here, as in the lower part, a distinct lineation occurs in the volcanic rocks parallel to the vertical contact between them and the limestone Figure 25_____Polished specimens of calcibreccia from a dike of vol- canic rock in area 7 on west side of Trout Creek (see fig. 23 for locations of specimens within dike). A, Stringer of irregularly banded calcibreccia (cb) composed almost entirely of carbonate breccia, even along contact with volcanic rocks of the Wapiti Formation (Tw). Banding is alined with stringer wall. B, Irregular pod of calcibreccia. Very irregular, wavy layering is not alined vertically with wall of dike. Photographs by Lowell Kohnitz.20 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING (fig. 23) but does not seem to be a dikelike feature. The lineation forms a zone about 7 m wide that dies out irregularly westward in the volcanic rocks and upward near the top of the limestone (fig. 23). The lineation seems to be a flowage feature apparently formed as the volcanic breccia flowed along the face of the limestone block and, as it did so, cut horizontal grooves in the limestone. Near the middle of the zone is a lens of brec-ciated limestone, probably of Madison Limestone, about 0.7 m wide and 7 m high, with a small prong of predominantly volcanic rock extending out horizontally (fig. 23), which is exposed in a nearly vertical face so that its horizontal dimension is not measurable. Much of the limestone breccia is composed of limestone fragments 0.2 to 0.5 cm long, some cobble size, 8 to 15 cm across, quite unlike the Heart Mountain fault breccia. No stringers of calcibreccia similar to those 100 m below occur in the volcanic rocks, nor are these rocks calcareous; the zone of lineation disappears about 25 m below the lens of brecciated limestone. Several additional small masses of calcibreccia as much as 0.7 m across occur in a large area of loose volcanic landslide debris near Trout Creek, 2 km northeast of area 7; their source is unknown. OTHER SITES OF CLASTIC DIKES In addition to the seven sites of clastic dikes of the Heart Mountain fault breccia that have been described above (areas 1-7, fig. 1), eight additional sites (Iocs. A-H, fig. 1) are known, information on which is summarized in the accompanying table (table 1). BRECCIA AND BRECCIATION PRODUCED BY THE HEART MOUNTAIN FAULT The Heart Mountain fault movement caused extensive brecciation of rocks in the lower part of the upper plate, most intense at the base and extending upward into shattered carbonate rock for as much as several Table 1.—Location of and brief data on other calcibreccia dikes Locality Location Country rock Dike size Remarks A SE. of Miller Mountain, SWV4NWV4 sec. 23, T. 9 S., R. 14 E., in Cooke City quadrangle. Wapiti Formation . 0.15 m wide, 5 m high. Calcibreccia dike terminates downward in Heart Mountain fault breccia, 0.3-1 m thick, altered to yellow brown color; fault breccia has sharp, nontransitional contact with underlying 65 m of Bighorn Dolomite, but upper contact with the Wapiti Formation is irregular, with a gradational zone of mixed carbonate and volcanic breccia at top. B 2 km due E. of Pilot Peak on north side of Fox Creek tributary, near foot of 65-m waterfall. Volcanic rocks ___ . 0.6-1 m thick, 20 m high. Dike begins at the Heart Mountain fault and curves irregularly upward; near the top a 3-m-long fork branches off the main dike and bluntlv terminates 1 m beyond the fork. About 200 m to NE. at the foot of a 50-m waterfall on Fox Creek, a calcibreccia dike 5-10 cm thick occurs near the base of volcanic rocks. C On north side of One Mile Creek, upstream from point where large fault block crosses creek and NE. of small block of Madison Limestone surrounded by Wapiti Formation. Wapiti Formation .. _ 0.15 m wide, 50 m long. Dike trends N. 30° W., dips 55° NE. At short distance to W. are several small calcibreccia dikes, each about 15 cm wide and 1-2 m long. D On north side of One Mile Creek, 0.8 km ENE. of locality C. - .0.3 m wide, 10 m long. Dike trends N. 10° E., dips 45° NW. E In bottom of valley, 0.5 km N. of One Mile Creek and 0.2 km ENE. of locality D. Some in broken and deformed limestone, others in the Wapiti Formation. In limestone: 0.5-5 cm wide or as irregularshaped bodies. In volcanic rock: irregular masses and stringers as wide as 0.3 m. The calcibreccia in the limestone is entirely carbonate rock, but that in volcanic rock contains some volcanic fragments. Lenses and stringers of calcibreccia occur along the very irregular but sharp contact between the limestone and volcanic rock. The volcanic rock contains a little hornblende and is calcareous in vicinity of the calcibreccia. The volcanic rock here appears to have been unconsolidated at the time it came in contact with the calcibreccia. F On divide between North Fork Crandall Creek and Blacktail Creek. Wapiti Formation _ 1 m wide and 15 m long; pieces scattered 150 m along strike. Dike striking S. 55° W. is at unusual height of about 170 m above the Heart Mountain fault. Fault blocks of upper plate are exposed 300 m to S. and 600 m to E. G Dikes are directly above the change in slope of the Heart Mountain fault from a transgressive to a former land-surface phase (fig. 1). Abrupt change in slope here may have favored accumulation of fault breccia that became the source of dike rock. tween Dead Indian Creek and west fork of Paint Creek, others 400 m and in ravine 600 m to SW. 0.15-0.3 m wide, 10 m long, with scattered fragments for 100 m SW. Dike 400 m SW. represented by numerous fragments 30 cm or more wide. H On ridge between two forks of Paint Creek, 0.5 km SE. of an abandoned CCC camp. Cathedral Cliffs Formation 0.3-0.5 m wide, 15 m or more long. Height of dike above the Heart Mountain fault is probably less than 25 m. Enclosing volcanic rock dips 25°SSW. is fractured ana striated, and displays some slickensides.MECHANISM OF DIKE INJECTION 21 tens of meters—a feature that has also been noted in the upper plate of the Muddy Mountain thrust (Longwell, 1922; Brock and Engelder, 1977). The abundant fault breccia remaining on the surface of tectonic denudation was part of the brecciation phenomenon associated with movement of the upper plate. The lower plate, on the other hand, has been thoroughly examined and nowhere shows any significant deformation that could be due to Heart Mountain faulting. This strange paradox is most probably related to the emplacement mechanism for the Heart Mountain fault. A particularly informative example of brecciation of rocks in the upper plate is the limestone mass described in the section "Area 3, North of Republic Mountain,” where calcibreccia dikes in its lower part and at the top have been described. This intensely brecciated block of limestone, which is composed mostly of Jefferson and a little Three Forks Formation at the top, lies on the Heart Mountain fault surface with the basal bed of the Bighorn Dolomite, which is dark gray here and has a petroliferous odor immediately below. The block of limestone was not only moved horizontally as a brecciated mass for some distance along the fault but also was displaced downward 100 m because the Bighorn Dolomite that normally underlies it is absent. How this movement was accomplished is unknown, but the consequent deformation of this upper-plate block is well preserved and will be described further as an aid in understanding the faulting process. The brecciated limestone block is about 25 m high and zoned roughly into three parts of about equal thickness and varying degrees of deformation (fig. 26). The lower zone consists at base of 2 m of carbonate fault breccia, finely comminuted and grading into about 6 m of overlying intensely brecciated limestone of the Jefferson Formation. Throughout this zone, some parts of the Jefferson are slightly less brecciated than others (fig. 27), and a crude horizontal lenticularity, resembling bedding, has been preserved. A few calcibreccia dikes occur in the lower zone, some of which have already been described in area 3. Near the top of this zone, vugs in less severely brecciated parts of the limestone block contain an asphalt residue, and some nearby breccia contains numerous blebs or specks of asphalt that give the rock a banded appearance. One indistinct calcibreccia dike in the lower zone also contains black asphalt specks. The middle zone, which consists of about 10 m of several somewhat massive beds of bluish-gray limestone, is quite thoroughly brecciated but exhibits the outward form and appearance of the original sedimentary beds; on fresh fracture it has a petroliferous odor. At the base of the upper zone, mostly unbrecciated, is a bed of limestone 2.5 thick containing many fine fractures, in places closely spaced and forming a crisscross pattern that gives a breccialike appearance, although deformation has not proceeded far enough to rotate the enclosed fragments. Above the basal bed in this zone are sheared and fractured rocks, apparently a mixture of rocks of the Jefferson and Three Forks Formations; the more massive limestone beds contain innumerable hairline fractures, and the thin-bedded limestone has been deformed by shearing and faulting. MECHANISM OF DIKE INJECTION An important clue to the origin of the clastic dikes of Heart Mountain fault breccia is provided by evidence that the volcanic country rock of the Wapiti Formation was unconsolidated at the time many of the dikes were intruded. Krill (1976) suggested a steam-injection model as a working hypothesis for the mechanism of dike injection, according to which the volcanic rocks were the source of heat. In this model, water in the calcibreccia was not vaporized immediately upon burial but only after burial beneath several volcanic flows (immediate vaporization is negated by the presence of dikes 20 m high passing through at least two distinct flows). Although this idea is interesting and imaginative, it does not seem to apply equally well at all sites, particularly those with dikes in both upper-plate and volcanic rocks. Voight (1973a) concluded that the clastic dikes injected into upper-plate rocks are somewhat older than those in volcanic rocks but did not state the basis for his conclusion. No evidence has been found, so far as I am aware, that clastic dikes of Heart Mountain fault breccia intruded upper-plate rocks at a different time from rocks of the Wapiti Formation. The calcibreccia dikes in the Wapiti Formation were evidently injected very soon after movement on the Heart Mountain fault had ceased, and so also the dikes in the upper plate, as shown by (1) the calcibreccia dike in a small block of the Three Forks Formation in area 1 that contains volcanic fragments from the adjoining Wapiti Formation, evidence showing that it was emplaced after both deposition of the Wapiti and Heart Mountain faulting; and (2) the undeformed dikes in the highly brecciated limestone mass of the upper plate in area 3, north of Republic Mountain, which unequivocally shows that dike injection occurred after deformation of the enclosing rock and thus after faulting. An alternative explanation for the calcibreccia dikes, proposed by Voight (1973b), is that dikes in the upper plate were injected as part of the Heart Mountain fault emplacement mechanism. Their presence was said to22 CLASTIC DIKES OK HEAR T MOUN TAIN FAUL T BRECCIA, NOR THWESTERN WYOMING Figure 26.—Diagrammatic cross section showing various degrees of deformation in brecciated limestone above Heart Mountain fault in area 3, north of Republic Mountain, cb, calcibreccia. Arrows show direction of movement on fault. 50 mm I I I I I I I I I I_________I Figure 27.—Polished specimen of brecciated limestone from 10 m above Heart Mountain fault in area 3, north of Republic Mountain. Degree of brecciation is not uniform, and many fragments are not rotated. Photograph by Lowell Kohnitz. "*** demand a fluid 'flotation’ mechanism in the mechanics of the Heart Mountain rockslide [his term], and by analogy, of South Fork and Reef Creek rockslides.” Several factors, however, indicate that this explanation is not correct. (1) The upper plate was under lateral tension rather than confining pressure, as shown by its having been broken up and pulled apart into numerous detached blocks during movement. In the absence of confining pressure, small upper-plate masses containing calcibreccia dikes such as those described here would simply have come apart and failed to retain the intrusions. (2) The dikes in the upper plate are not genetically related to the emplacement process because evidence indicates that they were injected some time after movement had ceased. (3) The dikes in volcanic rocks are younger than the Heart Mountain fault movement (Pierce, 1968; Voight, 1974, fig. 9). If the dikes in upper-plate rocks are not the same age but older and part of the faulting process, then different mechanisms are required for the fault breccia dikes in one type of country rock and those in another type, although the dikes are similar in all other respects.GEOLOGIC SIGNIFICANCE OF THE CLASTIC DIKES 23 Voight (1974) also cited the clastic dike of fault-zone material along the Muddy Mountain thrust in Nevada as evidence for the former existence of high fluid pressure along that fault. In a recent report, however, Brock and Engelder (1977) proposed that these clastic dikes are probably not related to high fluid pressure during faulting and from several lines of evidence concluded that the advance of the Muddy Mountain thrust could not have been aided by high pore pressure. If the premise be accepted that the calcibreccia dikes intruded both the upper-plate and volcanic rocks of the Wapiti Formation in the Heart Mountain fault area at essentially the same time, then a common mechanism for their injection seems far more likely than separate mechanisms. The model proposed here, which would satisfy all known constraints imposed by the observed data, assumes that lithostatic pressure was imposed by a rapidly accumulating overburden of volcanic rock. The sequence of events associated with dike injection may be reconstructed as follows. Once the surface of tectonic denudation had been created by catastrophic movement on the Heart Mountain fault, water-saturated carbonate fault breccia, from 0 to 4 m or more thick, was irregularly distributed over the fault surface. As volcanic rock was poured out onto this land surface, some of the calcibreccia was mixed with the volcanic rocks, some was caught up as irregular pods and lenses in the lower part of the Wapiti Formation, and some remained as irregular tabular bodies at the base of the volcanic rocks. These irregular bodies were unconsolidated, and so an unstable condition was formed, with the more mobile fault breccia below and the less mobile volcanic rocks above. As deposition of volcanic rocks continued, the smallest upper-plate blocks were immediately buried, and the somewhat larger ones were covered soon after. Lithostatic pressure on the unconsolidated fault breccia at the base of the upper plate increased with the thickening volcanic cover until the cohesive strength of the overlying rock was exceeded, and the calcibreccia was injected upward as dikes along fractures or lines of weakness or lesser consolidation, regardless of whether the overlying rocks were upper-plate rocks or volcanic rocks. The degree of lithostatic pressure needed to initiate intrusion of a dike into the Wapiti Formation would have depended on the degree of consolidation and the character of the volcanic rock, as well as on the fluidity of the calcibreccia; in some cases perhaps only a few tens or hundreds of meters of overburden may have been sufficient. GEOLOGIC SIGNIFICANCE OF THE CLASTIC DIKES The calcibreccia dikes and related features affirm some earlier conclusions regarding the Heart Mountain fault and furnish additional data on the events associated with the fault movement. It has already been established that the Wapiti Formation was deposited on the surface of tectonic denudation soon after the fault movement ceased because the bedding-plane phase (fig. 1) of the fault has not been altered by erosion. This conclusion can now be extended to include the transgressive and former land-surface phases of the fault because calcibreccia dikes occur in these areas as well. The significance of the dikes is that the fault breccia from which they were derived was almost immediately buried by volcanic rocks, otherwise it would have been eroded away. Rapid deposition of at least the basal part of the Wapiti Formation is now considered to have extended over the entire 75-km-long area where calcibreccia dikes are known to occur. Voight (1973a, p. 118-119) stated that "*** the presence of fault breccia-derived dikes injected within glide blocks seem to demand a fluid flotation mechanism for the Heart Mountain structure (Voight, 1973b).” His conclusion is based on the premise that the fault breccia-derived dikes (calcibreccia dikes of this paper) were injected as part of the Heart Mountain fault movement, but he presents no evidence for this assumption. Moreover, his mechanism of dike injection is invalid because the data presented here show that the clastic dikes were not injected as part of the fault movement but only after this movement had ceased and after basal rocks of the Wapiti Formation had been deposited. Likewise, the fault breccia-derived dikes are not indicative of the other fluid-pressure mechanisms that have been suggested: the high-fluid-pressure mechanism of Hubbert and Rubey (1959) or the volcanic-gas mechanism of Hughes (1970) and Prostka (1978, p. 435). The calibreccia dikes are useful in conjunction with the surface of tectonic denudation for a stratigraphic correlation of the various volcanic rock formations. Because this surface was exposed for only a very brief interval, the rocks deposited on it must be of the same age. The problem that arises in using this surface for stratigraphic correlation of the volcanic rocks is to determine whether these rocks were deposited on the surface or were fault-emplaced during the Heart Mountain fault movement, that is, whether the volcanic rocks have a depositional or a fault contact with the surface of tectonic denudation. If these volcanic rocks were fault emplaced, they should follow the well-known pattern of other upper-plate rocks and form fault breccia composed of volcanic rock fragments. In this case, the basal part of the rock unit in question should also show tectonic brecciation, although opinions may differ as to whether a given volcanic breccia is of tectonic or other origin. However, the presence of24 CLASTIC DIKES OF HEART MOUNTAIN FAULT BRECCIA, NORTHWESTERN WYOMING calcibreccia bodies can help to determine whether the rock is either depositional or fault emplaced because, although clastic dikes occur in both types of deposits, these dikes may exhibit differences that reflect the origin of a given rock unit. Thus, those dikes in volcanic rocks deposited on the surface of tectonic denudation may have a mixture of carbonate and volcanic rock in the dike wall (figs. 4, 6-8), and the basal part of the volcanic rock may contain lenses and irregular bodies of calcibreccia (figs. 10, 21, 22). SUMMARY The salient points revealed by the clastic dikes are: 1. They are widely distributed within the area of the Heart Mountain fault. They are more common within the bedding-plane phase of the fault, but a few also occur in the transgressive and former land-surface phases of the fault. 2. They occur in the upper plate and in volcanic rocks that were deposited very soon after movement on the Heart Mountain fault had ceased. 3. They were intruded into the upper-plate and vol- canic rocks at approximately the same time. 4. They confirm an earlier conclusion that the Heart Mountain fault movement was a cataclysmic event. 5. They were injected after movement on the Heart Mountain fault had ceased and therefore are not indicative of a fluid-flotation mechanism for the Heart Mountain fault, as suggested by Voight. 6. Injection of the dikes was caused by a rapidly in- creasing overburden of volcanic rocks that also rapidly increased the lithostatic pressure on the unstable fault breccia. 7. Wapiti volcanism began by rapid deposition of at least the lower part of the Wapiti over many square kilometers. 8. In some places where calcibreccia was abundant, deposition of the lower part of the Wapiti was so rapid that before the basal part was consolidated, the lithostatic pressure on it and the unstable calcibreccia became sufficient to cause both to flow upward as a dike. 9. Differences between calcibreccia dikes in indurated country rock and those in unconsolidated country rock can be used as an aid in distinguishing between volcanic rocks of the Cathedral Cliffs and Lamar River Formations that are fault emplaced by the Heart Mountain fault movement, and the Wapiti Formation that is depositional on the tectonic surface. 10. The high permeability of the extensively brec- ciated rocks in the lower part of the upper plate and the separation of the upper plate into numerous blocks and pieces indicate that the Heart Mountain fault movement was not aided by high pore pressure of water, air, or volcanic gas. In my opinion, the extensive brecciation of rock above the fault surface, which is observed to occur immediately above a line of seismic discontinuity separating a lower 350-m-thick layer predominantly of shale from an upper 500-m-thick layer of carbonate rock, may well have been caused by a catastrophic earthquake of vertical acceleration approaching 1 g. However, it remains possible that heat generated by rapid movement of the upper plate produced feteam (Goguel, 1969), which helped to reduce friction on the fault. ACKNOWLEDGMENTS The author is indebted to R. A. Loney and W. H. Nelson for examining many thin sections of the calcibreccia. This report is based on fieldwork performed over a period of several years, and I wish to thank D. H. Anspach, W. C. Barnes, J. H. Clark, K. R. Jayne, Allan Krill, C. C. McAneny, and J. I. Ziony, who served so capably and enthusiastically as my field assistants. The suggestions and comments of R. G. Schmidt and E. B. Ekren on an earlier draft of this paper are appreciated and have been most helpful. REFERENCES CITED Brock, W. G., and Engelder, T., 1977, Deformation associated with the movement of the Muddy Mountain overthrust in the Buffington window, southeastern Nevada: Geological Society of America Bulletin, v. 88, no. 11, p. 1667-1677. Bucher, W. H., 1933, Volcanic explosions and overthrusts: Transactions of the American Geophysical Union, v. 14, p. 238-242. -----1940, The geology of the Cody region: Transactions of the New York Academy of Sciences, ser. 2, v. 2, no. 7, p. 1-4. Dake, C. L., 1918, The Hart Mountain overthrust and associated structures in Park County, Wyoming: Journal of Geology, v. 26, no. 1, p. 45-55. Goguel, Jean, 1969, Le role de l’eau et de la chaleur dans les phenomenes tectoniques [The role of water and heat in tectonic phenomena]: Revue de Geographie Physique et de Geologie Dynamique, v. 11, no. 2, p. 153-163. Hague, Arnold, 1899, Description of the Absaroka quadrangle (Crandall and Ishawooa quadrangles)[Wyoming]: U.S. Geological Survey Geologic Atlas, folio 52, 6 p. Hewett, D. F., 1920, The Heart Mountain overthrust, Wyoming: Journal of Geology, v. 28, no. 6, p. 536-557. Hubbert, M. K., and Rubey, W. W., 1959, Role of fluid pressure in mechanics of overthrust faulting. I. Mechanics of fluid-filled porous solids and its application to overthrust faulting: Geological Society of America Bulletin, v. 70, no. 2, p. 115-166. Hughes, C. J., 1970, The Heart Mountain detachment fault—a volcanic phenomenon?: Journal of Geology, v. 78, no. 1, p. 107-116.REFERENCES CITED Krill, A. G., 1976, Petrography of Heart Mountain fault breccia and fault breccia dikes and related literature review: University of California, Santa Cruz, Department of Earth Sciences, senior thesis, 46 p. Longwell, C. R., 1922, The Muddy Mountain thrust in southeastern Nevada: Journal of Geology, v. 30, no. 1, p. 63-72. Nelson, W. H., and Pierce, W. G., 1968, Wapiti Formation and Trout Peak trachyandesite, northwestern Wyoming: U.S. Geological Survey Bulletin 1254-H, p. Hl-HU. Nelson, W. H., Pierce, W. G., Parsons, W. H., and Brophy, G. P., 1972, Igneous activity, metamorphism, and Heart Mountain faulting at White Mountain, northwestern Wyoming: Geological Society of America Bulletin, v. 83, no. 9, p. 2607-2620. Pierce, W. G., 1941, Heart Mountain and South Fork thrusts, Park County, Wyoming: Bulletin of the American Association of Petroleum Geologists, v. 25, no. 11, p. 2021-2045. ------1957, Heart Mountain and South Fork detachment thrusts of Wyoming: Bulletin of the American Association of Petroleum Geologists, v. 41, no. 4, p. 591-626. ------1960, The "break-away” point of the Heart Mountain detachment fault in northwestern Wyoming, in Short papers in the geological sciences: U.S. Geological Survey Professional Paper 400-B, p. B236-B237. ------1963a, Cathedral Cliffs Formation, the early acid breccia unit of northwestern Wyoming: Geological Society of America Bulletin, v. 74, no. 1, p. 9-21. ------1963b, Reef Creek detachment fault, northwestern Wyoming: Geological Society of America Bulletin, v. 74, no. 10, 1225-1236. ----1968, Tectonic denudation as exemplified by the Heart Mountain fault, Wyoming, in Orogenic Belts: International Geological Congress, 23d, Prague, Czechoslovakia, 1968, Report, Section 3, Proceedings: p. 191-197. ------1973a, Crandall Conglomerate, an unusual stream deposit, and its relation to Heart Mountain faulting: Geological Society of America Bulletin, v. 84, no. 8, p. 2631-2644. ------1973b, Principal features of the Heart Mountain fault and the mechanism problem, in De Jong, K. A., and Scholten, Robert, 25 eds., Gravity and tectonics: New York, John Wiley and Sons, p. 457-471. Pierce, W. G., Nelson, W. H., and Prostka, H. J., 1973, Geologic map of the Pilot Peak quadrangle, Park County, Wyoming: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-816, scale 1:62,500. Prostka, H. J., 1978, Heart Mountain fault and Absaroka volcanism, Wyoming and Montana, U.S.A., in Voight, Barry, ed., Rockslides and avalanches, 1—natural phenomena: New York, Elsevier, p. 423-437. Prostka, H. J., Ruppel, E. T., and Christiansen, R. L, 1975, Geologic map of the Abiathar Peak quadrangle, Yellowstone National Park, Wyoming and Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-1244, scale 1:62,500. Smedes, H. W., and Prostka, H. J., 1972, Stratigraphic framework of the Absaroka Volcanic Supergroup in the Yellowstone National Park region: U.S. Geological Survey Professional Paper 729-C, p. C1-C33. Stanley, R. S., and Morse, J. D., 1974, Fault zone characteristics of two well exposed overthrusts: The Muddy Mountain thrust, Nevada, and the Champlain thrust at Burlington, Vermont [abs. ]: Geological Society of America Abstracts with Programs, v. 6, no. 1, p. 78-79. Voight, Barry, 1973a, The mechanics of retrogressive block-gliding, with emphasis on the evolution of the Turnagain Heights landslide, Anchorage, Alaska, in De Jong, K. A., and Scholten, Robert, eds., Gravity and tectonics: New York, John Wiley and Sons, p. 97-121. ------1973b, Role of fluid pressure in mechanics of South Fork, Reef Creek, and Heart Mountain rockslides [abs.]: Geological Society of American Abstracts with Programs, v. 5, no. 2, p. 233-234. ------1974, Architecture and mechanics of the Heart Mountain and South Fork rockslides, in Voight, Barry, and Voight, M. A., eds., Rock mechanics: The American Northwest: University Park, Pa., Experiment Station, College of Earth and Mineral Sciences, Pennsylvania State University, p. 26-36.I RETURN EARTH SCIENCES LIBRARY *£.?Ckr^ /, * ^ ^007I : 1 / (T- *