• it . ■ • •'- • . • .«, / v ' -"'* : .• . - » I ." i ■ ^ . t*f "•«. . • . I i+ I "*» •■/ - s > * > California Division of Mines Special IRefeatt 54 K f-X£ YTWVCAUFORNIA V — — *> STATE OF CALIFORNIA GOODWIN J. KNIGHT. Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON. Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 GORDON B. OAKESHOTT. Chief \N FRANCISCO SPECIAL REPORT 54 OCTOBER 1958 AGE DETERMINATION OF SOME GRANITIC ROCKS IN CALIFORNIA BY THE POTASSIUM-ARGON METHOD By G. H. CURTIS, I. F. EVERNDEN, and ]. LIPSON Price 50f Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/agedetermination54curt AGE DETERMINATION OF SOME GRANITIC ROCKS IN CALIFORNIA BY THE POTASSIUM-ARGON METHOD By G. H. Curtis,* J. F. Evernden,* and J. Lipson * OUTLINE OF REPORT l'ai;e Jgbract 3 ntrodnction 3 uminary of the potassium-argon dating method 3 ,ge data and geological relationships 5 Group 1 5 Group 2 7 Group 3 9 Hscussion of results 10 iibliography _ 16 I llustrations 'igure 1. Map showing location of samples of granitic rock dated by potassium-argon method 4 2. Geologic map of southern Klamath Mountains 6 3. Geologic map showing granitic intrusions, portions of the Sierra Nevada batholith in the vicinity of Yosemite National Park 8 4. Map showing San Andreas fault in the central Coast Ranges 15 ABSTRACT Five intrusions in the foothill Sierra Nevada and Qamath Mountains, ten in the high Sierra Nevada, and ix in the Coast Ranges have been dated using the otassium-argon method. These three geographic groups all into two age groups or within two orogenic epochs, 'he earlier group, emplaced during the Nevadan orog- ny, spans the interval 133 to 143 million years and was mplaced in the Upper Jurassic later than the Ox- ordian-Kimmeridgian boundary and before the close of he Tithonian stage, some of the intrusions being em- laced at the same time as the nearby Tithonian rocks r ere being deposited. The later group was emplaced dur- le Santa Lucian orogeny in the Upper Cretaceous and pans the interval 78 to 95 million years. Comparison dth potassium-argon dates obtained from glauconite in Fpper Cretaceous-Eocene boundary sandstone (60 mil- on years) and from Albian-Cenomanian boundary andstone (98 million years) indicate that the Santa mcian orogeny occurred during the Cenomanian, Tu- onian, and Senonian stages of the Upper Cretaceous, -Inch is in harmony with the limited geologic data. The dates of the Nevadan intrusions suggest that the lolmes time scales are incorrect for the beginning and hd of the Jurassic period, the error probably being aused by Holmes' using too small a total thickness of urassic in his computations. A correction for this, based u published data, increases the length of the Jurassic rom 25 million years to a minimum of 40 million years nd decreases proportionately the lengths of the Per- sian, Triassic, and Cretaceous periods. I The Coast Ranges intrusions, previously thought by )me geologists to be pre-Jurassic in age, are in fault ontact with rocks of Jurassic and Lower Cretaceous ge but nowhere intrude them. They are locally overlain onconformably by sedimentary rocks of late Upper 'retaceous age. One of these intrusions, the Gabilan jluton, is in fault contact with the older Franciscan 'Department of Geological Sciences, University of California. J. Lipson was concerned only with the physical measurements on the several samples. He is not responsible for any of the geo- logical conclusions here presented. Work supported by the U. S. Atomic Energy Commission, Shell Development Company, and California State Division of Mines. formation for over 100 miles and is at one point less than 10 miles from a thick section of conformable early Lower Cretaceous to late Upper Cretaceous sedimentary rocks. These geological anomalies appear to be a result of large strike-slip displacement of the San Andreas fault system. A displacement of at least 300 miles along the San Andreas fault appears to have occurred since late Upper Cretaceous time. At the time of emplacement, the Coast Ranges intrusions were probably in the central part of a single fold-belt which included the pinions of the high Sierra Nevada and the Southern California batholith. INTRODUCTION This report describes the first results of a long range project. The data are already sufficient to suggest gen- eral solutions to several problems of California geology, and it is the purpose of this report to present and discuss these solutions. Potassium-argon dates have been obtained primarily from biotite-bearing granitic rocks ranging in composi- tion from quartz diorite to granite. Locations of these samples are shown on figure 1. They belong to three geographic groups: (1) five samples from individual plutons in the western foothills of the Sierra Nevada and southern Klamath Mountains, (2) ten samples from individual plutons in the central high Sierra (Evernden, Curtis, and Lipson, 1957), (3) six samples from indi- vidual plutons in the central Coast Ranges. Within each group, the ages of the individual plutons appear to be reasonably consistent, so that each group can properly be considered a genetic as well as a geographic unit. SUMMARY OF THE POTASSI UM -ARGON DATING METHOD Of the several isotopes of potassium, K 38 and K 41 are non-radioactive, K 40 is radioactive, and the status of K 39 is being evaluated at the present time. The potassium- argon dating method is based upon the transformation of K 40 to A 40 by K-electron capture. Approximately 88 percent of K 40 transforms to Ca 40 by electron emission, but a potassium-calcium dating technique has not been perfected due to the prevalence of normal or nonradio- genic Ca 40 in virtually all potassium-bearing minerals. Owing to the inert nature of argon, the normal or non- radiogenic content of A 40 in the crystal lattices of min- erals is very low. Atmospheric argon adhering to the surfaces of the mineral grains may be present in quan- tities exceeding the radiogenic argon in the crystal lattice but its evaluation is a simple process (Lipson, 1958). If none of the radiogenic argon developed from K 40 transformation has escaped from the crystal lattice since genesis of the mineral, there is therefore a way of measuring the age of the mineral. The accuracy of the age determination will depend upon the following factors: Knowledge of the Total Amount of Potassium in the Sample. In this laboratory, the potassium analyses have been made with a Perkin-Elmer flame photometer. If (3) Special Report 54 irSi Plutonic rocks Figure 1. Map showing location of samples of granitic rock dated by potassium-argon method. Age Determination by Potassium-Argon Method duplicate determinations of a sample do not cheek within 1.5 percent, they are rejected. Knowledge of the Relative Amounts of the Several Isotopes of Potassium in the Sample. The K 40 content s computed by measuring- the total amount of potassium md multiplying this quantity by the ratio K 40 /total K. This ratio has been investigated (Rankama, 1954) and as been found to be essentially independent of chemical process, so that a fixed value of the ratio can be used for all samples. Knowledge of the Rate at Which K J, ° Transforms to A i0 . The agreement of potassium-argon ages, computed )y using laboratory determinations of this rate, with •oneordant uranium ages from the same rocks (Wetheril it al., 1956) indicates the reliability of the value pres- ntly used. Determination of the Total Amount of A i0 in the Sample. Discussion of the apparatus used in this labo- ratory can be found in J. Lipson's report (1958). A measured amount of A 38 , termed the "spike," is added to the argon sample obtained from destruction of the mineral. Knowledge of the spike quantity and the isotope ratios in the mixed gas sample provides sufficient data for determination of the total A 40 in the sample. Experi- ments show that the technique used yields an evaluation of total A 40 to within 1 percent, assuming equal ac- uracy of spike measurement. The Validity of the Assumption That All Contaminat- ing Argon Has the Isotope Composition of Atmospheric irgon. This assumption is supported by two types of experimental evidence : the consistency of dates obtained using this assumption, even though the content of con- taminating argon may vary greatly, and the demonstra- tion that contaminating argon is more easily removed from mica and feldspar by heating than is their radio- genic component (Reynolds, 1957). The inference is that the contaminating argon is not argon within the lattice structure but is argon adhering to grain surfaces as a result of their exposure to the high argon atmosphere of the earth. Knowledge of the Rate at Which Radiogenic Argon [Ins Been Lost by Diffusion Since Formation of the Min- eral. Diffusion of argon out of a crystal lattice is incompletely understood. Diffusion losses are known to vary from mineral to mineral, in certain minerals be- coming so excessive that they are useless for dating. However, extensive work on biotite samples from pre- Cambrian granitic rocks indicates that argon loss from them is inappreciable for periods as long as 2 • 10 9 years. Biotite and muscovite, obtained from granitic rocks, were used to obtain all of the California dates reported here. The Degree to Which Argon Content of the Minerals Has Been Altered by Geologic Processes Since Its For- mation. It is believed that the K 40 /A 40 ratio of materials here discussed has not been affected by such processes. AGE DATA AND GEOLOGICAL RELATIONSHIPS Table 1 presents the experimental data upon which the age figures are based. Group 1 KA 81 Shasta Bally quartz diorite Age. 134 million years.* •An age of 97 million years was obtained by Gottfried (Kinkel et al., 1956) for the Shasta Bally batholith, using the Larsen method. As will be brought out, this date is incompatible with the geology. Location. The specimen was collected by N. E. A. Hinds 1 mile north of the town of Ono, Shasta County, California, at the northern end of the Sacramento Valley. Geologic Data. The Shasta Bally batholith (Hinds, 1933, p. 340) is the southernmost of the granitic masses exposed in the Klamath Mountains in northern Cali- fornia. It is approximately 30 miles long by 10 miles in maximum width, and intrudes rocks of Mississippian age and older. Within the Klamath Mountain complex and in southern Oregon are deformed sediments of Middle and Upper Jurassic age, so that it is reasonable to assign a post-lower Upper Jurassic age to the batholith. Lying unconformably on the southern part of the batholith is a thick sequence of fossiliferous sands, sandy shales, shales, and bouldery conglomerates which dip gently southward away from the higher parts of the Klamath Mountains. Anderson (1938, p. 47) found a faunal assemblage near the base of these beds which he correlated with a low position in the Valanginian stage of the European Lower Cretaceous. Beneath these Lower Cretaceous beds 13 miles to the southwest of the batho- lith, but lying on schists adjacent to the batholith, is the northern border of a sequence of unmetamorphosed Knoxville shale and conglomerate that contains fossils that show them to be correlative with the Portlandian stage of the Upper Jurassic of Europe. This sequence thickens southward to Redbank Creek, 28 miles from the batholith, where the Knoxville is approximately 16,000 feet thick. Where the Knoxville formation is overlain by Lower Cretaceous sediments there is only a slight an- gular unconformity between them. Southward this un- conformity becomes somewhat more marked, though no- where is it profound. Taliaferro (1944) has given the name "Diablan orogeny" to the crustal movements which produced the unconformity and assigns it to the very close of the Jurassic and beginning of the Creta- ceous. The presence of a major unconformity below the Knoxville that truncates the Shasta Bally batholith led Taliaferro and others to believe that the batholith was intruded before the Knoxville was deposited. These crit- ical relationships are shown on figure 2. As will be dis- cussed below, one's opinion of the time relationships of the Knoxville sediments and the batholith is very de- pendent upon the geologic concepts that he accepts. Hinds states that the batholith is composed of two types of quartz diorite, hornblende-dominant and bio- tite-dominant, which, because of their gradational char- acter, are probably phases of a single intrusion. The principal type throughout the mass is hornblende quartz diorite with a small percentage of biotite. Kinkel ( Kin- kel et al., 1956, p. 48) mentions that in the southern part of the batholith the border is darker than the in- terior, and much of the border phase is hornblende dio- rite. The specimen used for the age determination is the hornblende-rich variety. It shows only slighl weathering effects, but the biotite is extensively ehloritized, pre- sumably from deuteric alteration at an early time. Most of the chlorite was separated from the biotite with a magnetic separator after fine grinding. Unfortunately the effect of chloritization of biotite on potassium-argon age determinations is unknown. The chloritization proc- ess results in the removal of most or all of the potassium from the part of the biotite affected. We are assuming Special Report 54 Pre-Knox metosedim and volcani rocks 10 20Mi Figube 2. Geologic map of southern Klamath Mountains. that, owing to the much higher mobility of argon than potassium, the argon will be removed as completely as the potassium during destruction of the biotite lattice by chloritization. Work on other partly chloritized biotites appears to support this assumption. KA 98 Belden granodiorite Age. 135.7 million years. L oca t inn. The specimen was collected by R. L. Rose approximately 2 miles west of the town of Belden on the Feather River Highway in Plumas County. Geolngic Data. This specimen is from the most north- erly exposed granitic mass on the west side of the Sier- ran crest. To the north, the basement complex of the Sierra is covered by Tertiary and Quaternary volcanic rocks erupted from centers in the vicinity of Lassen Peak. The pluton is intrusive into slates of Carboniferous age and into metavolcanic rocks of possible Jurassic age. Auriferous river gravels of Eocene age rest unconform- ably on the mass in places. KA 102 and KA 103 Rocklin Granodiorite Age. 131 million years. (KA 102-130.6 million years, KA 103-131.5). Location. The sample dated was obtained from the large rock quarry at Rocklin in Placer County (sec. 19, T. 11 .V. R. 7 E., MD). Geologic Pain. The Rocklin granodiorite pluton is an elliptical mass of approximately 150 square miles which crops <>ut in the foothill belt of the Sierra Nevada in the part of the Auburn quadrangle. It intrudes the Mariposa formation of the Oxfordian-Kimeridgi stages of the Upper Jurassic as well as older rocks, a 1 is overlain unconf ormably by gently dipping marine b( t equivalent to the Campanian stage of the Upper C | taceous. According to Lindgren (1894), the pluton as a wh«| is composed of biotite hornblende granodiorite. He iik tions, however, that it exhibits considerable variati from place to place, showing a range in composition f r« I gabbro to quartz diorite. Our observations indicate th the pluton is actually composite in nature, and til some of the variations in composition observed by Lirl gren represent separate intrusions. This is certaii true of the quartz diorite in the vicinity of Horses! • Bar which is surrounded by granodiorite (see KA 97) I The specimen dated is biotite-museoviteJiornblenl quartz diorite. Whether this is a distinct intrusi sharply demarcated from the main mass of granodiori was not determined. Separate dates obtained from tl biotite and muscovite are 131.5 and 130.6 million yea J respectively. KA 97 Horseshoe Bar Quartz Diorite Age. 142.9 million years. Location. The sample dated was collected from t quarry in the NEi sec. 18, T. 11 N., R. 8 E., MD., a proximately 3 miles southeast of Loomis. Genlngic Data. The rock dated was called gabbro Lindgren and was considered to be a local variation the Rocklin granodiorite which surrounds it. He sho- two small patches of this rock on his geologic map (189 which crop out east of Loomis. Both of these patch appear to be part of a more or less continuous belt se eral miles in length. Within the granodiorite along southwest margin of the belt the ferromagnesian mi erals show strong alignment parallel to the contact \ tween the two rock types, suggesting relative moveme of the masses. Although the color of the rock is very dark, the ro should probably be called quartz diorite rather th; gabbro, for it contains abundant quartz. Hypersther hornblende, and biotite are the ferromagnesian minera the hornblende replacing and forming extensive reaetii rims around the hypersthene crystals. Clearly, equili rium was not obtained during the period of crystalliz tion. KA 120 Guadalupe Mountain Quartz Monzonite Age. 142.9 million years. Location. The specimen was collected from the si of a road through the southern end of the Guadalu Mountains in the Indian Gulch quadrangle (NE^ I 24, T. 6 S., R. 17 E., MD). Geologic Data. The Guadalupe Mountain pluton the southernmost mass date in Group 1. It intrudes tl Mariposa formation and older rocks. The specimen is the most potassic rock of Group containing equal parts of microcline and oligoclase. \ addition to quartz, the other major constituents a biotite and hornblende. Tectonic Environment of Group 1 Most geologists who have studied the Sierra Neva< have concluded that most of the deformation of the sec mentary rocks preceded emplacement of the plutons frl J Age Determination by Potassium- Argon Method licit that some deformation probably accompanied em. Taliaferro (1942), working mainly in the foothill It of the Sierra Nevada, states: "All observations show at the folding antedated the final emplacement of the eat batholiths. The writer has mapped a number of Ids in the Jurassic rocks of the Sierra Nevada and has aced them to the borders of stocks and off-shoots of the ain batholithic area and found them again on the op- isite side, sometimes as much as 8 miles away, with e same trends and the same formations exposed along em. Plutonic bodies are also found either cutting ross or guided by strong Upper Jurassic thrust zones. Jding and thrusting unquestionably preceded plutonic trusion but the length of the interval between the two ents is not known. Broadly speaking, they may be eon- lered as component parts of a revolution, but it must recognized that they may be separated by a consider- le interval." These observations are supported by those of Kinkel. all, and Albers (1956), working in the southern lamath Mountains. Concerning the Shasta Bally tholith, they state: "The Shasta Bally batholith is late irassic or early Cretaceous in age. It was not affected - the Nevadan orogeny and cuts direotly across folia- m formed during the Nevadan orogeny of late Jurassic early Cretaceous age. Also the Shasta Bally batholith ,s metamorphosed the Copley greenstone to amphibo- e, gneiss, and migmatite, but no retrograde meta- Drphism has been superposed on the altered zones by ter orogeny. ' ' Kinkel et al. (op. cit.) believe that emplacement of the ghtly older Mule Mountain batholith may have aecom- mied the latest phases of strong compression of sedi- entary rocks. Group 2 All of the specimens of Group 2 were collected from utons within Yosemite National Park. Their locations e shown on figure 3. These rocks were chosen because ey are well exposed and their relative ages had previ- sly been established by Calkins (1930) and by Rose 957) on the following field evidence: dikes and apo- tyses of the younger intrude the older ; fragments of e older are included in the younger ; aligned tabular inerals in the older are truncated by the younger ; and ten tabular minerals in the younger are aligned trallel to its margins. Of those plutons studied by Calkins, samples of seven Uected by P. Bateman of the U. S. Geological Survey ;re dated. We quote briefly from Calkins' paper con- rning field relationships of these intrusions. \ 133 Johnson Granite Porphyry Age. 82.4 million years. Geologic Data. "The intrusion is completely sur- unded by the much larger mass of Cathedral Peak anite, into which it has broken." \ 135 Cathedral Peak Granite Age. 83.7 million years. Geologic Data. "Wherever its periphery has been ex- ained, the Cathedral Peak granite is found to be in ntact with porphyritic facies of the Half Dome quartz onzonite. It has many characteristics in common with at rock, yet the boundary between them is everywhere sharp, and there is ample evidence that the Cathedral Peak granite is the younger of the two. ' ' KA 73 Half Dome Quartz Monzonite Age. 84.1 million years. Geologic Data. "The Half Dome quartz monzonite is younger than the Sentinel granodiorite, as is clear from the tongues which it sends far into that rock. On the other hand, it is older than the Cathedral Peak granite, whose mass it completely surrounds. ' ' KA 100 Sentinel Granodiorite Age. 86.4 million years. Geologic Data. ' ' At the west end of the belt crossing the [Yosemite] valley the Sentinel granodiorite is in con- tact with the Taft and El Capitan biotite granites, which are older ... a few tongues of granodiorite extend into the granite, which had been solidified before the gran- odiorite was injected. Near the longitude of Sentinel Dome and Yosemite Falls, it encloses a swarm of frag- ments of El Capitan granite. The granodiorite is under- cut by the Half Dome quartz monzonite which is slightly more recent and sends flat sheets into the granodiorite. ' ' KA 72 El Capitan Granite Age. 92.2 million years. Geologic Data. "The El Capitan granite is cut by all the adjoining intrusive masses excepting the gran- odiorite of the Gateway and some of the gabbro and diorite that occur near the lower end of the valley. It is therefore clearly among the oldest of the intrusive rocks of the Yosemite region." KA 71 Gateway Granodiorite Age. 92.9 million years. Geologic Data. "Its relation to the El Capitan gran- ite may be observed along the lower part of the Coulter- ville road, where fragments of the granodiorite are enclosed in the granite." KA 67 Arch Rock Granite Age. 95.3 million years. Geologic Data. "The Arch Rock biotite granite is flanked on both sides by the Gateway granodiorite but the age relationships of the two are unknown." Adequate specimens of the other intrusive bodies mapped by Calkins were unavailable to us. Three addi- tional Yosemite rocks have been dated. The mapping of these was done by R. L. Rose. These rocks are the Hoff- man quartz monzonite, the Sentinel pegmatite, and the Hoffman pegmatite, described below. KA 177 Hoffman Quartz Monzonite Age. 83.3 million years. Geologic Data. This pluton intruded the Cathedral Peak granite; nowhere does it come in contact with Johnson granite porphyry, so that the age relationship of the two has not been determined. KA 100 Sentinel Pegmatite Age. 86.4 million years. Geologic Data. This andalusite-bearing pegmatite in- truded hornfels and slates enclosed in Sentinel gran- odiorite. Special Report 54 ^V>>l'\-/f ^^"^-'i^VV^ ■>+ + + + + + + + + + KQii«»V?i\^ni) - l /.•••'.• : '« : v + + + + + + + + + + (^fcv^'r (^ \\j v* : '-f'»M+ + + + + + + + + + + /w-TM>- -k '^V/V„ u Av-v D 3^v 4. + + + + + + + + + + + + + + + + + + + + u m Wi^-j + + + + ,+ + + EXPLAN AT I ON + + + + + + + + + + + + + + + + + + + + + + + + + + ■< + + + + \ + + + V + + + x -c + + + + + + + + + + + + + + +■ + + + + + + + + + + + + + + + + + + + hp % h.V ?'-'J.,W \: the hi:!: + + hd Age Hoffmann pegmatife 769my Hoffmann quartz monzonite 83 3 Johnson gronite porphyry 82 4 Cathedral Peak granite 83.7 Half Dome quortz monzonite 84 l Sentinel pegmatite 864 ;iec v, Age 92.2my ,,j,gw;i und El Ca pi tan granite Gateway granodiorite 92.9 Arch Rock granite 95 3 Metamor phic rocks Undifferentiated complex Unmapped area vi'sV;'-.-: Sentinel granodiorite 884 O Sample location Figure 3. Granitic intrusions, portions of the Sierra Nevada batholith in the vicinity of Yosemite National Park. Age Determination by Potassium-Argon Method 9 KA 137 Hoffman Pegmatite Age. 76.9 million years. Geologic Data. The Hoffman pegmatite is a small body that cuts Hoffman quartz monzonite near its con- tact with Sentinel granodiorite. Tectonic Environment of Group 2 It has been shown that intense deformation was closely associated with the emplacement of Group 1, and it is reasonable to suppose that Group 2 might have been at- tended or preceded by strong deformation. Group 2 was intruded into rocks previously deformed by the Upper Jurassic (Nevadan) orogeny so that it is difficult to dis- tinguish a later period of folding ; however, Chandra (1953), Lyon (1955), and Talbot (personal communica- tion), in structural studies of the metasedimentary rocks of the basement complex of the Sierra Nevada have found two separate axes of folding in the Mariposa formation and have concluded that these rocks have been subjected to two separate periods of deformation. Group 3 KA 93 Point Reyes Granodiorite Age. 83.9 million years. Location. The specimen dated was collected by F. M. Anderson from "a quarry near the old Government Landings. ' ' Geologic Data. Granitic rocks ranging from quartz diorite to granodiorite crop out in the vicinity of the Point Reyes lighthouse and along the northern shores of the Point Reyes peninsula toward Tomales Point at the head of Tomales Bay. A few miles north of Tomales Bay at Bodega Head granitic rock is exposed which is probably an extension of the Point Reyes intrusion. All outcrops of the mass lie west of the San Andreas fault. Immediately east of the San Andreas fault only rocks of the Franciscan formation are to be found. In the vicinity of Drakes Bay the granodiorite is overlain non- conformably by a thin veneer of Eocene ( ?) and Miocene sedimentary rocks which thickens rapidly southward to 8,500 feet near Bolinas. The specimen dated contains 10 to 15 percent of orthoclase. Other specimens from the area contain very little or no orthoclase and are quartz diorite. "Whether there is more than one intrusion in the area has not been ascertained. Like so many of the intrusive rocks west of the San Andreas fault, this one is extensively faulted and brecciated. KA 74 Farallon Quartz Diorite Age. 89.5 million years. Location. The specimen dated was collected by G. D. Hanna from the largest of the Farallon Islands. Geologic Data. Only quartz diorite is exposed on the Farallon Islands. These plutonic outcrops have long been regarded by geologists as the ancient core of a mountain range which once extended along the coast of California and which supplied the sediments for the Franciscan formation and for many of the Cretaceous formations (Lawson, A. C, 1914; Taliaferro, N. L., 1943). KA 92 Montara Quartz Diorite Age. 91.6 million years. Location. The specimen dated was collected by A. C. Lawson from the Pilarcitos dam site approximtaely 20 miles south of San Francisco. Geologic Data. Probably more than one pluton com- poses the intrusive mass generally called "Montara granite," which crops 0U1 over an area of approximately 30 square miles on the San Francisco peninsula a few miles south of San Francisco and west of the San An- dreas fault. It receives its name from Montara Mountain, the bulk of which is composed of this rock. Darrow (1951) states that the Montara granite varies in composi- tion from quartz diorite to granite, the rock cropping out along the coast in the vicinity of Devils Slide being granodiorite. Earlier, Lawson (1914) pointed out the variation in composition of the Montara granite, stating that probably the principal type is biotite quartz diorite. Neither of these geologists mentioned the nature of the contacts between the different petrologic types. The oldest sedimentary rock resting on the Montara is Miocene in age and is clearly nonconformable. All older sedimentary rocks adjacent to the mass are in fault contact with it. KA 94 Santa Lucia Granodiorite Age. 81.6 million years. Location. The specimen dated was collected by A. C. Lawson from a quarry at Carmel Cove on Carmel Bay. Geologic Data. In the vicinity of Carmel Bay the principal type of granitic rock exposed is coarsely porphyritic granodiorite or quartz monzonite in which large phenocrysts of orthoclase are set in a medium- grained ground-mass of quartz, orthoclase. plagioclase, and biotite (Lawson, A. C, 1893-96). According to Trask (1926), this porphyritic type grades southward into nonporphyritic orthoclase-deficient biotite-hornblende quartz diorite, which constitutes the bulk of the granitic rocks exposed in the Santa Lucia Mountains. The Santa Lucia granitic rocks intrude a variety of schists, quartzites, gneisses, and crystalline limestone of unknown age collectively called the Sur series by Trask (1926, p. 127). The oldest unmetamorphosed sedimentary rocks resting nonconformably on the Santa Lucia com- plex are of late Upper Cretaceous age (Taliaferro, N. L., 1944, p. 509). The Franciscan formation is in fault con- tact with the complex along the southwest margin of the complex. KA 184 Gabilan Mesa Quartz Diorite Age. 83.8 million years. Location. The specimen dated was collected in sec. 29, T 17 S R 7 E., MD. ; on the road to the Pinnacles Na- tional Monument 3.2 miles east of its junction with the Metz road. Geologic Data, The Gabilan Mesa lies east of Salinas Valley. It is a broad, low, rolling mesa covering an area of approximated 250 square miles; it has been cut in granitic rock predominantly quartz diorite in composi- tion On the east the mass is bounded for 20 miles by the San Andreas fault, beyond which are sedimentary rocks ranging from Upper Jurassic to Pliocene in age. To the west it is bounded by the Salinas Valley. Toward the northern limits of the quartz diorite numerous small roof-pendants of crystalline schist are presenl which are probablv equivalent to the schists of the Sur series m the Santa Lucia Mountains to the west. The oldest sedimen- tary rock resting in depositional contact on the flanks ot the Gabilan Mesa is Miocene in age. 10 Special Report 54 KA 185 Santa Margarita Granodiorite Age. 84.1 million years. Location. The specimen dated was collected from an outcrop in the bottom of a creek a quarter of a mile north of the road to McKittrick 3.6 miles east of Santa Margarita (sec. 10, T. 29 S., R. 13 E., MD.). Geologic Data. The sample of the Santa Margarita intrusion dated is the most potash-rich of Group 3. Al- though it lias been called granodiorite, it contains slightly less orthoclase than plagioclase. Approximately 100 square miles of intrusive rocks are exposed east of Santa Margarita, and it is probable that more than one lilhologic type is present. Where sampled, the rock was highly fractured. The oldest unmetamorphosed sedimentary rocks lying nonconformably on the mass are Upper Cretaceous in age. Tectonic Environment of Group 3 None of the plutons of Group 3 have been found to intrude rock younger than the strongly deformed and highly metamorphosed Sur series whose age is now known to be pre-Lower Cretaceous. Within the central Coast Ranges in the southern part of the Santa Lucia Mountains, however, Taliaferro (1944) found evidence for two disturbances occurring about mid-Cretaceous time. The first of these, which is marked by an uncon- formity between the Marmolejo formation of very early Cretaceous age and the Jack Creek formation of early Upper Cretaceous age, was not so widespread or intense as the second disturbance which occurred between early Upper Cretaceous (post-Jack Creek formation) and late Upper Cretaceous (pre- Asuncion formation). To this latter orogeny Taliaferro (1944, p. 484) has given the name Santa Lucian. "In the Santa Lucia Range the evidence is clear and convincing; it was the strongest orogeny to affect this region between the deformation of the Sur series and the late Pliocene orogeny that so profoundly affected practically all of the Coast Ranges. ' ' DISCUSSION OF RESULTS The definition of two Mesozoic orogenies in California immediately raises the problem of the nomenclature to be used in defining them. It is unfortunate that the Nevadan orogeny, first identified in and named for the Sierra Nevada, should have been assigned a Jurassic age on the basis of evidence found elsewhere, for it is now known that the bulk of the granite in the Sierra was em- placed in the Upper Cretaceous. Further, the effects of the Upper Jurassic orogeny have all but been erased from the Sierra Nevada by the Upper Cretaceous intru- sions. One solution would be to drop the term "Nevadan orogeny" entirely, but its firm entrenchment in the Literature is too great; to which of the two Mesozoic orogenies should the term be applied? It would probably be less desirable to change the significance of the term than to abandon it, so there is little choice but to retain 'Nevadan orogeny" for events of the Upper Jurassic and to sever its connection with the region of its birth. The need for a new name for the major events of the Upper Cretaceous then arises. Again, the proper name is not obvious, bul we suggest "Santa Lucian orogeny." The term "Laramide" must be reserved for events of the late Cretaceous and Eocene. Witt i.-h and Bose (1913) European divisions of the Cretaceous and Jurassic systems. Danian Maestrichtian (Campanian Upper Cretaceous Senoian . A Santonian Turonian (Coniacian Cenomanian Albian Aptian (Barremian Lower Cretaceous Neocomian. - _ J Hauterivian | Valanginian [Berriasian Upper Jurassic-. Tithonian Kimmeridgian \Portlandian Oxfordian Callovian did not introduce a term to define the Upper Cretaceou orogeny for which they first discerned evidence in Lowe California; Larsen correlated it with the Nevadan ore geny at a time when he had no idea of the existence o two epochs of deformation and intrusion. Taliaferro ha applied the name "Santa Lucian orogeny" to a stron orogeny affecting lower Upper Cretaceous and olde formations of the Santa Lucia Mountains. The undesira ble aspects of applying this name to the Upper Creta ceous orogeny in the Sierra are that it suggests n« obvious geographic relationships to the Sierran granite of the area on the basis of field evidence, and it i defined in a region that has had a very complicated tec tonic history since emplacement of the granites. How! ever, rather than introduce a new term which would b a synonym for one already extant, we have adoptee "Santa Lucian orogeny" as the phrase connoting Uppe Cretaceous orogenesis along the Pacific Coast. Group 1. The data presented, if accepted as accurate lead to the following conclusions : a. The emplacement of the intrusions of Group spanned at least 12 million years (Rocklin granodioriti to Horseshoe Bar quartz diorite) beginning later thai early Kimmeridgian time and terminating before th( Lower Cretaceous. This orogeny was the first of two dis^ tinct Mesozoic orogenies to affect the Sierra Nevada area b. The Oxfordian-Kimmeridgian boundary as now de fined in California is older than 143 million years (Gua- dalupe Mountain quartz monzonite and Horseshoe Bai quartz diorite). c. The basal Cretaceous (Valanginian stage) as now defined in California is younger than 133 million years (Valanginian sediments lying upon Shasta Bally batho lith. d. The time line of 133 million years is either oldei than Tithonian sediments aggregating 16,000 feet inj thickness in California (Knoxville formation) or falls; somewhere within the time interval during which these 1 sediments were deposited. Absolute age considerations suggest the latter conclusion. Some of these conclusions appear to be at variance with widely held geologic beliefs concerning the geology of California and with the Holmes time-scale. In regard to (a), on the basis of stratigraphic evi- dence, both the Nevadan and the Santa Lucian orogenies were completed within one to three geologic stages Age Determination by Potassium-Argon Method 11 (Nevadan between lower Kimmeridgian and Valangi- nian, and Santa Lucian between lower Upper Cretaceous and upper Upper Cretaceous) or, in terms of absolute time, within 15 to 20 million years. It has been pointed out that folding largely preceded granitic emplacement during the Nevadan orogeny and that it is likely that it preceded granitic emplacement during the Santa Lil- ian orogeny also (two periods of folding in Mariposa formation). From the duration of intrusive activity in both of these orogenies, it can be inferred that the strong leformational phase was confined to relatively brief periods at their beginnings. Conclusions (b) and (c) above are in marked disagree- ment with the Holmes time-scales (Holmes, 1947). The Oxfordian and Kimmeridgian stages, as defined, are aotli in the Upper Jurassic, only the Portlandian and Purbeckian (elsewhere collectively called Tithonian) be- ing younger, while seven stages of the Jurassic are )lder. According to Holmes 'A time-scale, with the Ju- rassic extending from 140 to 167 million years, the Shasta Bally batholith (133 million years) should have >een emplaced fairly late in the Lower Cretaceous. This s incompatible with the field evidence, as early Lower Cretaceous sediments rest unconformably upon the bath- >lith. On the B time-scale (Jurassic from 127 to 152 nillion years), a rock 143 million years old should have )een formed in the Lower Jurassic ; whereas our results nclicate that 143-million-year-old rock is Upper Jurassic. Several explanations may resolve this problem : (i) The Mesozoic of the Holmes time scales is in need of modification. (ii) The Oxfordian-Kimmeridgian boundaiw is at the temporal middle of the Jurassic, as the paleonto- logical stages of D 'Orbigny are unequal in dura- tion. (iii) Rocks in California and Europe that have been correlated faunally are not the same age by ten or more millions of years. (iv) The age data are unreliable. An increasing mass data suggests that the age data re reliable, although it must be admitted that individual lates may be in error (see Evernden et al., 1957). Of he other three possibilities, (i) is almost certainly true, rhile resort to (ii) and (iii) becomes unnecessary if the problem is resolved in terms of (i). Holmes assigned a thickness of 22,000 feet of rock to be Jurassic in proportioning the time between the two ie points, St. Joachimstal pitchblende of late Carbon- ferous or Lower Permian age and Laramide pitchblende f Late Cretaceous or end-of-Paleocene age. This is robably less than half of the Jurassic thickness that hould have been used. In California, there are over 0,000 feet of Middle and Upper Jurassic, 16,000 feet of fhich belong to the Tithonian stage alone (Anderson, 945). If one sums the maximum figures given by Arkell Arkell, 1956) for the stages of the Jurassic, a figure f over 60,000 feet is obtained. This figure may be ex- essive due to its composite nature, but 40,000 feet is robably a conservative estimate of maximum thickness f the Jurassic throughout the world. We shall use this gure for illustrating the gross changes it causes in the ime-scale. We will use the following data: Top of Cretaceous— — 58 million years (Laramide pitchblende at top of Paleocene) Cretaceous thickness _ __71,000 feel (measured in Califor- nia) Jurassic thickness I0,000feet (conservative esti- mate ) Triassie thickness _ ,__25,000 feel (Holmes) Permian thickness . —18,000 feet (Holmes and Lower Permian age for St. Joachimstal pitch- blende) Lower Permian 220 million years Computations based upon these figures lead to the follow- ing time-scale : Top of Cretaceous 58 million years ago Cretaceous 75 million years Top of Jurassic 133 million years ago Jurassic 42 million years Top of Triassie 175 million years ago Triassie 26 million years Top of Permian 201 million years ago This scale is not entirely satisfactory, but it does illus- trate that a time-scale based upon a realistic estimate of Jurassic thicknesses is in much closer agreement with our results than those published by Holmes. Thus, it is our belief that there is no essential inconsistency in our results, but that they indicate the necessity of a revision in the generally held estimates of the length of the Jurassic. The relative age of the Shasta Bally batholith and the unmetamorphosed Knoxville sediments to the southwest is not obvious. If the granite is older than any of the Knoxville sediments, as field evidence suggests (Talia- ferro, 1943), the following schedule seems unavoidable: Millions of years Age of Shasta Bally batholith 133 Time to produce unconformity 8-12 Time to deposit 10,000 feet of fine-grained sediments__ 10-15 Time to produce unconformity between Knoxville and Lower Cretaceous sediments (Berriasian stage?) 2-5 Indicated age of the base of Valanginian sediments 101-113 This indicated age of Valanginian sediments is almost certainly in error. The age of the base of the Cretaceous has not yet been established by a physical method, but dates have been obtained in our laboratory for : Millions of years Paleocene-Cretaceous (Glauconite. California) 60 Maestrictian (Glauconite, Gulf Coast) 72 Lower Cenomanian (Glauconite, England) 95 Upper Albian (Glauconite, England) 101 Upper Albian (Feldspar, Crowsnest volcanics) 96 Prom these data coupled with maximum known thick- nesses of Lower Cretaceous (38,000 feet) and Upper Cretaceous (33,000 feet), it can be inferred that the base of the Valanginian is older than 125 million years. Age data, therefore, indicate that the Shasta Bally batholith is not older than all of the Knoxville sediments, the alternative being that the batholith was emplaced at some time during the period of deposition of the Knox- ville. If a magmatic origin for the Shasta Bally batho- lith is held, and the evidence favors it ( Kinkol, et al., 1956), this alternative explanation carries with it a num- ber of implications. The intrusion must have been em- placed slowly and with only local disturbance of the country rocks, for sediments being deposited within 10- 12 Special Report 54 Table 1 Number KA 81 KA 98 KA 102 KA 103 KA 97 KA 120 KA 137 KA 177 KA 133 KA 135 KA 73 KA 100 KA 68 KA 72 KA 71 KA 67 KA 93 KA 74 KA 92 KA 94 KA 184 KA 185 KA 78 KA204 KA 199 Name Weight (grams) %K Radiogenic A 40 (moles) Atmospheric A 4 (moles) AgeJ (10 6 yr.) Group 1 Shasta Bally quartz diorite Belden granodiorite Rocklin granodiorite (muscovite) Rocklin granodiorite (biotite) Horseshoe Bar quartz diorite Guadalupe Mt. quartz monzonite Group 2 Hoffman pegmatite Hoffman quartz monzonite Johnson granite porphyry Cathedral Peak granite Half Dome quartz monzonite Sentinel pegmatite Sentinel granodiorite El Capital! granite Gateway granodiorite Arch Kock granite Group 3 Point Reyes granodiorite Farallon quartz diorite Montara quartz diorite Santa Lucia granodiorite Gabilan Mesa quartz diorite Santa Margarita granodiorite Detrital biotite from Cretaceous sedimentary rocks Upper Senomanian sedimentary rocks _. Turonian (?) sedimentary rocks Valanginian sedimentary rocks 2.304 1.353 1.403 1.794 1.472 1.218 0.660 1.093 0.504 0.547 0.562 0.988 1.917 1.633 1.334 0.976 1.380 1.876 1.944 2.518 1.660 0.842 1.429 1.736 0.999 6.25 6.17 7.77 6.29 7.04 6.96 7.06 6.81 5.89 5.59 6.41 8.13 6.61 7.59 7.55 7.09 6.63 5.82 5.32 7.40 6.61 5.42 2.84 2.16 1.45 33.80X10- 19.70 24.96 26.02 26.01 21.33 6.20 10.74 4.23 4.44 5.25 12.17 19.43 19.83 16.25 11.46 13.21 17.12 16.56 26.34 15.94 6.66 6.37 6.94 3.69±£ 2.72X10- 3.88 12.09 3.47 1.74 2.60 3.56 9.39 2.91 7.64 3.19 3.16 1.91 4.02 1.51 1.65 5.79 3.35 31.39 33.34 1.22 3.80 11.01 5.57 2. 95 ±8% 134.4 135.7 130.6 131.5 142.9 142.9 76.9 83.3 82.4 83.7 84.1 86.4 88.4 92.2 92.9 95.3 83.9 89.5 91.6 81.6 83.8 84.1 90.9 106 150 ±8% t Ages were computed on the basis of X = 0.557 X 10~>° yr" 1 andX„ = 0.472 X 10" 9 yr 1 . 25 miles were predominantly fine-grained, and show no signs of concurrent folding. Regional warping was of the order of 5 miles of relative vertical movement within a distance of 30 miles horizontally. Rather than a slow accumulation of sediments derived from a worn down Landmass (Taliaferro, 1942, p. 87), the Knoxville forma- tion is a rapid accumulation of sediments synchronous with a late stage of the Nevadan orogeny, a stage during which the Klamath Mountains and the present foothill areas of the Sierra Nevada were being intruded by granite and were rising at a pace commensurate with their erosion and the filling of the adjacent trough. Al- though these sediments are predominantly fine-grained, there are 5,400 feet of sands and conglomerates in the central part of the formation (Anderson, 1945, p. 927). The generally fine-grained nature of the Knoxville sedi- ments can as well be ascribed to the lithology of the landmass from which the sediments were derived as to its topography; before metamorphism and intrusion, both the Klamath Mountains and the Sierra Nevada were composed of predominantly fine-grained sedimentary rocks. The si rati graphic evidence presented by Anderson suggests a northeastward onlap of the lower portion of the Knoxville (no information is available on conditions in the upper Knoxville sediments) while Murphy (Murphy, 1956) indicates similar conditions in the Lower Cretaceous sediments that overlap the batholith. ly, therefore, consider a situation in which the li was rising relative to sea level while the mean topographic elevation of the area was continuously de- creasing. The implication may be that marine planation with associated subsidence of the depositional area is a dominant factor in the rapid erosion of rising massifs. The data of Group 2 show that granite is emplaced and rises to sites of erosion concurrently with sedi- mentation in another portion of the same structural province. The only question in the Shasta Bally batho- lith area is the matter of scale. Group 2. The intrusions of Group 2, spanning period of 18 million years, are separated from the plu- tons of Group 1 by the Mother Lode zone of faulting. According to the Holmes B time-scale and to the glau- conite and volcanic dates mentioned above, this group was emplaced in lower Upper Cretaceous time. The agreement between field and experimental deter- minations of relative age of the various plutons is re- markable. It can be seen that the average interval of time between intrusions is approximately 2 million years. Field evidence indicates that most of these intrusions were almost completely crystallized at the time succeed- ing intrusions were emplaced. Except for the Hoffman quartz monzonite, which was so viscous it sheared along its margins during emplacement, most exhibit evidence! of having been mobile at the time of emplacement. Thus, crystallization required something less than 2 million; years in these masses, a figure which is in good agree-: ment with Larsen's (1945) estimate of 1 million years for crystallization to a depth of 6 kilometers in a batho- lith of this type. Age Determination by Potassium-Argon Method 13 The depth at which crystallization occurred is not known. It may be assumed that the Sierra was not static during these 18 million years of intrusive activity and that, because of uplift and erosion during this time, the older samples were more deeply buried at the time of crystallization than were the younger. Supporting this hypothesis is the fact that undeformed Upper Creta- ceous (Campanian) sediments along the western flank of the Sierra rest on the unconformity developed over these plutonic and related metamorphic rocks. The rela- tively short interval of time separating the last of the granitic intrusions from the encroaching Cretaceous sea suggests that uplift, erosion and deposition to the wesl kept close pace with granitic emplacement. It is prema- ture to say that this seemingly delicate balance of processes is not fortuitous. However, it is a fact that the Sierra remained a comparatively stable landmass throughout the remainder of the Cretaceous and all of Tertiary time, and did not begin to rise appreciably again until the Pleistocene. If the rate of uplift were a function of the rate of formation and intrusion of granite, the solution to at least one of the problems of granite emplacement, the room problem, would be at hand. It is suggested that, in view of the structural evidence which abounds in the granitic rocks of the Sierra (Cloos, 1936; Mayo, 1941; and Rose, 1957) and in view of the protracted time of emplacement of the batholith, room for this great mass was made slowly and in small increments by vertical uplift of the overlying sedimentary rocks which were stripped by erosion as rapidly as they rose. Possibly, some of the earliest granitic intrusions were at the sur- face by the time the last intrusion squeezed into place. Preliminary work on detrital biotite in the Upper Cre- taceous sediments of the west side of the San Joaquin Valley indicates that, in Turonian time, either metamor- phic or intrusive debris of the Upper Cretaceous orogeny to the east was already being poured westward (see dis- cussion of Group 3 ) . Group 3. Although there is no direct proof that the Group 3 plutons were intruded at the time of either of the lower Upper Cretaceous orogenies detected by Taliaferro in the Santa Lucia Mountains, the post-Al- bian age for the intrusions and for the disturbances makes it appear likely that the two events were closely related. When the nature of the sediments laid down after the Santa Lucian orogeny is taken into account, the correlation becomes almost certain. In view of the absence of deformed sedimentary rocks younger than Upper Jurassic in the Sierra Nevada and the indisputable geologic evidence that some of the Sierra Nevada-Klamath Mountain intrusions are late Jurassic in age, it is not surprising that the Upper Cre- taceous age of the great bulk of the Sierra Nevada bath- olith should have remained unrecognized until Larson et al. (1954) applied the lead-alpha method to Sierran intrusions. The Coast Ranges intrusions, however, are in the midst of fossiliferous Jurassic and Cretaceous sedimentary rocks and, to one unacquainted with the geology of the Coast Ranges, it must seem incredible that the Upper Cretaceous age of the plutons should have gone unrecognized for so long. The explanation for this is simple and two-fold. All of the contacts between these plutons and older Jurassic and Cretaceous formations are faults (Taliaferro, 1942, p. 85), and only in the highly metamorphosed Sur series may the intrusive re- lations be observed. The Sur series, too, is in fault con- tact with all formations older than high Upper Creta- ceous. Coupled with these spatial relationships is the fact that in the western Coast Ranges, in the vicinity of the Santa Lucia Mountains, conglomerates, in sedimentary rocks identified as Franciscan, contain abundant frag- ments of marble, schist, and plutonic types which Talia- ferro (1942, p. 85) believes were derived from the Sur series and associated intrusions. As long as there was reason to believe that the Coast Ranges intrusions were older than the Franciscan for- mation, no involved explanation was needed for the ex- tensive system of fault boundaries surrounding the plu- tons and the rocks they were thought to intrude : fault- ing of normal and thrust types had simply exhumed the ancient crystalline basement underlying the Franciscan and younger formations. Now that it is established that these intrusions are Upper Cretaceous in age, it becomes necessary to explain why they nowhere intrude the Up- per Jurassic and Lower Cretaceous formations in their immediate vicinity. The Gabilan pluton is bounded on the east by the San Andreas fault for a distance of 100 miles or more but it does not cross the fault at any point nor does the Franciscan formation show any signs of metamorphism adjacent to it across the fault. The Lower Cretaceous Marmolejo formation is nowhere known to be intruded by granitic rocks nor does it rest on them or the Sur series, it being found only in depositional contact on Franciscan-Knoxville rocks. In addition, the lithologic composition of the Upper Cretaceous sedi- ments on either side of the San Andreas fault is dis- tinctly different. The average of 19 mineral grain analy- ses of sandstones from the Asuncion formation in the Adelaida quadrangle (west of fault) yields the follow- ing proportion of constituents (Taliaferro, 1944, p. 492) : Percent Quartz GO Orthoela.se 8 Plagioclase 20 Mica 2 Rock fragments 10 The average of 9 mineral analyses of sandstones from the Upper Cretaceous Panoche sandstones of the Diablo Range (east of the fault) yields the following propor- tion of constituents (Briggs, 1953, p. 426) : Percent Quartz 28 Feldspar -38 Biotite 5 Hornblende 2 Rock fragments 10 .Matrix (grains < 0.053 mm)__ 17 Both of these series of sediments were obviously derived from crystalline masses but, just as obviously, they were derived from masses of markedly different character. The Asuncion sands are reasonably similar in composi- tion to the quartz diorites, quartz mica schists, and quartzites composing the bedrock complex west of the San Andreas fault, but the low quartz content of the Panoche sands su^-ests a source dominated by granodi- orite. The sample descriptions given earlier indicate the contrast in rock type of the Coast Ranges igneous rocks and those of the high Sierra Nevada. Available modal 14 Special Report 54 analyses and mineralogic descriptions of the plutonic ilifornia show that the Coast Ranges granitic ks have a dose affinity with those of the Southern Eornia batholith, both being predominantly tonalites or quartz diorites, while the rocks of the high Sierra Nevada arc largely granodiorites. The similarity of the (nasi Ranges and Southern California batholiths ex- tends to the metamorphic rocks as well as the igneous. I E one compares the descriptions of the Paleozoic quartz- ites, quartz-mica schists, and marbles in the Elsinore area i Larsen, 1948) with those of the Sur series of the Santa Lucia .Mountains (Reiche, 1937), one is impressed by their similarity, even to the presence in both areas of numerous basic sills and dikes now converted to am- phibolite. Finally, the profound unconformity associated with the Santa Lucian orogeny in the Santa Lucias is prac- tically nonexistent east of the San Andreas fault, even though the base of the thick Cretaceous section (in \\ 'art han Canyon ) is within 8 miles of the fault. In the northern Sacramento Valley, there is no major uncon- formity from the Valanginian to the Campanian (Kirby, 1943), though a marked change from siltstone to con- glomerate and coarse sandstone at the base of the Upper Cretaceous Venado formation indicates strong uplift nearby. Southward in the Diablo Range along the west side of the San Joaquin Valley, a slight unconformity becomes recognizable at the base of the thick (33,000- foot) Upper Cretaceous deposits in the vicinity of Ortig- alita Peak. The unconformity disappears again south- ward and is unrecognizable in Warthan Canyon. South of "Warthan Canyon, the unconformity becomes recog- nizable again and, as the southern end of the Sierra Nevada batholith is approached, it becomes profound. It seems incongruous that uninterrupted deep subsi- dence and sedimentation throughout the Cretaceous along almost the entire western flank of the northern San Joaquin Valley should have taken place while a st rong orogeny was occurring less than 10 miles to the west. The juxtaposition of Cretaceous sediments of different origin and the contrasting sedimentary histories on either side of the San Andreas fault, the presence of Upper Cretaceous granites in a complex of Jurassic and Lower Cretaceous sediments with not one intrusive re- lationship but only fault contacts, and the similarity of the Coast Range plutonic rocks and the Southern Cali- fornia batholith strongly suggest that there has been gross displacement of rock masses. The apparently anomalous situation is easily resolved if there has been Large strike-slip displacement along the San Andreas fault, for then the sedimentary and structural events recorded in the rocks on either side of the fault, though correlative in time, cannot be interpreted in terms of their present relative position. Owing to the change in strike of the San Andreas fault from the Salton Sea to the Diablo Range, the entire block west of the fault, if moved relatively southward about 350 miles, falls di- rectly in line with the trend of the Southern California batholith. This batholith was determined to be of Upper Cretaceous age as Long ago as 1913 by Wittich and Bose, who found evidence that the granitic rocks of Lower Eornia intrude sedimentary rocks of early Upper Cretaceous age and are nonconformably overlapped by rocks of late Upper Cretaceous age. The contemporane- ity of the Southern California batholith and the hig Sierra Nevada plutons has since been established 1 Larsen et al. (1954) by the lead-alpha method. This solution to some of the major problems of wes era California geology has long been advocated by son geologists and vehemently opposed by others. Dibblee ar Hill (1953) reviewed the question of extensive strik slip movement of the San Andreas fault and conclude that the amount since Jurassic time is 300 miles or mor We believe that this much movement or more has take place in the past 80 million years. It would appear tha during this same time, other faults have also had larg strike-slip displacements along them. The indicate mean rate of displacement along the San Andreas fau during the last 80 million years is 0.023 feet per yea in contrast with the present rate of 0.2 feet per year. The alternative interpretation of the tectonic histor of the central Coast Ranges supposes (a) little or n strike-slip movement on the San Andreas fault befor the Pleistocene; (b) homogeneity of structure acros the fault; (c) the existence of an ancient (pre-Jurassic crystalline landmass to the west of the San Andrea fault with the presently outcropping igneous rocks bein portions of that mass; and (d) the derivation of mos of the Jurassic and Cretaceous deposits of the Coas Ranges, both east and west of the fault, from that am cient complex. Evidence contrary to (a), (b), and (c) has alread; been presented. Evidence will now be given to sho'v that, with respect to (d) ; (1) the Upper Cretaceou sediments east of the fault were largely derived from plutonic mass (and associated metamorphic rocks) o Upper Jurassic to Upper Cretaceous age, the Uppe Cretaceous predominating in the sediments of Turoniai age and younger; (2) east of the fault there is no com pelling evidence that the debris of the Cretaceous sedi ments must have come from the west ; all unequivoca evidence is in agreement with derivation of this materia from an eastern source. In fact, the Upper Cretaceous sediments east of the fault cannot have been derivec from the crystalline rocks now west of it. Proof of (1) above is based on the ages of detrita biotite collected from the Lower to Upper Cretaceous sediments in the Diablo Range immediately east of the San Anelreas fault. The elates obtained are presumed to give the mean age of the source rocks at the time the sediments were deposited. As the results show (table 1) in the Lower Cretaceous the mean age of the rocks contributing elebris was Jurassic, and this detritus was almost certainly coming from crystalline rocks formed during the Nevadan orogeny ; during the Turonian. there must have been erosion of some materials formed during the Santa Lucian orogeny, while the Panoche sediments (upper Senonian) are composed largely of Santa Lucian elebris. Concerning (2), it seems desirable to review the evi dence and criteria that have been presented to support the case of western origin of the Cretaceous sediments To begin with, there is excellent evidence of a western source for much of the Asuncion elebris (Taliaferro, 1944, p. 487). Owing to its more shaley character in its eastern exposure and its greater abundance of conglomerates westward, Taliaferro believes that the sediments were de rived from a landmass to the west. No such positive evi- dence exists in the Cretaceous east of the fault. Briggs Age Determination by Potassium-Argon Method If, 1953a, 1953b), studying the geology of a portion of the dablo Range along the west side of the San Joaquin alley, supports Taliaferro's views for a western origin I most of the debris in the Upper Cretaceous Panoehe )rmation of that area. He states that there is a close etrographic relationship between the Panoehe and the .suncion sediments. However, as has been noted already, comparison of mineral grain analyses of the two feaations show striking differences, the bulk composi- on of the Asuncion being that of quartz diorite while lat of the sands in the Panoehe formation is closer to ranodiorite. The extreme contrast in the quartz/feld- oar ratio between the two series is to be noted, being pproximately 2/1 in the Asuncion and 28/37 in le Panoehe. These ratios remain essentially constant iroughout the formations. The concept of a western source must include the mdition that during the Upper Cretaceous a portion f the hypothetical landmass was composed of Fran- iscan-Knoxville sediments. However, glaucophane has ot been detected in either of Briggs' or Taliaferro's nalyses, nor in the Panoehe formation of Corral Hollow ear Livermore (Huey, 1948) nor in the Upper Creta- 30us beds in the San Benito quadrangle (Wilson, 1943) fig. 4). Taliaferro explains the absence of glaucophane com the Asuncion sand in the Adelaida quadrangle as eing due to the location of all of the samples to the )uth of the main outcrops of glaucophane-bearing mists. This may be true, although there are outcrops of laucophane schist in the San Simeon quadrangle due 'est of the sampled area, and, as this is believed to have een a high area (Taliaferro, 1943, p. 494), it is surpris- ig that none of the glaucophane appears in any of the imples. Daviess, in a detailed study of the heavy min- rals in three oil wells along the west side of the San Joa- uin Valley (Daviess, 1946), found only trace amounts P glaucophane in 7 of 51 samples of Upper Cretaceous ■ids. As glaucophane appears in minor amounts in letamorphic rocks adjacent to serpentine intrusions in he foothill belt of the Sierra Nevada (Taliaferro, per- Dnal communication), trace amounts could have come rom there. Not only is the easily recognized mineral laucophane all but missing from the Upper Cretaceous edimentary rock in all of these areas but, as Huey oints out (Huey, 1948), there is a general absence in tie Diablo Range of the rock types in the conglomerates hat are found in the Franeiscian formation and the iur Series. In view of the preponderance of metavolcanic types i many of the interbedded conglomerates in the Upper Iretaceous deposits, Briggs (1953, p. 434) has suggested hat the source was principally a volcanic terrain in diich granitic rocks were also present. It is probable, owever, that these pebbles, cobbles, and boulders of oleanic type, because of their fine grain, great hardness, nd extreme resistance to erosion, are present in an bundance out of all proportion to their relative amount a the terrain from which they were derived. Many of hese cobbles are undoubtedly second cycle materials. Briggs found staurolite in some heavy mineral sepa- ates and in some thin sections of the sands in the 'anoche formation to a stratigraphic depth of 11,500 eet. He believes that it originated in the Sierra Nevada, ilutton (1953) found no trace of staurolite in modern »eaeh sands between San Francisco and Pacific Grove, FIGURE 4. Map showing San Andreas fault in the central Coast Ranges. which tends to confirm Briggs' conclusion. The supposed absence of staurolite from lower in the section, however, hardly justifies Briggs' deduction that its first appear- ance at this depth represents the time at which the Sierra Nevada batholith was unroofed. Staurolite is not abundant in any of his samples and thin sections, appear- ing in only 3 out of 14 thin sections and 3 out of 4 heavy mineral separates above 11,500 feet. He failed to find staurolite in 21 thin sections and 3 heavy mineral sepa- rates below that depth. However, in a heavy mineral separate from sandstone near the base of the Upper Cretaceous in Warthan Canyon, we found abundant staurolite. Briggs asserts that observable coarsening of conglom- erates indicates a western source. He presents no evi- dence on this point, but simply quotes Huey (op. cit.) who says that one conglomerate exposed on the Alta- mont anticline shows westward coarsening. However, Snow (1956) discredits this observation of Huey and asserts that there is no discernible evidence of such westward coarsening. Briggs describes slump structures that seem to indi- cate movement of beds downslope from west to east. In describing this same phenomenon, Snow (op. cit.) points out that while megastructures may indicate a movement from west to east, microstructures in the same place may indicate movement in other directions. In one instance this was from northeast to southwest. It is evident that the case for western origin of the Panoehe debris is weak, whereas none of the unequivocal data obtainable are in disagreement with an eastern source. Any theory of a western landmass must, at least, be altered in light of the absolute ages obtained for both the plutons and the detrital biotite of the Creta- ceous sedimentary rock. Hi Special Report 54 According to a strike-slip theory, the post-Nevadan history of the areas intruded by (J roups 2 and 3 may be ,| as follows. In early Upper Cretaceous time, rocks of Hit' Santa Lucia Mountains were the central pari of a single bell which then included the now meta- morphosed rocks of the present high Sierra Nevada, Santa Lucia .Mountains, southern California and Lower California areas. Along' this belt the Upper Cretaceous Santa Lucian orogeny centered. Westward, the effects of the orogeny died out in a broad basin which had been accumulating sediment almost continuously since Upper Jurassic time or earlier. During the latter stages of the orogeny or shortly thereafter, a stress pattern, which we shall not attempt to define, caused northward and northwestward relative movement of the southern portion of this orogenic belt, the line along which movement was taking place being deflected westward around the buttress of the warped sunt hern end of the northern mass. The result of con- tinued movement along this line of fracture has brought the deformed rocks of the Santa Lucia Mountains far west of the original orogenic belt, and even west of the probable center of the Upper Cretaceous sedimentary basin. BIBLIOGRAPHY Anderson, F. M., 1945, Knoxville series in the California Meso- zoic: Geal. Soc. America Bull., vol. 56, pp. 909-1014. Anderson, F. M., 1938, Lower Cretaceous deposits in California and Oregon: Geol. See. America Special Paper 16, 339 pp. Arkell, W. J., 1956, Jurassic geology of the world, Oliver and Boyd Ltd., 806 pp. B8se, 10., and Wittich, E., 1913, Informe relativo a la explora- tion de la region norte de la costa occidental de la Baja California: Parergones del Inst. Geol. Mex. vol. 4, pp. 307-529. Briggs, L. I., 1953a, Geology of the Ortigalita Peak quadrangle, California: California Div. Mines Bull. 167, 61 pp. Briggs, L. I.. 1953b, Upper Cretaceous sandstones of Diablo Range, California: California t'niv. Pub. Geol. Sci. vol. 29, pp. 117 452. Calkins, F. C, 1930, The granitic rocks of the Tosemite region: U. S. Geol. Survey Prof. Paper 160, appendix. Chandra, I). K., 1953, Geology of the Colfax and Forest Hill quadrangles: Ph.D. thesis, Univ. Calif., Dept. Geology, Berkeley, 143 pp. Cloos, E., 1936, Der Sierra Nevada Pluton in Californian : Neues Jahrb. Min. Geol. Palaeont., Beil. Bd. 76, Abt. B., pp. 355-450. Harrow, K. L., 1951, The geology of the northwest part of the Montara Mountain quadrangle: M.A. thesis, Univ. California, Dept. Geol., Berkeley, 51 pp. Daviess, S. X., 1940. Mineralogy of late Upper Cretaceous, Paleo- cene, and Eocene sandstones of Los Banos district, west border of San Joaquin Valley, Calif.: Am. Assoc. Petroleum. Geologists Bull., vol. 30, pp. 63-83. Evernden, .1. P., Curtis, G. II.. and Lipson, J., 1957, Potassium- argon daiin- of igneous rocks: Am. Assoc. Petroleum Geologists Hull., vol. 11, pp. 2120-2127. I I'll. M. I... and Dibblee, T. \\\, Jr., 1953, San Andreas, Garloek and Big Pine faults, California: Geol. Soc. America Bull., vol. 64, til.. It:; 158. Hinds, X. E. A., 1933, Geologic formations of the Redding- Weaverville districts, northern California: California Jour. Mines and I ol. 29, pit. 77 122. Holmes, A.. 1947, The construction of a geologic time scale: Tra is., vol. 21, pt. I, pp. 118-152. Huey. A. S., 1948, Geology of the Tesla quadrangle. Californi; California Div. Mines. Bull. 140, 75 pp. Hutton, C. O., 1952, Accessory mineral studies of some Californ beach sands : U. S. Atomic Energy Comm. RMU-981, 112 pp. Kinkel, A. R„ Hall, W. E., and Albers, J. P., 1956, Geology ai base-metal deposits of West Shasta copper-zinc district, Shaa County, California : U. S. Geol. Surv. Prof. Paper 285, 156 pp. Kirby, J. M., 1943. Upper Cretaceous stratigraphy of west si< of Sacramento Valley south of Willows, Glenn County, Californi: Am. Assoc. Petroleum Geologists Bull., vol. 27, pp. 279-305. Larsen, E. S.. Jr., Gottfried, D„ Jaffe, H., and Waring, C. I 1954, Age of the southern California, Sierra Nevada, and Idal batholiths (abstract) : Geol. Soc. America Bull., vol. 65, p. 12T, Larsen, E. S.. Jr., 194S. Batholith and associated rocks of Ce rona. Elsinore, and San Luis Roy quadrangles, southern California Geol. Soc. America Mem. 29, 182 pp. Larsen. E. S., Jr., 1945, Time required for the crystallization < the great batholith of southern and Lower California : Am. Jou Sci., vol. 243-A, pp. 399-416. Lawson, A. O, 1914, U. S. Geol. Survey Geol. Atlas, San Frai cisco folio (no. 193) 24 pp. Lawson, A. O, 1893, The geology of Carmelo Bay : Univ. Cali Dept. Geol. Bull., vol. 1, pp. 1-70. Lindgren, W., 1894, U. S. Geol. Survey Geol. Atlas, Sacrament folio (no. 5) , 3 pp. Lipson, J., 1958, Age dating of sedimentary rocks: Geol. Sn America Bull., vol. 69, pp. 137-150. Lyon, R. J. P., 1954, Studies in the geology of the wester Sierra Nevada: Ph.D. thesis, Univ. Calif., Dept. Geology, Berki ley, 186 pp. Mayo, E. G., 1941, Deformation in the interval Mt. Lyell-M Whitney, California : Geol. Soc. America Bull., vol. 52, pp. 1001 1084. Murphy, M. A., 1956, Lower Cretaceous stratigraphic units 0} northern California : Am. Assoc. Petroleum Geologists Bull., vo 40, pp. 2008-2119. Rankama, K., 1954, Isotope geology, Pergamon Press, 535 pp. Reed, Ralph D., 1943, California's record in the geologic histor of the world : California Div. Mines Bull. 118, p. 107. Reiche, P., 1937. Geology of the Lucia quadrangle : Univ. Calil Pub. Dept. Geol. Sci., vol. 24, pp. 137-144. Reynolds, J. H., 1957, Comparative study of argon content ano argon diffusion in mica and feldspar: Geochim. et Cosmochim. Act; vol. 21, pp. 177-184. Rose, R. L., 1957, Geology of the May Lake area, Yosemite Na tional Park : Ph.D. thesis. Dept. Geol. Univ. Calif. Berkeley. Snow, D. T., 1957, The geology of the north east corner o Alameda County and adjacent portions of Contra Costa County M.A. thesis, Univ. Calif. Dept. Geol., Berkeley, 187 pp. Taliaferro, N. L., 1944, Cretaceous and Paleocene of Santa Lueii Range, California : Am. Assoc. Petroleum Geologists Bull., vol 28, pp. 449-521. Taliaferro, N. L., 1943, The Franciscan-Knoxville problem California : Am. Assoc. Petroleum Geologists Bull., vol. 27, ppi 109-219. Taliaferro, N. L., 1942, Geologic history and correlation of th< Jurassic of southwestern Oregon and California: Geol. Soc. Amer ica Bull., vol. 53, pp. 71-112. Trask, P. D., 1926, Geology of the Point Sur quadrangle. CalM fornia: Univ. Calif. Dept. Geol. Sci. Bull., vol. 16, pp. 119-186 Wetherill, G. W., Wasserburg, G. J., Aldrich, L. T., Tilton, G. R.. and Hayden, R. J., 1956, Decay constants of K 10 as determined by the radiogenic argon content of potassium minerals: Phvsica! Review, vol. 103, pp. 987-89. Wilson, I. F., 1943, Geology of the San Benito quadrangle. California: California Jour. Mines and Geologv, vol. 39, pp. 183- 270. ",-58 3500 printed LIFOK.NIA STATE PRINTING OFFICE -' ?i X i ' ' r . i vs v • '•'. - * j . 1 J • * ". r* .* - - ■ : 5" - * V •V ■J+- • *1 ' * 1 • .. ; *;>- ^ _ r . i \ «• • :- t * 1 > ,. • **? ■;&*<\.3Kk, >■■■ ■ • • . V . ' . ■ f . -