TEI-809 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY GEOLOGY OF THE WILLISTON BASIN, NORTH DAKOTA, MONTANA, AND SOUTH DAKOTA, WITH REFERENCE TO SUBSURFACE DISPOSAL OF RADIOACTIVE WASTES* By Charles A. Sandberg June 1962 Report TEI-809 This report is preliminary and has not been edited for conformity with Geological Survey format and nomenclature. *Prepared on behalf of the U.S. Atomic Energy Commission TEI-809 GEOLOGY OF THE WILLISTON BASIN , NORTH DAKOTA, MONTANA, AND SOUTH DAKOTA, WITH REFERENCE TO SUBSURFACE DISPOSAL OF RADIOACTIVE WASTES By Charles A. Sandberg UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Washington 25, D. C. June 12, 1962 Mr. Wo G. Belter Chief, Environmental and Sanitary Engineering Branch Division of Reactor Development U,So Atomic Energy Commission Washington 25, D. C. Dear Walt: Transmitted herewith are 15 copies of TEI-809, "Geology of the Williston basin, North Dakota, Montana, and South Dakota, with reference to subsurface disposal of radioactive wastes," by Charles A. Sandberg, June 1962, We plan to release this report to the Geological Survey open files. Sincerely yours, H. E. LeGrand y Chief, Radiohydrology Section Water Resources Division Digitized by the Internet Archive in 2019 with funding from University of Illinois Urbana-Champaign Alternates https://archive.org/details/geologyofwillistOOsand USGS - TEI-809 Distribution - AEC No, of copies Division of Reactor Development (W, G, Belter)---—-- 15 Division of Research (D. R, Miller)--—-—•-—---—— 1 Division of Raw Materials (R 0 D, Nininger)---———-— ——- 1 Division of Peaceful Nuclear Explosives (R, Hamburger)-——1 Hanford Operations Office (C. L, Robinson) —--1 Idaho Operations Office (John Horan)-—-—-—*— ——— 1 Oak Ridge Operations Office (H, M, Roth)-— -*--——«—-- 1 Savannah River Operations Office (Karl Herde)---————— 1 Office of Technical Information Extension, Oak Ridge—————— 6 Office of Operations Analysis and Planning, Washington——™—— 1 U,So Naval Radiological Defense Lab,, San Francisco-————- 1 Health Physics Division, Oak Ridge National Laboratory (E, G, Struxness)—.— ---»————-—--— 3 U,S, Bureau of Mines, Bartlesville, Oklahoma (J, W, Watkins)---- 1 Water Resources Division, Billings, Montana (F, A, Swenson)—---- 1 North Dakota Geological Survey (Dr 0 W, M, Laird)——--—-——- 1 South Dakcjta Geological Survey (Dr,-A, F, Agnew)———— 1 John E, Galley, Chairman, Subcommittee on Atomic Waste Disposal, AAPG— ——————————————————— 8 Earth Sciences Division, NAS-NRC (Linn Hoover)------————-—— 10 University of Texas, Austin (E., F 0 Gloyna)--————————— 1 General Electric Company, Richland, Washington (E, R, Irish)—— 2 University of California (W, J, Kaufman)-—— 1 Los Alamos Scientific Laboratory (C, W, Christenson)—1 Eo I, DuPont de Nemours and Company (C, M, Patterson)-——— 1 Lawrence Radiation Laboratory, Technical Information Division (Clovis G, Craig)——————————--—— 1 Waste Disposal Div., Oak Ridge Natl Lab„ (F, L. Parker)--———- 2 64 . M USGS - TEI-809 Distribution - USGS No. of copies Organic Fuels-—--------*-—-- 18 Northern Rocky Mountains----—-——-* 5 Alaskan-————-----—-- — 1 Regional Geophysics—- ——- --•-----—*— 4 Experimental Geochemistry and Mineralogy----—- 1 Theoretical Geophysics---------------- 2 Field Geochemistry and Petrology-----—-------- 2 Foreign Geology—-------———■————-—--——— 1 Library——■— -——»———>————■— -—----——— 3 Special Projects————————-—-—— —--—------ 30 Geologic Division--— --————-——--— - ————- 6 < 2 CONTENTS Page Abstract---——--- 6 Introduction——--- 8 Area of report— -----*- 8 Geologic setting—-—--—---*--- 9 Geographic setting------— 13 Acknowledgements and sources of data---—---- 15 Stratigraphy---------—— --- 18 Precambrian rocks------- 21 Deadwood Formation and related rocks------—- 23 Winnipeg Formation-- -—•— ------ 28 Red River and Stony Mountain Formations--—33 Stonewall and Interlake Formations-----— --—------——- 39 Devonian rocks-----------—--43 Bakken Formation----— ---— ---- 52 Madison Group-— ----r— -------— --—------- 56 Big Snowy Group ----— --— -- 64 Minnelusa Formation and related rocks-*----— 67 Opeche Formation and Minnekahta Limestone----— - 72 Spearfish and Saude Formations--------- 75 Post-Saude Jurassic rocks--- 82 Dakota Group and related rocks----— -- 94 Lower part of Colorado Group------- 100 . 3 Page Stratigraphy--Continued Belle Fourche Shale, Greenhorn Formation, and Carlile Shale- 104 Niobrara Formation and Pierre Shale-------—----- 108 Rocks between the top of Pierre Shale and surface---- 112 Structure-----—-------—-— ------— 116 Folds—————————————-----—— 116 Faults——--—~— -----119 Age of deformation-----—- ---■-- 120 Economic geology------------—- - - - - - -----•--- 122 Oil and gas--—------—-----—————————-------- 123 Lignite—————————————-——--— - - - - - 125 Uranium---—--- ———— — — —----——-- 126 Ground water—————— — — -126 Other resources—------—-—-—------—■ •— -128 Waste disposal possibilities-----—------•— ---—-•—• 128 Deep sandstone and carbonate reservoirs- —------—<—-- 130 Reservoirs in salt beds- ——————---. —134 Shallow shale reservoirs--—-----— -------- 135 Shallow sandstone reservoirs------— ---- 136 Conclusions—-—-----———---- - —---■----—- 138 References cited----- 140 4 ILLUSTRATIONS Page / Figure 1. Index map showing location of Williston basin and major structural features---— 10 2, Generalized geologic map of Williston basin and adjacent areas---— --------—•— 11 3, Oil and gas fields of Williston basin and adjacent areas— -- ———«—•-— 16 4, Structure contour map of Precambrian surface—19 5, Generalized stratigraphic section of sedimentary rocks in Central Williston basin-— In pocket 6 , Correlation chart of pre-Tertiary formations in Williston basin---—-----—--------- In pocket 7, Isopach map of Deadwood Formation and related 8 , Isopach map of Winnipeg Formation----— —--- 30 9, Isopach map of Red River and Stony Mountain Format ions------ 35 10o Isopach map of Stonewall and Interlake Formations'—— 41 11, Isopach map of Devonian System showing limit of salt member of Prairie Formation (Middle Devonian—---------—-—-—~ ™ 45 12, Isopach map of salt member of Prairie Formation— 48 13, Isopach map of Bakken Formation----- — 53 14, Structure contour map of base of Mississippian rocks---—--—■----- 54 15, Isopach map of Madison Group showing limit of salt beds--—-------— ----— —•--- 58 5 Page Figure 16, Aggregate thickness map of seven salt beds in Madison Group-—-———-—--■-«- 63 17* Cross section from southeastern Montana to south-central South Dakota——---■—- In pocket 18, Isopach map of Opeche Formation and Minnekahta Limestone showing limit of salt bed in Opeche Formation—■—— •—————----———*- 73 19, Isopach map of Spearfish and Saude Formations showing limit of Pine Salt-————--—------ 78 20* Isopach map of Pine Salt in Spearfish Formation- 80 21 * Isopach map of post-Saude Jurassic rocks showing limit of Dunham Salt-————-——--—--—-— 84 22 0 Isopach map of Dunham Salt——————————— 87 23* Structure contour map of top of middle member of Piper Formation (Jurassic)—————.—- 88 24* Isopach map of Morrison Formation and Dakota 25* Structure contour map of top of Fuson and Kootenai Formations (Cretaceous)------———-- 99 26 0 Isopach map of Newcastle Sandstone (Lower J Sand of informal usage) and Upper J and D Sands of informal usage---——-----———-—-- 102 27 * Isopach map of Belle Fourche Shale „ Greenhorn Formation 8 and Carlile Shale of Colorado 28* Isopach map of Niobrara Formation and Pierre Shal e109 6 GEOLOGY OF THE WILLISTON BASIN, NORTH DAKOTA, MONTANA, AND SOUTH DAKOTA, WITH REFERENCE TO SUBSURFACE DISPOSAL OF RADIOACTIVE WASTES By Charles A. Sandberg ABSTRACT The southern Williston basin, which underlies about 110,000 square miles in North Dakota, South Dakota, and eastern Montana, is part of a large structural and sedimentary basin. Its surface is a flat to gently rolling plain, standing about 1,500 to 3,500 feet above sea level and locally studded by a few high buttes. The sedi¬ mentary sequence that fills the basin has a maximum thickness of about 16,700 feet and rests on Precambrian metamorphic rocks at depths of 500 to 13,900 feet below sea level. It contains rocks of every geologic system from Cambrian to Quaternary. Rocks of Middle Cambrian through Middle Ordovician age are largely shale and sandstone, as much as 1,200 feet thick; rocks of Late Ordovician through Pennsyl¬ vanian age are largely limestone and dolomite, as much as 7,500 feet thick; and rocks of Permian through Tertiary age are predominantly shale and siltsone, as much as 8,000 feet thick. Pleistocene glacial drift mantles the northern and eastern parts of the area. Rocks of the Williston basin are gently folded and regional dips are 1° or less from the margins to the basin center. Dips on the flanks of the major anticlinal folds, the Nesson and Cedar Creek . 7 anticlines and the Poplar and Bowdoin domes, generally are about 1° to 3° except on the steep west limb of the Cedar Creek anticline. The basin was shaped by Laramide orogeny during latest Cretaceous and early Tertiary time. Most of the present structural features, however, were initiated during the Precambrian and reactivated by several subsequent orogenies, of which the latest was the Laramide. The most important mineral resource of the area is oil, which is produced predominantly from the Paleozoic carbonate sequence and largely on three of the major anticlinal folds, and lignite, which is present near the surface in Paleocene rocks, The subsurface disposal of radioactive wastes at some places in the Williston basin appears to be geographically and geologically feasible* Many sites, at which large quantities of wastes might be injected with minimal danger of contamination of fresh-water aquifers and oil-producing strata, are available* The strata and types of reservoirs that deserve primary consideration for waste disposal are the Winnipeg Formation of Middle Ordovician age as a deep salaquifer, the Permian to Jurassic salt beds as moderately deep units in which solution cavities might be created for storage, the thick Upper Cretaceous shale beds as shallow hydraulically fractured shale reservoirs, and the Newcastle Sandstone of Early Cretaceous age as a shallow shale-enclosed sandstone reservoir. 8 INTRODUCTION This geologic summary of the Williston basin is one of a series of reports on the sedimentary and structural basins of the United States, prepared by the Geological Survey for the Division of Reactor Development, U.S, Atomic Energy Commission. These reports summarize the geology of several major basins and make preliminary generalized evaluations of the possibilities for the subsurface disposal of radioactive wastes. This report evalutes four possible types of subsurface reservoirs for radioactive wastes in the Williston basin: (a) permeable sandstone and carbonate beds at moderate to great depths in the central part of the basin but at or near the surface on the margins, (b) salt beds which are present only at moderate to great depths, (c) thick beds of shale at shallow depths, and (d) permeable sandstone beds at shallow depths. (Shallow is here used for depths less than 5,000 feet; moderate, for depths between 5,000 and 10,000 feet; and great, for depths more than 10,000 feet,) 4 Area of report The Williston basin, one of the largest structural basins in North America, underlies approximately 200,000 square miles in North Dakota, northern and central South Dakota, eastern and north-central Montana, southern and central Saskatchewan, and southwestern Manitoba, This report considers only the southern or United States part of the viomoaoflmi , . . n 9 Williston basin, which constitutes more than half of this large area or about 110,000 square miles. The southern part of the Williston basin (fig* 1) is hereafter referred to simply as the Williston basin. The deepest part of the basin, designated on figure 1 the Central Williston basin, occupies about one-fifth of the report area and underlies about 22,000 square miles of northwestern North Dakota and northeastern Montana. The eastern and southern limits of the Williston basin are outlined by a belt in which the edges of Paleozoic and Mesozoic rocks onlap the Precambrian basement rocks. For this report, the eastern limit is arbitrarily placed between the -500-foot and -1,000-foot structure contours drawn at the base of Mississippian rocks (fig. 1). The southwestern and western limits of the basin are placed at the northeast flanks of the Black Hills uplift and an unnamed arch that connects the Black Hills and Central Montana uplifts, the north flank of the Central Montana uplift, and the east flanks of the Little Rocky and Bearpaw Mountains (fig, 1), Geologic setting Rocks at the surface in the Williston basin generally are flat lying or dip only a few degrees, They comprise predominantly nonmarine Tertiary rocks and predominantly marine Cretaceous rocks (fig’ 2), North and east of the Missouri River, shown as a sinuous line of county boundaries that passes through Pierre, S, Dak., ■ 10 11 1N0W rrm h- 'iT 11118 'S'iy u 5 <3: p 'CU iiii A-li. ' 12 Bismarck, N. Dak., and thence northwestward (fig. 2), the bedrock is mantled at most places by thick glacial drift. The thickness of this drift averages 200 feet but locally may be as much as 500 feet. Beyond the southwestern and western limits of the Williston basin Jurassic, Triassic, and Paleozoic rocks crop out on the flanks of the Black Hills, the Big Snowy Mountains of the Central Montana uplift, and the Little Rocky Mountains (figs. 1 and 2). Tertiary intrusive rocks are exposed in the Black Hills and the Little Rocky and Bearpaw Mountains (figs. 1 and 2). Precambrian metamorphic and granitic rocks form the core of the Black Hills. During the Paleozoic and early Mesozoic, the Williston basin area was at times an intracratonic basin or a shelf area that bordered a miogeosyncline much farther west. At other times the area was flooded by seas that followed a trough extending eastward from the miogeosyncline across the area of the Central Montana uplift and the Central Williston basin. For short intervals the entire Williston basin area was above sea level and subjected to subaerial erosion; infrequently thin continental deposits were laid down. In Cretaceous time, the Williston basin area was part of a huge epicontinental seaway that extended southward through Canada and the western conterminous United States. As the sea periodically transgressed westward and regressed eastward within this trough, some continental deposits were laid down largely in the western part of the area. Most Cretaceous deposition in the eastern part of the area was marine. Li : 13 During and following Laramide orogeny in latest Cretaceous and Tertiary time, the area of the Williston basin was for the most part above sea level and received continental deposits of sandstone, shale, and lignite. Sedimentation in the Williston basin area has been either conti¬ nental or in shallow epicontinental seas, whose depths probably were never much greater than 300 feet. Nevertheless, because of intermittent but continuing downwarping, a considerable thickness of sedimentary rocks has accumulated. The accumulation is greatest in the Central Williston basin. There the maximum thickness of sedimentary rocks is about 16,700 feet beneath the Killdeer Mountains, 20 miles directly south of the Nesson anticline (fig. 1), in northwestern North Dakota. Geographic setting The Williston basin area occupies the northern part of the Great Plains. The land surface is flat to gently rolling, but in places it is hilly or studded by solitary buttes. The glaciated area north and east of the Missouri River contains many small saline lakes. Rugged breaks or badlands occupy narrow belts along many major streams and in a few places, such as along the Little Missouri River in North Dakota, wider belts of badlands are present. The general land surface is 1,500 to 3,500 feet above sea level. Several isolated buttes in western South Dakota and southeastern Montana rise to a little more than 4,000 feet, but the elevation of the highest butte in North Dakota is 3,468 feet. Local relief exceeds 500 feet at only a few localities. nn I I n, , ’ ' ' 14 The report area is drained principally by the eastward- and southward-flowing Missouri River, its major tributaries, the Yellow¬ stone and Little Missouri Rivers, and its many smaller tributaries. The Missouri River enters the west edge of the area at an elevation of about 2,300 feet, drops 900 feet in approximately 700 miles, and leaves the south edge at an elevation of about 1,400 feet. The streams in north-central North Dakota flow northward into Canada. Almost all parts of the Williston basin, except for the badlands and some swampy glaciated areas, are traversed by a large network of improved and unimproved roads. The area is served largely by east-west routes of the Great Northern, Northern Pacific, and Chicago, Mil¬ waukee, St, Paul, and Pacific railroads. Parts of North Dakota and South Dakota are served also by the Minnesota, St. Paul, and Sault Ste. Marie and by the Chicago and Northwestern railroads. The population of the Williston basin (fig. 1), as compiled from the 1960 Census of Population, is about 525,000 persons, of whom about two-thirds live in North Dakota. This population is predominantly rural and the average population density is about 4,7 persons per square mile. The principal cities and their populations are; Bismarck, capital of North Dakota, 28,000; Minot, N. Dak., 31,000; and Pierre, capital of South Dakota, 10,000. Several major business and railroad centers with populations of 35,000 to 53,000 lie within a hundred miles of the outside limits of the Williston basin. These cities are Billings, Mont., Fargo and Grand Forks, N. Dak., and Rapid City, S. Dak. , 15 The climate is semiarid and subject to severe diurnal and seasonal temperature changes, Hot dry summers generally have many days with temperatures higher than 95°, Long cold winters may have several periods, as long as two weeks, of subzero temperatures, and readings of <=>20° to -40° are common. Cool wet springs commonly prevent heavy trucks from leaving the hard-surfaced roads. The economy has long been agricultural and is based largely on dryland wheat farming and stock grazing. However, since 1951, the Williston basin has been subjected to intensive petroleum exploration and petroleum exploration, production, and refining have become ^ “ * important economic factors. In 1960, oil production in the Williston basin was almost 40 million barrels from about 30 oilfields in Montana, about 70 oilfields in North Dakota, and 1 oilfield in South Dakota (fig. 3), Significantly, three-quarters of the oil production came from oilfields located on only three major structural features, the Cedar Creek and Nesson anticlines and the Poplar dome (fig, 1), Acknowledgments and sources of data This report draws heavily on the author’s background in the stratigraphy of the Williston basin. His experience was acquired during work since 1954 on the Geological Survey’s Williston basin Oil and Gas Investigations project, a part of the Interior Department’s program for development of the Missouri River basin. The cross section, correlation chart, stratigraphic section, and most maps ' I§ 17 pertaining to lower and middle Paleozoic rocks were compiled largely from heretofore unpublished data. However, time did not permit origi¬ nal compilation of the entire report; many of the other maps are from several published and unpublished sources. Some maps were modified to incorporate more recent data or to conform to the author’s personal interpretations; others were not changed except by adapting their formats. The generalized geologic map (fig, 2) is based on state geologic maps of Montana (Ross and others, 1955), North Dakota (Hansen, 1956), South Dakota (Petsch, 1953a), and Wyoming (Love and others, 1955), The oil and gas fields map (fig, 3) was compiled from maps by Dorothy Sandberg (1959) and Petroleum Information Corp, (1960), and subse- quently has been updated from various reports published by Petroleum Information Corp, and articles and reports in the periodicals, Williston Basin Oil Review and Montana Oil and Gas Journal, All other maps, unless original to this report, are credited directly below their titles, Where only the name and date of the source are stated, the map is essentially unmodified, Severn' leagues on the Geological Survey kindly permitted use of their unpublished maps, N, M, Denson and J, R„ Gill furnished a structure contour map of the Precambrian surface between the 102° and 106° meridians and between the 45° and 49° parallels, E, K, Maughan furnished a map of Permian formations in the Williston basin. . c£ 18 Special thanks are expressed to Barlow, Hammond, and Haun, Geologists, Inc, f who gave permission (written communication, May 21, 1961) to use parts of four unpublished Cretaceous isopach maps. Without this assistance, an adequate discussion of the Cretaceous rocks could not have been prepared in the short time available, STRATIGRAPHY The sedimentary rocks that fill the Williston basin attain a maximum thickness of about 16,700 feet and include parts of every geologic system from Cambrian to Quaternary. This thick sequence rests on an erosion surface of Precambrian metamorphosed igneous rocks at elevations ranging from 500 feet to about 13,900 feet below sea level (fig. 4), The sequence comprises three distinct lithologic assemblages. Rocks of Middle Cambrian through Middle Ordovician age are about 1,200 feet thick in the Central Williston basin and consist of clastic rocks, largely shale and sandstone, containing interbeds of limestone and limestone-pebble conglomerate. Rocks of Late Ordovician through Pennsylvanian age comprise a carbonate sequence about 7,500 feet thick there. They consist largely of limestone and dolomite but include one thick salt bed, seven thinner salt beds, and several thin beds of shale. Rocks of Permian through Tertiary age are about 8,000 feet thick in the Central Williston basin and consist predominantly of shale, containing interbeds and lenses of sandstone and siltstone in the lower and upper parts,and near the base, ' . - 19 Ui o -J < > z 35 LJ o h* a: Z X mm H Z h- £ Art O o O Li o U. 1 > LI 8-1 O a o in •a l(l LI Ui h- (A o _ z < $ hi z < 3 * cc o Q tA cc X i UJ < “ UJ V) Z < UJ 5 ' 108 Mowry Shale. The major source of sediments probably lay far southwest of the report area, as evidenced by the thickening of deposits in that direction at the southwestern margin of the basin (fig, 27), Currents only infrequently carried in coarser elastics to be deposited as interbeds of shaly sandstone or sandy shale. Niobrara Formation and Pierre Shale The upper part of the thick marine shale sequence of Late Cretaceous age (fig. 5) includes the Niobrara Formation of the Colorado Group and the conformably overlying Pierre Shale of the Montana Group in the eastern two-thirds of the Williston basin 0 The Niobrara Formation conformably overlies the Carlile Shale. Because the Niobrara-Pierre contact is difficult to pick consistently, the Niobrara is here considered with the Pierre rather than with the upper part of the Colorado Group. The Pierre Shale is conformably overlain by the Fox Hills Sandstone, the uppermost formation of the Montana Group. The arbitrary western limit of usage of the Pierre Shale is a line that denotes the eastern limit of the easternmost bed of sandstone interbedded with marine shale (fig. 28), West of this line, the Pierre Shale is equivalent to the Telegraph Creek Formation, Eagle Sandstone, Claggett Shale, Judith River Formation, and Bearpaw Shale of the Montana Group, in ascending order (fig, 6), . S' - . 109 r 000 tr o Ll < tr < UJ tr _j od < 2 x Q_ < X o < CL o CO o E E rK 0-1 - L _ /_ XN01AJ ;r OAM 110 The Niobrara Formation and Pierre Shale have a maximum thickness of about 2,500 feet in east-central and northeastern Montana (fig, 28) 0 They are thickest in a trough that extends southward through western North Dakota and eastern Montana, They thin gradually eastward and have a thickness of about 1,000 feet near the outcrop of the Pierre Shale in central North Dakota (fig. 28). The Pierre is exposed on the eastern and southern margins of the Williston basin and on the Cedar Creek anticline. The Niobrara Formation consists of medium- to dark-gray partly speckled calcareous shale interbedded with thin beds of argillaceous limestone, chalk, and bentonite. Its thickness in the Central Williston basin ranges from about 100 to 200 feet. The Pierre Shale consists of medium-gray to black bentonitic shale with thin interbeds of light-gray and yellowish-gray bentonite. Zones of silty or sandy shale in the Pierre probably were deposited contemporaneously with equivalent beds of sandstone farther west. The Pierre generally is not subdivided in the Williston basin, although as many as eight members are mapped in outcrops on the margins. Its thickness ranges from about 900 feet in central North Dakota to 2,300 feet in the Central Williston basin. Rocks of the Montana Group equivalent to the Pierre Shale are exposed in most of north-central Montana and on the Central Montana uplift. There they are several hundred feet thicker than the Pierre is in the Central Williston basin. However, they appear to thin - - Ill westward in most areas because of present-day erosion of the flat-lying Bearpaw Shale. The marine Telegraph Creek Formation, about 100 to 200 feet thick, consists of interbedded yellowish-gray thin-bedded sandstone, siltstone, and sandy shale. Its contacts are gradational and its lithologic character is transitional between the underlying Niobrara Formation and the overlying Eagle Sandstone. Both the Eagle Sandstone, about 100 to 300 feet thick, and the Judith River Formation, about 350 to 500 feet thick, consist in part of marine yellowish-gray and gray sandstone and siltstone and in part of marginal-marine and continental claystone, carbonaceous shale, and lignite. Each was deposited during an episode of eastward regression followed by westward transgression of the sea. The marine Claggett Shale, about 400 to 500 feet thick, and the marine Bearpaw Shale, about 700 to 900 feet thick, are lithologically similar to and depositionally continuous with corresponding parts of the Pierre Shale, They were deposited during episodes of maximum western extent of the Late Cretaceous sea. During deposition of the Niobrara Formation and Pierre Shale, the tectonic framework of the Williston basin area remained essentially unchanged from that of late Early Cretaceous time. However, the western shoreline twice migrated back and forth across the western third of the Williston basin area (Weimer, I960). Deposition of marine shale in the eastern two-thirds of the area was not interrupted. r - ■ - n i 112 although silty or sandy shale was laid down there during the shoreline migrations. Ash falls continued intermittently throughout the area. Rocks between top of Pierre Shale and surface Rocks of Late Cretaceous to Pleistocene age, locally as thick as 2,800 feet, overlie the Pierre Shale and its equivalents in part of the Williston basin area. Their thickest accumulation is in the Killdeer Mountains, 20 miles directly south of the Nesson anticline (fig. 1) in the Central Williston basin. Rocks between the top of the Pierre Shale and the surface comprise, in ascending order, the Fox Hills Sandstone and Hell Creek Formation of Late Cretaceous age, the Fort Union Formation of Paleocene age, the Golden Valley Formation of Eocene age, the White River Formation of Oligocene age, the Arikaree Formation of Miocene age, the Flaxville Gravel of Miocene or Pliocene age, and glacial drift of Pleistocene age. The Fort Union Formation comprises the Ludlow, Cannonball, Tongue River, and Sentinel Butte Members, in ascending order. The White River is considered to be a group, comprising the Chadron and Brule Formations, in ascending order, by some geologists. All post-Pierre strata, except for the Fox Hills Sandstone, a thin tongue in the Hell Creek Formation, and the Cannonball Member of the Fort Union Formation, are continental. The Fox Hills Sandstone consists largely of marine yellowish-gray and light-gray shaly bentonitic sandstone and silty or sandy shale. Its thickness ranges from about 100 to 350 feet in the Central Williston s v' i s Had 'fin.,) ' 113 basin. The Fox Hills is absent northeast of the Porcupine dome (fig. 1). The Fox Hills was deposited during the southward and eastward regression of the Late Cretaceous sea from the Williston basin area. The Hell Creek Formation, which disconformably overlies the Fcx Hills Sandstone, consists of a highly varied assemblage of nonmarine gray and yellowish-gray mudstone, siltstone, and sandstone (Cobban, 1952). It is 100 to 550 feet thick in the Central Williston basin. In some areas the lower part of the formation contains carbonaceous shale and lignite (Brown, 1952). The Hell Creek is continental except for a brackish-water tongue near the base in south-central North Dakota (Laird and Mitchell, 1942). This tongue was deposited during a very minor marine transgression after the sea had largely withdrawn from the Williston basin area. The Fort Union Formation, as much as 2,000 feet thick, conforma¬ bly overlies the Hell Creek Formation. The Fort Union consists largely of gray and yellowish-gray shale, claystone, and sandstone interbedded with lignite. In some areas it is divided into three members on the basis of the different average shade of rather indefi¬ nite stratigraphic zones. The Ludlow Member at the base is dark colored; the overlying Tongue River Member generally is lighter in color; the uppermost member, the Sentinel Butte, is dark colored. These three members of the Fort Union are continental, but the Cannonball Member which interfingers with the Ludlow Member in parts • • t. « • ' • • i . ~ 114 of North Dakota and South Dakota, is marine. The Cannonball Member has a maximum thickness of about 300 feet in North Dakota, but it thins westward to extinction in easternmost Montana. It consists of gray and yellowish-gray sandstone and shale, which were deposited during a minor transgression of the sea into the southern part of the Williston basin area. The continental Golden Valley Formation, about 175 feet thick, conformably overlies the Fort Union Formation in southwestern North Dakota. It is found in isolated erosional remnants between the Missouri River, at Bismarck, N. Dak., and the Montana line (Brown, 1952). It consist of interbedded purplish-gray carbonaceous shale and yellowish-orange and white claystone overlain by interbedded fine- to coarse-grained sandstone, shale, and claystone (Benson, 1949). The White River Formation, as much as 400 feet thick, uncon- formably overlies Eocene and older strata. It is present on high divides and isolated buttes in southeastern Montana, southwestern North Dakota, and western South Dakota (Denson and others, 1959). It consists of very light gray and pinkish-gray tuffaceous sandstone, conglomerate, claystone, and limestone and probably was deposited on flood plains and in lakes. The continental Arikaree Formation, as much as 300 feet thick, unconformably overlies the White River Formation on some of the highest divides and buttes. The Arikaree consists largely of yellowish-gray tuffaceous sandstone overlying a basal conglomerate. f*. ” • : . : ■ : f .v • c-: ' . . . ■ . •: , i : w •. ■■ 115 The Flaxville Gravel caps some of the highest divides in north-central and northeastern Montana but probably is not present in North Dakota. Its thickness generally is 30 feet or less. It was deposited on a flood plain and consists of unconsolidated or locally cemented sand and rounded gravel. Glacial drift mantles Tertiary and Cretaceous rocks north and east of the Missouri River in northern Montana, northern and eastern North Dakota, and eastern South Dakota (fig. 2). Its thickness averages about 200 feet but locally may be as much as 500 feet. Thin unconsolidated Quaternary deposits, including alluvium, colluvium, terrace gravel, landslide debris, and some sand dunes, are widely distributed. The Late Cretaceous sea withdrew southward and eastward from the Williston basin area during deposition of the Fox Hills Sandstone. During the latest Cretaceous and Tertiary the Williston basin area was alternately subjected to continental deposition and erosion, except for two brief marine incursions into the southern part in latest Cretaceous and early Paleocene time. The Continental Glacier periodically covered the northern and eastern parts of the area during the Pleistocene. Since its retreat, the Williston basin area has been subjected to widespread erosion accompanied by minor local deposition. ji. . ; 116 STRUCTURE The Williston basin is a large, relatively shallow elliptical basin, lying partly in midwestern Canada and partly in the con¬ terminous United States, Its axis trends about N. 10° W. in the southern part of the basin, the area considered in this report (figs, 1, 4, 14, 23, and 25). Rocks at the surface generally appear to be flat-lying because regional dips from the margins to the basin center are 1° or less. Most of the major folds are on the west side of the basin; the east and south sides contain only minor wrinkles. Dips on the flanks 6f major folds generally are about 1° to 3°, but dips on folds in the basin interior do not exceed 2°, The only known dips steeper than 3° in the Williston basin are adjacent to the uplifts and mountains bordering the western margin and on the west limb of the Cedar Creek anticline (fig. 1). The dominant structural grain trends N. 30° W., and secondary structural grains trend N. 55° to 60° E. and N. 2° E. Many linear structural features within the Williston basin trend roughly in one of these three directions. Folds The major folds in the Williston basin are the Cedar Creek and Nesson anticlines, the Poplar and Bowdoin domes, and the Sheep Moun¬ tain and Blood Creek synclines (fig. 1). The important secondary 117 folds are the Camp Crook, Plevna, and Sanish anticlines, the Freedom dome, and the Coburg syncline (fig. 1). The Cedar Creek anticline is a 125-mile long, northwest-plunging asymmetrical anticline with a steep west limb. It trends N 0 30° W. from northwestern South Dakota to east-central Montana (fig 0 1). Dips on its west limb range from about 4° to 30° SW 0 , but locally the surface rocks are nearly vertical where they reflect a subsurface fault. The Plevna anticline branches from the Cedar Creek anticline and parallels its west limb. The Sheep Mountain syncline borders the Plevna and Cedar Creek anticlines on the west. The Camp Crook anti¬ cline connects the southern part of the Cedar Creek anticline with the Black Hills uplift. Two small anticlines have been inferred west of the Plevna anticline, and two or more small anticlines have been inferred east of the Cedar Creek anticline (fig. 14). These anticlines also trend about N. 30° W. and probably are related to the Cedar Creek anticline. The Nesson anticline is a 75-mile long, 15-mile wide anticline that plunges southward just east of the basin center. It trends about N, 2° E. in northwestern North Dakota (fig. 1). A subsidiary fold, the Sanish anticline, branches from its east limb and trends S. 30° E., parallel to the Cedar Creek anticline. The Sanish anticline follows the trend of the ancestral Nesson anticline of Devonian time (fig. 11). The Poplar dome is an elliptical feature, 25 miles long from east to west and 15 miles wide in northeastern Montana (fig. 1). Its west I end lies at the projected trend of the Cedar Creek anticline. ■ 118 > t* \ . ' \ !. y ■. i The Bowdoin dome is a large subqjdrcular feature, about 65 miles i * i \ t long from east to west and about 50 miles wide in north-central Mon¬ tana (fig. 1). It is bordered on the southwest by the Coburg syncline, whose axis trends N. 30° W. The Western limit of the Williston basin is drawn around the east and south flanks of the Bowdoin dome by some geologists. The dome is here included within the Williston basin, however, as dips on any of its flanks rarely exceed 1°. The only steep dips that logically limit the Williston basin in north-central Montana are west of the Coburg syncline on the flanks of the Little Rocky and Bearpaw Mountains. The Freedom dome is a small elliptical feature, whose longer dimension is less than 10 miles. It lies north of the large Porcupine dome at the east end of the Central Montana uplift (fig. 1). Freedom dome is included in the Central Montana uplift by many geologists, although it is much lower than the other structural features on the north side of the uplift (figs. 23 and 25). The Blood Creek syncline lies between Freedom dome on the south and the Bowdoin dome and Little Rocky Mountains on the north. The syncline terminates abruptly, possibly because of a subsurface fault, at the north limb of the Cat Creek anticline of the Central Montana uplift (fig. 1). f 119 Faults The latest deformation of the Williston basin area resulted only in gentle folding, and faults are rare in the surface rocks, The only two known major surface faults are the Brockton-Froid fault zone and the Weldon fault, which parallel one of the secondary structural grains. Several large faults have J be * 124 |y,F Winnipeg Formation of Ordovician age, the Kibbey Formation of Missis- sippian age, the Amsden Formation of Pennsylvanian age, the Minnelusa Formation of Pennyslvanian and Permian age, the Piper, Swift, and Sundance Formations of Jurassic age, and the Dakota Group, Kootenai Formation, and Newcastle Sandstone of Cretaceous age. Significant oil shows have not been found in rocks below the Winnipeg Formation or above the Newcastle Sandstone or in the redbed sequence of Permian and Triassic age (fig. 5). These strata, therefore, are not at present regarded as potentially oil-bearing in the Williston basin area. Natural gas production is insignificant in the Williston basin area. Most of the natural gas is a byproduct of producing oil wells, and a large part of this gas is flared. Bowdoin and Cedar Creek are the only important gasfields (fig. 3)„ The Bowdoin field produces gas from two local beds of sandstone in the upper part of the Colorado Group of Cretaceous age and the Cedar Creek field produces gas from silty and sandy shale and shaly and sandy siltstone at two horizons, equivalent to the Eagle Sandstone and Judith River Formation, in the Pierre Shale of Cretaceous age (Billings Geological Society, 1958 ) 0 Plevna gasfield, just west of the Cedar Creek gasfield (fig. 3), has minor gas production from the Judith River equivalent. Only a limited market exists at present for natural gas produced in the Williston basin area, and hence there is little active explora¬ tion . 125 Lignite The area of outcropping Tertiary rocks in the Williston basin (figo 2) constitutes most of the Fort Union region of the Great Plains coal province (Trumbull, 1960). The Williston basin probably contains the largest lignite reserves of any area in the United States 0 North Dakota leads the United States in annual lignite production with about three million tons (Hainer, 1956) 0 Commercial deposits of lignite, in beds as much as 40 feet thick, are abundant and widely distributed in the Fort Union Formation of Paleocene age. Most of the lignite reserves are contained in the Tongue River Member. Thin, largely noncommercial beds of lignite are locally present at or near the surface in the Eagle Sandstone, Judith River Formation, and lower part of the Hell Creek Formation of Cretaceous age and in the Golden Valley Formation of Eocene age (Combo and others, 1949; Hainer, 1956; Rothrock, 1944). Thin local beds of bituminous coal may be present at depth in the Morrison Formation of Jurassic age. Lignite is mined in the Williston basin area largely by stripping, and this will probably continue to be the major method of mining. Underground mining, except at very shallow depths, appears unlikely. at least for some time to come. '• c 126 Uranium Uranium is present in carbonaceous shale and lignite beds of the Hell Creek Formation of Late Cretaceous age and the Fort Union Forma¬ tion of Paleocene age along the southwestern margin of the Williston basin in southwestern North Dakota* northwestern South Dakota* and southeastern Montana (Denson and others* 1959), Reserves of rock containing greater than 0,1 percent uranium are estimated to be in excess of one million tons (N. M, Denson and J, R, Gill* oral commu¬ nication* Jan, 31* 1962) e Lignite is not being commercially mined solely for its uranium content at present. Any future mining of uranium-bearing carbonaceous shale and lignite probably would be largely by stripping. Ground water Upper Cretaceous and Tertiary rocks as well as glacial drift and unconsolidated deposits yield potable water by pumping from shallow wells for farm and ranch consumption in many parts of the Williston basin. Sandstone beds in Upper Jurassic and Lower Cretaceous rocks are important aquifers* which flow potable water in the western part of the Williston basin near their intake areas in the Black Hills (Rothrock, 1944), In the vicinity of the Black Hills* the Deadwood* Winnipeg* and Minnelusa Formations and the Pahasapa Limestone locally serve as aquifers (Rothrock* 1944), Along the eastern margin of the basin, the most important aquifers are the Dakota Group and Newcastle ' 127 Sandstone of Early Cretaceous age 0 The thick Paleozoic carbonate sequence and the Winnipeg Formation probably contain many deep aquifers throughout the Williston basin,, Hydrodynamic studies suggest that formation waters in the thick Paleozoic carbonate sequence flow northeastward through the Williston basin under considerable hydrostatic head from intake areas in the Big Snowy and Little Rocky Mountains and Black Hills along the western margin 0 A similar direction of flow is postulated for waters in the Winnipeg Formation 0 Salinity studies indicate that waters in Devonian and Mississippian rocks become highly saline in the Central Williston basin probably in part through solution of salt beds and in part through addition of connate waters„ Salinity studies of waters in Cambrian to Silurian rocks indicate a similar increase in salinity in the Central Williston basin (Porter and Fuller s 1959) 0 A northeast movement of water accompanied by salt solution is taking place in Devonian rocks in the Saskatchewan part of the Williston basin (Milner, 1956)„ A number of brine springs, high in sodium chloride content, flow from Devonian rocks at the eastern margin of the Williston basin in Manitoba (Baillie, 1953 ) 0 Along the eastern margin of the Williston basin in North Dakota, however, aquifers in the Paleozoic carbonate sequence are truncated by Cretaceous aquifers, which probably absorb most of the flow. There the saline water may be freshened by the addition of upward-flowing water from vertical fractures in the underlying Precambrian rocks. ' I • . 128 Other resources The salt member of the Prairie Formation of Middle Devonian age probably contains sizable but unestimated reserves of sylvite at great depth in the Central Williston basin. Potash production from this formation at somewhat shallower depths has been under investigation since 1953 in the Saskatchewan part of the basin (Cheesman, 1958 ) 0 Undrained saline lakes in the glacial drift area of northwestern North Dakota contain high concentrations of sodium sulfate (Hainer, 1956). Although some Glauber's salt was produced and marketed in 1951, this resource is not at present being exploited. The surficial strata of the Williston basin area contain large deposits of clay, shale, sand, and gravel, some of which are being used in road construction and by local industries (Hainer, 1956; Rothrock, 1944). Deposits of bentonite, however, are generally too thin for commercial exploitation by present methods (Rothrock, 1944), WASTE DISPOSAL POSSIBILITIES Many formations in the Williston basin may be considered fair to good possibilities for the subsurface disposal of radioactive wastes. The only strata that probably do not contain potential storage reser¬ voirs are the Big Snowy Group, rocks between the top of the Minnelusa Formation and base of the Swift Formation except for interbeds of salt, and rocks between the top of the Pierre Shale and the surface (fig, 5) 0 Rocks of the first two sequences are believed to be unfavorable because M - ; : 129 of their lithologies. They comprise heterogeneous assemblages of shale, siltstone, sandstone, and limestone, in which fluid movement would be largely unpredictable. Rocks between the top of the Pierre Shale and the surface are believed to be less favorable because of economic, lithologic, and safety considerations. These rocks contain all the important lignite reserves of the area and many aquifers that are widely used for potable water. The post-Pierre rocks comprise a heterogeneous assemblage of discontinuous beds and lenses of sandstone, claystone, shale, siltstone, and lignite, in which fluid movement would be unpredictable. Furthermore, in some areas west and south of the Missouri River they are being deeply dissected by Recent headward erosion and in areas of glacial drift they are cut by buried pre-Pleistocene valleys. Wastes injected into these rocks, therefore, might leak into springs, lakes, streams and aquifers. The remaining formations of the Williston basin afford a large variety of possible reservoirs at a wide range of depths. Fdur categories of possible reservoir formations are considered: (a) permeable sandstone and carbonate beds at moderate to great depths, (b) salt beds at moderate to great depths, (c) thick shale beds at shallow depths, and (d) permeable sandstone beds at shallow depths. (Shallow is here used arbitrarily for depths less than 5,000 feet; moderate, for depths between 5,000 and 10,000 feet; and great, for depths more than 10,000 feet.) The formations recommended for con¬ sideration in each category are: (a) the Deadwood, Winnipeg, Bakken, 130 and Minnelusa Formations as deep sandstone and carbonate reservoirs; (b) the Pine and Dunham Salts and the unnamed salt bed in the Opeche Formation as salt-solution caverns; (c) the Belle Fourche, Carlile, and Pierre Shales and the Greenhorn and Niobrara Formations as shallow sand-fractured shale reservoirs; and (d) the Swift Formation and Newcastle Sandstone as shallow sandstone reservoirs (fig. 5), Other formations may afford fair to good possibilities for the storage of radioactive wastes, but they are disqualified from con¬ sideration for various reasons given in the discussion of each category. Deep sandstone and carbonate reservoirs The two sandstones of the Winnipeg Formation (fig. 8) deserve primary consideration, if deep-well disposal of low- to intermediate- activity radioactive wastes were attempted in the Williston basin. The Winnipeg lies about 13,000 to 15,000 feet below the surface in the Central Williston basin, where its salinity exceeds 150,000 ppm chloride (Porter and Fuller, 1959). Wastes could be injected into either sandstone at depths of 8,000 to 9,000 feet in eastern Montana and into the upper sandstone at depths of about 5,000 feet in western South Dakota with the expectation that they would flow northeastward into the Central Williston basin. Tighter cementation and increased clay content of the lower sandstone in the central part of the basin and gradation of the upper sandstone to shale northward in North Dakota 131 might provide porosity traps in the deepest part of the basin and attenuate the continued flow of injected wastes toward the eastern margin. In any event, the radioactivity of low-activity wastes probably would be greatly decreased as they migrate the approxi¬ mately 250-mile distance from the injection site to the eastern margin. Additional factors favoring consideration of the Winnipeg are its higher permeability relative to underlying and overlying strata and its stratigraphic position below all oil-producing reservoirs. Some features of the Winnipeg, however, are less favorablej (a) it is potentially oil-productive in areas such as the Nesson anticline, and (b) it probably is a fresh-water aquifer usable for potable water along the eastern margin of the basin, so the maximum possible rate of fluid movement would have to be determined and considered in re¬ lation to sorptive properties and rate of radioactive decay. Nevertheless, the Winnipeg probably presents fewer obstacles to the safe and predictable flow of radioactive wastes than any other deep salaquifer in the Williston basin. The Deadwood Formation (fig. 7) merits attention if deep dis¬ posal of radioactive waster in sandstone or limestone bodies intertonguing with shale is considered. Several favorable factors, which have been noted in regard to the Winnipeg Formation, such as depth, stratigraphic position, and salinity and flow of formation waters, apply also to the Deadwood. Another factor favoring the 132 Deadwood is its apparent lack of oil-producing potential. Furthermore, possible downward migration of wastes into the directly underlying granite wash or Precambrian rocks would be permissible, and the possibility of upward movement of wastes into the overlying porous Winnipeg is lessened by basinward thickening of the Deadwood north of the Black Hills. Some factors, however, weigh against disposal in the Deadwood without more detailed study? the thickness, size, shape, porosity, and permeability of its potential reservoirs are not available because of (a) lack of data, and (b) leakage into stratigraphically higher formations might occur on the flanks of undiscovered Precambrian monadnocks, similar to the one near Pierre, So Dak, Although an approximate location for a disposal site in the Deadwood cannot be suggested without more detailed studies of its facies changes, the Deadwood merits careful consideration as a possible reservoir formation because of its favorable stratigraphic position, The Bakken Formation (figs. 13, 14) deserves qualified considera¬ tion for the disposal of low- to intermediate-level radioactive wastes at moderate to great depths in the Central Williston basin 0 There its middle unit constitutes a potential carbonate reservoir, consisting of about 30 to 50 feet of silty dolomite and dolomitic siltstone and sandstone. This silty dolomite reservoir is capped and bottomed by the largely impermeable black carbonaceous shales of the Bakken (fig, 5), and the only likelihood of leakage of wastes would be through fractures in these shales. Two factors detract from consideration of / ■ 133 the Bakkens (a) it has recently been found to be oil-productive along with the underlying Three Forks Formation in west-central North Dakota, and (b) an underlying quartzitic sandstone at the top of the Three Forks produces oil from vertical fractures on parts of the Sanish and Nesson anticlines„ Nevertheless, in areas of slight structural closure, where its oil-producing potential has already been disproven and where vertical fractures are absent B the Bakken might provide a suitable reservoir 0 Permeable lenticular sandstones of the Minnelusa Formation at moderate to great depths might be used for waste disposal in the southern part of the Williston basin 0 Although the Minnelusa pro- duces oil in a field south of the Black Hills in South Dakota 9 it is not being actively explored in the Williston basin„ Sufficient well data are available, however, for a moderately detailed study of the size, shape, thickness, and reservoir characteristics of the sandstones in the upper part of its lower member and in its upper member 0 Most strata between the top of the Winnipeg Formation and the base of the Big Snowy Group should not at present be considered for the deep disposal of radioactive wastes in the Williston basin„ These predominantly carbonate rocks, which account for nearly all of the present oil production and most of the future oil-producing capacity of the Williston basin, are the Red River, Stony Mountain, Stonewall, Interlake, Winnipegosis, Dawson Bay, Souris River, Duperow, Birdbear, ■ 134 and Three Forks Formations and the Lodgepole and Mission Canyon Limestones and Charles Formation of the Madison Group (fig. 5), Apart from economic considerations, these formations consist of many minor facies, whose varied porosity and permeability could not be safely predicted, and contain few barriers to the upward or downward migration of fluids. Interconnection of oil reservoirs in adjacent formations has been demonstrated in several producing oilfields. Other oil-productive or potentially oil-productive formations that should not be considered for deep waste disposal are the Tyler and Amsden Formations. Reservoirs in salt beds The Pine and Dunham Salts and the unnamed salt bed in the Opeche Formation (figs, 18 , 20, and 22) of the Williston basin might be utilized for disposal of radioactive wastes in artificial solution caverns (National Academy of Sciences, 1957, p. 136), These three salt beds lie within an interval of about 500 to 800 feet at depths of 4,000 to 7,700 feet below the surface. The salt beds are thin, some¬ what discontinuous, and enclosed largely by redbeds. Their thinness and discontinuity probably would be advantageous in controlling the size and shape of solution caverns. Furthermore, any slight leakage from these caverns into surrounding strata would not be economically harmful as the redbeds are not considered to be potentially productive of oil. 135 The thick salt member of the Prairie Formation (fig. 12) and the seven salt beds within the Madison Group (fig. 16) probably should be disregarded as reservoirs for radioactive wastes. The salt member of the Prairie may contain large potash reserves and it is directly underlain and overlain by oil-producing formations, which might be contaminated if leakage of wastes occurred. Because of the great thickness and depth of the salt, solution caverns in the Prairie probably would be subjected to salt flowage. The thinner salt beds in the Madison are closely interbedded with oil-producing carbonate beds, which might be contaminated by the slightest leakage. Another deterrent to storage in the Prairie and Madison salt beds is the likelihood that they are being partly dissolved at present by the flow of formation waters in some areas. Shallow shale reservoirs The Belle Fourche Shale, Greenhorn Formation, and Carlile Shale (fig. 27), and the Niobrara Formation and Pierre Shale (fig. 28) offer possibilities for the disposal of small volumes of radioactive wastes in shallow shale reservoirs in North Dakota which could be hydraulically fractured. They constitute a sequence of marine shale, as much as 3,600 feet thick, and extend from the surface at the basin margins to a depth of about 5,000 feet below the surface in the Central Williston basin. They contain no significant beds of sandstone in North Dakota, where the shale sequence is interrupted only by thin silty, calcareous, ■ 136 and bentonitic zones. Although this sequence produces gas in three fields along the western margin of the Williston basin, it is not regarded as potentially productive of oil or gas in North Dakota, The Skull Creek and Mowry Shales are less favorable shale reservoirs because of their proximity to the Newcastle Sandstone and Dakota Group (fig. 5), two major sandstone aquifers, which yield potable water in some areas. Shallow sandstone reservoirs The Newcastle Sandstone (fig. 26) offers reservoir possibilities for the shallow disposal of radioactive wastes where it lies about 1,500 to 5,000 feet below the surface in eastern Montana and western North Dakota. In this area it forms lenses and pods of sandstone, as much as 120 feet thick, entirely enclosed by shale. The sandstone commonly is silty or shaly but probably has sufficient porosity and permeability for the injection of wastes. Although the contacts of the sand bodies with the enclosing shale apparently are gradational, little leakage of wastes into the shale is anticipated. The Newcastle produces oil in the nearby Powder River Basin but has yielded only minor oil shows in the Williston basin. Although it has an oil- producing potential, the Newcastle has proved to be unproductive in many areas, which could be studied in detail for possible disposal sites. The Newcastle Sandstone should not be regarded as a potential waste disposal reservoir in South Dakota and southern North Dakota, for in that area it serves as a major fresh-water aquifer. 137 Sandstone beds in the upper part of the Swift Formation of Montana and North Dakota merit secondary consideration as possible shallow sandstone reservoirs for waste disposal. These beds of sandstone are interbedded with and gradational to calcareous shale and siltstone, which probably would serve as fairly impermeable barriers to migration of injected wastes. The base of the Swift lies about 200 to 600 feet above the middle member of the Piper Formation (fig, 23), The sec- ondary rating of the Swift results from a lack of data on the thickness, distribution, and reservoir characteristics of its sandstones and on the exact location of the unconformity.between the Swift and the underlying Rierdon Formation. Furthermore, the Swift produces oil from several fields in northwestern and central Montana and hence has a higher oil-producing potential than the Newcastle in the Williston basin. Nevertheless, the Swift has many unproductive areas, which could be studied in detail as possible disposal sites. Correlative beds in the upper part of the Sundance Formation of South Dakota should not be considered as waste disposal reservoirs because they serve as important fresh-water aquifers adjacent to the Black Hills, The Morrison Formation (fig, 24) is not highly regarded as a possible reservoir formation because of its proximity to the Dakota Group and because it comprises a heterogeneous assemblage of discon¬ tinuous beds of claystone, shale, siltstone, and sandstone. However, in local areas, where the shape, thickness, and reservoir character¬ istics of individual sandstone bodies could be studied in detail, suitable sandstone reservoirs might be located. . ' 138 The Dakota Group (figs. 24, 25) should not be considered as a reservoir for radioactive wastes because it is an important fresh¬ water aquifer in some parts of the Williston basin and has a high oil-producing potential elsewhere, CONCLUSIONS The Williston basin offers geographically and geologically feasi¬ ble possibilities for the subsurface disposal of radioactive wastes. The thick and varied stratigraphic sequence contains almost all types of subsurface reservoirs now under consideration. Geographic factors such as low population density, relatively level land surface, and large road network, are favorable feature^- of the Williston basin. The size of the basin, moreover, is so great that many possible sites are available where large quantities of wastes probably could be injected with minimal danger of contamination of fresh-water aquifers and oil-producing strata. The strata and types of reservoirs that deserve primary consid¬ eration for waste disposal are the Winnipeg Formation as a deep salaquifer, the Permian, Triassic, and Jurassic salt beds as moderately deep solution caverns, the thick Upper Cretaceous shale beds as shallow hydraulically fractured shale reservoirs, and the Newcastle Sandstone as a shallow shale-enclosed sandstone reservoir. Detailed studies of the pressure gradient and salinity of forma¬ tion waters, based on drill stem tests, and of the porosity, ■ 139 permeability, and detailed stratigraphy of possible reservoir forma¬ tions, based on well cuttings and cores, should precede even the tentative location of disposal sites. Before final site selections are made, further complex studies and tests at tentative locations will be required to determine the flow of injected wastes as well as chemical compatibility to insure adequate and safe containment. It is difficult to escape the conclusion, however, that for maximum safety of population, preference in the selection of disposal sites should be given to unpopulated or to least populated areas. .. r - - ' J ' . ■ . J : ■ ■ . "■ ■ ' ; O’ . ; 1 r- - .. i . • ; • • ; • ! * .. 140 REFERENCES CITED American Commission of Stratigraphic Nomenclature, 1961, Code of stratigraphic nomenclature: Am. Assoc. Petroleum Geologists Bull., v. 45, p. 645-665. Anderson, S. B., and Hansen, D. E., 1957, Halite deposits in North Dakota: North Dakota Geol. Survey Rept, Inv, no. 28. Baillie, A, D., 1953, Devonian system of the Williston basin area: Manitoba Dept, Mines and Nat. Resources, Mines Br,, Pub 6 52-5, 105 p, Bateman, A. F., Jr., 1957, Structure contour map of the Wesson anticline, Williston basin, North Dakota: U.S. Geol. Survey open-file rept, Benson, W. E., 1949, Golden Valley Formation of North Dakota (abs.): Geol. Soc. America Bull., v. 60, p. 1873-1874. 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Guidebook, Black Hills Field Conf.: p, 49-55. _1956, Pre-Piper post-Minnekahta red beds in the Williston basin, in North Dakota Geol. Soc,, Williston Basin Symposium, 1st Internat., Bismarck, Oct, 1956: p 0 170-178 [1957], -^OF SUBSURFACE USASE FIGURE 6.- CORRELATION CHART OF PRE-TERTIARY FORMATIONS IN WILLISTON BASIN PENNSYLVANIAN ' PERMIAN >ssic[ JURASSIC WARREN PETROLEUM CO. I CURRY CENTER NW'4SE'4 SEC. 20, T IS. , R. 60 E. CARTER COUNTY, MONT. ELEV 3154KB AMERADA PETROLEUM CORP. I STATE N Wk»NW'4 SEC. 4, T. 14 N., R. 4 E. BUTTE COUNTY. S. DAK. ELEV 3028KB I HERNDON DRILLING CO. I SHINOST CENTER SE'/ 4 SE'/4 SEC. 26, T 8 N., R. 13 E. MEADE COUNTY. S. DAK. ELEV 2932KB POHLE-GILMARTIN I MAY SE'/ 4 NE'/ 4 SE'/ 4 SEC. 21, T. 4 N., R 18 E. HAAKON COUNTY, S. DAK. ELEV 2568KB PAUL G. BENEDUM I SCHAFFER CENTER NW'/ 4 NW'/ 4 SEC. 26, T 6 N., R 27E STANLEY COUNTY. S. DAK ELEV 2137KB GENERAL CRUDE OIL CO. I STRAKA N WV 4 NW'/ 4 N W'/ 4 SEC. 22, T 105 N., R. 72 W LYMAN COUNTY, S. DAK. ELEV I73IKB MONTANA jSOUTH DAKOTA - 48 Miles SPONTANEOUS POTENTIAL CURVE NORMAL RESISTIVITY CURVE GAMMA-RAY CURVE KB KELLY BUSHING TD TOTAL DEPTH FIGURE 17. -CROSS SECTION FROM SOUTHEASTERN MONTANA TO SOUTH-CENTRAL SOUTH DAKOTA TO 8600 ft PRECAMBRIANl cretaceous FIGURE 5. GENERALIZED STRATIGRAPHIC SECTION -1_______PALEOZOIC ----- _ _____ CAMBRIAN ORDOVICIAN SILURIAN riFvn n i a m 1 T-1-1--- T - ME S O ZOlC _CENOZOIC ERA SYSTEM SERIES "I 32 L2 OLIGOCENE >• cr < h- cr UJ PLEISTO¬ CENE MIOCENE EOCENE PALEO- CENE c n o u o < h- UJ ce o o in w> < cr r> —> UPPER GROUP FORMATION MEMBER GLACIAL DRIFT ARIKAREE FORMATION WHITE RIVER FORMATION GOLDEN VALLY FORMATION FORT UNION FORMATION SENTINEL BUTTE MEMBER TONGUE RIVER MEMBER CANNONBALL MEMRFR LUDLOW MEMBER HELL CREEK FORMATION MONTANA GROUP LOWER UPPER COLORADO GROUP FOX HILLS SANDSTONE PIERRE SHALE THICK¬ NESS IN FEET A/ GENERALIZED LITHOLOGY 0-500 0 -300 0- 400 0- 175 0-950 0-650 0 - 300 0 - 350 100-550 150-350 1500- 2300 NIOBRARA FM CARLILE SHALE STRATIGRAPHIC SUBDIVISION DISCUSSED IN THIS REPORT ROCKS BETWEEN TOP OF PIERRE SHALE AND SURFACE THICK¬ NESS IN FEET AJ 500- 2800 ISOPACH MAP SHOWN IN THIS REPORT NOT MAPPED NIOBRARA FORMATION AND PIERRE SHALE 100 - 200 -——— 200-500 -— GREENHORN FM BELLE FOURCHE SHALE MOWRY SHALE NEWCASTLE SS DAKOTA GROUP SKULL CREEK SHALE FALL RIV £M LA KOTA F~M MORRISON FORMATION SWIFT FORMATION RIERDON FORMATION MIDDLE TRIASS 1C PERMIAN PENN¬ SYLVANIAN PIPER FORMATION DUNHAM S£TT JL SAUDE FORMATION^ SPEARFISH FORMATION PINE SALT 1/ MINNEKAHTA LIMFSTONF OPECHE FORMATION |UN NAMED SALT 50-200 150-450 _ 50-200 1650- 2500 FIG. 28 BELLE FOURCHE SHALE, GREENHORN FORMATION, AND CARLILE SHALE LOWER PART OF COLORADO GROUP 400- MOO DAKOTA GROUP AND RELATED ROCKS 350-400 125 - 475-EI- 150-400 0 - 140 50-325 0-500 MINNELUSA FORMATION AMSDEN FORMATION CL Q_ LO CO ■S) T> UPPER TYLER FORMATION ZJ BIG SNOWY GROUP LOWER > UPPER MADISON GROUP HEATH FM OTTER FM KIBBEY SS 0-425 TT^i- 1 7 I / I POST - SAUDE JURASSIC ROCKS 250- 400 150-400 FIG. 27 NOT MAPPED FIG. 2 6 NOT MAPPED 800- 13 00 SPEARFISH ANDj SAUDE FORMATIONS 0 - 500 0 -150 0 - 100 0 - 100 0-225 CHARLES FORMATION MISSION CANYON LIMESTONE LODGE POLE LIMESTONE 0-275 525- 725 7A| / 1 / 7~ \ / \ r miiriiM iln mf irpTtrrr mfini ni | iifnnrrnpTr i in ill11in ii in nrnTTTTT OPECHE FORMATION AND MINNEKAHTA LjIMESTONE MINNELUSA FORMATION AND RELATED ROCKS 100-700 FIG. 21 FIG. 24 FIG. 19 FIG. 22 FIG. 20 0-475 0-750 BIG SNOWY GROUP IIIJITMIllll^llfllj 350- 775 BAKKEN FORMATION THREE FORKS FORMATION MIDDLE -LOWER JEFFERSON GROUP BIRDBEAR FM 525- 850 0-140 0-240 0-120 0-600 MADISON GROUP / I—/A -7 1 =\ 7 1/ 1 Bri3 T\ /a 1—/ ' ~ ' B ■ 5/J DUPEROW FM SOURIS RIVER FORMATION DAWSON BAY FORMATION MIDDLE LOWER UPPER MIDDLE LOWER UPPER PRAIRIE FM ELK POINT GROUP |WINNIPEGOSIS FM BEARTOOTH BUTTE (?) FM SALT M8R LOWER MBFi INTERLAKE FOR M A TION 2 / STONEWALL FORMATION 2/ STONY MOUNTAIN FM RED RIVER FORMATION WINNIPEG FORMATION MIDDLE DEADWOOD FORMATION 125-450 100-340 25-155 0-400 0- 125 50-300 o- 50 300- 1100 BAKKEN FORMATION 7-1 /— 7= ~l > — I JZZ EZ iTtTmC i „,l T 7" 1400- 2300 FIG. 18 NOT MAPPED NOT MAPPED FIG. 15 0-140 DEVONIAN ROCKS zxz 50-100 140-200 410-700 130-350 70-1020 v r n< 2® TTT 300- 2000 STONEWALL AND INTERLAKE FORMATIONS RED RIVER AND STONY MOUNTAIN FOR M ATIONS I / i~/ 7-V/ IV WINNIPEG FORMATION DEADWOOD FORMATION AND RELATED ROCKS Jj FIG. 16 FIG. 13 FIG. II 350- 1200 550- 900 130- 3 50 70-1020 FIG. 12 FIG. io FIG. 9 FIG. 8 FIG. 7 —^ -*---__ FIGURE 5, GENERALIZED STRATIGRAPHIC SECTION OF SEDIMENTARY ROCKS IN EXPLANATION ' °/ \° A/i 0 ° x ■ / C/\ 01 x O,/ O/S O^O/N o' GLACIAL DRIFT SANDSTONE SILTSTON E SHALE, CLAY, OR MUDSTONE BLACK SHALE IN PALEOZOIC OR LIGNITE IN TERTIARY — / \ / / ZLL LIMESTONE OR DOLOMITE SALT (HALITE ) OR ANHYDRITE G LAUCONITIC 5 * M f SILICEOUS A CHERTY o O o o c?o • CONGLOMERATIC SANDY SHALY OR ARGILLACEOUS 1000 FEET 500 1 / 1/ J/'Z 2 / o 500 SCALE IN CENTRAL WILLISTON BAS OF INFORMAL SUBSURFACE L STRUCTURE CONTOUR MAPS THESE HORIZONS SHOWN ON 25, 23, 14, AND4 , RESPECTI' AGGREGATE THICKNESS MAP 7 SALT BEDS CENTRAL WILLISTON BAS 12 112939589