GEOLOGY OF THE FOX HILLS FORMATION (LATE CRETACEOUS) IN THE WILLISTON BASIN OF NORTH DAKOTA, WITH REFERENCE TO URANIUM POTENTIAL by A. M. CVANCARA UNNERSITY OF NORTIl DAKOTA DEPARTMENT OF GEOLOGY GRAND FORKS, NORTII DAKOTA 58202 REPORT OF INVESTIGATION NO. 5S NORTH DAKOTA GEOLOGICAL SURVEY E. A. Noble, State Geologist 1976 PREPARED FOR mE U.S. ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION GRAND JUNCTION OFFICE UNDER CONTRACT NO. AT(05-1)-1633 G1O-1633-1 CONTENTS ABSTRACT ~ag~ INTRODUCTION . 1 ACKNOWLEDGMENTS .... 1 MATERIALS AND METHODS 2 STRATIGRAPHY ......................... .. .. 2 Definition and relationship to other rock units .. 2 Distribution . .. 3 Lithology and sedimentary structures .. 3 Persistence of lithologic units ... .. 7 Contacts .. ... .. .. ., 7 Thickness . .. ... .. 8 STRUcrURE ... 8 PALEONTOLOGY . 9 Fossil groups . 9 Occurrence of fossils · ..... , 9 AGE AND CORRELATION 10 DEPOSITIONAL ENVIRONMENTS .............................. 10 URANIUM POTENTIAL . · 12 General . .. ., 12 Fox Hills Formation . 13 REFERENCES . · 14 ILLUSTRATIONS Figure Page 1. Fox Hills and adjacent Formations in North Dakota (modified from Carlson, 1973) . 4 2. Schematic stratigraphic column of Fox Hills Formation in North Dakota (modified slightly from Erickson, 1974, p. 144). The Linton Member was named by Klett and Erickson (1976). 5 Plate 1. Northwest-southeast cross section (Dunn to Sioux Counties) of Fox Hills Formation in southwestern North Dakota . (in pocket) 2. Southwest-northeast cross section (Bowman to Pierce Counties) of Fox Hills Formation in western North Dakota (in pocket) 3. Southwest-northeast cross section (Adams to Burleigh Counties) of Fox Hills Formation in southwestern North Dakota (in pocket) 4. Isopach map of Fox Hills Formation in North Dakota (in pocket) ABSTRACT model is followed for the deposition of Fox Hills sediments. The lower Fox Hills, The Fox Hills Formation is a marine of the Trail City and Timber Lake and brackish sequence ofprimarily medium Members, is believed to represent the lower and fine clastics within the Late Cretaceous to upper shoreface of a barrier bar Montana Group. In the Williston basin of environment. The upper Fox Hills, of the North Dakota, four members (in ascending Iron Lightning and Linton Members, order) are recognized: Trail City, Timber suggests deposition within a deltaic Lake, Iron Lightning (with Bullhead and complex as does the overlying Hell Creek Colgate lithofacies), and Linton. The Fox Formation. Specific environments may Hills conformably overlies the Pierre Shale have included tidal channel, bay, estuary, and conformably and disconformably prodelta, and various environments underlies and interfingers with the Hell associated with a delta-plain and the Creek Formation; it occurs in about the seaward part of a distributary system. western two-thirds of the state. Principal Water depths may have been a few hundred lithology includes poorly consolidated fee t and less. sandy shale and siltstone (Trail City); fine­ Since the Hell Creek, Fox Hills, and to medium-grained, poorly consolidated Pierre Formations are at least in part and well-indurated sandstone (Timber penecontemporaneous, a variety of Lake); interbedded, poorly consolidated depositional environments were in areal sandstone and shale or siltstone (Bullhead jUx tap osition during Late Cretaceous time. lithofacies); medium-grained, poorly Streams originating or passing through consolidated, muddy sandstone (Colgate coastal plain bogs (of the Hell Creek) could lithofacies); and very fine- to fine-grained, have carried uranium ions (derived from indurated, siliceous sandstone (Linton), A volcanic materials) into bays, lagoons, or conspicuous volcanic ash occurs in the the open sea where they were deposited formation and rises in the section from syngenetically. Epigenetic uranium may central Emmons County to southeastern occur in finer clastics in the Fox Hills that Morton County. Lignite beds occur in the directly underlie a volcanic ash bed. subsurface in part of southwestern North Dakota. Considerable lithologic variation INTRODUCTION occurs in the subsurface away from the main outcrop area in the south-central part The 1Fox- Hills Formation is a marine of the state, and the members are difficult d brac£'ish unit of mostlr,. medi1!-!!WUl. to trace. The thickest recognized sequence 'f1!tLf1asticrdeposited primarily in the is 391 feet in northwestern Hettinger northern Great Plains during.....1a!e County; sequences thicker than 300 feet Q.etaJ:e_ous_ time. It conformably oyerlie_ also occur in two other areas-Dunn and 'tk Pierre S~e and_b.,pth c.Qn(orm3blx ai!ci Mercer, and Burke and Ward Counties. unconformaoly ~derlies the HelrCree Nearly 300 feet of Fox Hills has also been Formation. reported as far east as western Pierce This report summarizes the geology of County. Fox Hills rocks generally dip the Fox Hills Formation in North Dakota, about 10-20 feet per mile toward the the stratigraphy of which is based on center of the Williston basin; regionally, dip previous surface information and recent values may be considerably greater (few to subsurface data, and evaluates its potential many degrees on the flanks of the Cedar for uranium. Creek an tieline) or less. Principal macrofossils are bivalve and gastropod ACKNOWLEDGMENTS mollusks that occur primarily in limestone and sandstone concretions and indurated This study was supported by part of sandstone. A barrier bar (or island)-deltaic grant AT (05-1)-1633 from the u.s. 2 Atomic Energy Commission. The North may not always be present at the top of the Dakota State Water Commission provided Fox Hills (e.g., as indicated in pI. 2, section unpublished subsurface stratigraphic data 13) and the thicknesses gained from logs in for Dunn, Grant, Morton, and Sioux places may be less than they are in reality. Counties. The North Dakota Geological Thicknesses of the unit from logs was also Survey provided the use of mechanical logs partly gauged by a continuous plot of of oil and gas wells. C. G. Carlson, North values on a working map as the study Dakota Geological Survey, provided useful progressed. Depths to the top of the Fox aid and advice during the course of this Hills and other similar data are on We in study. F. D. Holland, Jr., Department of the Department of Geology, University of Geology, University of North Dakota, North Dakota. The letter designation in the reviewed the manuscript. legal description of well or outcrop localities is that of the North Dakota State MATERIALS AND METHODS Water Commission. The cross sections prepared for this STRATIGRAPHY report (pIs. 1-3) are, with One exception, based exclusively on recent unpublished Definition and Relationship and published subsurface data of the North to Other Rock Units Dakota State Water Commission because outcrop sections are rarely complete. These The Fox Hills Formation, originally subsurface data, although based on "Formation No.5" (Meek and Hayden, samples, are, in certain cases, difficult to 1856, p. 63), was named by Meek and use because of inconsistent or incomplete Hay den ( 1 8 6 1 , P . 4 1 9 , 4 2 7) for the descriptions. Contacts of the Fox Hills youngest marine sandy deposits in the were changed in several instances from western interior. The name is from Fox those placed by previous worurs. The Hills (or Fox Ridge), an ill-defined part of datum for the cross sections is the top of the divide between the Cheyenne and the Pierre Shale. The isopach map (pI. 4) is Moreau Rivers in Ziebach and Dewey based on the same type of Water Counties, north-central South Dakota. Commission data as well as on mechanical Waage (1968, f1£;. 1) has outlined the logs; about 200 control points were used approximate "historical type locality," and for this map. About 90 electric logs proposed a larger type or reference area (resistivity, spontaneous potential, and (ftg. 2) that includes the Fox Hills outcrop laterolog) from oil and gas wells were used in most of Ziebach and Dewel Counties to supplement the Water Commission and in Corson County south 0 the valley subsurface data. On these logs, the of Oak Creek. Pierre-Fox Hills contact was picked where Good stratigraphic discussions of the the curves gradually but notably began to formation are by Waage (1968) in the type depart from the shale line as they are area and by Feldmann (1972) and Erickson traced from the Pierre Shale into the (1971, 1974) in North Dakota. overly ing Fox Hills. In the Water Waage (1968, p. 18-48) has Commission wells, this contact was picked comprehensively discussed the history of so as to include shale and silty shale in the study of the formation in the type area, Pierre and siltstone and sandstone in the including the development of the Fox Hills. The upper contact was more classification and nomenclature of the unit difficult to pick consistently on the logs; it (his fIg. 4). His proposed revision of was generally taken where the spontaneous subdivision includes three members (in potential and resistivity decreased abruptly ascending order): Trail City, Timber Lake, at the top of what was interpreted to be a and Iron Lightning. Feldmann (1972, p. sandstone bed about 20 feet or more in 5-15) has traced the history of study of the thickness. A pronounced sandstone bed Fox Hills in North Dakota. His subdivision 3 of the formation recognized three members sandy shale or siltstone that passes upward (in ascending order), the Timber Lake, to brown- and greenish-gray-weathering, Bullhead, and Colgate. Erickson (1974, fIg. poorly consolidated sandstone (Fisher, 3) recognized a subdivision essentially that 1952, p. 10) with abundantly fossiliferous, of Waage (1968, fig. 5) with the addition of medium to dark gray, limestone another member, the Linton (Klett and concretions that weather light tan to light Erickson, 1976), at the top of the sequence gray. These concretions are as large as more (fig. 2). I rrefer to follow the subdivision than three feet in diameter. Erickson scheme 0 Erickson (1974) in North (1974, p.