Stratigraphy of the Lnyan Kara Group in the Black Hills Rjy KARL M

Stratigraphy of the lnyan Kara Group in the Black Hills rJy KARL M. WAAGE 20NTRIBUTIONS TO GENERAL GEOLOGY :7EOLOGICAL SURVEY BULLETIN 1081-B This report concerns work done on behalf if the U.S. Atomic Energy Commission znd is published with the permission of ·he Commission NIT ED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 19 59 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. G<W'ei'Rmen1: Prin~ Office Washington 25, D.C. - Price 40 cents (paper cover}. CONTENTS Page Abstract---------------------------------------------------------- 11 Introduction______________________________________________________ 12 Regional pattern of Early Cretaceous sedimentation___________________ 13 History of the Black Hills nomenclature______________________________ 18 Twofold lnyan Kara subdivision____________________________________ 26 Character of subdivision________________________________________ 26 Relation to existing terminology_________________________________ 28 Revision of nomenclature_______________________________________ 32 Stratigraphic summary_____________________________________________ 33 Lakota formation______________________________________________ 34 Northwestern sequence_____________________________________ 35 Morrison contact in the northwestern sequence________________ 38 Coal-bearing sequence______________________________________ 40 Eastern sequence__________________________________________ 43 Southernsequence_________________________________________ 45 Relationship of the Lakota sequences________________________ 46 Contact of the Morrison and Lakota formations_______________ 50 Age of the Lakota formation________________________________ 52 Contact of the Lakota and Fall River formations__________________ 52 Transgressive disconformity _____ _ _ _ _ _ __ _ _ _ _ _ _ ___ __ _ _ __ _ _ _ _ _ _ 52 Siderite zone______________________________________________ 55 Fall River formation___________________________________________ 58 Typesection______________________________________________ 58 General character and thickness_____________________________ 58 Southeastern and eastern Black Hills_________________________ 59 Northwestern Black Hills _____________________________ ------ 62 Age and environment______________________________________ 63 References cited___________________________________________________ 64 Measured sections of lnyan Kara rocks_______________________________ 68 Index------------------------------------------------------------ 89 ILLUSTRATIONS Page PLATE 2. Darton's type Fuson, Fuson Canyon _________________ Facing 23 FIGURE 5. Lithogenetic equivalents of lnyan Kara rocks_______________ 15 6. Darton's miscorrelation of the Graneros shale_______________ 22 7. lnyan Kara outcrop in the Black Hills region_______________ 27 8. Lakota sections in the northern Black Hills_________________ 37 9. Fan River sections in the southeastern Black Hills___________ 60 III CONTRIBUTIONS TO GENERAL GEOLOGY STRATIGRAPHY OF THE INYAN KARA GROUP IN THE BLACK HILLS By KARL M. WAAGE ABSTRACT Darton's subdivision of beds, originally called Dakota in the Black Hills, has proved difficult to apply outside of a limited area in the southeastern Black Hills in which the names were first applied. As early as 1930 the principal subdivisions, the Lakota, Fuson, and Fall River (Dakota of Darton) forma uons were placed in the Inyan Kara group because they could not be distin guished consistently as separate units. Early miscorrelation of the Fall River (Dakota) with the Dakota sandstone of southeastern Colorado has led to con fusion in the application of Darton's terminology outside of the Black Hills. Stratigraphic studies of the Inyan Kara group reveal a basic twofold litho genetic subdivision which has been recognized in equivalent beds elsewhere in the western interior region. Deposits of the lower part of this twofold divi sion are dominantly sandy sediments of varied continental facies and are allied lithogenetically with the underlying Morrison formation. Deposits of the upper part are dominantly sandy sediments of marginal marine facies allied litho genetically and gradational with the overlying marine Skull Creek shale. The contact of the two parts is a transgressive disconformity of regional extent marking the initial incursion of the Cretaceous sea. The subdivision and nomenclature of the Inyan Kara group is adjusted to conform to this twofold lithogenetic division by refining the definition of the Ii'all River formation so that it corresponds to the upper part, and by extend ing the term "Lakota" to include the entire lower part. The transgressive disconformity becomes the contact of the Lakota and Fall River formations. The Inyan Kara group is retained to include these two formations. The Minne \vaste, called a formation by Darton, is recognized as a local limestone mem ber of the Lakota. Use of the name Fuson as a member of the Lakota is considered permissible only where the Minnewaste limestone member is pres ent. Because of the strictly local nature of much of the Inyan Kara group neither its name nor the names Fall River and Lakota should be used outside of the Black Hills region. The sequence of Lakota rocks in the southern part of the Black Hills ls markedly ditierent from that in the northwestern part. Additional beds are added progressively at the base of the formation as it thickens eastward and southeastward. The relationship of these beds to the underlying Morrison is not completely understood, but they do not interfinger. The base of the Lakota 11 12 CONTRIBUTIONS TO GENERAL GEOLOGY is an admittedly arbitrary, indefinite, and inconstant boundary generally drawn at the base of the first appreciable sandstone bed above the Sundance forma tion, the local, distinctive Unkpapa sandstone excepted. The base of the first dark-gray or black claystone above the variegated marlstone beds of the Mor rison serves as a convenient contact in the absence of thick sandstone bodies. More precise definition of units in the complex of continental facies making up the Morrison-Lakota interval must await more detailed study of these closely related formations. Included plant remains indicate that the Lakota is Early Cretaceous in age. The contact of the Lakota and Fall River formations is a surface of dis conformity that can be found throughout the Black Hills. Its aspect varies from place to place depending on the rock types that are locally in contact ; <lark-gray laminated siltstone is the commonest basal Fall River rock and light-colored claystone or clayey siltstone the commonest upper Lakota rock. Scattered small spherulites of siderite characterize the upper several feet of the Lakota beneath the contact. The major features of Fall River rocks extend throughout the Black Hills area. The chief variations within the formation are the presence of a thin tongue of continental claystone and siltstone in the southern and eastern parts of the Black Hills and the increase in thickness of included massive sandstone subunits in the southern part. Fossils from the Fall River formation are not Oiagnostic as to age, but the formation is gradational with the overlying Skull Creek shale, which contains an Albian fauna. INTRODUCTION Field parties of the U.S. Geological Survey currently working in different parts of the Black Hills found the contacts between the Lakota, Fuson, and Fall River formations too ill-defined to 1nap consistently from place to place and questioned the validity of the Fuson as a formation. During July and August of 1955 and for 5 weeks in August and September of 1956, a special study of this problem was made for the Geological Survey, on behalf of the Divi sion of Raw Materials of the U.S. Atomic Energy Commission, to determine whether any suitable basis existed for subdividing the Inyan Kara group. Results of these studies are the subject of the present paper. Fieldwork in 1955, done with the able assistance of Copeland Mac Clintock, was concentrated largely in the Inyan l(ara outcrops of Crook County, Wyo., and a reconnaissance examination of these beds was made all around the Black Hills. \Vithout the constant help and guidance of the Geological Survey field parties working in the area and without the information they had already acctnnulated and generously gave, it would have been impossible to n1ake much prog ress with the study. I am particularly grateful to Charles S. Robin son, who suggested the study and to his coworkers and assistants, and to Garland Gott and the field parties under his supervision for help in acquainting me with the stratigraphy in the southern Black Hills. I am indebted to Henry Bell, also of the Geological Survey, STRATIGRAPHY OF THE INYAN KARA GROUP, BLACK HILLS 13 who furnished information on Lakota fossils; to Bruce MacPherson of the Atomic Energy Commission, who furnished drill core sam ples of the Lakota-Fall River contact beds; to V. Rama Murthy, graduate student at Yale University, who identified the spherulitic siderite from many samples; and to Daniel Barker, who assisted in the field the summer of 1956. For the opportunity to continue the study in 1956 and 1957 and for his willing help throughout the project I am much indebted to William J. Mapel of the Geological Survey. The desirability of consistent stratigraphic classification in an area where many detailed studies are underway in the highly variable Inyan J{ara beds has prompted presentation at

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