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Stratigraphy and Nomenclature of Some Upper Cretaceous and Lower Tertiary Rocks in South-Central Wyoming By]

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Stratigraphy and Nomenclature of Some Upper Cretaceous and Lower Tertiary Rocks in South-Central Wyoming By] Stratigraphy and NOmenclature of Some Upper Cretaceous and Lower Tertiary Rocks in South-Central Wyoming By]. R. GILL, E. A. MEREWETHER, and W. A. COBBAN ~ GEOLOGICAL SURVEY PROFESSIONAL PAPER 667 .. Regional stratigraphic studies and ammonite zonat~·on are used in interpreting complex intertonguing and subtle facies changes in· rocks of Late Cretaceous and early Tertiary age ... ) UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1970 ,:0: ! UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director .... Library of Congress catalog-card No. 72-604715 ... ·<. For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.. C. 20503- Price 65 cents (paper cover) • CONTENTS Page Page Abstract------------------------------------------- 1 Mesaverde Group and Mesaverde Formation-Con. Introduction and acknowledgments __________________ _ 1 Allen Ridge Formation _______________ - ______ ---- 24 Historical background ______________________________ _ 2 Pine Ridge Sandstone __ - ___ -------_--- ___ --_---_ 29 Almond Formation __________________________ - --- Southwestern Colorado _________________________ _ 2 31 Lew~ Shale _______________________________________ _ Northwestern Colorado _________________________ _ 2 36 Fox Hills Formation ____________ - _____ ----- _____ ---- 41 South-central Wyoming ________________________ _ 3 Medicine Bow Formation __ ---_---------_------ __ ---- 43 SuminarY-------------------------------------- 5 Foote Creek and Dutton Creek Formations ___________ _ 45 Steele Shale _____ -- ___ -_---- ___________ -- ___ - __ --- __ 7 Ferris Formation __________________________________ _ 46 Mesaverde Group and Mesaverde Formation __________ _ 11 Hanna Formation ____________________ - __________ ---- 47 Haystack Mountains Formation _________________ _ 12 References cited ________________ -_----_---_--- __ ---- 48 Index ____________________________________________ _ Rock River Formation _________________________ _ 20 51 ILLUSTRATIONS Page FIGURE 1. Index geologic map of south-central Wyoming _______________________ ----_--- ___ - ___ -_ 4 2. Stratigraphic diagram showing intertonguing relations of Upper Cretaceous rocks in south- central Wyoming-------------------------------------------------------------- 8 3-15. Photographs: 3. Haystack Mountains Formation _____________________________ ---_-- ____ - __ --_ 12 4. Hatfield Sandstone Member of Haystack Mountains Formation ________________ _ 13 5. Thin-bedded O'Brien Spring Sandstone Member of Haystack Mountains Forma- tion-------------------------------------------------------------------- 14 6. O'Brien Spring Sandstone Member__________________________________________ _ 14 7. Allen Ridge Formation at Pine Tree Gulch __________________________________ _ 25 8. Outcrop of an unnamed marine member of Allen Ridge Formation ______________ _ 25 9. Pine Ridge Sandstone _____________________________________________________ _ 30 10. Almond Formation-------------------------------------------------------- 33 11. Concretionary sandstone in Lewis Shale _____________________________________ _ 36 12. Lewis Shale--------------------------------------------------------------- 37 13. Cliff-forming sandstone of the Dad Sandstone Member of the Lewis Shale_- ____ -_ 38 14. Rocks of the Hanna Formation resting unconformably on the Medicine Bow For- mation----------------------------------------------------------------- 44 15. Hanna Formation unconformably overlying the Lewis Shale ___________________ _ 48 TABLE Page TADLE 1. Chart showing correlation of Upper Cretaceous rocks in south-central Wyoming and nearby areas---------------------------------------~--------------------------------- 6 m STRATIGRAPHY AND NOMENCLATURE OF SOME UPPER CRETACEOUS AND LOWER TERTIARY ROCKS IN SOUTH-CENTRAL WYOMING .,.. By J. R. GrLL, E. A. MEREWETHER, and W. A. CoBBAN ABSTRACT marine origin. It grades eastward into the dominantly nonsandy Upper Cretaceous rocks described and correlated in this report Pierre Shale. In southeastern Wyoming, the Mesa verde Group is conform­ are the Steele Shale, Mesa verde Group (consisting of the Rock River l!'ormation (new name), Haystack Mountains )formation ably overlain ·by the Lewis Shale. The Lewis, 2,200-2,600 feet thick, is chiefly dark-gray marine shale but contains sandy units. (new name), Allen Ridge Formation, Pine Ridge Sandstone, and In the Hanna Hasin, the Lewis is divided into three parts by the Almond Formation), Lewis Shale, E'ox Hills Formation, Medi­ eine Bow ll..,ormation, and the lower part of the Ferris Forma­ Dad Sandstone l\Iember, which is as much as 1,400 feet thick. ·The Fox Hills Formation conformably overlies the Lewis tion. '.rhe lower Tertiary rocks described consist of the upper Shale. It is chiefly ridge-forming yeJNowish-gray to light-brown part of the Ferris ll..,ormation of Paleocene age and the overlying shallow-water marine sandstone and ranges in thickness from Hanna ll..,ormation of late Paleocene and Eocene age. 'l'he oldest unit Jnvestigated, the Steele Shale, consists of 2,300- less than 200 to more than 700 feet. The Medicine Bow Formation, 3,000-6,500 feet thicl{, rests 8,800 feet of dark-gray marine shale that contains limestone conformably on the Fox Hills. Lenticular beds of sandstone, concretions, beds of bentonite as much as '5 feet thick, and· thin siltstone, and shale, ·and persistent beds of coal make up the layers of siltstone ami very fine grained sandstone. It becomes sandy upward nnd grades into the overlying Mesaverde Group. formation. Most <lf the formation is nonmarine, but the lower 'The Mesaverde Group, in the Rawlins-Medicine Bow area, con­ part contains brackish-water beds. sists of the following formations, from oldest to youngest: Hay­ The Ferris Formation, <lf nonmarine origin, conf<lrmably stack Mountains Formation, Allen Ridge Formation, Pine Ridge overlies the Medicine B<lw and is divisible into two parts. The Sandstone, and Almond Formation. The Haystack Mountains lower part, 1,100 feet thick, consists of conglomeratic sandst<lne, Formation comprises a sequence of mostly shallow-water marine sandstone, siltst<lne, ·and shale. The congl<lmeratic beds contain sandstone and shale that thins southwestwardly across south­ small well-rounded pebbles of resistant rocks such as chert and quartzite. The upper part, about 5,400 feet thicl{, is finer grained central 'Vyoming from 2,550 to 850 feet. It contains three persist­ • ent ridge-forming sandstone members interpreted as beach and than the lower part and consists of sandstone, shale, and coal. barrier-bar deposits. The oldest member, Tapers Ranch Sand­ The Hanna· F<lrmation unconformably overlies the Ferris or stone (new name) , is 270 feet thick ·and forms the base of the older formati<lns. It is as much as 7,000 feet thick and consists formation. The second member, O'Brien :Spring Sandstone (new of nonmarine congl<lmerate, sandstone, shale, and coal. The con­ name) is 220 feet thick and lies 1,350 feet below the top of the glomerates contain large cobbles of locally derived rocks. formation. The Hatfield Sandstone Member, as much as 167 The names Foote Creek and Dutt<ln Creek, recently proposed feet thick, lies 450 feet ·below the top. by H. J. Hyden, Harry McAndrews, and R. H. Tschudy (1965), Nonmarine sandstone, shale, and carbonaceous beds largely are abandoned. The Foote Creek is a remnant of the Medicine mal{e up the Allen Ridge Formation, but some sandstone and Bow Formation, and the Dutton Creek is part of the Hanna shale of brackish-water and marine origin are included. The Formation. Allen Ridge is conformable with the underlying Haystack Mountains Formation. It is as much as 1,500 feet thick. It inter­ INTRODUCTION AND ACKNOWLEDGMENTS tongues with the Rock River Formation, and in the Laramie Basin area, it is replaced by that formation. Regional stratigraphic and paleontologic studies and Dlsconformably overlying the Allen Ridge Formation is the detailed geologic mapping in south-central Wyoming Pine Ridge Sandstone, a white t<l light-gray nonmarine sand­ have demonstrated the need for revision of the strati­ stone 60-450 feet thick. The :formation contains carbonaceous graphic nomenclature for the rocks of Late Cretaceous shale and impure coal, ·and <lrdinarily f<lrms a conspicuous age. The ammonite zonation of the Upper Cretaceous tree-covered ridge. The Almond Formation, 30-575 feet thick, conformably over­ rocks in the Western Interior established by Cobban lies the Pine Ridge Sandstone. It is a sequence of sandstone, provides a framework for regional stratigraphic studies shale, and coal, representing fluviatile, brackish-water, and near­ ~nd can be used in interpreting the complex intertongu­ shore marine environments of deposition. Ing and subtle facies changes within these rocks. In this In the southeastern part of th4~ area (Laramie Basin), the Mesaverde Group consists of the Rock River F<lrmation and Pine report we describe some of these complexities and at­ Ridge Sandstone. The Rock River is a 1,500-foot-thick sequence tempt to make the nomenclature more compatible with of nonresistant sandstone and sandy shale of shallow-water that used in adjacent areas. 1 2 STRATIGRAPHY AND NOMENCLATURE, SOME CRETACEOUS AND TERTIARY ROCKS, SOUTH-CENTRAL WYO. We acknowledge the assistance of Drs. D. L. Black­ ment sandstone as the Point Lookout Sandstone and re­ stone, Jr., and R. S. Houston, of the University of Wyo­ ferred the three new formations to the Mesa
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