Stratigraphy and Composition of the Sharon Springs Member of the Pierre Shale in Western Kansas GEOLOGICAL SURVEY PROFESSIONAL PAPER 728 Stratigraphy and Composition of the Sharon Springs Member of the Pierre Shale in Western Kansas By JAMES R. GILL, WILLIAM A. COBBAN, and LEONARD G. SCHULTZ GEOLOGICAL SURVEY PROFESSIONAL PAPER 72 Stratigraphic and miner alogic study of one of the more distinctive and widespread shale units of Late Cretaceous age in the western interior of the United States UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600083 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 75 cents (paper cover) Stock Number 2401-2104 CONTENTS Page Abstract............................................. 1 Composition.......... ................................................................ 20 Introduction...................................... 1 Previous studies..... ............ .......... ............... 20 Acknowledgments............................. 3 Mineralogy .............................................................. 22 Historical background..................... 3 Niobrara Formation........................................... 23 Geologic setting................................ 5 Sharon Springs Member of the Pierre Shale. 23 24 Stratigraphic resume...................... 6 Upper members of the Pierre Shale. 24 Niobrara Formation................ 6 Bentonites............................................................. Basaluminite and hydrobasaluminite. .......... 26 Pierre Shale ...... ................... 7 Potassium feldspar....................... ................... 30 Sharon Springs Member. 7 Barite and apatite.............................................. 30 Weskan Member............... 7 Chemical composition................................................. 31 Lake Creek Member......... 9 Mode of occurrence of elements.----------. ------ 34 Salt Grass Member.......... 10 Leaching of surface samples...... ... ........... 37 Unnamed member............ 10 Bentonites............................................................. 37 Beecher Island Member... 10 Comparison with other shales...... .. 37 Sharon Springs Member................. 10 Major elements . 37 Dark soft shale unit................. 11 Minor and trace elements.......................... 38 Organic-rich shale unit........... 11 Measured sections...-.-.--------.- ............ ........ 39 Phosphatic shale unit. ... 14 References cited.................... ....... .............. 45 Rate of sedimentation.............. 16 Index.......... ........................................ ... 49 ILLUSTRATIONS [Plates follow index] PLATE 1. Mollusks from the Baculites obtusus zone. 2. Mollusks from the Baculites asperiformis zone. Page FIGURE 1. Map showing location of six measured sections 2 2. Paleogeographic map during latter part of Niobrara time 6 3. Chart showing time span of Pierre Shale in western Kansas and equivalent rocks in nearby States......... 8 4. Graphic section and electric log of Sharon Springs Member of Pierre Shale in Wallace and Logan Counties, Kans............................................................................................................................................... 12 5-8. Photographs showing: 5. Contact of Sharon Springs Member and Niobrara Formation...--......--- .. 14 6. Outcrop of dark soft shale unit of Sharon Springs Member--.-....... 15 7. Hard buttress-weathering shale of Sharon Springs Member. 16 8. Northeasternmost outcrop of Sharon Springs Member..-............-.....-................. 17 9. Diagram showing eastward and upward migration of organic-rich shale unit of Sharon Springs Member.. 17 10. Paleogeographic map of early stage of deposition of Sharon Springs Member 18 11-15. Photographs showing: 11. Gypsum rosettes............... ................................................................................................................................ 18 12. Irregularly shaped limestone concretions....--.-......--.--.----.--..... 18 13. Septarian limestone concretions........-.......-.-.............................--..--...---...-.-...................... 18 14. Cone-in-cone limestone concretions............................................................................. .........--- ..... 19 15. Contact of phosphatic and organic-rich shale units. ........................ .. 19 16. Paleogeographic map of late stage of deposition of Sharon Springs Member.....-..... 19 17. Electric and gamma-ray log correlation in Kansas, Colorado, Nebraska, and South Dakota. 20 ill IV CONTENTS FIGURES 18-22. Photographs showing: Page 18. Phosphatic shale unit............................................................................................................... 22 19. Contact of Sharon Springs and Weskan Members. ........................................................ 22 20. Phosphate nodule from phosphatic shale unit.................................................................... 22 21. Phosphate nodules from phosphatic shale unit.................................................................. 23 22. Contact of Sharon Springs and Weskan Members............................................................. 24 23. Diagram showing thickness and implied rates of sedimentation of Pierre Shale in western Kansas and other localities....................... ................................................................................... 28 24. Schematic structure of a montmorillonite layer....................... ....................................................... 30 25. Graph showing distribution of elements in sample cuttings from the Guy Holland test hole. 32 TABLES Page TABLE 1. Analyses of sample cuttings from the Sharon Springs Member of the Pierre Shale and the Smoky Hill Member of the Niobrara Formation from the Guy Holland test hole..................... 26 2. Analyses of samples from the Sharon Springs and Weskan Members of the Pierre Shale from outcrops near the Guy Holland test hole..... ...................................................................................... 28 3. Mineralogy of samples from the Pierre Shale... ...................................................................................... 30 4. Mineralogy of bentonites............................................... ................................................................................ 30 5. Approximate lower limits for visual detection of elements...................................................................... 31 6. Mean content and inferred occurrence of chemical elements and minerals in samples from the Pierre Shale and Niobrara Formation....................--....-----....-.-... --.................................................... 33 7. Correlation coefficients between elements and minerals in sample cuttings from the Guy Holland test hole and nearby outcrops-..-.......--......-... ........................................................................................ 34 STRATIGRAPHY AND COMPOSITION OF THE SHARON SPRINGS MEMBER OF THE PIERRE SHALE IN WESTERN KANSAS By JAMES R. GILL, WILLIAM A. COBBAN, and LEONARD G. SCHULTZ ABSTRACT The maximum thickness in Kansas is about 1,600 The Sharon Springs Member of the Pierre Shale is a wide­ feet in extreme northwestern Cheyenne County spread unit that can be recognized readily in outcrops by its (Merriam, 1963, p. 42). Elias (1931, p. 56-57) distinctive lithologic character and in the subsurface by elec­ divided the Pierre into the following members, from tric and gamma-ray well-log characteristics. The Sharon oldest to youngest: Sharon Springs, Weskan, Lake Springs Member is in Kansas, Nebraska, North and South Dakota, and parts of Colorado, Wyoming, Montana, and east­ Creek, Salt Grass, an unnamed member, and Beecher ern Utah. Its type locality is in Wallace County, Kans., where Island. the member is about 215 feet thick and consists of three units: The Sharon Springs Member of the Pierre Shale a lower dark soft shale unit 115 feet thick, a middle hard or­ is stratigraphically unique. It differs from the rest ganic-rich shale unit 90 feet thick, and an upper hard slightly of the Pierre Shale because of its dark color, resis­ phosphatic shale unit about 10 feet thick. The Sharon Springs contains abundant vertebrate remains but few mollusks. tance to erosion, richness in organic material, and Minerals in the Sharon Springs Member, as determined high radioactivity. At most localities, many ben- from well cuttings, are predominantly clay minerals but in­ tonite beds are in the lower part of the member, and clude about 25 percent quartz and several percent each of gray septarian limestone concretions are conspic­ pyrite, dolomite, organic matter, and, in the lower part where uous in many places. The member derives its name the member grades into the Niobrara Formation, calcite. The from the town of Sharon Springs in Wallace County most abundant clay mineral is mixed-layered montmorillonite- illite; there are lesser amounts of illite, kaolinite, and chlorite. near the west boundary of Kansas (fig. 1). Pyrite, dolomite, and chlorite are leached from outcrops. Ben- From 1956 to 1968, a detailed regional study of tonite beds in the Sharon Springs contain unusually large the stratigraphy, paleontology, mineralogy, and amounts of kaolinite and beidellite