Structural control of the distribution of subtidal to supratidal paleoenvironments of the Americus Limestone Member (lowermost bed) in eastern Kansas
by Jonathan C. Sporleder
Originally published in 1991 as Kansas Geological Survey Subsurface Geology Series 13. The complete report is available as an Acrobat PDF file (3.4 MB)
The Americus Limestone Member of the Foraker Limestone (Wolfcampian) in Kansas formed during transgression of an epeiric sea. Lateral differentiation of paleoenvironments resulted from differences in paleotopography of the seafloor, which overlies buried anticlinal structures. Eight paleoenvironmental stages of transgression are recognized in a typical vertical succession of lithofacies: (1) formation of limestones and shales under hypersaline, brackish, and subaerial conditions and deposition of lime sand under freshwater conditions preceding inundation and deposition of the Americus limestone; (2) deposition of a thin layer of shale during initial inundation; (3) transition from brackish to restricted marine paleoenvironments characterized by development of peloid-alga boundstones (stromatolites); (4) restricted marine paleoenvironments characterized by encrustations of Spirorbis-foraminifer-alga boundstones; (5) deposition of carbonates under conditions that were transitional from restricted marine to nearly normal marine, characterized by abundant Calcivertella foraminifers, ostracodes, and gastropods, and foraminifer-alga boundstone; (6) deposition of carbonates under more nearly normal to normal marine conditions characterized by grapestones, ooids, echinoids, crinoids, bryozoans, brachiopods, sponge spicules, pectinoid and myalinid bivalves, and a variety of foraminifers; (7) turbid marine conditions corresponding to the deposition of shale with some crinoids, brachiopods, and bivalves; and (8) formation of the upper limestones of the Americus under normal marine conditions, characterized by abundant fusulinids, crinoids, brachiopods, bryozoans, and bellerophontid gastropods. The lower limestone of the Americus indicates lateral differentiation of paleoenvironments with more nearly normal marine conditions toward the south. Differences in paleotopography of the seafloor resulted in high areas characterized by supratidal to relatively high-energy shallow- water paleoenvironments and low areas characterized by relatively low-energy deeper-water paleoenvironments. Paleotopographically high areas are generally characterized by packstones and grainstones, thin intervals of overlying shale, discrete plates of Spirorbis-foraminifer-alga boundstone, laterally discontinuous stromatolites with extensive fenestrae, rip-up clasts of boundstone, thick stromatolitic layers with extensive encrustations, high-domed stromatolites, and Globivalvulina foraminifers and pectinoid and myalinid bivalves. Paleotopographically low areas are characterized by lime mudstones and wackestones; thick intervals of overlying shale; and thin, flat-layered to low-domed stromatolites. Paleotopographically high areas of the ancient seafloor coincide with such structural features as the Nemaha anticline, the Alma-Davis Ranch anticline, the Bourbon arch, and the Beaumont anticline, indicating that these structural features were ultimately responsible for lateral differentiation of paleoenvironments either by differential compaction or by subtle tectonic movement. It should be possible to use these relationships in similar strata to locate potential anticlinal structures.
This report resulted from work done at the University of Kansas in partial fulfillment of the requirements for the degree of Master of Science. Roger Kaesler suggested and supervised the project and provided invaluable help in editing the manuscript. Paul Enos and Richard Robison served as committee members and provided numerous helpful suggestions and criticisms. Ron West, of Kansas State University, and Jim Chaplin, of the Oklahoma Geological Survey, reviewed the manuscript and provided constructive criticism. Robert Goldstein generously shared his knowledge concerning carbonate rocks. Larry Denver showed several outcrops within his study area as an introduction to the field relationships of the Americus Limestone Member. Ken Hood and Brian McNeice listened to some of my initial ideas and provided constructive criticism. Scott Johnsgard provided and helped with the examination of air photos. Frank Wilson, Brian Stevens, Dave Newell, Jim Anderson, and Mike Lambert, of the Kansas Geological Survey, provided information that was useful in finding outcrop localities. Alan Kamb generously provided specimens from the Kansas Museum of Invertebrate Paleontology. Brian McNeice, Burt Rowell, Byron Wiley, and Donna DeCarlo assisted with the photography. Jim Anderson, Donald Sprowl, and Paul Sporleder gave advice concerning the computer graphics. Mike Johnson helped with the fieldwork in Elk County. Jim Pilch picked ostracodes from crushed rock samples. Janet Sporleder provided support and also served as a field assistant.
Research facilities, supplies, and financial support were provided by the Geology Department of the University of Kansas and by a Haworth Memorial Tuition Scholarship.
Samples 227,000 to 227,022 have been reposited at the Kansas University Museum of Invertebrate Paleontology.
Kansas Geological Survey, Geology
Placed on web Sept. 8, 2011; originally published 1992.
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