Biostratigraphic Correlation of Eustatic Cyclothems (Basic Pennsylvanian Sequence Units) from Midcontinent to Texas and Illinois
HECKEL, P. H., University of Iowa, Iowa City, IA, J. E. BARRICK and D. R. BOARDMAN, Texas Tech University, Lubbock, TX, L. L. LAMBERT, University of Iowa, Iowa City, IA, W. L. WATNEY, Kansas Geological Survey, Lawrence, KS, and C. P. WEIBEL, Illinois State Geological Survey, Champaign, IL
The basic Midcontinent Pennsylvanian eustatic cyclothem (transgressive limestone, offshore shale, regressive limestone, nearshore/terrestrial shale/paleosol) has sequence stratigraphic counterparts (major eustatic cycles) in Texas and Illinois, in which the limestone members are poorly developed or localized because of proximity to detrital influx. The gray to black phosphatic facies of the offshore shales in all three areas, however, are characterized by abundant conodont faunas that not only are conspecific at each level across the entire region but also are diagnostic of individual cycles. These faunas, supplemented at several horizons by distinctive fusulinids and ammonoids, allow correlation of up to 16 of the major eustatic cycles at a time scale of about 400,000 years across a arge area of North America. This correlation can be refined to perhaps 100,000 years when the intervening minor eustatic cycles are matched between the biostratigraphically diagnostic horizons. Examples of the faunally diagnostic cycles include latest Desmoinesian Lost Branch Formation (Midcontinent), upper East Mountain Shale (Texas),
Lonsdale/West Franklin limestones (Illinois); early Missourian Hushpuckney Shale/Swope Limestone (Midcontinent), upper Salesville Shale (Texas), Macoupin Limestone (Illinois); middle Missourian Quivira Shale/Dewey Limestone (Midcontinent), mid-Posideon shale (Texas), Reel Limestone (Illinois); late Missourian Eudora Shale/Stanton Limestone (Midcontinent), upper Winchell/Merriman limestones (Texas), unnamed marine horizon in Charleston core (Illinois); earliest Virgilian Little Pawnee Shale/Cass/Haskell limestones (Midcontinent), lower Colony Creek Shale (Texas), Omega/Bonpas limestones (Illinois); early Virgilian Heebner Shale/Oread Limestone (Midcontinent), Finis Shale (Texas), Shumway Limestone (Illinois); and mid-early Virgilian Queen Hill Shale/Lecompton Limestone (Midcontinent), ecessity Shale (Texas), Bogota Limestone (Illinois). Similar successions of conodont faunas are reported in preliminary work on marine horizons in the Appalachians and in published work on the Russian platform augur well for eventual worldwide correlation of eustatic cycles on a time scale within the Milankovitch band of the Earth's orbital parameters during a period of time when glacial eustasy seems to have controlled inundation and withdrawal of the sea over large portions of the continents.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)