WROBLEWSKI, ANTON F.-J.
Department of Geology and Geophysics, The University of Wyoming, Laramie, WY 82071

Abstract: Depositional History Of Paleocene Foreland Basin Strata, Southern Wyoming

Paleocene strata (Hanna Formation) of southern Wyoming's Hanna and Carbon basins represent basinward progradation of two separate clastic wedges. The older of the two wedges represents an ~1,000 m thick, as yet unnamed member of the Hanna Formation and is differentiated from the rest of the unit in being dominated by mature paleosols and shallow (average < 2 m thick), laterally restricted, fine-grained fluvial channels with extraformational conglomeratic lag. Crayfish burrows (up to 4.5 m in length) and ichnofossils of terrestrial arthropods and plant roots are abundant throughout this succession. Midway through this lower member is an interval containing shallow channelforms filled with siltstone, mudstone and fine sand. Lag deposits are comprised of vertebrate fossils and extraformational clasts. Vertebrate fossils are restricted to these silty/muddy channels. Source areas for the lower member were uplifts to the northeast and southeast, as indicated by trough axes and pebble imbrication.

A dramatic rise in base level during the Tiffanian is marked by a lithologic shift from well-drained alluvial deposits to lacustrine units (including sandy and gravelly fluvio-deltaic systems) and poorly drained alluvial units. East and northeastern progradation of a younger clastic wedge is indicated by a change in paleocurrent indicators, suggesting a primarily southern source area (Medicine Bow and Sierra Madre mountains). The fluvial channel sandstone differs in being much coarser and arkosic. Invertebrate (gastropods and bivalves) body fossils are preserved in lacustrine shoreline sands and fluvial deltaic channels, but vertebrates are rare.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas