WROBLEWSKI, ANTON F. J., and RON STEEL, Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006

Abstract: Sandbody Variability and Classification in the Ferris and Hanna Formations (Maastrichtian-Danian): Implications for Timing of Retreat of the Western Interior Sea

Sandstone bodies within the Ferris (ca. 66-63 Ma) and Hanna (ca. 62-55 Ma) formations in Wyoming's Hanna and Carbon basins have been described as braided and/or meandering fluvial channel belts. New sedimentological and paleontological data suggest that many of these sand bodies can be reinterpreted as incised valley-fills (comprised of estuarine and fluvial channel belts as well as tidal flats), tidally-influenced fluvial, and distributary channels.

A newly recognized marine tongue in the Lancian (66-65 Ma) FF suggests local flooding during the latest Cretaceous. Sand bodies within this ca. 300 m-thick interval are single storey, < 5 m-thick, and consist of lower coarsening- and thickening-upward, rippled, accretionary strata, overlain and partially incised by a coarser-grained, trough cross-stratified channel form unit. Paleocurrents in these sand bodies are strongly unimodal, and marine trace fossils are preserved at the bases, suggesting that they represent distributary channels feeding into interdistributary bays.

Single storey tidally influenced fluvial channels and multistorey incised valley-fills in the middle FF and lower HF have more variable paleocurrents reflecting tidal processes and higher sinuosity. Brackish and marine ichnofossils (Thalassinoides, Ophiomorpha, and Teredolites) are found within these sandbodies, suggesting presence of elevated salinities in some of the systems. 

AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid