--> Abstract: Alluvial Architecture And Paleosols Of Lower Eocene Strata, Bighorn Basin, Wyoming, by M. J. Kraus, A. J. Pulham, and M. Dykstra; #90928 (1999).

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KRAUS, MARY J., ANDREW J. PULHAM, and MASON DYKSTRA
University of Colorado, Boulder, CO

Abstract: Alluvial Architecture and Paleosols of Lower Eocene Strata, Bighorn Basin, Wyoming

Preliminary results are presented of an analysis of the alluvial architecture and floodplain paleosols of the lower Eocene Willwood Formation in the McCoullough Peaks area of the Bighorn Basin. A detailed reconstruction of a 250m thick stratigraphic sequence covering approximately 3 sq mi allows lateral and vertical variations in the paleosols and alluvial architecture to be identified. Laterally-extensive paleosols form the stratigraphic framework for the alluvial architecture analysis. In particular, well-developed floodplain paleosols change upsection from purple paleosols with hydromorphic features to red paleosols with few hydromorphic properties. This suggests that soil drainage improved due to either a climate change and/or some change in the sub-regional Willwood fluvial system.

Sandstone bodies, predominantly channel deposits, can be grouped into two major categories: (1) laterally extensive, amalgamated sandbodies that have thicknesses on the order of 20m and WIT ratios of ~100 and (2) laterally restricted, isolated, ribbon-like sandbodies that are usually less than 10m thick and have W/T ratios of ~10. The large sandbodies are interpreted as the deposits of trunk rivers. The smaller sandbodies are interpreted as either feeders to the trunk systems or parts of avulsion complexes. Commonly, the ribbon sandbodies are stratigraphically closely spaced and separated by compound paleosols representing short intervals of soil formation. This pattern suggests that small feeder or crevasse channels episodically relocated to particular areas of the floodplain and that individual channels were short-lived. Paleosols are less well preserved around the trunk river complexes, but still provide information about likely dvnamics of these systems.

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