Mary Kraus¹, Andrew Pulham², Erich Heydweiller², Mason Dykstra³

(1) Univ of Colorado, Boulder, CO
(2) University of Colorado, Boulder, CO
(3) Anadarko Petroleum Corporation, Houston, TX

ABSTRACT: Alluvial Architecture and Subsidence Rates: Variability in the Early Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

Potential controls on alluvial architecture have been investigated in foreland basin deposits of the Big Horn Basin, Wyoming, USA. Two time equivalent sections that record contrasting sediment accumulation rates have been described in terms of sandstone types and geometry and in their paleosol attributes. Climatic factors and sediment yield for both areas are considered comparable, enabling investigations to concentrate on the effects of variable subsidence rates.

Area A has only half the stratigraphic thickness for the same time interval as Area B. Sandstone bodies in Area A are mostly single storey, ribbon channels and rare thin sandstone sheets that are encased in moderately mature to mature paleosols. Multistorey channels sandstones in Area A are relatively thin (10-12m) and comprise sheet-like units up to 1.5kms across. Vertical and lateral spacing between these candidate trunk channel systems is 100's to 1000's of meters.

In contrast, Area B has a greater variety of both small and large scale multistorey and single storey channel sandstones than in Area A. Paleosols in Area B are generally less mature than in Area A and are also more laterally variable. The multi-storey channel sandstones in Area B are up to several 10's meters in thickness and exhibit greater lateral variability in thickness than observed in Area A. The vertical and lateral spacing of individual channels and multistorey sandstones in Area B is much less than in Area A. This results in greater vertical connectivity of sandstones in Area B than occurs in Area A.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado