--> AAPG Bulletin; #90909 (2000)

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HEYDWEILLER, ERICH C., University of Colorado, Department of Geological Sciences, Boulder, CO

ABSTRACT: Controls on Sandstone Body Amalgamation and Alluvial Architecture in the Fluvial Willwood Formation, Bighorn Basin, Wyoming

A revival of alluvial stratigraphic modeling in recent years has led to debate over the relationship between sedimentation rate and avulsion frequency. This discussion has important implications for studies of alluvial architecture and reservoir geometry. Specifically, sandstone body geometry and connectedness are important considerations in prospecting and producing from reservoirs and aquifers found within ancient fluvial sediments.

Computer and physical models of fluvial systems have produced differing and sometimes conflicting results. This results from ill-defined constraints on certain variables used as input in the models. Avulsion frequency is one of the least constrained variables, primarily due to a poor understanding of the avulsion process and its causes. It is generally accepted that avulsion frequency is related to sedimentation rate, but the nature of the relationship is unclear. Field-testing is necessary to resolve this question, but as yet has produced no clear answer.

This study focuses on two time-equivalent sections of the Willwood Formation (early Eocene) that show significantly different sediment accumulation rates. Previous fieldwork has shown that the two sections contain different styles of alluvial architecture. Because of their time-equivalence, and therefore similar climatic regime, it should be possible to isolate the role of sediment accumulation in determining avulsion frequency and alluvial architecture. The current fluvial models will then be evaluated in view of these results.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid