Origins of Amalgamated Sand Bodies in the Lance Formation, Bighorn Basin, Wyoming

Jennifer McHarge
University of Wyoming, Department of Geology and Geophysics Laramie, Wyoming
[email protected]

Allogenic processes have long been considered the prime control on the distribution of channel bodies in areas influenced by base-level changes. However, the importance of autogenic processes on the construction of the stratigraphic record and the distribution of reservoir-scale features is becoming more appreciated, as is their influence on reservoir quality and distribution in alluvial basins.
Sand-dominated (high net-to-gross) intervals in the Lance Formation (Maastrichtian, Big Horn Basin, Wyoming) have been interpreted as incised-valley complexes, cut during sea-level lowstands and filled during subsequent sea-level rises. However, closely-spaced sand bodies in the Ferris Formation (Lance equivalent, Hanna Basin, Wyoming) are interpreted as aggradational in origin, and have been compared to autogenic avulsion stratigraphy produced in experimental basins. This study evaluates the Lance Formation in an effort to determine whether these high net-to-gross intervals are truly incised-valley fills resulting from sea-level changes, or if they were generated by another process.
The Lance Formation crops out in the western and southern margins of the basin, exposing relatively proximal and distal portions of the system. By comparing alluvial architecture between exposures, we evaluate similarities and differences from upstream to downstream. In particular, the size and geometry of sand bodies, scaled by paleoflow depth, may provide a means for determining the relative importance of base-level controls on of the observed architecture. Our preliminary results indicate that avulsion clusters dominate upstream, but that relative sea-level change may play a role in alluvial architecture in the downstream part of the system.

AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid