Michael W. Webb1, Ron J. Steel1
(1) University of Wyoming, Laramie, WY
ABSTRACT: Incised Paleovalleys in the Lance Formation, southwestern Bighorn Basin, Wyoming
The latest Cretaceous Lance Formation in parts of the southwestern Bighorn Basin can be interpreted as incised valley fills related to base-level falls during overall aggradation in the final regression of the Western Interior Seaway. Eastward-trending paleovalleys within the basal Lance Formation fed significant push-outs of the Fox Hills shoreline and were the source of sand bypassed onto slope and basin floor segments of the Lewis Sea. Outcrop photomosaics and measured vertical sections were employed to study reservoir geometries and heterogeneities in the basal Lance Formation. Channel-bar systems, channel belts, and valley fills have been delineated.
Channel-bar systems form units mainly of trough cross-bedded sandstones that vary in thickness from 0.5 to 3 meters and have limited lateral extent. Poorly preserved bars are common due to a combination of rapid channel switching and lack of grain size variation. Channel-bar systems contain no major internal flow barriers. Channel belts are composed of stacked channel-bar systems, vary in thickness from 3 to 12 meters, and cover areas on the order of square kilometers across the Gooseberry Creek study area. Channel belts are separated by significant thicknesses of fine-grained material that were deposited after avulsion events. Valley fills are the largest architectural element present, with probable widths of 2 kilometers and incision depths on the order of tens of meters. Valley fills are evidenced by drastic large-scale changes in sand/shale ratios and depositional style along strike.
The valley-fill elements in the Lance Formation and their linkage to the Fox Hills shorelines are the key to understanding deepwater sand emplacement in the Lewis Shale.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado