Several regionally extensive well log cross sections of the Cody Shale were constructed in the Powder River Basin, Wyoming. These sections are hung on the Ardmore Bentonite and include the Sussex, Shannon, Gammon and Niobrara intervals.

This stratigraphic interval is more than 1500 feet thick, dominantly consists of silty shale and contains several regionally extensive and economically important sandstones. Depositionally it represents the distal portions of multiple transgressive-regressive cycles within the Mesaverde Group which were deposited along the western margin of the Cretaceous Interior Seaway. Although most of the intervals show overall thinning in a basinward (east) direction, they do not generally display eastward prograding clinoform geometries. Several intervals however, display southward prograding clinoforms. This direction of transport is consistent with current transport directions measured in the sandstones, all of which show strongly dominant southward flow. In addition, some intervals show thinning and onlap onto what is interpreted as a paleo-high in the eastern part of the basin. Although there is no evidence that this paleo-high was emergent, it may have been prominent enough to influence shelf currents, especially during relative lowstands of sea level.

Numerous regionally extensive discontinuities occur throughout the section studied. While most of the discontinuities typically display little to no evidence of erosional incision, abrupt termination of stratigraphic intervals beneath them occurs locally. Commonly, terminations below several different discontinuities all occur in the same geographic area. We believe these patterns indicate non- deposition and differential erosion of strata occurred across topographically high areas bounded by periodically recurrent synsedimentary faults.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah