STRECKER, UWE, ALAN D. THOMPSON, GLEN E. CHRISTIANSEN, and RANDI S. MARTINSEN, University of Wyoming, Laramie, WY

ABSTRACT: Stratal Geometries in the Powder River Basin: Depositional, Erosional, or Combinations Thereof?

Sequence stratigraphic analysis of a siliciclastic succession (upper Niobrara Formation-Pierre Shale) in the southeastern Powder River basin has resulted in a reevaluation of Cretaceous

depositional and erosional stratal patterns. Previous workers interpreted resistivity log correlations in that area as representing either clinoform depositional topography or erosional topography related to the presence of a large incised valley. The results of this study support both primary depositional and secondary erosional topography.

Within a sequence stratigraphic context, the following depositional and hiatal stratal patterns were identified on the basis of resistivity log correlations: (1) onlap of the Gammon Ferruginous Member (GFM) onto a regionally extensive erosional surface floored by the Niobrara Formation, (2) downlapping clinoforms within the GFM, (3) thinning of topset beds within the GFM, (4) an erosional surface developed by the truncation of clinoforms on top of the GFM, (5) sediment infilling of relief (up to 30 m) developed on the erosional surface displaying a highly distinctive resistivity log signature, (6) onlap of the Ardmore-Pedro bentonite beds onto the lower part of the erosional surface, and (7) various downlaps in the Pierre Shale. The complex pattern of stratal geometries recognized in his study reflects a combination of primary clinoform depositional topography subsequently modified by erosional truncation and slumping.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.