ABSTRACT: Cambrian Extensional Tectonics and Seismic Stratigraphy of the Powder River Basin, Wyoming

Kenneth J. McMillen

Seismic sequence stratigraphic interpretation of regional set of multichannel seismic-reflection profiles from the Powder River basin, Wyoming, shows distinct Cambrian stratigraphic units that can be correlated to outcrop on the basin margins. The lowest unit consists of subparallel bottomset reflections correlating to the Middle Cambrian transgressive Flathead Sandstone. This lowest unit is overlain by sigmoid clinoforms that correspond to shale of the Depass Formation. Topset reflections correlate to the clastic-dominated Deadwood Formation. Assumed interval velocities of each unit were used to infer maximum Cambrian thickness in the basin center of 1700 ft (520 m), which is comparable to Cambrian sedimentary thicknesses in outcrop of 1000 ft (304 m) to the west, and 50 ft (152 m) to the east. Based on clinoform progradation direction, sediment transport was from the east and northeast.

The Cambrian stratigraphic units were used to identify and map a probable top Precambrian seismic reflection. The top Precambrian is offset by Middle to Late Cambrian normal faults in the basin. Some of the largest faults define grabens that have curvilinear trends of eastwest to northwest-southeast orientation in the southern part of the basin and a linear east-west trend in the northern part of the basin. Some graben-bounding faults die out in the Cambrian section. Normal faults that cut the near-top Cambrian reflection are oriented east-west and may cut across the older graben-bounding faults. The faulting resulted from Middle to latest Cambrian northeast-southwest extension or eastwest, left-lateral transtension within the Powder River basin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990