Tectonic Controls on Delta-Front to Basin Sedimentation: The Lewis Shale–Fox Hills Turbidite System, Northernmost Great Divide Basin, Wyoming

Donna S. Anderson
Colorado School of Mines, Golden, CO

The Lewis Shale-Fox Hills (Maestrichtian) turbidite system was the first “deepwater” system recognized in the Western Interior Cretaceous Seaway over twenty years ago. Lewis sandstones in subsurface of the Great Divide basin consist of turbidites deposited in slope, toe-of-slope and basinal settings that were fed from Fox Hills deltas to the north and northeast.

Tectonic controls on the coupled Lewis-Fox Hills system are under-appreciated. Outcrop study has long shown that Lewis-Fox Hills strata in the Bairoil area are younger than the basal Lewis Shale in the Great Divide basin to the southwest. Lewis-Fox Hills rocks in the Bairoil area also overlie a zone of progressive (?) unconformities within the Mesaverde Group. Previously thought to be local, the zone of unconformities extends west >60 km, at least to the southern end of the Wind River Range. Furthermore, subsurface correlations in the Great Divide basin consistently show that Lewis clinoforms are progressively rotated toward the north and northeast. All of these relationships point to subsidence within a mini-foreland basin created by an active thrust-margin.

Persistent tectonic uplift along the northernmost margin of the Great Divide basin created and maintained a high topographic gradient relative to the subsiding Lewis foreland basin to the south. Lewis-Fox Hills delta-front stratigraphic cycles in the Bairoil area suggest that sedimentation first bypassed, then kept pace, and finally overfilled accommodation space. Laterally restricted, convolute-deformed mouthbar platforms within these cycles suggest delta-front failures as a mechanism for delivery of thick gravity flows to the basin. High gradient conditions were needed to sustain the ignitive flows.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005