Abstract

The Sussex sandstone was deposited as part of an extensive muddy sheet which prograded southward and eastward into the central part of the Late Cretaceous Campanian seaway of western North America. Outcrop, core, and well-log data from the Wyoming portion of the Powder River basin show that the sandstone units of the Sussex interval are concentrated into large-scale northwest-trending bodies which have lobate lateral edges. A process-response depositional model has been developed to account for these sandstones by relating modern sedimentary processes to the physical characteristics of the lithologies, and then placing them within the proper stratigraphic and tectonic setting. The model invokes storm-generated and tidal currents to create and shape a series of sand-bar or ridge complexes. These bars were periodically breached by intense storm-generated currents, cutting channels which may subsequently have acted as tidal passes. The mouths of those channels are marked by washover fans and/or tidal deltas which give the overall complexes their lobate edges. The eastern and southern termini of the Sussex sheet are marked by an abrupt depositional slope as indicated by physical and biostratigraphic studies, and may be observed visually on seismic profiles. Petroleum accumulated along the updip pinchouts of the more porous bar and channel lithologies.

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