LOBE GEOMETRY AND LATERAL FACIES DISTRIBUTION IN THE UPPER CRETACEOUS PEAY SANDSTONE MEMBER (FRONTIER FORMATION), NORTHEAST BIGHORN BASIN, WYOMING

Several sandstone bodies, encased by marine mudrocks, are preserved within the Upper Cretaceous (Cenomanian-Turonian) Frontier Formation in the Bighorn Basin of Wyoming, where they are hydrocarbon-producing reservoirs. These are thought to have been deposited in coastal to shallow marine environments by eastward and southward progradation into the Cretaceous Western Interior Seaway. The Peay member is a top-truncated, composite, fluvial/deltaic-shallow marine sandstone body displaying an elongate geometry along depositional dip (SSE) and is lensoid across depositional strike (ENE-WSW), with abrupt lateral pinch outs from the core to flanks. Analysis of outcrop and subsurface from the Peay sandstone body’s core to its flanks reveal consistent, thus predictable changes in sandstone body thickness. Correlation of several depositional strike oriented stratigraphic sections shows the Peay passes over <15 km from a thick (<60 m), fluvial dominated, tidally influenced, core to a thin (<5 m), sheet-like, wave and tide dominated body at the flanks. The central lobe displays a deltaic mouth bar-deposit facies with current-generated structures (small scale cross-bedding (SSX-B), ripple cross-lamination (RXL), syneresis cracks (SC)) with evidence for tidal/storm influence (hummocky cross-stratification (HCS), interference ripples (IR)), with little to no bioturbation present. Toward the flanks, the sandstone thins to <20 m, comprising proximal delta front facies. This facies exhibits more tidal/storm influence (HCS, flat-low angle cross-bedding, IR, symmetrical wave ripples (SWR), mudstone drapes and partings) with lesser current influence (RXL, SSX-B, SC). The presence of a restricted trace assemblage (Cruziana/Skolithos ichnofacies) indicates turbid conditions in high energy environments. The flank margins (<5m) preserve HCS, RXL, climbing RXL, SC, IR and SWR and several siltstone partings, accompanied by a stressed Cruziana Ichnofacies assemblage. This represents a medial to distal delta front under the influence of waves, storms, and river outflow above storm base. Physical structures reveal a paleoflow direction orthogonal to the SSE axial trend. The Peay sandstone thus shows a complete delta front facies assemblage with lateral pinch outs in both directions.