Abstract

The Turonian (Cretaceous) Wall Creek Member, Frontier Formation and equivalent Turner Sandy Member, Carlile Shale, Powder River Basin, Wyoming were deposited on the western margin of the North American Seaway. The Seaway occupied the foreland of the Sevier Orogenic belt. The Wall Creek-Turner interval is up to 67 m thick and consists of shale, sandstone, and minor conglomerate and bentonite. Deposition occurred during rising sea level following an approximately 90 Ma lowstand. Sandstone and mudstone of the Wall Creek were deposited in thin shelf sand sheets 6-15 m thick up to approximately 120 km from equivalent shorelines. Shelf sand sheets dip gently seaward (<< 1 degrees ) and represent short regressive episodes. Sand moved offshore and downcurrent from a delta-strandplain system positioned just west of the western edge of the Powder River Basin. Delta-strandplain deposits were mostly eroded by subsequent transgression. Storm-generated flows parallel to slightly oblique to shelf contours are inferred to have been the most significant transport and sedimentation events. The dominant meteorological events likely were winter storms centered over the seaway. Core and outcrop show shelf sandstone of the Wall Creek in the Powder River Basin to consist of burrowed to bioturbated, medium-scale cross beds, near-horizontal laminated beds, and ripple-laminated beds. Reactivation surfaces and mud drapes are common. The lack of consistent periodicity across cross sets in foreset angle and thickness and unperiodic spacing of reactivation surfaces, mud drapes, and burrowed intervals suggest that tides were not an important transport process over most of the depositional area. The dominance of storm currents over tides is also indicated by variable bed thickness, structure type, grain size, and degree of burrowing vertically. Much of the mud separating beds and draping foresets likely represents sedimentation from river floods following storm events.

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