SHANNON SANDSTONE IN HARTZOG DRAW HELDT DRAW FIELDS (CRETACEOUS, WYOMING, USA) REINTERPRETED AS LOWSTAND SHOREFACE DEPOSITS

The lower Campanian Shannon Sandstone at Hartzog DrawHeldt Draw (Powde r River Basin, Wyoming) is interpreted in the literature as a shelf-ri dge complex deposited 160 km from the contemporaneous shoreline. This interpretation poses difficult problems, and does not discuss the sign ificance of bounding discontinuities preserved in the sandbodies. Cros s sections show that the Shannon Sandstone is present at several strat igraphic horizons, and is bounded by regionally extensive discontinuit ies. The presence of these bounding discontinuities suggests that fluc tuations in relative sea level were important in controlling geometry of sandbodies on the Shannon shelf. In the Hartzog Draw-Heldt Draw are a of Wyoming the base of the Shannon is a planar surface with respect to a lower bentonite marker and is characterized by the abrupt appeara nce of bioturbated sandstone on top of black basinal mudstone. Siderit e pebbles < 1 cm in diameter are common at this contact. In Hartzog Dr aw the bioturbated sandstone interval has an average thickness of 10 m and is separated from an overlying unit of cross-bedded fine to mediu m sandstone (up to 20 m thick) by a ubiquitous (50 cm thick) horizon o f mud clasts in a cross-bedded, highly glauconitic, medium sandstone. These three units are laterally continuous in Hartzog Draw except wher e removed by erosion. The top of the Shannon is marked by an abrupt ch ange from the cross-bedded sandstone or bioturbated medium sandstone t o laminated black mudstone. Mud clasts, siderite pebbles, chert grains , and glauconite mark this abrupt contact, suggesting erosional transg ression. This surface can be traced westward, where in many areas it i s a mud-on-mud contact marked by a lag of shell debris. This transgres sive surface, which terminates Shannon deposition, is characterized by at least one regionally extensive onlapping marker in the Hartzog Dra w-Heldt Draw area. The presence of depositional discontinuities in the Shannon Sandstone is suggested by (1) the sharp basal contact, (2) th e lateral continuity of individual facies, (3) the abrupt vertical fac ies changes and the association of mud clasts, siderite pebbles, and g lauconite accompanying these abrupt juxtapositions of facies, (4) the stratigraphic variability, and (5) the termination of Shannon depositi on by erosive transgression, marked by onlapping markers. These deposi tional discontinuities are the bounding discontinuities. These boundin g discontinuities have asymmetrical one-sided scour morphologies, with steeper landward margins that flatten basinward. These geometries and the linearity of the overlying sandstone and the troughs defined by t hese scoured surfaces suggest incised shoreface profiles truncated by erosive transgression. Thus, the Shannon Sandstone in Hartzog Draw-Hel dt Draw is reinterpreted as a lowstand shoreface deposit.