Abstract

The lower Campanian Shannon Sandstone at Hartzog Draw-Heldt Draw (Powder River Basin, Wyoming) is interpreted in the literature as a shelf-ridge complex deposited 160 km from the contemporaneous shoreline. This interpretation poses difficult problems, and does not discuss the significance of bounding discontinuities preserved in the sandbodies. Cross sections show that the Shannon Sandstone is present at several stratigraphic horizons, and is bounded by regionally extensive discontinuities. The presence of these bounding discontinuities suggests that fluctuations in relative sea level were important in controlling geometry of sandbodies on the Shannon shelf. In the Hartzog Draw-Heldt Draw area of Wyoming the base of the Shannon is a planar surface with respect to a lower bentonite marker and is characterized by the abrupt appearance of bioturbated sandstone on top of black basinal mudstone. Siderite pebbles < 1 cm in diameter are common at this contact. In Hartzog Draw the bioturbated sandstone interval has an average thickness of 10 m and is separated from an overlying unit of cross-bedded fine to medium sandstone (up to 20 m thick) by a ubiquitous (50 cm thick) horizon of mud clasts in a cross-bedded, highly glauconitic, medium sandstone. These three units are laterally continuous in Hartzog Draw except where removed by erosion. The top of the Shannon is marked by an abrupt change from the cross-bedded sandstone or bioturbated medium sandstone to laminated black mudstone. Mud clasts, siderite pebbles, chert grains, and glauconite mark this abrupt contact, suggesting erosional transgression. This surface can be traced westward, where in many areas it is a mud-on-mud contact marked by a lag of shell debris. This transgressive surface, which terminates Shannon deposition, is characterized by at least one regionally extensive onlapping marker in the Hartzog Draw-Heldt Draw area. The presence of depositional discontinuities in the Shannon Sandstone is suggested by (1) the sharp basal contact, (2) the lateral continuity of individual facies, (3) the abrupt vertical facies changes and the association of mud clasts, siderite pebbles, and glauconite accompanying these abrupt juxtapositions of facies, (4) the stratigraphic variability, and (5) the termination of Shannon deposition by erosive transgression, marked by onlapping markers. These depositional discontinuities are the bounding discontinuities. These bounding 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 these scoured surfaces suggest incised shoreface profiles truncated by erosive transgression. Thus, the Shannon Sandstone in Hartzog Draw-Heldt Draw is reinterpreted as a lowstand shoreface deposit.

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