Shannon Sandstone in Wyoming; a shelf-ridge complex reinterpreted as lowstand shoreface deposits

The Shannon Sandstone (Campanian) of Wyoming was formerly interpreted as two stacked shelf-ridge complexes. Sand was believed to have been transported from a time-equivalent shoreline 110-150 km to the west and reworked or molded into ridges at the depositional site. We show that this time-equivalent shoreline section at Lucerne, Wyoming, consists of not one shoreface sandbody, but two superimposed. They are both storm-dominated, and the lower one terminates in beach facies capped by root traces. There must have been a rise in relative sea level of at least 14 m to make accommodation space for the second shoreface sandbody. In the Salt Creek area and the adjacent subsurface Teapot Dome, there are two sandier-upward facies successions. In the lower succession there are abrupt vertical facies contacts between offshore bioturbated sandstones, thicker hummocky cross-stratified sandstones, and coarser cross-bedded sandstones. There is also evidence that the cross-bedded sandstones rest erosively on underlying facies. The trace fauna in the sandstones includes Macaronichnus segregatis, Rosselia socialis , and Ophiomorpha . The M. segregatis suggests a foreshore or upper-shoreface depositional environment, and R. socialis indicates a lower to middle shoreface; neither are characteristic of a shelf-ridge complex. The abrupt and probably erosive facies contacts, along with the trace fauna, suggest that the cross-bedded sandstones in this succession represent a shoreface deposit that formed during a stage of actively falling relative sea level. Overlying muddy bioturbated sandstones indicate that shoreface deposition was terminated by a transgression. The upper sandier-upward succession contains facies and trace fauna similar to those of the lower succession, and is also interpreted as a prograding shoreface. Rather than the sand moving 110-150 km from the shoreline and then being molded into shelf-ridge complexes at Salt Creek, we suggest that the shoreline moved into the Salt Creek area as the result of a fall in relative sea level. It was transgressed and moved back westward; a second fall of relative sea level moved the shoreline back to Salt Creek, and a final transgression terminated Shannon deposition.