Abstract

The Mancos Shale, Book Cliffs, eastern Utah, USA, represents the open-marine mudstones of the Cretaceous Western Interior Seaway and encloses the Prairie Canyon Member, which is located over 50 km seaward of interpreted contemporaneous highstand shoreline deposits in the Blackhawk Formation. Examination of the Member reveals that it does not wholly represent offshore deposition, as previously interpreted, but instead contains three nearshore facies associations: (1) tidally influenced fluvial channel fills, (2) fluvial-dominated delta fronts, and (3) weakly storm-influenced shorefaces. Tidally influenced fluvial channel fills are commonly stacked into multistory bodies that can be traced for tens of kilometers at discrete stratigraphic levels, defining incised-valley-fill networks. Four such incised-valley networks are identified at outcrop. Fluvial-dominated deltas and weakly storm-influenced shorefaces are eroded into by, and lie at the down-dip terminations of, incised-valley fills and are interpreted as forced regressive and lowstand shoreface deposits. One incised-valley fill appears to be onlapped by additional fluvial-dominated deltas, which represent pulses of shoreface progradation during an overall transgression. Forced regressive, lowstand, and transgressive shorefaces in the Prairie Canyon Member differ significantly from highstand shorefaces in the Blackhawk Formation. The former are sand-poor and weakly wave/storm-influenced, whereas the latter are sand-rich and wave-dominated. This change in shoreface style reflects increased mud supply and an enhanced embayment paleogeography during periods of relative lowering of sea level.

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