A sedimentolocical and sequence stratigraphic re-interpretation of the upper Cretaceous prairie canyon member ("Mancos B") and associated strata, Book Cliffs area, Utah, USA

The Mancos Shale, Book Cliffs, eastern Utah, USA, represents the open-marin e mudstones of the Cretaceous Western Interior Seaway and encloses the Prai rie Canyon Member, which is located over 50 km seaward of interpreted conte mporaneous highstand shoreline deposits in the Blackhawk Formation. Examina tion of the Member reveals that it does not wholly represent offshore depos ition, as previously interpreted, but instead contains three nearshore faci es 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 bodi es that can be traced for tens of kilometers at discrete stratigraphic leve ls, defining incised-valley-fill networks. Four such incised-valley network s are identified at outcrop. Fluvial-dominated deltas and weakly storm-infl uenced shorefaces are eroded into by, and lie at the down-dip terminations of, incised-valley fills and are interpreted as forced regressive and lowst and shoreface deposits, One incised-valley fill appears to be onlapped by a dditional fluvial-dominated deltas, which represent pulses of shoreface pro gradation during an overall transgression. Forced regressive, lowstand, and transgressive shorefaces in the Prairie Ca nyon Member differ significantly from highstand shorefaces in the Blackhawk Formation, The former are sand-poor and weakly wave/storm-influenced, wher eas the latter are sand-rich and wave-dominated. This change in shoreface s tyle reflects increased mud supply and an enhanced embayment paleogeography during periods of relative lowering of sea level.