WHAT TO DO WITH A SEA OF MUD: THE MANCOS SHALE REVISITED

Depositional and sequence stratigraphic models that use heterogeneity in mud-dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. We have used core, outcrop, wireline log data and select core-based geochemical and petrophysical analyses to re-examine the ~4000 ft. thick Mancos Shale in the Uinta Basin of Utah, U.S.A. using a modern understanding of mud deposition. Two depositional models for offshore deposition in the Cretaceous Western Interior Seaway have been developed: 1) a high accommodation, wave dominated shoreline of the Lower Blue Gate Member (Emery Sandstone and Niobrara equivalent) and 2) a low accommodation, river dominated shoreline model of the Juana Lopez Member (Ferron Sandstone equivalent). The prodelta, the mudbelt and the sediment-starved shelf sub-environments comprise the high accomodation offshore depositional system. The prodelta, the prodelta muddy fringe and the low angle offshore ramp comprise the low accommodation system. Each sub-environment contains characteristic facies assemblages that are reviewed herein. Sequence stratigraphically, the high accommodation system is dominated by aggradational to progradational stacking patterns. The low accommodation system is instead characterized by forced regression and lowstand fan deposition. Regional sequence stratigraphic correlations tie downdip offshore deposits that were previously undifferentiated to updip shallow marine deposits in the Book Cliffs succession from which seminal sequence stratigraphic models were developed, including the Ferron/ Frontier Sandstone, Emery Sandstone, Star Point Sandstone (Panther Tongue) and Blackhawk Formation. Differentiation of a thick, mudstone dominated, Cretaceous Western Interior Seaway deposit like the Mancos Shale provides a sedimentologic and stratigraphic model that can be used to decipher similar successions in analogous epicontinental seaway settings.