Hyperpycnal wave-modified turbidites of the Pennsylvanian Minturn Formation, north-central Colorado

The Pennsylvanian Minturn Formation in north-central Colorado exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. The formation records a wide range of environments including alluvial fan, fluvial, deltaic, and open marine settings. This field trip will examine outcrops of a remarkable ~20 to 35-m-thick, unconformity-bound unit with turbiditelike beds that presumably developed within the lower reaches of incised valleys. This unit consists of dark green shale and graded sandstone beds with tool marks produced by abundant plant material. The sandstone event beds contain evidence for strong unidirectional flows and the variable influence of storm-generated waves. Proximal deposits contain beds with evidence for wave-dominated combined flows, including well developed, large-scale hummocky crossstratification. Distal sections contain beds with reverse-to-normal grading and vertical successions of sedimentary structures that indicate long-lived waxing-to-waning unidirectional flows in conjunction with storm waves. We interpret these beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which waxed and waned in response to the rising and falling flood hydrograph. The focus of this trip will be the hydrodynamic interpretation of these different bed types, including their spatial and stratigraphic distribution.