Abstract

Turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds, but in most cases the evidence is circumstantial. Such flows must commonly travel through a field of oscillatory flow caused by wind-generated waves; little is known, however, about the interactions between waves and turbidity currents. We explore these interactions through detailed process-oriented sedimentological analysis of sandstone event beds from the Pennsylvanian Minturn Formation in north-central Colorado, U.S.A. The Minturn Formation exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. An ~ 20–35-m-thick, unconformity-bounded unit of prodelta deposits consists of dark green shale and turbidite-like sandstone beds with tool marks produced by abundant plant debris. Some of the sandstone event beds, most abundant at distal localities, contain reverse-to-normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows. In contrast, proximal deposits, in some cases less than a kilometer away, contain abundant beds with evidence for deposition by wave-dominated combined flows, including large-scale hummocky cross-stratification. We interpret the majority of these event beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which accelerated and decelerated in response to a rising and falling flood discharge. Additional support for this interpretation includes the following: (1) a variety of sole marks including flute and gutter casts, as well as tool marks made by relatively large (up to tens of centimeters across) woody debris (i.e., groove, prod, and chevron marks); (2) consistent unimodal orientation of sole marks and abundant ripple cross-stratification, which indicate strong downslope-directed flow; (3) a well documented sedimentological framework for the formation of fan-delta deposits adjacent to nearby highlands; and (4) plant fossils typical of middle- to high-elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits, which have a separate floral assemblage. Differences in grain sizes, vertical stratification sequences, and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects, the time history of hyperpycnal flows, and the interaction of these processes. The latter varied both spatially and temporally and produced a wide range of bed types, which are incorporated into a new conceptual model for storm-influenced hyperpycnal flows.