Abstract

Detailed sedimentological analysis of a prodeltaic turbidite unit within the Pennsylvanian Minturn Formation provides insight into the nature of fan-delta deposition, and allows reconstruction of paleogeographic and paleoclimatic conditions within a tectonically active cratonic basin. The formation, a thick succession of fan-delta deposits, exhibits a complex stratigraphic architecture of alluvial, fluvial, and deltaic lithofacies that were deposited in the Central Colorado Basin (CCB), U.S.A., a fault-bounded depression within the Ancestral Rockies. An ~ 20 to 35-m-thick, regionally extensive, unconformity-bounded prodelta unit was deposited during a large eustatic sea-level rise that temporarily interrupted coarse-grained fluvial-deltaic deposition. The unit consists of dark-green shale and sandstone event beds with tool marks produced by abundant plant debris. The sandstone event beds were previously interpreted as turbidites because they contain current ripples, parallel lamination, and sole marks consistent with deposition from turbidity currents in the absence of storm-generated surface gravity waves. We have found, however, that proximal prodeltaic deposits contain evidence for combined oscillatory and unidirectional flow (i.e., asymmetric hummocky cross stratification (HCS), quasi-planar lamination, and combined-flow ripple stratification), indicating deposition under large waves in relatively shallow water. Paleohydraulic calculations indicate that formation of the HCS beds requires a fetch of at least 75 km to generate the necessary wave periods. Given the position of the CCB in the lee of the ancestral Front Range relative to the prevailing trade winds, and the narrow seaway to the south, storms either tracked across the Cordilleran seaway from the west or formed in situ within the CCB. The former hypothesis is consistent with paleoclimate models that purport the development of summer monsoonal circulation patterns over western equatorial Pangea during the late Paleozoic, and regional observations of southeastward eolian bedform migration directions.