Pro-Deltaic Turbidites in the Late Cretaceous Foreland Basin of NW Colorado, USA

Turbidite sandstones encased in shelfal mudstones of the Mancos Shale of the Cretaceous Western Interior Basin of the USA have been reported in Utah, Colorado and Wyoming. This study adds to previous investigations and describes the sedimentology, distribution and interprets the origin of turbidite sandstones in the uppermost Blue Gate Member of the Mancos Shale, Rio Blanco County, NW Colorado.

The turbidite sandstones are very fine to lower fine grained and occur in discontinuous packets both parallel to and across paleoflow. In contrast to occurrences of similar turbidite sandstones in the Western Interior Basin, these turbidite sandstones can be mapped directly up dip into a delta, placing them between 5 and 14 km from their contemporaneous shoreline.

We conclude from our sedimentological investigations and areal distribution of the turbidites that they represent periods of high fluvial discharge that resulted in unstable conditions and gravity driven processes at the nearby deltaic coast. Magnitude of single sediment gravity flows and the down dip extent of their sandy deposits were likely limited by the scale and duration of floods during relatively wet climatic episodes in the hinterland. In this respect we concur with prior interpretations of similar sandstones in the basin as ‘pro-deltaic’ or ‘delta front’, but dispute studies that interpret that this type of ‘shelfal’ sandstone can result from turbulent flows that extended many 10s of kilometers or more from contemporaneous shoreline settings.