SEDIMENTOLOGY AND STRATIGRAPHY OF CAMPANIAN SANDSTONE BODIES IN THE WESTERN INTERIOR SEAWAY, USA

The roles of fluvial, wave, bottom current, shoreline morphology and topographic controls on sediment distribution in the Cretaceous Western Interior Seaway remains poorly constrained. However, there is growing recognition of their importance, based on studies of the numerous marine sandstones that occur throughout the Campanian of the U.S Western Interior. During our investigation of these controls on sediment distribution, twenty-one sandstone outcrops in Wyoming and Colorado were studied. Recurring facies associations were identified, and deposits were interpreted as: 1) paralic, tide-influenced coastal margins, 2) wave-dominated delta-fronts and shorefaces, 3) heavily bioturbated, glauconitic sandstones, and 4) traction current-dominated, tidally influenced, open-marine subaqueous delta and shelf deposits with prevailing southward paleocurrent directions. The wave-dominated and glauconitic facies were often reworked by a moderately diverse ichnofauna, while sparse bioturbation is seen in the cross-stratified shallow, coastal marine and shelf deposits. The distribution of lithofacies associations suggest a link to water depth, while ichnofacies associations are inferred to be related to proximity to river mouths. Recurrent southward paleocurrent directions in the sandstone bodies indicate strong, along-shelf sand-transport, which has not been previously documented in detail. The common occurrence of southward sediment transport, combined with sedimentary structures indicating strong currents, suggests significant reworking of the river-derived sediment by open-marine processes. Laramide-driven uplift and subsidence is thought to have been a key control on the amplification and acceleration of bottom and tidal currents in the north-to-south reworking of coeval river-derived sediments, as well as older, relatively coarse marine sediments on the shelf. The results of this study indicate the importance of tectonically-controlled basin physiography, north-to-south seaway orientation with a strong axial/longitudinal system, eustatic sea-level fluctuations, and autogenic responses on sediment dispersal in the Western Interior Seaway.