Sedimentology and sedimentary tectonics of the Salt Wash Member, Morrison Formation, Western Colorado

Abstract

The Brushy Basin Member of the Morrison Formation records a time of increased volcanic activity in the North American Cordillera during the Late Jurassic. Sedimentological and petrographic observations in the Brushy Basin, in conjunction with findings of widespread plutonic intrusion in the source areas, point to a volcanic pulse within the Cordilleran magmatic arc. This study investigated the subjacent Salt Wash Member, for the purpose of better constraining the timing of the volcanic pulse. Petrographic and statistical analyses of the Salt Wash sandstone identified statistically significant upsection trends in volcanic rock fragment and plagioclase feldspar at one of the four study areas. The remaining three study areas showed no upsection trends in sandstone composition that would reflect a pulse in volcanism during Salt Wash Member time. It is more likely that the Salt Wash was deposited during a time of volcanic quiescence leading up to the post-Nevadan Orogeny volcanic reactivation. Sedimentology and cementation patterns of the Salt Wash Member were also studied. Cathodoluminescence indicates that the member was well-flushed with shallow formation waters, thus preventing any calcite optical zoning. Luminescence intensity suggests that the Salt Wash Member sediments were cemented at varying depths and within differing Eh-pH regimes. Field-based sedimentological observations support a model of braided stream channel deposition across a semi-arid landscape with streamflow entering the basin from both the south and west.