SEDIMENTARY TECTONICS OF THE SALT WASH MEMBER, MORRISON FORMATION, WESTERN COLORADO

The Late Jurassic Morrison Formation (155-148 Ma) of western Colorado is represented by approximately 200 m of terrestrial sediments deposited within a back-bulge foreland basin depozone. Sediment was largely derived from the western fold/thrust belt and magmatic arc associated with an Andean-style continental margin. Recent petrographic analysis of the lower Brushy Basin Member of the Morrison has revealed sedimentological evidence of a Late Jurassic volcanic pulse in the source area. A clastic record of the volcanic pulse has yet to be fully described within the underlying Salt Wash Member, which may contain evidence of the pulse initiation. Petrographic analysis of sandstone framework grain percentages throughout the Salt Wash has documented fluctuations in the amount of volcanic detritus and classified the volcanic lithologies for provenance identification. These observed compositional fluctuations support a model of non-steady state volcanism in the Late Jurassic Cordilleran arc. Cathodoluminescent microscopy has been applied in order to provide a new level of detail regarding Salt Wash sandstone diagenesis and cementation patterns.