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Sedimentary Tectonics of the Salt Wash Member, Morrison Formation; western Colorado

Michael Robbins

Boston College, Dept. of Geology and Geophysics

Chestnut Hill, MA

[email protected]

Recent work in the lower Brushy Basin Member of the Morrison Formation in western Colorado has revealed the existence of a Late Jurassic tectonic pulse associated with the subduction of Farallon paleoplate. This pulse is evidenced by a grain percentage increase in plagioclase from 5% – 12% and an increase in volcanic rock fragments among total lithic fragments of 20% - 40% (Galli, 2003). The focus of this study is to delineate the extent of this pulse within the underlying Salt Wash Member. Fieldwork was conducted from June 5th, 2006 through July 12th, 2006 to determine channel geometry and primary sedimentary features and to collect sandstone samples of the Salt Wash Member at four sites. Localities included Trail Through Time, Fruita Paleontological Research Area, Echo Canyon and at Uravan CO. Sixty-seven sandstones have been sent for thin section production. Petrographic analysis of sandstone framework grain percentages will allow interpretation of the tectonic pulse within the Salt Wash and determine its temporal extent. Documented channel sandstone geometry and sedimentary features of this study reveal several indicators of flashy discharge and variable flow conditions within the Salt Wash Member. This evidence suggests a braided stream system during at least lower Salt Wash Member time for which a tectonic pulse within the Salt Wash may be the forcing factor.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid