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The First Absolute Ages and Chronostratigraphic Framework for the Muddy Portion of the Jurassic Entrada Sandstone in South-Central Utah

Toby S. Dossett, George Jennings, and Thomas H. Morris

The Jurassic Entrada Sandstone in south-central Utah is comprised of a mud-dominated, tidally-influenced siliciclastic shoreline system. Regional correlation is difficult because eolian sands from the Entrada erg system interfinger with the tidal deposits in the south and east, creating dramatic facies changes. Additionally, the thickness of the Entrada Sandstone increases nearly threefold from the northeast to the southwest. Chronostratigraphically significant surfaces created by volcanic ash deposits allow us to correlate stratal packages from various locations. Sections of the Entrada Sandstone surrounding the San Rafael Swell and Escalante, Utah have been measured. Samples from ash deposits in these sections have been collected in order to separate out minerals for 40Ar/39Ar ages and chemical correlation. Using the ages obtained from these minerals and preliminary electron microprobe analysis of biotites, candidate chronostratigraphic packages have been identified. These packages, in conjunction with updated models, reveal depositional relationships and aid in the paleogeographic interpretation and reconstruction of the Entrada Sandstone at different instances in time.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013