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Absolute Ages and Paleogeography of the Jurassic Entrada Sandstone, Utah: Implications for Heterolithic, Tidally-Influenced Systems


The first precise formation ages (40Ar/39Ar) of the Middle Jurassic Entrada Sandstone were derived from volcanic ash beds found in south-central Utah. Biotite crystals were processed and analyzed at the New Mexico Geochronology Laboratory in Soccoro, New Mexico. Seven ages ranging from 168.76 +/- 0.10 to 160.9 +/-0.3 suggest Bajocian(?) through Oxfordian(?) deposition. The ages were used to create chronostratigraphic divisions within the mud-dominated, tidally influenced Entrada Sandstone. Microprobe analysis of ash constituents were used to test the validity of ash bed correlations. Eight key sections were measured for facies analysis. Four of these sections contained ash beds that were used to create a chronostratigraphic framework. The stratigraphic packages were then used to develop detailed paleogeographic maps. The maps demonstrate the landscape evolution and resulting stratigraphic complexity of the Entrada system including potential stratigraphic and combination hydrocarbon traps. These data will provide insights for future climatic, oceanographic, and tectonic studies in the Entrada Sandstone and other heterolithic intertidal systems.