The first 40Ar/39Ar ages of the Middle Jurassic Entrada Sandstone were derived from tephra beds found in central Utah. Eight air fall ash beds in the Entrada Sandstone, with 40Ar/39Ar biotite ages ranging from 168.1 ± 0.2 to 160.8 ± 0.2 Ma, help to establish the age of Entrada deposition. They were also used to create the first chronostratigraphic divisions within the mudstone-dominated Entrada Sandstone. Statistical cluster analysis of chemical data from electron microprobe analyses of phenocrysts were used as a second line of evidence to test absolute age and stratigraphic correlations. The first direct correlations of two distinct air fall ash beds within Jurassic rocks were correlated using three criteria: (1) stratigraphic position, (2) absolute ages, and (3) mineral chemistry. These tephra beds were identified and correlated across significant lateral distances (~40 km) of the San Rafael Swell in central Utah, and one can be correlated farther southwest to Cannonville, Utah (~160 km) using absolute age relationships. This latter tephra bed allows for stratigraphic correlation across significant facies and thickness changes thereby establishing a regional framework that future studies can use to make more accurate and precise litho- and sequence stratigraphic correlations. Absolute ages from a tephra bed ~20 m below the J-3 unconformity provide a lower age boundary for formation of the J-3 surface. Mega- and microfossil assemblages in the overlying Curtis Formation together with the radiometric ages reported in this study indicate that the age of the Callovian-Oxfordian boundary in the 2004 geologic time scale (161.2 ± 4.0 Ma) is more correct than the current boundary age (163.5 ± 1.1 Ma) in the 2012 geologic time scale.



College and Department

Physical and Mathematical Sciences; Geological Sciences



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Entrada Sandstone, J-3 unconformity, tephrochronology, 40Ar/39Ar biotite, Callovian/Oxfordian



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