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.
| Identifer | oai:union.ndltd.org:BGMYU2/oai:scholarsarchive.byu.edu:etd-6314 |
| Date | 01 May 2014 |
| Creators | Dossett, Toby S. |
| Publisher | BYU ScholarsArchive |
| Source Sets | Brigham Young University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | http://lib.byu.edu/about/copyright/ |
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