Title

Bridger Formation Sandstones Used as an Indication of Tectonics in the Green River Basin and Western Wyoming

Degree Year

1999

Document Type

Thesis

Degree Name

Bachelor of Arts

Department

Geology

Abstract

Sandstone from the Eocene Bridger Formation of southwestern Wyoming can be used as a tool to constrain the timing and order of controversial tectonic events in the region. The key tectonic element in this region is the Wind River Range. Sandstones in the Bridger were derived from two source areas to the north, one being the basement rocks from the Wind River Range and the other volcanic rocks from the Absaroka Volcanic field (AVF). The abundance of volcanic grains increases upsection in the Bridger indicating that more volcanic material was carried through the Wind River Range. This evidence supports the theory that the southern Wind River Range was initially uplifted during the Laramide Orogeny, eroded throughout the Eocene, and uplifted during a second event in the Oligocene. This theory contrasts the traditionally accepted tectonic history of the Wind River Range which says the last uplift was during the Eocene Laramide Orogeny. The base of the Bridger has been dated at between 51 and 48 Ma (Groll and Steidtmann, 1987; Clyde et aI, 1997). The Bridger at Continental Peak in the northeastern Green River basin contains volcanic quartz which is believed to be from the AVF and constrains the timing of Bridger deposition.

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