By: Douglas A. Sprinkel
The interim geologic map of the east part of the Duchesne 30’x60′ quadrangle (year 3) consists of eighteen 7.5 minute quadrangles and is the third year of a multi-year project to map and compile the geology of this quadrangle. The quadrangle is located mostly in the western Uinta basin, with the northwest corner located along the southwest flank of the Uinta Mountains but the area mapped is centered on Roosevelt, Utah. The map area includes a variety of surficial deposits that range from historic to lower Pleistocene piedmont alluvium, stream alluvium, and glacial deposits. Bedrock map units include members of the Duchesne River formation, the upper two members of the Uinta Formation, and upper two members of the Green River Formation. Members of the Duchesne River Formation are (in descending order) Starr Flat, Lapoint, Dry Gulch Creek, and Brennan Basin. Members of the Uinta Formation are (in descending order) member C and member B. The sandstone and limestone facies and the saline facies are of the Green River Formation are exposed south of U.S. Highway 40 and south-southwest of Duchesne, Utah. Structural features include the axis of the Uinta Basin syncline (and associated folds), the Uinta Basin-Mountain boundary fault zone located in the northern part of the map area with evidence of movement in the Quaternary, and the Duchesne fault zone located in the southern part of the map area. The Duchesne 30’x60′ quadrangle also contains an array of geologic resources including minerals, phosphate, sand and gravel, and gilsonite, but energy resources are the most significant with the giant Altamont-Bluebell and Monument Butte fields included in the quadrangle.