BEDROCK GEOLOGY OF THE MT. TERRILL AND HILGARD MOUNTAIN 7.5' QUADRANGLES, HIGH PLATEAUS, UTAH

The Mt. Terrill and Hilgard Mountain 7.5' quadrangles are located in the northern Fish Lake Plateau of central Utah, in a transitional zone between the Colorado Plateau and Basin & Range geologic provinces. The quadrangles lie astride the divide between the Colorado and Great Basin drainage systems. The region is underlain by Cretaceous to lower Tertiary sedimentary units, Tertiary volcanic rocks, and mantled with an array of surficial deposits.

700 to 1000 m of late Cretaceous to Eocene age clastic and carbonate units, including the Price River, North Horn, Flagstaff, Colton, and possibly the Crazy Hollow formations, are exposed. Three Oligocene to early Miocene volcanic units-the porphyritic Johnson Valley trachyandesite, the phenocryst-poor Lake Creek trachyte, and the biotite-rich Osiris trachyte-form a 300 to 500 m package that unconformably overlies the sedimentary units.

The structural geometry of the northern Fish Lake Plateau is dominated by steeply-dipping normal faults that bound a suite of NNW to NE-trending grabens and half grabens. Grabens range from 5 to 12 km in length, and 1 to 2 km in width, with maximum displacements of <700 m. The oldest grabens form broad valleys with abundant colluvium; based on age relations from the southern Fish Lake Plateau, the oldest grabens in the Mt. Terrill and Hilgard Mountain quadrangles range may be as old as 5 Ma. Petes Hole is a distinctive, down-to-the west, half graben complex with an internal drainage consistent with Quaternary faulting that has yet to be integrated into the Lost Creek drainage.