Structural Geology of the Northern Part of Oxford Quadrangle, Idaho

The northern part of the Oxford Quadrangle, Idaho, includes parts of the Bannock Range to the west and the Portneuf Range to the east. These ranges are separated by Marsh Valley to the north and Cache Valley to the south. Red Rock Pass, the outlet of glacial Lake Bonneville, divides the two valleys. The mapped area is north of Preston, Idaho, and south of Downey, Idaho. The north-south dimension of the mapped area is 5.2 miles and the east-west dimension is 6.4 miles. The stratigraphic units, within the mapped area, are Precambrian, Paleozoic, and Cenozic in age. The oldest rock unit is the Pocatello Formation of late Precambrian age. Other Precambrian units, in ascending order, are the Papoose Creek Formation and the Caddy Canyon Quartzite. The Precambrian units are dominantly argillite and quartzite with some extrusive and intrusive metaigneous rocks. The Cambrian formations, in ascending order, are the Langston, Ute, Blacksmith, Bloomington, Nounan, and St. Charles Formations. The Cambrian units are dominantly limestone and dolomite, with interbedded shale. Ordovician formations consist of the basal Garden City Formation which is limestone and the Swan Peak Formation which is quartzite. The Tertiary Salt Lake Formation is dominantly tuff and conglomerate. The Quaternary is represented by colluvial deposits, Lake Bonneville Group, and alluvial deposits. Major structural features, within the mapped area, are thrust faults and normal faults. A major thrust fault places Middle Cambrian rocks on Precambrian rocks and eliminates a significant part of the stratigraphic column. Thrust faults are also present between Papoose Creek Formation and Caddy Canyon Quartzite, Bloomington and Nounan Formations, St. Charles and Garden City Formations, and Garden City and Swan Peak Formations. The evidence, within the mapped area, indicates eastward thrusting. The normal faults postdate the thrust faults and offset Salt Lake Formation. The normal faults generally trend north-northwest. These structural features are the result of two separate events. The thrust faulting and related deformation is late Jurassic to Eocene in age. The Basin and Range normal faulting began in late Eocene and continued to Holocene