Abstract

The succesion of Paleozoic strata in the Logan quadrangle, northern Utah, is one of the thickest and best exposed in the western United States. It includes 16 formations, representing every system of the Paleozoic except the Permian. Measurable sections of these formations have a total thickness of 23,200 feet. Here most of the formations are better exposed than in the area, contiguous to the Logan quadrangle at its northeasteastern corner, where Mansfield made his phosphate studies.

These formations are described in detail. The Swan Peak formation (Ordovician) is divisible into three members; the Jefferson formation (Devonian) is divided into two members—the Hyrum dolomite member below and the Beirdneau sandstone member above. For the Lower Devonian rocks of the area a new formation, the Water Canyon formation, is defined. The Upper Mississippian rocks, called the Brazer formation, are 3700 feet thick at one locality in the quadrangle, a much more complete exposure than at the type locality of the formation in the Randolph quadrangle to the east. Pennsylvanian rocks reach a total thickness of 6000 feet in this area. A stromatolitic limestone in the Wasatch conglomerate is named the Cowley Canyon limestone member.

The structures of the area include folds and overthrust faults produced in the Laramide deformation and high-angle faults of the Basin-and-Range type produced late in the Cenozoic. The major Laramide folds are the Logan Peak syncline and the Strawberry Valley anticline. Overthrusting appears to be responsible for separating the homoclinal masses that constitute the mountains of the west side of Cache Valley. The principal high-angle faults are defined as the East Cache, Hyrum, Wellsville, Dayton, and Clarkston faults.

A résumé of the geologic history of the area includes the recognition of two erosion surfaces—the Rendezvous Peak surface (late Tertiary) and the McKenzie Flat surface (Quaternary).

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