ABSTRACT

The Oquirrh and Phosphoria basins in northwestern Utah, northeastern Nevada, and southern Idaho are downwarped segments of the Cordilleran geosyncline superposed on a complex structural pattern of Precambrian and early and middle Paleozoic age.

The Oquirrh basin contains Pennsylvanian and Early Permian sedimentary rocks as much as 26,000 feet thick; the area of maximum sedimentation was in west-central Utah. Three principal units are recognized: a lower unit of cyclically bedded bioclastic limestone and sandy limestone, a middle unit of interbedded limestone and quartzite, and an upper unit of quartzite. The lower and middle units were mostly deposited in an offshore shallow water environment; the upper unit in offshore moderately deep water; these grade laterally both eastward and westward into shallow nearshore facies.

The Middle Permian Phosphoria basin was partly coextensive with the Oquirrh basin, but the area of maximum sedimentation was in northeastern Nevada and southern Idaho, where locally 3,500 feet of shale, cherty shale, chert, dolomite, and limestone accumulated. This facies was deposited in an offshore deep-water environment, favoring formation of thick sponge-spicule chert and cherty shale units; these grade southward and eastward into carbonates and shales that were deposited in shallow nearshore environment.

In Cretaceous time the Paleozoic and early Mesozoic rocks of northeastern Nevada and western Utah moved eastward on great thrust plates that extended from southern Utah into Idaho. Movement took place on the Willard-Charleston-Nebo thrust belt in the Wasatch Mountains. Westward continuations of these thrusts crop out in northwestern Utah and eastern Nevada. Imbricate thrusts and tear faults within the upper plate have resulted in complex distribution of late Paleozoic facies.

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