Throughout most of the eastern Great Basin major parts of the stratigraphic section are missing. In general effusive volcanic rocks of Early Tertiary age overlie strata ranging in age from Precambrian to Permian. Because sedimentary rocks of Mesozoic and early Cenozoic age are generally absent it has been difficult to interpret the geologic history of these eras. Individual units and sequences of the early Tertiary volcanic rocks have been correlated over most of the eastern Great Basin and western Colorado Plateau. These rocks provide a valuable datum for the deciphering of the structural and stratigraphic evolution of the region.

A study of the contact relationships of the volcanic datum to the older rocks indicates the existence of a linear positive element in western Utah, called herein the Sevier arch, during the late Mesozoic era. The axis of this arch trends generally northeast and is more or less parallel with the axis of the Manhattan geanticline of central Nevada and the early Cordilleran geanticline of eastern Nevada that became positive in Devonian and latest Paleozoic time respectively. The Sevier arch appears to be the third of a series of major upwarps developed by the eastward progression of the Paleozoic and Mesozoic orogenies.

It is convenient to describe the tectonic development of the Sevier arch in six stages. Stage I was represented by deposition of Triassic and Jurassic sediments westward as far as the early Cordilleran geanticline.

Stage II was initiated by linear uplift of the Sevier arch that extended from west-central Utah into southern Nevada. Material eroded from the arch was transported eastward into the Rocky Mountain seaway to form the thick clastic formations of latest Jurassic (?) and Cretaceous age.

Stage III was climaxed by regional thrusting that culminated the upwarping and folding of the previous stage. Highlands produced by this phase of the orogeny extended from southern Nevada northward at least as far north-central Utah. Erosion of these highlands resulted in deposition eastward of material that formed rocks of latest Cretaceous and early Tertiary age.

Stage IV resulted in continued erosion that reduced the region to one of mild topographic relief.

In stage V early Tertiary volcanics were deposited over the region as an extensive sheet.

In stage VI late Tertiary deformation superposed the characteristic basin and range type structure upon the structural features of late Mesozoic and early Tertiary age.

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