The Basin and Range Province in Utah, Nevada, and California

Abstract

In this report an attempt has been made to summarize and in places to interpret the published information that was available through 1938 on the geology of those parts of Nevada, California, and Utah that are included in the geologic province known as the Basin and Range province. This region includes most of the Great Basin, from which no water flows to the sea, as well as part of the drainage basin of the lower Colorado River. It is characterized by numerous parallel, linear mountain ranges that are separated from one another by wide valleys or topographic basins. All the major divisions of geologic time are represented by the rocks exposed in this region. The oldest are of pre-Cambrian age and crop out chiefly along the eastern and southern borders. They have been carefully studied at only a few localities, and the correlation and extent of the subdivision so far recognized is uncertain. There appear to be at least three series of pre-Cambrian rocks which are probably separated from one another by profound unconformities. Large masses of intrusive igneous rocks have been recognized only in the oldest series. During the Paleozoic era the region was a part of the Cordilleran geosyncline, and sediments were deposited during all of the major and most of the minor subdivisions of the era. There are thick and widespread accumulations of Cambrian and Ordovician strata, the maximum aggregate thickness possibly exceeding 23,000 feet. The eastern and western boundaries of the province were approximately those of the area of rapid subsidence within the geosyncline, though the axes of maximum subsidence oscillated back and forth during the two periods. The Silurian and Devonian seas, on the other hand, extended beyond the province and, possibly as a consequence, are represented by much thinner sections - of the order of 6,000 feet. At the end of the Devonian period the geosyncline was split by the emergence of a geanticline in western Nevada, and Mississippian and Pennsylvanian sedimentary rocks are known only in the central and eastern part of the province. They locally attain considerable thicknesses, however, as the combined thickness of the two series in western Utah approaches 24,000 feet. The geanticline appears to have been eroded by Permian time, as Permian strata have been recognized in most parts of the province except the southern, where another geanticline, which persisted into the Mesozoic era, began to rise coincidentally with the disappearance of the older one. Igneous activity was at a minimum throughout the Paleozoic era. Some volcanism appears to have occurred locally in the Carboniferous period, but the lavas and sills cannot yet be accurately dated and may be somewhat younger. The second geanticline, which began to form in Permian time, was greatly extended during the Mesozoic era and eventually caused the disappearance of the geosynclinal seas that had persisted throughout most of Paleozoic time. Its axis lay east of the earlier geanticline, and its gradual emergence resulted in the development of two depositional troughs within the province. The western trough was filled with more than 30,000 feet of sediments and interbedded volcanic deposits, which range in age from basal Lower Triassic through the Lower Jurassic. Deposition in this trough was terminated by a period of intense orogeny near the end of Lower Jurassic time. The deposits of the eastern trough are found only along the eastern border of the province; they consist largely of nonmarine sedimentary beds ranging in age from Triassic to Upper Cretaceous. Marine Lower Triassic, possible Middle Triassic, and Upper Jurassic sedimentary rocks have been found in a few places, and some pyroclastic rocks occur in Lower Triassic and Upper Cretaceous beds. The eastern seaway appears to have been closed to the south, for lagoonal deposits, such as salines, characterize the southern and southeastern extensions of the marine formations. In addition to the sur