THE BASIN AND RANGE PROVINCE IN. UTAH, NEVADA, AND CALIFORNIA1
By THOMAS B. NOLAN
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 sectionsof 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 surficial volcanic rocks, there are numerous extensive bodies of coarse-grained intrusive rocks. Some of these are clearly extensions of the late Jurassic or early Cretaceous intrusions of California, but some of those in Utah are best dated as early Tertiary. Most of the intrusive masses cannot be accurately dated, but practically all of them were emplaced after the intense local orogeny that affected individual areas at various times from the Lower Jurassic well into the Tertiary.
Cenozoic rocks underlie more than half of the province. Most of them, however, are either igneous rocks or nonmarine sedimentary rocks that are sparsely or not at all fossiliferous. As a result, definite age assignments for many of the rock units cannot be made, and correlations between the relatively few well-studied sequences are hazardous. The few marine sedimentary rocks are restricted to the southwestern border of the province. The province appears to have been a highland under going erosion throughout Eocene and Oligocene time. Continental and, in southern California, marine sedimentary deposits of Eocene age have been recognized only along the borders of the province, though volcanic rocks of this age may be represented throughout the area. Oligocene sedimentary rocks have been recognized in only one area, but here they appear to have been formed in a basin not unlike those existing at present. Several bodies of granular intrusive rocks in the eastern part of the province have been referred to either the Eocene or the Oligocene. By far the greater number of the known fossiliferous sedimentary rocks are assigned to the Miocene and Pliocene and are almost exclusively of continental origin, being composed of fanglomerates, silts, and saline beds that are in many respects like the beds now being deposited in the intermontane valleys. Volcanic debris commonly forms a large proportion of the sedimentary sequence and thick accumulations of lava flows and pyroclastic rocks and local intrusive masses are generally associated and interbedded with them. The widespread Quaternary sedimentary deposits in the province consist chiefly of lake beds and fanglomerates, but locally glacial deposits and river gravel and silt are well developed. Basalt flows and in places rhyolite flows are also found at several localities.
The Basin and Range province has been orogenically active throughout a large part of its history. The pre-Cambrian structural history is little known at present, though it appears that were in pre-Cambrian time at least three epochs of deformation.
Beginning with Cambrian time the record is much more complete. The geosynclinal sea which in the early part of the Paleozoic era covered most of the province and in which many thousands of feet of sediments were deposited was divided in late Devonian time by a rising arch or geanticline in western Nevada. Locally there was moderate deformation during the uplift, but by Permian time elevation had ceased and most of the positive area had been covered by marine sediments. Coincidentally with the degradation of this geanticline during the Permian epoch similar uplift began in eastern Nevada, and a land mass between two seas persisted there until early Jurassic time, when, instead of subsiding as the earlier one did, it became the site of intense diastrophism, marked particularly by large overthrust faults. To judge from the relatively meager evidence available, recurrent epochs of similar deformation continued into early Tertiary time, affecting an area that considerably exceeded that of the Basin and Range province. No regular pattern in space or time can yet be discerned for these epochs; in one area there may be evidence of only one episode of deformation, but in another area nearby there may have been several. The Mesozoic and early Tertiary folding and over-thrusting were succeeded closely by the initiation of the block faulting that has been the chief cause, directly or indirectly, of the present relief of the province. The faulting appears to have started at least by early Oligocene time and to have continued up to the present day. For several reasons the block faulting is regarded as an end stage of the more intense deformation that preceded it.
The Paleozoic and later structural history of the province leads to the suggestion that these events have been integral parts of a single orogenic cycle, during which the initial geosynclinal downwarping was successively followed by one or more broad geanticlinal uplifts, the last of which culminated in an epoch of folding and overthrusting, and a final stage of block faulting.
The province contains many mining districts, from which large quantities of the nonferrous metals and of nonmetallic materials have been obtained.
Last Updated: 28-Nov-2007