The present Great Basin embraces deposits of an inner (eastern) geosyncline and an outer (western) geosyncline. Disregarding the problem of naming and classifying these two features, attention will be focused on the better known eastern trough where an intensive search for oil is currently in progress. Outstanding features of the sedimentary section are: (1) preponderance of marine chemical rocks, (2) great thickness of certain units, and (3) evidences for mid-Paleozoic orogeny.

Base of the sedimentary section almost everywhere is a thick quartzite (Prospect Mountain-Brigham) which transgresses through time from late pre-Cambrian to late Cambrian. Above this is an accumulation of Cambrian carbonate rocks reaching a thickness of 10,000 feet in the central part of the trough. The Cambrian passes into Ordovician with little evidence of hiatus, and carbonate sediments (Pogonip-Garden City) continued to accumulate giving a complete record of the period up to about Chazy time. A useful key bed and one of the few clastic units, the Eureka-Swan Peak, was then laid down. Late in the Ordovician a second, thinner dolomite unit (Fish Haven-Hansen Creek) came into being.

The Silurian system is represented by a rather uniform, scantily fossiliferous dolomite formation (Laketown-Lone Mountain-Roberts Mountain) which accumulated during the Niagaran epoch. Thickness averages between 500 and 1,000 feet.

All epochs of Devonian time are represented, but Middle Devonian rocks are the most extensive. Key locality for the system is the Roberts Mountains region of Nevada where about 4,500 feet of Devonian rocks are present. Dolomite appears to be the dominant rock of the thinner sections. Much in the way of correlation of Devonian units in western Utah remains to be done.

The Mississippian is lithologically the most varied of the Paleozoic systems. Sediments reflect orogeny in west-central Nevada and range from conglomerates and redbeds on the west through quartzites, sandstones and black shales to limestones on the east. Maximum thicknesses are on the order of 5,000 feet. Numerous named and unnamed units remain to be more thoroughly studied. Continuous sedimentation from Mississippian into Pennsylvanian is indicated.

The most characteristic and striking Pennsylvanian unit is the fusulinid-bearing Oquirrh formation which shows crude cyclic deposition, involving a variety of chemical and clastic rocks. This unit may reach a thickness of 20,000 feet in the east-central Great Basin, but there is a decrease in thickness and loss of certain intervals westward. Again sedimentation disregarded systemic lines and Permian Wolfcamp fusulinids are found in the upper part of the Oquirrh.

Following the Oquirrh is a variety of thinner Permian units such as the Phosphoria, Supai, Kaibab, and unnamed formations in Nevada. Redbeds, gypsum, phosphate rock and massive chert are rather unusual constituents of this interval.

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