Paleozoic Rocks of North-Central Nevada

Recent geologic mapping and stratigraphic study in north-central Nevada has increased our knowledge of the Paleozoic stratigraphy and structural history of this complex area. The present paper summarizes the stratigraphy and outlines a suggested synthesis of the Paleozoic geologic history. Eastern Nevada differs greatly in stratigraphy and structural history from the western part. In eastern Nevada, roughly east of longitudes 116°–117°, the Paleozoic rocks from Middle Cambrian to Upper Mississippian are mostly limestone and dolomite with minor amounts of shale and quartzite. In central and western Nevada the correlative strata are predominantly clastic sedimentary rocks and chert, with intercalated volcanic rocks and pyroclastics. The eastern assemblage is on the order of 15,000 feet thick in most places; the western assemblage is much thicker, probably more than 50,000 feet. The depositional environments of the eastern and western assemblages were clearly different. The eastern assemblage is miogeosynclinal, the western eugeosynclinal. The two have been brought into contact by telescoping along a thrust fault of great magnitude—the Roberts Mountains thrust—which carried the western assemblage relatively eastward or southeastward over the eastern assemblage. In several places an assemblage that does not belong clearly to either the eastern or western assemblages, but includes elements of both, is recognized as transitional. Rocks of the transitional assemblage occur both in parautochthonous windows beneath the Roberts Mountains thrust plate of western assemblage rocks, and as slivers in the upper plate of the thrust. The broad geosyncline in which the three assemblages were laid down persisted, with local disturbances beginning in Late Cambrian, until the end of Devonian time. A belt along the 116°–118°meridians—the Antler orogenic belt—was the locus of intense folding and faulting that culminated in the Roberts Mountains thrust fault in Late Devonian or Early Mississippian time. From an emerged area along this belt coarse clastic rocks were deposited both eastward and westward; these grade laterally into finer sediments and limestone of normal marine facies. In the orogenic belt the sediments that were deposited on the deformed strata of the eastern, transitional, and western assemblages are designated the overlap assemblage. Orogenic movements continued along the belt in Pennsylvanian and Permian time, and throughout the Mesozoic, probably into Cretaceous time, causing further deformation of the previously folded and thrust-faulted rocks and of the sediments of the overlap assemblage as well. A correlation chart and representative stratigraphic sections of the several stratigraphic assemblages are included.