Paleozoic rocks are exposed over slightly more than one-quarter of the re~ion here designated as the Carlin-Pinon Range area, which includes four 15-minute quadrangles that cover about 925 square miles in northeastern Nevada. Paleozoic rocks crop out mostly along the Pinon Range, trending north through the central part of the area, in the northeastern part of the area, and in isolated ridges and hills. Strata ranging in age from Ordovician through Permian make up the stratigraphic section of Paleozoic units. Two prominent unconformities separate these units into three major sequences: (1) Ordovician to upper Upper Devonian, (2) Lower Mississippian to Middle Pennsylvanian, and (3) Upper Pennsylvanian to Upper Permian. Rocks forming sequence 1 consist essentially of two different facies and differ strikingly from the other two sequences. Sequences 2 and 3 are somewhat similar lithologically; the older parts of both are composed in part of sedimentary rocks deposited during major uplift of the source areas of the sediments. Formations in sequence 1 consist of an autochthonous carbonate assemblage deposited in a miogeosyncline, an allochthonous siliceous assemblage deposited in a eugeosyncline to the west, and an allochthonous transitional assemblage. The oldest strata in the carbonate assemblage are of Ordovician age, consist of gray dolomite with interbeds of shale in the upper part, and are assigned to the Pogonip Group; these beds, incompletely exposed in a very small area, are about 350 feet thick with their base unexposed. Overlying the Pogonip and also exposed over only a small area is the Eureka Quartzite, which consists of 70 feet of obscurely stratified, thin-and thick-bedded white quartzite. Regionally, the basal contact of the Eureka is an uncomformity; in the area of this report it is poorly exposed but seems conformable. The Hanson Creek Formation overlies the Eureka on what appears to be a conformable contact; regionally, however, this contact also is an unconformity. The Hanson Creek is composed of 150 feet of thin-to thick-bedded black and gray dolomite; a persistent yellowishweathering limy siltstone and silty limestone unit marks the top of the formation. Poorly preserved fossils from this formation suggest that it is both Middle and Late Ordovician in age. Ordovician units of the siliceous (western) assemblage in the mapped area are the Valmy and Vinini Formations. These two formations are time-equivalent and are similar l.ithologically in containing black shale and chert. The Valmy west of the area of the present report, however, also contains much quartzite and greenstone: On the basis of quartzite and greenstone content along with shale and chert., about. 150 feet of strata at the northern end of the Sulphur Spring Range is placed in t.he Valmy Formation. Graptolites date these beds as Middle Ordovician. The dominantly black chert, shale, and mudstone that make up the Vinini Formation in this area are probably AI A2 STRATIGRAPHY OF PALEOZOIC ROCKS, CARLIN-PINON RANGE AREA, NEVADA thickness of t...

Age and correlation. _______ ___---Conditions of deposition. ____________ m Fag-43 44 44 45 45 45 49 49

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Following a long tranquil interval in early Paleozoic time, rocks of the area were subjected to at least seven distinct deformational episodes as follows: (1) multiple subparallel thrusts of great displacement in late Late Devonian to early Early Mississippian; (2) deep subsidence in the area coupled with strong uplift close to the west edge of the area beginning in middle Early Mississippian; (3) regional uplift and local folding and eastward-directed thrusting in late Middle to early Late Pennsylvanian; {4) renewed uplift and permanent end of marine conditions in early Mesozoic; (5) igneous intrustion followed by intense folding, some of very large scale, and westward-directed thrusting climaxing in Late Jurassic to Early Cretaceous; (6) igneous intrusion and local folding in early Oligocene; and (7) normal faulting climaxing in Miocene. Evidence from this area suggests that the displacement on the Roberts thrust at this latitude was at least 110 km (68 mi) and probably much more. The leading edge of the upper plate of that thrust probably lay diagonally or irregularly across the area after movement ceased. Sedimentation was scarcely interrupted near this leading edge of the upper plate, but other areas nearby to the west were emergent and were deeply eroded immediately after emplacement. Mesozoic tectonism involving prolonged uplift, injection of large igneous bodies, volcanism, intense folding, and thrusting constitutes a complex orogenic event of major importance and is a principal orogenic event in the history of the area.

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