AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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Sonoma Orogeny and the Golconda Thrust

A thick oceanic sequence of Mississippian through Permian chert, limestone, conglomerate, siltstone, shale, lava flows and pyroclastics accumulated in a trough west of the Antler orogenic belt. This tectonic assemblage was emplaced along the Golconda thrust, at least 60 miles (100 km) eastward or inboard of the new continental margin, during the Late Permian through Early Triassic Sonoma Orogeny (Speed, 1983). Rather than a separate and isolated orogeny, perhaps the Sonoma Orogeny is better viewed as a final tectonic event in a deformational continuum which began with the emplacement of the Roberts Mountains allochthon in the Late Devonian.

The upper plate of the allochthon is composed of internally thrusted Pennsylvanian and Permian rocks which are deformed into broad symmetrical to overturned isoclinal folds with amplitudes of several thousand feet (Speed, 1977). Most of the folds are north-striking anticlines and synclines which are dominantly asymmetrical and overturned. Windows through the allochthon are not abundant and where present show that the effects of Golconda thrusting do not penetrate more than a few tens of feet below the thrust (Oldow, 1984). The Golconda allochthon is thrust onto the Roberts Mountains allochthon or the Pennsylvanian and Permian sediments which overlap the allochthon.

The structural effects of the Late Permian and Early Triassic Sonoma orogeny are prominent, but primarily affect the rocks which lie west of the evaluation area (Stewart and McKee, 1977). Within the evaluation area, the Golconda allochthon is represented by small exposures of the Pumpernickel and Havallah Formations exposed in the western Toiyabe, northern Shoshone, and Monitor Ranges, as well as by the Mississippian Schoonover Sequence in the Independence Mountains and scattered exposures of the Permian Reservation Hill Formation in northern Elko County (Kleinhampl and Ziony, 1985; Coats and Riva, 1983; Coats, 1985). The Schoonover is very similar to the Havallah and Pumpernickel which appear to be one sequence composed of five or six thrust sheets. The basal part of the sequence is composed of Mississippian to Middle Pennsylvanian siltstones, cherts and conglomerates, and the upper unit is mainly Late Pennsylvanian to Early Permian carbonate turbidites.

In the Independence Mountains, the Mississippian Schoonover Formation is thrust over the Ordovician Valmy Formation and Permian Edna Mountain Formation and is intruded by an undeformed 150 Ma pluton (Coats and McKee, 1972; Miller and others, 1981). These relationships bracket the age of the Golconda thrust as post-Upper Permian and pre-Jurassic. Fagan (1962) suggested that the sequence in the Independence Mountains was compressed along a southeast-northwest vector. Other probable exposures of Golconda thrusting in northern Elko County include allochthonous Permian Edna Mountain Formation in the Divide Peak area, and the Pennsylvanian-Permian(?) Reservation Hill Formation thrust over the Mississippian Mountain City Formation in the Mountain City Quadrangle (Coats, 1985).


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Last modified: 09/12/06