OVERTHRUST ORDOVICIAN AND THE NANNIE'S PEAK INTRUSIVE, LONE MOUNTAIN, ELKO COUNTY, NEVADA

Abstract

The Lower Paleozoic rocks of the Great Basin consist of an eastern facies, composed of carbonate rocks, in eastern Nevada and western Utah, and a western facies, composed of clastic rocks, cherts, and volcanic rocks, in central and western Nevada. Major thrust faults have moved the rocks of the western facies eastward over the rocks of the eastern facies. The trace of one such thrust fault is exposed on the flanks of Lone Mountain in Elko County, Nevada. The rocks of the eastern facies lie beneath the thrust fault and are subdivided into the autochthonous McClellan Creek sequence and the allochthonous Coal Creek sequence. The McClellan Creek sequence is composed of Devonian? limestones; it is successively overthrust by the Devonian fractured limestone of the lower plate of the Coal Creek sequence and by the Upper Devonian limestones, shales, and calcareous siltstones of the upper plate of the Coal Creek sequence. The rocks of the western facies lie above the thrust fault in a series of thrust slices. The name Basco formation is proposed for mappable lithic units in one of the thrust slices. The Basco formation is Ordovician and contains four members: (1) basal calcareous siltstone with a sandstone bed and lenses of altered peridotite, (2) lower chert, (3) shale, and (4) upper chert. About 1750 feet of the Basco formation is exposed within the thrust slice. The calcareous siltstone member of the Basco formation contains Early Ordovician graptolites, and the shale member contains Middle Ordovician graptolites. The Basco formation is overlain by Silurian siltstone. The thrust fault at Lone Mountain is domed up by intrusive rocks of probable Miocene age. The most striking of these is the Nannie's Peak intrusive, which forms the crest of Lone Mountain; the intrusive is arcuate in map view, and it is interpreted to be a dike. Related stocks and dikes crop out on the east flank of the mountain. North, west, and south of Lone Mountain, the Paleozoic rocks are overlain by Cenozoic conglomerate that grades upward into welded tuffs. Normal faulting may bound Lone Mountain along its eastern margin, and the desert valley to the east is floored with vitric tuffs and with younger alluvium. The vitric tuffs are probably of late Miocene age, and they seem to grade laterally into the welded tuffs that lie north, west, and south of Lone Mountain.