Geology of the west side of Peavine Mountain, Washoe County, Nevada

Abstract

The oldest rocks exposed on the west side of Peavine Mountain are Mesozoic metamorphosed sedimentary and volcanic rocks. The latter have tentatively been assigned to the Jurassic on fossil flora found in a black shale. Epidote surrounded by bleached haloes is common in the meta-volcanic rocks. Piedmontite, a manganese epidote, has been found in close proximity to quartz veins. A granodiorite stock intrudes the metamorphic complex. Earlier writers referred to the stock as quartz-monzonite, however, modal analysis has proven it to be granodiorite. Tertiary andesites lie unconformably on the deeply weathered granodiorite. The andesites, which range from basic pyroxene andesite with basaltic texture to hornblende— mica andesite, have been tentatively correlated with the Alta Andesite and the Kate Peak Andesite of the Virginia City quadrangle. Near surface alteration by acids has caused local bleaching of the andesites and granodiorioe in the southeastern portion of the mapped area. A reconnaissance map of the bleached areas in the southern half of the Reno quadrangle has been prepared. Lake deposits of the Pliocene Coal Valley Formation lie on a weathered surface of the Kate Peak Andesite without marked angular relationship. On Rakota Hill, northeast of Verdi, the Coal Valley diatomite and sandstone have been oxidized brick red. There are also unusual areas where they have been fused to black glass. Olivine basalts, the youngest volcanic rocks in the mapped area, overlie and, in part, interfinger with the Coal Valley Formation. The characteristic structural features of the Peavine area are block faults. The mountain is a tilted fault block with a topographic displacement of 2000 feet on the fault on the northeastern side. Most of the Basin-Range faulting is post-Coal Valley Formation and post-olivine basalt. Numerous surfaces which have been planed by the Truckee River have been tilted and deeply dissected by tributary streams. Pleistocene glacial gravels have been deposited on some of these surfaces.