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Publication Authorized by the Director, U. S. Geological Survey, Washington, D. C.

Cenozoic deposits of the Silver Peak region, western Nevada, and adjacent California consist principally of continental sedimentary and pyroclastic rocks of the Esmeralda Formation and lavas and tuffs of the Silver Peak volcanic center.

The sedimentary rocks comprise several thick sequences of tuffaceous volcanic sandstone and siltstone and interbedded air-fall tuff. These rocks were deposited in basins that coincide in a general way with the present valleys. Thick wedges of conglomerate and sandstone occur along the basin margins and reflect source areas.

Most of the sedimentary rocks were deposited under fluctuating fluviatile and lacustrine conditions, but paludal conditions prevailed locally. Abrupt facies changes and numerous local unconformities indicate that deposition was not uniform within a given basin.

The sedimentary rocks range in age from late Miocene to late Pliocene. The oldest reliably dated rocks in the Esmeralda Formation are 13.1 m.y. by K-Ar (Barstovian), and all sedimentary strata are younger than an ash-flow sheet dated at 21.5 m.y. by K-Ar. An air-fall tuff in the upper part of the section has a K-Ar age of 4.3 m.y.

Rocks of the Silver Peak volcanic center, 4.8 to 6.1 m.y. by K-Ar, crop out principally in the central part of the Silver Peak Range. Along the margins of the range, they overlie and interfinger with the various sedimentary sequences. The volcanic rocks are mostly rhyolite and trachyandesite tuffs and flows with subordinate basalts and andesites.

Basin and Range faulting began in late early to middle Miocene time and has continued intermittently to the present, but present topography was largely established by late Pliocene time.

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