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A formation of Early Cretaceous age composed of locally derived clastic rocks, including pebble to boulder conglomerate, siltstone, coarse graywacke, and finely crystalline limestone is exposed at several places in the central and northern part of the Jackson Mountains, Humboldt County, Nevada. This formation was folded and at two places probably completely eroded before deposition of the next younger unit--a pebble conglomerate composed of exotic pebbles of chert and quartzite derived from rocks of early Paleozoic age. This younger pebble conglomerate may be of Late Cretaceous or early Tertiary age and may be equivalent to rocks of similar stratigraphic position and lithologic character exposed over a considerable area of eastern Nevada and western Utah. Both of these co rse clastic formations have been overriden by a thrust sheet of Permian or older volcanic rocks. The dimensions of the thrust sheet are not known exactly but remnants are exposed over a 25-mile-long segment of the range. Upper Tertiary volcanic rocks exposed in the range are not involved in the thrusting.
The Cretaceous and younger rocks of the Jackson Mountains record a long period of orogenic unrest that included: (1) uplift of the source area of and deposition of the Lower Cretaceous rocks; (2) folding and beveling by erosion; (3) deposition of the exotic-pebble conglomerate; (4) thrusting of the Permian or older volcanic rocks over the two coarse clastic formations; and (5) later folding, faulting, and erosion providing the present outline of the range.
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