Scientific Investigations Report 2005–5288
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005–5288
The granitic magma of the Sierra Nevada pluton was intruded 63–138 Ma (million years ago) during the Cretaceous Period into sedimentary and volcanic rocks deposited 138–240 Ma during the Triassic and Jurassic Periods. The resulting granodioritic, metavolcanic, and metasedimentary rocks form the bulk of the Carson Range and the Pine Nut Mountains (fig. 3), and underlie the floor of Carson Valley (Moore, 1969, p. 18; Pease, 1980, p. 2). Basin and range faulting, which produced much of the present topography in Carson Valley, took place from 7 to 10 Ma (Muntean, 2001, p. 9), uplifting the Carson Range and the Pine Nut Mountains, and down dropping the floor of Carson Valley.
Prior to, and contemporaneous with the faulting, volcanic rocks and sediments were deposited during the Tertiary Period, 1.6–66 Ma. Through time, the sediments have become semiconsolidated. Volcanic rocks are exposed primarily on the extreme northeastern and southeastern ends of the valley (fig. 3). The semiconsolidated sediments are exposed primarily on the eastern side of the valley, but dip towards the west and probably are present beneath the entire valley. The semiconsolidated Tertiary sediments vary in their degree of compaction (Pease, 1980, p. 14), and vary in lithology from fine-grained and tuffaceous siltstone with isolated lenses of sandstone and conglomerate, to primarily sandstone and conglomerate (Muntean, 2001, p. 18–31). The coarser grained Tertiary sediments are exposed primarily on the southeastern part of the valley at the base of the Pine Nut Mountains (Muntean, 2001, p. 19). The aggregate thickness of the Tertiary sediments is estimated to exceed 3,000 ft (Muntean, 2001, pl. 5).
Throughout the Quaternary Period (present day–2 Ma), unconsolidated sediments (fig. 3) have been deposited on the valley floor by the Carson River and tributary streams surrounding the valley. These sediments generally are well-sorted sand and gravel, interbedded with fine-grained silt and clay from overbank flood deposits. Unconsolidated sediments deposited by tributary streams are coarse- to fine-grained, poorly sorted deposits, which form alluvial fans at the base of the mountain blocks.
The mountain blocks bounding Carson Valley are west-tilted structural blocks (Stewart, 1980, p. 113), with the valley occupying the down-dropped western edge of the Pine Nut Mountains block (Moore, 1969, p. 18). A steep, well-defined normal fault creates a 5,000 ft escarpment along the Carson Range on the west, whereas a diffuse fault zone on the eastern side of the valley divides the Pine Nut Mountains block into several smaller blocks (fig. 3). Continued westward tilting is shown by recent faulting along the base of the Carson Range (Pease, 1980, p. 15) and by displacement of the Carson River to the extreme western side of the valley floor (Moore, 1969, p. 18). A gravity survey by Maurer (1985) indicates the depth to the top of consolidated bedrock beneath the western half of Carson Valley is as much as 5,000 ft.
U.S.
Department of the Interior | U.S. Geological
Survey
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