Scientific Investigations Report 2006–5305

Scientific Investigations Report 2006–5305

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Geologic Setting

The distribution of geologic units in Carson Valley is shown in figure 5. The geologic units of Stewart and Carlson (1978) were grouped into metamorphic rocks of Jurassic to Triassic age, granitic rocks of Cretaceous age, volcanic rocks of Tertiary age, semi-consolidated sediments of Tertiary age, and alluvial fan, gravel, eolian sand, and basin-fill sediments of Quaternary age.

During the Cretaceous Period, 63 to 138 million years (m.y.) ago, the granitic magma of the Sierra Nevada pluton intruded into sedimentary and volcanic rocks of the Triassic and Jurassic Periods (138 to 240 m.y. ago). The resulting granodioritic and metavolcanic and metasedimentary rocks form the bulk of the Carson Range of the Sierra Nevada and the Pine Nut Mountains (fig. 5), and underlie the floor of Carson Valley (Moore, 1969, p. 18; Pease, 1980, p. 2). Basin and range faulting, which produced the present topography in Carson Valley, took place from 10 to 7 m.y. ago (Muntean, 2001, p. 9), uplifting the Carson Range and the Pine Nut Mountains, and down-dropping the floor of Carson Valley.

Contemporaneous with the faulting, volcanic rocks and sediments were deposited during the Tertiary Period and the sediments have become semi-consolidated. The volcanic rocks are exposed primarily on the extreme northeastern and southeastern ends of the valley (fig. 5). The semi-consolidated sediments are exposed primarily on the eastern side of the valley, but dip towards the west and probably are present beneath the entire valley. Geologic units mapped by Stewart and Carlson (1978) as older Quaternary alluvium on the eastern side of the valley likely are underlain by the Tertiary sediments at depths of 100 ft or less and, for this reason, are grouped with the Tertiary sediments on figure 5. The semi- consolidated Tertiary sediments vary in their degree of compaction (Pease, 1980, p. 14), and in their lithology, varying from fine-grained and tuffaceous siltstone with isolated lenses of sandstone and conglomerate, to sandstone and conglomerate (Muntean, 2001, p. 18-31). The coarser grained parts of the 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 to 2 m.y. ago), unconsolidated sediments (fig. 5) have been deposited on the valley floor by the Carson River and by tributary streams surrounding the valley. Unconsolidated sediments deposited by the Carson River generally are well-sorted sand and gravel, interbedded with fine-grained silt and clay from over-bank flood deposits. Unconsolidated sediments deposited by tributary streams are coarse- to fine-grained, poorly sorted deposits that 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 Mountain 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 is found on the eastern side of the valley, dividing the Pine Nut Mountain block into several smaller blocks (fig. 5). 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 (Moore, 1969, p. 18). A gravity survey by Maurer (1984) indicates the depth to consolidated bedrock beneath the western half of Carson Valley is as great as 5,000 ft.

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