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Preliminary geologic map of the Riverside Quadrangle, Clark County, Nevada

1992; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr92554

2332-4899

D.L. Hoover, Robert G. Bohannon, Frederic W. Simonds,

Geology and Paleoclimatology Research

Mormon Mountains are composed largely of Paleozoic carbonate rocks and minor Precambrian and Mesozoic rocks, whereas the Virgin Mountains are composed chiefly of Precambrian igneous and metamorphic rocks with lesser areas of Paleozoic carbonate rocks and Mesozoic clastic rocks in approximately equal amounts.Along the west side of the basin, Mormon Mesa, altitude about 560 m, is an erosional remnant of lacustrine and fluvial basin-fill deposits, the flattish top of which is underlain by an ancient, thick, resistant carbonate-rich soil (Gardner, 1972a).The Virgin River flows westward and southward from an altitude of about 550 m where it debouches from its gorge through the Virgin Mountains to about 335 m at its confluence with the Muddy River at the south end of Mormon Mesa.Piedmont plains rise steeply from the river to about 1,000 m altitude against the rugged Virgin and Mormon Mountains.Along the Virgin River at Littlefield, Ariz., near the east edge of the Mesquite basin, the mean annual temperature is 18* C and the mean annual rainfall is 16 cm (U.S.Department of Commerce, 1975).Identification of the fluvial and other alluvial deposits that post-date the Muddy Creek Formation are based mostly on topographic position, soil development, topographic expression, amount of dissection by washes that head within a deposit (internal dissection) and, to a lesser extent, on lithology and depositional environment These criteria are described in detail in Hoover and others (1981).Ages of surficial deposits have not been determined by numerical dating techniques; ages assigned to surficial deposits in this quadrangle are estimated based on similarity of desert varnish development, etching of carbonate clasts, packing of desert pavement, and the stage of soil carbonate development (Gile and others, 1966) to that observed for Quaternary deposits at the Nevada Test Site, about 170 km northwest of the map area, where many numerical ages have been determined (Hoover, 1989).The Riverside quadrangle lies in a transitional setting between the miogeosynclinal section of Paleozoic and Mesozoic rocks to the west in the southeastern Great Basin Province and the platform sequence of Paleozoic and Mesozoic rocks in the Colorado Plateau Province to the east.The quadrangle is in an area of east-to-west fades changes and westward thickening.The stratigraphic nomenclature established for rocks of the Colorado Plateau Province, chiefly in southern Utah and the Grand Canyon region, differs from that used regionally in the southern Great Basin.For example, more rock units are named and more geologic time is represented in the rock column of the Great Basin.In the Riverside quadrangle, we have incorporated geologic names from both of these provinces, and the nomenclatures have been interchanged.This choice was dictated by our assessment of the local stratigraphy and its similarity to other named sections.Certainly many alternative choices are possible.The Proterozoic rocks have been mapped and studied by Beal (1965).Where possible, we have tried to conform to his terminology.Parts of the rock descriptions used here are taken from Beal (1965), primarily mineralogical and textural descriptions. GEOLOGIC HISTORY AND STRUCTUREThe Mesquite basin is a large tectonic basin that was extensionally deformed during late Tertiary time and filled with a thick sequence of basin-fill deposits.The exposed part of the basin fill is the postextensional Muddy Creek Formation, which consists of several hundreds to many hundreds of meters of mostly well bedded, moderately consolidated silt and sand that are probably largely fluvial (Kowalk's and Everett, 1986).The Muddy Creek Formation is about 11.5 to 5.5 m.y.old.To the west and southwest of the map area the formation is composed mostly of lacustrine deposits.Muddy Creek lake was abruptly drained when the Colorado River was integrated through the present-day Lake Mead area.