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Geologic Quadrangle Map GQ–1770
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By Van S. Williams, Robert G. Bohannon, and D.L. Hoover

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The Riverside quadrangle lies along the eastern edge of the Basin and Range province in southeastern Nevada. The map area extends from the Virgin Mountains northward to the edge of Mormon Mesa, and is traversed in its northern part by the narrow, incised valley of the Virgin River. Most of the map area lies in the southwestern part of the Virgin River depression (Bohannon and others, 1993).

The Virgin River depression, about 28 km wide, lies between the Mormon Mountains (maximum altitude 2,260 m) and East Mormon Mountains to the northwest and the Virgin Mountains (maximum altitude 2,356 m) to the southeast and east. The Mormon Mountains are within the Sevier thrust belt, where a thin sheet of allochthonous lower Paleozoic strata tectonically overlies an autochthon consisting of Paleozoic and lower Mesozoic rocks. Detachment faulting associated with middle Miocene extension has produced strong local tilts, exposing Precambrian rocks at a few localities (Axen and others, 1990), notably in the East Mormon Mountains. The Virgin Mountains are entirely east of the thrust belt and they contain a complete, but dismembered section of autochthonous Paleozoic and Mesozoic strata. Strong uplift in the Virgin Mountains associated with middle Miocene extension and strike-slip faulting has resulted in the exposure of Proterozoic crystalline rocks along the entire southwest edge of the Virgin River depression.

Proterozoic rocks in the Riverside quadrangle and surrounding areas 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, primarily mineralogical and textural information, are taken from Beal (1965).

Version 1.0

Posted 1997

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