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Abstract

This one-day field trip gives an overview of the geologic framework of this region, with emphasis on its Quaternary geomorphologic and hydrogeologic aspects. Its route (Fig. 1) is northwest from Las Vegas, along an arm of Las Vegas Valley, the problematic trace of the Las Vegas shear zone, to where the shear zone disappears into the Tertiary volcanic field of southern Nevada. Enroute, we pass several late Pleistocene/early Holocene groundwater discharge areas that have been long inactive, and visit a typical one. Then we go west to one of many warm springs in Ash Meadows, a major regional groundwater discharge area, noting exposures of 2-3-m.y.-old volcanic-ash beds within a few m of the surface, indicating that the Amargosa Desert basin has been a pediment, not a depocenter, for at least the last 1 m.y.

Thence, we travel south along the Amargosa River to Tecopa Valley, the site of pluvial Lake Tecopa and the chief objective of this trip. Its badlands expose a very detailed record of Quaternary climatic change (many pluvial-interpluvial cycles and consequent erosion and groundwater cycles) and tectonism for the last 2.5 m.y. This record is relevant to climatic, groundwater, erosion, and tectonic issues at the Yucca Mountain, Nevada, proposed nuclear-waste repository. It also has important evidence on the significance of episodes of regional pedimentation versus alluvial-fan aggradation during the Quaternary in the Great Basin. After 8 stops in Tecopa Valley, we return to Las Vegas, briefly viewing more extinct groundwater-discharge areas of late Pleistocene-early Holocene age in Chicago and Pahrump Valleys.

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