Surficial Deposits of Yucca Flat Area, Nevada Test Site
Yucca Flat, an intermontane basin and piedmont area, is underlain principally by a series of coalescing fans of two types: (1) colluvial-alluvial fans, developed where minor streams emerge from the mountains; and (2) alluvial fans formed by major streams. Colluvial-alluvial fans consist of rubble or rubble mixed with gravel; their lithology reflects closely the adjacent bedrock, and in places they merge upslope into talus. Alluvial fans consist largely of coarse gravel on the piedmont but become finer-grained in the lowland, where they grade into the playa deposits of Yucca Lake. Chronologically, most of the fans are of Recent age, but some terraces and dissected fans are probably late Pleistocene in age. Older deposits, of Tertiary and Quaternary age, consist of thick fanglomerates in some places, and thin pediment deposits in other places; collectively, they contain boulder- to pebble-sized gravel, rubble, and sand. Locally, the fanglomerates consist of well-rounded gravel derived from Paleozoic rocks and angular rubble from Tertiary volcanics. The fanglomerates are now deeply dissected, but their distribution indicates that broad fan aprons once bordered the mountains.