QUATERNARY GEOLOGY OF THE IVANPAH 1:100,000-SCALE QUADRANGLE, CALIFORNIA AND NEVADA

As part of a USGS project to map Quaternary deposits in the arid southwest, this map was prepared to depict process and age characteristics of surficial deposits and generalized bedrock units. The 100,000-scale Ivanpah quadrangle, covering much of the Mojave National Preserve, California, and part of adjacent Nevada, consists of northeast-trending high-relief mountains and pediment domes separated by internally and externally drained alluvial valleys. Quaternary basaltic cinder cones and lava flows mantle pediments in much of the western part. Surficial geologic deposits were identified by process of deposition using geomorphic and sedimentologic features. Geomorphic position, surface roughness, pavement maturity, pedogenesis, and inset relations were used to differentiate deposit ages.

Geologic mapping, conducted using field methods and interpretation of remote-sensing images, identified Quaternary deposits cut by faults. Several faults cut middle to early Pleistocene deposits, such as in the Mid Hills. Others questionably cut old Quaternary deposits, such as near the town of Cima, in the lava fields, and near Old Dad Mountain. Younger faults exist in the northern Bristol Mountains, where several faults cut deposits as young as latest Pleistocene. Pliocene tectonism over much of the quadrangle is indicated by asymmetric mountain ranges and valleys that include tilted Miocene deposits, all cut by Pliocene faults.

Valley-axis floodplains and playas are sources for eolian sand and dust, as reflected in the sediments near sources such as Soda Lake and Kelso and Ivanpah valleys, where eolian-enriched alluvial deposits are common. Eolian-enriched alluvial deposits display a marked change in vegetation and animal habitat, commensurate with expected effects on soil moisture properties of the deposits. Groundwater-discharge deposits, created by past springs and wetlands, are present in several locations, providing markers of past groundwater levels associated with wetter climates.