FORMATION AND DESTRUCTION OF SEDIMENTARY BASINS ALONG SINISTRAL FAULTS, FORT IRWIN, CA, APPLIED TO GROUNDWATER AVAILABILITY

Geology, geophysics, and borehole investigations are shedding light on sedimentary basins formed along and between late Cenozoic faults of the Eastern California Shear Zone (ECSZ). These types of basins constitute the primary resource for groundwater at Fort Irwin National Training Center, northern Mojave Desert. Early Miocene volcanic rocks of the fringe of Eagle Crags field and sedimentary rocks overlie bedrock consisting of Mesozoic plutonic rocks and their Paleozoic wallrocks. Subsequent deposits, coeval with ECSZ fault activity, are distinctive in basins formed along east-striking sinistral faults.  Middle(?) to late Miocene clastic stream sediments, and Plio-Pleistocene and younger alluvial fan deposits that grade to basin-axis deposits, are punctuated by a few basalt flows and ash beds that provide chronologic constraints. Overall, basins formed in synclines adjacent to sinistral faults and in local down-sags, and were sourced mainly from highlands in the west, resulting in coarser sediments, and therefore more productive aquifers, most commonly found in western parts of basins. The basins are fragmented by splays and stepovers of the sinistral faults, which created popups and sags reflected in modern topography. Most faults are composed of two or three strands, and pairs of these strands in places frame persistent synclines and anticlines. Stepovers between strands in several places create positive flower structures, shifting fault slip to an adjacent strand. The subsurface geometry of these features is constrained by basement depth estimates from gravity modeling and by geometries revealed by airborne time-domain electromagnetic surveys. In addition, basin axis positions are interpreted from clast provenance, showing that depocenters have migrated laterally. Although complex, the resulting mosaic of fault compartments and sedimentary facies can be used in developing hydrogeologic framework and groundwater-flow models to simulate groundwater flow in and between basins and thus help to predict greatest likelihood for water quantity and quality needed for military operations at Fort Irwin.