GEOLOGY OF THE SADDLE PEAK HILLS 7.5’ QUADRANGLE, SAN BERNARDINO COUNTY, CALIFORNIA

The Saddle Peak Hills 7.5’ quadrangle (southeast corner of Death Valley National Park) was mapped at 1:24,000-scale as part of USGS EdMap #G11AC20185. Normal-fault bounded ranges expose bedrock in three locations: the Saddle Peak Hills, Sperry Hills and Salt Spring Hills. The quadrangle contains exceptional exposure of Mesoproterozoic to Cambrian sedimentary rocks including the newly revised Pahrump Supergroup (sensu Mahon et al., 2012) – the Crystal Spring Formation, the newly named Horse Thief Springs Formation, Beck Spring Dolomite, and the Kingston Peak Formation. Ediacaran to Cambrian rocks include the Ediacaran Noonday Dolomite up to the Middle-Lower Cambrian Cararra Formation. The Saddle Peak Hills contain exceptional exposures of a 300 m.y. (1070-770 Ma) unconformity between the Crystal Spring Formation and the overlying Horse Thief Springs Formation.

The quadrangle is located at the junction of three major tectonic provinces: the Basin and Range to the north and east, the Transverse Ranges to the west, and the Mojave Province to the east. Furthermore it is just north of the junction of two major transform faults, the Death Valley Fault Zone and the Garlock Fault. This has resulted in a complex, multi-stage structural history including four stages of Cenozoic extensional tectonism preceded by at least one generation of compressional tectonism. Compressional tectonism (C-1) is evident in south-southeast trending folds and is inferred to be Mesozoic in age. The first extensional (E-1) episode included down-to-the-west normal faults, which are generally flat lying at present. E-2 includes northwest-striking, down-to-the-northeast and down-to-the-southwest normal faults and extrusion of middle Miocene (ca. 12.6-12 Ma) trachytic dikes and volcanic rocks, and monolithologic breccias of landslide-origin. E-3 includes north-striking normal faults with both down-to-the-east and down-to-the-west displacement. E-4 includes the present north- and northwest-striking range-bounding normal faults inferred along the west sides of the Saddle Peak, Sperry and Salt Spring Hills. Effects of late Neogene transpression in the Avawatz Mountains may be evident in pervasive northwest oriented cleavage in the Kingston Peak Formation and in 12.6-12 Ma dikes throughout the quadrangle.