Tertiary Stratigraphic Units of Western Mojave Desert, California

Abstract

Scattered throughout the western Mojave Desert, mostly in the vicinities of Rosamond, Mojave, and Boron, are exposures of non-marine sedimentary and volcanic rocks of Tertiary age. These rocks rest on a deeply eroded surface of pre-Tertiary granitic and metamorphic rocks that form the crystalline basement complex of this region, and are overlain unconformably by alluvial sediments of Quaternary age. Where exposed the Tertiary rocks are moderately deformed and much eroded. With the exception of exposures in the foothills northwest of Mojave, the rocks of Tertiary age are mapped as a group, named the Tropico group, with a maximum exposed thickness of about 2,800 feet. This group is divided into several lithologic units of local extent. In most sections the lower unit is a pyroclastic formation, with associated rhyolitic intrusions and flow-breccias, and the upper unit is either fanglomerate or a sequence of carbonate rocks, clays, and sandstones of lacustrine and fluviatile origin. Several basalt flows occur locally in the group, mostly in the middle part. The group is non-fossiliferous except at one locality, where diatom remains in the upper unit suggest early Pliocene age. The lower unit is tentatively correlated with lithologically similar formations of known Miocene age in areas west and northwest. Formations in the Tropico group that are locally distinct are named the Gem Hill formation, Fiss fanglomerate, Bissell formation, Saddleback basalt, and Red Buttes quartz basalt. In the foothills northwest of Mojave the granitic basement is overlain by about 1,000 feet of terrestrial sandstones and clays named the Horned Toad formation. This formation yielded a mammalian fauna indicating middle or early Pliocene age, and is either younger than or correlative with the upper part of the Tropico group.