MIO-PLIOCENE PALEODRAINAGE CHANGES IN THE NE MOJAVE, CA INFERRED FROM PALEOCURRENT DATA AND COMPOSITION OF FLUVIAL CONGLOMERATES

Between the Eastern California Shear Zone and the southern Death Valley and Colorado River extensional domains, in the Old Dad Mtn., Seventeenmile Point, and Indian Spring 7.5' Quads, a 0.5-1km thick sequence of Miocene coarse clastic strata (Tc) provides evidence for multiple Neogene paleodrainage reversals. Tc consists of polymict conglomerates, breccias, and block glides with sparse interbedded volcanic rocks that can be divided into 7 unconformity bounded sequences, Tc1-7, based on composition, flow direction, stratal dip, and stratigraphic position.

Tc1 fills a SSW trending paleochannel cut into crystalline basement and is overlain with angular discordance by an ash flow tuff correlated with the 18.5Ma Peach Springs tuff (Tps). Clasts include well-rounded, polished, chattermarked quartzite boulders to 70cm probably sourced from late Proterozoic-early Cambrian quartzite. Paleocurrents flowed SSW. Tertiary volcanic clasts first appear in the upper 10m. Tilting of Tc1 to the SW occurred prior to deposition of Tps. The Tc1 paleochannel is cut by a post-Tps fault with 1.5km of dextral offset. Overlying Tps, Tc3 contains a clast assemblage with a high percentage of Jurassic metavolcanic rocks. Based on areal distribution, composition, clast size, and paleocurrents, Tc3 was deposited in a NNE flowing mid-late Miocene paleoriver that locally cut into crystalline basement. Paleodrainage reversal may coincide with subsidence to the north in southern Death Valley. Tc4 and Tc5 are more areally restricted than Tc3, but also show NNE flowing paleocurrents. They may have been deposited by tributaries to the larger Tc3 drainage. Tc6 overlies Tc3-5, and is overlain by latest Miocene-Pliocene basalt flows. The Tc6 clast assemblage contains mostly granitoid rocks sourced from the Teutonia Batholith. Paleocurrents indicate S flowing drainages into a depocenter in Ivanpah Valley or beyond. A fourth change in paleoslope is recorded by Plio-Quaternary basalt flows and drainages that flow W towards Soda Lake. In an area of the NE Mojave with limited exposure of Neogene rocks, detailed mapping and sedimentology of Neogene coarse clastic rocks documents previously unrecognized Miocene deformation and the rapid response of paleodrainages to changing base level in surrounding areas.