Extensional arc setting and ages of Middle Jurassic eolianites, Cowhole Mountains (eastern Mojave Desert block, California)

Mesozoic strata in the Cowhole Mountains, eastern Mojave block California, include 200–800 m of eolian quartz arenite (Aztec Sandstone) overlain by more than 575 m of silicic ignimbrites, lava flows, and minor sedimentary rocks (Cowhole volcanics). U-Pb zircon geochronologic data on a crystal-rich dacite lava flow in the Aztec Sandstone indicate that the sandstone is Middle Jurassic and is therefore age equivalent to backarc eolianites of the Temple Cap and Carmel Formations, not the Lower Jurassic Navajo Formation as previously assumed. Our U-Pb zircon data on two ignimbrites of the Cowhole volcanics indicate that they are the same age, within error, as the lava flow in the Aztec Sandstone at 170 3 Ma. New structural and stratigraphic data, together with published data, indicate syndepositional normal faulting and deposition of landslide blocks throughout deposition of this section. These results are consistent with our previous volcanologic studies in Middle Jurassic rocks of southern Arizona, southeastern California, and the central Mojave block, which document ignimbrite eruptions from calderas contemporaneous with deposition of craton-derived eolian sands, in extensional or transtensional intra-arc basins. Busby, C.J., Schermer, E.R., and Mattinson, J.M., 2002, Extensional arc setting and ages of Middle Jurassic eolianites, Cowhole Mountains (eastern Mojave Desert block, California), in Glazner, A.F., Walker, J.D., and Bartley, J.M., eds., Geologic Evolution of the Mojave Desert and Southwestern Basin and Range: Boulder, Colorado, Geological Society of America Memoir 195, p. 79–9