Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences in the central Mojave block

Sedimentologic, stratigraphic, and geochronologic data from strata of early Mesozoic age in the central Mojave block elucidate the paleogeographic and tectonic evolution of the magmatic arc in the southern U.S. Cordillera. A sequence of calcareous siltstone, volcaniclastic conglomerate, tuff, and quartzose sandstone records the transition from shallow-marine rocks of the Fairview Valley Formation to the subaerial Sidewinder volcanic series. Quartzose sandstones occur below, within, and above the transitional sequence and indicate that texturally mature, craton-derived quartz sand gained access to the arc during the initial stages of volcanism. U-Pb data indicate that explosive volcanism began at 179.5 3.0 Ma and continued until 151 1.3 Ma (Lower Sidewinder volcanic series). A rhyolite dike of the Independence dike swarm (Upper Sidewinder volcanic series) that postdates normal faulting and tilting of the ignimbrites yielded a U-Pb date of 151.9 5.6 Ma. The data define the age of extension and development of the angular unconformity between the Upper and Lower Sidewinder volcanic series at ca. 151 Ma. The data suggest that at least part, and possibly all, of the Fairview Valley Formation is late Early Jurassic in age. We correlate the Fairview Valley Formation with Mesozoic metasedimentary rocks in the Rodman Mountains and Fry Mountains, and at Cave Mountain to the east. Eolian quartz arenites in these sequences suggest a coastal environment coeval with the Navajo Sandstone on the Colorado Plateau. The reinterpretation of the shallow-marine rocks as Jurassic instead of Triassic suggests a period of uplift and erosion or nondeposition extending from the Early Triassic into the Early Jurassic, followed by a return to marine conditions. Shallow-marine conditions persisted until the beginning of arc volcanism in the late Early Jurassic time. Similarities to the early Mesozoic arc of the Sierra Nevada, together with the structural evolution of the region, suggest that the change from high-standing to lowstanding paleogeography reflects a large-scale tectonic control on relative sea level related to a period of intra-arc extension or transtension. Schermer, E.R., Busby, C.J., and Mattinson, J.M., 2002, Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences in the central Mojave block, 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. 93–115. E.R. Schermer, C.J. Busby, and J.M. Mattinson 94