New Hypothesis of Early Jurassic Paleogeography and Sediment Dispersal for Western United States

Abstract

Recent studies of both composition and sedimentary structures of Upper Triassic and Lower Jurassic clastic rocks in the Cordillera provide evidence for a modified paleogeographic reconstruction. The presence of marine Triassic rocks in north-central Nevada and of Jurassic mature quartz sandstones in western Nevada and southeastern California indicates that no continuous Mesocordilleran geanticline completely separated the miogeosynclinal and eugeosynclinal belts during early Mesozoic time. In southern Nevada, Upper Triassic and Lower Jurassic sedimentary strata considered typical of miogeosynclines are interstratified with conglomerates and volcaniclastic rocks derived from a rising volcanic island arc in the west in California. The only adequate source of pure quartz sand to form the Navajo and associated formations appears to have been Triassic and upper Paleozoic sandstones of the western craton in Montana and Alberta, where Middle Jurassic strata are regionally disconformable on middle and lower Paleozoic rocks. Crossbedding orientation data indicate that sand was transported south from the region parallel with the cratonic margin, and thence southwesterly into western Nevada and California. A broad marine carbonate bank lay farther northwest in Nevada. Much of the deposition of the pure sands probably was as very shallow marine sand waves, as well as in coastal eolian dunes formed by onshore winds.