SUBMARINE CANYON DEVELOPMENT DUE TO RAPID SUBSIDENCE WITHIN THE MANIOBRA BASIN ALONG THE SOUTHWESTERN CORDILLERAN MARGIN DURING THE EARLY-MIDDLE EOCENE

The paleogeography and tectonic development of the southwestern Cordillera during the Eocene remains an elusive topic. A key component to reconstructing the continental margin during this time is the identification of sedimentary basins that preserve evidence of marine shorelines and sediment provenance that can be linked to the continent. The >1000 m-thick Maniobra Formation, located along the southern margin of the Mojave Desert, represents a >25 km-long submarine canyon incised into underlying Hayfield granite between 53-46 Ma (Advocate, 1982). Lithofacies are grouped into 4 depositional environments: shallow marine shelf, submarine slope, submarine canyon channel, and submarine fan lobe. Shallow marine strata occur locally as thin (<5 m-thick) white sandstone above a deeply weathered granite basement. Marine slope strata consist of shale and thin (<20 cm thick) sandy turbidites and are interpreted to have occurred in bathyl water depths. Submarine channel strata occur as boulder breccia with interbedded sandstone and shale turbidites. They occur above the shallow marine strata and basement, locally obtain stratigraphic thickness of up to 250m ,and fill topographic relief incised above the basement. New flute cast measurements (n=21) from the bases of two turbidite flows within the basin axis trend due west. Canyon deposits grade laterally into slope strata to the north and south and define the channel width (~3 km). Canyon lobe strata are thin(decimeter-scale) sand/shale turbidite sequences that represent distal submarine fan lobes deposited out of channel. DZ spectra throughout the section are dominated by Paleoproterozoic age peaks (~ 1.7 – 1.79 Ga), with subordinate Late Jurassic (168 – 156 Ma) and Late Cretaceous (91 – 78 Ma) populations increasing upsection, and are consistent with a Mojave basement source. Development of this deep, submarine canyon above deeply weathered, sub-aerial basement requires rapid subsidence along the continental margin at 53 Ma. We speculate that a west dipping detachment structure may be hidden along the eastern basin margin. When we remove 90° of Miocene rotation, our data support development of a N-S directed submarine canyon formed along an E-W shoreline, which may have formed due to subsidence within the path of the Hess-conjugate during the Eocene.