LATE-MIOCENE TO EARLY-PLIOCENE TIDAL SEAWAY ALONG THE LOWER COLORADO RIVER VALLEY: EVIDENCE FROM LARGE-SCALE TIDAL BEDFORMS IN THE SOUTHERN BOUSE FORMATION BASAL CARBONATE MEMBER

Contrasting hypotheses for the depositional paleoenvironments of the late Miocene to early Pliocene southern Bouse Formation, south of Parker, Ariz., currently hinder progress toward a consensus on tectonic and paleogeographic reconstructions for the lower Colorado River region. The basal carbonate member of the southern Bouse Formation has been interpreted to record deposition in either a large inland lake isolated from the ocean, or a transgressive intertidal to subtidal marine embayment. To further test these hypotheses, we conducted a detailed sedimentologic study of the Bouse basal carbonate member in the SE Palo Verde Mountains, CA. Bouse basal carbonate in this area consists of (i) basal travertine encrusted on volcanic bedrock, (ii) bioclastic facies (focus of this study) that onlap travertine, and (iii) upper marl. Travertine formed during initial flooding of the basin. Bioclastic facies record a transition from alluvial-fan margin to subaqueous conditions and reflect the strong influence of inherited paleotopography on depositional processes. 15 lithofacies are grouped into two main facies associations: (1) siliciclastic-dominant tidally influenced alluvial fan-margin; and (2) carbonate-dominant tidal flat and subtidal dune field association. Paleocurrent data provide a clear record of bi-directional rectilinear flows that were oriented NW-SE and E-W around basin margins, and N-S in the basin axis. Large-scale (5 m thick) compound dunes have sigmoidal geometries, migrated primarily in the direction of ebb currents (E, SE, S), and are observed close to steep paleo-hills. Ebb-directed simple dunes are consistently larger (0.2-1.8 m thick) than flood-directed simple dunes (<0.25 m). The data record input of debris flows and sheet floods from local alluvial fans during deposition of bioclastic facies in marginal tidal flats and axial subtidal dune fields during progressive submergence and increasing water depths. We conclude that these deposits accumulated in a transgressive, ebb-dominated marine macro-tidal seaway prior to arrival of the Colorado River. Our paleogeographic reconstruction suggests that a narrow passage through the Chocolate Mountains at the Highway 78 pass provided a connection to the northern Gulf of California during deposition of the Bouse basal carbonate member.