STRUCTURAL AND PALEOTOPGRAPHIC CONTROLS ON DEPOSITION OF THE SOUTHERN BOUSE FORMATION IN THE SOUTHEASTERN PALO VERDE MOUNTAINS, CALIFORNIA

The late Miocene to early Pliocene Bouse Formation records conditions that existed prior to and during downward integration of the Colorado River (CR) to the Gulf of California. Interpretation of depositional environments of the southern Bouse Formation is debated, and is important for understanding regional tectonic evolution and origins of the CR. Bouse deposits in the southeastern Palo Verde Mountains display evidence for a complex interplay between pre-Bouse paleotopography, syn-Bouse tide-driven paleocurrents, and syn-depositional transtensional deformation that is unique to the study area. We used variable-scale geologic mapping integrated with detailed stratigraphic and lithofacies analysis to document major trends in stratal architecture in the SE Palo Verde Mts. The Bouse Formation in this area is divided into three members: (1) basal carbonate consisting of earliest encrusting travertine overlain by onlapping transgressive bioclastic and marl facies; (2) siliciclastic member comprising green claystone, red mudstone, and thick multi-story cross-bedded CR channel sandstone; and (3) upper bioclastic member which records re-flooding of the lower CR valley after the river first ran through it. Four detailed stratigraphic panels oriented NW-SE (perpendicular to the range front), integrated with numerous paleocurrent measurements, reveal the strong influences of pre-existing paleotopography and regional tilting on depositional processes and stratal architecture. Paleohills eroded in Miocene volcanic rocks were encrusted by Bouse basal travertine, and formed major barriers that created areas of high and low depositional energy. In areas between paleohills we observe thick tidal bar and dune deposits (up to 5 m thick) in bioclastic grainstone that display abundant tide-generated bi-directional cross bedding. Thickness and paleocurrent patterns show that inherited paleotopography focused and strengthened tidal rip currents that produced locally preserved giant bedforms. Stratigraphic panels show a distinct systematic SE-thickening wedge geometry within all members of the Bouse Formation, providing evidence for syn-depositional tilting to the southeast during late Miocene to early Pliocene deformation in a transtensional step-over of the Eastern California Shear Zone.