GEOLOGIC MAPPING OF COLORADO RIVER DEPOSITS IN IN THE SOUTHERN BLYTHE BASIN, BETWEEN EHRENBERG AND CIBOLA, ARIZONA AND PARTS OF CALIFORNIA

Colorado River (CR) deposits in the lower Blythe basin, from I-10 south to the Cibola area, have been the focus of recent (since 2013) and ongoing detailed geologic mapping. Arizona Geological Survey (AZGS) geologists have mapped at 1:24,000 scale in various 7.5-minute quadrangles and parts of adjacent areas, as part of the STATEMAP Mapping Program; we have collaborated with the USGS LOCO mapping project, and academic researchers working in the region. Here, we summarize the results of published and ongoing mapping, primarily in Arizona but including parts of California, with emphasis on late Miocene and early Pliocene Bouse Formation, early Pliocene Bullhead Alluvium, and late Miocene and Pliocene tributary deposits.

Detailed mapping of CR-related deposits since the first regional investigations of the 1960s and 1970s has documented many previously undocumented Bouse deposits. It has also provided insights into relationships between pre-, syn-, and post-Bouse deposits, including important basinwide stratigraphic and structural relationships between units laterally and vertically. In the southern part of the map area, thick and complex Bouse deposits are spectacularly exposed from the margins of the modern CR floodplain to adjacent mountains. Sand and gravel of the Bullhead Alluvium and tributary alluvium erosionally overlie Bouse deposits. Farther north, exposures from the valley axis to high on the adjacent piedmonts consist primarily of sand and gravel of the Bullhead Alluvium, with tributary deposits predominant near the mountains and locally interbedded with Bullhead deposits. Bouse basal carbonate deposits (primarily travertine/tufa) are extensively exposed on the basin margins, draped over or encrusting complex bedrock paleotopography. Multiple cross-sections that combine surface and subsurface geologic information help to interpret, correlate and discuss aspects of post-mid-Miocene basin architecture such as base-level, topography, paleo-landscape gradients, and areas of syn- and post-depositional basin subsidence. New dating methods have allowed for ongoing interpretations about the timing of deposition, boundaries and potential for sequence-stratigraphic history of Colorado River deposits.