CONTROLS ON MID-PLIOCENE (CA. 2.8 MA) PROGRADATION OF COARSE DETRITUS IN THE FISH CREEK-VALLECITO BASIN, SOUTHERN CALIFORNIA

A major goal of basin analysis is to distinguish between climatic and tectonic controls on progradation in non-marine sedimentary basins. The Late Neogene Fish Creek-Vallecito basin in southern California contains a well-exposed record of Pliocene gravel progradation during subsidence in the hanging wall of the West Salton detachment fault. Our analysis of this section includes detailed mapping, measured sections, magnetostratigraphy, paleocurrent data, conglomerate clast counts, stable isotopes, quantitative subsidence analysis, and study of paleo-erosion using Be isotopes. Basin reorganization occurred ca. 2.8 Ma when locally-derived streams and alluvial fans prograded across the western part of a large fluvial depocenter that previously filled from the east with sediment from the Colorado River. Paleocurrent and clast count data show that two modern drainages, Carrizo Creek in the south and Vallecito Creek in the northwest, were the primary sources for the locally-derived sediment. Punctuated, relatively slow progradation of these deposits began sometime before 3.3 Ma. At ca. 2.8 Ma progradation accelerated, blanketing the area with locally-derived sediment and ending deposition of Colorado River sediment in the western part of the supradetachment basin. The abrupt progradation event was, paradoxically, coeval with the onset of lacustrine conditions at the western margin of the regional “Borrego Lake”, which filled much of the Salton basin lowland at this time. In the Vallecito badlands, the lake setting was located at the distal downstream ends of the Carrizo and Vallecito Creeks.

Although rapid progradation corresponds with global climate deterioration ca. 2.8 Ma, we see no evidence for increased precipitation in the region (Peryam et al., 2009). Decay-corrected ¹⁰Be concentrations in quartz sediment collected from the basin support a gradual decline in paleo-erosion rate. These findings are not consistent with sediment flux as a driver of progradation of locally derived sediments into the basin. We infer that progradation likely was driven by a decrease in basin subsidence rate ca. 3.1 Ma. Calibrating the age of strata across the basin remains a challenge. Age interpretations and depositional models will continue to be refined by ongoing paleomagnetic and cosmogenic isotope investigations.