CONSTRAINING THE LONG-TERM SEDIMENTATION HISTORY USING BOREHOLES AT ANCIENT LAKE CAHUILLA, COACHELLA, CA

The southernmost ~100 km of the southern San Andreas fault (SAF) has an average ground rupturing earthquake recurrence interval of ~200 years over the last 1000 years based published paleoseismic studies. However, the most recent ground rupturing earthquake within this area occurred ~300 years ago, during the last highstand of the ancient Lake Cahuilla (LC) in the Salton Trough, California. The correlation between major SSAF earthquakes and LC highstands is debated and the relationship is difficult to establish due to the brevity of the records. In order to establish a correlation between the major rupture events along the SSAF and LC highstands, an extended lake sediment depositional history is needed. In this study, we have analyzed two continuous boreholes, 33.5 and 40 m deep from the Coachella paleoseismic site along the SSAF. The 33.5-m-deep core was collected within a small structural depression at 9 m elevation near a ~12-m-high shoreline of LC. The 40-m-deep core was collected southwest of the first core, outside the deformation zone. From these two boreholes, we interpret the long-term sedimentation history of LC using luminescence (p-IR IRSL) and grain size analysis to determine the timing and frequency of sediment supply, aggradation, and depositional hiatuses from layers in the cores. Our new results show that the borehole logs extend the long-term sedimentation history at the Coachella paleoseismic site to ≥ ~9 ka. These cores also exhibit different characteristics of sedimentary deposits, with potentially different numbers of fine grain sediment layers. Our work from both of these boreholes contributes to the further understanding of the long-term depositional and tectonic history along the SSAF.