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Constraining the long-term sedimentation history using boreholes at ancient Lake Cahuilla, Coachella, CA

Marina O. Argueta, Sourav Saha, Seulgi Moon, Nathan D. Brown, Thomas K. Rockwell, Katherine M. Scharer, Zoe Morgan, & Jenifer Leidelmeijer

Accepted October 10, 2022, SCEC Contribution #13399

The southernmost ~100 km of the San Andreas fault (SSAF) has been determined to have an average recurrence interval between ~116 and ~221 years within the last 1000 years based on previously examined trench records. However, the most recent ground rupturing earthquake within this area occurred ~300 years ago, coinciding with the last highstand of the ancient Lake Cahuilla (LC). The correlation between major rupturing events and LC highstands has been debated and the relationship has been difficult to establish due to the limited trench records only going back ~1-2 ka. 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 to the southwest of the first core, outside the deformation zone. Within these two boreholes, we interpret the long-term sedimentation history of LC using p-IR IRSL and grain size analysis to determine the timing and frequency of sediment supply, aggradation, and 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.

Argueta, M. O., Saha, S., Moon, S., Brown, N. D., Rockwell, T. K., Scharer, K. M., Morgan, Z., & Leidelmeijer, J. (2022, 10). Constraining the long-term sedimentation history using boreholes at ancient Lake Cahuilla, Coachella, CA. Poster Presentation at Geological Society of America Annual Meeting.