ANALYZING THE GRAIN SIZE DISTRIBUTION FROM SEDIMENTARY CORES IN THE COACHELLA VALLEY, CALIFORNIA

Coachella Valley in southern California is periodically filled by Lake Cahuilla (LC), an ancient freshwater lake that extended from the near the north end of then Salton Trough southward into Mexico multiple times during the Holocene. Prior studies suggested various stages of lake appearance and disappearance due to changes in the Colorado River course, along with minor fluvial contributions from the San Bernardino and San Jacinto Mountains. Radiocarbon dating has identified six instances of the lake reaching full capacity over the past 1100 years, yet we don’t fully know its sedimentation history earlier than that. Here, we present grain size distributions from two cores obtained from the Coachella Paleoseismic Site - located 9 m above sea level near the 13 m high stand of LC in the northeastern Coachella Valley. One core was taken from within the Coachella Structural depression (C1), and the other from the Mesquite Dune (C2) outside of it. Using luminescence dating, radiocarbon dating, and grain size analysis, these boreholes record the longest (past ~ 10.5 thousand years ago) chronology of sediment cores in this region. Preliminary data and trends suggest that the first core (C1) exhibits several sections of fine-grained clay or silty layers in the upper section, suggesting the presence of lake deposits within this sedimentary section. Silt and clay percentages increase with depth, with an increase in sedimentation rate starting at around 10,000 years ago, possibly due to the fluvial sediment contribution. Conducting thorough grain size distributions throughout the entire section will enable a more comprehensive exploration of the paleolake occurrences and their geological context.