PROVENANCE OF THE SANTA MONICA BASIN, CALIFORNIA: EFFECTS OF CHANGING SEA LEVEL ON DEEP-WATER CLASTIC DEPOSITION

The Santa Monica Basin, located in the southern California Borderland, is a closed source-to-sink system and currently the region’s major sink for coarse-grained sediment. Sea level has been shown to have a significant effect on the pathway by which clastic sediment is transported from land into the ocean. However, further study is required to better understand the impact that changing sea level has on the provenance, mineralogy, and texture of deep-water deposits. To address this, we present a multi-proxy provenance analysis on samples from fluvial, beach, and deep-water deposits to characterize bulk mineralogy, provenance, grain size, and texture. In total, 53 samples were analyzed and categorized into 6 regional groups. Detrital zircon U-Pb analyses show a high degree of similarity between the Santa Monica Basin samples (inter-sample R² ranges from 0.74-0.87). Multidimensional scaling shows the majority of basin samples to be similar to the Santa Clara River, with the exception of samples deposited from 0.48-0.70 kya, 3.25-4.15 kya, and ca. 12.5 kya that are most similar to Santa Ynez sources, the Ventura River, and Calleguas Creek, respectfully. Forward mixture modeling indicates the Santa Clara River to be the dominant source, with Calleguas Creek becoming most abundant at ca. 12.5 kya, and Santa Ynez sources becoming significant around 3.25-4.15 kya and 0.48-0.70 kya. Heavy mineral analysis indicates that basin samples deposited between 7-60 kya are most similar to Santa Clara River and Calleguas Creek. Samples deposited after ~4.15 kya are more similar to Santa Ynez sources, with larger quantities of spessartine/grossular garnet, chromite, and sulfides. Portable X-ray fluorescence results were found to be more influenced by depositional environment than provenance; relative to deep-water samples, rivers are enriched in Ca, Cr, Fe, Ni and Cu and beaches significantly depleted in these elements. Overall, results suggests that sedimentation into the Santa Monica Basin was not controlled by a single source but rather by an alternation of fluvial and littoral processes. Specifically, increased contributions from Santa Ynez sources during late Holocene time suggests the integration of littoral drift with Santa Clara River and Calleguas Creek fluvial sources during sea level rise.