143-15 Comparing Provenance Among Miocene-Aged Basins Along the San Andreas Fault System, Southern California, Through Sandstone Petrography and Geochemical Analysis

Abstract:

The San Andreas fault (SAF) system has translated and reorganized numerous sedimentary basins across southern California, including Miocene non‑marine deposits preserved in the Cajon Pass and Mill Creek areas of the San Bernardino Mountains. These deposits occupy a structurally complex portion of the SAF system and may record aspects of their original depositional environments as well as subsequent strike‑slip transport of 160–310 km along the southern SAF. This study aims to establish the provenance and geochemistry of the Cajon Valley and Mill Creek formations by pairing sandstone modal point count analysis with X‑ray fluorescence (XRF) and X‑ray diffraction (XRD) analyses. These datasets provide independent constraints on evolving Miocene sediment sources and enable tests of potential correlations with other displaced units along the SAF system.

To evaluate provenance and compositional variability, we analyzed nine sandstone samples from Units 2, 3, and 5 of the Cajon Valley Formation and three samples from the Mill Creek Formation using >400‑point count modal petrography following the Gazzi-Dickinson method. Preliminary results show systematic relationships between our new modal data and published QFL datasets from other Miocene basins in southern California. Both formations plot within quartz–feldspar–lithic fields consistent with continental block provenance, and several potentially correlative units—including the Branch Canyon Sandstone, Santa Margarita, Punchbowl, Paradise Springs, and Vasquez formations—exhibit similar feldspathic compositions marked by nearly equal proportions of plagioclase and potassium feldspar.

Lithic fragment compositions, however, show more limited correspondence. Mill Creek samples contain lithic assemblages characteristic of continental block sources and lack sedimentary lithic fragments, similar to Vasquez Formation samples. In contrast, Cajon Valley samples contain lithic populations dominated by magmatic arc fragments, a pattern shared by only a few published samples from other formations. These differences likely reflect distinct sediment dispersal pathways and variable source‑area weathering histories, despite similarities in feldspar proportions. XRF and XRD data processing is underway and will be used to further refine interpretations.

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