Document Type

Article

Publication Date

6-2-2025

Abstract

The Great Valley Forearc (GVF) basin of California, USA, preserves an extensive rock record of the Jurassic–Paleogene tectonic development of the California segment of the North American Cordillera. We present new U-Pb geochronology, zircon and whole-rock geochemistry, and petrographic analyses from the Great Valley Group (GVG), Franciscan subduction complex, and Coast Range Ophiolite (CRO) in the northern San Joaquin Valley to better understand the timing and location of initial forearc sedimentation, and how sediment routing systems may have evolved during Cretaceous time. Basal GVG strata of the Knoxville Formation were deposited ca. 145–140 Ma and are separated by an ~40 m.y. unconformity with overlying strata of the Upper Cretaceous Panoche Formation. Pre-Mesozoic zircon grains are present in both the Knoxville and Panoche formations, but are sparse (0%–7%) compared to other GVG sandstones. Zircon geochemistry records felsic igneous sources (Th/U 0.9–0.2) during both periods of deposition, and epsilon Hf signatures reveal a shift from juvenile to more evolved sources between Knoxville and Panoche deposition. Whole-rock geochemistry shows increasing compositional maturity from latest Jurassic crystallization of the CRO to Early and Late Cretaceous deposition of the GVG. Integrating these data, we present a tectonic model for the northern San Joaquin portion of the GVF basin from ca. 145 Ma to 80 Ma that documents the onset of basin deposition and details sediment pathways during the Early to earliest Late Cretaceous. In addition, we discuss potential drivers for the ~40 m.y. unconformity within the San Joaquin Valley and implications of this work for global forearc basin processes.

DOI

https://doi.org/10.1130/B38093.1

Publisher

Geological Society of America

Publication Information

GSA Bulletin

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