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Anatomy And Provenance Of A Deep-Water Boulder Conglomeratic Submarine Canyon In The Upper Cretaceous Panoche Formation (Cenomanian), Great Valley Group, San Luis Reservoir, Central California


Low water levels in the San Luis Reservoir in the southern Diablo Range of Merced County, CA, have exposed a deep-water boulder conglomerate succession approximately 1500 m thick along a 2 km transect within the Upper Cretaceous (Cenomanian) Panoche Formation of the Great Valley Group (GVG). Individual boulder conglomeratic units represent the thickest (up to 180 meters thick) and coarsest (up to 4.5 meter clasts) deposits ever recorded in the GVG. Fully characterizing these extraordinary deposits and placing them in a paleogeographic context adds to a steadily evolving history for the GVG in the San Joaquin basin. The dominant lithofacies associations include clast- and matrix-supported boulder and cobble conglomerate within a coarse sandy matrix, pebble and cobble traction carpets, medium to coarse-grained sand lenses containing abundant planar bedding, and thin-bedded turbidites marking breaks between periods of coarser-grained deposition. Basal surfaces of the conglomerate lithofacies are commonly sharp and erosive but with limited relief. In general, clast size does not display grading throughout the conglomerate lithofacies, however, the lowest conglomeratic unit in the system contains consistent 2-3 m clasts at the base. Published geologic maps indicate Panoche-aged conglomerate may extend approximately 18 km along strike to the northwest, roughly parallel to northwest-to-southeast directed paleocurrent measurements recorded on imbricated clasts. Therefore, although no canyon wall exposures were observed, we interpret the entire section to represent a strike-parallel submarine canyon exposure based on the style of deposition, thickness, continuity of deposits, and proportion of oversized boulder clasts. Combined provenance data from conglomerate clasts and mudstone geochemistry suggest sources at two difference scales. The oversized conglomerate clasts are dominated by hornblende gabbro, brecciated basalt, and keratophyre indicating a local ophiolitic source from nearby headlands contributing to the submarine canyon deposits. However, the dominant background source for the canyon is Sierran when compared to a more regional mudstone geochemical database. In addition, although Panoche-aged mudstone data generally overlap with San Joaquin basin GVG samples, our results fill a data gap that records a transition from Lower Cretaceous mafic trace element signatures to Upper Cretaceous felsic signatures.