AAPG Foundation 2019 Grants-in-Aid Projects

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The Upper Cretaceous Pigeon Point and Atascadero Formations: Sedimentology, Stratigraphy, and Detrital Zircon Geochronology Evidence for Tectonic History of the San Gregorio-Hosgri Fault

Abstract

The Upper Cretaceous Pigeon Point Formation outcrops along the Pacific Coast westward of the San Gregorio-Hosgri Fault (SGHF), containing a full spectrum of coarse-grained deep-water deposits. This study aims to develop a depositional model and evolutionary picture of the formation, improve the understanding of depositional processes of low- and high-density turbidity currents, and correlate the formation across the SGHF. We integrate both detailed stratigraphic measurements and geologic mapping at a sedimentation-unit scale to better understand the lateral and vertical changes of turbidite deposits and refer their depositional processes. Since the current location of the Pigeon Point Formation is bounded by the Pacific Ocean to the west and SGHF to the east, the depositional location and potential correlative formation(s) are difficult to determine. Previous studies suggest that the Pigeon Point Formation is correlated to the Upper Cretaceous Atascadero Formation. In this study, we use comprehensive provenance analyses of sandstone petrography, detrital zircon geochronology, and major and trace element mudstone geochemistry with a broader dataset from both formations to test whether both formations should have the same provenance and whether they might have been contiguous prior to faulting. Preliminary sandstone petrography agrees with the previous interpretation; however, mudstone geochemistry and detrital zircon geochronology reveal that the deep-water section of the Pigeon Point and Atascadero Formations have different provenance signatures. These results suggest that the Pigeon Point Formation might be correlated to other formation further south, implying a larger slip of the SGHF than currently suspected.