Petrography and Provenance of the Pismo Formation, San Luis Obispo County, California

The Pismo Formation is a Miocene-Pliocene oil-bearing sandstone that was\ud deposited following the erosion of a major unconformity at the top of the Miocene\ud Monterey Formation. Sediments change at the unconformity from the extremely lowclastic\ud content of the diatomites, porcellanites and dolomites of the Monterey Formation\ud to the massive, highly porous sandstone of the Pismo Formation. During a Pacific\ud Section AAPG field trip to view the Monterey Formation outcrops at Shell Beach near\ud the town of Pismo Beach in San Luis Obispo County, California questions were raised\ud about the tectonic history of the Pismo basin. Point-counting of grains from Pismo\ud Formation sandstones combined with a petrologic evaluation of rocks from possible\ud source areas show changes in provenance could be due to right-lateral movement along\ud strike-slip faults and uplift and erosion due to transpressional tectonic forces. Temporal\ud and spatial changes in the provenance of the Pismo Formation sandstones increase our\ud knowledge of patterns of tectonism and sedimentation during the Miocene and Pliocene\ud in south-central California