Abstract: Petrography and Provenance of the Miocene-Pliocene Pismo Formation, San Luis Obispo County, California

DOWNEY, DONALD, Chevron Overseas Petr. Inc., San Ramon, CA

The objectives of this study were to determine the provenance of sandstones of the Pismo Formation by correlating mineral grain suites from the sandstones with their source rocks and to evaluate changes in the spatial and temporal provenance relationships by interpreting the effects of tectonic uplift/unroofing of source areas, sea-level changes, and movement along strike-slip faults. Results from this study show that most of the Pismo Formation sands were derived from Atascadero Formation sandstones. Some input from the Franciscan Formation was identified in the western part of the basin. Obispo Formation and Morro Rock Formation volcanic rocks were also contributors. The tectonic patterns during the transtensional phase suggest slow subsidence of the basin and downcutting of successive source areas, punctuated by episodic local uplift of source areas along nearby strike-slip faults, followed by uplift during the transpressional phase and the addition of Monterey Formation diatomite and chert lithic fragments. During the Late Miocene and Pliocene, changes in Pismo Formation provenance reflect local uplift and erosion along proximal faults. Differential subsidence of sub-blocks within the Pismo syncline also appears to have controlled provenance patterns. This study shows that movement along the West Huasna fault was not significant enough to move different source areas into place. This confirms the study of McLean (1993), which defined piercing points across the West Huasna fault by correlating outcrops of Obispo Formation andesitic rocks, limiting estimates of Early Miocene to present right-lateral movement on the West Huasna fault to 5 to 8 kilometers.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California