Abstract

The Miocene Monterey Formation in the Pismo Syncline, San Luis Obispo County, California, is characterized by the following lithofacies: 1) diat-omite, 2) carbonate, 3) phosphate, and 4) siliciclas-tic siltstone. The Pismo Syncline is a relatively simple fold that served as a depositional basin during most of the Miocene and Pliocene. During deposition of the Monterey Formation, the Pismo Basin was separated into two parts by a positive volcanic high running parallel to the San Miguelito fault along the western edge of the Basin. That portion of the Basin northeast of the topographic high received 1500 meters of Monterey sediment, whereas the portion of the Basin to the southwest of the high received 300 to 600 meters of sediment.

The Monterey Formation in the central portion of the Pismo Syncline consists of a thick section of silty diatomaceous sediments. In contrast, the Monterey Formation along the western edge of the Basin (west of the topographic high) is characterized by carbonate and phosphatic-rich rocks, as well as diatomaceous rocks. The volcanic ridge acted as an effective barrier to siliciclastic sediments entering the Pismo Basin from the east. Current directional features in the Monterey rocks in the western portion of the Pismo Basin suggest that much of the calcareous, phosphatic and diatomaceous material was originally deposited or formed in situ on the slopes of the topographic high and was later swept downslope into the lower portions of the Basin.

In the Monterey Formation along the western side of the Pismo Syncline there is an upward trend of carbonate→phosphate→diatomite sedimentation. This vertical trend in sedimentation can be correlated to the Miocene sea level changes described by Vail and Hardenbol (1979).