PROGRESS TOWARD UNDERSTANDING THE STRATIGRAPHY AND RIGHT-LATERAL DISPLACEMENT OF UPPER MIOCENE ROCKS ALONG THE SAN ANDREAS FAULT IN CENTRAL CALIFORNIA

Recent field and biostratigraphic studies of upper Miocene strata in central California provide new information that bears on the history of right-lateral displacement along the San Andreas fault. Near Maricopa, the upper Miocene Bitterwater Creek Shale is exposed along the northeast side of the San Andreas fault and consists mainly of hard, siliceous shale with dolomitic concretions and turbidite sandstone interbeds. Diatom assemblages indicate that the Bitterwater Creek Shale was deposited about 8.0 to 6.7 Ma, coincident with the uppermost part of the Monterey Formation in other parts of California. The Bitterwater Creek Shale overlies fan-delta deposits of the upper Miocene Santa Margarita Formation, which in turn overlie siliceous shale of the Monterey Formation from which we obtained diatom assemblages dated at about 10.0 to 9.3 Ma. Huffman (1972, GSA Bulletin) noted that the Santa Margarita Formation contains granitic and metamorphic clasts derived from a source in the northern Gabilan Range, on the opposite side of the San Andreas fault, that has moved relatively northwestward by 254 ± 5 km of right-lateral displacement. Our new diatom ages suggest that this displacement occurred after 10 to 8 Ma.

Dibblee (1966, CDMG Bulletin 190) proposed that the Bitterwater Creek Shale near Maricopa was separated by about 80 km of right-lateral displacement from siliceous mudstone in the Pancho Rico Formation near Parkfield, on the opposite side of the San Andreas fault. However, we suggest that this hypothesis is incorrect because (1) diatoms from lower part of the Pancho Rico indicate deposition about 6.7 to 5.5 Ma, younger than the Bitterwater Creek Shale, and (2) the Pancho Rico Formation is lithologically unlike the Bitterwater Creek Shale.

In the upper Miocene Panorama Hills Formation of Dibblee (1962, Pacific Section AAPG Guidebook) on the northeast side of the San Andreas fault about 15 km west of Fellows, we found a new species of Forreria, a marine gastropod. The new species also occurs in the Pancho Rico Formation about 105 km to the northwest on the opposite side of the San Andreas fault. These Forreria localities were closer together prior to right-lateral motion along the fault, but whether they constrain the amount of displacement is uncertain because the paleogeographic distribution of Forreria is poorly known.