LATE MIOCENE-PLIOCENE DIATOMITES OF THE PETALUMA FORMATION, SONOMA COUNTY, CALIFORNIA

The Petaluma Formation (PF) interfingers with Donnell Ranch Volcanics, Sonoma Volcanics and contains several beds of diatomite in the Sonoma Mountain area. We sampled 17 diatomite localities for diatoms. Coupled with using dated tuffs and earliest/latest known occurrences of extinct diatoms, we have constrained both diatomite ages and units across the Rodgers Creek fault (RCF).

The earliest known diatomite in the PF occurred at the time of the Roblar Tuff (RT) (~6.25 Ma). Immediately west of the RCF at Crane Canyon, a diatomite conformable with the RT contains the following species: Eunotia naegelii (earliest known occurrence: 7 Ma); the extinct diatoms E. majuscula, Gomphonema blandicum, and Pinnularia legumen v. staurophara give a 4 Ma age. Further north at Fountain Grove, the identical package occurs again west of the, or perhaps within, the RCF.

A diatomite and tuff package exposed in Lichau Creek where it intersects Petaluma Hill Road, west of the RCF, gives an age range of 2-5 Ma based on the fauna Hantzschia amphioxys v. densistriata and Melosira areolata. The tuff was too altered to date. A similar fauna and age was collected and identified at Pressley Road east of the RCF and west of its intersection with Sonoma Mountain Road. Here the tuff has been dated at 4.64-4.65 Ma. Further east of the RCF and along Sonoma Mountain Road, the same package described previously crops out again and contains a similar fauna and therefore age.

At Donnell Ranch, east of the RCF and along the southwestern foothills of Sonoma Valley, a diatomite and tuff package contains Navicula radiosa v. acuta, N. subglobosa, Cyclotella subatomus, C. hagermanensis (first known occurrence: 5.5 Ma), and Cyclotella kansasensis (extinct at 4 Ma). The tuff was later dated at 5.2 Ma. 2-3 miles north at Grove road and Jack London State Park, and east of the RCF, a similar fauna was identified. From the Grove road locality, a tuff interbedded with this diatomite yields a 4.88 Ma age.

From these data, the dated tuff units attest our oldest diatom age calls per locality. A diatomite bloom of late Miocene to early Pliocene age occurs in the PF where it's exposed on either side of the RCF. Using the late Miocene and early Pliocene localities described above as a continuous diatomite horizon now possibly offset 12-15 km across the RCF since about 4 Ma, we suggest a slip-rate of 3-4 mm/yr.