COMPOSITION, AGE AND GEOGRAPHICAL EXTENT OF THE GLEN ELLEN, HUICHICA AND PETALUMA FORMATIONS, SONOMA AND NAPA COUNTIES, CALIFORNIA

The Glen Ellen (GEF), Huichica (HF) and Petaluma (PF) formations are located in Sonoma and western Napa Counties, California. These formations are fluvial and/or lacustrine with the exception of Petaluma Formation which has been demonstrated to contain documented estuarine facies in its lower, middle and upper. In addition to this difference in environment, the PF also differs from the GEF and HF in terms of conglomerate clast composition, location and age.

The PF (~4 - 9 Ma) occurs on both sides of the Rodgers Creek fault in the Sonoma Mountain area. It has three easily recognizable members: lower shale member; middle Franciscan Complex (FC)-derived conglomerate; and an upper Monterey formation derived conglomerate. Clast composition in the middle member is: 96-100% FC graywacke, chert, silicic volcanic and metasedimentary. Dominant clast composition in the upper member is both: 80% FC graywacke, chert and metasedimentary or 90% Monterey Group Claremont Chert. The addition of conglomerate beds dominated by Monterey-derived clasts typifies the upper PF conglomerates. All three members are present in the western portion of the formation, near and interfingering with the coeval Wilson Grove Formation. In the eastern portion of the formation, the PF is interbedded with Donnell Ranch and Sonoma Volcanic mafic, rhyolitic, pyroclastic rocks and diatomites. The upper member mapped along the western Sonoma Valley foothills east of the RCF and BVF contains two separate tuff beds dated at 5.2 Ma and 4.88 Ma by Ar/Ar method.

The HF (4.71 Ma and younger) is located in the southern Maacama Mountains at the Sonoma-Napa county line. The basal HF is coeval with the upper PF, but is separated from PF by the Sonoma Valley, Eastside Fault and difference in lithology. HF conglomerates are volcaniclastic, lack Monterey clasts and are derived from the Sonoma Volcanics from the Maacama Mountains. The nearest outcrop of Monterey is to the east in Carneros Valley. The lack of Monterey debris in the HF suggests the outcrops of Monterey in Carneros Valley were down-dropped, or buried during the Pliocene-Pleistocene.

The GEF (0.8-3.2 Ma) is located in northern Glen Ellen, Santa Rosa and the Maacama Mountains. The GEF contains basalt, andesite and obsidian clasts. Contacts with the PF in the Bennett Valley and Santa Rosa areas are everywhere faulted.