Sonoma Valley - Geology

Geology

The Sonoma Valley is part of the Coast Range Physiographic provence. Basement rocks that make up the valley at great depth are the Great Valley Sequence shale, sandstone and conglomerate deposited in a continental slope- to abysmal plain environment via turbidite flows. The Cretaceous Great Valley Sequence overlies and contacts the Franciscan Complex along the Coast Range Thrust. The Jurassic-Cretaceous Franciscan Complex includes crumpled, uplifted terranes that have resulted from the subduction of the former oceanic Farallon Plate under the North American continent. During late Miocene-Pliocene time (~10 to ~4 million years) the area was attended by volcanism (Late Miocene Tolay Volcanics and Late Miocene - Pliocene Sonoma Volcanics) which are interbedded with the late Miocene-Pliocene Petaluma Formation. The (~9 to 4 million year old) Petaluma Formation was a fresh-water river system flowing from east to west and through the volcanics. At that time, volcanic lava flows and river sands and gravels were actively deposited together, hence "interbedded lavas and gravels". The volcanoes may have been similar to island arcs. The Petaluma Formation is found in outcrop from Sears Point to Santa Rosa (through Sonoma Mountain) and as far west as Cotati where it interfingers with a marine sandstone called the Wilson Grove Formation. Gravels in the Petaluma Formation did not come from rocks located in Napa, but have been sourced to mountains east of San Jose, California. This does not mean rivers flowed northward from San Jose to Sonoma; rather, strike-slip movement along the Hayward-Sonoma Valley-Carneros fault system has dislocated present-day Sonoma County north and away from the mountains in San Jose where the basin formed.

The valley is drained by Sonoma Creek, whose headwaters rise in Sugarloaf Mountain State Park and discharge into the San Pablo Bay at the Napa Sonoma Marsh. Some of the principal tributaries to Sonoma Creek are Yulupa Creek, Graham Creek, Calabazas Creek, Bear Creek, Schell Creek and Carriger Creek.