VOLCANIC BOMBS DEFINE VENT AREA FOR LATE PLIOCENE SONOMA VOLCANICS

The Upper Pliocene Sonoma Volcanics (Osmont, 1905) occupy an area of about 50x65 kilometers north of San Francisco Bay in the wine country of Sonoma and Napa counties. The volcanic rocks total about 1,500 meters in thickness and consist of andesitic and rhyolitic flows (including obsidian), along with extensive tuffs and agglomerates. They are interbedded toward the west with the shallow marine Merced formation, and on the east with the continental Tehama formation. It is an area of extensive folding and faulting, and relationships are often obscure. To the best of my knowledge, no clearly defined vent areas have been defined, although several investigators have suggested that there were multiple sources, probably located where the Sonoma Volcanics are thickest, well to the east of the Pliocene marine embayment. With the identification of three classic examples of volcanic bombs east of Glen Ellen, CA, there is good evidence for the location of a late Pliocene volcano. Although it may have been more than a kilometer in height, it has eroded completely, except for its coarser remnant debris. Two textbook examples of volcanic bombs, weighing 10-15 kg, were recently identified on the Bouverie Land Preserve. In 1957 a larger bomb, of about 25 kg, was found in situ about one kilometer to the west, when I was an undergraduate at U.C. Berkeley. I had brought it to noted vulcanologist, Howell Williams, who stated that it would have been cast no farther than the lower slopes of the Pliocene volcanic edifice, which I name here, the Warfield Volcano as it overlaps the historic Warfield Ranch. Blockage by such Pliocene vents may have diverted the subsequent Sonoma Creek from the axis of Sonoma Valley, and diverted it westward to its present anomalous channel, along Warm Springs Road, where it is incised through hills comprised of the continental gravels and sands of the Pleistocene Glen Ellen formation.