ContentsPutah and Cache: Clear Lake or Lypoyomi

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Geology of Putah-Cache: Cenozoic Volcanic Rocks

Eldridge M. Moores and Judith E. Moores

Two sequences of volcanic rocks are present within the Putah-Cache watershed—the Sonoma Volcanics and the Clear Lake Volcanics.

Mt. Konocti, one of the most visible examples of the Clear Lake Volcanics, is an inactive volcano which still preserves its volcanic form. The Clear Lake Volcanics are mostly silica-rich volcanic rocks (e.g. obsidian) located in and around Clear Lake, but they also include some basaltic rocks as well. These volcanic rocks are about 10,000 years to 2.25 million years old. For the past million years or so, the main center of volcanic activity has been south and east of Clear Lake. Interbedded with the Clear Lake Volcanics is a Pliocene-Pleistocene sequence of lake and stream beds, approximately 2 km thick, the Cache Creek beds. It is thought to have been deposited in a fault-bounded depression (graben) east of present-day Clear Lake. It's not clear whether these sediments connected with the Tehama sediments to the east (e.g., in the Winters area). Pyroclastic deposits are rare in the Clear Lake Volcanics, unlike the Sonoma Volcanics.

The Sonoma volcanic rocks are very well developed in Napa and Sonoma counties (they constitute the principal rock source of the soils of the wine-producing region in and around Napa Valley). They also are present in the southwestern part of the Putah-Cache bioregion, e.g. on Mt. St. Helena, whose eastern flank is part of the region. These rocks are a diverse collection of volcanic rocks of varying composition with interlayered sediments. These rocks include many silica-rich volcanic rocks that represent the product of explosive eruptions and deposition from a hot volcanic cloud (pyroclastic deposits). Short-lived lakes and redistribution of volcanic rocks by stream action locally formed interlayered sandstones, conglomerates, and siltstones. The source region of the Sonoma Volcanic rocks is uncertain, but it possibly was near the northern end of Napa Valley not far from the town of Calistoga. Ages of these rocks range from 2.6 to 8 Ma (late Miocene to late Pliocene).

The Clear Lake and Sonoma Volcanics represent the northernmost occurrences of volcanic rocks exposed throughout the California Coast Ranges from the Transverse Ranges (north of Los Angeles) to Clear Lake. These volcanic rocks formed progressively from southeast to northwest through time, in concert with the northwest migration of the northwest tip of the the San Andreas Fault.

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