ContentsPutah and Cache: Winters

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Geology of Putah-Cache: Cenozoic Deposits

Eldridge M. Moores and Judith E. Moores

Near Winters, it's possible to see relatively recent (Cenozoic) rocks: sedimentary and volcanic deposits up to about 65 Ma, listed here from oldest to youngest.

1. Lower Tertiary (Eocene: 36-50 Ma) shallow water marine sandstones and shales form the top of the Great Valley sequence, explained in more detail in the Monticello Dam chapter.

2. Overlying the Eocene rocks is a discontinuously exposed Miocene (5-24 Ma) continental flood basalt volcanic unit, called the Putnam Peak Basalt, after exposures on Putnam Peak just a few km south of Putah Creek along the east side of Pleasants Valley Road. Large blocks of this volcanic unit are present in a large landslide along Putah Creek in the "Black Rocks" area just east of the mouth of the main Putah Creek Canyon. Similar rocks have been traced in the subsurface from here to Oroville, where they are exposed on Table Mountain. The source of these volcanic rocks is not well understood. One idea is that they originated as fissure eruptions (from cracks in the Earth's crust) east of the present Sierra Nevada in Plumas County. The Putnam Peak volcanic rocks are the same age and composition as those exposed in a vast area of eastern Washington and Oregon, called the Columbia Plateau lavas. Current thinking is that these latter rocks represent the results of a "hotspot" volcanic eruption, similar to the Hawaiian Islands, with the active hotspot or source presently under Yellowstone Volcano. The compositional and temporal similarity between the Putnam Peak and Columbia Plateau rocks makes it tempting to ponder whether there is a connection between these two regions.

3. A Pliocene-Pleistocene stream deposit, the Tehama Formation, forms the top of the Cenozoic rock sequence on the west side of the Sacramento Valley. These rocks, aged 1-3 Ma, were deposited by streams flowing eastward from the rising Coast Ranges into the Valley. Interbedded with these stream deposits are a few layers of a light-grey to tan volcanic tuff unit, the Putah Tuff; 1.8 Ma in age, exposed along Highway 128 near upper Solano Lake. This tuff unit, and other units were formed by air-fall deposits of volcanic eruptions probably from the north end of Napa Valley.

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