ContentsPutah and Cache: Monticello Dam

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Geology of Putah-Cache: The Great Valley Sequence

Eldridge M. Moores and Judith E. Moores

The Great Valley sequence of rocks are exposed along the western side of the central valley (e.g., along Putah Creek near Lake Berryessa and along Cache Creek and Bear Creek northwest of Rumsey). They were deposited in deep water between the ancestral Sierra Nevada to the east and the subduction zone, represented by the Franciscan rocks, to the west. These rocks, indicated on Figure 1, are of late Mesozoic age (140-65 million years old). They aggregate a 10-13,000 meter thick pile of sediments beneath the lower Cache and Putah watersheds. These rocks are upturned and generally display an interbedded series of less resistant shale and more resistant sandstones, which together give the landscape a striped look. The sandstones commonly show "graded bedding," from coarse sand grains at the base to finer grains at the top. This provides a means to interpret how the rocks were deposited and which is "younging" direction, the way up when the rocks were originally horizontal.

In the Putah Cache bioregion, most exposed Great Valley rocks trend (strike) generally N-NW and are inclined (dip) generally east, and they are older in the west and younger in the east. The general interpretation of these rocks is that they were deposited off the edge of the continent, in approximately 3000-5000 feet of water, by pulses of sediment-rich slurries that flowed off the continental shelf in response to some disturbance (possibly individual earthquakes). These slurries are called "turbidity currents" and the deposits, such as the Great Valley sequence, are called turbidites.

Many turbidites have casts of grooves, scour, or gouge marks on the bottom of sandstone beds that were made by a turbidity current in the underlying mud. These "bottom markings" sometimes show the direction that the turbidity currents moved when the deposits were formed. In the case of the Great Valley sediments, most bottom markings indicate that currents flowed from north to south. The roadcuts in Cache Creek Canyon have especially well-developed examples of such bottom markings, but the marks are also present near Monticello dam.

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