11.3: The Great Valley Sequence

Forarc Basin Sediments

When you drive along I-5 in central California, you encounter landscapes so flat that you could mistake them for the endless agricultural fields of the Midwest. But underneath the rectangular plots there exists an incredible story: buried as deeply as 12 km, there are the shattered fragments of a chain of giant volcanoes. Their eroded clasts, plucked apart by oxidizing amphibole and feldspars disintegrating into clay, washed down streams and out into the ocean. But because the Coast Range Ophiolite hindered movement of these clasts to the west, they settled in deep offshore basins, grain piled upon grain, filling what would become the Central Valley with the debris of a California that no longer exists. We call this configuration a forearc basin (Figure \(\PageIndex{1}\)) .

Gravity and water, over many millions of years, transferred the husk of dead volcanoes from their high loft in the east down into the Central Valley, piling thick sediment layers that would, in time, consolidate to become mudstones, sandstones, conglomerates. This is the Great Valley Sequence.

Figure \(\PageIndex{1}\): The Great Valley Sequence formed in the forearc basin between the volcanic arc of the Sierra Nevada and the Coast Range accretionary wedge. "Ancient Subduction Zone" by the National Park Service is in the public domain. Access a detailed description.

For most of the Central Valley, you cannot see these sedimentary rocks because you drive only on the most recent layer of compacted dirt. But as you approach the Coast Ranges’s eastern edge, the rocks of the Great Valley Sequence burst from the ground like breaching whales, turning upwards until they are near vertical. Of course, according to the principle of original horizontality, (see Geologic Time), these rocks too were once horizontal, and have since been tilted during the later uplift of the Coast Ranges (see Formation of the San Andreas Fault). In places such as the Monticello Dam near Lake Berryessa, these alternating layers of Great Valley Sequence sediment are stacked for hundreds of meters at an angle so near vertical that the strata might as well be trees in a forest.

Figure \(\PageIndex{2}\): This photograph shows an impressive rock outcrop of turbidites in the Venado Sandstone, located near Lake Berryessa in California. It highlights the classic alternating light and dark bands of sandstone and shale typical of deep-sea turbidity current deposits. The rock layers are steeply tilted, nearly vertical, due to tectonic deformation. "Lake Berryessa turbidites2.jpg" by Mikesclark via Wikimedia Commons is licensed under CC BY-SA 3.0.

Forming at the same time as the Great Valley Sequence, but to the west, a vast complex now scraped off the descending Farallon Plate and crumpled, twisting and torquing, unsettled in its new home. This is the time of the Franciscan.

Query \(\PageIndex{1}\)

Alt text: Diagram showing subduction of the Farallon Plate beneath North America, forming the Coast Range, Great Valley, and Sierra Nevada.

Alt text: Outcrop of the Great Valley Sequence at Lake Berryessa.