Great Valley Sequence

Great Valley Sequence Stratigraphic range: Late Jurassic through Cretaceous
Geologic map showing location of the Great Valley Sequence. Modified from Irwin (1990)
Type	sedimentary
Underlies	Cenozoic sedimentary fill
Overlies	basement
Thickness	up to 40,000 feet (12 km)
Lithology
Primary	shale and sandstone
Other	minor conglomerate
Location
Region	Great Valley-Central Valley of California
Country	United States
Type section
Named for	Great Valley-(Central Valley) of California
Named by	Baily, Irwin & Jones (1964)

The Great Valley Sequence of California is a 40,000-foot (12 km)-thick group of related geologic formations that are Late Jurassic through Cretaceous in age (150–65 Ma) on the geologic time scale. These sedimentary rocks were deposited during the late Mesozoic Era in an ancient seaway that corresponds roughly to the outline of the modern Great Valley (Central Valley) of California.

Although the total thickness of the Great Valley Sequence in places is upwards of 12 kilometres (39,370 ft), most of these rocks today are buried beneath the thick Cenozoic sedimentary fill of the Great Valley of California. A number of gas and oil wells do penetrate these rocks in the subsurface of the valley, and these same rocks also crop out extensively along the west margin of the valley and to a lesser extent along the northern and eastern margins. The sequence overlies the Franciscan Assemblage along the west margin of its extent, and onlaps onto granitic rocks of the Sierra Nevada in the east.

The rocks in Great Valley Sequence are largely shale deposited in a deep-marine setting with thick bodies of sandstone and minor conglomerate that represent ancient deep-sea submarine fans and submarine canyons. Individual sandstone beds within these fans and canyons are typically turbidites and related sediment gravity flow deposits, such as the chaotic, often coarse-grained deposits of submarine mudflows. By contrast, the shales represent clay that was originally suspended in the water column and slowly settled out on the deep ocean floor. Thus, the mudstones represent background sedimentation that took place continuously, whereas the turbidites and mudflow deposits, by and large, represent sudden events.