11.3: Physical Geology
The San Joaquin Valley is a sediment-filled depression, called a basin, that is bound to the west by the California Coast Ranges, and to the east by the Sierra Nevada. 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.
The Valley dates back more than 65 million years to the Mesozoic, when subduction was taking place off the coast of California. However, the plate tectonic configuration of western North America changed during the Tertiary, and the ancient trench that once characterized offshore California was transformed into a zone of right-lateral strike-slip motion that we know today as the San Andreas fault. Nonetheless, the Valley still retains many features that characterized it prior to formation of the San Andreas transform.
Because the San Joaquin Valley is bound to the west by the California Coast Ranges, which represent a zone of folding and thrusting, such as an accretionary pris, associated with the ancient subduction zone, and bound to the east by the Sierra Nevada, which represent the eroded roots of an ancient volcanic arc that was also associated with the subduction zone, some call the valley a remnant arc-trench gap.
The tectonic processes by which this arc-trench gap formed are complicated, as are the events by which the ancient trench became the San Andreas fault.
When the San Joaquin Valley first formed, it was an inland sea between two mountain ranges. This configuration remained even after formation of the San Andreas fault (below). However, as the volcanic cover of the Sierras was eroded off, the resulting sediment was dumped into the Valley below. At the same time, The Coast Ranges were also being worn down and dumped into the valley. Thus, the inland sea was filled to create the continental basin we know today.
When the basin was still an inland sea, diatoms and other plankton thrived in it, and when these marine organisms died, they accumulated on the basin floor to create organic-rich shales that include the Eocene Kreyenhagen, and Miocene Monterey Formations. The integrated effects of heat and time then acted on the buried organic matter within these shales to create oil, and the detritus eroded from the Coast Ranges and the Sierra Nevada provided reservoir rocks where the oil could accumulate
Tectonic Depositional Setting
The ancient marine basin that the Great Valley Sequence was deposited in very closely approximates the combined extent of the modern Sacramento Valley and San Joaquin Valley as delineated by the California Coast Ranges on the west side and the Sierra Nevada on the east. Geologists believe that the Great Valley Sequence represents the sedimentary fill of forearc basin formed along the convergent plate boundary that existed along the west coast of North America during the Jurassic and Cretaceous. During this time oceanic crust was being subducted beneath the continental margin prior to formation of the modern San Andreas Fault.
The eastern boundary of the forearc basin was a volcanic arc, a chain of ancient volcanoes located where the Sierra Nevada is today. Batholiths and metamorphosed volcanic rocks in the Sierra Nevada are the geologic record of this volcanic arc. As these ancient Sierran volcanoes were rapidly eroded they fed volcanic-derived sediment through submarine canyons westward to the ancient submarine fans of the Great Valley Sequence. Additional sediment was fed into the basin as the granitic roots of these volcanoes were uplifted and eroded. Therefore, the volcanic cover of the Sierra Nevada has been largely stripped off and used by nature to fill the Great Valley basin. Little sediment is believed to have been derived from the uplifted Franciscan rocks of the Coast Ranges.
The western boundary of this basin was created by the growth and uplift of an accretionary wedge consisting of sedimentary, volcanic, and metamorphic rocks scraped off the subducting plate. The uplift of this accretionary wedge acted like a dam to form the western side of a basin in which the Great Valley Sequence was deposited. The geologic record of this accretionary wedge are the metamorphic and sedimentary rocks of the Franciscan Assemblage that today make up much of the California Coast Ranges.
The Great Valley Sequence was largely deposited atop basaltic rocks representing a slab of oceanic crust of mid-Jurassic age which was incorporated into the North American continent and today is known as the Coast Range Ophiolite.
Morphology
The Great Valley Sequence contains several geologic formations. Because geologists studying this sequence in different areas have used different formation names over the years to describe the same packages of rocks, the nomenclature is complex and confusing. Parts of the sequence are well exposed at several places along a line of outcrops that extends for about 150 miles (241 km) along the west side of the Sacramento and San Joaquin valleys (both of which make up the Great Valley of California). These same rocks are also found in several deep wells in oil and gas fields located in the center of the valley. Geologists working these different areas independent of each other developed different sets of formation names for the different areas.
Some of the best outcrops for these rocks are on the west side of the southern Sacramento Valley at Putah Creek and Cache Creek canyons. The formation names used here extend to the subsurface where several gas wells in the center of the valley produce from the same rocks, and from younger rocks that overlie them. Although the younger rocks are not exposed in the canyons to the west, most geologists combine the formation names from the west side outcrops with the subsurface names from the gas fields to create a coherent set of names that is largely agreed upon for most of the Sacramento Valley.
The same is not true for the San Joaquin Valley to the south, where another area of well-known outcrops is found in the hills to the west of Coalinga. This area uses a completely different set of formation names for the same group of rocks as are found to the north. Yet another set of formation name is applied to Great Valley rocks penetrated by oil and gas wells in the subsurface northeast of Coalinga in the area near Stockton in the northern part of San Joaquin Valley. In addition, there are several lesser-known areas in the San Joaquin Valley that each have their own terminologies. All these names are firmly established in the geologic literature and attempts to set up a unified set of formation names have not met with success. An exception are the Jurassic-age rocks of the Great Valley Sequence, which most geologists assign to the Knoxville Formation, irrespective of location.