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.

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.

[2] 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.

[4][5][6] 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.

[7] The Great Valley Sequence contains a diverse and abundant assemblage of fossils that includes microfossils, plant remains, invertebrate marine creatures, and vertebrates.

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.