The Mesozoic rocks underlying the Tertiary cover in the Santa Maria basin, California, consist of an Upper Cretaceous sedimentary sequence overlying dismembered ophiolitic rocks of undetermined age. The ophiolitic rocks are ophicalcites, basalts, greenstones, dike and sill rocks, gabbros, and serpentenized peridotites in a Franciscan assemblage. The Upper Cretaceous rocks are sandstones, mudstones, and shales that were deposited in a submarine fan environment. They are characterized by tectonic disruption in the form of brecciation and chaotic mixing. These basement rocks of the Santa Maria basin were compared to rocks of similar age surrounding the basin in terms of petrography, sedimentary structures, and facies relations. Two major groups of strata were compared: relat vely undisturbed sedimentary rocks correlated to the Great Valley sequence and the disrupted Franciscan rocks of the Cambria, Point San Luis, and Pfeiffer Beach slabs. The Santa Maria basin basement rocks are similar to the latter group.

Mesozoic rocks underlying the Santa Maria basin were formed in a converging plate boundary regime on the inner trench-slope. The ophiolitic rocks and the overlying deep-water cherts and pelagic sediments were accreted in fold and thrust segments as the downgoing oceanic plate was subducted. The accreted material formed linear ridges which trapped younger sediments in a trench-slope basin. Sediments from several different source areas accumulated in these localized basins and were tectonically deformed with continuation of subduction in the trench. The rocks resulting from this process are called the Franciscan melange.

The Santa Maria basin formed above these Franciscan rocks following collision of the Farallon-Pacific spreading center with the trench off California in Oligocene time. The subsequent transform regime produced major right-lateral strike-slip fault movements and associated downwarping of the Santa Maria area. Deep-water, middle Miocene sediments were deposited in this newly formed basin in contact with the Mesozoic rocks.

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