Upper Cretaceous and Paleogene conglomerates in the Gualala basin of northern California likely preserve the westernmost record of sedimentation in California during the interval between cessation of Sierran batholith magmatism and inception of Neogene transform activity on the San Andreas fault system. Detailed chemical and H, O, Sr, Nd, and Pb isotope analyses of conglomerate clasts, as well as U/Pb zircon ages, identify two source types: (1) evolved granite, like those found in the Salinian block or eastern Sierra Nevada batholith, and (2) primitive oceanic arcs that were restricted to the western portions of the Mesozoic arc system. The provenance of the Gualala clasts can be entirely explained through derivation from the Sierra-Salinian-Mojave portion of the Mesozoic Cordilleran arc, and do not support models for extensive large-scale translation of the basin from low latitudes along a pre-Neogene transform fault system. Contemporaneous deposition of the contrasting lithologies in the Gualala basin is interpreted to reflect tectonic juxtaposition of the source terranes approximately 80 m.y. ago. Tectonic processes responsible for juxtaposition of the two source regions in the Late Cretaceous most probably involved zones of lithospheric weakness that later localized the San Andreas fault system in the Neogene. Chemical, isotopic, and age data tie the Gualala basin to North American continental basement in the Mojave-Salinian segment of the Mesozoic Cordilleran arc and imply minimal pre-Neogene translation of the basin contrary to the low latitude origin for the basin inferred from previous paleomagnetic and paleontologic studies.