| The fluvial Sycamore Formation was deposited between 8.5 Ma and 2.5 Ma in the northern Livermore basin, east of the San Francisco bay. Sediment in the formation records the evolution of the basin, as the plate margin in this area changed from a convergent to a transform system. The Finley Road, Blackhawk, and Roundhill petrofacies are defined here on the basis of variation in gravel composition within the Sycamore Formation. Tuffs and fossils within the formation provide time constraints for deposition of these petrofacies. The three petrofacies indicate successive changes in provenance to local sources, as uplift occurred first on the south side of the basin and then along the northeast edge, blocking deposition of Miocene Sierran volcanic material. Sediment was derived from the area of Mount Diablo, but not from the Franciscan core. The Sycamore Formation was deformed after 2.5 Ma, when the regional stresses changed from slightly extensional to compressional. |
