MARINE DEPOSITION AND NEOGENE BASIN SUBSIDENCE IN THE GULF OF CALIFORNIA-SALTON TROUGH

Exposures of Neogene marine sediments are relatively limited within the Gulf of California-Salton Trough region but provide important evidence bearing on the evolution of the gulf during Miocene and Pliocene time. Key stratigraphic sequences are present along the western flank of the Salton Trough, the eastern coast of Baja California, on islands in the gulf, and along the Pacific coast of mainland Mexico. Neogene sediments recovered from the gulf by dredging and DSDP drilling and available seismic reflection profiles provide additional insights. Biostratigraphic and radiometric ages together with lithostratigraphic and paleobathymetric data from selected stratigraphic sections allow application of backstripping techniques to derive rates and patterns of basin subsidence in contrasting crustal and tectonic settings. Slow rates of subsidence during mid Miocene rifting and non-marine deposition within the Gulf Extensional Province are typical of thermal decay. Analysis of sediments on Isla Maria Madre and at Punta Mita on the Nayarit coast illustrate that marine flooding in the gulf mouth area began ca. 10 Ma with rapid mechanical subsidence to bathyal water depths between 10-6 Ma. Marine deposits at San Gorgonio Pass, California and near San Felipe, Baja California indicate that subsidence to bathyal water depths also occurred in the northernmost gulf as early as 7-6 Ma. Widespread rifting and rapid subsidence to middle and lower bathyal water depths occurred from 5-2 Ma in concert with initiation, establishment and evolution of the Gulf of California transform-rift plate boundary. Evidence from Isla Maria Madre and analysis of stratigraphic sections in the San Jose del Cabo basin and DSDP Site 475 off Cabo San Lucas suggests that deformation of Miocene "proto-gulf" sediments occurred along the eastern margin of the gulf as crustal extension, spreading, transform motion, and basin subsidence accelerated to the west during Pliocene time.