PROVENANCE AND GEOCHEMISTRY OF SEDIMENTS IN THE NORTHERN GULF OF CALIFORNIA: RESULTS FROM A DEEP STRATIGRAPHIC RECORD FROM WELL SAMPLES

The timing of the earliest marine incursion in the northern Gulf of California–Salton Trough is not well defined, although it has important implications in the regional tectonics. The oldest Colorado River sediments in the Salton Trough are ~5.3 Ma and overly a latest-Miocene marine section. However, marine microfossils found in exploration wells (PEMEX) in the northern Gulf of California suggest a mid-Miocene marine incursion. We study sandstone and mudstone composition on drill cuttings from the PEMEX wells, and from modern fluvial sediments to investigate provenance and the onset of basin fill in the Tiburon and Wagner basins. Modern fluvial sources have distinctive petrofacies and Sm-Nd isotope ratios and define three end-members: (1) modern sediments from the Colorado River have quartzose petrofacies, older Nd model ages (1666 - 1316 Ma) and strongly negative ƐNd values (-15.4 - 12.5) , (2) sediments from northwestern Sonora (end-member 2) are characterized by younger Nd model ages (1296 - 1261 Ma) and less negative ƐNd values (-11.5 - -7.8), and (3) sediments from local sources in northern Baja California (end-member 3), which include significant volcanic detritus from local syn-rift volcanic fields, and present the youngest Nd model ages (1292 - 889) and high ƐNd values (-7 - -2.0), the sandstones from end-member 3 show lesser content of quartz and higher contents of P/K. Sandstone petrofacies and Sm-Nd isotope ratios from well samples varies within a narrower range compared to modern end-members and suggest variation in the relative contribution of each end-member, and/or mixing of terrigenous sediments by tides, waves and longshore currents. Modern Colorado River muds are isotopically enriched compared to Pliocene Colorado mudstone deposits and likely reflects the down cutting of the Grand Canyon. Our results indicate that Colorado River sediments are present within the upper 5000 and 2800 meters of the Wagner and Upper Delfin basins, respectively. In contrast Tiburon basin show mixing processes between sediments from Sonora and the Colorado River The deep occurrence of Colorado River sediments in these basins favor a late Miocene marine incursion and a rapid basin fill during Plio-Pleistocene time coeval with deltaic sedimentation across the rift zone.