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

The age of the earliest deltaic deposits of the Colorado R. in the Salton Through is Early Pliocene (Dorsey et al., 2007). Deltaic deposits prograded over a latest Miocene marine section that includes relatively thick gypsum deposits, which likely predate the arrival of the Colorado R. sands. However, marine microfossils found in industry drill cores from PEMEX in the northern Gulf of California [GoC] suggest a mid Miocene marine incursion (Helenes et al., 2009). To constrain the provenance of sediments in the Wagner, Upper Delfin and Tiburon basins located in the northern GoC and the arrival of Colorado R. sand and mud to these basins, we analyzed trace element and Sm-Nd isotopic composition of 40 shale samples from three exploratory wells with depths ranging from ~5500 to 3200 m. Petrofacies analysis in 25 sandstone samples from wells and 9 samples from modern fluvial sediments from Sonora and Baja California were also accomplished. Three modern source end-members can be defined: (1) modern sediments from the Colorado R. have quartzose petrofacies, older Nd model ages (1666-1316 Ma) and highly negative εNd values (-15.45 - -12.5). Contrasting with sediments from NW Sonora (end-member 2) and from local sources in northern Baja (end-member 3), which include significant volcanic detritus from local syn-rift volcanic fields. End-members 2 and 3 are characterized by a decrease in quartz content, higher lithic fragments, younger Nd model ages (1292-889 Ma) and lower εNd values (-9.46 - -2.01). 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 marine processes, principally, tides, waves and long-shore currents. The Sm-Nd isotope composition and model ages indicate that modern Colorado R. muds are derived from a more enriched source compared to Pliocene Colorado R. muds. Our preliminary interpretation indicate that modern Colorado R. incise more ancient rocks from the Grand Canyon in Arizona. Older deposits may represent erosion of younger rocks in the Colorado Plateau. Additional petrofacies analysis in progress, will constrain the arrival of Colorado R. sand to the Wagner, Upper Delfin, and possibly to the Tiburon basins in the south.