KEYNOTE: NEOGENE MICROPALEONTOLOGY AND EVOLUTION OF THE NORTHERN GULF OF CALIFORNIA

Geologic models locate Baja California next to mainland Mexico before the Oligocene and propose opening of the Gulf of California during Miocene times. Exploratory wells in the northern Gulf, drilled as much as 6000 meters of Upper Tertiary sedimentary rocks with previously unknown paleontological information. We present our findings on the micropaleontology of these rocks, and comment on their regional significance.

Outcrop data around the northern part of the Gulf, indicate marine environments during late Miocene times. 7.4-6.09 Ma in Salton Sea, and 7.09-6.93 Ma, near Santa Rosalia). Considering these ages and the volcanic history of Puertecitos and IslaTiburón, current models propose the start of the opening at around 6Ma. However, dinoflagellates and calcareous nannofossils from exploratory wells indicate marine sedimentation in this area starting in middle Miocene times (ca. 12 Ma) and continuing until the Pleistocene.

Subsurface data from the northern part of the Gulf indicates two stages for the opening of this part of the Gulf. The older stage in the eastern side, dominated by normal faulting, formed basins near Tiburon Island. Here, the influence of the Colorado River arrived by the end of the Pliocene. Farther to the north, marine sedimentation started in late Miocene times. The Colorado Delta influx dominated deposition during Plio-Pleistocene times in these northern basins. To the south, there is no information on Middle Miocene, marine sedimentation in the region. But, by late Miocene times, the central and southern parts of the Gulf received marine sedimentation in tropical environments. During Pliocene times, the Gulf reached its present configuration.