ANALYSIS OF FOSSILIFEROUS, VOLCANICLASTIC LIMESTONE FROM THE BASE OF THE UPPER MIOCENE BOLEO FORMATION OF SANTA ROSALIA BASIN, BAJA CALIFORNIA SUR, MEXICO

The Santa Rosalía Basin is located on the eastern side of the central Baja Peninsula. Sedimentary deposits of Upper Miocene Boleo formation represent the initial formation of the basin and the opening of the Gulf of California. Carbonate outcrops at the base of the formation were investigated to gain insight into the depositional environment and processes of diagenesis. The tilted limestone beds are now topographic highs and unconformably overlie basement of volcanic and volcaniclastic rocks. Six stratigraphic sections were measured and samples collected for petrographic, fossil, and SEM/EDS analyses. The carbonate rocks change up section from very fine, carbonate-cemented friable sandstone to more consolidated tuffaceous limestone. Volcanic lithic fragments are deposited as framework grains. The transport is not far from the source as indicated by the lack of rounding and the preservation of plagioclase crystals. The unit has been heavily altered due to Mn-Cu mineralization (Manto 4) and secondary diagenetic fluids. Volcanic clasts and pumice have altered to siliceous-aluminous smectite clays. Fossils and their preservation change vertically and across sections. Ostracod molds are dominant in the basal sections with siliceous and oxide replacements. Gastropods are altered with oxides in the middle section, and foraminifera have been replaced with dolomite in the upper areas. SEM/EDS scans of fossils extracted from each section show that their shells were replaced with iron oxides and coated with siliceous cement or dolomite. Abundant volcanic clasts deposited within the limestone are evidence of volcanic activity during the incursion of seawater at the early stages of the proto-gulf of California. Further geochemical, paleontological, and optical analyses will be helpful in developing a depositional model for the unit. This research was carried out as part of the NSF-funded Baja Basins Research Experience for Undergraduates.