BIOSTRATIGRAPHIC AND PALEOCEANOGRAPHIC RECONSTRUCTION OF THE LATE MIOCENE IN BAJA CALIFORNIA AND TRES MARIAS ISLANDS, MEXICO
PEREZ GUZMAN, ANA MARIA
Doctor of Philosophy
A reconnaissance of sediments containing radiolarians was performed for localities along Baja California and Tres Marias Islands. Only seven land-base sections (El Cien, Tortugas, Arroyo Tiburon, Cabo San Lucas-La Paz, San Felipe, Arroyo Hondo and Maria Cleofas) together with Experimental Mohole (the only offshore site) were considered worthy of pursuit in this study. The most significant siliceous deposition in the selected sections occurs during the Didymocyrtis antepenultima Zone (between 10.1 and 8.8 m.y.a.). This interval is also the major period of formation of siliceous rock sequences in California. All the sections studied herein are correlative with the tropical radiolarian zonation of Riedel and Sanfilippo (1978), and exhibit very little to poor correlation with the cold water radiolarian biofacies of Weaver et al. (1981). A refinement of Casey's hypothetical ocean (Casey et al., 1982) and his biogeographic categories was utilized in the paleoceanographic interpretation herein. The quantitative analyses of the sections provided enough samples to reconstruct the paleoceanography of the study area at two time intervals. The first one at 10 m.y.a., shows the intense development of a paleo-El Nino Current, producing high percentages of warm water radiolarians in all the basins. However, Arroyo Tiburon might have been influenced by the paleo-California Current, causing obstacle upwelling in this basin. The second reconstruction at 8.8 m.y.a. suggests that the opening of the Gulf of California occurred at that time, originating an oceanic front that could influence the movement of the paleo-California Current. This reconstruction (8.8 m.y.a.) showed a time slice of high upwelling conditions, where the paleo-California Current probably had a wide path, producing obstacle upwelling in the Vizcaino Peninsula, but the southernmost sections (Tres Marias Islands and Cabo San Lucas-La Paz) were mostly influenced by eastern tropical waters. In addition, partial synonomies of the radiolarians identified (including notes on their paleoecological considerations) and the radiolarian barren localities are given as Appendices I and II respectively.