DIATOM BIOSTRATIGRAPHY OF THE UPPER OLIGOCENE TO LOWERMOST MIOCENE SAN GREGORIO FORMATION, BAJA CALIFORNIA SUR, MEXICO

Abstract

Sixty biostratigraphically useful diatom taxa are tabulated from three stratigraphic sections of the upper Oligocene to lowermost Miocene of the San Gregorio Formation of Baja California, Mexico. The diatom flora is a mixture of low- and high-latitude species and allows correlation of key sequences at San Gregorio, La Ventana, and San Hilario. Three tropical diatom zones, the early to late Oligocene Rocella vigilans Zone, the late Oligocene Bogorovia veniamini Zone, and the latest Oligocene to earliest Miocene Rocella gelida Zone, are recognizable within the formation. Radiometric dates in the San Gregorio Formation, together with correlation of diatom zonal boundaries to the geologic time scale, provide multiple evidence of age.