Dinoflagellate cysts from the LB-5 core in the San Gregorio Formation (SGF) from the La Purísima area, Baja California Sur, Mexico, limit the age of the studied succession to Oligocene–Miocene (28–20Ma). Our results allow correlation of this core with the type locality and another well-dated sections of the SGF in the region, namely the nearby La Purísima section, with an unequivocal Oligocene–Miocene boundary (O/M – 23Ma). Lithology and dinoflagellate cyst data indicate mainly marine sedimentary environments, with a transgression from continental at the base to upper bathyal (200–500 m) in the O/M level (∼243 m), and a regression towards the top of the unit. Dinoflagellate cysts are virtually absent in some intervals, probably due to oxidation during diagenesis. The abundance of heterotrophic taxa during intervals with high cyst concentrations indicates that palynological preservation was adequate for quantitative analyses in the rest of the samples. Samples with dinoflagellate cyst concentrations >2000 cysts/gram of sediment (c/g sed), the abundance of heterotrophic taxa, and the presence of phosphoritic layers and diatomites indicate high biogenic productivity during the deposition of the SGF. Intense upwelling conditions in the area are probably associated with the cold events Mi-1 (∼23Ma) near the O/M and Mi-1a (∼21.7Ma) at ca. 170 m depth. Quantitative dinoflagellate cyst data in the LB-5 core suggest lower productivity (mean = 203 c/g sed) in the Oligocene than during the Miocene (mean = 848 c/g sed) interval. The absolute abundances of the Miocene intervals indicate similar dinoflagellate cyst concentrations in the LB-5 core and modern samples from the Pescadero Basin in the southern Gulf of California.