The 315 m of Rosario Formation exposed at the San Antonio del Mar (SADM) section (Baja California, Mexico) contains moderately-to-well preserved benthic and planktic foraminifera, calcareous nannofossils, and molluscs. Nannofossils suggest most of the SADM section was deposited within a narrow interval of the late Campanian (CC21-CC22), whereas foraminifera and molluscs suggest a younger maximum age (younger than the Globotruncana ventricosa Zone) and allow deposition over a longer interval of time. Planktic foraminifera at SADM represent common Tethyan taxa. They are largely restricted to the lower and middle portions of the section and comprise 0-~40% of foraminiferal assemblages. Stable isotopic analyses of Rugoglobigerina rugosa yield δ18Ov-PDB values from −2.27‰ to −2.82‰ corresponding to salinity-corrected paleotemperature estimates of 26–30°C for the Late Cretaceous eastern Pacific. These estimates are as warm as modern tropical temperatures and are similar to tropical paleotemperature estimates from δ18O analyses of exceptionally preserved Maastrichtian samples; however, they are considerably warmer than most tropical Campanian-Maastrichtian estimates. Benthic foraminifera indicate outer shelf paleo-depths with a slight increase in depth or decrease in benthic oxygen levels in the upper parts of the interval studied. The change in the benthic assemblage corresponds to an ~1‰ positive shift in benthic δO18, suggesting a relationship between benthic assemblages and an inferred increase in the local intensity of upwelling.

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