Abstract

Siliceous microfossils, mega-invertebrates and vertebrates are described from the marine Middle Miocene Los Indios Member of the Rosarito Beach Formation. The large and diverse marine vertebrate assemblage of sharks, rays, bony fishes, sea birds, dolphins, cetotheres, pinnipeds, a sea cow and a desmostylian, is here named the La Mision Local Fauna. Correlation with the (?)Hemingfordian/(?)Barstovian land mammal ages is tentatively made. Mollusks from the Los Indios Member allow correlation with the “Temblor” provincial molluscan stage, and silicoflagellates indicate a correlation with the Corbisema triacantha Zone, dated in the deep sea at 14–15 million years before present. This date is corroborated by radiometric dates on basalts stratigraphically above (14.3 ± 2.6 million years) and below (16.1 ± 2.1 million years) the Los Indios Member.

Preliminary paleoenvironmental analysis indicates that the fossil assemblages of the Los Indios Member lived under open marine, inner shelf conditions with subtropical water temperatures and clear, non-turbid water.