Biostratigraphy and depositional history of the tepetate formation at Arroyo Colorado (Early-Middle Eocene), Baja California Sur, Mexico

Abstract

An integrated study of Foraminifera, Ostracoda and lithostratigraphy was conducted on samples of the Tepetate Formation from the Arroyo Colorado locality, Baja California Sur (Mexico). Based on the planktonic Foraminifera, Acarinina pentacamerata (latest lower Eocene) and Hantkenina nuttalli (earliest middle Eocene) zones were recognized that indicate an age range from 51.2 to 48.4 Ma. Benthic Foraminifera suggest correlation with the Penutian to Narizian California stages, which agrees with the planktonic foraminiferal biozones. The sedimentary structures and microfossils throughout the sequence indicate deposition within a transgressive-regressive-transgressive cycle, including environments from the inner to outer marine shelf in depths shallower than 150 m. Gravity flows dominate the sequence, and storm-deposited beds are present in the middle of the section. Considering the nature of this formational unit, we recommend the designation of a composite stratotype, according to the North American Stratigraphic Code.