Early Neogene foraminifera and associated microfossils of the Cerro Tierra Blanca Member (El Cien Formation), Baja California Sur, Mexico

Abstract

The foraminifera and associated microfossils from the type section of the Cerro Tierra Blanca Member (El Cien Formation), supposedly of late Oligocene age, indicate these strata to be of latest Oligocene to late Miocene age. The analysis done from the sequence demonstrates severa! stratigraphic discontinuities that, associated with the scarce geological information from the area, are indicative of structural complexities not detected during the establishment of the type section. lt was impossible to determine the stratigraphical relation between the strata studied and the supposed two other members of the formation for the same reasons -at El Cien- as well as with other sequences that crop out at El Cien-San Hilario, La Purísima or San Juan de la Costa. The micropaleontological assemblage suggests initial subsidence and deposition of early Miocene volcanics and shelf marine clastics, followed by deposition of upper-bathyal diatomaceous sediments during earliest late Miocene. The observed microfossils are a mixture of low- and high-latitude species, such as the ones found at present, near the distal end of the California current off Baja California Sur, and indicate periods of low and high fertility. Based on the present results, it is not possible to determine if the El Cien Formation is a valid name for the beds surrounding El Cien, nor for those to the east.