Chronology of early Eocene marine and terrestrial strata, Baja California, Mexico

Abstract

We present a multidisciplinary study of the marine and terrestrial biostratigraphy, and paleomagnetism of early Eocene, intertonguing marine and continental strata from the Punta Prieta area, central Baja California, Mexico. The mammalian fauna is assigned to the Wasatchian North American Land Mammal Age, rather than the Clarkforkian assignment of earlier studies. Extensive molluscan assemblages from intertonguing marine strata are correlated to the restricted Capay Stage and therefore are middle Ypresian (middle early Eocene) in age. This correlation is supported by a benthic foraminiferal assemblage assigned to the Penutian/Ulatisian Stage, and the ostracode Buntonia (early Eocene first appearance). This study is only the second that provides direct correlation of early Paleogene marine and terrestrial biochronologies in North America. Our correlation indicates that part of the Wasatchian mammal age is correlative with the Ypresian age, as had been suggested previously from indirect evidence. All of the paleomagnetic sites from three sections are reversely magnetized; the associated middle early Eocene biochronologic information permits correlation of this reversed polarity interval to Chron C23R or the reversed polarity interval within Chron C24N. Wasatchian mammal assemblages now are known from northern Canada (Ellesmere Island) to central Baja; all available geochronologic information indicates that these assemblages are synchronous across broad, intracontinental areas. Our early Eocene paleomagnetic study provides the first early Cenozoic results from the Baja peninsula and the first direct constraint on the early Cenozoic plate-tectonic motion of the peninsula. Our results indicate that Baja California was in its pre-Gulf of California position, sutured adjacent to the northern Mexican mainland, prior to the middle early Eocene. These results conflict with some previous interpretations of the motion of Baja; discrepancies may be caused by failure to treat small terranes as discrete, potentially independent, mobile land masses.