Chronostratigraphy of the mammal-bearing Paleocene of South America

Abstract

Land mammal faunas of Paleocene age in the southern Andean basin of Bolivia and NW Argentina are calibrated by regional sequence stratigraphy and magnetostratigraphy. The local fauna from Tiupampa in Bolivia is ~59.0 Ma, and is thus early Late Paleocene in age. Taxa from the lower part of the Lumbrera Formation in NW Argentina (long regarded as Early Eocene) are between ~58.0–55.5 Ma, and thus Late Paleocene in age. A reassessment of the ages of local faunas from the Río Chico Formation in the San Jorge basin, Patagonia, southern Argentina, shows that the local fauna from the Banco Negro Inferior is ~60.0 Ma, making this the most ancient Cenozoic mammal fauna in South America. Critical reevaluation of the Itaboraí fauna and associated geology in SE Brazil favors the interpretation that it accumulated during a sea-level lowstand between ~58.2-56.5 Ma. All known South American Paleocene land mammal faunas are thus between 60.0 and 55.5 Ma (i.e. Late Paleocene) and are here assigned to the Riochican Land Mammal Age, with four subages (from oldest to youngest: Peligrian, Tiupampian, Itaboraian, Riochican s.s.). Based on the fact that notable taxonomic reorganization of Riochican faunas begins ~58.2 Ma, an Early (Peligrian, Tiupampian; 60.0–58.2 Ma) and Late (Itaboraian, Riochican s.s.; ~58.2-55.5 Ma) subdivision for this land mammal age is formally recognized. An interchange event for continental taxa between North and South America is identified between 58.2 and 56.5 Ma, which correlates with a sea-level lowstand and was probably enhanced by a major tectonic event beginning ~59.5 Ma that uplifted the Andean margin.