The oldest Andean synorogenic fossiliferous deposits of the Miocene Cerro Azul Formation, central Argentina

Abstract

Cerro Patagua is a locality with fossiliferous outcrops of the Cerro Azul Formation in La Pampa Province (central Argentina). First fossil remains were known in 1999 and assigned to the Chasicoan Stage/Age (Late Miocene, Tortonian) after the presence of the Octodontoidea rodent Chasichimys bonaerense. Later references to vertebrate remains were scarce and mostly limited to some mentioned taxa in broader faunal contexts. Now, we present a detailed taxonomic study of the entire mammal assemblage from Cerro Patagua, which evidences that most taxa are typical of the Chasicoan Stage/Age. A peculiar enterolith-like structure, recovered from loess levels, is also described, and several taphonomic aspects observed on fossils are related to their level of provenance. In addition, we offer stratigraphical interpretations of the Cerro Azul Formation and a geomorphological analysis of the Cerro Patagua area, and provide a detrital zircon dating of 12.3 ± 1.8 Ma. This result reflects the time interval 14.1–10.5 Ma, a temporal range that includes the Serravalian Stage (late Middle Miocene) and could indicate that the oldest synorogenic deposits could have begun to accumulate at the end of this stage. Even considering the minimum age of this interval (Tortonian Stage), this dating is the oldest obtained for the Cerro Azul Formation, compared with previous published data from Arroyo Chasicó locality (9.43–9.07 Ma). This assignment could explain that taxa from Cerro Patagua show affinities with Late Miocene mammal assemblages from central Argentina, as well as with older faunas from Patagonia and Bolivia.