New South American record of the Cretaceous–Paleogene boundary interval (La Colonia Formation, Patagonia, Argentina)

Abstract

The La Colonia Formation is an Upper Cretaceous to Paleogene shallow marine sedimentary unit in Chubut Province (Patagonia), Argentina that preserves important vertebrate and plant fossils. Despite good exposures and significant paleontological investigation, the precise age of the La Colonia Formation and its stratigraphic relationship to the Cretaceous–Paleogene (K–Pg) boundary are not well understood. The K-Pg boundary is associated with one of the largest mass extinctions in Earth's history yet there are few continental stratigraphic records of the boundary in South America, resulting in poor understanding of its effects there. This study combines magnetostratigraphy, detrital zircon uranium–lead (U–Pb) geochronology, and preliminary palynological results to constrain the age of the La Colonia Formation and its fossils more precisely and determine whether it preserves the K–Pg boundary. U–Pb ages of detrital zircons from the directly underlying Puntudo Chico Formation indicate that the base of the La Colonia Formation is younger than ~71.7 Ma. The studied palynological assemblage includes Quadraplanus brossus, which is restricted to the Maastrichtian in other southern hemisphere records. Paleomagnetic results show that the ~125 m Cerro Buitre Norte section of the La Colonia Formation and the overlying Cerro Bororó Formation has dominantly normal polarity except for a ~5 m zone of reversed polarity near the base that is interpreted to correlate to Chron C30R of the Geomagnetic Polarity Timescale (GPTS), and a ~13 m zone of reversed polarity near the top of the La Colonia Formation that is interpreted to correlate to Chron C29R. The presence of a magnetozone correlated to C29R in the La Colonia Formation suggests that it preserves the K–Pg boundary, making it an important target for continued paleontological collection to better understand the K–Pg extinction and recovery dynamics in South America.