A CENOZOIC FOSSIL RECORD FROM SOUTHEAST PERU IN A REVISED CHRONOSTRATIGRAPHICAL AND PALEOENVIRONMENTAL FRAMEWORK: EVIDENCE FOR CROSS-CONTINENT BIOGEOGRAPHIC PATHWAYS

Reconstructing the history of the Amazonian lowlands is hampered by the scarcity and quality of long sedimentary sequences. Outcrops are mainly exposed along river cuts or alongside the westernmost edge of the Amazonian basins, where they are uplifted and cut-in by present-day rivers. This has led to paleogeographical and paleoenvironmental reconstructions that are based on very limited datasets and short temporal ranges, eluding the exact pathways of evolution and timing of emergence of important faunal and floral groups.

Here we present a multiproxy analysis, including sedimentological, geochemical and paleontological data with improved chronostratigraphical control from a series of outcrops located along the Alto Madre de Dios river and its tributary, the Palotoa River, in southeastern Peru. The outcrops are part of the Cenozoic section affected by the Sub-Andean thrust front in the Madre de Dios basin, located northeastward of the Eastern Cordillera of southern Peru. A total of 8 detrital zircon samples were analyzed with LA-ICP-MS to provide radioisotopic age-control and sediment provenance data, and alongside reported apatite fission track dates and paleontological age constraints, allow for chronologically constraining a section covering ~58 - 3.7 Ma. Geochemical analyses for δ¹³C and δ¹⁵N on the organic fraction of paleosols are used to reconstruct paleoenvironment, as do δ¹⁸O and δ¹³C analyses on organic and inorganic carbonates to further constrain paleoenvironment and post-depositional processes. Several fossil assemblages were recovered along the transect, yielding a new primate genus, a diverse rodent fauna, as well as turtles, notoungulates, marsupials, crocodiles, along with other minor faunal components. A series of paleosoils are being analyzed for their palynological content, allowing us to track vegetational composition and changes throughout the section. Initial analyses of a major rodent assemblage dated at 18.9±0.7 Ma shows faunal similarities and links with the well-studied La Venta Fauna and Argentinian & Chilean rodent assemblages. Our findings further expand on initial reports of fossil data along the Alto Madre de Dios river, and elevate this section to an important locality for South American biogeographic reconstructions.