First early Oligocene plant macrofossils from Northern Patagonia (Río Negro, Argentina)

ARI IGLESIAS; MAURO PASSALIA; FLORENCIA BECHIS; VANESA LITVAK; JUAN IGNACIO FALCO; GABRIELLA ROSSETTO-HARRIS; PETER WILF; ARIANA PAULINA-CARABAJAL

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

The Paleogene is well known for major climatic and geological changes that aff ected ecosystems worldwide. During the latest Eocene-early Oligocene transition, changes in the composition of paleocommunities are associated with global decrease in temperature after the warmth of the Eocene, including abrupt cooling with the earliest Oligocene (Oi-1) glaciation. Fossil macrofl oras that can be precisely correlated to the Oi-1 event are best known from the Northern Hemisphere. In Patagonia (which was at that time adjacent to the expanding Antarctic ice sheet), early Oligocene sediments are represented mostly by marine or estuarine environments, and continental biotas are only known for southern Patagonia. Here, we provide the first report of an early Oligocene macroflora that occurs in a volcaniclastic sequence overlaying the Eocene Huitrera Formation, located only 1.6 km to the east of the well-known 47.8 Ma Río Pichileufu plant fossil locality. Our sample to date includes 900 specimens of plants and insects. The flora is dominated by Nothofagus leaves, with at least three different leaf species and associated reproductive structures, and otherwise a generally low angiosperm-leaf diversity. Cupressaceous leafy branches and broad-leaved Araucariaceae associated with ovuliferous complexes are noted among the conifers, and at least two fern species are represented. Rock samples were U-Pb dated at two different levels intercalated within the new fossiliferous section, both giving similar early Oligocene ages (~ 33.5 Ma) that correlate to the first half million years of the Oligocene and the Oi-1 interval.