The Río Leona Formation: a key record of the Oligocene flora in Patagonia.

BARREDA, V.; CÉSARI, S.; MARENSI, S.; PALAZZESI, L.

San Carlos de Bariloche

Congreso; 7º Internacional Organization of Paleobotany Conference; 2004

The Río Leona Formation is characterized by the presence of anatomical preserved trunks, leaves and palynomorphs in the Estancia 25 de Mayo, few kilometers to the south of the El Calafate city. This unit is made up of approximately 97 meters of conglomerates, sandstones, mudstones, carbonaceous shales and tuffs. They are arranged in a general fining upward trend, from conglomerates at the base to carbonaceous shales at the top, although minor reactivation cycles do exist. At the base, coarse clastics unconformably rest on top of green, fine-grained sandstones of the Eocene Man Aike Formation. At the top, the Rio Leona Formation is transitionally covered by the fossiliferous very fine-grained sandstones and mudstones of the latest Oligocene-early Miocene Centinela Formation. Deposits of the Río Leona Formation represent a progressive transition from high-energy, bed load to low-energy, suspension load fluvial systems. Basal conglomerates contain abundant transported tree trunks and the upper part bear some tree stumps preserved in living position on top of rootled sandstone intercalations interpreted as levee deposits. Imprints of leaves are preserved in the mudstones and the fine carbonaceous shales are bearing palynofloras. Three samples from these upper levels yielded organic debris dominated by woody particles, cuticles and non marine palynomorphs. The first results of the study of the trunks show the presence of at least two fossil wood morphotypes. Fossil wood with closest anatomical similarity to the southern Cunoniaceae family is assigned to the organ genera Weinmannioxylon Petriella. Nothofagoxylon Gothan is the other angiosperm component of the fossil forest. The available specimens of leaves are poorly preserved but allow the identification of several species of Nothofagus which dominates the association. The recovered palynological assemblages are well preserved but diversity is low. Angiosperms are dominant consisting mainly of Fagaceae species (Nothofagidites spp.), prevailing the fusca type. Podocarpaceae (Podocarpidites spp.), Proteaceae (Proteacidites spp.) and Myrtaceae (Myrtaceidites sp.) are less frequent. Fern spores are dominated by the Polypodiaceae (Polypodiisporites sp.) Fungal palynomorphs are abundant as well as fresh water algae of Botryococcus. A number of rare species related to Loranthaceae (Gothanipollis bassensis Stover), Ephedraceae (Equisetosporites notensis (Cookson) Romero) and possible Cunoniaceae (Rhoipites sp. comparable to recent Weinmannia L.). were also identified. A similar association, both in composition and relative frequencies, was described from the Early Oligocene of La Sara well (840-790 mbs. interval), in Tierra del Fuego Province. Similarities are also important with the latest Eocene?- Early Oligocene Sloggett Formation, also from Tierra del Fuego. Although no palynomorphs of stratigraphic value were recovered from the analyzed samples Nothofagus rainforests were not present in Patagonia until the Middle Eocene. From a paleoclimatic point of view the palynological assemblages composed by angiosperm of gondwanic origin, apparently without megathermal species would suggest cold temperate conditions. This sequence containing anatomical preserved stems, leaf remains and spore-pollen assemblages represent an important unit for the improvement of the knowledge of Oligocene paleofloras from southern Patagonia. Both sedimentological and paleobotanical records will allow the reconstruction of this past ecosystem.