Fluvial systems variations in the Rio Leona Formation: Tectonic and eustatic controls on the Oligocene evolution of the Austral (Magallanes) Basin, southernmost Argentina

The stratigraphy and sedimentology of the Oligocene Río Leona Formation (Austral/Magallanes Basin) is described and interpreted on the basis of a study of two sections located in the area close to Viedma and Argentino Lakes in southern Argentina. This nonmarine unit unconformably rests on top of the marine Eocene Man Aike Formation and is transitionally covered by the shallow marine, latest Oligocene–Early Miocene Centinela Formation. The Río Leona Formation is composed of varying percentages of conglomerates, sandstones, and mudstones, as well as thin but conspicuous levels of carbonaceous shales and volcaniclastic beds. The rocks represent sedimentation in different fluvial systems. Sedimentological data indicate a progressive change in the fluvial styles from high-energy braided systems to low-energy meandering and anastomosed systems. The landscape is envisaged to have evolved during the Oligocene from a high gradient piedmont area to a low-lying coastal plain. The Patagonian Sea flooded the area by the end of the Oligocene (22–25 My). Within the framework of the nonmarine sequence stratigraphic scheme for foreland basins, the Río Leona Formation deposits represent good examples of degradational, transitional, and aggradational systems tracks. The development of a basal unconformity and the erosion of the Eocene rocks suggest an initial uplift of the source area to the west. Changes in the fluvial systems indicate an increase of the accommodation space. The Oligocene evolution of this part of the Austral Basin probably reflects a combination of tectonic processes and eustatic sea-level changes. The former may be related to an episode of uplift in the southern Andes, followed by a period of tectonic quiescence and active volcanism with strong subsidence typical of foreland basin evolution. The latter may be linked to late Paleogene glaciation–deglaciation events in Antarctica.