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

SERGIO A. MARENSSI, CARLOS O. LIMARINO, ALFONSINA TRIPALDI Y LAURA I. NET

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elservier

Lugar: Columbia, USA; Año: 2005 p. 359 - 372

0895-9811

The stratigraphy and sedimentology of the Oligocene Río Leona Formation (Austral or Magallanes Basin) is described and interpreted based on the study of two sections located in the area close to Lakes Viedma and Argentino in southern Argentina. This non-marine unit unconformably rests on top of the marine Eocene Man Aike Formation and it 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, and thin but conspicuous levels of carbonaceous shales and volcaniclastic beds. These rocks represent sedimentation in different fluvial systems. Sedimentological data allow interpreting a progressive change in the fluvial styles from high-energy braided systems to low-energy meandering and anastomosed systems. The landscape of the area 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 (c.a. 22 to 25 My). Within the framework of the non-marine 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 erosion of the Eocene rocks suggest an initial uplift of the source area located to the West. Changes in the fluvial systems indicate an increase of the accommodation space in the basin. The Oligocene evolution of this part of the Austral Basin probably reflects a combination between 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 the foreland basin evolution. The latter may be linked to the late Paleogene glaciation-deglaciation events in Antarctica.