MACNBR   00242
MUSEO ARGENTINO DE CIENCIAS NATURALES "BERNARDINO RIVADAVIA"
Unidad Ejecutora - UE
capítulos de libros
Título:
Loessic and fluvial sedimentation in Sarmiento Formation pyroclastics, middle Cenozoic of central Patagonia
Autor/es:
BELLOSI E. S.
Libro:
The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia. Madden, R., A. Carlini, M. Vucetich & R. Kay (eds.)
Editorial:
Cambridge University Press
Referencias:
Lugar: Cambridge; Año: 2010; p. 274 - 288
Resumen:
The pyroclastic Sarmiento Formation (middle Eocene to early Miocene) at its type locality (Gran Barranca) is divided into six members, delimited by erosive and nondepositionalunconformities, and subordinate discontinuity surfaces. The sedimentary history during Sarmiento time was characterized by subaerial distal ash falls (tephric loessites), eolian and fluvial reworking and deposition, soil formation during landscape stability, and events of deep fluvial erosion. These processes were controlled by changes in climate, and subordinately by volcanism, subsidence, and sea-level variation. The Gran Barranca Member (middle Eocene) is mostly composed of tephric loessites deposited in subhumid to semiarid rolling plains, withephemeral ponds in areas of poor drainage. Similar depositional processes are also found for the Rosado Member (late middle Eocene), a strongly developed paleosol which records the driest and more stable landscape period during Sarmiento time. Coarse intraformational deposits and paleosols of the Lower Puesto Almendra Member (late Eocene) represent the installation of a braided, probably ephemeral fluvial system in a subhumid environment.A significant modification in sedimentation style is interpreted for the overlying Vera Member (late Eocene to earliest Oligocene). During this interval, laterally continuous and very thick loessic sediments accumulated rapidly, with few interruptions in a steady semi-arid environment. Subsequently, the development of an incised valley, related to a climate change (increase in rainfall) and probably to extensional faulting, eroded up to 100m of sediments.The infill deposits constitute the Upper Puesto Almendra Member (Oligocene). In a first stage, the valley was filled with fluvial sediments, accumulated in braided channels, with some eolian deposits. Basalt flows occupied some of these channels during a relatively short volcanic episode. The sedimentation of the second stage (late Oligocene) occurred in a meandering fluvial system under wetter conditions. A new period of valley incision and infilling  is recorded in the Colhue-Huapi Member (early Miocene). During Colhue-Huapi time, depositional settings evolved from braided fluvial and loessic in a subhumid seasonalclimate, to exclusively loessic in a semi-arid environment