Loessic and fluvial pyroclastic sedimentation in the Middle Cenozoic of Central Patagonia (Sarmiento Formation).

EDUARDO S. BELLOSI.

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia. Madden, R., A. Carlini & M.G. Vucetich (eds.)

Cambridge Univ. Press.

Lugar: Cambridge; Año: 2007;

 The pyroclastic Sarmiento Formation (Middle Eocene-Lower Miocene) at the type locality (Gran Barranca) has a cumulative thickness of 319 m, and is divided into six members, delimited by erosive and non-depositional unconformities, and subordinated discontinuity surfaces. Sedimentary history was characterized by subaerial distal ash falls (tephric loessites), eolian and fluvial reworking and depositation, pedogenesis during lapses of landscape stability, and events of deep fluvial erosion. These processes were controlled by climatic changes, and subordinately by volcanism, subsidence and sea-level variations. The Gran Barranca Member (Middle Eocene) is mostly constituted by tephric loessites originated in subhumid-semiarid rolling plains, with ephemeral ponded areas. Similar depositional processes are also interpreted to 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 (Upper 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 (Uppermost Eocene-Lowermost Oligocene). During this lapse, laterally continuous and very thick loessic sediments accumulated rapidly, with scarce interruptions in a steady semiarid environment. Subsequently, the development of an incised valley, related to a climate change (rainfall increase) and probably to extensional faulting, eroded a 100 m column 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, and scarce 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 (Lower Miocene). During this lapse, depositional settings evolve from braided fluvial and loessic (subhumid seasonal climate) to exclusively loessic in a semiarid environment.