Sedimentary evolution of Neogene continental deposits (رirihuau Formation) along the رirihuau River, North Patagonian Andes of Argentina

The sedimentary evolution of the Ñirihuau Formation (late Oligocene–middle Miocene) was studied along the southern margin of the Ñirihuau River, in the North Patagonian Andes. The 1300-m-thick section includes 15 epiclastic and volcaniclastic lithofacies which are grouped into five lithofacies associations: deep lacustrine, shallow lacustrine, fluvial channels, subaerial floodplains and volcaniclastic flows (lahar). Syn-eruptive and inter-eruptive stages are recorded along the Ñirihuau River section. The former consist of highly aggradational packages several tens of meters thick of ash-fall beds and lahar deposits. During inter-eruptive periods sedimentation took place mostly in shallow and deep lacustrine environments, with four cycles of lake expansion and contraction, and a minor proportion of fluvial deposits. Sedimentary supply originated from the northeast and northwest in the lower part of the unit through low to moderate sinuosity fluvial systems, flowing into a lake with high-gradient margins, and forming Gilbert-type deltas. The younger sections were sourced from the northeast, east and southeast, indicating changes in the basin morphology. Basic and intermediate volcanic rocks similar to those of the Ventana Formation (Oligocene) are interstratified at the beginning of the sedimentation. The syn-orogenic nature of the Ñirihuau Formation is evidenced by the changes in the basin shape, but mainly by the differences in styles and intensities of deformation between the Ñirihuau River section and the overlying outcrops of La Buitrera Hill, both separated by a folded unconformity.