Pliocene Piedmont Glaciation in the Río Shehuen Valley, Southeast Patagonia, Argentina

Abstract

There are few sites in southern South America where late Tertiary glacial sediments have been radiometrically dated. Glaciations occurred near the Miocene-Pliocene transition, during the mid-Pliocene (3.5 Ma), and after 2.1 Ma ago. In the Río Shehuen valley at least four river terraces older than 2.25 Ma correlate with terminal moraines and outwash plains. The age of the youngest Pliocene advance is bracketed by two radiometric dates; glaciofluvial sediments lie on a 3.0 Ma old basalt lava and merge into a river terrace that is covered by 2.25 Ma old basalts. A remnant of a terminal moraine of the second oldest advance still exists. This glacier advance extended over 160 km from the Southern Patagonian Icefield. The oldest glaciation occurred well before 3.0 Ma. It may be possible to correlate these pre-Pleistocene glaciations to glacial deposits covering the Meseta Desocupada north of the glacial basin of Lago Viedma (250 m altitude). There, at an elevation of 1500 m, previous research found a till lying between two basalt flows with an age of 3.5 Ma. Till, separated by soil formations, covers the plateau surface.