PLIOCENE GLACIATIONS IN THE LAGO VIEDMA AREA (SOUTHERN PATAGONIA, ARGENTINA)

Pliocene glaciations in Patagonia were described for the first time by Feruglio (1944) at Cerro del Fraile (Santa Cruz), but subsequent ⁴⁰Ar/³⁹Ar dating of associated volcanic rocks (Ton That et al., 1999; Singer et al., 2004; Rabassa, 2008) show that these glaciations are of earliest Pleistocene age, according to the presently accepted Pliocene/Pleistocene boundary at 2.6 Ma. Later, Fleck et al. (1972), Mercer (1976), and Mercer et al. (1975) identified and dated Pliocene glacial deposits in the Southern Andes (see Rabassa, 2008). This work showed that extensive piedmont glaciations occurred in the Pliocene, and some of them extended as far eastward as the major Pleistocene glacial events. Schlieder (1989) and Rabassa et al. (2011) also described and dated Pliocene glaciations in the Northern Patagonian Andes of Neuquén, and Wenzens (2000) dated Pliocene/earliest Pleistocene (?) glaciations north and east of Lago Viedma, with limiting dates of 3.0 and 2.25 Ma. We have revisited Mercer’s most important sites and developed detailed field studies and sampling for radiometric dating and paleomagnetic studies in Southern Patagonia. We have evidence of Pliocene glaciations north and east of Lago Viedma, at Meseta Chica, an unnamed tableland just east of Meseta Chica, Meseta del Viento, Estancia Punta de Lago, Estancia La Adriana, Meseta Escorial and Condor Cliff, east of Lago Argentino. All identified glacial deposits predate the Great Patagonian Glaciation (GPG), which itself may comprise more than one glaciation. Mercer (1976) obtained K/Ar ages of 3.55 ± 0.19 and 3.68 ± 0.03 Ma at Meseta Chica, 3.48 ± 0.09 Ma and 3.55 ± 0.07 Ma at Meseta Desocupada, and 2.79 ± 0.15 Ma at Condor Cliff. With the exception of Condor Cliff, Mercer’s samples yielded normal magnetization. One of the most important questions to be discussed once the chronology of the glacial events is finally established is why the Pliocene Patagonian glaciers were so extensive at the times when most of Earth was under much warmer climates during the Pleistocene. The scale of Pliocene glaciation in the Southern Andes may be related to paleogeomorphological and/or paleoclimate conditions, e.g a very flat, non-dissected piedmont landscape and/or an unusually cold regional climate.