Pliocene glaciations in the lago Viedma area (Santa Cruz Province, Argentina)

RABASSA, J., CLAGUE, J.J., BARENDREGT, R., MARTÍNEZ, O., ERCOLANO, B. CORBELLA. H., GRIFFING,C.

Cordoba

Congreso; XIX Congreso Geologico Argentino; 2014

Asociacion geologica Argentina - XIX Congreso Geologico Argentino

Pliocene glaciations in Patagonia were explicitly 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, for a general summary). This work showed that extensive piedmont glaciations occurred in the Pliocene, and some of them extended as far eastwards 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. Our group has revisited Mercer?s most important sites and developed detailed field studies and sampling for radiometric dating and paleomagnetic studies in Santa Cruz province. A summary of our paleomagnetic data and Mercer?s radiometric ages follows: (1) we have evidence near the surfaces of mesetas, north and east of Lago Viedma for several glaciations of Pliocene age. The glaciations predate the Great Patagonian Glaciation (GPG), which itself may comprise more than one glaciation. (2) Meseta Chica, north of Lago Viedma: till between two reversely magnetized basalt flows; (3) Meseta Desocupada, north of Lago Viedma: surface basalt flow is reversely magnetized; (4) Meseta del Viento, north of Lago Viedma: basalt and the lowest of four tills above basalt are reversely magnetized; (5) Estancia Punta De Lago, north of Lago Viedma: upper basalt flow below till is reversely magnetized; (6) Estancia La Adriana, east of Lago Viedma: upper basalt flow, which overlies glacial deposits, is reversely magnetized; (7) Meseta Escorial, east of Lago Viedma: basalt and underlying glacial sediments are reversely magnetized; (8) Condor Cliff, east of Lago Argentino: surface basalt has normal magnetization and is underlain by till and basalt, with the possibility that two tills are present at the site. 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. Malagnino (1995) assigned a late Pliocene age to the Chipanque Glaciation, following Mercer?s chronology. Strelin et al. (1999) described moraines beyond the GPG limit, overlying the Condor Cliff basalt, but these glacial events could date to the earliest Pleistocene. Critical basalt samples collected by our group are presently being dated; the results will be presented at the Congress. Perhaps one of the most relevant questions to be discussed once the chronology of the glacial events is finally established is why the Pliocene glaciers were so extensive at the times when most of Earth was under much warmer climates that during the Pleistocene. This may be related to paleogeomorphological conditions, very flat, undissected piedmont landscape or to unusually cold regional climate.