The Quaternary Succession in the RIO Blanco Basin, Cordon Del Plata, Mendoza Province, Argentina: An Application of Multiple Relative-Dating Techniques

Abstract

In the basin of Rio Blanco, which flows eastward from the Cordón del Plata west of Mendoza, Argentina, moraines and fossil rock glaciers record several periods of activity during and since the last major glaciation, the Vallectios (=Wisconsinan). Farther down the valley than the moraines of Vallecitos age are three older tills, one of which is exposed almost as far downstream as the junction with Rio Mendoza, more than 17 km beyond the outer Vallecitos moraines. No vegetation borders the modern glaciers and rock glaciers in this dry part of the Central Andes, and efforts to obtain datable organic material proved fruitless. A lense of coarse volcanic ash that overlies one of the tills was dated by zircon fission-track methods as between 100,000 and 200,000 years old. Relative dating (RD) techniques were used, therefore, in order to try to develop a stratigraphic order for the decosits. Granites are rare in the Rio Blanco basin and, although the quartzites and rhyolites that dominate the area weather, few effects of weathering on these boulders could be quantified. For deposits of Vallecitos age and younger, the most consistent data came from 1) morainal and rock glacier position and overlap, 2) sharpness of topography, 3) loess thickness, 4) soil profile characteristics, 5) vegetation cover, and 6) lichens. For the older (pre-last glaciation) deposits, data for comparison was provided by: 1) topographic position, 2) stratigraphic sequence, 3) soil profile characteristics and development, and 4) boulder weathering.