No abstract

Glacial geologic evidence and radiocarbon-dated glacial deposits provide evidence of glacier fluctuations during the last 5 ka years in the Andes. Radiocarbon-dated moraines, morphological and stratigraphical studies permit the reconstruction of glacier fluctuations during the Holocene in the Valenzuela area of the Mendoza Andes valley located at 35S, in the upper Rio Grande basin. The Holocene record in this part of the Andes is mostly unknown, so this research is relevant for palaeoclimatic reconstructions in South America. In the Rio Valenzuela, glacier variations were dated in the El Azufre and El Pefión valleys. The first Neoglacial advance occurred at c. 5700 yr BP and a very close re-advance of similar magnitude occurred at 4700-4300 yr BP which formed well-preserved lateral-terminal moraines. The first Neoglacial advance occurred in the El Pefión valley at c. 4400 yr BP and a well-preserved terminal moraine is observed. A second Neoglacial advance reached its maxima at c. 2500-2200 yr BP in the El Azufre and El Pefión glaciers. A third, corresponding to the ‘Little Ice Age’, culminated at c. 400 yr BP (C. AD 1451-1641 cal. yr) and 350 +60 yr BP (c. AD 1460-1644 cal. yr) in the El Azufre valley. The third Neoglacial advance is distinguished in the El Pefión valley based on morphological characteristics, colour, freshness of the glacial deposits and the proximity to the active glacier. It is indicated by the Amarilla I, II, III lateral moraines. An estimation of the magnitude of snowline depression is obtained comparing the altitude of the present snowline with that of the reconstructed snow line for the first Neoglacial and ‘Little Ice Age’ advances.

In the Río Mendoza valley, five Pleistocene drifts and one Holocene drift are distinguished by multiple relative-age criteria, including surface-rock weathering, development of rock varnish, moraine morphology, soil-profile development, and stratigraphic relationships. Several absolute ages suggest a preliminary chronology. During the oldest (Uspallata) glaciation, a system of valley glaciers flowed 110 km from the Andean drainage divide and 80 km from Cerro Aconcagua to terminate at 1850 m. Drift of this ice advance is older than a widespread tephra dated by fission-track at 360,000 ± 36,000 yr. During the Punta de Vacas advance, ice terminated at 2350 m, while during the subsequent Penitentes advance, the glacier system ended at 2500 m. A travertine layer overlying Penitentes Drift has U-series age of 24,200 ± 2000 yr B.P. The distribution of Horcones Drift, which is inferred to represent the last glacial maximum, delimits an independent ice stream that flowed 22 km down Horcones valley to 2750 m. A later readvance (Almacenes) reached 3250 m. Confluencia Drift is considered to be Neoglacial in age and extends downvalley to 3300 m. The moraine sequence is compared with those studied by Caviedes (1972) along Río Aconcagua on the Chilean flank of the Andes.