36Cl terrestrial cosmogenic nuclide dating suggests Late Pleistocene to Early Holocene mass movements on the south face of Aconcagua mountain and in the Las Cuevas–Horcones valleys, Central Andes, Argentina
The morphology, sedimentology and mineralogy of deposits that previously had been associated with glacial advances (the Penitentes, Horcones and Almacenes drifts) were reinvestigated and dated using the terrestrial cosmogenic nuclide (TCN) 36Cl. These results indicate that the deposits previously associated with the Horcones and Almacenes drifts are actually deposits of a rock slope failure from the southern face of Aconcagua mountain forming a debris–ice avalanche that were deposited 10 490±1120 years ago, while the deposits previously associated with the Penitentes drift is a rock avalanche from the Mario Ardito valley that deposited in the Las Cuevas valley 11 220±2020 years ago. Earlier in the Late Pleistocene a further rock–ice avalanche sourced from Aconcagua mountain and deposited in the Las Cuevas valley, predating related lake sediments with a calibrated 14C age of 14 798–13 886 years and travertine deposits with a U-series age of 24 200±2000 years. In addition, three further rock-avalanche deposits were dated that sourced from Tolosa mountain, having 36Cl mean ages of 14 740±1950 years, 12 090±1550 years and 9030±1410 years. No deposits of massive rock slope failures were found in those parts of the valleys that date younger, suggesting that climatic conditions at the transition from the Late Pleistocene to the Holocene, that were different from today's, caused the slopes to fail. Alternatively, the rock slope failures could have been seismically triggered. We suggest that the slope failures at the southern face of Aconcagua mountain have caused or contributed to a reorganization of glacial ice flow from Aconcagua mountain that might ultimately be the cause of the surging behaviour of the Horcones Inferior glacier today. Our results indicate that the glacial stratigraphy of this part of the Central Andes is still poorly understood and requires detailed mapping and dating.