Abstract
This paper reviews the genesis of a chaotic deposit in the Blanco River basin in the Argentine Andes that has been interpreted to be either of debris flow or glacial origin. A detailed sedimentological and geomorphic study of the deposit was undertaken to determine its origin. Glacial landforms in the source area of the deposit contrast markedly to those in neighboring valleys of the Blanco River basin. The lack of moraines and the presence only of rock glaciers in the headwaters of the Angostura Valley suggest that a glacier might have been destroyed by a rock avalanche in the Late Pleistocene. The deposit itself has the characteristics of rapid deposition by a huge (≤105 m3) rock avalanche sourced on the eastern slope of Mount Plata (5956 m asl). The rock avalanche travelled a distance of 26.6 km and descended 4700 m in altitude. Optically stimulated luminescence ages on alluvial sediments associated with the landslide deposits suggest that the rock avalanche occurred ~35-39 ka ago. The rock avalanche may have been triggered by an earthquake, given that the active Carrera fault system extends across the basin and there is a cluster of seven Late Pleistocene rock avalanches in the region.
Highlights
Researchers often argue about the origin of chaotic sedimentary deposits in high mountains (Abele, 1984; Hewitt, 1999; Hewitt et al, 2011)
The motivation for this study was understanding the genesis of a chaotic deposit that had been previously and ambiguously interpreted to be till (Wayne and Corte, 1983; Wayne, FIGURE 1 | Study area location at 33◦S: (a) the Blanco River Basin (BRB) on the eastern slope of the Plata Range and its relative position with a cluster of rock avalanches studied in the northern extreme of Plata Range
The deposit that motivated this study has been described as a greenish–grayish chaotic deposit found along the main branch of the Blanco River
Summary
Researchers often argue about the origin of chaotic sedimentary deposits in high mountains (Abele, 1984; Hewitt, 1999; Hewitt et al, 2011). Polanski (1953, 1963) emphatically argued that Pleistocene glaciers could not have reached below 1,800 m asl, whereas Groeber (1951, 1955) defended the existence of such extensive advances. This discussion continued two decades later when the “cenoglomerate,” a chaotic mass of boulders lacking internal structure in the Blanco River basin (BRB) which Polanski (1966) interpreted to be the deposit of a huge debris flow, was reinterpreted as till of the Angostura glacial advance (Wayne and Corte, 1983; Wayne, 1984). Based on relative methods (e.g., rock varnish on blocks, degree of weathering, and soil development), these authors proposed an Early Pleistocene
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