A catastrophic rock avalanche from the eastern hillslope of the Plata Mount, Central Andes

MOREIRAS, S.M.

Heidelberg

Conferencia; 23th Latin American Colloquium LAK- 2014, 25-27th March, Heidelberg. Germany.; 2014

Relict deposits of a eroded huge Pleistocene rock avalanche were identified in different areas of middle and lower basin of the Blanco River valley. The rock avalanche source area seems to be the eastern hillslope of the Plata Mount (6,200 m asl), monolithic composition of the deposits coincides with the Carboniferous rocks outcropping in this sector. We preclude that this cata-strophic event should remove and eliminate a pre-existing old glacier from this side of the mount. A recent morphological study evidences this mount face is lacking on uncovered ice bodies contrasting with surrounding areas over the 4,500 m in altitude. Identified relict deposits cover an approximately 64,000 km2 area with an estimated volume of 32x106 m3 which suggests a huger dimension for the original collapse. Debris material moved down slope 26 km descending topographically 4,400 meters in high. A low angle friction results from these parameters (H/D~0.17). The greenish-grayish deposit presents more than 98% of grey sandstones, conglomerates and phyllites belonging to El Plata Formation, Carboniferous in age. Clasts of an altered gabbro appear, as well, what contribute to the matrix greenish color. In this chaotic deposit greater boulders reach 3 m in diameter, while mean diameter is 0.50 m. Ma-trix content varies from 20 to 40% corresponding to middle sand in grain size.