Quaternary geological-geomorphological evolution of the uppermost course of the Rio Laja Valley.

The Quaternary geological history of the uppermost the Rio Laja valley show s a complex interrelation between volcanic, mass wasting, and alluvial and fluvial sedimentary processes. The valley was initially carved by Lower Ple istocene glacial action on a Tertiary basement (Cura-Mallin and Trapa-Trapa Formations, intrusives), over which four major Quaternary units were depos ited unconformably: the polygenic Quilleco alluvial cone, a Pleistocene vol canic sequence, and the products of the Antuco volcano and the Antuco volca nic avalanche. The Quilleco polygenic cone includes mixed intermediate and distal volcano-sedimentary facies which interfinger with the Pleistocene vo lcanic sequence derived from coeval stratovolcanos. The Antuco volcano is a mixed and composite andesitic to basaltic andesitic stratovolcano of basal t, which commenced its activity ca. 130,000 yr B.P. Its first constructive phase (Antuco 1) culminated at 9,700+/-600 yr B.P. with the lateral gravita tional collapse of the edifice, this event produced the major Antuco volcan ic avalanche which dammed the natural outlet of Lago del Laja and its tribu taries. The gravitational collapse was the final result of a Bandai-San typ e phreatomagmatic eruptive event associated with almost simultaneous wet tu rbulent pyroclastic base surges made up of black basaltic ash (Arenas Negra s de Trupan-Laja). The present Antuco volcano (Antuco 2) includes the erupt ive event that built the main cone with lavas and scoria falls and the erup tion of, at least, three pyroclastic flows, locally separated by mud-flow a nd colluvial deposits. Later, due to the rupture of the Lago dei Laja dam, the ash deposits were remobilized by debris flows which came down from the upper reaches of the river to the Central Depression where they formed a ma jor alluvial fan of approximately 50 x 60 km(2).