Resumen
The Quaternary geological history of the uppermost the Río Laja valley shows a complex interrelation between volcanic, mass wasting, and alluvial and fluvial sedimentary processes. The valley was initialiy carved by Lower Pleistocene glacial action on a Tertiary basement (Cura-Mallín and Trapa-Trapa Formations, intrusives), over which four major Quaternary units were deposited unconformably: the polygenic Quilleco alluvial cone, a Pleistocene volcanic sequence, and the products of the Antuco volcano and the Antuco volcanic avalancha. The Quilleco polygenic cone includes mixed intermediate and distal volcano-sedimentary facies which interfinger with the Pleistocene volcanic sequence derived from coeval stratovolcanos. The Antuco volcano is a mixed and composite andesitic to basaltic andesitic stratovolcano of basalt, 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 gravitational collapse of the edifice; this event produced the major Antuco volcanic avalancha which dammed the natural outlet of Lago del Laja and its tributaries. The gravitational collapse was the final result of a Bandai-San type phreatomagmatic eruptive event associated with almost simultaneous wet turbulent pyroclastic base surges made up of black basaltic ash (Arenas Negras de Trupán-Laja). The present Antuco volcano (Antuco 2) includes the eruptive event that built the main cone with lavas and scoria falls and the eruption of, at least, three pyroclastic flows, locally separated by mud-flow and colluvial deposits. Later, due to the rupture of the Lago del 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 major alluvial fan of approximately 50 x 60 km2.