Descripción: An ancient alluvial fan and its subsequent evolution are described. Ancient alluvial fan accumulations have been interpreted as the northern continuation of the rodados patagónicos of the Montemayor Plateau, a proposal that is contested. The methodological recognition of the ancient alluvial fan and its genetic and temporary distinction, were made from geomorphologic analysis and altimetric evaluation. Time assignment is a problem not yet solved. The methodology led to three different geomorphic systems or levels related to successive episodes of aggradation and erosion, with increasing lower positions, which coincide with a complex paleodrainage ancient alluvial fan. its successive developments are linked to proposed proto-rivers Chubut 1, 2, 3 and to a Simpson paleo-valley. Despite the mentioned limitations, we propose that the evolutionary model of ancient alluvial fan responds to a structure of cut and fill which is supported by the poly-formational composition of an ancient alluvial fan, and a gradual southward migration of the paleo-drainage. Based on geomorphologic analysis and altimetric distinction an upper level, composed of the remains of three plains (Pa, Pm and Pb), a middle level with two terraces (T1 and T2) and a complex lower level and five terraces (T3, T4, T5 T6 and T7) were recognized. Relict evidences of paleo-drainages are proposed based on the Estancia Paz and Don Morgan-Saraza paleo-valleys in the upper level. The age of the ancient alluvial fan based on the geomorphic evolution of the study area is tentatively assigned to an uncertain period between the late Pleistocene and late Sangamon interglacial at the end of the last glaciation.

Descripción: The region is situated in the Neuquén Cordillera at 38° 30'S latitud close to the international border. Geomorphic and structural features are presented which indicated a glacial morphogenesis for the depression that contains the Moquehue- Aluminé lakes. Previous works have identified a large volcanic complex related to Plio-Quaternary volcanic activity located at the east of the present volcanic arc. In this sector various caldera depressions and associated domic bodies have been identified. From the nine proposed calderas only the Pino Hachado caldera has been studied in detail. The Meseta del Arco and Nacimientos del Aluminé are disregarded as calderas. The erosional genesis of the steep scarps previously interpreted as caldera margins is also identified with glacial erosion. Facies and types of the local Pleistocene glaciation, its extension, their flow direction and its quite probable diffluence (Paso del Arco) through coeval glaciers situated north of the international border are also analyzed. Local characteristics of volcanic and eolian morphogenic and the mass-wasting process are described. Brief comments about pre-glacial drainage and the previous proposal of Bío Bío-Aluminé trough on the region are added. Transversal topographic-geological profiles discard west of the Bío Bío-Aluminé Fault a structural unit related to tectonic depression of Bío Bío-Aluminé trough and the Nacimientos del Aluminé and Moquehue calderas.