Hoja Geológica 4172-II, San Martín de los Andes, Provincias de Río Negro y Neuquén: Programa Nacional de Cartas Geológicas de la República Argentina 1:250.000

Abstract

The Geological Sheet 4172 II San Martín de los Andes includes a sector in the south of Neuquén province and a little portion in the northwest of Río Negro province. It occupies an area of about 9.000 km2 and is bounded by the parallels 40º and 41º of south latitude, meridian 70º 30’ of west latitude and the border with Chile. The Sheet encompasses part of the northern segment of the Cordillera Patagónica Austral, the southern end of the Precordillera Neuquina Sur, most of the Collón Curá trough and a little portion of the Macizo Norpatagónico. The oldest exposed rocks in the region are part of the crystalline basement; they are the Colohuincul Complex (Devonian/Carboniferous) in the mountainous region, while other units are located in a northsouth belt in the eastern sector of the sheet integrated by Cushamen (Devonian/Permian), Mamil Choique (Carboniferous/ Permian) and Lipetrén (Permian/Triassic) Formations. Continental sediments with great fossil content of Paso Flores (Triassic) and Nestares (Lower Jurassic) Formations, outcrop in areas surrounding the Piedra del Águila dam. Pyroclastic rocks of the Sañicó Formation (Upper Triassic/Lower Jurassic) and marine/continental sediments of the Piedra Pintada Formation (Lower Jurassic) outcrop in the extraAndean region. Meanwhile, in the mountains, volcanic and sedimentary rocks of the Montes de Oca Formation (Lower Jurassic) are intruded by granitoids of the Los Machis Formation (Jurassic/Cretaceous). A small outcrop of conglomerates and red sandstones of the Angostura Colorada Formation (Upper Cretaceous) is located on the southeastern tip of the sheet, and scarce showings of Cretaceous basalt appear to the west of Alicurá dam. The Paleogene, characterized by significant volcanic activity associated with the evolution of the Andean continental margin, is represented by large and thick volcanic outcrops of Huitrera, Ventana and Lolog Formations. Basaltic lava flows of Cerro Petiso (lower Miocene) formed structural plains in the region of the Limay River, and were covered by deposits of Maria Sofia Formation (Miocene) characterized by the presence of paleosols. During the Middle Upper Miocene the Collón Curá was filled with thick sediments, tuffs, ignimbrites and basalts of the Caleufú and Collón Curá Formations. Miocene rocks in the mountainous region are represented by granodiorites, granites and granitic porphyry belonging to the Coluco Formation, which intrudes the Colohuincul Complex and the volcanics of Ventana Formation. During the Pliocene there was a significant volcanic activity, with basaltic and andesitic emissions forming the Chapelco Formation in the montain range of the same name. In the eastern region of the sheet, large tracts of olivine basaltic flows of the Chenqueniyeu Formation are seen. Rhyolitic intrusive of the Upper Pliocene outcropping north and northeast of San Martín de los Andes integrate the Los Pinos Formation. During Pliocene Pleistocene olivine basaltic and andesitic flows of the Aseret and Paso Mirador Formations spilled in the Cordillera. In the Pleistocene, glacier-related deposits such as glacial, fluvioglacial and glacilacustrine sediments were deposited along the Cordillera, in areas adjacent to the lakes. Holocene basaltic lava flows spilled in several areas of the sheet, as in the Chapelco Range, in the Pampa de Alicurá and Cerro La Horqueta. Occasional showings of unconsolidated pyroclastic material of the Río Pireco Formation cover the relief in a large mountain sector. The stratigraphic record is completed with modern alluvial, landslides and colluvial deposits. The structure in the area of the sheet is well exposed along the Patagonian Andes, where important representative structures of the Andean compressive phase are observed. To the East, Collón Curá basin is located along the river of the same name, bounded on the east by the southern foothills of the Precordillera Neuquina Sur. Geomorphologically, the region has three clearly differentiated areas: a higher western area, along the Cordillera, dominated by glacial landscape, a central volcanic area and an eastern plateau shaped area. Mining of any kind is unknown for the area, except for those related with the use of activity gravel or sand to cover local needs.