The late Oligocene - Miocene Ñirihuau Formation interpreted as a foreland basin in the Northern Patagonian Andes

RAMOS, MIGUEL E.; ORTS, DARÍO; CALATAYUD BASUALDO FELIPE; FOLGUERA, ANDRÉS; RAMOS, VICTOR A.

Mar del Plata

Congreso; International Geological Congress on the Southern Hemisphere; 2010

Instituto Nazionale di Oceanografia e di Geofisica Spreimentale

A field study of the Ñirihuau basin southwest region, nearby the Maitén range, between the 42º00´S and the 42º20´S, has revealed new stratigraphic relationships. The structural analysis of this Andean segment, allowed a new interpretation of the tectonic setting of these Patagonian foothills. The stratigraphic sequence consists of four units: The base is represented by the Oligocene Ventana Formation, formed mainly by andesitic volcanic and pyroclastic rocks defining the Maitén eruptive belt (Rapela et al., 1988). Based on chemical analyses, these authors recognized this sequence as a typical arc succession. The shales, sandstones and coal beds of the Ñirihuau Formation of ~22-17 Ma rest in general conformably on the previously described volcanic piles (González Bonorino, 1973; Cazau, 1980). More recently, Paredes et al. (2009) made a detailed sedimentary analysis in the northern part of the Ñirihuao basin. Here, they differentiated a series of lithofacies associations in stratigraphical order: deep lacustrine, volcaniclastic flows, shallow lacustrine interbedded with Gilbert-type deltas, volcaniclastic flows, fluvial channels, and volcanics at the top.