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

RAMOS, M.E.; ORTS, D.L.; CALATAYUD, F.; FOLGUERA, A.; RAMOS, V.A.

Mar del Plata

Congreso; International Geological Congress on the Southern Hemisphere; 2010

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

A detailed study of the Ñirihuau basin southwest region, nearby the Maitén range, between the 42º00´S and the 42º20´S, has revealed in detail some stratigraphic relationships not previously described. The structural analysis of this sector located in the Patagonian foothills, together with those mentioned relationships, allowed a new interpretation of the tectonic setting of this Andean segment. The studied local sequence consists of four units: The base is represented by the Oligocene Ventana Formation, formed by mainly andesitic volcanic and pyroclastic rocks defining the Maitén eruptive belt (Rapela et al., 1988). These authors recognized this sequence as a typical arc succession based on chemical analyses. The shales, sandstones and coal beds of the Ñirihuau Formation of ~22-17 Ma rest 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. In this work, circumscribed to the southern part of the basin, we identified a series of sequences that leads to a comparison with the ones described by Paredes et al. (2009) to the north. Both sequences start with a thin carbonaceous section that could be associated with a relatively deep lacustrine association of 10 meter thickness, constituting an excellent stratigraphic marker through the basin. This is followed in the southern section by a 50 meter thick volcaniclastic sandtstone succession. The upper section begins at this site with liquefact levels, associated with Gilbert-type deltas, and floodplain deposits of 350 meter thickness. Finally a fluvial system is represented by a series of channels and flood plain deposits. In the southern sector a 50 meter thick conglomerate level is developed at the top, which is not present to the north. These sequences are exhumed at the eastern slope of the Maitén range which is represented as an east-verging thick-skinned block dismembered by a series of synthetic structures (eg. El Pantano thrust), and affected by a series of west-verging backthrusts (El Maitén thrusts). Basement shortening produced by these structures was transmitted to the upper sedimentary cover where a frontal syncline was formed, associated with thin-skinned deformation that developed fault bend folds associated with flexural slip (Figure 1) (Giacosa y Heredia, 2004a). In the eastern sector, four sets of progressive unconformities from the base to the top of the Ñirihuau Formation and the base of the Collón Cura Formation were observed. These were explained through a kinematic model applying the concept of growth strata bed-by-bed by kink-band migration (Suppe et al., 1997).  This finding implies that the basement structure was created at the time of both Ñirihuao and Collón Curá sedimentation and therefore that these constitute remnants of a proximal foreland basin. These progressive unconformities are usually found at the top wedge of the fold and thrust belt (DeCelles et al., 1996), implying that the Miocene orogenic front was located in the Maitén range.