The Rhyolitic Plateau of the Marifil Formation (Jurassic): A Gondwana Paleosurface in the Southeastern Portion of the Northern Patagonian Massif

Abstract

Along the southeastern border of the Northern Patagonian Massif of the provinces of Río Negro and Chubut, an extensive surface is presently called the “Rhyolitic” or “Ignimbritic Plateau.” This large geomorphological unit has a geographical extension which exceeds 50,000 km2 and it is located between 40°30′ and 44° lat. S and between the Atlantic Ocean coast and 67°30′ long. W. It is characterized by a smooth topography of low and rounded hills, shallow endorheic basins, and a poorly integrated drainage network. The drainage network is mostly nonfunctional and roughly coincident with the bedrock fracture system. Bedrock is almost exclusively composed of the acid volcanic and pyroclastic rocks of the Marifil Formation of Early to Middle Jurassic age. A significant proportion of the identified positive landforms present form and nature very similar to that of “bornhardts,” as defined by Twidale (Revista de la Asociación Geológica Argentina 62(1):139–153, 2007), basically for granites. Bornhardts are uncovered dome hills (Twidale, Revista de la Asociación Geológica Argentina 62(1):139–153, 2007) which are usually frequent in Gondwana landscapes (Fairbridge, Encyclopedia of geomorphology. Ronald, New York, 1968). Furthermore, the ubiquitous presence of “corestones” (isolated, large, rounded boulders), which are taken as indicators of an ancient, deep weathering front, supports the hypothesis that these paleosurfaces were generated by long-term, intense chemical weathering processes. The deep weathering would have occurred over at least 25 Ma, between the Middle and Late Jurassic, under a hot and moist paleoenvironment and under extremely stable tectonic conditions. The mobilization, denudation, and later sedimentation of the regolith/saprolite formed under such conditions would have taken place during several erosion episodes, mostly under tectonic forcing, between the Late Jurassic and the Late Cretaceous. The important clay and other secondary mineral accumulations (some of them significant sources of uranium) in the region would have a direct genetic relationship with the development of these paleosurfaces. From the Late Miocene onwards, the colder and drier conditions that were imposed in the region by the uprising Andes and the establishment of mountain glaciers and ice caps during numerous glaciations allowed the modification of this landscape by hydro-eolian processes which generated the widely distributed endorheic depressions (locally known as “bajos sin salida”) by deflation and occasionally reworked the surviving rocky hills by abrasion.