Geological outline of the basins in southern Argentina and their continuation off the Atlantic shore

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The stratigraphy, structural framework, geological provinces, and evolution of the southern Argentine territory and adjoining continental shelf are discussed. Four main basins (Salado, Colorado, San Jorge, and Magallanes) and several smaller ones occur in the region and are separated by positive areas. With the exception of several uplifts near the Andean region, most of these positive areas are cratonic. The southern hills of Buenos Aires, parts of the Tandil high, and scattered localities in the Deseado and northern Patagonia massifs are relics of Paleozoic basins. The basement, as considered here, is made up of Precambrian and Paleozoic rocks in the Salado and Colorado basins. In the San Jorge and Magallanes basins the basement includes younger rocks, as young as Late Jurassic in the latter area. The present-day basins are essentially post-Jurassic features. In Early Cretaceous time continental sediments, generally redbeds, were laid in the Salado and Colorado areas. In the San Jorge basin the Lower Cretaceous deposits are tuffaceous, and in the Magallanes basin they are marine. After a Subhercynian tectonic phase, the sedimentation continued during later Cretaceous time, being nonmarine except in the Magallanes basin. A reversal of the regional slope—from Pacific to Atlantic—occurred at the close of the Cretaceous, in many areas probably with no interruption of the deposition. As a consequence, the sea invaded the basins in the Paleocene, and during the Eocene the marine deposition continued in the Magallanes basin. There are no evidences of sediments of Eocene age in the areas of the Salado and Colorado basins; in the San Jorge basin the sediments are nonmarine. A withdrawal of the seas from the Argentine territory occurred during the Oligocene, according to recent data, and a general subsidence with marine deposition occurred in all the basins in the area in the Miocene. This subsidence was followed by a Pliocene emergence and, during the Quaternary, by several short-lived advances of the sea near the present-day Atlantic coast. The main tectonic features in the area are faults trending NW-SE or NNE-SSW, with subsidiary N-S and, to a lesser extent, E-W directions; thus the differential movements between the basement blocks formed by the faults gave origin to the basins. Successive tectonic phases, which, in part, were contemporaneous with the deposition, faulted and distorted the blocks. This block tectonics also has an important bearing on the igneous activity in Patagonia from Cretaceous time onward. Although the ‘modern’ fault pattern is essentially a post-Middle Jurassic feature, some faults are pre-Jurassic and others were formed along pre-existing tectonic lines. Arcuate trends in the sedimentary cover in several basins (Colorado, offshore; San Jorge) are very likely a manifestation of the above-mentioned differential movements, perhaps with some horizontal displacement. Close to the Andean area the tectonic lines show predominant N-S directions, with evidences of compressional forces near the Cordillera.