Abstract:

In central-western Argentina, the basement comprises Cambrian to Devonian sedimentary rocks, deformed and uplifted during the Late Devonian-earliest Carboniferous Chanic orogeny along the Paleo-Pacific margin of South America. Unconformably above basement, the Gondwana cycle comprises two unconformitybounded sequences. The Visean (350 Ma) to earliest Permian (275 Ma) Lower Sequence started with deposition in the Andean (or western) Calingasta-Uspallata Basin of valley-fill sediments. By the Namurian, alpine glaciation of a basement ridge, the Proto-Precordillera, fed sediment into the marine Calingasta-Uspallata Basin on the west and the nonmarine western Paganzo Basin on the east. The Paganzo Basin received additional sediment shed from basement highs further to the east. Mainly marine (but not glacial) sediment continued to be deposited in the Calingasta-Uspallata Basin into the Early Permian (275 Ma). At the same time, the Paganzo Basin expanded as dominantly nonmarine sediment encroached eastward over the craton. Tuff first appeared in the earliest Permian (-286 Ma) and reflects initial input from the magmatic arc on the Panthalassan margin. At the same time sediment overlapped the Proto-Precordillera. Southward, in the Andean San Rafael Basin, a similar Lower Sequence started with glacial sediment, probably in the Namurian, and likewise concluded at 275 Ma. In east-central Argentina, in the Sauce Grande Basin, deposition did not start until the latest Carboniferous (-290 Ma), with a marine glacial deposit capped by Tastubian transgressive glaciomarine sandstone and shale. At about 275 Ma, the mild extensional tectonics that had generated the Lower Sequence of quartzofeldspathic petrofacies from the craton, and sedimentary-lithic petrofacies from the Proto-Precordillera, gave way to convergent magmatic-arc tec- tonics that generated the volcanic and volcaniclastic Upper Sequence, which continued to the end of the Permian. In the magmatic arc, the Choiyoi Group (275 to 250 Ma) of mainly dacitic ignimbrites merged eastward with volcaniclastics in the nowcontinuous Calingasta-Uspallata/ western Paganzo foreland basin. Further east, in the Sauce Grande Basin, the Tunas Formation with tuffaceous interbeds conformably succeeded the glacial sediments. The tuffaceous interbeds reflect the southeastward swing of the arc at 34°S to parallel the axis of the Sauce Grande Basin 250 km away. The Gondwana cycle concluded with mild compressive deformation and uplift represented by an Early Triassic lacuna. Violent extensional deformation in the Middle and Late Triassic cut a swath of NW-trending grabens across northern Chile to Patagonia. The grabens filled with marine sediment in Chile and nonmarine alluvial-fan and then lacustrine-fluvial sediment in Argentina. The Late Paleozoic and Triassic succession was Anally capped by Late Jurassic and Early Cretaceous flood basalt.