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The Paleozoic end


South Atlantic Ocean
South Atlantic Ocean

The Paleozoic ended with the final amalgamation of Gondwana, which together with Laurasia to the north constituted the late Paleozoic supercontinent of Pangaea. Subduction beneath the western margin of Pangaea slowly ceased. The igneous rocks formed in the volcanic arc that developed along what is now the Cordillera Central between Chile and Argentina and then along the western continental margin, are transitional in nature—i.e., the composition of the rocks changes from primarily andesitic to predominantly rhyolitic. These changes in mineral composition indicate the passage from a subduction-related compressive regime to one of extensive magmatic activity and crustal extension. Vast sheets of magma, consisting of flood basalts and of rhyolite deposits up to 2.5 miles thick, covered the west from southern Peru to the border between Argentina and Chile. Farther north this activity was partially obliterated in Cenozoic times by the uplifting of the Andes and by volcanic cover.

The mosaic of continental blocks accreted to form the Pangaea supercontinent was unstable and remained amalgamated for only a few million years. Extensive sedimentary cover indicative of arid conditions was accumulated unevenly in the late Paleozoic basins. Desertic sandstones, mudstones, and tuffs of Triassic age (i.e., about 200 to 250 million years old) have preserved fossils of a rich fauna of dinosaurs and mammallike reptiles, as in the Ischigualasto basin of Argentina. A series of Middle to Late Triassic basins also developed through horizontal crustal extension during the early phases of Pangaea’s dispersal. These rifted basins largely followed the previous Paleozoic sutures along the western side of the continent. Crustal extension reactivated the inner part of the supercontinent as well, with an increase in subsidence in the Parnaiba and Paraná intracratonic basins, where deposits of Triassic age have been recovered from core samples.

The opening of the South Atlantic Ocean is recorded in a series of Mesozoic and Cenozoic basins that developed along the present Atlantic margin. Most of these basins have clastic red beds that date to the Late Jurassic and Early Cretaceous epochs (about 160 to 100 million years ago). North of Porto Alegre, Braz., are found evaporite salt deposits created in marine basins with restricted circulation. Sediments formed under less-restricted drift conditions began approximately 125 million years ago and are younger to the north, where they are mostly represented by clastic marine deposits. The basins of northern Brazil have carbonate deposits mixed with clastics. Deposits laid down in a restricted-circulation, anoxic (oxygen-poor) environment along the basins of Brazil and Argentina now contain abundant black shales rich in organic matter and are an important source of hydrocarbons. Open marine conditions have prevailed in the Atlantic basins since mid-Cretaceous times. The first open oceanic circulation between the South and North Atlantic oceans was established along the passive margin of South America in the Late Cretaceous Epoch (about 100 to 65 million years ago), though marine sediments had accumulated and been lithified in these basins for some time prior to that. "South America" © Emmanuel BUCHOT, Encarta, Wikipedia

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