Abstract

The major features of the geologic framework of the continent are outlined. Attention is called throughout the paper to the many similarities in the continental framework and geologic history of South America with those of North America.

The change in paleogeography and facies from Cambrian to Pliocene inclusive is pictured on 15 paleogeographic facies maps (Pis. 2–16) selected to show the principal sedimentary overlaps. Intervening changes and conditions, the progress of transgressions, and the fundamental transitions in the architecture of the continent that caused or conditioned the paleogeographic and facies changes are briefly reviewed.

The major events as interpreted from the stratigraphy, structure, and other features of the geology are:

  • 1. Late Proterozoic diastrophism.

  • 2. Upper Cambrian marine transgression and wide-spread late Cambrian-early Ordovician sedimentation, with less extensive deposits of middle and later Ordovician and early Silurian time.

  • 3. Main Caledonian orogenies of late Ordovician to Silurian, and a general withdrawal of the seas during much of the Silurian. The best-developed Silurian seaways, as in North America, are apparently in the eastern part of the continent.

  • 4. Marine advance in late Silurian and early Devonian. An extensive development of Lower Devonian sediments, which embrace the major part of the Devonian sediments of South America. However, a middle Devonian marine invasion spread through the Colombia-Venezuela and Amazon troughs and southward down the Cordilleran trough at least as far as 20° S. Lat.

  • 5. Upper Devonian sediments are unknown in South America. Mississippian marine sediments occur in the West-Central Argentine embayment and at least locally in the Colombian seaway; and they may yet be found in the Cordilleran trough of Peru and Bolivia, etc., where continental beds of Mississippian and probably also Pennsylvanian ages occur.

  • 6. Mid-Pennsylvanian marine sediments are known locally in the Cordilleran trough, but the first major marine advance following the Devonian began in the late Pennsylvanian and persisted through the early Permian. It covered approximately the same areas that were occupied by Devonian seas. Glaciation occurred south of about 20° S. Lat., and its deposits are interfingered with both marine and continental sediments in the southern areas. Less extensive, mainly continental sedimentation of later, possibly mid-Permian age, locally carrying glacial deposits in the south, rest disconformably on early Permian.

  • 7. Marine sediments of Upper Permian and Lower and Middle Triassic age are not recognized on the continent. Some plant- and coal-bearing beds in Peru are thought to be mid-Triassic.

  • 8. The sea advanced from the west during the Upper Triassic into the basins occupied by Permian sedimentation. The marine sediments interfinger with and change to a continental facies toward the margins of the advance. These continental elastics overlap beyond those of the Permian in many places.

  • 9. A new pattern of geosynclines, whose formation began along the belt of the present Cordillera in the Triassic, is outlined by Jurassic sediments whose main seas existed in the Lias, Bajocian, and Callovian ages. Barred basins developed in these geosynclines. These were sufficiently closed to deposit salt and anhydrite in the Oxfordian stage.

  • 10. A readvance of the sea in the late Jurassic, Tithonian, spread sediments of Tithonian and early Cretaceous Neocomian stages through somewhat more extended areas of the western geosynclines, opening southward through Tierra del Fuego, and also extending for the first time across the northern part of the continent.

  • 11. In the Aptian-Albian-Lower Cenomanian (Comanche) ages of the Cretaceous the Cordilleran seaways enlarged in the northern and northwestern areas and receded toward the new southern Patagonian embayment. A marine embayment appeared for the first time in the Bahia area of the Brazilian coast.

  • 12. A withdrawal of the seas in the Cenomanian was followed in late Cenomanian and Turonian time by a wide marine advance over the areas of Aptian to early Cenomanian sedimentation. Deposits of this sea are most prominently developed in the Turonian and lower Senonian stages. As a preliminary to the uplift of the Andes, the overall slope of the continental platform, which had been largely westerly during the Paleozoic and Lower Mesozoic, began to take a more level stand. As a consequence extensive, largely fresh-water lakes spread over the interior of the continent during the Upper Cretaceous.

  • 13. Two or more relatively minor to locally more severe movements affected the continent in the late Cretaceous. The first caused a retreat of the sea in the Upper Senonian. In many parts of South America, as in other parts of the world, a well-defined hiatus separates the Tertiary from the Cretaceous. However, the seas advanced into the deeper parts of the troughs and embayments to deposit sediments of late Senonian to Paleocene stages.

  • 14. The first major Andean orogenic movement of the Tertiary occurred in post-Paleocene, early Eocene time. A marine advance followed which spread widely in the middle Eocene, and there were further lesser retreats and prominent readvances in the upper Eocene.

  • 15. A second major Andean movement had its climax in early Oligocene. This was followed by sea advances of middle Oligocene to early Miocene time.

  • 16. The Oligo-Miocene deposits spread headward and inland and continued upward through the Middle Miocene into brackish and still more continental facies, while they remained marine in character in more restricted episeas of the continental margins.

  • 17. Upper Miocene time brought a further general retreat of seas and a drying-up of interior areas preliminary to a third major Andean orogenic-epeirogenic movement in late Miocene.

  • 18. The seas readvanced into certain local embayments and marginal areas in the Pliocene.

  • 19. Continental deposition continued along the Andean front in the Pleistocene as the fourth major Andean movement raised the Cordilleras to approximately their present conformation. Glaciation developed in the higher mountains and was general over southern Patagonia.

  • 20. Taphrogenic breakdown and general subsidence of continental border areas and belts, which began in the Mesozoic, became more pronounced in the late Tertiary and Quaternary.

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