• Alwine Bertels


Cretaceous At the end of the Cretaceous the configuration of South America was markedly different from that of today; the northwestern extreme was isolated from Central América whereas the southernmost region was probably connected with the Antarctic Peninsula. During the terminal Lower Cretaceous (Aptian) the African and South American plates began to separate; concomitantly the first marine Atlantic ingressions are registered; in the remainder Southamerican borders marine sedimentation took place during early Mesozoic and older times. In southern Perú, since the Turonian, marine beds are replaced by strata of continental origin; however in the northern part of the country, marine strata are recorded until the Maastrichtian. Similar events are registered in Ecuador, Colombia, Venezuela, Trinidad, some Brazilian and Argentine basins and Chile. In Perú, Ecuador and Colombia two sedimentary areas, separated by the Real and Central Ranges, of Ecuador and Colombia respectively, can be differenciated: the western area is characterized by deep water assemblages -common to those of the Pacific and Caribbean areas- whereas the eastern one is distinguished by the presence of the genus Siphogenerinoides. They constitute the Pacific-Caribbean and the Peruvian Geobioprovinces respectively. The northern part of the Southamerican Continent -in general the Caribbean area- is typified by the presence of larger foraminifera and abundant planktonic foraminifera; Venezuela, Trinidad and the Guayana basin are included in the Caribbean Geobioprovince. In Brazil, benthonic assemblages, particularly those recorded from the northeastern Sergipe/ Alagoas basin show strong endemic features; they integrate the Brazilian Geobioprovince. In Argentina Maastrichtian assemblages are recorded from the Neuquén/Colorado and Austral basins. The first one is distinctive because of its marked Navarroan faunal affinities; the second by its strong Australoasiatic and Pacific relations. In both basins, excluding cosmopolitan faunas, strata of the same age do not show common elements; thus the Argentine and the Argentine-Chilean Geobioprovinces are recognized. Tertiary (Paleocene-Eocene) In all southamerican basins, excluding the Recóncavo/Tucano (Brazil), Golfo de San Jorge and Noroeste (Argentina) and the Peruvian Geosyncline, the Cretaceous-Tertiary boundary is recognized. During this time interval, several sedimentary breaks, related with the Andean orogenic movements, are registered. In Perú, the Eocene assemblages show some distinctive features: those of Middle Eocene age, belong to the Peruvian Geobioprovince whereas the Upper Eocene•benthonics indicate close Pacific and Caribbean influences; this fact is probably related with the global higher temperatures during this interval. The Pacific-Caribbean Geobioprovince overreached its highest southermost extent, gaining latitudes as those located today in Central Chile regions. The Caribbean Geobioprovince persisted with its peculiar features: large forarninifera and carenate planktonics; it is integrated by Venezuela, Trinidad and the Guayana basin. In Brazil the faunal assemblage shows similarities with those of Midwayan age; it is possible to infer that the Brazilian Geobioprovince areal span reached to the north the Foz de Amazonas basin and to the south the Paraná basin. In Argentina, the assemblages of the Neuquén/Colorado and San Jorge basins show strong relationships with those of the Midway Group in the United States. They belong to the Argentine Geobioprovince. The Austral basin assemblages continued with its own distinctive strong australoasiatic and pacific affinities; during this time the Austral Argentine-Chilean Geobioprovince persisted. Oligocene-Lower Miocene In the ""Bolívar Geosyncline"", Venezuela, Trinidad, Guayana and some Brazilian basins such as Foz de Amazonas Barreirinhas and Recôncavo, continuous marine sequences are registered. In the Guayanas, Foz de Amazonas, Barreirinhas and Recôncavo basins the end of the lower Miocene marine sedimentary cycle concluded with shallow water assemblages. In other areas such as Ecuador and Colombia a hiatus, equivalent to the Globorotalia fohsi s.l. Zone is recorded. In Argentina the marine sedimentation persisted during the Oligocene and ended in the lowermost Miocene; its sediments are present in the Golfo de San Jorge and Austral basins; after this regression a neatly continental sedimentation prevailed. During Oligocene-lower and middle Miocene times the Pacific-Caribbean, Caribbean, North Brazilian, South Brazilian, Argentine and Austral Argentine-Chilean Geobioprovinces are recognized. The differentiation between a North and South Brazilian Geobioprovince is mainly based on the presence of larger foraninifera in the northern area of the country; they are absent from northeastern region assemblages. Upper Miocene-Pleistocene During the middle Miocene, benthonic assemblages in the northwestern part of the continent are common to those of the Caribbean, Peruvian and Pacific areas; since the Upper Miocene a neatly microfaunistic differentiation took place; since that time the Caribbean and Pacific Geobioprovinces are distinguished, being this fact related with the origin of favorable conditions for the free terrestrial faunal migration (Duque-Caro, 1972, 1975). North Colombia, Venezuela and Guayanas make up the Caribbean Geobioprovince. In Brazil, whereas the marine sedimentary cycle concluded at Lower or Middle Miocene times, in the southern region of the country (Pelotas Basin) a marine transgression took place at this interval; the assemblage shows features of subtropical-temperate ecological conditions, being remarkable the presence of the genus Amphistegina which in the Recent is restricted to lower latitudes. Similar events occurred in Northern Uruguay. Both are as belong to the South Brazilian Geobioprovince. In Argentine the first marine influences into the Paraná Basin are registered; although the water temperature was higher than today at the same latitude, they show cold influences; on the other hand the absence of Amphistegina and related genera allows to separate this area from the South Brazilian one: it is the Argentine Geobioprovince. In Central Chile the planktonic assemblage is predominantly composed of Globorotalia pachyderma (Ehrenberg) with sinistral coiling ; the area belongs to the Chilean Geobioprovince. In both -Argentina and Chile- during this time interval cold water courrents -Antarctic and Humboldt- respectively were operative; this fact is related with the opening of the Drake Passage.



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