The gondwana glaciation in Chile: Description of alleged glacial deposits and paleogeographic conditions bearing on the extension of the ice cover in Southern South America

Abstract

During the Late Paleozoic three main elevated areas can be recognized in southern South America: the Protoprecordillera, the Asunción Height and the present coastal area of Uruguay and of the Buenos Aires Province. The Protoprecordillera, located along the western margin of Gondwana and extending at least from present southern Bolivia to the tip of the continent, represented a major source for clastic deposits accumulated in continental intramontane and adjacent marine basins: the Septosyringothyris Sea to the west, the Chaco—Mesopotamia basin to the east and the Salta—Bolivia basin to the northeast. The Asunción Height located in present northeastern Argentina and Paraguay had a more reduced extension and separated the Chaco—Mesopotamia and the Salta—Bolivia basins to the west from the Paraná Basin to the east. The elevated area along the coastal area of Uruguay and of the Buenos Aires Province bounded the Paraná Basin to the East. Breccia deposits along the north-central Chilean coast originally considered to be of glacial origin have no typical glacial features and no similarities with the Gondwanan glacial deposits in Argentina, Bolivia and Brazil. They may be of Triassic age, and may correspond to detritus covering an erosion surface, cliff deposits, as well as fore-arc basin deposits. The alleged glacial deposits of the El Toco and La Caleta Formations have ages that preclude a glacial origin related to the Gondwana Glaciation. Further studies are needed to define the age and origin of the diamictites of Puchuncaví. The available paleomagnetic data for South America indicate that: (1) During the Late Paleozoic central Chile and western Argentina were located at distances greater that 30° latitude from the South Pole; (2) During the Early and Middle Carboniferous the present Chilean coast had an approximately E-W orientation and during the Late Carboniferous and Early Permian, a Ne-SW orientation. The proximity of these areas to the Late Paleonzoic South Pole do not demonstrate the existence of a glaciation. The fossil faunal and floral evidence in the Late Paleozoic deposits suggests general moderate to cold, but not polar, humid climates with seasonal variations. On the basis of this paleogeographic setting, mountain glaciations can be postulated for the Protoprecordillera and the Asunción Height. The east margin of the Paraná Basin and part of it was covered by the margin of a polar ice calotte centered in centered South Africa. These glaciations are responsible for the existence of abundant diamictite deposits known in the Late Paleozoic of southern South America. The Protoprecordillera formed a barrier for the humid oceanic westerly winds and separated two main climatic domains: a milder, rainy coastal domain and a more dry, continental-type eastern domain. There is no evidence supporting the existence of land emerging from the Septosyringothyris Sea along the present Chilean coastal area. The existence of Late Paleozoic glacial covers in southern South America can be attributed to two main factors: the relative movement of Gondwana and the South Pole, and the existence of elevated areas. The existence of a regional polar glaciation is, thus, rejected for the western part of southern South America.