Abstract

Glacial activity is recorded in rocks of Early, Middle and Late Carboniferous and Early Permian(?) age in Argentina and Bolivia, and in Carboniferous and Permian(?) strata of the cratonic Parana basin in Brazil, Uruguay and Paraguay. Direct evidence for glaciation in the four Andean basins consists of a single striated floor and common striated clasts in diamictites and associated strata. Many Andean diamictites are intercalated in sequences which contain marine invertebrate fossils and turbidites. There is abundant evidence in the form of sandstone inclusions showing soft sediment deformation that subaqueous mass movement took place during deposition. These rocks are therefore interpreted as resedimented till. Pebbly shale and varvelike rocks formed from ice rafting in marine and lacustrine environments. The glacial impress is slight in the continental facies as developed in fluvial strata of the Rio Blanco basin.

The dominant feature of the glacial Itararé Subgroup of the eastern border and of the Aqui-dauana Group of the western border of the Parana basin is fluvial reworking of diamictite during intervals between glacial advances. This extensive reworking has removed much primary evidence of glaciation but the presence of about 30 striated floors and surfaces within the sequence is unequivocal. Subaqueous mass movement and ice rafting also took place.

Ice centers apparently developed in rugged highlands within the Andean orogenic belt. Alpine or piedmont glaciers, or both, flowed downward onto narrow shelves, and till then moved into the marine portions of the basins by mudflow and sliding. The continental portions of the basins lay generally eastward of the marine areas and a topographically mature terrane still farther east was affected only slightly by glaciation, presumably because it was distant from glacial centers and lay at a relatively low elevation.

Continental glaciers of eastern South America probably radiated from centers located in South West Africa before separation of the continents, flowing northwestwardly onto the Brazilian shield and thence into the Paraná basin. Three separate lobes of ice occupied the Uruguayan shield, parts of Paraná and Santa Catarina states, and the state of São Paulo. A marine seaway occupied the basin center at times and was succeeded by a large body of fresh water at the time of glaciation of the western border of the basin. The southern portion of the Asunción arch was a glacial source which yielded debris northeastwardly from the western border after glaciation ceased in the east.

The Paraná sequences suggest 10 distinct ice advances separated by fluvial activity that may represent interglacial periods. This extensive glaciation resulted because the region was in high latitudes (as indicated by paleomagnetic data) through much of the Carboniferous and the Early Permian. The restricted glaciation of the Andean region was more closely controlled by elevation, possibly because of its position in somewhat lower latitudes.

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