ORIGEM DAS CACHOEIRAS DA BACIA DO ALTO PARANÁ

Abstract

Sculptured in the northern part of the Paraná Basin, specially in its basaltic rocks, the Rio Grande presents majestic falls and rapids, which are singular features of its physiography on its lower course. In this work were studied Marimbondo, Água Vermelha, Mutuca and Onça falls. Morphologically those falls have the same characteristics: the river is wide and shallow before them, and narrow and deep after them; they are constituted by complex groups of falls and rapids, which present a maximum lenght of 4 km and a maximum height of 30 m. Geologically, the falls are always associated with the basaltic rocks of Serra Geral Formation, being present also, although irregularly, lens of intra-trapiano sandstone and basaltic breccia. Statistical measures of directions of joints reveal the existence of preferential systems of rock fractures, specially around N-S, E-W, N30E and N30W. By this study, we may conclude: (1) all the falls have identical features and behavior, suggesting common processes in their origin and development; (2) there is a marked structural control of the falls and rapids and of all the involved drainage; (3) the origin and evolution of the falls are directly connected to the joint systems and also to the textural differences present in the rocks; (4) little faults, although not observed in the field, but indirectly inferred, could be another factor also responsible by the origin of those features; (5) comparative and correlative studies with other falls and rapids present in the Alto Parana Basin, suggest identical processes on the origin and development of all falls of that basin.