GEOLOGIA DA CHAPADA DOS PARECIS, MATO GROSSO, BRASIL

SETEMBRINO PETRI, VICENTE JOSÉ FULFARO

Resumo


The Parecis Plateau separates the Amazonas basin from the La Plata River basin, and thus is one of the most important watersheds in South America. It is made up of the Parecis Formation that lies in unconformity upon the Early Cretaceous Tapirapuã Basalt and crystalline rocks of Pre-Silurian age. The Parecis Formation begins with fine sandstones having an abundant clay matrix, followed by fine-to-medium grained, cross-bedded sandstones. To the south, several levels of conglomerate enveloping clayey sandstone lenses represent the upper part of the Formation, which in the north comprises a thick cross-bedded sandstone sequence. The summit of the plateau is covered by Tertiary gravel deposits that unconformably overlie the Parecis Formation. Directions of paleocurrents drawn from cross-bedding in the sandstones show a northward transport, which is also indicated by increasing thincknesses of the sandstone facies northward. The main source area for the whole formation would then be located to the south. The gravel deposits that rest in unconformity upori the sandstones show a paleocurrent directed to the south, based on pebble orientation and on the distribution of sedimentary bodies resembling old fanglomerates. An idea of the tectonic and paleogeographic evolution of this area may be obtained from these facts and also from knowledge of the stratigraphic column of the adjoining Paraná sedimentary basin. In the Late Cretaceous, fluviatile sedimentation was going on throughout the Parecis and Parana basins, which were separated by an arch, here named the Paraguay Arch, made up of Pre-Silurian metamorphic and sedimentary rocks of the Alto Paraguay and Cuiaba Groups. This arch is a great continental N-S tectonic structure separating two cratonic areas that is represented in the studied area by rocks of the Paraguay Geossyncline. To the south, this arch has a continuation through the N-S oriented Asunción Arch, now tectonically depressed in a graben where olivine basalts 46 m.y. old crop out. During the Eocene the Paraguay Arch underwent downfaulting that also gave rise to the present geomorphic situation and the initiation of the Amazonas and La Plata River basin. Fanglomerate deposits formed along the southern flank of the Parecis Plateau, the newly formed watershed of these two great basins. At this time climatic conditions were probably semiarid. Climatic oscillations and base level changes led to the present situation with downcutting through the gravel deposits by streams of the Paraguay River basin.

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