Caracterização geológica dos depósitos recentes na Serra do Tamanduá, NE do Quadrilátero Ferrífero

Abstract

The Quadrilátero Ferrífero is a region of globally known for its world class metal ore deposits. In this area, pre-cambrian units occur, locally covered by recent sedimentary units. Those sediments have been detailed studied since the 90’s, comprehended cartography, facies and biostratigraphic analysis. This paper studies one sedimentary basin located in Serra do Tamanduá, so called Bacia Bandeira. Aside from the geologic description and the mapping information given, three drillhole cores were described, in detail of stratigraphical profile of the encountered sediments. That information was unified, a wide drillhole was generated, which allowed the basin evaluation. Three stratigraphic units were proposed, so called units A, B and C. The unit A is the oldest, occurring in a restrict area in the central portion of the basin, consists in gravel to sandy material, with clayey portions and rich in caolinitic matrix and phyllite fragments from Archean units. The top of this unit is an organic sediment layer. It was interpreted as debris flow covered by lacustrine sediments. This organic-rich layer was further studied revealing a rich microfossil content, with upper Eocene age and comparable to other organic-rich units described in the region. The unit A is overlaid by unit B, which prevails in the central portion of the area and is composed of clay and sandy sediments, commonly gravel in a caolinitic matrix, fragments of phyllites and schists, besides quartz vein of Archean origin, with local occurrences of nodules and veins of manganese oxide, consisting in diamict-like and talus sediments. Successive debris flow is interpreted for this unit. Unit C occurs in outcrops, consisting in diamict-like sediments, in which matrix is clay sand silty, with clasts of Proterozoic units. This is a coluvionar material, with slope transportation from its source area, with episodic events interbedded with residual soils. In C, there is also lateritic soils and canga, representing pedogenetic processes. There is evidence of tectonic influence on the basin geometry, interpreted by sediments distribution and the sedimentary mechanism. This basin was tridimensional modelled, helping the comprehension of the spatial distribution of these sediments. There are remarkable the similarities of Bandeira Basin’s sediments to other basins in the eastern portion of Quadrilátero Ferrífero. This dissertation shows simple applied geological use of field investigation for the description for a Cenozoic sedimentary basin, comparing and correlating the studied units to other known.