ESTRATIGRAFIA E ORIGEM DO GRUPO BARREIRAS EM PERNAMBUCO, PARAÍBA E RIO GRANDE DO NORTE

J. M. MABESOONE, A. CAMPOS E SILVA, K. BEURLEN

Resumo


In the coastal region and some parts of the interior of Brazil, between the Amazon area and Rio de Janeiro, there occurs a non-fossiliferous sedimentary sequence of Cenozoic age, called Barreiras Group. Recently, its stratigraphy has been revised and its origin determined. Three formations can be distinguished: (1) Serra do Martins Formation of Early Tertiary age, (2) Guararapes Formation of Pliocene age, and (3) Macaiba Formation of Early Quaternary age, separated by unconformities. Every formation has its typical soil profile. As was generally assumed, the depositional environment should have been sand and mud flows, under semi-arid climatic conditions, as correlate sediments of the relief. Recent studies showed, however, another pattern which may lead to different interpretations. The Serra do Martins Formation, which covered almost entirely the areas of the States of Rio Grande do Norte and Paraiba, and parts of Pernambuco, is a deposit of fluvial origin on a plain relief, not necessarily being a pediplain. Lithologically it shows kaolins, quartzites and iron-cemented sandstones. After deposition the formation suffered a strong laterization resulting in crusts of ironsandstones. Due to later monoc1inal uparching, the greater part of the deposits were removed, remaining only small patches on top of tablemounts. The Guararapes Formation, most frequent and due to its colour most conspicuous, shows really a deposition in a semi-arid environment with sand and mud flows. Some river influence can also be detected. In a few outcrops flow structures are frequent. The whole sequence is related to the formation of the relief of the region (lower pediplain). On top occurs a weathering profile (Riacho Morno) with leaching and deposition of iron oxides at a lower level or in vertical columns. The upper Macaiba Formation is composed of a white-coloured sand to clayey sand, with a high kaolin content. The depositional environment was again fluvial, especially in fault troughs. The occurrence is therefore restricted. Its soil is a cover or yellowish to reddish fine sandy material (Potengi). The determination of the depositional environments was made by field observations including geomorphological analyses, grain-size and sand analyses, and mineralogical composition.

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