The geological evolution of the Amazon basin and its mineral resources

Abstract

1. lntroduction The general view concerning the geological development of the Amazon basin (Amazonia) spans aperiod of more than two billion years. We discuss the 'superstructure' - the 'geologica\' Amazon basin proper - which may be defined as an enormous depression, originating as an intercratonic basin on the Brazilian Shield, and its post -Precam brian fill of sediments and volcanics. The rocks of this 'superstructure' are situated within the Brazilian territory, except in the utter western part where they belong to eastern Colombia, Ecuador and Peru. The basement is the framework of the 'geological' basin and crosses the frontiers of the Guianas. The basement is entirely of crystalline rocks of the Precambrian. It is necessary to include these Precambrian rocks in the description because the framework commands a heigher position than the basin proper and, therefore, has been the soulrce area for the sediments deposited in the basin. Even today this framework supplies the sedimentary ami the solution load of the Amazon water and of its tributaries. Mineral resources. already of great importance for the world mining industry, occur both in the 'geologiea\' basin and in its framework. The prospection and exploitation of these important resources leads to problems for the ecology and the economic infrastrllcture: it is neeessary to bllild highways and railroads, bridges and airports, harbollrs and power-plants, settlements and factories within the virgin forests growing on the basin proper and on its crystalline framework. The geoscientific research of Amazonia started in 1870 with the U .S. Morgan- Expedition, led by geologist Hartt. The Palcozoic formations on both margins of the basin were discovered during that expedition. J.M. Clarke, O.A. Derby, F. Katzer and R. Rathblln have produced the paleontological base for the stratigra- phy of the Paleozoic between 1871 and 1903. Later, petroleum geologists A.I. de Oliveira and P. de Moura prospected for hydrocarbons in the basin. After the last war F.F.M. de Almeida, O. Barbosa, G. Ludwig, L. Loczy, J.c:. Mendes, L. Morales, S. Petri, J. Andrade Ramos and K. Voll brecht enlarged essentially our Sioli. H. (ed.). Thc Ama7on. LlmllOlogl ,md lanJ,capc cc"logl of;J might\ tropical river and its basin. ISBN 90-6193-1Il8-8. © 1984 Or W. Junk Puhlishers. [)ordrccht. Boston. Lanc;Jstcr. Printed in the :>Ietherlands. 16 knowledge about the geology of Amazonia. In 1970, K. Beurlen evaluated the results of former investigations and gave a general overview in his book Geologie von Brasilien. Between 1970 and 1980, a new generation of scientists, particularly Brazilians, completed our learning and reviewed former concepts by new and systematic research concerning the interpretation of mapping, sampling, drilling and geophysics, supported by modern methods like remote sensing, radar, geo- chemistry, sedimentology and so on. Aubouin, Geyer, Julivert and Stibane have researched the subandean regions of Colombia; Sauer, Putzer, Faucher and Tschopp of Ecuador; and Audebaud, Benavides, Koch and Ruegg of Peru. It was Ackermann who started the prospection for ore deposits in the rain forest in Amapa during the last war, and to whom we owe the discovery of the large manganese deposits of the Serra do Navio. After 1960, the search for ores was intensified. Petrobras carried out a costly drilling program, supported by mapping and geophysics with the aim of finding economic petroleum fields. In the last years a new cartographic atlas was created by the 'Projeto RAD AM' , later 'RADAM-Brasil', which is of great importance, too, for the geoscientific re- search in the forest -covered country. There are a lot of modern publications about various mineral resources: manganese (Dorr; Scarpelli), iron ores (Tolbert; Mascarenhas; Putzer; Rangei; Parada; Ruff), tin (Santos; Lobato; Grabert; Putzer; Kloosterman); diamonds (Barbosa & Ramos), bauxite (Grubb, Wolf & Ranger; Machado; Assad), and gold (SUDAM). The meritorious bibliography on geosciences in Amazonia, compiled by Loewenstein and his team (1969), comprises 1358 titles from 1641 up to 1964. In 1981 B.A. dos SANTOS published the book 'Amazönia Potential Mineral e Perspectivas de Desenvolvimento'. 2. The 'geological' basin of the river Amazon 2.1 Structure The giant intracratonic Amazon basin has its origin along an ancient zone of weakness within the Precambrian Shield, the last consolidation ofwhich occurred 600 million years ago. The basin, more than 3500 km long and 300 km to 1000 km wide, is stretched between two parts of the Precambrian Kraton: in the north the Guiana Shield and in the south the Central-Brazilian Shield (Putzer 1976), also called Guapore Kraton. These kratons consist of crystalline rocks, folded and metamorphosed by various orogenies between 600 m. y. and 3500 m. y. Geophysi- cal measurements and deep boreholes have proved the continuation of the basement below the basin proper. The basin is divided by three arches, i.e. highs, with sigmoid direction: in the west the Iquitos Arch ne ar Iquitos, crossing Acre; near the mouth of the river Purus, west of Manaus, the Purus Arch; and east of the mouth of the river Xingu, the Gurupa Arch. The arches have been proved by