Evolução geológica no cenozóico da região norte entre Açailândia (MA) e Ligação (PA)

Abstract

The studied area is located along the BR-010 Highway (Belém-Brasília) between the cities of Açailândia (MA) and Ligação do Pará (PA), in the southern part of the Bauxiti-bearing Province of Paragominas and has an extension of about 115 km. In it, two surfaces were distinguished, each one supported by a specific alteritic-sedimentary sequence. The highest surface corresponds to the tops of vast residual plateaus from 360 to 220 m in altitude and presents a slight slope to the north. The other surface, embedded in the first, culminates between 210 m, to the south, and 150 m, to the north, and shows a slight swell. The old lateritic-bauxitic cover that supports the higher surface is constituted by a saprolitic horizon originating from the alteration of clayey or archosean sandstones and claystones from the “Itapecurú Deposits” of the Upper Cretaceous. On the saprolite rests a powerful armor, essentially ferruginous in the extreme south, which, towards the north, is progressively enriched in gibbsite. The first signs of bauxitic levels, still diffuse, are found about 40 km north of Açailândia. This trend is confirmed to the north with the presence of increasingly clear and consistent bauxitic horizons, above and at the base of the ferruginous horizon. Superimposed on this cover and in extremely sharp contact, there is a thick clay capping, the Belterra Clay. The lateritic-bauxitic cover developed in three phases. During the first phase, the ferruginous armor was formed through the ferruginization of the Itapecurú sediments, during the Lower Tertiary. Afterwards, the armor was affected by partial degradation and physical reworking of its products. Remains of older alterites may have also been involved in this process that resulted in deposition of nodular to pseudopisolithic gravel and sandy clay. The third phase, essentially chemical, consisted of the generation of bauxite in areas that presented more favorable climatic and geomorphological conditions. This event probably occurred at the end of the Eocene. The exclusively ferruginous lateritic armor that supports the lower surface has as substrate a reddish clayey sandstone containing small lateritic fragments and quartz pebbles. The cuirass itself has a massive, columnar or nodular structure, and the degraded stone-layer can be found in several places. Its cap is formed by a yellowish clayey-sandy material, rich in quartz grains and disseminated letheritic fragments. This laterite probably formed in the late Pliocene – early Pleistocene. Its substrate can be correlated with the Barreiras Group, while its capping would correspond to the unit called Post-Barreiras further north, in the Ipixuna-Aurora region. The stone-layers quite frequent in the area are located in the same stratigraphic position as the more recent lateritic armor and resulted from in situ degradation of this armor, without significant physical rework. However, locally, colluvial deposits of the glacis type or pediments composed of fragments of ancient armor were identified. The following stages characterized the evolution of the two alteritic-sedimentary sequences: 1- formation of a ferruginous armor from the Itapecurú Deposits; 2- partial degradation of the armor; 3- Bauxitization in the Paleogene; 4- deposition of sediments that later change to Argila de Belterra; 5- major erosive phase and individualization of vast plateaus; 6- filling of the valleys and lowered areas separating the plateaus by sandy-clay sediments correlated with the Barreiras Group and degradation products of the old armor; 7- ferruginization of the sediments from step 6; 8- more or less accentuated degradation of the ferruginous crust from stage 7, giving rise to stone-layers; 9- deposition of clayey-sandy sediments correlated with the Post-Barreiras on the armoring of stage 7 and the stone-layers; 10- dissection and establishment of the current drainage network. Climatic variations and periodic tectonic (epirogenetic) reactivations throughout the Cenozoic were the main factors responsible for this evolution.