EVOLUÇÃO GEOLÓGICA DA PORÇÃO MERIDIONAL DA PROVÍNCIA BAUXITÍFERA DE PARAGOMINAS DURANTE O NEÓGENO/PLEISTOCENO (NOROESTE DA BACIA DO GRAJAÚ, NORDESTE DO PARÁ E EXTREMO OESTE DO MARANHÃO)

Abstract

The surface geology of the southern Paragominas Bauxite Province, which lies in the northwestern Grajau basin, essentially consists of two sedimentary-lateritic sequences. The oldest one is composed of siliciclastic deposits of the Cretaceous Itapecuru Group overlain by a lateritic/bauxitic mantle, which sustains the Paleogene Sul-Americana surface, and a clayey overburden – the Belterra Clay, whose top is marked by a fairly plane regional surface. The youngest sequence unconformably overlies the Itapecuru deposits in the low areas, which separate the plateaus of the Sul-Americana surface. The lower unit of this sequence consists of a reddish, poorly sorted and friable clayey sandstone up to 30 m thick, which contains dispersed laterite fragments and quartz pebbles, besides lateritic stone-lines. Although it is normally massive, the sandstone occasionally shows an incipient stratification. The reddish sandstone is overlain, in sharp contact, by a ferruginous duricrust up to 3 m thick, that sustains the Late Velhas surface of Neomiocene age. According to its degradation degree, the duricrust displays a columnar, blocky or nodular structure or a pisolitc facies. When completely degraded, the duricrust is transformed into a ferruginous stone-layer. Conglomeratic accumulations, composed of laterite blocks, pelitic fragments and strongly subordinate quartz pebbles locally occur, corresponding to colluvial deposits resulting from the dismantling of the oldest residual cover. The ferruginous duricrust is overlain in sharp contact by a homogeneous, yellowish sandy clay, up to 5 m thick, that contains minute laterite fragments. The surface which marks the top of this overburden is situated 60 to 80 m below the surface sustained by the Belterra Clay. The reddish sandstone represents a continental deposit originated by successive debris-flows and is correlated with the Pirabas and Barreiras Formations of Neo- Oligocene to Mesomiocene age. The ferruginous horizon, interpreted as a water-table laterite, was probably formed during the Neomiocene, while the clayey sandy overburden, comparable with the continental Pos-Barreiras sediments described to the north, in the Bragantina Platform, should be of Pliocene age.