Mineralogia e geoquímica dos sedimentos de fundo do rio-lago Tapajós-Pa.

Abstract

The Tapajós River experienced changes in its sedimentary load after the sea level in the Middle Holocene. The river’s mouth had its sedimentary load altered due to the difference of the density of its waters in relation to those of the Amazonas River, which created a physical barrier and drowned the mouth of the Tapajós River between the cities of Aveiro and Santarém. Thus, this part of the Tapajós River is nowadays better described as Lake Tapajós. Aiming the characterization of the Lake Tapajós, as well as the geological source of its sediments, samples from the water-sediment interface and from boreholes (90 cm) were collected and submitted to sedimentological, mineralogical and geochemical studies. The results show that the samples of the water-sediment interface are predominantly sandy, whereas the boreholes profiles are silty and composed by quartz, kaolinite, illite, muscovite, with goethite, gibbsite, anatase and smectite, showing a common mineralogical composition found in the terrains surrounding the lake. The occurrence of smectites suggests they are from the following different sources: Amazonas River, rocks of the sedimentary basin and partially weathered rocks from the Lake Tapajós upstream. The heavy mineral assemblage, predominantly composed by ultra-stable minerals, indicates recycling of these minerals, originally from primary sources as igneous and metamorphic rocks. The preferential distribution of sandy sediments in the margins and silty and clay in the central part of the lake, as well as the cyclical variation in the chemical composition along the borehole profiles, mainly in the southern portion of the lake, reinforces the lacustrine environment at least in the last 90 cm depth.