CARACTERIZAÇÃO MINERALÓGICA DOS SEDIMENTOS DO IGARAPÉ DIABINHO (FORMAÇÃO SOLIMÕES) NO ESTADO DO ACRE – PROVENIÊNICA E IMPLICAÇÕS AMBIENTAIS

Abstract

Igarapé Diabinho, situated in the central region of the state of Acre, is a tributary on the right bank of the Envira river, belonging to the Juruá river basin. Its U-shaped grooved channel is carved in sediments known as the Solimões Formation, of Late Miocene–Pleistocene age. The rocky exposures along Diabinho stream demonstrate a succession of massive silt-clay sediments, covered by other compact sandy silts cemented by carbonates. These sediments are partly overlain by alluvial (fluvial-lacustrine) sediments from the Quaternary, in the region where this stream dominates. For the study, samples collected in three profiles along a ravine on the right bank of Igarapé Diabinho were investigated, in an area used for picnics during the dry season, that is, with slight human disturbance. To assist in identifying the probable source areas of these sediments, and determine their weathering and transport conditions, granulometric analyzes were carried out, investigating the mineralogical composition of the sediments, their heavy minerals, and the mineralogy of the sediments’ clay fraction. The sediments are predominantly silts with little clay, being composed of quartz, feldspars (albite and microcline), clay minerals (smectite, illite, and kaolinite), muscovite, and chlorite. As a result, the sediments were deposited in mild paleoclimatic conditions under weak chemical weathering, indicated by the predominance of unstable minerals, such as amphiboles, feldspars, biotite, epidote, garnet, apatite, and titanite. The dominant presence of aluminosilicates (kyanite, andalusite, sillimanite, garnet, and micas) and epidote indicates that these sediments had as their primary source metamorphosed pelitic rocks. A second assemblage composed of tourmaline, zircon, apatite, titanite, and even ilmenite points to possible contributions from basic and intermediate igneous rocks that are currently observed in Serra do Divisor on the borders of Brazil and Peru. In conclusion, these findings contribute to implications in the paleoenvironmental context by providing insights into the probable source areas of the sediments, determining the weathering and transport conditions, and identifying the mineralogy of the sediments.