SEDIMENTOLOGIA, ESTRATIGRAFIA E MINERALOGIA DA FORMAÇÃO SOLIMÕES NO IGARAPÉ DIABINHO AFLUENTE DO RIO ENVIRA FEIJÓ-ACRE

Abstract

The Igarapé Diabinho, is a tributary on the right bank of the Envira river, which is part of the Juruá river basin in the central region of the State of Acre (municipality of Feijó). It is in a U-type valley, carving out the sediments known as the Solimões Formation, considered as of Late Miocene to Pliocene age. In this stream, as well as in many regions of Acre, these sediments are partly covered by alluvial (fluvial-lacustrine) sediments from the Quaternary. The rocky exposures along the Diabinho stream show a succession of massive silt-clay sediments, green to cream in color, with vertical columnar concretions, scattered irregular calcite concretions, and gypsum venulations. These sediments are in turn covered by other compact sandy silts cemented by friable carbonates or even partially covered by a layer of horizontal discoid concretions of calcitic limestone. It is common in this area to locally occur accumulations of remains of fossil bones of vertebrates and plant trunks, constituting a basal conglomerate. For this study, three vertical stratigraphic profiles were described approximately 15 meters apart and rock samples were collected in addition to fossil bones along the profiles, which were subjected to granulometric analysis and mineralogical identification with the aid of X-rays diffraction (XRD), transmitted light optical microscopy and scanning electron microscopy (SEM/SED) with semiquantitative analysis. The sediments have a silty-sandy grain size with little clay, manganese, and carbonate concretions. The main minerals identified in these sediments by XRD and optical microscopy in thin sections were quartz, smectite, kaolinite, illite, albite, microcline, muscovite, chlorite, and hematite. In fragments of fossil vertebrate bones, scanning electron microscopy and semiquantitative analyses allowed the identification of the presence of apatite as the main constituent in addition to the presence of calcite, rhodochrosite, siderite, goethite, hematite, and gypsum.