Gênese e distribuição das silicificações nos paleossolos e rochas sedimentares aflorantes em Manaus

Abstract

In Manaus region outcrop sedimentary units Novo Remanso (Miocene Average) and Alter do Chão (Cretaceous / Paleocene - Miocene). In the latter occur bodies silicified, currently called Sandstone Manaus. The few studies that They addressed the origin and distribution of these bodies are based only on the petrographic description, without recognizing soil and / or silcretes microstructures or systematic survey samples. In order to understand the genesis, the processes involved, the stages diagenetic and distribution of outcropping silicified bodies on the outskirts of Manaus, this He characterized his work macromorphology, micromorphology, petrography and chemistry. Were found 13 outcrops, all in riverbanks, with lateral extension ranging between 98 to 928 m, and a thickness of up to 3.0 m. nine sedimentary facies were characterized in study sites, associated with a braided fluvial system: Cm, Aa, Ap, At, Al, Am, Afb, Pm and Pb. The silicificações range from one to four facies in outcrop, and were not only seen on facies At. Feature pedogenetic present in facies Afb and Pb confirms multiple sequences paleosols developed in seasonal climates in humid periods and dried. vertisols and ultisols were recognized and may be the last stratigraphically a layer tab at the local geology. The composition of major oxides, determined by fluorescence X-rays, it exhibits the predominance of silicified lithotipes oxides SiO2, Al2O3 and Fe2O3, and a lesser proportion of P2O5 and TiO2, and those not silicified low SiO2 content and high Al2O3 and TiO, and the last major oxide strongly weathered samples. Samples with microstructures and silicification SiO2 concentrations> 80% were denominated silcretes. The chemical characterization found agrees with the mineralogy determined by X-ray diffraction, consisting of quartz, kaolinite, haloysita, hematite, ferrihidrita, goethite, anatase and Vivianita. Petrographically, the lithologies are classified as quartzoarenitos and sublitoarenitos, medium to fine, some silty to conglomeratic, presenting also paleosols. Grain subangular to sub, often with floating contacts, point and longitudinal rare subordinates confirm the open packaging indices. Porosity mainly móldica, intergranular and intragranular. composition mainly Quartz, organic matter, some opaque, rare skeletal feldspars and traces of muscovite, biotite, rutile and zircon, and igneous and metamorphic quartz as fragments of rock. It is indicated arcosiana initial nature in these sandstones, now mature and submaturos. diagenetic processes of incipient mechanical compression have been identified, dissolving feldspar and quartz, kaolinite autigênese, opal and iron oxides, and the first the main cement of sandstones, hydration-dehydration, pedogenesis, and neomorfismo from amorphous to Chalcedonian opal and microcrystalline quartz. THE silicification has several species of silica: opal, chalcedony and microcrystalline quartz. two diagenetic stages were identified: shallow eodiagênese and telodiagênese with absence of mesodiagenesis. These would be developed in surface drainage lines and / or the swing area of ​​the water level of shallow aquifers under the influence of seasonal climate wet and dry, with availability of organic matter. At least two events silicification have been associated with one or both diagenetic stages. occurrences silicificações of the subsurface area described in drilling wells may be same nature from those observed in outcrops, and later moved neotectonics by vertically, or it may have been caused by fluctuation of the water level groundwater. Because of the high occurrence of paleosols outcropping in the region is proposed the Geossolo Manaus term for this set of paleosols