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Título

FERRUGINOUS SUPERGENE HORIZONS IN THE BARREIRAS FORMATION: INSIGHTS FROM WEATHERING PROCESSES ON THE SOUTHEASTERN BRAZILIAN ATLANTIC COAST

Texto do resumo

The Barreiras Formation (Fm.) is a continental and shallow-marine sequence that extends along the Atlantic margin from Rio de Janeiro to the Amazon. In southeastern Brazil, a striking feature is the extensive ferruginization evidenced by numerous duricrust layers. This study aims to investigate the ferruginous horizons and their transformations within a sequence of the Barreiras Fm. constituted by fine to very coarse-grained sandstones with tabular geometry interbedded with mudstones, in the south of the state of Espírito Santo. Survey activities included the identification of representative profiles for stratigraphic description, petrography and sampling, with three profiles selected: two on the coastal tablelands and one on the crystalline basement. The samples were analyzed by X-ray diffraction and oriented polished thin sections were examined in transmitted and reflected light and complemented by energy-dispersive X-ray spectroscopy. Mineralogical and micromorphological results evidenced seven supergene horizons, from the bottom to top: (i) a continuous massive duricrust (MD), composed mainly of quartz grains disseminated in a ferruginous micromass, characterizing a close to single-spaced porphyric c/f related distribution, (ii) a fragmentary duricrust (FD), showing similar groundmass to the MD and iron depletion pedofeatures towards fragments edges, (iii) two layers of platy duricrust (PD) separated by a sandstone with goethite micromass, (iv) a clayey level (C) that exhibits opaque grains and muscovite disseminated in a fine undifferentiated b-fabric, (v) a sand-clayey mottled (SCM) exhibiting an open porphyric c/f distribution, hypocoatings and relict iron mottles. (vi) a nodular duricrust (ND) characterized by ferruginous nodules with dispersed quartz grains disseminated in a friable sand-clayey matrix and (vii) Oxisol (SC). Quartz, hematite, kaolinite and goethite are the main mineral phases identified in all horizons. The micromorphology of most horizons also shows zircon, epidote, biotite, muscovite and feldspar dispersed in an iron-rich aluminous micromass. In the upper part of the Barreiras Fm. occurs the nodular horizon (ND), whose ferruginous nodules gradually decrease in quantity and size towards the top, leading to the formation of a YR Oxisol. The spatial distribution and correlation between the ferruginous horizons in the three profiles indicate a ferruginization process progressing from the continent towards the coast, probably driven by climatic changes and fluctuations in the water table and sea level. Subsequent weathering processes, associated to pronounced deferruginization, led to a gradual transformation of the ferruginous horizons into well-developed pedogenetic material, suggesting in situ degradation. The evolutionary processes can be summarized as: (1) deposition of detrital sediments, (2) subsequent influx of iron-rich fluids leading to absolute ferruginization, and (3) degradation of the ferruginous horizons giving rise to the formation of pale zones and the Oxisol (SC) at the top, which is lithologically and chemically affiliated with the Barreiras Fm. This study was financed in part by the CAPES Foundation, financial code 001, the National Council for Scientific and Technological Development (CNPq) and the Federal University of Ouro Preto (UFOP/PROPPI/PPGECRN).

Palavras Chave

Absolute accumulation; Degradation; Duricrust; Barreiras Formation; Oxisol.