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Título

THE THIN AND WIDESPREAD CARBONATE BRECCIAS OF THE IRATI FORMATION (PARANÁ BASIN): SUBAERIAL EXPOSURE OR BURIAL DISSOLUTION?

Texto do resumo

A thin and widespread Lower Permian carbonate breccia horizon of the Irati Formation (Paraná Basin) is usually described as the lithostratigraphic boundary between the overlying Assistência and the underlying Taquaral Member. The breccias are the first carbonate layers directly overlying the 13 m-thick Taquaral Shale. The breccias are found throughout the Paraná Basin over a length of more than 1,100 km. They are the most prominent features underlying the well-known anoxic and mixed siliciclastic-carbonate succession of the Assistência Member, which develops in a homoclinal ramp. The breccias have been described as tepee structures that occur in association with evaporitic minerals (anhydrite and gypsum). The previous authors interpreted the breccia formation in a hypersaline environment with periodic subaerial exposure and evaporite-displacing intrasediment growth. However, a detailed sedimentological and petrographic description of several drill cores revealed well-preserved pyrite framboids in the matrix of the breccia and siliciclastic matrix of black shale of the above beds. The presence of these controversial sedimentary features motivates this work, the primary aim of which is to better describe the breccia beds and propose a formation model. The cores described in this work are from the central part of the Paraná Basin. They are crackle breccias, matrix-rich mosaic breccias, clast-supported and matrix-supported chaotic breccias. The carbonate breccias are composed of polymict and monomict dolostone clasts. They are interconnected and range from chaotic, mosaic to crackle breccias with a progressive variation of fitting framework which gradually transitions to non-brecciated carbonate at the top and base. Crackle breccias are well-fitted clasts with no evidence of significant displacement and rotation, cemented by calcite, dolomite and rarely pyrite. Mosaic breccias show a high degree of dislodgment and rotation. They are angular to semicircular, mostly broken rock fragments. Mechanical seepage of siliciclastic or marl matrix within the fractures is common. Clast- supported chaotic breccias have a preferential orientation conformable to the bedding and exhibit soft-sedimentary deformation. Matrix-supported chaotic breccias have elongated clasts or with irregular shape and black to brown siliciclastic matrix predominate. The overall matrix observed in the breccias is (i) dolo- to peloidal micrite, (ii) sandy to silty siliciclastic (terrigenous infill) and (iii) a varied mixture forming marls, from clayey to carbonate-rich. The above evidence points to post-depositional dissolution, possibly related to rising fluid under overburden pressure. The underlying 13-m thick shale of the Taquaral Member acts as a geochemical barrier, concentrating fluids and dissolution in the overlying breccia bed. Nevertheless, subaerial exposure cannot yet be ruled out. There is a possibility that post-depositional brecciation was triggered by initial subaerial dissolution. Previously δ13C and δ18O analyzes are inconclusive. Further data are required to better define the origin of the breccias.

Palavras Chave

mixed siliciclastic–carbonate sedimentary system; Dissolution breccia; Lower Permian; Dissolution-collapse breccia; post-depositional brecciation