Paleoambiente e quimioestratigrafia da Formação Serra do Quilombo, Neoproterozóico da Faixa Paraguai Norte, regiões de Cáceres e Nobres (MT)

Abstract

After the last criogenian glaciation (ca. 635 Ma), large carbonate platforms were developed in several cratonic regions of the Earth and, despite of the intense dolomitization of these deposits, many of paleoceanography and sedimentary processes can still be rescued, contributing to the understanding of the paleoenvironment and the isotopic geochemistry of the Neoproterozoic seawater. One of the most important testimonies of this period in Brazil is the Serra do Quilombo Formation, upper portion of the Araras Group, exposed in the northern Paraguay Belt, southern of the Amazonian Craton. This unit has been interpreted in its type section in the Caceres region, Mato Grosso, as deposits of shallow to moderately deep platform influenced by storms and earthquakes. The stratrigraphic and facies revaluation in combination with C, O and Sr isotopes data in the Caceres (type-section) and Nobres regions, allowed enlarging the paleoceanography and paleoenvironmental interpretations, aiming at building a chemostratigraphic framework for the unit. The 140 m thick of the study succession includes the Serra do Quilombo Formation in sharp basal contact with the Guia Formation and the gradual contact, to the top, with the Nobres Formation. In this succession were described five depositional facies that correspond the organic matter rich laminated dolostone (Dl), massive to laminated dolostone (Dml), hummocky/swaley to planar stratified sandy dolostone (DAh), sandy/oolitic dolostone with wave cross laminations (DAl) and matrix-supported dolomitic breccia (BDm). The sedimentary facies were grouped into two facies associations (FA) that indicate environments of deep to moderately shallow carbonate platform (FA1: Dl and Dml) and storm influenced shoreface (FA2: DAh, DAl and BDm). The Serra do Quilombo Formation comprises a shallowing upward succession, which represents the progradational record of a highstand system tract, in a homoclinal carbonate ramp, installed on the southern part of the Amazonian Craton during the Ediacaran. C and O isotopic analyses were performed in 141 samples (among laminated limestones and dolostones, sandy/oolitic dolostones and clasts, cement and matrix of dolomitic breccias), of which 6 were also selected for the Sr analysis. Major (Ca, Mg and Fe) and trace (Rb, Sr and Mn) elements contents, performed on 20 samples, assisted in the selection of those for Sr isotopic analysis and evaluation of the primary nature of the isotopic signal of study carbonates. The δ13C values, interpreted as representing the original sea water, showed a trend ranging from negative, around -2 ‰ at the base of the succession (Guia Formation) to above 0‰ at the top of the succession. The type-section of the Serra do Quilombo Formation presents a homogeneous C-isotope curve, with δ13C values around 0 ‰, which allows its perfect correlation with other sequences, for example, its section reference in Nobres, far about 200 km. The 87Sr/86Sr follow the general C-isotopes trend, increasing towards the top of the succession, ranging from 0.7077 to 0.7083, directly related to the environmental shallowing and the consequent influx of siliciclastic grains recorded in the top portion of the unit (AF2). The δ13C and δ18O data obtained for the dolomitic matrix-supported breccias (BDm) indicate, in general, a similarity between the isotopic signal of both matrix and clasts, suggesting a sindeposicional and primary process. On the other hand, for the dolomitic cemented breccia, the values obtained for the sparry dolomite cements indicate lower values of δ13C and, notably, of δ18O in relation to the clasts, suggesting that its origin is secondary, probably resulting from the interaction with meteoric water or hydrothermal fluids during the breccias formation. The establishment of a moderately deep to shallow carbonate platform environment with C isotopic patterns, tending to positive values, is clearly distinct of the postglacial deposits of the base of the Araras Group, with strongly negative values of C, and may represent the restoration of normal sedimentation conditions. Likewise, the growing trend of Sr isotopic ratio indicates the increasing in the influx continental in a scenario completely devoid of glacial influence (Snowball/slushball Earth hypothesis). Sr values indicate ediacaran age, between 560 and 580 Ma for the Serra do Quilombo Formation deposition.