Estudo das relações entre o arcabouço estratigráfico e as alterações diagenéticas observadas na seção devoniana da Bacia do Paraná

Abstract

This work aimed to discuss the relationship between the stratigraphic framework and the diagenetic evolution observed in the Devonian Section of the Paraná Basin based on the concept of sequence stratigraphy. The present study made use of data from samples obtained from six boreholes located on the southeast and on the north regions of the Basin. Five boreholes located on the southeast region, in Tibagi and Ponta Grossa counties in Paraná State are positioned in the Praguian/Ensian interval (Gaugris and Grahn, 2006). One borehole located on the north region in Dom Aquino County in Mato Grosso State is positioned in the Givetian/ Frasnian interval (Mendlowicz Mauller, 2008). The diagenetic evolution of the studied sandstones was inferred through petrographic analyses, X-rays and sweep electronic microscopy studies. The different conditions in which sediments were deposited, led to the investigation of the depositional variations that originated the distinct sedimentary facies observed in the Early/Late Devonian Section of the Paraná Basin. In this context, the role and distribution of the diagenetic constituents as berthierine and parautochthonous glauconite in the stratigraphic framework are of great importance. The peculiar factors responsible for the occurrence of these two minerals point out to important environmental inferences. Berthierine is related to environments near deltas and estuaries, whereas glauconite is related to deep water environments. Based on these characteristics, diagenetic units were proposed and their distribution was apparently controlled by the stratigraphic framework. Eodiagenetic constituents whose origin is closely connected to the environmental changes caused by the relative variation of the sea level were used in this study. It was possible to correlate eodiagenetic minerals that are usually formed in transgressive conditions. The study of the mineral assembly and the diagenetic evolution indicated that the shallow- marine environments, registered in the Devonian Section of the Paraná Basin, favored conditions for the organic matter decomposition and concomitantly the occurrence of oxireduction reactions of metal oxides, mainly iron oxides. These shallow-marine environments, so frequent in a Transgressive System Tract, would have produced a characteristic mineral assembly constituted of siderite, pyrite, glauconite and berthierine observed in the studied sandstones. So, the study of the Devonian sandstones of the Paraná Basin enabled an efficient correlation between the key surfaces and the system tracts identified for the Devonian third and fourth order sequences and the diagenetic evolution.