Caracterização sedimentológica e estratigráfica em rochas microclásticas do Devoniano da Bacia do Paraná

Abstract

The Devonian microclastic rocks of the Paraná Basin (Ponta Grossa Formation) are well known from a geochemical, organofaciological and paleontological point of view. However, due to the difficulty in observing their particles and structures and, consequently, the need to use more advanced analysis techniques, interferences are, in general, poorly integrated from a sedimentological point of view, especially if positioned in a sequential stratigraphic framework. Based on this, the present objective study of sedimentological (on macro and microscopic scales) and high-resolution stratigraphic characterization of a 3rd order transgressive-regressive (T-R) sequence identified in the drilling core of well 2-TB-1-PR, acquired on the eastern edge of the Paraná Basin. This sequence extends between ~170 and 210 m depth of the core, and is formed by thick layers of mudstones, rocks with lenticular and flaker heterolithic bedding and medium to coarse siltstones. Considering the textural aspects and the bioturbation index, seven sedimentary facies were defined (Al, Ar, He, Heb, Hf, Sb and Sib), which, based on observations in thin section, were developed into seven microfacies (A1, A2, A3, H1, H2, S1 and S2). The facies and microfacies indicated position in a shallow marine, shelf environment, under constant action of storm waves. The laminated and striped-bedded mudstones of the Al and Ar facies depict the lowest energy deposition of the proof succession, deposited under higher water depths, indicating that even below the base level of storm wave action, sedimentation occurs dynamically with constant input of sludge from the remobilization of sediments from the internal to the external platform in high-energy events. On the other hand, the sediments of the Sb and Sib facies represent the highest energy deposition, establishing that closer conditions, above the base level of normal wave action, are not reached and deposition is concentrated in the transition region between the shoreface and offshore. The vertical relationship and facies stacking pattern allowed the identification of five facies successions (SF-A, SF-B, SF-C, SF-D and SF-E), closely related to the 3rd order stratigraphic framework. SF-A occurs exclusively at the base of the study interval, marking the beginning of the transgression. While SF B, SF-C, and SF-D intersperse along the transgressive systems tract (TST) and the regressive systems tract (TSR), similar depositional contexts occur repeatedly, controlled by sea level oscillation. The highest water depth conditions are reached in SF D, where the Al and Ar facies predominate. SF-E, on the other hand, occurs at the top of the study interval, marking the highest energy deposition conditions, which occurs in a regressive context. The facies successions indicate a high-frequency cyclicity, internal to the 3rd order T-R sequence, which is highlighted by the behavior of the gamma-ray curve, used in this case study to map 4th order T-R cycles. These bring even greater detail to the study interval and allow the identification of complete cycles associated with a single facies (e.g. Air), highlighting the importance of integrated studies between facies and microfacies for a better understanding of essentially clayey successions, something fundamental to improve understanding of the evolution of underlying mud systems and the variables of these rocks can be presented in the geological record.