Análise estratigráfica de alta resolução em bacias continentais: um exemplo na formação Serra dos Lanceiros - RS

Abstract

Today, there are many researches about sequence stratigraphy applied on continental basins. However, high-resolution studies are uncommon on these basins. The definition of high-frequency cycles (4th, 5th, or 6th orders) in stratigraphy is important for the delimitations of rock’s internal heterogeneities. The implication is directly related to the quality of the reservoir and its hydrocarbon production, for example. For a high resolution analysis it is important to take into account that the stacking patterns are characterized by cyclic succession bounded by stratigraphic key surfaces (e.g. Maximum Flooding Surface). Thus, the high-resolution cycles may shown the same trend found in minor resolution analyses (e.g. 3rd order). So, this dissertation describes facies, facies associations, and progradationals and retrogradationals cycles for pre-Cambrians deposits of Serra dos Lanceiros formation, on the occidental Camaquã basin (Caçapava do Sul/RS). Based on faciologicals data acquisition of outcrops, identification, description, and stacking patterns interpretation following highresolution sequence stratigraphy basic assumptions – this study aims to correlate facies associations nature that composes the pro and retrogradationals (P & R) cycles. The results of this work comprise 100 meters of rock in vertical section, and 4 facies associations associated with different depositional environments, like Faceis Association of Prodelta, Frente Deltaica Distal, Frente Deltaica Proximal (and of Distributional Channels. The stratigraphic cycles identified were divided into 5 different orders (2nd, 3rd, 4th, 5th, and 6th orders). The 2nd order cycle comprises every succession established, representing the progression of the depositional environments identified, with the main influence of tectonic efforts related to the uplift of Caçapava do Sul High. The 3rd order cycles were traced by the main trends of coarsening-upward and fining-upward of the sections that are influenced by long-term climate changes. The 4th order cycles marks the limit between the influence of autocyclic factors and allocyclic generated by the internal dynamics of depositional environments influenced by climatic trends. 5th and 6th order cycles are autocyclic and marked by the internal tendencies of the rocky packages or internally to the packages. On this stratigraphic scale, the possible discussion is about sediment input and flow velocity, according to the precepts of Stokes' law. The depositional model is based on the comparison with depositional models proposed for river-influenced deltas, considering the peculiarities of ¬¬¬Precambrian sedimentation. The deltaic system marked the advance of the typical Precambrian interlaced fluvial system over the lacustrine system, passing through the prodelta phases, with the advancement of the delta front and stabilization of distributive channels with braided fluvial style. The data collected from this work need more data surveys in the area for the validation of these results.