Estudo diagenético aplicado ao Membro Maruim da Formação Riachuelo , na parte terrestre da sub-bacia de Sergipe, Brasil

Abstract

The Maruim Member of the Riachuelo Formation (Neoalbian), in the terrestrial part of the Sergipe Sub-basin, contains shallow water facies composed mainly of rudstone/grainstone with oncolites and oolites, characterized by the low content and variety of bioclasts. The correlation of the outcrops and the integration of the petrographic, cathodoluminescence, SEM and geochemical (elemental and isotopic study) analyses allowed the reconstruction of the diagenetic history of the studied interval. Carbonates of the Maruim Member are completely affected by diagenetic processes of the eogenetic, mesogenetic and telogenetic phases. The dolomitization was one of the main diagenetic products of the eogenetic phase and it replaces all or part of the limestones of the Maruim Member. The dolomitization is concentrated at the top of the depositional cycles described in the study area and it gradually decreases towards their base. The relationships between porosity and dolomitization were studied with basis on the comparisons of the crystalline dolomite fabric through the studied outcrops. The isotopic results of the dolomites indicate that the dolomitization process occurred from the reflux of brines in a slightly hypersaline environment (penesaline environment). The areas closest to the contact with the brine, source of the dolomitizing fluids, exhibit lower porosity development because there would have occurred processes of super dolomitization (Carapeba Quarry). In these areas, the carbon and oxygen isotopic signature is very positive (δ13C between 2.37 and 4.83 , δ18O between 0.61 and 3.92 ), indicating that the late diagenetic processes would not have altered much the original isotopic signal. The dolomites generated in the areas farthest from the brine source of the dolomitizing fluids (Massapé, Inorcal I, Inorcal II, Inhumas and San Antonio Quarries), exhibit a greater development of porosity and have an isotopic composition of carbon and oxygen of more negative values (δ13C between -5.66 and 2.61 , δ18O between -4.25 and 0.38 ). Moreover, in these quarries the isotopic signature of the dolomites is also altered by processes of dedolomitization. Diagenetic cements precipitated during the mesogenetic phase was responsible for the obliteration of the primary and secondary porosity of the Maruim Member limestones. Further, the late diagenetic cements calcitized the dolomite and partially closed its secondary porosity. The porosity of the carbonate rocks is also greatly reduced by mechanical and chemical compaction. Dissolution was the only process that led to the generation of secondary porosity in the telogenetic stage, although in very low proportions. The dolomitic facies are those that present greater development of secondary porosity as a result of the dissolution processes in the telogenetic environment. The dissolution comprises one of the last diagenetic events identified in the studied interval.