Microbial Mat Microbiofacies Analysis of the Pitanguinha Lagoon (Região dos Lagos, RJ, Brazil)

The term ‘microbiofacies’ has most commonly a petrographic connotation for faciologic studies of carbonate rocks with plenty of fossil grains in microscale, for what it might be more appropriate the term ‘biomicrofácies’; however this term is used here with a connotation of a biofacies of microbial nature. The occurrence of microbialites in several lagoons in the southeastern coast of Rio de Janeiro State (namely Região dos Lagos) has been revealing extremely important for the study of carbonate processes and facies of microbial origin, which gained evidence after the discovery of the “Presalt” petroleum province in Brazil, of this putative nature. Among these lagoons, the Lagoa Pitanguinha (here under study) present not only microbial mats containing carbonate particles, but also stromatolites, thrombolites and oncoids. This study characterises the microbial mats (microbialites) from a biosedimentological perspective, in terms of their morphologies and textures, in the field, and in terms of their structures and composition, by microscopy (petrographic microscope), in order to establish a microbiofacies classification scheme. The Lagoa Pitanguinha (lagoon) was formed during the Holocene, as a result of a marine regression, that isolated it from the sea by a set narrow strandplains, and developed hypersalinity conditions in its waters due to the regional climatic aridity and the invasion of marine salt wedge. In this context, four microbiofacies were possible to be identifed (MBF-C, Coloform; MBF-Po, Polygonal, MBF-Pu, Pustular; MBF-O, Oncoidal) during field observations in August 2014 – under a high level of the lagoon conditions (rainy year), but hypersaline – and in January 2015 – under conditions of great aridity and greater hypersalinity. The microbiofacies show to be peculiarly controlled by increasing hypersalinity, as lagoonal water level drops, gradually exposing their margins during drought periods. Dehydration of some of the microbial mats in subaqueous environment which is been hypersalinised (synaeresis) seems to be a precursor mechanism of the cracking observed in many of the mats, which may be enhanced or remodeled by the subsequent exposure and subaerial dehydration.