Print this pageAnalysis of Recent Microbialites from the Coastal Lagoons of Rio De Janeiro State (Brazil) from Biosedimentological and Geomicrobiological Perspectives
Microbialites are organosedimentary deposits formed by the activity of cyanobacteria, which involve algal mats, stromatolites, thrombolites and oncoids/oncolites. They’ve been part of the geological records since the Archean until present day. The study of microbialites can focused either in the interaction between the microbial organisms and their environment, particularly the geochemical mobility of elements and mineral formation (Geomicrobiology) or in the sedimentary particles and rocks derived from the microbial activity (Biosedimentology, a Comparative Sedimentology branch). Also, their study addresses geological problems like the origin and evolution of early life, the origin of expressive iron formations, and even the search for indications of extraterrestrial life. In respect their economic significance, their potential as reservoir rocks can be highlighted by the recent giant hyrocarbon discoveries in the Aptian “Pre-salt” of Santos basin and present-day analogs for the reservoirs of this new exploratory frontier are particularly scarce. Insofar, recent microbialites can be seen in series of hypersaline coastal lagoons in Rio de Janeiro State (Brazil), at the onshore edge of Santos and Campos basins, which may present themselves as potential analogs for the pre-salt discoveries. These microbialite occurrences show a complete evolutionary spectrum from biofilms to algal mats to a stromatolite-thrombolite-oncoidal plex (STOP). Algal mats vary morphologically (bubble, film, gelly, smooth, polygonal, pustular and tuffed), and contain Ca-carbonate grains (clots, peloids, micro-oncoids/spherulites etc.); whereas the STOP shows a peculiar microstratigraphy, characterized by an association of truly stromatolitic (convex to crenulated, thin laminae), thrombolitic (micritic coagules and peloids, dissolution cavities and borings) and micro-oncolitic microfacies. Dissolution (micro-caves) and bioerosion (serpulid and vermitid borings) are responsible for the enhanced STOP porosity. Terrigenous clastics (micro- and macroclastics) and allochemical sediments (foraminifera, ostracods and microgastropods) are also present in varying proportion in some of the microfacies. Tides, wind regime, rainy seasons, freatic level, water chemistry and their own microbiocoenosis are the controlling factors analyzed in the studied lagoons.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil