Sucessão estratigráfica e paleoambientes dos microbialitos proterozóicos do membro sumidouro, formação lagamar, grupo Vazante, na fazenda Sumaré, em Lagamar - MG

Microbialites (irregular agglutinated grains, laterally continuous mats and stromatolites) occur in small, nearly continuous outcrops over a ~60 m-thick carbonate interval of the Sumidouro Member, Lagamar Formation, Vazante Group, Meso-Neoproterozoic, on the Sumaré Farm, in Lagamar (MG, Southeastern Brazil). Diversified stromatolites formed under shallow, high energy conditions predominate and exhibit frequent lateral and vertical changes, including probable bioherm borders. In the lower part of the interval, coniform columnar stromatolites (Conophyton), representative of the deepest and/or calmest settings, are common. Higher up, narrow subcylindrical unbranched forms become abundant and may grade to forms with subparallel dichotomous or multiple divergent branches. The microbialites are apparently organized in shallowing upward cycles

Microbialitos da formação Codó (Aptiano Superior) às margens do rio Tocantins em São Miguel do Tocantins (TO)

The southwestern region of the São Luís-Grajaú Basin has a rare outcrop of the Codó Formation (upper Aptian) with seven outstanding microbialite bioherms along the left margin of the Tocantins river, near Imperatriz (MA). Resting on sandstones of the Grajaú Formation, the Codó Formation presents: 1) a 20 cm thick basal calcilutite with gypsite pseudomorphs and some fossil tree stems; 2) metric dark shales with carbonate nodules and thin intercalated carbonate layers, enclosing some microbial laminites; 3) a 2 cm thick upper breccia composed of microbialite fragments and other carbonate clasts, with halite hoppers on the top; 4) the carbonate bioherms, which partially overlie the extensive shales and interrupt them laterally, as well as the breccia. The bioherms in the northern part of the outcrop are thicker (<2 m) and have interbedded dark shales, whereas the southern are thinner and continuous in the vertical direction. In general, they are composed of irregular gently to strongly wavy microbial laminites, sometimes with pseudocolumnar to conical lamination. All microbialites with highest synoptic relief (<20 cm) look like columnar stromatolites on weathered lateral expositions. In plan view, the horizontal sections of these microbialites are circular to slightly elliptic, sometimes forming very small channels (N60W) filled with fine breccia. The highest bed of the northern bioherm has mixed microbial laminites and columnar stromatolites, where intercolumnar spaces were filled with microbialite clasts, fish bones, plant fragments and very small probable crustacean coprolites. Several fractures and deformation in this upper bed indicate an initial brecciation process probably caused by subaerial exposure. In microscopic scale, the lamination is smooth, diffuse, defined by subtle granulation differences of very fine granular calcite crystals within micrite, but oxide levels, dissolution surfaces or thin precipitated calcite veneers...

Estratigrafia do Andar Alagoas na sub-bacia do Tucano Norte, Bahia

Pós-graduação em Geociências e Meio Ambiente - IGCE

Estratigrafia do Andar Alagoas na sub-bacia do Tucano Norte, Bahia

Pós-graduação em Geociências e Meio Ambiente - IGCE