Guilherme Pederneiras Raja Gabaglia

Microbialitos gerados em ambiente deposicional lacustre apresentam grande diversidade de estruturas e texturas internas. A organização e a caracterização do arranjo formado por esses atributos correspondem ao que se denomina intrabioarquitetura. Este trabalho tem o objetivo de identificar os principais tipos de intrabioarquiteturas nos microbialitos da Sequência Balbuena IV (Daniano, Bacia de Salta, Argentina) em escala mesoscópica (100 – 10-2 m) e a interpretação cíclica da recorrência dos componentes que formam estas intrabioarquiteturas, em escala microscópica. Foram reconhecidos sete padrões intrabioarquiteturais, assim denominados: Microbialito Aglutinante de grão grosso (MA-gg); Microbialito Bandeado Fasciculado/Aglutinante de grão fino (MBFA-gf); Microbialito Pseudo-microcolunar Aglutinante de micrita (MPMA-m); Microbialito Arbustiforme Microgrumoso Aglutinante de grão fino (MArbMA-gf); Microbialito Dendriforme Microgrumoso Aglutinante de grão fino (MDMA-gf); Microbialito Bandeado Fasciculado/Aglutinante de grão fino com esferulito (MBFA-gf,esf) e Estromatólito Microgrumoso/Aglutinante de grão grosso (EMA-gg). Essas intrabioarquiteturas foram agrupadas e relacionadas a três tipos de microciclos deposicionais. Os microciclos possibilitam organizar o registro microbial em escala meso- e microscópica. Essa organização cíclica está relacionada às variações climáticas, que afetam o nível de base, o aporte sedimentar e os processos biológicos e geoquímicos do sistema lacustre. Portanto, foi possível associar a formação dos microbialitos a períodos de maior ou de menor aridez no ambiente deposicional.

Uppermost Jurassic limestones of the South‐East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave‐induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave‐cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave‐induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present‐day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair‐weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100‐m‐thick wedge, which prograded over 3 km and was built up by the stacking of 5‐ to 20‐m‐thick cross‐stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high‐velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel‐like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave‐induced currents was quasi‐permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft‐sediment deformed on a hurricane‐dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.