Arquitetura faciológica e petrografia de depósitos de marés Proterozóicos

Abstract

The Morro do Chapéu Formation (Espinhaço Supergroup) is in the central portion of the São Francisco Craton, covering a large area of the Chapada Diamantina, Bahia. Composed of basal conglomerates overlain by fine sandstones, graded to intercalated sandstones and siltstones, interpreted as a wave-dominated platform system in which for a given moment has the action of the tides increased due to morphological changes. The response of the gravitational pull generated by the Earth-Moon-Sun system on the oceans, coupled with the rotation of the Earth, results in the phenomena known as tides. Tidal sedimentary deposits record the periodic change in the speed and direction of currents between high and low tides, producing characteristic sedimentary structures. The present study of the tide-dominated sedimentary deposits of the FMC has as its main object the characterization of the processes originating from these currents, aiming to describe the dynamics of the sedimentary processes of the depositional system and to understand the variation of the tides during the Neoproterozoic. For this purpose, six Virtual Outcrop Models - three panels and three halfblocks with two faces - and petrographic analyses of five thin sheets were performed. Thus, characterizing and classifying the bedforms originated from tidal processes as a Composite Dune Complex, with NW predominant paleocurrents, and architectural elements reflecting a frontal accretion, showing a migration towards the basin depocenter. Due to the superposition arrangement of the architectural elements - composite macroforms, simple macroforms, accretionary sand dunes and erosional sand dunes - it is possible to observe a decrease in energy, sedimentation rate, accommodation and erosion decreasing in the direction of the ebb tide. Considering the similar sizes of Proterozoic bedforms to those of today, but tidal currents have similar velocities to those of today, however, several authors propose models where the orbit of the Earth-Moon system has been moving away over time. The Quartz-sandstones of the FMC present high textural and mineralogical maturity, with quartz cementation has as a source of silica the release of this through pressure dissolution, this process also generates stylolites.