Evolução sedimentar recente ao longo de uma plataforma continental contrastante de sedimentação
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Data
2017-04-05
Autores
Silva, Alex Evaristo da
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Universidade Federal do Espírito Santo
Resumo
The present thesis is study about the sedimentary and morphological evolution in continental shelf section with distinct influence between sedimentary input and dispersion. The north-central continental shelf of Espírito Santo (Brazil) has contrasting features such as the presence of delta lobe and incised valley with exposed channels, as well as the mixed sedimentary environment marked by terrigenous dominance in the coastal areas and by rhodoliths and carbonate gravels in offshore areas. Were analyzed 186 surface sediment samples from published works, 6 piston corer that are deployed in 730 samples, and 485 km and 920 km of seismic lines boomer and stratabox source respectively. Sediments surface analysis showed the presence of distinct sedimentary regimes resulting from current and past hydrodynamic processes. Distal mud belt suggests high energy events as responsible for resuspension and redistribution of fine sediments. The surface heterogeneity is also recognized in subsurface sediments, where six sedimentary environments were recognized: fluvio-estuarine with continental dominance; inner estuarine; beach or river mouth; marine continental shelf, terrigenous shelf and prodelta. Transgressive ravinement and maximum flooding stratigraphic surfaces were observed throughout the piston corer. In terms of seismic stratigraphy, eight seismic units were identified. The pattern distribution of these units condition two distinct evolutionary models for the study area. The sedimentary evolution in the north indicates the construction of carbonate environment during MIS3, high development of transgressive fluvial deposits and presence of Doce River regressive lobe. In the south, incised valleys domain were observed, with decrease in their filling towards the basins, which suggests two distinct moments of sea level rise. The low development of the seismic units around the incised valleys suggests limited sedimentation during transgression, restricted to fluvial channel. After continental shelf flooding, carbonate sedimentation dominated the external shelf, with greater development to the south. Difference between north and south was probably conditioned by continental and continental shelf physiography and by variation of past and present sedimentary input. The Doce submerged delta presented an elongated geometry similar to main fluvial systems of the word, which indicates significant longitudinal sedimentary transport and fine sedimentary composition, corroborated by elongated orientation to the north of the sedimentary plume, and by sand-muddy submerged delta composition. Two depocenters and two seismic subunits were observed near river mouth and associated with shift of the channel. Delta sub- merge delimitation is important to scale the potential impact area of the tailings mud that reached the mouth of the Doce River on November 22 (2015) and spread over a wide area of the continental shelf. Geological faults were recognized on the continental shelf, mainly along 42 m isobaths with the same SSW-NNE orientation of lineaments in the continent. The distribution of physiographic elements, such as lobed features, high and low topographic areas, channels, escarpments and erosive grooves show distinction of the sedimentary processes along the continental shelf. Lobed features were found in the limits of the inner-medium and medium-outer continental shelf, which suggests significant sedimentary contribution during the last transgression. High topographic and escarpment were interpreted as possible paleo-coastline in the middle shelf, while distinction at height of the topographic highs on the outer shelf indicates greater development of carbonate constructions to the north. The topographic highs associated with incised valleys in the south were interpreted as reef growing along their margins. The results indicate that distribution and preservation of sedimentary environments installed on the continental shelf during Upper Pleistocene-Holocene were mainly conditioned by fluvial input, either as a direct contribution of terrigenous sediments or by inhibition in the establishment of carbonate communities. The continental geomorphology also seems to be an important indicator of the characteristics found in the continental shelf.