Sedimentary dynamics of a subtropical tidal flat sheltered inside a coastal channel (Araçá Bay, SE Brazil)

Abstract

The sedimentary patterns of Araçá Bay and the adjacent sector of the São Sebastião Channel (SSC) were determined by grain size parameters and carbonate content. The inner bay contains two intertidal flat facies, with very fine to coarse sand and very coarse silt respectively. The outer bay is a subtidal flat facies with unimodal very well sorted and very fine sand. In the adjacent area of the SSC, the central and northern sectors correspond to a sublittoral muddy facies, composed of poorly to very poorly sorted and very coarse to medium silt, and the southern sector corresponds to a sublittoral sandy facies, with very poorly sorted and very fine to coarse sand. The source of sand and very coarse silt are relict terrigenous deposits; coarse silt and medium silt are supplied to the inner bay and the SSC by the intense summer rainfall; and the input of fine and very fine silt is related to suspended sediment from the inner shelf due to storm waves coming from the south. In contrast to seasonal grain size variations in the SSC and the intertidal flat, grain size of the subtidal flat is homogeneous throughout the whole year, denoting an intense sediment transport by local waves and tidal currents. In summary, Araçá Bay is a particular case of sheltered tidal flats, controlled by tides and waves with a minor fluvial influence, but mainly composed of terrigenous sediments. According to this sedimentary characterization, the sensitivity of Araçá Bay to oil spills is reclassified. The practical absence of fine sediments in the subtidal flat determines its low potential to accumulate pollutants, while the intertidal flat and the adjacent SSC present a higher potential. Consequently, this study shows that sensitivity to oil spills of sandy tidal flats and muddy tidal flats must be classified separately.