Geomorfologia, sedimentação e processos atuantes na Lagoa de Maricá, Rio de Janeiro

Abstract

The Maricá Lagoon in the city of Maricá, Rio de Janeiro State, belongs to a barrier-lagoon system formed in the Pleistocene. The geology of this lagoon is poorly known, so, this paper focus on the characterization of its morphology and sedimentation, as well on the understanding of the lagoon dynamics. For that, bathymetric data and 72 samples from the lagoon margin and bottom were acquired. Several analyses were conducted, such as grain size, particle morphology (roundness and surface textures), composition and organic matter content. The results show that the bottom of the Maricá Lagoon is mainly flat reaching maximum depth of 2 meters at its central portion. From the water line, depth gradually increases differently at the northern and southern margins: the northern margin shows a smooth profile gently inclined towards the bottom as the southern margin is steeper. In the northern margin, the 1,5 meter depth (where sand changes to muddy sands) is reached between the distance of 320 to 720 meters from the water line, as in the southern margin, this same depth is reached at about 160 to 300 meters. The lagoon bottom is composed basically by sandy silt and silty sand, mostly deposited by rivers. At the northeast site, an accumulation of silty sand (plus clay) is forming a delta by the Mumbuca River. The northern lagoon margin is formed by poorly sorted gravelly sand and sandy gravel derived from the erosion of Precambrian outcrops as well as from urbanization. At the sea side, the margin is formed by moderately well sorted coarse to medium sands similar to the barrier's sands. In general, the sands are quartz rich with minor amounts of feldspars, micas, heavy minerals, in addition to shells and shell fragments. There is a striking difference in the roundness between the sands of the northern margin, which are angular, and the well rounded sands of the southern margin. Storm winds and overwash processes have transported sands from the barrier to the lagoon throughout the time. The winds also generate waves capable of reworking lagoon margin and bottom sediments. Fluvial input, strong winds and waves are the main processes responsible for the lagoonal sedimentary dynamics, as well as give rise to the distribution of the various sediments types observed in the Maricá Lagoon.