O fundo marinho da baía da Ilha Grande, RJ: o relevo submarino e a sedimentação no canal central

Abstract

Based on high-resolution seismic data (sub-bottom profiler 7,0 kHz and side scan sonar 100 kHz) and geological bottom samples, interesting aspects of the Ilha Grande bay seabed were investigated. The seabed along the eastern-central channel of the bay was characterized by eight bottom sedimentary patterns. Such patterns showed variations in the texture, in the bio-debris content and in the acoustic back-scattering of the sediments. Among these patterns, lithobioclastic muds are closer to the land, terrigenous muds are closer to Ilha Grande and sands are far eastward from the central channel. Sediment analyses indicated a low energy environment predominantly in the lithobioclastic muds area, subject to some random events of higher energy. It seems to influence the area by selecting the bottom sediments, besides the submarine topography. Observed evidences of modern facies transitions from sub-environments dominated by higher energy agents (waves) to facies dominated by lower energy agents (tides, bottom currents) were related to the complex submarine topography and to the coast line orientation. The boundary geometry among the ecofacies also suggests a hydrodynamic control by wave action and bottom currents. Evidences of these combined effects of waves and currents may be approached in a future study, based on the observed bedforms. The observed characteristics of the eastern-central channel seabed of the bay indicate that oscillations on the level of energy may be acting on the sediments sorting. Therefore regularity of the seabed has an important role in relation to the sediment environment on the Ilha Grande bay central channel.Keywords: high resolution seismic; sediments; bathymetry; ecofacies; Ilha Grande bay. RESUMOAspectos interessantes do relevo submarino e da sedimentação na baía da Ilha Grande foram investigados com base em sísmica de alta resolução (perfilador de 7,0kHz e sonar de varredura lateral 100 kHz), e amostras de sedimentos. O fundo marinho na região centro-leste do canal central da baía pode ser caracterizado por oito padrões sedimentares superficiais de fundo (ecofácies), que refletem variações na textura, no teor de biodetritos e no caráter acústico dos sedimentos. A disposição dos contatos entre as ecofácies sugere que existam padrões dominados por agentes de maior energia, e padrões dominados por agentes de menor energia, relacionadas à topografia submarina e à orientação da linha de costa. Ecofácies areno-lamosas (tipos transitórios) estão associadas a estruturas sedimentares (formas de fundo) obtidas nos registros de sonar. Tais estruturas estão localizadas em profundidades em torno de 16m, próximo a Conceição de Jacareí, onde ocorre uma transição textural de leste para oeste de um padrão de fundo grosso para um padrão fino. É possível que a ação de forçantes oceanográficos sobre as ecofácies, contribuam para o entendimento da sedimentação na baía. Assim, as feições observadas (faciológicas e morfodinâmicas) do fundo marinho, sugerem que oscilações no nível de energia do meio, p...

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