| Journal: | Revista brasileira de geofisica |
| Database: | PERIÓDICA |
| System number: | 000279678 |
| ISSN: | 0102-261X |
| Authors: | Belo, Wellington Ceccopieri1 |
| Institutions: | 1Petroleo Brasileiro S.A, Ilha do Fundao, Rio de Janeiro. Brasil |
| Year: | 2002 |
| Season: | Ene-Abr |
| Volumen: | 20 |
| Number: | 1 |
| Pages: | 17-30 |
| Country: | Brasil |
| Language: | Portugués |
| Document type: | Artículo |
| Approach: | Experimental, analítico |
| English abstract | Based on high-resolution seismic data (sub-bottom profiler 7,0kHz and side scan sonar 100kHz) 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. 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 were approached based on the observed bedforms. Such sedimentary structures are predominantly "mega ripples" located in depths around 16 m, nearby Conceição de Jacareí. There is a textural transition of the bottom sedimentary patterns from westward to eastward. Their crests are NE-SW and ESE-WNW oriented and plenty variable in the form (straight line, wavy, forked, discontinuous). Their heights ranged from 0.2 to 0.55 m with medium value around 0.35 m. Based on the bedform parameters (wavelength and height) and by using diagrams relating the medium diameter of the grains, to the depth and to the type of the structure, the speed of the flow up to 1,0 meter from the bottom (U100) was estimated. It varied from 46 cm/s (medium sands) to 34 cm/s (very fine sands) - from eastward to westward. The medium speed of the stationary flow (U), differently, varied from 54 cm/s to 64 cm/s, respectively. Four studies concerning to wave action indicated that high-energy waves usually observed in the area can contribute to the genesis of the observed bedforms being a good evidence that they could be active |
| Portuguese abstract | Com base em sísmica de alta resolução (perfilador de 7,0kHz e sonar de varredura lateral 100kHz), e amostras de sedimentos, foram investigados aspectos interessantes do fundo marinho na baía da Ilha Grande. Podem-se evidenciar efeitos combinados de ondas e correntes agindo sobre o fundo marinho, a partir dos índices de ripple (RI) e de paralelismo (PI2) de estruturas sedimentares (formas de fundo) observadas nos registros de sonar. Estruturas estas associadas a padrões sedimentares de fundo (ecofácies). Tais estruturas são predominantemente "mega ripples" localizadas em profundidades em torno de 16m, próximo à Enseada de Conceição de Jacareí onde ocorre uma transição textural de fundo de leste para oeste de um padrão de fundo grosso para um padrão fino. Suas cristas são orientadas a NE-SW e a ESE-WNW e bastante variáveis na forma (retas, onduladas, bifurcadas, descontínuas). Suas alturas variam de 0,2 a 0,55m, com valor médio em torno de 0.35m. O comprimento de onda varia de 10 a 60m. Velocidades estimadas - unidirecionais para correntes de fundo e orbitais para ação de ondas - com base em parâmetros geológicos das estruturas observadas, sugerem uma possível gradação na magnitude das velocidades (35-80cm/s) na região dos padrões sedimentares de transição de fundo. Assim, as feições observadas (faciológicas e morfodinâmicas) do fundo marinho indicam que ondas e correntes ao interagirem com a geometria da baía podem provocar oscilações no nível de energia do meio, atuando no grau de selecionamento dos sedimentos e caracter |
| Disciplines: | Geociencias |
| Keyword: | Geofísica, Sismología y vulcanología, Sísmica de alta resolución, Ecofacies, Bahía de Ilha Grande, Brasil |
| Keyword: | Earth sciences, Geophysics, Seismology and volcanology, High resolution seismic, Ecofacies, Ilha Grande bay, Brazil |
| Full text: | Texto completo (Ver HTML) |
