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Title: Geologia, geomorfologia e sismoestratigrafia rasa da plataforma continental ao largo da Laguna de Araruama RJ
Keywords: Geologia Física;  Geologia Marinha;  Geomorfologia;  Laguna de Araruama (RJ)
Issue Date: 1-Oct-2004
Abstract: This work involved analysis of surface sediments the morphology of bottom and subbottom as well as the structuring of the sediments of the continental shelf adjacent to the Lagoon of Araruama (RJ). The results allowed the geological characterization of the area and the elaboration of an evolutionary model starting on the Upper Miocene-Pliocene (?). The used batimetrics data were obtained from nautical charts of the DHN; the sedimentological data from the DHN(BNDO) and PGGM banks, and the seismic profiles were collected during the GEOMAR XVI (1980), CENTRATLAN I (1981), GEOMAR XX (1982) surveys, and the new seismic data were collected by the cruise Diadorim (2003). The interpretation of the seismic profiles allowed the limit of the acoustic basement at about 115 m isobaths, and also the reccognition of the sedimentary thickness, was identified ranging from 0 m at the NW to more than 200 m at the SE of the studied area. The different morphologies of the acoustic basement and the similarity to the exposed land topography suggests that it is constituted by crystalline and of volcanic rocks, similar to the ones found in the island of Cabo Frio. Four sedimentary sequences (I, II, III and IV) were identified limited by four unconformities (DI, DII, DIII and DIV). The ages of the main geological events identified in this work were estimated through correlation among Azevedo Jr. (1991) and Silva (1992): The Sequence ( I ) would be Miocenic and the unconformity (DII) could correspond to Gray mark, dated as Mio-Pliocene. Sequence ( II ) would have been formed in the Neo-Pliocene, while (DIII) could correspond to the mark P , with 500.000 years old. The Sequence ( III ) is related to Pleistocene and the unconformity (DIV) would be related to the regressive event that preceded the 123.000 year-old transgression. From these estimates, it is considered that the current morphology of the inner, mid and outer shelves was molded from the transgression begun approximately 18.000 years B.P., when the sea level was about 130m below the current, based on Corrêa (1990). Twenty-six paleo-channels were identified and the dimensions and depths analysis of the thalwegs suggest the migration of the channels like the system of Araruama Lagoon during the past sea level fluctuation. XIX The interpretation of the morphology and of the surface sediments distribution revealed that there are two different domains: from the coast to about 105 m the bottom is characterized by the gradient of 1:220 (0,26º), by the parallelism to the sub-parallelism of the contours of the isobaths and by the terrigenous sediments; the second domain from approximately 105 m to the lower limit of the area is characterized by the gradient of 1:1550 (0,036º), by the irregular pattern of the isobaths contours and by the carbonate sediments. Two areas of canalizations were observed, one at NW of the area and other along isobaths ranging between 100 and 115 m, which can be related to the downwelling currents from SW storm waves. It is suggested that this same process is responsible for the canalizations between the isobaths 90 and 100 m below the present sea level, that were active at about 15.000 years B.P.
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