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Impactos da elevação do nível médio do mar em ambientes costeiros : o caso do estuário do Sado
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Autores
Resumo(s)
Reconstitui-se a evolução recente do estuário do Sado e plataforma continental
contígua baseada em mapas históricos, sonar de varrimento lateral e sísmica de alta
resolução.
O estuário do Sado é separado do oceano pela península de Tróia, tem um prisma de
maré de 2,1x108m3 e uma área submersa de ~200km2, cujo ~¼ corresponde ao seu
domínio externo dominado pelo delta de vazante submarino e ~¾ ao domínio interno.
Entre 1968 e 1995/2002, tanto a área estuarina, como os volumes do prisma de maré e
do espaço de acomodação sofreram reduções totais de 6%, 7% e 8%, respectivamente.
O domínio interno teve taxas de sedimentação positivas, com um decréscimo
acentuado a partir de 1979. No estuário externo verificou-se acumulação para o
período de 1968-1979, invertendo-se para o período 1979-1995/2002, em que a erosão
focalizada no lobo frontal do delta passa a ser dominante, com perda de 150x106m3 de
sedimentos (14%) entre 2002-1930, resultante da redução do prisma de maré.
Identificaram-se 3 superfícies sísmicas principais S1-S3, individualizando 4 unidades
US1-US4. US1 corresponde ao actual delta do estuário, cuja idade máxima estimada é
7,5kanos. US2 é uma unidade transgressiva, cuja base se associa ao ravinamento da
transgressão Holocénica. US3 tem uma idade estimada de 60-25kanos e é interpretada
como correspondendo a barreiras costeiras progradantes que ocorrem dos 26m aos
85m de profundidade e são entalhadas por duas plataformas costeiras, definindo uma
faixa do fundo do mar marcada por quebras de declive, fundos rochosos e/ou uma fina
película de US2. US4 é heterogénea, incluindo sedimentos do Plio-Plistocénico ao
Miocénico.
A evolução da plataforma desde os 60kanos é reconstituída com base na relação
estabelecida entre curvas de variação do NMM publicadas e estruturas interpretadas
como paleo-linhas de costa marcadas por quebras de declive aos 35-45m (~60-
50kanos e ~13kanos), 65-75m (~55-25kanos), 80m (~35-25kanos e ~11kanos) e 95-
100m (~16kanos).
The recent evolution of the Sado estuary and adjacent continental shelf is reconstructed based on historical maps, side scan sonar images and high resolution seismics. The Sado estuary is separated from the ocean by the Tróia peninsula, has a tidal prism of 2,1 x108m3 and a submerged area of ~200km2, with ~¼ corresponding to its external domain dominated by the submarine ebb delta and ~¾ corresponding to the internal domain. Between 1968 and 1995/2002, the estuarine area, the tidal prism volumes and the accommodation space suffered total reductions of 6%, 7% and 8%, respectively. The internal domain had positive sedimentation rates, with a rapid decrease since 1979. In the external estuary accumulation occurred during the 1968- 1979 period, with a reversal between 1979-1995/2002, when focused erosion in delta frontal lobe became predominant, with a sediment loss of 150x106m3 (14%) between 2002-1930, as consequence of the tidal prism reduction. Three main seismic surfaces were identified S1-S3, separating 4 units, US1-US4. US1 corresponds to the current estuary delta, whit a maximum estimated age of 7.5ky. US2 is a transgressive unit, deposited above the Holocene transgressive ravinement surface. US3 has an estimated age of 60-25ky and is interpreted has corresponding to prograding coastal barriers occurring from 26m to 85m depth, and are etched by two coastal platforms which define a stretch of the sea bottom characterized by slope breaks, rocky bottoms and/or a fine layer of US2. US4 is heterogeneous, including sediments from Plio-Pleistocene to the Miocene. The shelf evolution since 60ky is reconstructed based on the relation between the published MSL change curves and the structures interpreted as paleo-coastlines indicated by slope breaks at 35-45 (~60-50ky and ~13ky), 65-75m (~55-25ky), 80m (~35-25ky and ~11ky) and 95-100m (~16ky).
The recent evolution of the Sado estuary and adjacent continental shelf is reconstructed based on historical maps, side scan sonar images and high resolution seismics. The Sado estuary is separated from the ocean by the Tróia peninsula, has a tidal prism of 2,1 x108m3 and a submerged area of ~200km2, with ~¼ corresponding to its external domain dominated by the submarine ebb delta and ~¾ corresponding to the internal domain. Between 1968 and 1995/2002, the estuarine area, the tidal prism volumes and the accommodation space suffered total reductions of 6%, 7% and 8%, respectively. The internal domain had positive sedimentation rates, with a rapid decrease since 1979. In the external estuary accumulation occurred during the 1968- 1979 period, with a reversal between 1979-1995/2002, when focused erosion in delta frontal lobe became predominant, with a sediment loss of 150x106m3 (14%) between 2002-1930, as consequence of the tidal prism reduction. Three main seismic surfaces were identified S1-S3, separating 4 units, US1-US4. US1 corresponds to the current estuary delta, whit a maximum estimated age of 7.5ky. US2 is a transgressive unit, deposited above the Holocene transgressive ravinement surface. US3 has an estimated age of 60-25ky and is interpreted has corresponding to prograding coastal barriers occurring from 26m to 85m depth, and are etched by two coastal platforms which define a stretch of the sea bottom characterized by slope breaks, rocky bottoms and/or a fine layer of US2. US4 is heterogeneous, including sediments from Plio-Pleistocene to the Miocene. The shelf evolution since 60ky is reconstructed based on the relation between the published MSL change curves and the structures interpreted as paleo-coastlines indicated by slope breaks at 35-45 (~60-50ky and ~13ky), 65-75m (~55-25ky), 80m (~35-25ky and ~11ky) and 95-100m (~16ky).
Descrição
Tese de doutoramento, Geologia (Geologia Económica e do Ambiente), Universidade de Lisboa, Faculdade de Ciências, 2010
Palavras-chave
Paleontologia Quaternário Nível do mar Plataforma continental Zonas costeiras Estuário do Sado (Portugal) Teses de doutoramento - 2010