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Caracterização da planície litoral da Manta Rota entre Vila Real de Santo António e Monte Gordo
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Esta dissertação tem como principal objetivo a caracterização sedimentar e geomorfológica da planície litoral da Manta Rota localizada entre Vila Real de Santo António e Monte Gordo (Algarve). Os estudos anteriores sobre esta área do sotavento algarvio descrevem aspetos da morfologia e da dinâmica da linha de costa adjacente ao estuário do rio Guadiana, mas focaram-se essencialmente na evolução daquela desembocadura estuarina. A planície litoral da Manta Rota é constituída por cordões dunares fixados por um coberto vegetal e uma praia de areia, e corresponde a um sistema progradante. Neste trabalho, estudaram-se amostras arenosas colhidas em dois perfis transversais, que atravessam as dunas da Mata Nacional de Vila Real de Santo António e a praia oceânica. Amostraram-se essencialmente as cristas dunares e sedimentos atuais, de praia. Recolheram ainda amostras no sapal de Vila Real de Santo António. Determinaram-se as características texturais, o teor em CaCO3, o teor em Fe de lixiviados e o pH do sedimento, no caso de materiais arenosos. Nos sedimentos vasosos de sapal determinou-se também o teor de matéria orgânica. As areias da planície litoral são médias a finas, bem a moderadamente bem calibradas, pouco carbonatadas e essencialmente subalcalinas. São constituídas fundamentalmente por grãos de quartzo, a que se somam bioclastos (com predominância de restos de bivalves) e minerais pesados (principalmente ilmenite e magnetite). Efetuou-se ainda o mapeamento geomorfológico do campo dunar instalado sobre a planície litoral onde se definiram três a tipologia de dunas: dunas secundárias, primárias e embrionárias; utilizando como critério de classificação a percentagem de superfície coberta de vegetação; para além das dunas caracterizou-se ainda uma zona de sapal e dois lagos artificiais. Para complementar esta análise integraram-se os resultados com dados cartográficos e bibliográficos disponíveis. A caracterização sedimentar e geomorfológica permitiu deduzir que existem duas zonas distintas na planície. A zona norte, com dunas secundárias mais antigas, onde as areias são mais grosseiras, menos carbonatadas e menos ferrosas, com interdunares mais amplos e cristas menos contínuas paralelas à linha de costa atual. A zona sul, que engloba dunas secundárias mais recentes, dunas primárias e, o sistema de duna-praia atual, com sedimentos mais finos e teores mais elevados em CaCO3 e Fe; os interdunares nesta zona são menos espaçados e as cristas dunares têm maior continuidade espacial com orientação E-W. Estes dois grupos de dunas estão separados por uma zona de transição que tem características intermédias. Por outro lado, a morfologia das dunas mostra que a planície progradou a velocidades diferentes: foi possível distinguir uma fase de progradação lenta e outra de progração rápida no grupo norte, e uma fase de progradação lenta na zona de transição. A variação do diâmetro médio das areias de duna pode estar associada a alterações do regime hidrodinâmico costeiro enquanto a variação do teor em CaCO3 e Fe relacionam-se com a depleção em Fe e Ca por lixiviação, induzida pela circulação da água meteórica no interior dos corpos dunares. Combinando estes resultados com os dados cartográficos disponíveis na bibliografia, conclui-se que as dunas da planície litoral ter-se-ão formado depois de 6200 BP e provavelmente após 5000 BP. De acordo com a informação cartográfica, as cristas do grupo norte e da zona de transição formaram-se antes de 1876. No grupo sul distinguiram-se várias etapas de formação. Entre 1876 e 1976 formaram-se cinco cordões de dunas secundárias, entre 2000 e 2005 duas dunas primárias e, após 2005, formou-se o sistema de duna-praia atual.
The main goal of this study is the sedimentary and geomorphological characterization of the coastal plain of Manta Rota; located between Vila Real de Santo António and Monte Gordo (Algarve). Previous studies about the area have focused essentially on the morphology and morphodynamics of the Guadiana estuary, with which the coastal plain exchanges sediment. Sand samples were collected along two crossshore profiles over the dunes of Mata Nacional de Vila Real de Santo António and ocean beach. Samples were collected in the dune ridges, beach, and a small saltmarsh expansion, and analyzed in the laboratory for textural characteristics, CaCO3 content, iron in leachates. Organic matter was also determined in muddy sediment. The sands of the strandplain are fine to medium in size, well to moderately well sorted, low in carbonates and mainly subalkaline. They are composed mainly by quartz grains, bioclasts (bivalves) and heavy minerals (among which ilmenite and magnetite dominate the opaque population). The strandplain is composed of progading foredune ridges covered and stabilized by vegetation. Furthermore, the geomorphological mapping of the dune field was performed, and dune forms were classified as primary, secondary or embryo, based on the percentage of vegetation coverage. The saltmarsh and two artificial lakes were also identified and mapped. To complete the analysis, the results were integrated with available cartographic and bibliographic data. Based on the sedimentary and geomorphological analysis, it was possible to distinguish between two areas: North and South. In the North area the secondary dunes are older, with larger interdune troughs and discontinuous ridges parallel to the present-day coast. The sand is coarser and yielded has lower concentration of carbonates and iron. The South area includes younger secondary dunes, primary dunes, and the current dune-beach system. The dune throughs in this region are smaller, and the ridges are more continuous and present E-W orientation; the sediments are finer, with higher contents of carbonates and iron. The North and South areas are separated by a transition zone with mixed characteristics. The morphology of the dunes indicates that the progradation occurred at different velocities: one period of slower and one period of faster progradation were recognized in the North area and a period of slow progradation in the transition zone. The differences in grain size, the dunes sand may be associated with variations of the hydrodynamic regime contemporaneous of the progradation whereas the differences in CaCO3 and Fe are explained by post-depositional depletion through leaching of the sand, promoted by percolation of meteoric water. Combining these results with data about the coastline and other elements available in the literature, one can conclude that the dunes of the strandplain were formed after 6200 BP and probably after 5000 BP. According to the cartographic information, the ridges of the North group and the transition zone were formed before 1876. In the South area, three stages of dune formation can be distinguished: from 1876 to 1976, five secondary dunes formed; between 2000 and 2005, two primary dunes; and after 2005, the current dune-beach system developed.
The main goal of this study is the sedimentary and geomorphological characterization of the coastal plain of Manta Rota; located between Vila Real de Santo António and Monte Gordo (Algarve). Previous studies about the area have focused essentially on the morphology and morphodynamics of the Guadiana estuary, with which the coastal plain exchanges sediment. Sand samples were collected along two crossshore profiles over the dunes of Mata Nacional de Vila Real de Santo António and ocean beach. Samples were collected in the dune ridges, beach, and a small saltmarsh expansion, and analyzed in the laboratory for textural characteristics, CaCO3 content, iron in leachates. Organic matter was also determined in muddy sediment. The sands of the strandplain are fine to medium in size, well to moderately well sorted, low in carbonates and mainly subalkaline. They are composed mainly by quartz grains, bioclasts (bivalves) and heavy minerals (among which ilmenite and magnetite dominate the opaque population). The strandplain is composed of progading foredune ridges covered and stabilized by vegetation. Furthermore, the geomorphological mapping of the dune field was performed, and dune forms were classified as primary, secondary or embryo, based on the percentage of vegetation coverage. The saltmarsh and two artificial lakes were also identified and mapped. To complete the analysis, the results were integrated with available cartographic and bibliographic data. Based on the sedimentary and geomorphological analysis, it was possible to distinguish between two areas: North and South. In the North area the secondary dunes are older, with larger interdune troughs and discontinuous ridges parallel to the present-day coast. The sand is coarser and yielded has lower concentration of carbonates and iron. The South area includes younger secondary dunes, primary dunes, and the current dune-beach system. The dune throughs in this region are smaller, and the ridges are more continuous and present E-W orientation; the sediments are finer, with higher contents of carbonates and iron. The North and South areas are separated by a transition zone with mixed characteristics. The morphology of the dunes indicates that the progradation occurred at different velocities: one period of slower and one period of faster progradation were recognized in the North area and a period of slow progradation in the transition zone. The differences in grain size, the dunes sand may be associated with variations of the hydrodynamic regime contemporaneous of the progradation whereas the differences in CaCO3 and Fe are explained by post-depositional depletion through leaching of the sand, promoted by percolation of meteoric water. Combining these results with data about the coastline and other elements available in the literature, one can conclude that the dunes of the strandplain were formed after 6200 BP and probably after 5000 BP. According to the cartographic information, the ridges of the North group and the transition zone were formed before 1876. In the South area, three stages of dune formation can be distinguished: from 1876 to 1976, five secondary dunes formed; between 2000 and 2005, two primary dunes; and after 2005, the current dune-beach system developed.
Descrição
Tese de mestrado em Geologia do Ambiente, Riscos Geológicos e Ordenamento do Território, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, em 2018
Palavras-chave
Planície litoral Cordões dunares Análise morfossedimentar Sedimentos Evolução litoral Teses de mestrado - 2018