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Sea level changes and neotectonics: some examples in Portugal (Arrábida and Southwest)
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A dinâmica litoral herdada é um tema recorrente na geomorfologia portuguesa. O reconhecimentodessa dinâmica passa pela identificação dos seus indicadores: formas de relevo e depósitos litoraiscorrelativos, presentes nas áreas hoje emersas e submersas. À semelhança do que sucedepresentemente, também no decurso do Quaternário, o litoral seria diversificado, com planícies arenosascom sistemas de praia-duna, sistemas de barreira, arribas com ou sem plataforma rochosa de sopé,estuários e terraços marinhos (elementos da plataforma litoral). Porém, esses indicadores nem sempresão de fácil identificação. No que respeita às formas de erosão - arribas com a respectiva sapa eplataforma rochosa de sopé - raramente se reconhecem, por serem formas antigas que foram sujeitas,desde a sua formação, à dinâmica geomorfológica, nomeadamente a movimentos de massa,desfigurando-as e dificultando a sua identificação. O estudo e enquadramento geomorfológicopormenorizado de muitos dos escarpados litorais, considerados previamente como arribas, tem reveladotratarem-se de escarpas de falha. No que respeita aos depósitos, (i) estes mudam lateralmente de fácies,(ii) as condições ambientais de génese dos depósitos quaternários mudaram e, em muitos casos, elesforam erodidos e/ou remexidos na sequência das variações da linha de costa que acompanham asflutuações do nível do mar e (iii) a actividade neotectónica, já reconhecida, pode tê-los mudado da suaposição original, fracturando-os, balançando-os ou mesmo levando à sua destruição. Por essas razões,as conclusões de um trabalho levado a efeito numa região não deverão ser extrapoladas, porque osvários compartimentos tectónicos no litoral não têm tido sempre o mesmo comportamento, como tmvindo a mostrar os diversos estudos precisos de pormenor recentemente divulgados. As correlaçõescronológicas são, também por isso, difíceis de estabelecer. Os dois casos apresentados permitemilustrar a complexidade da dinâmica litoral, mas também as dificuldades de correlação e deextrapolação dos resultados, nomeadamente como consequência da actividade de neotectónico nolitoral de Portugal continental.
The inherited coastal dynamics is a recurrent theme in Portuguese geomorphology. The recognition of this dynamics goes through the identification of its indicators: landforms and correlative coastal deposits, present in the present-day emerged and submerged areas. In comparison to what happens nowadays, also during the course of the Quaternary, the coast would have been diversified, with sandy plains with beach-dune systems, barrier systems, cliffs with or without rock platforms at their bottom, estuaries, and marine terraces (elements of the coastal platform). However, these indicators are not always easily identifiable. Relative to the types of erosion –cliffs with their notches and the rock platforms at the bottom –these are seldomly recognized, because of being ancient landforms that were subjected, since their creation, to geomorphologic dynamics, namely mass movements, damaging them and making difficult their identification. The detailed geomorphologic study and framework of many of the coastal escarpments, previously considered as cliffs, has shown them to be scarp faults. As to the deposits, (i) these change facies laterally, (ii) the environmental conditions in the genesis of the Quaternary deposits changed and, in many cases, they were eroded and/or remixed preceding the changes of the coastline that followed the sea level fluctuations and (iii) the already recognised neotectonic activity may have shifted them from their original position, fractured, tilted or even destroyed them. For these reasons, the conclusions from a study carried out in a region must not be extrapolated, because the different coastal tectonic sections do not always have the same behaviour, as have shown many recently published precise and detailed studies. The chronologic correlations are, therefore, difficult to establish. The two cases presented, allow show the complexity of the coastal dynamics, as well as the difficulty in correlating and extrapolating results, n amely as a result of the activity of the coastal neotectonics of Portugal mainland .
The inherited coastal dynamics is a recurrent theme in Portuguese geomorphology. The recognition of this dynamics goes through the identification of its indicators: landforms and correlative coastal deposits, present in the present-day emerged and submerged areas. In comparison to what happens nowadays, also during the course of the Quaternary, the coast would have been diversified, with sandy plains with beach-dune systems, barrier systems, cliffs with or without rock platforms at their bottom, estuaries, and marine terraces (elements of the coastal platform). However, these indicators are not always easily identifiable. Relative to the types of erosion –cliffs with their notches and the rock platforms at the bottom –these are seldomly recognized, because of being ancient landforms that were subjected, since their creation, to geomorphologic dynamics, namely mass movements, damaging them and making difficult their identification. The detailed geomorphologic study and framework of many of the coastal escarpments, previously considered as cliffs, has shown them to be scarp faults. As to the deposits, (i) these change facies laterally, (ii) the environmental conditions in the genesis of the Quaternary deposits changed and, in many cases, they were eroded and/or remixed preceding the changes of the coastline that followed the sea level fluctuations and (iii) the already recognised neotectonic activity may have shifted them from their original position, fractured, tilted or even destroyed them. For these reasons, the conclusions from a study carried out in a region must not be extrapolated, because the different coastal tectonic sections do not always have the same behaviour, as have shown many recently published precise and detailed studies. The chronologic correlations are, therefore, difficult to establish. The two cases presented, allow show the complexity of the coastal dynamics, as well as the difficulty in correlating and extrapolating results, n amely as a result of the activity of the coastal neotectonics of Portugal mainland .
Descrição
Palavras-chave
Coastal indicators Sea level changes Tectonics
Contexto Educativo
Citação
Ramos-Pereira, A. (2008). Sea level changes and neotectonics: some examples in Portugal (Arrábida and Southwest). Estudos do Quaternário/Quaternary Studies, (5), 31-37. https://doi.org/10.30893/eq.v0i5.57
Editora
Associação Portuguesa para o Estudo do Quaternário