The origin of the Pedra Furada sandstone tubular structures (South of Lisbon, Portugal) |
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| Authors: | T.M. Azevê do, M.M. Abreu, M. Azevedo Coutinho,J. Figueiras |
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| Affiliation: | aCentro de Geologia, Faculdade de Ciências, Univ. de Lisboa, Ed. C6, 4° Piso, Campo Grande, 1749-016 Lisboa, Portugal;bDepartamento de Ciências do Ambiente, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal;cDepartamento de Engenharia Civil e Arquitectura, Instituto Superior Técnico, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal |
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| Abstract: | The Pedra Furada is a 12 m wide, 18 m high outcrop feature showing hundreds of ferruginised sandy tubes and looking in part like a giant organ. In this paper the origin of the tubes is explained on the basis of geochemical, petrographic and microscopic (optical and electronic) analytical data. The tubes are considered to represent vertical escape channels for overpressured water, exhibiting inward decreasing grain size due to water velocity gradients inside the escape channels. The ferruginisation is due to iron oxides associated with colloidal/clayey fine sediments and to goethite formed from solution. The overpressure of water may be due to seismically fluidised beds below the Pedra Furada outcrop or to artesian water ascent. In both cases, fault rupturing may have played a major role in the focussing of the ascending flow. |
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| Keywords: | Sandstone tubular structures Ferruginisation Water escape structures Water escape sedimentation |
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