Estuarine–fluvial floodplain formation in the Holocene Lower Tagus valley (Central Portugal) and implications for Quaternary fluvial system evolution |
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| Institution: | 1. Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, Aichi 463-8560, Japan;2. Department of Electronics Engineering, Kobe City College of Technology, 8-3 Gakuen-higashimachi, Nishi-ku, Kobe, Hyogo 651-2194, Japan;1. Department of Botany, Institute for Coastal and Marine Research, Nelson Mandela University, Summerstrand South Campus, Port Elizabeth 6031, South Africa;2. SARChI Chair: Shallow Water Ecosystems, Nelson Mandela University, Summerstrand South Campus, Port Elizabeth 6031, South Africa;3. South African Institute for Aquatic Biodiversity, Grahamstown 6139, South Africa;4. Department of Ecology and Resource Management, University of Venda, Thohoyandou, Limpopo, South Africa;5. South African Environmental Observation Network, Elwandle Coastal Node, Ocean Science Campus, Nelson Mandela University, Port Elizabeth 6031, South Africa;6. Department of Zoology, Institute for Coastal and Marine Research, Nelson Mandela University, Summerstrand South Campus, Port Elizabeth 6031, South Africa;1. Department of Applied Science, National Hsinchu University of Education, 521, Nanda Rd., Hsinchu City 300, Taiwan;2. Department of Natural Resources and Environmental Studies, National Donghua University, 1, Sec. 2, Da Hsueh Rd., Shoufeng Hualien 974, Taiwan;3. Department of Geosciences, National Taiwan University, 1, Sec. 4, Roosevelt Rd., Taipei 106, Taiwan;4. YIC Geological Office, 2F, 201, Zhonghua Rd., Magong, Penghu, 880, Taiwan |
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| Abstract: | We present a brief synthesis of the Quaternary fluvial record in the Lower Tagus Basin (central Portugal), concentrating on factors controlling infill and incision. The Holocene part of the record forms the focus of this paper and guides the questioning of the basic assumptions of the established Quaternary fluvial evolution model, in particular the link between sea-level change and fluvial incision-deposition. We suggest that several incision-aggradation phases may have occurred during glacial periods. Major aggradation events may overlap with cold episodes, while incision appears to concentrate on the warming limb of climate transitions. The complex stratigraphy of the Quaternary record in the Lower Tagus valley is influenced by repeated base-level and climate changes.This paper submits the first chronostratigraphic framework for valley fill deposits in the Lower Tagus area. Sea-level rise forced aggradation and controlled deposition of the fine-grained sedimentary wedge underlying the low-gradient Lower Tagus floodplain. Investigations have focused on the lower Muge tributary, where rapidly aggrading estuarine and fluvial environments were abruptly established (～8150 cal BP) as sea level rose. Base level at the valley mouth controlled the upstream extent of the fine-grained backfill. Tidal environments disappeared abruptly (～5800 cal BP) when the open estuary at the Muge confluence was infilled by the Tagus River. The decrease and final still stand of sea-level rise led to floodplain stabilisation with peat (～6400–5200 cal BP) and soil formation (～5200–2200 cal BP). Localised renewed sedimentation (～2200–200 cal BP) is linked to human activity. |
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