The Tajo River, the 10th largest river in Europe, drains part of the western passive margin of Europe that includes multipletectonic elements of the Iberian plate. Modern fluvial sand composition in the Tajo River drainage basin reflects the nature ofthe source region, which lies in the central part of the Iberian Peninsula. Four fluvial petrographic provinces (A, B, C and D) canbe established in the Tajo drainage basin, corresponding well with the four principal structural units drained: (1) the IberianRange; (2) the Hesperian massif; (3) The Tertiary Tajo basin; and (4) the Neogene Santarem–Lisboa basin.Province A corresponds to the Tajo River head and is characterized by quartzolithic sedimenticlastic sands (Qm67F4Lt29 andRs79Rg3Rm18). These sands have been derived from diverse Mesozoic siliciclastics and carbonates of the Iberian Range.Province B appears in the upper reaches of the Tajo River course and is quartzofeldspathic (Qm57F34Lt9) with diverse rockfragments (Rs34Rg23Rm43). Sources are Hercynian granitoids and metasediments and Neogene clastics and minor carbonates ofthe Tertiary Tajo basin. Province C extends along the middle course of the Tajo River with quartzofeldspathic metamorphiclasticsand modes (Qm60F33Lt7; Rs4Rg21Rm75). The sources are metamorphic rocks intruded by plutonites of the Hesperian Massif.Province D is quartzofeldspathic (Qm55F39Lt6) with a dominance of phaneritic rock fragments (Rs5Rg33Rm62) and correspondsto the lower reaches of the Tajo River, where siliciclastic deposits of the Neogene Santarem–Lisboa basin are the main sources.Sands plot on provenance-discrimination diagram (QmFLt) within the recycled-orogen field (Tajo River head) and continentalblockfields (upper, middle and lower course). In addition, we have proven the usefulness of the RsRgRm diagram to discriminatethe defined fluvial provinces, originating from heterogeneous parent-rock textures and mineralogy.Climate does not exert any strong influence on the petrogenesis of the Tajo River drainage basin sand, and erosion in thesource areas may be described in terms of weathering-limited denudation regime. By contrast, mixing with tributary supplies isthe main process that modifies composition in the Tajo River sand. The establishment of fluvial provinces related to the mainbedrock structural units reflects the great relevance of tributaries from each province in the generation of the Tajo River sandand the low significance of inherited sandy load from previous provinces.0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2004.05.019* Corresponding author. Tel.: +39 984 835528; fax: +39 984 835319.E-mail addresses: emilia.lepera@irpi.cnr.it (E. Le Pera)8 arribas@geo.ucm.es (J. Arribas).Sedimentary Geology 171 (2004) 261–281www.elsevier.com/locate/sedgeoThe abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of theTajo River drainage basin faithfully represents the relative abundance of granodiorite+monzogranite bedrock exposure ineach subbasin. Metasedimentary and metamorphic bedrock outcrop area is overrepresented by metamorphic rockfragments (RsRgRm%Rm) in both sand from tributaries and from the Tajo main trunk river. This is manifested bydifferences of 18–23% between means of metasedimentary outcrop area and the RsRgRm%Rm mean in the sand fromProvinces B and C+D, respectively. Sedimentary (mainly carbonate) outcrop area is underrepresented or not representedby sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This underrepresentation causes a differencebetween means of sedimentary outcrop area and the RsRgRm%Rs mean of 21%, 17.1% and 10.7% in sands fromProvince A, B and C+D, respectively. This fact confirms the rapid loss of these grains during transport because of theirlabile nature.

Sand composition in an Iberian passive margin fluvial course: the Tajo River

LE PERA, Emilia;
2004

Abstract

The Tajo River, the 10th largest river in Europe, drains part of the western passive margin of Europe that includes multipletectonic elements of the Iberian plate. Modern fluvial sand composition in the Tajo River drainage basin reflects the nature ofthe source region, which lies in the central part of the Iberian Peninsula. Four fluvial petrographic provinces (A, B, C and D) canbe established in the Tajo drainage basin, corresponding well with the four principal structural units drained: (1) the IberianRange; (2) the Hesperian massif; (3) The Tertiary Tajo basin; and (4) the Neogene Santarem–Lisboa basin.Province A corresponds to the Tajo River head and is characterized by quartzolithic sedimenticlastic sands (Qm67F4Lt29 andRs79Rg3Rm18). These sands have been derived from diverse Mesozoic siliciclastics and carbonates of the Iberian Range.Province B appears in the upper reaches of the Tajo River course and is quartzofeldspathic (Qm57F34Lt9) with diverse rockfragments (Rs34Rg23Rm43). Sources are Hercynian granitoids and metasediments and Neogene clastics and minor carbonates ofthe Tertiary Tajo basin. Province C extends along the middle course of the Tajo River with quartzofeldspathic metamorphiclasticsand modes (Qm60F33Lt7; Rs4Rg21Rm75). The sources are metamorphic rocks intruded by plutonites of the Hesperian Massif.Province D is quartzofeldspathic (Qm55F39Lt6) with a dominance of phaneritic rock fragments (Rs5Rg33Rm62) and correspondsto the lower reaches of the Tajo River, where siliciclastic deposits of the Neogene Santarem–Lisboa basin are the main sources.Sands plot on provenance-discrimination diagram (QmFLt) within the recycled-orogen field (Tajo River head) and continentalblockfields (upper, middle and lower course). In addition, we have proven the usefulness of the RsRgRm diagram to discriminatethe defined fluvial provinces, originating from heterogeneous parent-rock textures and mineralogy.Climate does not exert any strong influence on the petrogenesis of the Tajo River drainage basin sand, and erosion in thesource areas may be described in terms of weathering-limited denudation regime. By contrast, mixing with tributary supplies isthe main process that modifies composition in the Tajo River sand. The establishment of fluvial provinces related to the mainbedrock structural units reflects the great relevance of tributaries from each province in the generation of the Tajo River sandand the low significance of inherited sandy load from previous provinces.0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2004.05.019* Corresponding author. Tel.: +39 984 835528; fax: +39 984 835319.E-mail addresses: emilia.lepera@irpi.cnr.it (E. Le Pera)8 arribas@geo.ucm.es (J. Arribas).Sedimentary Geology 171 (2004) 261–281www.elsevier.com/locate/sedgeoThe abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of theTajo River drainage basin faithfully represents the relative abundance of granodiorite+monzogranite bedrock exposure ineach subbasin. Metasedimentary and metamorphic bedrock outcrop area is overrepresented by metamorphic rockfragments (RsRgRm%Rm) in both sand from tributaries and from the Tajo main trunk river. This is manifested bydifferences of 18–23% between means of metasedimentary outcrop area and the RsRgRm%Rm mean in the sand fromProvinces B and C+D, respectively. Sedimentary (mainly carbonate) outcrop area is underrepresented or not representedby sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This underrepresentation causes a differencebetween means of sedimentary outcrop area and the RsRgRm%Rs mean of 21%, 17.1% and 10.7% in sands fromProvince A, B and C+D, respectively. This fact confirms the rapid loss of these grains during transport because of theirlabile nature.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11770/149565
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