%0 Journal Article
%A Arribas Mocoroa, José
%A Critelli, Salvatore
%A Le Pera, Emilia
%T Composition of modern stream sand derived from a mixtureof sedimentary and metamorphic source rocks(Henares River, Central Spain)
%D 2000
%@ 0037-0738
%U https://hdl.handle.net/20.500.14352/57077
%X The Henares River, central Spain, flows westward from the Iberian Range (Mesozoic sedimentary rocks) under semiaridclimatic conditions. In the middle and lower reaches, the Henares River receives sediment from three tributaries(Cañamares, Bornova and Sorbe rivers) that drain the Central System (Paleozoic crystalline rocks, low-grade metamorphicrocks, and minor amounts of Mesozoic sedimentary rocks). Modern sands from the Henares River and itstributaries offer an excellent opportunity to evaluate the importance of lithology and physiography in determining detritalmodes from mixed metamorphic and sedimentary source terrain. Sand modes from the Henares River and its tributariesare quartzolithic. They plot in a restricted area on a QmFLt diagram, with low contents of feldspar and variable amountsof quartz, and carbonate, metamorphic and minor siliciclastic lithic fragments. Higher feldspar abundances in theCañamares and Bornova stream sands are related to the erosion of gneissic rocks. By contrast, sands from SorbeRiver are low in feldspar, reflecting the absence of coarse metamorphic sources. Thus, proportions of bedrock lithologiesin the drainage sub-basins are the main control on detrital modes of the tributaries. In addition, slope acts on the sandproductivity of source rocks. Sedimentary source rocks in the upper reaches of the three tributaries have a poorproductivity in the carbonate lithic fragments (Lsc), compared with the Lsc productivity of sedimentary sources locateddownstream, with higher relief. The percentage of bedrock types in the source area versus petrographic indices (Lm/L,Lss/L and Lsc/L) provide a useful contrast between source and sand composition in the tributaries. Thus, the highcontent of metamorphic lithic grains in the lithic grain population over-represents this lithology (slate plus schist) at thesource terrain. Lithic grains from sedimentary clastic sources generate few recognizable grains (Lss) in the lithic grainpopulation. Local supplies from carbonate sources may produce important increases of Lsc in short reaches of thechannels. Sand compositions in the Henares River seem to be very homogeneous in a QmFLt diagram, with onlyminor differences caused by the supplies from the tributaries. These differences can be modeled in terms of end-membermixing processes between Henares detritus and detritus from the relevant tributary. These mixing processes are moreevident when lithic grain contents (LmLvLs and LmLssLsc diagrams) are compared. Changes produced by inputs fromthe Cañamares, Bornova and Sorbe rivers are not permanent along the Henares course. An important homogenization of Henares sand composition takes place by the mixing of tributary deposits with previously deposited Henares Riverterraces. Abrasion during transport does not appear to cause significant changes in the sand composition along theHenares River.
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