Publication: Composition of modern stream sand derived from a mixture
of sedimentary and metamorphic source rocks
(Henares River, Central Spain)
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Publication Date
2000
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Elsevier
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Abstract
The Henares River, central Spain, flows westward from the Iberian Range (Mesozoic sedimentary rocks) under semiarid
climatic 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 metamorphic
rocks, and minor amounts of Mesozoic sedimentary rocks). Modern sands from the Henares River and its
tributaries offer an excellent opportunity to evaluate the importance of lithology and physiography in determining detrital
modes from mixed metamorphic and sedimentary source terrain. Sand modes from the Henares River and its tributaries
are quartzolithic. They plot in a restricted area on a QmFLt diagram, with low contents of feldspar and variable amounts
of quartz, and carbonate, metamorphic and minor siliciclastic lithic fragments. Higher feldspar abundances in the
Cañamares and Bornova stream sands are related to the erosion of gneissic rocks. By contrast, sands from Sorbe
River are low in feldspar, reflecting the absence of coarse metamorphic sources. Thus, proportions of bedrock lithologies
in the drainage sub-basins are the main control on detrital modes of the tributaries. In addition, slope acts on the sand
productivity of source rocks. Sedimentary source rocks in the upper reaches of the three tributaries have a poor
productivity in the carbonate lithic fragments (Lsc), compared with the Lsc productivity of sedimentary sources located
downstream, 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 high
content of metamorphic lithic grains in the lithic grain population over-represents this lithology (slate plus schist) at the
source terrain. Lithic grains from sedimentary clastic sources generate few recognizable grains (Lss) in the lithic grain
population. Local supplies from carbonate sources may produce important increases of Lsc in short reaches of the
channels. Sand compositions in the Henares River seem to be very homogeneous in a QmFLt diagram, with only
minor differences caused by the supplies from the tributaries. These differences can be modeled in terms of end-member
mixing processes between Henares detritus and detritus from the relevant tributary. These mixing processes are more
evident when lithic grain contents (LmLvLs and LmLssLsc diagrams) are compared. Changes produced by inputs from
the 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 River
terraces. Abrasion during transport does not appear to cause significant changes in the sand composition along the
Henares River.