J. Arribas et al., Composition of modem stream sand derived from a mixture of sedimentary andmetamorphic source rocks (Henares River, Central Spain), SEDIMENT GE, 133(1-2), 2000, pp. 27-48
The Henares River, central Spain, flows westward from the Iberian Range (Me
sozoic sedimentary rocks) under semiarid climatic conditions. In the middle
and lower reaches, the Henares River receives sediment from three tributar
ies (Canamares, Bornova and Sorbe rivers) that drain the Central System (Pa
leozoic crystalline rocks, low-grade metamorphic rocks, and minor amounts o
f Mesozoic sedimentary rocks). Modern sands from the Henares River and its
tributaries offer an excellent opportunity to evaluate the importance of li
thology and physiography in determining detrital modes from mixed metamorph
ic and sedimentary source terrain. Sand modes from the Henares River and it
s 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 fel
dspar abundances in the Canamares and Bornova stream sands are related to t
he erosion of gneissic rocks. By contrast, sands from Sorbe River are low i
n feldspar, reflecting the absence of coarse metamorphic sources. Thus, pro
portions of bedrock lithologies in the drainage sub-basins are the main con
trol on detrital modes of the tributaries. In addition, slope acts on the s
and productivity of source rocks. Sedimentary source rocks in the upper rea
ches of the three tributaries have a poor productivity in the carbonate lit
hic fragments (Lsc), compared with the Lsc productivity of sedimentary sour
ces located downstream, with higher relief. The percentage of bedrock types
in the source area versus petrographic indices (Lm/L, Lss/L and Lsc/L) pro
vide a useful contrast between source and sand composition in the tributari
es. Thus, the high content of metamorphic lithic grains in the lithic grain
population over-represents this lithology (slate plus schist) at the sourc
e terrain. Lithic grains from sedimentary elastic sources generate few reco
gnizable grains (Lss) in the lithic grain population. Local supplies from c
arbonate sources may produce important increases of Lsc in short reaches of
the channels. Sand compositions in the Henares River seem to be very homog
eneous in a QmFLt diagram, with only minor differences caused by the suppli
es from the tributaries. These differences can be modeled in terms of end-m
ember mixing processes between Henares detritus and detritus from the relev
ant tributary. These mixing processes are more evident when lithe grain con
tents (LmLvLs and LmLssLse diagrams) are compared. Changes produced by inpu
ts from the Canamares, Bornova and Sorbe rivers are not permanent along the
Henares course. An important homogenization of Henares sand composition ta
kes place by the mixing of tributary deposits with previously deposited Hen
ares River terraces. Abrasion during transport does not appear to cause sig
nificant changes in the sand composition along the Henares River. (C) 2000
Elsevier Science B.V. All rights reserved.