Geochemistry of the sands of the Allier river terraces, France

S.B. Kroonenberg; M.L. Moura; A.T.J. Jonker

S.B. Kroonenberg Department of Soil Science and Geology, Agricultural University, P.O. Box 37, 6700 AA Wageningen, the Netherlands
M.L. Moura Department of Soil Science and Geology, Agricultural University, P.O. Box 37, 6700 AA Wageningen, the Netherlands
A.T.J. Jonker Department of Soil Science and Geology, Agricultural University, P.O. Box 37, 6700 AA Wageningen, the Netherlands

Keywords: major and minor elements, geochemistry, sedimentary petrography, fluvial deposits, multivariate statistics, grain size, density sorting, provenance, weathering

Abstract

The distribution of 24 major and minor elements has been studied in 66 sand samples of different grain sizes from six terrace levels along a stretch of 40 km along the Allier river, which drains an area mainly underlain by granitic and gneissic Hercynian basement and alkalibasaltic Cenozoic volcanic rocks in the French Central Massif. The river sands show a large spread in SiO2 63-97%). Principal component analysis shows two factors, F1 composed of Ti, Mg, Fe, Mn, Ca, P, Ni, Cr, V, Sr, Nb and Zr, together with -Si, mainly elements from basaltic components, and F2 composed of K, Na, Al, Rb and Ga, mainly elements of alkalifeldspar and micas. Within single terrace levels F1 varies mainly due to lateral and downstream density sorting of basaltic rock fragments, and F2 due to increasing concentration of micas in finer-grained samples. Variations in F1 between terrace levels reflect partly uplift and erosion history, partly increasing contribution of fluvioglacial basalt-rich sediment in glacial times and of basement-rich sediment in interglacial times. Weathering of basaltic components with increasing sediment age is reflected in decreasing Ca/Ti and Mg/Fe ratios with terrace height. The results show that the geochemical study of unconsolidated fluvial sands is a rapid and useful tool next to classical sedimentary petrography.