Reconstructing the depositional history of Pleistocene fluvial deposits based on grain size, elemental geochemistry and in-situ 10Be data

Abstract

Constraining depositional phases in braided river deposits can be challenging if only based on sedimentology or geochronology. In this paper, we explored how different data sources can provide complementary information on the depositional history of Pleistocene fluvial deposits. The study was realized on the Zutendaal gravels that outcrop in Northeastern Belgium. In an 8 m high exposure of gravel sheets, we collected bulk samples that were processed for grain size, elemental geochemistry and cosmogenic radionuclides. After dry sieving, we derived the D10, D50, D90, gravel and sand content. From the elemental geochemistry of the bulk samples, we calculated the Ti/Zr and Cr/Zr ratios as proxies for provenance, the Al2O3/SiO2 and Ba/Sr ratios for hydrolysis, and the MnO, Fe2O3 concentrations and the Fe2O3+MnOAl2O3 ratio for oxidation. The 10Be concentration-depth profiles are informative to identify phases of landscape stability characterized by depositional hiatuses. To verify the existence of any aberrant variation in 10Be concentration associated to provenance, the in-situ 10Be concentrations were contrasted with provenance and weathering indices. Differences in grain size, elemental geochemistry and in-situ 10Be between sedimentary units were evaluated based on Mann-Whitney tests, while Spearman correlation tests informed us on the co-variation between data sources. The outcrop in the Zutendaal gravels contains three depositional units with significant differences in grain size and chemical weathering extent, and representing distinct 10Be accumulation phases. The absence of correlation between provenance and oxidation proxies indicates that weathering is essentially intraformational or post-depositional. Sedimentary hiatuses were identified based on fining upward sequences and 10Be concentration-depth profiles, whereby local deviation in 10Be concentration at the top of the fining upward sequence provided information on the duration of the hiatus. The case study illustrates how co-variations in weathering, provenance, granulometry indices and 10Be concentration are informative for the depositional history of fluvial sediments, and landscape stability during the Pleistocene.