Assessment of a locally-sourced loess system in Europe: The Swiss Jura Mountains

Abstract

Loess of Northern, Central, and Eastern Europe cover large areas and can be of considerable thickness. However, this already well-described loess belt did not reach the Swiss Jura Mountains. Nevertheless, local deflation, transportation, and deposition processes were efficient during the Late Glacial Period. Alpine moraines and outwash deposits on the Swiss foreland worked as deflation sources under the action of winds. Aeolian particles were deposited on the easternmost ridge and summits of the Jura Mountains, forming a thin loessic cover (<50 cm), which was generally eroded and/or incorporated in periglacial sediments. Jura “loess” is enriched in Alpine silicate minerals and constitutes a clear allochthonous input in the context of Jura Mountains, where Mesozoic limestones, and their weathering products, compose the quasi-entirety of autochthonous material. The Jura loess, already recognised for more than 40 years, still needs to be characterised more accurately. Five study sites were chosen along an 85 km long transect on the easternmost ridge of the Jura Mountains. A soil pit was dug and analysed at each site, in order to identify loess layers. Grain-size distribution curves of loess permitted to discriminate four particle subpopulations, which are related to various aeolian episodes/sources and post-depositional weathering processes, respectively. The mineralogical composition is dominated by Alpine components. However, high amounts of kaolinite in the clay fraction of loess layers point to a more local origin, thus indicating the contribution of Jura moraines as potential deflation sources. Finally, the geochemical composition of loess reflects the preponderant influence of Alpine minerals.