Discovery of a landscape-wide drape of late-glacial aeolian silt in the western Northern Calcareous Alps (Austria): First results and implications

Abstract

Aeolian deposits record palaeoenvironmental conditions and may coin soil properties. Whereas periglacial loess is extensively investigated for ~200years, the study of the intramontane wind-blown deposits of the Alps has just stuttered along. Herein, we describe a drape of polymictic siliciclastic silt interpreted as an aeolian deposit that veneers extensive areas in the western Northern Calcareous Alps (NCA), from kames terraces near valley floors up to last-glacial nunataks.The NCA — part of the Eastern Alps mountain range — consist mainly of Triassic carbonate rocks; these are overlain by deposits of the Last Glacial Maximum (LGM) and its deglacial-paraglacial aftermath (e.g., glacial tills, fluvio-lacustrine successions, alluvial fans, scree slopes) — and a regional drape of polymictic silt newly described herein. The drape is typically a few decimeters in thickness and slightly modified by soil formation; it consists mainly of well-sorted silt of quartz, feldspars, phyllosilicates (muscovite, chlorite, biotite), amphiboles and, rarely, calcite or dolomite. The drape is unrelated to the substrate: it overlies carbonate bedrock and — in lateral continuity — abandoned deposystems such as colluvial slopes of redeposited till, kames, alluvial fans, scree slopes, and rock avalanche deposits. The drape was spotted from near the present valley floors up to LGM nunataks, over a vertical range of some 2000m; it is also present in catchments of the NCA that were not overridden by far-travelled ice streams and that lack metamorphic rock fragments. Two OSL quartz ages of the drape from two distinct locations (18.77±1.55ka; 17.81±1.68ka) fall into the early Alpine late-glacial interval shortly after the collapse of pleniglacial ice streams; this fits with geological and geomorphological evidence, respectively, that the drape should be of early late-glacial age, and that it accumulated during a specific interval of time.In the NCA, localized minor deposition of aeolian dust is documented — by other authors — from plateaus deglaciated only during the late-glacial to Holocene; no evidence, however, exists for another phase of similarly widespread aeolian deposition such as that which gave rise to the described regional drape of silt. Intense aeolian transport and deposition was probably a direct consequence of the liberation of huge amounts of unsorted sediment during deglacial ice collapse, perhaps combined with climatic aridification. This provides a hitherto unappreciated element of the deglacial to paraglacial phase: intramontane dust storms. Because of its large extent and the availability to OSL dating, the aeolian drape provides an excellent geochronological marker level identified in terrestrial post-glacial successions of the Eastern Alps. Because of its fine-grained siliciclastic composition, the drape gives rise to widespread development of atypical Cambisols (on carbonate bedrocks) with comparatively high water storage capacity and nutrient supply.