Erosion and filling of a glacially overdeepened trough in the northern Alpine Foreland of Switzerland during the last 300'000 years
Abstract
As the major weather divide in Europe, the Alps represent one of the most interesting areas for understanding past climate change and its impact on continental environments. However, our knowledge of the Quaternary environmental history of the region is still rather limited, especially for the time preceding the last glaciation of the Alps. The sedimentary filling of overdeepened troughs - elongated depressions of subglacial origin cut into both unconsolidated sediments and bedrock - are excellent archives for various environmental reconstructions. Here, we present the results of a study of the glacially overdeepened Wehntal trough, northwest of Zurich, mainly based on multi-proxy data and luminescence chronology of a 93.6 m long sediment core, which has been drilled down to Miocene bedrock in the village of Niederweningen. This core represents one of the very few records in the northern Alpine Foreland that provides crucial insights into timing of erosion and infilling history of pre-Eemian glacially overdeepened structures. The recovered succession is interpreted from bottom to top as: in-situ molasse bedrock, overlain by diamictic till during Marine Isotope Stage (MIS) 8. It is suggested that this extensive ice advance caused the final erosion of the bedrock. The till merges seamlessly into laminated, carbonate-rich, fine-grained siliciclastic proglacial lake sediments, which accumulated also during MIS 8. This unit was deposited proximal to a calving glacier-front as confirmed by the presence of numerous dropstones. The overlaying lake sediments are comparable to the former unit, but the absence of dropstones indicates a more distal lake facies and thus, a retreat of the glacier lobe during MIS 7. Low organic carbon contents point to almost no biological activity and suggest cold environmental conditions. The continuous succession is now interrupted by a hiatus that covers almost the complete MIS 6, which is believed to represent a major glaciation of the Alps. Although a striking drop in carbonate content and shear strength, as well as an increase in grain size can be noticed in the topping sequence, no obvious erosional features are visible in the sediment fabric. The top of the unconformably overlying unit documents a gradual infilling of the palaeolake and onset of biological productivity due to climate warming during early MIS 5. The prominent environmental change culminates in the abrupt accumulation of peat during late MIS 5e. During early MIS 3, the peat was flooded, producing silty sediments, which show relative high organic carbon contents. Thereafter, the palaeolake was filled again, resulting in the accumulation of fossil-rich peat c. 45 ka ago, which was finally covered with local alluvial silts and sands.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMEP51B0552D
- Keywords:
-
- 4914 PALEOCEANOGRAPHY / Continental climate records;
- 4926 PALEOCEANOGRAPHY / Glacial;
- 4936 PALEOCEANOGRAPHY / Interglacial;
- 4942 PALEOCEANOGRAPHY / Limnology