Distribution, geometry, age and origin of overdeepened valleys and basins in the Alps and their foreland

Abstract

Overdeepened valleys and basins are commonly found below the present landscape surface in areas that were affected by Quaternary glaciations. Overdeepened troughs and their sedimentary fillings are important in applied geology, for example, for geotechnics of deep foundations and tunnelling, groundwater resource management, and radioactive waste disposal. This publication is an overview of the areal distribution and the geometry of overdeepened troughs in the Alps and their foreland, and summarises the present knowledge of the age and potential processes that may have caused deep erosion. It is shown that overdeepened features within the Alps concur mainly with tectonic structures and/or weak lithologies as well as with Pleistocene ice confluence and partly also diffluence situations. In the foreland, overdeepening is found as elongated buried valleys, mainly oriented in the direction of former ice flow, and glacially scoured basins in the ablation area of glaciers. Some buried deeply incised valleys were generated by fluvial down-cutting during the Messinian crisis but this mechanism of formation applies only for the southern side of the Alps. Lithostratigraphic records and dating evidence reveal that overdeepened valleys were repeatedly occupied and excavated by glaciers during past glaciations. However, the age of the original formation of (non-Messinian) overdeepened structures remains unknown. The mechanisms causing overdeepening also remain unidentified and it can only be speculated that pressurised meltwater played an important role in this context.

A geological snapshot from the front of the Northern Calcareous Alps: Well Obermoos TH-1, Salzburg, Austria

“…We tentatively argue for a tributary of the paleo-Salzach river draining Alpine basin further to the north (Donadel et al, 2014). The encountered coarse-grained facies is very different from the common facies of the Salzach Valley fill near the city of Salzburg mainly comprising lacustrine fines / mudstones (Kramer and Kröll, 1977;Brandecker, 1974;van Husen, 1980;Preusser et al, 2010).…”

Quaternary evolution of the inner Riss Valley, Tyrol (Austria) – an integrated sedimentological and geophysical case study

“…Such a bipartite valley fill is found in many glacially carved valleys (e.g. Brückl et al, 2010;Preusser et al, 2010).…”

“…For example, the central Inn Valley in Tyrol was some 250 m higher at the end of the Middle Würmian some 30 ka ago and the river valley was two to three times wider than today . The Last Glacial Maximum (LGM) resulted in large-scale erosion of these former sediments, whereby the depth of this subglacial erosion is poorly known in most valleys of the Alps (Preusser et al, 2010). Subsequent to the LGM large quantities of clastic sediment were deposited during the meltdown of the ice masses .…”

Interaction of various flow systems in small alpine catchments: conceptual model of the upper Gurk Valley aquifer, Carinthia, Austria

“…Thick layers of coarsegrained (gravel and sand) alluvial sediments fill the glacially overdeepened alpine valleys. Postglacial landslides are a consequence of pressure release after deglaciation (Preusser et al 2010). Particularly in narrow alpine valleys such as the presented upper Gurk Valley, these landslides create temporal barriers for the surface runoff.…”

“…In the resulting temporal lakes, lacustrine sedimentation takes place and leads to the development of fine-grained (silt and clay) layers. Changing conditions with sequences of alluvial and lacustrine sedimentation lead to a heterogenous internal structure of the sedimentary valley fill (Preusser et al 2010;Bleibinhaus and Hilberg 2012;Salcher et al 2012) with strong influences on the aquifer structure and the groundwater flow dynamics within the valley floor.…”