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

We present the results of a study examining Quaternary sediments in the central and inner Riss Valley of the Karwendel Mountains, Tyrol. By using geological mapping, seismic and geoelectrical surveys, as well as shallow drilling and radiocarbon dating we investigated the sedimentary evolution of this previously poorly studied area. At Großer Ahornboden, seismic data reveal a glacially carved bedrock surface, buried beneath up to 140 m of sediments. The lowest sediment sequence consists of consolidated sediments showing high seismic velocities (2750 m/s) and reaching a thickness of 115 m. The overlying uncompacted sequence yielded lower velocity values (500 to 1100 m/s) and is interpreted as Holocene valley fill, with sediment derived from Rissbach river, small talus screes and debris-flow fans from tributary creeks. Up to 10-m-thick deposits of a Holocene paleolake, dammed by a large Lateglacial terminal moraine ridge, are present at the northern rim of Großer Ahornboden. Radiocarbon dates constrain the duration of this lake to between 10.5-10.2 and 5.5-5.3 cal ka BP. Lacustrine sediments pinch out in the subsurface just below the small fan leading down from Tränkkarl. In the central Riss Valley over 100 m thick, consolidated sediments of a proglacial delta are exposed. This delta complex consists of bottomset silts and thick foreset gravel and is overlain by diamict forming poorly developed moraine ridges of Lateglacial origin.