Colluvial filling of a glacial bypass channel near the Chiemsee (Stöttham) and its function as geoarchive

For more than a hundred years the Chiemgau area in Southern Bavaria has been well known for its remarkably well developed polymorphic glacial landscape, including large moraines, ice-scour lakes, glacial melt out terraces and kettle lakes. Indeed this area provides landscape features internationally recognized as being typical of glacial geomorphology. Nevertheless, it has more recently been suggested that one of these kettle lakes, the Tüttensee, was actually formed by a meteorite impact; some of the glacial paleo-channel fill at a site near Stöttham, close to the Chiemsee, is argued to be made up of a mixture of ejecta from this impact event and sediments from a resulting tsunami. The event is thought to have taken place in the Celtic Period. In our study we investigate the paleo-channel fill at Stöttham using a combination of various field sedimentological and pedological methods together with extensive laboratory investigations, all with the aim of characterizing the genesis of the relevant paleosoils and sediments. We also use both relative and absolute dating techniques, including sediment and archaeological stratigraphy, radiocarbon dating of charred organic matter and optical stimulated luminescence dating using quartz to provide a chronostratigraphy for the paleo-channel fill. At its base the paleo-channel contains a well developed soil of late glacial age; the channel was then progressively filled with gravels and soils developed on finer-textured sediments. The sediment texture and colour, the clast orientation, and the numerous archaeological artefacts all provide clear evidence for continuous colluvial processes occurring over thousands of years. Four independent dating techniques all support a consistent stratigraphy starting in the Late Glacial and continuing until Medieval times. The layer that has been previously suggested to be a mixture of ejecta and tsunami sediments was dated to between 3.7 ± 0.2 ka and 4.3 ± 0.3 ka by the various methods, i. e. to the Bronze Age Period. The luminescence studies using quartz grains suggest complete bleaching of the grains by daylight exposure prior to sedimentation, provide additional arguments for a rather slow geomorphological erosion and transport process such as colluviation, prior to deposition. Taken together, the sedimentological, pedological and chronological data argue strongly against any catastrophic geomorphological event as the origin of the channel fill at Stöttham. On the contrary, the studies provide consistent evidence for colluvial processes being the main mechanism for filling the channel during the Holocene; such processes are often linked to settlement phases in this area.