Großer Ahornboden, Karwendel

It is the symbol of the Karwendel Mountains and its Alpenpark, the Großer Ahornboden, declared a natural monument in 1927. Located just north of the prominent east-west trending cliffs of the Karwendel Hauptkamm the Great Ahornboden forms the head of the Riss Valley.
As part of a long-term geological mapping project of the Rissbach Valley we have started to study the Quaternary history of the conspicuously wide and flat Ahornboden, home of some 2200 sycamore maple trees. Dendrochronological studies have shown that the old population of these trees dates back to the 17^th century.
We have conducted a series of field studies to elucidate the subsurface of the Großer Ahornboden, several of them as part of student practicals of the Master´s programme in Earth Sciences. In late 2011 and again this summer students performed electrical resistivity measurements along profiles across this valley, supervised by Prof. Robert Scholger, a geophysicist of Leoben University. These geoelectrical studies demonstrated overdeepening along the south-north axis of the Großer Ahornboden. Subsequent reflection and refraction seismic studies, carried out collaboratively with Werner Chwatal (Technical University of Vienna), refined this picture and allowed to map the base of the Quaternary valley fill at ca. 150 m below the present valley floor getting shallower towards the South. The seismic images revealed a textbook-like example of a U-shape valley cross-section characteristic of glacially carved valleys. Most of the valley fill appears to be rather highly compacted gravel, but the top ca. 10-15 m are unconsolidated sediment. These top sediments provide detailed insights into the geologically youngest history of this area with is also relevant for the current vegetation in general and the maple trees in particular. Previous studies have shown that these trees root in fine-grained sediments and are buried by up to 1.5 m of gravel derived from debris-flows of the Engergrundbach. This creek has repeatedly devastated the plain and the adjacent Engalm pastures in historical times. Our ongoing studies of the clayey silts beneath the gravel layer revealed the existence of a lake in the central and northern part of the Great Ahornboden, which existed at least during the 4^th millennium BC. The lakebed sediments are more than 8 m thick in the northern part as shown by ram and driving core sounding. Ongoing research, partly carried out within the Master thesis work of David Mair from our group focuses on the environmental implications of the onset and termination of this palaeolake. Pollen studies conducted in collaboration with Prof. Klaus Oeggl will allow to address the question since when the Großer Ahornboden has been used as a summer pasture for cattle.