MASS MOVEMENTS WITHIN CONGLOMERATES IN THE KAUFBEUREN AREA (BAVARIA, SOUTH GERMANY) – IDENTIFICATION AND RISK PREDICTION

Since the deadly rock fall in Stein an der Traun in 2010, mass movements within conglomerates are in the focus in south Germany. The area around Kaufbeuren is characterized by Pleistocene conglomerates, which formed from fluvioglacial meltwater gravel bodies from different ice-ages (Würm, Riss, Mindel, Günz, and Donau glaciation). They directly overlay flat-lying Neogene pelitic sediments from the Upper Freshwater Molasse (Obere Süßwassermolasse, North Alpine Foreland Basin). The entire succession is draped here by Würm and Riss glacial tills in varying thickness.

The gravel bodies are stratigraphically differentiated by their relative elevation, with the highest deposits being the oldest (Donau glaciation) and today’s valleys fill being the youngest (Würm glaciation). The total difference in height between the base of the oldest and youngest gravels is about 120 m. The pre-Würmian gravels are irregularly and often only loosely lithified by carbonate cements, thus forming highly porous conglomerates. They attain a thickness of up to 40 m, consisting predominantly of carbonaceous clasts. Grain sizes are usually around 4 to 8 cm.

The conglomerates form steep escarpments; in contrast, the underlying pelites are characterized by lower slope angles, and partly undulating smoother land surfaces. The top of the pelites is an ideal slide horizon, especially if weathered and wet. Thus, the slopes formed by the conglomerates are often affected by partly astonishing large mass movements, with the degree of glacial or glacio-fluvial erosion and the position of the Quaternary base being the most important controlling factors. Mass movements are relatively frequent along the margins of glacial spillways. Also, older, higher elevated conglomerates are more often affected than younger, lower lying conglomerates, which are stabilized by Würmian sediments covering the top Molasse boundary.

Financed by the Bayerisches Landesamt für Umwelt and the European Union, our aim is to map the area and to survey the mass movements occurring here. Based on the outcrops of the conglomerates, the position of their base (top Tertiary), the degree of oversteepening during the last ice age, and on the frequency of existing slides, the slopes are given different classes indicating the risk of future mass movements.