THE GEOLOGY OF THE SPITZINGSEE AREA – AN EXAMPLE FOR THE CONNECTION OF STRATIGRAPHY, TECTONICS AND ASPECTS OF ENGINEERING GEOLOGY

The area around the Lake Spitzing and the village of “Spitzingsee” (Oberbayern) is not only a popular region for hiking and skiing in the vicinity of Munich, but also gives excellent insights into the geological structure of the Bavarian Alps. Hardly in another area is the combination of geological structure, utilization of surface, touristic development and slope instabilities more evident than here.

The Spitzingsee area is situated within the “Bavarian Synklinorium”, a double syncline system of the Lechtal nape, limited by wide dolomite zones in the north and the south. Caused by the folding of the sedimentary rocks the stratigraphy from Triassic to the lower Cretaceous is obvious in different facies types. The geological construction is connected closely with the relief, the vegetation and the utilization of man: Limestone and dolomite stick out as jagged, bare rocks and peaks and show the tectonic structure in an excellent way. In contrast on the soft, weatherable marlstones develop fruitful, but very wet meadows often used as pastures and being suitable for touristic development by building hiking trails and skiing courses.

With regard to the tectonic structure the Spitzingsee area is a key for the interpretation of folded mountain belts. Actual field mapping showed that the folded areas are not only dominated by overthrusting and diagonal faults. Also fractures vertical to the folding axis (so called A/C faults) are more important as up to now estimated appearing as numerous cross cutting N-S faults. Their development corresponds with changes of facies in the Kössen Formation and can be explained with the different deformation of bedded series rich of marlstones on the one hand and massive limestones on the other hand.

Typical for the alpine area are different types of slope instabilities as a result of the Pleistocene oversteepening of slopes. In unfavorable dipping conditions for example, the calcareous beds drift downslope on top of soft, weathered marl layers. Weathering processes continuously accumulate loose material, which may be transported down the slope in discrete events. Actual examples are the debris flow near the “Grundalm” and the “Krottenthal landslide”, a debris flow which caused wide attention in 1996.