Relations between geomorphic and geologic framework and sinkhole distribution of the South Franconian Alb, Germany

The South Franconian Alb is a well known karst area in Southern Germany. It comprises mainly of a slightly inclined plateau intersected by a few rivers and numberless dry valleys. Main rocks are limestones and dolomites of Jurassic Age.

Numerous sinkholes occur within the area. Unfortunately, till now the data collection is fragmentary done for the whole area. Consequently, the sinkhole distribution is incomplete and very heterogenously spread. Nevertheless, to gain insights into the background of sinkhole distribution and the associated geologic, geomorphic and land use conditions the available data were compiled. Different local archives, the available geologic and topographic maps of the South Franconian Alb were searched for sinkhole informations, on the other hand digital elevation models from selected areas were detected for hollow shapes or depressions to estimate the maximum quantity of possible dolines. For all detected objects both verified sinkholes and unclassified depressions the geomorphological environment, cover deposit, host rock, rock facies, stratigraphy and land use were listed.

First results show great discrepancies for the sinkhole distribution related to land use. More than 90% of the detected objects are located within forests although forested areas cover only 30-50% of the South Franconian Alb. Thus, most of former sinkholes were destroyed by agricultural or other activities. Furthermore, historic mining activities (stone-age chert mining, historic mining pit areas for iron mining, small local quarries) have also changed the sinkhole distribution. Due to such anthropogenic overprinting of the landscape an automatic detection of dolines from digital elevation models requires a very critical assessment.

Geologically, sinkhole occurrences are closely related to the host rock distribution or rock facies. About 60% of sinkholes are hosted by dolomites, massive reefal or thick-bedded limestones whereas dolines within platy or thin-bedded rocks occur more rare.