Mapping large-scale thickness variations in the Helvetic domain (Switzerland) using a new semi-automated method

We present semi-automatically generated maps showing the variability of the stratigraphic thickness of the Helvetic Kieselkalk, a siliceous limestone-dominated geological unit which is widely exposed across different Helvetic nappes over more than 300 km along the Swiss Alps. The Helvetic Kieselkalk is commonly extracted to produce hard rock aggregates for the national road and railway infrastructure. The deposition of this unit onto the European (Helvetic) continental margin during the Early Cretaceous was affected by normal faulting, which lead to strong lateral thickness variations.

The Python and MATLAB approach used to create the thickness maps was developed as part of a Switzerland-wide mineral resource mapping project, funded by the Swiss Geological Survey (swisstopo). It is designed to rapidly generate large-scale map overviews of the stratigraphic thickness by analysing geological vector data such as the GeoCover dataset in Switzerland. The same approach is currently used in the framework of the swisstopo-funded Swiss Alps 3D project. There, automatically extracted and validated thickness data are used to improve the quality of the large-scale 3D geological model of the Swiss Alps.

Our results highlight an increase in thickness of the Helvetic Kieselkalk along the strike of the Alps from ca. 100 m in the western Helvetics (Wildhorn Nappe) to up to 1000 m in the eastern Helvetics (Drusberg and Säntis nappes). The depositional thickness was certainly affected by burial, folding and faulting during the formation of the Helvetic nappes. Nevertheless, two distinct thickness jumps indicate the presence of three sedimentary basins in east-west direction with a half-graben-like geometry. These thickness jumps coincide with present-day nappe boundaries and suggest that the inherited basin geometry influenced the formation of the Helvetic nappes.

The large-scale thickness maps and the improved undestanding of the paleogeography and tectonic evolution are helpful to identify stratiform mineral occurrences with favourable geometry and to refine 3D geological models.