Einfluß organischer Substanzen auf die verwitterungsbedingte Entfestigung von Peliten am Beispiel der Posidonienschiefer

Summary

Organic content significantly influences strength and weathering behavior of rocks. This correlation has been demonstrated with laboratory tests and field observations for the Lower Toarcian Posidonia Shales of southwestern Germany (Lias epsilon).

Two mechanisms influence the weathering processes: Physical disintegration during drying and Wetting cycles, and chemical oxidation of the long-chain organic molecules linking the mineral components. The weathering behaviour of the Posidonia Shales and other pelitic rocks with low carbonate contents is controlled by a threshold value of organic carbon. Slaking occurs if the amount of organic carbon is below approximately 4 percent (fig. 2). When the organic content is higher, slaking does not take place, but bedding-parallel exfoliation can be observed (fig. 1). Weathering profiles are not highly developed, due to the large weathering resistance of the Posidonia Shales. It is usually possible to reach almost unweathered bedrock at a depth of around 1 to 2 meters below ground surface. A distinct dependence of rock strength on organic content has been proven by uni-axial compression tests (fig. 4).