Verwitterungsabhängige geochemische, mikrostrukturelle und geomechanische Eigenschaften halbfester Tonsteine der oberen Röt-Folge (Rhön)

Summary

The weathering State of lower Triassic shales (Upper Bunter, Rhoen mountains, Germany FR) was characterized by mineralogical, geochemical, micro structural and geomechanical methods comparing materials of near-surface weathering profiles and cores from a 50 m deep bore hole (fig. 1). The clay fraction increases towards the Earth’s surface. The predominant clay minerals are illite, chlorite and corrensite. The mobility of major cations, determined by selective extraction methods, increases with increasing weathering intensity. Due to the dissolution of (dolomitic) carbonates and of iron Compounds the stability of intergranular bonds is loosened and is not compensated by newly formed minerals. It results in a mixture of unweathered mineral aggregates and plastified clay matrix The depth distribution of the stability of the major minerals and the tendencies of mineral transformations were confirmed by use of the thermodynamic Computer model WATEQF (fig. 2) and microprobe analysis.

The interrelations between the changes of geomechnical properties (e.g. cohesion, compressibility, shear strength, creep and failure behavior (tab. 1 and 2)) and the complex mineralogical, geochemical and microstructural processes (fig. 3 – 5) during weathering are described and their causes are discussed.