Mesozoic-Tertiary weathering and soil formation on slates of the Rhenish Massif, Germany

Abstract

Weathering of Paleozoic slates and sandstones of the Rhenish Massif in western Germany, which occurred during the Upper Mesozoic and Tertiary under warm, humid tropical to subtropical climatic conditions, led to the formation of a regolith up to 150 metres thick. In flat upland areas of the Hunsru¨ck and Eifel, where the tectonic uplift was relatively weak, remnants of this weathering mantle, more or less truncated by Upper Tertiary and Pleistocene erosion, are still preserved. On overlapping sections and drill cores of an autochthonous paleosoil and the saprolite underneath, the morphological, geochemical and mineralogical zonation of a complete profile of the weathering mantle has been reconstructed. A fossil pre-Oligocene Plinthitic Acrisol, covered by Oligocene fluvial sediments, displays characteristics formed under a high groundwater table. On the other hand the underlying saprolite can be subdivided into a lower reduction horizon and an upper oxidation horizon, each of them more than 40 meters thick. This indicates a polygenesis of the weathering mantle during the Tertiary, due to descending groundwater tables caused by the beginning tectonic uplift of the Rhenish Massif and drier climatic periods during the Middle Tertiary. During the Upper Oligocene the deposition of kaolinitic clays derived from removed horizons of the kaolinitic soils started in fault-block depressions. Periglacial processes during the Pleistocene led to a further removal of the weathering mantle and to the deposition of superficial layers on top of the autochthonous remnants of the saprolite. Presently the kaolinitic sediments as well as the saprolite are exploited as kaolin deposits.