Indian laterites, and lateritious residues of southern Germany: A petrographic, mineralogical, and geochemical comparison

The Bohnerz Formation of the Swabian Alb (southern Germany) currently exist s as a series of isolated pockets of iron rich pebbles, often pisolitic in nature and typically set within a kaolinitic matrix. These pockets represen t infillings of karst solution pits located on the higher elevations of the Swabian Alb. A history of exploitation of the Bohnerze for their iron indi cates their existence has long been recognized, however, their origin and e volution has remained a source of debate. Petrographical, mineralogical, an d geochemical comparison between the Bohnerz and Indian laterite clearly de monstrates that their origin is consistent with the development of a widesp read lateritic weather;ng residuum in southern Germany during the Cretaceou s and early Tertiary. The Bohnerz Formation therefore represents the residu es of a palaeo-weathering regime, and the last vestiges of a once extensive tropical weather;ng surface. Moreover the long term geomorphological evolu tion of the entire region is best explained in terms of a phase of Late Cre taceous - early Tertiary etchplanation which was terminated by erosion and stripping as a consequence of the climatic change since the later Eocene an d early Oligocene. The geomorphology of southern Germany is clearly a lands cape palimpsest controlled and influenced by regional tectonics associated with the adjacent Alpine orogeny, by Cretaceous 'greenhouse' conditions, an d by dramatic climatic changes at the end Cretaceous and during the Tertiar y.