EOCENE-PLEISTOCENE GOETHITE PRECIPITATION AND EXPOSURE HISTORY OF THE BOHNERZ FORMATION, GERMANY AND SWITZERLAND

The Bohnerz Formation consists of reddish to yellowish clays and contains abundant ferruginous pisoliths. It is widespread in Central Europe in karstic fissures and depressions on Jurassic limestones as well as in the form of paleosols. These deposits have previously been interpreted as being Cretaceous to Early Eocene. We measure (U-Th)/He formation ages as well as cosmogenic ³He in goethite from pisoliths to test this hypothesis. We analyzed pisoliths from a paleosol in Switzerland as well from several fissure locations across Southern Germany. We found that the formation ages of all analyzed pisoliths are between 41 Ma and 2 Ma, providing evidence that the Bohnerz Fm. is younger than expected from previous interpretations. The pisoliths from the paleosol record continuous goethite precipitation from 41 Ma to 17 Ma, when the paleosol was covered by a conglomerate. Only few ages are younger than 17 Ma, recording only minor goethite precipitation in the paleosol after being covered. The exposure age based on cosmogenic ³He concentrations is at least 5 Ma. Pisoliths from fissures in Southern Germany have formation ages of 14-2 Ma. They are consistently younger than the biostratigraphic ages, showing that these pisoliths grew in the fissure filling after it was deposited. We map the paleo-extent of the Bohnerz Fm. in Central Europe based on published studies to show that Fe-rich kaolinitic soils covered all of the Central European limestone plateaux. We show that the precipitation of goethite is linked to the formation of the palesol. The age of goethite precipitation corresponds to the biostratigraphic age range of fissure fillings. We therefore conclude that the Bohnerz Fm. formed between Late Eocene to Pleistocene.