EVIDENCE FOR >5 MA PALEO-EXPOSURE OF A PALEOSOL IN THE BOHNERZ FORMATION, SWITZERLAND

Soils are usually seen as transient deposits. Their formation, burial, and erosion has been linked to changes in tectonic and climatic conditions. Previous studies found that the exposure ages of modern soils in stable environments are on the order of 10 ka to 100 ka. We measured cosmogenic ³He in goethite pisoliths from the Bohnerz formation (Siderolithikum). A cross-section of the paleosol is exposed in a quarry near Lohn, Switzerland. The paleosol consists of red clay of about 3 m thickness developed on deeply weathered Jurassic limestone and overlain by early Miocene conglomerate. It contains an abundance of ferruginous pisoliths of 1-5 mm diameter. We sampled a vertical profile of pisoliths at 10 cm intervals. They are composed mostly of goethite, which is a He-retentive phase. The ³He concentration of individual pisoliths was measured by degassing the sample. The maximum exposure age at the surface of the paleosol is around 5 Ma. The concentration of 3He decreases with depth. The (U-Th)/He formation ages of the pisoliths range from 40 Ma to 18 Ma. These results indicate that the clay soils of the Bohnerz formation may have been continuously exposed at the surface for more than 5 Ma. The existance of the overlying conglomerate demonstrates that this is true paleo-exposure. Modern exposure ages of similar magnitude have only been reported from tectonically and climatically stable regions, such as the Dry Valleys of Antarctica. The Alb region of Switzerland and Germany, however, has been subject to tectonic, morphologic, and climatic changes during the time of exposure. We propose that clay soils may be stable for millions of years despite environmental changes.