Evidence for >5 Ma paleo-exposure of an Eocene–Miocene paleosol of the Bohnerz Formation, Switzerland

Abstract

We obtained (U–Th)/He formation ages and cosmogenic 3He concentrations for pisoliths from a paleosol of the Bohnerz Formation (Siderolithic) of Central Europe. The paleosol is exposed in the Almenbühl quarry near Lohn, Canton Schaffhausen, Switzerland. The paleosol consists of red clay of 3–4 m thickness developed on deeply weathered Jurassic limestone and overlain by Early Miocene conglomerates. The (U–Th)/He formation ages of the pisoliths are between 50 Ma and 8 Ma, with most ages being older than 17 Ma. There is a sharp decline in the frequency of ages at the time of burial of the paleosol at 17 Ma. These ages are inconsistent with the previous assumption that the Bohnerz Formation formed in a Cretaceous to Early Eocene laterite in a tropical climate. We propose that the Bohnerz Formation more closely resembles Terra Rossa soils, which do not require a tropical climate to form. The 3He concentration in the pisoliths is roughly constant with depth throughout the paleosol at 300 Matoms/g. We interpret this as the result of soil convection during cosmic ray exposure. The minimum exposure duration at the surface of the paleosol is ∼5 Ma. A simple model of soil convection shows that the true exposure duration of the paleosol is approximately 10–20 Ma. These results indicate that the clay soils of the Bohnerz formation were continuously exposed at the surface for millions of years. Since the paleosol was covered by conglomerate since 17 Ma, the 3He measured here was produced by cosmic ray exposure before burial. Cosmogenic 3He concentrations measured in fine-grained soil iron-oxides (<1 μm) are similar to those measured in pisoliths. This might indicate that fine-grained iron-oxides are retentive to helium and might be used for studying the formation and cosmic ray exposure of modern soils and paleosols.