THE DEVELOPMENT OF IRON CRUST LATERITIC SYSTEMS IN BURKINA-FASO, WEST-AFRICA EXAMINED WITH IN-SITU-PRODUCED COSMOGENIC NUCLIDES

We have investigated the development of iron crust laterites on the stable West African Craton in northern Burkina Faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles. Lateritic soils develop in tectonically stable, slowly eroding, tropical environments and are a major component of the Earth's surface. To examine processes affecting laterite formation, we determined Be-10 and Al-26 in samples of quartz from three sites representing two sequential and connected iron crust laterite systems. Results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3-8 m Myr-1. In addition, data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth-dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss. Results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years. Quartz cobbles, presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial-colluvial material, have Be-10 distributions that suggest that the lowland lateritic surface may have formed during an erosive episode, presumably associated with a wetter climate, roughly 300 kyr BP. These results illustrate the practicality and the potential of the use of in-situ-produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in-situ chemical weathering and mechanical transport.