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 st able West African Craton in northern Burkina Faso using cosmogenic rad ionuclides produced in situ in quartz veins and pebbles. Lateritic soi ls develop in tectonically stable, slowly eroding, tropical environmen ts and are a major component of the Earth's surface. To examine proces ses affecting laterite formation, we determined Be-10 and Al-26 in sam ples of quartz from three sites representing two sequential and connec ted 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 i ron crusts demonstrate that depth-dependent distributions of these nuc lides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss. Results from sections of the lowland laterit ic 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 mat erial, have Be-10 distributions that suggest that the lowland lateriti c surface may have formed during an erosive episode, presumably associ ated with a wetter climate, roughly 300 kyr BP. These results illustra te the practicality and the potential of the use of in-situ-produced c osmogenic nuclides for understanding the history of formation of later ites and for differentiating between systems formed through in-situ ch emical weathering and mechanical transport.