Chapter 5. Albertine Rift basin lakes

The Albertine Rift is dominated by Lakes Albert and Edward, which together represent two of the great rift lakes of East Africa. Rift basin lakes form one of the most obvious geomorphological features in an early continental rift basin, as crustal extension proceeds, causing the rift valley floor to subside and diverting the regional drainage pattern into the depocentre. Modern-day open lacustrine sedimentation in Albertine Rift lakes is dominated by deposition of organic-rich clays and diatomites, with fan deltas and turbidites delivering clastics from the flanks and long axes of the rift basins, respectively. Oxidation colours in the clays can be used to indicate bottom-water anoxia and seasonal mixing. Field evidence from Plio-Pleistocene rift-fill sediments demonstrates that diatomite horizons mark lacustrine transgressions, with deeper clay intervals recording elevated uranium (U) and thorium (Th) concentrations. Alkaline lake marly limestones, with algal mats, were discovered in two areas of onshore Lake Edward and preserve exceptionally high U and Th signatures. Similar values were identified in lacustrine clays of northern Lake Albert and suggest the development of restricted alkaline lake conditions within the Albertine Rift during its geological evolution were more common than previously acknowledged.