Chapter 8. Albertine Rift Fluvial Systems and Terrestrial Rift Valley Environments

Abstract

Whilst Albertine Rift marginal lacustrine and deltaic depositional environments each produce characteristic lithofacies that together can form a complex stratal architecture, ≥80% of the onshore area of these early continental rift basins is actually dominated by fluvial systems and their associated rift valley terrestrial environments. Albertine Rift rivers today can be classified as flank fan drainage, flank drainage rivers or long-axial systems. Dependent upon slope gradient and sinuosity, and reflecting the bed or suspended load transported, alluvial channels can cycle through four main stages before entering a delta distributary system at the lake shoreline. Higher energy flows closer to rift margins are dominated by gravel bed loads, transitioning to mixed and then suspended loads as gradients decrease out across the rift valley floor. Typical fluvial geomorphological features such as riffle-pool sequences, channel and side bars, meanders, point bars and ox-bow lakes are accompanied by development of characteristic ichno- and rhizo- fabrics in riparian sands and interfluve silts and clays. The four different fluvial stages can be recognized in onshore Pleistocene - Holocene sedimentary successions of both Lake Edward and Lake Albert, with well-sorted fluvial sand bodies, containing termite nests, correlating with petroleum reservoir intervals in the subsurface.