Chapter 7. Albertine Rift basin deltas

One of the most obvious geomorphological features that can form at a coastline is a delta. In the Albertine Rift, at the long-axial shorelines of lakes away from rift-border faults at the basin flanks, and where there is a gentle, offshore topographic dip to the rift valley floor, then a long-axial delta system (LADS) can form. Low-level aerial photography demonstrates that these have a relatively high sediment discharge over a remarkably short period of time, prograding tens of metres in only a few years. This produces an elongate delta finger lobe, with associated characteristic geomorphological features, such as dextral and sinistral spits, barrier and mouth bars. Yet, no delta finger lobes are more than a few kilometres long, indicating that incremental movement along rift-bounding and intrabasinal faults change the dip of the rift valley floor and cause forced avulsion in rivers and rapid abandonment of a LADS. Together, these factors produce easily identifiable intervals within Pleistocene–Holocene rift-fill stratigraphy that display coarsening-up parasequence sets within an overall 3–5 m deltaic package. Typically, these will be stacked in the stratigraphy between recognizable marginal lacustrine sediments, such as sand berms and runnel lagoons.