Tectonic Controls on the Development of Rift-Basin Lakes and Their Sedimentary Character: Examples from the East African Rift System

Tectonic style and history exerted a strong influence on the development of East African Rift lakes. Fundamental rift-basin geometry is controlled, in large part, by extension rates and the angles of bounding faults. Where the ratio of vertical to horizontal displacement is large and climatic conditions result in fluvial discharge into the lake greater than evaporation, conditions are favorable for the development of deep, long-lived lacustrine systems. Where this ratio is lower, climatic conditions may only periodically support the formation of relatively shallow lakes. The timing and magnitude of rift margin uplift also exert an influence upon drainage patterns and therefore affect the discharge of water and sediment into the basin. Rift lakes with high-angle, high-relief margins tend to be filled by relatively fine-grained sediments with coarse elastics confined to rift margins and in the rift axis only at rift segment terminations or as basin plain turbidites. Basins with low-angle, low-relief boundary faults are commonly dominated by coarse elastics that may be deposited throughout the rift lake. Lakes in relatively short rift segments with well developed axial drainage systems flowing into them tend to be shallower and coarser-grained than those without them. Lake Tanganyika is a long, deep, relatively fine-grained lake, in a humid environment, characterized by steep, high-angle boundary faults with high vertical-to-horizontal displacement ratios and limited axial input. On the other hand, the southwestern Turkana basins (Upper Oligocene-Middle Miocene) are examples of half-grabens characterized by low-angle boundary faults and relatively low vertical-to-horizontal displacement ratios. They are dominated by coarse-grained fluvial deposits with inter-bedded shales which were deposited during episodic, shallow lacustrine conditions. The present Lake Turkana developed during two separate rifting episodes and has a complex history. Lake Rukwa appears to be a hybrid of the end members and serves as an example of how rift-lake systems can evolve with time. As is typical of continental rifts, its early history (Neogene) was dominated by coarse-clastic deposition. During the Plio-Pleistocene, lacustrine conditions, characterized by the deposition of muddy sediments, prevailed. The modern lake, which is in an arid setting, is dominated by well developed axial drainage systems which maintained sedimentation rates at levels greater than or equal to tectonic subsidence rates and is consequently shallow and sand-dominated.