An inter-basinal comparison of the sedimentology of Late Holocene to recent sediments in the Rift Valley, Lake Turkana, Kenya

Abstract

Grain size variations, sediment chemistry and general mineralogical assemblages of sediments in Lake Turkana reflect provenance. Allogenic sediments in Lake Turkana are mainly supplied by the Omo and Kerio-Turkwel Rivers. Minor inputs are from seasonal streams and strong southeasterly winds. The depth profiles of the grain size distributions in lake sediment cores exhibit enantiomorphism, which is interpreted as being diagnostic both of shifts in the equilibrium energy regime of the transporting media and of the dominant provenance of particular size grades within the specific basins of the lake. The North Sub-basin is dominated by fine-grained sediments, which reflect the texture of the volcanic rocks of the Omo River drainage basin. The Central Sub-basin sediments reflect, as sources, the coarser metamorphic terrane of the Kerio-Turkwel Rivers drainage basin. Kaolinite and fine-grained iron oxides are brought into the lake mainly by the two large fluvial input systems: the Omo River in the North Sub-basin and the Kerio-Turkwel Rivers in the Central Sub-basin. Some fine-grained overflow of this material makes its way into the South Sub-basin. Illite in the North and Central Sub-basins is strongly related to transport of material from near-shore sediments and, in the Central Sub-basin and northern reaches of the South Sub-basin, from the Kerio and Turkwel Rivers input. Smectite and calcite are mainly authigenic. In the South Sub-basin, however, the relatively coarser detrital particles are derived from silt and sand-sized in situ biogenic (calcitic and siliceous) debris and ˦olian-transported particles from regions southeast of the lake. The ˦olian fraction accounts largely for the ubiquitous and distinct very fine sand size grade, and consists of quartz, feldspar and blue-green amphiboles.