Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana) eastern Rift Valley, Kenya

Tertiary sediments around Lake Rudolf (now Lake Turkana) in the East African Rift Valley have yielded abundant palaeontological and palaeoanthropological remains. The present study provides a basis for interpreting the ancient lake environment and furthering our knowledge of rift valley lacustrine deposits. Bottom sediments in Lake Rudolf are fine-grained (average 71% clay) well laminated and have montmorillonite, kaolinite and illite as the principal clay minerals. The sediments are relatively poor in silica (40–45%) but rich in Fe₂O₃ (10%). Both mineral proportions and chemical composition change systematically over the area of the lake and delineate four sedimentological provinces: (1) iron-rich, silty kaolinitic muds (Omo Delta); (2) iron-rich, fine-grained montmorillonite muds (North Basin); (3) silty montmorillonite muds rich in Na₂O and K₂O (Central Delta); and (4) argillaceous calcite silts (South Basin). Omo Delta and North Basin sediments are derived from the volcanics of the Ethiopian plateau; the source of Central Delta sediments is the Precambrian metamorphic terrain of the rift valley margin; the South Basin has a restricted detrital input. The water in the lake is alkaline (pH 9.2) and moderately saline (TDS = 2500 p.p.m.). Comparisons with influent water from the Omo River indicate a 200-fold concentration for the lake water. Models based on equilibrium between sediments and water column account for most of the non-conservative chemical components in the lake water. Sedimentation rates are high (about 1 m per 1000 years) and the dominance of detrital sediments makes Lake Rudolf unusual in comparison with other closed-basin lakes in the African Rift Valley although some similarities with ancient rift valley deposits are suggested.