The mineralogy and sedimentology of recent subaqueous Sediments in lake Turkana, Kenya

Abstract

This study was carried out between January 1990 and January 1992. The objectives were to determine the sedimentology, mineralogy and chemistry of the sediments, with particular regard to their interbasinal variations, and to determine the relative importance of the different prO\z~ance areas and transportation modes on their ~ ~ composition. The influence of the lake water on sediment composition was also investigated. Lake Turkana has a pH of 9.2 and a salinity of 2.50/00. The water column is well mixed by strong diurnal winds, and has a uniform temperature of 25-260 C and dissolved oxygen concentration ranging from 70-100% saturation. It is a Na- HC03 lake. The major source of allogenic sediment supply is the Omo river drainage area. Secondary sources are the Kerio- Turkwel drainage area and sources to the south east of the lake. Grain size increases southward from the Omo river delta. The North Basin profundal sediments are silty clays, the Central Basin sediments are clayey silts and the South Basin sediments are silts.,_. Larger grain sizes in the Central Basin as compared to the North Basin is attributed to differences in the source area geology. North Basin sediments are largely derived from the Omo river drainage basin which is dominated by fine grained volcanic rocks, while the Central Basin sediments are derived largely from the Kerio- Turkwel drainage basin, which is dominated by coarser grained metamorphic rocks. Relatively larger grain sizes in the South Basin is mainly due to higher concentrations of silt-sized biogenic and aeolian sediments as a result of decreased dilution by fluvial detritus due to its distance and isolation from the fluvial input sources. The grain size distributions downcore exhibit .enantiomorphism, which is interpreted as being diagnostic of shifts in the equilibrium energy regime of the transporting media. It has been used to determine the dominant provenance of particular size grades. The sediments consist of a kaolinite-illite-montmorillonite mineral assemblage. Kaolinite and illite are supplied mainly by the Omo and KerioTurkwel rivers respectively. Montmorillonite is mainly authigenic, being derived from the alteration of volcanic ash and glass, and precipitating from colloids in solution. It alters diagenetically to saponite, a Mg-rich smectite, as a result of interaction with pore waters. The clay minerals serve as important regulators of the chemical composition of the lake and pore waters due to their high cation exchange capacity and strong adsorption properties. They thus limit the availability of certain ions in solution, restricting the formation of some authigenic minerals which may otherwise be saturated in the water column. The aeolian fract.ion consists of a metamorphic assemblag~e of '.. -:-.: quartz, feldspar and blue-green amphiboles which constitutes the ubiquitous and distinct very fine sand size grade. Other authigenic minerals include low Mg-calcite, which accounts for about 10% of the total sediment. This value is increased in the South Basin by biogenic calcite (upto 25% of total sediment). Biogenic silica is also important in the South Basin, being contributed to by diatoms. "