| dc.description.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.
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