Quaternary diatomaceous sediments and the geological evolution of lakes, Turkana, Baringo and Bogoria Kenya Rift Valley
Quaternary lacustrine sediments are described from three contrasting areas within the Kenya Rift Valley. Descriptions are given of the mid-Pleistocene Olorgesailie Formation at Olorgesailie. (southern-, Kenya Rift Valley), a series of lacustrine sediments deposited between the mid-Miocene and present in the Baringo District (central Kenya Rift Valley) and finally of Quaternary'(largely Holocene) deposits at East Turkana (northern Kenya Rift Valley). A wide range of environments are represented by these deposits including offshore and littoral lacustrine, deltaic and alluvial situations. Emphasis is placed on the examination of lacustrine and lake marginal sediments. Diatom assemblages found. in these deposits are described for the first time. These have 'been studied using optical and scanning electron microscopy. The. relationships between diatom assemblages and sedimentary facies are examined and evolutionary trends in certain diatoms are discussed. The contemporary ecology of diatoms at East Turkana is discussed and a review is given of diatom ecology and lake classification in East Africa. Diatoms are used to indicate transgression-regression. cycles during the Holocene, and palaeoecological conditions through the Quaternary. Mapping in conjunction with some altimetric data is used to indicate the location, extent and height of several Holocene lacustrine still-stands. Geochemical and sedimentological data is presented for the Holocene deposits at East Turkana and in the Baringo District. Several erosional and depositional processes operating at East Turkana are briefly discussed. A classification of Holocene environments at East T»rksna is presented. The palaeogeography of the northern Kenya Rift Valley and the development of diatom floras during the Holocene is discussed. Data presented here and in the literature is considered and reviewed from a palaeoclimatic viewpoint. The development of Lakes Turkana, Baringo and Bogoria through the Quaternary are also considered. Conclusions are drawn as to the value of diatoms in palaeoecology and stratigraphy.