Abstract
Summary It has previously been considered that the sedimentary rocks of the Koobi Fora basin, NE of Lake Turkana, lie disconformably upon older volcanic strata, local volcanism and sedimentation being separated in time. Volcanic and sediment relationships observed at the boundary of the basin reveal that major Pliocene tholeiitic flood volcanism took place after the initiation of proto-Lake Turkana. Significant lacustrine and fluviatile deposits lie beneath lavas in the upland margin of the northern part of the basin. These are lithologically similar, and may be laterally equivalent, to strata outcropping along the northern edge of the basin and on the southeastern flank of the Kokoi upland. The fine-grained lacustrine rocks formed during an extended period of slow sedimentation. The age of Lake Turkana, therefore, may be significantly greater than 4.3 Ma, which is recorded from the lowermost tuff bed within the Koobi Fora basin (McDougall 1985). Further lacustrine sediments form thin but laterally persistent intercalations between flood lavas. Concurrence of large scale subaerial eruptions and lacustrine sedimentation is explained by a model of volcano-tumescence and deflation. The early Pliocene lake was relatively shallow and of greater extent than present-day Lake Turkana. Swellings of the land surface preceding volcanic eruptions E of the lake produced significant withdrawals of the shoreline westwards. Subsidence, following lava outpourings, allowed transgression of the lake towards the former shorelines.
Published Version
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