A contributon to the geology of the Kavirondon Rift Valley

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Summary: The paper describes some of the geological work of the 1947 British-Kenya Expedition to Rusinga Island and other sites in western Kenya where Miocene anthropoids have been found. The pre-Miocene floor of the region consists of gneisses (Basement System) in the east, separated by the Nandi fault from altered basic igneous rocks (Nyanzian System) and post-Nyanzian granitic rocks in the west. Relics of a peneplain supposed to be Mesozoic are preserved on high residuals; about 1300–1500 feet lower is another surface, the sub-Miocene peneplain, which is correlated with the main (Buganda) peneplain of Uganda. In early Miocene times this surface was gently warped (there was no great pre-Miocene Kavirondo Valley) and extensive shallow lakes were formed. In the region of the present Kavirondo Rift Valley, normal sedimentation was soon augmented by the explosive eruption of carbonatite, ijolite and granitoid blocks, lamprophyric tuffs, melanite-nephelinite-agglomerate, and nephelinite-agglomerate. Nepheline-basalts were erupted from great volcanoes; phonolite flooded the area from the east. During these times normal faulting and earthquake shocks led to sub-lacustrine landslips and slumping. Local erosion is proved by intra-Miocene unconformities, formerly supposed to be thrusts. Another period of erosion followed, when the “ end-Tertiary peneplain ” was developed on the softer beds. In late Pliocene or earliest Pleistocene times the region was dislocated by powerful rift movements. The Kavirondo Rift Valley was depressed between faults and flexures, relatively to the regions to the north and south, while these regions were uplifted and tilted. The peneplains now slope away from the shoulder of the Eastern or Gregory Rift Valley towards the basin of Lake Victoria, where the waters were impounded by a transverse warp. These great movements do not seem to have caused any local structures suggestive of lateral compression; normal faulting is the rule. Yet a considerable negative isostatic anomaly was determined at Kisumu by Bullard. On the Elgeyo escarpment, which forms part of the western wall of the Gregory Rift Valley, Miocene beds are again exposed. They are intermediate in facies between those of Kavirondo and Turkana. The Elgeyo escarpment, like those bounding the Kavirondo Rift Valley, is a fault-scarp due to post-Miocene faulting.

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