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Ancient Fluvial Systems
Fluvial Facies of the Plio-Pleistocene Koobi Fora Formation, Karari Ridge, East Lake Turkana, Kenya
Four major lithofacies have been described and interpreted (Vondra and Bowen, 1976) as a complex of lacustrine, transitional lacustrine, deltaic and fluvial environments of an embayment of paleo Lake Turkana. Fluvial environments are recorded by the lenticular conglomerate, sandstone and mudstone facies of the Upper Member of the Koobi Fora Formation which consists of four interfingering subfacies: 1) the interbedded basalt conglomerate and pebbly mudstone, 2) the lenticular basalt clast conglomerate, 3) the polymictic conglomerate and sandstone, and 4) the interbedded sandstone and tuffaceous siltstone subfacies. These are well exposed along the Karari Escarpment and represent 1) alluvial fan channel and debris flow; 2) high-energy channel — bar core, gravel sheet; 3) lower energy channel — bar-side, transverse bar, and point bar; and 4) floodplain depositional environments, respectively.
Body geometries, sequences of primary structures and textural and compositional variations by which the subfacies are interpreted are well exemplified by the Upper Member. Lateral upstream-downstream (longitudinal) and channel-floodplain (transverse) as well as vertical subfacies relationships indicate that a series of ephemeral streams entered the basin from the volcanic highlands to the northeast and east, forming a belt of alluvial fan deposits along the basin margin. A high-energy, low sinuosity braided river system flowed southwestward across a broad alluvial plain toward paleo Lake Turkana. This system gave way vertically to a low energy, high sinuosity meandering river system as the embayment was filled with sediment by a prograding delta complex.
Episodic volcanism surrounding the Lake Turkana basin is manifest throughout the sedimentary sequence by interbedded volcaniclastic units, and reflects the tectonically active Plio-Pleistocene setting of the region.
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