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Examples of sedimentation in the rift lakes of Eastern Africa - Tectonic versus Climatic controls: A comparison between the Eocene - mid Miocene Lake Lokichar and the mid Miocene - Present Lake Turkana in Northern Kenya. Consequences on hydrocarbon prospects

Abstract : Extension processes have dominated the evolution of the northern segment of the Kenya Rift from Eocene to Present-day. Several typical N-S half-graben basins have been created during two major phases of extension during Eocene times then mid-Miocene times, each phase being characterized by migration of extension processes from west to east then south to north, resulting in the development of two elongated half-grabens, i e. the Lokichar Basin of Eocene - mid-Miocene age, and the Turkana Basin of mid-Miocene - Present age. Sedimentation processes in these two rift basins have been primarily controlled by their typical half-graben morphology as well as by climatic conditions prevailing on each basin during its development, i.e. tropical humid conditions for the Lokichar Basin, and semi-arid conditions for the Turkana Basin. Large lateral clastic inputs are issued from fault-controlled watersheds, i.e. the major border faults - Lokichar Fault for the Lokichar Basin, Murua Rith-Lapur Fault for the Turkana Basin - characterized by a Precambrian basement-dominated watershed for Lokichar, and a volcanic-dominated watershed for the Turkana Basin. Tropical climate and associated vegetation prevailed in the Lake Lokichar watershed from Eocene to lower Miocene resulting in quartz-rich, well-sorted sand inputs in shallow lake waters and deltaic platforms. In the Turkana Basin, semi-arid conditions induced poorly sorted conglomerates and sandstones inputs, transported by an ephemeral lateral fluvial network and deposited in shallow depth littoral platforms. The nature of the axial clastic inputs in both lakes were mainly controlled by the fault structure at the watershed as well as by its lithology, issuing well-sorted quartz-rich sands for the Lokichar Basin, and mainly silt and mud from the volcanic-dominated far watershed for the Turkana Basin. Major differences in basin sedimentation for both the Lokichar and Turkana Basins concern the deep lake basin environment. Thick (100m's), black, organic-rich shales form 2 major sedimentary units deposited in the Lokichar Basin during 2 lake highstand periods in the Eocene and Oligocene. Organic matter resulting from freshwater algii indicates the existence at these times, of a deep freshwater lacustrine environment with well-marked anoxic conditions at depth, which is in line with high rainfall conditions at the watershed. Offshore lake sedimentation in the Turkana Basin was controlled by semi-arid climate conditions, resulting in major inputs of fine-grained sediment with poor organic content from axial and lateral origins. Organic-rich mudstones only occurred in the Turkana Basin during high lake levels that characterized the Plio-Pleistocene period. Thereby, climate conditions play a major role in rift basin sedimentation by controlling the sorting and petrography of inflowing sediments, as well as influence offshore sedimentation, where organic-rich deposits characterize deep lakes/high lake level conditions that can be linked to short or longer humid climatic conditions. Precise knowledge of such conditions may greatly help to identify lake basins favourable for hydrocarbon prospects.
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Contributor : Isabelle Dubigeon <>
Submitted on : Wednesday, February 1, 2012 - 1:40:33 PM
Last modification on : Friday, April 5, 2019 - 8:17:31 PM

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  • HAL Id : insu-00665198, version 1

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Peter Thuo, George Muia, Jean-Jacques Tiercelin. Examples of sedimentation in the rift lakes of Eastern Africa - Tectonic versus Climatic controls: A comparison between the Eocene - mid Miocene Lake Lokichar and the mid Miocene - Present Lake Turkana in Northern Kenya. Consequences on hydrocarbon prospects. 5th International Congress of Limnogeology, Aug 2011, Constance, Germany. ⟨insu-00665198⟩

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