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Abstract:

Lake Bogoria is a saline (100 g l–1 TDS), alkaline (pH- 10.3) meromictic lake that lies in an asymmetric half-graben in the semi-arid central Kenya Rift. The lake is fed by about 200 hot springs and ephemeral streams draining a volcanic catchment. Despite its shallow depth (11 m), the brine is stratified and stable meromixis prevails. Sedimentation along the border fault is dominated by major slope failures, many earthquake-induced, and youthful fan deltas. Small deltas on the ramp and axial platforms are slowly prograding lakeward. Within the lake, the littoral zone is dominated by siliciclastic sediments, many deposited by density flows. In the deeper, central part of the lake, sedimentation is mainly chemical-organic, with some detrital fines introduced as hypopycnal plumes. Cores spanning the last 30,000 years reveal organic muds, which formed during periods of intermediate to high lake levels and high microbial productivity, interbedded with evaporites (trona, nahcolite) that were precipitated during arid phases of low lake level. A large suite of authigenic minerals, including sodium silicates and zeolites, is present in the lake sediments. The evidence from Lake Bogoria demonstrates the complex interplay between climatic change and tectonics in a young rift lake, and confirms the important role played by hydrothermal springs in sedimentation in continental rifts.

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