Lake Bogoria Basin, Kenya Rift: A Case Study for Climatic Versus Tectonic Controls on Sedimentation

Michael, TALBOT¹, TIERCELIN Jean-Jacques² (1) Bergen University, Bergen, Norway (2) Rennes University/CNRS, Rennes, France

The Bogoria Basin is a structurally complex Plio-Pleistocene half-graben, today occupied by the 17km long, 12m deep saline, alkaline Lake Bogoria. Despite its small size, the basin displays a wide variety of features illustrating the influence of climate and tectonics on lacustrine sedimentation. The basin is bounded to the east by a N-S-trending border fault and to the west by a zone of dense grid faults. To the north and south, it is closed by transverse faults, the northern boundary coinciding with the margin of a lozenge-shaped alluvial plain, the surface expression of a deep (up to 8 km) basin. These structural features exert a primary control on the drainage network, and thus a significant proportion of the basin's water and siliciclastic sediment supply.

Quaternary climatic variations have resulted in lake-level fluctuations with amplitudes of 10-20 m. During highstands Lake Bogoria transgresses to the north, flowing over the structural barrier separating it from Lake Baringo, possibly forming a single, large freshwater lake. In addition to extensive alluvial-fan and deltaic deposits, highstands are characterised by microbial and molluscan carbonate accumulations. Basin segmentation occurs during lowstands, first by separation from the Baringo Basin, and then, in periods of severe aridity, with the emersion of Bogoria's north and south sub-basins. The lake's deep central basin is characterised by mainly organic-rich mud accumulation. Cores provide no evidence of emersion here, but primary Na-carbonates indicate periods of hypersalinity. Hydrothermal springs may maintain a permanent waterbody in the deepest part of the basin during even the most arid periods.