PLEISTOCENE PALEOLAKES OF THE SOUTHERN KENYA RIFT: A TRANSITION FROM FLUVIAL TO GROUNDWATER RECHARGE AND ITS IMPACT ON SEDIMENTATION

Several paleolakes occupied the southern Kenya Rift (SKR) during the Pleistocene. This was a period of axial volcanism (flood trachytes, volcanoes), followed by grid faulting that created the present rift morphology. As the rift depression deepened, the depocenter moved southward from the apical region of the ‘Kenya Dome’. These tectonic changes modified the regional hydrology and the character of the paleolakes, while under variable climatic control.

The Munya-wa-Gicheru (MWG) paleolake (1.9–1.65 Ma) was dilute with mainly fluvial recharge (overfilled). To the south, the Olorgesailie (OL) paleolakes (~ 1.2–0.5 Ma) were dilute to moderately saline (balanced-filled), and were often contemporary with the Oloronga paleolake in the Magadi-Natron basin (~ 0.8–0.3 Ma). These paleolakes were fed by rivers and groundwater. Following a phase of faulting, lacustrine sedimentation migrated to the Magadi-Natron basin, where it continues today with hot springs providing inflow. Since 100 ka, Magadi basin lake water has ranged from dilute to hypersaline (balanced- and under- filled) with a gradual increase in salinity.

Lacustrine sedimentation in the SKR reflects changing climate superimposed on an evolving rift. Hydrology and hydrogeology were major controls of the lacustrine sedimentary record. With entrenchment of the rift floor, changing accommodation and uplifted rift shoulders, much of the rift became internally drained but several basins had axial drainage. Paleolakes periodically overflowed into other lakes. Uplifted rift shoulders enhanced a rain-shadow effect, increasing aridity on the rift floor. More of the lake recharge then derived from groundwater and springs, which were often geothermally heated. As a result, the character of lacustrine deposition changed from clastic and diatomaceous (MWG: fluvial recharge) to diatomaceous and carbonate (OL: mixed recharge) to silica and evaporite (Magadi: groundwater recharge). Crustal thinning at Magadi led to silica-rich hot springs that promoted chert formation, and the release of CO₂ which enhanced formation of trona.

While tectonics and climate control lacustrine sedimentation in volcanic rifts, the evidence from the SKR shows that the sediment record is intimately linked to changing sources of inflow that modify the basin hydrochemistry.