Pleistocene stratigraphy and sedimentation in the Magadi-Ewaso Nyiro Basins, South Kenya Rift

The Magadi and Ewaso Nyiro basins lie in the South Kenya Rift and preserve a one-million-year sedimentary record. Chronostratigraphic and lithostratigraphic classifications have been confused with hierarchical use of terms and naming procedures failing to conform with the rules of the International Commission on Stratigraphy. Here, we split the Oloronga Beds into the Oloronga Formation and Orpirikata Formation. The Chert Series was renamed the Green Beds, which we assign to the Oloika Formation. The Orkaramatian Beds are dropped in favour of the Ngare Nyiro Beds, which has precedence. However, we raise it to formation status. The Evaporite Series is renamed the Magadi Formation, and the High Magadi Beds are raised to the High Magadi Formation.

The 37 facies preserve a predominantly lacustrine environmental record and represent several distinct water bodies. These were initially dominated by shallow, fresh and oxic conditions, with lakes becoming deeper, more saline, anoxic and meromictic after 720 ka. A separate freshwater lake formed along the base of the Nguruman Escarpment to the west after 600 ka due to the growth of north–south horsts that diverted the Ewaso Nyiro and other cross-rift rivers south towards Lake Natron. Periodically, extreme euxinia developed. By about 380 ka, continued basin breakup into several sub-basins led to increasingly hypersaline waters in the Lake Magadi area and fresher shallow lakes to the west. From 105 ka until today, hypersaline lakes precipitated trona, reflecting ascending CO₂ along faults and an overall increase in aridity that was punctuated by wetter periods.