QUATERNARY ENVIRONMENTS OF THE MAGADI BASIN: GEOCHEMICAL AND MICROFOSSIL STRATIGRAPHIC VARIABILITY

Thin (< 5 m) discontinuous outcrops of Quaternary sediments provide a fragmentary record of deposition in the Magadi basin. The oldest deposits are the Oloronga Beds (tuff, limestone, zeolitic silt, chert), which rest on Magadi Trachytes (~ 1.4–0.7 Ma) and accumulated in fresh to saline lakes. The overlying Green Beds (silt, zeolitic tuff and chert) formed in saline lakes and playas. The late Pleistocene-Holocene High Magadi Beds (zeolitic silt, tuff, laminites, fish beds, chert) accumulated in an alkaline to fresh water lake.

Two sets of cores collected by the Hominin Sites and Paleolakes Drilling Project (HSPDP) reached trachyte at ~ 194 and 133 m and document a more continuous environmental history of up to ~ 1 Ma. The cores include limestone, zeolitic laminated and massive mud, chert, trona, gravel and sand. Na₂O and LOI in core MAG14-2A increase in the upper 60 m with SiO₂, TiO₂ and K₂O decreasing, reflecting trona. MgO and CaO decline upwards. P₂O₅ peaks in the lower 20 m. Au is most concentrated in the top 40 m with Ag, As, Ba, Be and Mo concentrated between ~40–154 m. High U concentrations are present in chert and between 82–89 m.

Sediment data suggest alkaline conditions. However, diatoms between 43–132 m indicate greater variability. Floras include fresh water planktonic taxa (Aulacoseira granulata, A granulata v. valida, A granulata v. angustissima, A. agassizi) and/or saline planktonic and facultative planktonic species (Thalassiosira faurii, T. rudolfi, Cyclotella meneghiniana), with less common benthic taxa. Both fresh and saline waters are implied, possibly reflecting episodic floods with fresh water overlying saline water.

Pollen studies show five major vegetation zones. Zone 1 (below 162 m) includes Afromontane and woodland taxa (5–20%) with common aquatics and Poaceae. Zone 2 (152–132 m) reflects an expansion of dry montane forest. Relatively abundant Cyperaceae and Typha suggest proximity of marshy, fresh waters and periodic flooding, consistent with the appearance of diatoms. Zone III includes local pollen taxa, which are more common than regional species. Zone IV includes mixed vegetation (forest, bush, grass). Zone V suggests open grassland/woodland. Fungal spores support pollen inferences. Both pollen and mineralogical data suggest an overall trend to drier climates.