ABSTRACT

Recent studies of rhizoliths from the Koobi Fora Formation (Plio-Pleistocene) of East Lake Turkana, Kenya, indicate that their shapes vary with the depositional environment of their host sediment. Vertical rhizoliths are associated with channel-bar and overbank deposits of fluvial origin. These rhizoliths represent the tap roots of phreatophytes that lived in well-drained, upland settings. Horizontal rhizoliths predominate in facies ascribed to beaches, lagoons, and floodplains associated with ancient Lake Turkana. They are produced by water-emergent aquatic macrophytes and coastal grasses that grow in saturated or poorly drained soil environments.

The rhizoliths consist of sparry and micritic calcite cements whose petrography and geochemistry reflect differences in diagenesis between the two types of root systems. The vertical roots contain little organic debris and have poorly developed meniscus and microstalactitic cements. These attributes reflect precipitation of the calcite in well-oxygenated, well-drained vadose conditions. In contrast, the cements of the horizontal root systems lack textures indicative of vadose conditions, contain abundant clay and plant debris, and display Mn concentrations as high as 4.5 cation percent. These features confirm that the horizontal rhizoliths developed in chemically reduced, water-saturated conditions. Bacterial decay of the plant debris maintained reduced Eh conditions while also supply ng abundant Mn²⁺ to the pore solutions. Groundwater that had moved through volcanic strata may have also contributed Mn²⁺. The Mn may have been incorporated in the calcite cements through a combined process of chemisorption and ionic substitution of Mn²⁺ as well as by inclusion of manganese oxides.