CALCRETES OF THE LAKE MAGADI AREA, SOUTHERN KENYA RIFT VALLEY

Pleistocene to Recent calcretes, which are common in the semi-arid parts of the Kenya Rift Valley, provide clues to interpret regional paleoclimate and paleohydrology. At Lake Magadi and Olorgesailie calcretes mantle the land surface, inhibiting erosion of the Pleistocene lake sediments. At Lake Magadi an extensive calcrete up to 50 cm thick lies at the interface between the Oloronga Beds (OB; 780 to 300 ka) and the overlying High Magadi Beds (HMB; ~12–9000 y BP) and represents about 300,000 years without net sediment accumulation. Based on its chemistry and field relationships, the commonly pisolitic calcrete was formed within the OB. Chondrite-normalized REE patterns for the calcrete are consistent with sediment and fluid derivation from the Magadi Trachytes, indicating local small-scale drainage patterns similar to those today. Alternatively, sediment could be derived from the trachytes of the Suswa volcano ~80 km to the north which would imply a hydrology much different from today. Based on calcrete microfabrics and stable isotope data the dominant process of formation was through periodic calcite precipitation from intrasedimentary pore fluids under hot and dry conditions following a wet period, implying climate seasonality. Pisoids formed through cycles of circumgranular cracking around calcite-rimmed lithic or monocrystalline clasts, induced partly due to pressure of calcite crystallization and root growth. Evidence for biotic activity during formation is scarce; rare fungal hyphae and spores were detected under SEM. These calcretes underwent several cycles of dissolution and reprecipitation until porosity lowered so that water flowed laterally over the cemented surface to produce subhorizontal laminae at the top of the calcrete. The calcretes were denuded and represent a ravinement surface that predated the HMB lake. Olorgesailie calcretes are chemically similar to their surrounding sediments and likely pedogenic in origin. Biogenic microfabrics are present with rare calcified plant fragments. REE data indicate a primary trachyte sediment source with a secondary basaltic source. Stable isotope data suggest a semi-arid climate with intermittent drier periods during which the calcretes formed within the soil.