MIOCENE PALEO-OXISOL-VERTISOL FORMED ON BASALT RECORDS LONG DURATION OF TROPICAL WEATHERING IN LAKE VICTORIA REGION OF WESTERN KENYA PRECEDING RAPID SUBSIDENCE AND ONSET OF LAKE-BASIN FORMATION

A 50 m thick section at Ngira, near Karungu on the shores of Lake Victoria in western Kenya, commences with a 7.6 m thick, early Miocene weathering profile that formed during a prolonged period of pedogenesis. The profile consists of saprolitized basalt overlain by two packages of basalt-derived colluvium. The initial soil was a smectite-dominated Vertisol, as evidenced by slickensides, coarse wedge peds, and relict birefringent clay microfabrics and smectite. However it was converted through progressive pedogenesis to an Oxisol dominated by kaolinite, that is highly depleted of all base cations, has abundant Fe concentrations (pisoids, redox concentrations around macropores and ped faces), and with highly variegated color mottle patterns that reflect extensive ferruginization. MAP estimated using CIA-K (1309 mm/yr + 172 mm and CALMAG (1538 mm/yr + 108 mm), as well as MAT estimated using salinization (16.4^oC + 3.6 ^oC) are all probably too low given the high Mafic Alteration Index (89) and Index of Lateritization (39). The thickness of the Oxisol and the degree of weathering indicates that this paleosol likely reflects stable land surfaces for 10s to 100s of thousands of years. We postulate that rapid subsidence, perhaps associated with slip on a high-angle fault associated with the onset or progression of the Nyanza Rift, buried the paleosol and formed a nascent lake basin. After deposition of 3 m of shoreline sands and carbonate oncoid conglomerate beds, fluvio-lacustrine sandstones and claystones that include weakly developed paleo-Entisols and vertic paleo-Inceptisols follow, recording brief episodes of pedogenesis associated with lake lowstands. Fluvio-lacustrine paleosol mineralogy is dominated by 2:1 phyllosilicates (smectite). Vertebrate faunas (including both terrestrial and abundant aquatic fossils) are preserved within lacustrine claystones and fluvial channel sandstones. Rapid fault-induced subsidence or rise in lake level resulted in deposition of deep-water mudstones and turbidite sands and silts, which dominate the upper 25 m of the section. This study demonstrates polypedogenesis and inferences regarding onset of abrupt tectonic and climatic events, and emphasizes the contrasts between landscape stability of the Oxisol and the poorly developed soils in the fluvio-lacustrine facies.