STRATIGRAPHIC REVISION OF THE EARLY MIOCENE DEPOSITS ON RUSINGA ISLAND AND ITS EFFECT ON INTERPRETATIONS OF PALEOCLIMATE AND EVOLUTION
New work on the oldest sediments on the island shows that the type section of the Wayando Formation is actually stratigraphically equivalent with the upper member of the Kiahera Formation, making the Kiahera the oldest sediments on Rusinga. The Kiahera Formation is composed of a number of facies across three different members: Nyamita Member, Ukowe Member, and Rondo Member. The facies suggest either fluvially- or volcanically-dominated systems that record the evolution of the neighboring Kisingiri Volcano. Paleosols found within the fluvially-dominated sediments are either calcic Protosols, Vertisols, or argillic Calcisols. These paleosol types suggest (1) lack of equilibrium between the soil and climate (Protosols), (2) climate that is seasonal (Vertisols), or (3) well-drained conditions with greater rates of evaporation than precipitation (Calcisols). Ar-Ar incremental heating of biotites and magnetostratigraphy suggests that the entire Kiahera Formation was deposited over a relatively short period of time from ~18.5-18.3 Ma. When we compare the paleosol types seen in the Kiahera Formation with paleosols reported from other contemporaneous sites in eastern Africa, there is strong evidence for fluctuations in seasonality, which may indicate the onset of the East African Monsoon. This suggests that the early diversification of apes at many sites in eastern Africa took place in climates driven by seasonal precipitation with changing intensity.