STRATIGRAPHIC REVISION OF THE EARLY MIOCENE DEPOSITS ON RUSINGA ISLAND AND ITS EFFECT ON INTERPRETATIONS OF PALEOCLIMATE AND EVOLUTION

Decades of geological and paleontological research on Rusinga Island, Lake Victoria, Kenya have made its fossil record pivotal for interpreting floral and faunal evolution in eastern Africa during the early Neogene. Previous paleontological research has largely focused on the fossil primates, as well as more than 100 species of vertebrates preserved in sedimentary rock, and most of the geological research has focused on the fossiliferous Hiwegi Formation. This has meant that most of the interpretations about the paleoclimate and paleontological record are limited to only part of the Rusinga sequence, which affects our ability to fully assess the interplay of climate and evolution in eastern Africa during the early Miocene.

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.