Paleoenvironmental reconstructions of the early Miocene Kiahera Formation, Rusinga Island, Lake Victoria, western Kenya

Abstract:

Longstanding research on Rusinga Island, Lake Victoria, Kenya has yielded a wealth of fossil plants and mammals that have been pivotal in understanding evolution of early apes; however, the majority of this research has been focused on the fossil-rich Hiwegi Formation. While important primitive ape, catarrhine primate, and other mammal fossils have been documented from most of the formations on the island, the lack of consistent fossil preservation for many of the formations has caused them to be understudied. Here we present new geological research from two formations on Rusinga and Mfangano Islands: the Kiahera and Wayando Formations. Previous research argues that the Wayando Formation was a pre-eruptive deposit at the base of the sequence and was overlain by the volcaniclastic Kiahera Formation. However, our detailed geological mapping and stratigraphic analyses reveals that the Wayando Formation type section is actually equivalent to the upper member of the Kiahera Formation, and not a unique formation. Further, deposits on the neighboring Mfangano Island mapped as Wayando Formation are older than the Kiahera Formation strata on Rusinga, and are instead a stratigraphic unit informally defined as the Makira Series. Thus, the Wayando Formation is not a valid formation and does not occur on either island. Sedimentological, stratigraphic, and paleopedologic analyses of the four members of the Kiahera Formation demonstrate that the formation represents changes in style of volcaniclastic deposition, including both intervals of major eruptions and volcanic quiescence. During the quiescence, deposition indicates rivers and affiliated paleosol overbank deposits that are dominated by Vertisols, and a rare Calcisol. We interpret the Vertisols to represent a seasonal climate with periods of water budget saturation and deficit while the Calcisol represents higher rates of evaporation. Analyses of fossil vertebrates, fossil leaves and tree trunks, and the paleosols, indicate that the Kiahera Formation sampled a mixture of habitats including both more closed woodland to forest ecosystems and more open woodland to shrublands suggesting evidence for environmental change through time.