THE PALEOVEGETATION AND PALEOECOLOGY OF MIDDLE MIOCENE NYAKACH FORMATION, WESTERN KENYA

The eastern African Middle Miocene is often seen as dominated by open landscapes and a seasonally arid wooded savanna biome, influenced by local rifting and climate shifts during the Mid Miocene Climatic Optimum (MMCO) and the Middle Miocene Climatic Transition (MMCT). In western Kenya, the Nyakach Formation lies on the southern flank of the Nyanza Rift, characterized by fluvial and volcaniclastic deposits with abundant Middle Miocene fossils. Our geologic surveys reveal lithological differences between sites east of Sondu town and west of the town. Eastern sites (Kamogool, Kamairoon, Chepetet, Kapletet, and Cherwa) are comprised of fluvial channels, moderately to poorly developed paleosols, and both airfall and fluvially reworked tuffs. These channels vary in size and are composed of polymictic conglomerates and sandstones with common quartz and granodiorite clasts. Paleosols have vertical root traces, clay coatings, slickensides, wedge peds, and occasional carbonate nodules, while the tuffaceous units are often pedogenically altered. Western sites, Pundo and Kadianga, are distinct from each other and from the eastern sites. Pundo consists of a large fluvial channel complex with conglomerates and sandstones. Kadianga mainly contains sandstone and conglomerate beds with interbedded tuffs and rare fine-grained beds. Interestingly, clasts at Kadianga are primarily basalt, with less quartz and no granodiorite evidence. Despite these lithological differences, vertebrate fossils are similarly distributed, mostly found in coarse sandstones and conglomerate beds across sections. Preliminary Ar-Ar dates from Kadianga and Kamogool tuffs suggests these sites are contemporaneous with nearby Maboko Formation sites. Phytolith analyses from 21 samples indicate a dominance of forest indicators and woody dicots, with moderate grass and monocotyledon morphotypes, suggesting an ecosystem of open grassy wooded forests. Ongoing work aims to refine paleosol paleoclimate estimates using newer models and δ¹³C analysis of soil organic matter. These findings collectively suggest that the Middle Miocene Nyakach Formation represented an open forest biome in a seasonally dry, fluvially dominated landscape, somewhat different from traditional views of Middle Miocene ecosystems.