Samuel N. Muteti

The Nyakach Formation of western Kenya provides a rare Middle Miocene (ca. 14-16 Ma.) terrestrial record bearing the monkey, Victoriapithecus macinnesi, and associated vertebrate faunas. Here we integrate sedimentology, clay mineralogy, paleosol geochemistry, phytoliths, and δ13C of soil organic matter to refine the age, depositional setting, and paleoecology of the Nyakach Formation. Stratigraphic and lithological evidence indicates subaerial deposition within a rift-margin fluvial system influenced by episodic volcanism, with eastern and western sites reflecting multiple sediment sources and variable degrees of reworking. Smectite-rich, base-saturated paleosols with high CaO concentrations point to carbonatitic parent material and a seasonally variable, but overall humid hydroclimates. Proxy-based precipitation estimates from paleosol geochemistry yield subhumid–humid conditions (ca. 1100–1400 mm/yr), which are substantially wetter than earlier reconstructions, but similar to consensus values from the Miocene Model Intercomparison project (MioMIP1). Phytolith assemblages dominated by forest-indicator morphotypes, together with δ13C value indicating predominantly C3 vegetation and only limited C4 biomass, support the reconstruction of a warm, tropical, seasonally wet forest with localized grassy openings. These findings highlight a heterogeneous but forest-dominated ecosystem during the Langhian stage, shaped by volcanism, tectonism, and fluvial dynamics, and provide refined ecological context for Middle Miocene primates and faunas inhabiting the Nyanza Rift region.