PALEOENVIRONMENTS AND PALEOCLIMATE OF EARLY MIOCENE FOSSILIFEROUS STRATA AT MORUOROT, WEST TURKANA, KENYA

Early Miocene, hominoid-bearing fossil sites across eastern Africa represent a wide range of habitats and climates, from perhumid rainforest to semiarid woody grassland. This investigation focuses on the Moruorot site complex (~ 17.5 Ma), which consists of fossiliferous, terrestrial strata of the Lothidok Formation in West Turkana, Kenya, that currently lack detailed paleoenvironmental reconstruction. The stratigraphic interval at Moruorot includes the Moruorot, Kalodirr, and Naserte Members, which are underlain by the Kalokol Basalts and capped by a ~ 2 Myr unconformity. The Moruorot Member contains basalt flows interstratified with colluvium and fanglomerate that give way to interbedded fanglomerate, fluvial channels, alluvial paleosols, lahars, and biotite tuffs. Fluvial channels vary from single story conglomeratic to multistory sandy channels, neither of which have clear indicators of sinuosity. The Kalodirr Member is marked by an increase in reworked and airfall tuff, sandy sheetflow deposits and wider fluvial channels, and rare paleosols formed on volcaniclastic silts and sands. The Naserte Member contains multistory, sinuous fluvial channels and lahars that contain cobbles reworked from the underlying members. Reworked tuffs containing pumice and lapilli are increasingly common into the Naserte Member. Based on stratigraphic analysis and existing K-Ar dates of the Lothidok Formation, we document the earliest occurrence of Afropithecus, which is roughly contemporaneous with Simiolus fossils and predates the first appearance of Turkanapithecus. Most vertebrate fossils at Moruorot occur in red paleosols and sand-dominated fluvial channels in the upper Mourorot Member after the initiation of local volcanism. A predominance of woody vegetation is supported by relatively low δ¹³C values of paleosol organic matter (~ -27‰), large root traces in paleosols, and calcified fruits and branches in lahars. Vertic and rare calcic features in paleosols are consistent with highly seasonal subhumid paleoclimate that could have supported a woodland biome. The strata at Moruorot document progressive infilling of a graben and concomitant evolution of a local volcanic center, with environmental change driven primarily by sediment supply and basin gradients rather than paleoclimate change.