Geochronology and sedimentology of the Buluk fossil site, East Turkana, Kenya: Hominoid fossil–bearing strata within the Miocene climate optimum

The Buluk Member of the Bakate Formation in the eastern Turkana Basin of northern Kenya includes the remarkable Buluk fossil site that preserves the remains of primitive apes and Old World monkeys, along with a rich faunal assemblage of terrestrial vertebrate fossils. The age of the site has been interpreted to be either middle Miocene based on its biostratigraphy or early Miocene (older than 17.4 Ma to 16.3 Ma) based on whole-rock K-Ar analyses on tuffs and basalt. Further, the depositional environments of the Buluk Member are poorly constrained. This study demonstrates that the Buluk Member primarily consists of a meandering river complex with fluvial channel deposits, pedogenically modified floodplain deposits, and rare lacustrine deposits that include oxbow lakes. Sedimentological analysis of the Buluk fossil sites and taphonomic assessments of the faunal remains indicate that the Buluk faunal assemblage accumulated primarily within fluvial channels through a variety of pathways, leading to differential weathering and fossil preservation. Combined magnetostratigraphy and 40Ar/39Ar geochronology indicate that all major mammal fossil localities at Buluk were deposited at the end of the early Miocene between ca. 16.6 Ma and 16.11 Ma, making Buluk the only known terrestrial strata that document the onset of the Miocene climate optimum (16.7–14.7 Ma) in eastern Africa.