EARLY MIOCENE PALEOENVIRONMENTS NEAR KARUNGU, WESTERN KENYA

New systematic surveys of early Miocene outcrops near Karungu, Western Kenya, have produced thousands of fossils, including a range of reptiles, turtles, fish, and mammals. A small but diverse assemblage of hominoids was recently discovered at the Kachuku and Ngira localities, reinforcing taxonomic resemblances with early Miocene fauna at localities on nearby Rusinga Island. However, the Karungu sites include a more aquatic and non-arboreal faunal signal. In this investigation, we documented the stratigraphy, sedimentology, and paleopedology at the Kachuku locality as part of the Research on East African Catarrhine and Hominoid Evolution (REACHE) project. A 75 m thick stratigraphic section at Kachuku was correlated 1.5 km NW to the Ngira locality. The basal stratigraphic unit at both localities is a lateritic paleosol parented on alluvium and basaltic colluvium. The colluvium was linked to an intact mafic paleo-inselberg subjacent to the Ngira outcrops, which is onlapped by the basal lateritic paleosol and capped by fluvio-lacustrine strata that can be traced between the two localities. At Kachuku, an expanded section below these marker beds fills a ca. 20 m thick paleovalley cut into the basal laterite. Paleocurrents taken on paleovalley-fill fluvial channels trended north, whereas fluvial channels at both localities flowed ENE following paleotopographic planation, parallel to the axis of the failed Nyanza rift. Fluvio-lacustrine paleosols at Ngira and Kachuku were similar to moderately developed Inceptisols, Vertisols and Alfisols. Mean annual precipitation (MAP) and temperature (MAT) were estimated using paleosol geochemical transfer functions. MAP values indicate sub-humid to semi-arid conditions and MAT estimates suggest a mesic climate. Given the presence of fluctuating soil drainage, vertic features, and pedogenic carbonates in the paleosols, precipitation was likely seasonally variable and relatively low. The warm temperatures and limited, seasonal precipitation suggests the climate was unlikely to have supported a closed-canopy forest, which has been reconstructed for some early Miocene sites on Rusinga Island. This interpretation is consistent with faunal reconstructions at Karungu and suggests that early Miocene hominoids were able to exploit a diverse range of habitats.