Sedimentological and palaeoenvironmental study from Waregi Hill in the Hiwegi Formation (early Miocene) on Rusinga Island, Lake Victoria, Kenya

Palaeontological deposits on Rusinga Island, Lake Victoria, Kenya, provide a rich record of floral and faunal evolution in the early Neogene of East Africa. Yet, despite a wealth of available fossil material, previous palaeoenvironmental reconstructions from Rusinga have resulted in widely divergent results, ranging from closed forest to open woodland environments. Presented here is a detailed study of the sedimentology and fauna of the early Miocene Hiwegi Formation at Waregi Hill on Rusinga Island, Kenya. New sedimentological analyses demonstrate that the Hiwegi Formation records an environmental transition from the bottom to the top of the formation. Lower in the Hiwegi Formation, satin-spar calcite after gypsum in siltstone deposits are interpreted as evidence for open hypersaline lakes. Moving up-section, carbonate deposits – interpreted previously as evidence of aridity – are actually diagenetic calcite cements, which preserve root systems of trees, suggesting a more closed environment; further up-section, the uppermost palaeosol layer contains abundant root traces and tree-stump casts, previously reported as evidence of a closed-canopy forest. These newly interpreted environmental differences are reflected by differences in faunal composition and abundance data from Hiwegi Formation fossil sites R1 and R3. Taken together, this work suggests that divergent palaeoenvironmental reconstructions in previous studies may have been informed by time-averaging across multiple environments. Further, results demonstrate that during the early Miocene local or regional habitat heterogeneity already existed. Rusinga’s Hiwegi Formation varied both spatially and temporally, which challenges the interpretation that a broad forested environment stretched across the African continent during the early Neogene, transitioning later to predominately open landscapes that characterize the region today. This result has important implications for interpretations of the selective pressures faced by early Miocene fauna, including Rusinga Island’s well-preserved fossil primates.