An unusual occurrence of the trace fossil Vagorichnus preserved in hydrothermal silica at Lake Baringo, Kenya Rift Valley: Taphonomic and paleoenvironmental significance

An unusual occurrence of the ichnogenus Vagorichnus is documented in the sublacustrine hydrothermal silica cement of conglomerate at Lake Baringo in the central Kenya Rift. The outcrop with ichnofossils lies at Soro on the NE shoreline of Ol Kokwe island, a Pleistocene volcanic centre in Lake Baringo. There, onshore and offshore hot-springs discharge hot and boiling (83-97.5 °C), alkaline Na-HCO3 water, steam and volcanic gases (mainly CO2). The ichnofossils are preserved in full relief in diatomaceous opaline silica cement precipitated between immature gravel clasts. The trace fossils comprise horizontal and subhorizontal burrow-systems with straight, curved, and loosely meandering segments, which locally show poorly defined constrictions. The preserved features are diagnostic of the ichnospecies Vagorichnus anyao. Vagorichnus, a feeding structure (fodinichnion), may have been produced by insect larvae (Diptera, Coleoptera or other insects), which are common in modern East African lakes including Lake Baringo. A probable body fossil of a Diptera larva is preserved in one of the burrows. Hydrothermal silica is an unlikely host for macroscale ichnofossils, but the soft silica that precipitated near hot-spring vents on the shallow gravelly lake-floor was cool enough for colonisation by microbes and higher organisms. The trace-makers burrowed into cohesive, but still soft, silica; the trace fossils are therefore bioturbation structures rather than bioerosion structures. The soft silica contained organic matter from phytoplankton and detrital plant debris, and was locally covered by benthic microbial mats, all of which were potential food sources for the burrowers. The presence of Vagorichnus near modern and former hot springs confirms the broad ecological, hydrological and environmental conditions represented by this ichnogenus, and shows that Vagorichnus is not restricted to a specific environment, substrate, or type of lake basin.