PALEOENVIRONMENTAL RECONSTRUCTION OF THE LOCHERANGAN MIOCENE LACUSTRIAN FOSSILIFEROUS SITE, KENYA

Reconstructing the paleoecology of primates is essential for understanding the role the environment and climate had in their evolution. This study examines a Miocene section in Locherangan, ~17 km west of Lake Turkana, Kenya. The section (~50m) is underlain by basalts and is comprised of tuffs, sand-, silt- and clay- stones. Preliminary work dated a tuff in the middle of the section to 17.5 ± 0.3 Ma. This study uses geologic facies analysis, paleosol observations, and whole-rock geochemistry to provide insights into past environmental conditions and refine the paleogeography. Paleosol samples were analyzed using XRF to infer changes in parent material (TiO2/Zr), salinity (Na2O/Al2O3), and redox {(FeO+MnO)/Al2O3}. The basal reddish-brown silty clay to clay paleosols have high TiO2/Zr ratios (0.004-0.011), low Na2O/Al2O3 (0.020-0.037), and high (FeO+MnO)/Al2O3 (0.571-0.667) and are interpreted as poorly drained fluvial deposits. The middle of the section is characterized by olive-colored deposits with higher clay content which alternate between laminated mudstones preserving ostracods and fish fossils, and paleosols forming on this lacustrine parent material. These have lower TiO2/Zr ratios (0.004-0.008), elevated Na2O/Al2O3 (0.037-0.059), and low (FeO+MnO)/Al2O3 (0.331-0.549) and are interpreted as fluctuating lacustrine margin. The top of the section consists of coarser deposits with a return to higher TiO2/Zr ratios (0.002-0.004), lower Na2O/Al2O3 (0.032-0.042), and higher (FeO+MnO)/Al2O3 (0.331-0.549), interpreted as a return to fluvial depositional environment. We used two proxies that predict mean annual precipitation (MAP) using paleosol subsoil geochemistry: CALMAG (for vertisols only) and a machine-learning model (RFMAP1.0). CALMAG yields a mean of 1093±108 mm yr-1, and RFMAP1.0 yields a mean of 1336±440 mm yr-1, both indicating that Locherangan had a subhumid to humid precipitation regime. Overall, these indicators show a decrease in rainfall that does not track with changes in the depositional environment suggesting a decoupling between these variations and climate. Our geochemical proxies suggest changes in parent material, soil salinity, anaerobiosis, and depositional environment that can be linked to landscape changes likely forced by topographic and/or tectonic factors.