EVIDENCE FOR SHIFTING BASE LEVELS AND CLIMATIC VS. TECTONIC CONTROLS ON THE FOSSIL RECORD OF THE OKOTE MEMBER, KOOBI FORA FORMATION, EAST TURKANA

Combined information from sedimentology, stable isotopes, and trace fossils allows unusually detailed paleoenvironmental reconstruction of the Ileret Tuff Complex (ITC), an 8-9 m unit within the Okote Mb dated at 1.53-1.51 Ma. The ITC preserves abundant vertebrate fossils and trackway surfaces, with two hominin taxa represented by body fossils and at least one hominin (probably Homo erectus) recorded in multiple footprint levels. Fluvial and lake margin deposits, including three widespread volcaniclastic units, alternate with paleosols in the ITC and indicate multiple cycles of deposition and subaerial exposure within the 20 kyr time interval. Vertebrates are preserved in bedded fluvial, deltaic, and lake margin sediments and less commonly in the paleosols. The ITC is an unusually rich source of fossil primates, diverse bovids, suids, and other ungulates. Systematic surface fossil surveys in ITC exposures indicate spatial differences in faunal composition across small distances (~5 km), with more aquatic vertebrates toward the west and greater diversity of terrestrial mammals toward the east. There is an upward trend through the ITC in Area 1A from more aquatic reptiles and fish to increased terrestrial components. Stable isotopes in ITC paleosol carbonate indicate dominant C₄ (grass) with some C₃ (bush, trees) vegetation. δ¹³C analyses of tooth enamel from a wide range of mammalian herbivores show diets dominated by C₄ vegetation, with relatively few C₃ taxa. The combined faunal and stable isotope evidence from the ITC supports reconstruction of the habitat as grassland with areas of woodland and bush. The preserved trackways can be attributed to numerous mammals as well as crocodile and large birds that indicate the proximity of a large body of water. This is consistent with the sedimentology of the trackway surfaces, which formed near a relatively stable base level likely associated with a lake. Okote Mb. lacustrine deposits 50 km to the south provide additional evidence for an extensive lake between ~1.6 and 1.4 Ma in the northern Turkana Basin. The presence of this lake and the short-term transgression and regression cycles in the ITC represent two different scales of tectonic and/or climatic controls on deposition that interacted to form the Okote Mb. and preserve its rich paleontological record.