GEOLOGICAL CONTEXT OF FOSSIL PRESERVATION AND HUMAN EVOLUTION IN THE TURKANA BASIN, KENYA – PERSPECTIVES ON THE PAST, PRESENT, AND FUTURE

Physical environments and climates helped to shape vertebrate evolution in Africa over the past 7 Ma years. Hominins and other organisms preserved in East African Rift (EAR) basins were subject to physical, chemical, and ecological processes resulting from tectonics, volcanism, topographic complexity, and regional climate variability. Each East African rift basin has somewhat different environmental conditions, resources, and depositional processes, as also would have been true in the past. The Turkana Basin is unique in terms of its size and geographic position between uplifted regions of Ethiopia and Kenya. This physiographic system was initiated during early stages of EAR mantle-driven tectonics, including a Miocene fluvial system that flowed eastward into the Indian Ocean along the Cretaceous Anza rift. Further development of the EAR established the Turkana Basin as a relatively low tropical region with a semi-arid climate but abundant water from rivers sourced in highlands to the north and south. Faulting, uplift, sediment accumulation and vulcanism shaped basin topography and drainage systems, while also burying and preserving abundant vertebrate and other organic remains. Within the large-scale rift basin setting, Plio-Pleistocene sedimentary sequences around the Turkana Basin document multiple cycles of lake transgression and regression. Fluvial systems periodically dominated deposition as well, including a possible through-flowing drainage to the Indian Ocean. Decades of geochronological work on volcanic strata and magnetostratigraphy provide the essential framework for calibrating the paleontological record. The Omo Group formations include correlated stratigraphic sequences around the basin that provide high resolution evidence for paleoenvironments inhabited by hominins and associated faunas and floras over the past 4.5 Ma. Detailed micro-stratigraphy of hominin discovery sites, paleontological and archaeological excavations, and ichnofossil occurrences reveal a wide range of sedimentary and taphonomic processes responsible for the basin’s extraordinary fossil record. The quality of the hominin record varies in terms of specimen completeness and time-space averaging, but ecological fidelity with respect to original habitats at sub-basin to local scale is often preserved.