Fingerprinting facies of the Tuff IF marker,with implications for early hominin palaeoecology,Olduvai Gorge,Tanzania

The top of Tuff IF marks the upper boundary of the Plio-/Pleistocene Bed I succession exposed in Olduvai Gorge, NE Tanzania, a tephrostratigraphic interval that is well known for remarkable Oldowan archaeological and vertebrate fossil assemblages, including early hominins. Geochemically, Tuff IF is characterized by a relatively consistent silica-undersaturated trachytic to phonolitic composition that relates both in terms of numerical ages (1.79 Ma) and compositional constraints to Olmoti volcano, located 22–38 km E of Olduvai Gorge.Measured Tuff IF sections are characterized by a thick unwelded pyroclastic flow in proximal settings and a succession of surges and ashfalls that interfingers with fluvio-lacustrine deposits of the Olduvai Basin in the medial to distal settings. Only in those areas with a relatively low topographic gradient, in distal reaches and beyond the toe of an Olmoti-sourced volcaniclastic fan, did the Tuff IF marker develop a typical threefold subdivision comprising: (1) primary surges and minor fallout, (2) reworked pumice units as lateral correlatives of proximally emplaced pyroclastic flows and (3) a succession of mass flows in conjunction with aeolian and fluvially reworked units.The pyroclastic marker unit is thus highly heterogeneous with regard to its vertical and lateral facies architecture and this has immediate effects on its preservation potential and in situ burial of fossils and stone artifacts. Even though the volcanically-related environmental perturbations were probably more severe at the eastern lake margin, the evidence for at least temporary freshwater sources and trees suggests environments conducive to hominin activities, but not during emplacement of Tuff IF pyroclastic flows and surges. This inference is supported by the presence of rich Oldowan stone artifact assemblages immediately preceding and following the deposition of Tuff IF, but only extremely sparse archaeological traces from one site in Tuff IF, restricted to the upper, fluvially reworked portion of Tuff IF.Tuff IF facies record the maximum of a regressive (drying) cycle that correlates with regional marine arid indicators. An ecological crisis covering ～ 2000–3500 a of time during Tuff IF deposition resulted not only from the lethal effects of explosive volcanism but also from its coincidence with a pronounced period of climate induced drought. These combined effects appear to have made at least the eastern basin uninhabitable by hominins and other vertebrates for most of the time during which Tuff IF accumulated.