Compositional and textural correlations between Olduvai Gorge Bed I tephra and volcanic sources in the Ngorongoro Volcanic Highlands, Tanzania

Abstract

Tephra layers from Bed I of Olduvai Gorge, Tanzania, are correlated directly to their source volcanoes in the nearby Ngorongoro Volcanic Highlands (NVH) based on composition, texture, and age. Tephra in lower Bed I are more silicic than the upper Bed I tephra, and most contain quartz. The Naabi Ignimbrite from lower Bed I is compositionally similar to an ignimbrite exposed close to Ngorongoro Crater. None of the sampled tephra from the Ngorongoro Crater wall is an exact match for the Naabi Ignimbrite or other lower Bed I tephra, although overall composition and mineral assemblages suggest a common source. The uppermost Bed I tephra, Tuff IF, contains at its base a pyroclastic surge component rich in trachytic lava fragments that match (in texture, mineral assemblage, and mineral composition) with trachytic lavas exposed in the crater wall of the NVH Olmoti volcano. Tuff IF is compositionally similar to other upper Bed I tephra (especially Tuffs IB, IC, and IE), so Olmoti is the most likely source for most upper Bed I marker tephra. Geochemical correlations indicate that widespread ignimbrites and airfall tephra erupted from Ngorongoro during Olduvai lower Bed I time, reaching at least 40 km from source. Upper Bed I records a switch in NVH volcanic activity from Ngorongoro to Olmoti, coinciding with the first appearance of stone artifacts in the Olduvai record. Olmoti continued to supply pyroclastic deposits to the Olduvai Basin until the end of Bed I.