KBS Tuff dating and geochronology of tuffaceous sediments in the Koobi Fora and Shungura Formations, East Africa

Abstract

Plio–Pleistocene lacustrine and fluvial sediments of the Koobi Fora Formation along the east shore of Lake Turkana in northern Kenya (see Fig. 1) and thicker fluvial sediments of the Shungura Formation in the Lower Omo River Basin 100 km to the north have become widely recognised because of their rich hominid and artefact assemblages, together with associated vertebrate fauna1–3. Over the past decade, tuffaceous horizons within these two sedimentary formations have been mapped4,5 and dated by K–Ar6–11 and fission-track techniques12 to provide chronologic control supporting studies of hominid and faunal evolution and archaeology. We report here new 40K–40Ar dates and revised values for previously published dates which give a mean age of 1.8±0.1 Myr for the KBS Tuff. This estimate suggests contemporaneity between the KBS Tuff and Tuff Units H2 and H4 in the Shungura Formation, lower Omo River Basin, Ethiopia, and with Bed I at Olduvai Gorge.