Geology and chronology of the Ndutu and Naisiusiu type sites: implications for Middle and Later Stone Age occupations at Olduvai Gorge (Tanzania)

Olduvai Gorge (northern Tanzania) preserves a rich and continuous paleoanthropological record spanning the past two million years. While its Early Stone Age occupations have been intensively studied, the Middle and Later Stone Age sequences, represented by the Ndutu and Naisiusiu Beds, remain less understood. This study presents new geological and geochronological data from the type localities of the Ndutu and Naisiusiu Beds. Using optically stimulated luminescence (OSL), we establish a revised chronological framework for these sedimentary units. The Lower Ndutu Beds, composed of fluvially reworked conglomerates and sands capped by a trachytic tuff and silts, are now dated between 152.9 ± 11.6 ka and 122.9 ± 8.3 ka. The Upper Ndutu Beds include archaeologically rich sandy and silty facies and are dated between 86.9 ± 4.5 ka and 70.8 ± 10.4 ka. At the Naisiusiu type site, a sequence of fluvial, sheetwash, and aeolian deposits yielded OSL dates ranging from 65.3 ± 4.4 ka to 32.8 ± 2.2 ka, suggesting that the LSA occupation at Olduvai began significantly earlier than previously assumed. These results provide refined age estimates for the Middle and Later Stone Age deposits at Olduvai Gorge and confirm a relatively rapid transition between these two technocomplexes during the Late Pleistocene. The data strengthen regional correlations with sites like Nasera and Mumba, providing critical chronological resolution to ongoing debates about the tempo and mode of behavioural evolution in eastern Africa.