Paper No. 6
Presentation Time: 2:30 PM


MCHENRY, Lindsay J., Geosciences, University of Wisconsin- Milwaukee, 3209 N Maryland Ave, Milwaukee, WI 53211,

Olduvai Gorge exposes a well-preserved record of Pleistocene archaeological, paleoanthropological, and ecological change, with sites distributed over the 20km basin and a time frame from 2 Ma to recent. The fluvio-lacustrine Olduvai deposits also include primary and reworked tuffs and volcaniclastic sandstones, which document compositional changes in the neighboring Ngorongoro Volcanic Highlands and serve as a stratigraphic framework for the Olduvai archaeological record.

In Olduvai Bed I, differences in mineral assemblage and the geochemical composition of augite, hornblende, feldspar, and titanomagnetite can be used to fingerprint individual rhyolitic and trachytic tuffs and identify specific stratigraphic intervals based on single-grain geochemistry of volcaniclastic sandstones.

Electron microprobe analysis of mineral and glass samples of Bed II tuffs and volcaniclastic sandstones from sections in the eastern, central, western, and side gorge reveal a change in volcanic source towards more nephelinitic compositions. A consistent, high-Mg augite composition across most Bed II tuffs and volcaniclastic sandstones points towards a common source, but also complicates the fingerprinting of individual tuffs or stratigraphic intervals based on phenocryst composition. Feldspar is not abundant but consistently covers a wide range from high-K anorthoclase to intermediate plagioclase within most samples. Only the Bird Print Tuff (high-Ca plagioclase) and Tuff IID (lower Mg augite at its base) are uniquely identifiable based on phenocryst composition. Volcanic glass, where preserved, provides a compositional contrast between lower Bed II tuffs (phonotephrite), the Bird Print Tuff and Tuff IIC of middle Bed II (basalt), and Tuff IID (trachyte).

The identification of these tuffs in close association with the archaeological sites HWK and MNK helps constrain the relative timing of the end of the Oldowan lithic technology and the emergence of the Acheulean at Olduvai. This also allows for a landscape archaeological approach to the study of the origins of the Acheulean.