2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 14
Presentation Time: 11:40 AM


MCHENRY, Lindsay J., Department of Geosciences, UW-Milwaukee, 3209 North Maryland Avenue, Milwaukee, WI 53211, lmchenry@rci.rutgers.edu

The Bed I strata (2.03-1.78 Ma) of Olduvai Gorge, Tanzania, preserve a well-known record of artifacts and hominin and vertebrate fossils interbedded with tephra layers derived from the nearby Ngorongoro Volcanic Highlands (NVH). These tephra help constrain the ages and relative stratigraphic positions of archaeological sites within the Olduvai Basin. A revised stratigraphic framework for Bed I based on the compositional correlation of these tephra has been recently presented (McHenry, 2004). Identifying sources for Olduvai tephra could help reconstruct the landscape and potential volcanic hazards that affected hominids at the time.

A change in tephra composition within Bed I indicates a likely change in volcanic source. The lower tephra are quartz-bearing rhyolites; the upper tephra are trachytes. The Naabi Ignimbrite, the oldest Olduvai tephra, can now be compositionally correlated to an ignimbrite proximal to Ngorongoro Crater, a large NVH volcano. Its anorthoclase, Fe-rich augite, and aenigmatite compositions and the presence of quartz allow this correlation. Ngorongoro is the only NVH volcano sampled that contains quartz, making it a likely source for the lower Bed I tephra.

The Upper Bed I tuffs are trachytic and lack both quartz and feldspathoid phenocrysts, though NORM calculations on glass suggest slightly silica-undersaturated compositions. Tuff IF, the tephra that marks the boundary between Olduvai Beds I and II, is now correlated to a thick, welded ignimbrite proximal to Olmoti volcano in the NVH. Tuff IF has distinctively high-Ba (up to 3.5 wt. %) anorthoclase feldspar and contains amphibole, unlike the other Bed I anorthoclase-bearing trachytic tephras. This mineral assemblage links Tuff IF closely to Olmoti. Its volcaniclastic surge component, found in the eastern and central “Junction” parts of the Olduvai Basin, also contains abundant trachytic lava fragments. A trachytic lava in the Olmoti crater wall contains the same textures and minerals (augite + anorthoclase + amphibole + Ti-oxide + aenigmatite + sodalite) as the lava fragments found within the Tuff IF surge. The anorthoclase in this lava is distinctively Ba-rich, a feature not shared with other Olduvai tephra or NVH volcanoes. These observations, along with new geochronological data (Mollel, 2005), help to tie Tuff IF directly to Olmoti.