2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 2
Presentation Time: 9:00 AM-6:00 PM


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

Bed II is a crucial part of the early Pleistocene record of Olduvai Gorge, Tanzania. Its deposits laterally include lacustrine, lake margin, fluvial, wetlands, and volcaniclastic fan environments, with a wide variety of hominin traces. Vertically, Bed II includes the transitions from humid to more arid conditions (with associated faunal changes), from Homo habilis to Homo erectus, and from Oldowan to Acheulean stone tools. Bed II is stratigraphically and environmentally complex, with facies changes, faulting, and unconformities, making site-to-site correlation using physical mapping alone over the ~20 km of its exposures difficult.

Bed II also contains a record of explosive volcanism from the nearby Ngorongoro Volcanic Highlands. Bed II tephra are thinner, less evenly preserved, and more reworked than those of older and better studied Bed I, but can still be used for correlation in some instances. Six marker tephra (Tuffs IIA-IID, Bird Print Tuff (BPT), and Brown Tuffaceous Siltstone (BTS), after Hay, 1976), plus additional localized tephra, were collected from sites throughout the gorge and analyzed by electron microprobe for phenocryst (feldspar, hornblende, augite, titanomagnetite) and glass (where available) composition. Using central “Junction” Locality 44 as a type section, compositional comparisons can be made between sites.

At present, Tuff IIA cannot be distinguished compositionally from other “tongues” of the Lemuta Member of which it is a part, though the Lemuta Member is overall compositionally distinguishable from underlying and overlying tephra. Tuffs IIB and IIC are highly contaminated and reworked at most sites (making correlation using them difficult), and the BTS is identifiable using hornblende but only locally preserved. The BPT and Tuff IID, however, are widespread and can be identified using their phenocryst compositions (high-Ca plagioclase and high-Ti hornblende, respectively). So far, Tuff IID is confirmed at Locality 80 to the west and at some sites to the east (as mapped by Hay, 1976), but was originally misidentified at Localities 6 and 91. These correlations are an important first step for establishing a basin-wide, high-resolution tephrostratigraphic framework for Bed II, and a regional framework for potentially Bed II equivalent sites nearby (Peninj, Manyara, Laetoli).