Tuff fingerprinting and correlations between OGCP cores and outcrops for Pre-Bed I and Beds I/II at Olduvai Gorge, Tanzania

摘要

Sediment cores retrieved from the Pleistocene Olduvai Basin by the Olduvai Gorge Coring Project (OGCP) provide a high resolution record of tuffs and other volcaniclastic deposits, together with a lacustrine sedimentary record full of paleoenvironmental indicators. Correlating tuffs between the cores and outcrops at Olduvai, where these tuffs are identified at paleoanthropologically important sites, is critical for applying the new paleoenvironmental data to the conditions under which hominins lived. Tuffs and other volcaniclastic deposits from three cores were analyzed for mineral assemblages and glass and mineral major element compositions (feldspar, augite, hornblende, titanomagnetite, and glass where possible) to compare to published geochemical fingerprint data, based on marker tuffs from outcrop equivalents at Olduvai Gorge. In combination with stratigraphic position, these mineralogical and geochemical data were used to correlate between the cores and outcrops, providing direct temporal tie-lines between the cores and sites of paleoanthropological interest. Direct correlations are most certain for Olduvai Bed I, where all major tuff markers from outcrop are identified for one or more of the three core sites, and for the upper part of the underlying Ngorongoro Formation, which includes the Coarse Feldspar Crystal Tuff (CFCT) and Naabi ignimbrites exposed in the oldest Pleistocene exposures of the Western Gorge. Also characterized were the mineral and glass compositions of tuffs and ignimbrites pre-dating the oldest exposed outcrop units, extending our record of explosive events from the Ngorongoro Volcano. While no specific correlations can be confirmed between individual Bed II tuffs in the cores and in outcrops, correlations are possible between the cores themselves (using newly identified tuff compositions), and some potential correlations (non-unique, based on individual mineral phases) between core and outcrop can be used in conjunction with other stratigraphic tools to help constrain the intervals in question.