VOLCANIC SOURCES FOR OLDUVAI GORGE TUFFS: IMPLICATIONS FOR GEOCHRONOLOGY AND THE EFFECT OF VOLCANISM ON HOMININ PALEOENVIRONMENTS

Olduvai Gorge, Tanzania preserves an exquisite record of Pleistocene hominin and other fossils and stone tools in a context that allows for paleoenvironmental reconstruction. Additionally, these fossiliferous deposits are interlayered with tuffs that can serve as a stratigraphic framework and source for phenocrysts for radiometric dating. Tuffs from Olduvai Bed I derived from the nearby Ngorongoro Volcanic Highlands (NVH) volcanoes Ngorongoro (lower Bed I rhyolites) and Olmoti (upper Bed I trachytes) (McHenry et al., 2008). The proximity of these volcanoes (~20 km) allowed for thick deposits including surges, flows, and primary airfall tuffs, which would at times dominate the landscape and undoubtedly presented a significant hazard to the hominid residents of the basin (e.g. Stollhofen et al., 2008).

Except for Tuff IID, the Bed II and younger Olduvai tuffs are less continuous, finer grained, more reworked, and more contaminated, likely representing a more distant source. Most were redeposited by fluvial or eolian processes, and likely presented less of a direct volcanic hazard. The mineral assemblage also changes, indicating a more nephelinitic source. In particular, augite compositions from Bed II tuffs are more Mg-rich than those of Bed I and the previously studied NVH volcanoes (McHenry et al., 2008; Mollel et al., 2009, 2011). Less known, younger volcanoes in the northern NVH (Elanairobi/Embagai, Loolmalasin, Kerimasi) are likely candidates. These volcanoes are largely “blocked” from Olduvai by the volcanic edifice of Olmoti, which would have prevented the deposition of proximal (e.g. surge) deposits but permitted airfall deposition. All have nephelinites documented among their eruptive products (Dawson, 2008).

We will compare the bulk composition and phenocryst compositions of newly collected lavas and tephra from Elanairobi/Embagai and Kerimasi to Bed II and younger tuffs. If these volcanoes (or Loolmalasin: Mollel and Swisher, 2012) do not yield high-Mg augite or other mineralogical features of Bed II and younger tuffs, a more distal, non-NVH volcanic source may need to be considered. If a correlation can be made, proximal deposits in the highlands could be easier to date than the often highly contaminated Bed II distal tephra, providing improved age control for the notoriously poorly dated Bed II.