A TUFF JOB: LINKING CORES TO OUTCROP USING TEPHROSTRATIGRAPHY AT PLEISTOCENE OLDUVAI GORGE, TANZANIA

The Olduvai Gorge Coring Project (OGCP) recovered 612 m of core from four boreholes (1A, 2A, 3A, and 3B), including abundant volcanics interlayered with lake and lake margin sediments. Contained tuffs provide an excellent opportunity to tie the paleoclimatic record emerging from the core to the Pleistocene paleoanthropological record of Olduvai Gorge, Tanzania.

Tuffs were sampled from cores 1A, 2A, and 3A, and glass shards and mineral grains were hand-picked and analyzed by electron microprobe. Glass, feldspar, augite, hornblende, and Ti-magnetite compositions (and the presence/absence of other phases, including quartz, aenigmatite, and ilmenite) were compared to previously analyzed outcrop samples.

At present, the strongest correlations between the core and outcrop are in Olduvai Bed I. All major Upper Bed I tuffs (Tuff IF, Ng’eju Tuff, and Tuffs IE, ID, IC, and IB) are identified in at least one core, though no single core appears to contain them all. Tuff IF is prominent and easily recognized in Cores 2A and 3A, and confirmed based on distinctive Ba-rich anorthoclase and abundant hornblende (though glass is altered). IF is absent in Core 1A, where an incision cuts out part of lowermost Bed II and Upper Bed I. However the underlying Ng’eju tuff is present in all three cores, with its variable-composition plagioclase, abundant hornblende, and high-Al Ti-magnetite. Tuffs IE and IC have similar compositions but can be distinguished stratigraphically when plagioclase- and augite-dominated Tuff ID is identified between them, as it is in Core 1A. At present, Tuff IB is only confirmed in Core 2A.

Core 2A also includes a long record of Lower Bed I (and older) volcanics, including more than 18 pyroclastic units not previously known from outcrop. The Naabi ignimbrite, which marks the base of the outcrop exposures at Olduvai, is identified at ~100 m drilling depth, based on its assemblage including aenigmatite, high-Fe anorthoclase and augite, and ilmenite. The core continues another 135 m through older, previously unreported lacustrine and volcaniclastic materials documenting the early evolution of the nearby Ngorongoro volcano.

Future work will target older core sections, Bed II, and smaller tuffs within Bed I to develop more ties to help relate the core paleoenvironmental record more directly to Olduvai’s rich cultural record.