CORES RECORD PLEISTOCENE NGORONGORO VOLCANIC HISTORY, OLDUVAI GORGE, TANZANIA

The Olduvai Gorge Coring Project (OGCP) recovered 612 m of core from four boreholes into the Olduvai basin depocenter, sampling volcanics sourced from the Ngorongoro Volcanic Highlands interspersed with Pleistocene paleolake deposits in a fan-delta. Together they help constrain the hominin and paleoenvironmental record of Olduvai Gorge, Tanzania. Cores 2A, 3A, and 3B recovered material below the oldest levels exposed in outcrop, expanding the record of lacustrine and volcanic deposition to about 2.24 Ma. Core 2A provides the longest record, with two major pulses of Ngorongoro ignimbrites, surge and airfall tuffs, and lahars (the Ngorongoro Volcanic Formation), which were sampled and analyzed by EPMA for glass and phenocryst (feldspar, augite, titanomagnetite, hornblende) compositions and dated by the ⁴⁰Ar/³⁹Ar method.

Equivalents of units previously described in outcrop at or near Olduvai, including the Coarse Feldspar Crystal Tuff (CFCT), Naabi ignimbrite, and Orkeri ignimbrite, are identified in the upper pulse of the Ngorongoro fan-delta using stratigraphic position, age, textural characteristics, distinctive mineral assemblages, and mineral and glass compositions. The Orkeri ignimbrite, identified at 110 mbs in Core 2A (2.047 ± 0.003 Ma), is the oldest core tephra for which an outcrop equivalent is known. The upper pulse of the Ngorongoro fan-delta continues down to 165 mbs, documenting a record of explosive Ngorongoro volcanism previously not accessible. Lake sediments from 165 to 177 mbs separate the upper and lower Ngorongoro pulses. The lower pulse continues down to 204.4 mbs, recording the earliest evidence of Ngorongoro volcanism. Here, the oldest directly dated tuff, at 202.72 mbs in Core 2A at the base of the lower Ngorongoro pulse, yielded 2.1671 ± 0.0019 Ma.

The coarse nature of most of these volcaniclastic units, along with their dominantly rhyolitic/trachytic glass compositions, facies relationships, and paleotransport indicators of outcrop analogs, support the interpretation that they derive from Ngorongoro Volcano directly to the southeast of the Olduvai Basin. These older units have the potential to overlap, both compositionally and temporally, with the Naibadad Formation tuffs at Laetoli, a hominin-bearing locality to the southwest of Olduvai.