A CORE-BASED ENVIRONMENTAL AND DEPOSITIONAL HISTORY FOR PLEISTOCENE PALEOLAKE OLDUVAI, TANZANIA, BASED ON MINERAL ASSEMBLAGE

Paleolake Olduvai was a prominent feature during deposition of the famed stone tools and fossils of Olduvai Gorge, and its sediments provide paleoenvironmental context. In 2014, the Olduvai Gorge Coring Project (OGCP) drilled four cores at three sites into the paleolake depocenter between Fifth and FLK Faults, totaling 612 meters. These cores reveal that the lake considerably pre-dated outcrop exposures, and was more laterally extensive and less ephemeral than previously believed. The fine-grained sedimentary intervals of Cores 1A, 2A, and 3A were sampled at 32-cm intervals for a variety of paleoenvironmental proxies. A split of each sample was powdered and analyzed by X-ray Diffraction (XRD) to establish the bulk mineral assemblages. Changing detrital mineral assemblages record changing sediment sources whereas authigenic mineral assemblages provide proxies of the lake conditions.

Zeolites, carbonates, clays, feldspars (authigenic, detrital, and volcanic), and quartz comprise most of the mineral assemblage. The oldest lake deposits (of the newly-defined Naibor Soit Formation, starting at 2.24 Ma) lack carbonate and authigenic feldspar, but abundant zeolites (analcime near the Core 2A base, then phillipsite and/or chabazite) indicate at least moderately saline-alkaline conditions. Detrital quartz and plagioclase reveal sediments derived from eroded volcanic and metamorphic sources to the south and west. Lake deposition became restricted by the westward progradation of a volcaniclastic fan from Ngorongoro volcano. When the lake expanded again, carbonates were still rare and chabazite and phillipsite were the dominant zeolites, reflecting less saline-alkaline conditions. A second, thicker fan progradation of Ngorongoro volcanic material separates this lake phase from the overlying Olduvai Bed I paleolake deposits. The Lower Bed I paleolake contains abundant carbonates but lacks zeolites or authigenic feldspar, reflecting fresher conditions. Above the Bed I basalts and through Lower Bed II, carbonates gain greater importance, and zeolites and authigenic feldspar reflect fluctuations in lake conditions between moderately and highly saline-alkaline. While calcite is the most common carbonate, some intervals contain dolomite and aragonite-rich episodes, particularly while basaltic volcanism dominated sediment sources. The Olduvai Bed I and II paleolake, which overlaps in time with evidence of hominin occupation in the basin, reveals cyclical wet and dry episodes in its mineral assemblages and other proxies.