BULK MINERALOGY AS A PROXY FOR PALEOGEOGRAPHY AND PALEOLAKE CONDITIONS: A CORE-BASED XRD RECORD FROM OLDUVAI GORGE, TANZANIA

Olduvai Gorge, Tanzania, incises through and exposes paleoanthropologically important strata, including classic Pleistocene hominin sites and lacustrine deposits that preserve a complementary paleoclimatic record. The Olduvai Gorge Coring Project (OGCP) recovered four cores in 2014 (612 m total), targeting more lacustrine deposits. These cores provide a nearly continuous (and less weathered) record of the Olduvai basin. Lacustrine portions of these cores were sampled at 32 cm intervals for multiple proxies. Reported here are preliminary results of bulk sample X-ray Diffraction (XRD) determined mineral assemblages. Samples were air dried, powdered using an agate mortar and pestle, and analyzed by XRD. Minerals were qualitatively described as dominant (4), abundant (3), present (2), rare (1), and absent (0), based on relative peak heights.

The Olduvai paleolacustrine record is dominated by the following minerals, though relative abundances differ over time. Smectite and/or illite clays (not fully characterized in the bulk analysis), quartz, calcite, dolomite, feldspars (K-feldspar, plagioclase, and anorthoclase), and zeolites (analcime, phillipsite, chabazite). Smaller amounts, or less frequent occurrences, are observed for augite, other zeolites (e.g. erionite), and pyrite, and others.

At Olduvai, different sediment sources can be tied to specific drainage patterns. The western basin is sourced from Precambrian quartz and feldspar-rich metamorphic rocks (quartzite, gneiss), while the eastern basin abuts Pliocene-Pleistocene volcanoes, which supplied volcanic glass (now altered), feldspar, and augite. The relative influence of these sediment sources helps show the changing paleogeography of the basin; more significant input from western fluvial sources is inferred for quartz-rich intervals, while the presence of anorthoclase and augite (and absence of quartz) indicates sediments from the east. When authigenic minerals dominate (clays, zeolites, carbonates), they preserve a record of changing lake conditions. For example, zeolites record saline-alkaline conditions, while pyrite indicates reducing conditions. This mineralogical record can help reconstruct both the paleogeography (sediment source) and lacustrine conditions (authigenic mineralogy) of the Olduvai paleolake.