A PERSISTENT LATE PLEISTOCENE SALINE-ALKALINE LAKE AT OLDUVAI, TANZANIA, AS RECONSTRUCTED BY THE SEDIMENTOLOGY AND MINERALOGY OF CORES

The 2014 Olduvai Gorge Coring Project (OGCP) recovered cores from three sites within the Pleistocene Olduvai basin covering the last ~2.2 Ma, providing a record of the lake that dominated the landscape alongside the famous hominid sites of Olduvai Gorge, Tanzania. Paleolake Olduvai was a more continuous and longer lasting feature of the landscape than expected, as the cores revealed lake sediments older than those exposed in outcrop and more lacustrine facies for the younger Olduvai Beds. This study focuses on the younger lake deposits, correlative to Olduvai Beds III-IV (~1.14 Ma – 0.64 Ma, Deino et al. 2021).

Prior to coring, Paleolake Olduvai was known from outcrops in Olduvai Gorge and a few minor fault exposures to the north. Lake sediments are exposed for Beds I-II (2.03 – 1.14 Ma), but Bed III-IV exposures are more fluvial and eolian, with lake facies only exposed at Loc. 204 to the north. The scarcity of lake deposits was previously interpreted to indicate increasingly arid conditions. However, Core 3A (north of Olduvai) intersected the depocenter for Beds III-IV and contains mostly offshore lake sediments (claystones), and Core 2A (closer to Olduvai) intersects more sandy claystones showing a shallower lake environment. To better constrain the conditions within the Bed III-IV lake, samples were collected every 32 cm from finer sediment intervals from Cores 2A and 3A, powdered, and analyzed by X-ray Diffraction (XRD). The most common minerals were clays (mostly illite, occasional smectite), quartz, calcite, feldspars (K-feldspar and/or albite, rare anorthoclase), and analcime. Some samples also contained dolomite, other zeolites, or accessory phases of likely volcanic origin.

Analcime and illite dominated the diffraction patterns for all Core 3A samples from Beds III-IV, with only occasional quartz. Core 2A samples also had analcime and illite along with more ubiquitous quartz and feldspar, consistent with more detrital input from fluvial systems draining the Precambrian metamorphics to the southwest. The consistent abundance of analcime shows that conditions within the lake were persistently saline-alkaline, with few fresher intervals. This contrasts with the lake deposits of Bed I, where cyclical changes in zeolite presence and abundance track alternating fresher and more saline-alkaline conditions.