Lake Olduvai was a saline, highly alkaline closed-basin lake in which lacustrine sediments of Bed I and lower Bed II of Olduvai Gorge were deposited between ca. 1.92 and 1.70 Ma. The lake consisted of a lacustrine area, the central basin (CB), surrounded by lake-margin terrain that was alternately flooded and dried. The lacustrine deposits consist largely of authigenic clay and carbonate minerals, zeolites, K- feldspar, chert, and pyrite.
Claystones of the CB and most of the eastern lake-margin (ELM) terrain consist chiefly of Mg-rich authigenic illite, smectite, and interlayered illite-smectite (I/S). These clays were formed penecontemporaneously with detrital-clay deposition by interaction of the detrital clays with the saline, highly alkaline water. The Lake Olduvai clays average ∼65%–70% Mg2+, SiO2, and alkali ions from lake water, and the remaining 30%–35% is detrital clay. Lacustrine clays of the ELM indicate that this area was flooded by saline water for substantial periods of time. Hominids occupied the ELM at times of lower lake levels, when the clays were dried.
Calcite is the most abundant carbonate mineral and occurs chiefly as sand-size euhedral crystals dispersed in claystone and less commonly as calcite-crystal limestones and nodules. The sand-size calcite crystals were formed diagenetically at shallow depths, possibly from a gaylussite and/or micritic calcium carbonate precursor. Dolomite occurs chiefly in the form of dolostones, some of which are probably of replacement origin.
The oxygen isotope composition of lake water and pore fluid of the CB varied over ∼11‰, as determined from calculated values of authigenic minerals. Clay minerals were formed chiefly in the more saline waters. Sand-size calcite crystals were generated mostly from less saline pore fluids, and limestone nodules were formed chiefly from dilute pore fluid. Dolostones were formed from fluids of varying salinity.
Hydrogeology of the central basin was complex, and postdepositional fluid movements are documented by differing values for different authigenic minerals of the same samples. Pore fluids moved within the lake deposits for a considerable period of time after deposition, as shown by δ18O values of quartz and K-feldspar formed at different times. Downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits.
Lake Olduvai became overall shallower during the deposition of Bed I, and the subaerial exposure increased, leading to occasional playa conditions. These changes very likely resulted from increasing aridity and tectonic activity. The lake that deposited lower Bed II was larger than that responsible for upper Bed I, and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the Bed I lake.