Lake Olduvai (~1.92-1.70 Ma) was a shallow closed-basin lake as much as ~15-20 km in average diameter. Lacustrine deposits are chiefly waxy clay but also include tuff, dolostone, and limestone. These deposits are *90% authigenic minerals, nearly all of which are diagenetic: Mg-rich smectite, celadonitic illite, K-feldspar, zeolites, calcite, dolomite, fluorite, magadiite-type chert, and trona and gaylussite replaced by calcite. These minerals indicate high concentrations of Na and CO₃ and a high pH (9.5-10). Salinities were overall higher in the central basin than in water flooding lake-margin terrain during high lake levels. Evaporative concentration was determined as the difference between average oxygen isotope composition of meteoric water (~-7‰) and calculated d¹⁸OH₂O values of authigenic minerals. A temperature of 15°C was used for calculating fractionation factors. Calculated d¹⁸OH₂O values of authigenic minerals in the central basin vary over 11‰, suggesting that salinities varied widely. Clay minerals are early diagenetic and were formed chiefly from the more saline waters. d¹⁸OH₂O values for clay minerals of the central basin are +0.6 to +5.2‰, averaging +2.3‰,, compared to –2.3 to +2.5‰, averaging –1.0 ‰ for lake-margin clays. Dolostones of the central basin formed from waters of differing salinity, with calculated water values averaging -0.1‰ for one group and –5.2‰ for the other. Hydrogeology of the central basin was complex, and post-depositional fluid movements are indicated by differing calculated d¹⁸OH₂O values for different authigenic minerals from the same samples.