2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 12
Presentation Time: 11:00 AM


HAY, Richard L., Geosciences, Univ of Arizona, 4320 N Alvernon Way, Tucson, AZ 85718-6180 and KYSER, T.K., Geological Sciences, Queen's Univ, Kingston, Ontario, K7L 3N6, rhay@dakotacom.net

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.