PALEOCLIMATE RECORDS FROM AUTHIGENIC CLAYS AND ZEOLITES: EXAMPLES FROM THE PLEISTOCENE OLDUVAI GORGE, TANZANIA, AND PLIOCENE CHEMERON FORMATION, BARINGO BASIN, KENYA (Invited Presentation)

Paleolimnology is hindered by limited deposition or poor preservation of biotic records in many Quaternary lacustrine stratigraphic sections. In such settings, mineralogy and geochemistry may provide the only paleoenvironmental proxies. Recent efforts throughout the East African Rift and associated basins are providing new datasets allowing a regional perspective on authigenic silicate accumulation. Here we compare silicate diagenesis and paleoclimate records from two paleolake basins – the Olduvai Basin in northern Tanzania, and the Baringo/Tugen-Hills Basin of Central Kenya. The Olduvai lake clays contain a record of fluctuating saline, alkaline conditions spanning from about 1.92-1.78 Ma. Elemental, octahedral, and stable isotopic compositions of authigenic clay minerals provide evidence of freshening events strongly in phase with orbital precession. Sediments of the Chemeron Formation in the Baringo/Tugen Hills Basin spanning from about 3.4-2.6 Ma are dominated by dioctahedral detrital clay minerals, with abundant authigenic zeolites, including chabazite, phillipsite, heulandite/clinoptilolite, and analcime, often interbedded with diatom-rich intervals. Vertical zeolite facies changes suggest a long-term directional increase in salinity, with superimposed high-frequency fluctuations. The greatest amplitude of environmental variability appears to coincide with peak orbital eccentricity, suggesting an important role for eccentricity modulation of precession cyclicity. In both of these depositional settings, biotic records are discontinuous, hence these mineralogical records provide important evidence of changes in moisture availability, surface soils, and weathering products.