AUTHIGENIC CLAY MINERALS IN GEOARCHAEOLOGICAL SETTINGS: OVERVIEW AND EXAMPLE FROM THE PLEISTOCENE OLORGESAILIE FORMATION, KENYA

Authigenic clay minerals form in many depositional settings of geoarchaeological interest. These sheet silicates can be highly reactive, yielding variable phases and compositions that reflect the chemistry of the waters in which they precipitated. They can be especially valuable where other proxies (e.g. microfossils) are absent. Size fractions <0.1µm are generally needed to avoid detrital, glass, or diatom fragments. They are well-known features of highly saline and alkaline basins; a growing body of evidence suggests they are important in other settings as well.

Here we provide the first detailed mineralogical and geochemical descriptions of clays of Members 1 (~1.2-0.99 Ma) and 7 (~0.9-0.8 Ma) of the Olorgesailie Formation. The basin sits in the southern Kenya Rift, yielding remarkable assemblages of Acheulean artifacts and vertebrate fossils, including hominin specimen KNM-OG 45500 from Member 7. Clastic and diatomaceous sediment of fluvial, lacustrine, and wetland origins dominate, with several marker tuffs dated by single-crystal 40Ar/39Ar.

XRD shows abundant clay minerals, with smectites dominating. Glycolated 001 peaks are at 17.5-18.0Å with rational basal spacings, though some Member 7 clays had significant illitic interstratification. 060 peaks of randomly oriented samples were in the region 1.506-1.511Å, consistent with dioctahedral aluminum-rich smectite. Electron microprobe yields an average chemical formula for the clays of (Si3.76 Al0.24)(Al0.86 Ti0.04 Fe0.68 Mg0.42)(Ca0.01 Na0.32 K0.26). These observations are in contrast to the kaolinitic and halloysitic soils of the area, and are consistent instead with authigenic mineral formation in alkaline lake or wetland waters with moderate salinity. Some samples had octahedral Mg ~0.8, approaching the Mg-rich smectites known from Beds I and II in saline, alkaline Olduvai Gorge. Overall, however, Mg content indicates significantly lower lake or wetland salinities than Olduvai. Continued study of these clay minerals across paleolandscapes and through time will provide an additional source of paleoenvironmental information for this important paleoanthropological locality.