NEW INSIGHTS INTO SPATIAL VARIABILITY OF RIFT BASIN PALEOSOLS USING SEDIMENTOLOGY, PALEOPEDOLOGY, AND SOIL GEOMORPHOLOGY, OLDUVAI GORGE, TANZANIA

The sediments of Olduvai Gorge, Tanzania contain a rich record of paleosols that vary spatially and temporally but can be divided into time slices using dated tuffs and studied in a paleolandscape context. The sediments were deposited in a semi-arid closed rift basin containing paleo Lake Olduvai with volcanoes supplying volcaniclastic material to an alluvial fan system on the east and quartzofeldspathic fluvial sediments derived from weathered basement rocks on the west. The shallow saline-alkaline paleo lake and groundwater levels in the surrounding upland fluctuated with Milankovitch-driven climatic fluctuations. The paleosols record a paleocatena related to both landscape and drainage.

The rift basin paleolandscape (at ~ 1.8 Ma) was reconstructed using sedimentology, paleopedology, and soil geomorphology. Differences in topography, depositional environment, parent material, and depth to the water table are reflected in the development of different soil types across the landscape. Earlier studies found calcium-carbonate-rich silty Aridisols on the interfluves and floodplain of the fluvial plain west of the paleolake, whereas, red silty Andisols developed on the volcaniclastic alluvial fan to the east. The Andisols were cumulative paleosols (tephra and debris were periodically added). They contained abundant biogenic structures, redoximorphic mottles, and zeolite crystal silt formed during water table fluctuations.

Here we present preliminary data on clay-rich paleosols (Vertisols) developed on the lake sediments in the center of basin. The Vertisols have well-developed lentil peds often greater than 20 cm in length and associated slickensides as well as stress cutans. The Vertisols show variations in color, ped types, and amount of volcaniclastic input that make it unlikely these represent simple soils. These Vertisols reveal an entirely new soil-forming environment previously unstudied at Olduvai Gorge shown by the differences in grain size as well as unique vertic features, which contrasts strongly with the Aridisols and Andisols found on basin margins. Further research into the bulk geochemistry and bulk mineralogy of these paleosols will provide considerably more information on the spatial variation and transition between environments within the paleocatena.