QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION DISTINGUISHED CAREER AWARD: PALEOSOL DIVERSITY IN THE OLDUVAI BASIN, TANZANIA: EFFECTS OF GEOMORPHOLOGY, PARENT MATERIAL, AND GROUNDWATER ON SOIL DEVELOPMENT

The Olduvai Basin is located at 3ºS in the East African Rift System in northern Tanzania. The basin is ~ 30 km wide and 100 m deep and is known for its fossil and cultural record of early hominins. In addition, the basin contains a rich and diverse record of paleosols that vary spatially and temporally. Fortuitously, the stratigraphy can be divided into time slices using dated tuffs and studied in a paleolandscape context. Sediments were deposited in a semi-arid closed basin containing a playa lake, Lake Olduvai. Volcanoes supplied volcaniclastic material to an alluvial fan system on the eastern side and quartzo-feldspathic sediments were transported by rivers from weathered basement rocks on the western side. The shallow saline-alkaline lake in the basin center and groundwater aquifers in the surrounding uplands fluctuated with Milankovitch-driven climatic (precession) cycles of ~20,000 years. The rift basin paleolandscape during the Early Quaternary is reconstructed using sedimentology, stratigraphy and paleopedology. Standard field descriptions of physical and biogenic structures are combined with the analyses of stable isotope of carbonates, whole rock geochemistry of sediments, and clay mineralogy that reveal a broad diversity paleosols. There are three distinct types of paleosols that record a paleocatena related to both landscape and drainage differences. Red tephra-rich paleo Andisols developed on the volcaniclastic alluvial fan to the east of the paleolake, calcium-carbonate-rich, silty paleo Aridisols developed on the interfluves and floodplain of the fluvial plain on the west, whereas clay-rich soils (paleo Vertisols) developed on subaerially exposed lake sediments in the center of basin. Variances in geomorphology, depositional environment, parent material, and depth to the water table are reflected in the development of contemporaneous but distinctly different soil types recognized in high-resolution reconstruction of the paleolandscape through time.