CYCLES OF EROSION AND DEPOSITION IN THE PLEISTOCENE OLORGESAILIE BASIN OF SOUTHERN KENYA AND THEIR IMPACT ON THE PALEOANTHROPOLOGICAL RECORD

The geological history of the Olorgesailie Basin in the southern Kenya Rift Valley shows the impact of tectonics and climate on fossil and artifact preservation over the past million years. The Olorgesailie Formation, between ~1.20 to 0.49 Ma, consists of ~80m of diatomite, volcaniclastics, fluvial deposits and paleosols deposited on volcanic basement north of Mt. Olorgesailie, an extinct Pliocene central rift volcano. Lacustrine sediments make up over 70% of the thickness but represent less than 25% of the time spanned by this formation. Two later formations, the Olkesiteti and the Oltepesi, occur both north and west of Mt. Olorgesailie and together span the period from ~ 360 to <64 Ka. These represent multiple cycles of valley incision and filling with approximately 30m of fluvial, volcaniclastic, and lacustrine sediments. All three formations preserve archeological and paleontological sites, with age control provided by interstratified volcaniclastic units. At least 26 different archeological levels occur throughout the sequence. A major episode of valley formation occurred between 490 and 360 Ka and represents a shift in base level likely caused by tectonic activity in the Koora Graben west of Mt. Olorgesailie. Other periods of marked erosion occurred between 210-190 Ka, 150-120 Ka and during the last 10 Ka. Throughout the entire depositional sequence, discrete levels with artifacts and fossils represent relatively short time intervals in which conditions were favorable for accumulation and burial, primarily on soil surfaces, slowly aggrading floodplains and in active channels. Stable isotope analysis of pedogenic carbonate as well as characteristics of the sediments and preserved biota provide paleoenvironmental evidence for these temporary landsurfaces. In contrast, the paleoenvironmental signals preserved within most of the lacustrine and volcaniclastic deposits were recorded during tectonically or climatically controlled intervals of rapid sedimentation. Consequently, the paleoenvironmental record based on the dominant sediments in the three formations is partially de-coupled from the paleoanthropological record. This has important consequences for interpreting environmental controls on the paleoecology of hominins and associated faunas.