AN OUTCROP RECORD OF RAPID LANDSCAPE TRANSFORMATION IN THE PLEISTOCENE OLORGESAILIE BASIN, EAST AFRICAN RIFT SYSTEM, SOUTHERN KENYA

The latest Pleistocene history of the East African Rift in southern Kenya is recorded by the Oltulelei Formation, which overlies the Olorgesailie Fm. (1.2 – 0.5 ma) and includes archeological sites and fossil remains between ~320 ka and ~50 ka. Sedimentological links to the Koora Graben core (ODP-OLO12-1A), ~20 km to the south (Koora Plain), reflect shifting connections between rift sub-basins. Synthesis of the Oltulelei Fm. depositional history shows that the Olorgesailie region and the northern Koora Graben formed three basins that were periodically connected and disconnected over the past 500 ka. Varying degrees of linkage between these sub-basins reflect a changing balance of sediment input and accommodation space controlled by climate, volcanism, and periodic movement of faults that displaced the volcanic basement. Following erosion of the Olorgesailie Fm. between ~500 and ~320 ka, the Oltulelei Fm. was deposited during three major cycles of aggradation and erosion, with fluvial, lacustrine and wetlands deposits preserved in fault-controlled areas or temporarily blocked drainage valleys. Geochronological calibration of the Oltulelei Fm., based primarily on Ar-Ar dates of interbedded tephras, shows erosion to deposition shifts in the physical landscape on time scales of 10⁴ – 10⁵ yrs. Periods of dissection alternating with filling of valleys up to 12 m deep were driven by a combination of southern Kenya rift tectonics and climate dynamics. Removal of large volumes of sediment required increased precipitation and through-flowing drainage into the lower base level of the Koora Graben, southwest and south of Mt. Olorgesailie. Regional volcanism resulted in episodes of volcaniclastic deposition, including one major influx between 225 and 207 ka that covered the faulted and dissected topography of the Olorgesailie Basin and overflowed into the northern Koora Graben. Portions of the Oltulelei Fm. correlate geochronologically with the ODP core, but in general the Olorgesailie and Koora Graben regions operated as semi-independent depositional basins. This works to the advantage of climate reconstructions for the paleontological and archeological records, as the ODP core includes lacustrine sediments representing times when erosion and subaerial landscapes prevailed in the Olorgesailie Basin to the north.