Abstract A sequence of lacustrine, volcaniclastic, and alluvial sedimentary deposits that record the past million years of the history in the Olorgesailie Basin, southern Kenya, provide an example of how tectonics, climate, and volcanism affect sedimentation in a rift valley. A series of radiometric dates on volcanic materials through this sequence permits relatively fine-scale calibration of the timing and duration of volcanic input to the depositional system, transgressive-regressive cycles of the lake, and intervals of valley cutting and filling. The Olorgesailie Formation, accumulated between 0.992 and 0.493 Ma, consists of relatively pure diatomites, reworked diatomites, primary volcanic and reworked volcaniclastic units, and alluvial deposits (clays, silts, and sands with several well-developed paleosols) that bear a rich archeological and paleontological record. After 0.493 Ma, increased tectonic activity initiated a series of valley cutting and filling cycles that continue into Recent times. A working hypothesis attributes the formation of the paleolake to a barrier on the southwest side of the basin, large-scale lacustrine versus alluvial phases of the Olorgesailie Formation to variations in subsidence rates operating on a time scale of 10 4 -10 5 yr, and transgressive-regressive cycles within these sedimentary packages to wet-dry climate cycles on time scales of 10 3 -10 4 yr. Episodes of volcanism were superimposed on these patterns but did not have significant long-term effects on the depositional system.

Abstract Lake Olorgesailie was one of several large paleolakes known to have existed in the Kenya Rift Valley during the Pleistocene. The Olorgesailie Basin coversan area of about 100 km in the southern Kenya Rift,25 km northeast of Lake Magadi (Figure 1). TheOlorgesailie Formation includes a wide range of lacustrine, fluvial, and colluvial sedimentary rocks.Gregory (1921) first described the deposits from the area, which he attributed to a Miocene "Lake Kamasia."In 1943 Louis and Mary Leakey discoveredabundant Acheulian hand axes in the deposits(Leakey, 1952), which led to further archeologicalstudies by Isaac (1968), and more recently by Potts(1989, 1994). Shackleton (1955, 1978) undertookdetailed mapping of the main archeological site.Some of his results were included in the Kenya GeologicalSurvey report of Baker (1958), who assigned the sedimentary rocks to the "Olorgesailie LakeBeds." Baker and Mitchell (1976) later referred to them as the "Legemunge Beds." Isaac (1977) placedthe deposits in the "Olorgesailie Formation," which he defined as "a series of well-stratified diatomites,pale yellowish volcanic siltstones, and claystones,and subordinate quantities of brown siltstones and volcanic sands." Marsden (1979) described the generalsetting and history of the Olorgesailie Formation. Owen (1981) and Owen and Renaut (1981) describedthe sedimentology, facies, and diatom stratigraphy of the Olorgesailie Formation, and attempted to reconstruct the sequence of depositional environments.Bye (1984) and Bye et al. (1987) examined aspects of the geochronology and diagenesis of the formation.In this paper, we summarize the main features of theOlorgesailie Formation, incorporating new data on553the lateral variations in facies and the diatom distributionin the paleolake.