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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 104-105 yr, and transgressive-regressive cycles within these sedimentary packages to wet-dry climate cycles on time scales of 103-104 yr. Episodes of volcanism were superimposed on these patterns but did not have significant long-term effects on the depositional system.

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