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The Oltulelei Formation of the southern Kenyan Rift valley; a chronicle of rapid landscape transformation over the last 500 k.y.

Anna K. Behrensmeyer, Richard Potts and Alan Deino
The Oltulelei Formation of the southern Kenyan Rift valley; a chronicle of rapid landscape transformation over the last 500 k.y.
Geological Society of America Bulletin (October 2018) 130 (9-10): 1474-1492

Abstract

The last half-million years of geological history of the East African Rift in southern Kenya is recorded by the Oltulelei Formation, which is newly described in this paper. Well-dated depositional cycles in this formation reveal how tectonics and climate shaped rift valley sedimentation, as well as the landscapes inhabited by human ancestors. The Oltulelei Formation (ca. 320 to ca. 36 ka) overlies the Olorgesailie Formation (1.2-0.5 Ma) on an erosional unconformity and is divided into three successive units, the Olkesiteti, Oltepesi, and Tinga Members. The stratotype for the formation is exposed in the Olorgesailie Basin, north and northwest of Mount Olorgesailie. The (super 40) Ar/ (super 39) Ar dates on interbedded tephras provide geochronological controls on the timing of three major depositional cycles. Lateral variability in the Oltulelei Formation shows that the Olorgesailie Basin and the northern Koora Graben formed three rift subbasins that were periodically connected and disconnected over the past 500 k.y. Erosional phases removed large volumes of sediment from the Olorgesailie Basin, requiring through-flowing drainage to a lower base level southwest and south of Mount Olorgesailie. Aggradational phases were primarily fluvial, with siliciclastic and volcaniclastic sediment filling valleys and burying dissected landscapes. Tufa, shallow lacustrine, and wetlands deposits also occur and indicate elevated water tables in fault-controlled sumps or blocked drainages. We propose that connections and disconnections between rift segments were driven by tectonic forces, primarily faulting of volcanic basement rocks and periodic influxes of volcanic sediment, combined with climatic conditions that enabled erosion and transport of large volumes of sediment into and out of the subbasins. The Oltulelei Formation preserves a record of a dynamic change in the physical landscape of the southern Kenya Rift, with major shifts in erosion versus deposition on time scales of 104-105 yr. This study provides new, well-calibrated information on sedimentation in active rift settings as well as an outcrop-based, three-dimensional, basin-scale geological history that can be integrated with emerging drill-core paleoclimate records from southern Kenya. The stratigraphic record preserved in the Oltulelei Formation advances understanding and poses new questions about how tectonics and climate shaped middle to late Pleistocene faunal turnover and the transition in southern Kenya from Acheulean to Middle Stone Age technology.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 130
Serial Issue: 9-10
Title: The Oltulelei Formation of the southern Kenyan Rift valley; a chronicle of rapid landscape transformation over the last 500 k.y.
Affiliation: Smithsonian Institution, National Museum of Natural History, Department of Paleobiology, Washington, DC, United States
Pages: 1474-1492
Published: 201810
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 41
Accession Number: 2018-101423
Categories: StratigraphyStructural geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2018082
Illustration Description: illus. incl. 2 tables, geol. sketch map
S02°00'00" - S01°00'00", E36°00'00" - E36°45'00"
Secondary Affiliation: Berkeley Geochronology Center, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201853
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