40Ar/39Ar Age Calibration of the Lacustrine Sediments at Kariandusi, Central Kenya Rift
Abstract
The Kariandusi site in the Nakuru-Elmenteita Basin, central Kenya Rift, contains a 31 m thick sequence of well-stratified diatomites and tuffs. It is significant in the overall late Cenozoic climate picture of East Africa in that Kariandusi is one of three sites (along with Olorgesailie, southern Kenya, and Afar, Ethiopia) that record a major wet period at ∼1000 ka. The diatomite assemblage records the complete history of a large, deep lake, from lowermost units containing benthic-epiphytic flora, followed by deep-lake genera, and succeeded in the higher parts of the section by an assemblage that indicates lake regression with higher alkalinity. The diatomite rests directly on the widespread Gilgil trachyte lava flows, dated at Kariandusi by laser 40Ar/39Ar total-fusion of anorthoclase phenocrysts at 0.985 ± 0.003 Ma (1σ ).The main diatomite is conformably overlain by a 0.7 m thick tephra horizon that apparently represents a direct airfall into the lake. The age of this unit by single-crystal 40Ar/39Ar dating of anorthoclase is 0.973 ± 0.003 Ma. It is succeeded by an intercalated sequence of diatomites and tephra about 4 m thick. Previous 40Ar/39Ar dating (Deino and Potts, 1990, JGR 95:B6: 8453-8470) indicates that the Kariandusi diatomite sequence was deposited at approximately the same time as lacustrine sediments in the lower part of the Olorgesailie Formation (bracketed between 1000-980 ka), 135 km to the south within the Kenya Rift. Limited tephra correlation based on trace-element analysis of glass separates suggests that the Olorgesailie lacustrine sequence may be equivalent to the upper part of the Kariandusi diatomite sequence, or slightly younger. Further work, including dating, tephra correlation, and diatomite assemblage studies, may eventually permit precessional-level assignment of the lacustrine phases at the two sites.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.U21A0703D
- Keywords:
-
- 9305 Africa;
- 9604 Cenozoic;
- 3344 Paleoclimatology;
- 1035 Geochronology