A high-resolution tephrochronological framework for the Okote Tuff Complex, Turkana Basin, Kenya
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2025
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Melbourne Authors
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School of Earth Sciences
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PhD thesis
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© 2025 Ashley Maria Savelkouls
Abstract
The Turkana Basin is a paleoanthropologically significant region within the East African Rift System. This long-lived basin records evidence of the earliest human fossils and associated archeological tools within its Plio-Pleistocene sediments. Known as the Omo Group, these sediments are characterised by fluvial and lacustrine deposits interbedded with volcanic tephras (known regionally as tuffs). These volcanic tuffs provide the building blocks for understanding the temporal evolution of the Turkana Basin via absolute dating. On the eastern shores of modern Lake Turkana lies the Koobi Fora Formation, where the discovery of the earliest stone tools prompted detailed study. Within this formation, the early Pleistocene Okote Member (1.6 to 1.4 Ma) is notable for an increase in the preservation of hominin fossils and a significant increase in interbedded tuffs. The Okote Member outcrops in three primary locations—Karari Ridge, Ileret, and Koobi Fora Ridge. Although these sites represent different depositional environments, they are temporally linked by key marker beds that constitute the Okote Tuff Complex. This complex contains more than 25 closely spaced, geochemically distinct tephra layers, which provide an unparalleled opportunity to refine the chronology of the Okote Member. This thesis refines the tephrostratigraphy of the Okote Tuff Complex using a combined approach of geochemical fingerprinting and high precision 40Ar/39Ar geochronology. Due to the fluvial deposition of the tuffaceous sequences, pumices are frequently observed reworked upwards throughout the stratigraphy of the Okote Member. Therefore, accurate geochemical characterisation of the tuffs and their subsequent volcanic pumices is required to appropriately assign the eruption age to a single tuff. Geochemical fingerprinting of the volcanic products using major and trace element compositions is combined with multivariate data analysis to correlate tuffs and pumice clasts. Six distinct compositional groups are identified in the Okote Tuff Complex that range from trachydacites to rhyolites. The Northern Ileret, Black Pumice and Morutot tuffs yield compositions that are enriched in Al2O3 (>11wt.%) and typically contain plagioclase feldspar. The Lower Ileret, Ileret, Lower Koobi Fora, Morte, Koobi Fora and Karari Blue tuffs yield similar major-element compositions that are differentiated by minor and trace elements (i.e., Cl, Ti, Zr, Nb). Trace element analyses are compared to potential volcanic sources of the Okote Tuff Complex, suggesting the caldera-forming eruptions of the Main Ethiopian Rift as a primary candidate for the source volcanism. Following geochemical characterisation, 40Ar/39Ar geochronology is applied to single crystals within the pumice clasts. Due to the advent of high-precision mass spectrometry (<0.1% uncertainty), significant dispersion (up to 100 ka) can be observed from a single eruption rendering traditional weighted mean age estimations inappropriate for these datasets. A Bayesian approach is taken to estimate the ‘true’ age of eruption, based on the assumption that the eruption age must post date crystallisation. For the case of K-feldspars, the crystallisation distribution typically forms an exponential distribution. Revised and new Bayesian 40Ar/39Ar age estimates for 8 tuffs in the Okote Tuff Complex including the Karari Blue (1.49257 +/- 0.00180 Ma), Koobi Fora (1.49430 +/- 0.00140 Ma), Morte (1.51038 +/- 0.00070 Ma), Lower Koobi Fora (1.51561 +/- 0.00090 Ma), Ileret (1.51716 +/- 0.00052 Ma), Lower Ileret (1.52607 +/- 0.00166 Ma), Black Pumice (1.542 +/- 0.010 Ma), and Morutot (1.62136 +/- 0.00098 Ma) tuffs. Subsequently, a combined approach of geochemical characterisation and 40Ar/39Ar geochronology is used to resolve the stratigraphic complexities of the Okote Member and provide a new Age-Depth model. Resulting in a new chronological framework for the Koobi Fora, Ileret and Karari Ridge regions. This improves previous age depositional models by an order of magnitude allowing for the age recalculation of several key paleoanthropological sites including hominin footprint site ET-2022-103-FE-22 (1.562 +/- 0.026 Ma), KNM-ER 47000 (Paranthropus boisei; 1.521 +/- 0.006 Ma) and KNM-ER 804 (Homo erectus; 1.539 +/- 0.013 Ma). These novel and improved ages provide a robust tephrochronology for future research including anchoring tuffs for paleoclimate and magnetostratigraphy studies. Furthermore, this study has implications for regional tephra correlations and contributes to the tephrochronology of the East African Rift System.
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Geochemistry, Geochronology, Ar-Ar, Volcanology, Turkana Basin, Plio-Pleistocene, Koobi Fora Formation, Okote Tuff Complex