U-Pb Dating of Oligocene-Miocene Petrified Wood Carbonates from the Turkana Basin, Kenya

Abstract:

New field and geochronologic data from Oligocene-Miocene localities from the Turkana Basin in northern Kenya establishes a stratigraphic framework and produces new ages for units through novel dating of fossil wood carbonate. Petrographically, the petrified wood is variably preserved; some hand samples preserve internal wood structure, while others are internal or external molds that preserve wood geometry and bark impressions. The preservation of such textures requires early permineralization, and thus carbonate ages should approximate the time of sedimentation. The minerals that preserve the wood are also varied, with both calcite and dolomite present. Many of these carbonate petrified wood samples have favorable U and Pb isotope systematics, and all of the samples we have measured thus far yield ages consistent with existing and new Ar-Ar ages of intercalated basalt and ash deposits. Besides improving basin geochronology, the sizes and types of trees may provide additional insight into Turkana paleoclimate. Carbonate clumped isotope analyses of fossil tree dolomite suggest thermal fluids were responsible for at least the most successful U/Pb sample from Kalodirr West. This sample has U concentrations in the 100’s of ppm, ²³⁸U/²⁰⁶Pb ratios of nearly 500, and very well-behaved systematics, giving an age of 14.74 +/- 0.62 Ma. The uranium is reduced, and strongly correlated with Zr and Hf. Sr is anti-correlated with U, but ⁸⁷Sr/⁸⁶Sr is unchanged through several orders of magnitude change in concentration. This rich geochemistry and the complex carbonate mineralogical associations make Late Cenozoic wood in the Turkana Basin unique worldwide, and begs larger questions about the role of tectonics in their fossilization. Work is ongoing to further characterize the mineralogy through synchrotron XRF techniques and a variety of stable isotope analyses (e.g., S, B, Li)