RECOGNIZING HOT-SPRING ACTIVITY AND WATER INPUT TO THE PLIOCENE CHEMERON FORMATION LACUSTRINE SYSTEM IN THE CENTRAL KENYA RIFT VALLEY

The long (~228 m) ICDP Hominin Sites and Paleolakes Drilling Project HSPDP-BTB13-1A drill-core from the Baringo Basin, Kenya, consists of the of the upper part of the Pliocene Chemeron Formation (~5.3–2.6 Ma) and represents mainly fluvial/alluvial and cyclical lacustrine packages with diatomaceous and laminated dark-brown mudstones sometimes brecciated and cross-cut by sedimentary dikes. The injection features are filled with brecciated material and cement, and are associated with alteration and discolouration along fractured zones. The timing of some of the features can be constrained within the corresponding stratigraphic package, which represents a full precession-scale cycle of lake-level rise-and-fall. This indicates that the degree of hydrothermal activity may be associated with lake-level, and that the fluids may have influenced lake hydrochemistry. We hypothesize that during periods of intermediate lake level, hydrothermal fluids with distinctive solutes undergo limited dilution within the lake, can influence mineral precipitation and elemental geochemistry of sediments, and promote development of microbial deposits. We used X-ray fluorescence data (scanned at 1 cm resolution) to characterize the geochemical signature of the observed features, and multivariate statistics including Principal Component Analysis (PCA) and Unconstrained Cluster Analysis to help recognize lacustrine deposits influenced by hydrothermal fluids. Although nearly ubiquitous in modern rift lakes, hydrothermal influence in ancient examples is not widely recognized due to limited evidence in the sedimentary record. Here, we use visual observation of the BTB13 core, X-ray fluorescence geochemistry, and statistical analysis to develop an approach to recognize the influence of hydrothermal fluids in cyclical lacustrine successions.