Sequence-stratigraphic and X-ray fluorescence characterization of lake transgression-regression cycles at the late Pliocene to Pleistocene transition, central Kenya Rift Valley

The late Pliocene to early Pleistocene Baringo Basin long drill core (ICDP HSPDP project) from the central Kenya Rift records cyclical fluctuations from deep lake to marginal environments between ~3.3-2.6 Ma, intercalated with thick packages of alluvial sediments. High-resolution sedimentology and ichnology (cm-scale) were used to recognize discontinuities and packaging of facies representing changing paleoenvironments. Sequence stratigraphy was applied to delineate discrete chronostratigraphic surfaces representing transgression (TS), forced regression (RS), and lowest base-level (SB) through the succession. Elemental composition measured by continuous scanning X-ray fluorescence at 1 cm intervals corresponds to stratigraphic packaging. A sharp decrease in K and Ti is observed at most transgressive surfaces, followed by progradation with gradually increasing K and Ti, and forced regression with a sharp increase in K and Ti. Together with Fe, these clastic indicators characterize the patterns of sediment supply to the lake. Lake expansion is represented by high Si/Ti, low magnetic susceptibility, and a high ratio of incoherent/coherent scattering (a bulk measure of organic matter and sediment density). A high-resolution Bayesian stratigraphic age model based on ³⁹Ar/⁴⁰Ar dating of tuffs and magnetostratigraphic boundaries (Deino et al., 2019) was then applied to determine the periodicity of events. Sequence boundaries and transgressive surfaces in lacustrine intervals are spaced at a precessional scale ~23 ka, with the deeper lakes corresponding to 100 ka eccentricity maxima, and sets of lake expansion-contraction cycles corresponding to highs in 400 ky eccentricity. Some packages may also record the influence of ~40 ky obliquity, from ~3.15-2.95 Ma. Precession-scale lake cycles are observed from ~3.18 Ma, associated with high 100 ky eccentricity. The most cyclic and regular packages, from ~2.75-2.58, represent an increase in water and sediment input to the basin corresponding to the highest amplitude insolation maxima. A major change in environments at ~3.04 Ma, from alluvial-dominated to fluctuating deeper lakes, may be due to regional or global climatic influences which fostered greater precipitation/reduced evaporation during lacustrine intervals.