PALEOLAKE TRANSGRESSION AND REGRESSION CYCLES IN THE PLIOCENE CHEMERON FORMATION AT TAMALAKWO, KENYA RIFT VALLEY

Tamalakwo is an understudied outcrop in the Chemeron Formation from the Pliocene of the central Kenya Rift Valley, which pertains to a critical interval of time for hominins and is older than the ICDP long drill core from this area (Hominin Sites and Paleolakes Drilling Project Baringo-Tugen Hills-Barsemoi; HSPDP-BTB13-1A). Coarsening-upwards cycles in the sediments at Tamalakwo are assessed to determine how they reflect paleolake transgression and regression, and whether the environmental fluctuations relate to astronomical cycles. Hand logs of a ~40 m measured stratigraphic section were digitized to produce plots of grain-size, facies, and mud percentage. Facies were interpreted, and a log of interpreted environmental zones was produced to compare with published Laskar curves for 15N June insolation of the likely corresponding time interval (~3.3–3.6 Ma) established from field relationships. X-ray fluorescence analysis of outcrop samples at 50 cm intervals helped to quantify changes in the elemental compositions of lithologies. To determine if the facies changes and interpreted lake-level fluctuations were caused by orbital control on precipitation, we extrapolated the calculated sedimentation rates from the age model developed for the HSPDP-BTB13-1A core. Statistical correlations between the datasets and Laskar insolation curves indicate that the lake-level fluctuations were likely related to insolation-driven precession-scale cyclicity. The cycles from this outcrop also illustrate the impact of 100 ky eccentricity on the paleoenvironments of Pliocene rift basins in central Kenya.