ANALYSIS OF ENVIRONMENTAL CONDITIONS OF AN EARLY MIOCENE PERENNIAL RIVER SYSTEM IN EAST AFRICA USING SEDIMENTOLOGY, STRATIGRAPHY, AND GEOCHEMICAL PROXIES

During the late Oligocene and early Miocene in East Africa, Hominoidea began to evolve and diversify from Old World monkeys, in turn making it an extremely important time period for beginning to understand the link between our ancestors and the environment. Throughout this time, East Africa experienced significant tectonic and climatic change, and we aim to determine the influence of each driving force on the paleoenvironment. We recreated the Critical Zone at Loperot (Kenya, west Turkana), a perennial river system that dates back to ~17Ma, using a multi-proxy approach that coupled sedimentology, stratigraphy, and quantitative geochemistry. To estimate climate and weathering patterns we used quantitative proxies (e.g., CIA-K, CALMAG) based on elemental ratios and found that (consistent with a decrease in paleosol chroma values) both mean annual precipitation and base/loss weathering ratios increase slightly upsection. Flow direction of cross beds changes from eastward to northward upsection, concurrent with a decrease in the thickness of paleosol units, an increase in quartz content and average grain size of sand units, and the disappearance of key minerals (e.g., gypsum and illite in paleosols, K-feldspar in sands). Much of this data can be explained by the formation of a normal fault beginning at or around the time of deposition of Unit 17. Before Unit 17, the Loperot river was a single-catchment river flowing East. Following the uplift of a horst to the east of Loperot around Unit 17 the river diverted northwards and was fed by more catchments. This uplifted horst could have upset weather patterns in the region, creating a more humid climate on the windward side. This analysis of environmental conditions at Loperot suggests that it was undergoing both tectonic and climatic changes and that tectonics may have directly affected localized climate during the early Miocene in East Africa.