REFINED STRATIGRAPHY AND PALEOENVIRONMENTAL INTERPRETATION FOR THE FOSSIL-BEARING LOWER MIOCENE LOCHERANGAN SECTION, WEST TURKANA, KENYA

Detailed sedimentological description allows for refined characterization of paleoenvironments. A precise understanding of paleoenvironment, in turn, can offer insight into the climate conditions that influenced early hominoid evolution. This approach was applied to the hominoid-bearing Lower Miocene Locherangan locality west of Lake Turkana in northern Kenya. The primates present, Afropithecus turkanensis and Simiolus enjienssi, emerged in an environment that was wetter than the modern savannah environment, and paleoenvironmental reconstructions can determine the local climatic conditions to which these early potential ancestors were adapted. Locherangan was previously described and vertebrate fossils were excavated in the late 1980s, but no detailed environmental interpretation was conducted in association with this work. The middle of the section was chronologically constrained at 17.5 ±0.3 Ma from a K/Ar date on alkaline feldspar from a tuffaceous unit. High-resolution section measurement and lithological description undertaken in the summer of 2016 documented 50.3 m of section exposed in a 100 square meter area. The sequence is dominated by fine-grained lithologies, including ostracod-bearing shales, with abundant fossil fish, reptiles, and coprolites. Sandstones occur less frequently, but preserve most of the fossil mammal assemblage. Previously unrecognized organic rich strata, bentonites, fossiliferous units including plant impressions, and repeated episodes of pedogenesis indicate a complex environment through time and suggest a significant depositional system. Locherangan represents one of few localities from the Lower Miocene in Turkana and is thus an important piece of the basin’s dynamic and poorly understood Miocene history. Altogether, the increased resolution attained in this locality provides valuable environmental context for this important time in hominoid evolutionary history and both corroborates and expands upon current understanding of the region’s development throughout the Miocene.