STRATIGRAPHIC ANALYSIS AND PALEOENVIRONMENTAL INTERPRETATION OF AN EARLY MIOCENE PRIMATE FOSSIL SITE IN THE TURKANA BASIN; WESTERN KENYA

Since 2012, the Loperot Miocene Project has collected a unique assemblage of primate fossils in west Turkana, including species of apes and monkeys previously not associated. As of yet, there has been no systematic analysis of the stratigraphy surrounding the fossiliferous layers or an attempt to interpret the ancient environment at this site. Our study aims to reconstruct the Early Miocene landscape in which this exceptional assemblage of primates lived.

We measured ~28 m of stratigraphic section at site LpM4 and identified 30 separate units. Samples were analyzed through X-ray diffraction (XRD) and optical microscopy to determine bulk and clay mineralogy. Interpretation of the stratigraphic section revealed that there are at least eleven fining-up cycles (each with sand at the base and red or green clay at the top). The sand units are arkosic or quartzose and often contain trough crossbedding, while the clay units are primarily smectitic, red or green, and contain peds and rhizoliths suggesting they are paleo-Vertisols. The fining-up cycles (from fluvial sand to paleosol clay) indicate a dynamic environment that alternated between a meandering river and its floodplain. Fossil fauna in the region include crocodiles, turtles, fish and a whale, suggesting that (when present) this river was large and at one point connected to the ocean. Mineralogical analysis of the river sands indicates an increase in quartz content and a decrease in feldspar up-section. We interpret this as a shift in sediment source or an increase in humidity (and weathering of feldspar) over time. The dip direction of crossbed foresets changes from E-W to N-S up-section, and may indicate a tectonic shift in the region that altered the river’s course and possibly its sediment source over time.

Our detailed stratigraphic analysis identified at least eleven fining-up cycles of a meandering river at Loperot and suggests a shift in the river’s course through time, indicating that this region was dynamic and changed rapidly throughout the Early Miocene. This study adds more detail about the environmental conditions that affected our Early Miocene primate ancestors in the western Turkana Basin.