THE GEOCHEMISTRY OF TEPHRA FROM LATE MIOCENE FOSSIL SITES IN SOUTHERN KENYA

Lemudong'o Formation exposures in the Southern Kenya Research Project (SKRP) pose an exciting opportunity to expand paleontological and geological investigations in the late Miocene (around 6 million years ago) to interpret the timing and environmental context of human origins. The formation contains Neogene volcanics interbedded within lacustrine, fluvial, and alluvial deposits. Proposed regional stratigraphic ties linking sections are based on lithostratigraphic evidence and previous dating efforts. Despite the abundance of explosive volcanic material in the area, we currently lack geochemical studies of interbedded tephras due to the infancy of our work at SKRP. Analyzing the geochemical fingerprint of tephra deposits using glass geochemistry to establish marker beds is crucial for broadening and dating paleoenvironmental reconstructions and interpreting paleoecology in the basin.

Here we present the first geochemical data from the Lemudong’o Formation. Eight samples from Lemudong’o sites 2 and 3 were prepared for electron microprobe analysis by wet sieving and acid washing. We analyzed 15 major and minor elements on 20-30 glass shards/pumice per sample. Results show that tephra deposits are rhyolite to dacite in composition, and one sample from Lemudong’o 3 is a trachyte. Intra-sample geochemical variability is high for most samples (e.g., Fe₂O₃ ranges from 6-10 wt%). No correlations were suggested by these results. We are currently analyzing tephra samples collected at the nearby sites of Enamankeon and Siloma at SKRP hypothesized to correlate to the Lemudong’o Formation. Results of these geochemical investigations will ultimately test proposed stratigraphic correlations and generate a tephrostratigraphic framework for the Lemudong’o Formation.