USING PHENOCRYST COMPOSITIONS TO DISCRIMINATE TUFFS OF BED III AND YOUNGER, OLDUVAI GORGE, TANZANIA

Olduvai Gorge is a very important paleoarchaeological site that exposes ~2 million years of Pleistocene sediments, hominin fossils, and stone tools. The sediment that makes up these rocks was sourced primarily from coeval volcanism from the numerous volcanoes within the Ngorongoro Volcanic Highlands (NVH) just east of the gorge. As such, there are many tuff marker units interspersed throughout the Olduvai stratigraphy that have been used for correlation to expand paleoenvironmental reconstructions of this hominin landscape. The older tuffs of Bed I and Bed II have been chemically fingerprinted using phenocrysts that exist in solid solution, but this has yet to be attempted for the tuffs of Bed III and younger. The same minerals that were used to fingerprint the older tuffs are present in the younger tuffs, and thus they can be similarly analyzed through Electron Probe Microanalysis (EPMA) to create a database of all of the marker tuffs throughout the stratigraphy. A challenge with this exists in the nature of deposition of these tuffs. A majority of the tuffs in Bed I and Bed II were deposited in relatively undisturbed lacustrine settings, making them laterally continuous in the stratigraphy. The tuffs in Bed III and younger were deposited in more dynamic settings, such as riverbeds and floodplains, making them generally laterally discontinuous in the stratigraphy. They also have a higher potential of including detrital contaminants that should not be used for fingerprinting purposes. As such, a further goal is to differentiate minerals within these tuffs that are volcanic in origin and formed during the time of deposition of Bed III and younger from minerals that are detrital contaminants from other, older sources. The objective of this research is to provide the detailed chemical analyses of Bed III and younger that have recently been established for Bed I and Bed II. This will then allow the upper beds to have modern mineralogical analyses that can be related to volcanic sources from the NVH and be used to make correlations and expand paleoenvironmental reconstructions in the younger beds.