Characterization and correlation of altered Plio-Pleistocene tephra using a 'multiple technique' approach: Case study at Olduvai Gorge, Tanzania

A "multiple technique" approach, using major and minor element compositions of phenocryst and glass (where available), was used to develop geochemical fingerprints for the major Bed I tuffs at Olduvai Gorge. A geochemical type locality was selected for each of the major tuffs for comparison to outlying regions. Using alteration-resistant phenocrysts, the tuffs were correlated between fresh and altered exposures in diverse depositional environments. A new high-resolution stratigraphic framework for Bed I is presented, including correlations into western Olduvai where the tuffs had been previously misidentified.; These correlations have implications for volcanic source and basin development. Individual Olduvai tuffs are now correlated with ignimbrites and lavas proximal to Ngorongoro (Lower Bed I) and Olmoti (Upper Bed I). The rock fragments in Tuff IF are correlated to a trachytic lava in the Olmoti crater wall. The identification of a Lower Bed I tuff (Tuff IA) below the Bed I basalts in eastern Olduvai suggests that the eruption of the basalts was not responsible for the westward movement of the saline-alkaline lake in the Late Pliocene, as this event occurred before the deposition of Tuff IA.; The Upper Bed I tuffs are more alkaline than previously thought; Tuff IF is a phonolite and thus silica-undersaturated. The change in volcanic source (from Ngorongoro to Olmoti) and composition (from rhyolite to trachyte) between Lower and Upper Bed I marks a fundamental lithological transition within Bed I. It is therefore suggested that this compositional change permits a more formal stratigraphic characterization of Lower Bed I from Upper Bed I, designated here as Bed 0. Tuff IA and the Bed I basalts mark this transition.; A pilot study on the Olduvai Bed II tuffs (nephelinites and foidites) and augitic sands did not find useable geochemical fingerprints. The minerals in these tuffs and sands are too variable within each sample and too similar between samples for mineral-based tephrostratigraphy.; Bulk geochemical analyses of altered tephra show considerable variability, and cannot be used for correlation between different environments. The mobility of major, minor, and trace elements during alteration varies depending on the type of alteration, with different patterns of enrichment and depletion for zeolitic and clay alteration.