Linking the tephrostratigraphy of the Pleistocene Naibadad Beds across the Laetoli area, Olduvai basin, and Ngorongoro volcano (Tanzania) using geochronology and tephra composition

Abstract While geological and paleoanthropological studies at Laetoli have focused on the relatively fossiliferous Ndolanya and Laetolil beds, Laetoli’s younger Naibadad and Olpiro beds provide an important record of Pleistocene volcanism, tectonics, and landscape evolution in northern Tanzania. This study documents the mineralogical and geochemical compositions of their tephra using EPMA of glass and phenocrysts, and their ages using 40 Ar/ 39 Ar geochronology. Naibadad Beds tephra is rhyolitic or trachytic, compositionally distinct from the underlying Ndolanya and overlying Olpiro beds in their mineral assemblages and glass and phenocryst compositions. The Naibadad Beds can be divided into chronostratigraphic clusters as follows: Lower (2.189–2.154 Ma), Middle (2.115–2.104 Ma), and Upper (2.036–2.004 Ma). Most Naibadad Beds tephra could not be compositionally differentiated, although the basal Naibadad Beds tuff is unique in having both trachytic glass and andradite garnet. The uppermost Naibadad Beds tuff at Locality 23 has rhyolitic glass and aenigmatite like Olduvai Gorge’s Naabi ignimbrite and a similar age (2.033 Ma and 2.004 Ma, respectively), although they differ in feldspar and augite composition and are likely not from the same eruption. The lack of direct correlatives between Olduvai and Laetoli, which both derived tephra from Ngorongoro over the same time interval, is likely explained by paleotopography.