The Lake Natron Footprint Tuff (northern Tanzania): volcanic source, depositional processes and age constraints from field relations

The Engare Sero Footprint Site situated on the southern shore of Lake Natron in northern Tanzania has been reported to host one of the best preserved sets of fossilized hominid footprints in the world. However until now there has been no detailed characterization and age determination of the footprint bearing strata (the Footprint Tuff). Here we combine field observations with geochemical and mineralogical analyses and measurements of the anisotropy of magnetic susceptibility to constrain depositional processes the role of reworking and the volcanic source for the Footprint Tuff. We find that the footprint bearing horizon consists of volcanic ash fall that has been slightly reworked by water and that this was produced during a voluminous eruption of the Oldoinyo Lengai volcano. The unit which covered the footprints and helped to preserve them consists of the wind blown material from the same eruption mixed together with locally derived detrital material. We can constrain the ash horizon to be of Holocene age based on: (i) the location of the Footprint Tuff within the regional stratigraphy (ii) previous age determination of an ash layer that can be correlated with the Footprint Tuff and (iii) the regional climatological history of the area. The ash horizon was probably deposited around 11000 10500 years ago but could potentially be even younger than this.