Abstract
Freshwater limestones are uncommon in the sedimentary record of the East African Rift System. Recent research in the Olduvai Basin, Tanzania has revealed an extensive carbonate unit ca 1.8 Ma in age that varies spatially in thickness, petrography, trace element and rare earth geochemistry, C and O isotope values and freshwater microfossils. Five distinct lithologies are recognized. Four reflect deposition in carbonate‐rich environments that occurred at the same time in a catena‐like pattern over a heterogeneous landscape: spring, wetland, freshwater lake/pond and saline playa lake. The fifth is carbonate nodular soil that formed by paedogenesis on areas of topographically higher terrane. The δ18O of the carbonate, as negative as −6.5‰, indicates that the groundwater is meteoritic derived. The origin of the carbonate comprising this thick, extensive, isochronostratigraphic deposit has not been determined, but it is believed that carbonatite volcanism may have supplied elevated levels of carbonate to the basin which then entered the groundwater system. The carbonate was deposited just prior to a major trachyte‐phonolite eruption of Mount Olmoti, and the deposition of a basin wide tuff, Tuff IF. The region is seismically active today and seismic events were likely associated with Olmoti volcanism. The widespread carbonate unit may have preceded the Olmoti eruption and formed from a recurring seismically induced increase in aquifer porosity and permeability. The carbonate‐enriched groundwater episodically discharged onto the land surface, interacting locally with both fresh and saline ponded water.
An extensive freshwater limestone in the Olduvai rift basin appears to be directly linked to carbonatite volcanism. An ash introduced a slug of carbonatite across the basin that dissolved and quickly entered the groundwater system. The limestone was formed simultaneously in five different sedimentary environments from calcium‐enriched groundwater that flowed onto the surface.