WIDESPREAD FRESHWATER CARBONATE IN THE OLDUVAI BASIN, A PRE-CURSOR TO A MAJOR VOLCANIC ERUPTION IN EAST AFRICA RIFT SYSTEM
Geologic mapping revealed that some of the carbonate was formed from groundwater flowing from faults that transect the basin. d18O values of the carbonate, as light as - 6.5‰, indicate freshwater. The groundwater was likely sourced from rainfall on adjacent volcanic highlands to the east. Petrographically, the limestone consists of three facies: (1) mm-to-cm scale nodules, with crystal rosette or bundle interiors; (2) marls with silt-to-fine-sand sized angular detrital grains in mottled micrite; and (3) homogeneous, very fine-grained micrite. Shelly fauna, such as ostracod fragments, are uncommon in the mottled micrite, and very rare to absent from the nodules and homogenous micrite. Geochemically, the homogenous micrites are enriched in Mg and Sr relative to the other facies. The 3 facies reflect environments that varied from subaerial to ponded water. We speculate that the widespread limestone/dolostone deposit formed from a seismically-induced increase in groundwater discharge that episodically flowed onto the surface. The region is seismically active today and seismic events were likely associated with the Olmoti eruption that ultimately produced Tuff IF.