WIDESPREAD FRESHWATER CARBONATE IN THE OLDUVAI BASIN, A PRE-CURSOR TO A MAJOR VOLCANIC ERUPTION IN EAST AFRICA RIFT SYSTEM

Freshwater limestones are uncommon in the sedimentary record of the East African Rift System, particularly in pre-Quaternary rocks. This dominantly arid region with no carbonate (karst) source rocks supports clay, sodic or silica-rich lacustrine sedimentation and localized groundwater-fed tufa deposits. Carbonatite eruptions have occurred throughout the region. We report on an extensive carbonate unit at Olduvai Basin, Tanzania, that varies spatially in thickness, petrography, geochemistry, C and O isotopes, and has with both abiotic and biotic facies. It is intercalated with lake claystones and was deposited just prior to a major eruption of Olmoti volcano (~ 1.80 Ma) spewing Tuff IF into the Basin. The tuff is a silica-undersaturated trachytic to phonolitic member that was comprised of thick unwelded pyroclastic flows in proximal settings and a succession of surges and ash falls distally. The eruption took place during the drying limb of a precession cycle. These cycles (~ 23 ka) controlled the magnitude of rainfall and fluctuation of alkaline Lake Olduvai, a playa, which frequently expanded and flooded the lake margin on a variety of time scales.

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.