THE CHEMICAL, PHYSICAL AND MICROBIAL ORIGINS OF PLEISTOCENE CHERTS AT LAKE MAGADI, KENYA RIFT VALLEY

Lake Magadi, famous for its trona and chert, lies at ~600 m elevation in the southern Kenya Rift Valley. The lake, a hydrologically closed, hypersaline, alkaline, intermittently dry pan, is fed by ephemeral streams and hot springs that lie on N-S faults along its margins. The basin hosted several precursor lakes from ~800 ka until today, recorded by sediments that crop out around the lake. These include the zeolitic Green Beds (GB: ~100-40 ka), exposed discontinuously in the axial trough, which contain abundant chert originally attributed to a magadiite precursor (Eugster, 1969). Behr (2002), however, showed that some silica precipitation involved prokaryotes. This study re-examines the origins of the GB cherts. Two groups are recognized: one ‘bedded’, the other ‘intrusive’.

Bedded cherts are cm-scale, quartzose stromatolitic cherts with structures that typify an evaporative playa-like setting including desiccation cracks, crystal pseudomorphs (trona, gaylussite), microbial laminae (couplets, triplets) and ichnofossils. These cherts cap zeolitic tuff, greenish silt and black mudstone, commonly in cycles 0.5 to 2 m thick.

Younger reddish quartz cherts intrude the lacustrine silts and bedded cherts forming meter-scale dikes and domal mounds oriented along N-S faults and fractures. Quartz breccias in the mound cores are commonly mantled by microbial laminites that lie parallel to the external surface of the mounds. Mounds are locally parallel to inferred paleoshorelines. Thick (> 5 m) massive cherts containing oncoids are found in places. Subaerial terraced sinter mounds are present northeast of the lake (NE Lagoon).

The bedded GB cherts show no diagnostic indicators of a magadiite precursor and limited evidence for carbonate replacement. Many sedimentological features imply silica precipitation, possibly as a gel, in an ephemeral playa fed by Si-rich hot springs. Intrusive cherts show evidence of both physical emplacement (brecciation) while partly solid and cementation by a fluid or gel. Some dikes and intrusive cherts can be linked to seismic events. Other cherts, however, may be subaqueous silica bioherms linked to thermal fluid discharge along N-S faults and fractures. Some silica bioherms resemble microbialites in Archean cherts in color and morphology.