U-TH DISEQUILIBRIUM DATING OF LAKE MAGADI CHERTS
Cherts in the Magadi cores formed by replacement and cementation of siliceous gels, magadiite, evaporites (trona), diatomaceous deposits, and carbonate shell beds (ostracodes and gastropods). Early chertification is indicated by randomly-oriented plant and insect fragments, and “labyrinth” patterns that indicate lithification of uncompacted siliceous gels, in which sediment grains were displaced by air. Magadi cherts incorporate significant amounts of uranium but little initial thorium as shown by their high 230Th/232Th and 234U/232Th ratios. Ages for such samples can be calculated from measurement of 230Th/234U and 234U/238U. Ten cherts in core 1A, from 24 m to 88 m in depth, range in age from 11.8 to 285 ka. Nine cherts from core 2A, from depths of 36 to 107 m, range in age from 12.4 to 267 ka. Three correlated stratigraphic intervals from core 1A and 2A have U-Th ages that overlap. The HSPDP Magadi cores confirm a shift from an early shallow dilute lake stage, evolving to a narrow, at times anoxic, stratified, saline lake. The suite of chemical sediments (zeolites, chert, magadiite, and trona) indicates alkaline saline lake conditions for much of the history of the Magadi Basin. Chert dates from trona layers show the first evaporites were deposited in the Magadi basin ~119 ka, much earlier than previously thought.