Inorganic bedded cherts from the Magadi area, Kenya

Abstract

The High Magadi beds can be divided into a lower and an upper sequence, separated by a bed rich in Tilapia nilotica remains. The lower sequence contains one or more magadiite horizons. Through digging of trenches it has been possible to establish that the cherts of the “Chert Series” are stratigraphically equivalent to the magadiite horizons. The transition magadiite-chert has been observed in many places and may occur over a distance of less than 50 cm. It is accomplished by removal of sodium by percolating waters. The resulting bedded chert may preserve the fine lamination present in magadiite. The conversion is accompanied by many textural and structural features such as large desiccation polygons, buckling, reticulation, extrusion, casts of mud-cracks and calcite.

The precipitation of magadiite is postulated to have taken place in annual increments in a stratified lake at the brine-epilimnion interface. The maximum duration is estimated at 4,000–6,000 years. Solubility measurements indicate that most Magadi waters at the present time are supersaturated with respect to magadiite and kenyaite, though no precipitation has been observed. The presence of chert chips in intraformational gravels indicates that the conversion to chert was already initiated during the magadiite precipitation interval.

The inorganic mechanism for the formation of bedded chert observed at Magadi is suggested to apply also to the Dales Gorge Member of the Precambrian Brockman Iron Formation of W. Australia. Evidence for magadiite as chert precursor is found in the depositional rates, the presence of riebeckite and in textures such as macules and cross-pods. The proposed model is extended to include Precambrian banded iron formations in general and the restriction of these formations to the early history of the earth is explained by the likely compositions of waters from closed basins.