East African Rift stromatolites

Abstract

Summary The East African Rift System contains many lacustrine basins in which biologically precipitated stromatolite carbonates are found. The development of these carbonates results from low-Mg calcite precipitation by cyanophytes and bacteria; the binding and trapping of sediments are not involved. A general outline for the distribution of stromatolites has been established by analysing data from different sites in East Africa. In the fluviatile environment, travertines are associated with a biotype of flowing waters with high calcium content. During exceptional rainy seasons, inundation of floodplains promotes seasonal growth of oncolites. Chimneys and pool-rim dams associated with hydrothermal springs may be sub-lacustrine, or may occur along lake shores. In the lacustrine environment, stromatolites form shorelines that indicate former high stands. The study of Quaternary stromatolites and their modern equivalents reveals their exceptional potential for reconstructing sedimentary palaeoenvironments in rifts. Stromatolites represent a particular continental carbonate facies, somewhat analogous to marine reefs. Furthermore they constitute an important source of other geological data: material for isotopic dating; determining altitudes of former lake levels; and providing evidence for hydrological and physico-chemical conditions in palaeolakes. All these may be combined to assist palaeoclimatic reconstruction.