Lacustrine carbonates from Lake Turkana, Kenya: A depositional model of carbonates in an extensional basin

Lake Turkana, northern Kenya, serves as a long-lived end-member extensional basin that preserves a volcanically mediated, mixed siliciclastic and carbonate system. Stratigraphic data from the southeastern margin of Lake Turkana reveal a suite of carbonate facies assemblages. Lithologies of this system include mudstone, siltstone, sandstone, conglomerate, diatomite, carbonate mudstone, wackestone, grainstone, packstone, and boundstone. The boundstones are subdivided into framestones (stromatolites or oncoids) and display upward growth from a nucleus where found in association with cobbles and shells and bindstones (laminar tufa) where not. Three discrete, large (up to 7.5 m high), stacked, mounded boundstones are interpreted as subaerial spring deposits based upon overall shape, surficial texture, and orientation with respect to mapped faults and the distribution of modern springs. Petrographic analysis reveals peloids, ostracods, microbial sheaths, gastropods, pelecypods, diatoms, and detrital grains that include quartz, volcanic glass, and volcanic lithic fragments. All grains are micritized, and fossils tend not to retain their original microstructure. Some lithologies have porosities as high as 39%, and permeability averages 4 mD as a result of the dominance of moldic porosity and microporosity. The combination of lithologic data and facies associations is combined to produce a depositional model that is useful for comparison with ancient rift basins that host economic quantities of hydrocarbons. The controlling factors for carbonate development in Lake Turkana include the structural framework, water depth, and lake chemistry.