DEPOSITIONAL ENVIRONMENTS OF LACUSTRINE CARBONATES OF THE SOUTH BASIN OF LAKE TURKANA, KENYA

Lake Turkana, northern Kenya, serves as a long-lived end-member extensional basin that preserves a suite of carbonate facies assemblages, useful for comparison with ancient rift basins that host economic quantities of hydrocarbons. This study focuses on lacustrine carbonate strata of the Plio-Pleistocene Koobi Fora Formation that accumulated along the flexural margin of the southeastern border of Lake Turkana. A variety of carbonate lithologies are present, including microbialites such as laminar tufa, stromatolites, stromatolite mounds, and oncoids. Petrographic analysis of the numerous carbonate facies revealed allochems that include peloids, ostracods, microbial sheaths, gastropods, pelecypods, diatoms, and detrital grains that include quartz and volcanic glass, and volcanic lithic fragments. The microbial growth is highly dependent on the availability of substrate, which typically includes shell material or basalt pebbles and/or cobbles. The microbially-mediated carbonates form stromatolites when found in association with basalt cobbles and laminar tufa deposits when no substrate is available. Here we present a depositional framework of the carbonate facies of the volcanically-mediated, mixed siliciclastic and carbonate system of the South Basin of the lake. Fluvial processes helped to provide the necessary dissolved chemistry and substrate for the deposition of carbonate facies during lake highstands. Stromatolites formed where stream channels prograded into the lake and provided a cobble substrate for nucleation. Smaller oncoids formed nearshore where shell material served as nuclei. Thick, discrete, mounded stromatolites indicate the presence of spring seeps that may have significantly enhanced microbialite growth rates and lateral extent.