Lacustrine Basins as Mixed Siliciclastic-Carbonate Systems: Examples from the East African Rift

Many lacustrine basins are mixed siliciclastic-carbonate systems, and may contain important hydrocarbon source rocks. Lacustrine extensional basins are characteristically dominated by siliciclastic deposits, on account of the high-relief generated on faulted rift margins, and associated basin subsidence. Predictive models of siliciclastic reservoir facies in extensional basins are grounded in our understanding of structural controls of drainage systems, although stacking patterns and lithofacies are commonly complicated by the interplay between subsidence and climate. Developing a systematic understanding of the original environments of deposition of lacustrine carbonate reservoir facies is complicated by the remarkable geochemical variability exhibited by relatively dilute waters found in most large lakes. Coarse-grained lacustrine carbonate sequences exhibit marked variability in different depositional settings in modern lakes, as well as in space and time in ancient successions. Here I present recent examples of mixed-system and carbonate accumulations from several of the Great Lakes of the East African Rift Valley, which is the most extensive modern example of a network of affiliated tectonic lakes. Lakes such as Tanganyika, Malawi, Albert, and Edward are large freshwater lakes covering several discrete structural basins, and are long-lived features that as depositional sinks have survived ongoing and occasionally intensive extensional deformation, as well as severe climate crises that resulted in near-desiccation. Smaller lakes of the rift, such as Lakes Bogoria, Baringo, and Nakuru are at present experiencing extreme lowstand conditions, and are alkaline and saline. Carbonate facies observed in these basins include algal bioherms, stromatolites, or microbialites; ostracode and chara silt beds; ooid beaches and shoals; and bioclastic grainstones composed dominantly of gastropod remains. All the basins have accumulated carbonate sequences at times during their long histories, but chemical/biochemical sedimentation has been frequently interrupted during wet climate intervals, or by major changes in their catchments.