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Influence of Extension and Climate Variability on Facies Architecture: Case Studies of Lacustrine Carbonates in East Africa and the Basin and Range


Lacustrine basins contain extensive and high-quality hydrocarbon source rocks, and the Cretaceous subsalt basins of the South Atlantic host significant quantities of hydrocarbons in non-marine carbonates. Most lacustrine extensional basins are characteristically dominated by siliciclastic deposits, on account of the high-relief on faulted rift margins, and associated deep-basin subsidence. Accordingly, predictive models of siliciclastic reservoir facies in continental rifts are grounded in our knowledge of structural controls of drainage systems, although stacking patterns and lithofacies variability can be complicated by climatic processes. Lacustrine carbonates respond to similar controls as siliciclastic facies in many systems, yet the accumulation and preservation of carbonates in continental rifts is further complicated by the immediate catchment bedrock composition, which strongly influences the geochemical properties of the lake water column. Important limnological processes include water column mixing, turbidity, and nutrient dynamics, and the resident biological community also plays a critical role in determining the carbonate facies assemblage. The high-amplitude and high-frequency changes in base level observed in many mid- and low-latitude lake basins control shoreline positions, and significantly influence deep-basin sedimentation rates, sediment mixing, and carbonate facies stacking patterns. The strongly asymmetric morphology of most extensional basins also leads to preferential of positioning and stacking of certain carbonate facies. The presence or absence of magmatic systems driving subaerial or subaqueous hydrothermal spring systems in continental rifts can also strongly influence the chemical attributes of the lake system and resulting carbonate facies. This presentation offers data-intensive perspectives of extant lake basins in both the western and eastern branches of Africa's Great Rift Valley and the Basin and Range Province of the western U.S. Basin case studies range from saline, hydrologically-closed, and volcanically-mediated systems such as Lake Abhe (Djibouti), Lake Turkana (Kenya), amagmatic Walker Lake (Nevada), to deep, hydrologically-open lakes such as Malawi and Tanganyika, both in in the largely amagmatic western branch of the East African Rift.