Lake Tanganyika provides an excellent opportunity for understanding tectonic and climatic influences on sedimentation in a rift lake. Each tectonic setting within rift half-graben basins generates a predictable range of lithofacies architectures.
Escarpment-margin (boundary-fault) drainages are small and steep, producing small fan deltas and thick, although not broad, sublacustrine fan complexes. Most water and sediment derived from escarpments is diverted away from the rift basin, although it may reenter along an adjacent half-graben basin margin.
Drainages crossing hinge ramps or platforms are larger and well integrated. Deltaic sands on these platforms may form sheets or be channelized into older alluvial valleys. Delta positions are poorly constrained by structure, and portions of the platforms may be clastic-sediment bypassed.
Fault-bounded interbasinal ridges, termed accommodation zones, are clastic-sediment starved. They are predominantly areas of pelagic sedimentation but may become areas of littoral carbonate accumulation at appropriate lake levels.
Rift-axial streams drain moderately large areas under very low gradients. Their deltaic positions are highly constrained by rift structure, providing for abundant clastic-sediment supply across the axis. A predominance of interflows and underflows generates strong density currents across most lake margins.
Asymmetry in lithology and strata thickness is the result of lake-level fluctuations interacting with varying rates of sediment accumulation, much of which is structurally influenced. Differences in sequence geometry have implications not only for interpreting ancient rift-lake deposits but also for deposition of economically viable reservoir facies and their juxtaposition with source rocks and caprocks. Several environments deserve more attention as exploration analogs than they have previously received. Platform and axial-margin sand bodies (clastic and carbonate), accommodation-zone carbonates, and turbidites or contourites derived from platform or axial sources all have considerable potential as reservoir facies.
Figures & Tables
Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation