Coarse-Clastic Facies and Stratigraphie Sequence Models from Lakes Malawi and Tanganyika, East Africa
Christopher A. Scholz, Bruce R. Rosendahl, 1990. "Coarse-Clastic Facies and Stratigraphie Sequence Models from Lakes Malawi and Tanganyika, East Africa", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
Download citation file:
Seismic-reflection data from Lakes Malawi and Tanganyika in the western branch of the east African rift system reveal a variety of coarse-grained depositional facies. These facies include fan deltas and slope aprons adjacent to border faults, deep-water sublacustrine fans and channel systems, lowstand deltas, and an array of clastic and carbonate littoral deposits. Each is located in specific areas within half-grabens and develops at specific times within the cycle of lake-level variation. Rift lakes in tropical settings are highly sensitive to level fluctuations. High-amplitude and high-frequency lake-level variations may cause the resulting depositional sequence and facies architecture to be more complex than on passive margins. Controls on sequence development, such as sediment supply and lake-level variation, may be more closely coupled than on passive margins. Progradational clinoform depositional packages are uncommon in these basins probably because of the small size of catchments relative to lake surface areas and because of high slope gradients on the basin margins. Erosional truncation surfaces are more readily observed in these seismic data than are downlap surfaces.
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