Structural And Stratigraphic Features Of Southern Lake Tanganyika, Tanzania
Malosha, Shigela J.
Geoscience Masters Degree, 2011
University of Adelaide
Lake Tanganyika rift basin is located along the western arm of the East African Rift System (EARS). It is a north-south-trending continental rift basin, extending from Burundi in the north to Zambia in the south, and is one of the most dominant structural features of the EARS. The lake has a length greater than 700 km, an average width of 50km, and a maximum water depth of 1.5km. The study area is the Beach Energy (Beach) Production Sharing Agreement (PSA) area in the Tanzanian portion of the basin and covers an area of approximately 7,000 km2. The dominant features in the Tanganyika rift basin are a number of alternating asymmetric half-graben which are bounded by large planar normal faults.
Interpretation of a sparse seismic grid shows the basin-ridge-basin configurations in the northern domain of the Lake Tanganyika South area, and the asymmetric half-graben patterns into the southern sub-basin. Three types of structures have been recognized: Major axially parallel border fault blocks; Bounding major normal fault blocks; and intra-basinal faults that act as major transform zones. The interpretation of 2D seismic data from Lake Tanganyika south basin also shows three main structural areas that defined by major faults, these are; Northwest-Southeast, North-South, and Northeast sub-basins.
Four key depositional sequences were defined in the north and central sub-basins of the study area based on seismic facies characteristics. Climatically induced changes in lake level appear to have forced sequence development. Sequence I deposits indicate acoustic basement units form the base of each sequence and spread over the underlying, irregularly shaped unconformity in the study area. The overlying sequence is defined as sequence II and is interpreted to be primarily deep sub-lacustrine fans which are mostly lens-shaped units. Sequence III composed of deep lacustrine units with seismic facies that alternating with thin discontinuous, medium-amplitude reflectors. The youngest Sequence IV shows onlap, and erosional truncation which is a manifestation of sub-aerial exposure and erosion. The combined results interpreted from 2D seismic sections and high resolution magnetic and gravity indicate three different domains in the study area (northern, central and southern) and confirm that the structural style in these sub-basins is asymmetric and shows rectangular-shaped half-graben structures, linked by transfer zones, and underlain by a thick (> 4 km) sequence of rift-related sediments.