This study examines the relationship between depositional environment and sedimentary organic geochemistry in Lake Malawi, East Africa, and evaluates the relative significance of the various processes that control sedimentary organic matter (OM) in lacustrine systems. Total organic carbon (TOC) concentrations in recent sediments from Lake Malawi range from 0.01 to 8.80 wt% and average 2.83 wt% for surface sediments and 2.35 wt% for shallow core sediments. Hydrogen index (HI) values as determined by Rock-Eval pyrolysis range from 0 to 756 mg HC g −1 TOC and average 205 mg HC g −1 TOC for surface sediments and 228 mg HC g −1 TOC for shallow core samples. On average, variations in primary productivity throughout the lake may account for ~33% of the TOC content in Lake Malawi sediments (as much as 1 wt% TOC), and have little or no impact on sedimentary HI values. Similarly, ~33% to 66% of the variation in TOC content in Lake Malawi sediments appears to be controlled by anoxic preservation of OM (~1–2 wt% TOC), although some component of the water depth–TOC relationship may be due to physical sediment transport processes. Furthermore, anoxic preservation has a minimal effect on HI values in Lake Malawi sediments. Dilution of OM by inorganic sediment may account for ~16% of variability in TOC content in Lake Malawi sediments (~0.5 wt% TOC). The effect of inputs of terrestrial sediment on the organic character of surface sediments in these lakes is highly variable, and appears to be more closely related to the local depositional environment than the regional flux of terrestrial OM. Total nitrogen and TOC content in surface sediments collected throughout the lake are found to be highly correlated (r 2 = 0.95), indicating a well-homogenized source of OM to the lake bottom. The recurring suspension and deposition of terrestrial sediment may account for significant amounts of OM deposited in offshore regions of the lake. This process effectively separates denser inorganic sediment from less dense OM and allows terrestrial OM to preferentially be transported farther offshore. The conclusion is that for the organic carbon content in these regions to be elevated a mixed terrestrial-lacustrine origin is required. The hydrodynamic separation of mineral and organic constituents is most pronounced in regions with shallow bathymetric gradients, consistent with previous findings from Lake Tanganyika.

Abstract Modern lake basins set within active continental rifts provide useful analogs for exploration efforts in ancient extensional basins that are known to be rich in hydrocarbons. Lake Albert is one of the Great Lakes of Africa and is located at the northern end of the western branch of the East African rift system. This large, but comparatively shallow, eutrophic, and probably geologically ephemeral lake basin serves as an end-member example of the modern tropical lake systems that occupy this extensional province. Seismic reflection and gravity data sets indicate that the basin contains a maximum of 5 km (3.1 mi) of synrift, dominantly lacustrine sedimentary fill, in two subbasins separated by a midbasin high. In contrast to other large rift basins in the western branch of the rift valley, the Lake Albert Rift is not a highly asymmetrical half-graben basin, but instead has subsided nearly symmetrically and continuously in the late Cenozoic along two extensive boundary fault systems on either side of the basin. Seismic sequences from across the basin were correlated to borehole stratigraphy from a deep well drilled on the Ugandan margin. These observations suggest that the basin has experienced a long-term change from a continuously open lacustrine, possibly deep lake system in the Miocene or early Pliocene, to an alternating shallow lacustrine and fluvial system in the mid and late Pleistocene. This history of basin evolution has led to the development of a rich hydrocarbon system.

Abstract Deposits from the modern highstand lacustrine deltas in Lake Malawi offer an excellent opportunity to test models of deltaic sedimentation in a tectonically active setting and to examine variations in sand-body geometry along the axis of a large lake that has significant gradients in physical processes. In 1991 we initiated a coring project in five of the largest deltas in the lake to describe, for the first time, the shallow-water environments of deposition and processes of sedimentation. Percussion drill holes through the lower delta plain in the Linthipe and Dwangwa (shoaling margin) deltas revealed moderate to extreme lithologic variability with sand units up to 15 m thick. Deposited in an alluvial or shallow subaqueous deltaic setting, the sediments ranged from clay to gravel, dark green to brown to orange in color, and contained sections with significant amounts of organic material. Units of gravel up to 1 cm in diameter were recovered, and 60 to 70 percent of the recovered sediments consisted of at least 75 percent sand and gravel. Silts and clays occurred in units up to about 1 m thick, usually in the middle section of each sequence. Although the age, and thus the sediment accumulation rate, is unknown, it is likely that the cored sections (20-26 m deep) represent a few hundred to a few thousand years of accumulation.

Abstract 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.