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Abstract

Lake Tanganyika is a well-known example of a large anoxic lake that acts as a sink for organic matter. More than 400 grab samples and 25 cores were collected to study distribution of organic facies in the Bujumbura and Rumonge subbasins of the northern lake area. The overall organic richness of the lake sediments (up to 12% TOC) is favored by anoxic conditions, which prevail below about 100 m. This situation is the combined result of (1) organic productivity, (2) lake geometry (deep and narrow), which is controlled by rift architecture and hinders water circulation, and (3) equable warm tropical climate, promoting stable water stratification.

The distribution of organic facies in bottom sediments shows considerable and rather complex lateral variability. Sedimentological and organic geochemical evidence (Rock-Eval, kerogen <513C, hydrocarbon composition) suggests that organic variability is controlled mainly by depositional processes—gravity- flow deposits, containing organic matter that reflects (in terms of concentration and properties) source area environment, interfinger with autochthonous organic-rich pelagic/hemipelagic muds. The importance of redepositional processes at the basin scale probably is a response to the lake bottom's rugged topography, which is controlled by rift tectonism. Such a situation, with its implications in terms of organic heterogeneity, must be kept in mind when considering source rocks deposited in similar continental rift settings.

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