Formation of euxinic lakes during the deglaciation phase in the Early Permian of East Africa
Thomas Kreuser, Gebretinsae Woldu, 2010. "Formation of euxinic lakes during the deglaciation phase in the Early Permian of East Africa", Late Paleozoic Glacial Events and Postglacial Transgressions in Gondwana, Oscar R. López-Gamundí, Luis A. Buatois
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The continental glaciation of Gondwanaland in the Late Carboniferous–Early Permian left traces in many places in southern and eastern Africa. This paper focuses on the last glacial advance and consecutive deglaciation leading to the formation of large euxinic lakes with high concentrations of organic matter. The Idusi Formation in the Tanzanian Ruhuhu Basin (initiating the Karoo cycle, which extends into the Triassic) provides the type section for this depositional sequence. It is subdivided into a lower Lisimba Member, the basal unit of glacial origin, and an upper Lilangu Member, characterized by postglacial black shale and rhythmites as evidence of a climatic amelioration on a large regional scale in Africa. Thickness and facies variations are attributed to a pronounced paleotopography as the result of scouring glaciers and local tectonic events. There is a gradual change between the members, reflecting a continuous climatic amelioration and change of sediment supply. The lacustrine environment was terminated by the onset of braided stream deposition (Mpera Sandstone Member); an erosional unconformity between the units marks the start of initial rifting in the Early Permian. This is followed by the development of extensive coal swamps in a temperate climate, where organic matter predominated over clastic supply.
Periglacial deposits with tillites and rhythmites, containing dropstones, are overlain by glaciolacustrine laminites intercalated with glaciofluvial marginal deltaic sediments. Deglaciation provided water and accommodation space for the evolution of extensive anaerobic stratified lakes, which were the focus of prolific deposition of organic matter. This black shale may contain up to 11% TOC (total organic carbon) content. Eventually, the lake became shallower and was succeeded by alluvial fan deposition. The duration of the glaciation and deglaciation was ~20–25 m.y., and the lacustrine phase lasted ~4–5 m.y. These ages have been verified by palynology (Granulatisporites confluens Oppel zone).
The hydrocarbon potential of the black shale was estimated by Rock-Eval pyrolyses. Hydrogen index, maximum temperature (Tmax), and vitrinite reflection were used to determine kerogen type, maturity stage, and subsidence history. A promising potential with respect to gaseous hydrocarbon generation was detected from both the euxinic black shale and the overlying coals. A comparison with other Tanzanian Karoo basins reveals similar conditions in TOC values and temperature history. The wide regional extent of the anaerobic lacustrine black shale of the deglaciation event in several eastern and southern African basins evinces a similar climatic and regional tectonic framework in the pre-breakup phase of Gondwanaland during the Early Permian. This period of time may be of some importance in the future when the economic potential with respect to hydrocarbon generation of the Permian basins is scrutinized in more detail.