Tectonic and climatic controls of lacustrine sedimentation in pre-rift and rift settings in the Permian-Triassic of East Africa

Lake sediments in the Ruhuhu Basin, Tanzania, and other East African basins have a similar facies evolution for particular time slices of the Permian and Lower Triassic. The Ruhuhu Basin exhibits three lacustrine phases related partly to climate and partly to tectonic setting. Two pre-rift lacustrine stages — post glacial and swampy lacustrine phases — are followed by major rifting in the Upper Permian. Postgacial lakes developed in pre-Karoo depressions were fed initially by meltwater and later by runoff and grounwater associated with climatic amelioration. The following swampy lacustrine episode developed from fluvial to lacustrine conditions with alternating clastic and organic input. Associated micritic carbonates and gypsum indicate high evaporation, and playa clay mineral associations provide evidence for poor drainage and saline, alkaline lake waters.The Upper Permian lake was characterized by fine clastics and biogenic carbonates. Facies include littoral clastics and turbidites, stromatolites, oolites and deeper water laminites. Early diagenetic cherts, chloritization and the absence of kaolinite indicate highly alkaline lake water during regressive phases. Stable isotopic evidence supports lake differentiation into hydrogeologically open and closed sub-basins.Two phases of rifting (Lower-Upper Permian; Upper Permian-Lower Triassic) are recognized in several Karoo basins. Rift evolution and lake formation are intimately related. The first rifting episode was characterized by local extension of depositional areas. Half-graben basinal asymmetry and permanent lacustrine conditions became established. The second episode was regional, and was characterized by further extension of depositional area, a basal unconformity, and a hiatus between the uppermost Permian and lowermost Triassic units. Climate was the main controlling factor during the Early Permian lake development, whereas structural constraints strongly influenced vertical and lateral facies development in the Late Permian/Early Triassic lakes.