Rift-basin sedimentation: responses to climate, tectonism and volcanism. Examples of the East African Rift

Abstract

Rift-basin sedimentation appears to be closely related to the influence of phenomena such tectonics, volcanism, and climate. The continent-scale structure of the East African Rift offers numerous examples of sediments types, sedimentary bodies or sedimentary environments showing clearly their dependence on these phenomena. Thus the distribution and the longevity of the troughs which compose the rift depend directly on tectonics and volcanic activity. Evolution of these troughs towards “sedimentary basins” is related to the permanence of the tectonic depression thus authorizing the deposition of a sedimentary sequence. Examples from the Afar Depression or the Gregory Rift show clearly cases of interruptions or removals of sedimentation related to the migration of axes of tectonic activity.Looking at sedimentary formations of Recent age in the East African Rift, it appears that the variety of tectonic, volcanic and climatic conditions along the 4000 km of horsts and grabens influences the sedimentation which can be in teh form of clastic, organic, evaporitic or metalliferous deposits. Such facies are associated to form sedimentary bodies or environments such as piedmont accumulations, alluvial fans and fan deltas, fluvio-deltaic systems, and lacustrine basins.Examples of Recent to Modern environments have been chosen in the Eastern and Western Branches of the East African Rift, Afar Depression, Ethiopian and Gregory Rifts, Kivu Trough, Tanganyika and Malawi Troughs. They clearly illustrate the close correlations existing between sedimentation, tectonics, volcanism and climate at different scales of time and space in such a complicated structure as the East African Rift.