Lacustrine littoral landforms in drylands: example of the wave-dominated clastic shorelines of Megalake Chad (Africa, Sahara Desert, Holocene)
Theatre 2 (Basement Level)
221 Bouverie Street, Theatre 2
221 Bouverie Street, Carlton
In a desert, aeolian dunes and dune fields are the most typical large-scale accumulation of sand, whereas any littoral landform, especially in endorheic basins, is most probably the least expected one. In the southern part of the Sahara Desert, the largest warm climate desert on Earth, prominent fossil littoral morpho-sedimentary structures are preserved. These coastal landforms are highly diversified and include beach ridges-and-swales, spits, barriers, wave-dominated deltas, wave-ravinement surfaces, and tombolos. Together, they allow outlining the paleo-shoreline of a single, very large, lake (350,000 km²) known as Megalake Chad. This megalake corresponds to the last highstand of Lake Chad, currently the largest freshwater lake of Sahara-Sahel region (~10,000 km2). Megalake Chad is one of the most dramatic evidence of the climate-driven environmental changes that occurred during the African Humid Period (~11–5 kyrs). This climatic optimum led to the reactivation of the drainage systems of the Sahara, that in turn favoured the development of a savanna landscape, and the human occupation of this previously barren region. The littoral landforms of Megalake Chad, also provides key-markers to infer the prevailing alongshore drift, and thus the paleowind regime that have been then tested with numerical simulations of the hydrodynamics. Megalake Chad is a wind-driven water body, where the main process for the distribution of clastic sediments at basin-scale is the result of wind-induced currents, both at the shoreline and at the bottom of the lake. Other littoral landforms, less well-developed and distributed at lower elevations, provide a record of the lacustrine forced-regression, subsequent to the termination of the climatic optimum. In the Australian deserts, several large paleo-lakes have developed through time in similar contexts (e.g. Lake Eyre, Lake Frome). Cross-comparisons between Megalake Chad and the megalakes of Australia, as well as other case study around the world, will help characterising and understanding lacustrine littoral landforms preserved in drylands from a significant data set.
Dr Mathieu Schuster, CNRS Researcher