Quaternary wadi, lacustrine, aeolian depositional cycles and sequences, Chott Rharsa basin, southern Tunisia

Chott Rharsa is a tectonically-controlled play a lake basin, located in Tunisia, along the northern Sahara margins between the Atlas Mountains and the Saharan Craton. Wadi- versus aeolian-dominated flanks of the basin present two distinct records of geomorphic and sedimentologic response to global climatic change, as modulated by large- and small-scale changes in regional water balance. Differences between these areas illustrate distinctions between the responses of systems to external forcing mechanisms versus preservation of those responses in the stratigraphic record, and an apparent differential sensitivity to climatic change The wadi-dominated northeast side of the basin, located along the flanks of the tectonically-active Atlas Ranges, provides a high-amplitude, low frequency record of four major wadi-lacustrine cycles during the last two glacial-interglacial periods. Individual cycles are preserved as a series of mappable terraces and incised valley fills, with facies successions that represent lowstand, transgressive, highstand, and early regressive parts of each lacustrine cycle. Available chronological data suggest lacustrine highstands correlate with interglacial/interstadial warm periods, whereas major episodes widespread basin-floor deflation, and valley incision correlate to orbitally-forced glacial or stadial cool periods. In spite of clear evidence for spatially-variable uplift or subsidence of individual palaeovalley axes, the wadi-lacustrine record remains robust, which further supports the conclusion that sedimentary rhythms reflect the major climate changes of the last 200 ky. However, syndepositional tectonism has partitioned the basin into topographic highs and subbasins, and will ultimately control the locations where the wadi-lacustrine record will be eroded away or preserved as part of the foreland basin fill. Aeolian landforms and deposits dominate the southern margins, but the long-term history is mostly one of deflation and sediment removal to downwind regions farther south and west (Grand Erg Oriental). However, remaining aeolian-lacustrine strata provide a high-resolution record of multiple high frequency humid-arid cycles since the last full-glacial maximum (ca. 20 ka). Aeolian units were mostly deposited as water tables fell during the drying phase of each cycle when large parts of the playa floor were exposed to deflation, whereas dune field stabilization by vegetation and/or gypsum crust formation occurred during humid periods when the water table rose. Location on the ‘cratonic’ margins of the basin, coupled with the overall long-term negative sediment budget, suggests limited prospects for preservation of an aeolian stratigraphic record in the basin fill.