Climatic and tectonic controls on quaternary eolian sedimentary sequences of the Chott Rharsa Basin, southern Tunisia
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This dissertation presents an investigation of climatic and tectonic controls on Quaternary eolian sedimentary sequences of the Chott Rharsa Basin on the northern margin of the Sahara Desert, in southern Tunisia. This basin, which lies within the larger Atlas foreland basin of North Africa, is a structurally controlled depocenter created by Miocene - Pleistocene compression associated with the Atlas Orogeny. Alluvial fans and fluvial systems are present on the northern margin of the basin, a continental sabkha occupies the basin center, and eolian deposits characterize the southern margin. Mapping of the southern margin of the basin, combined with thermoluminescence dates from eolian deposits, reveal late Quaternary millennial-scale changes in eolian activity and stabilization. Specifically, the Chott Rharsa record contains four phases of eolian sand accumulation that occurred around 12.2, 10.0, 7.5, and 6.2 - 5.6 ka B.P. Lacustrine deposition occurred sometime between the eolian accumulations of 10.0 and 7.5 ka B.P., and sabkha deposition occurred sometime between the eolian accumulations of 7.5 and 6.2 ka B.P. There was also an episode of pedogenic gypsum crust development following the 6.2 - 5.6 ka B.P. eolian accumulation but before the onset of deflationary conditions that prevail today. From this record, which is of greater chronologic resolution than has been established previously from other Saharan eolian settings, it is determined that tectonic and structural features control locations of depocenters and locations of major sediment sources while climate exerts a greater influence on the nature and rates of stratigraphic accumulation. Humid times of higher water table positions and more abundant vegetation correspond with preservation of a stratigraphic record via lacustrine/sabkha deposition and stabilization of eolian deposits. Arid times of lower water table positions and less abundant vegetation coincide with eolian activity and also with destruction of the stratigraphic record via greater deflation and removal of stabilization agents. Furthermore, comparison of the Chott Rharsa record with other eolian-based records indicates that late Quaternary Saharan eolian activity has been relatively synchronous and that timing of Saharan eolian activity is coincident with major oceanic and atmospheric changes, suggesting the existence of global forcing mechanisms.