Chott el Djerid, Tunisia

Chott el Djerid (translates as "Lagoon of the Land of Palms") is the largest in a zone of chotts formed where a series of regional aquifers emerge across a region of discharge playas at the northeast extremity of the Bas Saharan Artesian Basin. It defines a groundwater sump region in a tectonically quiescent area of the North African Craton. The encompassing chott-hosting depression extends over much of the Saharan desert into Morocco and Libya and encloses the northern end of the Grand Erg Oriental. Like the interior salt lake basins along the southern edge of the Great Artesian Basin of Australia, the Djerid and other nearby chotts are deflationary rather than accretionary depressions. These sumps have lost something like 15-20m of late Quaternary fill via eolian deflation to the adjacent Sahara desert erg.

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Chott el Djerid, and other nearby chotts, Tunisia, showing the extensive saline mudflats that characterises the lowermost part of this inland depression. A surface geology map of the saline mudflat in northeastern portion of this region is given below (Scaled Bing® image mounted in MapInfo)

Chott Djerid, with an elevation typically more than 10 metres below sea level on the salt flats, has an area of approximately 5,360 km2 in a drainage basin with an area of 10,500 km2. Mean annual rainfall for the area is 80-140 mm, the mean annual temperature is 21°C and evaporation, which is highest between May and September, has a mean annual value of 1500 mm. It has an elongated arm (Chott Fejej) that stretches eastward toward the coastal city of Gabes. Chott el Djerid is the largest in a zone of chotts formed where a series of regional aquifers emerge to create a region of discharge playas along the northeast extremity of the Bas Saharan Artesian Basin. It defines a groundwater sump region in a tectonically quiescent area of the North African craton along the northern edge of the Saharan erg. The encompassing chott-hosting depression extends over much of the Saharan desert into Morocco and Libya and encloses the northern end of the Grand Erg Oriental.

The lowest and more central part of the deflationary depression that is Chott el Djerid is now a halite pan outlined by a thin, ephemeral halite crust atop Quaternary clays and sand aquifers that host saline pore waters. It is surrounded by a gypsum mudflat, which in turn pass marginward into a broad evaporitic mudflat and dry sandflat. Ephemeral carnallite accumulates as efflorescences and rare intercrystalline cements in the uppermost parts of ephemeral halite crusts in lowermost, more central portions of the saline pan facies. Gypsum petees and adhesion ripples cover much of the mudflat surface surrounding the halite-encrusted part.

Parts of the sandflat are transitional into aeolian-overprinted sediments with characteristic wind-blown sand streaks, wind-rippled sand sheets mounds and shadow dunes, all of which are cross-cut by wadi channels that feed surface water into the central depression during times of flood. Like the interior salt lake basins along the southern edge of the Great Artesian Basin of Australia, the chotts are deflationary rather than accretionary depressions. They have lost something like 15–20 m of late Quaternary fill via eolian deflation to the adjacent Sahara desert erg.

No evaporite beds occur in the 10 m of eolian-dominated Pleistocene sediments that lie atop the Miocene unconformity and beneath the ephemeral modern salt crust cap to the chott. The deflation-dominant outflow hydrology of the chott offers little preservation potential in terms of salts deposited in a surface salt crust. Quaternary salts cannot remain long enough in the stratigraphy to be buried. That is, periodic flooding and ongoing deflation mean there is no potential for potash preservation in this style of saline hydrology. Its sump hydrology is simply not saline enough, even though a thin salt crust cap extends for hundreds of kilometres over most of the chott surface. 

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Chott el Djerid, Tunisia (after Warren, 2016). A) Locality showing Zone of Chotts as regional groundwater outflow zone. B) Regional hydrology. C) Facies in northeastern corner of Chott el Djerid (after Bryant et al., 1994a).

Even though most of the present-day chott surface is deflationary, there is some local accretion around spring mounds, which are point sources of water escape from underlying artesian aquifers. Interestingly, the region around Chott el Djerid salt pan includes some of the most extensive gypsum beds in Africa). Regional chemical analyses show a marine signature to this pedogenic gypsum and by association the rare potash efflorescences. The constituent sulphate ions are leached by deeply circulating meteoric groundwaters moving through Cretaceous and Triassic marine evaporite beds. That is, the ultimate salt source to the solutes in the nonmarine chott depression is seawater, it is just that the cycling of the marine solutes into the depression was delayed by more than 100-150 million years, while the salt was stored in sediment beds.

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View from the road

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