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Field seminar to the areas around chotts Jerid and El Rharsa, Tunisia

This excursion examines the sedimentary environments and dynamics of a system of inland desert lakes, their marginal sabkhas and surrounding aeolian and fluvial tracts. The area offers a unique opportunity to examine an easily accessible, modern analogue to the transitional, Silverpit-Leman facies belt of the Permian gas basin of western and central Europe. It also has major significance in the understanding of continental series elsewhere (e.g. the Trias argilo-gréseux of Algeria). Chott Jerid is located some 120km west of Gabès and is the largest (c.6,500km²) of a number of desert lakes which border the southern foothills of the Atlas Mountains and the northeastern margin of the Grand Erg Oriental.

The central part of Chott Jerid is a halite saltpan bordered by concentric zones in which gypsum flats give way to a c.15km-wide sandy sabkha, the surface of which is adhesion-rippled and blistered by evaporite growth. Marginal facies form a broad sandflat which surrounds the chott to the east, west and south. Its surface is characterized by aeolian sand streaks and windrippled sheets, slipfaceless sand mounds and small shadow dunes and is, in some places, incised by shallow, ephemeral channels which feed the chott during periods of flooding. The main feeder systems are from the north where chotts Jerid and El Rharsa are bordered by alluvial plains draining the Atlasic foothills. These foothills are flanked by low-relief alluvial fans which form proximal facies to axial, braided-channel drainage systems such as Oued El Melah.

Sandflats to the south of Chott Jerid are bordered by marginal aeolian dunefields, the outer edges of which are characterized by low-relief, isolated sand mounds. These mostly lack well-defined slipfaces and are separated by sabkhas. There is a transition into central parts of the dunefields in which aerodynamic bedforms develop avalanche surfaces, increase in height and link to form barchanoid, sinuous-crested, transverse dunes with maximum crestline heights of the order of 10m. Large, compound duneforms are present in the Jebil area, where a similar transition from external dunes to transverse ridges (of 50m to 100m height) with superimposed, sinuous-crested dunes defines the northern flank of the main sand sea of the Erg Oriental.  

There is a spectacular example of a fossilized Pleistocene dunefield near El Franig on the southwestern side of Chott Jerid. Dunes are of the order of 6m in height, have simple transverse geometries and wavelengths of c.120m. Interdune areas are enclosed and up to c.1km in length. The upper surfaces of the dunes were stabilized by vegetation growth and gypsum pedogenesis during a pluvial phase which probably marked a major expansion of the chott system.

Chott El Rharsa is located to the northwest of Chott Jerid and is a smaller desert lake whose central part lies below sea-level. Fluvial systems feeding the lake terminate in distributary-fan complexes which are visible on Landsat imagery. Fluvial processes operating on the terminal discharge site of Oued El Melah interact with a small aeolian dunefield and this locality forms a complex sedimentary system whose deposits are modified by groundwater influence and associated salt-growth. The largest fan complex is located on the northern side of the lake, covers an area of some 500km² and is fed by a river system which debauches onto the fluvial plain via Faum El Khinga. A very distinct terminal lobe at the southeastern end of the main fan formed during a high-magnitude flood which destroyed the ancient town of Tamerza in 1969. Pleistocene lake-highstand deposits are also exposed in the area, as are examples of pedogenic gypcretes.

The chotts flood (to varying degrees) annually but are normally dry from mid April to mid October. The weather in May and October is less stable than at other times of the year, with stronger winds and associated sand storms, reversing winds and increased rainfall. These periods provide the best chance of witnessing the dynamics of the depositional system (modification of dune patterns, fluvial activity etc.) and also avoid the heat of summer when temperatures average 32^oC and can reach 45^oC.

  Further information and a provisional itinerary may be obtained from:

Dr Lou Macchi,

Reservoir Associates International, Laburnum House, Shocklach, Cheshire SY14 7BT, U.K.

Tel: +44 1829 250562, fax: +44 1829 250444