Spring mounds in southern Tunisia

Abstract

Summary Chotts Djerid and Fedjadj lie in the arid zone of southern Tunisia, and are chemically active, artesian environments which give rise not only to extensive emergent groundwater gypsum and salt deposits, but also to landforms of more complex architecture. In some geological situations belts of spring mounds up to 25 or 30 m high straddle former high-level late Pleistocene lake shorelines. Spring mounds are circular, gypsiferous or calcareous, volcano-like structures fed by point sources of artesian water rising from aquifers in the Continental Intercalaire and Complexe Terminal aquifer series. Similar mounds have been identified around playas in the USA, Australia, Egypt, Iraq and elsewhere. In some mounds a central well is occupied by a small pond with water conduits in the base, whilst others are dry and degraded. In these latter, water is only present at depth and wind erosion and redeposition are moulding the landforms. The morphology of spring mounds is determined by the balance between aquifer pressure head, output, chemical content, net evaporation, aeolian activity, and plant colonization. Their stratigraphy comprises alternations of crystalline materials, wind-blown gypsiferous sand, wash and splash deposits, and organic layers arranged in overlapping sequences which dip outwards from the water source. Such deposits offer a potentially sensitive index of environmental changes in the late Quaternary and Holocene.

Pitted cones and domes on Mars: Observations in Acidalia Planitia and Cydonia Mensae using MOC, THEMIS, and TES data

“…The locus of spring activity can shift with time, producing multiple pits in a single mound. The spring mounds described by Roberts and Mitchell [1987] from southern Tunisia are a good morphologic analog to the domes and cones we have observed; albeit with heights at the low end of the range observed for the Acidalia pitted cones. Some of the Tunisian mounds are described as consisting of cemented gypsiferous sand, organic fragments, eolian sand lenses, and ''gypsiferous rubble''.…”

“…The distinct albedo and variable thermal properties of the cones and domes is therefore not consistent with the pingo hypothesis. In summary, we find the observed character- Pike and Clow [1981] (these authors grouped tuff rings and tuff cones together; these features have been separated here on the basis of their table of features); 4, Gurney [1998]; 5, Kholodov [2002] (figures from his descriptions of cone shaped mud volcanoes in Azerbaijan; many mud volcanoes described by Kholodov have different dimensions); 6, Roberts and Mitchell [1987] (measurements averaged from examples cited in paper).…”

“…The constructs around springs and geysers can have a variety of sizes and shapes, including sizes and shapes similar to the cones and domes seen in the Acidalia region. In their study of clusters of spring mounds in southern Tunisia, Roberts and Mitchell [1987] note that these features can range from 1 to 30 m in height and can achieve diameters of 500 m. Terrestrial spring deposits commonly have pits at their summits. These can be small holes or broad depressions that held water in a surficial basin.…”

Pans, Playas and Salt Lakes

“…These range from pans and playas where the groundwater table intersects the basin surface ( Figure 15.2(a)), accompanied by surface evaporite accumulation and evaporative effects, as in the Chotts of Tunisia (e.g. Roberts and Mitchell, 1987;Bryant et al, 1994) , to those where the water table lies at depth (Figure 15.2(b)). These latter features are usually clay-floored and percolating groundwater plays a major role in deep weathering and eluviation.…”

The Chott El Djerid, Tunisia: Observation and Discussion of a SAR Phase Signature Over Evaporitic Soils

“…It is a flat area, with a mean altitude of 15 m, centered at 33.73 • N−8.40 • E. The chott El Djerid forms the northeastern extremity of the Bas Sahara artesian basin. Aquifers emerge at the surface of the Chotts Trough region, through a thin clay layer of the Quaternary age [18], allowing temporary flooding of the playa depressions in winter. The mean annual rainfall for the area is around 100 mm, while evaporation has a mean annual value of 1500 mm.…”