A number of theories have been put forward to explain the formation of gypsum crusts in south-central Tunisia. They can be considered as two types; those that rely on upward movement of gypsum into the surficial materials from gypsiferous Cretaceous and Tertiary substrates and those that rely on an aeolian input from nearby playas. This may be followed by downward movement to form a subsurface crust, which is subsequently exhumed. In order to assess these disparate models, detailed information on the distribution and abundance of gypsum in this area is needed, which is far beyond the scope of any field-based survey. However, gypsum has distinct spectral reflectance characteristics, allowing discrimination from multispectral remotely sensed imagery. The Landsat Thematic Mapper is especially suited to this, as it has a band sensing in the short-wave infrared, where gypsum has a strong absorption feature. Linear mixture modelling is a new image processing technique which estimates the proportions of scene components in an image, and this technique is used to produce a map showing the amount of gypsum within each 30 m pixel. This indicates that the main concentration of gypsum is aeolian sands on the southern side of Chott Fedjaj. Secondary concentrations are found corresponding to outcrops of gypsiferous Cretaceous and Tertiary strata, which are covered by gypsum crusts (gypcretes). A model is proposed which describes a cycle of gypsum, some of which is weathered from Cretaceous and Tertiary evaporitic deposits and transported by surface and ground water into the large salt playas or Chotts, where it forms evaporitic crusts. From here it is deflated, forming sand dunes on the surrounding terrain, where further solution and reprecipitation results in consolidation. We conclude that several different processes are involved in gypsum crust formation in the Tunisian Southern Atlas.
