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Abstract:

In January 1990, an exceptional rainfall event in southern Tunisia caused the Chott el Djerid, an ephemeral salt playa, to fill with water. Under an arid climate, the ephemeral lake on Chott el Djerid evaporated to dryness in ten months. During March, May and September 1990 we sampled the lake brines. Chemical analysis of the major solutes showed that the dilute waters that flow into the Chott el Djerid basin (groundwater, wadis and aquifer waters) have a consistent chemistry, generally saturated with respect to gypsum. This may result from the uniform basin geology, which is made up of Cretaceous, Mio-Pliocene, and Quaternary sediments, dominated by marine evaporites. Potassium was conserved throughout the evaporation sequence, suggesting the saturation of sorption surfaces within the playa. With increasing evaporation, the precipitation of gypsum and halite are predicted and observed, the final, most-concentrated, brines being saturated with respect to sylvite. XRD analyses of salt crusts from the Chott el Djerid reveal a mineral assemblage of gypsum, halite and carnallitite (carnallite with halite). The overall nature of both the predicted and observed salt phases suggests that the main control on the geochemistry of the playa is the recycling of ancient marine evaporites.

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