Bahiret el Biban is a restricted lagoon in an arid climate. The general geological setting is briefly analysed, especially with respect to the relations between the water body and its evaporitic bordering areas, i.e. supratidal sabkhas.

The geochemical behaviour of the lagoon waters has been studied over a 2-year period following floodings. In spite of some exchange with the open sea, there is a general tendency towards increasing salinity, calcium sulphate impoverishment and a consequent increase of the Mg/Ca ratio. Thus, unless large floodings occur, the basin waters undergo an evaporitic evolution.

Sedimentation and early diagenesis are controlled largely by this geochemical pattern, but also by exchange with the open sea and by the water depth. Organic matter derived from the sea is deposited on entering this highly saline basin and locally produces sediments which could eventually become source rocks.

Bahiret el Biban is a possible model which may be of considerable interest in the reconstruction of the early stages of the evolution of evaporitic basins.