The Jir Formation, of Eocene age, consists of gypsum, anhydrite, halite and dolomite. The thickest sequence is developed in and adjacent to a major graben system in the western part of the Sirte Basin. The Basin was domed, initially faulted and eroded during the late Mesozoic, probably due to drift of the African plate over a fixed mantle hotspot during the early Cretaceous (Van Houten, 1983). The area was open to the sea of the Sirte Gulf as a basin at the very beginning of the Eocene, and then became isolated lagoons, when the Dahra Ridge began to function as a platform or barrier bar with a lagoon to the southwest. The barrier permitted a regular influx of warmed and sulphate-enriched brines to maintain continuous evaporite precipitation. For simplifiction, the term 'Jir Formation' is used here for the major evaporite unit of middle Eocene age, both at surface and in the subsurface. 'Rawaghah Formation' is used both for the dolomites of the 'Rawaghah Member' at the surface and for the underlying 'Facha Dolomite', a major reservoir in the subsurface. The type-section of the Jir Formation is at Wadi Faras, in the hills of the Jabal Waddan. Here, at the basin margin, the formation consists of at least 52 m of secondary gypsum and dolomite. In the subsurface, in the Zalla Trough or Meulagh Graben, to the southeast, the Formation comprises up to 1000 m of anhydrite and halite with dolomite. The evaporites at Wadi Faras are of shallow water lagoon and sabkha origin, with foraminifer-rich carbonates indicating periodic marine influx. In the Zalla Trough laminated anhydrite has been formed also in a shallow lagoon. Halite was deposited in a closed salt-lake in the central part of the basin. In the late Eocene marine water flooded the basin and carbonate sedimentation followed, with coccoliths and foraminifers in the basin centre and shallower facies at the margin. Celestite in the Jir Formation was formed by replacement of calcium sulphate, Brecciation of chert in the region is interpreted as resulting from evaporite solution and collapse. The stages of evaporite sedimentation in the area studied were: started with 1) the precipitation of carbonates followed by 2) sulphates then 3) chlorides. In Auversian times, at the end of the middle Eocene, marine water flooded the basin and carbonate sedimentation with coccoliths and foraminifers followed. The dolomites of both the Jir and the underlying Rawaghah Formation are stoichiometric, non-ferroan and have low Na content. The dolomites formed in two stages by different mechanisms: 1) A penecontemporaneous phase of replacement of calcium carbonate resulted in finely crystalline dolomite (less than 20 μm crystal size). This occurred in relatively arid lagoon and sabkha environments in which Mg-rich brines were developed. 2) A late phase of diagenetic replacement subsequently occurred in a zone of seepage-reflux. Reflux dolomitization took place when Mg-rich sea water or modified sea water was seeped through porous and permeable carbonate sediments. Normally only seawater, in contrast to continental brines, contains sufficient magnesium to cause massive dolomitization.