From its inception during the early Miocene, the Suez Rift has been dominated by marine sedimentation. New high-resolution biostratigraphic and sedimentologic analyses of synrift deposits have resulted in the recognition of late Burdigalian-early Langhian brackish water and lacustrine deposits in the Wadi Abu Gaada-Gebel Gushia area, Sinai Peninsula. The Abu Gaada section is unique because: (1) it is an anomalously thick section of non-calcareous shale and mudstone in the Lagia Member of the Ayun Musa Formation; and (2) the mudstones contain an abundant microflora consisting of marine and nonmarine diatoms and freshwater algae that indicate they were deposited in a freshwater to brackish water environment. The abundant freshwater and shallow marine algae include the nonmarine diatoms Aulacoseira Thwaites, Fragilaria construens (Ehrenberg) Grunow, Synedra ulna (Nitzsch) Ehrenberg, and SurrirellaTurpin, as well as the freshwater algae Botryococcus Kutzing and Pediastrum Meyen. Shallow marine diatoms are represented by Actinoptychus Ehrenberg, Actinocyclus ehrenbergii Ralfs, Paralia sulcata (Ehrenberg) Cleve, Rhaphoneis Ehrenberg, and HyalodiscusEhrenberg. Correlations with other coeval stratigraphic sections in the Sinai Peninsula indicate that brackish water and lacustrine deposition was localized in the Wadi Abu Gaada-Gebel Gushia area. This is explained by uplift and tilting of the bounding faults separating the Wadi Abu Gaada-Gebel Gushia block from adjacent rift blocks where normal marine conditions prevailed. The increased freshwater influence was the result of a rising water table during a period of rising sea level. As transgression continued, the barrier that separated the Wadi Abu Gaada depression from marine waters was eventually breached. Brackish water then filled the depression. Eventually a hypersaline lagoon formed in what previously had been a fresh waterlake, depositing the Markha Anhydrite. Correlative brackish water/lacustrine shales also have been found in the subsurface of the Gulf of Suez and thin coeval deposits are present on its southwestern margin. This reflects that the local occurence of these depositional environments was controlled by structural reorganization of fault blocks in the Suez Rift in response to the “mid-clysmic” tectonic event.