Marine Miocene sedimentation on the broad platform of the northern Western Desert of Egypt was controlled tectonically by slight oscillations and local small undulations and warps of the sea bottom. Sedimentation ceased in post-middle Miocene times when intensified tectonism uplifted the whole platform to form El Diffa Plateau of the present Western Desert. Three lithostratigraphic units--a lower one belonging to the Moghra Formation and two upper units belonging to the Marmarica Formation--are delineated in the Miocene succession exposed across most of the northwestern part of the Western Desert of Egypt. All three units are marine, and generally they are a lower coquina-marlshale member, a middle chalky member, and an upper fossiliferous limestone-chalk member. In the S wa Depression area of the Western Desert, the constant thickness of the middle member helps to define the structural configuration of the depression, which is an elongate asymmetric and irregular basinlike syncline. This gentle syncline has a west-east extension of more than 80 km (50 mi) and forms the middle part of the great geomorphic arc of the Jaghbub-Siwa-Qattara depressions of the northern Western Desert.

It is suggested that during post-middle Miocene uplift, hinge faulting and rejuvenation of old faults occurred in the basement underlying the Miocene blanket sediments. This resulted in sags and downwarps of the overlying Miocene beds and finally produced large, tectonically controlled surface depressions like that of Siwa. In the exposed Miocene succession around Siwa Town, along the main trough of the Siwa syncline, the higher beds with relatively lighter overburden are much more deformed than the beds underlying them. This deformation is ascribed to internal stresses generated within a localized core of the irregular Siwa syncline by the squeezing and upward buckling of the freer part of the downwarped sediments, possibly above or along a small decollement.

In the wet post-Miocene climate, the morphotectonic depression of Siwa and its associated tectonically induced jointing and other surface fractures immediately captured and localized the surface drainage. Water erosion within the chemically vulnerable Miocene sediments of the initial depression proceeded rapidly and greatly enlarged it. There is conflict of opinion concerning the agents and mechanisms responsible for the creation of the Siwa and other large oasis depressions in the Western Desert; however, this study proves that initial tectonism and subsequent ancient water erosion were the most important factors. Wind deflation had a relatively late and minor role. Exsudation is likewise unimportant except perhaps in the late dry stages.