干旱区生态环境知识资源中心

The Duwi Formation (late Cretaceous) represents the first onset of fully marine conditions and comprises sediments that were confined by a preexisting depression enclosed by the Dakhla uplift in the west and Kharga uplift in the east. The Duwi Formation overlies unconformably the Qusseir Formation and underlies conformably the Dakhla Formation. The Duwi Formation could be subdivided vertically into three remarkable informal lithostratigraphic units. The Duwi Formation comprises a heterogeneous suite of shallow marine rocks (phosphorite, shale, mudstone, marlstone, glauconite, and dolomite). A model has been presented for the mode of phosphorite accumulation. This model includes aspects from both the traditional upwelling phosphogenic model and key elements from the non-upwelling model. The role played by high-energy events (e.g., internal waves) in phosphorite accumulation was achieved by the interplay between the physicochemical and hydrodynamical processes. Internal waves are thought to be the triggering mechanism for the density flows. Internal waves and induced bottom currents are sporadic but strong enough to bring sediments into suspension. Accumulation of phosphorites by high-energy waves is evidenced by the predominance of many types of bedform sequences. Such type of bedforms are represented by fining upward, firm grounds, erosive pockets and pot holes, planar lags, imbrication, sorting, and fragmentation. The Duwi Formation has been divided into several meter-scale cycles of three types (gradual, non-gradual, and truncated). This represents the higher frequency sea-level fluctuations forming shallowing-upward cycles that were arranged into the larger regressive megasequence during a third-order sea level change.