Lithostratigraphy, sedimentology, and cyclicity of the Duwi Formation (late Cretaceous) at Abu Tartur plateau, Western D
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ORIGINAL PAPER
Lithostratigraphy, sedimentology, and cyclicity of the Duwi Formation (late Cretaceous) at Abu Tartur plateau, Western Desert of Egypt: evidences for reworking and redeposition Abdalla M. El Ayyat
Received: 29 May 2013 / Accepted: 24 October 2013 # Saudi Society for Geosciences 2013
Abstract 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 Electronic supplementary material The online version of this article (doi:10.1007/s12517-013-1169-x) contains supplementary material, which is available to authorized users. A. M. El Ayyat (*) Geology Department, Faculty of Science, Assiut University, Assiut 71516, Egypt e-mail: [email protected] A. M. Ayyat e-mail: [email protected]
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. Keywords Abu Tartur . Phosphorites . Black shales . Paleogeography . Internal waves . Cyclicity
Introduction The Abu-Tartur plateau lies 600 km southwest of Cairo and 50 km west of El Kharga Oasis, Western Desert (Fig. 1). It is located between Dakhla Oasis to the west and Kharga Oasis to the east (Fig. 2). The southern edge of the plateau overlooks the Nubia plain to the south, whereas it gently tilts northward forming the general surface of north Western Desert. The name of the Abu Tartur deposit is derived from the vast Abu Tartur plateau of which the western, southern, and eastern escarpments to the depression below bear the outcrops of
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