Alluvial karstification and paleodoline development in Eocene limestones, a case study from west Sohag City, Egypt: impl
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ORIGINAL PAPER
Alluvial karstification and paleodoline development in Eocene limestones, a case study from west Sohag City, Egypt: implications for causes and impacts Tawfiq Mohamed Mahran 1 & Abdallah Mohamed Hassan 1 Received: 27 September 2018 / Accepted: 8 March 2019 # Saudi Society for Geosciences 2019
Abstract The Late Oligocene pre-Eonile paleoriver flows northwesterly over the Eocene Limestone Plateau in the east central Western Desert, west of the Nile Valley (Sohag), building an alluvial plain and leading to the karstification of the limestone bedrock, as paleoerosional surfaces and paleodolines. Knowledge of this alluvial karstification is completed with the detailed studies of the sedimentary evolution of the fluvial system and its contemporary deposition. Analysis of the sedimentological and geomorphological features allows discussing the natural environmental conditions that favored karst in the past and its main genetic mechanisms. The architectural elements of the fluvial deposits indicate that they were deposited in a gravel-dominated braided fluvial system, characterized by high availability of water and sediment supplies. The current study proves that genesis of paleodolines was mainly related to this high water supply. Some of them were progressively filled by syn-sedimentary deformed unconsolidated deposits. Such deformation is due to dissolution of the underlying sagging synclinal limestone, conditioning the location of sinking waters where preferential dissolution and later suffosion took place. A model for the evolution of the paleodoline fills is proposed and interpreted in three different stages: (1) gravitational processes developed and disorganized gravels were remobilized and dragged toward the created paleodolines; (2) flooding of the paleodolines acted as lakes where Gilbert-type deltas and gravel slope deposits accumulated, exhibiting several unconformities interpreted as a result of continued subsidence of the paleodoline related to dissolution; and (3) nondeformed fluvial facies sealed the whole deposits, marking the end of the karstification process. Combinations of different types of factors that could cause karstic subsidence and associated hazards include soluble rock lithology, paleoclimate, and deformational structure. Keywords Karstification . Nile Valley . Paleodolines . Geomorphology . Geohazards . Pre-Eonile
Introduction Karst is a geomorphologic feature that is formed in soluble rocks such as carbonates and evaporites. These rocks include distinctive landforms related to dissolution characteristics (White 2002; Ford and Williams 2007; Youssef et al. 2018). Karst features are formed in these soluble rocks by processes of surface weathering, stream sinking, cave development, faEditorial handling: Beatriz Badenas * Abdallah Mohamed Hassan [email protected] 1
Geology Department, Faculty of Science, Sohag University, P.O.Box: 82524, Sohag, Egypt
vorable climatic conditions, hydraulic gradient of waters and tectonic uplift (White 1988, 2007), or by conduits or cavitie
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