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Carbonate lithofacies of the lower member of the El Hefhuf Formation (Turonian, Coniasian-Santonian) in the Bahariya Oases, Western Desert of Egypt comprise dolostone, burrowed dolostone, cherty dolostone, calcareous dedolostone, dolomitic quartzose lime-mudstone, and caliche. The dolostone and cherty dolostone formed in intertidal to supratidal environments, while dedolostone and caliche were formed during subaerial exposure. The dolomitic quartzose lime-mudstone was deposited in a restricted subtidal environment. Stable isotope analyses of the dolostone lithofacies are consistent with dolomitization by normal marine to evaporatively enriched dolomitizing fluids with only slight mixing of meteoric water. Therefore, dolomitization occurred in the intertidal and supratidal environments immediately following deposition. The dolostone and cherty dolostone lithofacies display pronounced cyclicity at the sub-meter scale, with individual cycles consisting of one (monolithic), two (diad) or at most three (triad) lithofacies. Most of all cycles show evidence of subaerial exposure at the top, such as brecciations, and many cycles are capped by calcareous palaeosol layers (caliche). In the absence of evidence for cycle periods at Milankovitch-scale frequencies, we reject orbital forcing as the cause of the cyclicity and suggest instead autocyclic depositional processes in the peritidal environment as the primary control on cyclicity, possibly modified by eustatic fluctuations at periods longer than the cycles recorded here. The significance of this study is to suggest formation of dolomite cycles due to high-frequency in sea level with intermittent subaerial exposure. [ABSTRACT FROM AUTHOR]

Abstract: The type section of the Lower Cenomanian Bahariya Formation at Gebel El-Dist (Bahariya Oasis, Western Desert), Egypt, comprises claystones, mudstones, siltstones and sandstones deposited in fluvial-deltaic coastal plain, lagoonal, estuarine and shallow marine environments. The formation is characterized by an abundance of ferruginous sandstones that locally weather to form prominent iron crusts. These centimeter to decimeter-scale ferruginous horizons display a continuum of features ranging from unaltered sandstone with a pervasive ferruginous matrix to distinct ironstone beds with massive, nodular, vesicular and pisolitic textures. Ferruginous sandstone typically occurs at the tops of sandstone beds, or bracketing the base and top of beds, in the fining-upward cycles of deltaic plain deposits in the lower part of the formation and on a low-energy fluvial floodplain in the middle of the formation. Indurated ironstone beds occur mainly as the caps of coarsening-upward cycles of prograding shoreface sediments through much of the formation. We interpret the ironstone crusts as ferricretes, formed by iron accumulation that resulted from the oxidation and precipitation of soluble iron or colloids transported in the sediment load or by groundwater. This accumulation possibly took place at the water table or possibly below the water table at the fresh water/saline water interface. However, base-level fall and subsequent subaerial exposure of the sediments resulted in reworking and pedogenic modification of some of the iron-impregnated horizons. [Copyright &y& Elsevier]