Origin of ferricretes in fluvial-marine deposits of the Lower Cenomanian Bahariya Formation,Bahariya Oasis,Western Desert,Egypt

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.