Microbially-Mediated Ooidal Ironstones, El Bahariya Depression, Egypt : Sedimentology, Mineralogy and Geochemistry

The present study deals with the sedimentology, mineralogy and geochemistry of the Middle Eocene ironstones of the northeastern part of the Bahariya Depression, Western Desert, Egypt. It also focuses on the microbial role in the precipitation of these ironstones. The ironstone succession rests unconformably on three latertitized Cenomanian paleohighs (e.g. El Gedida, El Harra and Ghorabi mine areas). The ironstone succession is composed of autochthonous/para-autochthonous facies rich in ferruginous ooids, oncoids and skeletal particles. The ironstone facies assemblages are stacked in two shallowing-upward sequences, separated by an unconformity. The lower sequence shows great lateral facies variations. Each sequence starts with lagoonal burrow-mottled hematitic mud-ironstone or tidal flat yellow goethitic stromatolitic ironstones that represent deposition from suspension in low energy water conditions. These facies grade upward into shoals/megarippled pack- to grain-ironstones, representing deposition during agitated tidal conditions and/or storm hydrodynamic forces. Lateritic iron ores were formed as a result of subaerial weathering of the ironstone sequences.