Concentrated and lean oolites: examples from the Nubia Formation at Aswan, Egypt, and significance of the oolite types in ironstone genesis

Summary Two textural types of oolitic ironstones occur in the Late Cretaceous Nubia Formation at Aswan, Egypt. These are: (1) relatively thin bedded (5–20 cm) muddy or ‘lean’ oolites containing up to 300oids, proto-ooids and peloids scattered in a matrix of ferruginous mud, and (2) thicker beds (as much as 2.5 m thick) of ‘rich’ or ‘concentrated’ oolites comprising 60–800oids. Sedimentologic and textural attributes of the ‘concentrated’ oolites suggest that these represent ooids transported and redeposited away from their site of formation. In fact, in ironstone-bearing successions elsewhere the concentrated oolites are either associated with winnowed sandstones or, in sand-poor deposits, they substitute for sandstone. Together the ooids and the detrital sand form bar and sand wave complexes in upward shoaling marginal marine sediments. In unbioturbated or less bioturbated occurrences these sandbodies of ferruginous ooids are replete with primary sedimentary structures indicative of strong current transport. Thin beds of similar ferruginous ooids in sequences of off-shore carbonate or argillaceous basinal mudstone, such as those in the Silurian Clinton Group of eastern United States, are storm deposits characterized by basal scours and hummocky cross-stratification. The muddy or ‘lean’ oolites, on the other hand, are commonly associated with prograding subtidal mudflat deposits. Textural characteristics of these ironstones suggest that they probably represent the primary sites of ooid formation. Different stages of ooid formation preserved in these ‘lean’ ironstones and their relationship with the concentrated oolites provide information which is significant for a more realistic and comprehensive genetic modelling for the Aswan ironstones. In the model proposed here the deposition of muddy, ferruginous sediments on prograding, near-shore, subtidal mudflats begins during the waning stages of detrital sediment supply. The ooids are formed on these shallow-water mudflats by mechanical accretion of detrital kaolinitic clay and precipitated iron hydroxide minerals, mostly around faecal pellets or flocculated peloids, under the influence of gentle wave or current agitation. Periodic high-energy events, such as storm-generated or tidal currents, transport the ooids to off-shore areas to be reworked there into oolitic sand bar and shoal complexes. Following deposition, the kaolinitic ooids, peloids and matrix are transformed into berthierine under a chemically reducing diagenetic environment.