Ichnofabric analysis of the shallow marine Nukhul Formation (Miocene), Suez Rift, Egypt: implications for depositional processes and sequence stratigraphic evolution

The shallow marine, early, syn-rift, Miocene, Nukhul Formation, Suez Rift, Egypt, is highly bioturbated and allows relationships between changes in trace fossils and ichnofabrics within a shallow marine depositional system to be documented and placed in a high resolution sequence stratigraphic framework. Seven ichnofabrics are present in a succession of interfingering, calcareous mudstones and calcarenites forming coarsening-up units of up to 30 m thick, bounded by marine flooding surfaces. The units grade upwards from a basal mudstone package with bed parallel concretions and a Planolites–Chondrites ichnofabric (offshore), through a coarsening-up succession of alternating calcarenites and mudstones with Thalassinoides–mottled sediment (offshore transition), Ophiomorpha irregulaire (lower shoreface), Ophiomorpha nodosa–Thalassinoides (lower middle shoreface), Thalassinoides–Taenidium (middle shoreface) and O. nodosa (upper shoreface) ichnofabrics. Gastrochaenolites (hardground) ichnofabric is separate, as it is not genetically related to the other ichnofabrics. Ichnofabric development is primarily controlled by depositional environment, e.g. bottom water oxygenation, sediment type, food abundance and energy level, which control substrate colonisation, sedimentation rate. flooding surfaces are generally well-cemented and marked by distinctive epifaunal and infaunal colonisation and can be traced out from proximal to distal settings over distances of >5 km. The epifaunal colonisation in proximal settings consists of abundant oysters and corals with the substrate below marine flooding surfaces containing abundant Thalassinoides and Ophiomorpha isp. Abundance and diversity of epifauna and trace fossils and burrow size decreases distally into the basin. In the most distal settings, epifaunal colonisation is absent and only Planolites and Chondrites colonise the basinal mudstone. Marine flooding surfaces in the most distal settings are poorly cemented, but are marked by carbonate concretions 10–15 cm below the surface.