Depositional models and sequence stratigraphy of the middle Miocene clastic/carbonate deposits in the Esh El-Mellaha range, southwest Gulf of Suez rift basin, Egypt

Abstract

The Langhian was a time of clastic/carbonate deposition over many fault blocks in the Gulf of Suez due to rising eustatic sea level coupled with a warming trend. These deposits are of great interest as they are important hydrocarbon reservoirs. A detailed study on facies organization, depositional geometries and internal architecture of surface analogues helps to realize the reservoir heterogeneity. A good example is preserved in the Mellaha block that represents 80–100 m thick clastic/carbonate deposits. It comprises three facies associations indicating simultaneous accumulation in subaqueous fan-delta, nearshore lagoon and carbonate platform depending on whether they occur in footwall or hangingwall sites. The associations and environments are depicted in two depositional models that display clear insights into their characteristics. The Abu Gerfan facies association, dominated by conglomerate, denotes deposition in a footwall-derived coarse-grained fan-delta built into a shallow hangingwall basin closely adjacent to the eastern Mellaha faults (Depositional model I). The architecture of the fan-delta deposits is strictly linked to the relative movement across the fault system and the sea-level rise in the hangingwall basin. The fan-delta conglomerate grades distally and upward to limestone of the Gharamul facies association that implies deposition in a coastal lagoon. The Abu Shaar facies association is characterized by carbonates dominated by corals, microbialites and rhodoliths. It marks a reef-rimmed carbonate platform developed on a submerged footwall high at the southern end of the block due to its double asymmetry (Depositional model II). This platform is characterized by sigmoidal-bedded fringing reefs initiated on the block crest that pass westward into horizontally-bedded lagoon facies with scattered patch reefs, and eastward into inclined-bedded reef talus. Synsedimentary block tilting, eustatic sea-level rise and global warming have imposed major controls on the growth of the Abu Shaar platform. The facies associations are attributed to three rift tectonic events that record two sequences with a dominance of transgressive and highstand stratal units. These sequences are consistent with globally defined Langhian sequences, which indicate that sediment accumulation was largely controlled by eustasy with a strong overprint of rift tectonics. This result allows better regional correlations, particularly with the Miocene Mediterranean carbonate platforms.