Structure, sedimentation, and basin dynamics during rifting of the Gulf of Suez and north-western Red Sea

Abstract

The Gulf of Suez formed as the northern segment of the late Oligocene-early Miocene Red Sea rift. Rare occurrences of basaltic dikes that cut and flows that are interstratified with the oldest syn-rift strata, a basal red bed sequence (Abu Zenima-Nakheil formations), suggest that rifting had initiated prior to ∼25 Ma. The oldest palaeontologically datable strata are of Aquitanian age (Nukhul Fm.), and are older than ∼21 Ma. Nearby basal strata in north-west Saudi Arabia are Chattian age (∼27 Ma). Apatitie fission-track data suggest that the earliest phase of Gulf of Suez and northern Red Sea rifting may have begun ∼34 Ma, but there is no dated sedimentary record of this. The areal distribution of the basal syn-rift units was largely controlled by the geometry and timing of movement of the early rift-fault system, which was strongly influenced by pre-existing structures in the Pan-African basement complex. In the early Burdigalian, the Gulf of Suez and northern Red Sea entered a period of rapid subsidence and increase in marine water depths, resulting in the widespread deposition of Globigerina marls (Rudeis and Kareem formations) in axial areas. Fault density decreased during the Burdigalian to Langhian, and most extension was accommodated by movement on a few large, basin- or block-bounding faults. By the Serravalian, connection between the Gulf of Suez and Mediterranean basins became restricted and sedimentation rapidly changed to laterally continuous evaporites (Belayim and South Gharib Formations). In the late Tortonian, renewed influx of normal marine water occurred from the southern Red Sea, and deposition switched to mixed evaporite-marginal marine settings (Zeit Fm.). The Zeit sediments differentially loaded the underlying South Gharib massive salt, driving diapirism and intra-sediment slump faulting that were generally focused along Middle Miocene fault trends. Basinal sedimentation patterns during the Messinian, Pliocene and Quaternary were largely controlled by the geometry of the evolving salt ridges. Subsidence became increasingly focused along the axis of the rift during this time.

Analysis of fault geometries, fault kinematics and sedimentation patterns indicates that the early Nukhul phase of rifting occurred during north-east-south-westerly directed extension. The existence of pre-existing north-north-east-trending basement structures, however, caused local development of N10–20°E transfer faults, and local rhombic basin geometries that other workers have interpreted as pull-apart basins. As rifting progressed, the N10–20°E faults were abandoned and younger transfer faults became predominantly rift-normal. In the Middle Miocene, the relative movement between Africa and Sinai shifted to the Gulf of Aqaba transform boundary, and extension rates across the Gulf of Suez decreased dramatically. During the late Pleistocene, the regional stress field changed to a N15°E extension direction and a new system of faults began to evolve.

The early phase of pre-Nukhul and Nukhul extension in the Gulf of Suez was accompanied by relatively minor basin subsidence. Between 19 and 16 Ma, within the Rudeis Fm., subsidence rates increased several fold. This increase in subsidence, and accompanying sedimentation rate, followed quickly after the onset of significant rift shoulder uplift, which according to apatite fission-track analysis occurred at ∼23-21 Ma. Many workers have therefore inferred that Gulf of Suez rifting began gradually, and culminated in the Rudeis. Our observations suggest, however, that the early Nukhul faulting and stratal rotation reflect significant regional extension during the Aquitanian. The extension was distributed across broad regions and accommodated by movement on many, closely spaced faults. Eventually, distinct master faults evolved and footwall uplift to the larger structures became a principal factor controlling erosion and regional sedimentation patterns. Initially, subsidence rates were relatively low, perhaps reflecting the inherent strength of the pre-rift lithosphere. The onset of rapid subsidence eventually occurred a few million years later, and most of the basin was flooded with deep marine waters. The Gulf of Suez is therefore best modelled with a non-instantaneous relationship between extension and subsidence.