Geology of Egypt: The Northern Red Sea

Abstract

The Red Sea and Gulf of Aden constitute parts of the Afro-Arabian rift system that are in the most advanced stages of continental break-up. These basins have therefore received extensive scrutiny in the geoscientific literature, but several aspects of their evolution remain enigmatic. Many of their most important features lie beneath several kilometers of water, in places covered by several kilometers of evaporite deposits, and along international political boundaries. All these factors greatly complicate the acquisition and interpretation of both subsurface wellbore and geophysical datasets. Much of our understanding of the evolution of the Red Sea has therefore relied on the integration of outcrop geology and land-based analytical studies with these more difficult to obtain marine observations. While stratigraphic, radiometric and structural data indicate that extension and rifting initiated in the southern Red Sea during the Late Oligocene (~28–25 Ma), the start of rifting in the northern Red Sea is more difficult to constrain due to paucity of rift-related volcanism and reliable biostratigraphy of the oldest syn-kinematic sedimentary strata. A regional NW-SE trending alkali basalt dike swarm, with associated extensive basalt flows in the vicinity of Cairo, appears to mark the onset of crustal-scale extension and continental rifting. These dikes and scarce local flows, erupted at the Oligocene-Miocene transition (~23 Ma) and coeval with similar trending dikes along the Yemen and Saudi Arabian Red Sea margin, are interbedded with the oldest part of the paleontologically dated siliciclastic syn-rift stratigraphic section (Aquitanian Nukhul Fm.), and are associated with the oldest recognized extensional faulting in the Red Sea. Bedrock thermochronometric results from the Gulf of Suez and both margins of the Red Sea also point to a latest Oligocene onset of major normal faulting and rift flank exhumation and large-magnitude early Miocene extension along the entire length of the Red Sea rift. This early phase of rifting along the Egyptian Red Sea margin and in the Gulf of Suez resulted in the formation of a complex, discontinuous fault pattern with very high rates of fault block rotation. The rift was segmented into distinct sub-basins with alternating regional dip domains separated by well-defined accommodation zones. Sedimentary facies were laterally and vertically complex and dominated by marginal to shallow marine siliciclastics of the Abu Zenima, Nukhul and Nakheil Formations. Neotethyan faunas appeared throughout all of the sub-basins at this time. During the Early Burdigalian (~20 Ma) tectonically-driven subsidence accelerated and was accompanied by a concordant increase in the denudation and uplift of the rift shoulders. The intra-rift fault networks coalesced into through-going structures and fault movement became progressively more focused along the rift axis. This reconfiguration of the rift structure resulted in more laterally continuous depositional facies and the preponderance of moderate-to-deep marine deposits of the Rudeis, Kareem and Ranga Formations. The early part of the Middle Miocene (~14 Ma) was marked by dramatic changes in rift kinematics and sedimentary depositional environments in the Red Sea and Gulf of Suez. The onset of the left-lateral Gulf of Aqaba transform fault system, isolating the Gulf of Suez from the active northern Red Sea rift, resulted in a switch from orthogonal to oblique rifting and to hyperextension in the northern Red Sea. The open marine seaway was replaced by an extensive evaporitic basin along the entire length of the rift from the central Gulf of Suez to Yemen/Eritrea. In Egypt these evaporites are ascribed to the Belayim, South Gharib, Zeit and Abu Dabbab Formations. Evaporite deposition continued to dominate in the Red Sea until the end of the Miocene (~5 Ma) when a subaerial unconformity developed across most of the basin. With the onset of seafloor spreading in the southern Red Sea, Indian Ocean marine waters re-entered through the Bab el Mandab in the earliest Pliocene and re-established open marine conditions. During the Pleistocene, glacial-isostatic driven sea-level changes resulted in the formation of numerous coral terraces and wave-cut benches around the margins of the Red Sea, Gulf of Suez and Gulf of Aqaba. Their present elevations suggest that the Egyptian Red Sea margin has been relatively vertically stable since the Late Pleistocene. While there is general agreement that full seafloor spreading, producing well-defined magnetic stripes, has been occurring in the southern Red Sea since ~5 Ma, there is ongoing debate whether and when lithospheric break-up has occurred in the northern Red Sea. Industry wellbore and seismic data demonstrate that continental crust extends at least several tens of kilometers offshore from the present-day coastline, and that the northern Red Sea is a non-volcanic rifted margin. On the basis of integrated geophysical, petrological, geochemical and geological datasets, we contend that true, laterally integrated sea-floor spreading is not yet manifest in the northern Red Sea.