1
The Red Sea Miocene Evaporite Basin

ABSTRACT

The African and Arabian plates separated in the Neogene, creating the elongated gulf known as the Red Sea. It is characterized by a thick Middle to Upper Miocene salt basin overlying continental and marine rift sequences onshore and along the continental shelf. The salt layer extends to the axial trough, where oceanic crust spreading centers are recognized. The Southern and Central parts of the Red Sea have segments of oceanic crust without salt, resulting in the African and Arabian conjugate margin salt basins split by oceanic spreading centers. The Northern Red Sea has a thick salt layer almost continuous from the Egyptian margin to the conjugate margin in northwest Saudi Arabia. The Gulf of Suez between Egypt and the Sinai Peninsula has salt layers overlying a rifted continental crust. The Gulf of Aqaba, between the Sinai Peninsula and Arabia, has a narrow, deep trough formed by a transform fault zone in the region adjacent to the Midyan Basin in Saudi Arabia.

The Red Sea and the Gulf of California are young oceanic basins formed by continental rifting and oceanic crust associated with embryonic spreading centers. The Gulf of California is connected to the Pacific Ocean via the Gulf of California (Chapter 12 in Mohriak, 2025). The Red Sea is an elongated gulf that communicates with the Gulf of Aden and the Indian Ocean via the Strait of Bab-el-Mandeb (Figure 1.1). Marine incursions and brine evaporation resulted in the accumulation of thick evaporite layers in the Middle to Late Miocene, forming the conjugate margin salt basins along the Red Sea, extending from the Arabian and African margins (Bosworth et al., 2005; Orszag-Sperber et al., 1998).

The northern movement of the Arabian Plate causes oblique rifting and oceanic crust propagation in the Red Sea. Plate movements correspond to the convergence of Arabian and Eurasian plates in the Meso-Cenozoic and the divergence of African and Arabian plates in the Neogene. The pole of rotation for the divergent motion is located north of Turkey, resulting in the anticlockwise rotation of the Arabian Plate relative to the African plate (Figure 1.1). As the Arabian plate moved along the transcurrent fault zone associated with the Dead Sea transform fault zone (DSTF), plate readjustments resulted in transtensional structures in the Red Sea depression. The DSTF zone links the spreading center in the Northern Red Sea with the transcurrent fault zone that links the Gulf of Aqaba to the Taurus Mountains in Turkey (Bosworth et al., 2005). The NE-SW fault zone extends across the Dead Sea and Sea of Galilee depressions and forms a compressional deformation zone in the Palmyrides fold-thrust belt in Syria (Chapter 11 in Mohriak, 2025).

The northeastern African and western Arabian Peninsula coastlines are parallel to a central depression (axial trough) in the Red Sea (Figure 1.1). The axial trough is well-developed in the Southern Red Sea, forming bathymetric lows where water depths exceed 2,000 m, based on regional bathymetric datasets (e.g., GEBCO, 2019) and high-resolution bathymetry (e.g., Augustin, Feldens, et al., 2016). In the Northern Red Sea, the axial trough is not as conspicuous because of the greater sedimentary input, resulting in a smooth bathymetric profile across the conjugate margins (Augustin, Feldens, et al., 2016; Augustin, Zwan, et al., 2016; Augustin et al., 2019, 2021; Bosworth et al., 2005; Cochran, 1983, 2005; Cochran & Martinez, 1988; Martinez & Cochran, 1988). The observed and inferred mid-ocean ridges within these axial troughs are locally offset by transform fault zones, such as the SW-NE Zabargad Transform Fault Zone (Figure 1.1).

The DSTF zone in the Northern Red Sea links the spreading ridge and the Gulf of Aqaba across the Dead Sea depression to the Palmyride fold-thrust belt and the Bitlis-Zagros suture zone in Iran. In the Gulf of Aden, the transform fault zones trend NE-SW, offsetting the active spreading ridge that separates the Arabian from the Nubian plates (e.g., Leroy et al., 2012; Mohriak & Leroy, 2013). In the Red Sea, the transform fault zones are fewer and smaller, but the Zabargad transform fault zone is well-defined by gravity and magnetic anomalies (Augustin, Feldens, et al., 2016; Augustin, Zwan, et al., 2016; Augustin et al., 2014, 2019; Bonvalot et al., 2012; Sandwell et al., 2014).

Figure 1.1 The topobathymetric map of northern Africa, the Arabian Peninsula, and Eurasia shows the main tectonic elements in the Eastern Mediterranean, Red Sea, Gulf of Aden, Arabian Gulf, and the Zagros Chain in Iran. The simplified map shows the interpretation of faults, subduction zones, and spreading centers by thin white lines, and the Arabian plate motions by white arrows. The eastern Mediterranean Sea is subducting under the Anatolian plate as the African plate moves toward the north. The convergence between the Arabian and Eurasian plates formed the Zagros fold-thrust belt and subduction zones extending to the Gulf of Oman. The Dead Sea transform fault zone (DSTF) links the spreading center in the northernmost Red Sea to the northern Arabian plate in Anatolia, forming the Bitlis-Zagros Suture Zone. The crushed zone in the Zagros chain accommodated a significant amount of northeast-directed plate motion along the left-lateral motion of the DSTF during the Neogene. The Gulf of Aden, between Yemen and Somalia, is characterized by NE-SW transform fault zones that offset the Sheba ridge, an E-W spreading center advancing toward the Afar region.

Credit: Mohriak and Leroy (2013)/with permission of The Geological Society of London.

Salt deposits in the Red Sea are recorded both onshore and offshore, as indicated by gypsum outcrops in the Midyan Basin in northwest Saudi Arabia, thick evaporite layers drilled by the Deep Sea Drilling Project (DSDP) scientific boreholes in the deep water region, and petroleum industry exploration boreholes drilled on the continental margin (e.g., Lindquist, 1998; Hughes et al., 1999). The salt deposits are recorded on land along a narrow strip extending from the coastline to the rift border fault. In the offshore region, the salt layers extend from the continental shelf to the main axial trough (Almalki, Betts, & Ailleres, 2015; Almalki et al., 2016; Lowell & Genik, 1972; Mitchell et al., 2022). This chapter focuses on the genesis and distribution of Miocene evaporite sequences, ranging from the southernmost salt province near the Strait of Bab-el-Mandeb to the northernmost salt occurrences in the Gulf of Suez and the Midyan Basin in northwestern Saudi Arabia (Figure 1.1).

1.1. INTRODUCTION TO THE RED SEA GIANT SALT BASIN

The separation of African (Nubian) and Arabian plates in the Neogene formed the Red Sea salt basin, a classical giant salt province overlying the rift troughs developed along the incipient divergent margins (Almalki, Betts, & Ailleres, 2015; Almalki et al., 2016; Warren, 2006, 2016). The extremely thick evaporite layers were deposited from the Middle to the Upper Miocene (Figure 1.2).

The origin of the Red Sea rift basin is linked to the anticlockwise rotation of the Arabian Peninsula relative to the Nubian plate, with the Euler pole of rotation determining the opening direction and transforming fault zones. The plate motions involve a complex arrangement of blocks and tectonics processes, including seafloor spreading, continental transform fault zones, subduction zones, continental collision and suture zones, and intraplate magmatism (Aldaajani et al., 2021; Courtillot et al., 1987; Viltres et al., 2022). According to magnetic anomalies and plate reconstructions (e.g., Chu & Gordon, 1998; Delaunay et al., 2022; Issachar et al., 2022; Viltres et al., 2022), the Euler pole of rotation for opening the Red Sea is located in the Mediterranean Sea (23° E-31.5° N).

The Red Sea is a narrow gulf with a length of almost 2,000 km and a width of more than 300 km in some places (Almalki, Betts, & Ailleres, 2015; Bosworth et al., 2005; Rasul et al., 2015, 2019). This gulf is considered a paradigm for the development of ancient salt basins formed on divergent margins, such as the South Atlantic salt basin (Lowell & Genik, 1972; Stockli & Bosworth, 2019). The South Atlantic salt basins formed along the Brazilian and West African margins in the Early Cretaceous, and include a tectonic phase known as the "gulf phase," where Late Aptian evaporite sediments were deposited above the rift phase sequences (Kukla et al., 2018; Mohriak, 2014, 2015, 2019; Ojeda, 1982).

In the South Atlantic continental margins, the rift phase corresponds to a fluvial-lacustrine continental environment associated with active extensional faults. The Neocomian to Barremian syn-rift deposits are overlain by Aptian siliciclastic and carbonate sediments, forming the sag basin. The sag basin deposits are overlain by the thick Late Aptian evaporite layers that extend from the rift border fault to the boundary with the oceanic crust (Mohriak, 2014). In the South Atlantic, as the continental margins drifted apart, the salt...