Stratigraphy and evolution of emerged Pleistocene reefs at the Red Sea coast of Sudan

Journal of African Earth Sciences 114 (2016) 133e142

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Stratigraphy and evolution of emerged Pleistocene reefs at the Red Sea coast of Sudan Basher Hamed a, *, Robert Bussert b, Wilhelm Dominik b a b

Faculty of Petroleum and Minerals, Al Neelain University, Khartoum, Sudan €t Berlin, Berlin, Germany Institute of Applied Geosciences, Technische Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 October 2015 Received in revised form 9 November 2015 Accepted 11 November 2015 Available online 25 November 2015

Emerged Pleistocene coral reefs constitute a prominent landform along the Red Sea coast of Sudan. They are well exposed with a thickness of up to 12 m and extend over a width of about 3 km parallel to the coastline. Four major reef units that represent different reef zones are distinguished. Unit 1 is located directly at the coastline and is assigned to the rock-reef rim, while unit 2 represents the reef-front zone. Unit 3 is attributed to the reef-flat zone and unit 4 to the back-reef zone. The stratigraphic position and age of the four units respectively the facies zones are based on field relationships and d18O analysis. Results of d18O analysis of coral, gastropod and bivalve samples were correlated to previous age dating of correlative reefs in Sudan and other parts of the Red Sea region. Estimation of reef ages was mainly based on d18O values of the reef-front zone (unit 2) and the observed sedimentary succession of the reefs. d18O values of two Porites coral samples from the reef-front zone strongly suggest equivalent ages of 120 and 122 ka that correspond to marine isotope stage MIS 5.5. Based on d18O values and the field relationship to the reef-front zone, ages of reef-flat zone (unit 3) and back-reef zone (unit 4) could be assigned to MIS 9 and MIS 7 respectively. MIS 5.1 is suggested for the reef-rock rim (unit 1). The relationship of the reef zones to individual MIS might be explained by the predominance of a specific zone during a certain stage, while other facies were less well developed and/or later eroded by wave action. The reef unit most distal from the recent coastline formed during interglacial stage MIS 7, while former studies assign this unit to interglacial stage MIS 9. Unique flourishing, high diversity and excellent preservation of corals in the back-reef unit of MIS 7 reflect growth in troughs landward of the oldest reef-flat formed during previous interglacial stage MIS 9. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Pleistocene reefs Red sea coast Marine isotope stages Sudan

1. Introduction Emerged Pleistocene coral reef terraces form a prominent landform of the Red Sea coast. Significant efforts were taken to determine their precise ages in order to evaluate relative sea-level changes in the region (Mansour and Madkour, 2015). The morphology and position of the reefs is primarily controlled by its tectonic setting in a rift basin as well as by eustatic sea-level changes (Dullo and Montaggioni, 1998; Emmermann, 2000). Correlation and dating of emerged reef terraces along the Red Sea coast have the potential to enhance the understanding of the tectonic, palaeogeographic and palaeoclimatic history of the region, as well as to reconstruct sea-level changes during the Pleistocene. The

* Corresponding author. E-mail address: [email protected] (B. Hamed). http://dx.doi.org/10.1016/j.jafrearsci.2015.11.011 1464-343X/© 2015 Elsevier Ltd. All rights reserved.

sedimentary evolution and the ages of the Pleistocene reef terraces along the Red Sea coast have been discussed by several authors, for example Dullo (1990), Strasser et al. (1992), Hoang and Taviani (1991) and Plaziat et al. (1998, 2008). In Egypt, Sudan and Djibouti, a few coral reefs were dated before the eighties of the last century (e.g. Butzer and Hansen, 1968; Veeh and Giegengack, 1970; Faure et al., 1980). A review of the scientific progress in the study of Pleistocene reefs at the coast of the Red Sea is given by Plaziat et al. (2008). Most previous age dating indicated that emerged Pleistocene terraces along the Red sea coast are composed of four main reef units which refer to the four marine isotope stages of the interglacial periods MIS 5.1, MIS 5.5, MIS 7 and MIS 9. Ages related to isotope stages 1, 5e (5.5), 7 and 9 are determined for the reef terraces at Gulf of Aqaba where their d18O variation indicates at least four major warm-wet climatic periods (El Asmar, 1997). The lowermost reef gave ages of 87.6, 86.6 and 57.6 ka assigned to MIS 5a (5.1) by Plaziat et al. (2008). Lower reefs assigned to the

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maximum highstand stage MIS 5.5 have been identified by Dullo (1990) of age 110e118 ka, Hoang and Taviani (1991), 125e138 ka, Strasser et al. (1992), 140 ka, Plaziat et al. (2008), 128e135 ka and by Manna (2011), 122.8 ka. Reefs that formed during MIS 7 (190e250 ka) have been determined by Hays et al. (1976), Gvirtzman and Buchbinder (1978), Al-Rifaiy and Cherif (1988) and Dullo (1990). El Moursi et al. (1994) additionally considered the middle reef to have formed during MIS 7 (170e230 ka). Age dating of the oldest reefs resulted in ages of 270e350 ka (Strasser et al., 1992) and >300 ka (Hoang and Taviani, 1991) which correspond to interglacial stage MIS 9. Dullo (1990), Strasser et al. (1992) and Plaziat et al. (1998) considered the reef to have formed during MIS 9 as the most distal from the coast. This investigation will indicate that this does not correspond to the sedimentological situation of the emerged Pleistocene reefs along the Red Sea coast of Sudan. The Sudanese Red Sea coast is of major historic and geological importance, for its coral reef was already interpreted by Darwin (1842) as uplifted within a modern period (Plaziat et al., 2008). In Sudan, fossil coral reefs represent a series of almost continuously raised reef terraces that characterize most of the shoreline of the Red Sea coast that extends for about 700 km between Egypt and Eritrea (Fig. 1). Sestini (1965) stated that the reefs are forming emergent reef terraces 2 m, 4 m, 7 m and 9e10 m above sea-level. He explained their position above the present sea-level by eustatic movements of the Red Sea. The reefs form well defined mounds or ridges aligned parallel to the coast, and are composed of corals and other carbonate components of Pleistocene age (Schroeder and Mansour, 1994). An attempt to determine the age of these reefs by radiocarbon dating of Tridacna shells was performed by Berry et al. (1966). He distinguished seven reef stages and estimated their ages from >0.1 to 2.1 ka. Boreholes drilled between Marsa Arus and Marsa Arakiyai, about 40 km north of Port Sudan city, demonstrate reef limestone more than 15 m thick, entirely of Pleistocene age (Whiteman, 1971). Hoang et al. (1996) investigated the stratigraphy, the tectonic and palaeoclimatic setting of Pleistocene coral reefs using uranium-series dating. Although Sudan was the place of the first radiocarbon dating of corals in the Red Sea region, conducted by Berry et al. (1966), this research was only followed by one Th/U dating (Hoang et al., 1996). Recently, the first author noted that the Pleistocene reefs in Sudan generally form four main reef zones, which include reef-rock rim, reef-front, reefflat and back-reef. The main objectives of this paper are to construct the stratigraphic framework of the reefs, and to reveal their evolution in dependence on sea-level changes, based on results of the study of field relationships and facies of the reefs, and the estimation of absolute ages of the Pleistocene reefs using d18O dating. 2. Methods Field work on Pleistocene reef terraces along the Red Sea coast of the Sudan (Fig. 1) included the study of lithofacies, sedimentary structures and fossils. Sampling was systematic and tried to cover vertical and lateral facies changes. The reef-flat, back-reef, frontreef and reef-rock rim zones were distinguished in the field based on their relative position, morphology and fossil content. A primary identification was done for the reef fauna of the zones, which include reef builders, binding and encrusting organisms. Stable isotope composition (d18O) was determined on a total of 32 coral, gastropod and bivalve samples that represent the different facies zones of the Pleistocene coral reefs in the study area. Sample preparation and analysis were performed at the Institute of Geophysics and Geology, University of Leipzig. Using a hand-held microdrill with a 0.5 mm steel drill bit, carbonate powders of specific species were produced. The carbonate powders were dissolved with 100% phosphoric acid (density > 1.9, Wachter and

Hayes, 1985) at 75  C using a Kiel III online carbonate preparation line connected to a MAT 253 mass spectrometer. Values are reported per mil relative to V-PDB (Vienna Pee Dee Belemnite) assigning a d18O value of 2.20‰ to NBS19. Reproducibility was checked by replicate analysis of 14 laboratory standards and is better than 0.036 for d18O (1s). Following pioneer work of Emiliani (1955), oxygen isotopes (d18O) in foraminifera from deep-sea sediments were used to establish a pattern of sea surface temperature that extended back to 600 ka and was marked by repeated asymmetric cycles. The overall pattern was found to match cycles predicted from astronomical theory by Milankovitch (Hays et al., 1976). Since theses cycles (orbital parameters) are constant and their frequency is known, they provide a basis for timing the cycles now referred to as marine isotope records or stages (Walker, 2005). Berger (1978) instated that because the basic frequencies of the cycles are known, they can be used to calculate the age of each isotopic stage. Walker (2005) also considered that the age of each stage can be calculated by extrapolating back from the present day. Such approach was first used by Imbrie et al. (1984) who achieved a time scale of 800 ka, known as the SPECMAP time scale, by amalgamation of several isotopic records (stacked records) to known frequencies of the astronomical variables. Other orbital tuned timescales were developed for the Middle and Early Pleistocene (Shackleton et al., 1990), while for example the technique has also been used by Ding et al. (1993), Ruddiman and Raymo (2003). The stages number from MIS 1, the Holocene, back to at least MIS 63 with an absolute age approaching 1.8 Ma (Montaggioni and Brathwaite, 2009). Oxygen stable isotopes data were used to investigate the age of different reef facies by comparison to the global mean SPECMAP d18O curve (Imbrie et al., 1984) and to marine isotope stages in the Red Sea recorded by Hemleben et al. (1996). 22 coral and 10 mollusc samples (some in Fig. 2) collected from 10 different sites of emerged reefs in the study area were dated using oxygen isotope measurements and compared with ages published by Berry et al. (1966) and Hoang et al. (1996). Table 1 shows the analytical results obtained from the total of 32 samples. The calcite content of the samples varies from <1% to 8%, except for eight samples which contain calcite of up to 96%. To avoid contamination by calcite in measured samples, a hand-held microdrill was used under a high-magnification binocular microscope to produce powder from molluscs and coral fragments that contain only aragonite. In order to convert the d18O values of the samples to the global mean d18O curve, the vital fractionation factor of 0.3‰ determined by Jasper and Deuser (1993) was used. These authors calculated the vital effects for the petropod species Creseis acicula in the MidAtlantic Ocean. Hemleben et al. (1996) computed the mean offset between C. acicula and the d18O value of foraminifera species Globigerinoides ruber that was obtained from core KL 11 offshore Sudan. The stratigraphic framework of the reefs was constructed by determining relative ages based on field relationships and sedimentary succession, and from correlating the data to the global mean d18O curve (Fig. 3). Results were compared to other Pleistocene reefs along the Red Sea coast. To determine equivalent ages of the reefs and marine isotope stages (MIS), oxygen isotope records of the 32 samples were correlated to the SPECMAP time scale (Imbrie et al., 1984). The isotopic events within MIS 5 which refer to 5a, 5c and 5e correspond to 5.1, 5.3 and 5.5 (Walker, 2005). Results obtained were correlated to the core KL 11 oxygen isotope records of the Red Sea constructed by Hemleben et al. (1996). The analytical results show that the oxygen isotope values (d18O) for 28 samples range from 4.73‰ to 0.79‰, while four samples did not yield any values. Most ages are not reliable due to their high

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content of calcite. However, of the obtained data, two samples representing the reef-front zone (unit 2) give d18O values of 1.92‰ and 2.17‰ (No. 6 and 28 in Fig. 3). These values can be correlated to the lowest negative values in the global mean d18O curve of SPECMAP (Imbrie et al., 1984). The values correspond to a peak of maximum interglacial stage MIS 5.5 and do not occur in other interglacial stages. Therefore, corresponding age and marine isotope stage MIS 5.5 of the two values are considered to be reliable, and are used as a base to estimate the age of other units. 3. Results Recently, the first author established a reef zonation of the Pleistocene reefs in the study area and interpreted their depositional environment (Hamed, 2015). Four main reef zones and a beach rock facies are distinguished. From the sea towards the land these are beach rock, reef-rock rim (unit 1), reef-front (unit 2), reefflat (unit 3), and back-reef (unit 4). In the study area, pre-recent beach rock has a limited and sporadic occurrence restricted to the intertidal zone. It is present exclusively in the lower slope of the reef-front zone (Fig. 4A). The rock is coarse-grained and consists mainly of moderately to poorly sorted, variously-cemented bioclasts. Dominant bioclastic grains in the rock are coralline algae, bryozoans and mollusc shells. Porites and Galaxea coral fragments are common. Coralgal grainstone/ rudstone has been identified for these reef-derived bioclasts that accumulated in a high-energy setting (Hamed, 2015). Of three gastropod samples collected from beach rocks of two localities, one did not yield any value while the other two gave d18O values of 0.10‰ and 0.68‰. According to the field relationship, beach rock forms the youngest rock type of the reefs in the study area. Therefore it is suggested that the d18O values of the samples correspond to MIS 1 (Holocene). The reef-rock rim terrace (unit 1) is positioned on average about 1.5 m above mean sea-level. It is located close to the coastline in Salak and Dungunab areas. The reef-rock rim shows well developed old benches that represent isolated terraces forming an outer reef rim (Fig. 4B). In the Arkiyi area, the reef-rock rim laterally overlaps the reef-front zone (described below) in the foot of the slope (Fig. 4C). In most other areas, the reef-rock rim generally forms, together with the other reef zones, homogenous reef terraces that show a regular gentle slope towards the sea (Fig. 4D). The regular morphology of the Pleistocene reef terraces at the Red Sea coast of Saudi Arabia was ascribed by Dullo (1990) to different erosional planations. Hamed (2015) showed that the reef-rock rim consists primarily of a coralgal-foraminifera bindstone. A gastropod and a Galaxea coral that represent the reef-rock rim (unit 1) gave d18O values of 0.69‰ and 3.80‰ respectively. The d18O value of the gastropod ( 0.69‰) could correspond to an age of 76 ka suggesting deposition during MIS 5.1. The reef-rock rim onlaps the lower slope of the reef-front zone (interpreted as having formed during MIS 5.5, see below). This advocates deposition of unit 1 during marine isotope stage MIS 5.1. The reef-front zone (unit 2) is represented by impressive, very well preserved massive coral knolls which extend close to the recent shoreline. The knolls either form a discontinuous isolated reef strip or ridge parallel to the coastline or exist as a homogenous massive reef pavement with a reef-rock rim facing seaward, and a reef-flat and back-reef facing towards the land. The isolated strips of the reef-front zone form terraces along the coastline, up to 6 m thick and 100 m to about 250 m wide in Beleib and Arkiyai areas.

Fig. 1. Landsat image of the study area showing the sample localities and other locations mentioned in the text.

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Fig. 2. A & B. Porites corals that show columnar and massive growth forms characterize the reef-front zone in Marsa Darah (B) and Marsa Arkiyai (A). Samples from these localities give d18O values of 1.92‰ and 2.17‰ respectively, which correspond to MIS 5.5. C. Rhinoclavis gastropod of the reef-rock rim yielded d18O values of 0.69‰ that might correspond to around 76 ka., Marsa Salak D. Bivalve shell (white arrow) surrounded by a Galaxea coral, Marsa Arus. The bivalve shell showed a d18O value of 1.30‰ that could be equivalent to ages of around 192 ka.

The coral community of the reef-front is dominated by very large massive colonies of Porites columna. Other reef builders are stony corals in form of large massive and domal colonies of Platygyra, Favia, Favites and pipe packed-like Montastraea. Porites/stony coral framestone is the dominant rock facies. Thirteen coral samples representing unit 2 were analysed, of which twelve samples gave d18O values that vary from 1.92‰ to 4.73‰, while one sample did not yield any value. Two samples of Porites corals that gave d18O values of 1.92‰ and 2.17‰ are considered the most reliable and likely correspond to 120 and 122 ka and the deposition during the last interglacial maximum high-stand stage MIS 5.5. Ten samples of the reef-front which yielded d18O values between 2.63‰ and 4.73‰ might correspond to the same marine isotope stage when plotted on the d18O curve. The reef-flat zone (unit 3) is very well defined in the study area. It is characterized by a prominent reef pavement that extends over wide areas along the coast. On average, it is about. 1 km wide and attains a height of 8e10 m in Shinab and Delwin areas. Its thickness gradually decreases to the north where it reaches 5 m in the Oseif area whereas it decreases to less than 3 m in the Marsa Marob area in the northernmost part of the study area. Onlap of the reef-flat zone by the reef-front zone is well developed in the Shinab area (Fig. 4E). In comparison to other reef zones in the study area the reef-flat is more variable in composition. From bottom to top, it is composed of whole fossils wackestone/floatstone, branched Porites bafflestone, bivalve floatstone and branched Porites framestone. Two coral samples, a Tridacna bivalve and a gastropod representing the reef-flat zone give d18O values of 0.54‰, 2.93‰, 0.79‰, 3.30‰ and 3.86‰, respectively. Field evidence suggests that the reef-flat zone represents the oldest reef

generation in the study area. Deposits of the back-reef zone (unit 4) are excellently exposed in the study area where they laterally onlap the reef-flat zone (unit 3). Further landward, older wadi deposits form their base. The backreef zone occurs at a height of up to about 10 m a.s.l. and is 300e400 m wide in Beleib and Arus areas. Its sediments are characterized by the occurrence of fresh, excellently preserved large arborescent coral colonies, including Galaxea facicularis of pipe-pack growth form and thickly branched Caulastrea connate. Of eight coral and bivalve samples of the back-reef zone, two bivalve samples give d18O values of 1.30‰ and 1.03‰. These values are considered to be equivalent to ages of around 192 and 240 ka respectively which indicate deposition during marine isotope stage MIS 7. Five coral samples give d18O values of 3.29‰, 3.83‰, 4.06‰ 4.39‰, and 4.57‰, while one coral sample did not yield a value. These values might also correspond to MIS 7. The suggested stage MIS 7 for this unit is also consistent with the outcrop position of the back-reef zone below the reef-front zone. 4. Discussion 4.1. Beach rock In this study, the beach rock is assigned to the Holocene. In other areas of the Red Sea, e.g. at the Red Sea coast of Egypt (Strasser et al., 1992) age dating of sub-recent beach rock is not consistent with its field occurrence, because Tridacna shells of the beach rock zone yielded ages likely much too old for their stratigraphic position. Strasser et al. (1992) suggested that this might reflect reworking of

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Table 1 d18O analytical results obtained from a total of 32 coral and mollusc samples collected from ten different sites of emerged reefs in the study area. Age and marine isotope stage equivalent to the samples with* mark are considered as a base to estimate ages of other facies zones. Sample

Location

Sample type

Facies zone

Aragonite (%)

Calcite (%)

d18O ( ‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Beleib Beleib Beleib Beleib Beleib Dara Dara Oseif Oseif Shinab Shinab Salak Salak Arkiyai 1 Arkiyai 1 Arkiyai 1 Arkiyai 1 Arus Arus Arus Little Inkefal Little Inkefal Little Inkefal Little Inkefal Arkiyai 2 Arkiyai 2 Arkiyai 2 Arkiyai 2 Arkiyai 2 Arkiyai 2 Arkiyai 2 Dungunab

Galaxea coral Porites coral Galaxea coral Bivalve Favites coral Porites coral Bivalve Porites coral Montastre coral Tridacna bivalve Gastropod Gastropod Galaxea coral Porites coral Favites coral Favites coral Montastrea coral Bivalve Galaxea coral Porites coral Gastropod Gastropod Favites coral Porites coral Gastropod Gastropod Porites coral Porites coral Favia coral Favites coral Porites coral Porites coral

Back-reef Back-reef Back-reef Back-reef Back-reef Reef-front Reef-flat Reef-front Reef-flat Reef-flat Reef-flat Rock rim Rock rim Reef-front Reef-front Reef-front Reef-front Back reef Back-reef Back-reef Beach rock Beach rock Reef-front Reef-front Beach rock Beach rock Reef-front Reef-front Reef-front Reef-front Reef-front Reef-flat

86 55 57 97 86 95 89 96 47 88. 77 66 79 88 98 97 90 99 93 78 3 20 96 82 67 60 87 97 91 52 97 10

4 5 5 0 6 2 4 2 43 20 2 15 8 0 0 1 6 0 2 3 96 0 0 3 25 25 1 0 2 33 1 87

4.06 3.29 No gas 1.03 4.39 1.92* 0.54 2.63 2.93 0.79 3.30 0.69 3.80 3.76 4.29 3.96 4.73 1,30 4.57 3.83 0.10 No gas 3.86 3.89 No gas 0.68 3.52 2.17* No gas 4.41 3.03 3.86

shells from underlying sediments, or could be due to exposure and diagenetic modification of the original isotopic composition. The wave notches that cut into an uplifted reef sequence 1e2 m a.s.l. in the Gulf of Aqaba and in Saudi Arabia have been dated as 4.2 to 6.5 ka of age (Reiss and Hottinger, 1984; Dullo, 1990) and thus correspond to the Holocene sea-level highstand (Strasser et al., 1992). Whiteman (1971) reported that a 2 m bench commonly occurs at the shore of Sudan, dated as 2.6e2.1 ka, which also correspond to the Holocene. 4.2. Unit 1: reef-rock rim Despite similarity in position and elevation that has been reported for coral reef terraces along the Red Sea coast by several authors, there are differences in their ages and interpretation. In Sudan, Hoang et al. (1996) stated that no evidence was found of a shoreline that formed during stages 5c (5.3) and 5a (5.1), and higher than present sea level except the 5e (5.5) last interglacial stage that was found at 2e6 m. However, field investigation by the first author shows a terrace of the reef-rock rim that occurs as well-defined wave cut notches and emerged beaches up to 1.5 m a.s.l. Based on radiocarbon dating of algal mat sediments that extend up to 2 m a.s.l. in the Gulf of Aqaba, Neev and Friedman (1978) concluded that the eastern coast of the Sinai was uplifted during the late Holocene. Although Siddall et al. (2003) agreed that tectonic uplift could explain the position of the lower reef, but suggest that the uplift rate in the area is likely too low. Thus, Parker et al. (2012) propose that it is more likely that the lower, younger reef terrace formed during a lower sea-level highstand within MIS 5e. Reiss and Hottinger (1984), Dullo (1990) and Strasser et al. (1992) have interpreted the lowest reef terrace (~1e2 m above

present day sea-level) as a mid-Holocene wave-cut platform. In contrast, Parker et al. (2012) believe that in some parts of the Red Sea this may be true, but in most cases the terrace is not a Holocene wave-cut feature, based on the lack of erosional features and borings in the fossil terrace. Plaziat et al. (2008) suggest that the lower reef terrace formed during one of the MIS 5e (5.5) sea-level highstands, which corresponds better to the sea-level curve of Rohling et al. (2008). However, oxygen isotope ages of gastropods in this study suggest that the reef-rock rim terrace (unit 1) likely formed around 76 ka (MIS 5.1). Therefore, and based on field relationships, it likely represents the youngest emerged reef terrace in the study area. This coincides with the position of the reef-rock rim zone close to the present coastline and its occurrence above deposits of the reef-front (likely of MIS 5.5 age, see below). 4.3. Unit 2: reef-front zone Uplifted beaches and coral reefs of MIS 5.5 (Eemian) are well known and one of the most common landforms along the Red Sea coast. These last interglacial high-stand reefs have been identified in many areas of the Red Sea coastal plain, for example in Egypt (Hoang and Taviani, 1991; Gvirtzman et al., 1992; Strasser et al., 1992; El Moursi et al., 1994; Taviani, 1998; Plaziat et al., 2008; Parker et al., 2012), Saudi Arabia (Dullo, 1990; Manna, 2011), Eritrea (Bruggemann et al., 2004) as well as in Afar and the Gulf of Aden (Faure et al., 1973; Hoang et al., 1980). The palaeoclimatic conditions during this interval allowed scleractinians to occupy higher latitudes than at present, possibly due to higher global sealevels producing larger shelf areas, and because of slightly higher sea-water temperatures (Gischler et al., 2009). Sudan was the place of the first radiocarbon coral dating in the

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Fig. 3. d18O values of coral and mollusc samples plotted against the global mean d18O curve of SPECMAP time scale (Imbrie et al., 1984). Equivalent ages and the marine isotope stage of two samples (marked by a circle) which represent the reef-front zone (unit 2) are considered as a reference to estimate ages of other facies zones. d18O or delta-O-18 is a measure of the ratio of stable isotopes oxygen-18/oxygen-16. ka: 1000 years before present. VPDB: Vienna Pee Dee Belemnite standard. MIS: marine isotope stage. 1e32: number of sample in Table 1.

Red Sea region that produced a date of 91 ± 5 ka (Berry et al., 1966). This measurement was interpreted as the age of a last interglacial reef growth on a stable coast situated 9 m above present sea level (Plaziat et al., 2008). Radiocarbon dates of Berry et al. (1966) indicate an age of more than 37 ka for the emerged reef that is positioned 16 m above mean sea level in Maghersum Island (Whiteman, 1971). However, these authors follow the interpretation of Fairbridge (1958, 1961), who believed that the elevation of the reef suggests an age of >100 ka. Moreover, Gvirtzman and Friedman (1977) mention that the three terrace units at the Red Sea coast of Egypt probably express major sea-level highstands which occurred between 85 and 125 ka (lower), 200e250 ka (middle) and more than 250 ka (highest). For the youngest coral formation along the Sudanese Red Sea coast, situated 2e6 m a.s.l. Hoang et al. (1996) reported Th/U ages of 125e142 ka. These dates seem to place reef growth in marine isotope stage 5e (MIS 5.5) of the last interglacial and suggest that it is coeval with a major reef growth period present all along the Red Sea coast, from Egypt in the north to Eritrea and Djibouti in the south. Plaziat et al. (2008) concluded that “during the high-level isotopic substage (MIS 5.5) of late Pleistocene, a rapid lowering of sea level, short and limited to about ten meters, was detected and associated with a glacioeustatic episode of global influence“. However, foraminifera

associations indicate that these reefs were deposited during three phases, caused by intra-interglacial sea-level changes and local tectonic activity (Parker et al., 2012). This study confirms the presence of MIS 5.5 reefs at the Red Sea coast of Sudan. The samples that strongly suggest equivalent ages of 120 and 122 ka (MIS 5.5) represent large massive Porites corals which form most of the reef-front zone (unit 2) positioned about 6 m a.s.l. Besides huge colonies of columnar and massive Porites corals (Fig. 4A), the coral community also includes large massive and domal Platygyra, Favia, Favites, Hydnophora and Montastraea. Communities and growth forms of the corals suggest that they grew in a relatively deep-water setting down to a water depth of about 10 m (Montaggioni and Braithwaite, 2009). This is in coincidence with the last higher interglacial sea level highstand of MIS 5.5 in the Red Sea that was 8 m higher than today, according to Plaziat et al. (1998). At the Egyptian Red Sea coast, the last interglacials reefs onlap and entirely mask the previously uplifted and partly eroded older reefs with subvertical to subhorizontal contacts (Strasser et al., 1992). Similar reef relationships are very clear in the study area. In the Marsa Abu Fanadir area (Fig. 5A), massive corals of the reeffront zone (MIS 5.5) onlap older branched Galaxea corals that characterize unit 4 (back-reef). Onlap of deposits of the reef-front

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Fig. 4. A. Beach rock (of mid-Holocene age) onlaps the lower part of the reef-front slope (MIS 5.5) in reef pools exposed to wave and storm action at Marsa Arkiyai. B. The reef-rock rim (MIS 5.1) is represented by isolated terrace notches. The terrace represents a well-developed old coastal plain around Salak area. C. A reef-rock rim (MIS 5.1) laterally onlaps a reef-front ridge (MIS 5.5) in the foot of the slope in Beleib area. D. General view of the emerged Pleistocene reefs in the Maras Shinab area. The reef zones form homogenous reef terraces that show a regular gentle slope towards the sea. E. The reef-front zone onlaps the reef-flat zone, seaward in Shinab area.

zone on sediments of the reef-flat zone (the oldest reef) is very obvious in the Marsa Shinab area (Fig. 5B). 4.4. Unit 3: reef-flat zone The oldest reef unit in the study area is the reef-flat (unit 3). This is indicated by its field relationship to underlying and overlying deposits. In Oseif and Marob areas, the deposits of the reef-flat are unconformably overlain by alluvial gravel terraces (Fig. 5C). These terraces (classified as Pleistocene to Recent sediments of the Upper Clastic Group by Robertson Research, 1986) indicate deposition in shallow streams which prograded during regression. This suggests that deposition of the reef-flat sediments was followed by a drop in sea-level and regression of the shoreline (El Moursi et al., 1994). The variable altitude of the reef-flat terraces in different localities

strongly suggests that it was caused by differential tectonic uplift, especially along the general strike trends of the stream channels (Marsas). The reef-flat zone laterally is overlain by the back-reef zone (MIS 7). Landward, this situation is clear in Mars Shinab and Delwin areas (Fig. 4E). Massive coral ridges interpreted as deposits of the reef-front zone (MIS 5.5) laterally onlap and vertically overlay the reef-flat in most of the studied sites. Therefore, d18O values of the samples that represent the reef-flat were correlated to MIS 9 in the global mean d18O curve of SPECMAP. Bosworth and Taviani (1996) reported an age of 426 ka for the oldest coral terraces in Zabargad island, at the Red Sea coast of southern Egypt. In the study area, Hoang et al. (1996) have reported ages of 253 ka and >300 ka for the older reef unit, which they interpreted as MIS 7 ages instead of MIS 9 (Plaziat et al., 2008).

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Fig. 5. A. Massive corals of the reef-front zone (MIS 5.5) vertically onlap patches of older branched Galaxea corals that characterize the back-reef zone (MIS 7) in the Marsa Abu Fanadir area. B. Onlap of deposits of the reef-front zone on sediments of the reef-flat zone (the oldest reef) in Marsa Shinab area. C. Reef-flat terraces (MIS 9) overlain by Pleistocene to Recent alluvial gravels of the Upper Clastic Group in the area west of Marsa Marob. D. The reef-flat terrace in Marsa Shinab area is transsected by normal faults. E. Sediments of the back-reef zone (MIS 7), to the right, are onlapping reef-flat terraces (the older unit of MIS 9) in Marsa Delwein area.

Following the later interpretation and mainly based on field relationship, the reef-flat zone (unit 3) can be assigned to interglacial highstand stage MIS 9. Similar ages have also been reported for probably correlative coral reef terraces along the Red Sea coast in Egypt (Gvirtzman et al., 1992; Strasser et al., 1992; Gvirtzman, 1994; El Moursi et al., 1994; Plaziat et al., 1998; Taviani, 1998) and Saudi Arabia (Dullo, 1990). Plaziat et al. (1998) indicated that the initial reef has been truncated and lowered for several meters by a series of small faults. They also stated that this structural lowering has facilitated the marked next marine transgression, which allowed discontinuous corals to grow. Small faults can also be traced in the reef-flat terraces in the study area (Fig. 5D).

4.5. Unit 4: back-reef zone Field relationship indicates that the reef-front zone (MIS 5.5) onlaps the back-reef zone (unit 4). Therefore, marine isotope stage MIS 7 is proposed for the back-reef zone, with ages of about 192 and 240 ka. Similar ages of emerged coral reefs are widespread in the Red Sea area and are reported from Egypt (Hoang and Taviani, 1991;

Strasser et al., 1992; El Moursi et al., 1994; Gvirtzman, 1994; Choukri et al., 1995; Taviani, 1998), Saudi Arabia (Dullo, 1990) and Eritrea (Hoang et al., 1974; Conforto et al., 1976). Based on its stratigraphic relationship to other reef zones, the back-reef zone (unit 4) in the study area can be considered an equivalent of the intermediate sequence and middle cycle described by Strasser et al. (1992) and El Moursi et al. (1994) in the northern part of the Red Sea coast. Only minor relics of these terraces were recorded in both studies. The present study suggests that along the Red Sea coast of Sudan, these terraces represent primarily back-reef deposits. In the study area, they are excellently preserved, easily accessible and easy to correlate. The back-reef (unit 4) is marked by a dominance of fresh, well preserved large arborescent Galaxea and Caulastrea coral colonies (Gig. 2D). Patches of coral colonies are overgrown by massive reeffront corals (likely of MIS 5.5) in the Marsa Abu Fanadir area (Fig. 5A). However, in sections Marsa Shinab and Marsa Delwein, the sediments of the back-reef zone onlap reef-flat terraces (MIS 9) seaward in a height of about 10 m a.s.l. (Fig. 5E). This outcrop of

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Fig. 6. Scheme of the emergent reef terraces at the Red Sea coast of Sudan illustrating the spatial relationship of zones and their assigned MIS ages. m.s.l.: mean sea level.

questionable age assignment to MIS 7 has been indicated and discussed in Zabargad Island, near the border between Egypt and Sudan (Hoang and Taviani, 1991). The records for the MIS 7 interglacial interval indicate that sea-level was lower than those of both MIS 9 and MIS 5, or the present sea-level (Imbrie et al., 1984; Rohling et al., 2009). Thus, the abnormal altitude of the MIS 7 reef may result from tectonic uplift (Hoang and Taviani, 1991; El Moursi et al., 1994). 4.6. Sea-level influences on reef evolution Based on field investigations of sedimentary successions and age dating, an interpretation scheme of the emerged Pleistocene reefs at the Red Sea coast of Sudan is constructed. Fig. 6 illustrates the relation of reef units to different marine isotope stages. Unit 3 (reef-flat dominated) represents the oldest zone, with MIS 9 suggested for their formation. Landwards, the reef-flat terraces are covered by alluvial fan deposits. The reef-flat was affected by faults and thereby troughs formed landward. These troughs provided accommodation space for the deposition of the unit 4 (back-reef dominated) during the following transgression. During interglacial stage MIS 7, troughs that occur in the lee side of the unit 3 were flooded and unit 4 corals developed. During the next maximum sea-level highstand (MIS 5.5) primarily unit 2 (reef-front dominated) formed. Landward unit 2 onlaps older deposits of unit 3. This confirms that unit 2 (reef-front dominated) constitutes the youngest major transgressive deposit in the study area. Unit 1 (reef-rock rim dominated) onlapped the bottom slope of unit 2 during a minor sea-level rise (MIS 5.1). Age dating and patterns of reef units indicate that the discrete reef zones can be assigned to individual marine isotope stages. This could be ascribed to the predominance of a particular zone during a certain stage. Other zones likely were less well developed and probably subsequently largely eroded. 5. Conclusion Field investigation and d18O dating of emerged Pleistocene reef terraces along the Red Sea coast of Sudan allow to construct their stratigraphic framework and to correlate sea-level changes to the formation of the different facies zones. Interpreted stratigraphic positions were correlated to existing ages of Pleistocene reefs in Sudan and other parts of the Red Sea. The reef-flat (unit 3) is interpreted as the oldest zone that formed during transgressive stage MIS 9. It is overlain by coarse-grained alluvial fan deposits, which indicates a phase of glacio-eustatic lowering of the sea-level. Faulting of the coastal area produced N-S, NNE-SSW, and NE-SW trending troughs parallel to the coast which locally served as sediments traps (Sestini, 1965). Several consequent troughs formed in the back of the older reef-flat terraces and provided

accommodation space for the formation of the next reef generation. The second reef generation that built up during the following transgressive phase of MIS 7 corresponds to the back-reef zone (unit 4). Most of the back-reef sediments were deposited in troughs behind the inner part of the older reef-flat zone (unit 3). The reef-front zone (unit 2) formed during maximum interglacial stage MIS 5.5 and onlaps the older reef-flat zone (unit 3) assigned to MIS 9. It also onlaps relics of the back-reef zone (unit 4) that formed during MIS 7. This confirms that the reef-front zone constitutes the youngest major transgressive sequence. The reefrock rim (unit 1) onlapped the reef-front sediments during minor interglacial period MIS 5.1. In comparison to previous studies and models in the region, a major difference in the stratigraphy of the reefs in the study area is the interpretation of the formation of the back-reef zone (unit 4) during MIS 7. The reef unit, most distal from the recent coastline, formed during MIS 7, whereas most previous studies assign it to MIS 9. Unique flourishing, high diversity and excellent preservation of corals in MIS 7 reefs reflect their growth in troughs landward of the reef-flat. The relationship of the reef zones to individual MIS might relate to the predominance of a specific zone during a certain stage, while other facies were less well developed and/or later eroded by wave action. Acknowledgements This work was financially support by the Al Neelain University, Sudan, and the Institute for Applied Geosciences, Technical University, Berlin. Much gratitude is to Prof. Dr. Mohamed Saeed, former vice president of Al Neelain University for his tremendous support. We are grateful to Prof. Dr. Thomas Brachert, Institute of Geophysics and Geology, University of Leipzig, for his allowance to perform oxygen isotope analyses in his lab, his assistance during analysis and for very useful discussions that helped tremendously to interpret the results. References Al-Rifaiy, I.A., Cherif, O.H., 1988. The fossil coral reefs of Al-Aqaba, Jordan. Facies 18, 2019e2230. Berger, A., 1978. Long-term variations of calorific insolation resulting from the earth's orbital elements. Quat. Res. 9, 139e167. Berry, L., Whiteman, A.J., Bell, S.V., 1966. Some radiocarbon dates and their geomorphological significance, emerged reef complex of the Sudan. Z. für Geomorphol. 10, 119e149. Bosworth, W., Taviani, M., 1996. Late quaternary reorientation of stress field and extension direction in the southern Gulf of Suez, Egypt: evidence from uplifted coral terraces, mesoscopic fault arrays, and borehole breakouts. Tectonics 15, 791e802. Bruggemann, J.H., Buffler, R.T., Guillaume, M.M., Walter, R.C., Von Cosel, R., Ghebretensae, B.N., Berhe, S.M., 2004. Stratigraphy, palaeoenvironments and model for the deposition of the Abdur Reef limestone: context for an important archaeological site from the last interglacial on the Red Sea coast of Eritrea.

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