Detection of the Submerged Topography Along the Egyptian Red Sea Coast Using Bathymetry and GIS-Based Analysis

ARTICLE IN PRESS The Egyptian Journal of Remote Sensing and Space Sciences (2013) xxx, xxx–xxx

National Authority for Remote Sensing and Space Sciences The Egyptian Journal of Remote Sensing and Space Sciences

www.elsevier.com/locate/ejrs www.sciencedirect.com

Detection of the submerged topography along the Egyptian Red Sea Coast using bathymetry and GIS-based analysis

Moawad Badawy Moawad *

Department of Biogeochemistry, Max Planck Institute for Chemistry, 55128 Mainz, Germany Department of Geography, Faculty of Arts, Ain Shams University, Cairo, Egypt

Received 5 October 2012; revised 29 November 2012; accepted 12 December 2012

KEYWORDS Abstract A long time ago the Red Sea was only known by small-scale bathymetric, magnetic Red Sea; anomaly maps and a few seismic reflection or refraction profiles. Therefore, detection of the major Submerged landforms; submerged coastal features was unattainable. This study is based on the integration of different data Bathymetry; sets of topography and bathymetry (e.g. the global bathymetry data set, the SRTM DTEDÒ 2, the Egypt soviet military topographic maps of scale 1:200.000 and the US army topographic maps of scale 1:250.000) to reveal the main submarine landforms that marked the continental shelf and its related slopes along the Egyptian Red Sea Coast from latitude 27°430N to the Egyptian-Sudanese border at latitude 22°000N. The study deduced that the continental shelf is noticeably influenced by the surface fault system extending eastward into the main Red Sea depression, showing the continental edge mostly like a fault-scarp of 60° anticlockwise fault plane. Sea ridges and subbasins were distinguished at the lower toe of the continental slope, which seem to be a result of a regional fold system. Two sea peaks of extinct volcanoes were recognized. Two types of submarine canyons were recognized as deep incised Messinian canyons and shallow canyons. The deep incised canyons (500 m bsl) carve the continental edge with remarkable steep walls. They might be formed as a result of the Messinian event (5.59 Ma). The shallow canyons are mostly developed during the Pleistocene lower sea level (90–130 m bsl) where the major wadis cut their water courses through the continental shelf. Some individual submerged deltas were identified, showing a close relationship with the present-day drainage system, although they were supposed to be produced by an ancestor drainage system. Notable submarine terraces were recognized at depths 20–25, 50–75, and 100–120 m bsl that are in agreement with the generalized global curve of sea-level rise since

* Tel.: +49 (0) 6131 305 6502; fax: +49 (0) 6131 305 6019. E-mail addresses: [email protected], moawad1974@yahoo. com. Peer review under responsibility of National Authority for Remote Sensing and Space Sciences.

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the LGM (23–18 ka bp). It is expected that results of this study will be helpful for shedding light on the geomorphologic evolution of the continental shelf and its surrounding landmass. Ó 2012 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction many horsts and fault blocks. Uchupi and Ross (1986) used seismic reflection profiles to distinguish oblong low relief fea- Depths of the Red Sea have been of great interest owing to the tures (gently folded structures) in the northern Red Sea, which great controversy concerning structure and evolution of the appear to have resulted because of the intrusion of the Mio- Red Sea as the best example of a young ocean that evolved cene evaporites. They divided the rift of the northern Red from an intracontinental rift to an axial oceanic trough (Guen- Sea topographically into narrow shelves, which are less than noc et al., 1988). A long time ago, the northern Red Sea was 50 m bsl, marginal slopes with reliefs of about 600 m bsl only known by small-scale bathymetric and magnetic anomaly and the main trough below 600 m depth. The central part maps (Allan, 1970; Laughton, 1970; Hall, 1977) and by a few of the main trough consists of northwest trending highs and seismic reflection or refraction profiles (Drake and Girdler, lows truncated at the north by the Gulf of Aqaba trend. 1964; Knott et al., 1966; Phillips and Ross, 1970; Tewfik and Moreover, submarine terraces and canyons have been dis- Ayyad, 1982; Guennoc et al., 1988). Therefore, neither the sub- tinguished in several localities along the Red Sea Coast, e.g. merged coastal features nor the continental shelf has got seri- Sharm El Sheikh, Gulf of Elat, Sudan, and Saudi Arabia (Em- ous attentions. The Admiralty chart of the Red Sea (C. 6359) ery, 1963; Gvirtzman et al., 1977; Fricke, 1983; Reaches et al., published in 1965 and its updates and/or reprinted versions 1987). Embabi (2004) inferred from bathymetric lines on topo- show many sporadic points that are not convincing or ade- graphic maps, scale 1:1000.000 small deltas along the Egyptian quate to ascertain the major submerged coastal topography. Red Sea Coast below the sea level in front of the main wadis. Another bathymetric map of scale 1:1000.000 was published In the central continental shelf extends from Safaga to El Qu- by the International Hydrographic Bureau, Hydraulic Depart- seir, Moawad (2008) distinguished probable submarine ter- ment-UK in 1970. The map was made of 500 m depth interval, races (100, 50, and 20 bsl), submerged like-deltas and showing many steep cliffs along the coastlines and somewhat incised canyons of 500 m depth. close to the inshore. Marginal deeps were revealed as well on both sides of the northern Red Sea that may have considerable 2. Study area tectonic significances. The accuracy of the map was poor as no radio aids covered the area and very few soundings were con- The Egyptian Red Sea Coast extends southwards from the trolled by satellite navigation (Laughton, 1970). Guennoc et al. southern limits of Gulf of Suez (27°430N) to the Egyptian- (1988) established new bathymetric maps of scale 1:250.000 at Sudanese border (22°000N), some 790 km (Fig. 1). It seems latitude 26° N with a 100 m contour interval based on the base to be straight for the most parts due to the tectonic genesis maps of Pautot et al. (1986) and Ba¨cker et al. (1975) and re- of the Red Sea itself as a major part of the African Rift Valley. vised using regional sea-beam surveys. The maps were com- The transition zone between the Gulf of Suez and the northern pleted with 12 kHz or 3.5 Hz echo-sounder profiles done Red Sea is avoided in this study, not to be confused with the during the ‘‘Geomerouad’’ and ‘‘Mines’’ cruises. Recently, Red Sea topography. The study area is a good example of a the RlV Meteor Cruise M44/3 in 1999 investigated some places passive continental margin as no collision or subduction is tak- in the northern Red Sea using a hydrosweep bathymetric mul- ing place and tectonic activity is minimal. It is generally char- ti-beam system and the narrow-beam parasound sediment acterized by narrow continental shelves as there is a little echosounder (Pa¨tzold, 2000). sediment transport by the drainage system or redistribute by On the one hand, Shukri and Higazy (1944) referred to the the longshore currents. Red Sea as probably unique to its very irregular bottom topog- General oceanography reveals that capillary waves (60.6 m) raphy (excluding the Gulf of Suez), as nature and distribution are the most common throughout the year and the averaged of its bottom deposits are unmatched in other seas. Jokela wave height ranges from 0.6 to 1.5 m (Moawad, 2008). The (1963) revealed that the Red Sea bottom topography is rough wave occasionally exceeds 3 m whereas the wind speed in- except below the shelves. He distinguished narrow shelves in creases above the normal average (6.15 and 4.5 m s1 in Hurgh- the northern Red Sea; wide shelves and narrow median valley ada and El Quseir, respectively, Climatological Normals of in the south. Braithwaite (1987) stated that depths of hundreds Egypt, 1975). It rarely exceeds 5 m during the gale wind as of meters are generally reached within a few kilometers of the the wind speed is +17.49 m s1, which mostly occurs during shoreline and there is commonly no effective continental shelf. winter and early spring (Edwards and Head, 1987). In the northern Red Sea, Tewfik and Ayyad (1982) referred to In addition, the study area spans from the micro-tidal envi- the presence of northwest trending negative gravity anomalies, ronments. Tide is semi-diurnal, has two high and two low which are believed to represent subbasins. These subbasins are waters per tidal day (Edwards and Head, 1987). Neap tide generally parallel to the coastline and separated by NW-SE ranges from 0.83 to 0.6 m in Hurghada and Halaieb corre- fault trend that is the main structural element in the area. spondingly. Tide amplitude is significantly low; varies between Other cross faults are perpendicular to the former trend. How- 0.46 and 0.48 m in winter and summer, respectively and the ever, Said (1990) assumed that the Red Sea marginal homo- maximum amplitude is about 0.5 m owing to the seasonal cline dips gently eastward from the shore and is made up of changes during the winter months (Behairy et al., 1992). How-

Figure 1 Location and bathymetry of the study area. (Boxed areas showing locations of Fig. 3). ever, tidal water inundates the coastal low lands up to a few scale 1:200.000, the US Army topographic maps of scale hundred meters (Moawad, 2008). As the longshore currents 1:250.000 and the ETM+ Landsat images. are mainly governed by the prevailing winds, they are generally The global bathymetry data set represents an estimation of very weak. The average velocity of the longshore currents is seafloor topography. The data were obtained from shipboard about 0.12 m s1 (Edwards and Head, 1987). In other words, depth soundings combined with gravity data derived from role of the coastal processes is mostly insignificant. satellite altimetry. The final production is a gridded represen- The main objectives of this study are to recognize the major tation of seafloor topography for all ice-free ocean areas within submarine landforms that marked the continental shelf and its 72° latitude. The depth data were obtained by screening 6905 related slopes, their characteristics and evolution. The study surveys from the NGDC (Marine Trackline Geophysics CD- may be helpful for further researches, as well to construct a gen- ROM version 3.2), the Scripps Institution of Oceanography eral overview about the geomorphologic characteristics and evo- and Lamont-Doherty Earth Observatory databanks, and other lution of the continental shelf and the Juxtaposition landmass. data using quality control procedures based on those of Smith (Amante and Eakins, 2009). The data were obtained from the 3. Data and methods interactive database management system GEODAS (GEOphysical DAta System) developed by the National The study depends on five data sets of topography and Geophysical Data Center (NGDC) (http://www.ngdc.noaa. bathymetry, which have been integrated and/or overlaid for gov/mgg/image/2minrelief.html). Data are available in ASCII the purposes of data analysis. The data sets are the global raster grid format including arc header using spatial resolution bathymetry data set from Smith and Sandwell (1997), the 2 min along latitudes and longitudes (about 4 km at the equa- SRTM DTEDÒ 2, the Soviet Military topographic maps of tor and 2.6 km at 45° latitude).

SRTM DTEDÒ 2 is defined as a uniform grid of elevation 2006) and cells of high flow accumulation were used to identify values, spaced at three-arc second (approximately 90 m) inter- the main channels. ETM+ Landsat images were used to define vals. Elevations are rounded to the nearest integer meter and accurately the pour points (outlets) of the main hydrographs are referenced to mean sea level defined by the WGS84 and to visually verify the output streams. Stream network EGM96 (Earth Gravitational Model 1996) geoid. SRTM ele- was derived from the flow accumulation layer by a threshold vations represent the elevations of the reflective surface (e.g. value of P150 cells, which was preferred as the resulting tree canopy, building roof, bare ground) for the radar return streams are likely corresponding to a network obtained from beam and have not been reduced to bare Earth. Accuracy of traditional methods using topographic maps of scale 1:50.000 the SRTM data is 16 m absolute vertical error (90% linear er- (Tarboton et al., 1991; Moawad, 2008). Streams were extracted ror, with respect to the reflective surface), 20 m absolute hori- by assigning the flow accumulation layer to integer values P1 zontal error (90% circular error) and 10 m relative vertical for a stream and no-data for background. Thereafter, streams error (90% linear error) (NIMA, 2000). SRTM DTEDÒ 2 were ordered according to Strahler (1957). For the purposes of was preferred for many reasons, i.e. (i) the ability of identifica- data representation, major and trunk streams are only repre- tion, delineation, and elevation determination of water bodies sented while the lower orders (i.e. orders 1st, 2nd and 3rd) were that meet the minimum size criteria, (ii) coastlines and sea level excluded. are set to 0 m, (iii) double-line drains (rivers) greater than 183 m in width can be delineated, (iv) islands are delineated if greater than 300 m in length or, for smaller islands (down 4. Results of data interpretation to14.400 m2); if at least 10% of the island’s elevations are 15 m or more above the surrounding water. Odd points (spikes The submarine topography in the study area has been divided or wells) in the elevation data were removed and voided out if into six major divisions (Figs. 2 and 3). They are: (i) continen- they exceeded the mean elevation of the surrounding (eight) tal shelf and continental slope; (ii) subbasins; (iii) sea ridges neighboring elevation posts by 100 m or more. and/or undulated seabed; (iv) sea mounts and knolls; (v) sub- Bathymetry lines were firstly digitized from thirteen topo- marine canyons; (vi) submerged deltas. The following para- graphic map sheets produced by the Soviet military of scale graphs shed light on their general characteristics. 1:200.000 and another eight topographic map sheets produced by the U.S. army of scale 1:250.000. Generally, contour interval varies from 10 m onshore and 100 m offshore. The lines 4.1. Continental shelf and continental slope were converted into points using ArcGIS v.10. Next, a raster surface was processed using ordinary kriging interpolation The continental margin is the submerged outer edge of the procedure. This procedure is preferred to determine error(s) continent. It is generally divided into two subdivisions: the of the global bathymetry data as the mean altitude varies continental shelf and the continental slope. The continental greatly between the landmass and the surrounding continental shelf starts from marks of the high water on the shore (littoral margin. Therefore, the procedure attempts to have a mean zone) and exhibit noticeable breaks in declivity from gentler residual error equal to or near zero and predict the semivari- slope near the shore to one that is much steeper (continental ances multiplied by the ordinary kriging weights based on slope) beyond the shelf break. Along the Egyptian Red Sea the variance of the vector data (Lloyd, 2007). This was carried Coast, depths of the shelf range from a very few meters out using 4518 control points derived from the topographic (1 m) down to 120 m bsl. It is mostly covered by coral com- maps. Then, these points were input into the geostatistical ana- munities. Width of the shelf changes locally, varying from a lyst to create a spherical semivariogram. After that, a predic- few kilometers (5 km) to tens of kilometers (46 km). The wid- tion surface was created and differences between the global est part ranges from 24 to 46 km in the area that extends from bathymetry data and the DEM derived from the topographic Ras Benas to the southern tip of El Foul Bay (15 km long) maps were determined along some shared localities. The pre- (Fig. 4). diction surface from kriging was subtracted from the global The continental shelf can be considered as tectonic and to a bathymetry data to get a new DEM with the error removed. lesser degree depositional margin. Its accretion is constrained The mean of the prediction error was estimated at 0.00453 primarily by sediment supply since role of the coastal processes and the RMSE (root-mean-square error) was estimated at is mostly insignificant (Moawad, 2008). However, deposition is 23.28. It is known that these values decrease as the number predominantly restricted to submerged deltas and some reef of nearest neighbors increase (Lloyd, 2007). Finally, bathyme- communities. try data (derived from topographic maps and the error re- Topography of the continental shelf is not completely flat. moved DEM) were integrated together to create the final The present-day platform slopes eastward gradually (2°–9°) digital elevation model using spatial resolution 100 m. Golden between the mean low water to the further limit of the breaker Software Surfer 9.11 was preferred for better representation of zone toward the offshore. It is mostly covered by coral reefs the data in 3D perspective views. that run parallel to the coastline (Unlimited Media, 2004). However, major wadis were extracted from the SRTM The breaker zone is defined by the outer limit of the fringing DTEDÒ 2 to clarify their relation to other submerged land- reefs as it acts as a line of breaker. Therefore, most of the forms (e.g. submerged deltas and submarine canyons). This waves are forced to be broken in the vicinity of the coastline was done by determining the directions that water will flow between 50 and 400 m in average at depths 0.5–1.5 m bsl. out of each cell to its steepest down-slope neighbor (flow direc- The platform is often disturbed by inter-lagoons (pool-like) tion) relying on the 8D method (O’Callaghan and Mark, or mud flats (Fig. 5). These lagoons have generally oval or cir- 1984). Flow accumulation function was applied to tabulate cular shapes (Moawad, 2008), and they are of few meters in for each cell the number of cells that will flow to it (Chang, depth (2 m bsl).

Figure 2 Major submerged topography along the Egyptian Red Sea Coast. (Source: extracted by visual and digital interpretation of the bathymetric data set).

Another interesting feature of the shelf is St. John’s cave at and some other small-scale sinkholes and submerged caverns. Mersa Alam that is reported by many divers in St. John’s reefs. Nowadays, development of the modern reef communities led The cave is a slightly misleading name it seems since scientifi- to shallow the shelf with numerous shoal reef areas that act cally it seems to be one of the many fissures that marked the as an extensive trapping of sediments. continental shelf at depths of 6to25 m bsl. Such fissures Regional tectonic pattern of the continental shelf is mainly are known by leader divers as tunnels or passages, of which influenced by the surface faults extending eastward into the many fissures have no direct access to the sea surface, although main Red Sea depression. Tewfik and Ayyad (1982) revealed there are always small openings to allow light penetration. that the Red Sea marginal homocline dips gently eastward The sea level dropped approximately 100 m below its pres- from the shore and it is made up of many horsts and fault ent-day level during the last glacial maximum (LGM) (23– blocks. Further offshore, seismic reflection data indicate a rel- 18 ka bp, Gasse, 2000). It is assumed that a broad part of atively narrow NW trending zone of pre-middle Miocene the shelf was exposed and subjected to subaerial erosion and horsts and tilted fault blocks. Ahmed (1972) revealed that in resulted in the development of Karst forms beside shallow can- late Pliocene and Pleistocene times there was renewed block yons and the present-day embayments (locally known as faulting accompanied by igneous intrusions. As a result, the sharms) as the wadi system carved the continental shelf. This Red Sea depression is characterized by rectilinear faults may explain the presence of the present-day coastal lagoons bordering blocks, which were rising or sinking concurrent with

Figure 3 3D perspective views showing the main submarine topography along the Egyptian Red Sea Coast. (Locations shown in Fig. 1).

the Neogene deposition. In many cases the structural axes are and/or 700 m bsl. The slope is generally narrow (1 km width) shifted laterally, probably as a result of E–W cross faults where it descends abruptly toward the deep seaﬂoor and looks (Tewﬁk and Ayyad, 1982). mostly like a fault-scarp. The form of the continental slope ex- The continental slope starts from the edge of the continen- poses normal faults trending NW–SE matching the main trend tal shelf (shelf break) and declines progressively down to 500 of the Red Sea trough with a fault plan of 60° anticlockwise

Figure 4 Topographic cross sections showing the main physiography of the continental shelf. (Locations shown in Fig. 1).

Figure 5 Geomorphology of the area surrounding sharm wadi Naqarah (left) (After Moawad, 2008) and Landsat ETM+ panchromatic image showing inter-reef lagoons at Shaab Safaga (right).

and steep slope (Fig. 4). Said (1990) cited that the major faults Sometimes width of the continental slope attains about are attributed to the initial rifting of the Red Sea and they 8 km as a result of the submarine deltaic deposits descending experienced tectonic events at the edge of the continental shelf into the seaﬂoor. No distinctive gently-sloping transition (e.g. uplift, volcanic activity) in the later stage. (continental rise) between the continental slope and the deep

Based on gravity data Tewfik and Ayyad (1982) recognized 4.3. Sea ridges many subbasins parallel to the coastline and separated by structural ridges. Using digital and visual image interpreta- Ridges are common in the vicinity of the lower tip of the con- tions 12 subbasins were recognized from the digital elevation tinental slope mostly without exceptions. Two types of ridges model. They vary in shape between longitudinal and semi-cir- can be distinguished based on their altitudes. The first one is cular (Fig. 2). They run parallel to the continental shelf except the low ridges that rise a few tens of meters above the sur- the subbasin numbers seven and 11 that run from east to west. rounding seafloor (Figs. 3 and 4). They range in elevation from Generally, the subbasins are restricted to the lower tip of the 400 to 600 m bsl. The second is the higher ridges that ex- continental slope from the east and to the sea ridges from ceed +100 m above the surrounding seafloor (Fig. 4). Summits the west with the exception of the subbasin number eight that of the ridges are generally round and differ greatly in depth lies on the continental shelf near El Foul Bay. Depth of the from 300 to 600 m bsl. The lower base of the ridges differs subbasins ranges generally between 196 m (subbasin number as well locally, ranging from 500 to 700 m bsl. Based on eight) and 853 (subbasin number nine). They vary as well in their symmetry, they might be inferred as a result of a regional width from 12 to 40 km and the general physiographic fold system. In the vicinity of the continental slope, probable

Figure 6 Topographic cross sections of the subbasins. (Locations shown in Fig. 2).

Figure 7 3D perspective views showing the sea mounts. (Locations shown in Fig. 2). normal faults separate the sea ridges from the continental 4.4. Seamounts and knolls slope with an estimated dip of a fault plane of 60°. Investigation of the geological maps revealed that some off- Seamounts are other distinctive features within the large struc- shore islands (e.g. Safaga, Sernaka Island, Mukawwa, Zabar- turally complex segment of the Red Sea. Twelve seamounts gad and the Rocky Island) are built up of the same rock were recognized, which rise surprisingly from the seafloor formation(s) as the neighboring landmass. The islands consti- and peaking below sea level (not reaching to the water’s sur- tute the small merged areas that are standing from the top of face). They range in elevation between 100 and 800 m bsl the sea ridges owing to the regional upward of the continental and have mostly circular shapes, which seem to be a result margin. Marginal upward might be a result of fault reactiva- of small volcanic cones. Seamount numbers one and two tions and major folding during the formation of the Red Sea shown in Fig. 2 in specific are distinguished as independent trough. Jokela (1963) suggested that episodic compression oc- features since they exhibit typically conical topography with curred in the direction of the Red Sea axis in pre-Oligocene very steep sides. They rise abruptly from the sea floor of time and was expressed by folding of soft sediments along 600 and 800 m depth, respectively up to 350 and the Egyptian shelf and possibly by uplift of mountains in Tur- 450 bsl in the same order, and seem to be extinct volcanoes key. Simultaneously, tension perpendicular to the sea axis with round peaks (Fig. 7). Other minor features could be dis- formed fractures parallel to the axis, some of which served tinguished as sea knoll (seamounts with flat top) as they rise as volcanic and hydrothermal vents. Marshak et al., 1992 re- gradually from the undulated seafloor, mostly due to the regio- vealed that Zabargad Island was affected by the final phase nal folding system and upthrust faulting. These knolls play an of tectonism resulting in an array of high angle faults and asso- important role in exposing tips of some reef pillars as semi- ciated folds. Together resemble palm structure typical of cross atoll or off shore islands such as Al Akhawein islands, which fracture zones in rift. The southwest corner of the island was is situated 67 km offshore east of El Quseir. The two islands subjected to large recumbent folds. It appears to be an uplifted are likely the exposed tips of two massive reef pillars that rise fragment of sub Red Sea lithosphere and it is essentially the re- from the sea. The pillars cover the pinnacles of two undersea sult of upward motion of mantle derived ultramafic bodies knolls rising from about 300 m bsl (Moawad, 2008). (Bonatti et al., 1983).

Figure 8 Examples of the deep incised Messinian submarine canyons.

Figure 9 The close relationship between the present-day Red Sea drainage system and the submarine canyons. (Extracted from the DEM, boxed areas approximately show locations of Fig. 11).

Alignment of the seamounts with the major axis of the con- flexure zone of the eastern margin of the Red Sea south of lat- tinental shelf suggests relative synchronicity of volcanic erup- itude 19° N(Coleman, 1974). Khedr (1984) cited that the west- tion with the fault and fold systems influencing the ward migration of the asthenosphere is leading to gentle continental edge. Although an absolute date is not yet avail- emergence of the continental margin, faulting and volcanic able, assumption of synchronicity suggests the seamount as extrusion in the middle Miocene. pre- and/or middle Miocene features. Said (1962) assumed that volcanism occurred in the compression zones of Egypt during the pre-Oligocene period of activity, but appears to have 4.5. Submarine canyons ceased after the Mio-Pliocene relaxation of stress in this area (Jokela, 1963). Coleman (1974) stated that during the Miocene The submarine canyons are the most conspicuous erosional time, volcanic activity continued in Afar Depression (Eritrea, forms marking the outer continental margin along the Egyp- Djibouti and the entire Afar Region of Ethiopia) and in wes- tian Red Sea Coast. They carve the deep fissures along the con- tern Saudi Arabia. However, he cited as well that during the tinental slope, which are probably produced by an ancestor formation of the axial rift (trench) in the Red Sea in the early drainage system. Shepard and Dill (1966) argued that such Pliocene (5 Ma); numerous submarine volcanoes contributed canyons appear to have had several origins and producing dis- pryoclastic rocks and lava. As a result, many alkaline olivine tinctive types. They can be distinguished within the study area basalt eruptive centers developed along the hinge line of the on the basis of their morphology as follows:

Figure 10 Shapes and depths of the submerged wadi canyons.

4.5.1. Deep incised canyons (Messinian canyons!!) Longitudinal profiles of the canyons are generally concave Deep incised canyons are the most common features that sig- and sloping strongly seaward from the head of the canyons nificantly break the continental slope. They reach 500 m be- to the deep sea. Although, the incised canyons sometimes fol- low the present-day sea level and distinguished by remarkable low the major structural features seaward from the adjacent steep walls that seem to have been cut in the bedrock (Fig. 8). land as shown in Fig. 3, they seem to be the result of a great In some cases, head of the canyons may be traced to the upper geological event that influenced the continental margin since part of the continental slope at depths 100 and 150 m bsl. they are common along the Egyptian Red Sea Coast mostly

Please cite this article in press as: Moawad, M.B., Detection of the submerged topography along the Egyptian Red Sea Coast using bathymetry and GIS-based analysis, Egypt. J. Remote Sensing Space Sci. (2013), http://dx.doi.org/10.1016/j.ejrs.2012.12.001 ARTICLE IN PRESS Detection of the submerged topography along the Egyptian Red Sea Coast using bathymetry 13 without exceptions. Moawad (2008) showed some incision 4.5.2. Submerged shallow canyons canyons spreading along the coastal area extending between The submerged lower water courses of the wadis exhibit some- 1 Safaga and El Quseir. Siroko (2008, personal communication) times small-scale canyon-like shapes, which marked mainly the believed that they are a result of the late-Miocene Messinian continental shelf and, to a lesser extent, the continental slope. salinity crises (Messinian Event!!) owing to the sudden discon- The canyons are relatively shallow as the depth varies from a nection of water supply between the Atlantic Ocean and the few meters to tens of meters (2to70 m bsl). They are char- Mediterranean sea. The Mediterranean water level dropped acterized by more or less symmetrical winding V-shaped and/ down to 1500 m below its present-day level, entailed a consid- or U-shaped (Fig. 10). Some smaller-scale tributaries, which erable increase of salinity and intense erosion of all Mediterra- have been identified from the hydrological model, are entering nean rivers such as the Rhone and Nile canyons. The erosive the canyons from both sides. The resemblance of submarine phase is supposed to have occurred between 5.59 and canyons to river-cut canyons on land and the juxtaposition 5.50 Ma and followed by deposition of non-marine sediments of many land and sea canyons has always provided a strong between 5.50 and 5.33 Ma (Krijgsman et al., 1999). Stoffers argument that at least the heads of the submarine canyons and Ross (1974) argued that there was a large relief differences were originally cut by rivers (Shepard and Dill, 1966). Mor- between the Red Sea depression and the surrounding land dur- phometry of such shallow canyons reveals that they are prob- ing Miocene time. Therefore, the Red Sea depression received ably developed during the Pleistocene lower sea level some 90– marginal clastic sediments (2–3 km thick) interfingering with 130 m below the present sea level (Reaches et al., 1987). As a evaporites (3–4 km thick) and the Red Sea depression contin- low raised beach borders the present-day coastline (1.5 m), ued to subside (Ahmed, 1972; Coleman, 1974). In context, it can be inferred that an episode(s) of cut and fill related to Gillmann (1968) inferred rapid erosion at the Yemen-Hail the sea level changes influenced the shallow canyons. Role of and Ethiopian arches during Miocene. It can be deduced that subaerial erosion as well was proposed by Gvirtzman et al. an old drainage system was the major carrier of such sedi- (1977). He assumed that the lowest erosional base level was ments, especially during middle-Miocene time as the climate 120 m bsl in the Red Sea during the last glacial maximum was warm humid (Khedr, 1984). As a result, a great bulk of (LGM). the Tertiary sedimentary rocks was swept out from the Mio- In general, position and configuration of the submarine cene highlands in the Eastern Desert of Egypt. Sedimentologic canyons are controlled by multiple factors, including structural 0 0 evidences obtained from Jabal El Rusas (25°11 –34°46 E) clar- fabric, tectonism, sea-level variations, and sediment supply ified that the lower unit unconformably overlies the basement (Laursen and Normark, 2002). However, idea of marine origin rocks. The unit consists of 2–3 m orthoconglomerate with of the canyons as supposed by Shepard (1972) is improbable in angular and round clastic, which range from large boulders the Red Sea as no large active marine erosion has been found to granular. Khedr (1984) assumed that sedimentation of this throughout the geological evolution of the Red Sea. Even the unit begun in middle Miocene. Small pockets of the Tertiary idea of turbidity currents that suggested by Daly (1936) is un- sedimentary rocks are still preserved in the morphotectonic likely since there is no powerful current in the Red Sea, as far depressions in the Central Eastern Desert of Egypt such as Ja- as known to date. It is believed that the role of marine erosion 0 bal Duwi (26°11 N–34°E) (Moawad, 2008). might have been restricted to some form modifications. Although, role of the recent fluvial process is insignificant, Fricke (1983) denoted that flash floods often of catastrophic nature, which influence the Red Sea region, carry large 4.6. Submerged deltas amounts of sediments into the sea. Accordingly, he suggested that occasionally gravity flows fed initially by sediments from The general configurations of the bathymetric lines reveal sub- aligned wadis are a major agent in cutting the deep submarine merged fan-like shapes of different sizes either partly of fully canyons into steep edges of the narrow Red Sea shelves. Fig. 9 extended as a part of the continental shelf. The fan-like shapes shows a close relationship between the present-day drainage are located seaward of the present-day drainage system. By networks and the bathymetry. It is notable, as an example, following some criteria of the alluvial fan definitions (e.g. mor- that many wadis were debouching into El Foul Bay. The wadis phology of the bathymetric lines, location at topographic are located some 25 km westward of the head of El Foul Can- breaks), Embabi (2004) and Moawad (2008) assumed that they yon. The major trend of the streams and their tributaries is of represent submerged deltas of an ancestor Red Sea drainage W–E orientation. The main wadis range from 100 to +200 km system in the Eastern Desert of Egypt. Toes of the deltas are long and their heads are incised in the basement rocks. An- plunging down into the deep sea for 500 m below the other evidence obtained from Jabal Abu Shaar (27°220N– present-day sea level. Individual deltas are bounded laterally 33°35E) and Jabal El Rusas (25°110N–34°460E) revealed that by incised canyons, troughs, channels, and therefore the bathy- the present-day drainage system is typically superimposed metric lines became more crenulated. The canyons carving the (overprinted) on the basement rocks. Therefore, the present- submerged deltas play a significant role in sediment transport day wadi system is mostly inherited from an older one that mostly during catastrophic flash floods. Sometimes, the lateral may be back to pre-middle Miocene as the climate was shifted boundaries are less distinctive between the adjacent deltas as in in Oligocene toward a Sudano-Guinean type of savannah (Le the case of delta wadi El Ambagi that seems to be a result of Houe´rou, 1997). some coalescing deltas (Fig. 11a). Correlation between individual wadis is not fully conform- able, as the present-day drainage system might have been 1 Prof. Dr. Frank Sirocko: department of climate and sediments subjected to meandering, shifting during the Pleistocene lower ‘‘Johannes Gutenberg-Universita¨t-Mainz, Germany (personal sea-level and/or tectonic events. However, some prolonged communication). channels have been traced in the shallower part of the shelf

Figure 11 3D perspective views showing the inferred submerged deltas related to the main present-day drainage system. (Locations shown in Fig. 9). such as wadi Queih (Fig. 11a) that seems to have cut the con- type of continental margin, relief and tectonic setting of the tinental shelf during the maximum low stand of sea-level. The hinterland, continental shelf-slope relief, nature of basinal landward side of the delta is found just beyond the point where areas and basin size, distance from the source region, width the recent trunk stream of wadi Queih is debouching into the and gradients of the shelf-slope, and type and amount of sed- Red Sea. iment supplied. The second group includes the secondary con- Generally, the submerged deltas seem to be constructed trols, i.e. climatic nature and vegetation of the source area, and from consolidated alluvial deposits, as processes of diagenesis high rainfalls during sea-level changes. that resulting in compaction, cementation, and lithification re- Marine terraces are one of the most important recognized quire millions of years to transform the sediment into sedimen- features that marked the submerged deltas as flat surfaces of tary rocks (National Research Council, 1996). Stow et al. different extents. During Pleistocene and Holocene, the Red (1985) and Kolla and Macurda (1988) determined the factors Sea experienced dramatic eustatic changes resulting in a se- controlling the development of such submarine system into quence of raised coral terraces and alluvial terraces. Moawad two groups. The first group includes the primary controls i.e. (2008) distinguished three submarine terraces at 20, 50,

16 ka bp). The sea level rose rapidly about 10–15 m in less than Table 1 Locations of some submarine terraces below sea- 500 years during the MWP-1A0 and followed by another in- level. crease about 16–24 m during the MWP-1A. Terrace 50– Location Coordinates Terraces (m) 75 m bsl is probably in accordance with the melt-water pulse- Sharm El Arab 26°5802600N–33°550200E20 1B (11.5–11 ka bp) when the sea level may have jumped by W. Naqarah 26°410100N–34°000900E 20–25 28 m. Terrace 20–25 m bsl is mostly formed during the fourth W. Safaga 26°3802700N–33°5904000E 20, 50, 100 melt-water pulse (MWP) 1C (8.2–7.6 ka bp) that is in accor- W. Queih 26°2101500N–34°0905700E 50–75, 100–120 dance with the Flandrian Transgression. It is believed that 0 00 0 00 W. Abu Hamra 26°17 7 N–34°12 14 E20 the shallow submarine canyons were probably formed during 0 00 0 00 Um Gheig 25°43 57 N–34°33 26 E 20–25 the LGM and the present-day embayments were submerged during the Flandrian Transgression. The submarine terraces in the study area are in accordance and 100 m bsl, which are mostly a result of the last glacial with the submarine terraces recorded in southern Sinai at 120, maximum (LGM) low sea-level (100–120 m bsl). It was as- 60, and 20 m bsl (Gvirtzman, 1994). Terraces 100–120 and 50– sumed in this study that the terraces are in agreement with 75 m bsl are in agreement with terraces 90 and 60 m bsl in the generalized curve of sea level rise, which has been proposed Sharm El Sheikh and Ras Burka (Fricke, 1983; Reiss and by Gornitz (2007), since the last ice age. Based on the analysis Hottinger, 1984) and terraces of Elat (90–50 m bsl) (Reaches of bathymetric data, notable submarine terraces were found in et al., 1987). Terraces of Elat have likely developed between different locations and levels along the coast as shown in 70–50 ka bp, when the sea level was 100 to 60 m below the Table 1 (Fig. 12). They constitute mostly ﬂat narrow surfaces present sea-level (Reaches et al., 1987). and differ locally in depth and width. In comparison with the Evidences obtained from the neighboring landmass clariﬁed generalized curve of sea level rise since the LGM (23–18 ka that the wadis cut through the present-day alluvial fans exhib- bp), it can be inferred that the terrace 100–120 m bsl may be iting several pairs of alluvial terraces that are in correspon- in correspondence to the last glacial maximum (LGM) in the dence with the global Pleistocene eustatic sea-level changes age range of melt-water pulse (MWP) 1A0 and 1A (19– (Table 2). It is worth mentioning that the peripheral parts of

Figure 12 Topographic cross sections showing the main physiography of the submarine terraces below the present-day sea-level.

Table 2 Altitudes of the fluvial terraces along the Egyptian Red Sea Coast. Wadi Coordinates Heights (meters above the wadi floor) Safaga 26°3601800N–33°5604300E 2.5–3, 6–7 Gasus 26°3304500N–33°5904100E 5–6; 12–13 Abu Safi 26°2603300N–34°050400E 5–6 Abu Hamra 26°1601900N–34°100100E 6–8; 15 Hamraween 26°1404200N–34°1003800E 5–6; 13–15 Queih 26°210500N–34°0604400E 10–12, 15–16 El Quseir El Qadim 26°0901200N–34°1403600E 5–6 An-Nakheil 26°0802900N–34°0802600E 1–1.5 El Ambagi 26°0602400N–34°1402500E 1–1.5; 4.5–5; 10–12; 12–21; 30–40; 60 Um Khariga 25°304800N–34°500000E 1–2; 6–7; 12–14 Seifan 25°60500N–34°5103800E 2–3; 4; 6; 10 Mubaraka 25°3003200N–34°3804800E 1–2; 8–14; 20–30 Um Gheig 25°4301600N–34°3202400E 1–2; 5–8; 13; 20–30 Source: Arvidson et al., 1994 and field work.

Please cite this article in press as: Moawad, M.B., Detection of the submerged topography along the Egyptian Red Sea Coast using bathymetry and GIS-based analysis, Egypt. J. Remote Sensing Space Sci. (2013), http://dx.doi.org/10.1016/j.ejrs.2012.12.001 ARTICLE IN PRESS 16 M.B. Moawad the NW Red Sea rift were inactive during the last 150– Two types of submarine canyons were recognized as deep in- 120 ka bp (Hoang and Taviani, 1991; Arvidson et al., 1994; cised canyons and shallow canyons. The deep incised canyons El Moursi et al., 1994; Plaziat et al., 1995; Orszag-Sperber carve the continental edge with remarkable steep walls that et al., 2001) and the coast attained its present-day shape mostly seem to have been cut in the bedrock down to depth after the Flandrian transgression during the last 8–6 ka bp 500 m bsl. They might be formed as a result of the Messinian (Goudie, 2004). event (5.59 Ma) owing to the sudden disconnection between the Atlantic Ocean and the Mediterranean during late Miocene 5. Discussion and conclusions times. Additionally, it is estimated that the shallow canyons are mostly developed during the Pleistocene lower sea level A long time ago the Red Sea was only known by small-scale (some 90–130 m bsl) where the major wadis cut their water bathymetric, magnetic anomaly maps and a few seismic reflec- courses through the continental shelf. Some individual sub- tion or refraction profiles. Therefore, detection of the major merged deltas were identified based on the configuration of submerged coastal features was unattainable. In this study the bathymetric lines and their location at the topographic submerged features along the Egyptian Red Sea Coast from breaks. A close relationship has been found between the pres- latitude 27°430N to the Egyptian-Sudanese border at latitude ent-day drainage system and the deltas. It is inferred that the 22°000N were identified based primarily on the global bathym- present-day drainage system is mostly a successor to an ances- etry data set from Smith and Sandwell (1997), bathymetry ob- tor drainage system that was mostly developed during middle tained from topographic maps of scale 1:200.000 and the US Miocene as the climate was turned toward warm humid (Khe- army topographic maps of scale 1:250.000. The Surface cre- dr, 1984) or pre-middle Miocene as the climate was shifted to- ated from the topographic maps has many gaps owing to lack ward a Sudano-Guinean type of savannah since the Oligocene of detailed bathymetric lines. Therefore, the global bathymetry (Le Houe´rou, 1997). data were integrated to overcome the problem of lack of infor- During Pleistocene and Holocene times the Red Sea experi- mation. Spatial resolution of the global bathymetry data is enced dramatic eustatic changes resulting in a sequence of about 2 min along latitudes and longitudes. Linear regression raised coral terraces and alluvial terraces. Three main submar- analysis reveals that the spatial accuracy ranges in the study ine terraces were identified at depths 20–25, 50–75, and 100– area from 3.3 km at latitude 22° N to 3.1 km at latitude 28° 120 m bsl. Generally, they are in agreement with the general- N. That means the data are only adequate for recognizing fea- ized curve of sea level rise since the LGM (23–18 ka bp). tures that have an extent of P3.1 km. Although Kriging inter- Hence, it is deduced that terrace 100–120 m bsl is equivalent polation procedure was used to create a smooth surface of cell to LGM, terrace 50–75 m bsl ranges in age between 11.5 and size 100 m, the procedure does not increase accuracy of the 11 ka bp and terrace 20–25 m bsl is mostly formed during the foot print of the original data. However, valuable information Flandrian Transgression (8.2–7.6 ka bp). The Red Sea attained has been obtained that manifest the main characteristics of the its present-day level mostly during the last 5–4 ka bp and the major submarine forms. Extracting smaller-scale features (e.g. formation of the present-day estuaries may also be in accor- estuaries, coral reefs, and submerged shallow water-courses) dance with this period. It is expected that results of this study may need another data source (e.g. detailed multi-beam or will be helpful for shedding light on the geomorphologic evolu- echo-sounding survey) or enhanced digital image processing tion of the continental shelf and its surrounding landmass. techniques that can be applied to high-resolution satellite images (e.g. ASTER, Quickbird, Orbview-2). The latter field Acknowledgements is open for more innovative approaches to estimate bathymetry from optical remote sensing data on the basis of numerical The author would like to acknowledge the anonymous review- models and algorithms. The Egyptian Red Sea Coast in spe- ers for their valuable comments and suggestions to improve cific deserves further researches to manifest the characteristics the quality of the manuscript. of the micro-submerged coastal landforms. The major objective of this study is to clarify the general characteristics of the major submerged features. 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