We examine the evolution of the northwestern Red Sea, Egypt, by study of the Quseir–Umm Gheig subbasin. The subbasin records two main tectonic events. The first event is related to development of Late Cretaceous synclinal basins due to sinistral movement along the reactivated Najd fault system. Evidence for this includes: (1) the Cretaceous basins are concentrated mainly in the central Eastern Desert, which represents the main influence zone of the Najd fault system, (2) folds are not everywhere parallel to the faults and their axes are curvilinear, (3) the faults dislocated the axial plane of the synclines, (4) the Cretaceous basins occur in an en-echelon arrangement, (5) there is a difference of 20° between the orientation of the sinistral strike-slip shear zones and the associated en-echelon synclinal folds, (6) principal stress directions are delineated by subhorizontal σ1 and σ3 and subvertical σ2, (7) sheared conglomerate is detected in the Nubia Formation, (8) minor overturned folds and minor NE-vergent thrusts occur in the Duwi and Dakhla Formations, and (9) there is a predominance of NE-SW normal faults in Cretaceous–Eocene sequences. The second event is related to the sinistral movement along the NNE-SSW Aqaba–Dead Sea transform and dextral movement along Queih and Hamrawin shear zones. This movement was synchronous with northeast extension of the Red Sea. The structures developed during this movement include: (1) NW-trending extensional faults, (2) extensional fault-related folds in Miocene-Pliocene deposits, and (3) buckle folds in Pliocene and post-Pliocene sequences. Buckle folds were developed during NW compression associated with sinistral movement along NNE-SSW strike-slip faults. Gypsiferous shale-rich beds in Miocene-Pliocene rocks played the main role in development of fault-related folds and buckle folds in the Quseir–Umm Gheig subbasin.
It is generally accepted that the main Red Sea extension started 30 m.y. ago during the late Oligocene–early Miocene and reactivated the steep NW-trending late Pan-African shear zones (McKenzie et al., 1970; Meshref, 1990; Moustafa, 1997; Purser and Bosence, 1998; Khalil and McClay, 2002, 2009). The initial rift occurred in response to the NE separation of the Arabian plate from the African plate (Nubia), and basins within the Red Sea rift were generally asymmetric, 60–80 km wide half grabens (Bosworth et al., 2005). The extension direction was N60°E during the late Oligocene to Miocene (Bosworth and McClay, 2001). The formation of Cretaceous basins and orientation of rift-related normal faulting were strongly controlled by the presence of a preexisting Precambrian fault zone (Dixon et al., 1987; Bosworth, 1994; Younes et al., 1998; Ghebreab, 1998; Ghebreab and Talbot, 2000; Younes and McClay, 2002; El Shemi and Zaky, 2001; Khalil and McClay, 2002, 2009; Gawthorpe et al., 2003; Bosworth et al., 2005; Guiraud et al., 2005; Jackson et al., 2006a, 2006b) such as the NW-trending shear zone of the Najd fault system (Davies, 1984; Stern, 1985).
The Cenozoic Red Sea rift belongs to a rift system that includes the East African rift in the south and the Gulf of Aden and the Gulf of Suez in the north (Bosworth et al., 2005; Guiraud et al., 2005; Kinabo et al., 2007). These rifts were initiated in the late Oligocene (Rupelian) to Miocene in several small, en-echelon, approximately E-W– to ESE-WNW–trending basins in the Gulf of Aden province (Fantozzi and Sgavetti, 1998; Watchorn et al., 1998), and they fragmented the Arabian-Nubian Shield (Martinez and Cochran, 1988).
Several rifting mechanisms have been proposed for the Red Sea (reviewed in detail by Ghebreab, 1998); they include: (1) prolonged normal faulting (e.g., Lowell and Genik, 1972), (2) lithospheric thinning by faulting and dike injection (Berhe, 1986), (3) diffuse extension followed by brittle deformation (e.g., Martinez and Cochran, 1988), (4) lithospheric simple shear (Voggenreiter et al., 1988), (5) combinations involving detachment faults and prolonged magmatic expansion (Bohannon, 1989; Bohannon and Eittreim, 1991), (6) asymmetric rifting (Dixon et al., 1989), and (7) pull-apart basin(s) (e.g., Makris and Rhim, 1991). The major differences between the various models center on the relative timing of updoming, rifting, and magmatism, and whether the rifting was active and driven by a mantle plume or passive and due to lateral extension of the lithosphere leading to reactive effects in the mantle (Ghebreab, 1998). The models that invoke graben-horst formation along steep normal faults are supported by the earlier semibrittle stage of extension that corresponds to the predicted low-angle simple shear zone through the lithosphere (Ghebreab, 1998).
In order to examine the evolution of Red Sea continental rifting, Bosworth et al. (2005) distinguished three phases of rifting: (1) late Oligocene–early Miocene rift initiation; (2) early Miocene main synrift subsidence; and (3) middle Miocene onset of the Aqaba–Dead Sea transform. The Red Sea rift initially included the present Gulf of Suez, Bitter Lakes, and Nile Delta region on the continental margin of North Africa (Bosworth and McClay, 2001).
A magnetic trend analysis carried out for the Gulf of Suez–Red Sea region from both regional and residual magnetic maps (Meshref, 1990) indicates the presence of the following regional magnetic trends: (1) Gulf of Suez–Red Sea, Erithrean, or Clysmic (NW) trend of mid-Tertiary age, (2) meridional or East African (N-S) trend of Precambrian age, (3) trans-African, Qena-Safaga, Idfu-Mersa Alam, or Aualitic (NE-SW) trend, (4) Tethyan, Mediterranean, or Sheikh Salem (E-W) trend of Paleozoic–Jurassic age, (5) Najd (WNW) trend of Precambrian age, (6) Atalla (NNW) trend of Precambrian age, and (7) Gulf of Aqaba, Dead Sea, or Aqaba (NNE) trend of mid-Tertiary age.
The study area occurs in the Eastern Desert of Egypt between Quseir and Umm Gheig along the Red Sea coast (Fig. 1). The Quseir–Umm Gheig region contains the southernmost exposures of the pre–Red Sea rift stratigraphic section of the uplifted Egyptian continental margin. The well-exposed Cretaceous–Pleistocene stratigraphic successions (Figs. 1 and 2) are subdivided tectonically into two major categories (Said, 1990): the prerift sequence (Precambrian to Eocene) and the synrift sequence (Oligocene–Pleistocene). These stratigraphic units were remarkably affected by the tectonic evolution of rifting. The prerift structures are variably enhanced and disrupted by the synrift structures. The structural architecture and tectonic evolution of the northwestern part of the Red Sea are still not fully understood. Understanding and reconstruction of the tectonic evolution of the northwestern Red Sea and evaluation of the Quseir–Umm Gheig subbasin, Cretaceous basins, and fault-related folds are the main objectives of this study. This study was based on detailed field mapping using aerial photographs (1:40,000) and Landsat images (1:250,000) and analysis of field and structural data collected from both the prerift and synrift rocks.
STRATIGRAPHY AND STRUCTURAL FRAMEWORK
Precambrian Shear Zones
The sequence of structural events in the Precambrian rocks of the central Eastern Desert of Egypt (Fritz et al., 1996, 2002; Shalaby et al., 2005; Abd El-Wahed, 2007, 2008, 2009; Abd El-Wahed and Abu Anbar, 2009) is as follows: (1) oblique island-arc accretion accompanied by NNW-SSE shortening produced low-angle NNW-vergent thrusts, open folds, imbricate structures, and thrust duplexes in the Pan-African nappe (low-grade volcano-sedimentary rocks), (2) a ENE-WSW compression event created NE-vergent thrusts, folded the NNW-vergent thrusts, and produced NW-trending major and minor folds in the Pan-African nappe, and (3) sinistral shearing related to the Najd fault system developed along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the core complexes. Sinistral shearing produced steeply dipping mylonitic foliation and plunging folds in the NNW- and NE-vergent thrusts.
The NW-striking strike-slip faults (650–540 Ma) of the Najd fault system represents one of the major sinistral shear zones that cut through the Arabian-Nubian Shield (Moore, 1979; Stern, 1985, 1994; El-Rabaa et al., 2001; Johnson and Kattan, 2001). It developed in the crust of central Arabia during the Proterozoic postorogenic stage as a result of convergence of a continental fragment from the east that was accompanied by E-W compression and N-S extension in the form of an escaping block (Stern, 1994).
Many NW-striking strike-slip shear zones (Fig. 2) have been recognized in the Precambrian rocks of the central Eastern Desert, e.g., Meatiq, Sibai (Fritz et al., 1996, 2002; Abd El-Wahed, 2008, 2009, and references therein), Hamrawin, and Queih shear zones (Abdeen et al., 1992; Moustafa, 1997; Abdeen and Greiling, 2005). Both sinistral and dextral movements are documented along the Hamrawin and Queih shear zones (Abdeen et al., 1992). Left-lateral slip on these faults was related to movement along the Najd fault system, whereas right-lateral slip was related to rifting in the northern Red Sea in the late Oligocene (Moustafa, 1997).
Quseir-Umm Gheig Subbasin and Duwi Accommodation Zone
The term “accommodation zone” refers to a complex zone of faulting that accommodates along-strike change in both the fault dips and in subbasin polarity within a rift system (similar usage to that of Bosworth, 1985; Jarrige et al., 1990; Faulds and Varga, 1998; Khalil and McClay, 2002, 2009; Younes and McClay, 2002). They are also known as transfer zones. Transfer zones exist between two parts of different structural style, such as between neighboring half grabens of different dip directions, and they represent the areas through which throw is changed from the bounding fault of one half graben to that of the next (Jarrige et al., 1990; Moustafa, 1997). Many authors divide the Gulf of Suez–northwestern Red Sea rift into four subbasins or half grabens separated by tectonic accommodation zones (e.g., Moustafa and Fouda, 1988; Jarrige et al., 1990; Moustafa, 1997; Khalil and McClay, 2002, 2009). The area between Safaga and Quseir along the Red Sea coast (Fig. 2) is occupied by the northern and southern parts of two major subbasins, named here Safaga and Quseir–Umm Gheig subbasins, respectively (Fig. 2). The two subbasins are separated by the Duwi accommodation zone. Each subbasin consists of a number of rift blocks. Safaga subbasin contains five rift blocks (Gebel Um Tagher, Mohamed Rabah, Gebel Gassus, El Um Huetat, and Gebel Wasif blocks) showing constant NW dip and an average dip of 18°.
Detailed mapping shows that the Quseir–Umm Gheig subbasin is delimited to the south by the NW-trending Queih and Hamrawin shear zones. Seven rift blocks down faulted against Precambrian rocks with a NE dip (average dip of 20°) are located in the study area. These are the Gebel Um Hammad–Gebel Duwi, Anz-Ambagi, Gihania, Gebel Atshan, Zug El Bahar, Gebel Hamadat, and Sharm El Bahari blocks (Figs. 2 and 4). The Quseir–Umm Gheig subbasin and its rift blocks are dominantly elongated northwestward. The main elongation of these rift blocks changes from NW-SE in Quseir–Umm Gheig subbasin to approximately N-S in Safaga subbasin. Directions of dip in Cretaceous sediments of the Gebel Atshan and Hamadat fault blocks are different, e.g., NE, E, SW, W, and NW, delineating the shape of NNW-plunging Atshan and Hamadat synclines (Figs. 5A and 5B). Bedding in Eocene–Pleistocene formations dips mainly NE, forming a NE-facing monocline dissected by extensional normal faulting and strike-slip faulting (Fig. 5C and 5D). In general, the amount of dip decreases northeastward, from 40° close to the Precambrian rocks to nearly horizontal close to the Red Sea shore.
Dip angles in pre-Miocene and Miocene sediments (Fig. 6) range between 15° to 25° but locally reach up 40°, especially in the Nubia Sandstone. In Pliocene rocks, these values decrease gradually to 7°–14°, then continue to around 5° in Pleistocene raised beaches (Figs. 4 and 6).
The observed structures in the Quseir–Umm Gheig subbasin include fault blocks and subblocks, major synclines in the Cretaceous–Eocene sequence, NE-SW and NW-SE normal faults, a major monocline constituting the Quseir–Umm Gheig subbasin, NNE-SSW, NE-SW, and NW-SE strike-slip faults, minor anticlines and synclines in Miocene rocks, and buckling and gypsum folds in Miocene-Pliocene rocks.
Late Cretaceous Synclines
Two main Cretaceous basins are present in the study area: Hamadat–Zog El Bohar basin and a part of Duwi-Nakheil-Atshan basin (Figs. 2 and 4). The axes of these basins trend mainly NW-SE and N-S, subparallel to the main trend of the major shear zones in the Precambrian rocks. Deflection of their axes from NW-SE to N-S may be assigned to sinistral movement along NNE-SSW and NE-SW strike-slip faults. The arrangement of these synclines suggests en-echelon orientation. These basin-shaped synclines are occupied by four fault blocks (Gihania, Gebel Atshan, Zog El Bohar, and Gebel Hamadat blocks) and four major asymmetric, doubly plunging synclines in the Atshan and Hamadat areas (Figs. 4 and 5). These synclines have been described previously by many authors (e.g., Moustafa, 1997; Khalil and McClay, 2002, 2009), and they represent landmarks in the central Eastern Desert of Egypt. They are described as extensional fault-related folding (Khalil and McClay, 2002, 2009) and as Late Cretaceous basins nucleated as small pull-apart basins by reactivation of the Najd lineament (Bojar et al., 2002). They trend NNW to NW and plunge gently N and NW near their southern end and gently SSE and SE farther north.
The Atshan syncline represents the southern continuation of the Wadi El-Nakheil syncline. The Nakheil syncline is characterized by a curvilinear axial-trace where its strike changes from NW-SE at the northern part to NNW-SSE at its southern part (Fig. 4). The Atshan syncline is asymmetric: the W-dipping limb is steeply dipping compared to the NE-dipping limb. The closure of the southern termination of the Nakheil syncline plunges 20° to north, and a major NNE-SSW oblique-slip fault dislocated it from the NE-dipping limb of the Atshan syncline. The southern nose of the Atshan syncline plunges 30° to north. Stereographic plots of the Atshan syncline and the southern termination of the Nakheil syncline (Fig. 6) show moderate scattering of poles to bedding with NNW- and SSE-plunging shape of folds.
The Hamadat fault block consists of three separate, doubly plunging asymmetric synclines bordered by a NW- to NNW-trending border normal fault dipping steeply SW. The axes of these synclines plunge NNW and SSE, parallel to the major trend of the border fault (Figs. 4 and 5). The W- and SW-dipping limbs are steeply inclined compared to the NE-dipping limbs. The SW-dipping limb of the northern syncline dips steeply (30°–70°) compared to the other synclines. The arrangement of these three synclines suggests an en-echelon pattern. The northern syncline represents the larger one and consists of a Cretaceous–Oligocene sequence, where the Nakheil Formation exists in the trough of the syncline (Fig. 5A and 5B). This formation is absent in the other two synclines. The northern and southern synclines are steeply plunging compared to the central one. The central syncline is highly eroded, and its axis is displaced from that of the southern syncline by the Wadi Essel strike-slip fault. There are two narrow transverse anticlines between the three synclines (Khalil and McClay, 2002). A stereographic plot of the Hamadat synclines (Fig. 6) shows strong scattering of poles for bedding due to the doubly plunging character of folds.
Wadi Aswad Subblock
The Wadi Aswad subblock is a part of the Zog El Bohar fault block (Figs. 2 and 4) and consists of Cretaceous–Eocene rocks that rest unconformably over Nubia Sandstone. This sequence includes the Quseir, Duwi, Dakhla, and Thebes Formations. The contact between the Duwi and Dakhla Formations is marked by black shale. This subblock forms an S-shaped topographic feature clearly visible on Landsat images (Fig. 7). The orientation of bedding changes from N60°W/20°NE in the northern part of the subblock to N45°W/20°NE in the central part and to N20°E/22°SE in the southern part (Fig. 4). The northern and central parts are separated from the southern part by a NW-SE dextral strike-slip fault. This fault hinders the continuation of the Duwi and Dakhla Formations to the southern part of this rift subblock. The Dakhla Formation contains abundant gypsum veins that exhibit high plastic deformation compared to the Duwi and Thebes Formations, which exhibit only brittle deformation. Both the northern and central parts are characterized by the presence of minor imbricated NE-vergent thrusts (Fig. 8A) and NW-trending overturned (Fig. 8B) and recumbent folds developed only in Dakhla shale. The observed overturned fold is dissected by a series of minor thrusts. These thrusts cut across competent Duwi Formation and die out downward and upward (Fig. 8C). The overturned folds in Dakhla shale can be interpreted as detached folds over a ductile décollement conditioned by the presence of a thick unit of Paleocene shales underlying the Eocene succession and overlying the Cretaceous deposits. On the other hand, the black shale and Thebes Formation are clearly dislocated by the NNW-striking normal faults (Fig. 8D).
Geometry and Kinematic Data of the Faults
A series of NE-SW–trending normal faults is remarkably well-exposed in the Cretaceous–Eocene sequence at Wadi Quseir El Qadim (Fig. 8E) and Wadi Essel. Also, scarce NE-trending reverse faults are also observed in the Duwi Formation interbedded with chert at Wadi Essel (Fig. 8F). These faults strike N50–55°E and dip commonly 55–65°SE and less commonly to the NW (Fig. 6). Kinematic indicators such as rock facets, slickolites, and normal separation of stratigraphic units indicate dominantly normal slip. Several NE-trending normal faults are also recorded in Pleistocene rocks of Wadi Sharm El Bahari, Wadi Umm Gheig, and around Quseir city.
Slickenside data from the NW-SE–trending normal faults recognized in the Miocene-Pleistocene rocks indicate dominantly dip-slip movement (Figs. 4, 5, and 6). The strike of faults and dominance of normal slip are concordant with the direction of extension that led to the opening of the Red Sea. Faults in the Quseir–Umm Gheig region can be subdivided into three groups related to three phases of extension: (1) NW-SE–trending normal fault bordering the Cretaceous–Eocene sedimentary basins, (2) two systems of extensional normal faults, the Border fault system and Coastal fault system (fault terminology after Sharp et al., 2000), and (3) strike-slip and oblique-slip faults.
The Cretaceous–Eocene basins occupied by the Gihania, Atshan, and Zog El Bohar blocks (Figs. 2 and 4) are bordered by two normal faults linked together to form a V-shaped basin (Fig. 4). One of these faults strikes NNE-SSW and dips toward WNW, whereas the second fault strikes NNW-SSE and is inclined toward ENE. NNE-SSW–trending normal faults are not observed in Miocene-Pleistocene rocks. The Hamadat syncline is bordered by two major NW-SE–trending normal faults forming one of the remarkable half grabens in the central Eastern Desert of Egypt. The normal faults bordering these basins form a conspicuous zigzag pattern marking the eastern edge of the Precambrian outcrop. The extensions of the normal faults are hindered by the effect of the strike-slip faults.
The Coastal and Border fault systems include the major normal faults separating Phanerozoic rocks from NE and SW (Fig. 6) and are largely dissected by NE-SW, NNE-SSW, and ENE-WSW strike-slip faults. Khalil and McClay (2002) determined the stratigraphic throws of both Coastal and Border fault systems to be 0.5–2 km and 1.5–3.5 km, respectively, on the basis of topographic offset of Precambrian rocks and prerift strata.
Coastal and domino-style normal faults are moderately to steeply seaward-dipping, and most show strike parallel to the Red Sea axis. They include several master and intrablock faults. They control the contacts between different rock units, especially between Cretaceous and Miocene and between lower Miocene and middle Miocene rocks. Within the monocline, major and minor normal faults strike NW-SE and dip steeply 60°–80° to NE or SW (Fig. 6).
Two main master faults are traced in the Quseir–Umm Gheig region, one exists between the middle and upper Miocene rocks, and the other is situated between the middle Miocene rocks and the older sedimentary rocks (e.g., lower Miocene, Cretaceous, and Precambrian rocks). They are named here Quseir (QMF) and Umm Gheig master faults (GMF), respectively. They extend northwest for ∼50 km in the area between Quseir and Wadi Umm Gheig. These master faults are dissected and dislocated by sinistral movements along strike-slip faults. The secondary structures observed adjacent to these master faults are fault-related folding of meter scale (10–300 m) and minor normal faults. A series of NW-SE–striking normal faults has been observed, including oppositely dipping normal faults (Figs. 9A and 9B), steeply dipping (60°–80°) (Fig. 9C), scarce listric (fault dip decreases with depth) (Fig. 9D), convex upward (fault dip increases with depth) normal faults, and reverse faults (Fig. 9E). Footwall meter-scale anticlines and hanging-wall synclines (Fig. 9D) were developed due to overlapping of NE- and SW-dipping macroscale normal faults (Fig. 9C). The dips of intrablock normal faults increase toward the master faults. These faults are either synthetic or antithetic to the master faults and form a series of horsts, grabens (Figs. 8A and 8B), and step-like normal faults (Fig. 9F) of variable length. A clear example of a major horst and graben system exists along the northern bank of Wadi Sharm El Bahari, where Ranga Formation, Precambrian rocks, Nubia, Qusier, Um Mahara, and Abu Dabbab Formations are faulted against each other (Figs. 4 and 5C). Displacements along these faults range from a few centimeters to tens of meters. Kinematic indicators, such as rough facets, slickenlines, and normal separation of stratigraphic units indicate normal slip. The presence of a listric normal fault is interpreted in the cases where the hanging wall is tilted more steeply than the footwall, whereas a convex-upward fault is inferred where tilting of the hanging wall is less than that of the footwall. Movements along listric normal faults created rollover or reverse drag folds. The geometry of fault-related folds and minor drag folds is largely controlled by dip angle and dip direction of normal faults. Minor gently dipping normal faults were also observed. These faults were developed originally as steep faults and then rotated to their present shallow dip by domino-like block tilting (e.g., Faulds et al., 2002) of their footwall and hanging wall. A few of the minor reverse-separation faults were observed in Miocene and Pliocene rocks, but major reverse faults were not observed. These faults probably developed by tilting of some antithetic SW-dipping normal faults.
NW-trending normal faults were observed in Quaternary sediments and recent terraces. Many of these faults are related to recent earthquakes affecting the northwestern coast of the Red Sea (El Shemi and Zaky, 2001).
Several major and minor NE- to NNE-trending sinistral strike-slip faults and some NW- to NNW-trending dextral strike-slip faults were observed (Fig. 4). Strike-slip and oblique-slip faults appear to be younger than the oldest Coastal and Border normal faults and dextral strike-slip faults. Many of the NE-trending faults in the Quseir–Umm Gheig subbasin, initiated as normal faults in the old Cretaceous–Eocene sequence, appear to have been subsequently reactivated as strike-slip or oblique-slip faults.
Three major dextral faults were observed, named Wadi Ambaji, north Wadi Sharm Al Bahari, and north Wadi Zareib dextral faults. Dextral offsets of geomorphologic features and stratigraphic units across Wadi Ambaji and Wadi Sharm Al Bahari strike-slip faults are generally a little over 500 m.
In the Quseir–Umm Gheig subbasin, sinistral strike-slip displacements dominate and dislocate Coastal and Border normal faults and dextral strike-slip faults. A series of major NE-trending steep strike-slip faults of this phase offsets structures in the Precambrian and Cretaceous–Miocene rocks and dissects the Quseir–Umm Gheig subbasin into a number of fault subblocks. The main trends of these faults range between N20°E and N80°E. They are related to the Aqaba–Dead Sea transform fault, and their second-order shear zones developed in the middle Miocene. They follow the old Precambrian NE faults and major fractures. These faults dislocated all the rock units from Precambrian rocks to Pleistocene rocks and controlled the following structural and topographic features: (1) the morphology of the Red Sea coast and the exposed Precambrian rocks, (2) the development of sharms at the entrance of the main wadis (large sharms [“bays”] in the study area, such as those at the entrance of Wadi Ambaji, Wadi Sharm Al Bahari, Wadi Sharm Al Qibli, Wadi Wizr, and Wadi Umm Gheig, are largely developed at the point of intersection between two conjugate strike-slip faults), and (3) the main courses of the major wadis (the most prominent sinistral strike-slip faults in the Quseir–Umm Gheig subbasin are those running along Wadi Aswad, Wadi Essel, and Wadi Umm Gheig). The NE-trending Wadi Aswad strike-slip fault (more than 16 km in length) dislocates (offset over 1 km) the Precambrian rocks, Nubia Sandstone, and the two normal faults bordering the Atshan fault block. The ENE-trending Wadi Essel strike-slip fault is ∼22 km in length and accommodates ∼1 km of horizontal displacement. It dislocates the major shears, normal faults, and lithological contacts in Precambrian rocks, stratigraphic units, and normal faults in the Cenozoic rocks. Also, this fault displaced the southern part of the Hamadat fault block from its northern part. The strike of this fault is ENE-WSW along Wadi Essel and in the Precambrian rocks between Zog El Bohar and Hamadat blocks, and then it changes its strike to WNW-ENE coinciding with northern extension of the Sibai shear zone in Precambrian rocks to the west of the Hamadat fault block (Fig. 4). The ENE-trending Wadi Umm Gheig strike-slip fault delineates the southern extension of the Sharm El Bahari fault block. There is a NNE-SSW oblique-slip fault that cuts across Wadi Ambaji, dislocating Precambrian and Cretaceous rocks, and relocating the axial plane of the Nakheil syncline and the Cretaceous–Eocene sequence of Gebel Atshan (Fig. 4). This fault has left-handed displacement (up to 500 m) and may be reactivated along an old NE-SW normal fault.
Definition and Classification
Faulds et al. (2002) summarized the mechanisms of normal fault-related fold development as follows: (1) movement on listric normal faults (i.e., rollover or reverse drag folds) (e.g., Groshong, 1989; Dula, 1991; Xiao and Suppe, 1992), (2) displacement gradients on normal faults (e.g., Schlische, 1992, 1995; Janecke et al., 1998), (3) along-strike overlap of oppositely dipping normal faults within accommodation zones (e.g., Morley et al., 1990; Faulds and Varga, 1998), and (4) isostatically induced flexures in footwalls of major normal faults (e.g., Spencer, 1984; Wernicke and Axen, 1988).
Schlische (1995) identified six categories of fault-related folds (in Khalil and McClay, 2002): (1) hanging-wall fault-bend folds generated by changes in fault dip (i.e., rollover anticline and ramp synclines; e.g., McClay, 1990), (2) normal drag folds formed by frictional resistance along the normal fault plane (e.g., Twiss and Moores, 1992; Peacock et al., 2000), (3) reverse drag folds, where the hanging-wall beds and the footwall beds flex the fault surface downward and upward, respectively (e.g., Barnett et al., 1987), (4) transverse folds generated by displacement variation along the strike of the fault (the axes of these folds are perpendicular to the fault surface), (5) fault-propagation folds produced by folding of a propagating fault-tip line (e.g., Allmendinger, 1998; Hardy and McClay, 1999; Corfield and Sharp, 2000; Sharp et al., 2000; Khalil and McClay, 2002; Willsey et al., 2002; White and Crider, 2006), and (6) compactional drape folds produced by differential compaction of sediments over a preexisting extensional fault scarp.
Kilometer-scale fault-propagation folds have been recognized in the Gulf of Suez rift margin (Moustafa, 1987; Gawthorpe et al., 1997; Sharp et al., 2000), Rhine graben (Maurin and Niviere, 2000; Lopes Cardozo and Behrmann, 2006), northwestern Red Sea (Khalil and McClay, 2002); Gulf of California (Willsey et al., 2002), the Smørbukk area, North Sea (Corfield and Sharp, 2000), southwest Iceland (Grant and Kattenhorn, 2004), Sirt Basin, Libya (Fodor et al., 2005), and Modoc Plateau, northeastern California (White and Crider, 2006). Fault-propagation folds have received much attention due to the frequent occurrences of hydrocarbons associated with the folds themselves and the underlying fault blocks.
Fault-Related Folds in Quseir–Umm Gheig Subbasin
Extensional fault-propagation folds are abundant in accommodation or transfer zones (e.g., East African rift and Gulf of Suez, northwestern Red Sea). The Miocene–Pliocene rocks of the Quseir–Umm Gheig subbasin are dissected and dislocated by a series of NW-SE–trending normal faults. Both oppositely dipping, listric and convex-upward normal faults are observed. The length of these faults ranges between a few tens of centimeters to several kilometers. The throw of these several-kilometer-long faults decreases rapidly toward the tip, suggesting elliptical slip distributions. Fault-parallel monoclinal folds are present beyond the tip, and axes are offset into the hanging wall like those described from the Modoc Plateau, northeastern California (White and Crider, 2006). Normal-fault propagation in the Quseir–Umm Gheig subbasin is accompanied by flexing of Miocene–Pliocene beds beyond the Quseir master fault-tip line, resulting in development of several kinds of extensional fault-related folds. These folds together constitute an along-strike fault-parallel monoclinal fold between middle and upper Miocene rocks.
Three types of NW-trending extensional fault-related folds were observed in the study area: (1) forced folds or extensional fault-propagation folds, (2) compactional drape folds, and (3) normal drag folds. Forced folds were formed between a series of oppositely dipping normal faults that characterize the horst and graben tectonics of the Quseir–Umm Gheig subbasin.
Several footwall anticlines and hanging-wall synclines were observed within this fault-parallel monoclinal fold and were observed to be well-developed mainly in the Samh Formation (late Miocene–early Pliocene), especially in the area between Wadi Sharm El Bahari and Wadi Wizr. Two meter scale, NW-SE–trending fault-related folds were observed along a small stream on the southern bank of Wadi Sharm El Qibli. They represent the footwall anticlines of the master fault. Their outcrops consist of gypsiferous sandy shale and fine-grained argillaceous sandstone capped with reefal limestone of the Gabir Formation. Reefal limestones were not affected by folding but only acquire the shape of the underlying fold and were clearly fractured and brecciated. The open anticline (Fig. 10A) is asymmetric; its SW-dipping limb dips at 40°, and its NE-dipping limb dips at 24°. It contains a thick bed of shale in its core, followed upward by thin layers of sandstone, sandy shale, shale, and reefal limestone. The other anticline is also asymmetric as well; its SW limb dips at ∼35°, and its NE limb dips at 80° changing to 15°. The upper Miocene–lower Pliocene rocks exposed around these folds are characterized by predominance of NW-trending oppositely dipping normal faults. The NE-dipping limb is dislocated by a minor NE-dipping normal fault, whereas the SW-dipping limb is displaced by a SW-dipping normal fault (Fig. 10B).
A macroscale NW-trending hanging wall syncline was recorded in the lower Miocene multicolored sandstone and minor shales of the Ranga Formation between Wadi Sharm El Bahari and El Qibli. It is a strongly open symmetric fold with wavelengths up to 50 m (Fig. 10C) developed in the hanging wall of the Umm Gheig master fault. Dip measurements on the lower Miocene rocks along the northern flank of Wadi Sharm El Bahari (Fig. 4) indicate 7 km extension for this syncline, although its central part was dislocated by the Sharm El Bahari strike-slip fault and obliterated by weathering processes. Several minor NW-SE–trending normal faults were recorded in the limbs of this syncline. In the upper Miocene rocks, some other minor synclines were also observed associated with the hanging wall of listric normal faults (Fig. 9D). These hanging-wall synclines are typical of fault-related folds developed by change in dip of fault by depth.
A few minor drape folds were also observed in upper Miocene rocks. They are characterized by development of the steeply dipping and intermittently reverse faulted monoclinal flank, accompanying the major normal faults between the middle and upper Miocene rocks. The main characters of these folds are that they are bounded by normal faults on one side and reverse faults on the other side (Fig. 11A); however, sometimes they are bounded by normal faults either on both sides, and the displacement of the normal fault is parallel to the steeply dipping flank. Sometimes, the steeply dipping flank is broken and displaced when the normal faults were reactivated by recent movements or present-day earthquakes (Fig. 11D). The presence of a few drape folds in upper Miocene rocks explains the occurrence of some reverse faults in the same places as the normal faults.
Both reverse and normal drag folds were sporadically observed and usually associated with NW-striking reverse faults. The best development of these folds was recorded in an outcrop along the contact between middle and upper Miocene rocks along the southern bank of Wadi Sharm El Bahari, where they are present as a series of oppositely dipping, NW-trending normal faults and a few reverse faults. Minor reverse drag folds (i.e., rollover) developed when the beds constituting the hanging wall bent downward relative to those of the footwall due to upward movement of the hanging wall (Fig. 10C). The normal drag folds (Fig. 11D) were formed when footwall beds were dragged down and hanging-wall beds were dragged up along the fault surface as a result of frictional resistance along the normal fault plane.
The evaporite deposits are remarkable features along the Red Sea coast. They are distributed as separated lobes deposited in isolated grabens and consisting of gypsum beds interbedded with sand, clays, marls, shales, and carbonates. The major and minor NE-trending folds in the upper Miocene and Pliocene Red Sea evaporites show characteristics of typical buckle folding. Sehim (1994) studied and regionally traced these buckle folds in the area between Gebel El Zeit in the north and Wadi Umm Gheig in the south (∼250 km along the Red Sea coast).
Naturally strained shale-sulfate multilayers and minor gypsum folds include chevron, symmetrical, asymmetrical, open, tight (Fig. 11E), overturned, and noncylindrical isoclinal folds. The disposition of these minor folds and the arrangement of their hinge lines between the limbs of alternate domes and basins resemble syncline-anticline alternation in enterolithic gypsum folds, which developed by vertical expansion during rehydration (e.g., Gandin et al., 2005). Folding may be open, and it often develops into small thrusts. The presence of thin shale layers between gypsum beds acts as a lubricant facilitating folding. These folds are only preserved in shale-sulfate multilayers, and they are completely absent in the overlying and underlying beds. The axes of these folds trend mainly NE-SW. The major buckling folds include both mode 1 and 4 of Ghosh et al. (1993). The nonplane, noncylindrical folds consist of a number of parallel arc-lengths (∼1 and 5 km) characterizing both upper Miocene and Pliocene rocks, and they are easily traced from correctly processed Landsat and conventional aerial photographs. They exist as a number of anticlines and synclines plunging toward the northeast (Figs. 7 and 11F).
Buckling is a well-known folding process that results from compression parallel to the layers (Ramberg, 1964), and it gives rise to regular waveforms, the lengths of which depend on both thickness and viscosity or strength contrast between layers (Lan and Hudleston, 1996; Espina et al., 1996; Woodward, 1997; Sengupta et al., 2005). Buckling is common in a single layer and multilayers.
In the study area, the well-exposed nonplane, noncylindrical buckle folds are described from Wadi Quseir El Qadim, Wadi Aswad, and Wadi Essel. Buckle folds of domes and basin patterns are best developed in the large evaporite lobe between Wadi Essel and Wadi Sharm El Bahari. They occur as marked domes (Fig. 11G) and half domes formed by vertical upward movement of evaporites that consequently pushed up the overlying Pliocene and Quaternary sediments. The observed domes occur close to the Red Sea shoreline, where they are of variable dimension and represent hazardous places for building construction and urban development.
Relation between Najd Fault System and Cretaceous Basins
An important structural characteristic of faults belonging to the Najd fault system is the formation of second-order strike-slip motion, which was often accompanied by extensional movements responsible for development of pull-apart basins (El-Rabaa et al., 2001). Such basins act as sites for deposition of molasse sediments. The Najd fault system is a major component in the geological framework of the Precambrian rocks in Egypt and Saudi Arabia and is regarded as the last significant structural event that affected these rocks (Moore, 1979; Stern, 1985, 1994; El-Rabaa et al., 2001; Johnson and Kattan, 2001; Abd El-Wahed, 2007, 2008, 2009; Abd El-Wahed and Abu Anbar, 2009). NW-trending sinistral shear zones bound the core complexes (e.g., Sibai, Meatiq, Hafafit) in the central Eastern Desert of Egypt and represent the northwest contiguity of the Najd fault system (Fritz et al., 1996, 2002; Abd El-Wahed, 2007, 2008, 2009).
The Cretaceous synclinal basins in the study area are described as extensional fault-related folding resulted from along-strike displacement variations on the individual fault segments during late Oligocene–Miocene extension of the Red Sea (Khalil and McClay, 2002). On the other hand, they have been interpreted as pull-apart basins developed during Cretaceous sinistral strike-slip tectonics with subhorizontal σ1 (ENE/WSW) and σ3 (NNW/SSE), and subvertical σ2, as evidenced from fission-track and paleostress data (Bojar et al., 2002).
According to Khalil and McClay (2002), folding of the Upper Cretaceous–lower Oligocene sequence was produced by folding of a propagating fault-tip line where half grabens produced synclines. This hypothesis does not satisfactorily explain all of the tectonic features of the Cretaceous basins for the following reasons: (1) it is difficult to accept the existence of as many half grabens as synclines, and each half graben contains more than one syncline; (2) the Cretaceous basins contain Nubia Sandstones, which have a wide range of ages, from Cambrian to Cretaceous (Moustafa, 1987; Jackson et al., 2002, 2006a, 2006b), and this indicates that the nucleation of the basins might have been initiated earlier than Cretaceous time; (3) the main Red Sea extension started in the late Oligocene–early Miocene and not in Early Cretaceous, as cited by Khalil and McClay (2002); (4) some of the folds axes are curvilinear, such as those of Nakheil and Atshan synclines; and (5) the strike-slip faults dislocated the axial plane of the Nakheil syncline.
The development of Cretaceous basins can be better explained in the context of strike-slip tectonics. The axes of the basins and the synclinal folds trend mainly NW-SE and NNW-SSE parallel to the major normal faults bordering them. An analysis of the geological map of the study area allows us to recognize the typical features of a wrench zone in the Quseir–Umm Gheig subbasin during Late Cretaceous time. The present-day normal faults bordering the synclinal basins were originally developed during wrench tectonics and enhanced during rift tectonics. The basins and the axes of synclinal folds show an en echelon arrangement; their orientation in respect to the main wrench fault and the dislocation of their axial planes by faults indicate a left-lateral displacement along the main wrench fault.
The main features that support the development of the Cretaceous pull-apart basins during sinistral wrench tectonics are:
(1) All the Cretaceous basins are concentrated in the area between Safaga and Umm Gheig (Fig. 2) in the central Eastern Desert, which represents the main influence zone of the Najd fault system (de Wall et al., 2001; Abd El-Wahed, 2007, 2008, 2009).
(2) The synclinal fold axes are parallel to the major faults. Transpressive deformation along vertical faults has two components: one is parallel to the strike-slip faults, and the other is perpendicular (Mount and Suppe, 1987; Ali Kassim et al., 2002). If the movement along faults occurs without dragging of the sedimentary cover, the stress component parallel to the faults is only responsible for the lateral displacement, i.e., of the slip, whereas the perpendicular component is responsible for the development of thrusts, anticlines, and synclines striking parallel to the faults (e.g., Ali Kassim et al., 2002). The parallelism between synclinal folds and the major shear zones depends mainly on orientation and intensity of σ1 (ENE to WSW). The curvilinear shape and the sudden change in orientation of the Nakheil syncline axis may have developed later and can be interpreted as a drag structure related to sinistral shearing during final stages of deformation.
(3) Major NW-SE–trending Precambrian shear zones (such as Sibai, Meatiq, Hamrawin, Queih shear zones) occur in the neighborhood of the synclinal basins. A final phase of Neoproterozoic basin formation resulted in deposition of molasse sediments in isolated, pull-apart basins (e.g., Umm Seleimat, Queih, Kareim, Atawi, El-Miyah, and Igla Basins) caused by strike-slip movements on faults of the Najd fault system (Abd El-Wahed, 2004, 2009).
(4) There is a difference of 20° between the orientation of the sinistral strike-slip shear zones and the associated en echelon synclinal folds.
(5) En echelon arrangement of Cretaceous basins and en echelon pattern of synclines are observed in the same basin (e.g., Hamadat). In progressive simple shear deformation, there is usually a 45° angle between the principal stress axis of the strain ellipsoid and the main shear plane; this orientation facilitates the formation of en echelon folds.
(6) The principal stress axes are subhorizontal σ1 and σ3, and subvertical σ2, as presented by Bojar et al. (2002).
(7) Sheared conglomerates from the Nubia Formation were incorporated into a major vertical sinistral shear zone, including vertical foliation and horizontal lineation (Bojar et al., 2002).
(8) Minor overturned folds and minor NE-vergent thrusts occur in Dakhla shale (Paleocene) of the Aswad subblock.
(9) There is a predominance of NE-SW normal faults in Cretaceous–Eocene sequence.
Tectonic Regime of the Fault-Related Folds in Miocene–Pliocene Rocks
Fault-related folds in Quseir–Umm Gheig subbasin were formed synchronous with the main episode of Red Sea extension and include a series of meter-scale folds produced in extensional tectonic environments. These include: (1) extensional fault-propagation folds, (2) compaction drape folds, and (3) normal drag folds. Meter-scale folds adjacent to large normal faults can be interpreted as a part of a breached monocline formed as a result of fault-propagation folding. These fault-related monoclinal folds are formed by warping of middle Pliocene beds beyond major-scale master normal faults.
Normal master faults nucleated at depth within the Precambrian rocks and then propagated upward into Phanerozoic cover and caused the development of master fault–related monoclinal folds within the overlying Miocene–Pliocene sequence. Layer-parallel slip occurred in shale and sandy shale units at depth, and a normal fault, which splays from the master fault, deformed limestone and sandstone units. The presence of thick shale beds in the study area increased competency of bending of beds. Also, all the recorded meter-scale fault-related folds are observed in shale-rich beds. The ductile character of shale- and sandy shale–dominated units of the Abu Dabbab and Samh Formations played the main role in the formation of fault-related folds in the Quseir–Umm Gheig subbasin. Layer-parallel slip and ductile flow occur in shale and sandy shale units, whereas sandstone and limestone units are faulted, fractured, and brecciated. The same setting has been described previously from the western margin of the Dead Sea rift (Gross et al., 1997), from the Sirt Basin, south-central Libya (Fodor et al., 2005), and the Gulf of Suez (Jackson et al., 2006a, 2006b), and has been explained as due to the ductile flow of mudstone units. The monocline that developed early in the process was later breached by meter-scale folds through continued upward propagation of the master fault at the different stages of monocline development. Generally, flexing increases in the area between Wadi Sharm El Bahari and Wadi Wizr and then dies out to the north and south with thinning of shale beds and thickening of limestones and sandstone beds. The disappearance of the fault-related monocline and its associated fault-related synclines and anticlines is due to thinning of shale and sandy shale beds in the Miocene–Pliocene sequence. Therefore, variations in deformation style were largely controlled by variations in shale and sandy shale intercalations.
The flexing disturbed the gentle regional dip of the Miocene–Pleistocene sequence and led to the development of large, gently dipping parts and small, moderately to strongly dipping segments. The change from gently dipping to steeply dipping limbs is abrupt and marked by development of meter-scale fault-related synclines and anticlines. These folds are associated with a series of NW-SE–trending macro- and micro-scale normal faults developed mainly in the hanging walls of the master faults. Some of these normal faults may have been overturned or rotated to appear as steeply dipping reverse faults. Rotation of normal faults occurred with the progressive rotation of strongly dipping segments of the fault-related monocline. Normal drag folds usually developed in the hanging walls of these rotated normal faults. Additional breaching increased displacement along the master fault and may have led to detachment of strongly dipping segments from gently dipping parts (see Fig. 10F).
Origin of Evaporite Folds and Buckling
The NE-trending evaporite folds in the upper Miocene–Pliocene sequence indicate NW-SE compression. Sehim (1994) discussed the mechanisms of these folds and concluded that the strain axes of these folds are parallel to those of the NNE-SSW strike-slip faults. This conclusion is supported in the present study by the following features: (1) the axes of these folds trend NE-SW and moderately plunge (20–30°) toward the NE; (2) the folds are asymmetric, their limbs have dip angles between 15° and 30°, and the folds extend along the entire Red Sea coast, forming a series of anticlines and synclines; (3) the intensity of folding increases with increasing thickness of gypsiferous shale beds in the upper Miocene–Pliocene sequence where the presence of shale increases the ductile flow of the beds and facilitates folding; (4) folds are not restricted to evaporite and gypsiferous shale beds but extend to the overlying clastic sediments; and (5) folds are also observed in areas with no evaporites but with the presence of shale. According to the structural features observed in this study and those recorded by Sehim (1994), the evaporite folds and the buckle folds are syntectonic folds developed during deposition of the upper Miocene–Pliocene sequence. These folds may have developed due to NW-SE compression associated with differential movement along sinistral NNW-SSE strike-slip faults and their second-order shears.
Three major tectonic events constitute the deformation history of the Quseir–Umm Gheig subbasin: D1 represents the oldest deformation event and was related to Late Cretaceous sinistral movement along the reactivated Najd fault system; D2 includes NE extension of the Red Sea in late Oligocene–early Miocene and a middle Miocene deformation event associated with NW-SE dextral and NNE-SSW sinistral strike-slip faults; and D3 is a late Pliocene–early Pleistocene deformation event associated with development of buckling and evaporite folds.
During D1, the study area was subjected to ENE-WSW compressive movement in the Late Cretaceous. This movement was accompanied by reactivation of sinistral movement along the NW-SE–trending major shear zone in the Precambrian rocks. Such movement was accomplished by formation of the en echelon–oriented pull-apart basins (Fig. 12A). These basins were filled with a thick Cretaceous–late Oligocene sequence composed of Nubia (Cretaceous?), Quseir (Campanian), Duwi (Campanian–Maastrichtian), Dakhla (Maastrichtian–Paleocene), Esna (late Paleocene–early Eocene), Thebes (early–middle Eocene), and Nakheil (late Oligocene) Formations. The axes of these basins trended mainly NW-SE and were deflected to N-S due to the effect of sinistral movement along NNE-SSW strike-slip faults (D2). Development of Late Cretaceous basins and deformation of the Late Cretaceous–Eocene sequence during wrench tectonic events are supported by: (1) arrangement of the Late Cretaceous basins in an en echelon system, (2) the axes of the synclinal basins, oriented at low angle to the wrench fault, (3) subhorizontal σ1 and σ3 and subvertical σ2 orientations, (4) the presence of NE-vergent thrusts and minor overturned folds in Dakhla shale of the Aswad subblock, and (5) the presence of NE-SW normal faults in the Cretaceous–Eocene sequence.
The D2 deformation event was related to the rifting of the Red Sea initiated in the late Oligocene–early Miocene. The synrift sediments rest unconformably over the lower–middle Eocene limestone of the Thebes Formation. The lower Miocene rocks are mainly conglomerates and varicolored sandstones deposited in an alluvial-plain environment.
In the early Oligocene, rifting was initiated in the form of several small, en echelon E-W– to ESE-WNW–trending basins in the Gulf of Aden province (Fantozzi and Sgavetti, 1998; Watchorn et al., 1998). In late Oligocene–early Miocene time, rifting had extended to Afar and all over the Red Sea system (Bosworth et al., 2005; Guiraud et al., 2005). The N-trending Pan-African weak zones or stress guides played a role in the redirection of the Red Sea rift propagation. During the middle Miocene, the Red Sea was subjected to NW compressive movement (Jarrige et al., 1986; Richert et al., 1986). This movement generated dextral transpression along NW-trending Queih and Hamrawin shear zones and sinistral movement along the NNE-SSW Aqaba–Dead Sea transform (Fig. 12B). Both synthetic and antithetic conjugate strike-slip faults were developed and oriented at low and high angles to the wrench fault, respectively. They include NNE-SSW, NE-SW, and ENE-WSW synthetic and antithetic faults. This event was associated with the NE extension of the Red Sea and reactivation of NW-SE–trending border normal faults marking the contacts between the prerift sediments and the Precambrian rocks.
In the middle–late Miocene transition, the Red Sea switched from rift-normal movement to highly oblique extension parallel to the transform (Bosworth et al., 2005). This probably produced minor compression and uplift in the northernmost Gulf of Suez (Patton et al., 1994), and, to some extent, isolated the northern Red Sea from invasion of marine waters of the Mediterranean. This was accompanied by sudden changes in sedimentation processes to prevalent evaporites of the Abu Dabbab Formation. A series of NW-trending coastal normal faults was developed during Miocene–Pliocene time. Normal master faults developed at depth within the Precambrian rocks and then propagated upward into the overlying Miocene–Pliocene sequence and caused development of the fault-related monoclinal fold. Macro- and micro-scale normal faults splayed from the master faults, and fault-propagation folds developed within the monoclinal folds.
During the late Miocene–Pliocene transition, an additional movement occurred along the NNE-SSW sinistral strike-slip faults and their second-order shears. This movement was accompanied by NW-SE compression that commenced with development of NE-SW folds in upper Miocene evaporites and lower Pliocene rocks (Fig. 12B). Also, buckling of Pliocene and post-Pliocene sediments occurred during this event. Buckling and NE-SW evaporite folds are important structural features throughout the Quseir–Umm Gheig subbasin and the entire Red Sea coast.
In the Quseir–Umm Gheig subbasin, the presence of extensional faults, extension gashes, joints in Pleistocene rocks, and uplift of upper Pleistocene and Holocene coral terraces indicates that movement along the NNE-SSW Aqaba–Dead Sea transform trends and normal extensional faults is still active to the present time (Fig. 12C). Active strike-slip faulting in Quseir–Umm Gheig subbasin generally poses a greater earthquake hazard than the movement on normal faults.
The authors would like to express their deep thanks to Adel R. Moustafa, Department of Geology, Ain Shams University, Cairo, Egypt, for his valuable and constructive comments on an earlier version of the manuscript. We are also grateful to Samir Kamh, Department of Geology, Tanta University, Tanta, Egypt, for his kind support in the field. The manuscript was substantially improved by the helpful comments of James P. Evans and an anonymous reviewer.