ABSTRACT

Egypt’s Late Oligocene–Early Miocene Nukhul Formation was deposited during the earliest geological evolution of the Gulf of Suez and Red Sea Rift System. In this paper the formation is cast as a depositional sequence based on published sections, and correlated across the Gulf of Suez and northern Red Sea. The resulting correlations indicate that deposition was initiated in local grabens by the oldest continental clastics of the lower member of the Nukhul Formation, the Shoab Ali Member. The member overlies the Suez Rift Unconformity, a term proposed for the entire Red Sea. Although this member can attain a thickness of ca. 1,000 ft (305 m) locally in grabens, it is generally absent over horsts. Sedimentary facies of the member are interpreted as indicating an initial alluvial-fluvial setting that evolved to an estuarine and coastal setting.

The upper part of the Nukhul Formation records a regional shallow-marine transgression, which can be subdivided into three correlative Upper Nukhul members. These sediments are absent over the highest paleo-horsts, but reach up to 900 ft (275 m) in thickness in grabens. In the southern Gulf of Suez the Ghara Member represents the Upper Nukhul members. In places it consists of four cycles, each of which starts with an anhydrite bed and is overlain by deposits of mixed lithology (sandstone, marl, and limestone). The four cycles are interpreted as transgressive-regressive subsequences that can be correlated across ca. 60 km in the Gulf of Suez. The Ghara Member correlates to Saudi Arabia’s Yanbu Formation, which consists of massive salt in wells drilled on the Red Sea coastal plains. The Yanbu Salt is dated by strontium-isotope analysis at ca. 23.1–21.6 Ma (earliest Aquitanian).

The Nukhul Formation is capped by the Sub-Rudeis Unconformity or correlative Rudeis Sequence Boundary, and overlain by the Rudeis Formation. The Nukhul Formation is here proposed as the Nukhul Sequence and defined in the Wadi Dib-1 Well, wherein it consists of Nukhul subsequences 1 to 10 (in descending order, ranging in thickness between 33–84 m). The lower six Nukhul subsequences 10 to 5 are characterized by shale-to-sandstone cycles of the Shoab Ali Member, and the upper four are represented by the cycles of the Ghara Member. The 10 subsequences are interpreted as tracking the 405,000 year eccentricity signal of the Earth’s orbit and to span ca. 4.0 million years between ca. 25.0 and 21.0 Ma.

INTRODUCTION

This paper argues for the proposition that sequence stratigraphy can be used to characterize depositional systems in rift basins (see GeoArabia’s Suez Debate, 2011, v. 16, no. 1, p. 13). It focuses on the oldest syn-rift deposits in Egypt’s Gulf of Suez to understand the earliest structural and depositional evolution of the Red Sea Rift System (Figures 1 to 3). This task is not straightforward because these deposits are named and described in terms of several parallel and oftentimes conflicting stratigraphic schemes that typically apply to local study areas (Figures 1 to 3). Therefore one aspect of this paper is to review some of these schemes so as to resolve synonyms. This simplification of terminology aids in the compilation of an important dataset and the identification of a regional sequence-stratigraphic framework that can be correlated across most – if not all – of the Red Sea Rift System.

Saoudi and Khalil (1986) were the first authors to name and define the continental clastics that were deposited in the oldest syn-rift grabens of the Gulf of Suez as the Shoab Ali Member of the Nukhul Formation. The member is overlain by deposits that are more widely distributed and defined in terms of three members; here referred to as the Upper Nukhul members: (1) October Member (mainly clastics), (2) Ghara Member (mainly evaporites and clastics), and (3) Gharamul Member (mainly clastics and carbonates) (Figures 2 and 3). In the paper these members are described in a lexicon style, and the formation is recast as a transgressive-regressive depositional sequence, the Nukhul Sequence. In the final part of the paper the Nukhul Sequence is dated between ca. 25.0–21.0 Ma by the calibration of its subsequence in an orbital-forcing, glacio-eustatic time scale (Matthews and Al-Husseini, 2010).

SHOAB ALI MEMBER, NUKHUL FORMATION

Nomenclature and Type Section of Shoab Ali Member

The oldest syn-rift rock unit in the Gulf of Suez was named the “Shoab Ali Member” of the Nukhul Formation by Saoudi and Khalil (1986). They defined the member in the Well GH 385-1 between log depths 6,980–7,980 ft (1,000 ft, 305 m thick, Figures 2 and 5c). As discussed in this paper their choice of this well for the type section is not considered adequate because its upper contact is not defined in the same well. The Wadi Dib-1 Well is considered a better choice and discussed at the end of the paper.

The Nukhul Formation (Shoab Ali and Upper Nukhul members), together with the Rudeis and Kareem formations, are assigned to the Gharandal Group (named after Wadi Gharandal, Figure 2) or pre-Evaporite Group (EGPC, 1964, 1996; El-Gezeery and Marzouk, 1974). In some parts of the Gulf, the formations attributed to the Gharandal Group are differently named and defined: Nukhul, Mheiherrat, Hawara, Asl and Ayun Musa formations (EGPC, 1996; Figure 3). The nomenclature for the non-marine or coastal undifferentiated clastics that pass to the Gharandal Group carry other names that are not reviewed here (see El-Gezeery and Marzouk, 1974).

Boundaries of Shoab Ali Member

In the type well the Shoab Ali Member overlies the pre-rift limestones of the Lower Eocene Thebes Formation (Figure 4; Saoudi and Khalil, 1986). The contact is here referred to as the Suez Rift Unconformity; it is synonymous to:

As will be shown in the discussion of the Nukhul’s age, the lower part of the Shoab Ali Member and its correlatives are Late Oligocene in age and so the term “pre-Miocene unconformity” is incorrect. The preferred term, “Suez Rift Unconformity”, avoids age interpretations (pre-Miocene unconformity) and ambiguities resulting from competing formation and/or group names above the unconformity (e.g. Sub-Nukhul unconformity or Sub-Abu Zenima unconformity, Figure 3). The dilemma of choosing between age versus positional naming for the unconformity is further highlighted in the paper by Saoudi and Khalil (1986). They show the age of the Shoab Ali Member as entirely Early Miocene but then add an unnamed Oligocene-aged wedge in Figure 4 and in the Zeit Bay-1 Well (Figure 5c).

The contacts between the Shoab Ali Member and the Upper Nukhul members are not defined in the same localities or type wells (Figure 4) by Saoudi and Khalil (1986). They consider the Shoab Ali Member to be conformably overlain by the Ghara, Gharamul or October members. As discussed in this paper the relationship between the Shoab Ali and October members is not adequately documented. For example, in Figure 2, type well GH 385-1 is shown in the middle of a locally thick Shoab Ali isopach (presumably a graben), whereas in Figure 5c it is shown on a paleo-horst. It seems more likely that in Figure 5c the correlation of the members is incorrect across the undocumented fault between wells GH 385-1 and GH 376-1.

Where the Shoab Ali Member is absent the Upper Nukhul members or younger formations overlie the pre-rift strata. In Figure 5, Saoudi and Khalil (1986) show the anhydrite marker beds (Ghara Anhydrite 1 to 4) in some places onlap paleohighs, and in others to merge with higher-up Ghara markers. These inconsistencies occur where no borehole control exists. The interpretation of onlap is here favored because, as discussed below, the anhydrite beds are interpreted as lowstand deposits in restricted basins, whereas the overlying clastics are interpreted as more widespread flooding units.

Lithology and Thickness of Shoab Ali Member

Saoudi and Khalil (1986) described the Shoab Ali Member as composed mainly of sand and sandstone, generally loose, colorless or pink or yellow, fine- to medium-grained, becoming coarser near the bottom. The sandstone is well- to fairly well-sorted, subrounded, with streaks of reddish brown shale, which is barren of fauna. They reported that the sand and sandstone are porous and constitute an excellent reservoir. The member tested oil in many wells in the southern Gulf and is considered one of the most important hydrocarbon reservoirs. They correlated and mapped the Shoab Ali Member as a rock-time unit across the southern Gulf of Suez (Figures 2, 4 and 5). In their map it is thickest along the southeastern coastal flank of the Gulf of Suez (e.g. 1,016 ft, 310 m in Wadi Dib-1 Well). They considered it to be absent in the central and northern Gulf of Suez (Figure 2), an interpretation that is believed to be incorrect in the present paper (see below).

Correlatives and Synonyms of the Shoab Ali Member

The Shoab Ali and Upper Nukhul members can be recognized in many localities in the Gulf of Suez and Red Sea basins but they are not so-named. By stratigraphic position immediately above the Suez Rift Unconformity, clastic lithology and continental depositional setting, several rock units are here considered synonyms of the Shoab Ali Member (Figure 3):

The Shoab Ali Member by lithology and stratigraphic position correlates to the oldest syn-rift rocks in Saudi Arabia (Montenat et al., 1988; Purser and Hötzl, 1988), specifically to the Al Wajh Formation of the Tayran Group (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005; Figure 3). It also correlates to Sudan’s Hamamit Formation (Bunter and Abdel Magid, 1989a, b), and Eritrea’s Dogali Formation (Savoyat et al., 1989). In Yemen’s Red Sea the undifferentiated stratigraphic column does not allow a correlation to a specific unit (e.g. Beydoun 1989).

Shoab Ali Member in the Nukhul’s Type Section at Wadi Nukhul

The Shoab Ali Member and Upper Nukhul members crop out in Wadi Nukhul, after which the formation is named and where its type section is defined (Figures 2 and 6, after Wescott et al., 1996). In this key outcrop the oldest syn-rift succession is 310 m thick and was described and named in several studies but not in a consistent manner. Indeed as noted above, Saoudi and Khalil (1986) did not recognize the Shoab Ali Member this far north suggesting their study area was restricted to the southern subsurface part of the Gulf of Suez (Figure 2). The following review shows how the Wadi Nukhul type section is presented by several authors (Figure 3).

Historical Definition of Nukhul Formation

According to the EGPC Stratigraphic Subcommittee (El-Gezeery and Marzouk, 1974), the Nukhul Formation was first defined by Pooley (unpublished report, 1947) south of Wadi Nukhul in the west-central Sinai Peninsula (Figure 2, 29°01′30″N; 33°11′30″E). El-Gezeery and Marzouk (1974) reported that it is 74 m thick and consists of a basal conglomerate, overlain by alternating varicolored shales and marls (occasionally “salty”) with calcareous sands, sandstones and conglomerate interbeds. Saoudi and Khalil (1986, in their figure 1) show Pooley’s 1947 Nukhul Formation in Wadi Nukhul to only consist of 197 ft (60 m) and do not show any conglomeratic facies. The present author has not seen Pooley’s report and suspects that these definitions either do not extend down to the Suez Rift Unconformity or are incomplete (60 m or 74 m compared to 310 m thick in Figure 6, Wescott et al., 1996).

Units 1 to 3 of the Nukhul Formation

Wescott et al. (1996) measured and described the Wadi Nukhul section starting above the pre-rift Lower Eocene Thebes Formation (Figures 2 and 6). They referred to the Suez Rift Unconformity as Paleontological Terrace T00 (here T00) and the Nukhul Formation as Biosequence S10 (here S10). They also interpreted the depositional setting of the Wadi Nukhul succession as follows:

  • Nukhul Unit 1 (165 m thick): From the top of the Eocene Thebes Formation (Suez Rift Unconformity) to 165 m, the section consists of unfossiliferous, non-burrowed, pebbly sandstones and mudstones. They interpreted the depositional environment as early syn-rift infill in a generally sinuous fluvial, and overbank with eolian settings. A brackish lagoonal setting is interpreted between 92–100 m, and an angular unconformity is identified at 150 m.

  • Nukhul Unit 2 (68 m thick): From 165 to 233 m, the section consists of burrowed, finer-grained sandstones and mudstones containing large numbers of low-diversity coquinas. The surface at 165 m marks the transition from fluvial to estuarine and coastal facies.

  • Nukhul Unit 3 (77 m thick): The uppermost section, between 283 and 310 m, comprises fine clastics that contain a more diverse fauna of echinoids, bivalves, and gastropods; the setting is interpreted as shallow marine.

A comparison to the three Nukhul units to the members of Saoudi and Khalil (1986) indicates that continental Unit 1 and possibly Unit 2 correlate to the Shoab Ali Member, whereas marine Nukhul Unit 3 correlates to the Upper Nukhul members (Figures 3, 4 and 6). It seems possible to speculate that Nukhul Unit 3 may be the Nukhul Formation sensu Pooley’s 1947 definition because they are comparable by thickness: 60 or 74 m versus 77 m. Nukhul units 1 and 2 (together 233 m, 764 ft thick), when correlated to the Shoab Ali Member, imply this member was deposited much further north than shown in the map of Saoudi and Khalil (1986; Figure 2).

Nukhul units 2 and 3 may also correlate to the Nukhul Formation in the Hammam Faraun Fault Block (Figure 2). At this locality the formation consists of mudstones and sandstones that were deposited in offshore, shoreface and estuarine settings (Carr et al., 2003; Jackson et al., 2006).

Group A and Nukhul Formation

Ott d’Estevou et al. (1986) also described the section in Wadi Nukhul and correlated it to other sections including the Abu Zenima section (Figures 2, 3 and 7). Their correlations shows the lower part of the Nukhul’s type section correlates to the Abu Zenima Formation. They adopted the regional scheme presented in Montenat et al. (1986a, b, c, 1988, 1998) in which the Nukhul Formation corresponds to Group A, consisting of subgroups A1, As and Ae, as follows:

  • Subgroup A1 (also Subgroup Ar in Thiriet et al., 1986) is the oldest syn-rift unit and also known as ‘Red Series’, “Red Bed Series” and “Série Détritique Rouge”. It is here considered synonymous to the Shoab Ali Member.

  • Subgroup As lies conformably on Subgroup A1, and is also referred to as “marine deposits”. It is here considered synonymous to the Gharamul Member.

  • Subgroup Ae also lies conformably on Subgroup A1 and is the lateral equivalent of Subgroup As. It is also referred to: (1) “lacustrine deposits” in the western coastal plains of the Gulf of Suez, (2) “early evaporites” in the Abu Ghusun area (Montenat et al., 1986a, b, c, 1988, 1998) and Safaga area along the Red Sea coastal plains (Thiriet et al., 1986); or (3) “Nukhul evaporite”, “lower evaporites” or “earlier sulfate deposits” (Orszag-Sperber et al., 1986). It is here considered synonymous to the Ghara Member.

The above synonyms are evident in the Wadi Nukhul section by correlating the descriptions of Ott d’Estevou et al. (1986) to Nukhul units 1–3 of Wescott et al. (1996; Figures 3, 6 and 7). In the scheme presented by Ott d’Estevou et al. (1986) and Montenat et al. (1986a, b, c, 1988, 1998) the Rudeis Formation correlates to Group B as represented in the sections at Gebel Ekna and Wadi El Qaa (Figures 2, 3 and 7). Their scheme consists of four groups (A to D from oldest to youngest; each bounded by regional angular unconformities) and was adopted in a series of outcrop studies by geologists from Elf and Total (now Total), CNRS, University of Paris, Ecoles des Mines de Paris, and the Institut Geologique Albert de Lapparent (IGAL). The studies were published (mostly in French) in 1986 by the GENEBASS Group.

Besides the cross-section shown in Figure 7, groups B–D and their subgroups are described (in various degrees of detail) in numerous map areas (Figure 1b) but not discussed in the present paper: Gharamul (Ott d’Estevou et al., 1986); Gebel Zeit and Gebel Mallaha (Prat et al., 1986); Safaga (Thiriet et al., 1986); Gebel Duwi (Jarrige et al., 1986); Quseir (Roussel et al., 1986); Ras Honkorab and Abu Ghusun (Montenat et al., 1986b); and Ras Banas (Burollet, 1986).

Historical Subsurface Type Section of the Nukhul Formation

The above discussion of the Nukhul’s type section suggests that the term “Nukhul Formation” was probably limited by Pooley (unpublished report, 1947) to only represent the upper marine part of the formation (Nukhul Unit 3, subgroups As and Ae, Upper Nukhul members). He may have considered the continental lower part (Nukhul units 1 and 2, Subgroup A1, Shoab Ali Member) as pre-rift or equivalent to the syn-rift Abu Zenima Formation as in some subsequent papers (e.g. Patton et al., 1994; Jackson et al., 2006).

The above speculations regarding Pooley’s definition of the Nukhul Formation may have been extended into the subsurface of the Gulf of Suez by Waite and Pooley (1953, inSchlumberger, 1984). Based on the interpretation of wireline logs from four wells, they informally named the marine facies of the Nukhul Formation, from base up: (1) Ras Matarma member (lower calcareous sandstone); (2) Sudr member (lower shale); Nebwi member (upper calcareous sandstone); and (4) Khoshera member (upper shale). The present author has not seen this report, but concludes that these members, being of marine origin, correlate to the Upper Nukhul members, probably the clastics of the October Member. It is unclear whether the lowest Ras Matarma member overlies the Shoab Ali Member or the Suez Rift Unconformity.

Tayran Group, Saudi Arabia

The Group A–D scheme in Egypt of Montenat et al. (1986a, b, c, 1988, 1998), and other authors noted above, was compared to the succession in the Midyan Peninsula of Saudi Arabia by Le Nindre et al. (1986) and Purser and Hötzl (1988). These authors recognized that the oldest syn-rift clastics in both regions are continental and concentrated in grabens. They also noted that in both regions, the basal clastics are overlain by relatively thin (ca. 20 m) evaporites and carbonates (subgroups Ae and As = Upper Nukhul members). However, Purser and Hötzl (1988) could not correlate these units across the Red Sea because of their apparently incompatible age interpretations: radiometric dating of intercalated basalts in Egypt’s continental Subgroup A1 (= Shoab Ali Member) indicated a Late Oligocene–earliest Miocene age, which ruled out a correlation to the supposedly Late Oligocene-aged carbonates in Saudi Arabia (incorrectly interpreted as Chattian by Dullo et al., 1983).

Susequently, a robust biostratigraphic correlation from the Gulf of Suez and Egyptian coastal plains across the northern Red Sea was given in a series of papers by Hughes and Filatoff (1995), Hughes et al. (1999) and Hughes and Johnson (2005) (Figure 3). They defined Saudi Arabia‘s oldest syn-rift continental clastics as the Al-Wajh Formation, and overlying, laterally correlative carbonate- and evaporite-dominated units as the Musayr and Yanbu formations, respectively. All three formations are assigned to the Tayran Group of Saudi Arabia’s Red Sea. On fossil evidence they correlated the Musayr Formation to Egypt’s Gharamul Member and the Yanbu Formation to Egypt’s Ghara Member. These correlations confirm that the structural and depositional evolution of the Red Sea was the same on both sides: basal continental clastics passing to shallow-marine deposits.

Offshore Red Sea

In the offshore Egyptian Red Sea the Nukhul Formation was not encountered in exploratory wells, possibly because they are generally located on basement horsts (Barakat and Miller, 1986; Tewfik and Ayyad, 1986; Miller and Barakat, 1988). For example, Miller and Barakat (1988) reported that three Esso wells (RSO T′95-1, RSO X′94-1 and RSO B″96-1) that targeted large structures encountered igneous rocks of Proterozoic age below the Rudeis Formation, without passing through the typical shallow-marine Nukhul lithologies and fauna of the Gulf of Suez. Nor were any pre-rift sedimentary rocks encountered in wells drilled in the offshore Red Sea anywhere.

Depositional Environment of Shoab Ali Member

As discussed above, in the Wadi Nukhul type section, the depositional setting of Unit 1 of the Nukhul Formation is mainly fluvial-alluvial and that of Unit 2 is estuarine and lacustrine (Figure 6, Wescott at al., 1996; Krebs et al., 1997). Richardson and Arthur (1988) arrived at the same conclusion based on the member containing feldspathic sandstones, non-calcareous red shales and polymictic conglomerates. West of the Gulf of Suez, the coeval sediments are non-marine sands and gravels containing abundant silicified trees and shells of fresh-water gastropods (Said, 1962).

The Shoab Ali Member’s correlative Al Wajh Formation in the Midyan Peninsula of Saudi Arabia (Figures 1 and 3) was deposited in a similar fluvial-alluvial to brackish, fresh-water setting according to Hughes et al. (1999) and Hughes and Johnson (2005). They interpreted the latter setting based on the presence of the fresh-water alga Pediastrum spp. and the benthonic foraminifera Ammonia beccarii, together with charophytes and unornamented ostracods in the subsurface.

UPPER NUKHUL MEMBERS

The Upper Nukhul members represent the laterally equivalent, mainly marine units deposited after the Shoab Ali Member and before the Rudeis Formation. They correspond to the Ghara and Gharamul members but not necessarily to the October Member as defined by Saoudi and Khalil (1986). These three members were discussed above so as to separate them from the Shoab Ali Member; to avoid repetition they are briefly discussed below.

The isopach of the Nukhul Formation was contoured by both Saoudi and Khalil (1986) and Richardson and Arthur (1988). The one shown in Figure 8 is reproduced from the latter paper because it shows the thickness of the formation exceeds 200 m in Wadi Nukhul (310 m thick in Figure 6) whereas the former authors’ map shows it as absent. These differences are probably due to both the lateral extent of their respective study areas and how they defined the Nukhul Formation. Both maps show that the formation extends across the entire Gulf of Suez and attains a maximum thickness of ca. 2,500 ft (700 m) in local basins, but it is absent over several horsts.

Ayyad and Stewart (1992) estimated the average thickness of the Upper Nukhul members as ca. 200 m thick. The cross-sections in Figure 5 indicate the thickness of the Ghara Member ranges from nil to ca. 270 m. The distribution of the Nukhul Formation and its members in the October Field area is particularly illustrative of how depositional setting and structural movements interacted (Figure 9, Dolson et al., 1996). In this area the Nukhul Formation is absent over the October structural high (crest of tilted block, Youssef, 2011), and ranges in thickness from ca. 0–800 ft (0–244 m) in the surrounding area. In the map area, according to EGPC (1996, p. 88), the Nukhul Formation consists of clastics intruded by volcanic sheets and dykes. These volcanoclastic deposits filled the earliest syn-rift structural lows and onlap the high block (EGPC, 1996). The lower infill sediments are here considered the Shoab Ali Member with the Upper Nukhul members onlapping the crestal “bald” area of the October Field.

Further south along the Red Sea coastal plains (Figure 1), the Upper Nukhul members may correlate to the middle part of Sudan’s Hamamit Formation (Carella and Scarpa, 1962; Sestini, 1965; Bunter and Abdel Magid 1989a, b), and the lower part of Eritrea’s Habab Formation (Savoyat et al., 1989). In Yemen the undifferentiated stratigraphic column does not allow a correlation to a specific unit (e.g. Beydoun, 1989).

Ghara Member, Nukhul Formation

Nomenclature, Synonyms and Correlatives: (1) Upper Nukhul members (this paper); (2) Subgroup Ae, lacustrine deposits, early evaporites or Nukhul evaporite (Montenat et al., 1986a, 1988); (3) lower evaporates or earlier sulfate deposits (Orszag-Sperber et al., 1986); and (4) Yanbu Formation in Saudi Arabia (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005).

Type Section: Well GS 391-2 (Figures 4, 5b and 5c), between log depths of 11,808–12,410 ft (602 ft, 184 m thick, Saoudi and Khalil, 1986).

Lithology: In the area south of latitude 29°27′N (south of GS 276-1 Well) the Ghara Member mainly consists of white and hard anhydrite layers inter-bedded with sandstones, gray marl, calcareous shale and limestone. In the southern Gulf of Suez, the member contains up to four, semi-regional, evaporite marker beds (numbered 1–4 from youngest to oldest; Figures 4 and 5, Saoudi and Khalil, 1986; Richardson, 1988). The anhydrite marker beds decrease from four in the southwest to one in the north. According to Richardson (1988), the anhydrites exhibit replacement textures of limestone in some wells, whereas in others the upper part of the evaporite interval is inter-bedded with siltstone and dolomite containing fragments of anhydrite.

In the Midyan Peninsula in Saudi Arabia, the Ghara-correlative Yanbu Formation is a massive laminated anhydrite, and in the subsurface near Yanbu city (Figure 1a), it consists of massive salt (many 10s of meters thick, Hughes and Johnson, 2005). In the subsurface, the evaporite intervals are often inter-bedded with siliciclastics.

Boundaries: The Ghara Member lies conformably over the Shoab Ali Member, and is unconformably overlain by the Lower Rudeis Member (Saoudi and Khalil, 1986; Richardson, 1988) or correlative Mheiherrat Formation (EGPC, 1996).

Gharamul Member, Nukhul Formation

Nomenclature, Synonyms and Correlatives: (1) Upper Nukhul members (this study); (2) Subgroup As or “marine deposits” (Montenat et al., 1986a, 1988, 1998); (3) Nukhul Carbonate equivalent (Hughes et al., 1992; Hughes and Beydoun, 1992); and (4) Musayr Formation in Saudi Arabia (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005).

Type Section: Gebel Gharamul (28°01′N, 33°08′E; Figures 1 to 3).

Lithology: mainly reefoidal limestone (Saoudi and Khalil, 1986), fossiliferous carbonates and shales (Richardson, 1988).

Boundaries: conformably overlies the Shoab Ali Member or Eocene limestone, and is unconformably overlain by the Rudeis Formation (Saoudi and Khalil, 1986; Richardson, 1988) or correlative Mheiherrat Formation (EGPC, 1996).

October Member, Nukhul Formation

Nomenclature, Synonyms and Correlatives: (1) Upper Nukhul members (questionably in this paper, see Depositional Environment below); and (2) Nukhul Sand equivalent (Hughes et al., 1992; Hughes and Beydoun, 1992). No equivalent for this member is given in the stratigraphic scheme of Montenat et al. (1986a, b, c, 1988, 1998).

Type Section: Rudeis-4 Well (Figures 1 to 3) between log depths of 8,470–8,960 ft (Saoudi and Khalil, 1986). In the type sections of the Shoab Ali Member its contact relationship with each of the Upper Nukhul members are not defined in the same wells or localities (Figure 3). It is therefore not possible to determine if the October Member occurs above the Shoab Ali Member or duplicates it in part.

Lithology: mainly sand, sandstone, conglomerates, shales and occasional limestone (Saoudi and Khalil, 1986; Richardson, 1988). See Depositional Environment below.

Boundaries: conformably overlies the Shoab Ali Member or the Eocene limestone (Richardson, 1988), and is unconformably overlain by the Rudeis Formation (Saoudi and Khalil, 1986). See Depositional Environment below.

Depositional Environment of the Upper Nukhul Members

October Member

Saoudi and Khalil (1986) show the thickness of the Shoab Ali Member as ca. 1,000 ft in its type well GH 385-1 (Figures 2, 4 and 5c); in the present paper the distinction between the Shoab Ali and October members is believed to be incorrect for several reasons.

  • (1) In Figure 2 they show the type well is located within a locally thick isopach of the Shoab Ali Member implying the well was drilled in a graben. However, in their cross-section, faithfully reproduced in Figure 5c, the member is shown as forming a paleohigh rather than a graben.

  • (2) In Figure 5c they interpret a normal fault as separating the Shoab Ali paleohigh from the Ghara Member. The fault is depicted with a throw of ca. 800 ft but terminates in the Shoab Ali Member without cutting the pre-rift section.

  • (3) The Nukhul Formation has nearly the same thickness on both sides of the interpreted normal fault; it seems more likely that the Ghara Member in Well GH 376-1 passes laterally by facies change (without a fault) to marginal marine clastics in Well GH 385-1.

The distinction between the Shoab Ali and October clastics is difficult to establish by lithology alone. It appears that the former is continental and the latter marine as discussed in the Wadi Nukhul type section (Figure 6).

Ghara and Gharamul Members

The Ghara anhydrite beds were deposited in highly restricted marine settings in isolated fault-bounded grabens in the southern Gulf of Suez (Figures 5 and 10, Saoudi and Khalil, 1986; Richardson, 1988). The evaporites of the correlative Yanbu Formation in Saudi Arabia are interpreted by Hughes et al. (1999) to have been deposited in localized salinas. Palynofloras from the intra-evaporitic sediments include the halophytic pollen Retiperiporites spp. but lack marine indicators; no microfauna are present.

The Gharamul Member (Subgroup Ar) consists of fetid dolomitic algal laminites and stromatolitic domes deposited in a restricted shallow-marine environment (Montenat et al., 1986a; Orszag-Sperber et al., 1986). The water was warm and brackish as indicated by the presence of coral patch reefs, shallow-water ostracods, oysters, shallow-marine foraminifera such as Miogypsinoides sp., Miogypsina sp., Elphidium crispum and Amphistegina sp., as well as many species of the bivalve pectenid (Saoudi and Khalil, 1986; Richardson and Arthur, 1988; Schütz, 1994).

RUDEIS SEQUENCE BOUNDARY

Nomenclature, Synonyms and Correlatives: (1) Rudeis SB or Sub-Rudeis Unconformity (this paper); (2) pre-Rudeis unconformity, post-Nukhul unconformity, post-Nukhul event (Beleity, 1984; Evans, 1988); (3) Unconformity II or Sequence Boundary SB 1.5 (Ayyad and Stuart, 1992); (4) Paleontological Terrace T10 (here T10) separating biosequences S10 (Nukhul Formation) and S20 (Lower Rudeis Member or Mheiherrat Formation; Wescott et al., 1996; Dolson et al., 1996; Ramzy et al., 1996; Krebs et al., 1997); and (5) Cyclolog marker GS-2 or Rudeis-1 event (Nio et al., 1996).

Stratigraphic Position: The Rudeis Sequence Boundary is positioned between the Nukhul Formation and Lower Rudeis Member (= Mheiherrat Formation; Figure 3). Over paleohighs, where the Nukhul and/or pre-rift Eocene and older formations are absent, the Rudeis SB passes to the Sub-Rudeis Unconformity and merges with the Suez Rift Unconformity.

Interpretation: In the Gulf of Suez, the Nukhul Formation is sharply overlain by the Rudeis Formation, which represents a major transgression above their boundary (Beleity, 1984; Evans, 1988; EGPC, 1996). Wescott et al. (1996) and Krebs et al. (1997) interpreted this surface as a “transgressive unconformity, or a major flooding surface within a second-order sequence”. Youssef (2011) considered the Nukhul Formation as the lowstand systems tract of the Suez Supersequence. He interpreted a higher-order sequence boundary at its top (T10) and an MFS near the base of the Rudeis Formation.

Evans (1988) described the Sub-Rudeis Unconformity at the Wadi Gharandal and Gebel Zeit outcrops as prominent, and suggested a eustatic origin. He tentatively correlating it to the global sea-level drop at ca. 21.0 Ma named Sequence Boundary SB TB1.5 in Haq et al. (1988). In Saudi Arabia, the upper contact of the Tayran Group (= Nukhul Formation) with the overlying Burqan Formation (= Rudeis Formation) is described as disconformable or unconformable (Hughes et al., 1999; Hughes and Johnson, 2005).

In the present paper the Nukhul/Rudeis boundary is referred to as the Rudeis Sequence Boundary (where it occurs above the Nukhul Formation) or Sub-Rudeis Unconformity (where the Nukhul is absent). The interpretation of the boundary as eustatic in origin requires a study of the Rudeis Formation, a subject that is targeted for a forthcoming paper (Al-Husseini, in preparation).

AGE OF THE NUKHUL FORMATION

Shoab Ali Member and Suez Rift Unconformity

In the Sinai Peninsula (Figures 2, 6, 7 and 11), the Suez Rift Unconformity separates the oldest syn-rift Shoab Ali Member (Subgroup A1, Nukhul Unit 1, Abu Zenima Formation) from the pre-rift Eocene limestones or the Lower Oligocene Tayiba Formation. Radiometrically dated basalt flows and dikes provide direct age constraints for these units and the Suez Rift Unconformity.

In the Wadi Nukhul type area, the pre-rift Eocene limestone is cut by a basalt dike, 15–20 m wide, that is dated as 26.0 ± 0.6 and 25.0 ± 0.5 Ma (Figures 2, 7 and 11; Ott d’Estevou et al., 1986). In the Abu Zenima area, the lower part of the Nukhul Formation (= Abu Zenima Formation = lower part of Shoab Ali Member, Figures 7 and 11) consists of more than 30 m of varicolored sandy marls juxtaposed by a fault against the Eocene limestone. The marls are concordantly overlain by a basalt flow (less than 40 m thick) that is dated at 21.95 ± 0.5 Ma and 22.15 ± 0.5 Ma. The top of the basalt is an erosional surface overlain by sandy marls and shales (= upper part of Shoab Ali Member). These ages constrain the maximum depositional age of the Shoab Ali Member to younger than ca. 26.0 and older than 22.0 Ma.

Other age datings from the Red Sea and Sinai coastal plains suggest the basalt flows occurred throughout the time interval between ca. 26.0–22.0 Ma (Figure 11). At Sharm El Bahari and Sharm El Qibli near Quseir city, Plaziat et al. (1998) correlated the Red Bed Series, Abu Ghusun Formation and Subgroup A1 (= Shoab Ali Member). Basalts flows in these units are dated at 24.9 ± 0.6 Ma (Roussel et al., 1986) and 22.6 ± 0.5 Ma (El-Haddad, 1984). At Gebel Monsill in the Gharamul area, a K-Ar age date of 24.7 ± 0.6 Ma was reported for a basalt dike that cuts into the pre-rift Cretaceous strata below the Nukhul Formation (Plaziat et al., 1998).

Similar age windows for the basalts are also reported in several papers as between (Figure 11): (1) 24.0–19.0 Ma (Evans, 1988); (2) 23.3–21.7 Ma (Schütz, 1994); (3) 24.0–22.0 Ma (Patton et al., 1994); (4) younger than 27.0 Ma (Bosworth and McClay, 2001); and 23.0 ± 2.0 Ma (Jackson et al., 2006). These estimates are based on radiometric studies cited in several papers (e.g. Garfunkel and Bartov, 1977; Steinitz et al., 1978; Steen, 1984; Montenat et al., 1986a, b, c; Mousa, 1987). The estimated age of basalt emplacement in Egypt essentially coincides to dated Red Sea volcanics in Saudi Arabia and Yemen between ca. 26.0–20.0 Ma (Chazot et al., 1998).

The age of the Miocene/Oligocene boundary is estimated at 23.8 Ma (Berggren et al., 1995), 23.3 Ma (Harland et al., 1991) or 23.03 Ma (Gradstein et al., 2004; Ogg and Ogg, 2006), thus implying the oldest basalt emplacement (start of rifting and the lower part of Shoab Ali Member) is Late Oligocene (Chattian) in age.

Dating the Shoab Ali as extending into Late Oligocene based on radiometric data, however, is not confirmed by fossil evidence in the Gulf of Suez and northern Red Sea. In Saudi Arabia the age of the Al Wajh Formation (= Shoab Ali Member) is interpreted as Early Miocene (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005). They based this age interpretation on the presence of acanthaceae-type pollen (evolution at base Miocene), Fenestrites spinosus sp. (evolution at base Miocene), and the absence of charred gramineae cuticle (not recorded in basal Miocene). Echinoids found within the siliciclastics are related to those considered typical of Miocene rocks in the Gulf of Suez (D. Hamama, 1998, inHughes and Johnson, 2005).

In their figure 40, however, Hughes and Johnson (2005) show the position of their Lower Miocene palynomorph assemblage immediately below the salt of the Yanbu Formation (= Ghara Member) suggesting the biostratigraphic samples represent the age of the uppermost Al Wajh Formation (= Shoab Ali Member). In the same figure, the Yanbu Salt is given strontium-isotope dates between 23.1–21.6 Ma (Cocker and Hughes, 1993) implying the uppermost Al Wajh Formation (Shoab Ali Member) occurs near the Oligocene/Miocene boundary (23.03 Ma, Figure 11).

A Late Oligocene–earliest Miocene age for the Shoab Ali Member seems consistent with that for the age of the oldest syn-rift sediments in the southern Red Sea. Hughes et al. (1991) identified calcareous nannofossil Heliocosphaera recta in the basal part of the Eritrean offshore Thio-1 Well (Figure 1). They considered this nannofossil to represent Upper Oligocene Zone NP25 of Martini (1971) and Berggren et al. (1995), which Ogg and Ogg (2006) calibrated between 27.27–23.03 Ma in the Late Oligocene Chattian time.

Ghara and Gharamul Members

At Abu Ghusun and the Ras Honkorab (Figure 1), marl beds near the top of Subgroup Ae (= Ghara Member) yielded Aquitanian planktonic microfauna Globigerinoides trilobus primordius,G. quadrilobatus, G. praebulloides and G. gr. Ciperoensis s. (Montenat et al., 1986b; Plaziat et al., 1998). At Wadi Gasus in the south Safaga area (Figure 1), Subgroup Ae is ca. 100 m thick (= Rosa Member of Ranga Formation = Ghara Member) and contains three well-bedded gypsum beds with a thickness of ca. 2–6 m each. They are separated by green marls and siltstones that yielded diatoms and Mediterranean foraminifera of Aquitanian age (Orzag-Sperber et al., 1986).

The Saudi Arabian Musayr Formation (= Gharamul Member) yielded the benthonic foraminiferal genera Miogypsinoides and Miogypsina including Miogypsina tani that indicate an Early Miocene age (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005). M. Simmons (2011, written communication) indicated that this is a key occurrence that implies an age of latest Aquitanian–Early Burdigalian and not older than planktonic foraminiferal Zone N5.

Planktonic Foraminifera (N and P) and Calcareous Nannofossil (NN) Zones of the Nukhul Formation

In the southern Gulf of Suez, Scott and Govean (1986) identified Globigerinoides altiapertura in the upper part of the Nukhul Formation (probably Upper Nukhul members) that is attributed to zones N5 and N6 (Figure 11). It underlies the Rudeis Formation, which is assigned to the Globigerinoides ruber Zone (N6 and younger zones). Evans (1988) attributed to Scott and Govean (1986) additional identifications of planktonic foraminifera from the Nukhul type section that he believes may belong to zones N4 and N5 of Blow (1969). Age indicative species include Globigerinoides trilocularis (N4 and younger N zones), and Globigerina ciperoensis (P18 to N4). M. Simmons (2011, written communication) noted that if correctly identified the latter species would indicate an earliest Miocene age.

Schütz (his figure 31, 1995) correlated the Nukhul Formation to planktonic foraminiferal zones N5 to mid-N7 and calcareous nannofossil zones NN1 to mid-NN4. The GUPCO Stratigraphy Team (unpublished chart, 2002) correlated it to zones N5 and/or older-than-N5 and NN2. The upper boundary of the Nukhul Formation, the Rudeis SB, is considered a hiatus spanning zones late-N4 and N5 and late-NN2 between ca. 21.0−18.6 Ma (Evans, 1988). Other age estimates for the Rudeis SB include 21.0–19.0 Ma (Ayyad and Stuart, 1992) or near the Aquitanian/Burdigalian boundary at ca. 20.0 Ma (Wescott et al., 1996; Krebs et al., 1997).

Summary of Age Estimates

The data in Figure 11 summarizes the wide range of possible ages for the Nukhul Formation. The biostratigraphic zones suggest an Early Miocene age (Aquitanian and Early Burdigalian). The strontium-isotope age of ca. 23.1–21.6 Ma for the Yanbu Formation (= Ghara Member) implies an Early Aquitanian age for the Upper Nukhul members. The Shoab Ali Member based on the ages of syn-depositional basalt flows and by stratigraphic position below the Upper Nukhul members would have an age of ca. 26 to 23 to possibly 22 Ma, thus spanning the Oligocene/Miocene boundary. These ages compare well with those shown in the unpublished Working Operations Chart of the Gulf of Suez (Figure 11). In the following section these age estimates are compared to those inferred by a calibration in an orbital time scale (Matthews and Al-Husseini, 2010).

ORBITAL INTERPRETATION OF THE NUKHUL SEQUENCE

Wadi Dib-1 Well Revisited

The Wadi Dib-1 Well (Figures 2 and 5c, 27°52′47″N, 33°17′17″E) was drilled in 1977 and 1978 by Chevron Oil Company of Egypt. It encountered the top of the uppermost Ghara Anhydrite 1 bed at 9,753 ft and the Proterozoic Basement at 11,602 ft (datum taken at elevation of Kelly Bush at 325 ft above sea level). The lithology from cutting samples was described by Chevron’s geologists M. Raslan, C. Allen and Z. Ebeid (Figure 12) and by Saoudi and Khalil (1996) and Richardson (1988). Based on the lithological description by Chevron’s geologists 10 Nukhul subsequence are here interpreted. The lower six correspond to the Shoab Ali Member (1,106 ft, 310 m thick) and upper four to the Ghara Member.

The top of the Nukhul Sequence is taken at the top of Subsequence 1 at 9,700 ft such that the Ghara Member is 885 ft (270 m) thick, as consistent with the cross-section of Saoudi and Khalil (1986, Figure 5c). Above the Nukhul Formation, the Rudeis Formation (705 ft, 215 m thick) consists entirely of sandstone, and is overlain by the Kareem Formation at 8,995 ft. The Kareem Formation (313 ft, 95 m thick) is recognized because its lower Rahmi Member contains its three characteristic anhydrite beds (Rahmi Anhydrite 1 to 3, see Al-Husseini et al., 2010).

The oldest four Nukhul subsequences are each characterized by a basal shale bed or shale-dominated interval (ranging in thickness from 10–60 ft, 3–20 m) that passes to sandstones with minor siltstone beds (ranging in thickness from 56–216 ft, 17–66 m). The shale is gray to brownish-gray and changes from non-calcareous in the lowermost Subsequence 10 to slightly calcareous in the fourth-up Subsequence 7. The next two subsequences 6 and 5 each start with calcareous shale inter-bedded with meter-thick beds of dolomite. These lower two units are each ca. 30 ft (10 m) thick and each passes to massive sandstones (112 and 144 ft, 34 and 44 m thick, respectively).

The youngest Nukhul subsequences 4 to 1 (Ghara Member) each starts with an anhydrite bed or an interval dominated by anhydrite beds (ranging in thickness from 57–132 ft, 17–40 m, Figure 12). The anhydrite is described as white to light gray, frequently mottled, hard, crystalline to soft and sucrosic. The anhydrite beds pass to sandstones ranging in thickness from 56–144 ft (17–44 m). Nukhul subsequences 1 to 4 are correlated by Saoudi and Khalil (1986) from the Wadi Dib-1 to Zeit Bay-1 wells (located ca. 15 km apart in the Gebel Zeit Graben), and again from wells GH 391-2 to GH 376-1 in the southern offshore Gulf of Suez (Figures 2 and 5c).

The Ghara anhydrites extend across ca. 60 km in the southern Gulf of Suez and correlate to the Yanbu Formation in Saudi Arabia’s Midyan Peninsula and Yanbu Salt in the Yanbu Graben, located some 200 and 1,000 km away, respectively (Figure 1). They represent periods of arid climatic conditions across the two sides of the Red Sea Rift System. The intervening facies (sandstones, marls and carbonates), on the other hand, reflect more temperate climates with pluvial-fluvial systems that powered the transport of sandstones into the rift basin. These same arid to pluvial-fluvial cycles would also apply to the shale-sandstone cycles forming Nukhul subsequences 10 to 5 of the Shoab Ali Member.

The rhythmic climatic changes as interpreted by the Nukhul subsequences are here considered stratons: transgressive-regressive depositional sequences that tracked the 405,000 (405 Ky) eccentricity signal of the Earth’s orbit. The ten stratons range in thickness from 33 to 84 m and average 57 m. Matthews and Al-Husseini (2010) estimated that stratons can vary in duration from 285–505 Ky and to average to 405 Ky over an interval of 14.58 My (one Orbiton consisting of 36 stratons). The difference in thickness may therefore be attributed to different durations for the subsequences and/or changes in the rate of subsidence. Converting the 10 subsequences with the 405 Ky/subsequence chronometer, the Nukhul Sequence represents ca. 4.0 million years of deposition in the Wadi Dib-1 Well.

The orbital time scale was previously used to estimate the age of the Kareem Sequence Boundary at 16.1 Ma (Figure 11; Al-Husseini et al., 2010). Between the Kareem SB and Rudeis SB, the Rudeis Formation is believed to represent 12 stratons such that its deposition lasted 4.86 million years between 21.0–16.1 Ma (Al-Husseini, in preparation). This calibration implies the Rudeis SB has an age of 21.0 Ma and the Suez Rift Unconformity, being ca. 4.0 My older, would have an age of ca. 25.0 Ma, as approximately consistent with the ages of the oldest syn-depositional basalts (ca. 26 Ma).

The maximum flooding surface of the Nukhul Sequence is taken at the base of Ghara Member (below Ghara Anhydrite 4), which marks the first marine transgression in the Gulf of Suez (Figures 11 and 12). The Nukhul MFS is ca. 1.6 million years (four stratons) older than the estimated age of the Rudeis SB (ca. 21.0 Ma), implying its age is ca. 22.6 Ma in the earliest Aquitanian time (younger than 23.03 Ma). This age estimate is one straton (400 Ky) younger than that of the Oligocene/Miocene boundary and occurs within the strontium-isotope dating window of 23.1–21.6 Ma for the Yanbu Salt.

In the chart “Cenozoic Biostratigraphy – Global and North Sea” (Ogg and Ogg, 2006), the Nukhul MFS correlates near the boundary of Polarity Chronozones C6C and C6B (22.56 Ma), within Planktonic Foraminifera Zone N4 (22.96–21.08 Ma), near base of Calcareous Nannofossil Zone NN2 (22.82–18.06 Ma). It closely correlates to the prominent flooding surface MFS Aq1 (undated in chart falling at ca. 22.3 Ma) between sequence boundaries Ch4/Aq1 (23.14 Ma) and Aq2 (21.44 Ma).

CONCLUSIONS

The Late Oligocene–Early Miocene Nukhul Sequence represents the oldest syn-rift time-rock unit in the Gulf of Suez. In the Wadi Dib-1 Well, the sequence overlies the crystalline basement of probable Proterozoic age and its base, the Suez Rift Unconformity, has an estimated orbital age of ca. 25 Ma in Late Oligocene (Chattian) time. The Nukhul Sequence is bounded above by the Rudeis Sequence Boundary, or correlative Sub-Rudeis Unconformity, with an estimated orbital age of 21.0 Ma. In the Wadi Dib-1 Well the sequence is 1,902 ft (580 m) thick and consists of Nukhul subsequences 1 to 10 in descending order, with thicknesses ranging from 33–84 m. The ten subsequences are interpreted as stratons that tracked the 405 Ky eccentricity signal of the Earth’s orbit. The older six Nukhul subsequences 10 to 5 correspond to the Shoab Ali Member that was deposited initially in fluvial-alluvial settings and then in estuarine-coastal settings. The younger four Nukhul subsequences 4 to 1 were deposited during a regional marine transgression and correspond to the Upper Nukhul members (Ghara, Gharamul and October members). The Nukhul maximum flooding surface (Nukhul MFS) is positioned between the Shoab Ali and Upper Nukhul members and orbitally dated at ca. 22.6 Ma.

ACKNOWLEDGEMENTS

This paper is part of a study conducted by the author, M. Dia Mahmoud and Rob K. Matthews in 2004 as the Red Sea Orbital Stratigraphy project. The author would like to thank ExxonMobil, Saudi Aramco and Shell for supporting the study. The author thanks E. Blanc, G.W. Hughes and M. Simmons for providing helpful comments, K. Breining for proof-reading the manuscript and GeoArabia Designer Arnold Egdane for designing the manuscript.

ABOUT THE AUTHOR

Moujahed I. Al-Husseini founded Gulf PetroLink in 1993 in Manama, Bahrain. Gulf PetroLink is a consultancy aimed at transferring technology to the Middle East petroleum industry. Moujahed received his BSc in Engineering Science from King Fahd University of Petroleum and Minerals in Dhahran (1971), MSc in Operations Research from Stanford University, California (1972), PhD in Earth Sciences from Brown University, Rhode Island (1975) and Program for Management Development from Harvard University, Boston (1987). Moujahed joined Saudi Aramco in 1976 and was the Exploration Manager from 1989 to 1992. In 1996, Gulf PetroLink launched the journal of Middle East Petroleum Geosciences, GeoArabia, for which Moujahed is Editor-in-Chief. Moujahed also represented the GEO Conference Secretariat, Gulf PetroLink-GeoArabia in Bahrain from 1999–2004. He has published about 50 papers covering seismology, exploration and the regional geology of the Middle East, and is a member of the EAGE and the Geological Society of London.

geoarabi@batelco.com.bh