Late Oligocene–Early Miocene Nukhul Sequence, Gulf of Suez and Red Sea

GeoArabia, 2012, v. 17, no. 1, p. 17-44 Gulf PetroLink, Bahrain

Moujahed I. Al-Husseini

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.

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a 35° 40°E 45° b 33°E 34° 35° JORDAN Gulf of Sinai Suez Gulf of JORDAN Sinai Abu Rudeis Aqaba 30°N 29°N Gulf of SAUDI Suez ARABIA Gulf of Aqaba Abu Durba Ras Maqna Gharib Midyan 28° Port Gharamul Safaga Gebel Zeit Gebel Mallaha EGYPT SAUDI ARABIA 25° 25° 27° Hurghada Ras Banas Yanbu Safaga Gebel Arabian Shield Duwi Quseir Jeddah 26° EGYPT Red Sea Mecca Red Sea 20° 20° Marsa’ Alam Port Sudan 25° Ras Honkorab SUDAN Abu Ghusun N 0 100 Ras Banas Jizan 24° ERITREA Km Khartoum Sana’a 15° Asmara Thio-1 15° Figure 1: (a) The Red Sea Rift System YEMEN extends from the northern Gulf of N Suez to the Bab Al Mandeb Strait. 0 200 (b) Locations mentioned in the paper. Bab Al Mandeb ETHIOPIA Gulf of Km DJIBOUTI Aden 35° 40° 45°

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.

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ISOPACH MAP, SHOAB ALI MEMBER, GULF OF SUEZ

30°N 32°30'E 33° 33°30' 34° 34°30'

30° 0 ft 500 N 1,000 North Darag 0 50 1,500 km 2,000 Sudr Contours in feet City Outcrop 29°30' Ras Matarma Well Asl Oil field Sinai 29°30' Peninsula

Hammam Faraun Zaafarana Figure 7 Abu Zenima 29° Wadi Gharandal Ras Wadi Nukhul “J” Budran (Figures 6 and 7) North October 29° (Figure 9) Abu Rudeis Sidri Issran Amer North Offshore October Belayim El Qaa Marine Feiran North Amer Gebel Ekna Belayim Ras Amer Land Ras Fanar 28°30' Proterozoic SG 300 Bakr Bakr West H Basement Hana Bakr West K Ramadan 28°30' Ras Gharib South Ramadan El Ayun GS 327 Kareem July Badri El Tor Umm El Yusr Morgan Kheir Nessim GS345 SB 339 Shukheir Gamma 28° Younis Sinai/GHM Waly (356) Amal GH (376) GH 385-1 Gebel Zeit Ras El Ush GS (356) 28° Gharamul 50 Sidki Shoab Ali 0 1000 Wadi Dib-1 (Figure 12) 1,500 Zeit Bay-1 500 Abu Girfan Hilal Zeit East Zeit Bay 0

Gemsa ? Geisum ? 500 Tawila

27°30' Gemsa-Southeast 1000

Proterozoic Esh El-Mallaha Basement Esh 27°30' El-Mallaha

Hurghada 32°30' 33° 33°30' 34° 34°30'

Figure 2: Thickness of the Shoab Ali Member as contoured at 500 ft interval by Saoudi and Khalil (1986). In their map the member does not extend north of about 28°N. Also shown are locations mentioned in the paper.

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Wadi Nukhul, Gulf of Suez, Gulf of Suez, Egypt Saudi Arabia Sinai, Egypt Egypt

Montenat et Saoudi and Khalil Pooley al. (1998) and Wescott Hughes and (1986) (1947) GENEBASS et al. (1996) Johnson (2005) Group (1996) T50 Shagar Ras Mbr Budran S50 Mbr Kareem T40 Wadi Waqb Middle Fm Formation Rahmi Mbr Lagia

un Musa Fm S40 Mbr Ay

Not T30 Reviewed Upper Asl Fm Member S30 Burqan Formation Hawara MIOCENE Fm

Rudeis Formation T20 Lower Mheiherrat S20 Member Fm T10 Lower Gharandal Group 3 As Ae

Nukhul Fm

A Nukhul nbu Formation Subgroup Subgroup Marine Unit 23.1–21.6 Ma

Ghara Member Formation October Member Ya Musayr Formation Gharamul Member Biosequence

? Group S10 ayran Group Unit T Nukhul Formation Subroup Abu 2 Shoab Ali A1 Al Wajh Zenima Member Red Formation OLIGO- Fm CENE Beds Unit 1 T00 Suez Rift Unconformity Figure 3: Stratigraphic schemes for the Nukhul Formation discussed in the paper. The present study follows Saoudi and Khalil (1986) and Richardson and Arthur (1988) and divides the Nukhul Formation into the continental clastics of the Shoab Ali Member, and the Upper Nukhul members consisting of the October (mainly clastics), Ghara (mainly evaporites) and Gharamul (mainly carbonates) members.

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:

(1) pre-Nukhul hiatus, event or unconformity (Evans, 1988; EGPC, 1996); (2) Unconformity I, Sequence Boundary 1 (SB1), pre-Miocene unconformity, early-clysmic or clysmic event (Ayyad and Stuart, 1992);

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COMPOSITE SECTION, NUKHUL FORMATION, GULF OF SUEZ

Bio Strat Rock Units Lithology Sonic Log Age Zone

Rudeis 1 2 3 Costata Formation

1 2 3 11,808 sp. sp. sp. Elphidium Ostracoda Miogypsina Ghara Member (GS 391-2 Well) October Member (Rudeis-4 Well) Gharamul Member (G. Gharamul)

12,410 6,980 Lithology Anhydrite

Sandstone

Conglomerate

Gharandal Group Limestone EARLY MIOCENE

Nukhul Formation Marly limestone

Shale

Figure 4: Composite section of the Nukhul Formation No Fauna

(GH 385-1) showing its four members

Shoab Ali Member (reproduced from Saoudi and Khalil, 1986). Note the boundaries between the Shoab Ali Member and the Upper members are not defined in the same wells or outcrops. In the present paper the base of the oldest syn-rift Shoab Ali Member is referred to as the 7,980 ft Suez Rift Unconformity and Oligocene Suez Rift Unconformity identified below the unnamed Eocene Oligocene Wedge shown by Saoudi and Khalil (1986).

(3) Paleontological Terrace T00 (here T00; Wescott et al., 1996; Dolson et al., 1996; Ramzy et al., 1996; Krebs et al., 1997; Youssef, 2011). (4) Jackson et al. (2006) referred to the Suez Rift Unconformity as the “base syn-rift unconformity”. They mapped it in the Hammam Faraun Fault Block above the uppermost (youngest) pre-rift Lower Oligocene Tayiba Formation. The Tayiba Formation is only preserved in the hanging walls to several intra-block fault zones and a few isolated localities in their study area (Figure 2).

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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):

(1) In part the Abu Zenima Formation at outcrop in the western Sinai Peninsula (Hantar, 1965, in Patton et al., 1994; El-Heiny and Martini, 1981; El-Heiny and Morsy, 1992; Plaziat et al., 1998; Bosworth and McClay, 2001). In the Hammam Faraun Fault Block, Jackson et al. (2006) characterized the volcaniclastic Abu Zenima Formation as continental clastics interbedded with local lenticular basalt units (up to 22 m thick), and followed other authors by placing the Nukhul Formation above it (e.g. Patton et al., 1994; Carr et al., 2003). In the present paper, and as discussed below, the Abu Zenima is considered a volcaniclastic equivalent of the lower part of the Shoab Ali Member. (2) Subgroup A1 of Group A, basal Red Series or Red Bed Series in the Sinai Peninsula and

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Egypt’s Red Sea coastal plains (Montenat et al., 1986a, b, c, 1988, 1998) or Subgroup Ar (Thiriet et al., 1986). (3) Lower part of Umm Abbas Formation and Rosa Member of the Ranga Formation along Egypt’s Red Sea coastal plains (Gindy, 1963; Issawi et al., 1981; El-Gezeery and Marzouk, 1974; Montenat et al., 1998). (4) Lower part of Abu Ghusun and Nakheil formations along Egypt’s Red Sea coastal plains (El-Akkad and Dardir, 1966).

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, 29o01’30”N; 33o11’30”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.

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(a) STRATIGRAPHIC CROSS-SECTION, NUKHUL FORMATION, GULF OF SUEZ (c) STRATIGRAPHIC CROSS-SECTION, NUKHUL FORMATION, GULF OF SUEZ

Northwest Southeast West East GS 276-1 Sabra-1 GS 351-1 Morgan-8 GS 327-1 GS 347-1 Ghara M-1 GH 376-1 Shoab Ali-1 Wadi Dib-1 Zeit Bay-1 OO 89-1 GS 391-2 GS 392-2 GH 376-1 GH 385-1 A A' C C' ft 0 0 0

1 200 200 200 1 200 Ghara Mbr 2 2 400 400 pre-Miocene 400 Ghara 400 3 Member

Nukhul Thickness in feet 600 Fm 600 3 A 4 600 600 Gebel Zeit GS 276-1 Suez Rift Unconformity High 4 800 28°30' Shoab Ali 800 800 Mbr Sabra-1 1,000 pre-Miocene B KK 85-1 28°30' 1,000 1,000 Sinai Shoab Ali J-3 Member GS 351-1 1,200 R-5A Morgan-8 GS 327-1 R-1 Alef-1 pre-Miocene 1,200 Nukhul 1,200 33°20' Formation Gulf of Suez 1,400 Sandstone marl, GS 313-3 Sinai shale and limestone LL 87-1 28 1,400 1,400 ° Continental clastics

paper they are believed to be isochronous units and to onlap the Gebel 1,200 Zeit High. The fault interpretation between GH 376-1 and GH 385-1 is Pre-Miocene here believed to be incorrect; instead the Ghara Member is suspected to 1,400 pass laterally to the October Member without any fault. See facing page for continuation.

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(a) STRATIGRAPHIC CROSS-SECTION, NUKHUL FORMATION, GULF OF SUEZ (c) STRATIGRAPHIC CROSS-SECTION, NUKHUL FORMATION, GULF OF SUEZ

Northwest Southeast West East GS 276-1 Sabra-1 GS 351-1 Morgan-8 GS 327-1 GS 347-1 Ghara M-1 GH 376-1 Shoab Ali-1 Wadi Dib-1 Zeit Bay-1 OO 89-1 GS 391-2 GS 392-2 GH 376-1 GH 385-1 A A' C C' ft 0 0 0

1 200 200 200 1 200 Ghara Mbr 2 2 400 400 pre-Miocene 400 Ghara 400 3 Member

GS 347-1 1,600 Thickness in feet Gulf of Suez Anhydrite bed or correlative marker 1,600 Shoab Ali 1,600 28° Member 28° N GS 345-1 Wadi Dib-1 33° Zeit Bay-1 Ghara M-1 1,800 1,800 Amal-6 Sandstone marl, shale and limestone C GH 376-1 Gulf of Suez GS 373-1 Continental clastics N 2,000 0 20 km 2,000 GS 382-3A Anhydrite bed or OO 89-1 GS 382-1A correlative marker 0 25 km Shoab GS 391-2 GH 376-1 28° 2,200 2,200 33°30' A' Ali-1 GS 392-2 pre-Miocene GS 391-2 0 15 km GS 392-2 C' Ashrafi-1 Suez Rift GH 385-1 33°30' B' 2,400 Unconformity N 2,400 (b) Northwest Southeast Oligocene 33° 2,600 2,600 87-1 Figure 5 (Continued): KK 85-1 J-3 R-5A Alef-1 R-1 GS 313-3LL GS 345-1Amal-6 GS 373-1 GS 382-3AGS 382-1AGS 391-2GS 392-2 Ashrafi-1 B B' ft 0 A comparison to the three Nukhul units to the members of Saoudi and Khalil (1986) indicates that 1 continental Unit 1 and possibly Unit 2 correlate to the Shoab Ali Member, whereas marine Nukhul Nukhul 2 1 200 200 Unit 3 correlates to the Upper Nukhul members (Figures 3, 4 and 6). It seems possible to speculate 3 2 Ghara that Nukhul Unit 3 may be the Nukhul Formation sensu Pooley’s 1947 definition because they are Member 400 Pre-Miocene 400 comparable by thickness: 60 or 74 m versus 77 m. Nukhul units 1 and 2 (together 233 m, 764 ft thick), Thickness in feet 3 when correlated to the Shoab Ali Member, imply this member was deposited much further north Suez Rift Unconformity 600 600 than shown in the map of Saoudi and Khalil (1986; Figure 2). 4 Figure 5: Cross-sections AA’, BB’ and CC’ in the southern Gulf of Suez Nukhul units 2 and 3 may also correlate to the Nukhul Formation in the Hammam Faraun Fault showing the Shoab Ali and Ghara members of the Nukhul Formation Anhydrite 800 Shoab Block (Figure 2). At this locality the formation consists of mudstones and sandstones that were (reproduced from Saoudi and Khalil, 1986). In the cross-section CC’ in Ali Marker Mbr Beds deposited in offshore, shoreface and estuarine settings (Carr et al., 2003; Jackson et al., 2006). Figure 5c the four anhydrite marker beds are interpreted to merge into 1,000 Marker 1 in the OO 89-1 Well by Saoudi and Khalil (1986). In the present paper they are believed to be isochronous units and to onlap the Gebel 1,200 Group A and Nukhul Formation Zeit High. The fault interpretation between GH 376-1 and GH 385-1 is Pre-Miocene Ott d’Estevou et al. (1986) also described the section in Wadi Nukhul and correlated it to other here believed to be incorrect; instead the Ghara Member is suspected to sections including the Abu Zenima section (Figures 2, 3 and 7). Their correlations shows the lower 1,400 pass laterally to the October Member without any fault. part of the Nukhul’s type section correlates to the Abu Zenima Formation. They adopted the See facing page for continuation.

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NUKHUL FORMATION, WADI NUKHUL 310 Flooding Distal shoreface Surface LEGEND 160 Distal Sandstone alluvial fan 300 Shale

150 Conglomerate Offshore Angular Evaporites Unconformity 290 Limestone 140 Horizontal beds

280 Planar cross beds Distal 130 Trough cross beds

shoreface 3 Fluvial, overbank Ripple laminae

270 Unit paleosols Wavy beds 120 Offshore Climbing ripples Glossi- fungites 260 Hummocky cross beds Ravine- ment Contested beds 110 Distal shoreface Fossils 250 Trace fossils

100 Rip-up clasts Brackish lagoon/lake Offshore 240 S Stromatolites Bioturbation 90 Ravine- Siltstone ment 230 1 Shoal Sand/shale Tidal flats 80 Unit Shell fragments

Lagoon Depth in meters 220 Figure 6: Type section 70 Tidal channel of the Nukhul Formation in Wadi Tidal flat 210 Nukhul (reproduced 60 from Wescott et al., Upper 2 1996; Krebs et al., estuary 200 channel 1997). In the present complex Unit Continental, 50 paper the base of the deposits/ formation is named fluvial, 190 aeolian, the Suez Rift paleosols Unconformity; 40 continental Unit 1 Estuary 180 and near-shore Unit 2 Glossi- are correlated to the fungites 30 Shoab Ali Member of the Nukhul 170 Flooding Formation, and surface 20 offshore Unit 3 to the (continued from left) Gharamul Member of the Nukhul Formation. See 10 Figures 3 and 7 for (T00) stratigraphic Marine Sequence Suez Rift Boundary Unconformity correlations. 0 m EOCENE

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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, in Schlumberger, 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

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North Gebel Ekna

600 33°10'E 33°20' 33°30' Abu Zenima Wadi Gharandal Wadi Nukhul 550 Ras 29°N Budran El Qaa N Plain 0 15 Abu Rudeis 500 km Sidri B 28°50' October Belayim Group 450 Marine Feiran El Qaa

Rudeis Formation Gebel Ekna 28°40' 400 Belayim Land

350 Northeast Gebel Ekna Sub-Rudeis Unconformity Wadi Nukhul 300 Southwest Gebel Ekna

As Eocene 3 Unit 250 Limestone Subgroup

Upper Members Siltstone

A Abu Zenima Sandstone 200

2 Volcanics

Foraminifera Unit

Note: Vertical scale 150 adjusted using Wadi Nukhul (Figure 8)

A1 22 Ma Suez Rift Eocene Nukhul Formation or Group Ali Member 100 Unconformity

1 Figure 7: Cross-section showing Subgroup

Shoab correlation of Group A along the Unit eastern coastal plain of the Gulf of Suez 50 Abu Zenima Formation (reproduced from Ott d’Estevou et al., 1986). This section shows that Group A is equivalent to the Nukhul Formation 0 m as defined in Wadi Nukhul (Figure 6, Eocene 26–25 Ma Eocene Wescott et al., 1996; Krebs et al., 1997). It also shows that the volcaniclastic Abu Zenima Formation correlates to the lower part of the Nukhul Formation.

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).

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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

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ISOPACH MAP, NUKHUL FORMATION, GULF OF SUEZ

30°N 32°30'E 33° 33°30' 34° 34°30'

75 30° 0 m 100 100 50 75 200 75 50 100 200 0 300 400 29°30' 500 50 600

100 700 75 29°30' 50 Sinai Peninsula

29°

50 200 75

29°

100

50 0

100 75 0 28°30' Proterozoic 100 75 Basement 400 75 0 28°30' 50 100

75 200 200 0 50 0

0 100 0 0 50 28° 75 50

50 0 0 100 300 28° 700 400 Proterozoic

Basement 0

100 0 0

100 27°30' 0 50 N 75 0 50 100 27°30' km 75 75 Red Sea Contours in meter 50 50

50 32°30' 33° 33°30' 75 0 34° 34°30'

Figure 8: Thickness of the Nukhul Formation as contoured by Richardson and Arthur (1988).

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NUKHUL FORMATION ISOPACH, OCTOBER FIELD, GULF OF SUEZ

a 0 ft. 400 Wadi Tayiba 100 200

60 300 0 400 500 500 Wadi Nukhul 600 North 700 200 October Field 600 300 500

400 400 Platform 0 F Well J 0 30 300

C 10 400 0 300 300 200 H B 200 100 A N G 0 10 E

km October Field Contours in feet

MHEIHERRAT FORMATION ISOPACH, OCTOBER FIELD, GULF OF SUEZ b 0 ft Wadi Tayiba 200 400 600 800 800 1,600 1,400 1,000 1,000 Wadi Nukhul 1,200 1,200 1,200 1,000 1,400 1,400 800 1,600 600 1,600 400

North F October Field J 1,600 Figure 9: Thickness of (a) 1,400 1,200 Nukhul Formation, and (b) D 1,000 Lower Rudeis Member 600 (Mheiherrat Formation) in C the area of the October and North October oil fields 1,600 1,400 H B 1,200 (see Figure 2 for location; October A reproduced from Dolson et Field G al., 1996). The Nukhul N 800 0 10 E Formation and Lower Rudeis Member are absent km 1,400 1,200 over the crest of the Contours in feet 1,000 October Field.

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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 29o27’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 (28o01’N, 33o08’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).

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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).

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30°

29°30'

29°

Al-Husseini 29°

ANHYDRITE ISOLITH, NUKHUL FORMATION, GULF OF SUEZ

32°30'E 33° 33°30' 34° 34°30' 0 ft Gulf of 28°30'N Suez 100 200 400 100 28°30' 0 Sinai 100 Peninsula Northern Limit of Anhydrite Facies 0 0 Proterozoic 28° 100 Basement

0 0 28° 200 100 400 0 0 Proterozoic 200

Basement 20

Ras Mohamed

200

27°30' N 0 0 0 50 Red Sea km 27°30' Contours in feet

32°30' 33° 33°30' 34° 34°30'

Figure 10: Cumulative thickness of the anhydrite beds in the Ghara Member of the Nukhul Formation (reproduced from Saoudi and Khalil, 1986).

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

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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, in Hughes and Johnson, 2005).

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Al-Husseini

Ali Member) Ali (Shoab Abu Zenima Formation Zenima Abu

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21.9 16.0 20.5 25.2

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Upper Members

Al-Husseini

This Study

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Kareem SB

(in preparation)

Al-Husseini et al.

Rudeis Formation

Kareem Sequence

Aq2

Suez Rift Unconformity

21.44 23.14

Ch4/Aq1

22.3

Aq1

MFS

NN4 NN3 NN2

17.95 18.28 22.82 23.03

NP25

ime Scale

N3 N4

N5 N8 N7

NN2

22.96 21.08 17.59

16.97

n Burdigalia Aquitanian Chattian

Ogg and Ogg (2006)

Geological T

N: Planktonic Foraminifera

NN: Calcareous Nannofossil

Mid

20.43 23.03 15.97 Early Miocene Early Oligocene

Miocene

Langhian

15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 25.0

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In their figure 40, however, Hughes and Johnson (2005) show the position of their Lower Miocene

Ali Member) Ali (Shoab Abu Zenima Formation Zenima Abu palynomorph assemblage immediately below the salt of the Yanbu Formation (= Ghara Member)

rking

21.9 16.0 20.5 25.2

18.2

Chart

Upper Lower suggesting the biostratigraphic samples represent the age of the uppermost Al Wajh Formation

Rudeis Rudeis Nukhul

Member Member

Formation

Operations (= Shoab Ali Member). In the same figure, the Yanbu Salt is given strontium-isotope dates between

Ae BASS Group BASS Subgroup Ghara GENE 23.1–21.6 Ma (Cocker and Hughes, 1993) implying the uppermost Al Wajh Formation (Shoab Ali is in

Ali and 21.0 Member) occurs near the Oligocene/Miocene boundary (23.03 Ma, Figure 11). NN2 orbital-

MFS, and Nukhul Formation Nukhul A Late Oligocene–earliest Miocene age for the Shoab Ali Member seems consistent with that for

Shoab

(Ghara N5 and older and N5 the age of the oldest syn-rift sediments in the southern Red Sea. Hughes et al. (1991) identified

Chart calibrated Al-Husseini,

GUPCO 25.0

tuned calcareous nannofossil Heliocosphaera recta in the basal part of the Eritrean offshore Thio-1 Well is

Aquitanian

Gulf of Suez

a N6 (1986) Scott and Govean and Scott Nukhul Formation N5 and N5 Formation Nukhul Nukhul (Figure 1). They considered this nannofossil to represent Upper Oligocene Zone NP25 of Martini and ca. (1971) and Berggren et al. (1995), which Ogg and Ogg (2006) calibrated between 27.27–23.03 Ma in

Biostratigraphic Zones on the members

Late N4 and N5 and N4 Late the Late Oligocene Chattian time. continental Rudeis SB Rudeis Sequence

21.0

18.6 the between based Ghara and Gharamul Members

(Matthews Nukhul Formation N4 to N6 to N4 Formation Nukhul Nukhul surface,

Evans (1988) ago

Nukhul At Abu Ghusun and the Ras Honkorab (Figure 1), marl beds near the top of Subgroup Ae from scale study

Date

Salt (= Ghara Member) yielded Aquitanian planktonic microfauna Globigerinoides trilobus primordius, G. nbu Salt and biostratigraphic assignments.

Upper Arabia Saudi Cocker and Hughes (1993) Hughes and Cocker

Strontium The u nb Ya

Ya quadrilobatus, G. praebulloides and G. gr. Ciperoensis s. (Montenat et al., 1986b; Plaziat et al., 1998). At years

21.6

23.1 flooding time 1:

1 Wadi Gasus in the south Safaga area (Figure 1), Subgroup Ae is ca. 100 m thick (= Rosa Member of

present (1994) Ranga Formation = Ghara Member) and contains three well-bedded gypsum beds with a thickness

marine transition Schülz of ca. 2–6 m each. They are separated by green marls and siltstones that yielded diatoms and

21.7 23.3 Figure the million forcing the

the Mediterranean foraminifera of Aquitanian age (Orzag-Sperber et al., 1986).

maximum to Patton et al. (1994) al. et Patton at A A

24.0 22.0 The Saudi Arabian Musayr Formation (= Gharamul Member) yielded the benthonic foraminiferal genera Miogypsinoides and Miogypsina including Miogypsina tani that indicate an Early Miocene

Evans (1988) Evans age (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005). M. Simmons

19.0 24.0

2010). placed Member Gharamul) at ca. 22.6 Ma. The figure shows radiometric dates for basalts, strontium age for the (2011, written communication) indicated that this is a key occurrence that implies an age of latest

5 5 GENEBASS Group GENEBASS 5 6

5 (Million years) Aquitanian–Early Burdigalian and not older than planktonic foraminiferal Zone N5.

26.0

25.0

± 0. ± 0. ± 0. ± 0.

21.9 22.1

et al. (1998) al. et emen, Chazot Chazot emen, Y and Arabia Arabia Saudi Saudi

Radiometric Dates for Basalts Planktonic Foraminifera (N and P) and Calcareous Nannofossil (NN) Zones

20.0 26.0 of the Nukhul Formation Nukhul Sequence Nukhul 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 Ali N5 and N6 (Figure 11). It underlies the Rudeis Formation, which is assigned to the Globigerinoides

Ghara

16.1 21.0 22.6 25.0

Gharamul ruber Zone (N6 and younger zones). Evans (1988) attributed to Scott and Govean (1986) additional

(2010)

Member

Shoab

Upper Members

Al-Husseini

This Study

Rudeis SB Nukhul MFS MFS Ng10 Kareem SB identifications of planktonic foraminifera from the Nukhul type section that he believes may

(in preparation)

Al-Husseini et al.

Rudeis Formation Kareem Sequence 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.

Aq2

Suez Rift Unconformity

21.44 23.14

Ch4/Aq1

22.3

Aq1 Schütz (his figure 31, 1995) correlated the Nukhul Formation to planktonic foraminiferal zones N5

MFS 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

NN4 NN3 NN2

17.95 18.28 22.82 23.03

NP25 ime Scale 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

N3 N4

N5 N8 N7

NN2

22.96 21.08 17.59

16.97 include 21.0–19.0 Ma (Ayyad and Stuart, 1992) or near the Aquitanian/Burdigalian boundary at ca.

n Aquitanian Chattian Burdigalia 20.0 Ma (Wescott et al., 1996; Krebs et al., 1997).

Ogg and Ogg (2006)

Geological T

N: Planktonic Foraminifera

NN: Calcareous Nannofossil

Mid

20.43 23.03 15.97 Oligocene Early Miocene Early Summary of Age Estimates

Miocene

Langhian 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 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 25.0 strontium-isotope age of ca. 23.1–21.6 Ma for the Yanbu Formation (= Ghara Member) implies an

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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, 27o52’47”N, 33o17’17”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

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NUKHUL SEQUENCE, WADI DIB-1 WELL 10,763 9,700 ft

10,800 9,753 Subsequence Subsequence 6 1 9,800 9,810 10,875

10,900 10,912

9,900

11,000 9,940

Subsequence Subsequence 7 2 10,000

11,100

11,128 10,074 11,150 10,100

10,138 11,200

Subsequence Ghara Member Subsequence 10,200 3 8

11,300 Ali Member 11,320 Shoab 10,300 10,310 11,378 11,400 Subsequence 9 11,455 10,400 11,485 11,500 10,454 Subsequence 10 Subsequence 10,500 4 11,588 RST 11,600 11,602 Basement TST

10,586 Nepheline Syenite Anhydrite 11,602–12,010 10,600 Anhydrite/shale interbedded 12,010 Dolomite Diorite Subsequence Sandstone

12,010–12,062 Ali Member 5 10,700 Siltstone

Shoab Shale Total Depth 12,062 ft 10,729 Basement 10,763 Figure 12: Lithology of the Nukhul Sequence in the Wadi Dib-1 Well as described by Chevron’s well site geologists M. Raslan, C. Allen and Z. Ebeid. Six Nukhul shale-sandstone subsequence are here interpreted in the Shoab Ali Member between 11,602–10,586 ft. The Ghara Member occurs between 10,586–9,753 ft and contains four anhydrite-dominated intervals. See Figure 2 for location and Figure 5c for correlation.

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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.

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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. [email protected]

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