ABSTRACT

The Miocene Kareem Formation in the Egyptian Gulf of Suez, and its equivalent formations throughout the Red Sea (250–550 m thick), contain one of the most important petroleum reservoirs in these highly faulted rift basins. They present a difficult exploration target, particularly over the shelves of the sparsely explored Red Sea for several reasons: (1) water depth exceeds one kilometer, (2) they underlie thick evaporites (including salt exceeding one kilometer in thickness), (3) they are difficult to image by conventional seismic techniques, and (4) their lithology is laterally variable and difficult to predict (anhydrite, carbonate, sandstone, shale and marl). The target Red Sea formations are best controlled by boreholes in the Gulf of Suez, where the Kareem Formation and its members are characterized by various synonymous units. A review of representative data and interpretations shows that the formation and its members are better understood when considered as a third-order, transgressive-regressive (T-R) depositional sequence, named the Kareem Sequence in the Middle East Geologic Time Scale (ME GTS). The Sequence is bounded above by the Belayim Sequence Boundary (Sub-Belayim Unconformity) and below by the Kareem Sequence Boundary (Sub-Kareem Unconformity), both corresponding to major sea-level lowstands. It contains the Arabian Plate Langhian Maximum Flooding Surface Neogene 30 (MFS Ng30) at the top of the Kareem Maximum Flooding Interval (MFI). Its lower Rahmi Member forms the majority of the transgressive systems tract (TST). The Kareem MFI and regressive systems tract (RST or HST) occur within the upper Shagar Member. The paleontology of the Formation is characterized by Planktonic Foraminiferal Zone N9 and in recent papers also N8, and Calcareous Nannofossil Biozone NN5, but the Formation’s assignment to Miocene stages (Burdigalian, Langhian and Serravallian) is unresolved in the literature.

In this paper, the Kareem Sequence is interpreted in terms of Kareem subsequences 1 to 6. At semi-regional scales (10s of km), the older three are each represented by an anhydrite bed (Rahmi Anhydrite 1 to 3, each c. 10 m thick) overlain by deep-marine deposits (shale, marl and carbonate, 10s of meters thick). Subsequences 4 to 6 are represented in El Morgan field (Kareem A to C units), and in representative boreholes, by three deep-marine shale/marl units, each of which is overlain by a regressive shallow-marine sandstone unit. The Kareem Sequence is correlated to third-order orbital sequence DS3 1.1 with a depositional period of ca. 2.43 million years between ca. 16.1 and 13.7 million years before present (Ma), or numerically the latest Burdigalian, Langhian and earliest Serravallian (Langhian: 15.97–13.65 Ma in GTS 2004; 15.97–13.82 Ma in GTS 2009). The six subsequences are correlated to the orbital 405,000 year eccentricity cycle (referred to as Stratons 40–35 or DS4 1.1.1 to 1.1.6). The older three subsequences form the transgressive systems tract; the fourth contains the maximum flooding interval MFI (ca. 14.9–14.7 Ma) in its lower part. The regressive systems tract starts in the upper part of the fourth subsequence and encompasses subsequences 5 and 6. The orbital architecture of the Sequence provides a simplified framework for predicting lithology and reservoir development.

The six Kareem subsequences carry the orbital-forcing glacio-eustatic signal. During low eccentricity, Antarctic ice-making and global sea-level drops, the northernmost Gulf of Suez and Bab Al Mandeb Strait restricted marine circulation in the Gulf and Red Sea rift basins. The resulting evaporitic setting was associated with the deposition of the Rahmi Anhydrite 1 to 3 beds and exposure over paleohighs. The deeper-marine deposits above the three Rahmi Anhydrite beds, and those of subsequences 4 to 6 reflect high eccentricity, Antarctic ice-melting, global sea-level rises, pluvial conditions at low latitudes (10–30oN), and open-marine circulation in the Red Sea. During pluvial periods, fluvio-deltaic systems prevailed over the mountainous rift shoulders and coastal plains and carried massive clastics into the Gulf and Red Sea Basins.

INTRODUCTION

The Upper Oligocene? and Miocene syn-rift succession in the Gulf of Suez and Red Sea basins constitutes a complete petroleum system that extends across the sparsely explored African and Arabian shelves of the Red Sea (Figure 1a; Barakat, 1984, 1990; Barakat and Miller, 1986; Barakat et al., 1997; Miller and Barakat, 1988; Beydoun, 1989; Beydoun and Sikander, 1992; Hughes and Beydoun, 1992; Barnard et al., 1992; Mitchell et al., 1992; Salah and Alsharhan, 1996; Lundquist, 1998; Heath et al., 1998). In the Gulf, the Miocene Kareem Formation houses one of the main petroleum reservoirs, which is sourced from Lower Miocene and older Mesozoic rocks and regionally sealed by massive Miocene evaporites (Lelek et al., 1992; Tewfiq et al., 1992; EGPC, Egyptian General Petroleum Corporation, 1964, 1996; Barakat et al., 1997; Salah and Alsharhan, 1997; Alsharhan, 2003). The lateral equivalents of the Kareem Formation are the most prospective hydrocarbon reservoirs throughout the Red Sea shelves as proven in Saudi Arabia’s Red Sea Burqan and Midyan fields (e.g. Cole et al., 1995a, b; Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005; Alsharhan and Salah, 1997; Polis et al., 2005; Figure 1a).

In the Gulf of Suez, the Kareem Formation is extensively sampled by boreholes but is still difficult to correlate because it is highly faulted, lithologically very heterogeneous and inadequately imaged by seismic data (e.g. Abd El-Naby et al., in press). Correlating the Formation beyond the Gulf into the Red Sea is difficult, not only for the same reasons, but also due to very sparse borehole control. These challenges are further compounded by the characterization of the formations in terms of numerous and oftentimes confusing and/or conflicting stratigraphic schemes (Figures 2 to 4). Therefore one of the main objectives of this paper is to reconcile these schemes in terms of a sequence stratigraphic framework that can clarify its reservoir development and lateral distribution.

Figure 3 shows one of the first attempts to apply a combined eustatic and structural interpretation for the Gulf’s syn-rift Miocene section (Webster and Ritson, 1984). The study used the sea-level curve of Vail et al. (1977) to distinguish between structural and eustatic unconformities, and recognized the top of the Kareem Formation as a sequence boundary. Subsequent studies of the Gulf of Suez attempted to tie its regional Miocene stratigraphy to the global eustatic cycles of Haq et al. (1988; Figure 4); notably Richardson and Arthur (1988), Evans (1988), Patton et al. (1994), Purser and Bosence (1998), Bosworth et al. (1998) and Bosworth and McClay (2001) and references therein. Several of these schemes are exactly reproduced in Figure 4 to illustrate the many differences that prevail in the literature. In particular, the structural/eustatic interpretation and regional correlation of most hiatuses is not adequately resolved.

This paper starts with a literature review of the Kareem Formation covering its lithostratigraphic nomenclature, biostratigraphy and sequence stratigraphy. It shows that the Kareem Formation has been implicitly recognized by many authors as a third-order, transgressive-regressive (T-R) depositional sequence, here proposed as the Kareem Sequence for the Middle East Geologic Time Scale (Al-Husseini, 2008). It then seeks to break its stratigraphic architecture into subsequences, which may be correlated to fourth-order, orbital-forcing eccentricity (405 Ky cycles of Laskar et al., 2004) here named stratons (Matthews and Frohlich, 2002; Al-Husseini and Matthews, 2008).

KAREEM FORMATION, GHARANDAL GROUP

Nomenclature and Type Section

The National Stratigraphic Sub-Committee of Egypt defined the Kareem Formation in the Gulf of Suez in 1974. It is the youngest formation of the Gharandal Group, which includes the Rudeis and Nukhul formations, and divided into the lower Rahmi Member and the upper Shagar Member (Figure 2). The type section is defined in the borehole Gharib North-2 (Figure 1b; 28o25′35″N, 32o54′11″E, spudded June 23, 1945) between 1,310.6–1,571.2 m (250.6 m thick).

Synonyms of the Kareem Formation

Besides the formal Kareem Formation and its Rahmi and Shagar members, several lithostratigraphic schemes are used to characterize this interval in different provinces of the Gulf of Suez (e.g. Montenat et al., 1986a; Hosney et al., 1986) or by operating companies (e.g. EGPC, 1996).

Montenat et al. (1986a, 1988, 1998) divided the syn-rift Miocene rocks in the outcrops adjacent to the Gulf of Suez and Egyptian Red Sea into four groups: from oldest to youngest A–D, each of which is bounded by an angular discordance of regional extent. In their scheme (Figure 2), the lower part of Group C was correlated to the offshore Kareem Formation and given a Serravallian age (e.g. Montenat et al., 1988, their figure 2, p. 164). This stratigraphic scheme was adopted in a series of detailed outcrop studies by geologists from Elf and Total (now merged as Total), CNRS, University of Paris, Ecoles des Mines de Paris, and the Institut Géologique Albert de Lapparent (IGAL) (e.g. Burollet, 1986; Jarrige et al., 1986a, b, 1990; Montenat et al., 1986b; Ott d’Estevou et al., 1986a, b; Prat et al., 1986; Roussel et al., 1986; Thiriet et al., 1986). This four-group stratigraphic scheme was compared to the northern Saudi Arabian Red Sea succession by Le Nindre et al. (1986) and Purser and Hötzl (1988).

The informal term “Kareem/Rudeis” formation was proposed by the National Stratigraphic Sub-Committee of Egypt (1974) in places where the Kareem and Rudeis formations are undifferentiated (Figure 2). A. Youssef (2009, written communication) proposed disqualifying this term because the lithostratigraphy and biostratigraphy of the two formations are presently well established in the Gulf.

In 1974 the National Stratigraphic Sub-Committee of Egypt introduced the informal “Ayun Musa member” of the Kareem Formation to correspond to a unit free of anhydrite beds that combines the Rahmi Member and the underlying mainly non-calcareous shale of the Mreir Member (in the uppermost part of the Rudeis Formation; Figure 2). In subsequent literature (e.g. EGPC, 1996; Wescott et al., 1996; Dolson et al., 1996; Krebs et al., 1997), the Ayun Musa was raised to a formation and divided into the lower Lagia Member (or Markha Member) and upper Ras Budran Member (Figure 2).

In many localities the Lagia (Markha) Member contains the same anhydrite beds that are the defining lithology of the Rahmi Member in type section, and so the terms “Rahmi, Lagia” and “Markha” are commonly interchanged and are synonyms in part or completely (e.g. EGPC, 1996; Ramzy et al., 1996). Similarly, the upper Ras Budran Member correlates closely to the Shagar Member (EGPC, 1996) or “Upper Kareem” (Ramzy et al., 1996), and these two terms are also synonyms in part to the Shagar. Throughout this paper, the formal names “Kareem, Rahmi” and “Shagar” are adopted, and the in-part synonyms of previous authors are shown in parenthesis.

The informal term “Globigerina marl” has been widely used in the Red Sea to apparently encompass the Rudeis and Kareem formations (e.g. Evans, 1988; Beydoun, 1989). A. Youssef (2009, written communication) emphasized that it should not be applied to the Lagia (Rahmi, in part) Member because it is completely barren of marine microfossils and hence does not represent open-marine deposition of marls.

Hughes et al. (1992) reported that in some Gulf of Suez boreholes, the Kareem Formation is predominantly represented by sandstone or carbonate, instead of marl and evaporite. They proposed the terms “Kareem Sand equivalent” (KS) and “Kareem Carbonate equivalent” (KC) where these lithologies exceed 50% of the section. The carbonate facies of the Kareem Formation have also been named Kharaza, Esh el Mallaha, Bal’ih and the Chaotic Breccia members in the Abu Shar Platform (Cross et al., 1998).

Thickness of the Kareem Formation

The thickness of the Kareem Formation is 256.2 m in the type section, and ranges between c. 250 and 550 m in the Gulf of Suez Basin (Scott and Govean, 1986; Evans, 1988; Richardson and Arthur, 1988). The greatest known thickness of 550 m occurs in the hanging wall near oil-producing Platform C in October field (Dolson et al., 1996) (Figure 1b).

Lithology and Members of the Kareem Formation

In the type section, the Kareem Formation consists of white to light grey massive anhydrite beds in the lower part, and shales, mostly grey, highly calcareous, grading to marl, with occasional grey argillaceous limestone intercalations (National Stratigraphic Sub-Committee of Egypt, 1974). Its two members are described below.

Rahmi Member, Kareem Formation

Synonymous: Markha Member of the Kareem Formation, in part or completely Lagia Member of Ayun Musa Formation (Figure 2) and “Markha Evaporite”.

Type Section: The Rahmi Member of the Kareem Formation is defined in borehole Rahmi-2 (Figure 1b; 28°44′07″N, 32°47′05″E, spudded November 24, 1960) between 863–1,025 m (162 m thick) (National Stratigraphic Sub-Committee of Egypt, 1974).

Lithology: In the type section, the Rahmi Member consists of anhydrite, milky white, light grey, hard, with calcareous greenish grey, soft, sticky shale interbeds (National Stratigraphic Sub-Committee of Egypt, 1974). Dolson et al. (1996) reported that in many regions the Rahmi (Lagia) Member includes three distinct anhydrite beds; the base of the lowermost anhydrite bed defines the base of the Kareem Formation (Figure 5, Butler et al., 1984). The top of the Rahmi Member is picked at the top of the uppermost anhydrite bed.

The Rahmi (Markha) Member was briefly described in the Nessim field by EGPC (1996, p. 218; Figure 1b) where it is 73–79 m thick. EGPC recognized that it represents three evaporitic cycles. In all boreholes drilled in the area a 9-m-thick anhydrite bed was encountered in the lowermost part of the Member. Another two anhydrite beds occur at the base of the intermediate Rahmi (Markha) sandstone bed and at the top of the Member. The three anhydrite beds are here referred to Rahmi Anhydrite 1 to 3 in ascending order (Figure 5), and together with their lateral equivalents represent the basalmost parts of Kareem subsequences 1 to 3.

Kareem subsequences 1 to 3 are partly expressed as cyclolog markers in the Gulf of Suez (Figure 6). Nio et al. (1996) correlated two markers that correspond to the Kareem and Belayim sequence boundaries (their GS6 and GS8 markers, respectively). In the borehole EE85-5B, the basalmost Rahmi anhydrite bed (at c. 1,950 m) has mirror gamma-ray (repeated with reverse polarity) and cyclolog signatures, and the latter signature can be correlated to borehole X80-1 at the base of the Rahmi Member. Nio et al. (1996) also correlated their GS-7 cyclolog marker, which corresponds to the top of the Rahmi Member. A third marker in X80-1 in the middle of the Rahmi Member (at 700 m) has no correlative in the EE85-5B borehole. The three markers in X80-1 (at about 780, 700 and 650 m) are here interpreted as the basal units of Kareem subsequences 1 to 3. The intermediate Kareem Subsequence 2 in X80-1 is believed to merge with either of the two subsequences or may be absent in EE85-5B borehole.

Shagar Member, Kareem Formation

Synonymous: Ras Budran Member of Ayun Musa Formation (Figure 2), and in part “Ras Budran Clinoforms” or “Upper Kareem”.

Type Section: The Shagar Member of the Kareem Formation is defined in borehole Shagar-1 (Figure 1b; 28o09′07″N, 33o03′57″E, spudded November 24, 1960) between 58–269 m (211 m thick) (National Stratigraphic Sub-Committee of Egypt, 1974).

Lithology: In the type section the Shagar Member consists of richly fossiliferous, grey-green, calcareous shale and grey marls with occasional white limestone intercalations (National Stratigraphic Sub Committee of Egypt, 1974) (Figure 5, Butler et al., 1984). The architecture of this Member is discussed below in the context of sequence stratigraphy.

Upper Boundary of the Kareem Formation: Belayim Sequence Boundary

The boundary between the Kareem Formation and overlying evaporites of the Baba Member of the Belayim Formation is a regionally mappable unconformity named in the literature the pre-Belayim unconformity or post-Kareem unconformity, sometimes associated with the Belaym event (Beleity, 1984; Evans, 1988; EGPC, 1996). It is here formally referred to as the Sub-Belayim Unconformity.Ayyad and Stuart (1992) used borehole logs and core descriptions to correlate the Unconformity across the middle part of the Gulf of Suez as their unconformity VI or sequence boundary 13 (sb13). As noted above, Nio et al. (1996, Figure 6) correlated the Unconformity to their GS8 cyclolog marker.

In the giant El Morgan field, EGPC (1996; p. 35–50, Figure 1b) documented the Sub-Belayim Unconformity, here expressed as an erosional trough that separates the North and South El Morgan anticlinal culminations. The trough cuts into the Kareem Formation and is infilled by thick salt and anhydrite of the Baba Member of the Belayim Formation. In this field, the Kareem is divided into Units A to C in descending order. Kareem Units A and B are absent in the trough, whereas the lowermost Kareem Unit C is continuous between the two culminations. Over the crest of the South El Morgan anticline, isopach and log data revealed a NW- to WNW-trending erosional channel that cuts into Kareem Units A and B. In the North El Morgan anticline, Ramzy et al. (1996) documented a 120-m-deep erosional valley cut into the Kareem Formation, which is filled by salt and anhydrite of the Baba Member.

The depth of the channels in North and South El Morgan, and between them, reflects a relative sea-level drop in excess of 100 m, which followed the deposition of the Kareem Formation. The lowstand persisted after their incision as evident by the deposition of massive salt and anhydrites of the younger Baba Member. This conclusion supports the correlation of the Sub-Belayim Unconformity to a global sea-level drop as shown in the curve of Vail et al. (1977) and proposed by Webster and Ritson (1984, Figure 3). The same interpretation was arrived at by Evans and Moxon (1988) and Evans (1988) who concluded that the pre-Belayim event marks the isolation of the Gulf of Suez as an evaporitic basin, beginning in the Early Serravallian. Alway et al. (2002) and R. Alway (2003, written communication) interpreted the Sub-Belayim Unconformity as a sequence boundary, here named the Belayim Sequence Boundary (Belayim SB).

Lower Boundary of the Kareem Formation: Kareem Sequence Boundary

The Kareem Formation unconformably overlies the Rudeis Formation and the intervening boundary is named in the literature as the pre-Kareem unconformity, Kareem-Rudeis unconformity, or the post-Rudeis unconformity, and is associated with the Kareem event (EGPC, 1996). It is here formally referred to as the Sub-Kareem Unconformity. Ayyad and Stewart (1992) used borehole logs and core descriptions to correlate this Unconformity across the middle part of the Gulf of Suez as their unconformity V or sequence boundary 11 (sb11). As noted above, Nio et al. (1996) identified the Sub-Kareem Unconformity as the GS-6 cyclolog marker (Figure 6).

Ayyad and Stewart (1992) interpreted the Unconformity to represent the transition from maximum occurrence of upward-shoaling followed by the basinward retreat of the sea. They added that the retreat preceeded the development of hypersaline ponds and lakes, in which the Rahmi anhydrite beds were precipitated and/or Kareem alluvial fan clastics were deposited.

In this paper, like the Sub-Belayim Unconformity, the Sub-Kareem Unconformity is also interpreted as due to a drop in global sea level, here referred to as the Kareem Sequence Boundary (Kareem SB). In basinal areas the Boundary becomes a correlative conformity between the clastics of the Rudeis Formation (or the lower Lagia shale) and Rahmi Anhydrite 1 at the base of Kareem Formation. Over emergent paleoghighs, the Kareem SB is expressed as a local unconformity that cuts into the Rudeis Formation and one or more of the Rahmi Anhydrite beds may be absent by non-deposition. In a rapidly subsiding basin such as the Gulf of Suez, the Kareem SB frequently switches from a local angular unconformity over emergent horsts or the crests of tilted blocks, to a correlative conformity in submerged grabens and basins. Importantly, over paleohighs, its expression as a local unconformity should not be misinterpreted as due to a rising structure or regional uplift of the entire basin. This concept also applies to other regional sequence boundaries such as the “mid-Clysmic (mid Rudeis)” unconformity” that divides the Rudeis Formation (Figure 6).

A. Youssef (2009, written communication) disagreed in one important aspect with the proposed position of the Kareem SB. He considered the Lagia non-calcareous shale together with the overlying Rahmi Anhydrite 1 to represent a single regressive cycle. His interpretation is opposite to the one adopted here and by other authors (Wescott et al., 1996, see their figure 9; Krebs et al., 1997).

Depositional Settings of the Kareem Formation

Based on core studies, Richardson and Arthur (1988) concluded that the Rahmi (Markha) anhydrite beds constitute a series of well-developed, sabkha cycles deposited in very shallow to emergent conditions. In the Wadi Feiran outcrop section (Figure 7), Wescott et al. (1996) and Krebs et al. (1997) interpreted the base of the two anhydrite beds as sequence boundaries, and the anhydrites as lowstands. They interpreted the passage from the anhydrite beds to the overlying limestone and organic-rich shale to represent rising relative sea level and the transgressive systems tract. They placed the higher-order flooding surface at the passage from the organic-rich shale to an organic-poor shale with the latter representing the highstand systems tract.

Dolson et al. (1996) noted that ‘chicken wire’ textures in the anhydrite, and the stromatolitic limestones immediately above the lower Rahmi (Lagia) anhydrite bed (Figure 7), indicate shallow-water conditions for the depositional setting of the anhydrite. They reported a marked drop in abundance and diversity of microfauna and flora during the evaporitic phases as seen in the Tanka-3 borehole (Figure 8). They believed that the evaporitic lowstands correlate with unconformities updip along the exposed basin margins, or over structural paleohighs within basinal areas (e.g. South Zeneima-1 borehole in Baba Plain, Figures 1b and 5).

According to A. Youssef (2009, written communication) the Rahmi shale units are mainly non-calcareous and the Rahmi Member does not contain marls, marine limestone or marine microfossils. Accordingly he believes that there is no evidence for significant marine influence (transgression) during the deposition of the Rahmi Member. He envisages the shale to have been deposited in a restricted setting flushed by fresh waters from river systems, which was followed by the precipitation of anhydrite in the final stage of each cycle. However, as seen in Wadi Feiran and based on examination of lithologies in several boreholes, the present authors can report that both limestone and highly calcareous shale beds do indeed occur between the three Rahmi Anhydrite beds. The presence of these lithologies is taken here to represent sea level rising starting from the base of each Rahmi Anhydrite followed by marine flooding.

Hughes et al. (1992) interpreted the transition from the Rahmi (Markha) to the Shagar Member as a sharp rise in relative sea level, and interpreted the depositional setting of the Shagar Member as deep to very deep marine. According to Scott and Govean (1986), megafossil assemblages from the Shagar Member (Nuculana, Pecten zitteli, Flemingostrea and Aturia) suggest muddy, deeper-shelf deposition.

They added that towards the top of the Kareem Formation, marine conditions persisted because the uppermost marl cycles correlate over large areas in the Gulf of Suez, until the sharp break to evaporites that define the base of the Belayim Formation. A. Youssef (2009, written communication) considered the deeper-water setting may have also been due to an abrupt increase of subsidence due to a regional tectonic event.

The provenances for the Kareem clastics, which include highly arkosic coarse sandstone, were located on both sides of the Gulf of Suez (Ayyad and Stewart, 1992; Schütz, 1994). Examples of clastic provenances include fan belts built-out of near the Belayim Land field and the southern end of the El Qaa Plain towards the El Morgan field (Schütz, 1994) (Figure 1b). Schütz (1994) reported that at Belayim Marine field, the Shagar Member sands scoured deeply into the underlying strata and cores show debris flow and turbidites. He added that the sandstone geometries in front of the Belayim Marine scarp indicate sharp channeling and submarine fans.

Regional Correlation of the Kareem Formation

The Kareem Formation correlates to rock units throughout the Red Sea Basin. It was encountered, and so-named, in the deep Esso (now ExxonMobil) exploration boreholes in Egypt’s Red Sea (Barakat and Miller, 1986; Miller and Barakat, 1988). It correlates to Sudan’s Red Sea Khor Eit Formation (Carella and Scarpa, 1962; Sestini, 1965), which Bunter and Abdel Magid (1989) proposed renaming as the Kareem Formation. It correlates in part to Eritrea’s Red Sea Habab Formation (Savoyat et al., 1989), and in part to the “Globigerina marl” in Yemen’s Red Sea (Beydoun, 1989).

Hughes and Filatoff (1995), Hughes et al. (1999) and Hughes and Johnson (2005) on the basis of biostratigraphy correlated Egypt’s Kareem Formation to the Jabal Kibrit Formation in Saudi Arabia’s Red Sea (Figure 2). On the basis of lithology, they correlated: (1) Rahmi (Markha) and Shagar members to the An Numan and Umm Luj members of the Jabal Kibrit Formation, respectively; (2) carbonates of the Wadi Waqb Member of the Jabal Kibrit Formation, to carbonate facies of the Kareem Formation: Kharaza, Esh el Mallaha, Bal’ih and the Chaotic Breccia members (Cross et al., 1998); and (3) fine clastics of the Shukeir Member of the Kareem Formation to the Dhaylan Member of the Wadi Waqb Formation.

Paleontology of the Kareem Formation

Most authors characterized the paleontology of the Gulf of Suez formations in terms of Planktonic Foraminiferal Zone (abbreviated N) and Calcareous Nannofossil Biozone (NN) (Figure 9, Table 1; see reviews of Gulf of Suez biostratigraphy inRichardson and Arthur, 1988, and Schütz, 1994, covering many other studies including Steinenger and Roegl, 1981; El-Heiny and Martini, 1981; Evans, 1988).

El-Heiny (1982) reported that planktonic foraminiferal associations from the shales of the Kareem Formation and sandy shale interbeds of the overlying Belayim Formation can be assigned to the Orbulina suturalis – Globorotalia fohsi peripheroronda Zone (N9–N10) and G. siakensis Zone (N11). Scott and Govean (1986, based on communication with F. Sullivan) narrowed the correlation of the Kareem assemblages exclusively to Biozone N9 on the basis of Globorotalia scitula, Praeorbulina glomerosa, Orbulina universa, Orbulina suturalis and Orbulina bilobata, and Biozone NN5 by the presence of Sphenolithus heteromorphus, which spans NN3–NN5, and Discoaster bouweri, which spans NN5–NN18. Similarly, Evans (1988) correlated the Kareem assemblages to the N9 and NN5 biozones and the first appearance of Orbulina suturalis (N9) to the base of the Formation. A. Youssef (2009, written communication) noted that Globorotalis scitula, Orbulina universa, Orbulina suturalis and Orbulina bilobata are long-range taxa extending from top Belayim to top Asl; the Praeorbulina glomerosa s.l. however appears in the lower part of the Ras Budran Member to top Lagia (c. 30 m in most cases).

In the Gebel Zeit outcrop area (Figures 1b and 4), Evans and Moxon (1988) calibrated the Belayim Sequence Boundary (their pre-Belayim event) between ca. N9/N10 and ca. NN5/NN6. At this locality, a series of massive gypsum/anhydrite beds interstratified with thin shales (c. 200–250 m thick), dip at about 15–20o SW. The youngest consistently datable inter-evaporite marl yielded an NN7 assemblage of calcareous nannoplankton, and a foraminiferal assemblage as young as N12, corresponding to the Belayim Formation (S. Fahmy, personal communication inEvans and Moxon, 1988). The Belayim Formation unconformably overlies the “Globigerina marl” (50–100 m thick), which yielded assemblages associated with the middle part of the Rudeis (NN4) through Kareem (N9, NN5) formations. The Belayim Sequence Boundary is highly weathered and planktonic foramnifera from this zone are highly hematitic, suggesting subaerial reworking (Sub-Belayim Unconformity). In Wadi Gharandal (Figure 1b), Evans (1988) reported that the upper NN5 zone (and possibly the lower NN6 zone) and N10 are absent, confirming the position of the Sequence Boundary at NN5/NN6 and N9/

N10.

Hughes and Filatoff (1995), Hughes et al. (1999) and Hughes and Johnson (2005) correlated the Kareem and Saudi Arabia’s Jabal Kibrit formations to biozones N8 and N9, and NN5 (Figures 2 and 9, Table 1). Biozone N8 was based on the first downhole occurrence of Praeorbulina glomerosa curva and the absence of Orbulina species in the lower part of the Shagar Member. The upper part of the Member yielded Globigerinoides sicanus, Orbulina suturalis, O. bilobata, Praeorbulina glomerosa circularis, P. transitoria, P. glomerosa glomerosa and P. glomerosa curva and assigned to “lower N9” Biozone. Biozone NN5 was based on calcareous nannofossils Sphenolithus heteromorphus, in the absence of Helicosphaera ampliaperta. They interpreted the age of the carbonates of the Wadi Waqb Member of the Jabal Kibrit Formation (i.e. Kareem Formation) as Middle Miocene based on the occurrence of the benthic foraminifera Borelis melo (Dullo et al., 1983).

In summary, the Kareem Formation is consistently correlated to biozones N9 and NN5 by all the above authors (Table 1). The works of Hughes and Filatoff (1995), Hughes et al. (1999) and Hughes and Johnson (2005), however, extended the Kareem into N8.

Stage Assignment of the Kareem Formation?

Based on biostratigraphy, the Kareem Formation was assigned to Lower Miocene (i.e. Burdigalian) by the National Stratigraphic Sub-Committee of Egypt (1974), Serravallian in upper Middle Miocene (Montenat et al., 1988), and Upper Langhian – ?Lower Serravallian (El-Heiny, 1982; see also authors in Figure 4). These inconsistencies probably arise because N8, N9 and NN5 are correlated differently to the Miocene stages in geological time scales. For example (Figure 9 and Table 1), in three successive generations of Miocene geological time scales:

In Figure 9, by aligning N9 and NN5 according to GTS 2004 (Ogg and Ogg, 2004), the Kareem Formation would essentially correlate to the Langhian Stage. When N8 is included (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005) the Formation correlates to Upper Burdigalian and Langhian. Hughes (2009, written communication) suggested adopting the N8/N9 boundary as the Burdigalian/Langhian (Lower/Middle Miocene) Boundary as originally defined by Blow (1969). He stressed that this is contrary to the Langhian extending from the middle of N8 (i.e. within the Burdigalian) to the top of N10 (Ogg and Ogg, 2004). He argued that because the dating the Kareem is based on planktonic foraminifera, from which the N zonations are derived, the Shagar Member should be assigned to the Langhian. He noted that the definitions of the Burdigalian and Langhian cause confusion when used to subdivide the Miocene.

A. Youssef (2009, written communication) reported that in the Gulf of Suez the range of the Praeorbulina glomerosa glomerosa and P. glomerosa circularis coincides with the basal part of the Ras Budran Member (base Shagar), and the range of P. glomerosa curva covers most of the upper part of Asl Formation. So the total range of the index fossils spans the lower part of Ras Budran Member, Lagia Member and most of the Asl Formation. He added that some authors assign this total range to the Langhian Stage but doubted that the Ayun Musa Formation extends into latest Burdigalian.

BIOSEQUENCES AND TERRACES, GULF OF SUEZ

Wescott et al. (1996) and Krebs et al. (1997) used paleontological data (principally foraminifera, calcareous nannofossils, algae, pollen and spores, dinoflagellate cysts and siliceous microfossils) from 18 Gulf of Suez boreholes and a biostratigraphic graphic correlation technique to construct a Miocene and Pliocene terrace-biosequence framework (Figure 2). In this framework the succession is divided into paleontological sequences (denoted “S”) separated by stratal surfaces and stratigraphic discontinuities (hiatuses, lacunae, regressive surfaces, condensed sections, ravinements, referred to as paleontological terraces and denoted “T”). Dolson et al. (1996) and Ramzy et al. (1996) adopted a similar terrace-biosequence (T-S) framework and conducted a more comprehensive study involving approximately 200 boreholes, 12 outcrop sections, 2-D and 3-D seismic data, borehole logs, dipmeter and paleomagnetic data. The T-S framework is apparently based on a biostratigraphic scheme that is specialized for the Gulf of Suez, and the biozones N and NN are not explicitly mentioned in these four papers. As such a direct comparison to other biostratigraphic schemes reviewed above is not possible. However, in the summary figures of Wescott et al. (1996) and Krebs et al. (1997), the Kareem (Ayun Musa) Formation is shown to span the latest Early and Middle Miocene (Figure 7).

Below the Belayim Formation (S60), the Kareem (Ayun Musa) Formation was assigned to S40 and S50 by Wescott et al. (1996) and Krebs et al. (1997), exclusively to S40 by Dolson et al. (1996) and either way by Ramzy et al. (1996). All four papers positioned the Kareem (Ayun Musa) Formation between T50 above, and T30 below (Figure 2). The lithostratigraphic nomenclature for the succession encompassing the Kareem and underlying Rudeis formations varies considerably across the four papers but appears to support the synonyms adopted here (Figure 2). Importantly, all these authors implicitly recognized the Kareem (Ayun Musa) Formation as a T-R sequence; indeed Dolson et al. (1996) stated that locally it “has all the characteristics of a true sequence as defined by Vail et al. (1977).”

Terrace T50: Belayim Sequence Boundary

In the T-S scheme, T50 at the top of S40 (Dolson et al., 1996) or top of S50 (Wescott et al., 1996; Krebs et al., 1997; Ramzy et al., 1996) is the same unconformity or sequence boundary in all these papers (Figure 2): it separates the Belayim and Kareem (Ayun Musa) formations and is the Belayim Sequence Boundary (Figure 10).

Terrace T30: Kareem Sequence Boundary

In the Wadi Feiran section, the syn-rit Miocene succession overlies the pre-rift Eocene Limestone (Figure 7). Between the Eocene Limestone and the lowermost Rahmi (Lagia) anhydrite bed, a c. 90-m-thick section of clastics was identified as the “Asl Fan Delta” and overlying “Asl Shale” (Dolson et al., 1996). These two units correspond to the informal Asl member of the Rudeis Formation of the National Stratigraphic Sub-Committee of Egypt (1974), later shown as the Asl Formation (EGPC, 1996) (Figure 2). At this locality the syn-rift succession below the Asl Formation is absent (i.e. Hawara, Mheiherrat and Nukhul formations, Figures 2 and 4) and its lower boundary corresponds to the Red Sea Rift Unconformity (Terrace T00), which separates the syn- and pre-rift rocks. In the Wadi Feiran section, the T30c and/or T30d minor terraces correspond to the base of the lowermost Rahmi (Lagia) anhydrite bed and represent the Kareem Sequence Boundary (Figure 10). They occur near the Lower/Middle Miocene Boundary implying the Kareem Sequence Boundary is latest Burdigalian in age.

Biosequence S40

Dolson et al. (1996) characterized the lower part of their S40 (i.e. in part Rahmi and Lagia members) as a transgressive systems tract (Figure 2); they stated: “Regional erosional surfaces, overlain and onlapped by discrete evaporitic lithologies, formed during lowstand events. The basin-centered [Rahmi] Lagia evaporites are similar to lowstand systems tracts which can occur due to relative sea level falls in closed carbonate rich basins (Sarg, 1988). The [Rahmi] Lagia Member … behaves like a transgressive systems tract, onlapping the basin shoulders and high structures.”

The three Rahmi Anhydrite beds, which mark the lowstands of the older three Kareem subsequences 1 to 3, are semi-regional correlative markers that pass upwards (and gradually and laterally towards the basins) to carbonates and/or open-marine outer-neritic shales and marls (Figures 5, 6 and 10). They indicate three sea-level rises that formed a step-wise transgression that onlapped paleohighs (e.g. El Morgan and October fields) and the uplifted margins of the Gulf (Dolson et al., 1996; EGPC, 1996).

In the Wadi Feiran section (Figures 1b and 7), two subsequences were interpreted in the Rahmi (Lagia) Member (Wescott et al., 1996; Dolson et al., 1996; Krebs et al., 1997). The lower subsequence (c. 36 m thick) consists of a basal Rahmi (Lagia) anhydrite bed, a stromatolite bed and is capped by a sand/shale unit. The upper subsequence (c. 35 m thick) consists of a second Rahmi (Lagia) anhydrite bed overlain by a sandstone bed and capped by shale. A third subsequence starts with siltstones and contains the “Lagia MFS” (T40), which provides the “downlap surface” for the Ras Budran Clinoforms of Dolson et al. (1996).

At Wadi Feiran only two anhydrite beds occur, whereas at other localities the Rahmi (Lagia) Member contains three (Figures 5 and 7). It seems likely that the oldest Kareem Subsequence 1 is absent by non-deposition in this paleo-high locality along the margin of the Sinai Peninsula (Figure 10). In the Tanka-3 borehole (Figures 1b and 8) three Kareem subsequences are manifested as anhydrite-marl/shale cycles (Kareem subsequences 1 to 3). The thinner intermediate anhydrite resembles by log character the anhydrite seen in boreholes Baba-2 and South Zeneima-1 (Figure 5). The correlation between the latter two boreholes suggests that where all three are present, Subsequence 1 is generally thicker than Subsequence 2 and 3, as also interpreted in Tanka-3 (Figure 7).

Lagia MFS and Terrace T40

In Wadi Feiran and Tanka-3, Dolson et al. (1996) correlated the Lagia MFS to the T40 Terrace as shown in Figures 7 and 8. These authors and Ramzy et al. (1996) stated that T40 divides the Kareem (Ayun Musa) Formation into two mappable “genetic” sequences. Ramzy et al. (1996) added that in El Morgan field “the T40 surface, when identified by biostratigraphy, always occurs in hot gamma-ray shales [high GR] which show high abundant and diversity foraminiferal populations. This suggests it is actually a maximum flooding surface (MFS) consisting of condensed section formed in a marine setting”. Ramzy et al. (1996) further stated: “a regionally extensive marine shale overlies the T40 Terrace and is mappable throughout El Morgan field. The shale coarsens upward into fanglomerates which have a lobate geometry.” These descriptions simultaneously use the concepts of maximum flooding surface (Lagia MFS or T40 Terrace, genetic sequence) and maximum flooding interval (marine shale unit, biotic plume, hot GR shale interval, condensed section).

A. Youssef (2009, written communication) further cautioned that several high gamma-ray shale intervals occur within the Ras Budran Member (Figure 8). In Tanka-3 he identified nine including T40, all of which he interpreted as higher-order flooding intervals. He also differed with the interpretation of the above authors by interpreting the equivalent of the T40 Terrace as a sequence boundary that is nearly immediately transgressed by the Shagar (Ras Budran) Member. In the present paper the third-order Kareem MFI is positioned in the lower transgressive part of Kareem Subsequence 4 with its MFS corresponding to the Lagia MFS (T40) as picked by Dolson et al. (1996; Figures 7, 8 and 10).

Biosequence S50

Whereas Wescott et al. (1996), Krebs et al., (1997) and sometimes Ramzy et al. (1996) identify S50 between T40 and T50 (Figure 2), Dolson et al. (1996) consider the T40–T50 interval as the upper part of S40. Regardless of nomenclatural conventions, it seems evident that the succession between T40 (Lagia MFS) and the Belayim Sequence Boundary contains the Kareem regressive (highstand) systems tract (Figures 8 and 10) above the Kareem MFI. Indeed, Dolson et al. (1996) stated that at Wadi Feiran (Figure 7) the section above the T40 Terrace “forms clastic prograding wedges similar to a highstand systems tract using ‘Vail’ terminology”.

The interpretation of the Kareem RST (between T40 and T50) is further supported, for example, by the stratigraphic geometry around local paleohighs in the October and El Morgan fields (Figure 10). Dolson et al. (1996, their figure 12) reported that across the crest of October field the Rahmi (Lagia) anhydrite beds onlap around the high paleostructures but are absent by non-deposition over them. The paleostructures were only buried during the sea-level rise corresponding to the Lagia MFS (T40). The Kareem Formation on the crestal part of the October field is c. 330 m thick (Dolson et al., 1996; EGPC, 1996) and forms the regressive systems tract. A similar stratigraphic geometry is also reported in El Morgan field by Ramzy et al. (1996, their figure 8). They found that over the field the T40 Terrace directly onlaps the T30 Terrace (Kareem Sequence Boundary) and the Rahmi anhydrite beds are absent (Figure 10).

The number of subsequences above Subsequence 4 may be deduced in El Morgan field (Figure 1b). As noted previously, EGPC (1996, p. 35–50, and 174–183) divided the Kareem Formation, c. 300 m thick in this field, into three coarsening-upwards clastic sequences (from base-up Units C to A, Figure 10). Each unit consists of a T-R sequence represented by open-marine mudstone (TST) above which a progradational deltaic sand lobe (RST) was deposited. Because the Rahmi anhydrite beds are absent over this field, the oldest Unit C would correlate by stratigraphic position to Kareem Subsequence 4 (Figure 10). Kareem Units B and A would then represent two more subsequences: Kareem Subsequence 5 and 6. In the Tanka-3 borehole (Figure 8), the sonic log and foraminiferal diversity curves also suggest several higher-order flooding intervals above the Kareem (Lagia) MFI (Kareem Subsequence 4). However, the entire Kareem Formation is not shown in this figure, and as explained by A. Youssef (2009, written communication) the Shagar (Ras Budran) Member contains as many as nine high-frequency parasequences. His parasequences are here believed to be higher in order than subsequences 4 to 6.

KAREEM SEQUENCE

Based on the observations and interpretations reviewed above, it seems evident that the Kareem Formation has been implicitly recognized by numerous authors as a depositional sequence: the Kareem Sequence (Figure 10).

  • It manifests a cycle of rising and then falling relative sea level;

  • It is bounded by sequence boundaries caused by sea-level drops: Kareem Sequence Boundary (Sub-Kareem Unconformity) and Belayim Sequence Boundary (Sub-Belayim Unconformity);

  • The Rahmi (Markha and Lagia) Member represent a step-wise rising relative sea level forming part of the transgressive systems tract (TST). The Kareem TST onlaps paleohighs and is absent by non-deposition over prominent structures and the rift shoulders.

  • The Kareem Maximum Flooding Interval (MFI) represents a major flooding with the isochronous Terrace 40 or Lagia MFS as its top surface. Over prominent paleohighs it generally represents the first flooding interval.

  • The Shagar Member (in part), and specifically the Ras Budran Clinoforms form the regressive systems tract (RST).

Correlation to Global Cycles

As noted in the Introduction, some authors have attempted correlations between the Gulf of Suez formations and global cycles (Figures 3, 4 and 9). In particular, most authors correlated the Kareem Formation to global cycles TB 2.3 and/or TB 2.4 of Haq et al. (1988). In the Haq et al. scale, Late Burdigalian – Langhian TB 2.3 was estimated to have lasted one million years (16.5–15.5 Ma) with an MFS at 16.0 Ma, and was correlated to Biozones N8, late NN4 and early NN5. Their Serravallian TB 2.4 was estimated to have lasted 1.7 My (13.8–15.5 Ma) with an MFS at 15.0 Ma, and was correlated to Biozones upper N8, N9, N10 and mid and upper NN5. Cycles TB 2.3 and TB 2.4 were approximately recognized by Abreu and Anderson (1998) based on isotope events. They showed an approximate correlation between TB 2.3 and a eustatic cycle between isotope events MLi-1 and MSi-1 that bound the Langhian Stage, and a possible correlation between TB 2.4 and an Early Serravallian cycle between isotope events MSi-1 and MSi-2.

As reviewed in the section on Paleontology, the Kareem Formation is assigned to N8, N9 and NN5 by Hughes and Filatoff (1995), Hughes et al. (1999) and Hughes and Johnson (2005). In the Haq et al. (1988) scale, the Kareem Sequence would therefore correlate to Late Burdigalian – Langhian TB 2.3 and the transgressive part of Serravallian TB 2.4, rather than just TB 2.4 as proposed by several authors as shown in Figure 2 (Bosworth and McClay, 2001; Douban et al., 2002).

A major source of confusion regarding correlations to the Haq et al. (1988) chart arises because subsequent publications by Berggren et al. (1995) and GTS 2004 (Ogg and Ogg, 2004) correlated Biozone N9 to the Langhian instead of Serravallian (Figure 9). This revision would move TB 2.3 almost entirely into the Langhian and approximately align it with two sequences between SB Bur5/Lan-1 (16.97 Ma) and SB Lan2/Ser1 (14.24 Ma) that correlate to N8 and N9, upper NN4 and lower NN5 in GTS 2004 (Ogg and Ogg, 2004; Figure 9 and Table 2).

The two GTS 2004 sequences are estimated to have a depositional period of 2.73 My and occupy Late Burdigalian and most of the Langhian Stage (15.97–13.65 Ma, 15.97–13.82 in GTS 2009). However, as noted in the section on Stage Assignment, the Kareem does not correlate to NN4 and cannot therefore be extended to SB Bur5/Lan-1 (16.97 Ma). As discussed in the section on Orbital Calibration the Sequence best correlates to TB 2.3 and the eustatic cycle between isotope events MLi-1 and MSi-1 that essentially bound the Langhian Stage.

Arabian Plate Maximum Flooding Surfaces

Sharland et al. (2001) mispositioned Middle Burdigalian MFS Ng20 (18.0 Ma) in the Middle Miocene (Langhian and Serravallian) Jabal Kibrit Formation of Saudi Arabia, which was correlated to the Kareem Formation by Hughes and Filatoff (1995), Hughes et al. (1999) and Hughes and Johnson (2005). It should be repositioned within the Rudeis Formation. Sharland et al. (2001) also mispositioned Upper Langhian MFS Ng30 (15.0 Ma) in the Serravallian Kial Formation of Saudi Arabia, which is correlated to the Belayim Formation (Hughes and Filatoff, 1995; Hughes et al., 1999; Hughes and Johnson, 2005). Langhian MFS Ng30 should be correlate to Terrace T40 at the top of the Kareem MFI and its correlative in the Jabal Kibrit Formation (Figure 10).

ORBITAL CALIBRATION AND INTERPRETATION

In the Middle East Geologic Time Scale (ME GTS 2008, Al-Husseini, 2008), the Kareem Sequence was correlated to the youngest third-order orbital sequence (denoted as DS3 1.1; Figure 10) of second-order Sequence DS2 1. DS2 1 occurs between second-order sequence boundaries SB2 1 (ca. 16.1 Ma) and SB2 0 at ca. 1.5 Ma. Third-order orbital sequences are predicted to typically consist of five, six or seven fourth-order consecutive sequences, each of which lasted ca. 405,000 years (405 Ky, named stratons and denoted DS4) (Matthews and Frohlich, 2002; Al-Husseini and Matthews, 2008). In the following discussion Kareem subsequences 1 to 6 are correlated to DS4 1.1.1 to 1.1.6 (Stratons 40–35) that constitute the nominal third-order DS3 1.1 Sequence, which lasted ca. 2.43 million years between ca. 16.1 and 13.7 Ma.

DS3 1.1 approximately correlates to TB 2.3 cycle of Haq et al. (1988) with revised biozones. It best correlates to the eustatic cycle between isotope events MLi-1 and MSi-1 that essentially bound the Langhian Stage (Abreu and Anderson, 1998).

Age of Kareem Sequence Boundary

The Kareem Sequence Boundary is correlated to SB2 1 at ca. 16.1 Ma, a model-predicted, second-order regression and sea-level lowstand caused by a major polar glaciation (Figure 10). SB2 1 corresponds to the lowest eccentricity of the Earth’s orbit in the Miocene Epoch (Figure 11). It has an age ca. 100 Ky older than the Burdigalian/Langhian Boundary (15.97 Ma in GTS 2004) and ca. 200 Ky younger than base N8b (16.27 Ma) in GTS 2004 (Ogg and Ogg, 2004, Figure 9). It is 300 Ky younger than SB 9 in Offshore Louisiana (16.4 Ma, Hentz and Zeng, 2003, using the time scale of Berggren et al. 1995), and one million years younger than SB Bur5/Lan-1 (16.97 Ma) of GTS 2004 (Ogg and Ogg, 2004, Figure 9).

Age of Belayim Sequence Boundary

The Belayim Sequence Boundary is correlated to SB3 1.2 at ca. 13.7 Ma, a model third-order regression and sea-level lowstand caused by a polar glaciation and low eccentricity (Figures 10 and 11). The age of SB3 1.2 falls between the age estimates for the Langhian/Serravallian Boundary in GTS 2004 (13.65 Ma) and GTS 2009 (13.82 Ma). The age estimates of SB3 1.2 and SB Ser2 (13.53 Ma, Ogg and Ogg, 2004) differ by ca. 15–20 Ky suggesting a direct correlation.

Kareem Subsequences 1 to 6

Kareem subsequences 1 to 3 are interpreted as fourth-order orbital sequences (DS4 1.1.1 to 1.1.3; Stratons 40–38) and the transgressive systems tract of the Kareem Sequence (Figure 10). The three Rahmi Anhydrite beds were deposited during arid periods, when the Red Sea and global sea level were at lowstands. During these time intervals circulation in the Red Sea was restricted by submerged or emergent sills at the northernmost tip of the Gulf of Suez and the Strait of Bab Al-Mandeb (Figures 1a and 11). The silled configuration for the Red Sea and Gulf of Suez, for all its Miocene evaporites, was also proposed by Orszag-Sperber et al. (1998). The sill model implies that sea level in the Red Sea and Gulf of Suez may have even been at times lower than the global level during the three highly evaporitic phases (Figures 11). G. Hughes (2009, written communication) noted that the Red Sea sill model would have allowed it to replenish any basinal water that evaporated. Whereas normal marine water could enter above the submerged sills, high-density brines could not exit and would therefore pond at the floor of the basin.

Following the arid phases represented by Rahmi Anhydrites 1 to 3, the highstands of subsequences 1 to 3 represent successive, step-wise increases in Red Sea and global sea levels. In Wadi Feiran only two Rahmi anhydrite beds occur suggesting the oldest flooding of Kareem Subsequence 1 (Straton 40 or DS4 1.1.1) did not transgress this area (Figures 7 and 10).

Kareem subsequences 4 to 6 are interpreted as ca. 405 Ky orbital sequences (DS4 1.1.4 to 1.1.6; Stratons 37–35). The Kareem MFI is placed in the TST of Sequence 4 (Straton 37, ca. 14.9–14.7 Ma) such that its MFS correlates to the Lagia MFS or T40 corresponding the Arabian Plate Langhian Ng30 (ca. 14.7 Ma) (Figure 10). In GTS 2004, the Kareem MFI straddles the boundaries between N8–N9 and NN4–NN5, suggesting a relationship between global biozones and the Kareem biotic plume (Figure 8). The thirdorder Kareem RST starts in the RST of Subsequence 4 and continues in subsequences 5 and 6 (Figure 10). Global sea level apparently rose very rapidly during Subsequence 4, such that the Red Sea sills gave way to open-marine circulation (Figures 11). The setting in the region was then strongly pluvial over the mountainous rift shoulders and coastal plains, allowing fluvial-deltaic systems to transport clastics into the margins of the rift basin. In deeper waters, turbidites, shales, marls and carbonates were deposited.

The proposed depositional model for the Red Sea is believed to represent global glacio-eustasy and orbital forcing. The sill hypothesis suggests that emergent or submerged dams at both extremities further reduced water circulation during global lowstands. Very low eccentricity translated to major sequence boundaries (unconformity), moderate eccentricity to evaporites and high eccentricity to marl/shale in deep basins, and sandstone and carbonate in more proximal or shallow settings. The changes from arid to pluvial conditions may also be related to eccentricity.

RECOMMENDATIONS

The narrative order of this paper shows that stratigraphy traditionally starts with: (1) the definition of a formation and its members based on lithology, (2) identification of its fossil assemblages, (3) correlation of the assemblages to standard biozones (e.g. N, NN), (4) then to stages, (5) then ages in million years using for example GTS 2004 (Gradstein et al., 2004). (6) Then comes empirical sequence stratigraphic architecture (TST, RST/HST, MFS, SB, terraces, biosequences), and finally (7) an attempt to correlate local sequences to global eustatic charts. This traditional route is difficult to follow in the Gulf of Suez.

As shown for the Kareem Formation and its members/units, the traditional route involves first reconciling lithostratigraphic synonyms, which over the years can multiply to such an extent that the name and definition of a rock unit becomes difficult to recognize. The fossil assemblages of the Formation are shown differently across papers, and cannot be consistently correlated to geological time scales and eustatic charts, which in themselves change across vintages. Standardization of nomenclature and chrono-sequence charts on a frequent basis are therefore a must-practice if this approach is to be followed. But even when due-diligence is practiced, fundamental results like the stage assignment of the Kareem Formation remains ambiguous and its lithostratigraphic characterization confusing.

An alternative strategy to consider may be: (1) define the third-order sequence by its sequence boundaries and maximum flooding interval. (2) Use biostratigraphic data and radiometric ages (not available for the Kareem Formation) to find its correlative third-order orbital sequence. (3) Break out its fourth-order (straton) architecture where best expressed (e.g. anhydrite to marl/shale, or shale to sandstone subsequences). (4) Stratons can then be counted and numbered according to their positions in longer-period orbital sequences (third- and/or second-order) and converted to age (Ma). (5) Characterize the lithology of the sequence in terms of stratons in space and time.

CONCLUSIONS

The Kareem Formation in the Gulf of Suez and its Red Sea lateral equivalents are the leading petroleum reservoir targets in these complex rift basins. They present a difficult exploration target due to poor seismic imaging and great water depths, particularly in the frontier Red Sea Basin. In the literature they have been represented in terms of various stratigraphic schemes, which do not adequately clarify their chrono-sequence stratigraphy, lithology and reservoir potential. This paper argues that an important first step is to explicitly recognize the Formation as the Kareem Sequence, a third-order transgressive-regressive depositional sequence, bounded by the Belayim and Kareem sequence boundaries. Based on representative data and previous works, the Kareem Sequence evidently consists of Kareem subsequences 1 to 6. In many places, the older three are represented, by anhydrite to marl/shale subsequences, and the younger three by marl/shale to sandstone subsequences.

The six Kareem subsequences are correlated to 405 Ky orbital-forcing cycles, named Stratons 40–35, that together form nominal third-order sequence DS3 1.1. The depositional period of the Kareem Sequence is estimated at ca. 2.43 million years between ca. 16.1 and 13.7 Ma, which numerically implies a latest Burdigalian, Langhian and earliest Serravallian age (Langhian: 15.97–13.82 Ma in GTS 2009). The Sequence best correlates to the global eustatic cycle between isotope events MLi-1 and MSi-1 that bound the Langhian Stage (Abreu and Anderson, 1998). It contains the third-order Langhian Kareem Maximum Flooding Interval (Kareem MFI, ca. 14.9–14.7 Ma by orbital calibration) with its top correlating to Arabian Plate Langhian MFS Ng30 (ca. 14.7 Ma by orbital calibration, 15.0 Ma inSharland et al., 2001). The Kareem MFI numerically encompasses the boundaries of Planktonic Foraminiferal Zones N8–N9 and Calcareous Nannofossil Zones NN4–NN5 in GTS 2004 (Ogg and Ogg, 2004), well within the apparent resolution of the Kareem’s biostratigraphic constraints.

The simplified stratigraphic model in Figure 10 offers a geological framework for predicting the lithology and reservoir potential of the Sequence. The regressive sandstone units in the Kareem subsequences offer the best reservoir targets along the shelves of the Red Sea, particularly where river systems were active along the coastal plains. The conceptual glacio-eustatic model shows how the orbital-forcing signal may have been stratigraphically recorded in the Red Sea and Gulf of Suez basins (Figure 11).

ACKNOWLEDGEMENTS

This paper is part of a study conducted by the authors in 2004 as the Red Sea Orbital Stratigraphy project. The authors would like to thank ExxonMobil, Saudi Aramco and Shell for supporting the study. The authors thank R. Alway, G.W. Hughes, R. Johnson, J. Mattner and A. Youssef for providing helpful comments. They also thank Nestor Buhay II, GeoArabia’s Production Manager and GeoArabia Designer Arnold Egdane for designing the manuscript.

ABOUT THE AUTHORS

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 AAPG, AGU, SEG, EAGE and the Geological Society of London.

geoarabi@batelco.com.bh

M. Dia Mahmoud has more than forty three years of experience in petroleum exploration and development in Egypt, Saudi Arabia, and adjacent countries. The experience of Dia includes: about 18 years with Saudi Aramco Exploration in Saudi Arabia, 8 years with Gupco, the Gulf of Suez Petroleum Company in Egypt, along with 16 years self-employed as the President of Geopex Limited (GEO Petroleum and Exploration Services in Egypt) and the Managing Director of Spectrum-Geopex Egypt Limited. Since 1975, Dia is a certified member of the AAPG, SEG, and EAGE along with EPEX in Egypt.

diasg@tedata.net.egdiam@spectrum-Geopex.com.eg

Robley K. Matthews is Professor of Geological Sciences at Brown University, Rhode Island, USA, and is general partner of RKM & Associates. Since the start of his career in the mid 1960s, he has had experience in carbonate sedimentation and diagenesis and their application to petroleum exploration and reservoir charcterization. Rob’s current interests center around the use of computer-based dynamic models in stratigraphic simulation.

rkm@brown.edu