ABSTRACT

The Saudi Arabian Red Sea stratigraphy consists of a variety of lithologies that range from evaporites, deep- and shallow-marine siliciclastics and carbonates, biostratigraphically constrained to range from the Late Cretaceous, Campanian, to Late Pliocene. The succession consists of pre-rift Mesozoic and Palaeogene sediments, and syn-rift and post-rift late Palaeogene and Neogene sediments. Three main episodes of shallow-marine carbonate deposition can be determined, including those of the earliest Early Miocene Musayr Formation of the Tayran Group later Early Miocene Wadi Waqb Member of the Jabal Kibrit Formation and of the Pliocene Badr Formation of the Lisan Group. The Midyan area of the northern Red Sea offers a unique window into the Cretaceous and Miocene succession that is otherwise only present in the deep subsurface. The sediments are of hydrocarbon interest because of the presence of source rocks, siliciclastic and carbonate reservoirs. The Wadi Waqb reservoir is hosted within the Wadi Waqb Member of the Jabal Kibrit Formation, and is of latest Early Miocene to possibly earliest Middle Miocene age. It is considered to have formed a fringing reef complex that formed a steep, fault-influenced margin to a narrow platform, similar to Recent coralgal reefs of the Red Sea.

The Wadi Waqb Member is exposed on the east and west flanks of the Ifal Plain. The bioclasts are considered to have traveled as a submarine carbonate debris flow 25 km from their presumed source to the east and possibly the west, and consist mostly of rhodoliths, echinoid and coral fragments together with the benthonic larger foraminifera Operculinella venosa, Operculina complanata, Heterostegina depressa and Borelis melo. The planktonic foraminifera include species of Globigerina, Globigerinoides and Praeorbulina; no specimens of the Middle Miocene planktonic foraminiferal genus Orbulina have yet been encountered in the thin sections. The presence of Borelis melo melo, and of B. melo curdica within the region, indicates a latest Early Miocene age. No specimens of the age-equivalent larger benthonic foraminiferal genera Miogypsina or Lepidocyclina have been observed, and this is consistent with evidence from the Wadi Waqb equivalent carbonates elsewhere in the Red Sea and Gulf of Suez.

In the east, the Wadi Waqb is represented by discontinuous fringing rhodolith and coral reefs that are welded to steep cliffs of granitic basement. In Wadi Waqb, located in hills that form the western margin to the Ifal Plain, the Wadi Waqb carbonates consist of packstones containing autochthonous planktonic foraminifera and abundant shallow-marine microfossils that are considered to have been derived from the basement-founded rhodolith and coral reefs in the east. The Wadi Waqb reservoir is located beneath the central part of the Ifal Plain, approximately midway down a ramp between the in situ rhodolith-coral reefs and the mixed allochthonous and autochthonous facies at Wadi Waqb. The reservoir contains biofacies similar to those exposed in Wadi Waqb, and indicative of a deep-marine environment, in excess of 50 m water depth. The Wadi Waqb carbonates display sedimentological and petrographic features that closely resemble those described from stratigraphically equivalent carbonates from the localities along the west coast of the Gulf of Suez, including Abu Shaar, where three depositional facies have been defined. It is apparent that these shallow-marine carbonates were established along the west and east rift margins of the Red Sea-Gulf of Suez rift complex prior to their dislocation during the Late Miocene and Pliocene by the left-lateral Aqaba faulting.

INTRODUCTION

Saudi Arabia is well known as the host of the world’s largest onshore oil field. Its producing oil and gas fields are located within the Palaeozoic to Mesozoic carbonates and siliciclastic reservoirs of central and eastern parts of the Kingdom. The Red Sea provides another petroleum province on the west flank of the country, for which exploration activity commenced in 1924 with oil shows in the Farasan Islands, although the Gemsa Oil Field was discovered in the Gulf of Suez as early as 1869 (Lindquist, 1998). Apart from Richard Burton’s (1878) brief exploration activities associated with a search of gold mines in Midyan, and along the Red Sea coast from near 27°30’N to 28°N, geological investigations of the Midyan region, and especially offshore Red Sea and Ifal Plain, by AUXERAP (1967), are documented as unpublished reports in the Ministry of Petroleum and Mineral Resources, Jeddah. The Lower Miocene carbonates of the study area and their mineralization were described by Motti et al. (1982). Brown et al. (1989) summarise the early lithostratigraphic investigations of Miocene carbonates in the Red sea region and include carbonates that may include the Wadi Waqb, owing to the presence of Borelis melo, in the Raghama Group. Geological mapping and numerous exploration and development wells extending from the Midyan area, in the extreme north, to Jizan, in the south, have revealed the varied relationships within the Neogene succession, which consists of evaporites, carbonates, fine and coarse siliciclastics. The Midyan region (Figure 1) of northwest Saudi Arabia provides a unique window into the entire Red Sea stratigraphic succession with which the subsurface succession encountered during exploratory drilling, can be readily compared.

The biostratigraphy, lithostratigraphy and palaeoenvironments of the various lithostratigraphic units of the Red Sea and Gulf of Aden were studied as part of a World Bank project and documented in confidential country-specific reports by Robertson Research International in 1989 for which the non-confidential aspects were published in a special publication of the Journal of Petroleum Geology (O’Connor, 1992). In support of exploration activities, the Saudi Arabian Red Sea lithostratigraphy was revised to incorporate surface and subsurface sedimentological and biostratigraphic data (Figure 2; Hughes and Johnson, 2005). In the Saudi Arabian Red Sea source rocks are represented by the Burqan Formation, with potential in the Jabal Kibrit, Kial and Mansiyah formations. Reservoir seals are plentiful and include intra-formational mudstones of the Burqan Formation and mudstones and evaporites of the Jabal Kibrit, Kial and Mansiyah formations. Reservoir facies are present in siliciclastics and carbonates of the Burqan, Jabal Kibrit, Kial and Mansiyah formations, including the Wadi Waqb Member, which is the focus of this study.

The Wadi Waqb Member is well exposed in the region, at Wadi Waqb, Ad Dhubaybah and Al Khuraybah, and has been analysed for its biocomponents from these exposures as well as from cored subsurface samples. The study was able to conclude that the member consists mostly of limestones, with locally developed dolostones, containing mixed allochthonous shallow-marine biocomponents and autochthonous deeper-marine planktonic foraminifera. It is of latest Early Miocene to ?earliest Middle Miocene, and was deposited in a variety of environments that spanned from a proximal, fringing rhodophyte-coral reef complex into a shallow reef talus setting that passed laterally down a steeply dipping ramp setting into moderately-deep middle-shelf conditions that supported planktonic foraminifera. Its distribution is discontinuous and mostly confined to the northern Red Sea, where it is considered to be located on rotated fault blocks (Bosence et al., 1998) created during various phases in the tectonic evolution of the Red Sea. Evidence of delta fans to turbidite deposits with coarse and slumped materials are observed in the NW of this area (near Maqna) above the upper Miocene (Langhian) Globigerina marls.

The purpose of this paper is to document the microfacies of the Wadi Waqb Member, as studied in the east, central and west flanks of the Ifal Plain and to suggest a palaeoenvironmental model that will best explain the lateral microfacies variations. The resulting ramp-style depositional model should assist understanding of the distribution of porosity within the Wadi Waqb carbonates and contribute towards the exploration and development program of this important hydrocarbon reservoir.

GEOLOGICAL BACKGROUND

The geological evolution of the Red Sea and Gulf of Suez has attracted much attention owing to its unique insight into relatively early continental splitting, with significance for better understanding of mature basins such as the Atlantic Ocean. Of the numerous accounts of the evolution of the Gulf of Suez, Red Sea and Gulf of Aden, that of Bosworth and McClay (2001) provides the most succinct summary. Tectonic episodes related to the opening of the Red Sea and its modification by the Aqaba left-lateral strike-slip fault include events that would have established an irregular, fault-bounded submarine topography upon the highest points of which shallow-marine carbonates, such as the Wadi Waqb proximal facies, would be expected to commence sedimentation. A similar situation is also envisaged to account for the localised distribution of the Musayr carbonates of the Tayran Group (Hughes and Johnson, 2005). The region includes a variety of sediments that were deposited in response to a variety of controls including regional and local tectonics and eustasy, of which the latter is probably controlled by orbital-forcing as discussed by Al-Husseini et al. (2010) and Al-Husseini (2012). The evolution can be summarised in the following events arranged in descending stratigraphic order:

  • Renewed subsidence, combined with possibly eustatic sea-level rise caused deep-marine sedimentation of the Ifal Formation across parts of the region during the Pliocene, and shallow clastic sedimentation of the Ifal Formation elsewhere. The base of this event was termed the Rift-Drift transitional event by Bosworth et al. (2005).

  • During the latest Middle Miocene, the basin was relatively shallow and climatic control caused a renewal of basement-derived siliciclastic deposition of the Ghawwas Formation across the entire region infilling the basin.

  • Differential widening of the Red Sea and abandonment of the Gulf of Suez occurred at around 13 Ma, but possibly at 11 Ma (Reilinger and McClusky, 2011). Rotational movement and constriction of the northern Suez arm led to gradual closure of the northern Gulf of Suez and initiation of intermittent, then long-term hypersaline evaporite sedimentation of the Mansiyah Formation through most of the Middle Miocene. This process was probably linked to orbitally-influenced eustatic sea-level changes as outlined by Al-Husseini et al. (2010). Accompanying rapid subsidence simultaneous with a eustatic fall of sea level combined with oceanic restriction at the Bab el Mandeb equivalent led to regional precipitation of submarine evaporites.

  • Regional tectonic readjustment occurred, probably in mid-early Miocene, approximately at the same time as the mid Clysmic event of the Gulf of Suez and equivalent to planktonic Zone N7. This created local highs of rotated fault-bounded basement blocks on which shallow marine carbonates of the Wadi Waqb Member were developed, with periodic transport of the shallow marine sediments into the adjacent deep-marine basins. The Wadi Waqb carbonates extend along the Saudi Arabian flank of the Red Sea from Midyan to at least as far south as Wadi Jerba in the Ubhur area, south of Jiddah (Moore and Al-Rehaili, 1989; Mandurah, 2009).

  • Rapid subsidence with regional deep-marine conditions and sedimentation of the Burqan Formation during the Early Miocene along the entire narrow rift basin from Ethiopia–Yemen to Egypt.

  • Deposition of patchy shallow, normal-salinity marine carbonates of the Musayr Formation of the Tayran Group on local basement highs in areas away from siliciclastic input, in the northern Red Sea and Gulf of Suez during the earliest Miocene.

  • Late Oligocene opening of the southern Red Sea with shallow marine invasion from the Gulf of Aden and Gulf of Suez, forming a rectilinear, narrow marine connection between the Mediterranean Sea and Indian Ocean. Fluviatile, estuarine, lacustrine and hypersaline salinas of the Al Wajh and Yanbu members of the Tayran Group.

  • Middle Oligocene opening of the Gulf of Aden with shallow marine invasion from the Indian Ocean.

  • Weakening of the plate along the proto Gulf of Aden and Red Sea lineament due to the presence of a localised mantle plume hot spot in the vicinity of present day Afar.

  • Possible emergence and removal of pre-Neogene sediments except in local depressions.

  • Possible crustal stretching and/or uplift due to drag and buoyancy, respectively, above the mantle plume.

  • Northeast movement of the African-Arabian plate due to drag above converging mantle convection cells at the Zagros suture.

LITHOSTRATIGRAPHY OF THE SAUDI ARABIAN RED SEA

Regional synthesis of the Red Sea lithostratigraphy was hindered by isolated studies by countries bordering the Red Sea and Gulf of Suez, each of which used a unique lithostratigraphic nomenclature. One of the goals of the World Bank Red Sea-Gulf of Aden project (O’Connor, 1992) was to integrate the basin-wide data set for all countries bordering the Gulf of Suez, Red Sea and Gulf of Aden, following the preliminary regional comparison by Beydoun (1989). The most comprehensive preexisting study was an unpublished, confidential multi-client study of the Gulf of Suez by Robertson Research International (UK) in 1984.

The lithostratigraphy of the Saudi Arabian Red Sea closely resembles that of the Tertiary of the Gulf of Suez (Hughes and Beydoun, 1992), and an equivalent scheme was erected by Saudi Aramco (Hughes and Johnson, 2005). With reference to Figure 2, the Red Sea rift-associated formations are clearly characterised by contrasting lithologies, for which chronostratigraphic and palaeoenvironmental biostratigraphic evidence is well established. The initial opening of the Saudi Arabian Red Sea is represented by the shallow-marine sediments of the Lower Miocene Tayran Group, in which siliciclastic, evaporite and carbonate members are termed the Al Wajh, Yanbu and Musayr respectively. The Musayr contains the co-occurrence of species of the larger foraminiferal genera Miogypsinoides and Miogypsina enables assignment to the Early Miocene Te5 to basal Tf1 East Indian Letter Stages of Adams (1970, 1976, 1984), equivalent to the Aquitanian to basal Burdigalian, using the calibration by Renema (2007). The Burqan Formation consists of deep-marine, typically planktonic foraminifera-rich, calcareous mudstones with scattered sands of debris flow origin deposited during the Early Miocene. Planktonic zones N4–N8 of Blow (1969) are present within the Burqan.

The overlying Maqna Group (Figure 3) consists of the Jabal Kibrit and Kial formations. The onset of episodic restricted, hypersaline conditions commenced in the latest Early Miocene, in the Jabal Kibrit Formation that includes calcareous deep-marine mudstones, submarine evaporites and siliciclastic units. Carbonates within this formation are termed the Wadi Waqb Member, and are the focus of this paper. The Jabal Kibrit Formation spans the Early–Middle Miocene boundary, and is overlain by deep-marine mudstones, submarine evaporites, limestones and siliciclastics of the Kial Formation. The evaporite and deep-marine mudstone association of the Jabal Kibrit and Kial formations characterise the Maqna Group. A thick, predominantly evaporite, submarine succession is termed the Mansiyah Formation, and is of undifferentiated but post-planktonic Zone N9 (Blow, 1969) Middle Miocene age. It is overlain by a predominantly siliciclastic, but partly evaporitic unit of Upper Miocene age, called the Ghawwas Formation. The Pliocene is represented by shallow- and deep-marine carbonates of the Badr Formation and shallow-marine siliciclastics of the Ifal Formation.

Sedimentological, biostratigraphic, lithostratigraphic and palaeoenvironmental aspects of the Wadi Waqb Member have been documented by Kamal and Hughes (1993, 1995), Hughes et al. (1999) and Hughes and Johnson (2005). Within the region, the Wadi Waqb carbonates are considered to have siliciclastic equivalents, especially south of the Midyan region and offshore to the coastal carbonates. These variations are considered to represent regional palaeoenvironmental variations with carbonates preferentially localted on basement block highs. Miocene carbonates equivalent to the Wadi Waqb Member have been studied in some detail in the Gulf of Suez and northwestern Red Sea, where they have been termed the Nullipore Rock and characterised by coralline algae then attributed to Lithothamnion (Sellwood and Netherwood, 1984; Tawadros, 2001; Bosworth and McClay, 2001). The Wadi Waqb Member has not been subdivided into discrete sedimentological units in Saudi Arabia, but equivalent carbonates in the Egyptian Red Sea have revealed a number of component lithostratigraphic units (James et al., 1988; Coniglio et al., 1996). An attempt to compare the Wadi Waqb lithological variations with those in Egypt will be discussed later in this paper.

The Wadi Waqb has been studied extensively in the Midyan region of northwest Saudi Arabia, both in the exposed sections and from exploration wells in the southern part of the Midyan area. Additional exposures remain to be examined and will, doubtless, add considerably to the depositional model interpreted in the present paper. The exposures studied in detail include those at Ad Dhubaybah (Figures 1, 14, 15, 16), Aynunah region (Figures 1, 17, 18a, 19) and at Wadi Waqb (Figures 1, 20, 21).

AGE OF THE WADI WAQB MEMBER

Biostratigraphic evidence for the age of the Wadi Waqb carbonates is difficult owing to the scarcity of easily identified age-diagnostic microfossils. Calcareous sediments that underlie and overlie the Wadi Waqb do, however typically contain age-constrained microfossils and enable the age of the Wadi Waqb to be determined on its stratigraphic position. Stratigraphic ranges of planktonic foraminifera used in the Red Sea are based on the ranges of Kennett and Srinivasan (1983). The Wadi Waqb carbonates overlie the Burqan Formation, dated as Early Miocene on the presence of Globigerina ciperoensis atypica, and assigned to planktonic foraminiferal Zone N7. The youngest Burqan in the region is also of latest Early Miocene age, Zone N8, based on the presence of Praeorbulina species in the absence of Orbulina species. The Wadi Waqb carbonates are overlain by planktonic foraminifera-bearing sediments that provide excellent chronostratigraphic indicators for the Jabal Kibrit Formation and the overlying Kial Formation. These formations contain the subspecies of Praeorbulina and Orbulina, which are especially useful for the discrimination of upper and lower parts of planktonic foraminiferal Zone N9, of early Middle Miocene age (Hughes et al., 1992, Hughes and Filatoff, 1995; Filatoff and Hughes, 1996), but the presence of Orbulina has not been confirmed in the Wadi Waqb.

The absence of confirmed Orbulina species, and definitive calibration with well-dated Kial exposures hinders correlation of the Wadi Waqb with the “Nullipore carbonates” of the Gulf of Suez considered as a facies of the Kial-equivalent Belayim Formation. The benthonic foraminifera Borelis melo melo and Borelis melo curdica provide evidence for an latest Early Miocene to earliest Middle Miocene age in shallow-marine, lagoon carbonates (Adams, 1970, 1976, 1984), although B. melo melo is present in the Wadi Waqb, B. melo curdica has yet to be found. Using this evidence, the upper part of the Jabal Kibrit Formation hosts the Early–Middle Miocene contact at the boundary between zones N8 and N9 lower.

The age of the Wadi Waqb is concluded to be late Early Miocene based on the presence of Borelis melo melo, the planktonic foraminifera Globigerinoides sinacus, and Praeorbulina glomerosa, in the absence of Orbulina spp. and stratigraphic position beneath the Kial Formation for which planktonic foraminiferal evidence provides an upper Zone N9, earliest Middle Miocene age. At Ad Dhubaybah and Al Khuraybah the presence of the Borelis melo within the uppermost samples provides a potential latest Early to earliest Middle Miocene age (Adams, 1970, 1976, 1984). Orbulina species are yet to be discovered in the Wadi Waqb Member, but a planktonic foraminiferal specimen that closely resembles Biorbulina bilobata or an oblique thin section through Praeorbulina transitoria in a sample from the Wadi Waqb section would indicate uppermost Zone N8 and, if Orbulina spp. were present, a latest Early Miocene age. It is of interest to note that the larger benthonic foraminiferal genera Lepidocyclina and Miogypsina have not been identified in the Wadi Waqb Member, although they would be expected in the reef complex and distal lagoon respectively (Boudagher-Fadel, 2008), and would have assisted in age refinement.

WADI WAQB MEMBER BIOCOMPONENTS AND THEIR PALAEOENVIRONMENTAL SIGNIFICANCE

The information presented in this section represents a summary of general knowledge concerning environmental aspects of the various biocomponents of the Wadi Waqb, as recorded during semi-quantitative analysis of randomly-oriented thin sections. The Wadi Waqb carbonates contain a moderately high diversity of microfossils that include, in order of decreasing abundance, calcareous or coralline algae, benthonic foraminifera, planktonic foraminifera and various microfossil fragments (Hughes, 2008). In the subsurface carbonates of central Ifal, localised dolomitisation has destroyed most microfossils except for the rhodophyte fragments and some agglutinated foraminifera. In addition to microfossils, inorganic fragments are present that include angular quartz and hornblende grains. The photomicrographs have been grouped into their locations so as to better display the respective biofacies with their occurring microfossils. Those from Ad Dhubaybah and Khuraybah/Aynunah regions are illustrated in Figure 4. Microfossils from subsurface wells S4, S1 and M5 are illustrated in Figures 5 and 6, 7 to 9, and 10 and 11, respectively. Representative microfossils from the Wadi Waqb exposure in traverse WWA of the Wadi Waqb exposure are illustrated in Figure 12.

Calcareous Algae

Calcareous red algae form the dominant fossils within many Miocene reef carbonates (Halfar and Mutti, 2005) and are especially well represented in the Wadi Waqb carbonates. They belong to the Class Rhodophyceae, Order Corallinales, Family Corallinaceae. The cosmopolitan character of coralline algae is reflected in their widespread occurrence throughout a large array of facies and biogeographical settings in the fossil record, since the appearance of the group in the Early Cretaceous (Arias et al., 1995). Their skeletons, composed of high-Mg calcite that impregnates the cell walls, confer on these algae a high preservation potential (Bosence, 1991). A major obstacle to their study is the difficulty of applying standard taxonomic criteria to random thin sections of the calcareous algal fragments (Braga et al., 1993, 2010; Braga and Aguirre, 1995; Braga, 2003). The validity of hundreds of fossil species names is questionable since they have been based only on the cell sizes of thallus fragments or other features of uncertain taxonomic significance (Bosence, 1983; Braga and Aguirre, 1995). The illustrations and original diagnoses of many fossil genera and species are inadequate from a modern perspective and the interpretations of particular taxa by later authors do not always coincide. Within the region, calcareous algal taxonomy focussed on general ultrastructural configuration (Elliott, 1955, 1956; Souaya, 1963a, b; Johnson, 1964; Basson and Edgell, 1971; Buchbinder, 1977), whereas modern taxonomy relies mostly on the character of the calcified reproductive cells or conceptacles. Variations in morphology are considered to be environmentally influenced adaptations, as summarised by Kundal (2010).

The rhodophyte microfossils present in the Wadi Waqb carbonates, as illustrated in Figures 4, 5, 7 to 10 and 12, await detailed taxonomy but a pragmatic approach has been followed. The densely laminated forms preserved in limestones and dolostones display thin, encrusting growth styles as well as stubby finger-lime structures that may be branched. Both forms closely resemble Lithothamnion spp. as illustrated from Middle Miocene carbonates of Australia (Martin et al., 1993). Such is the difficulty of using the existing diverse and confusing status of rhodophyte taxonomy that similar lamellar forms are termed Lithothamnium corallinaeforme by Pisera and Studencki (1989) and Lithothamnium spp. by Flugel (2010, his plate 54.3). The approach by J.C. Braga is more useful, providing that the conceptacles are visible and upon which he has established species definitions for Neogene Lithophyllum (Braga and Aguirre, 1995). For various genera of the Subfamily Melobesioideae, Brandano and Piller (2010) is a useful reference but similarly based on the presence of conceptacles for which the Wadi Waqb forms seem to be sadly lacking. The more coarsely chambered and thicker layered forms are more readily assigned to the genus Lithophyllum, as illustrated by Johnson (1964) and Buchbinder (1977).

In the western Tethys, Oligocene carbonate-facies and algal assemblages indicate a dominance of Lithothamnion species in the shallower environments, while Mesophyllum is most abundant in deeper platform settings (Kroeger et al., 2006; Braga et al., 2010; Kundal, 2010). Martin et al. (1993) state that a Sporolithon-Lithothamnion facies is distinguished from a Mesophyllum-dominated facies, although both facies are considered to have grown at depths of tens of meters and below normal wave base as similar facies are present between 30 m and 80 m in the Indo-Pacific. They detected two phases of rhodolith growth in storm-influenced settings, in which calm episodes enabled active growth and high-energy episodes caused partial rhodolith destruction. Specimens examined by Braga (written communication, 2010) indicate a predominance of Mesophyllum within the Wadi Waqb. In gross form, coralline algae have been divided into two groups, branched geniculate and non-branched, encrusting non-geniculate, although this division does not constitute a taxonomic grouping. Red algae have been extensively studied in the Bahamas (Bergman et al., 2010) where they are important sediment producers and binders, with an annual growth rate up to 2.2 cm and an annual calcium carbonate production of up to 3.6 kg per square meter. They become locally dominant below 50 m, but are present as deep as 290 m in the Bahamas.

Calcareous green algae belonging to the Dasycladaceae and that resemble Clypeina digitata are also present, but generally uncommon and seen as transverse sections across the branched laterals. Calcareous green algae preferentially colonise low-energy lagoons, at water depths shallower than 50 m (Freile et al., 1995). In the opinion of Carannante et al. (1995) the dominance of rhodolith-dominated biofacies “rhodalgal” facies over “chlorozoan-chloralgal” facies is indicative of cooler, darker, deeper, eutrophic-tending and rather anomalous environmental conditions possibly related to upwelling of cooler water.

Benthonic Foraminifera

Benthonic foraminifera identified within the Wadi Waqb carbonates are typical of those present in Miocene carbonates of the Mediterranean (AGIP, 1982) and the genera compare well with those in Recent carbonates of the Mediterranean Indo-Pacific region (Cushman, 1932, 1933, 1942; Todd, 1965; Belford, 1966; Murray, 1991). Representative benthonic foraminifera from the Wadi Waqb are illustrated in Figures 4 to 6, and 9 to 12. They include simple textularid agglutinated species, a variety of simple miliolid forms attributed to Quinqueloculina and Spiroloculina species, complex walled miliolids assigned to Borelis melo melo, smaller rotalids and larger rotalids that include Sphaerogypsina globula, Operculinella venosa, Operculina complanata and Heterostegina depressa. The encrusting species Planorbulina acervalis is also present.

Benthonic foraminifera are known to preferentially occupy certain submarine conditions, of which depth is the most useful for palaeoenvironmental interpretation. Water depth is responsible for variations in many environmental factors, including temperature, salinity, hydraulic energy, sediment fabric, light penetration and presence of endosymbionts, phytal or non-phytal substrate, salinity and abundance of land-derived grains, dissolved oxygen, nutrient content, trace element concentration and isotopic enrichment. The depth and palaeoenvironmental preferences of Recent shallow-carbonate platform complexes have been summarised by Murray (1991, 2006) and Boudagher-Fadel (2008, her figure 7.5), and in the Red Sea, Reiss (1977, his figures 24), Hansen and Buchardt (1977) and Badawi et al., (2005) provide depth ranges for common benthonic and planktonic foraminifera. As many Neogene benthonic foraminifera have Recent morphological equivalents, such studies on the following selected Recent foraminiferal genera provide excellent means of calibrating the life habitats of Miocene foraminiferal biofacies of the Wadi Waqb.

The relatively large fusiform, complex-walled miliolid Alveolinella lives on the sediment surface (epifaunal) or clinging, typical of algal-covered carbonate gravels within a depth range of 5–100 m in the inner shelf, lagoon. The disc-like complex-walled miliolid Amphisorus is epifaunal and clinging, and is typical of carbonate sediment within a depth range of 0–50 m in the nearshore lagoon. The lenticular rotalid form Amphistegina is epifaunal upon coarse carbonates, within a depth range of 0–130 m in coral reefs and lagoons. Of the two species considered common in the Indo-Pacific (Resig, 2004), A. lobifera typically dominates shoals in less than 5 m of water (Hallock, 1984) and is relatively thick in cross-section (Hallock and Hansen, 1979; Hallock, 1979; Hallock and Glenn, 1986). A. lessonii is prevalent on reef slopes at 5–20 m water depth and is thinner than A. lobifera. The complex walled discoid miliolid Archaias is epifaunal, clinging and associated with plants within a depth range of 0–20 m in the inner shelf. The complex-walled spherical to subspherical miliolid Borelis is epifaunal, free and occupies algal-coated substrates, seagrass and coarse sediment within a depth range of 0–40 m in lagoons and reefs. The keeled hyaline lenticular form Elphidium is epifaunal, free and lives on vegetated sand within a depth range of 0–50 m in the inner shelf. The non-keeled form lives within the mud and sandy sediment (infaunal) within marshes and lagoons of the inner shelf.

The complex chambered compressed lenticular larger foraminiferal genus Heterostegina is epifaunal, free and associated with plants in muddy carbonate sediment and hard substrates within a depth range of 0–130 m on the shelf or in a lagoon. In the Pacific, this form is found between 31–38 m (Hughes, 1985). The simple compressed lenticular larger foraminiferal genus Operculina is epifaunal on carbonate sediment within a depth range of 0–130 m in the lagoon and shelf, typically low-energy and with medium light. Increase in the diameter-to-thickness ratio of Amphistegina and Operculina has been found to be related to increased turbulence, in a shallow-marine setting, but a lower ratio with thinner tests is found in decreased light intensity associated with increased water depth (Renema, 2005). The less compressed lenticular larger foraminiferal genus Operculinella is epifaunal on carbonate sediment within a depth range of 0–130 m in lagoons or on the shelf. The complex chambered laterally compressed miliolid Peneroplis is epifaunal, and clinging upon plants and hard substrates within a depth range of 0–70 m in lagoons and the innermost shelf. The simple, rather unorganized chambered hyaline genus Planorbulina is epifaunal and attached immobile upon hard substrates in high-energy settings within a depth range of 0–50 m in the inner shelf. The simple chambered miliolid Quinqueloculina is epifaunal, free or clinging on plants or sediment in lagoon and shelf settings.

The spatial relationships of many of these genera is illustrated, with reference to standard facies belts by Hallock and Glenn (1986, their figure 2). Their model would place the larger foraminiferal genera Amphistegina, Operculina, Operculinella and Heterostegina, together with the robust calcarinid genus Sphaerogypsina within standard facies belts 5–2, spanning the reef, foreslope, toe of slope and open shelf down to approximately 100 m. Reiss and Hottinger (1984) significantly mention the slightly hypersaline tolerance (up to 41 parts per thousand salinity) of these larger foraminifera in the Gulf of Aqaba. Euphotic, mesophotic and oligophotic terms have been defined by Pomar (2001) and Hallock and Pomar (2008) to describe the depth zones related respectively to zones with (a) photosynthesis and hypercalcification-supporting light typically less than 30 m depth, (b) photosynthesis-supporting but not hypercalcifying light typically 20–70 m depth, and (c) light only sufficient to support rhodophyte, corals and flat larger benthonic foraminifera at depths greater than 70 m.

The absence of Lepidocyclina species and Miogypsina may be explained by slightly hypersaline conditions within the restricted marine conditions of the proto Red Sea, compared with the normal salinity open-marine conditions favoured by both genera. Conditions conducive for Miogypsina are known to have existed during deposition of the earlier carbonates of the Musayr Formation (Hughes and Johnson, 2005).

Planktonic Foraminifera

Planktonic foraminifera are locally common but are nowhere found without some of the benthonic microfossils considered in this section. The underlying Burqan Formation is characterised by rich Early Miocene planktonic foraminiferal-dominated assemblages. Their study is mostly based on disaggregated, picked residues and species identification is relatively easy. In the Wadi Waqb carbonates, the strongly lithified nature of the sediment restricts planktonic foraminiferal identification to thin-section analysis. There are very few publications that assist Neogene planktonic foraminiferal identification and this procedure must rely on an interpretation of high-quality scanning electron photomicrographs such as those provided by Blow (1969), Kennett and Srinivasan (1983), Bolli and Saunders (1985) and Iaccarino (1985). There is certainly a need for an atlas of Neogene thin-section photomicrographs in the style of Postuma (1971) and Sliter (1989). Photomicrographs of representative planktonic foraminifera from the Wadi Waqb are illustrated in Figures 4, 5, 9, 11 and 12.

For the globigerinids, discrimination at generic level is relatively simple, based on the presence of the supplementary sutural apertures, and on this basis both Globigerinoides and Globigerina species have been confirmed within the Wadi Waqb carbonates. Globigerina bulloides/praebulloides and the supplementary apertured Globigerinoides quadrilobatus/trilobatus have been identified. Identification of genera within the Praeorbulina-Orbulina lineage is more difficult, as the areal apertures of Orbulina and the sutural apertures of Praeorbulina are not easily observed in the studied randomly-oriented thin sections. Orientation of the thin section also controls the ease at which the penultimate globigerine chambers are determined to be partly or completely enveloped by the final spherical chamber leading to the presence of Orbulina species not being confirmed. Specimens of Praeorbulina glomerosa have, however, been identified although their subspecies have remained unidentifiable with confidence. Their presence within the Wadi Waqb carbonates is sometimes difficult to detect by inexperienced observers, leading to inaccurate biofacies and palaeoenvironmental determination when only the shallow-marine benthic foraminifera were identified. In one sample from the Wadi Waqb exposure, a specimen that resembles the bilobate species Biorbulina bilobata is preserved, but this could represent a section through the similarly profiled Praeorbulina transitoria.

Planktonic foraminifera have not been recovered from the basal siliciclastic Al Wajh Formation of the Tayran Group (Figure 2), where the presence of the Neogene benthonic foraminifera Ammonia beccarii, charophyte oogonia and thin, unornamented ostracods provide evidence for freshwater to marginal-marine conditions. In a recent study, however, planktonic foraminifera of Upper Oligocene to Lower Miocene (Hewaidy et al., 2012) have been documented from the Gulf of Suez. Calcareous mudstones of the Burqan, Jabal Kibrit and Kial formations typically contain abundant planktonic foraminifera, of variable diversity.

Insights to the palaeobathymetric preferences of Neogene planktonic foraminifera are mostly based on their recent morphological equivalents for which the pioneering work is that of Bé and Tolderlund (1971), and for which a summary of recent studies is given below (Sen Gupta, 2003). The maximum abundance of living planktonic foraminifera is considered to be present within euphotic near-surface waters between 10 and 50 m depth below which they gradually decline in response to a decrease in the availability of nutrients, prey and for photosymbiont-bearing forms, light penetration. In the depositional model of Hallock and Glenn (1986), planktonic foraminifera are considered to typify substrates below 100 m water depth and to represent their standard facies 1, considered to be basinal. Shallow-water species are typically spinose, thin-walled and generally small and include species of Globigerinoides and several of Globigerina for which morphological equivalents are common in the Wadi Waqb carbonates. In water depths of between 50 m and 100 m, larger specimens of spinose planktonic foraminifera are present and include Globigerina bulloides and the spherical form Orbulina universa for which the morphological near-equivalent genus Praeorbulina is present in the Wadi Waqb carbonates. These forms are found together with smooth, non-spinose forms for which Neogene equivalents are not considered to be represented on the Wadi Waqb carbonates. It is of interest to note that in nearshore environments the influence of depth can persist even when physiochemical conditions are equivalent to an open-marine setting.

Corals

Corals are very well represented in the Wadi Waqb carbonate exposures along the east flank of the Ifal Plain. At these localities, they are mostly in growth position and form very large compound coral colonies (Figure 13). In the exposures of Wadi Waqb along the west flank of the Ifal Plain, although coral fragments are clearly present, none are in growth position. Photomicrographs of coral fragments are illustrated in Figures 4, 5, 7 and 12. The most comprehensive study of Miocene corals of the Red Sea (Purser et al., 1996) states that the reef core facies is characterised by the presence of branched forms of Stylophora regulata and subsidiary Tarbellastraea spp., Montastrea alloiteaui, Porites and solitary mussids. The off reef facies is poorly coralliferous and dominated by rhodoliths. With reference to the illustrations of Purser et al., 1996, their figure 5) corals similar to Diploastrea (Thegioasteraea) sp., Tarbellastraea sp. and Stylophora sp. are present within the Wadi Waqb carbonates in growth position in Wadi Waqb exposures along the eastern flank of the Ifal Plain. In thin section, the coral fragments are poorly preserved owing to their aragonitic composition, and are mostly present as moulds.

The growth rate of modern corals varies among species, but in general, declines in deeper water. The highest growth rates are associated with shallow-water, branching corals, such as Acropora palmate and Acropora cervicornis, followed by “finger” corals such as Porites porites, “head” corals, such as Montastraea annularis) and finally platy corals, such as Agaricia spp. (http://geology.uprm.edu/ Morelock/rfbuild.htm). Most of the corals seen in the Wadi Waqb are massive or “head” corals, and are typical of deeper water setting.

The large compound, vertically growing corals visible in the Wadi Aynunah-Al Khuraybah region represent in situ coral growths within the reef wall or fore-reef region. In a study of coral palaeoenvironments, Morsilli et al. (2012) state that the concept of the modern tropical carbonate factory is usually associated with the primarily biologically controlled carbonate production that occurs in warm, well-illuminated, oligotrophic, near-surface waters of the tropics and subtropics. These settings are typically dominated by a range of photosynthetic autotrophs, such as calcareous green algae, and by organisms with photosynthetic symbionts including corals and larger benthonic foraminifera whose sediment association is known as photozoan (sensuJames, 1997).

Associated Microfossils

In addition to the microfossils described above, the Wadi Waqb carbonates contain echinoid plates and spines, branched bryozoa, ostracods, brachiopod fragments, bivalve and gastropod fragments. Echinoid plates and spines are easily identified by their straight extinction in crossed polarized light and their often accompaniment by syntaxial cement overgrowths. A rather distinctive, robust and relatively large echinoid spine is characteristic of the Wadi Waqb, and displays an undulating margin indicative of the presence of spine ribs. Ostracods and brachiopod fragments are typically well preserved, but the aragonitic gastropod and bivalve fragments are mostly preserved as moulds. Photomicrographs of representatives of the associated microfossils are illustrated in Figures 4, 5, 6, 7, 10 and 12.

BIOFACIES AND PALAEOENVIRONMENTS OF THE WADI WAQB MEMBER IN THE MIDYAN AREA

In the Midyan area, carbonates of the Wadi Waqb Member are well exposed along the east flanks of the Ifal Plain and form isolated exposures on the west flank, such as at the isolated hills of Ad Dhubaybah (Figure 14). Along the east flank, the carbonates form small hills of Ad Dhubaybah, and scattered exposures southeast of Aynunah and Al Khuraybah. South of Khuraybah, the carbonates form an almost continuous exposure upon the basement to beyond Aynunah and are locally preserved along the eastern Red Sea coast at least as far as Jiddah (Moore and Al-Rehaili, 1989; Mandurah, 2009). In the central area of the Ifal Plain, the Wadi Waqb is not exposed and can only be analysed from cored samples of exploratory wells. Along the west flank of the plain, the Wadi Waqb Member is well exposed in the southern hills at the Wadi Waqb type section in Wadi Waqb. The following discussion applies the environmental preferences of the various microfossils and microfossil fragments, as discussed above, to the biofacies present at the widely separated studied sections.

Ad Dhubaybah

The Wadi Waqb carbonates are exposed at the isolated outcrop of Ad Dhubaybah (Figures 14 to 16) where their contact with the basement is concealed by desert sand. At Ad Dhubaybah, the Wadi Waqb carbonates are exposed as three distinctive isolated hills that rise from the plain so that their contact with the underlying basement is not visible, although the granitic basement is clearly visible approximately 1 km to the east. They are well bedded when viewed from a distance, and exhibit a low dip towards the east. They are deeply incised by narrow sinuous wadis. On the east side of the main hill, the thick exposure is almost vertical and consists of a lower unit of poorly bedded abiotic quartz and lithic sandstones ca. 25 ft (8 m) thick, followed by a succession of carbonates ca. 30 ft (9 m) thick (Figure 16b) (see Hughes and Johnson, 2005, their figure 80) with which the contact is transitional that pass upwards into calcareous, fine-grained quartz sandstones that contain abundant miliolid foraminifera assigned to Quinqueloculina spp. The basal carbonates contain a distinctive echinoid biofacies (see Figure 16c) and sub-vertical crab or shrimp tunnel cf. Ophiomorpha (Figures 16c and 16d).

On the west side of the hill, the lower part of the succession is concealed by a scree slope on the west side of the main hill and the upper, exposed, section consists of rubbly, rhodolith-dominated, poorly to well-cemented carbonates that display a nodular, poorly bedded appearance. This unit may be a time equivalent of the Wadi Waqb Member as it is overlain by coarse limestones of the Wadi Waqb that are rich in rhodophytes and corals. Borelis melo melo has been identified within the sediments collected from the west flank of the outcrop (Kamal and Hughes, 1993; Hughes and Johnson, 2005). Its Recent morphological equivalent Neoalveolina pygmaea, indicates a derivation from water depths within the range of ca. 17–24 m (55–78 ft) (Said, 1950a). Other foraminifera include Elphidium crispum, Planorbulina complanata, Quinqueloculina spp., Sphaerogypsina globula and rotalids. Rare planktonic foraminiferal genera Globigerina and Globigerinoides are also present at one level considered to represent a maximum flooding event within the succession. It is of interest that on the Egyptian Red Sea, a similar Miocene outcrop, Gebel Abu Shaar displays a quartz sand-dominated facies interpreted as “platform interior facies” that is overlain by a coral-dominated “platform edge facies” (Aissaoui et al., 1986).

Samples collected up the ca. 59 ft (18 m) high cliff on the west side of Ad Dhubaybah (Table 1, samples M10-40 to M10-49) display a coral and rhodolith-dominated grainstone succession with common fragments of bivalves, bryozoa and echinoids. Foraminifera are mostly benthonic and include Quinqueloculina spp. and Borelis melo, Sphaerogypsina globula and Textularia spp. Rare planktonic foraminiferal genera Globigerina/Globigerinoides are present in the upper part of the section (sample M10-49) and their presence above the sole presence of Operculinella venosa suggests possibly deeper conditions. Although no in situ corals were seen, the succession suggests proximity to a fringing reef and predominantly shallow-marine conditions, except for the uppermost part. A shorter section was sampled to represent a possible back-reef environment (Table 1, samples M10-54 to M10-59). This section also includes fragments of corals and rhodoliths, bivalves and echinoids. Subangular quartz grains are well represented and indicate greater proximity to the granitic basement than the fore-reef succession described above. Foraminifera are rare and of significantly lower diversity than the fore-reef succession and include only species of Quinqueloculina and Textularia.

Wadi Aynunah

On the northwest side of the Wadi Aynunah gorge is located a well exposed section through the Wadi Waqb carbonates and their contact with the underlying basement (Figures 17 and 18a) (Hughes and Johnson, 2005, their figure 81). A study of this exposure by Hussein and Al-Ramadan (2009) has revealed four microfacies of which two carbonate microfacies are associated with upper and middle reef-front environments, one siliciclastic microfacies with a flash flood setting and one carbonate microfacies with a lower reef-front environment.

Samples M10-75 to M10-109 (Table 2) contain rich microfacies that include fragments of articulated coralline rhodophytes, encrusting rhodophytes, articulated dasyclad algae gastropods, coral, benthonic foraminifera including Borelis melo melo, Sphaerogypsina globula, Quinqueloculina spp., Rotalia spp., Textularia spp., Operculinella venosa and Heterostegina suborbicularis. Of interest is the presence of the dasyclad alga Halimeda sp. in the lower part of the succession (below sample M10-87). Planktonic foraminifera include Globigerina spp. and Praeorbulina glomerosa, but re-concentrated in the upper part of the succession. This biofacies is considered to represent a lower shallow-marine succession that is overlain by moderately deep-marine conditions that supported species of the planktonic foraminifera Globigerina and Praeorbulina, and compares well with the upper part of the Ad Dhubaybah fore-reef succession where planktonic foraminifera are also present. All of the benthonic biocomponents are considered to have been transported into this environment as downslope transported debris from a shallow-marine, rhodophyte and coral reef, probably by storms. The presence of Borelis melo melo, if compared with its Recent morphological equivalent Neoalveolina pygmaea, indicates a derivation from water depths within the range of 17 to 24 m (Said, 1950a). Lithic fragments and the consistent presence of small subangular quartz grains indicate proximity to the granitic basement. This exposure contains a biofacies that contains moderately high microfossil diversity with a variety of benthonic foraminifera and rare planktonic foraminifera that suggest slightly deeper conditions than those interpreted for the Ad Dhubaybah reef complex. The Wadi Waqb carbonates in the Aynunah area and along cliffs north of Wadi Aynunah contain large and diverse corals in growth position (Figure 13).

The steeply-dipping succession is considered to probably represent depositional dip in a fore-reef talus environment, rather than a fault-rotated block. The Wadi Waqb carbonates are unconformably overlain by near-horizontal beds of sandy limestones that may represent an extensive lagoon that prograded over the main Wadi Waqb. These beds are related to the Khuraybah Formation as defined by Clark (1986), and cap the main Wadi Waqb succession along the coastal strip in this region (Figure 19). A similar succession from the Khuraybah area (Table 3) was sampled in 1991 by the author but the accurate sample heights are unavailable. Of significance is the common and consistent presence of planktonic foraminifera within the lower part of this rhodophyte-dominated grainstone succession. The consistent presence of angular quartz grains, including those samples with planktonic foraminifera, suggests deep but nearshore conditions as would be expected on the steeply dipping flanks of the basin in a similar fashion to the present Red Sea nearshore marine environment.

Ifal Plain Subsurface

In the subsurface the Wadi Waqb carbonates contain a mixture of shallow-marine benthonic foraminifera and red algal debris in which planktonic foraminifera are locally present. This association and the distribution of depth-sensitive planktonic foraminifera has been used to determine that the site of deposition was located within middle-shelf water depths, of between 50 m and 75 m, into which was transported debris from a predominantly rhodophytic reef complex.

A number of significant features are evident, and include the consistent presence of encrusting coralline red algal fragments, assigned to the rhodophyte genus Lithophylum, and echinoid fragments and the scarcity of dasyclad algal fragments and of miliolids assigned to Quinqueloculina spp. The episodic presence of the benthonic foraminiferal genus Amphistegina is noteworthy. Species assignment is difficult in thin section, as the sutural details are not visible. Nevertheless, the absence of papillae would suggest that the species represented is most likely to be A. lessonii, based on the criteria described by Larsen (1976). This species ranges from 0 to 75 m water depth in the Gulf of Aqaba, with an increase in abundance shallower than approximately 40 m. Scattered, rare occurrences of coral fragments are present in the lower part, and increase to a consistent presence in the upper 50 ft (16 m) of the section. Planktonic foraminifera, including Globigerina spp. and Globigerinoides spp. are evident at five levels within the section.

The micropalaeontological assemblages suggest that the Wadi Waqb carbonates analysed in the subsurface were deposited on the flanks of a rhodophyte and coral reef from which were continuously shed rhodophyte fragments, probably as storm deposits. Although rare planktonic foraminifera are currently recorded from depths as shallow as 10 m in the Gulf of Aqaba (Erez and Gill, 1977) the planktonic foraminiferal biofacies include spinose, non-keeled genera that are typical of the upper 50 m water depth (Bé and Tolderlund, 1971). Their localised presence suggests episodes of elevated sea level when hydraulic energy levels dropped and the supply of allochthonous debris was reduced. Orbulina/Praeorbulina spp. were not recorded, and this may assist in confining the water depth to shallower than 50 m (Berger, 1969; Reiss et al., 1974).

The localised presence of Amphistegina spp., often coincident with the planktonic foraminiferal pulses, is consistent with deeper conditions, as this genus is presently known to form over 97% of faunas in the Gulf of Suez and Red Sea (Said, 1950b), and over 60% of foraminiferal faunas at 45 m in the Gulf of Aqaba (Frenkel, 1974). The lenticular to platy profile of the specimens suggests that deeper water conditions are represented, as none of the globular shallow forms were noted, following the studies by Larsen (1976). Diverse foraminiferal assemblages and dasyclad algae typical of lagoons are poorly represented, and suggest that the allochthonous material was mainly derived from the fore-reef or main reef (Carozzi et al., 1976).

Wadi Waqb

Three sections were measured and sampled in Wadi Waqb (Figures 20, 21a and 21b), this being the type section for the Wadi Waqb Member. Two traverses have been made of which WWA is the measured and sampled section to the south, and WWB is the measured and sampled section a short distance to the north.

The tributary wadi of Wadi Waqb (28°11’33.96”N, 34°44’49.86”E) (traverse WWA) displays the most continuous and extensive succession of carbonates of the Wadi Waqb Member. The results of semi-quantitative micropalaeontological analysis, together with chronostratigraphic, lithostratigraphic and palaeoenvironmental interpretation with local biofacies of thin sections from samples WWA 1 to WWA 96 are displayed in Table 4 and summarised in Table 5. The section has an irregular distribution of samples over the 385 ft (117 m) section, owing to the presence of exposure-obscuring scree over parts of the measured cliff.

The succession consists of wackestones and packstones containing fragments of coral, rhodoliths, echinoids, bryozoa and bivalves. In contrast to the successions described from the Ad Dhubaybah, Wadi Aynunah and subsurface locations, the Wadi Waqb section contains well-represented dasyclad fragments, the benthonic foraminifera Borelis melo melo, Operculinella venosa with species of Globigerina and Globigerinoides planktonic foraminifera. The vertical distribution of the benthonic bioclasts suggests a succession of possibly transported shallow-marine debris flows that were deposited into a deep-marine setting that contained well-represented planktonic foraminifera. The consistent presence of the dasyclad alga that resembles Halimeda spp. implies a very shallow source for these debris flows. When compared with the other locations described above, the Wadi Waqb carbonates at this location clearly display the richest planktonic foraminifera and suggest that this location represents the deepest depositional environment. The head of a southern Wadi Waqb tributary valley provides a viewpoint towards the location of the subsurface samples collected from a Saudi Aramco exploration well (Figure 22).

The succession at Wadi Waqb is interpreted to represent a deep-marine setting, in which planktonic foraminifera were accumulating under low-energy conditions. This situation was occasionally interrupted by episodes of high-energy debris flow deposition, containing shallow-marine bioclasts that were derived from an algal-reef setting. Unfortunately, the present fractured carbonates make any indication of the direction of such allochthonous beds difficult. The confirmed presence of in situ shallow-marine coralgal carbonates representative of fringing reefs along the east flank of the Ifal Plain present an obvious potential source. Recent work by King Fahd University of Petroleum and Minerals has discovered, however, in situ coralgal sediments at the western head of the Wadi Waqb valley and a local, westerly-derived source is now a strong additional contributary probability (Richard Collier, Leeds University, oral communication). The biofacies analysed from Ad Dhubaybah represent the shallowest of those studied in the entire Midyan region.

DEPOSITIONAL MODEL FOR THE WADI WAQB CARBONATES

Exposures of the Wadi Waqb carbonates are scattered, except for the almost continuous coastal rim of carbonates south of Aynunah, in the southeast of the Midyan area. The coastal fringe carbonates have been examined at a few localities, where they are seen to lie directly upon basement. The abundance of large compound corals and rhodoliths and proximity to the palaeoshoreline strongly suggests a narrow band of fringing reefs. The lithology underlying the carbonates exposed at Wadi Waqb remains poorly understood, although recently examined sections west of Wadi Waqb display the carbonates overlying basement (Richard Collier, University of Leeds, United Kingdom oral communication 2013, as part of an ongoing study by the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia). Clearly, the carbonates at Wadi Waqb were not deposited as fringing reefs, even though they may have been redeposited a short distance from fringing or basement-capping local submarine highs. The presence of planktonic foraminifera in the carbonates collected from Wadi Waqb and also in the subsurface samples from the Midyan region (Hughes and Johnson, 2005) implies that the shallow reef facies of the Wadi Waqb Member were located adjacent to deep water into which the reefal debris was periodically transported, either by storms or by earthquakes in this highly seismically active area. The sharp palaeobathymetric contrast between the shallow reefs and the adjacent deep-marine mud-dominated sediments resemble that which exists along the present day Gulf of Aqaba and Red Sea coastline (Reiss, 1977; Hottinger, 1977; Auras-Schudnagies et al., 1989; Hottinger et al., 1993; and Siccha et al., 2009).

Palaeoenvironmental interpretation of the various Wadi Waqb biofacies has been guided by reference to the palaeobathymetric distribution of various Recent calcareous algae and foraminifera recorded from the Red Sea by Reiss et al. (1977) (Figure 23) and from idealised Tethyan Neogene studies summarised by Boudagher-Fadel (2008) (Figure 24) and Pedley (1998). The few isolated, small and widely-separated exposures of Wadi Waqb carbonates hinder accurate reconstruction of the entire depositional palaeoenvironment. While the basement proximity and coral constructed fabrics of the eastern exposures would geomorphologically suggest a rimmed shelf, in the style of Carozzi et al. (1976), Moore (2001) and Ahr (2008), the rhodolith-dominated microlithofacies compare well with the Miocene “foramol/rhodalgal ramp” of Pomar et al. (2004), with an additional proximal coral reef bioherm biofacies. Such fringing reefs are well described by Hayward (1982, 1983) and Rouchy et al. (1983).

The Miocene reef carbonates of the Philippines (Carozzi et al., 1976) include, however, a coral-red algal reef barrier and increase the potential of their model as an analogue for the Wadi Waqb carbonates. Microfossils of the Philippine carbonate model of Carozzi et al. (1976) are more diverse than those of the Wadi Waqb, especially that of the benthonic foraminiferal diversity in which the more open-marine genera including Lepidocyclina, Spiroclypeus, Miogypsina and Austrotrillina are present (these genera have never been recorded in the late Early to early Middle Miocene sediments of the Red sea). The back-reef lagoonal sediments, coralgal reef, and associated offshore calcareous siliciclastic sediments, suggest a rimmed, distally steepened platform rather than a ramp. The “middle ramp” biofacies of Pomar et al. (2004) compares well with those if the Wadi Waqb Member, including abundant red algae and larger foraminifera such as Heterostegina, Operculina, and Amphistegina, is considered to represent a deep oligophotic setting below wave base. Their outer ramp biofacies includes proximal, middle and deep sub-biofacies of which the latter includes planktonic foraminifera and equates well with the deeper biofacies of the Wadi Waqb carbonates. A summary of the interpreted depositional environments is displayed in Table 6, with the interpreted distribution of the various microfossils placed with reference to the rimmed shelf model of Moore (2001) is displayed in Figure 25.

In the deeper-marine assemblages of the Ifal subsurface and in the measured section in Wadi Waqb, the relationships of the deeper-marine planktonic foraminiferal-dominated assemblages and the shallower benthonic assemblages could be explained by at least three depositional situations. The first situation would require a deep-marine setting into which shallower marine, allochthonous bioclasts could be transported by a storm or destabilization of the sediment by an earthquake possibly related to fault-block tilting in this tectonically active region and be termed an event bed such as described by Martin et al. (1993) and by Bosence et al. (1998). These beds would be expected to have a clear-cut base and a normally fine-grading upper part returning gradually into a planktonic-dominated biofacies. This situation has been documented in debris flows of similar age in the Gulf of Papua, where the shallow-marine derived microfossils of the Darai Formation were transported into the planktonic-dominated deep-marine sediments of the Puri Formation (Hughes, 1988; confidential report for the World Bank Gulf of Papua project). The second situation would be created by a eustatic sea-level fall, resulting in a fall of base level and active erosion and downslope transport of shallow-marine sediments. The succession of beds containing such transported beds might be expected to be more gradational upwards, with a gradual increase in the proportion of shallow-marine bioclasts as sea level approached its lowest. The third situation would be autocyclic, and the presence of shallow-marine bioclasts could be explained simply by progradation of a shallow-marine setting into deeper-marine sediments. Current knowledge of the bed relationships would suggest that the shallow-marine bioclasts were transported as periodic catastrophic events triggered by storms or seismic activity and relate to the first of the scenarios discussed above. The following summary provides evidence upon which this conclusion is based, although it must be considered that one or all of these situations could have been active at different times during deposition of the Wadi Waqb carbonates.

The micropalaeontological variations between the Ad Dhubaybah, Al Khuraybah, subsurface and Wadi Waqb locations can be expressed in Table 4. Palaeoenvironmental interpretation has been based on the various microfossils. Of these, certain benthonic foraminiferal species are typical of a shallow to moderately deep lagoon. The rhodophyte fragments suggest deeper marine conditions and the presence of planktonic foraminifera provides evidence of even deeper marine conditions. Variations in their relative proportions within the biofacies are used to interpret varied palaeoenvironments for the Wadi Waqb Member, as discussed below.

The Ad Dhubaybah section is characterised by biofacies that typify the inner ramp although the presence of abundant rhodophytes would suggest some proximal middle ramp influence. As the distance from the steep basement cliffs provides a finite width of the original carbonate platform at this location, it is evident that the Ad Dhubaybah actually represents a rhodophyte-dominated reef front that protected a narrow lagoon. At Al Khuraybah the biofacies more closely resembles a middle ramp environment, with large compound corals well represented together with rhodophyte fragments. Planktonic foraminifera are locally common and indicate moderately-deep marine conditions, at least deeper than 25 m (Reiss et al., 1977). The predominance of planktonic foraminifera within the subsurface of the south-central Ifal Plain and also in the Wadi Waqb WWA and WWB successions are consistent with the proximal outer ramp. It is noteworthy that deep-marine benthonic foraminifera are absent to rare, and are considered to represent exclusion due to environmental instability related to the episodic input of shallow-marine bioclasts and bioclastic debris. Pedley suggests approximate widths of 10 km for the inner ramp, 12 km for the middle ramp and more than 22 km for the outer ramp, based on Miocene ramps of the Mediterranean. It is clear that the middle ramp of the Midyan region was wider than this suggested range. The Wadi Waqb carbonates provide an example where micropalaeontological analysis and the identification of planktonic foraminifera especially, have provided critical palaeoenvironmental information that would not necessarily be evident in routine core-based sedimentology.

With reference to Figure 1, and the evidence presented in the preceding section, these locations are considered to represent progressively deepening palaeoenvironments that range from a shoreface to very shallow lagoon (Ad Dhubaybah) to a moderately deep, middle shelf setting with planktonic foraminiferal access (Al Khuraybah, subsurface and WWA). It is probable that shallow carbonates existed west of Wadi Waqb, and to have shed debris eastwards (Dr. Richard Collier, University of Leeds, oral communication). The biofacies identified in the present study have enabled a basic palaeoenvironmental map to be drawn (Figure 25), using the principles described by Carozzi et al. (1976) and Pedley (1998), especially the distribution and relative abundance of planktonic foraminifera. Figure 25 and Table 6 illustrate the palaeoenvironments interpreted for each of the studied locations in the Midyan region.

Transport of shallow and relatively shallow carbonate ramp bioclasts into a deeper setting can be triggered by a variety of situations, of which variations in accommodation, cementation, gradient, eustacy, weather or tectonic activity are of most importance. Within the tectonically active Red Sea region, it is not inconceivable that the shallow bioclasts were destabilized by periodic earthquakes and that this may have been the prime reason for their destabilisation. This is an especially attractive mechanism when one considers that the rhodoliths would not necessarily have colonized the shallow lagoons of the fringing reefs. Of the other candidate causes, the relatively protected nature of the Red Sea may preclude, but not necessarily exclude, their origin by storms. Highstand shedding (Schlager et al., 1994) seems unlikely as the frequency of the allochthonous debris is probably too high to be related to elevated carbonate productivity and shedding due to the resulting decreased accommodation during highstands. Interpretation of the remarkably well preserved specimens of foraminifera may be explained by the simulated destruction of 81% of the benthonic foraminiferal species Amphistegina gibbosa by physical abrasion over the equivalent of transport over a distance of 34 to 100 km (Peebles and Lewis, 1991). This would imply a possible transport distance less than 34 km, if similar abrasion rates had prevailed. In a mud-laden debris flow, it could be assumed that physical abrasion would be less, and that this estimation of distance travelled could be significantly conservative.

It is of interest to note that mid ramp to lower ramp facies may not display responses to productivity rates related to sea-level variations (Brandano and Piller, 2010, Brandano et al., 2010). The location of rhodolith-dominated facies in the middle to lower ramp oligophotic zone is considered to reduce their sensitivity to variations in light availability or hydraulic energy resulting from sea-level variations. These authors also consider the Lower and Middle Miocene inner ramp productivity to be insufficiently high to generate superfluous bioclastic material to be exported to the middle and outer ramp settings. Marrack (1999) noted that rhodoliths in tidally dominated beds below 12 m water depth appear to move only occasionally due to bioturbation and severe storms. Episodic transport of shallow bioclasts into the deeper-marine setting may indeed be triggered by severe storms or seismic activity. Storms are an accepted dominant process for sediment export on many reefs. The passage of Hurricane Hugo over the island of St. Croix in 1989 (Hubbard, 1992) removed at least 2 million kg of sediment were calculated to have been transported from the St. Croix reef during a 10 hour hurricane event, compared with a normal estimated transport of 1.8 million kg in a century (Hubbard, 1992).

In the Wadi Waqb carbonates of the subsurface beneath the Ifal Plain and also in the Wadi Waqb exposures on the west flank of the plain, the character of the shallow-marine bioclasts within the planktonic foraminiferal-bearing mudstones may provide a clue to determine if they represent periplatform talus, or debris flows “debrites” and turbidites as summarised by Ahr (2008, p.126). Steep slopes are more likely to shed rock falls, debrites and coarse grain traction flows, whereas gentle slopes are associated with the generation of turbidites, rhythmites and soft sediment slumps that move as suspension flows. Debrites are characterised by being the products of sediment gravity flow composed of large clasts supported and carried by a mud-water mixture, that are poorly sorted and internally structureless. The Wadi Waqb carbonates of the central and western Ifal region with their poorly sorted bioclasts, including large corals, would typically fall into this category. To gain some indication of the extent of such displaced shallow-marine debris within the deep-marine setting, Crevello and Schlager (1980) state that in the Bahamas, gravity displaced sediment contains more than 70% of shallow carbonate fragments with an areal extent up to 400 sq km.

COMPARISON WITH RHODOPHYTE-DOMINATED PLATFORMS OF THE REGION

Early to Middle Miocene carbonate platform with similarities to that envisaged for the Wadi Waqb Member are well exposed in the Mediterranean, Gulf of Suez and Red Sea regions (Bosence and Pedley, 1982; Franseen et al., 1996; Buchbinder, 1996; Janson et al., 2010; van Buchem et al., 2010). With special relevance to the Wadi Waqb carbonates of northern Saudi Arabia are the excellent exposures of Miocene platform carbonates in the Gulf of Suez. These include the Abu Shaar-Esh Mellaha, Sharm el Bahari, Sharm el Luli and Zug al Bohar carbonates (Purser et al., 1996; Figure 18b). The Abu Shaar carbonates, situated on the southwest side of the Gulf of Suez, have been examined in considerable detail (Coniglio et al., 1988; James et al., 1988; Alsharhan and Salah, 1994; Coniglio et al., 1996; Purser et al., 1996; Bosence et al., 1998; Bosworth and McClay, 2001) since their first description by Gregory (1906). Biostratigraphic evidence provides an Early Miocene for the Abu Shar carbonates as exposed in Gebel Abu Shaar el Quibli. This complex has been considered by some to represent informal members of the Rudeis Formation (James et al., 1988) and equivalent to the Burqan Formation of Saudi Arabia, but later workers consider its age to span the Early to Middle Miocene, Burdigalian to Langhian (Purser et al., 1996) based on the presence of Borelis melo (Magne, 1978). The sandy facies with Pecten-type lamelli branch at the top of the main carbonate and underlying the Langhian marls is likely to be Burdigalian by comparison to other localities.

The latter age interpretation is more consistent with that considered for the Wadi Waqb Member. These “fault-block carbonate platforms” are considered to have formed as a carbonate platform complex that developed on the crests of a series of domino-style fault blocks, and subsequent syn-depositional fault block rotation caused by rejuvenation of the rift-margin fault complexes (Burchette, 1988; Bosence et al., 1998).

The Abu Shaar complex includes four informal members that represent successive changes in the depositional environments (James et al., 1988; Coniglio et al., 1996). The Kharasa forms the lowermost member and consists of transgressive conglomerates and fossiliferous dolomitic sandstones that overlie the basement and pass upwards into an open platform, mixed carbonate-terrigenous carbonate succession. The Esh el Mellaha member consists of a succession of reef and fore-reef carbonates that partly onlap the Kharasa member. The Bali’h member is a succession of well-bedded shallow subtidal and tidal-flat dolomitised limestones with evidence of former evaporites. A chaotic mix of breccias and bedded sediments of the Chaotic Breccia member overlies the Esh el Mellaha member (James et al., 1988). Of these, it is possible to compare the Kharasa member with the siliciclastics of the Usayliyah Formation exposed in the hills east of Aynunah in the Midyan area and possibly represented at the foot of the east flank of Ad Dhubaybah. The Esh el Mellaha member is equivalent to the rhodolithic and coral carbonates that form the bulk of the Wadi Waqb Member along the east flank of the Ifal Plain in Midyan. The Bali’h member may be equivalent to the flat-lying sandy carbonates that cap the rhodolith and coral limestones of the eastern Ifal Plain exposures. There is no equivalent of the deep-marine facies of the Wadi Waqb Member in the published data on the Egyptian carbonate exposures.

ASPECTS OF DIAGENESIS

With the exception of the study by Coniglio et al. (1988, 1996), the diagenetic aspects of the Wadi Waqb carbonates have received little detailed attention. As microfossil preservation has been affected by diagenetic replacement at some localities, a few aspects of diagenesis will be considered. The aragonitic parts of microfossils have all been dissolved and provide mouldic porosity, and intergranular porosity is also locally preserved (Hussein and Al-Ramadan, 2009). Calcite cement has occluded much of the primary porosity. Echinoderm plates and spines are commonly overgrown by syntaxial calcite spar (Evamy and Shearman, 1965). Although most microfossils are not preserved in the subsurface, rhodophyte fragments and ghosts of planktonic foraminifera are locally detected.

Although the exposed Wadi Waqb carbonates do not display evidence for dolomitisation, those of the Abu Shaar complex in the Gulf of Suez are known to be dolomitic (El-Haddad et al., 1984; Aissaoui et al., 1986; Coniglio et al., 1988, 1996). In the subsurface of the Midyan area, the carbonates are extensively dolomitised and planar-e micromozaic and sucrosic forms are present (Figure 7). Coniglio et al. (1996) suggest that dolomitisation of the Abu Shaar carbonates was an early diagenetic process within the mixing zone during post-depositional uplift and percolation of hypersaline fluids through the carbonate performing aragonite dissolution and dolomite replacement. The thick layer of evaporites that overlie the Wadi Waqb Member in the subsurface would provide an obvious source of magnesium-enriched fluids that would encourage dolomitisation. Warren (2006, his figure 5.32) describes a mechanism for dolomite “aprons” within evaporite-prone basins. One aspect lightly considered by Coniglio et al. (1996) may be even more significant than percolation by supersaturated fluids from the overlying Mansiyah, and envisages hydrothermal genesis for which the saddle dolomite provides firm evidence. The intensive block-faulting of the sub-Wadi Waqb granitic basement during the Late Miocene to the present day, envisaged to have taken place in response to the Aqaba transpressional structural regime, would have provided ideal sources of hydrothermal fluids and fracture conduits from the basement into the Wadi Waqb.

CONCLUSIONS AND FUTURE RESEARCH

Shallow-marine, rhodophyte-dominated carbonates, of latest Early Miocene to possibly earliest Middle Miocene age, are well exposed in the Mediterranean, Gulf of Suez and northern Red Sea regions. They are especially well exposed in the northern and eastern flanks of the Saudi Arabian Red Sea where they have been mapped as the Wadi Waqb Member of the Jabal Kibrit Formation. These carbonates have been proved to contain commercial quantities of oil and gas in the subsurface of offshore and onshore Gulf of Suez and in the Midyan area of the Saudi Arabian Red Sea.

Benthonic and planktonic foraminiferal evidence in the Midyan region has proved, until now, to be similarly insufficiently sparse to confidently confirm the presence of latest Early to earliest Middle Miocene carbonates. Within the relatively short chronostratigraphic extent of the Wadi Waqb in the Midyan region, these normally well-preserved and diverse biofacies display evidence for simultaneously existing shallow- to deep-marine depositional environments as well as events that testify to down-slope transport of shallow-marine bioclasts into the deeper-marine setting. In subsurface locations where dolomitisation has replaced most biocomponents, rhodophyte fragments have mostly proved resistant and provide palaeoenvironmental information. Such palaeoenvironmental studies, on a more regional basis, should assist in segregating palaeoenvironmental variations that were caused in response to eustacy versus those that were caused by structural readjustment expected in this highly tectonically active region.

The predominance of mud-supported carbonates and cementation has reduced the porosity of the Wadi Waqb carbonates, although hydrocarbon-significant porosity is preserved. The common presence of originally aragonitic biocomponents has led to mouldic porosity, with intra-particle porosity also preserved. Where not occluded by cement, inter-particle porosity is also preserved. Localised dolomitisation in the subsurface has provided inter-crystalline porosity. No permeability comments can be deduced.

In view of the limited geographic extent of the Wadi Waqb carbonates in the studied area of the Midyan region, future research on the numerous additional exposures of the Wadi Waqb Member along the eastern flank of the Saudi Arabian Red Sea is expected to provide additional information on lateral variations of the shallow-marine lithofacies. In addition to possibly revealing significant new information on the regional biofacies variations, it would also provide important lithofacies characterisation that may lead to better understanding the distribution of possible Wadi Waqb hydrocarbon reservoir facies in the Red Sea subsurface. We are aware that these carbonates were located on basement highs, but additional localities where such organic buildups developed are yet to be located in the study region, although the head of Wadi Waqb may be a candidate location (Richard Collier, oral communication). Depositional models established for the Wadi Waqb Member, and based on the well exposed variety of depositional environments, could provide insights to improving the understanding of similar settings within the Mesozoic carbonates of Saudi Arabia, and thereby improve models for predicting the distribution of reservoir facies variations.

There is much to be discovered regarding the paragenesis of the Wadi Waqb carbonates, and its varied influence on those carbonates that represent in situ accumulations versus those that are products of down-slope transport. Causes of dolomitisation of the carbonates, and its regional distribution are yet to be fully understood, especially where this has been influenced by hydrothermal fluid injection adjacent to re-activated basement blocks. Various investigations for metalliferous ore deposits have commented superficially on the relationship between mineralisation and the fractured carbonate host (Motti et al., 1982).

To refine the age of the Wadi Waqb Member, serial thin sections of the subspherical planktonic foraminifera should be made to identify subspecies of Praeorbulina and search for the presence of Orbulina species. Similarly, a search for Borelis melo curdica within the carbonates, as found in the Gulf of Suez, would further constrain the age. Strontium, carbon and oxygen-isotope analysis may also assist in such chronostratigraphic refinement. Of potentially biostratigraphic and high-resolution palaeoenvironmental significance would be research into the taxonomy of rhodophytes of the Red Sea, as the Midyan region has proved to provide rather limited examples. Additional exposures would hopefully provide specimens where the taxonomically significant reproductive cells may be developed.

ACKNOWLEDGEMENTS

The samples analysed by the author were collected during a series of short field trips to the Midyan area, and also from cored samples from subsurface sections related to exploration reconnaissance and drilling by Saudi Aramco, for which permission to publish is acknowledged. Readers will understand the limitations that confidential company data have necessarily restricted publication of some data. The results were presented in a poster at GEO 2010. The assistance of my colleagues during the fieldwork is acknowledged, and include Abdulkader Afifi, Mohettin Senalp, Robert Johnson, Tom Connelly and Rami Kamal. Recent fieldwork, as part of the Wadi Waqb study in the Midyan area, by King Fahd University of Petroleum and Minerals (KFUPM) has provided new insights following discussions with Dr. Khalid Ramadan (KFUPM) and Dr. Richard Collier (University of Leeds, UK). I thank my colleagues Nassir Naji, Nigel Hooker and Bob Lindsay for reviewing an earlier version of this paper. GeoArabia’s Assistant Editor Kathy Breining is thanked for proofreading the manuscript, and GeoArabia’s Production Co-manager, Nestor “Nino” Buhay IV, for designing the paper for press.

ABOUT THE AUTHOR

Geratnt Wyn Hughes is currently a Micropalaeontology Consultant and Director of Applied Microfacies Limited, based in North Wales, having recently left his position after 21 years as Senior Geological Consultant in the Biostratigraphy Group of Saudi Aramco’s Geological Technical Services Division. He gained considerable experience in the integration of micropaleontology and microfacies with sedimentology to support exploration activities and assist reservoir characterization, and has an extensive publication record. He gained BSc, MSc, PhD and DSc degrees from Prifysgol Cymru (University of Wales) Aberystwyth, UK, and in 2000 he received the Saudi Aramco Exploration Professional Contribution award, in 2004 the best paper award, and in 2006 the GEO 2006 best poster award. His biostratigraphic experience, prior to joining Saudi Aramco in 1991, includes 10 years with the Solomon Islands Geological Survey, and 10 years as Unit Head of the North Africa-Middle East-India region for Robertson Research International. He maintains links with academic research as an Adjunct Professor of the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia and visiting lecturer at various United Kingdom universities and the University of Urbino micropalaeontology summer school. He is an Associate Editor for the AAPG, Saudi Journal of Earth Sciences, GeoArabia reviewer, and a member of the British Micropaleontological Society, the Grzybowski Agglutinated Foraminiferal Society, SEPM Society for Sedimentary Geology, the Cushman Foundation for Foraminiferal Research and Geoscience Wales. He has recently been accepted as a Scientific Associate of the Natural History Museum in London.

wynapgwilym@gmail.com