ABSTRACT

The Midyan region provides a unique opportunity in which to examine exposures of the Upper Cretaceous and Neogene sedimentary succession. Recent investigations have yielded new interpretations of its depositional environments, stratigraphic relationships, and structure. In this paper, all the lithostratigraphic units of the Midyan succession are considered to be informal in advance of an on-going process of formalization.

The region is bounded to the north and northeast by mountains of Proterozoic rocks and to the west and south by the Gulf of Aqaba and the Red Sea, respectively. The Wadi Ifal plain occupies most of the eastern half of the region, beneath which is a thick sedimentary succession within the Ifal basin. The oldest sedimentary rocks are the fluviatile Upper Cretaceous Adaffa formation and marine siliciclastics and carbonates of the lower Miocene Tayran group, unconformable on the Proterozoic basement. The Tayran group is unconformably overlain by the deep-marine lower Miocene Burqan formation that, in turn, is overlain by marine mudstones, carbonates, and evaporites of the middle Miocene Maqna group. The poorly exposed middle Miocene Mansiyah and middle to upper Miocene Ghawwas formations consist of marine evaporites and shallow to marginal marine sediments, respectively. The youngest rocks are alluvial sands and gravels of the Pliocene Lisan formation.

A complex structural history is due to Red Sea Oligocene-Miocene extension tectonics, and Pliocene-Recent anti-clockwise rotation of the Arabian Plate relative to Africa on the Dead Sea Transform Fault. The Upper Cretaceous succession is a probable pre-rift unit. The Oligocene?-Miocene syn-rift 1 phase of continental extension caused slow subsidence (Tayran group). Syn-rift 2 was an early Miocene phase of rapid subsidence (Burqan formation) whereas syn-rift 3 (early to middle Miocene) was another phase of slow deposition (Maqna group). The middle to late Miocene syn-rift 4 phase coincided with the deposition of the Mansiyah and Ghawwas formations. The Lower Pliocene to Recent succession is related to the drift (post-rift) phase during which about 45 kilometers of sinistral movement occurred on the Dead Sea Fault.

The structural control on sedimentation is evident: the Ifal basin was formed by east-west lithospheric extension; pull-apart basins occur along major left-lateral faults on the eastern coast of the Gulf of Aqaba; and basin-bounding faults controlled deposition of the Burqan, Ghawwas, and Lisan formations. Pliocene to Recent earth movements may be responsible for activating salt diapirism in the Ifal basin. Extensive Quaternary faulting and regional uplift caused the uplift of coral reefs to at least 6 to 8 meters above sea level.

INTRODUCTION

The Midyan Peninsula lies within the angle formed by the Gulf of Aqaba and the Red Sea (Figures 1 and 2). The Gulf of Aqaba is the surface expression of the southern segment of the Dead Sea Transform Fault that connects the Red Sea rift zone with the Bitlis-Zagros subduction zone of southern Turkey. Offset magnetic anomalies (Hatcher et al., 1981) and sedimentary contacts (Quennell, 1958; Hagras et al., 1988) indicate between 100 and 105 kilometers (km) of left-lateral offset since the Cretaceous.

Within the Peninsula is exposed a Cretaceous to Neogene lithostratigraphic succession (Figure 3) that can be directly related to the one described from the Red Sea by Hughes and Beydoun (1992) and to the stratigraphic scheme established informally by Saudi Aramco (Hughes and Filatoff, 1995; Johnson et al., 1995). High, rugged mountains of Proterozoic basement rocks form the northern and eastern boundary to the region and also the core of the Maqna massif in the west. Wadi Ifal supplies basement-derived sediments that are transported onto the Ifal plain and mask the underlying Cretaceous to Neogene deposits of the Ifal basin. The sedimentary rocks are exposed around the margins of the Ifal plain and on the flanks of the Maqna massif.

Recent interest in the geology of the Midyan region follows from hydrocarbon exploration along the Saudi Arabian Red Sea coast. It was recognized that the region offers a unique opportunity to examine the lithofacies and biofacies that would otherwise be accessible only by drilling. Seismic and drilling activities by Saudi Aramco in the 1990s proved the existence of hydrocarbon-bearing carbonate reservoirs in the Midyan region (Kamal and Hughes, 1995). Fieldwork and satellite image interpretation in 1997 and 1998 concentrated on the structural relationships, lithofacies and biofacies variations of the exposed limestones in order to improve the understanding of the reservoir facies framework within the carbonate sequences.

The 1:250,000-scale geologic map of the Al Bad’ quadrangle (Clark, 1986) provided the most comprehensive description of the geology of the Midyan region prior to the Saudi Aramco investigations. Combining the results of the recent ground investigations and the interpretation of satellite imagery produced a revised geologic map of part of the Midyan Peninsula (Figure 4). The lithological descriptions of exposures are based on recent work by Saudi Aramco geologists, supplemented by the Explanatory Notes that accompany Clark’s map. As the Proterozoic basement is of limited reservoir significance, it was not considered in any detail in the present study.

The Ifal basin is one of several Oligocene-Pliocene half-grabens on the western margin of the Arabian Plate that developed in response to two episodes of rifting and drifting. Sedimentation in the basin probably began with deposition of the Late Oligocene to early Miocene Tayran group (Figure 3) and was followed by deposition of the early Miocene Burqan formation and the early middle Miocene Maqna group (Jabal Kibrit and Kial formations). A marked angular unconformity is present at the base of the Jabal Kibrit formation in outcrop and in the subsurface. Evaporites characterize the Kial formation and the overlying late middle Miocene Mansiyah formation. Faulting was active during early and middle Miocene times and became a controlling influence of sedimentation during the deposition of the middle to late Miocene Ghawwas formation. Uplift of the Pliocene and Pleistocene Lisan and Ifal formations, together with fault reactivation, indicate a significant tectonic event in the Ifal basin in response to movement on the Dead Sea Fault system.

Previous Work

The geology of the Midyan region has attracted attention since the 19th century (Burton, 1878) when exploration for gold was the incentive. Bogue (1953) and Philby (1956) investigated the mineral resources of the region. Although numerous geological investigations had been made of the sedimentary rocks in the adjacent Egyptian Red Sea and Gulf of Suez areas, the first examination of the sedimentary succession in Midyan was by Richter-Bernburg and Schott (1954) followed by Kahr and Agocs (1962) and Agocs and Keller (1962). Regional investigations, including the compilation of geological maps, were by Bramkamp et al. (1963), Powers et al. (1966), and Drysdall (1978). A renewed interest in the region’s mineral deposits resulted in studies by the Japanese Geological Survey and British Steel Corporation. The Midyan Peninsula was mapped by Clark (1986) at 1:250,000-scale. He lists the many studies made of the Proterozoic basement.

Stratigraphic studies, including a concentration of interest in the extensive evaporite deposits, have been made by Croisille (1967), Skipwith (1973), Bigot and Alabouvette (1976), Shearman (1979), Boardman (1980), Brosset (1980), Remond and Teixido (1980), Motti et al. (1980, 1982), Le Nindre (1981), and Vial et al. (1983). Detailed stratigraphic investigations include those by Dullo et al. (1983), Clark (1986), Jado et al. (1990), and Taj (1991) together with unpublished reports by Saudi Aramco geologists. Structurally focussed investigations are few, but include Bayer et al. (1987, 1988), Montanet et al. (1988), Salah and Alsharhan (1996), and Gardner et al. (1996).

Between 1967 and 1976, ten deep exploratory wells were drilled to investigate the petroleum geology of the Saudi Arabian Red Sea (Figure 5) and a further six wells were drilled between 1982 and 1984 in the Yanbu region. The rocks penetrated by these wells were readily equated with the standard lithostratigraphy of the well-explored Gulf of Suez, but no serious attempt was made at the time to establish a formal stratigraphic succession.

Recent Work

Saudi Aramco made the most recent and intensive investigation into the petroleum geology of the Red Sea between 1992 and 1993. Fourteen exploratory wells were drilled and detailed field observations made in the Midyan region and in the Al Wajh, Yanbu, Jiddah, Ghawwas, and Jizan areas (see Figure 5). The results of the Saudi Aramco work have been documented either as Saudi Aramco internal reports (Hughes and Kamal, 1993 a-c; Kamal and Hughes, 1993 a, b; Johnson et al., 1995) or published by Hughes and Filatoff (1995), Kamal and Hughes (1995), Filatoff and Hughes (1996), and Hughes et al. (1998a, b). Most of the fieldwork was in the Midyan region (wholly within the area of the Al Bad’ 1:250,000-scale quadrangle mapped by Clark (1986)) as it has the most comprehensive exposures on the Red Sea coast of Saudi Arabia (Figure 2). This work led to the establishment of the informal lithostratigraphic scheme shown as Figure 3.

The nomenclature of the lithostratigraphic succession is varied, but Hughes and Beydoun (1992) suggested a regional scheme based on the similarity of the succession to the well-studied Gulf of Suez and the successions drilled or exposed in the Red Sea region. An informal scheme was later suggested for use by Saudi Aramco (Hughes and Filatoff, 1995; Johnson et al., 1995) that has been followed in the present work. Figure 6 compares earlier Saudi Arabian Red Sea lithostratigraphic schemes with the current scheme.

The aims of the present study have been to establish a lithostratigraphic scheme and depositional history for the Midyan region. A stratigraphic lexicon has been constructed that takes into account information from the wells drilled elsewhere in the Saudi Arabian Red Sea and its onshore margins (see Figure 3). In this, it builds on the work of Hughes and Filatoff (1995).

In late 1997, Saudi Aramco in conjunction with the Research Institute of the King Fahd University of Petroleum and Minerals (KFUPM), conducted a field trip to investigate carbonates that are the surface equivalents of the Wadi Waqb reservoir in the Midyan oil and gas field (KFUPM unpublished report, 1998). The fieldwork also provided the ground control for a structural synthesis of the region that incorporated satellite images derived from the Landsat Thematic Mapper (Figure 2). The structural analysis focused on the western part of the Ifal basin. The geological map shown as Figure 4 was the result of the fieldwork and structural interpretation.

In the context of recent work in the Midyan region, a new seismic technique for imaging the pre-Mansiyah fault blocks in the Midyan region is discussed by Mougenot and Al-Shakhis (1999) in this issue (p. 445 to 464).

LITHOSTRATIGRAPHY, BIOSTRATIGRAPHY AND PALEOENVIRONMENTS

The lithostratigraphic sequence that had been established for the Saudi Arabian Red Sea region also took into account the lithostratigraphy of neighbouring countries (see Karpoff, 1956; Skipwith, 1973; Remond and Teixido, 1980; Bokhari, 1981; Le Nindre, 1981; Dullo et al., 1983; Moltzer and Binda, 1984; Schmidt and Hadley, 1984; Clark, 1986; Le Nindre et al., 1986; Srivastava and Binda, 1991). The present scheme incorporates this pre-1990s stratigraphic nomenclature together with the results of the recent

Saudi Aramco studies of outcrop and subsurface data (Hughes and Filatoff, 1995; Johnson et al., 1995). However, according to the rules of stratigraphic nomenclature (Hedburg, 1961), the lithostratigraphic scheme remains informal as neither the publications by Hughes and Filatoff (1995) nor by Johnson et al. (1995), despite their detailed information, provided the comprehensive details of the designated type sections. G.W. Hughes and R.S. Johnson are in the process of formalizing the lithostratigraphic nomenclature adopted by Saudi Aramco and used informally in this present study. Therefore, the lithostratigraphic units of the entire Midyan succession, irrespective of authorship, are identified as (lower case) ‘group’, ‘formation’, and ‘member’ in order to indicate their present informal status.

In the following discussion (and in order to improve clarity of regional similarities), reference is made to the well-documented equivalent lithostratigraphic units of the Gulf of Suez (Figure 3), as suggested by Hughes and Beydoun (1992). The terms ‘early’ and ‘late’ are restricted to indicate relative time designation whereas ‘lower’ and ‘upper’ are applied to equivalent time-stratigraphic units (Bates and Jackson, 1980; Haile, 1987). As none of these designations are formalized for the Miocene (Barron et al., 1985), lower case lettering is used.

Basement

The Proterozoic basement borders the Midyan region to the north and northeast and also crops out in the core of the Maqna massif to the west of the Ifal plain. It consists of ultramafic, metavolcanic, and metasedimentary rocks and granitic plutons, that have been intruded by basalt, rhyolite, and dolerite dikes (Figure 7). The basement has been dated at about 600 to 700 million years old (Gardner et al., 1996) and is considered to have been formed along an accreting Proterozoic volcanic arc. Exploration drilling in the Ifal basin shows that the basement rocks are fractured and leached. The granitic character of the basement indicates a continental rift origin for the proto-Red Sea, rather than spreading and the generation of oceanic crust (Bosworth, 1993; Sultan et al., 1993).

Suqah group

The Suqah group consists of the Adaffa, Usfan, and Matiyah formations of which only the Adaffa formation is recognized in the Midyan region. The group includes essentially all the pre-rift Upper Cretaceous to Paleogene successions exposed in the Saudi Arabian Red Sea area (Figure 3). Johnson et al. (1995) named the group after Wadi as Suqah near Jiddah.

Adaffa formation

The oldest sedimentary sequence exposed in the Midyan area has a Late Cretaceous age based on macrofossils.

Name:Clark (1986) named his Adaffa formation after Al Adaffa village in Wadi ash-Sharmah in the Aynunah area of southeast Midyan. The formation now described corresponds to the lower part of Clark’s formation as much of his outcrop, including the fossiliferous calcarenite, has been reassigned to the Miocene Maqna group. Prior to the recent study, the sequence was assigned to the ‘Un-named Clastic Unit’ unit by Hughes and Filatoff (1995) (Figure 6).

Regional Equivalent: The group is the siliciclastic age equivalent of various carbonate units in the Gulf of Suez, such as the Duwi (Lower Campanian) and Sudr (Upper Campanian-Maastrichtian) formations.

Type Section: Aynunah graben in southeast Midyan (28°05'45"N; 35°16'3.4"E) (Clark, 1986).

Lithology: Clark estimated the Adaffa to be possibly 200 meters (m) thick (see Figure 8). The section is made up of a thin basal conglomerate overlain by approximately 300 feet (ft) of yellow to reddish-brown cross-bedded sandstone and gray-green shale. The conglomerate contains granite cobbles and pebbles, phosphatic nodules, dinosaur and turtle bones, and petrified wood fragments. The sandstone is a well-sorted quartz arenite (Afifi et al., 1993; Milner et al., 1993).

Distribution: The distribution of the formation is not known in detail but it is present in outcrop in Midyan and at depth in the Jiddah area. Similar rocks crop out in the Wadi Azlam basin on the Red Sea coast between Duba and Al Wajh.

Contact Relationships: Clark stated that the siliciclastics in the lower part of his Adaffa formation had a faulted contact with the Proterozoic basement and, although the upper contact was not clearly defined, suggested that it was conformably overlain by the Oligocene Sharik formation. In the present study, neither the lower nor the upper contacts of the Adaffa were visible due to sand and gravel covering much of the low-relief exposures. Preservation of the Adaffa in the southeast corner of the region may result from its location in an area of post-depositional faulting and graben development, as suggested by a Saudi Aramco unpublished seismic interpretation by G.S. Ferguson. There is no evidence for the Usfan and Shumaysi formations that have been described only from the Jiddah region (Karpoff, 1956; Al Shanti, 1966).

Age: Clark (1986) cites microfossil evidence of Andrieff (Le Nindre et al., 1981; Redmond and Teixido, 1980) to support a Late Cretaceous to Eocene age. In addition, in the Aynunah region, a thin bed of limonitic sandstone at the top of the sandstone succession contains bones of a sauropod (Titanosaurid) dinosaur and turtle plates (Ref: 28°05'45"N; 35°16'3.4"E) (Milner et al., 1993). The co-existence of Albian-Maastrichtian dinosaur bones and Cretaceous to Recent turtle plates provides an Albian to Maastrichtian age for the Adaffa formation. A similar assemblage of dinosaur and turtle remains, together with crocodiles, lungfishes and theropods, has been found in Sudan and given a Late Cretaceous age (Buffetaut et al., 1990). A Late Cretaceous microflora has been found in similar rocks approximately 290 km to the south at Wadi Azlam (Clark, 1986; Giot et al., 1980).

Paleoenvironment: A fluvial depositional environment has been suggested for the formation by Afifi et al. (1993). In Sudan, the fossil assemblages similar to those of the Adaffa formation have been interpreted as indicative of a lacustrine to braided-river environment (Buffetaut et al., 1990).

Tayran group

The group consists of the Al Wajh, Musayr, and Yanbu formations and is associated with the early period of Red Sea rifting. Recent studies of surface and subsurface data have clarified previously inconsistent classification schemes.

Name:Bokhari (1981) first assigned the name (after Jabal Tayran; Figure 2) to his Tayran formation.

Regional Equivalents: Correlation with the Nukhul Formation of the Gulf of Suez and the Red Sea is based on the lithological and paleontological similarities (Sellwood and Netherwood, 1984; Hughes et al., 1992; Hughes and Beydoun, 1992; Hughes and Filatoff; 1995; Ravnas and Steel, 1998).

Lithology: The group consists of conglomerates, sandstones, shallow-marine carbonates and, in the subsurface, anhydrite. Bokhari (1981) divided his formation into three members: the lower Wadi al Hamd member of red, siliciclastics; the upper Wadi al Kils member, a shallow-marine carbonate unit; and the Wadi Telah member, a laterally equivalent mixed siliciclastic-carbonate unit. Bokhari’s formation formed part of the Raghama group (Richter-Bernburg and Schott, 1954) that included the entire ‘Oligocene-Miocene’ succession (Skipwith, 1973; Brown et al., 1989) (Figure 5). In the subsurface, it is made up of anhydrite and was referred to as the Sharik formation by Clark (1986).

The Tayran group, as currently defined, displays lateral and vertical lithological variations. The siliciclastic Al Wajh formation makes up the entire group at certain subsurface localities but, as exposed in the Midyan, is overlain by carbonates of the Musayr formation. In the subsurface of the Midyan region and elsewhere in the Saudi Arabian Red Sea area, the Al Wajh siliciclastics are overlain by anhydrite of the Yanbu formation.

Distribution: In the Midyan region, the group is exposed along the northeastern margin of the Ifal plain and in the Jabal ar Risha area. Regionally, the Al Wajh and Yanbu formations are present in the Al Wajh and Yanbu basins.

Contact Relationships: The group is unconformable on Proterozoic basement rocks both onshore and offshore.

Age: Recent biostratigraphic studies (Hughes and Filatoff, 1995; Filatoff and Hughes, 1996) suggest an early Miocene age for most of the group with definite evidence of an early Miocene age for the Musayr formation. However, Jado et al. (1990), provide Late Eocene macropaleontological evidence for pre-rift siliciclastics at a small exposure 10 km north of the coastal town of Maqna. If correct, this may provide evidence for the presence of the Shumaysi formation, or time-equivalent sediments, within a succession currently attributed to the Al Wajh formation in the Midyan region.

Paleoenvironment. The group represents the early syn-rift stages in the evolution of the Red Sea, and the progression from a siliciclastic to carbonate depositional regime results from the progressive regional increase of marine influences.

Al Wajh formation

The formation represents the oldest unit of the Tayran group exposed in the Midyan region, with the exception of the rocks described by Jado et al. (1990). The formation consists of red siliciclastics that are barren of fossils, although microfossils have been recovered from its subsurface equivalent. The mineralogical and lithological components of the siliciclastic rocks and their proximity to underlying and adjacent exposures of the Proterozoic granitic basement indicate a direct derivation from the basement. Clark (1986) provisionally assigned the succession to the Sharik formation of Remond and Teixido (1980).

Name:Hughes and Filatoff (1995) published the name ‘Al Wajh formation’ following its use by Saudi Aramco in detailed lithological and biostratigraphic studies on samples from several shallow boreholes in the Al Wajh basin. The formation was previously named the Wadi al Hamd member of the Tayran formation by Dullo et al. (1983) (Figure 6).

Regional Equivalents: Probably equivalent to the Bathan formation described from the Jiddah region by Brown et al. (1989). When Hughes and Filatoff (1996) named the formation, the regional equivalent of the Al Wajh formation in the Midyan region was not defined, although its similarity to the Shoab Ali Member of the Nukhul Formation (Saoudi and Khalil, 1984) of the Gulf of Suez was noted.

Type section: Al Wajh South-1 (AWSO-1) Saudi Aramco exploration well, coastal Saudi Arabian Red Sea.

Lithology: The formation consists of poorly sorted sandstone and conglomerate. The basal conglomeratic sandstone unconformably overlies the Proterozoic basement (Johnson et al., 1995). It contains several rock types, notably of granitic composition, but chert pebbles are also present in the subsurface. Dullo et al. (1983) recognized a maximum thickness of 114 m (350 ft) for what is now the Al Wajh formation, whereas in the subsurface of the Saudi Arabian Red Sea as much as 600 m (1,900 ft) has been measured (Johnson et al., 1995). At Ad Dubaybah (28°27'09.9"N; 35°05'19.8"E) a total thickness of 93 ft is exposed beneath limestone of the Musayr formation.

Distribution: Widespread in the Red Sea coastal basins, though largely displaced by the Jizan Volcanics in the south. The formation is well exposed in Midyan at several localities in the northern part of the Ifal plain and in the Jabal as Safra region. North of Al Bad’, sandstone and conglomerate (Figure 9) are well exposed in a roadcut at 28°35'27.7"N; long 35°03'07"E. At some localities (e.g. 38°32'30.8"N; 35°0.0'37.5"E), oyster shells are preserved within thin beds of poorly sorted and bedded reddish brown sandstone. At Ad Dubaybah, (28°27'09.9"N; 35°05'19.8"E) well-bedded sands, with large-scale cross-bedding are exposed beneath carbonates of the Maqna group. Cross-bedded, very friable sandstones are 18 ft thick in Wadi Waqb (28°11'19.3"N; 34°44'07.8"E), where they rest directly on the basement.

They are overlain at this locality by oyster-bearing, blocky packstones and grainstones of the Musayr formation.

Contact Relationships: The Al Wajh formation is the lowermost unit of the Tayran group and overlies the Proterozoic basement. It is conformably overlain by carbonates of the Musayr formation in Wadi al Hamd (28°23'55.9"N; long 34°54'4.2"E).

Age: No palynological evidence has been obtained for the age of the formation in the Midyan region. However, an early Miocene age (Hughes and Filatoff, 1995) was established in the type section based on the presence of Acanthaceae-type pollen (evolution at base Miocene), Fenestrites spinosus (evolution at base Miocene), and the absence of charred Gramineae (not recorded in basal Miocene). Echinoids recently found within the siliciclastics at Ad Dubaybah are related to those considered typical of Miocene rocks in the Gulf of Suez (D. Hamama, Cairo University, personal communication, 1998). Similar echinoids found north of Maqna in the extreme northwest of the Midyan region have been assigned a Late Eocene age based on their similarity to species from the upper Shumaysi formation of the Jiddah region (Jado et al., 1990). Further sampling is necessary to prove or refute the Paleogene age, as such an age would prove the existence of an additional pre-rift lithostratigraphic unit in northern Midyan.

Paleoenvironment: Fluvio-lacustrine with marginal marine (possibly estuarine) pulses. A brackish to freshwater paleoenvironment in Midyan is indicated by the presence of the freshwater alga Pediastrum spp. and the benthonic foraminifera Ammonia beccarii, together with charophytes and unornamented ostracods in the subsurface. Le Nindre et al. (1986) concluded that the sediments are of fluvial to alluvial fan origin, and Johnson et al. (1996) similarly categorized the basal conglomeratic sandstone.

The formation was deposited during the early stages of slow subsidence and the first effects of a gradual marine transgression.

Yanbu formation

Name:Hughes and Filatoff (1995) named the formation for a subsurface succession of evaporites in the Yanbu region of western Saudi Arabia.

Regional Equivalents: Ghara Member of the Nukhul Formation (Saoudi and Khalil, 1984) in the Gulf of Suez and Egyptian Red Sea; and subsurface in the Yemeni and Somali Gulf of Aden (Hughes and Beydoun, 1992).

Type Section: Saudi Aramco exploration wells near Yanbu, central Saudi Arabian Red Sea. Lithology: Halite and anhydrite

Distribution: The formation is not exposed along the Red Sea coast but halite and anhydrite have been intersected in exploration wells in the Yanbu basin and north to Midyan (Cocker and Hughes, 1993). However, only one of the wells (both shallow and deep) drilled in the Midyan region contains anhydrite (20 ft thick) in the expected stratigraphic position (Johnson et al., 1995). Scattered exposures of anhydrite, considered to represent the Yanbu formation by Ferguson et al. (1993), are probably evaporites of the middle Miocene Maqna group or Mansiyah formation. They display an identical satellite image signature to those of the Maqna group. Excavations in the vicinity of the anhydrite exposures revealed Proterozoic basement and proved their superficial position and Mansiyah identity.

Contact Relationships: In the subsurface, the formation consists of anhydrite and halite that overlie the Al Wajh siliciclastics and in places are interbedded with them. It is typically overlain by the Burqan formation.

Age: An early Miocene age is assigned to the formation based on its stratigraphic position subsurface and a palynoflora similar to that of the Al Wajh formation. Strontium isotope dating of anhydrite laminae and crystals in a thick halite sequence gave an absolute age of 22 to 23 million years (Cocker and Hughes, 1993).

Paleoenvironment: The deposition of the formation is probably related to the early stage in the opening of the Red Sea. The purity of the evaporites argues against a sabkha origin and they probably formed in localized hypersaline ponds in a salina setting. Palynofloras from the intra-evaporitic sediments include the halophytic pollen Retiperiporites spp. but lack marine indicators; no microfauna are present. Orszag-Sperber et al. (1992) suggested that the formation of the evaporites was related to the beginning of Red Sea rifting and the tectonic control of restricted marine conditions, and not to a fall in sea-level.

Musayr formation

Name:Clark (1986) informally named the carbonates in the Jabal Musayr region as the Musayr formation. They had previously been considered as the Wadi al Kils member of the Tayran formation by Dullo et al. (1983) (Figure 6).

Regional equivalents: The Gharamul Member of the Nukhul Formation in the Gulf of Suez (Saoudi and Khalil, 1983).

Type section: An almost complete section of well-bedded carbonates dip at 33° to the northeast (N35°E) in a 323 ft section in Wadi al Hamd.

Lithology: Basal calcareous sandstone overlain by skeletal grainstone and packstone carbonates; rich in both macro- and micro-fauna.

Distribution: The formation is restricted to the Midyan region. A distinctive carbonate succession crops out on the flanks of Jabal Musayr, where its contacts with the underlying Al Wajh formation are well exposed (Figure 10). The Musayr limestone forms distinctive escarpments on the northern flank of Jabal Tayran. The carbonates are particularly well exposed and accessible in Wadi al Hamd (28°23'55.9"N; 34°54'04.2"E) and also in Wadi al Kils. At Wadi al Hamd, the formation directly overlies the basement, and the transitional basal beds consist of calcareous sandstone that grade vertically into a sequence of skeletal grainstones and packstones. At this locality, the gorge wall is characterized by recessive oyster beds that alternate with more resistant massive beds (Figure 11) (Kamal and Hughes, 1993a). At the Jabal Musayr locality, large compound corals derived from the Musayr formation are scattered along the foot of the mountain.

Contact Relationships: The formation conformably overlies the Al Wajh formation and is unconformable on the Proterozoic basement. It is unconformably overlain by the Burqan formation. There is no evidence that the Musayr formation overlies evaporites of the Yanbu formation.

Age: The presence of the benthonic foraminiferal genera Miogypsinoides and Miogypsina (Figure 12), including Miogypsina tani, within the carbonates indicate an early Miocene age, equivalent to the Tertiary Upper Te Letter Stage (Adams, 1970; Boudagher-Fadel and Banner, 1999). The Late Oligocene age suggested by Dullo et al. (1983) is based on larger benthonic foraminiferal specimens misidentified as Nummulites fichteli, but their illustrated thin sections (Dullo et al., 1983; Figures 1 to 4) are assigned to Operculinella venosa in this study.

Paleoenvironment: The basal calcareous sandstones were deposited in an intertidal environment. The oyster beds, corals, and miogypsinid assemblages in the overlying carbonate rocks indicate a warm, shallow-marine depositional environment, such as a shallow-marine carbonate platform or lagoon. In contrast to the other formations of the Tayran group, dinoflagellate cysts are prominent in the palynofloras, and are typical species of Systmatophora and Polysphaeridium.

Burqan formation

The Burqan formation is a thick succession of deep-marine calcareous mudstones with thick sand interbeds. It is well exposed in the Midyan region west of Jabal Rughama flanking the Maqna massif and along the Gulf of Aqaba coast (see Figure 4). It has not been assigned to a group.

Name: From the Burqan (offshore Midyan) exploration wells. The formation was informally called the ‘globigerina marls’ in unpublished (non-Saudi Aramco) oil company reports (see also Figure 6 for previous naming of the Burqan formation). The intensive investigation by Saudi Aramco has justified clarification of the previous informal lithostratigraphic assignment. The term Nutaysh (Clark, 1986) is retained as the sand-dominated Nutaysh member of the Burqan formation, and is readily distinguished from the equivalent mudstone-dominated lithology of the Subayti member (Johnson et al., 1995).

Regional Equivalents: The formation is time-equivalent (based on biostratigraphic evidence) to the lower part of the Rudeis Formation of the Gulf of Suez and Egyptian Red Sea (Abdine, 1979; Hughes and Beydoun, 1992; Hughes et al., 1992).

Type section: The type section was established by Saudi Aramco using the subsurface succession in the Burqan-3 well, situated offshore Midyan (Johnson et al., 1995).

Lithology: Turbidites, conglomerates, and sandstones of the Nutaysh member characterize the basal part of the sequence, especially in the west. Elsewhere, the upper part of the exposed section and most of the southeastern outcrops are composed of calcareous mudstones (Figure 13) of the Subayti member. Clark (1986) estimated a total thickness of 400 m for the entire formation. The sandstone forms thick, massive, poorly consolidated beds as much as 5 m thick (Figure 14). The calcareous mudstones are typically soft and ramified by veinlets of anhydrite. Sandstone is particularly well exposed in the western area of the region in road-cuts along the ‘old Maqna road’ (from 28°20'53.8"N; 34°46'38.5"E to 28°20'04.6"N; 34°45'26.5"E). The thickness of the Burqan formation is variable in the north-central part of the study area but generally thins southward due to extensive erosion that occurred after the deposition of the formation.

Distribution: The Burqan formation is well exposed north of the Al Bad’ to Maqna road, on the Gulf of Aqaba coast south of Maqna, and along the western and eastern boundaries of the Proterozoic basement that forms the core of the Maqna massif (Figure 4).

Contact Relationships: The deep-water turbidites of the Burqan formation lie with apparent disconformity on the Tayran group but are unconformable on the Proterozoic basement. At Jabal ar Risha (28°17'00"N; 34°45'21"E) (see Figure 2), sandstones of the Nutaysh member conformably overlie oyster-bearing carbonates of the Musayr formation. At this and other localities within the region, the Burqan formation is overlain by anhydrite of the Kial formation of the Maqna group. Such contacts are also well exposed south of the Maqna road, on the western slope of Jabal Rughama (28°26'01.5"N; 34°56'45.9"E). The formation is well-bedded, and dips radially away from the exposed Proterozoic basement core of the Maqna massif. It is apparent that during deposition of the Burqan formation, localized basement highs were exposed, and sites of non-deposition continued until late early Miocene times when the carbonates of the Wadi Waqb member (Jabal Kibrit formation, Maqna group) were deposited. In Wadi

Waqb (28°11'19"N; 34°44'07.8"E), carbonates of the Musayr formation are overlain by 105 ft of interbedded soft marl and hard calcareous sandstone of the Burqan formation.

Age: An early Miocene age has been assigned, based on the presence of age-diagnostic planktonic foraminifera and calcareous nannofossils (Hughes et al., 1992; Hughes and Filatoff, 1995). They are: Planktonic Foraminifera Zone N7, Globigerina ciperoensis forma atypica; Zone N6, Globquadrina praedehiscens; Zone N5 Globigerinoides primordius, and Calcareous Nannofossil Zone NN4, Helicosphaera ampliaperta; NN3, Sphenolithus belemnos; and NN2, Triquetrorhabdulus carinatus. Although of limited age significance, numerous echinoids are present within shale exposed at 28°17'13.5"N; 34°48'11.1"E.

Paleoenvironment: The presence of certain benthonic foraminifera, including Bathysiphon taurinensis, hispid Uvigerina spp. and Nodosaria spp., and the generally high diversity planktonic and deep-marine benthonic foraminiferal assemblages, indicate a predominantly bathyal depositional environment for the mud-dominated succession of the Nutaysh member. Well-developed trails as trace fossils occur at the base of many of the beds (Figure 15). The sandstones interbedded with the mudstones typically contain allochthonous penecontemporaneously transported shallow-marine microfossils.

Sequences of the Burqan formation in the belt of northwestern outcrops in Midyan were interpreted as deep submarine-fan deposits (Ferguson and Senalp, 1993). Several sediment sources are suggested. In the northwest, proximal turbidites display decreasing grain size towards the southeast where they become distal fan turbidites. However, in the southeastern outcrop, flute marks on the base of beds exposed at the head of Wadi Waqb (28°12'58.4"N; 34°44'40.6"E) indicate transport of sediment from the southeast. At this locality, the beds dip northwest (N35°W) at 32°. Near here is exposed a 15-ft-thick bed of massive sandstone.

Maqna group

A distinctive feature of the Midyan area is the extensive blanket of gypsum and anhydrite that covers much of the central part. The evaporite-bearing succession has been named the Maqna group. It consists of the Jabal Kibrit formation (and Wadi Waqb member) of early to middle Miocene age, and the middle Miocene Kial formation (Hughes and Filatoff, 1995; Johnson et al., 1995). In the subsurface, the carbonate and siliciclastic facies Umm Luj member (Johnson et al., 1995) underlies the Wadi Waqb member of the Jabal Kibrit formation.

Name: The first published use of the name (after the town of Maqna on the Gulf of Aqaba) was by Hughes and Filatoff (1995) following in-house usage by Saudi Aramco since 1992. (See Figure 6 for previous naming of the group in the Midyan area.)

Regional Equivalents: The group is regionally equivalent to the combined Belayim and Kareem formations of the Gulf of Suez and other areas of the Red Sea (Hughes et al., 1992; Hughes and Beydoun, 1992).

Type Area: Although well exposed in the Midyan region, the definition of the group has been based upon the better-understood subsurface stratigraphic relationship. The subsurface succession consists of relatively thin units of interbedded anhydrite or halite and calcareous mudstones.

Lithology: The group consists of exposed deep-marine carbonates, shallow-marine carbonates, and anhydrite and, in the subsurface, deep-marine mudstones and anhydrite. At Jabal Rughama, anhydrite is interbedded with soft, calcareous mudstones (Figure 16) that overlie Burqan mudstones. The thickness of the interbedded mudstones varies along the length of the exposure, most probably due to slumping of the anhydrite cap. A.A. Laboun (Saudi Aramco, personal communication, 1998) has confirmed that some of the repeated mudstone-anhydrite sections are the result of a series of gravity slides of the poorly consolidated mudstones. However, there are also undisturbed alternations between siliciclastics and interbedded anhydrite of the Kial formation (Figure 17).

Distribution: The group is exposed along the eastern and western margins of the Ifal plain in the Midyan region, and is present in the subsurface along much of the Saudi Arabian Red Sea.

Age: The group was dated as early to middle Miocene, based on the presence of age-diagnostic planktonic foraminifera and calcareous nannofossils (Hughes and Filatoff, 1995). Samples collected during the present investigation and from the subsurface confirm this age. The occurrence of charred Gramineae cuticle in the palynological assemblages (the richest and most diverse in the Neogene succession) has served to differentiate the Maqna group from underlying units.

Jabal Kibrit formation

The Jabal Kibrit formation is represented in outcrop within the Midyan region only by the carbonate facies Wadi Waqb member (see below) but, elsewhere, the siliciclastic Umm Luj member (Johnson et al., 1995) occurs beneath the Wadi Waqb member (Figure 3). The equivalent Miocene carbonates of the Egyptian Red Sea adjacent to Midyan have received considerable attention in recent years (Aissaoui et al., 1986; James et al., 1988; Shaaban et al., 1997; Youssef, 1997).

Name:Hughes and Filatoff (1995) first published the name (after Jabal Kibrit, Figure 2) following in-house usage by Saudi Aramco since 1992. In the present understanding of the stratigraphy of the Midyan region, the carbonates of the Khuraybah formation and the siliciclastics of the Usayliyah formation, as defined by Clark (1986) in the Aynunah area, are assigned to the Jabal Kibrit formation. Clark’s Usayliyah formation is exposed to the west of the main outcrop of the Adaffa formation (28°07'43"N; 35°12'01"E) and overlies the Khuraybah formation.

Type Section: Midyan-1 (MDYN-1) Saudi Aramco exploration well (see Figure 4 for location).

Regional Equivalents: The Jabal Kibrit formation is equivalent to the Kareem Formation of the Gulf of Suez and other areas of the Red Sea (Hughes et al., 1992; Hughes and Beydoun, 1992). The siliciclastics assigned to the Usayliyah formation by Clark (1986) are considered to be possibly equivalent to the Umm Luj formation of Johnson et al. (1995).

Lithology: The Jabal Kibrit formation consists of calcarenite, marl, sandstone, siltstone, and shale. The calcarenite and marl in these sections could be a distal equivalent of the Wadi Waqb member.

Distribution: The formation (represented by the Wadi Waqb member) crops out in Midyan in the Wadi Waqb, Ad Dubaybah, and Al Khuraybah areas. The formation is widely distributed subsurface in Midyan and elsewhere along the Saudi Arabian Red Sea.

Contact Relationships: Unconformably overlies the Burqan formation.

Age: An early to middle Miocene age was interpreted from the subsurface presence of planktonic foraminifera belonging to Zones N9 and N8 (see Figure 3) in the upper unit of shales and claystones (Hughes and Filatoff, 1995). Planktonic Foraminiferal Zone N9 (earliest middle Miocene) was based on the first downhole occurrence of Praeorbulina glomerosa, P. transitoria, and Globigerinoides sicanus, in the presence of Orbulina suturalis and Orbulina bilobata. Zone N8 (latest early Miocene), was based on the first downhole occurrence of P. glomerosa curva in the absence of Orbulina species. Calcareous nannofossils Sphenolithus heteromorphus, in the absence of Helicosphaera ampliaperta indicated Zone NN5 (Hughes and Filatoff, 1995). The analysis of samples collected during the present investigation confirms the age.

Paleoenvironment: From subsurface evidence, the Jabal Kibrit sediments are considered to have been deposited under deep-marine outer neritic to possible upper bathyal conditions, as indicated by the rich and diverse assemblages of planktonic and benthonic foraminifera (Hughes and Filatoff, 1995). However, coarse-grained facies may be the products of penecontemporaneous downslope transport from a shallow-marine source (see Wadi Waqb member).

Wadi Waqb member

The Wadi Waqb member of the Jabal Kibrit formation is a carbonate unit that is an important hydrocarbon reservoir in the Midyan region and also occurs in outcrop. Kamal and Hughes (1995) investigated the unit briefly. The carbonates were previously included in the Al Bad formation by Dullo et al. (1983). Several exposures mapped by Clark (1986) as being equivalent to limestones of the Musayr formation have now been identified as belonging to the Wadi Waqb member. In particular, the assemblages of planktonic and benthonic foraminifera in the Musayr formation limestones and the Wadi Waqb member differ greatly both in terms of biostratigraphy and environmental indicators.

Name:Kamal and Hughes (1995) mapped a carbonate unit on the southwestern flanks of Jabal Kibrit, believing its outcrop to be regionally extensive but the recent fieldwork has shown that the exposures are limited to the Wadi Waqb vicinity. Biostratigraphic evidence from Hughes and Kamal (1993b, 1993c) confirmed that the Ad Dubaybah (Figure 18) and Al Khuraybah (Figure 19) carbonates mapped by Clark (1986) as belonging to the Musayr and Khuraybah formations, respectively, are stratigraphically equivalent to those at Jabal as Safra, and belong to the Wadi Waqb member.

Type Section: The best exposures of the Wadi Waqb member are in a small-unnamed wadi immediately east of Wadi Waqb (28°12'19"N; 34°45'49"E). A total thickness of 677.4 ft has been measured and intensively sampled but faults may have caused discrepancies within the section. Figure 20 is of an exposure in a wadi adjacent to the type section.

Lithology: The carbonates of the Wadi Waqb member are typically rich in corals and rhodoliths. East of the mouth of Wadi Aynunah near Al Khuraybah (Figure 19), the succession is 205 ft thick, and consists of large compound corals and Mya-like bivalves set within mudstones, wackestones, and dolomitic limestones. The compound corals progressively increase in number upward through the measured sections. The youngest beds of the member contain well-developed stromatolites (for example, at 28°11'26.7"N; 34°44'37.1"E). Bedding attitudes vary, but generally strike at about N100°E, with a dip of 45° to 50°S.

Distribution: The carbonates exposed at Ad Dubaybah (28°27'09.9"N; 35°05'19.8"E) (see Figure 18) and at the mouth of Wadi Aynunah near Al Khuraybah (28°04'50.2"N; 35°11'36"E) (see Figure 19), have similar assemblages of foraminifera and are considered to be equivalents of those in the Wadi Waqb region. The Wadi Waqb member also forms the upper part of the two hills of Jabal as Safra, north of Al Bad’ (Figure 21) (28°31'02"N; 35°01'54.9"E).

Contact Relationships: The Wadi Waqb carbonates are unconformable on the Proterozoic basement in the Wadi Waqb and Khuraybah (see Figure 19) localities, and are unconformable on siliciclastics of the Al Wajh formation in the Jabal as Safra area north of Al Bad’ (Figure 21). In outcrop, they are everywhere directly overlain by anhydrite assigned to the Kial formation (Figure 20). However, subsurface in the Midyan region, the Wadi Waqb carbonates overlie basinal mudstones of the Burqan formation with apparent conformity and are overlain by basinal mudstones of the upper part of the Jabal Kibrit formation. The difference between surface exposures and subsurface intersections suggests a significant variation in the post-depositional paleoenvironment.

Age: The presence of the benthic foraminifera Borelis melo (Dullo et al., 1983) indicates a middle Miocene age. The well-preserved specimens of B. melo (Figure 22a) were discovered in carbonates collected from the small wadi east of Wadi Waqb in the recent investigation (Hughes, in preparation). Carbonates at Ad Dubaybah and Khuraybah also contain B. melo (Kamal and Hughes, 1993c; Hughes and Kamal, 1993c). The matrix of the deep-marine carbonates at Wadi Waqb contains planktonic foraminifera assigned to Praeorbulina spp. (Figure 22b), and is therefore of late early Miocene to early middle Miocene age (Zones N8-N9).

Paleoenvironment: The Wadi Waqb member subsurface in the Midyan field and at the Wadi Waqb outcrops, consists of a deep-marine, planktonic foraminiferal-bearing wackestone matrix containing transported allochthonous bioclasts (Figures 22c and 22d) derived from a shallow-marine carbonate setting (Kamal and Hughes, 1995). Possible sources for the shallow-marine components are considered to be similar to the shallow-marine, coral- and rhodolith-dominated carbonates that are now exposed along the eastern margin of the Ifal plain at Ad Dubaybah and Khuraybah (Figure 2) (Kamal and Hughes, 1993a; and Hughes and Kamal, 1993b, 1993c). At Ad Dubaybah, the carbonates rest directly upon moderately well-bedded sandstones that resemble distal facies of the Al Wajh formation. However, the sandstones may represent a proximal shallow to marginal marine facies of the Jabal Kibrit formation as the Proterozoic granitic basement lies within a short distance to the east. The carbonates in the Khuraybah-Aynunah region rest directly upon basement rocks, and contain large corals in growth positions.

Kial formation

Name: The formation was informally named by R. Kamal (Saudi Aramco) after Kial village adjacent to the Midyan oil and gas field where the formation is well developed. The name was first published by Hughes and Filatoff (1995).

Regional Equivalents: The formation is equivalent to the Hammam Faraun, Feiran, Sidri, and Baba members of the Belayim Formation of the Gulf of Suez and the Red Sea (Hughes et al., 1992; Hughes and Beydoun, 1992).

Type Section: Midyan-1 (MDYN-1) Saudi Aramco exploration well (Figure 4).

Distribution: The Kial formation is widely distributed along the Saudi Arabia Red Sea but is best exposed along the escarpment on the western flank of Jabal Rughama in Midyan. At a locality south of the road to Maqna (28°26'01.5"N; 34"56'45.9"E), the Burqan formation is overlain by at least four well-defined ‘benches’ of anhydrite belonging to the Kial formation (Figure 16). Resistant anhydrite forms the tops of the benches whereas the underlying soft, friable calcareous mudstone beds are recessive. The thickness of the anhydrite beds is apparently uniform but that of the mudstones varies and it may be that the appearance of multiple beds is the result of repeated gravity sliding of a single anhydrite bed and its underlying mudstone. This may explain some, but not all, of the repeated exposures along the west flank of Jabal Rughama, as interbedded siliciclastics and anhydrite are confirmed in the Jabal Kibrit (Figure 17) and Jabal Musayr areas.

Lithology: The formation consists at the surface and in the subsurface of four members (Hughes and Filatoff, 1995; Johnson et al., 1995). From top to bottom they are: member 1, siliciclastics and minor carbonate, sandstone, and mudstone; members 2 and 4 (equivalent to the Feiran and Baba members of the Belayim Formation), evaporites (anhydrite with local halite), and thin beds of mudstone and siltstone; member 3, generally calcareous mudstone, with minor carbonate and sandstone (Figure 17). Some members pinch out along strike and it is difficult to distinguish them from each other or from the overlying Mansiyah formation.

Contact Relationships: Conformable on the Jabal Kibrit formation of the Maqna group (Hughes and Filatoff, 1995) and unconformable on the Burqan formation.

Age: A middle Miocene age is based on planktonic foraminifera and calcareous nannofossils in calcareous mudstones in the subsurface. Orbulina suturalis, Praeorbulina glomerosa circularis and P. glomerosa glomerosa (Hughes and Filatoff, 1995) belong to Planktonic Foraminifera Zone N9, and the calcareous nannofossil Sphenolithus heteromorphus (but without Helicosphaera ampliaperta) is indicative of Zone NN5 (Figure 3).

Paleoenvironment: Moderately deep marine, with shallow-carbonate platform facies locally developed. The interbedded relationship of evaporites with planktonic foraminifera-bearing mudstones in the subsurface suggests deposition in a deep episodically hypersaline submarine environment.

Mansiyah formation

The Red Sea Miocene subsurface succession in Saudi Arabia is characterized by a thick deposit of gypsum, anhydrite, and halite named the Mansiyah formation.

Name: The formation is named after the Mansiyah-1 (MNSY-1) exploration well north of Jizan. The Mansiyah formation in the Midyan area has, at various times, been considered as part of the middle Raghama formation of Skipwith (1973), the upper part of the Al Bad formation (Dullo et al., 1983), part of the Bad formation of the Raghama group of Clark (1986), the Raghama formation of Brown et al. (1989), and within the Bad formation of Jado et al. (1990).

Regional Equivalents: The formation is equivalent and lithologically comparable to the South Gharib Formation of the Gulf of Suez and other areas of the Red Sea (Hughes et al., 1992; Hughes and Beydoun, 1992).

Type Section: In the Mansiyah-1 Auxerap exploration well (Hughes and Filatoff, 1995; Johnson et al., 1995) drilled at a coastal location 40 km north of Jizan (Figure 5).

Distribution: The eastern dip slope of Jabal Rughama is characterized by a blanket of white anhydrite that may be part of the Mansiyah formation, but could also represent part of the Kial formation. A similar extensive exposure of anhydrite blankets the western and southern flanks of Jabal Mundassah (Figure 23). The Mansiyah formation is well represented in the subsurface but it is difficult to distinguish in outcrop from the underlying Kial formation. On the geological map (Figure 4) it is differentiated from the Kial formation only in the Jabal Kibrit area of the extreme southwest.

Lithology: The Mansiyah formation consists of massive halite, gypsum, and anhydrite, with thin interbeds of calcareous shale and mudstone, and subordinate sandstone and siltstone.

Contact Relationships: It overlies the Kial formation of the Maqna group with apparent conformity, although some faulted unconformable contact relationships are present in areas adjacent to halokinetic glide planes. It is overlain by the Ghawwas formation with probable conformity.

Age: The formation has been dated as middle Miocene based on its stratigraphic position (in the absence of age-diagnostic biostratigraphic evidence) above the biostratigraphically constrained basal middle Miocene Kial formation, and beneath the middle to upper Miocene Ghawwas formation. In the Gulf of Suez, marine diatoms indicate the presence of middle Miocene sediments within the overlying Zeit Formation. Palynological age criteria in the inter-evaporitic siliciclastics are as for the Jabal Kibrit formation but supplemented by the presence of the dinoflagellate cyst species Systematophora ancyrya/placacanatha (extinction at top middle Miocene); Pentadinium laticinctum (extinction near top middle Miocene) (Hughes and Filatoff, 1995).

Paleoenvironment: The depositional environment was moderate to very deep marine and conditions of extremely hypersalinity led to the precipitation of evaporites. A marine environment is indicated by the presence of dinoflagellate cysts in inter-evaporitic shales; anoxic conditions are suggested by an abundance of pyrite-impregnated amorphous kerogen (Hughes and Filatoff, 1995). The hypersaline episode was probably regional in extent and, based on the traditional silled deep-basin model, resulted from the isolation of the Red Sea from the Gulf of Aden and/or the Mediterranean (Hughes and Beydoun, 1992; Crossley et al., 1992).

Ghawwas formation

The Ghawwas formation is a thick succession of interbedded fine and coarse-grained siliciclastics, and thin beds of anhydrite, which have been intensively studied by Saudi Aramco. The Ghawwas and overlying Lisan formations and Quaternary sediments are combined on the geological map (Figure 4).

Name:Hughes and Filatoff (1995) named the Ghawwas formation. In Midyan, it was previously included within the middle Raghama formation (Skipwith, 1973), the upper part of the Al Bad formation (Dullo et al., 1983), the Bad formation of the Raghama group (Clark, 1986), the Raghama formation (Brown et al., 1989), and the Bad formation (Jado et al., 1990).

Type Section: The type section was established by Saudi Aramco using the subsurface succession in Ghawwas-1 (GHWS-1) exploration well (Figure 5) in the southern part of the Saudi Arabian Red Sea (Johnson et al., 1995).

Regional Equivalents: The formation is equivalent to the Zeit Formation of the Gulf of Suez and other areas of the Red Sea (Hughes et al., 1992; Hughes and Beydoun, 1992).

Lithology: The Ghawwas formation consists of conglomerate, sandstone, minor claystone, carbonates, and local evaporites.

Distribution: The formation is distributed widely along the Saudi Arabian Red Sea.

Contact Relationships: It overlies the Mansiyah formation with probable conformity and is unconformably overlain by the Pliocene-Pleistocene Lisan formation (Johnson et al., 1995).

Age: A middle to late Miocene age is based on diatom evidence within the region together with the stratigraphic position of the formation above rocks regionally dated as middle Miocene (Hughes et al., 1992; Hughes and Filatoff, 1995). A Late Neogene age is suggested by the relatively common occurrences of Plumbaginaceae-, Papilionaceae- and Nyctaginaceae-type pollen.

Paleoenvironment: Regional studies indicate that the Ghawwas formation was deposited under a variety of shallow-marine to marginal-marine environments, perhaps with the periodic development of sabkha conditions. Marine microfauna, for example, are scarce (Hughes and Filatoff, 1995; Filatoff and Hughes, 1996) but freshwater Pediastrum spp. and Anthoceros-type spores are common.

Lisan formation

The Lisan formation is a fluviatile succession that was deposited during the opening of the Gulf of Aqaba during the Early Pliocene. It is not distinguished from the Ghawwas formation on the geological map (Figure 4).

Name: The formation was named by Clark (1986) to replace the term ‘Ifal formation’ of Bokhari (1981), and by Jado et al. (1990).

Regional Equivalents: Warden (siliciclastic) and Shagara (carbonate) formations of the Gulf of Suez.

Type Section: Western edge of the Ifal plain (Clark, 1986). In the subsurface, the reference section is in Midyan-1 (MDYN-1) Saudi Aramco Red Sea exploration well.

Distribution: Crops out on the eastern margin of Jabal Rughama and underlies much of the Ifal plain. Present elsewhere in the subsurface along the Saudi Arabian Red Sea.

Lithology: The formation consists of rhythmic, poorly consolidated fluviatile sandstones and conglomerates. Clark (1986) described thin beds of gypsum in the southern part of the Midyan area. Concentric ridges that emerge from the Ifal plain may represent part of the general Ghawwas-Lisan-Quaternary succession (Clark, 1986), and be due to the deep-seated diapiric movement of evaporites. In the Midyan subsurface, the formation consists of coarse- and fine-grained siliciclastics and carbonates.

Contact Relationships: The formation is disconformable, probably unconformable, on the Ghawwas formation. Clark (1986) described an unconformity between the Lisan formation and the Mansiyah formation on the east side of Jabal Rughama.

Age: A Pliocene age is related to the stratigraphic position of the outcropping formation above the late Miocene Ghawwas formation. In the subsurface, a Pliocene to Pleistocene age is based on planktonic foraminifera and calcareous nanofossils (Hughes and Filatoff, 1995). The fossil evidence is as follows:

Planktonic Zone N21 (Late Pliocene) based on the first downhole occurrence of Globigerinoides extremus; Nannofossil Zone NN17 (Late Pliocene) based on the first downhole occurrence of Dicoaster pentaradiatus; Nannofossil Zone ?NN16–15 (Late to Early Pliocene) based on the first downhole occurrence of Reticulofenestra pseudoumbilica and of Sphenolithus moriformis. The ‘clastic’ unit is typically near-barren of both palynmorphs and kerogen. The ‘carbonate’ unit has yielded sparse dinocyst assemblages typified by Tuberculodinium vancampoae.

Paleoenvironment: In outcrop, the formation consists mostly of fluviatile and delta-plain deposits. Along the southern flank of Jabal Kibrit, gypsum beds and limestones contain echinoids and bivalves that indicate shallow-marine depositional conditions. Subsurface indications are of sedimentary environments ranging from supratidal, through intertidal and shallow-marine carbonate to upper bathyal, based on the variable character of the foraminiferal populations.

Pleistocene sediments

The Ifal plain and extensive areas between the hills west of Jabal Rughama are covered with undifferentiated sands and gravels, and locally with sand dunes. Seven uplifted carbonate reef terraces are exposed along the coast south of Maqna village and are similar to ones on the western coast of the Gulf of Aqaba and the southern shore of the Midyan region. Similar terraces have been described by Al-Rifaiy and Cherif (1988) from Al-Aqaba in Jordan where they were attributed to four major cycles of reef development caused by eustatic changes in sea level. However, in the Midyan region, tectonism is considered to have provided an additional control on the development and present distribution of the terraces. The highest terrace is 120 ft above sea level and may correspond to those in the area of Ra’s Sheikh Humaid at the southern extremity of the Midyan Peninsula.

STRUCTURAL HISTORY OF THE IFAL BASIN

A structural interpretation of the Landsat Thematic Mapper image that covers part of the onshore Ifal basin was made during the recent investigations. The geological map (Figure 4) shows the distribution of anticlines; primary, secondary, and third-grade faults; and lineaments related to bedding. Most of the Landsat image interpretation has been verified by ground-control surveys.

The Arabian Plate has rotated anti-clockwise away from Africa, resulting in the opening of the Red Sea. The Gulf of Suez was formed by the northward propagation of the Red Sea opening in Late Oligocene to early Miocene times by rifting and the formation of pull-apart basins (Courtillot et al., 1987). Makris and Henke (1992) suggest that the thinned continental crust underlying the eastern side of the Red Sea is the result of continental stretching between the mobile Arabian Plate and the stable African Plate. They attribute the entire Red Sea as a response to wrench faulting that eventually culminated in the Dead Sea strike-slip fault.

In middle to late Miocene times, left-lateral fault movements began in the Gulf of Aqaba and along the Dead Sea rift. Left-stepping along the major left-lateral strike-slip faults was observed at two locations (LS1 and LS2, Figure 2) on the eastern coast of the Gulf of Aqaba and suggests the formation of a releasing bend and additional extension. The northeast-trending en echelon anticlines in the study area are consistent with the major left-lateral strike-slip fault system in the Gulf of Aqaba. West to west–northwest-trending right-slip (synthetic) shear faults (R1, R2 in Figure 4) and north–northwest-trending left-slip (antithetic) shear faults (L1 in Figure 4) in the southwestern part of the study area are the products of a simple shear-stress field. An offset in the drainage along the R1 and R2 faults indicate right-slip movement. Slickensides on the fault surface (L1) that brings the Burqan formation against the Musayr formation (Figure 24) indicate normal left-lateral oblique movement. These observations indicate a regional left-lateral strike-slip event. Numerous faults and overturned anticlines along the line of the major left-lateral strike-slip faults were observed on the eastern coast of the Gulf of Aqaba (Figures 25 and 26). The Dead Sea Fault system caused extensive fracturing in the Miocene-Quaternary section (Figures 27 and 28) including the reservoir rock of the Jabal Kibrit formation.

Purser et al. (1990) and Filatoff and Hughes (1996) divided the Red Sea succession into discrete regional depositional events. Several related tectonic episodes have been suggested for the Red Sea region, ranging from the informal scheme of Filatoff and Hughes (1996) to the structurally based schemes of Girdler and Southern (1987), Guennoc et al. (1988), and Girdler (1990). In the present scheme, the geological history of the Midyan region is summarized as pre-rift and syn-rift events. They represent a simple subdivision of interpretative events that are directly linked with the natural subdivision of the lithostratigraphic succession, in a style similar to the ‘M’, ‘P’ and ‘Q’ grouping suggested by Jado et al. (1990).

Pre-rift

The pre-rift episode refers to the time interval before activation of the Red Sea rift, as defined by Purser et al. (1990) in the northwestern part of the Red Sea. It includes the entire pre-Neogene, possibly pre-Upper Oligocene, succession. During the pre-rift episode, Arabia remained part of the African Plate but evidence of a topographic depression along the proto-Red Sea exists within the Upper Cretaceous and lower Tertiary successions as far south as Jiddah. This southerly embayment is attributed by Basahel et al. (1982) to the depression caused by the reaction between the Nubian Shield to the west and the Hail Arch to the east. Purser and Hotzl (1988) also noted the sedimentological similarity of Cretaceous and Paleogene sediments between the northeastern and northwestern margins of the Red Sea.

In the Midyan area, the pre-rift succession is represented by the Upper Cretaceous Adaffa formation. Montenat et al. (1988) commented that the paleogeographic features of the pre-rift succession trend east-west and northeast. They do not, therefore, have the N140° Clysmic orientation of the Gulf of Suez and cannot be used to imply the existence of a proto-rift as suggested by Sellwood and Netherwood (1984). The present interpretation is consistent with that of Purser et al. (1990) who restrict the use of ‘protorift’ to the early stage of rifting.

Syn-rift succession

The onset of the syn-rift episode is here defined as the event that separated pre-rift sedimentation from deposition in response to the rifting of the Red Sea. This succession consists of the Tayran group, Burqan formation, Maqna group, and the Mansiyah and Ghawwas formations. Four subdivisions of the syn-rift succession are defined and can be traced throughout the Red Sea region and equate with Rift Stage I of Bayer et al. (1988). Filatoff and Hughes (1996) suggested that the Tayran group, the Burqan formation, and the Magna group sediments are the product of the early syn-rift phase, as the post-Maqna evaporites of the Mansiyah formation are in marked contrast to any of the earlier sediments. In the present study, the arbitrary use of ‘early’ and ‘late’ syn-rift is abandoned in favor of numbered episodes that can be recognized regionally.

Syn-rift 1

This early phase of Oligocene?-Miocene rifting caused slow subsidence. Sedimentation in the Ifal basin began in the early Miocene with the deposition of the marginal and shallow-marine sediments of the Tayran group. Slow subsidence rates characterize this episode that was termed the ‘protorift’ by Purser et al. (1990). Girdler and Southern (1987), Guennoc et al. (1988), and Girdler (1990) related it to continental extension.

Syn-rift 2

The Burqan formation was deposited in response to rapid subsidence and the creation of a deep, steep-sided marine basin. From northwest to southeast, a proximal fan grades into mid-fan and distal-fan depositional environments (Ferguson and Senalp, 1993). The distribution of the sediments and their increase in thickness towards the northwest in Midyan suggest asymmetric graben-type fault control. Bounding faults are located on the northern margin (at Jabal as Sabil in Figure 2) and along the northwestern edge of the basin. Major faults dip to the east, southeast and south, and the basin is tilted towards the northwest.

Syn-rift 3

The onset of restricted marine conditions, as indicated by the regional deposition of evaporites of the Maqna group, suggests a possible decrease in the subsidence rate. This event has been used to mark the lower boundary of the syn-rift 3 episode. In the Midyan area, however, carbonates characterize the base of the episode as the basal Maqna marine evaporites are not developed. Steeply dipping surfaces related to reactivated faults controlled the distribution of the reef carbonates of the Wadi Waqb member. Basement highs and islands probably existed within the area during and after the deposition of the Burqan formation. Penecontemporaneous downslope transport from localized shallow-marine sources created different depositional settings adjacent to the deep-marine facies of the Wadi Waqb member. The fault-controlled Wadi Waqb carbonates preferentially accumulated in the Midyan field and the Jabal Kibrit area. Rift-associated major extensions took place in the Red Sea region during the syn-rift 2 and 3 episodes, and was followed by the onset of drifting as a result of early sinistral shear movement on the Dead Sea Transform Fault (Girdler and Southern, 1987; Guennoc et al., 1988; and Girdler, 1990).

Syn-rift 4

The base of the thick succession of anhydrite of the Mansiyah formation is a well-defined regional event that is taken to define the base of the syn-rift 4 episode. It includes the terminal hypersaline environment that gradually developed during the deposition of the Maqna group. A moderately deep to very deep-marine environment is suggested for the Mansiyah formation at the beginning of this phase, previously considered as a late syn-rift phase by Filatoff and Hughes (1996). The deposition of shallow-marine mixed siliciclastics and evaporites of the Ghawwas formation resulted from basin infilling during the late Miocene. This onset of shallow and marginal marine sedimentation may be related either to the global eustatic sea-level fall during the late Miocene, or to a decrease in the rate of vertical subsidence of the Red Sea region. This decrease in the subsidence rate would conform to the concept of a contemporary tectonic hiatus as proposed by Girdler and Southern (1987), Girdler (1990), and Guennoc et al. (1988).

Post-rift (drift)

This episode is an Early Pliocene event that coincides with the post-rift stage of Purser et al. (1990) and with the Rift Stage II of Bayer et al. (1988), and which has been considered as a transitional phase between continental and oceanic rifting by Cochran and Martinez (1988). It is believed to be responsible for the triangular shape of the Midyan region that lies within the zone bounded by the Red Sea rift margin and the new plate boundary between Sinai and Arabia. The tectonic episode coincides with the cessation of Red Sea rifting and the onset of Aqaba-related transtensional subsidence. There is no evidence for the rifting of the Pliocene succession within the Midyan area, and Bayer et al. (1988) explain the continued subsidence during this phase to rotation, or oblique drift of the Arabian Plate in response to the movement on the Dead Sea Fault zone. This episode is considered by Girdler and Southern (1987), Guennoc et al. (1988), and Girdler (1990) to represent a phase of rejuvenated sinistral shear movement along the Aqaba transform, and renewed drift.

It is noteworthy that Girdler and Southern (1987), Guennoc et al. (1988), and Girdler (1990) consider the 107 km of total shear movement to be composed of an early to middle Miocene displacement of 62 km and an Early Pliocene to Recent displacement of 45 km.

STRUCTURAL SYNTHESIS

Three major fault systems are present within the Ifal basin (Figure 2). The Ifal East Fault and Ifal Fault systems bound the triangular-shaped basin on its northeastern and southwestern sides, respectively. Within the Basin, the Ifal Central Fault trends approximately north-south. The area between the Ifal Central and Ifal East faults is known as the Midyan East Terrace.

The Ifal Central Fault has been active since the middle Miocene. Available seismic data indicate that this west-facing fault was active during deposition of the Ghawwas formation (KFUPM, 1998) and probably the Lisan formation as well, but this observation needs further investigation. Initiation of the Ifal East Fault is considered to have begun later and the fault was active during much of the

Pliocene-Quaternary. At least four alluvial fans are related to this fault system. They channel into each other and onlap the eastern foothills of the Ifal basin beyond the study area, and indicate continuous uplift. Landsat imagery indicates recent movement on the Ifal East Fault system. Non-deposition, uplift, and subsequent erosion reduced the thickness of the Miocene section on the Midyan East Terrace.

The Ifal Fault is a northeast-trending major down-to-the-southeast normal fault located along the eastern margin of the Maqna massif between Al Bad’ and Jabal Kibrit. The fault was active or reactivated after the deposition of the Lisan formation. A northeast-trending fold, truncated by this fault, may indicate that the fault also had a lateral strike-slip component (KFUPM, 1998). Active faulting and uplifting continued into recent times. Small-scale, pull-apart sub-basins are present along the right and left strike-slip faults to the south and west of Jabal Kibrit, and southwest of Al Bad’, respectively (KFUPM, 1998). Alluvial terraces indicate an extensive amount of uplift (Figure 29). Pliocene-Quaternary beach rock and reef build-ups along the coast of the Gulf of Aqaba (Figure 30) and conglomerates along drainage channels in the region, were elevated 6 to 8 m during this time.

CONCLUSIONS

The Midyan region provides a unique view of the Saudi Arabian Red Sea subsurface stratigraphy. The various outcropping lithostratigraphic units relate to the succession established in the subsurface.

The Al Wajh formation compares well with the subsurface, but the overlying Yanbu evaporites are absent in the Midyan exposures. The Musayr carbonates are, on available evidence, restricted to the Midyan area. The exposures of the Burqan formation provide critical information for determining the associated sedimentary processes, especially those that were responsible for the deposition of the extensive, thick sandstone beds that have reservoir significance.

The previously problematical Wadi Waqb carbonate is now considered to be a member of the Jabal Kibrit formation (latest early to earliest middle Miocene), and to have unconformable contacts with the Proterozoic basement and the Al Wajh formation. These stratigraphic relationships are not present in the subsurface where the carbonate overlies siliciclastics of the Burqan formation. The carbonates exposed along the eastern flank of the Ifal plain are now considered to represent the shallow-marine equivalents of the deep-marine carbonates that form the reservoir rock within the Midyan region, and are not part of the lower Miocene succession as previously mapped. The exposures of this unit in the Wadi Waqb area contain an admixture of autochthonous planktonic foraminifera-bearing deep-marine carbonates with allochthonous shallow-marine bioclasts. Although there is evidence that gravity sliding is responsible for duplication of interbedded siliciclastic-anhydrite beds at some localities, certain exposures display undisturbed alternations of siliciclastics and interbedded anhydrite. Anhydrite is a distinctive feature of the Midyan region. It is present in the Kial and Mansiyah formations.

Several tectonic phases are defined that fall into the categories of pre-rift, syn-rift, and post-rift (or drift). The pre-rift succession displays lineaments that are not in accordance with the Red Sea tectonic trend, and are considered to pre-date the formation of the Red Sea rift. The rift and drift-associated successions extend from the early Miocene (possibly Late Oligocene) to approximately the top of the Miocene succession. Field evidence has confirmed that the Jabal as Sabil basin-bounding fault controlled the deposition of the Burqan formation and that the Ifal Central Fault controlled the deposition of the Ghawwas formation.

The Pliocene-Pleistocene post-rift episode is considered to represent drift in response to the transtensional tectonic controls exerted on this region by the Dead Sea Transform Fault. The eastern margin of the Ifal basin is bounded by the Ifal East Fault that controlled the deposition of the Lisan formation. The Ifal Fault, which is related to the Dead Sea Fault zone, was synchronous or possibly younger than the Ifal East Fault and is responsible for the uplift of the western part of the Midyan region. During this time, the reactivation of older faults caused diapiric movement of the evaporites of the Mansiyah formation.

Quaternary uplift is indicated by the emergence of coral-reef limestone to at least 6 to 8 m above sea level along the Gulf of Aqaba coast and southern onshore flanks of the Midyan region. Deeply dissected alluvial terraces also testify to the recent and continued uplift of the region.

ACKNOWLEDGEMENTS

The authors express their thanks to Saudi Aramco for permission to publish this paper. The results of a joint investigation by Saudi Aramco and the Research Institute of the King Fahd University for Petroleum and Minerals (KFUPM) have been made accessible for incorporation into this paper. In addition, we thank R.S. Johnson and the following geologists from Saudi Aramco whose comments, reports and discussion helped to improve our understanding of the geology of the Midyan region: T.C. Connally, R. Kamal, M. Senalp, A.M. Afifi and A.A. Al-Laboun. The co-operation and kind assistance of K. Al-Hinai, N.A. Al-Honaid and M.A. Khan of the Remote Sensing Unit of the Research Institute of KFUPM provided new insights into the geological interpretation of the region. Dr. O. Varol (Varol Research, U.K.) was responsible for all calcareous nannofossil age determinations. We also thank the staff of Gulf PetroLink for expert help particularly with the graphics.

ABOUT THE AUTHORS

Geriant Wyn Hughes is a Consultant Geologist with the Research and Development Division of Saudi Aramco. He received his BSc, MSc, and PhD degrees from Prifysgol Cymru (University of Wales), Aberystwyth, UK. His nearly 30 years of experience in biostratigraphy include 10 years as a field geologist/biostratigrapher with the Solomon Islands Geological Survey and 10 as a biostratigraphic consultant and unit head of the Middle East-India region with Robertson Research based in Singapore and Wales. Professional activities are focussed on the integration of micropaleontology with sedimentology to enhance sequence stratigraphic interpretations of Saudi Arabian carbonate rocks. Wyn maintains links with academic research through his activities as an external examiner for the University of Wales and as an Adjunct Professor of the King Fahd University of Petroleum and Minerals. He is a reviewer for GeoArabia, a member of the British Micropaleontological Society and of the Dhahran Geoscience Society and a Fellow of the Cushman Foundation for Foraminiferal Research.

Dogan Perincek received his MSc in Geology and PhD in Petroleum Geology from the University of Istanbul in 1972 and 1978, respectively. He worked with the Turkish Petroleum Corporation as a Structural Geologist and Petroleum Explorationist between 1975 and 1989. Dogan also worked with King Fahd University of Petroleum and Minerals (KFUPM) as a Research Scientist, as an Explorationist with Mobil Exploration and Huffco Turkey Incorporated, as a Geophysicist with the Geological Survey of Victoria (1992), Geological Survey of Western Australia (1995) and World Geoscience Co. Ltd. (1997). Dogan rejoined KFUPM in late 1997. He is a member of the American Association of Petroleum Geologists, the Australian Society of Exploration Geophysicists, the Petroleum Exploration Society of Australia, and the Dhahran Geoscience Society.

David Grainger has recently joined GeoArabia as Geoscience Editor. From 1978 he was Senior Geological Editor at the Saudi Arabian Deputy Ministry for Mineral Resources. Before his career change to geological editing, he worked as a regional mapping and mineral exploration geologist in East and North Africa, Canada, Iran, Australia, Papua New Guinea, and Antarctica. He also lectured at Sunderland Polytechnic, UK. David obtained his BSc in Geology from Nottingham University, UK in 1962 and an MSc in Mineral Exploration from Imperial College, London in 1966. He is a member of the Institution of Mining and Metallurgy, a fellow of the Geological Society on London, and a member of the European Association of Science Editors.

Abdul-Jaleel Abu-Bshait is employed by the Geological Research and Development Division of Saudi Aramco. He received his BSc in Geology from the University of Southern California. His main areas of interest are the characterization of carbonate reservoirs and FMI image interpretations. He is a member of the American Association of Petroleum Geologists and of the Dhahran Geoscience Society.

Abdul-Rahman Mohammed Jarad is a Lecturer in the Earth Sciences Department of the King Fahd University of Petroleum and Minerals (KFUPM). He was employed in the Geology and Minerals Division of KFUPM’s Research Institute from 1991 to 1998. He received a BSc in Geology from KFUPM in 1991 and an MSc in 1996. Abdul-Rahman’s professional interests include the application of geostatistics to ore deposit evaluation and reservoir characterization. He is a member of the Saudi Society for Earth Sciences, the Dhahran Geoscience Society, and the American Association of Petroleum Geologists.