Late Ediacaran to early Cambrian (Infracambrian) Jibalah Group of Saudi Arabia

The first version of the Neoproterozoic – Phanerozoic Middle East Geologic Time Scale was published in 2008 with a plan to improve its accuracy in subsequent versions (Al-Husseini, 2008). An important part of the scale that proved difficult to construct is the Neoproterozoic – Cambrian time interval, which is represented by the oldest sedimentary rocks in the Middle East. Accordingly, a separate chart was dedicated to this interval and a series of papers are being written to document its rock units. The documentation was started in 2010 by defining a regionally correlative early Cambrian time-rock unit: the Asfar Sequence bounded below by the lower Cambrian Angudan Unconformity and above by the middle Cambrian Burj Sequence Boundary (Al-Husseini, 2010). The present paper extends the documentation below the Angudan Unconformity by reviewing the descriptions and definitions of the oldest sedimentary (and relevant igneous) rocks in Saudi Arabia: the late Ediacaran – early Cambrian (Infracambrian) Jibalah Group.

The Jibalah Group was first recognized in 1965 by geologists from France’s Bureau de Recherches Géologiques et Minères (BRGM) during the 1:100,000 scale mapping and mineral exploration for the Directorate General of Mineral Resources, Kingdom of Saudi Arabia (currently Saudi Geological Survey). The group crops out in several isolated and far-apart basins in the Arabian Shield, and several formation names and definitions have been proposed for the group’s rock units. In particular, Delfour (1967, 1970) of the BRGM, and Hadley (1974, 1986) of the United States Geological Survey (USGS) proposed different formation names in areas located ca. 400 km apart (Figure 1). Their schemes are commonly cited in the literature but remain uncorrelated. This paper reviews these pioneering field studies to show how their formations may be cast into a generalized chrono-stratigraphic framework that better clarifies the geological evolution of the Jibalah Group (Figure 2 in enclosed Chart).

The first part of the paper describes the Jibalah Group including the primary authors, distribution, boundaries, and nomenclature for its formations and members. This is followed by a lexicon-style discussion of each formation and its members including measured sections. The lexicon closes with the Sub-Siq Unconformity (corresponding to the regional Angudan Unconformity), which is overlain by the lower Cambrian Siq Sandstone (Asfar Sequence, Al-Husseini, 2010). The final sections review the age and evolution of the group, and how it may correlate to other groups in the Arabian Shield (Figure 2 in enclosed Chart).

The spelling and rank of the Jibalah Group has varied in DGMR-SGS, BRGM and USGS reports since it was first introduced as the “Jibala Formation” by J. Delfour in 1967 (unpublished BRGM report). It has been referred to as the “J’Balah Group” (Delfour, 1970), “Jubaylah Group” (Hadley, 1974; Brown et al., 1989) and correctly as the “Jibalah Group” (Hadley, 1986). The Jibalah Group was not included in the stratigraphic lexicon of Saudi Arabia (Powers, 1968) and should not be confused with the Jurassic “Jubaila Formation”.

The reports describing the Jibalah Group by the three geological surveys (DGMR-SGS, BRGM and USGS) are not widely referenced in the literature. The description of the group, presented here, is mainly taken from the BRGM Bulletin (Delfour, 1970) and the report authored by Hadley (in 1972 as a USGS open-file report, and in 1974 as a DGMR Bulletin). The Jibalah Group was also mapped at a 1:250,000 scale in several quadrangles and briefly described in their explanatory notes. This paper includes some stratigraphic aspects from the notes to illustrate the heterogeneity of the group’s rock units across quadrangles (Figure 1): Nuqrah (Delfour, 1977), Afif (Letalenet, 1979), Halaban (Delfour, 1979a), Wadi Ar Rika (Delfour, 1980), Al Hissu (Delfour, 1981), Ad Dawadimi (Delfour et al., 1982), Al Wajh (Davies, 1985), Wadi Ash Shu’bah (Quick and Doebridge, 1986), Sahl Al Matran (Hadley, 1986) and Shaghab (not shown in Figure 1; Grainger and Hanif, 1989).

The Jibalah Group crops out in isolated regions up to ca. 20 km wide and 100 km long, which are interpreted as pull-apart basins along the Najd Fault System (Figure 1, Delfour, 1970; Hadley, 1974; Moore, 1979; Delfour, 1979b; Brown et al., 1989). Each basin was described as a synclinorium (Delfour, 1970, 1977) or taphrogeosyncline (Hadley, 1974). Some basins have been named in the literature (sometimes spelled differently) and these names are adopted in this paper (Figure 1).

The Najd Fault System consists of four named fault zones (Figure 1), from south to north: (1) Ruwah Fault Zone; (2 and 3) Rika Fault Zone and its northwestern continuation as the Qazzaz Fault Zone; and (4) Halaban Fault Zone (Moore, 1979; Brown et al., 1989; Nehlig et al., 2002). The Halaban Fault Zone is sometimes referred to as the Halaban-Zarghat Fault Zone (e.g. Johnson, 2003). The Jibalah Group does not crop out along the southern Ruwah Fault Zone, which is mainly characterized by older intrusive rocks emplaced as gneiss domes. It is unclear whether the Jibalah Group is absent along this fault zone due to non-deposition or erosion. The Ruwah Fault Zone is interpreted to merge with the NS-trending Ad Dafinah Fault Zone and is sometimes referred to as the Ruwah-Ad Dafinah Fault Zone (Johnson and Kattan, 2001).

Along the Rika Fault Zone, the Jibalah Group crops out in four main basins (Figure 1), and their stratigraphy was summarized in the explanatory notes of the 1:250,000 quadrangle maps based on more detailed BRGM reports (not seen by the present author).

• (1) Kibdi Basin: The southernmost Jibalah outcrop area (ca. 4 x 23 km) in the Arabian Shield is found in this basin (named after Jabal al Kibdi; Ar Rika Quadrangle, Leca and Al-Shanti, 1972; thickness not cited).

• (2) Bir (Bi’r) Sija Basin: Further northwest in the Afif Quadrangle, Letalenet (1979) divided the Jibalah Group in this basin, which is ca. 6 x 30 km, into eight units (Figure 2 in enclosed Chart). The group overlies the Shammar Group and is ca. 2,500 m thick.

• (3) Sukhaybarah Basin (also spelled “Sukhaybarat”): This basin straddles the Afif and Al Hissu quadrangles and consists of southern and northern sub-basins, each ca. 80 km long. The southern sub-basin is narrow (ca. 5 km) whereas the northern one is ca. 18 km wide (BRGM report by J. Aguttes and M. Duhamel, inDelfour, 1970, 1981). The total thickness of the group is not cited but would exceed that of the Jifn Formation with an estimated thickness of ca. 2,500 m.

• (4) Hawaqah Basin: Southeast of Al Hanakiyah city, three Jibalah outcrops are located in the northwestern corner of the Al Hissu Quadrangle (Delfour, 1970, 1981; thickness not cited). These outcrops occur in grabens that are separated by NW-trending faults, and together are referred to as the Hawaqah Basin or Al Hanakiyah Basin.

In the Khaybar Quadrangle (Delfour and Dhellemmes, 1980), the Tertiary Volcanics cover the Arabian Shield, and the Rika Fault Zone is mainly recognized from geophysical data. Further northwest of the Tertiary Volcanics, the NW-trending Qazzaz Fault Zone emerges on-trend as the continuation of the Rika Fault Zone. The Qazzaz Fault Zone intersects the Red Sea coast and continues as the Duwi Fault Zone in Egypt (Brown et al., 1989).

In the northwestern Arabian Shield, the Jibalah Group is recognized as such and is mapped in the Sahl Al Matran, Al Wajh and Shaghab quadrangles (Figure 1; Shaghab not shown). In the Wadi al ‘Ays Quadrangle the upper part of the Hadiyah Group may correlate to the Jibalah Group (see below). The Jibalah outcrops in these four quadrangles are associated with informally named basins.

Along the Halaban Fault Zone (Figures 1, and Figure 2 in enclosed Chart), the Jibalah Group crops out in five major basins.

• (1) Antaq Basin: This basin, ca. 8 x 45 km in extent, is named after Jabal Antaq (also spelled “Hentag”). It is located 120 km east of Afif city in the Halaban Quadrangle (BRGM report by J. Bois, inDelfour, 1979a) and is the southernmost basin along the Halaban Fault Zone in the Arabian Shield. Here the Jibalah Group unconformably overlies a dioritic basement and reaches a thickness of ca. 2,500 m (Nettle, 2009).

• (2) Khutayfah Basin: Further northwest from the Antaq Basin an unknown thickness of the Jibalah Group crops out along the Halaban Fault Zone in an area of ca. 8 x 60 km in the Ad Dawadimi and Miskah quadrangles (Delfour et al., 1982; Hutin, 1983; Brown et al., 1989).

• (3 and 4) Al Jifn and Nuqrah Basins: In the Nuqrah and Wadi Al Shu’bah quadrangles the group is exposed in the ca. 10 x 130 km Al Jifn Basin (also spelled “Jifan”) that extends southeastwards into the Nuqrah Basin. At the intersection of the two basins, at Jabal Umm Al ‘Aisah (also spelled “Umm Leissah” or “Umm Al Aissah”), Delfour (1970, 1977) described an unfaulted reference section for the Jibalah Group, some 3,000 m thick (see Figure 5).

• (5) Zarghat Basin: The northwesternmost extent of the Jibalah Group outcrop, along the Halaban Fault Zone, occurs west of Zarghat city in this basin (Delfour, 1970). Further west the Tertiary Volcanics cover the Arabian Shield, and the northern continuation of the Halaban Fault Zone is unknown.

Four more Najd basins occur along unnamed NW-trending faults between the Halaban and Rika fault zones (Figure 1; Delfour, 1970, 1979b).

• (1) Al Hissu Basin (Bi’r Al Hissu, sometimes spelled “Hisw”) is ca. 3 x 30 km and located north of Bir al Hissu city in the Al Hissu Quadrangle. Here the Jibalah Group exceeds 1,300 m in thickness and unconformably overlies a granitic basement.

• (2) Sumamaiyah Basin: Further northwest, in the Nuqrah Quadrangle, the Jibalah Group crops out in this basin (sometimes spelled “Summamaiyat” or referred to as the Bir or Bi’r Arja Basin; BRGM report by J.R. Chapelain, 1970, inDelfour, 1970, 1977). The basin is ca. 3.5 x 30 km in areal extent (thickness not cited).

• (3) Jibalah Basin: This basin, named after Jabal J’Balah, continues northwest from the Sumamaiyah Basin and has a similar lateral extent (BRGM report by R. Campi and C.A. Lafoy, inDelfour, 1970; see Figure 6).

• (4) Suwan Basin: South of Jabal As Suwan, a small Jibalah outlier crops out in this basin between the Tertiary Volcanics, which further west cover the Khaybar Quadrangle (Delfour and Dhellemmes, 1980; thickness not cited).

Sub-Jibalah Unconformity: In the northern part of the Arabian Shield, the Jibalah Group overlies the Shammar Group, which mainly consists of rhyolites, coeval granites or older granitic plutons (Figure 2 in enclosed Chart, Figures 3 and 4). In most outcrops the basal conglomerates of the Jibalah Group are derived from the Shammar Group and older rocks thus establishing the contact as a regional unconformity (Delfour, 1970). Elsewhere, the group unconformably overlies a basement consisting of a variety of older metamorphic and plutonic rocks. The unconformity separating the Jibalah Group from older rocks is here referred to as the Sub-Jibalah Unconformity.

Sub-Siq Unconformity (Angudan Unconformity): The lower Cambrian Siq Sandstone is the oldest-known formation to have been deposited after the Jibalah Group (Hadley, 1974; Brown et al., 1989; Figure 2 in enclosed Chart). The unconformable relationship between the Jibalah Group and Siq Sandstone at outcrop is documented in the Mashhad area (Figures 3 and 4, Hadley, 1974). In this area the flat-lying basal conglomerates of the Siq Sandstone rest on the dipping beds of the Muraykhah Formation, and the Jifn Formation of the Jibalah Group is absent.

The Sub-Siq Unconformity in Saudi Arabia was correlated across three other countries (Iran, Jordan and Oman) and proposed as the Angudan Unconformity in the Middle East Geologic Time Scale (Al-Husseini, 2010). Miller et al. (2008) referred to this unconformity as the Afro-Arabian Peneplain (AAP). The Siq Sandstone (Tayma Group) is assigned to the Asfar Sequence and correlates to the lower Cambrian Salib Arkosic Sandstone of Jordan (Al-Husseini, 2010). In Jordan, the unconformity is also characterized as a peneplain surface that separates the fault-bounded Neoproterozoic - lower Cambrian Safi Group from the overlying Salib Arkosic Sandstone (Andrews, 1991). See section on the Jifn Formation for further discussion.

Three measured sections are here reviewed to illustrate the Jibalah Group. The first is located in the Mashhad area, where Hadley (1974, 1986) defined the Rubtayn, Badayi and Muraykhah formations (Figures 1 to 4). The second is located in the Al Jifn Basin, where Delfour (1970, 1977) defined the Umm Al ‘Aisah Formation and overlying Jifn Formation (Figures 1, 2 and 5). The third section is located in the Jibalah Basin, where Delfour (1970, 1977) originally defined the group (Figures 1, 2 and 6).

A comparison of the two sections described by Delfour (1970, 1977) and Hadley (1974, 1986) indicates (Figures 2 to 6): (1) the Rubtayn, Badayi and Muraykhah formations, together, correlate by lithology and stratigraphic position to the Umm Al ‘Aisah Formation; and (2) the Jifn Formation of Delfour is missing in the Sahl Al Matran area. In this paper, the sedimentary rocks of the Jibalah Group are divided into three formations: (1) Rubtayn Formation; (2) Muraykhah Formation; and (3) Jifn Formation. This proposed nomenclature combines the formations of Delfour and Hadley into a generalized stratigraphic scheme that is relatively more complete across the Arabian Shield.

The Jibalah Group contains mafic lava flows at various stratigraphic intervals, and is intruded by subvolcanic mafic dikes and sills. These lava flows reach a thickness of ca. 150 meters in the Mashhad area, where it was defined by Hadley (1974) as the Badayi Formation. Therefore this name only applies where the andesite-basalt unit occupies this stratigraphic position. In some quadrangles, differently named rock units are assigned to the Jibalah Group, or differently named groups may correlate, in part, to the Jibalah Group (see below).

Locality: Mashhad area and Sahl Al Matran Quadrangle (Figures 1 to 4).

Authors and Nomenclature:Hadley (1974) defined and mapped the Rubtayn Formation in the Mashhad area, and described it in terms of four facies at Jabal Rubtayn and Jabal Nadah (also spelled “Na’adah”; Figures 1 to 4). Subsequently, in 1986, he divided the formation in the southeastern part of the Sahl Al Matran Quadrangle into three informal members, from base-up: (1) ‘Volcanic Conglomerate Member’, (2) ‘Polymictic Conglomerate Member’ and (3) ‘Sandstone Member’ (Figure 2 in enclosed Chart). No localities or type sections for these three members are given by Hadley (1986). He also introduced the undivided Rubtayn Formation and undivided Jibalah Group.

The Rubtayn Formation is here considered synonymous to the Mataar Formation of the Jibalah Group in Al Wajh Quadrangle (Davies, 1985; Miller et al., 2008) and the lower Antaq succession of the Jibalah Group in the Antaq Basin (Nettle, 2009). It may correlate to the informal lower Fatima member (Nebert et al., 1974) and corresponding Baqar Formation of the Fatima Group in the Makkah Quadrangle (Moore and Al-Rehaili, 1989; Grainger, 2001).

Lithology and Thickness (Hadley, 1986): This informal member, ca. 700 m thick, is exposed in the southeastern part of the Sahl Al Matran Quadrangle, but not in the Mashhad area (Figures 3 and 4). Most of the clasts are of volcanic origin and derived from the Proterozoic Matran Formation (see below). They are generally less than 20 cm in diameter and set in a coarse-grained framework of volcanic detritus. In the southeastern part of the quadrangle, andesite-basalt flows, similar to the Badayi Formation (see below), occur within this member.

Boundaries: The Volcanic Conglomerate Member unconformably overlies volcanic rocks of the Proterozoic Matran Formation of the Al Ays Group (older than the Shammar Group). The Volcanic Conglomerate Member is overlain by the Polymictic Conglomerate Member; the contact is not described.

Lithology and Thickness (Hadley, 1986): This informal member is up to ca. 1,500 m thick in the southeastern part of the Sahl Al Matran Quadrangle. The clasts are poorly sorted, rounded to subrounded and attain a diameter of up to one meter. A polymictic matrix surrounds the clasts. This member apparently corresponds to the “Boulder Conglomerate Facies” of the Rubtayn Formation (Figures 3 and 4). The Boulder Conglomerate Facies, ca. 75 m thick at Jabal Rubtayn, consists of rounded to well-rounded clasts made up of a large variety of Proterozoic rock types. Pebbly to very coarse-grained, calcite-cemented lithic sandstone surrounds the clasts. The upper part of the conglomerate gradually becomes finer grained and better stratified and passes upwards into lithic sandstone.

Boundaries: In the Mashhad area, the Boulder Conglomerate Facies (presumed correlative of the Polymictic Conglomerate Member) overlies the Mashhad Formation of the Shammar Group. The Polymictic Conglomerate Member is overlain and interbedded with the Sandstone Member of the Rubtayn Formation.

Lithology and Thickness (Hadley, 1986): This informal member attains a thickness of up to ca. 1,000 m in the southeastern part of the Sahl Al Matran Quadrangle. It consists of reddish-brown, medium-grained, uniformly bedded sandstone and siltstone, interbedded with polymictic conglomerates. The sandstones are well bedded, 2–15 cm thick, and commonly have ripple-marked surfaces. The lithology is principally arkose to arkosic greywacke; the grains are subangular to subrounded set in a matrix of microcrystalline quartz and calcite.

The Sandstone Member apparently corresponds to the three facies described in ascending order above the “Boulder Conglomerate Facies” in the Mashhad area (Figures 3 and 4):

• Sandstone Facies (ca. 200 m thick) consists of uniform tabular beds, many of which show graded bedding. Beds are generally about 5 cm thick, and vary from 3–20 cm. Its reddish brown to deep red color is due to the presence of hematite suggesting an oxidizing depositional environment. No cross-bedding and other paleocurrent indicators were observed.

• Red Bed or Siltstone and Mudstone Facies (40 m thick) consists of deep red to maroon, thinly bedded (1–3 cm), fine-grained siltstone and massive mudstone. Ripple marks and other sedimentary structures are generally absent.

• Pebble Conglomerate Facies (60 m thick) occur sharply but conformably above the Red Bed Facies. The unit consists of 0.5–1 m thick beds of pebble conglomerate commonly separated by thin beds of very coarse-grained, generally tabular sandstone beds. The pebbles (typically less than 10 cm in diameter) were sourced from rhyolite of the Shammar Group.

Boundaries: Lower boundary, see Polymictic Conglomerate Member of the Rubtayn Formation. The Rubtayn Formation is sharply but conformably overlain by the Badayi Formation in the Mashhad area.

Locality: Mashhad area, Sahl Al Matran quadrangle (Figures 1 to 4).

Author:Hadley (1974, 1986).

Lithology and Thickness (Hadley, 1974): The formation in Jabal Rubtayn (150 m thick) and Jabal Nadah (120 m) consists of well-bedded, 1–3 m thick, flow units of amygdaloidal basalt. The flows are rubbly at their base and amygdales are found throughout. Macroscopically, the rocks are highly porphyritic (see figure 10, in Hadley, 1974).

Brown et al. (1989) compared the Badayi basalt to volcanic rocks associated with large intra-continental rifts. They noted that lava flows throughout the Jibalah Group are petrographically different from older volcanic rocks (i.e. Shammar rhyolite). They are composed of andesite, dacite, alkaline basalt (mugearite), and rhyolite or minor lithic tuffs. They are often porphyritic, often with large (2 cm) plagioclase phenocrysts, and are amygdaloidal. Amygdale minerals include quartz and calcite, with lesser amounts of barite, celestite and nepheline (Delfour, 1970).

Boundaries: In the Mashhad area, the Badayi Formation is conformably underlain by the Rubtayn Formation and conformably overlain by the Muraykhah Formation.

Locality: Mashhad area, Sahl Al Matran Quadrangle (Figures 1 and 3).

Authors and NomenclatureHadley (1974, 1986): The Muraykhah Formation is here considered in part or completely synonymous to the 300–500 m thick “Cherty Limestone” unit in the upper part of the Umm Al ‘Aisah Formation of the Jibalah Group of Delfour (1970, 1977; see Umm Al ‘Aisah Formation). It is also considered synonymous to the Dhaiqa Formation of the Jibalah Group in Al Wajh Quadrangle (Davies, 1985; Miller et al., 2008) and upper Antaq succession of the Jibalah Group in the Antaq Basin (Nettle, 2009). It may correlate to the informal middle Fatima member (Nebert et al., 1974) and corresponding Shubayrim Formation of the Fatima Group in the Makkah Quadrangle (Moore and Al-Rehaili, 1989; Grainger, 2001).

Lithology and Thickness: In the Mashhad area the thickness of the Muraykhah Formation is 330 m at Jabal Rubtayn and 370 m at Jabal Nadah (Figures 3 and 4). The succession above the Badayi Formation starts with a basal conglomerate (10 m thick), which Hadley (1974) assigned to the Muraykhah Formation. It has the same appearance as the pebbly conglomerate facies (uppermost facies) of the Rubtayn Formation. The rest of the formation consists of carbonates, inter-bedded calcareous shale (mudstone), siltstone, andesite and conglomerate derived from rhyolite.

Hadley (1974, 1986) divided the Muraykhah Formation into two main facies: (1) lower cherty, non-dolomitic facies (135 m in both jabals); and (2) upper dolomitic, chert-poor facies, 135 m thick at Jabal Rubtayn and 175 m at Jabal Nadah (Figure 4). In the measured sections in Figure 4, a ca. 20 m interval consisting of siltstone and mudstone occurs between the two carbonate-dominated intervals. In the following discussion the facies are given the status of informal members.

• “Cherty Limestone Member”: The limestone (135 m thick) consists of buff and gray, finely laminated beds, about 5 cm thick, as well as non-laminated beds up to one meter thick. In some beds the laminations are undulose and highly irregular and probably related to laminated algae stromatolites. In other beds the laminations are head-shaped and have the form of laterally linked hemispheroidal stromatolites. Common sedimentary structures include ripple drift and small-scale current lamination. Interbedded with the limestone are thin beds of fine-grained, buff to greenish-gray calcareous siltstone and shale. Irregular to tabular beds, up to 60 cm thick, are formed by black chert.

• “Siltstone and Mudstone Member”: ca. 20 m thick, see above and Figure 4.

• “Dolomitic Limestone Member”: This informal member (135–175 m thick) consists of a buff colored dolomitic limestone, commonly separated by thin, evenly bedded calcareous siltstone and shale (generally 15–30 cm thick). Surrounding the dolomite is buff-weathering laminated calcilutite. Faint outlines of ripple drift are common in many beds indicating it was current sorted. Desiccation cracks are generally poorly developed, indicating periodic exposure. The occurrence of flat-pebble conglomerate beds indicate shallow scouring. A pebble conglomerate (2 m thick) and andesite bed divides the Dolomitic Member in Jabal Nadah. The conglomerate consists of angular dolomite and rhyolite pebbles enclosed in a fine-grained limestone matrix and suggests periodic exposure during the deposition of the Dolomitic Limestone Member.

Boundaries: The lower boundary of the Muraykhah Formation (10 m pebble layer) is conformable with the underlying Badayi Formation in the Mashhad area. Here the Muraykhah Formation is unconformably overlain by the lower Cambrian Siq Sandstone (Figures 3 and 4, Hadley, 1974). In a complete stratigraphic succession, the Muraykhah Formation is overlain by the Jifn Formation (see below); the contact between the Muraykhah and Jifn formations is described below.

Some authors have considered the possibility that the carbonates of the Muraykhah Formation may have been deposited in a lacustrine rather than marine setting (e.g. Johnson, 2003; Miller et al., 2008). In the present paper, the carbonates are interpreted to represent a regional marine flooding event that occurred after the deposition of the continental Rubtayn Formation (Figure 2 in enclosed Chart). The interpretation of a marine environment is based on the following studies.

As part of an extensive BRGM feasibility study to mine phosphates, Hutin (1983) prepared a report on the carbonates of the Muraykhah Formation in the Mashhad area and the central Arabian Shield (area encompassed from Bir Sija Basin in the southeast to the Zarghat Basin in the northwest, Figure 1). The study involved surveying 21 sections across the formation (total of 4,460 m) and examining 138 thin sections. In some localities Hutin (1983) recognized the trend from the lower cherty carbonate to the non-cherty dolomitic limestone, as noted by Delfour (1970) and Hadley (1974; Figure 4). Hutin (1983) recognized five main facies: (1) silicified carbonate and chert; (2) carbonates; (3) detrital rocks; (4) evaporites; and (5) volcanic rocks.

The carbonates form the major part of the Muraykhah Formation and are further characterized by Hutin (1983) as: (1) laminated limestone (usually fetid algal laminite); (2) thin-bedded limestone; (3) brecciated and conglomeratic limestone; (4) massive limestone; (5) dolomite; and (6) dolomitic limestone. The evaporites occur as layers of gypsum, several centimeters thick, interbedded in the carbonates and as loose fragments on the surface. Hutin attributed the brecciation of some chert bands to the possible leaching of the underlying gypsum beds. Hutin concluded that the Muraykhah Formation was deposited in a structurally unstable, shallow-marine to littoral setting with continuous volcanic activity. He tentatively concluded that the open sea was located north of the Arabian Shield, whereas an evaporitic setting prevailed to the east.

Further east in the Antaq Basin (Figure 1), Nettle (2009) conducted a detailed stratigraphic study of the Rubtayn Formation (lower Antaq succession, ca. 2.0 km thick) and Muraykhah Formation (upper Antaq succession, ca. 500 m thick). He interpreted the depositional setting of the Rubtayn Formation as fluvially influenced, non-marine to restricted marine. In the overlying Muraykhah Formation he recognized eight facies and interpreted their depositional settings as follows: (1) ash bed – volcanism; (2) carbonate – supratidal to shallow subtidal; (3) polymictic conglomerate – fluvial; (4) interbedded fine sand and silt – foreshore; (5) rippled medium sand – fluvial or deltaic; (6) massive fine to medium sand – deltaic with storm events; (7) hummocky cross-stratified sand – subtidal; and (8) shale and laminated silt – the shale representing a deep subtidal setting.

Nettle (2009) used these facies to interpret approximately 22 fourth-order depositional cycles, each ca. 10–30 m thick, in three measured sections that span about 12 km in the Antaq Basin. The facies recorded in the three sections indicate the northern part of the basin was proximal whereas the southern part was increasingly distal. Based on the evidence for tidal influence, the occurrence of Ediacaran fossils and δ¹³C and δ¹⁸O values, Nettle (2009) concluded that the Antaq Basin recorded an overall marine transgression. He concluded that the basin was intermittently connected to a much larger sea to the south and ruled-out the interpretation that it was an isolated fault-controlled lake.

Locality: Jabal Umm Al ‘Aisah, located between the southeastern part of the Al Jifn Basin and northwestern part of the Nuqrah Basin in the Nuqrah Quadrangle (Figures 1 and 5).

Authors and Nomenclature: The Umm Al ‘Aisah Formation was defined by Delfour (1970, 1977). By stratigraphic position it corresponds to the combined Rubtayn, Badayi and Muraykhah formations of Hadley (1974, 1986) (Figure 2, enclosed Chart).

Thickness and Lithology: In the Al Jifn Basin, the Umm Al ‘Aisah Formation begins with the “Lower Conglomerate”, which is ca. 500 m thick. This unit consists of cobbles and boulders of granite and rhyolite, volcanic sandstone, and more rarely of quartz and mafic rocks. The Lower Conglomerate is here correlated to the Polymictic Conglomerate Member of the Rubtayn Formation. It is overlain by a 320-m-thick unit of alternating beige, gray and black cherty limestone, and beige, gray, green and yellow argillaceous and calcareous shale. Gray, red and black chert occurs as irregular beds, lenses or scattered fragments in the limestone layers. This succession is here referred to as the “Cherty Limestone” unit and correlated to the Muraykhah Formation.

In the Jibalah Basin (Figures 1 and 6; Delfour, 1970), the Cherty Limestone unit (Muraykhah Formation) is ca. 200 m thick and encased between volcanic rocks dated by Brown et al. (1989, see below). The basal conglomerate (Rubtayn Formation) is ca. 120 m thick in the measured section.

Boundaries: In the Al Jifn Basin area the Umm Al ‘Aisah Formation overlies the Neoproterozoic Murdama Group, which is older than the Shammar Group, and is overlain by the Jifn Formation.

Localities: The Jifn Formation is defined in type area at Jabal Umm Al ‘Aisah located between the southeastern part of the Al Jifn Basin and northwestern part of the Nuqrah Basin (Figure 5; Delfour, 1970, 1977, 1981). The Jifn Formation is absent in the Sahl Al Matran Quadrangle, where Hadley (1974, 1986) described the lower part of the Jibalah Group (Figures 1 to 4). It may correlate to the informal upper Fatima member (Nebert et al., 1974) and corresponding Daf Formation of the Fatima Group in the Makkah Quadrangle (Moore and Al-Rehaili, 1989; Grainger, 2001).

Author:Delfour (1970, 1981).

Thickness and Lithology: In the Al Jifn Basin, above the Cherty Limestone unit of the Umm Al ‘Aisah Formation, the Jifn Formation starts with the “Upper Conglomerate” unit overlain by alternating decimeter-thick beds of sandstones, shale and minor conglomerates (ca. 1,870 m thick in Figure 5). The formation also contains lenses of carbonates and andesite-basalt flows. Red-brown sandstones constitute the dominant lithology and their grain size varies considerably (conglomerate to siltstone). They show sedimentary structures including ripple marks and graded bedding. The siltstones are finely stratified in alternating beds (millimeter to centimeter, sometimes decimeter thick) and terminate in ripple marks before passing to shale. Occasionally, the siltstones are intercalated within lenses of carbonates. The shales are soft, argillaceous, varicolored (brown, green and yellow), sometimes carbonaceous and organic rich.

The Jifn Formation is recognized in only a few basins in the central part of the Arabian Shield. In the Hawaqah Basin the Jifn Formation exceeds 2,000 m in thickness (Delfour, 1981). It begins with fine-grained sandstone and siltstone and continues with a succession of inter-bedded sandstone, tuffaceous sandstone and andesite-basalt flows. The upper part of the Jifn Formation consists of several-decimeter-thick beds of sandstone and conglomerates, with pebbles up to 5 cm in diameter. The upper unit is found in a NW-trending, narrow belt (32 x 1 km) in the northeastern part of the basin. The succession is frequently interrupted by reworked andesitic or basaltic tuff and by epiclastic deposits that reflect contemporaneous volcanism. In the northern Sukhaybarah Sub-basin, the Jifn Formation has an estimated thickness of 2,500 m (Delfour, 1981). It is composed mainly of relatively thin beds of olive-green, brown and purplish-gray fine-grained sandstone, commonly argillaceous or carbonate-cemented, alternating with sandstone and shale. A gritty limestone is commonly present but remains subordinate.

Lower Boundary: In the type area at the western foot of Jabal Umm Al ‘Aisah the “Upper Conglomerate” forming the lower part of the Jifn Formation unconformably overlies the “Cherty Limestone” unit (Muraykhah Formation) of the Umm Al ‘Aisah Formation (Figure 5). In the Hawaqah Basin (Al Hissu Quadrangle), the stratigraphic contact occurs between the fine-grained sandstone and siltstone of the Jifn Formation and the “Cherty Limestone” unit (Muraykhah Formation) of the Umm Al ‘Aisah Formation (Delfour, 1981).

Upper Boundary, Sub-Siq Unconformity (Angudan Unconformity): In the type section at Jabal Umm Al ‘Aisah, the upper boundary of the Jifn Formation with the Siq Sandstone is not documented. Evidence that the Siq Sandstone was the first formation to be deposited after the Jifn Formation is found in the outcrop map of the Arabian Shield. East of Madina city, Brown et al. (1989) highlighted a large outlier of the Siq Sandstone (250 sq km; centered at 24°30'N, 40°40'E), which occurs along a major Najd Fault Zone. It surrounds the Jibalah Group outcrop in the Hawaqah Basin in the Al Hissu Quadrangle (Delfour, 1981).

Based on radiometric studies conducted in the 1960s and 1970s, the Jibalah Group was dated between 580–530 Ma (e.g. Fleck et al., 1976). In 2002, Nehlig et al. reviewed about 500 dates (U-Pb, Rb-Sr, K-Ar and Ar/Ar) in the Arabian Shield and showed that three main maxima occur at 730–720 Ma, 640–630 Ma and 590–580 Ma. They concluded that a transition from the “Suturing Orogeny” to “Extension” occurred across the Arabian Shield at ca. 600 Ma. The extensional phase continued to ca. 545 Ma, or later, and is characterized by alkali magmatism and the formation of the Najd (Jibalah) pull-apart basins.

Some authors, however, consider the Jibalah Group to be no younger than ca. 580–570 Ma (e.g. Johnson, 2003; Allen, 2007). They cite two U-Pb zircon dates obtained by Kusky and Matsah (2003):

• (1) 624.9 ± 4.2 Ma from the basement of the Al Jifn Basin as the oldest limit for the group; and

• (2) 576.6 ± 5.3 Ma from an undeformed felsite dyke that intrudes the Jibalah Group as its youngest age limit. The following review cites radiometric data collected from the 1960s–1970s for the Jibalah and Shammar groups, recent zircon geochronological data (Nicholson et al., 2008; Miller et al., 2008; Nettle, 2009), and attempts an approximate calibration (i.e. ± 5–10 My) of the Jibalah Group and its formations.

The age of the Sub-Jibalah Unconformity is younger than that of the underlying Shammar Group, which was dated by Rb-Sr and K-Ar at 620 Ma (Delfour, 1970). In the Mashhad area, Fleck et al. (1976) obtained K-Ar dates of 567 ± 6 and 581 ± 7 Ma for the Shammar rhyolite. Brown et al. (1989) reported dates for the Shammar Group of 633 ± 15 Ma, and 555 ± 25 Ma; and for a comagmatic series as ca. 620–550 Ma, with a culmination at ca. 570 Ma (Fleck et al., 1976; Aldrich et al., 1978; Schmidt et al., 1979).

In the Nuqrah Quadrangle, Delfour (1970, 1977) reported that seven samples from the Jabal Bidayah Granite, which intrudes the Shammar Group, yielded a whole-rock Rb-Sr isochron of 570 ± 8 Ma, and later recalculated as 590 ± 8 Ma by Delfour (1981). The Jabal Furqayn Batholith, which intrudes the Shammar Group in Al Hissu Quadrangle, yielded a whole-rock Rb-Sr isochron of 606 ± 14 Ma (Delfour, 1977, 1981)

At Jabal ar Rihadah, a granite that lies unconformably below the Jibalah Group was dated by a seven-point, whole-rock Rb-Sr isochron at 577 ± 15 Ma (Baubron et al., 1976, in Brown et al., 1989). Granite beneath the Jibalah Group in the Jibalah Basin yielded a whole-rock Rb-Sr of 574 ± 28 Ma (Figure 6, Calvez et al., 1983, in Delfour, 1982). In the Al Wajh Quadrangle, the Jibalah Group is unconformable on granitoids dated at ca. 609 Ma (SHRIMP zircon ages from the Hadb, Ash Shab and Warid complex granitoids, unpublished SGS report by Kennedy et al., inMiller et al., 2008).

The ages of the Shammar rhyolites and post-tectonic granites (alkali granites) overlap between ca. 620–580 Ma, and center at ca. 600 Ma (Nehlig et al., 2002). They have the same age as the Aqaba Granite Complex near the Gulf of Aqaba in Jordan (Figure 2 in enclosed Chart), which was dated between 595–585 Ma by Ibrahim and McCourt (1995). Taking into consideration these various radiometric dates, the present author approximates the age of the Sub-Jibalah Unconformity at ca. 585 Ma.

In the Ar Ridayniyah area, Ad Dawadimi Quadrangle, post-tectonic granophyre dikes with compositions ranging from trondhjemite to granodiorite yielded on six samples (Rb-Sr) a date of 558 ± 10 Ma (Calvez et al., 1983, in Delfour, 1982). A sample from the Badayi Formation at Jabal Rubtayn (Mashhad area) gave a whole-rock Rb-Sr date of 540 ± 18 Ma (Hadley, 1974). Brown et al. (1989) reported collecting two samples from volcanic rocks in the section at Jabal Jibalah (Figure 6), but disqualified one sample. Here the volcanic rocks occur above and below the Cherty Limestone unit of the Umm Al ‘Aisah Formation (Muraykhah Formation), and the stratigraphic position of the accepted sample is not clear. It gave a whole-rock K-Ar date of 548 ± 19 Ma for the alkalic andesite flow (sample 69, 25°33′N, 40°45′E, table 8 of Brown et al., 1989). The sample was also analyzed by J. Sutter (written communication inBrown et al., 1989) using a ⁴⁰Ar/³⁹Ar spectrum and yielded a plateau date of 558 ± 6.6 Ma, the error representing two standard deviations.

As noted above Nettle (2009) divided the Jibalah Group in the Antaq Basin into the lower Antaq succession (Rubtayn Formation, ca. 2.0 km thick) and upper Antaq succession (Muraykhah Formation, ca. 500 m thick). He reported a date of 573 ± 12 Ma (U-Pb, detrital zircons) for a sample taken from a polymictic conglomerate layer (ca. 100 m thick) located 1.5 km above the base of the Rubtayn Formation. This layer may be coeval to the Varanginian (Gaskiers) Glaciation dated between ca. 590–580 Ma (Halverson et al., 2005) or ca. 582 Ma (Bowring et al., 2007). The time required to deposit the Rubtayn Formation may be estimated by converting its thickness to time. Assuming a typical syn-rift conversion rate of 100 m/My and a thickness of 2,000–3,000 m would suggest ca. 20–30 My for the deposition of the Rubtayn Formation. An age between 585–560 Ma is assumed to be consistent with the ⁴⁰Ar/³⁹Ar date of the overlying basalt-andesite flows in the Jibalah Basin (ca. 558 ± 6.6 Ma, Brown et al., 1989).

Nicholson et al. (2008) reported that tuff beds within the Muraykhah Formation yielded by SHRIMP analyses of zircons (U-Pb) dates ranging between 600–588 Ma, and to cluster around 594–591 Ma. Allen (2007) noted that in a previous conference presentation in 2003 by Nicholson and Janjou the samples were reported as detrital zircons, and that these would therefore only constrain the age of the Muraykhah Formation as younger than ca. 600 Ma. Miller et al. (2008) obtained U-Pb dates of 599 ± 4.8 and 570 ± 4.8 Ma from detrital zircons taken from a thin diamictite interval in the middle of the Dhaiqah (Muraykhah) Formation in the Dhaiqa Basin. In the Antaq Basin, the Muraykhah Formation contains two ash beds dated at 568 ± 11 Ma and 584.8 ± 9 Ma (Nettle, 2009).

Miller et al. (2008) also compared the δ¹³C_carb data from the Dhaiqa (Muraykhah) Formation to that of other studies (Le Guerroué et al., 2006; Halverson et al., 2007) and concluded the formation could have any one of the following ages: (1) ca. 600 Ma; (2) ca. 570–560 Ma; or (3) ca. 551–542 Ma. The present author interprets the 22 fourth-order cycles recognized by Nettle (2009) as stratons that tracked the ca. 405 Ky orbital eccentricity signal of the Earth (Matthews and Al-Husseini, 2010). In such a calibration the Muraykhah Formation in the Antaq Basin would represent ca. 8.9 million years. When subtracted from 560 Ma the top of the Muraykhah would have an age of ca. 551 Ma. The age interval ca. 560–551 Ma falls between options (2) and (3) of Miller et al. (2008) noted above.

This formation is up to 2,500 m thick and was mainly deposited in a continental setting. Its thickness when converted at 100 m/My corresponds to 25 My in the time interval ca. 551–526 Ma. This interval overlaps the estimated age of the combined Ara and Nimr groups in Oman (ca. 547520 Ma, Forbes et al., 2010). The end of Jifn deposition corresponds to the Sub-Siq Unconformity at ca. 526 Ma, which falls in the estimated age range for the correlative Angudan Unconformity between 530–520 Ma.

Aldrich et al. (1978, inBrown et al., 1989) reported a basalt dike that was exposed to weathering prior to the deposition of lower Paleozoic sandstones (Siq Sandstone) gave a K-Ar whole rock age of 532 ± 15 Ma. A porphyritic andesite sill (or dike 60 km northwest of Al-Madinah) was dated at 525 ± 16 Ma using the same method. They also dated a gabbro plug intruded into a Najd shear zone, where there was movement after the Jibalah Group was deposited. The chilled edge forms a rim on a crater at Wadi ar Rika (28°28'N, 44°34'E) and gave a K-Ar whole-rock date of 513 ± 17 Ma.

In several quadrangles, rock units that appear to be coeval to the Jibalah Group were only tentatively correlated to it or assigned to groups and/or formations with different names. This practice was mainly due to the great distances that separate the outcrops and inconclusive dating of the rocks.

The Fatima Group crops out in the NE-trending Wadi Fatima near the city of Jiddah (Nebert et al., 1974; Moore and Al-Rehaili, 1989; Grainger, 2001; Figure 1). It overlies granitic basement dated at ca. 773 Ma and lavas within the group have been dated at ca. 580 Ma (K-Ar) and 680 Ma (Rb-Sr). Gorin et al. (1982) tentatively correlated the Fatima Group to the Jibalah Group, Saramuj Conglomerate of Jordan, Huqf Supergroup of Oman, Ghabar Group of Yemen, Inda Ad Series of Somalia and the Hormuz Series of the Gulf. The Fatima Group consists in ascending order of the Baqar, Shubayrim and Daf formations.

The Baqar Formation ranges in thickness from 35 to 130 m. It consists of volcaniclastic conglomerate and epiclastic rocks, mainly of arkosic to tuffaceous sandstones. The overlying Shubayrim Formation (109–160 m thick) consists of carbonates interbedded with tuffaceous sedimentary rocks. Collenia- and Conophyton-type stromatolites occur in the limestone and are commonly silicified. The Daf Formation is 488 m thick and its upper part is eroded. The base is marked by a 9-m-thick green, vesicular basalt flow. The lower part consists of siltstone, fine-grained sandstone and subordinate volcaniclastic rocks and lava flows. The upper part is a heterogeneous assemblage of tuffs, ignimbrites, volcanic breccia and minor lava flows, and subsidiary interbedded mudstone, sandstone and conglomerate.

The correlation of this group to the Jibalah Group by stratigraphic position, thickness and lithologic correlations suggests: (1) Baqar to Rubtayn; (2) Shubayrim to Muraykhah; and Daf to Jifn.

In this quadrangle (Figure 1), the Hadiyah Group consists of the predominantly volcanic Siqam Formation, and the overlying sedimentary Tura’ah and Aghrad formations (Kemp, 1981). The latter two formations may, in part, be coeval to the Jibalah Group. The Hadiyah Group is best preserved in the As Sard Basin in the southeast of the quadrangle from where it can be followed in a NW-trending strongly deformed zone. It also crops out in the northwest of the quadrangle between Wadi al Jizl and Wadi Amudan.

Siqam Formation, Hadiyah Group: The formation (ca. 2,000 m thick) consists largely of basaltic to andesitic volcanic rock, ranging from structureless to partly intrusive. It includes thick amygdaloidal, pillowed units characterized by nodes, amygdales of pale-green chalcedony, and aquagene breccia and tuff. A persistent, thick, silicic unit, dominantly ash flow tuff, occurs near the top of the formation, and other thin silicic volcanic and probably epiclastic deposits occur throughout. The Siqam Formation has been correlated to the Mashhad and Nadah formations of the Shammar Group by Kemp (1981), implying the overlying Tura’ah and Aghrad formations are coeval to the Jibalah Group.

Tura’ah Formation, Hadiyah Group: The formation is divided into the Jammazin and overlying Qaraqah members, together 3,400 m thick in a measured section in the eastern part of the As Sard Basin. The formation rests with slight unconformity on a smooth and gently undulating surface at the top of the Siqam Formation. The Jammazin Member (1,600 m thick) consists dominantly of light to dark green sandstone, interpreted as deep-water deposits. A thin unit of medium to thin-bedded silicic ash-tuff is common at the base. In places medium to thick-bedded sandstone, with medium- to large-scale cross-bedding occurs. Limestone occurs at or near the base in a number of places and includes several meters of laminated algal rocks in one locality. The Qaraqah Member (1,800 m thick) begins with flat-laminated sandstone and silicic laminite inter-bedded with ripple-marked red beds. Most of the member consists of red beds that are generally thin-bedded, rippled, medium-grained sandstone to mudstone.

Aghrad Formation, Hadiyah Group: The formation (2,100 m thick) is confined to the core of the As Sard Basin. It starts abruptly with pebble to cobble conglomerate (120 m thick, consisting of silicic and basaltic to andesitic volcanic rock), which contains sparse sandstone lenses; near the top a thick cross-bedded sandstone unit occurs. The overlying 450-m-thick unit is a broadly upward-thinning and –fining sequence of mainly red-gray to red shallow-water sedimentary rocks. It commences with thick-bedded sandstone and ends with thin-bedded siltstone and mudstone. The rest of the formation consists predominantly of monotonous gray to red-gray, poorly bedded, planar cross-bedded, sheet sandstone with a few thin pebbly to conglomeratic beds at its base.

In this quadrangle (Figure 1), Davies (1985) described the Mataar Formation (silicicastics) and conformably overlying Dhaiqa Formation (carbonates), but did not assign them to the Jibalah Group. Brown et al. (1989) reported that strata belonging to the Jibalah Group may crop out in this quadrangle, near where the middle zone of the Najd Fault System (Qazzaz Fault Zone) intersects the Red Sea. Miller et al. (2008) assigned the Mataar-Dhaiqa succession to the Jibalah Group and studied it in the Dhaiqa Basin. Two more basins in this quadrangle, Salih and Dawqah, contain siliciclastics and rhyolite flows (Misya and Salih formations) but not apparently the Jibalah Group. The Mataar Formation, as well as the Misya and Salih formations, are unconformable on granatoids dated at ca. 609 Ma (SHRIMP zircon ages from the Hadb, Ash Shab and Warid complex granitoids, unpublished SGS report by Kennedy et al. in Miller et al., 2008).

The Mataar Formation, named after Wadi al Mataar, consists of polymicitc conglomerates and arkosic sandstones and attains a thickness of ca. 100–150 m in the Dhaiqa Basin (Davies (1985; Miller et al., 2008). The conglomeratic clasts are poorly sorted, subangular to subrounded in shape, and vary in size. They range up to one meter and consist of outsized boulders of rhyolite, dacite, andesite, monzogranite, and abundant alkalic granite. Intercalated units of pebbly litharenite and arkose are common in the upper part of the formation. According to Miller et al. (2008) the nature of clast support (matrix versus clast) is not obvious, but matrix support is indicated for the outsized boulders. They add that the unit is consistent with molasse or fanglomerate, but matrix-supported outsized boulders also suggest a glacial diamictite.

The thickness of the Dhaiqa Formation is ca. 150 m according to Davies (1985) to as much as 300 m according to Miller et al. (2008). The following discussion follows from similar descriptions by these authors for the formation. The formation begins conformably above litharenite of the Mataar Formation as intermittent limestone beds, followed by more-or-less continuous limestone deposition throughout the remainder of the unit. The limestone is thin-bedded and laminated. It contains chert nodules, well-preserved fossil bioherms, fetid stromatolitic horizons with carbonaceous intercalations. The carbonaceous beds pass upwards into fine-grained limestone breccias, formed by partial reworking of the stromatolitic beds.

Miller et al. (2008) recognized at ca. 180 m, above base of the Dhaiqa Formation, a distinct 2–3 m thick sequence of glauconitic arenite followed by poorly sorted polymictic conglomerate. They reported that clasts in this intra-Dhaiqa conglomeratic sequence range up to 50 cm in size and consider this unit a diamictite because its sedimentary origin is unclear. Carbonates resume abruptly above the diamictite sequence with an algal boundstone bed, with abundant peloids and late chert cement. Apart from its sharp basal contact, the bed lacked sedimentologic and geochemical characteristics of cap carbonates (e.g. unusual textures or cements, teepee or roll-up structures, negative δ¹³C_carb). A large oncolite (ca. 15 cm in diameter) with an igneous clast nucleus (ca. 5 cm in diameter) occurs in the float above the diamictite layer. This suggests that algal colonization directly ensued from diamictite deposition, and that moderate energy levels existed to move the accreting oncolite. The overlying carbonate sequence, examined in less detail, consists largely of flaggy (thin splitting) limestone beds with parallel laminations.

The Farra’ah and Naghr formations crop out north of Wadi al Farra’ah in the Shaghab Quadrangle (Grainger and Hanif, 1989). The quadrangle is located immediately north of the Al Wajh Quadrangle but is not shown in Figure 1. The Farra’ah Formation consists of conglomerate, rhyolite flows, and breccia, which lie unconformably on slightly metamorphosed volcanic rocks. Grainger and Hanif (1989) considered the Naghr Formation as equivalent to the Jibalah Group, and to have a local name because of its isolation from the outcrops of definite Jibalah Group. It consists of folded sandstones with horizons of conglomerate, siltstone, claystone and limestone, overlain with marked unconformity by sub-horizontal arenaceous rocks of the Siq Sandstone. This correlation suggests that the underlying Farra’ah Formation may correlate to the Shammar Group.

The enclosed Ediacaran – Cambrian Middle East Geologic Time Scale 2011 is a work-in-progress aimed at converging towards a regional correlation (Figure 2 in enclosed Chart). The first step in constructing it involved defining and correlating the early Cambrian Asfar Sequence between Iran (Lalun Sandstone), Jordan (Salib Arkosic Sandstone), Oman (Andam Formation) and Saudi Arabia (Siq Sandstone) (see Al-Husseini, 2010). This paper takes the second step by documenting the Jibalah Group of Saudi Arabia in terms of three formations; in ascending order: (1) continental clastics of the Rubtayn Formation (ca. 2,000 m thick in Antaq Basin); (2) carbonate-dominated Muraykhah Formation (up to 300–500 m thick); and (3) mainly clastic Jifn Formation (up to 2,500 m thick). In the enclosed Chart the group is hung from the base of the Asfar Sequence (Sub-Siq or Angudan Unconformity at ca. 530–520 Ma) and its formations are approximately time-calibrated down to the Sub-Jibalah Unconformity at ca. 585 Ma. The estimated age of the Jibalah Group remains essentially unchanged since 35 years ago at ca. 580–530 Ma (e.g. Fleck et al., 1976).

The author thanks Yves-Michel Le Nindre, Denis Vaslet and Joerg Mattner for their assistance in collecting many of the reports and maps cited in the paper. He thanks Abdulkader Afifi, Jacques Delfour, Gordon Forbes, John Powell and Robert Stern for their important suggestions. The interpretations and correlations in this paper and accompanying chart represent the view of the author but not necessarily that of the above named colleagues. Kathy Breining and Heather Paul-Pattison are thanked for proofreading the text and Arnold Egdane for designing the paper for press.

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

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