The Permian-Triassic Khuff Formation crops out in central Saudi Arabia along a N-S belt, some 1,200 km in length. It is 171.4 m (562.2 ft) thick in the type section and divided into five members; from oldest to youngest: Ash Shiqqah (formerly Unayzah member of the Khuff Formation), Huqayl, Duhaysan, Midhnab and Khartam. The base of the Khuff Formation is recognised as a regional unconformity (Pre-Khuff Unconformity), and the top of the formation is defined by the conformable contact with the overlying Sudair Shale. The Ash Shiqqah Member is tentatively dated as ?Middle Permian ?Capitanian (?Midian) based on the presence of the fusulinids Monodiexodina kattaensis and Reichelina sp. The Huqayl Member is tentatively dated as ?Late Permian ?Wuchiapingian (?Dzhulfian) based on an assemblage of smaller foraminifers dominated by Pseudomidiella cf. labensis, Earlandia? spp. and Neodiscus aff. qinglongensis. The Duhaysan Member is dated as Late Permian (Wuchiapingian-Changhsingian) based on Hemigordius baoqingensis, Graecodiscus cf. kotlyarae, “Dentalina” hoi, and Colaniella cf. minuta. The sMidhnab Member is dated as Changhsingian (Dorashamian) based on Paradagmarita sp. and “Glomospirella spirillinoides” and the disappearance of the genera Nankinella and Globivalvulina. Within the continental facies in the upper part of the Midhnab Member, incised channels facies yielded the Late Permian Midhnab Flora. The Lower Khartam Member is also dated as latest Permian (Dorashamian) based on the presence of several species of Paradagmarita and “Nodosaria” dzhulfensis in the lowest beds of the member, and the ostracods Paraparchites spp., Knoxiella spp. and Kloedenellitina sp. throughout the Lower Khartam Member. The Upper Khartam Member is assigned to an Early Triassic (‘Scythian’) age based on the occurrence of the characteristic annelid Spirorbis phlyctaena.

The Khuff Formation, in central Saudi Arabian outcrops, consists of four regional depositional sequences (DS PKh, DS PKm, DS PKk and DS TrS), each containing a maximum flooding interval (MFI, with the same designation). The oldest sequence DS PKh (named after Permian-Khuff-Huqayl) consists of the Ash Shiqqah and overlying Huqayl members. The basal sequence boundary corresponds to the Pre-Khuff Unconformity (PKU) and represents the onset of the first Permian flooding event recorded in the outcrops of Saudi Arabia. The massive gypsum, gypsiferous claystone, and solution breccias in the Ash Shiqqah Member are correlated to the subsurface Khuff-D Anhydrite. The MFI PKh is positioned in the basal part of the Huqayl Member, and is followed by the regressive evaporitic palaeoenvironments of the Huqayl Member.

The DS PKm (named after Permian-Khuff-Midhnab) starts with the subtidal to littoral deposits of the Duhaysan Member, over an erosive surface upon DS PKh; and ends with the regressive supratidal to continental deposits at the upper part of the Midhnab Member. The MFI PKm is located at the base of the Midhnab Member, where limestones yielded an abundant marine fauna including cephalopods and brachiopods. The terminal Permian DS PKk (Permian-Khuff-Khartam) corresponds to the Lower Khartam Member. The basal SB PKk is marked by a return to marine subtidal conditions after the continental break at the end of DS PKm. The MFI PKk is characterised by marine fauna including abundant Permian ostracods, bactritids, and locally cephalopods. The DS TrS (named after the Sudair Shale Formation) starts with the littoral, tidal to intertidal deposits of the Early Triassic Upper Khartam Member of the Khuff Formation, and ends with the closed basin, clayey to evaporitic rocks of the Early Triassic Sudair Shale.

The Permian-Triassic Khuff Formation crops out in central Saudi Arabia along a N-S belt that is some 1,200 km in length (Steineke and Bramkamp, 1952; Steineke et al., 1958; Powers, 1968). The formation was mapped during the 1980s by the Deputy Ministry for Mineral Resources, Ministry of Petroleum and Minerals of Saudi Arabia (DMMR, now the Saudi Geological Survey – SGS), and the French Geological Survey BRGM (Figure 1). This paper provides a synthesis of these studies together with new data and interpretations. It starts with a description of the Pre-Khuff Unconformity (PKU) in outcrop and subsurface. Next the five formal members of the formation are described in the type section in the Ad Dawadimi quadrangle (Figures 1 and 4). The following section reconciles various conflicting lithostratigraphic definitions attributed to the names “Unayzah” and “Ash Shiqqah”. This is followed by a lateral correlation of the five members; first northwards and then southwards along the strike of the outcrops. The correlation includes several published well data from central Saudi Arabia.

The study next presents the palaeontology and ages of the five Khuff members and the underlying Unayzah Formation. This section includes ongoing biostratigraphic analysis of foraminifers and algae by D. Vachard and J. Gaillot (Vachard et al., 2002, 2005), of ostracods by S. Crasquin-Soleau (Crasquin-Soleau et al., 2004, 2006), and flora by J. Broutin and M. Berthelin (Broutin et al., 2002; Berthelin, 2002; Berthelin et al., 2005). Throughout the paper and figures, all stages and ages are related to the proposed global geological time scale of the Permian Period (Jin et al., 1997; Menning et al., 1997; Menning, 2001a, b; Gradstein et al., 2004) (Figure 2). The final section describes four regional Khuff depositional sequences (DS) and their corresponding maximum flooding intervals (MFI).

The Khuff Formation, first recognised in publication by Steineke and Bramkamp (1952), was formally defined by Steineke et al. (1958) near ‘Ayn Khuff (between 24°58’36”N, 44°41’48”E and 24°53’12”N, 44°42’48”E) in the Ad Dawadimi quadrangle in central Saudi Arabia (Figure 1). The definition of the Khuff Formation was later amended by Powers et al. (1966) and Powers (1968). In the type section, the formation is 171.4 m (562.2 ft) thick, and its base is at the unconformable contact between a lower sandy phase of the Khuff and the underlying massive sandstone of the Saq. The top is placed at the level where Khuff limestone and dolomite are in sharp contact with red and green gypsiferous shale of the overlying Sudair Shale. A more complete reference section was documented in Wadi Ar Rayn quadrangle (Figures 1 and 3: between 23°32’45”N, 45°34’30”E and 23°43’N, 45°42’E; R.A. Bramkamp, E.L. Berg, R.L. Meyers and W.T. Short, inPowers et al., 1966). At this locality, the Khuff Formation overlies the Proterozoic Basement.

A revised type section of the Khuff Formation was proposed by J.M. Brosse, Y.M. Le Nindre and D. Vaslet (in Delfour et al., 1982) in the Ad Dawadimi quadrangle (Figures 1 and 4). They divided the formation into five informal members; from oldest to youngest (Figures 4 and 5): Unayzah (hereafter Ash Shiqqah Member of Khuff Formation), Huqayl, Duhaysan, Midhnab, and Khartam. These five Khuff members were adopted and mapped (at a scale of 1:250,000; Figure 1) across central Saudi Arabia by the DMMR and BRGM, during the exploration for minerals.

In 1987, Y. Le Nindre, J. Manivit and D. Vaslet synthesised the lithostratigraphic, sedimentologic and biostratigraphic results of the DMMR and BRGM mapping program, together with additional analytic data, in three PhD thesis at Paris University (in French), and in four BRGM publications (Le Nindre et al., 1990a, b; Manivit et al., 1990; Vaslet, 1990). These syntheses included revisions to the lithostratigraphic units, palaeoenvironments and sedimentologic interpretations, and biostratigraphic data that was published in the Explanatory Notes to the Geosciences Map series published by the DMMR (Figure 1).

The definition of the members of the Khuff Formation in Saudi Arabia varies according to author, and from the surface to subsurface (Powers et al., 1966; Powers 1968; Delfour et al., 1982; Le Nindre et al., 1990b; Al-Jallal, 1995; Alsharhan and Nairn, 1997; Alsharhan, 2006). This nomenclatural inconsistency causes confusion; especially in the case of the term “Unayzah” which is known as the “Unayzah member of the Khuff Formation” (Delfour et al., 1982), as well as to the much older “Unayzah Formation” (Al-Laboun 1982, 1986, 1987; El-Khayal and Wagner, 1983, 1985; Senalp and Al-Duaiji, 1995). In order to avoid confusion, this paper will propose the “Ash Shiqqah Member” to replace the “Unayzah member of the Khuff Formation” of Delfour et al. (1982).

The Khuff Formation in central Saudi Arabia unconformably overlies the Proterozoic Basement (Figure 6) or Lower Palaeozoic deposits (e.g. Powers et al., 1966; Powers, 1968; Le Nindre et al., 1990b). From north to south (see Figure 7 for longitude), the Pre-Khuff Unconformity (PKU) can be seen continuously between 27°40’N (Al Khuwayr, in the Baq’a quadrangle) and 19°57’N (Bani Ruhaya, in the Wadi Tathlith quadrangle). This angular unconformity reflects a period of regional uplift and compressional tectonic activity that occurred during the Carboniferous (Powers, 1968; Wender et al., 1998; Le Nindre et al., 2002; Al-Husseini, 2004).

Beds below the Pre-Khuff Unconformity in outcrop (Figure 7) are enhanced by a pedogenic lateritic alteration surface (Figure 8) that is developed over central Saudi Arabia (El-Khayal and Wagner, 1983, 1985; Le Nindre et al., 1990b; Senalp and Al-Duaiji, 1995). According to Le Nindre et al. (1990b), depending on the pre-Khuff substratum, this lateric layer is represented either by a duricrust containing goethite pisoliths, or by an iron crust (10–80% silica, 2-15% Al2O3 and 4–76% Fe2O3). The main outcrops of the pisolitic duricrust are located between Jal al Khuffiyat (27°10’N) and Wadi Ar Rayn area (23°45’N), corresponding to a palaeohigh in the Khnaygu’iyah area where the Ash Shiqqah Member is locally reduced to a single sandstone bed (Figures 5 and 7).

The Khnaygu’iyah palaeohigh is located on the axis of the broader Central Arabian Arch (Figures 1, 7 and 9). The crest of the arch is expressed by the subcrop of the Pre-Khuff Unconformity, and consists of the Proterozoic Basement at its center and successively younger Palaeozoic formations along its flanks (Figure 7). In the subsurface, the Unayzah Formation was deposited around the periphery of the arch but not on its crest (Figure 9).

In the north of the Ad Dawadimi quadrangle (Figure 1), Delfour et al. (1982) defined the informal Unayzah member (hereafter Ash Shiqqah Member) of the Khuff Formation above the Cambrian-Ordovician Saq Sandstone Formation (Figure 4). This definition was later revised by Le Nindre et al. (1990b). In the Khuff Formation type section, the Ash Shiqqah Member is 35 m (114.8 ft) thick, and consists of four subunits; from base up:

  • (1) 10.5 m (34.4 ft) emerald-green, purplish red, or varicolored silty claystone, with secondary gypsum interbedded in its lower part by yellowish or gray silty dolomite;

  • (2) 2.5 m (8.2 ft) white, fine-grained sandstone with cross-bedding;

  • (3) 15 m (49.1 ft) red and green silty claystone interbedded with pinkish to yellowish dolomite, locally slumped and tinted red by iron oxides;

  • (4) 7 m (22.9 ft) clayey, localy finely bioclastic dolomite, with beige patina, algal laminites, and fenestrate structures.

The Ash Siqqah Member becomes thinner to the southeast, and along the eastern edge of the quadrangle it consists of only 2 m (6.5 ft) or less of sandy dolomite.

Overlying the Ash Shiqqah Member, the type section of the Huqayl Member of the Khuff Formation is also defined in the Ad Dawadimi quadrangle (Figures 4 and 5; Delfour et al., 1982, Le Nindre et al., 1990b) along a traverse at Safra Huqayl from 24°44’25”N, 44°39’08”E to 24°45’37”N, 44°39’15”E. In the type section (Figure 4), the Huqayl Member is 34.2 m (112.2 ft) thick, and elsewhere in the quadrangle ranges from 30–40 m (98.4–131.2 ft). This member is divided into lower and upper parts and six subunits. The Lower Huqayl Member consists of three subunits; from base up:

  • (1) 6 m (19.7 ft) of bluish bioclastic intraclastic dolomite with a cream patina, locally silty, and peloidal dolomite containing oolitized bioclasts, silty, bioclastic dolomite with a clear patina, overlying an intraclastic dolomitic bed;

  • (2) 4 m (13.1 ft) of green or yellow gypsiferous clay;

  • (3) 2.7 m (8.9 ft) of blue-gray clayey dolomite containing algal laminites and dolomite with a fenestrae structure containing pseudomorphs after gypsum and anhydrite. The Upper Huqayl Member consists of three subunits; from base up:

  • (4) 2.9 m (9.5 ft) bluish, intraclastic and bioclastic dolomite, commonly arranged in tidal channels, overlain by laminated dolomite;

  • (5) 17.6 m (57.7 ft) white gypsiferous clay and fine-grained cherty dolomite, interrupted by a few bioclastic and oolitic dolomitic layers;

  • (6) 1.0 m (3.3 ft) blue-gray clayey dolomite containing algal laminites with fenestrate structures, and showing very bioturbated beds (burrows), locally containing chert nodules or showing dissolution breccias or pseudomorphs of gypsum and anhydrite.

In outcrop, the benches of the Huqayl Member are locally dislocated and slumped due to collapse phenomena associated with the dissolution of anhydrite intervals known in the subsurface. Above the Huqyal Member, the type section of the Duhaysan Member is defined at Jabal Duhaysan (Delfour et al., 1982; Le Nindre et al., 1990b) where it is 13.4 m (44 ft) thick between 24°40’35”N, 44°45’58”E and 24°40’37”N, 44°47’46”E. The Duhaysan Member consists of three subunits (Figures 4 and 5); from base up:

  • (1) 3.7 m (12.1 ft) gray dolomitic calcarenite, with pebbles, intraclasts, and bioclasts (Figure 10), overlain by white bioclastic peloidal dolomite showing current ripples and pseudomorphs of gypsum and anhydrite;

  • (2) 5.6 m (18.4 ft) of white, gypsiferous, dolomitic clay;

  • (3) 4.1 m (13.5 ft) blue-gray, fine-grained peloidal limestone with bioturbations, algal laminite, overlaid by white clayey dolomite with fenestrae structures.

The Midhnab Member, which is an approximate synonym of the Midhnab shale and informal unit 3 (Figure 3) that were discarded by Powers (1968), was reintroduced by Delfour et al. (1982) and emended by Le Nindre et al. (1990b) in the Ad Dawadimi quadrangle (Figure 1). The latter authors measured a type section along Wadi Maghib, near Khuff town, along a traverse from 24°55’28”N, 44°36’47”E to 24°58’20”N, 44°38’19”E, where it is 57.9 m (189.9 ft) thick (Figures 4 and 5). Throughout this quadrangle, the Midhnab Member is fairly uniform in thickness at about 60 m (196.7 ft), and consists from base up:

  • (1) 2 m (6.6 ft) conglomeratic limestone with pebbles of dolomite and abundant bioclasts (bed-on-bed reworking) and bluish-gray platy limestone rich in marine fossils;

  • (2) 9 m (29.5 ft) alternating gypsiferous platy dolomitic claystone and beds of blue-gray bioclastic limestone and dolomite;

  • (3) 46 m (150.8 ft) yellow to blue gypsiferous clay, interspersed with beds of clayey dolomite;

  • (4) 3 m (9.8 ft) light-gray, lacustrine limestone containing charophyte remains.

Delfour et al. (1982) adopted the Khartam Member (Figures 4 and 5), a synonym for the discarded Khartam limestone or unit 4 (Figure 3) of Powers (1968), to describe the mainly calcareous upper part of the Khuff Formation. The type section of the Khartam Member (Le Nindre et al., 1990b) is located northeast of Khuff town in the Ad Dawadimi quadrangle, where the member is about 37 m (121.3 ft) thick along a traverse from 24°58’20”N, 44°38’19”E to 24°59’52”N, 44°40’42”E.

The Lower Khartam Member, 14 m (45.9 ft) thick, comprises from base up:

  • (1) 10.9 m (35.7 ft) coquina and peloidal limestone overlain by blue, laminated, dolomitic clay rich in pellets and terrigenous material;

  • (2) 3.1 m (10.1 ft) ochreous, ferruginous, bioclastic dolomite and dolomitic claystone. The Upper Khartam Member is at least 23 m (75.4 ft) thick, and the missing upper section is estimated at about 10 m (32.8 ft) thick, and comprises from base up:

  • (3) 15 m (49.2 ft) cream oolitic and peloidal limestone with local megaripples and tidal channels (Figure 11). Stromatolitic constructions are locally developped in the middle part of the unit (Figure 12). This facies is entirely calcitized (the calcite ooids corresponding to rounded debris of entirely recrystallised dasycladacean algae) (Figure 13);

  • (4) 6.5 m (21.3 ft) of alternating dolomitic coquina limestone and cream clayey dolomite;

  • (5) 1.5 m (4.9 ft) massive oolitic limestone bed at the top of the outcrops. The upper 10 m (32.8 ft), which are probably clayey and soft, are hidden by Quaternary deposits.

The boundary of the Khuff and Sudair formations is not visible in the type section (Delfour et al., 1982).

The reference section for the basal Ash Shiqqah Member (previously the ‘Unayzah member’) of the Khuff Formation is proposed near the town of Unayzah in the Buraydah quadrangle, some 150 km to the north of the Khuff type section (Figures 1, 1416). At this locality (26°05’10”N, 43°59’24”E), Manivit et al. (1986) measured a 70-m-thick (229.6 ft) section above the Ordovician Qasim Formation. Manivit et al. (1986) and Le Nindre et al. (1990b) considered this section as the ‘Unayzah member’ (i.e. Ash Shiqqah Member), and divided it into four subunits (Figures 1416) These authors considered the base of this section to be close to the base of the Khuff Formation.

In contrast, Al-Laboun (1982, 1987) interpreted part of the same section as the older Unayzah Formation (Figure 16) (26°06’N, 43°58’E). Senalp and Al-Duaiji (1995), also near Unayzah town (Figure 1), defined subunit 3 of Manivit et al. (1986) as the basal Khuff clastics or the Ash Shiqqah Member of the Khuff Formation (Figure 16). They assigned basal subunits 1 and 2 of Manivit et al. (1986) to the Unayzah Formation and subunit 4 to the Huqayl Member of the Khuff Formation. Senalp and Al-Duaiji interpreted a sequence boundary between subunits 1 and 2.

In a later study in the Ash-Shiqqah area of the Buraydah quadrangle, M. Senalp and A. Al-Duaiji (written communication, 2001) proposed the basal Khuff clastics (Ash Shiqqah Member of the Khuff Formation in their 1995 paper) as the Ash Shiqqah formation in a type section (26°22’44.6”N, 43°54’13.8”E; located 1.5 km south of the flour mill on the west side of the Buraydah-Hail highway). They divided the Ash Shiqqah formation into lower and upper units (Figure 16) separated by an unconformity associated with incised channels. They described the lower Ash Shiqqah Member to consist of argillaceous and extensively burrowed dolomitic algal limestone at the base (one metre thick) followed by 10.3 m (33.8 ft) of interbedded yellowish gray marl and dark gray, laminated, fissile, and organic-rich shale.

In 1997, a field trip by teams from Saudi Aramco and BRGM allowed D. Vaslet and M. Senalp to compare their observations. As a result, Vaslet proposed revising the lower boundary of the Khuff Formation in the Buraydah quadrangle, and to place it above the top of subunit 2 (Figure 16) at the change in colour between dark red for the lower part and yellowish or gray-green for the upper part. The Ash Shiqqah Member is thus limited to subunits 3 and 4, and only 42.5 m (139.4 ft) thick. This revision is supported by the presence of pedogenetised sandstone and claystone at the top of subunit 2 (Figure 16) as observed by Vaslet in 1996, while re-sampling the Khuff Formation for palaeomagnetic studies (Torcq et al., 1997), and in 1998 while re-sampling the Unayzah Flora with J. Broutin. D. Vaslet proposed correlating this pedogenetic event with the ferruginised crust observed southwards in other sections at the lowest part of the Ash Shiqqah Member (Figure 7).

Based on the above considerations, the four subunits of Manivit et al. (1986) are here interpreted (Figures 14 and 16) from base to top:

  • (1) Unayzah Formation: estimated as 23 m (75.4 ft) green and purplish-red very pedogenetised claystone; only the top 2 m (6.6 ft) of the claystone are visible in outcrop. The remainder of subunit 1 was observed over a thickness of 20 m (65.6 ft) in trenches dug to channel water (Figure 17). The claystone overlies very pedogenetised sandstone. The top part of the claystone is very laminated and has produced a rich flora (Unayzah Flora, described by El-Khayal et al., 1980; Lemoigne, 1981a, 1981b; El-Khayal and Wagner, 1983, 1985; Hill and El-Khayal, 1983; Broutin et al., 1995; and see section “Fossils and Age of the Khuff Formation”);

  • (2) Unayzah Formation: 4.5 m (14.8 ft) conglomeratic sandstone and medium-grained, slightly consolidated sandstone, channelled into subunit 1;

  • (3) Ash Shiqqah Member: 32 m (105 ft) alternating gray claystone with gypsiferous intervals and green dolomitic claystone; the upper part displays intervals of fine-grained sandstone locally interpreted as tidal channels, and containing ligneous debris and ripple marks (Figure 18);

  • (4) Ash Shiqqah Member: 10.5 m (34.4 ft) carbonate assemblage of beige laminated (possible algal films) dolomite and bioturbated and bioclastic clayey dolomite.

In this section, the lateral variations of the members of the Khuff Formation are described. The order of presentation proceeds member-by-member, and northwards from the type section in the Ad Dawadimi quadrangle (Figure 1), and then southwards (Figure 5).

Ash Shiqqah Member: Northwards of the Type Section

To the north, the Ash Shiqqah Member progressively increases in thickness. In the Al Faydah quadrangle (Vaslet et al., 1985a), the member is 47 m (154.2 ft) thick at 25°20’N (Safra as Sark-Al Faydah; Figures 5, 9 and 19). The lower half of the member comprises predominantly green silty claystone with secondary gypsum, but also contains dolomite beds and 5 m (16.4 ft) above the base, dark-gray fine-grained bedded sandstone with ripple marks. The upper half of the member is more calcareous, consisting in clayey algal laminated dolomite with fenestrate structures, finely bioclastic dolomite, and rare sandstone beds.

In the Buraydah quadrangle (Manivit et al., 1986), the Ash Shiqqah Member reaches a thickness of 42.5 m (139.4 ft) in the reference section at 26°05 (Unayzah, Figures 5, 1416). Further north, at 26°27’N, in the Buraydah Ash Shiqqah-At Tarafiyah section (Figure 5), Manivit et al. (1986) identified four subunits (37.5 m or 123 ft thick) that they considered to be the “Unayzah member”. Here subunit 1 (3.5 m or 11.5 ft thick), of which the basal contact with a ferruginous crust is visible, consists of reddish pedogenetised clay and sandstone. This subunit corresponds to the lower part (subunits 1–3) of the reference section near Unayzah town (Figures 5 and 16), and is here attributed to the Unayzah Formation (sensu subsurface). The Ash Shiqqah Member is limited to the subunits 2–4 of Manivit et al. (1986), consisting of:

  • (2) 12 m (39.4 ft) thick, yellow to green clay and yellow dolomite;

  • (3) 4 m (13.1 ft) thick microconglomeratic sandstone channel containing silicified tree trunks as much as 5 m (16.4 ft) long and aligned with the bedding;

  • (4) 15 m (49.2 ft) alternation of green clay and yellow manganese rich dolomite which is locally collapsed and presents intense solution breccias (Figure 20).

Further north at 26°52’N (Figure 1), subunits 1–4 are about 60 m (196.8 ft) thick (Manivit et al., 1986). Sandstone facies clearly predominate over clay facies in the first 52 m (170.6 ft), occurring either as bedded sandstone or as channels of coarse-grained and microconglomeratic sandstone containing fragments of silicified wood. Fishbone structures and inclined overturned beds are observed in the bedded sandstone. The upper part is, however, finer grained (silty and sandy). The carbonate sequence (8 m or 26.2 ft thick) of subunit 4 at the top, is similar to the section at Unayzah town, but is interbedded with a layer of brown conglomeratic sandstone (2 m or 6.6 ft thick).

North of 26°55’N (Figure 1), in the Qibah quadrangle (Robelin et al., 1994) and in the Baqa quadrangle (Vaslet et al., 1986), outcrops of the Ash Shiqqah Member are rare (Jal al Khuffiyat at 27°10’N; Al Khuwayr at 27°40’N), but the facies remain comparable to those observed at the latitude of Qusayba.

Ash Shiqqah Member: Southwards of the Type Section

Southwards from the type section, the clayey component of the Ash Shiqqah Member decreases abruptly, and is only 10 m (32.8 ft) thick at the latitude of Jabal Duhaysan (24°40’N; Figures 5 and 21). The clayey facies completely disappears at 24°27’N (Jabal ash Shuqran-Rijm ad Dahawi), where the member is only represented by 2 m (6.5 ft) of intraclastic dolomite alternating with sandstone beds and solution breccia horizons together with pseudomorphosed gypsum nodules (Figures 9 and 22). This thinning occurs over the Central Arabian Arch (Powers et al., 1966). South of the Arch, the member again increases in thickness and clayey facies.

In the Ad Dawadimi quadrangle (Delfour et al., 1982), between 24°25’N and 24°10’N (in the vicinity of the Central Arabian Arch), the thickness of the member is 1.5–3.25 m (4.9–10.6 ft). Here, the substratum displays complex horst and graben structures, variably covered by detrital deposits attributed to the underlying Unayzah Formation (coarse- and medium-grained bedded sandstone, locally bioturbated), pre-dating the lateritic episode. The Khnaigu’iyah area contains copper and zinc mineralisation (Le Nindre et al., 1990b), local copper occurrences being located in the Ash Shiqqah Member at As Sfayrat (24°12’N). In this locality, the member comprises:

  • (1) 2 m (6.5 ft) sandstone containing rounded fragments of malachite-bearing clayey tuffite that channels the underlying bedrock also composed of cupriferous tuffite;

  • (2) 0.5 m brown sandy dolomite;

  • (3) 0.75 m brown crystalline dolomite containing columnar stromatolites about a metre wide, capped by a layer showing mud cracks; these stromatolites are impregnated with copper mineralisation, and the dolomitic layers show marked Sr and Ba anomalies.

South of the Central Arabian Arch, the thickness of the Ash Shiqqah Member progressively increases to 10 m or more. In the Darma’ quadrangle (Manivit et al., 1985a), the member is 12 m (39.4) thick at Al Quway’iyah (24°07’N, Figure 5), beginning with fine-grained sandstone with a sparitic cement and ended with green silty claystone, clayey dolomite, and pitted sparry dolomite.

West of the town of Al Quway’iyah, Senalp and Al-Duaiji (1995) described a similar succession (8 m, 26.2 ft thick) to the one measured by Manivit et al. (1985a). The former authors considered this succession as the Unayzah Formation, and not the Ash Shiqqah Member. They attribute this interpretation to the detection of a red-coloured, oxidised, weathered zone at the top of the succession, below the Khuff limestone.

Also at the Quway’iyah area, Cole et al. (2000) describe a well-exposed section from basement to the uppermost part of the Huqayl Member in road cuts on the Riyadh-Mecca highway. Its basal clastic succession consists of well-developed, trough cross-bedded, red and green sandstone interpreted as point-bar deposits. The exposures oscillate between clastic and carbonate sedimentation. The overlying 3–5 m (9.8–16.4 ft) of cross-bedded, red and green feldspathic sandstones, coarse clastics, and marls contain interbedded dense, dolomitic mudstones 10 cm thick. Large salt hoppers are present in the marls.

Located in the Darma’ quadrangle, the SHD-1 well (Figure 1; 24°13’40”N, 45°37’30”E; Manivit et al., 1985a; Le Nindre et al., 1990b) encountered the five Khuff members proposed by Delfour et al. (1982). The Khuff Formation in this well is 272.1 m (892.4 ft) thick (encountered between 413.0 m and 685.1 m), and overlies the Proterozoic granodiorite basement (Figure 23). In this completely cored well, the Ash Shiqqah Member is 28 m (91.8 ft) thick and encountered between 657.0 m and 685.1 m. In this well the member is thicker than in nearby outcrops, some 40 km to the west and this may in part be due to the presence of undissolved evaporites (Manivit et al., 1985a). The member consists from base up of:

  • (1) 1 m (3.3 ft) conglomerate and coarse sandstone in an argillaceous matrix, sometimes carbonaceous, with pyritic nodules and fine ligneous debris;

  • (2) 10 m (32.8 ft) argillaceous dolomite with anhydrite nodules;

  • (3) 7 m (23 ft) of massive anhydrite;

  • (4) 10 m (32.8 ft) of emerald-green, dolomitic shale with fine beds of dolomite and anhydrite.

In the Wadi Ar Rayn quadrangle (Vaslet et al., 1983), the Ash Shiqqah Member is composed mainly of emerald-green silty, gypsiferous claystone (Vaslet et al., 1983). The total thickness ranges from 11–55 m (39.1–180.4 ft) depending on the development of the clay layers (Figures 1 and 5). The basal part of the member is 1–2 m (3.3–6.6 ft) thick, and consists of a sandstone bed. In the south of the quadrangle it yielded silicified fossil wood debris and has the characteristics of reworked granitic sand in a fluviatile to littoral margin environment. In this southern area (Ayn al Minjur), a 3 to 4 m thick massive gypsum bed (?anhydrite at depth), occurs in the lower part of the gypsiferous claystone unit of the Ash Shiqqah Member (Figure 24). Locally, yellowish or gray, fine-grained dolomite occurs in rare layers in the lower third of the Ash Shiqqah Member, but the upper third of the member is dolomitic with cream coloured, dolomite; and gray, very bioturbated, granular dolomite. The top of the member is stromatolitic in the north and center of the quadrangle, and oncolitic in the south. In the southern area of Wadi Ar Rayn quadrangle, the upper part of the Ash Shiqqah Member is deformed by intense collapse structures and sliding.

Also in the Wadi Ar Rayn quadrangle (Figure 1), Cole et al. (2000) measured a section attributed to the Khuff Formation at 23°33’33”N and 45°33’14”E that overlies the weathered basement. The lowermost 38-m-thick (124.6 ft) unit consists of a thin (some inches to one foot thick, about several cm to 30 cm) basal conglomeratic sandstone overlain by green gypsiferous shales, and dolomite. At the nearby Gazelle Hill, the basal unit consists of 25–50 cm (about 1–2 ft) of red, poorly sorted, feldspathic, trough cross-bedded sandstone and coarse clastics that overlies a regolith of highly weathered granitic basement. These clastics are succeeded abruptly by 3–5 m (9.8–16.4 ft) of red and green marls overlain by 3 m (9.8 ft) of interbedded thin sandstones and sandy dolomites followed by 10 m (32.8 ft) of finely laminated dolomites.

Located in the Wadi Ar Rayn quadrangle, the Wadi Birk-2 well (Figures 1 and 7; 23°14’7”N, 45°47’13”E) was drilled directly in the Khuff outcrop. In this well, a section of about 21 m (68.9 ft) of clastics was encountered above the granitic Proterozoic basement. This section is referred to as the basal Khuff clastics, and from base up it consists of (Cole et al., 2000):

  • (1) 5 ft (1.52 m; 570–565 ft) red siltstone;

  • (2) 32 ft (9.8 m; 565–533 ft) green and black shales with a one foot sandstone stringer between 565–564 ft, and a 5 ft sandstone stringer between 562–537 ft;

  • (3) 2 ft (0.61 m; 533–531 ft) friable red claystone;

  • (4) 20 ft (6.1 m; 531–511 ft) mottled dolomudstone;

  • (5) 10 ft (3 m; 511–501 ft) green and black shales.

Above the clastic section another 10 m (32.8 ft) of bioclastic dolomites was encountered below an anhydrite bed, about 8 m (26.2 ft) thick, attributed to the Khuff-D Anhydrite (Ferguson and Chambers, 1991).

In the Wadi al Mulayh quadrangle (Figure 25; Manivit et al., 1985b) the Ash Shiqqah Member increases in thickness to 36 m (118.1 ft). This member was not recognised in the study by Al-Aswad (1997; Figure 26). Close to the Wadi al Mulayh (22°25’N), the lithology remains similar, with white sandstones with rare crustacean thoracic shields remains and abundant plant matter (Jabal Umm Mus’ham in Figure 5), and emerald-green claystone at the base (5-7 m, 16.4-52.5 ft) in which occur metre-thick beds of massive gypsum (anhydrite at depth?), and a succession of clayey or bioclastic dolomites, bioturbated and rarely stromatolitic (22°50’N).

The Ash Shiqqah Member is unknown in outcrop south of 22°10’N.

Huqayl Member: Northwards of the Type Section

In the Al Faydah and Buraydah quadrangles (Figures 1 and 5), the Huqayl Member maintains relatively homogenous lithology and thickness (33–51 m or 108.2–167.3 ft) and its division into the lower and upper units. Some minor changes of facies however do occur. For example, oncolites locally replace the oolites at the base of subunit 1. At 26°05’N (Unayzah town, Figure 27), traces of pedogenesis in claystone of subunit 2 are observed together with extensive solution breccias which yielded, locally, south of Unayzah town, some pinkish gypsum nodules of pseudomorphosed anhydrite. North of 26°05’N, subunit 3 contains local algal laminites, and the upper Huqayl Member contains a terrigenous fraction in the dolomites. Parts of the member (mainly subunits 3 and 6) are collapsed between 25°20’N (Al Faydah) and 26°25’N (Buraydah).

North of 26°40’N, in the Baqa quadrangle, the Huqayl Member is masked by surficial deposits. Only gray clayey dolomites crop out in disaggregated beds at 27°15’N (Shari) and east of Al Khuwayr (27°35’N).

Huqayl Member: Southwards of the Type Section

To the south in the Darma’, Wadi Ar Rayn, and Wadi Al Mulayh quadrangles, the HuqayI Member is relatively constant in thickness (30–35 m, 98.4–114.8 ft) as far as 22°N. The lithologies are also very homogeneous, and the lower and upper parts of the member are everywhere clearly distinguished.

Variations in lithologies are minor: the Lower Huqayl becomes enriched in peloids, and the base (subunit 1) displays cross-bedding and reworked mud galls toward 23°40’N (Ar Rayn), with oncolitic bioclasts and algae toward 23°10’N. In the vicinity of Wadi Al Mulayh (22°22’N), subunit 4 at the base of Upper Huqayl, contains an accumulation of Gymnocodiaceae (red algae) layers, overlain by an assemblage of cream-colored, in place gypsiferous, dolomicrite containing algal layers.

In the Sulayyimah quadrangle (Vaslet et al., 1985b), at 21°55’N, an outcrop of clayey gypsiferous dolomite and brown fine-grained dolomite containing peloids, lithoclasts, and algal films is also assigned to the Huqayl Member.

In the SHD-1 well (Figure 23), the Huqayl Member is 54 m (177.1 ft) thick and consists, as in outcrops, of two units (Manivit et al., 1983, 1985a; Le Nindre et al., 1990b). From the base, the lower unit consists of dolomite with bioclasts, dolomitic claystone and anhydrite, and finegrained dolomite and massive anhydrite. The upper unit starts with intraclastic and bioclastic dolomite, argillaceous dolomite, followed by nodular dolomite with massive anhydrite.

Duhaysan Member: Northwards of the Type Section

The thickness of the Duhaysan Member varies from 25.5 m (83.6 ft) in the Al Faydah quadrangle, to 28 m (91.8 ft) in the Buraydah quadrangle (Figure 5). In many localities, the member is marked at the base (subunit 1) by a metre-thick layer of reworked lithobioclastic calcarenite (Figure 19). This layer is overlain by layers of laminated bioclastic peloidal limestone rich in marine fauna and peloidal calcarenite alternating with bioturbated clayey dolomite. At 25°55’N (Midhnab), bioclastic calcarenite forms tidal channels in subunit 3.

The Duhaysan Member is unknown in outcrop north of 26°40’N (north of Buraydah).

Duhaysan Member: Southwards of the Type Section

The Duhaysan Member is only 6 m (19.8 ft) thick between 24°30’N and 24°N over the Central Arabian Arch, in the Ad Dawadimi and Darma’ quadrangles (Figure 5), where it appears to be mainly carbonate (bioclastic, dolomitic, and peloidal limestones; clayey limestone incorporating bioclastic dolomitic horizons). A number of benches of limestones contain marine fossils including conic-shaped fossils attributed to Bactritidae (Manivit et al., 1990).

In the SHD-1 well, the Duhaysan Member is 41 m (134.5 ft) (Figures 1 and 23; Manivit et al., 1985a). It comprises thin bioclastic limestone (3.5 m, 11.5 ft) that is overlain by marly dolomite interbedded with massive anhydrite beds.

In the Wadi Ar Rayn quadrangle (Vaslet et al., 1983), the member becomes thicker southwards, reaching 18 m (59 ft) at 23°N (Wadi Ar Rayn), where it is still essentially carbonate. The intraclastic and bioclastic dolomites, at the base, display cross-bedding; peloidal bioclastic dolomite dominates, and the succession ends with laminated, variably dolomitic gypsiferous claystone. The upper part of the member incorporates a layer of limonitic dolomitic sandstone south of 23°20’N.

The thickness of the member then decreases southwards in the Wadi Al Mulayh quadrangle, being only 6 m (19.7 ft) thick at 22°10’N. With the exception of the reworked layer at the base, the Duhaysan Member also becomes predominantly clayey and rich in gypsum, comprising gray-dolomitic and laminated-gypsiferous claystone.

Farther south, in the Sulayyimah quadrangle only one outcrop is attributed to the Duhaysan Member by Vaslet et al. (1985b) at 21°50’N. It comprises gray limestone containing rare phosphatic bioclasts and some rounded micritic lithoclasts.

Midhnab Member: Northwards of the Type Section

A section measured in the Al Faydah quadrangle (Vaslet et al., 1985a) at 25°55’N about 5 km north of Midhnab (Figure 5), enables comparison of the type section described at Khuff with facies in the area where the Midhnab shale was defined by Powers (1968). The member is 70 m (229.6 ft) thick, and the facies are slightly more clayey than in the type section. From bottom to top, it comprises:

  • (1) 2 m (6.6 ft) conglomeratic limestone containing pebbles of dolomite and abundant bioclasts (bed-on-bed reworking) (Figure 28);

  • (2) 16 m (52.5 ft) alternating bluish bioclastic platy limestone, rich in marine fauna (Figure 29), and yellow clayey limestone containing several layers of bioclastic, locally oolitic, cherty calcarenite;

  • (3) 45 m (147.6 ft) yellow dolomitic gypsiferous claystone, incorporating beds of clayey dolomite, bioclastic dolomite containing chert nodules, and sandy layers less than one metre thick, showing inclined bedding toward the top;

  • (4) 7 m of calcareous claystone and green claystone as well as sandy channels containing plant debris (Midhnab flora of Jal al Khartam, Vaslet et al., 1985a; Broutin et al., 2002) and floated tree trunks in the topmost part (Figure 30).

Further north, in the Buraydah quadrangle (Manivit et al., 1986), the facies of the Midhnab Member remain calcareous and clayey as far as 26°05’N (Jal Khartam) where the thickness is 72 m (236.2 ft). The topmost plant-bearing clayey facies (subunit 4), is locally eroded by megachannels of sandstone containing claystone galls, plant remains and silicified tree trunks (Figure 31).

At 26°30’N, along Jal al Khartam escarpment, the topmost clayey and sandy parts of the Midhnab Member contain the charophyte deposit described by Hill and El-Khayal (1983) in the upper part of the Khuff Formation.

The northernmost outcrop of the Midhnab Member is located at 26°55’N (An Naghabiq), where only the uppermost 8 m (26.2 ft) is visible. These comprise fine-grained sandstone (4 m, 13.1 ft) with inclined bedding, arranged in large festoon structures indicating a southeastward flow direction. The megachannels cut into green and blue-gray claystone (4 m, 13.1 ft) containing oxidised plant debris at the top.

Midhnab Member: Southwards of the Type Section

In the southern Ad Dawadimi quadrangle (Delfour et al., 1982), the Midhnab Member is 47 m thick at Rijm ad Dahawi (24°26’N) and further south in the Darma’ quadrangle (Manivit et al., 1983) is estimated to reach 76 m (249.3 ft) with comparable facies to the type section.

In the SHD-1 well, the Midhnab Member is 88 m (288.6 ft) thick (Manivit et al., 1983; 1985a), and consists from base up:

  • (1) 2 m (6.6 ft) bioclastic and lithoclastic calcarenite;

  • (2) 18 m (59 ft) dolomite and gray and black dolomitic claystone, with at the top finegrained peloidal calcarenite;

  • (3) 53 m (173.8 ft) of continuous dark gray dolomite and argillaceous dolomite with nodules of anhydrite (Figure 32) and gypsum; and 5 m (16.4 ft) fine dolomite with stylolitic joints;

  • (4) 11 m (36.1 ft) bioclastic, fine-grained calcarenite with gray claystone at the top.

The Midhnab Member becomes thicker in the Wadi Ar Rayn quadrangle (Vaslet et al., 1983), and is 92 m (301.8 ft) at 23°35’N. The lower 30 m (98.4 ft) is carbonate, comprising bivalve coquina and intraclastic limestone interspersed with gray dolomite, clayey dolomite, and laminated algal? dolomite. The upper 62 m (203.4 ft) is clayey and gypsiferous, and intercalated with thin layers of bioclastic dolomite, commonly very bioturbated.

In the Wadi Al Mulayh quadrangle (Manivit et al., 1985b) at 22°47’N (Jabal Umm Mus’ham), the carbonate unit at the base is 35 m (114.8 ft) thick, the facies remaining the same. It is overlain by 30 m (98.4 ft) of dolomitic and gypsiferous claystone containing beds of clayey or bioclastic dolomite. The succession is capped by gray vermiculate micritic limestone of lacustrine origin.

At 22°11’N (Urayq al Munsaraqah), the thickness of the Midhnab Member is reduced to 30 m (98.4ft). The clayey upper unit is only 22 m (72.1 ft) thick, and the top comprises beds of carbonate sandstone and siltite. Farther south, the few rare Midhnab Member outcrops in the Sulayyimah quadrangle (around 21°30’N) consist of yellow, red, and green, gypsiferous dolomitic claystone with rare beds of white or pink bedded dolomite.

Khartam Member: Northwards of the Type Section

In the Al Faydah quadrangle (Vaslet et al., 1985a), the Khartam Member becomes progressively thicker (Figure 5). At Khashm Khartam (25°57’N), the total thickness is 50 m (164 ft) and the Lower Khartam is comprised from bottom to top:

  • (1) 10.5 m (34.4 ft) yellow dolomite overlain by green or dark-red gypsiferous claystone intersected in the middle by platy, and clayey limestone;

  • (2) 3 m (9.8 ft) microsparitic limestone and silty claystone. The upper Khartam is more massive and forms the carbonate frame of the cuesta of Jal al Khartam, and it consists of:

  • (3) 14 m (45.9 ft) cream oolitic and peloidal limestone in massive beds with numerous pronounced stylolitic joints; and local stromatolitic patches;

  • (4) 13 m (42.6 ft) bioclastic (coquina of bivalves) dolomitic limestone containing interbeds of clayey platy limestone and sparitised dolarenite (reddened surfaces);

  • (5) 9.5 m (31.2 ft) cream oolitic and bioclastic limestone, and gray dolomite with clayey interbeds, and capped by gray/red bioclastic dolosparites.

Further north, in the Buraydah quadrangle (Manivit et al., 1986), the Khartam Member is uniform in thickness as far as 26°15’N; it then thins to 36 m north of Wadi ar Rimah (Jal al Aswad). The lower clayey and dolomitic unit contains rare sandy beds, and from 26°05’N (Jal al Khartam) the upper unit begins with quartz dolomite and fine-grained sandstone in massive beds with clayey interbeds and sand-ball structures (Figure 31). Cross-bedding is observed in the overlying oolitic facies, the oolites being commonly weathered out in outcrop. Stromatolites patches are located from place to place in subunit 4 of the upper Khartam. At 26°30’ N, the uppermost beds contain oncolites.

The northernmost outcrop identified as the Khartam Member is at 26°55’N (An Naghabiq), where the Lower Khartam (3.5 m, 11.5 ft) consists of green and red very silty claystone with exsudated gypsum; only the base (8 m, 26.2 ft) of the upper Khartam is visible, comprising cream oolitic limestone with inclined bedding and rare white clayey intercalations.

Khartam Member: Southwards of the Type Section

In the Darma’ quadrangle (Manivit et al., 1985a), the Khartam Member increases slightly in thickness to about 60 m (196.8 ft) at Jabal Sufah (24°20’N) where the base is not seen in outcrop. The facies remain comparable to those in the type section, although the carbonate facies (upper Khartam) are thicker (52 m, 170.6 ft). The upper part of the Lower Khartam consists of blue claystone containing ammonoids and of ochre bioclastic dolomite.

In the SHD-1 well, (Manivit et al., 1983, 1985a; Le Nindre et al., 1990b), the Khartam Member is 61 m (200.1 ft) thick, and consists mostly of carbonates (Figure 23). The Lower Khartam consists of 14 m (45.9 ft) of bioclastic arenitic calcarenite. The upper Khartam consists of 30 m (98.4 ft) of clayey dolomite locally bioclastic, followed by a basal oolitic dolomite with anhydrite nodules (1 m or 3.28 ft), followed by 16 m (54.4 ft) of bioclastic dolomite.

Above the Khartam Member, the SHD-1 well encountered an undated section considered as the Sudair Formation by Manivit et al. (1985a); or the Khuff and upper part and Sudair Shale Formation by Al-Jallal (1995). It consists from base up: (1) 18 m (59 ft) of interbedded gray gypsiferous clay, sulfated dolomite, sulfurous gypsum, and massive halite in 1-m-thick beds; (2) 57 m (187 ft) of essentially dolomitic limestone with sandy layers containing bioclasts and inclusions of spheroidal anhydrite; and (3) 236 m (774.1 ft) of variegated green, or brick red and purplish, silty claystone and gypsum.

Farther south, the thickness of the member decreases progressively to 58 m (190.3 ft) at Wadi Ar Rayn (23°40’N), where the Lower Khartam (20 m, 65.6 ft) is clayey and carbonate, with bioclastic limestone intercalated in bluish carbonate, gypsiferous claystone, and brown dolomite. South of 23°15’N, the member becomes enriched in terrigenous material, such as purple claystone and micaceous sandstone. The Upper Khartam (38 m, 124.6 ft) consists of oolitic limestone (calcitized dolarenite), friable and cross-bedded, overlain by coquina, beige peloidal limestone with clayey interbeds, and a topmost bed of bioclastic oolitic limestone (Al Huwwah section, Figure 5).

In the Wadi Al Mulayh quadrangle, at 22°47’N (Jabal Umm Mus’ham), the Khartam Member is only 37 m (121.4 ft) thick. The Lower Khartam (15 m, 49.2 ft), entirely dolomitic and variably clayey or bioclastic, is rich in algae. The upper Khartam (22 m, 72.2 ft) is of oolitic facies.

Although of limited extent, the best outcrops of the Khuff Formation between 22°N and 21°N in the Sulayyimah quadrangle (Vaslet et al., 1985b), are those of the Khartam Member. Towards 21°55’N, in addition to carbonate oolites, ferruginous oolites and coquina facies occur. Towards 21°25’N, the member begins with a reworked layer of lithobioclastic calcarenite over the gypsiferous claystone of the Midhnab Member. It comprises bluish fine-grained laminated limestone, and ends with a gypsiferous clayey-dolomite unit.

Undifferentiated Khuff Formation, South of Latitude 21°N

In the Wadi Tathlith quadrangle (Figure 1), the Khuff Formation overlies the Juwayl Member of the Wajid Formation (Kellogg et al., 1986). Evans et al. (1991) correlated the Juwayl Member to the Unayzah Formation of subsurface (e.g. Ferguson and Chambers, 1991), and the Al Khlata Formation of Oman (Hughes Clark, 1988). Kellogg et al. (1986) noted that the Khuff Formation begins with 20 m (65.6 ft) of pale- to mouse-gray, fine-grained dolomite, poorly stratified and in places brecciated in appearance. This basal dolomite contains numerous diffuse occurrences of chert together with thin beds of pinkish, fine-grained sandstone, but contains no fossils.

A Khuff sequence that is several metres thick and probably fairly close to the unexposed contact with the underlying pre-Khuff strata, occurs at 20°49’20”N and 44°55’00”E. The lowest rocks consist of one metre (3.3 ft) of white marl, the base of which is not visible; overlain by a metre-thick layer of white dolomicrite containing grains of silty quartz. This sequence is succeeded upwards by 3 m (9.8 ft) of beige or pinkish bioclastic dolomicrite containing bivalve debris, algal fragments, rare foraminifers, and one cephalopod (orthoceratid), lithoclasts of dolomite fragments, and small grains of silty quartz. Overlying the dolomite is at least 70 cm (2 ft) of regularly bedded, fine-grained, feldspathic quartzarenite.

To the south of ‘Irq al Wadi (20°06’30”N, 44°56’15”E), silicified tree trunks are exposed in an area mostly covered by superficial deposits.

The contact with the Early Triassic Lower Sudair shale is not exposed.

Located south of the Sulayyimah quadrangle, the Sulayyil-1 well (Figures 1 and 33; 20°27’40N, 45°34’32E; also Sulayyil D-1 inEvans et al., 1991; or DEW-Su, unpublished Italconsult report, 1969, inAl-Laboun, 1993), encountered the Khuff Formation. Italconsult divided this formation from base up into three members (Figure 33): (1) Ruwayha; (2) Dawasir; and (3) Kumdah.

The Ruwayha member unconformably overlies the Faw formation. Based on regional considerations, the Faw formation probably corresponds to : (1) Juwayl Member of the Wajid Formation of Kellogg et al. (1986); (2) subsurface Unayzah Formation in Evans et al. (1991); or/and (3) Ash Shiqqah Member of the Khuff Formation.

The lower Ruwayha member is about 50 m (164 ft) thick. Near its base it consists of cream, microcrystalline dolomite. The remaining Ruwayha section is described as interbedded, light red and white, fine- to coarse-grained, poorly sorted, friable, sometimes dolomitic cemented sandstone, argillaceous siltstone and red to varicolored shale. Frequent chert is found in sandy-silty, dolomitic, cemented beds in the Ruwayha member.

The Dawasir member is about 150 m (492 ft) thick and consists mostly of gypsiferous or anhydritic, dolomitic shale, except for a 20-m-thick (65.6 ft) dolomite bed in its middle part. The Kumdah member is about 50 m (164 ft) thick and, near its base it consists of sandstone with silicified wood trunks.

Summary of the Lithostratigraphy of the Khuff Formation

The Khuff Formation outcrops in central Saudi Arabia comprise five members. The Ash Shiqqah Member (formerly Unayzah member), consists of terrigenous sediments, generally fine-grained (claystone), interbedded with sandstone intervals, and give way upwards to bioclastic clayey dolomite. The greatest thicknesses are recorded to the north and south of the Central Arabian Arch (Khnaigu’iyah palaeohigh) a persistent relief of central Arabia at the beginning of the Khuff deposition. This palaeohigh is overlain by the stromatolite-bearing dolomite layers at the top of the Ash Shiqqah Member.

The lithology of the Huqayl Member is fairly homogeneous from north to south. The constant facies and thickness encountered are remarkable when compared to the variations in thickness observed in the underlying Ash Shiqqah Member. Two sequential units (calcarenite and gypsiferous claystone with subsurface evaporite) are clearly distinguished, marking the cyclic character of these deposits. The presence of marine fauna in calcarenite layers from the base of the lower unit, together with their extensive distribution, is indicative of the transgressive marine character of this member.

The Duhaysan Member contains the first truly calcareous or calcareous-dolomitic facies of the Khuff Formation. The base is marked by bed-on-bed reworking, accompanied by numerous marine fossils. Carbonate facies persist throughout the greater part of the member, giving way locally (southwards) to increasingly clayey facies. As in the case of the HuqayI Member, the effect of the Central Arabian Arch was a reduction in thickness, accompanied by a decrease in the claystone fraction throughout the middle outcrop sector. The terrigenous fraction of the Duhaysan Member is very weak.

The Midhnab Member is easily distinguishable in outcrop from north to south, and displays a rather homogenous vertical distribution of facies. It has been mapped continuously, for almost 700 km between 27°N and 21°N. All the sections yield at the base reworked calcareous or calcareous-dolomitic facies. These facies are accompanied by numerous marine fossils, and overlain by a fossiliferous carbonate succession evolving toward gypsiferous and dolomitic claystone facies. The upper part of the member locally displays plant bearing claystone, sandstone channels containing fossilised driftwood, or lacustrine limestone.

Over a total outcrop length of 700 km, the Khartam Member shows remarkably constant, thick oolite and peloid and bioclastic facies. The Lower Khartam Member is slightly clayey and dolomitic or sandy at the base, and overlies the detrital, gypsiferous, or lacustrine facies at the top of the Midhnab Member. The Upper Khartam Member represents a return to marine carbonate facies.

In this section, the stages of the proposed global Permian System are tentatively adopted (Jin et al., 1997; Menning, 2001a, b; Gradstein et al., 2004). Where a stage is cited by an author, the approximate equivalent global stage or epoch (Late, Middle, Early) is added in parenthesis (Figure 2).

The biostratigraphic framework of the Khuff Formation, in central Saudi Arabia, is mainly based on small benthic foraminifers, algae and ostracods that were observed in the collected samples. These are the only organisms that establish a sufficiently detailed biostratigraphic framework. The following data pertaining to algae, foraminifers and ostracods represent a complete revision based on the ongoing systematic description by D. Vachard and J. Gaillot (Vachard et al., 2003, 2005) on foraminifers, and by S. Crasquin-Soleau on ostracods (Crasquin-Soleau et al., 2004, 2006). These data were acquired previously during the DMMR and BRGM mapping project of the Saudi Arabian outcrops (Figure 1), and recently in further studies of the Khuff Formation outcrops in central Arabia (D. Vaslet, J. Broutin and M. Halawani in 1999; D. Vaslet and Y.M. Le Nindre in 2002).

Other marine fossils are locally relatively abundant although of no stratigraphic value in dating the units of the Khuff Formation. They are, however, important for palaeoenvironmental reconstructions and sequence stratigraphic interpretations. These fossils include bivalves, nautiloids (Chirat et al., 2006), unidentified bactridids, orthocones, ammonoids, echinoderms, bryozoans, and gastropods. Rare brachiopods were recently discovered in the lower part of the Midhnab Member (Angiolini et al., 2006). Despite intensive searching, only a single specimen of a conodont has been recently discovered (Nicora et al., 2006) in a reworked calcarenite at the lower part of the Midhnab Member in Darma’ quadrangle. Abundant microscopic fish teeth are found in the Khuff carbonates, but at the present time they have no stratigraphic significance.

The Khuff and pre-Khuff macroflora has important palaeobiogeographic significance but no chronostratigraphic value. The Unayzah Flora (El-Khayal, 1980; El-Khayal and Wagner, 1983, 1985; Hill and El-Khayal, 1983; Broutin et al., 1995) found in the Unayzah Formation, below the Ash Shiqqah Member (Figures 5, 14–16), and the Midhnab Flora (Hill and El-Khayal, 1983; Vaslet et al., 1985a; Broutin et al., 2002; Berthelin, 2002; Berthelin et al., 2006) in the upper part of the Midhnab Member (Figures 5, 14 and 19), do not provide precise ages. Most Khuff outcrops are too weathered to preserve microflora, and this type of data originated mainly from samples taken from boreholes.

Ash Shiqqah Member (formerly Unayzah member) of the Khuff Formation: ?Middle Permian Epoch, ?Capitanian Stage

The definitions and ages of the lithostratigraphic units within the Ash Shiqqah reference section, in the Unayzah locality, has been the subject of controversy during the past two decades (Figures 1, 14–16). This is mainly due to the lack of reliable chronostratigraphic controls based on: (1) plant remains in continental deposits of the lower part (Unayzah Formation); and (2) rare benthic foraminifers in the upper shallow-marine part (Ash Shiqqah Member of the Khuff Formation). In the following discussion, the fossils and age interpretations of Unayzah Formation and Ash Shiqqah Member are reviewed together.

In the Buraydah quadrangle, the Unayzah Flora from the clastic unit just below the Pre-Khuff Unconformity (Figures 5, 14 and 16), was tentatively dated as ?Late Carboniferous-?Early Permian by El-Khayal et al. (1980). Al-Laboun (1982, 1987) proposed this stratigraphic interval as the lower part of the Late Carboniferous-Early Permian Unayzah Formation (Figure 16). Later studies at the same Unayzah locality, based on additional plant material by Lemoigne (1981a, b) and El-Khayal and Wagner (1983, 1985), indicated that the age of the Unayzah Flora is probably Middle Permian, Murgabian (Wordian) (Lemoigne, 1981a, b; Broutin et al., 1995).

According to Broutin et al. (1995), the Unayzah Flora is a true Cathaysian-Euramerican mixed flora including important Cathaysian forms such as Lobatannularia lingulata Halle, L. cf. heianensis Kodaira, Fascipteris hallei (Kawasaki) Lee et al., Gigantonoclea sp., Pecopteris phegopteroides Feistmantel, Qasimia schyfsmae (Lemoigne) Wagner and El-Khayal and associated with “northern elements” mainly belonging to Filicophyta and Cordaites. According to Broutin et al. (1995) the Dadoxylon spp., figured by Lemoigne (1981b), corresponds clearly to the Euramerican Araucarioxylon genus. Both macro- and microfloral data (Fauconnier inLe Nindre et al., 1990b) indicate a late Murgabian (Wordian) age. More recently, however, J. Broutin (written communication, 2002), reviewed all the former studies and concluded that Middle Permian, Kubergandian (Roadian) and/or Murgabian (Wordian) ages are both possible for the Unayzah Flora.

The Ash Shiqqah Member of the Khuff Formation is poorly dated. In the Buraydah quadrangle (Manivit et al., 1986), the carbonate subunit at the top of this member contain foraminifers, including Paradagmarita flabelliformis Zaninetti et al. This species was not encountered in our new investigation in the Huqayl Member. It can be confused with Sengoerina argandi Altiner, from the Midian (Capitanian) of northeastern Turkey (Altiner, 1999). Taxa of weak biostratigraphic value were identified in our samples: Permocalculus sp., Globivalvulina sp., Nankinella sp., “Glomospira” sp., Cornuspira sp., Hemigordius sp., Pachyphloia sp.

Near the Ar Rayn town locality (Figure 1), Vaslet et al. (1983) noted that the Ash Shiqqah Member is poor in benthic foraminifers (rare Ammodiscus sp., Hemigordius sp. and Glomospira sp.), in gymnocodiacean algae Permocalculus sp., and in microproblematica Aeolisaccus sp. Among the bioclasts are bivalves, gastropods, and rare phosphatic fish debris. A ?late Middle or early Late Permian age was assigned by Vaslet et al. (1983).

Near Wadi Al Mulayh (Figure 1), Manivit et al. (1985b) noted in the lowest sandstone unit of the member, some thoracic shields of palaeocrustaceans identified as Tropidocaris sp., bearing no age value. In the well SHD-1 (Manivit et al., 1985a), the top of the Ash Shiqqah Member is characterised by the appearance of abundant and varied (15 species) monosaccate and bivesiculate pollen, marking a renewal of the microflora.

According to G.W. Hughes (written communication, 2002) the Ash Shiqqah Member may pass in part to the Khuff-D Member in subsurface (Figure 9). Cuttings from the Khuff-D contain the foraminifers genus Hemigordiopsis dated as Middle Permian (late Capitanian) by Nestell (written communication to G.W. Hughes), but with an early Midian (early Capitanian) acme (Vachard et al., 2002, 2003). Fusulinids Reichelina sp. and Monodiexodina kattaensis (Schwager) recovered from the

Khuff-D Member in the Butabil well, located in Saudi Arabia near the Oman border, confirm this Midian age, although Nestell interpreted Reichelina spp. as late Capitanian to Dorashamian (Changhsingian) and Monodiexodina kattaensis as early Wordian. In fact, Reichelina is known from the base of the Midian (Leven, 1993), and M. kattaensis is associated with a Codonofusiella in the Pakistani Amb Formation (Mertmann and Sarfraz, 2000), which is Wordian in age according to conodonts (Wardlaw and Pogue, 1995; and L. Angiolini, personal communication, 2004) and to foraminifers (Vachard et al., 2002).

It therefore seems likely that the Ash Shiqqah Member (and the subsurface Khuff-D Member), is Capitanian (Midian, and probably early Midian). The Late Permian Wuchiapingian (early Dzhulfian) age, suggested by Vachard et al. (2002), seems unlikely due to the local presence of Monodiexodina kattaensis, compared to the complete absence of keriothecal fusulinids in the Late Permian (Sheng, 1992).

Huqayl Member: Late Permian Epoch, ?Wuchiapingian Stage

The local appearances of new taxa or reappearances of Lazarus-taxa that are characteristic of the Huqayl Member (Figure 34) are Pseudomidiella cf. labensis Pronina-Nestell in Pronina-Nestell and Nestell, Earlandia? sp. and Neodiscus aff. qinglongensis Wang. Neodiscus is not significant since it appears in the Early Permian (Grozdilova, 1956). Earlandia?/Aeolisaccus are relatively common during the Changhsingian in Hungary (Berczi-Makk et al., 1995), and seem to reappear in the early Dzhulfian of Greece (L. Angiolini, personal communication, 2004; D. Vachard, personal communication, 2004). Although recently described, the genus Pseudomidiella might be a good marker of the late Changhsingian in the northwest Caucasus and in the Himalaya (Pronina-Nestell and Nestell, 2001). However, Pseudomidiella also appears in the early Wuchiapingian (early Dzhulfian) (sample GL252 of Hydra Island, Greece; D. Vachard, personal communication, 2004). Based on these considerations, the Huqayl Member may be Wuchiapingian (Dzhulfian) as suggested in Figure 34.

In the type section, the Huqayl Member sometimes contains algae Gymnocodium bellerophontis (Rothpletz) Pia and Permocalculus fragilis (Pia) Elliot, that reveal a Middle to Late Permian age (A. Roux, unpublished report inDelfour et al., 1982). Nevertheless, only Permocalculus were observed in our material, with the species P. solidus (Pia), P. tenellus (Pia) and P. digitatus Elliot. The common presence of Earlandia?, Permocalculus, and Neodiscus, in the Huqayl Member indicates high salinity, and emphasises the biologic confinement of this member (D. Vachard, oral communication in Vaslet et al., 1985; Vachard et al., 2003). A cone-shaped marine fossil identified as bactritid, was also found in the upper subunit of the member, together with brachiopod debris, gastropods and bivalves.

To the north, in the Midhnab section (Vaslet et al., 1985), the upper part of the member (subunit 5) yielded Robuloides cf. gourisiensis Reichel, a genus and species characteristic of the Dzhulfian-Dorashamian (Wuchiapingian-Changhsingian), accompanied by Globivalvulina cf. graeca Reichel, “Glomospira” sp., and Nankinella sp., the stratigraphic range of which is from the early Murgabian to the Dzhulfian-Dorashamian (early Wordian to Changhsingian).

In the Unayzah and Buraydah areas (Figure 1), the Lower Huqayl Member contains ostracods, calcareous algae, molluscs, and bryozoans and (in the Unayzah and Buraydah sections, subunit 1) the foraminifers Pseudomidiella cf. labensis. Another significant foraminifer, Paradagmarita flabelliformis Zaninetti et al., is located in this basal bench of the Huqayl Member in the Buraydah section (Manivit et al., 1986). Paradagmarita flabelliformis is late Dzhulfian (Wuchiapingian) to early Dorashamian (Changhsingian) in age (Altiner, 1981; Lys, 1984). The genus is only considered as early Dorashamian (Changhsingian) by Vachard et al. (2002), but is present up to the late Changhsingian with Paradagmarita sp. 1 (Pronina-Nestell and Nestell, 2001, plate 4, figure 12), and it can appear previously in the Dzhulfian (J. Gaillot, personal communication, 2004).

To the south, the Huqyal Member yielded limited biostratigraphic data. In the Wadi Al Mulayh section (Manivit et al., 1985b), the base of the Upper Huqayl (subunit 4) contains some foraminifers known from the entire Middle and Late Permian (e.g. Lys, 1984), including Lasiodiscus minor Reichel and L. tenuis Reichel, associated with a bactritids-bearing bed.

In the well SHD-1 (Manivit et al., 1985a), the same pollen microflora prevails between the base of the Huqayl and the top of the Midhnab members. At the base of the Huqayl Member, the occurrence of a microfloral association including Lueckisporites virkkiae Potonié and Klaus, Klausipollenites schaubergeri Potonié and Klaus, Kraeuselisporites cf. wargalensis Balme, and Potoniesporites novicus Bhardwaj as well as Cordaitina gunyalensis Pant and Srivastava in the Duhaysan and Midhnab members supports the Late Permian Wuchiapingian (Dzhulfian) age (D. Fauconnier inLe Nindre et al., 1990b). The association is congruent with the presence of the foraminifers Robuloides cf. gibbus Reichel from the base of the member (subunit 1) and Baisalina pulchra Reitlinger in subunit 2.

Duhaysan Member: Late Permian Epoch, Wuchiapingian or Changhsingian Stages

The biozonal characteristics of the Duhaysan Member are as follows: “Dentalina” hoi Trifonova, Hemigordius baoqingensis Wang in Zhao et al., Graecodiscus cf. kotlyarae Pronina-Nestell in Pronina-Nestell and Nestell, and Colaniella cf. minuta Okimura (Figure 34). Locally, “Dentalina” hoi is present up to the Triassic. Hemigordius baoqingensis and Graecodiscus cf. kotlyarae were described in the late Changhsingian, but the exact range is poorly known due to numerous convergences for the Miliolida during the Late Permian. The exact range of Colaniella is discussed by several authors. It is restricted to the Changhsingian (Zhao et al., 1981; Lin et al., 1990), or appears before, i.e. in Midian or Dzhulfian (Capitanian or Wuchiapingian) (Jenny-Deshusses and Baud, 1989; Mertmann and Sarfraz, 2000; Vachard et al., 2002, 2003).

The Duhaysan Member also contains the foraminiferal association of Lasiodiscus minor, Globivalvulina kantharensis Reichel, Hemigordius sp., Pachyphloia sp., and Robuloides sp., Paradagmarita monodi Lys and P. flabelliformis of Dzhulfian-Dorashamian (Wuchiapingian-Changhsingian) age. Nankinella sp. is reworked in the lowest bed of this member. Ostracods have recently been described in the middle and upper parts of the Duhaysan Member (Crasquin-Soleau et al., 2004, 2006) including: Knoxiella cf. infirma Shi, Sulcella sulcata Coryell and Sample, and Sargentina transita (Kozur). The latter two were described in the Wordian Khuff Formation of Oman (Crasquin-Soleau et al., 1999). The rest of the biozone bears no stratigraphic precision and is made of debris of algae (see Okla, 1992), gastropods (Bellerophon sp.), bryozoans, echinoderms, fish teeth, and small nautiloids of the genus tentatively attributed to Caelogastroceras aff. mexicanum (Girty) (written communication, J. Roger, 1985, inLe Nindre et al., 1990b).

In the well SHD-1 (Manivit et al., 1985a), the foraminifers (Geinitzina postcarbonica Spandel and Hemigordius permicus Grozdilova), and the microflora association (including five additional species compared to the Huqayl Member) suggest no precise age.

The Duhaysan Member, due to its position between the ?Wuchiapingian Huqayl Member and the Changhsingian Midhnab Member is here attributed to the Wuchiapingian or Changhsingian (Figure 34). In the middle part of the member, in the Buraydah area, the foraminifer Graecodiscus cf. kotlyarae is common as the whole porcelaneous foraminifers in the modern hypersaline environments. It is accompanied by rare very small, primitive Colaniella cf. minuta. The level might correspond to the base of the local Changhsingian.

Midhnab Member: Late Permian Epoch, Changhsingian Stage

The Midhnab Member is characterised in its lower part by the foraminifer Paradagmarita and in its upper part by a level with “Glomospirella spirillinoides” (Grozdilova and Glebovskaya) sensu Wang inZhao et al. (1981). This taxon of Miliolida must be created, because it is structurally different from the fusulinid Brunsia spirillinoides (Grozdilova and Glebovskaya), although perfectly homeomorph. The Midhnab Member is also characterised by successive disappearances: e.g. Globivalvulina spp. and Nankinella sp. (Figure 34). According to the foraminifer association, the Midhnab Member is tentatively attributed to the Changhsingian (Dorashamian) (Figure 35). The Midhnab Member also yielded an ostracod assemblage including Hollinella herrickana (Girty) (Crasquin-Soleau et al., 2004 and 2006) of imprecise Permian age.

In the type section and in the Midhnab area (Vaslet et al., 1985b), the basal limestone (subunit 1) of the Midhnab Member contains debris of echinoderms, gastropods (Bellerophon sp.), and ostracods associated with gymnocodiaceans and dasycladacean algae (Permocalculus sp. and Mizzia sp.; see Okla,1992). This lowest unit of the Midhnab Member has also yielded abundant small nautiloids (Caelogastroceras aff. mexicanum (Girty) and Aphelaeceras sp., according to Le Nindre et al., 1990b), bactritids (Figures 29, 3638), and rare brachiopods. The brachiopods have been identified by L. Angiolini (Angiolini et al., 2006) as Kotlaia sp., and a new genus of the Terebratulida subfamily Dielasmatinae, very close to the genus Dielasma. The two taxa are also described in the Khuff Formation of Oman (Angiolini and Bucher, 1999; Angiolini et al., 2004), and are known from Middle to Late Permian ages. The gypsiferous clay of subunit 3 yielded several benthic foraminifers including Globivalvulina graeca Reichel, G. vonderschmitti Reichel, Paradagmarita sp., Geinitzina sp., Pachyphloia sp., Calcitornella sp., and Pseudotristix sp., the latter assemblage being characteristic of the late Dzhulfian-Dorashamian (Wuchiapingian-Changhsingian) (Altiner, 1981; Lys, 1984). The lower Midhnab Member contains also rare fusulinid debris identified as Nankinella minor Sheng, as well as Lagenidae foraminifers such as Protonodosaria sp., “Dentalina” hoi, “Nodosaria” spp., etc.

The lower clayey and calcareous part of the Midhnab Member (subunit 3) is poorer in organisms, containing the foraminifers Globivalvulina vonderschmitti Reichel, and Paraglobivalvulina mira Reitlinger, associated with some other bioclasts. These species existed from the late Murgabian (Wordian) or early Dzhulfian (Wuchiapingian) until the late Dorashamian (Changhsingian).

The lacustrine limestone described locally at the upper part of the Midhnab Member (top of subunit 4) contains charophytes and gastropods (Delfour et al., 1982; Vaslet et al., 1985; Le Nindre et al., 1990b). Laterally in the sandstone and clayey facies at the Jal al Wata locality north of Buraydah, Late Permian plants were discovered at the top of the unit by Hill and El-Khayal (1983) who described the new species of charophyte Palaeonitella tarafiyensis. This Midhnab flora has been extensively studied by Broutin et al. (1995, 2002), Berthelin (2002), and Berthelin et al. (2006), along the Jal al Khartam escarpment in the Al Faydah and Buraydah quadrangles (Figures 1 and 3).

The Midhnab flora associations vary according to the different palaeoenvironments encountered in the upper part of the Midhnab Member. The flood plain environments contain associations of Euramerican elements such as vegetative and fertile shoots of conifers (Pseudovoltzia liebeana Florin, Culmitzschia sp.), mixed with Cathaysian elements (Discinites sp. cf. D. orientalis Gu and Zhi, Pelourdea sp., P. hallei Gu and Zhi). They also contain (Wadi al Batin area, Buraydah quadrangle) typical leaves and fructifications of Gondwanian glossopterids (Glossopteris decipiens Feismantel, G. formosa Feismantel, Alberia sp., Phyllotheca australis Brongniart). The glossopterids were discovered for the first time in Saudi Arabia, where they occur interminged with the Euramerican and Cathaysian representatives.

The deltaic environments of the upper part of the Midhnab yielded flora associations dominated by sphenophytes including arborescent leaves and cones of lycophytes (Lobatannularia multifolia Kon’no and Asama; L. heianensis Kawashi), wood fragments of gymnosperms, and rare coniferous fertile shoots (Voltzia affinities). The swampy palaeoenvironments contain associations dominated by coniferous leafy shoots (Ullmania bronnii Goeppert, U. frumentaria Goeppert, Ullmania sp., Culmitzschia sp.), and pteridophytes (Pecopteris chihliensis Stockmans and Mathieu, Taenopteris sp., and Lepidostrobus sp.).

The microfloral associations, dominated by gymnosperm bisaccate pollen, are in accordance with a Late Permian age and very similar in composition to the coeval palynologic assemblages described in the Salt Range in Pakistan (Balme, 1970) and Australia (Foster, 1979; Backhouse, 1991).

Floras and microfloras both confirm a Late Permian age for this part of the Khuff Formation.

To the north (Unayzah and Buraydah sections), the basal layers of the Midhnab Member have also yielded Paraglobivalvulina mira Reitlinger, Pseudotristix solida Reitlinger, Geinitzina reperta Bykova and Pachyphloia pedicula Lange known from the late Murgabian (Wordian) to the Dorashamian (Changhsingian). The carbonate marl in subunit 2 contains an abundant fauna. Foraminifers include Hemigordius permicus Grozdilova, Pachyphloia pedicula Lange, Paradagmarita flabelliformis Zaninetti and others, and Robuloides lens Reichel, all typical of the Late Permian.

To the south, in the southern part of the Ad Dawadimi quadrangle (Safra ad Dumaythiyat), a single specimen of a nautiloid described as Tirolonautilus gr. hoernesi (Chirat et al., 2006) was recovered from the bluish-gray clayey bioclastic limestone of subunit 2 of the Midhnab Member. Other macrofauna includes bactritids, bryozoans, bivalves (Myophoridae) and gastropods. In the Darma’ quadrangle (Jabal al Amshiyah), the reworked bioclastic and lithoclastic calcarenite of subunit 1 of the Midhnab Member, has yielded a single specimen of a conodont described as “Jinogondolella” cf. altaduensis by Nicora et al. (2006). The Late Capitanian age recorded for some species of this genus is however questionable here due to the high probability of reworking of this corroded, broken and conferred species of conodont. In the Wadi Al Mulayh, the lowest unit of the member is a bioclastic limestone (phosphatic bone debris) containing foraminifers (Cornuspira sp., cf. Meandrospira sp., Globivalvulina graeca Reichel, and Lasiodiscus minor Reichel), associated with abundant gymnocodiacean algae and bactritids. Another abundant foraminiferal association is located in the clayey marl of subunit 2, including Pseudotristix cf. solida Reitlinger, Geinitzina reperta Bykova and Nodosaria dzhulfensis Reitlinger, and it is indicative of Dorashamian (Changhsingian) and Early Triassic. Midhnab subunit 2 also yielded the ostracod Hollinella herrickana (Girty) as well as numerous ostracod tests observed in thin sections.

In the well SHD-1 (Manivit et al., 1985a), the foramniferal association includes: Dagmarita chanakchiensis Reitlinger, Globivalvulina vonderschmitti Reichel, Hemigordius ovatus Grozdilova, Pachyphloia iranica Bozorgnia and Paradagmarita flabelliformis Zaninetti from the base; and the microflora association (renewed with 14 species compared to lower units) includes: Alisporites tenuicorpus Balme and Klausipollenites schaubeigeri Potonié and Klaus. These associations support a late Dorashamian (Changhsingian) age for the lower part of the Midhnab Member (J. Broutin, written communication, 2002). Manivit et al. (1985a) assigned the Midhnab Member to the Late Permian based on the microflora species Alisporites tenuicorpus Balme, Klausipollenites schaubergeri (Potonie and Klaus), and K. wargalensis Balme, together with Densoisporites playfordii (Balme), Dettmann.

In the Khuff type section, the coquina beds of the Lower Khartam Member contains debris of echinoderms, gastropods, bivalves, ostracods, algae, and fish teeth, together with phosphatic remains, and unidentified cephalopod in the subunit 2. A foraminiferal association found in the lowest intraclastic and bioclastic layer of the member includes Nodosaria dzhulfensis Reitlinger, “Dentalina” hoi Trifonova, Geinitzina reperta Bykova, and Ichtyolaria latilambata Sellier de Civrieux and Dessauvagie and several species of Paradagmarita (Figures 34 and 35). This association of foraminifer and ostracods allow assigning a Late Permian (Changhsingian) age for the Lower Khartam Member.

The biophase of the Upper Khartam Member consists essentially in undetermined rounded carbonate algae and stromatolites.

To the north (Al Faydah and Buraydah quadrangles), the Lower Khartam contains bivalves, large bactritids, nautiloids and rare foraminifers (Climacammina sp.). According to (Crasquin-Soleau et al., 2004 and 2006) the presence of ostracods of the Paraparchitidae family (Paraparchites spp.) and Kloedenellacea superfamily such as Knoxiella sp., Kloedenellitina sp., and K. infirma Shi, are also in favour of a Late Permian age for the Lower Khartam Member. Other Permian ostracod forms are also found in the Lower Khartam, such as Arqoviella permiana Gerry and Honigstein and Sulcella suprapermiana Kozur. Crasquin-Soleau et al. (2004) described two new species of ostracods (Arqoviella arabica and A. khartamensis) from the Lower Khartam Member in the type section of Jal al Khartam.

The Upper Khartam (subunit 4) is characterised by the appearance of the annelid Spirorbis phlyctaena (Brönimann and Zaninetti) (Figure 39) associated with bioclasts of brachiopods, bivalves, and rare ostracods including Langdaia cf. soboblonga Wang described in the Early Triassic of South China (Crasquin-Soleau et al., 2004). The annelid S. phlyctaena is characteristic of the Early Triassic (late Griesbachian substage of the Scythian according to Altiner et al., 1980; Zaninetti et al.,1981) (Figure 35). In the Buraydah quadrangle, ostracods with thick tests are present up to the top of the Midhnab, but are replaced by ostracods with thin tests in the Upper Khartam unit as described in thin sections by Manivit et al. (1986).

To the south, an isolated specimen of nautiloid found in the Ad Dawadimi quadrangle (Jabal al Amshiyah), described as Tirolonutilus feltgeni n. sp. (Chirat et al., 2006), occurred in the laminated dolomite of the subunit 1 of the lower Khartam. A genus recorded from the Dzhulfian and Dorashamian in the Tethyan areas. In the Darma’ and Wadi Ar Rayn quadrangles, the Lower Khartam contains Late Permian foraminifers such as Geinitzina sp., Lenticulina sp., Hemigordius sp and Robuloides lens Reichel, associated with sponge spicules, ostracods, bivalves and large bactritids 10 to 15 cm long. The foraminifers Pachyphloia sp. and Neohemigordius sp. were found at the base of the Khartam Member in the south of the Wadi Ar Rayn quadrangle.

In the well SHD-1 (Manivit et al., 1985a), the microflora species that existed below the base of the Khartam Member completely disappear in it, and mark a rupture with earlier successions. The Upper Khartam yielded the Triassic Scythian serpulid Spirorbis phlyctaena Bronnimann and Zaninetti (Manivit et al., 1985a).

The boundary between the Permian (Changhsingian) and Triassic (Early Scythian) is thus located within the Khartam Member, and tentatively placed at the sequence boundary at the base of the Upper Khartam. This position for the Triassic/Permian Boundary remains identical to the boundary assigned by Le Nindre et al. (1990b) who placed this limit at the base of the Upper Khartam with the appearance of Spirorbis phlyctaena. The Khartam Member is thus assigned to a Late Permian (late Changhsingian) to Early Triassic (Early Scythian) age (Figure 35).

The regionally transgressive Khuff Formation is dominated by low-energy carbonate and evaporite deposits that represent relatively confined environments of the mid-littoral domain (Figures 40 and 41). In outcrop, along a N-S strike in central Saudi Arabia, the formation is subdivided into four main depositional sequences (DS; see Figures 4 and 5), each characterised by: (1) a basal sequence boundary (SB) corresponding to a significant break in sedimentary deposition; (2) transgressive system tract (TST); (3) maximum flooding interval (MFI); and (4) highstand system tract (HST).

The first depositional sequence DS PKh (named after Permian-Khuff-Huqayl) includes the Ash Shiqqah and the Huqayl members. Its basal SB corresponds to the Pre-Khuff Unconformity (PKU). The Ash Shiqqah Member is interpreted as the TST, while the HST consists of two sabkha sequences that evolved from intertidal to subtidal environments (Lower and Upper Huqayl). The first Late Permian flooding event over central Arabia is represented by the flooding interval MFI PKh. The flood PKh (located in the basal part of the Huqayl Member, Figure 4) was followed by the regressive evaporitic palaeoenvironments of the Huqayl Member (Le Nindre et al., 1990b). The subtidal environments of the second sabkha sequence (Upper Huqayl Member) may represent a subsequence and contain a flooding interval of limited extent (Figure 4). The DS PKh corresponds closely with Khuff cycle 1 of Al-Aswad (1997), and may contain MFS P30 of Sharland et al. (2001) (Figure 26).

The second depositional sequence DS PKm (named after Permian-Khuff-Midhnab) starts with the subtidal to littoral deposits of the Duhaysan Member (TST), over an erosive surface (SB) upon DS PKh. The SB is marked by reworked deposits. DS PKm ends with the regressive supratidal to continental deposits of the upper part of the Midhnab Member (Figure 4). The MFI PKm is clearly located in the outcrops at the base of the Midhnab Member with abundant marine fauna (Le Nindre et al., 1990b): Changhsingian foraminifers (Vachard et al., 2003, 2005), brachiopods (Angiolini et al., 2006), nautiloids (Chirat et al., 2006), and bactritids. The MFI PKm represents the maximum extent of the marine domain in central Saudi Arabia (Figures 4 and 41). The Midhnab MFI PKm is followed by the regressive HST that ends in the continental deposits at the upper part of the Midhnab Member. The DS PKm corresponds to Khuff cycles 2 and 3 of Al-Aswad (1997), and may contain MFS P40 of Sharland et al. (2001, 2004) (Figure 26).

The third depositional sequence DS PKk (named after Permian-Khuff-Khartam) corresponds to the Lower Khartam Member and represents the terminal Permian (Changhsingian) depositional sequence in the outcrops of central Saudi Arabia (Figure 4). The basal SB is marked by a return to marine subtidal conditions after the continental break at the end of DS PKm. The transgressive reworked facies at the base of the Khartam Member represents the TST and MFI PKk, with marine fauna including abundant Permian ostracods (Crasquin-Soleau et al., 2004), bactritids, and locally cephalopods (Chirat et al., 2006). The HST PKk is probably incomplete, being partly eroded by the dolomitic sands at the base of the Upper Khartam Member.

The DS PKk could be correlated with the lowest part of Khuff cycle 4 of Al-Aswad (1997; Figure 26). The MFI PKk has the same position as MFS Tr10 of Sharland et al. (2001), and so MFS Tr10 should be positioned higher in the section. The MFI PKk could correspond to a similar event recognised in the latest Permian of the Zagros Mountains (Insalaco et al., 2006).

The fourth depositional sequence (DS TrS, named after the Sudair Shale) starts with the littoral, tidal to intertidal deposits of the Early Triassic Upper Khartam Member of the Khuff Formation, and ends with the closed-basin, clayey to evaporitic rocks (HST) of the Early Triassic Sudair Shale (Le Nindre et al., 1990b). The Sequence Boundary is marked by an abrupt change in facies between the subtidal deposits of DS PKk and the intertidal to beach deposits of DS TrS (Figure 4). This erosional boundary could represent the Triassic/Permian Boundary in central Saudi Arabian outcrops. The MFS Tr10 is interpreted as an Early Triassic Scythian event (Stephenson, 2003; Sharland et al., 2004), and it should be positioned at least in the Upper Khartam Member or in the lower part of the Sudair Shale. This maximum flooding event has not been characterised clearly in the outcrops of central Saudi Arabia.

Depositional Sequence DS PKh: Ash Shiqqah and Huqayl Members

Above the Pre-Khuff Unconformity, the TST of DS PKh corresponds to Ash Shiqqah Member (Figures 40 and 41), which marks the progressive passage from the continental domain to lagoonal environment. In spite of the poor biofacies, the depositional palaeoenvironment and the polarity of the sequence still show the transgressive character of the Khuff Formation. The Ash Shiqqah Member exhibits very different facies north and south of the Central Arabian Arch (Figure 40). North of the arch, clastic coastal plain and salt-marsh environments dominated. South of the arch, brackish to evaporitic facies interpreted as salina deposits, correspond to the flooding of the former coastal plain.

To the north of the Central Arabian Arch, red and green claystone, fine-grained manganiferous, silty azoic dolomite and burrowed dolomite were successively deposited over a basal palaeosol. The first beds of carbonate mudstone are discontinuous, floating in clayey sediment, and display plastic deformation structures due to subsidence or slumping. In the Unayzah area, channels of quartzose sand channelled in coastal plain to lagoonal clayey unit, bearing gypsum pseudomorphs. The biophase is very rare, marked by benthic foraminifers, or poorly preserved. These facies are indicative of low-energy lagoonal-brackish environments with abnormal salinity under continental influences.

To the south, in the Darma’, Wadi Ar Rayn, and Wadi Al Mulayh quadrangles, Manivit et al. (1985a, 1985b), Vaslet et al. (1983), and Cole et al. (2000) interpreted the basal clastics and mixed clastic and dolomite series of the Khuff Formation, to indicate a gradual environmental change from fluvial to shallow marine with intertidal influences (Figures 40 and 41). Between the continental and marineend of the Ash Shiqqah Member, a transitional coastal plain environment with hypersaline lagoons was established. Conditions oscillated between peritidal marine (laminated dolomites) and marginal marine to coastal plain (marls and sandstones with salt hoppers). Continuing arid conditions led to the establishment of extensive evaporite platform deposits throughout the lower part of DS PKh. Passage to a supra- or medio-littoral domain at the top of the Ash Shiqqah Member, is shown by deposits locally progressing as far as constructed forms, such as columnar stromatolites.

The marine bioclastic facies at the base of the Huqayl Member (Figure 40) corresponds to MFI PKh. It represents an intertidal domain with biocalcarenite and locally intraclastic tidal channels. This marine interval represents the first transgressive deposits over the Central Arabian Arch. These carbonate beds are widespread and continuous. They represent a sharp break between the heterogeneous facies below and the homogeneous marine succession above.

The HST of DS PKh (Figures 4 and 40) corresponds to the Huqayl Member; it is composed of two higher-order sabkha sequences. Each of these two regressive sequences begins with marine bioclastic deposits and ends either with laminated mudstones containing gypsum pseudomorphs, or with anhydrite and solution breccias. The basal parts of both the Huqayl cycles are marked by a fairly rich microfauna. The base of the second sequence consists of an oncolitic marker layer. The alternation of facies is indicative of an oscillating depositional regime ranging from external lagoon to sabkha.

Carbonate sands, containing pellets and porcelaneous foraminifers, give way to deposits with a restricted fauna of molluscs that are silted up and encrusted by annelids. Abundant gymnocodiacean algae occupy an intermediate position, and tend to be reworked. Subtidal environment in the second sequence (Upper Huqayl) may represent a secondary flooding event (Figure 4). Burrowed mudstone commonly precedes paler-coloured laminated deposits. In the upper parts of the two sequences, sedimentary structures include laminations, mud cracks, mud chips, planar stromatolites, solution breccias, and sulfate pseudomorphs; all criteria suggest a low-energy mid-littoral zone such as a tidal flat and supra-littoral evaporitic domains.

Depositional Sequence DS-PKm: Duhaysan and Midhnab Members

The start of DS PKm is marked at the base of the Duhaysan Member by erosional and transgressive marine wedges (Figures 4 and 40). In this significant channelling phase, the underlying carbonate mud of the Upper Huqayl Member was reworked as mud galls in calcarenorudite, indicating an increased energy level (Duhaysan subunit 1). The sequence is marked by an abundant and varied, clearly marine fauna consisting of bryozoans, algae, echinoderms, brachiopods, gastropods, bactritids and cephalopods (nautiloid embryos). In north-central Saudi Arabia, the upper part of the Duhaysan displays a return to proximal and even supratidal conditions with carbonate claystone and dolomite. To the south of the Central Arabian Arch, terrigenous discharge and gypsiferous claystone reflect increased confinement.

The pelagic faunas (bactritids and nautiloids) associated with MFI PKm extended over considerable areas from north-central to south-central Saudi Arabia. Although this flood was the most widespread inundation of the Arabian Platform during Late Permian, it was very short-lived and followed by rapid confinement of the palaeoenvironments. In contrast to the TST of the Duhaysan Member, a new trend emerged in MFI PKm with the increased rareness or dispersion of the more-marine organisms (bryozoans) and the accumulation of bactritids. Measurements on thanatocenotic accumulations of the conic-shape bactritids, indicate a N175° direction, probably parallel to the palaeoshore. Wave ripples are commonly associated with these deposits.

Above MFI PKm, the rest of DS PKm indicates a clear regressive trend up to continental deposits in the upper part of the Midhnab Member. From the lower third of the Midhnab Member (subunit 2), the fauna becomes rarer but the algae persist, giving way upwards in the unit to rare fish debris and sulfate pseudomorphs. This general confinement was subsequently further manifested by the constant evaporitic character of the HST of DS PKm. At the top, very proximal facies containing intraclasts, fish debris, and stromatolites, is overlain by plant-bearing claystone (Midhnab flora) and sandstone channels in a swamp environment (Berthelin et al., 2006). In places, lacustrine limestone containing charophyta oogones, marks the continental trend at the top of DS PKm.

Depositional Sequence DS PKk: Lower Khartam Member

DS PKk corresponds to a terminal Permian marine incursion (Lower Khartam Member, Figures 4 and 40). It started with renewed open-marine influences followed by a rapid transgression that culminated with MFI PKk (located just a few metres above the major break between the Midhnab and Khartam members). The MFI PKk is characterised by echinoderms, bryozoans, and cephalopods associated with foraminifers (diversity of the genus Paradagmarita), ostracods and phosphatic debris. The extension of the marine episode (with cephalopods) is limited to the northern edge of the Central Arabian Arch. In the Buraydah area, according to Le Nindre et al. (1990b), the depositional environment of the Lower Khartam is interpreted from subtidal to lagoonal and mid-littoral.

Depositional Sequence DS TrS: Upper Khartam Member and Sudair Shale

The Depositionnal Sequence TrS starts with the Upper Khartam Member of the Khuff Formation and ends within the Sudair Shale Formation. In the Lower Khartam Member, the facies comprises

grainstone with commonly very planar oblique bedding, stromatolite mats (intertidal environments), and coquina. The dolarenites that predominate are composed of fine, sorted cortoids (proto-oolites) with a dolomitic bioclastic nucleus originating from biodegradation of undetermined carbonate algae. Several elemental sequences comprising the three facies are superposed, and indicate a beach environment extending from the upper part of the infralittoral domain to various levels of the midlittoral domain (foreshore to backshore), separating the lagoon from the sabkha. In the infralittoral deposits (Upper Khartam Member, subunit 4), Spirorbis phlyctaena is virtually constant. This sedentary annelid must have lived (much as the present-day genus) as an epiphyte over algae that are no longer preserved; it is associated with ostracods and, locally, with echinoderms. Also, due to a change in environment, a relatively abundant fauna of ostracods with a thin test appeared at this time.

The passage to a different type of sedimentation between the Lower and Upper Khartam members, underlines a discontinuity between the two depositional sequences. This change may be due to the progradation by beaches with recycling of locally available material (carbonate algae debris). Beach construction could have introduced an erosional unconformity and a change in the depositional mechanism. The diachronous aspect may have caused the depositional break to become even greater landward from the initial shoreline. This sequence boundary is believed to represent the Triassic/Permian Boundary in the outcrops of the Khuff Formation in central Saudi Arabia.

Although still showing marine influence (beds of bioclastic dolomite) in the lower part, the TST facies of DS TrS evolved throughout the remainder of the Early Triassic Scythian (Sudair Shale) towards supralittoral facies of sabkha type. These facies are essentially represented by transitional ostracod dolomite, then by gypsiferous claystone containing beds of massive gypsum, and in the SHD-1 well, some halite (Figure 23). The passage to the Sudair Shale Formation is progressive. Despite the presence of finely-oolitic and peloidal grainstone, an irreversible evolution continued toward sulfate deposits, initially interstratified in the algal mats, and subsequently as massive gypsum.

The Khuff Formation holds the world’s largest non-associated gas reserves in many giant fields located in Bahrain, Iran, Qatar, Saudi Arabia and the United Arab Emirates (Al-Jallal, 1995). The Khuff outcrops in central Saudi Arabia offers one the most complete and representative exposures of this commercially important reservoir. This study presents a regional synthesis of previous and new lithostratigraphic and biostratigraphic data collected from this formation, and presents it in a chronostratigraphic and sequence stratigraphic framework. The study extends stratigraphically from below the Pre-Khuff Unconformity to the Sudair/Khuff Boundary, and geographically along the N-S outcrop region of about 1,200 km; the study also includes several wells.

The Khuff Formation consists of five members; from oldest to youngest: Ash Shiqqah (replaces the obsolete “Unayzah member”), Huqayl, Duhaysan, Midhnab and Khartam. The Ash Shiqqah Member is tentatively dated as ?Capitanian (?Midian) based on the presence of the fusulinids Monodiexodina kattaensis and Reichelina sp. and appears to correlate to the subsurface Khuff-D Member. The overlying Huqayl Member is tentatively dated as ?Wuchiapingian (?Dzhulfian) based on an assemblage of smaller foraminifers dominated by Pseudomidiella cf. labensis, Earlandia? spp. and Neodiscus aff. qinglongensis. The Ash Shiqqah and Huqayl members constitute a depositional sequence (named DS PKh) with an age of ?Capitanian-Wuchiapingian (?late Middle Permian and Late Permian).

Another Late Permian Khuff sequence is represented by the Duhaysan and overlying Midhnab members (DS PKm). The Duhaysan Member is dated as Late Permian (Wuchiapingian-Changhsingian) based on to Hemigordius baoqingensis, Graecodiscus cf. kotlyarae, “Dentalina” hoi, and Colaniella cf. minuta. The Midhnab Member is dated as Changhsingian (Dorashamian) based on Paradagmarita sp. and “Glomospirella spirillinoides” and the disappearance of the genera Nankinella and Globivalvulina. Incised channels at the top of the Midhnab Member contain continental deposits that yielded Late Permian flora. The top of the Midhnab Member corresponds to the maximum intra-Khuff sea-level lowstand but does not represent the Triassic/Permian Boundary.

The Triassic/Permian Boundary is positioned between the Upper and Lower Khartam members. The Lower Khartam Member is dated as latest Permian due to the presence of several species of Paradagmarita spp. and “Nodosaria” dzhulfensis in its lowest beds, and the ostracods Paraparchites spp., Knoxiella spp. and Kloedenellitina sp. throughout the unit. The Lower Khartam Member appears to represent a single sequence (DS PKk). The Upper Khartam Member is dated as Early Triassic (‘Scythian’) based on the appearance of the characteristic annelid Spirorbis phlyctaena. The Upper Khartam Member is interpreted as the start of Triassic sequence that extends into the Sudair Formation (DS TrS).

This study is based on geological field data acquired in the 1980s as part of the framework for the geological mapping of central Saudi Arabia by the Saudi Arabian Deputy Ministry for Mineral Resources (DMMR, now the Saudi Geological Survey, SGS) and the French Bureau de Recherches Géologiques et Minières (BRGM). Some of the work presented here was published in the 1980s in a series of Geoscience Maps, and in PhD thesis at the University of Paris VI, France. Dr. M.A. Tawfiq, President of the Saudi Geological Survey, and the SGS-DMMR and BRGM are acknowledged for supporting this work. More recently, in 2002, additional field data from the Khuff Formation outcrops was acquired during a field trip organised by Saudi Aramco. We thank Saudi Aramco geologists R.K. Al-Dakhil, I. Al-Jallal, R.A. Kamal, A.A. Tawil, I. Billing, R.G. Demaree and G.W.G. Hughes for their help during the 2002 field trip and the many useful discussions. We also thank Beicip geologists B. Murat and E. Pluchery for their contributions in the field and for providing us with the photograph in Figure 22. The manuscript was greatly improved as a result of various reviews (at different times over several years) by D. Casey, R. Davies, J. Filatoff, G.W.G. Hughes, J. Mattner, P. Osterloff, P. Sharland, M. Simmons, M. Stephenson and C. Strohmeinger. We are very grateful for the palaeontological descriptions provided by L. Angiolini for brachiopods, R. Chirat for nautiloids and A. Nicora for conodont. We also thank T. Vachard for her technical assistance. Finally the GeoArabia team, and especially Heather Paul, are thanked for the preparation of the text and the final design of the paper.

Denis Vaslet is Head of the Geology and Geoinformation Division at the Bureau de Recherches Géologiques et Minières (BRGM), the French Geological Survey. He has 30 years of experience in the geology of the Middle East. From 1977 to 1979 Denis was involved in geological mapping and phosphate prospecting in Iran for the Geological Survey of Iran and the National Iranian Oil Company. From 1979 to 1991, he was responsible for the Cover Rocks mapping program in Saudi Arabia for the Saudi Arabian Deputy Ministry for Mineral Resources. Denis has been involved in the complete lithostratigraphic revision of the Phanerozoic rocks of central Saudi Arabia, for which he received his Doctorate of Sciences from the University of Paris in 1987. He is currently in charge of geological and geophysical mapping both in France and overseas, and for the production and distribution of digital geological information at BRGM. Denis remains involved in several research projects in the sedimentary geology and stratigraphy fields within the Arabian Peninsula.

Yves-Michel Le Nindre has more than 10 years of experience in the geological mapping of the Phanerozoic rocks of Saudi Arabia. He received his Doctorate of Sciences from the University of Paris in 1987. Yves-Michel’s dissertation was on the sedimentation and geodynamics of Central Arabia from the Permian to the Cretaceous. He is currently working with the Bureau de Recherches Géologiques et Minières on sedimentary basin analysis and modelling, particularly in hydrogeology, and is also involved in present-day littoral modelling.ym.

Daniel Vachard is a Researcher at the French National Centre of Scientific Research (CNRS). He is with the UMR 8014 of the University of Lille (France). His PhD thesis (1974) was a study of the Palaeozoic of southern France, and his Doctorate of Sciences at Dijon University (1980) concerned the Carboniferous-Permian biostratigraphy of Afghanistan. His work deals mainly with five topics: (1) Fusulinids; (2) Palaeozoic smaller foraminifers; (3)Palaeozoic algae sensu lato (including pseudo-algae and other incertae sedis); (4) carbonate microfacies; and (5) Carboniferous-Permian biostratigraphy and palaeobiogeography. Daniel has contributed to the development of reference scales for the Carboniferous and Permian of numerous areas (e.g. southern France, Afghanistan, Libya, Morocco, Sumatra, Carnic Alps, Oman and Mexico). His interest in the Late Permian of Saudi Arabia began with an unpublished report in 1976 for the French Geological Society (BRGM).

Jean Broutin is Professor of Palaeobotany and Palaeoecology at the University of Paris, and is a specialist of Late Paleozoic micro and macroflora. His field of research extends over both the northern and southern margins of the Tethys Ocean, from Europe to the Urals, and from Brazil to Morocco and Oman. Jean has developed associations between palaeobotany and palaeoenvironments, so that analysis of the dynamic distribution of the plant cover can be used to determine palaeoclimatic and biogeographic changes through time. In this respect, palaeobotanical data are used for testing global geodynamic and palaeoclimatic modelling.

Sylvie Crasquin-Soleau is a Micropalaeontologist at the French National Centre of Scientific Research in Paris, France. She received a PhD in Palaeontology from Lille University in 1986 where her research focused on Lower Carboniferous ostracodes from North of France and the Rocky Mountains in Canada. Sylvie’s work presently focuses on Permian and Triassic marine ostracodes of the Palaeo-Tethys area. Her emphasis is on ostracod palaeoecology, palaeobiogeographic analysis and palaeoenvironment reconstructions in Oman, Turkey, Saudi Arabia, Iran, Thailand and south China.

Martine Berthelin is currently studying Permian floras and microfloras of the Arabian Peninsula in preparation of a PhD thesis. One of her main aims is to understand the climates of the past and the kinetics of ancient vegetation so as to provide a tool for testing palaeoclimate and palaeogeographic modelling. She uses biogeochemical studies for characterising fossil organic molecules and determining their biological origin in relation to plant remains of fossiliferous beds.

Jérémie Gaillot received his MSc in Biodiversity of present-day and fossil ecosystems in 2002 from the University of Science and Technology, Lille, France. After his studies, he participated as a Junior Geologist in a synthesis project on the biostratigraphy of the Upper Dalan Formation of Iran. In June 2003, Jérémie began his PhD on the Biostratigraphy and Palaeoecology of the Khuff Formation of the Arabian Platform at the University of Lille 1, funded by Total. The main focus of his PhD is the sequential foraminiferal biostratigraphy of epeiric platforms and the mechanisms of mass extinction at the Permian-Triassic Boundary.

Mohammed Halawani is a Geologist in the Saudi Geological Survey (SGS), which is part of the Saudi Arabian Ministry of Petroleum and Mineral Resources. He received a BSc from the Faculty of Earth Sciences at King Abdulaziz University, Jeddah in 1981. After graduating he worked on various projects in the Arabian Shield. Since 1990, he has been involved in the mapping of Phanerozoic rocks, especially in the Palaeozoic of northern and northwestern Saudi Arabia. Mohammed is presently working on the Palaeozoic stratigraphy of Saudi Arabia.

Moujahed Al-Husseini founded Gulf PetroLink in 1993. 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 (1971), MSc in Operations Research from Stanford University (1972), PhD in Earth Sciences from Brown University (1975) and Program for Management Development from Harvard University (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 from 1999-2004. He has published about 30 papers covering seismology, exploration and the regional geology of the Middle East, and is a member of the AAPG, AGU, SEG, EAGE and the Geological Society of London.