ABSTRACT

This Note presents a formal update to the Middle East Geologic Time Scale 2008 (ME GTS) for the Late Triassic and Jurassic rock units of Saudi Arabia. It reviews their lithostratigraphic nomenclature, ranks and stage assignments, and proposes names for third-order chrono-sequences as compiled and/or interpreted from the published literature. The review starts with the Late Triassic (Late Norian – Rhaetian) Minjur Sandstone of the Buraydah Group, with the Triassic – Jurassic (TrJ) boundary positioned at its top. The Minjur Sandstone consists of two units, here ranked as members, which are interpreted as the Late Norian – ?Early Rhaetian Lower Minjur Sequence (Lower Minjur Member) and ?Late Norian – Rhaetian Upper Minjur Sequence (Upper Minjur Member). The Early Jurassic Unconformity - Hiatus (Hettangian, Sinemurian and Pliensbachian stages) separates the Minjur Sandstone from the Toarcian Marrat Formation of the Jurassic Shaqra’ Group. The Shaqra’ Group consists of seven formations, from base-up: (1) Marrat Formation consisting of the Lower, Middle and Upper units, here ranked as members. (2) Dhruma Formation consisting of eight units (D1–D5, Wadi ad Dawasir “delta”, D6 and D7). The Dhruma units have been named in the literature as Balum Member (D1 and lower part of D2 units), Dhibi Limestone Member (upper part of D2 unit), Uwaynid Member (D3), Barrah Member (D4), Mishraq Member (D5), ‘Atash and Hisyan members (D7); the D6 and Wadi ad Dawasir “delta” units are not formally named as members. (3) Tuwaiq Mountain Limestone consisting of the Baladiyah (T1 unit), Mysiyah (T2 unit) and Daddiyah (T3 unit) members. (4) Hanifa Formation consisting of the Hawtah and Ulayyah members. (5) Jubaila Limestone with J1 and J2 units. (6) Arab Formation consisting of D to A members. (7) Hith Anhydrite consisting of Main Hith Anhydrite Member and Manifa Reservoir/Member.

The seven formations of the Jurassic Shaqra’ Group are interpreted as 11 third-order chrono-sequences: (1) Early Toarcian Marrat Sequence B (Lower and Middle Marrat members) and Mid- to ?Late Toarcian Marrat Sequence A (Upper Marrat Member), the latter containing Arabian Plate maximum flooding surface MFS J10. (2) Bajocian Lower Dhruma Sequence (Balum Member and Dhibi Limestone), formed by the Balum and Dhibi subsequences, the former containing MFS J20. (3) Late Bajocian – Mid-Bathonian Dhruma Sequence B (Uwaynid, Barrah, Mishraq members, and Wadi ad Dawasir “delta” unit), with the Mishraq containing MFS J30. (4) Late Bathonian – early Mid-Callovian Dhruma Sequence A (unit D6, ‘Atash and Hisyan members), with the Hisyan containing MFS J40. (5) Mid- to Late Callovian Tuwaiq Sequence (Tuwaiq Mountain Limestone) containing an undesignated MFS at the base the Daddiyah Member (T3 unit). (6) Early and Mid-Oxfordian Hawtah Sequence (Hawtah Member of Hanifa Formation) containing MFS J50. (7) Late Oxfordian – ?Early Kimmeridgian Ulayyah Sequence (Ulayyah Member of Hanifa Formation) containing MFS J60. (8) Kimmeridgian Jubaila Sequence (Jubaila Limestone) containing MFS J70. (9) Arab-D Sequence (Arab-D Member inclusive of the Arab-D Anhydrite) containing an undesignated MFS in the Arab-D carbonate. (10) ?Kimmeridgian – ?Tithonian Arab-C and B Sequence (Arab C and B members) containing fourth-order MFS J80 and J90 in the lower carbonates of the members; and (11) ?Kimmeridgian – Tithonian Arab-A - Main Hith Sequence (Arab-A Member and Main Hith Anhydrite below the Manifa Reservoir/Member) containing fourth-order MFS J100 in the Arab-A Member. The Tithonian Manifa Member (upper part of the Hith Anhydrite Formation) is interpreted as heralding a transgression (fourth-order MFS J110), which deposited the Late Jurassic – Early Cretaceous Sulaiy Formation of the Thamama Group. The Jurassic – Cretaceous (JK) boundary is placed in the Sulaiy Formation by stratigraphic position. Based on the Geologic Time Scale GTS 2004 and Arabian Orbital Stratigraphy (AROS), the ages in million years before present of the sequence boundaries and maximum flooding surfaces are estimated for these chrono-sequences.

INTRODUCTION

The Middle East Geologic Time Scale (ME GTS) was launched in late 2008 with the objective of capturing the region’s time-rock units in a multi-country chronostratigraphic framework (Al-Husseini, 2008). In particular, transgressive-regressive (T-R) depositional sequences (DS), bounded by sequence boundaries (SB) and containing maximum flooding surfaces (MFS), were targeted as the preferred correlative units (chrono-sequences) to populate the ME GTS charts. Since its publication, Énay et al. (2009) interpreted a previously unrecognized major Early Bathonian regression followed by a late Mid-Bathonian hiatus in central Saudi Arabia. This new interpretation, together with those from previous studies, provide the opportunity to update the Late Triassic – Jurassic chrono-sequences of Saudi Arabia in the context of the ME GTS, the Geologic Time Scale GTS 2004 (Gradstein et al., 2004) and Arabian Orbital Stratigraphy (AROS, Al-Husseini and Matthews, 2006, 2008).

In order to better appreciate the evolving relationships between lithostratigrapic, biostratigraphic and sequence stratigraphic definitions, nomenclature and interpretations, this Note provides a brief review of the Late Triassic – Jurassic rock units of Saudi Arabia. For each unit, the Lexicon of Saudi Arabia (Powers, 1968) is used to credit the pioneering authors and their works. Next, the refinements and/or revisions resulting from the mapping conducted in the 1980s–1990s by the Saudi Geologic Survey (SGS, previously Deputy Ministry of Mineral Resources – DMMR) and French geologic survey (BRGM) are summarized (Figures 1 and 2). The main objective of the Note is to identify those rock units (formation, member, unit) that may form third-order T-R sequences, their ages and supporting bibliography (Table 1). The ages and positions of Arabian MFS (Sharland et al., 2001) in Saudi Arabia are noted and revisions are proposed where appropriate (Table 2).

PREVIOUS STUDIES

In 1968, Powers published the Lexicon of Saudi Arabia wherein he compiled the nomenclature and definitions of lithostratigraphic rock units and their biostratigraphy from unpublished Aramco reports (dating back to the 1930s), papers by Aramco and other geologists, as well as his observations at outcrop and subsurface. Since then, many refinements and/or revisions have been made; particularly during the 1980–1990s when the DMMR-SGS and BRGM mapped Saudi Arabia’s Phanerozoic cover rocks in detail. They published 1:250,000 scale geological maps, each covering a 1.0oNS by 1.5oEW quadrangle, accompanied by Explanatory Notes. The key quadrangles in which the Upper Triassic and Jurassic rocks crop out, in part or completely, are shown in Figure 1a; in order of publication: Wadi ar Rayn (Vaslet et al., 1983), Sulayyimah (Vaslet et al., 1985a), Al Faydah (Vaslet et al., 1985b), Darma’ (Manivit et al., 1985a), Wadi al Mulayh (Manivit et al., 1985b), Buraydah (Manivit, et al. 1986), Shaqra’ (Vaslet et al., 1988), Ar Riyadh (Vaslet et al., 1991), Qibah (Robelin et al., 1994) and Turubah (Lebret et al., 1999). Figure 2 shows the of Saudi Arabian uppermost Triassic and Jurassic rock units as compiled in 2001 by Fischer et al. (based on the Explanatory Notes of quadrangle maps in Figure 1a).

Since the publication of the Lexicon (Powers, 1968), the stage assignments for most rock units have either been confirmed, refined or revised by biostratigraphic studies. The assignments adopted here are based on the interpretations summarized by Fischer et al. (2001), Hughes (2004a, b, c. 2006), Hughes et al. (2008) and Énay et al. (2007, 2009). The assignments in these recent papers were based on previous studies of ammonites (Énay and Mangold, 1985, 1994; Énay 1987; Énay et al., 1986, 1987), nautiloids (Tintant, 1987), brachiopods (Almeras, 1987), ostracods (Depêche et al., 1987), nannofossils (Manivit, 1987; Varol inHughes, 2006), gastropods (Fischer et al., 2001), foraminifera (Hughes, 2004a, b, c) and calcareous algae (Okla, 1987; Hughes, 2005). Besides these papers, extensive paleontological data and bibliographies are also reported in Manivit et al. (1990a, b), Le Nindre et al. (1990), doctoral thesis (Le Nindre, 1987; Manivit, 1987; Vaslet, 1987) and Explanatory Notes of quadrangle maps (Figure 1a). R. Énay (2009, written communication) noted that in Figure 2 the Oxfordian - Kimmeridgian boundary should be higher and the Hypselocyclum zone is incorrect.

In terms of chrono-sequence stratigraphy, Le Nindre et al. (1990) were the first to interpret the Mesozoic T-R sequences in central Saudi Arabia outcrops, and to correlate them to the global cycles of Haq et al. (1988). Subsequent T-R schemes followed their approach (Al-Husseini, 1997; Le Nindre et al., 2003; Haq and Al-Qahtani, 2005), or interpreted maximum flooding surfaces (Sharland et al., 2001), or biosequences (Hughes, 1996, 2004a, b, c, 2006; Hughes et al., 2008). The most recent study by Énay et al. (2009, Figure 3) revised the sequence stratigraphic interpretations for the Saudi Arabian Middle Jurassic Epoch (Le Nindre et al., 1990, 2003).

ORBITAL FORCING MODEL

In Saudi Arabia, radiometric data are not generally available for absolute age dating of rock units in million years before present (Ma). To convert interpreted biostages to ages involves using global geological time scales (e.g. Sharland et al., 2001; Haq and Al-Qahtani, 2005). For the stage boundaries of the Upper Triassic and Jurassic systems, the GTS 2004 ages are highly uncertain with standard deviations of up to + 4.0 million years (My). A different orbital-forcing approach to address this challenge was first proposed by Matthews and Frohlich (2002), and remains in testing as the Arabian Orbital Stratigraphy hypothesis (AROS, Al-Husseini and Matthews, 2008; Al-Husseini, 2008).

Based on orbital-forcing numerical simulations, Laskar et al. (2004) found the dominant term in the model of the Earth’s eccentrical orbit has a period of 405,000 years (405 Ky). They predicted it to be a stable chronometer that should be manifested in the geological records, and therefore to provide an absolute geological time scale back to at least 250 Ma (end of Permian Period). Al-Husseini and Matthews (2008) proposed the 405 Ky cycle as the fourth-order depositional sequence (denoted DS4 in AROS; Figure 4). This discrete period contrasts to the empirical view of fourth-order cycles (e.g. Vail et al., 1991), which are incorrectly believed to have a continuous range of periods between 100–500 Ky. To emphasize the importance of the 405 Ky cycle they named it the straton.

Laskar et al. (2004) also highlighted two beats with periods of ca. 1.2 and 2.4 My and stated: “because of the occurrence of these beats, the detection of the resonant state in the geological data can be possible, as one has to search now for phenomenon of large amplitude in the geological signal.” Based on numerical modeling of orbital-forcing, Matthews and Frohlich (2002) found that longer-period sequences are formed as beats by 5, 6 or 7 consecutive stratons, and proposed their corresponding discrete periods (2.025, 2.43 and 2.835 My) as third-order (denoted DS3, Figure 4). Matthews also found that pairs of DS3s combine into double-DS3 with a period of 4.86 My (12 stratons), and three double-DS3 form very long sequences with a period of 14.58 My (denoted DS2, consisting of 36 stratons). Second-order DS2 1 started at ca. 16.1 Ma with its basal boundary denoted SB2 1. Whereas stratons, and the 1.2 My and 2.4 My beats, have been recognized in geological studies as orbital cycles (e.g. Laskar et al., 2004; Palike et al., 2006), depositional sequences with much longer periods have not been given an orbital age significance.

In this Note, these long-period orbital sequences are tentatively identified in the Late Triassic and Jurassic succession of Saudi Arabia. Where third-order sequences are interpreted, the MFS is approximately dated at the mid-age of the sequence (Tables 1 and 2, Figure 5).

MINJUR SANDSTONE, BURAYDAH GROUP AND TRIASSIC – JURASSIC TrJ BOUNDARY

Authors and Nomenclature: In the Lexicon of Saudi Arabia, Powers (1968) credited the identification of the Minjur rock unit to 1945–50 unpublished Aramco reports (unnammed authors, and R.A. Bramkamp, 1950), and to Steineke and Bramkamp (1952). The first formal definition was by Bramkamp and Steineke (in Arkell, 1952) with its type section in the Wadi ar Rayn quadrangle (Figure 1a). Other mentioned authors are Thralls and Hasson (1956, 1957), Steineke et al. (1958) and Powers et al. (1966). In the Wadi ar Rayn quadrangle, Vaslet et al. (1983) divided the Minjur Sandstone into a lower assemblage (99 m thick) and upper one (115–143 m), separated by a regularly channeled boundary; here referred to as Lower and Upper Minjur members (Figure 5). The Minjur Sandstone Formation was assigned to the uppermost position of the Buraydah Group by Vaslet at al. (1988), and the two members were mapped and described in other quadrangles (Figure 1a).

Boundaries: The Minjur Sandstone unconformably overlies the Triassic Jilh Formation of the Buraydah Group (Figure 5). In the Qibah and Turubah quadrangles (Figure 1a), the basal part of the Minjur Sandstone is distinguished by a marker layer with silicified tree trunks up to 3 m (10 ft) long, and some basal units contain coarse clasts up to 2 cm in diameter that mark the soles of channel infill (Robelin et al., 1994; Lebret et al., 1999). The base Minjur is considered a sequence boundary, not only because it is characterized by channels that cut into the Jilh Formation, but also because it marks an abrupt switch from the marine setting of the Jilh Formation to the Minjur’s continental setting.

The Minjur Sandstone is unconformably overlain by the Lower Jurassic Toarcian Marrat Formation of the Shaqra’ Group (Figure 5). The Minjur – Marrat boundary corresponds to the Lower Jurassic Unconformity – Hiatus, which spans the earliest Jurassic Hettangian, Sinemurian and Pliensbachian time in Saudi Arabia (Vaslet et al., 1983, 1988, 1991; Manivit et al., 1985a, b, 1986; Al-Husseini, 1997; Sharland et al., 2001). The Early Jurassic Hiatus has been interpreted in terms of rift-shoulder uplift along the margins of the Arabian Plate (see review of plate-tectonic reconstructions in Al-Husseini, 1997; Sharland et al., 2001). It may also be, in part, related to a a period of global sea-level lowstand (Haq et al., 1988). Indeed, Price (1999) highlighted the Pliensbachian time as a possible episode of cold to sub-freezing polar climate (presumably a polar glaciation) implying the later part of the Early Jurassic Hiatus may have been due to a global sea-level lowstand.

LithologyPowers (1968) described the Minjur Sandstone, 315 m (1,033 ft) thick in type section, as a monotonous succession of clastic beds deposited in a continental setting (Figure 2).

Stage Assignment: Upper Triassic, Upper Norian and Rhaetian (Figure 5). On biostratigraphic evidence, Le Nindre et al. (1990) assigned the uppermost Jilh Formation to probable upper Middle Norian. Two meters above the Jilh – Minjur boundary, Robelin et al. (1994) identified a Norian palynological assemblage. Therefore the lower part of the Minjur Sandstone is assigned to the Upper Norian. Powers (1968) considered the Minjur Sandstone as Upper Triassic to Lower Jurassic; however, on biostratigraphic evidence, Vaslet et al. (1983, 1988) and Manivit et al. (1985a, b, 1986) restricted the Minjur Sandstone to the Upper Triassic (Upper Norian and Rhaetian).

LOWER AND UPPER MINJUR SEQUENCES

In the Darma’ quadrangle (Figure 1a), Manivit et al. (1985a) interpreted the Minjur Sandstone (227 m, 745 ft thick) as two fining-upwards depositional sequences (corresponding to the Lower and Upper Minjur members, Figure 5).

Lower Minjur Sequence (c. 137 m, 449 ft thick) begins with unsorted, gravelly, cross-bedded sandstone containing fairly steep sets and a channeled surface, and showing traces of leached silty paleosols and ferruginization. It ends with medium-grained, well-bedded sandstone with ferruginous crusts at various levels.

Upper Minjur Sequence (90 m, 295 ft thick) begins with conglomeratic sandstone channeled into the underlying sandstone [Lower Minjur Member], and continues with graded, medium- to fine-grained sandstone bodies, commonly separated by white to purplish sandy siltstone; some beds display traces of substantial bioturbation. White pedogenic clay and white sandstone appear near the top of the exposures, which end with gray and purplish silt, commonly altered to duricrust.

Age Calibration: In AROS, Al-Husseini and Matthews (2008) correlated the base Minjur Sandstone to second-order sequence boundary SB2 14 (205.6 Ma) implying a Late Norian age (older than the Norian – Rhaetian boundary at 203.6 + 1.5 Ma in GTS 2004). The Lower and Upper Minjur sequences are correlated to DS3 14.1 and 14.2 between ca. 205.6 and 200.8 Ma (Figure 5 and Table 1). In GTS 2004, the Triassic – Jurassic (TrJ) boundary is dated at 199.6 + 0.6 Ma, such that the orbital calibration is consistent with the Late Norian and Rhaetian age for the Minjur Sandstone interpreted by biostratigraphy. The corresponding MFS3 14.1 and 14.2 have ages of ca. 204.4 and 202.0 Ma, if respectively dated in the middle of the two sequences (Table 2).

MARRAT FORMATION, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) credited the identification of the Marrat rock unit to an unpublished report by M. Steineke (1937), and the first formal definition to Bramkamp and Steineke (in Arkell, 1952) with the type section in the Shaqra’ quadrangle (Figure 1a). Powers et al. (1966) described a better-exposed reference section at Khashm ad Dhibi in the Darma’ quadrangle (Figure 1a), and defined the informal lower, middle and upper Marrat, of unspecified status. Other mentioned authors include Steineke and Bramkamp (1952), Thralls and Hasson (1956, 1957) and Steineke et al. (1958). The formation and three members were mapped with modifications by Manivit et al. (1985a) and described in several quadrangles (Figure 1a).

The Shaqra’ Group was introduced by Vaslet et al. (1988) to encompass seven Jurassic formations discussed in the following sections (Figures 2 and 5). The Marrat occupies the lowermost position of the Group. The Group, together with the lower part of the Sulaiy Formation (Thamama Group), constitutes the Jurassic System in Saudi Arabia. The Jurassic – Cretaceous (JK) boundary is not documented in Saudi Arabia, and is placed by stratigraphic position within the Sulaiy Formation (Powers et al., 1966; Powers, 1968).

Boundaries: The Marrat Formation lies unconformably on the Minjur Sandstone (Lower Jurassic Unconformity, see Minjur Sandstone) (Figures 2 and 5). The Middle Jurassic (Bajocian – Callovian) Dhruma Formation unconformably overlies the Toarcian Marrat Formation, with the Aalenian Hiatus (Aalenian Unconformity) separating their depositional periods. Haq et al. (1988) interpreted a major sea-level lowstand during the Aalenian time, which Le Nindre et al. (1990) and Al-Husseini (1997) correlated to the stratigraphic gap between the Marrat and Dhruma formations.

Lithology: The Marrat Formation is mainly composed of limestone but partly sandstone and shale (Figure 2); it maintains its identity in well sections in the Rub’ al-Khali and Eastern Province of Saudi Arabia, (Figure 1, Powers, 1968). In the Darma’ quadrangle (Figure 1a), Manivit et al. (1985a) revised the interpretation of the Marrat reference section of Powers (1968) and described it in terms of the three units, together 126 m (413 ft) thick, here considered members.

The Lower Marrat Member (47 m, 154 ft thick) begins with 12 m (39 ft) of medium-grained sandstone with a matrix of green clay, and ocher and green claystone. This is overlain by a dolomitic succession with mudcracks, stromatolites, and solution vesicles, in turn overlain by clayey gypsiferous silt and sandstone with a carbonate cement. The unit ends with beds of dolomitic bioclastic limestone.

The Middle Marrat Member (40 m, 131 ft thick) begins with 3 m (10 ft) of bioturbated and bioclastic dolomite and limestone containing an abundant macrofauna. This is overlain by 25 m (82 ft) of reddish-brown laminated claystone and green gypsiferous clayey siltstone that is capped by a fine-grained sandstone. The clayey facies lie directly on the Lower Marrat in the center and north of the quadrangle. A second interval of green and brown claystone above the sandstone is associated with limestone containing a restricted fauna.

The Upper Marrat Member (39 m, 128 ft thick) begins with bioclastic limestone in thin beds, and brown claystone with ammonites and microgastropods. This is overlain by 17 m (56 ft) of clayey limestone, cream limestone containing lamellibranchs and brachiopods and with nodular and then uniform jointing, and a bed of oolitic limestone. The unit ends with a 12-m-thick (39 ft) sequence of massive gypsum incorporating broken slabs of bioclastic limestone, and foliated claystone and silt, which Powers (1968) misassigned to the lower part of Dhruma Formation.

Stage Assignment: Lower Jurassic, Toarcian Stage (Arkell, 1952 inPowers, 1968) (Figures 2, 3 and 5). In his review of biostratigraphy, Fischer et al. (2001) noted that on faunal content, the upper part of the Lower Marrat and Middle Marrat members are Lower Toarcian, whereas the basal part of the Upper Marrat Member is Middle Toarcian. These assignments were based on ammonites (Énay et al., 1987) except for the Lower Marrat Member, which yielded none (R. Énay, written communication).

Reservoir: The upper part of the Formation contains the Marrat Reservoir in eastern Saudi Arabia (Ayres at al. 1982) (Figure 5).

MARRAT SEQUENCES A AND B

The interpretation of the Marrat’s depositional settings by Powers (1968) suggests that the Lower and Middle Marrat members forms one depositional sequence, and the Upper Marrat Member another (Figure 5). Powers (1968) described the Khashm ad Dhibi sections as follows:

“The lower Marrat [36.5 m, 120 ft thick] comprises two units: at the top limestone and dolomite; in the lower part increasing amounts of shale, sandy shale and calcareous sandstone. The upper limestone interval is fossiliferous, … the lower sand and shale interval is barren, showing no trace of a marine influence. While the lower Marrat varies considerably in thickness, most of the gain or loss appears to take place below the limestone interval.

The middle Marrat [41.8 m, 137 ft thick] consists mostly of dark brick-red shale; to the south this interval becomes increasingly sandy; to the north and east in the subsurface, the red shale is replaced by green shale and/or limestone. Where present in shale facies, the entire interval appears to be non-marine representing flood plain and tidal flat deposits.

The upper Marrat [24.2 m. 79 ft thick] is aphanitic limestone with streaks of shale toward the base; shallow-water nature of these beds is demonstrated by abundance of echinoid spines; thickness is variable. Increases in thickness of the upper Marrat appear to take place at the expense of the underlying shale for their combined thicknesses remain relatively constant.”

Marrat Sequence B is interpreted to consist of the Lower and Middle Marrat members. The transgressive systems tract (TST) corresponds to the barren, non-marine shaley lower part of the Lower Marrat Member (Powers, 1968). The maximum flooding interval (MFI, dolomite and fossiliferous limestone units) occurs at the top of the Lower Member sensuPowers (1968) or straddles their boundary sensu Manivit et al. (1990). The Middle Marrat Member (flood plain and tidal flat deposits) is interpreted as the regressive systems tract (RST).

Marrat Sequence A consists of a TST and MFI represented by the aphanitic limestone (bioclastic limestone in thin beds, and brown claystone with ammonites and microgastropods) in the lower part of the Upper Marrat Member. The RST corresponds to the 12-m-thick massive gypsum in the upper part of the Upper Marrat Member (Manivit et al., 1990), which was misassigned by Powers (1968) to the lower part of Dhruma Formation.

Age Calibration:Sharland et al. (2001) positioned Middle Toarcian MFS J10 (185.0 Ma) in the lower part of the Upper Marrat Member (i.e. Marrat Sequence A); its age was revised to 181.0 Ma in GTS 2004 (Simmons et al., 2007; see summary inAl-Husseini, 2007). If the Marrat Formation is confined to the Toarcian Stage, then its depositional age would be between 183.0 + 1.5 and 175.6 + 2.0 Ma in GTS 2004. In AROS, the two Marrat sequences correlate to orbital sequences DS3 13.5 and 13.6 between 181.3 and 176.5 Ma (Figure 5 and Table 1), as consistent with intra-Toarcian time. Their corresponding MFS3 13.5 and 13.6 have ages of ca. 180.1 and 177.7 Ma; the latter correlates by position to MFS J10 (181.0 Ma), but with a much younger age (177.7 Ma, Table 2).

DHRUMA FORMATION, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) credited the identification of the Dhruma rock unit to an unpublished report by M. Steineke (1937), and the first formal definition to Bramkamp and Steineke (in Arkell, 1952). Other mentioned authors include Steineke and Bramkamp (1952), Thralls and Hasson (1956, 1957), Steineke et al. (1958) and Powers et al. (1966). In Powers (1968), the type section of the Dhruma Formation, located at Khashm ad Dhibi in the Darma’ quadrangle (Figure 1a), was divided into informal lower, middle and upper Dhruma, of unspecified rank. The Dhibi Limestone Member occurs in the upper part of the lower Dhruma, and the ‘Atash and Hisyan members form the upper Dhruma (Figures 2, 3 and 5). Vaslet et al. (1983) further divided and mapped the formation as seven units (D1 to D7) in the Wadi ar Rayn quadrangle. Manivit et al. (1990) adopted the D1–D7 units and revised the definition of Powers (1968) at the type section. The D1–D7 units were mapped in other quadrangles (Figure 1a), and most recently units D3–D7 were reinterpreted by Énay et al. (2007, 2009), with the Wadi ad Dawasir “delta” isolated as a separate unit between D5 and D6 (Figure 3).

Hughes (2006) modified the lithostratigraphic definition of the lower Dhruma units by naming the entire D2 unit as the Dhibi Limestone Member, and unit D1 as the Balum Member (Figure 5). In this Note, the Balum Member encompasses the D1 unit and lower part of D2 unit such that the original definition of the Dhibi Limestone is preserved (Powers, 1968; Vaslet et al., 1983). In Hughes (2006) units D3 to D5 are named as the Uwaynid, Barrah and Mishraq members, respectively. The Wadi ad Dawasir “delta” and Dhruma D6 units are not designated as members in Hughes (2006).

Boundaries: The Dhruma unconformably overlies the Marrat Formation (Aalenian Hiatus – Aalenian Unconformity; Figures 2 and 5). Powers (1968) reported that the top of the Dhruma Formation is also an unconformity; he wrote: “Truncation is slight, almost everywhere involving Tuwaiq Mountain Limestone resting on the gently eroded surface of the Hisyan Member. At Jauf and Safaniya, however, all but the pre-Dhibi part of the formation has been removed and the lower Dhruma in these two areas is overlain respectively by the Tuwaiq Mountain and Hanifa formations.”

Lithology: At the type section in the Darma’ quadrangle (Figures 1a and 2), the Dhruma Formation was initially measured at 374.5 m (1,228 ft) (Powers, 1968) but later remeasured to 447 m (1,466 ft) by Manivit et al. (1990). Powers (1968) described the Formation as mainly composed of carbonate in the subsurface, carbonate and claystone in the central part of the outcrop belt area, and siliciclastics in outcrops to the north and south. In northeastern subsurface Saudi Arabia (Jauf and Safaniya oil fields, Figure 1b), darker, more argillaceous, presumably deeper-water, limestone and gray-black shale occur.

Lower Dhruma, Units D1 and D2, Balum and Dhibi Limestone Members:Powers (1968) reported that the lower Dhruma is 127 m (417 ft) thick in type section, and usually between 60–120 m (197–394 ft) at outcrop; Manivit et al. (1990) measured its thickness as 143 m (469 ft; unit D1 = 57 m = 187 ft, and D2 = 86 m = 282 ft thick). The Dhibi Limestone Member (30–35 m, 100–115 ft thick) occurs in the upper part of the lower Dhruma (upper part of unit D2), which at outcrop and subsurface passes from limestone downwards into partly calcarenitic limestone with interbedded shale (D1 and lower part of D2 units, Balum Member) (Powers, 1968).

Middle Dhruma, Units D3 to D6, Uwaynid, Barrah and Mishraq Members: The middle Dhruma, 193 m (633 ft) thick, consists of unit D3 (Uwaynid Member: 42–52.5 m, 138–172 ft thick), unit D4 (Barrah Member: 26–44 m, 85–144 ft thick), unit D5 (Mishraq Member: 41–47 m, 135–154 ft thick) and unit D6 (28–55.5 m, 92–182 ft thick) (Manivit et al., 1990). Prior to the study by Énay et al. (2009), the middle Dhruma was believed to represent relatively continuous deposition without major stratigraphic gaps (Figure 2). The most significant stratigraphic break within the Dhruma Formation was previously interpreted between the middle and upper Dhruma (D6 – D7 boundary) and associated with a Late Bathonian – Early Callovian hiatus (e.g. see discussion in Le Nindre et al., 1990; Al-Husseini, 1997; Fischer et al., 2001; Sharland et al., 2001; Hughes, 2006). The biostratigraphic interpretation by Énay et al. (2007, 2009) introduced a separate chrono-stratigraphic unit between D5 and D6: the upper Lower and Middle Bathonian Wadi ad Dawasir “delta” (Figure 3). They showed a hiatus following the “delta” unit, and essentially closed or reduced the duration of the Late Bathonian – Early Callovian stratigraphic gap (between D6 – D7).

Upper Dhruma, Unit D7, ‘Atash and Hisyan Members:Powers (1968) divided the upper Dhruma (111 m) into the ‘Atash Member (c. 40 m, 131 ft thick) and overlying the Hisyan Member (47 m = 154 ft at type section, ranging from 20–145 m = 66–476 ft in thickness), together corresponding to Dhruma unit D7 (Figure 2). Manivit et al. (1990) measured thicknesses for the ‘Atash at an approximately uniform 26 m (85 ft) and the Hisyan at between 57–85 m (187–279 ft) in the Darma’ quadrangle.

Stage Assignment: Unit D1 and the basal part of D2 are Lower Bajocian (Balum Member), the Dhibi Limestone Member is Upper Bajocian, Unit D3 (Uwaynid Member) Upper Bajocian, D4 (Barrah Member) and D5 (Mishraq Member) Lower Bathonian, Wadi ad Dawasir “delta” upper Lower and Middle Bathonian, D6 Upper Bathonian, and D7 (‘Atash and Hisyan) Lower and lower Middle Callovian (Figures 2, 3 and 5).

Reservoir: The middle Dhruma contains three reservoirs in eastern Saudi Arabia (Figure 5); their correlations to outcrop are not firmly established: Faridah Reservoir possibly to unit D3, Sharar Reservoir possibly to unit D4, and Lower Fadhili Reservoir to unit D6 (Haq and Al-Qahtani, 2005; Hughes, 2006). The ‘Atash Member correlates to the Lower Fadhili Reservoir in the Fadhili and Qatif fields (Figure 1b; Powers, 1968; Hughes, 2006).

LOWER DHRUMA SEQUENCE

Following the Aalenian Hiatus, the oldest part of the Dhruma Formation marks the start of a regional transgression in earliest Bajocian time. The upper boundary of unit D2 (top Dhibi Limestone) was interpreted as a sequence boundary by Énay et al. (2009; Figures 3 and 5) and coincides with a lithologic and electrical log break (Powers, 1968). Hughes (2006) interpreted the Balum Member and Dhibi Limestone (sensuPowers, 1968, and this Note) as two T-R cycles: Balum and Dhibi subsequences. His interpretation is consistent with Powers (1968) in which the “lower Dhruma unconformity” constitutes the lower boundary of the Dhibi Limestone (i.e. in the middle of unit D2). It is expressed by regional erosion that cuts out the upper beds of the lower shaley interval (D1 unit). These considerations suggest that the lower Dhruma forms a composite T-R sequence – Lower Dhruma Sequence – consisting of two higher-order T-R cycles.

Age Calibration:Sharland et al. (2001) positioned Lower Bajocian MFS J20 (175.0 Ma) at the top of unit D1 (within the Balum Member); its age was revised to 171.0 Ma in GTS 2004 by Simmons et al. (2007; Al-Husseini, 2007). The Dhruma transgression arrived in central Saudi Arabia in earliest Bajocian, estimated in GTS 2004 at 171.6 + 3.0 Ma; the average GTS 2004 age correlates precisely with the predicted AROS age of SB3 12.3 (171.6 Ma) (Figure 5 and Table 1). This age correlation places the start of the Dhruma transgression within DS3 12.3. The older long-lasting sea-level lowstand (Aalenian Hiatus) correlates to DS3 12.1 and 12.2 between 176.5 and 171.6 Ma, approximately the Aalenian Stage (175.6 + 3.0 to 171.6 + 3.0 Ma). The Lower Dhruma flooding surface MFS J20 (171.0 Ma) is correlated to MFS3 12.3 at ca. 170.4 Ma (Table 2).

DHRUMA SEQUENCE B

The recent study by Énay et al. (2007, 2009) interpreted a sequence from the base of the middle Dhruma (base D3, base Barrah) through unit D5 (Mishraq), culminating in the Wadi ad Dawasir “delta” regression (Figure 3). The deposition of the “delta” was followed by an hiatus in late Mid-Bathonian. Dhruma Sequence B corresponds to this newly interpreted sequence (Figure 5). The letter “B” in “Dhruma Sequence B” is used here (rather than middle Dhruma sequence) so as to distinguish it from the middle Dhruma, which also includes unit D6. Énay et al. (2009) interpreted a Lower Bathonian MFS in the middle of D5 (Mishraq), the RST in the upper part of D5 and capped Dhruma Sequence B with the “delta” unit.

Age Calibration: The MFS of Énay et al. (2009) is consistent with the stage assignment and stratigraphic position of Lower Bathonian MFS J30 (168.0 Ma) of Sharland et al. (2001) with revised age of 167.5 Ma in GTS 2004 (Simmons et al., 2007; Al-Husseini, 2007). The Mid-Bathonian hiatus (Énay et al., 2009) can be approximately dated by GTS 2004 by assuming the mid-point age for the Bathonian Stage (halfway between 164.7 and 167.7 Ma) implying ca. 166.2 Ma. This estimate compares well with the age of SB3 12.5 (166.7 Ma) (Figure 5 and Table 1). Dhruma B Sequence correlates to AROS sequence DS3 12.4 between ca. 169.2 and 166.7 Ma. MFS J30 (167.5 Ma) correlates to MFS3 12.4 at ca. 168.0 Ma (Table 2).

TUWAIQ MOUNTAIN LIMESTONE, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) credited the identification of the Tuwaiq Mountain Limestone rock unit to an unpublished report by M. Steineke (1937), and the first formal definition to Bramkamp and Steineke (in Arkell, 1952). The type section is located northwest of the capital Riyadh in the Darma’ quadrangle (Figure 1a). Other mentioned authors include Steineke and Bramkamp (1952), Thralls and Hasson (1956, 1957), Steineke et al. (1958) and Powers et al. (1966). Three informal lithostratigraphic units (T1, T2 and T3) were defined and mapped by Vaslet et al. (1983) in the Wadi ar Rayn and other quadrangles (Figure 1a). Manivit et al. (1985a) retained the section of Powers (1968) and adopted the T1–T3 units during the mapping of the Darma’ quadrangle. Further lithostratigraphic and sedimentologic studies, carried out in the vicinity of Riyadh, have been published by Okla (1987) and Moshrif (1987) and references therein. The T1, T2 and T3 units were named as Baladiyah, Mysiyah and Daddiyah members, respectively in Haq and Al-Qahtani (2005) and Hughes (2006) (Figure 5).

Boundaries: The Tuwaiq Mountain Limestone lies unconformably above the Dhruma Formation (see Dhruma Formation; Figures 2 and 5). It is overlain by the Hanifa Formation with the boundary informally named the pre-Hanifa unconformity (Powers, 1968).

Lithology:Powers (1968) reported that the Tuwaiq Mountain Limestone (203 m, 666 ft thick at type section) consists of mainly dense aphanitic limestone with some layers of calcarenitic limestone and calcarenite (Figure 2); it becomes soft and marly in the lower 35–40 m (100–131 ft) interval. At outcrop, the clean shelf carbonates form a great plate of coral-bearing, dense, pure limestone, at the base of which is a thin non-coralliferous well-bedded chalky unit. In the subsurface, the lower part of the formation is made up of gray, calcite-cemented calcarenites and calcarenitic limestones; the upper massive aphanitic limestone interval tends to become progressively more argillaceous and impure going eastward in the subsurface.

Stage Assignment: The stage assignments (and thicknesses) of the three Tuwaiq Mountain Limestone members are Middle Callovian for Daddiyah Member (unit T1; 32 m = 105 ft), Middle Callovian for Mysiyah Member (unit T2, 56 m = 184 ft) and Middle to Upper Callovian for Baladiyah Member (unit T3, 96 m = 315 ft) (Vaslet et al., 1983; Énay et al., 2009) (Figures 2, 3 and 5).

Reservoir:Powers (1968) reported that the oil-bearing Hadriya Reservoir occurs in the upper part of the Formation at Abu Hadriya field (Figures 1b and 5). Ayres et al. (1982) reported the Upper Fadhili Reservoir occurs in the lower part of the Formation, and Hughes (2006) correlated the Upper Fadhili Reservoir to the Baladiyah Member (unit T1) and the Hadriya Reservoir to the Daddiyah Member (unit T3).

DHRUMA SEQUENCE A AND TUWAIQ SEQUENCE

Hughes (2006) interpreted the upper Dhruma (unit D7, ‘Atash and Hisyan members) and Baladiyah Member (unit T1) as a T-R cycle. Énay et al. (2009, with Hughes as coauthor), on the other hand, interpreted a much longer sequence to consist of a TST in D6 and lower part of D7 with a Middle Callovian MFS in the middle of the Hisyan Member (D7, Figure 3). They interpreted the RST in the upper part of the Hisyan Member and the entire Tuwaiq Mountain Limestone. Their Dhruma–Tuwaiq Mountain Limestone sequence appears to ignore the significance of the top Dhruma unconformity between these two formations in subsurface (Powers, 1968, see above). It also does not account for the MFS at the base of upper Middle Callovian Daddiyah Member (unit T3, Le Nindre et al., 1990; Figure 3). These considerations argue that Dhruma units D6 and D7 (‘Atash and Hisyan members) form one third-order sequence – Dhruma Sequence A, whereas the Tuwaiq Mountain Limestone another – Tuwaiq Sequence (Figure 5 and Table 1).

Age Calibration: The MFS in the Hisyan Member (unit D7, Énay et al. (2009) is consistent by stratigraphic position and stage assignment with Middle Callovian MFS J40 (162.0 Ma) of Sharland et al. (2001), with revised age of 162.5 Ma in GTS 2004 (Simmons et al., 2007; Al-Husseini, 2007). In AROS, Dhruma Sequence A and the Tuwaiq Sequence correlate to DS3 12.5 and 12.6 (Figure 5 and Table 1). Together these two sequences extend form Late Bathonian to end-Callovian (Énay et al., 2009) and span ca. 5.1 My in average GTS 2004 time (ca. 166.3 Ma to 161.2 Ma). In AROS, they span a period of 4.86 My between 166.7 and 161.9 Ma. The MFS of Dhruma Sequence A is correlated to MFS3 12.5 at ca. 165.5 Ma, and MFS J40 (162.5 Ma) but with an older age. The MFS of the Tuwaiq Sequence (Le Nindre et al., 1990) is correlated to MFS3 12.6 at ca. 163.1 Ma (Table 2).

Polar Glaciation during the Callovian – Oxfordian Transition

Dromart et al. (2003) interpreted a latest Callovian – Oxfordian severe cooling event based on analysis of sea-surface temperatures and isotopic thermometry. They concluded that this event climaxed in the latest Callovian and lasted about 2.6 My. It had a pronounced asymmetrical pattern, consisting of an 0.8 My temperature fall and a 1.8 My stepwise recovery. They correlated the latest Callovian climax to a sea-level drop of between 40–80 m (130–260 ft) and a hiatus following the deposition of the Tuwaiq Mountain Limestone. Tremolada et al. (2006) also recognized this cooling event from a study of calcareous nannofossil assemblages. In terms of average GTS 2004 time, the Callovian – Oxfordian boundary (161.2 Ma) is 700 Ky younger than to SB2 11 (161.9 Ma) implying a Late Callovian age for this second-order sequence boundary and polar glaciation (Figure 5 and Table 1).

HANIFA FORMATION, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) credited the identification of the Hanifa rock unit to an unpublished report by M. Steineke (1937). The unit was first published by Kerr (1951), and formally defined by Bramkamp and Steineke (in Arkell, 1952) in the Darma’ quadrangle (Figure 1a). Other mentioned authors include Steineke and Bramkamp (1952), Thralls and Hasson (1956, 1957), Steineke et al. (1958) and Powers et al. (1966). Vaslet et al. (1991) formally defined the Hawtah and overlying Ulayyah members of the Hanifa Formation (obsolete H1 and H2 units) in the Ar Riyadh quadrangle (Figure 1a) and the members were also mapped and described in other quadrangles. The sequence stratigraphy of the Hanifa Formation was interpreted by Moshrif (1984, and references therein).

Boundaries: The Hanifa Formation lies disconformably upon the Callovian Tuwaiq Mountain Limestone at outcrop (Figures 2 and 5), and in the subsurface it corresponds to the pre-Hanifa unconformity of Powers (1968). At outcrop and subsurface the Jubaila Limestone lies conformably upon the Hanifa Formation (Powers, 1968; Vaslet et al., 1991).

Lithology: The Hanifa Formation, 113 m (371 ft) thick in the Ar Riyadh quadrangle (Figure 1a, Vaslet et al. 1991), attains a thickness of 180 m (590 ft) in the far eastern Rub’ al-Khali and along the coast of the Arabian Gulf (Figure 1b, Powers, 1968). It consists of a lower muddy carbonate unit and an upper stromatoporoid and lagoonal carbonate unit (Figure 2).

Stage Assignment: Oxfordian to possibly lowermost Kimmeridgian (Vaslet et al., 1991; Figures, 2 and 5).

Reservoir:Powers (1968) reported that the Hanifa Reservoir (60–90 m, 197–295 ft thick) occurs in the upper part of the formation in Abqaiq, Abu Hadriya, Berri, Khurais, Khursaniyah and Qatif oil fields (Figure 1b). Hughes (2006) correlated the Hanifa Reservoir to the upper part of the Ulayyah Member.

HAWTAH SEQUENCE

The Lower and Middle Oxfordian Hawtah Member (up to 66 m, 217 ft thick) is bounded by sequence boundaries and corresponds to the Hawtah Sequence, (Vaslet et al. 1991; Mattner and Al-Husseini, 2002; Al-Husseini et al., 2006; Hughes et al., 2008; Figure 5).

Age Calibration: The Hawtah Sequence contains MFS J50 (Sharland et al., 2001) repositioned from Middle to Lower Oxfordian and redated from 156.0 to 159.0 Ma by Simmons et al. (2007; Al-Husseini, 2007). The age of the Hawtah Sequence is intra-Oxfordian between 161.2 + 4.0 and 155.7 + 4.0 Ma in GTS 2004. It was interpreted as a short third-order orbital sequence (DS3 11.1, 2.025 My, five stratons) with a depositional age of 161.9–159.9 Ma (Al-Husseini et al., 2006; Figure 5 and Table 1); the age for MFS3 11.1 is ca. 160.9 Ma (Table 2) and correlated to MFS J50 by position.

ULAYYAH SEQUENCE

The Upper Oxfordian to possibly Lower Kimmerdigian Ulayyah Member (up to 74 m, 243 ft thick) is bounded by sequence boundaries and corresponds to the Ulayyah Sequence (Vaslet et al. 1991; Mattner and Al-Husseini, 2002; Al-Husseini et al., 2006; Hughes et al., 2008; Figure 5).

Age Calibration: MFS J60 (Sharland et al., 2001) was revised from Lower Kimmeridgian to Upper Oxfordian by Simmons et al., 2007) and redated from 154.0 to 155.25 Ma (see Al-Husseini, 2007). The age of the Ulayyah Sequence is Late Oxfordian – ?Early Kimmeridian, essentially its top is near-to or younger-than their boundary at 155.7 + 4.0 Ma in GTS 2004. It was interpreted as a long third-order orbital sequence (DS3 11.2, 2.835 My, 7 stratons) with a depositional age of 159.9–157.0 Ma (Al-Husseini et al., 2006; Figure 5 and Table 1) suggesting an older average age (ca. 157.0 Ma) for the Oxfordian – Kimmeridian boundary than estimated in GTS 2004 (155.7 Ma). The age for MFS3 11.2 is ca. 158.4 Ma and correlated to MFS J60 by position (Table 2).

JUBAILA LIMESTONE, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) credited the identification of the Jubaila Limestone rock unit to an unpublished report by M. Steineke (1937). It was first published by Steineke and Bramkamp (1952), and first formally defined by Bramkamp and Steineke (in Arkell, 1952) in the Darma’ quadrangle (Figure 1a and 2). Other mentioned authors include Thralls and Hasson (1956, 1957), Steineke et al. (1958) and Powers et al. (1966). At outcrop, it consists of informal units J1 (50 m, 164 ft thick) and J2 (35 m, 115 ft thick), which are separated by a prominant disconformity or unconformity (Manivit et al., 1985a, b, 1986; Vaslet et al., 1991), and mapped and described in other quadrangles (Figure 1a).

Boundaries: At outcrop and subsurface, the Jubaila Limestone occurs conformably between the Hanifa Formation below, and Arab Formation above (Powers, 1968; Vaslet et al., 1991; Figures 2 and 5).

Lithology: In the outcrop belt, the Jubaila carbonates in central Saudi Arabia (Figure 2) pass into sandstones to the south and northwest, whereas in the subsurface the Formation consists of moderately deep-marine carbonates in the lower part that are overlain by a shallow-marine stromatoporoid-associated assemblage (Powers, 1968; Hughes, 2006).

Stage Assignment: Based on nautiloids (Tintant, 1987) and endemic ammonites that resemble Lower Kimmeridgian species, the Jubaila Limestone is considered Lower Kimmeridgian (Énay et al., 1987; Fischer et al., 2001; Hughes, 2006; Figures 2 and 5).

Reservoir: The Arab-D Reservoir consists of the upper part of the Jubaila Limestone (not established as correlative to the J2 unit at outcrop) and overlying carbonates of the Arab-D Member (Powers, 1968; Ayres et al., 1982; Meyers et al., 1996; Figure 5). Powers (1968) reported that the Arab-D reservoir is, by far, the most important reservoir discovered in the Arabian Peninsula. It contains significant quantities of oil at Abu Sa’fah, Abqaiq, Dammam, Fadhili, Ghawar, Khurais, Khursaniyah and Qatif fields (Figure 1b).

ARAB FORMATION, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) reported the Arab rock unit was considered a zone by several authors (Weeks, 1949; Eicher and Yackel, 1951; Thralls, 1955; Hasson, 1955, unpublished reports), or a formation by R.A. Bramkamp (1951, unpublished report) and others (Thralls and Hasson, 1956, 1957). The first formal definition was by Steineke et al. (1958) in the Dammam-7 well (Figure 1b), as adopted in Powers et al. (1966) and Powers (1968). The Formation was mapped and described in several quadrangles (Figure 1a).

Boundaries: At outcrop and subsurface, the Arab Formation occurs conformably between the Jubaila Limestone below, and Hith Anhydrite above (Powers, 1968; Vaslet et al., 1991; Figures 2 and 5).

Lithology: The Arab Formation (c. 54 m = 177 ft at outcrop, c. 100–180 m = 328–590 ft in subsurface) consists in ascending order of three carbonate-evaporite cycles, formally named Arab-D to Arab-B members and the carbonate Arab-A Member (Powers, 1968; Figures 2 and 5). The Arab-A Member is overlain by the Hith Anhydrite. In most Saudi Arabian outcrops, the anhydrite beds of the Arab and Hith formations are dissolved, and the carbonates are brecciated and collapsed (Powers, 1968; Vaslet et al., 1991).

Stage Assignment: The microfaunal associations of the Arab Formation suggest a Kimmeridgian to Tithonian stage assignment (reviews by Fischer, 2001; Hughes, 2006). Sharland et al. (2001) assigned all four Arab carbonates (A to D) to Lower Kimmeridgian (Figures 2 and 5).

Reservoir: Arab-D Reservoir, see Jubaila Limestone. The carbonates of the Arab-C, B and A members correspond to the Arab-C, B and A reservoirs (Figure 5; Powers, 1968; Ayres et al., 1982; Meyers et al., 1996). Powers (1968, Figure 1b) reported that significant quantities of oil are present in the Arab-C Reservoir at Abqaiq, Abu Hadriya, Abu Sa’fah, Berri, Dammam, Khursaniyah, Manifa, and Qatif fields; a minor accumulation has been found at northern Ghawar field. The Arab-B Reservoir contains oil in Abu Hadriya, Berri, Dammam, Khursaniyah, Manifa, and Qatif fields. The Arab-A Reservoir (synonymous with the Arab-A Member) contains significant amounts of oil in Abu Hadriya, Berri, Dammam, Khursaniyah, and Manifa; minor accumulations occur in north Qatif and Abu Sa’fah fields.

JUBAILA SEQUENCE AND ARAB-D SEQUENCE

The Jubaila Limestone and Arab-D Member may each form one third-order sequence, separated by a subtle sequence boundary (Figure 5). Hughes (1996), based on bioevent stratigraphy, interpreted the Arab-D Reservoir (upper part of the Jubaila and Arab-D carbonate) in terms of biozones that reflect a transgression and highstand. He interpreted the Arab-D Anhydrite as a lowstand, but was uncertain as to whether the sequence boundary should be positioned at its top or base. In this Note, sequence boundaries are interpreted at the base of the Arab Formation and at the top of the Arab-D Anhydrite, such that the Jubaila Limestone and Arab-D Member each forms a T-R sequence. This interpretation follows Powers (1968) who reported:

“In well sections, the Arab Formation is defined to include four main cycles of deposition each of which started with shallow-water, normal marine carbonate and closed with precipitation of nearly pure anhydrite, probably first deposited as gypsum. The boundary between the carbonate units and their capping anhydrites is known to be diachronous in at least two cases and perhaps it is in all. This, coupled with the fact that each carbonate-anhydrite cycle taken as a whole approximates a time-stratigraphic unit, prompted redefinition of the lower three members to include their anhydrite caps (Powers et al., 1966).”

Age Calibration: Upper Kimmeridgian MFS J70 (Sharland et al., 2001) was positioned in the lower part of the Jubaila Formation with an age revised from 152.75 to 152.25 Ma (Simmons et al., 2007). The Jubaila Limestone and the Arab-D Member are of Kimmeridgian age (Hughes, 2004a, b, c, 2006) implying an age between 155.0 and 150.8 + 4.0 Ma in GTS 2004. In AROS, the Jubaila and Arab-D Sequences are correlated to DS3 11.3 and 11.4 between 157.0 and 152.2 Ma (Figure 5 and Table 1), as approximately consistent with the Kimmeridgian Stage. Their respective MFS would correlate MFS3 11.3 (ca. 155.8 Ma) and 11.4 (ca. 153.4 Ma; Table 2). MFS3 11.3 correlates by position to MFS J70, but the Arab-D MFS3 11.4 does not have a designated Arabian Plate MFS.

HITH ANHYDRITE, SHAQRA’ GROUP

Authors and Nomenclature:Powers (1968) credited the identification of the Hith rock unit to an unpublished report by R.A. Bramkamp and T.C. Barger (1938). It was amended by R.W. Powers et al. (1964, unpublished report), and formally defined in Powers et al. (1966) at Dawl Hith, north of the capital Riyadh (Figure 1a). The Hith Anhydrite Formation is divided into the main Hith anhydrite and overlying Manifa Reservoir (Powers, 1968). The latter was first named the Manifa zone; with the discovery of oil at Manifa, in 1957, it was more formally designated as the Manifa Reservoir in Powers (1968). In this Note, the Main Hith Anhydrite and Manifa Reservoir are ranked as members.

Boundaries: The Hith Formation conformably overlies the Arab Formation (Powers, 1968; Figures 2 and 5). The contact with the overlying Sulaiy Formation is a possible disconformity, taken at the change from limestone breccia (solution-collapse) below, to evenly bedded oolite calcarenite above (Powers, 1968).

Lithology: The Hith Anhydrite Formation, 90 m (295 ft) thick at the type section, represents the evaporitic part of the Arab-A and Main Hith carbonate-evaporite cycle (Powers, 1968; Figures 2 and 5). Powers (1968) reported that characteristically, the lower four-fifths (usually between 60–120 m, 197–394 ft thick) of the formation is massive, bedded anhydrite with minor and insignificant intercalations of aphanitic limestone, dolomite or calcarenite. Much of the anhydrite gives way to salt at Haradh and Khurais fields, and in the southwestern Rub’ al-Khali (Figure 1b). He informally named this unit as the main Hith anhydrite (here Member). The Manifa Reservoir (Member) occurs in northeastern Saudi Arabia, between the sharply defined top of the Main Hith Anhydrite and the base of the tight aphanitic limestone of the Sulaiy Formation. The Reservoir/Member, on average c. 18 m (59 ft) thick, is mainly oolite calcarenite with variable amounts of nodular anhydrite, bedded anhydrite and aphanitic carbonate.

Stage Assignment: The Hith Anhydrite is considered Tithonian based on stratigraphic position. Based on strontium-isotope analysis, G. Grabowski (written communication inAl-Husseini and Matthews, 2006) interpreted the age of the Hith Anhydrite as Early to Mid-Tithonian.

Reservoir: The Main Hith Anhydrite forms the regional caprock of the Arab reservoirs, and in particular the Arab-A Reservoir. Power (1968) reported that the Manifa Reservoir has been identified at Ghawar, Khurais, Manifa and fields to the north (Figure 1b).

ARAB-C AND B SEQUENCE AND ARAB-A - MAIN HITH SEQUENCE

The Arab-C and B members form two carbonate-evaporite T-R cycles (Figures 2 and 5), each believed to be a combinations of fourth-order sequences (stratons), and together apparently form the third-order Arab-C and B Sequence. The Arab-A and Main Hith Anhydrite form the Arab-A - Main Hith Sequence (Hith Anhydrite below the Manifa Reservoir/Member).

Age Calibration:Sharland et al. (2001) positioned Upper Kimmeridgian MFS J80 (151.75 Ma), MFS J90 (151.25 Ma) and MFS J100 (150.75 Ma) in the Arab-C to Arab-A reservoirs, respectively. Their ages were slightly revised by Simmons et al. (2007, Table 2).

In AROS (Figure 5 and Table 1), the Arab-C and B Sequence corresponds to DS3 11.5 between 152.2–150.1 Ma (short sequence, 2.025 My, 5 stratons) with the main MFS3 11.5.2 at ca. 151.6 Ma in the Arab-C carbonate. MFS4 11.5.4 of the Arab-B cycle is tentatively dated ca. 150.8 Ma. The Arab-A and Main Hith Sequence corresponds to DS3 11.6 between 150.1–147.3 Ma with MFS3 11.6 at ca. 148.5 Ma. The MFS J100 was picked in the Arab-A carbonate, which is the lowermost straton of DS3 11.6 (denoted DS4 11.6.1) with MFS4 at ca. 149.9 Ma (Table 2).

Polar Glaciation during the Tithonian

Price (1999) presented evidence that supports an episode of cold or sub-freezing polar climate during the Tithonian time. He hypothesized that the Earth’s Tithonian climate may have involved a relatively steep Pole-to-Equator temperature gradient. The Arab and Hith evaporites were deposited in a low paleo-latitudinal Arabian Plate (between the Equator and 20° South, see review in Al-Husseini, 1997) and reflect a hot and arid setting. Evaporitic conditions prevailed during sea-level lowstands, most probably due to restriction caused by a rimmed margin along the edge of the Neo-Tethys Ocean. The upwards increasing proportion of evaporites-to-carbonates (from the Arab-D Anhydrite towards the Main Hith Anhydrite) implies an aridity climax occurred during the deposition of the Main Hith Anhydrite in mid-Tithonian time. The arid conditions were probably due to the increased locking-up of atmospheric moisture in polar ice.

BASE MANIFA RESERVOIR AND SULAIY TRANSGRESSION

The base Manifa Reservoir/Member marks the end of deposition of massive Jurassic evaporites in the Arabian Plate (Figure 5). This member is not recognized at outcop, where it may be either brecciated due to dissolution of the underlying Hith Anhydrite, or absent by non-deposition (Powers, 1968). The Manifa Reservoir corresponds to the first transgressive straton (DS4 10.1.1) immediately preceeding the regional transgression of the Late Jurassic – Early Cretaceous Sulaiy Formation (R.K. Matthews, 2007, written communication).

Age Calibration:Sharland et al. (2001) positioned Middle Tithonian MFS J110 in the Manifa Reservoir with an age of 147.0 Ma. In AROS, second-order SB2 10 is estimated to have an age of 147.3 Ma and correlated to base Manifa (Figure 5 and Table 1), and the estimated age of MFS410.1.1 (straton) is ca. 147.1 Ma (Table 2).

AGE COMPARISONS

In Table 1 and Figure 5, Arabian sequence boundaries, orbital SBs and their age estimates from GTS 2004 are compared. They are all consistent with GTS 2004 ages if one standard deviation is invoked. This criterion, however, is not a meaningful measure of accuracy for the present exercise because most deviations exceed several million years. When only average GTS 2004 ages are considered, approximate age correlations are evident in several cases:

  • A double inequality occurs for the two sequences of the Minjur Sandstone. The top should be older than 199.6 Ma (TrJ boundary) and the base older than 203.6 Ma (Norian – Rhaetian boundary); indeed this pair of sequences has AROS ages that meet these two constraints.

  • Another double inequality is noted for the Toarcian Marrat Formation (average age between 175.6–183.0 Ma in GTS 2004), as consistent with its two Marrat sequences predicted between 176.5–181.3 Ma.

  • Base Dhruma Formation, base Bajocian (171.6 Ma) and SB3 12.3 (171.6 Ma) correlate precisely and correspond to the start of the Dhruma transgression following the Aalenian Hiatus.

  • SB3 12.5 (166.7 Ma) correlates to the Mid-Bathonian hiatus following the Wadi ad Dawasir “delta” regression (Figures 3 and 5). Its age should be intra-Bathonian and between 164.7 and 167.7 Ma, as the case.

  • Base Hawtah Sequence dated as near base Oxfordian (161.2 Ma) should be younger than SB2 11 (161.9 Ma) corresponding to the climax of the latest Callovian polar glaciation (Dromart et al., 2003).

The main difference between GTS 2004 average time and AROS time occurs for the age of top Hanifa Formation. It is near the Oxfordian – Kimmeridgian boundary at 155.7 Ma in GTS 2004, which is 1.3 My younger than the AROS estimate (157.0 Ma). Age comparisons in the Kimmeridgian and Tithonian times cannot be made because of biostratigraphic uncertainties in dating most of the Arab members and Hith Anhydrite. Whereas the Arab-D Member is considered Kimmeridgian (Hughes, 2006; Sharland et al., 2001); the Arab-C to A members are considered Early Kimmeridgian (Sharland et al., 2001) or possibly Tithonian (Hughes, 2006).

Table 2 and Figure 5 summarize the Arabian Plate MFS (Sharland et al., 2001) and AROS MFS for the Late Triassic and Jurassic periods of Saudi Arabia. Reasonable age correlations (+ 1.0 My) are evident in several cases (e.g. MFS J20, J30, J80, J100 and J110), but most differ by several million years.

CONCLUSIONS

The emergent second-order pattern in Figure 5 resembles, to some extent, the orbital template in Figure 4, inspite of masking by the major hiatuses in the Early Jurassic and Aalenian times. Second-order DS2 14 starts with the continental third-order sequences DS3 14.1 and 14.2 (Lower and Upper Minjur sequences) but then the Early Jurassic Hiatus masks DS3 14.3 to 14.6. Second-order DS2 13 continues with the Hiatus through DS3 13.1 to 13.4, until the third-order 13.5 and 13.6 transgressions deposited the Toarcian Marrat sequences B and A. The Aalenian Hiatus, together with the Dhruma and Tuwaiq Mountain Limestone span the Arabian Mid-Jurassic Epoch, which in average GTS 2004 time lasted ca. 14.4 My (between 175.6 and 161.2 Ma) – it compares closely in age and duration with DS2 12 (between 176.5–161.9 Ma) lasting 14.58 My.

A complete second-order sequence DS2 11 emerges in the succession consisting of the Hanifa, Jubaila, Arab formations and Main Hith Anhydrite Member. It spans the Late Jurassic Epoch except for the Late Tithonian (in average GTS 2004 time between 161.2 and ca. 147.0 Ma) for ca. 14.2 Ma – again close to the predictions for DS2 11 (161.9 and ca. 147.3 Ma) for 14.58 My. Importantly, the sequence boundaries of DS2 11 are both implied polar glaciations: the lower corresponds to the Callovian – Oxfordian polar glaciation (SB2 11) interpreted by Dromart et al. (2003) and the upper to the Tithonian glaciation (SB2 10) implied by Palike et al. (2006).

Six double third-order sequences (duration of 4.86 My) are interpreted in the studied interval: (1) one pair in Minjur, (2) another pair in Marrat, (3) Lower Dhruma and Dhruma B, (4) Dhruma A and Tuwaiq, (5) Jubaila and Arab-D, and (6) Arab-C and B, and Arab-A and Main Hith Anhydrite. Each of the 12 constituent sequences (2.025, 2.43 and 2.85 My) contains at least one third-order Arabian Plate maximum flooding surface (Sharland et al., 2001), except for the Tuwaiq and Arab-D sequences. The latter two sequences, however, have been recognized as sequences with MFSs by other authors (e.g. Le Nindre et al., 1990). The MFS in the Arab-C, B and A and Manifa members are more likely to be fourth-order (stratons).

To resolve the third-order sequences into stratons (405 Ky fourth-order sequences) requires much more data and modeling – particularly along the dip direction from the outcrop belts of central Saudi Arabia to the paleomargin of the Neo-Tethys Ocean located in the subsurface of the eastern part of the Arabian Plate. Finding these stratons can provide a much better correlation between sequence stratigraphy and an absolute geological time scale.

ACKNOWLEDGEMENTS

The author thanks R.K. Matthews for many discussions and for the paramatric forward modeling of lithofacies for the Upper Jurassic Arab and Hith succession in Saudi Arabia and the United Arab Emirates. His studies are part of a consortium proposal to model the higher-order sequence stratigraphy of this succession. The author thanks Raymond Énay, Wyn Hughes and Joerg Mattner for their helpful comments. The author also thanks GeoArabia’s Nestor Buhay II for designing the paper.

ABOUT THE AUTHOR

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

geoarabi@batelco.com.bh