The Middle East Geologic Time Scale (ME GTS) seeks to document and age-calibrate Arabian Plate transgressive-regressive (T-R) depositional sequences using: (1) Geological Time Scale of the International Commission on Stratigraphy (GTS), and (2) Arabian Orbital Stratigraphy time scale (AROS). AROS is based on an orbital-forcing glacio-eustatic model that offers three orbital clocks to date T-R sequences: (1) Stratons @ ca. 405 Ky; (2) Dozons @ ca. 4.86 My (12 stratons); and Orbitons @ ca. 14.58 My (36 stratons, three dozons). The Earth today is in Orbiton 0, which started ca. 1.5 Ma (SB 0); the ages of lower boundaries of orbitons can be estimated with the formula SB n = n × 14.58 + 1.5 Ma. This scheme was used to calibrate the Arabian Plate’s Mid-Permian to Early Triassic Khuff sequences, which contain one of the largest gas-bearing carbonate reservoirs in the World.
The Khuff and equivalent formations have been interpreted by several authors in terms of six long-period sequences in outcrop belts and subsurface sections (Khuff sequences KS6 to KS1 in ascending order). Their type sections are briefly reviewed with emphasis on their boundaries, higher-order architecture and stage assignments. The age calibration starts at the basal Khuff Sequence Boundary (Khuff SB, Sub-Khuff Unconformity) defined in a type section in Al Huqf outcrop in Oman. Above the Khuff SB (ca. 268.9 Ma) the type sections of the oldest Khuff sequences KS6 (ca. 268.9–264.0 Ma) and KS5 (ca. 264.0–259.1 Ma) are defined in Oman and interpreted to each consist of twelve subsequences (stratons) with the predicted architecture of two consecutive dozons. By biostratigraphy they span the Mid-Permian (Guadalupian Epoch), Wordian and Capitanian stages.
Type-Sequence KS4 (ca. 259.1–254.2 Ma) is defined in Iran and corresponds to the Wuchiapingian Stage. The Iranian type-Khuff Sequence KS3 (ca. 254.2–249.4 Ma) contains nine subsequences (stratons) grouped between two major exposure surfaces. By correlation to the Changhsingian Stage and Permian/Triassic Boundary (PTrB) type section in South China, it is interpreted as a dozon with three missing stratons. Khuff Sequence KS2 (ca. 249.4–247.8 Ma) contains the PTrB with an orbital age of ca. 249.0 Ma, compared to 251.0 ± 0.4 in GTS and 249.0–253.0 Ma by radiometric dating in its type section. Khuff sequences KS2 and KS1 contain 13 subsequences (stratons) between ca. 249.4–244.1 Ma spanning latest Permian and Early Triassic. The boundary of the Khuff with the overlying Sudair Formation, Sudair Sequence Boundary, is defined in Borehole SHD-1 (Central Saudi Arabia) and calibrated at ca. 244.1 Ma falling near the age of the Early/Mid-Triassic Boundary in GTS. The enclosed Chart shows a work-in-progress correlation of the six Khuff sequences across the Arabian Plate.
The Middle Permian – Lower Triassic carbonate reservoirs in the Arabian Plate contain one of the World’s largest gas accumulations with total reserves of ca. 2,000 trillion cubic feet (TCF) (Figure 1a). These reservoirs produce non-associated gas and condensate from Iran’s Dalan and Kangan formations and the coeval Khuff Formation (Steineke, 1937, inPowers, 1968) in several Arabian countries (see enclosed Chart). In the World’s largest North Field - South Pars gas accumulation, these reservoirs have an effective thickness of ca. 300 m over an area of ca. 10,000 square kilometers in Iran and Qatar (Figure 1a). Across the Arabian Plate the Permian – Triassic reservoir zones are named differently and their high-resolution chrono-stratigraphic correlation remains unresolved.
Al-Jallal (1994, 1995) was amongst the first to propose Plate-wide nomenclature and a regional framework for correlating the Khuff and equivalent formations. Sharland et al. (2001, 2004) presented Arabian Plate correlations using third-order maximum flooding surfaces with three in the Middle and Upper Permian (MFS P20, P30 and P40) and three in the Lower Triassic (Tr10, Tr20 and Tr30). These frameworks have provided important regional constraints for tying locally interpreted transgressive-regressive (T-R) sequences (e.g. Strohmenger et al., 2002; Osterloff et al., 2004; Vaslet et al., 2005; Alsharhan, 2006; Insalaco et al., 2006; Al-Husseini, 2008; Maurer et al., 2009; Koehrer et al., 2010) and biostratigraphic zones (e.g. Stephenson et al., 2003; Stephenson, 2006; Angiolini et al., 2004; Vachard et al., 2005; Crasquin-Soleau et al., 2006; Chirat et al., 2006).
Two recent papers have advanced the understanding of the Khuff and equivalent formations much further by documenting the architecture of their higher-order cyclo-stratigraphy. The first study spans the Upper Permian and Lower Triassic in the Fars-Zagros subsurface and outcrops in Iran (Insalaco et al., 2006; Figure 1a). The second covers the Middle Permian to Lower Triassic in the Saiq Plateau of Al Jabal al-Akhdar in Oman (Koehrer et al., 2010; Figure 1b). These two papers, taken together with other sequence-stratigraphic studies, provide an important opportunity to recalibrate the Middle East Geologic Time Scale (ME GTS 2008) much more accurately (Al-Husseini, 2008; see Web-Lexicon at www.gulfpetrolink.com and enclosed Chart: Middle East Geologic Time Scale 2010: Middle Permian to Early Triassic Khuff sequences).
OVERVIEW OF ME GTS 2010: MIDDLE PERMIAN TO EARLY TRIASSIC
To calibrate T-R sequences in ME GTS two time scales are used. The first is taken from the book A Geological Time Scale GTS 2004 (Gradstein et al., 2004) and the website of the International Commission on Stratigraphy (ICS, see also GTS 2009, www.stratigraphy.org). It is shown in the left column of the Chart and referred to as GTS in the paper. The second is the Arabian Orbital Stratigraphy time scale (AROS; Matthews and Frohlich, 2002; Immenhauser and Matthews, 2004; Matthews and Al-Husseini, 2010). Throughout this paper, the AROS time scale is compared to radiometric age estimates of stage boundaries in GTS, including the Permian/Triassic Boundary (PTrB) as defined in South China and the Arabian Plate.
Key sections from three regions (Fars-Zagros, Saiq Plateau and subsurface Interior Oman) are used to define high-resolution type sections for the Mid-Permian to Late Triassic sequences (see AROS Chrono-Sequence column in Chart). In most other Arabian Plate countries, the chrono- and higherorder sequence stratigraphy of this interval have not been published in similar detail. In the Chart, lithostratigraphic units and/or long-period sequences from these parts of the Plate are tentatively shown in their most likely spatio-temporal positions (van Bellen et al., 1959-2005; Szabo and Kheradpir, 1978; Rabu et al., 1993; Al-Jallal, 1994, 1995; Sharland et al., 2001, 2004; Strohmenger et al., 2002; Weidlich and Bernecker, 2003; Vaslet et al., 2005; Al-Hadidy, 2007; Alsharhan, 2006; Knaust, 2009; Maurer et al., 2009).
In some localities, stage assignments are shown in color as indicated by the original authors (Chart). For many long-period sequences their sequence boundaries (SB), maximum flooding intervals (MFI) and possible hiatuses are tentatively positioned so as to align with better-resolved type sections. Representative thicknesses of units and sequences are cited so as to aid the correlations.
One of the fundamental results of the orbital-forcing model of Laskar et al. (2004) is that the highestamplitude term in the Fourier representation of the Earth’s eccentricity has a stable period of ca. 405,000 years (405 Ky). They predicted that it provides an accurate orbital clock to at least as far back as the Permian/Triassic Boundary (PTrB) at ca. 250 million years before present (Ma). The 405 Ky orbital cycle, referred to as long eccentricity, produces sea-level cycles with a comparable period that control the deposition of T-R sequences. To distinguish them from empirically defined fourth-order sequences that range between 100–500 Ky, the orbital sequence is named the Straton.
Besides identifying stratons, most orbital-forcing research has generally focused on cyclo-stratigraphic patterns, mainly in Cenozoic (especially the Late Pliocene and Pleistocene) and Cretaceous sediments, in the bandwidth of ca. 20 Ky to 2.4 My (e.g. Zachos et al., 2001; Pälicke et al., 2006). Yet much longer-period eustatic cycles dominated the pre-Pliocene time, and they are recognized throughout the Phanerozoic Era (e.g. Haq et al., 1988; Hardenbol et al., 1998; Sharland et al., 2001; Haq and Al-Qahtani, 2005; Miller et al., 2005; Haq and Schutter, 2008). For example, for the interval ca. 20–90 Ma, Miller et al. (2005) stated: “Spectral analysis of our sea-level records shows that variations occur with an as-yet-unexplained, persistent ca. 3-My beat and a second primary period varying from 6 to 10 My.” The orbital-forcing explanation for these long-period T-R sequences is provided below.
Straton: ca. 405 Ky T-R Sequence
Individual stratons require a name so they can be uniquely identified in the stratigraphic record. The proposed nomenclature assigns the integer multiplier of 0.405 My that is closest to the age of a straton’s lower sequence boundary (Matthews and Frohlich, 2002). The Earth is today near the end of Straton 1 (Matthews and Al-Husseini, 2010); it started at ca. 0.3 Ma and integer one is the closest multiplier of 0.405 to 0.3 Ma. Similarly, Straton 2 started at ca. 0.7 Ma and multiplier 2 of 0.405 Ky yields the closest number (0.8 Ma) to 0.7; and so the integer naming of stratons goes back in time.
The enclosed Chart shows the numbering of stratons from the Mid-Permian (Late Roadian) to early Mid-Triassic (Anisian). Significant model sea-level peaks corresponding to rapid transgressions and/or sea-level peaks are shown in blue, i.e. maximum flooding interval (MFI) containing maximum flooding surface (MFS) (see table 2inMatthews and Al-Husseini, 2010). Shown in red are sea-level lowstands and likely positions of sequence boundaries (SB).
Identifying a straton, or a series of stratons, in the stratigraphic record does not give absolute time. Instead stratons are like a stratigraphic tuning fork or chronometer and imply a time duration when they are not separated by significant hiatuses (longer than 405 Ky; Matthews and Frohlich, 2002). To estimate the absolute age of stratons, Matthews and Al-Husseini (2010) discuss how they group into recognizable multi-million-year sequences, all formed by groups of stratons (Chart).
Orbiton: ca. 14.58 My T-R Sequence
The most important grouping of stratons is caused by the tuning of the 20 leading Fourier terms of Laskar et al. (2004): it produces a significant sea-level drop every ca. 14.58 My, the duration of 36 stratons (36 × 0.405 My; Matthews and Al-Husseini, 2010). To avoid confusion with empirically defined second-order sequences, which are attributed to global tectonism, the 14.58 My T-R sequence is named the Orbiton. Like stratons, orbitons are geologic clocks but with added advantages: they not only last much longer but they can be dated and recognized by their prominent sequence boundaries (SB) and architecture. Their SBs represent regional regressive surfaces (disconformity, or unconformity/hiatus in proximal settings) caused by significant sea-level drops due to polar glaciations. The predicted age of the lower boundary of Orbiton Zero (SB Zero) is ca. 1.5 Ma; older orbitons (n) are predicted to start at SB n = n × 14.58 + 1.5 Ma. In the Chart, Orbiton 18 occurs between SB 18 (ca. 264.0 Ma) and SB 17 (ca. 249.4 Ma) and consists of the 36 stratons 652–617.
Dozon: ca. 4.86 My T-R Sequence
The second predicted grouping pattern consists of 12 consecutive stratons that build a T-R sequence with a period of ca. 4.86 My (Matthews and Al-Husseini, 2010). Again, to avoid confusion with empirical third- and second-order sequences, the 4.86 My T-R sequence is named the Dozon. Three dozons build one orbiton and are designated A to C in ascending order (Chart). Their SBs also represent significant regional regressive surfaces. In the Chart, Dozon 19C occurs between SB 19C (ca. 268.9 Ma) and SB 18 (same as SB 18A ca. 264.0 Ma) and consists of the 12 stratons 664–653. Also shown in the Chart are the numbering of stratons based on their positions within orbitons and dozons. The parallel naming system provides the criteria for predicting MFIs and SBs within any Phanerozoic orbiton.
In the Chart a model sea-level curve for Dozon 19C is shown next to the numbering scheme to illustrate typical sea-level fluctuations. Intermediate sequence boundaries (e.g. Straton 6) can occur within dozons resulting in their break-up into shorter-period third-order sequences consisting of an integer number of stratons: sometimes two (ca. 0.8 My), three (ca. 1.2 My) or four (ca. 1.6 My), but more commonly five (stratons 1 to 5, ca. 2.0 My, short), six (ca. 2.4 My, nominal) or seven (stratons 6 to 12, ca. 2.8 My, long). The grouping of stratons into third-order sequences may depend on local conditions (e.g. accommodation space, sediment supply, etc.) resulting in different interpretations of third-order SBs and MFSs. It also depends on the parameters selected for the glacio-eustatic model (Matthews and Al-Husseini, 2010).
The long third-order sequence (ca. 2.8 My) is predicted to occur frequently and may provide the missing explanation for the “as-yet-unexplained, persistent ca. 3-My beat” recognized by Miller et al. (2005). Moreover the durations of one (ca. 4.86 My) and two consecutive dozons (ca. 9.7 My) may correspond to the other primary beats varying from ca. 6 and 10 My of Miller et al. (2005).
NOMENCLATURE FOR KHUFF SEQUENCES
Several papers have defined Arabian Plate Mid-Permian to Early Triassic T-R sequences adopting the term “Khuff” (abbreviated “KS”; e.g. Strohmenger et al., 2002; Vaslet et al., 2005; Insalaco et al., 2006; Alsharhan, 2006; Knaust, 2009; Maurer et al., 2009; Koehrer et al., 2010). Each Khuff Sequence is here given a type section from a published paper in which the biostratigraphy and high-resolution architecture are best documented. The type sequences are named in the Chart’s “Chrono-Sequence” column and proposed for adoption across the Arabian Plate. For each sequence a short summary is given in terms of type locality, sequence boundaries (SB), architecture including maximum flooding surface (MFS) or interval (MFI), stage assignment and regional correlation. In the Chrono-Sequence column SBs are named after the overlying sequence.
The term “sequence” is adopted here as in the original paper without implying any particular duration or empirical order. The term “subsequence” is used to imply one straton. For example, Koehrer et al. (2010; Chart) divided the Saiq Plateau section into six Khuff sequences (KS1 to KS6 top down) and 36 cycle sets (KCS 1.1 to KCS 6.4). Their cycle sets (each ca. 5–25 m thick) consist of between 3–10 cycles or parasequences. They reported that in large parts of the outcrop, the cycle sets are the “most obvious and easiest to recognize order of cyclicity” and estimated their durations as ca. 400 Ky. In this paper their cycle sets KCS 3.1 to KCS 5.12 are referred to as subsequences to imply stratons; however, others like KCS 6.1 to KCS 6.4 are not because they are exceptionally thick and appear to be groups of stratons as discussed below.
PLATE-TECTONIC SETTING OF THE PERMIAN – EARLY TRISSIC ARABIAN PLATE
In the Late Carboniferous and Early Permian the Arabian Plate formed part of the Pangea Supercontinent (e.g. Stampfli et al., 2001; Sharland et al., 2001; Konert et al., 2001; Ruban et al., 2007; Muttoni et al., 2009; see references therein). In the Mid-Permian, the Cimmerian terranes of the Middle East (Afghanistan, Iran’s Sanandaj-Sirjan, Alborz or Northwest Iran, Lut or Central Iran, and Turkey or Anatolia) started to break away from the eastern margin of the Arabian Plate, and the Neo-Tethys Ocean started to form along the Zagros Suture and Gulf of Oman (Figure 1a). The mainly carbonate/evaporite platform that developed on the rapidly subsiding Arabian Plate is represented by the Khuff and equivalent formations: (1) Saiq Formation and Lower Mahil Member in Oman outcrops, (2) Dalan and Kangan formations of Iran, and (3) Chia Zairi and Mirga Mir formations of Iraq (Chart). The stratigraphic architecture of the platform is characterized as layer-cake as evident by isochronous marker beds that can be correlated over several 100s of kilometers (Al-Jallal, 1994, 1995; Osterloff et al., 2004; Forbes et al., 2010; Chart).
In the Saih Hatat outcrops (Wadi Aday located south of Oman’s capital Muscat; Figure 1), the initiation of the Neo-Tethys Rift is manifested by massive lava beds inter-bedded with mixed clastics and carbonates in the lowermost Saiq Formation. Rabu et al. (1993; Figure 2) documented the Mid-Permian horst-and-graben system that developed in this area adjacent to the new margin. They also interpreted the interaction of this extensional structural setting and sea-level cycles to produce TR sequences (Figure 2). The lava beds of the oldest Saiq Sq1V Unit are not dated by radiometric techniques but based on biostratigraphic interpretations the carbonates above them are Mid-Permian (Wordian, Rabu et al., 1993; see below discussion in Khuff Sequence KS6).
Prior to the deposition of the Khuff and equivalent formations, in early Mid-Permian time (Roadian?), the Arabian Plate was in a continental setting. The surface upon which the oldest Khuff-equivalent transgressive deposits were laid down is named the Khuff Sequence Boundary (Khuff SB). Over many uplifted features inherited from the Mid-Carboniferous tectonic event the Khuff SB passes to the Sub-Khuff Unconformity (e.g. Central Arabian Arch and Al Huqf Anticline, Figure 1a; Al-Husseini, 2004; Vaslet et al., 2005; Faqira et al., 2009). Along the eastern margin of the Plate some regions may have also been highlands associated with pre-rift thermal doming and/or syn-rift rift shoulders of the Neo-Tethys Ocean (Figure 1a). In most highlands the hiatus represented by the Sub-Khuff Unconformity typically spans the late Proterozoic or early Paleozoic to Mid-Permian time.
In basinal regions the transition from continental to marine settings was more continuous. In particular along the edge of the Al Huqf Anticline (Figures 1, 3 and 4), the transition appears to be without a significant stratigraphic break and thus offers a type locality for the definition of the Khuff SB (see next section).
KHUFF SEQUENCE BOUNDARY (KHUFF SB)
In a complete stratigraphic succession in Oman, the Khuff Formation overlies the Gharif Formation (Figures 3 and 4, Chart). The Gharif/Khuff transition is best seen in the Al Huqf outcrops in southern Oman (Broutin et al., 1995; Crumeyrolle et al., 1997; Angiolini et al., 2004; Figures 1a, 3 and 4), where the depositional environment changes from:
(1) Fluviatile setting (Upper Gharif Unit A);
(2) Hiatus associated with erosional unconformity as manifested by channels cut into Unit A. The channels are several kilometers wide, about 10 m deep and represent a drop in base level (relative sea level).
(3) Infilling of the channels in a coastal plain/estuarine setting (Upper Gharif Unit B).
(4) Transgressive shoal-barrier setting (Khuff Unit C);
(5) Open-marine carbonate platform of the Khuff Formation.
The start of the Khuff Transgression is represented by Khuff Unit C with its base taken as the Khuff Sequence Boundary (Khuff SB).
A similar stratigraphic relationship for the Khuff Transgression occurs in Al Jabal al-Akhdar in Oman (Figure 1b, compare Figures 3 to 5, Rabu et al., 1993). Here the Saiq Formation (Glennie et al., 1974; equivalent to Permian part of Khuff Formation) overlies Neoproterozoic and Cambrian rocks, and the Sub-Khuff (Sub-Saiq) Unconformity represents erosion of a high feature of likely Mid-Carboniferous – Early Permian age. The Lower Saiq Member starts with a basal conglomeratic sandstone (0–20 m thick) deposited in a fluviatile setting in lowlands, which is capped by a zone characterized by carbonaceous flakes, hemetite and chlorite pisoliths. This lowermost unit is here correlated to the subsurface Gharif Formation and Unit A in Al Huqf. The overlying units of the Lower Saiq Member consist of the Red Siltstone (0–20 m thick), which is correlated to the Red Beds (Unit B) in Al Huqf. In turn the Red Siltstone passes to inter-bedded carbonates and fine clastics (0–15 m) followed by the massive carbonates of the Upper Saiq Member. In this region the Khuff (Saiq) SB is positioned at the top of the Red Siltstone.
Osterloff et al. (2004) suggested a possible correlation between Gharif Unit B at outcrop to Gharif Cycle 8 in the uppermost Gharif Formation in subsurface Oman. Guit et al. (1995) named this subsurface unit (ca. 10 m thick) as the Gharif Red Beds. Gharif Unit B may also correspond to the subsurface Khuff Transition Unit of Stephenson (2006). In North and Central Oman, cores taken from Cycle 8 show a change from fluviatile sandstones to lagoonal shales that reflect tidal/estuarine environments as in Unit B (Figure 4; coastal plain in Figure 3). Osterloff et al. (2004) suggested that Cycle 8 may represent either the lowstand or transgressive systems tract of the overlying Khuff carbonates, or a separate higher-order sequence. In the present paper the latter interpretation is followed such that the Khuff SB is taken at the Gharif/Khuff Boundary.
KHUFF SEQUENCE KS6
Type Section and Definition (Figures 1 and 6): Above the Gharif Formation (Khuff SB), three Khuff T-R cycles (depositional sequences DS P17, P18 and P19) were first recognized in subsurface Oman by A. Al-Harthy (2000, PDO unpublished report inOsterloff et al., 2004). They constitute the Lower Member of the Khuff Formation and were correlated across most of subsurface Interior Oman by Osterloff et al. (2004). Together they form Khuff Sequence 6 with its type section taken in Sayyala-29 (Figure 6).
Boundaries: In Sayyala-29, the Khuff SB corresponds to the Gharif/Khuff Boundary. The upper sequence boundary of Sequence KS6 (SB KS5) is taken at the top of the continental shale bed that caps the Lower Khuff Member or its lateral equivalents (Figure 6). The shale bed is overlain by the massive carbonates of the Middle Khuff Member (Khuff Sequence KS5, see below). In the Al Huqf region the Khuff Formation is truncated by Triassic and younger unconformities and only the lowermost part of Sequence KS6 is preserved (Figures 3 and 4, Chart).
Architecture: Sequence KS6, 144 m thick in Sayyala-29 (Figure 6), ranges in Oman from 0 to a maximum of 325 m in Lekhwair-70 (Figure 1, Osterloff et al., 2004). In Sayyala-29, Osterloff et al. (2004) picked three flooding surface within maximum flooding intervals MFI P17, MFI P18 and MFI P19 (Figure 6; the MFIs are shown here as intervals that enclose the surfaces as depicted in Osterloff et al.’s figure 26). The three sequences were correlated across subsurface Interior Oman, with MFI P19 (corresponding to the Khuff Marker Limestone, KML, ca. 5–10 m thick) being the easiest to recognize in most boreholes (Osterloff et al., 2004).
Towards southwest Oman Sequence KS6 passes to massive shale but the three MFIs generally persist as correlative markers (especially MFI P19, KML, see Osterloff et al.’s figures 27 and 28). To the north towards the Rub’ Al-Khali (Figure 1a) Sequence KS6 becomes much thicker and consists mostly of massive carbonate (see cross-sections inOsterloff et al., 2004). Sayyala-29 is located in a marginal setting and recorded higher-order shale/carbonate cycles, which the present authors interpret as 12 subsequences (designated LK1 to LK12 in ascending order; “LK” for Lower Khuff is used to avoid confusion with nomenclature for KS6 of Koehrer et al., 2010). In Sayyala-29, the thickness of the subsequences ranges from 8–16 m and averages ca. 12 m. Several are characterized by a lower shale bed (lowstand) overlain by a limestone unit (flooding and regression). Most can be approximately recognized by the cyclical patterns of the density electric log.
Stage Assignment:Osterloff et al. (2004) assigned Khuff Sequence KS6 to the Mid-Permian (Wordian – Capitanian). Stephenson (2006) assigned the basal Khuff Transition unit to OSPZ6 (Oman-Saudi Arabia Palynozone 6) and interpreted the lower part of OSPZ6 as probably Wordian with its upper part extending into Capitanian.
Angiolini et al. (2004) reviewed the biostratigraphy of the Gharif and Khuff formations in the Al Huqf outcrop (Figure 3). Gharif Unit B contains a rich microflora (Gharif paleoflora of Broutin et al., 1995), which is not diagnostic of a precise age. They considered it ?Roadian – ?Lower Wordian by its stratigraphic position below the Khuff Formation, which yielded marine fossils of Wordian age (Figure 3). P. Osterloff (2003, written communication) considered Unit B to range in age from Kungurian (Early Permian) to Early Wordian.
Regional Correlation of Khuff Sequence KS6
In the Chart’s Chrono-Sequence column the Khuff SB is positioned near the Roadian/Wordian Boundary. Sequence KS6 can be correlated to most Middle East localities by its stratigraphic position above the Sub-Khuff Unconformity or Khuff SB (Al-Jallal, 1994, 1995; Sharland et al., 2001, 2004; Alsharhan, 2006), and its great thickness (exceeding 300 m in places; e.g. Lower Dalan in Iran, Szabo and Kheradpir, 1978; Khuff sequences 7 and 6 in UAE, Strohmenger et al., 2002; see Chart). In Iraq it correlates to the lower part of the Zinnar Member of the Chia Zairi Formation at outcrop (van Bellen et al., 1959-2005) and in subsurface to the Chia Zairi basal clastics and Member CH3 in reference well Jabal Kand-1 (Al-Hadidy, 2007, his figure 30).
In the Khuff outcrop belt and over the Central Arabian Arch in Saudi Arabia, Khuff Sequence KS6 is absent by non-deposition and nearly the entire Mid-Permian is not represented (Chart; Vaslet et al., 2005; D. Vaslet and Y.-M. Le Nindre, written communication, 2003). Several key correlations are discussed in more detail below.
Saiq Plateau, Oman: In this locality (Figure 1b), the Saiq Formation starts with the basal fluviatile clastics of the Lower Member (17 m thick), which unconformably overlie Proterozoic and Lower Paleozoic formations (Figure 5; Rabu et al., 1993). Koehrer et al. (2010) interpreted Khuff Sequence KS6 to start in the upper part of the Lower Saiq Member (above Khuff SB) and to end at SB KS5. They interpreted Sequence KS6 in terms of four cycle sets (KCS 6.1–KCS 6.4 top down, Chart) and tentatively assigned it to the Wordian Stage. Two of their cycle sets (KCS 6.3 and KCS 6.2, each ca. 50 m thick) are much thicker than the 5–25 m range they reported for all the Khuff cycle sets. Their Sequence KS6 has a thickness of 147 m, which is nearly identical to that in Sayyala-29 (144 m) wherein the subsequences are 8–16 m thick (Figure 3). In the Chart their Sequence KS6 is tentatively re-interpreted in terms of 12 subsequences instead of four and correlated to the type-Khuff Sequence KS6.
Wadi Sahtan, Oman: The Saiq Formation at Wadi Sahtan in Oman (Baud et al., 2001) overlies Proterozoic and Lower Paleozoic rocks (Figure 1b and Chart). It was divided into Cycles A to C base up. On the basis of stratigraphic position and thickness, Saiq units A1 (upper part of Basal Clastics, up to 20 m thick), A2 (carbonates, ca. 100 m thick) and A3 (carbonates, ca. 100 m thick) may correlate to Sequence KS6. Their combined thickness (ca. 220 m) is comparable to that of Sequence KS6 elsewhere.
Wadi Aday, Oman: The relationship between Khuff Sequence KS6 and extensional tectonism along the Tethyan rift margin can be approximately established at the structurally unstable Saih Hatat (Wadi Aday) outcrop region of Oman (Figures 1b and 2, Chart).
In the Masqat map area (fromLe Métour et al., 1986a,b, 1993), Sq1V starts with a conglomerate (ca. 1–20 m), comprising pebbles (1 mm to 5 cm) of quartz, quartzite and dolomite in a quartz-mica matrix. The discontinuous beds of conglomerate decrease upwards and alternate with shale, locally spotted by hydroxides and iron carbonates or carbonaceous material. The dm-scale beds of these immature deposits contain numerous crystals (quartz, oligoclase-andesine and subordinate potash-feldspar) and lithic clasts probably derived from marine volcaniclastic deposits (reworked silicic ash). Locally, in the southeast of the Masqat map area, Sq1V includes lava flows of dacite and recrystallized porphyritic trachyandesite. The thickness of Sq1V probably does not exceed 50–60 m in the Masqat map area. On the border with the Sib map area, it reaches some 80 m. In the west in the Quryat map area, it is only 5-30 m thick, but its thickness increases rapidly eastward and probably attains several 100 m in the Wadi al Hulw area (Figure 1b). At the extreme northeast of the Al Jabal al-Akhdar (Figure 1b), carbonates of the uppermost part of Sq1V furnished a faunal assemblage attributed to the late Murgabian (Wordian) and Midian (Capitanian) (Rabu et al., 1993; D. Vachard, 2001, written communication).
In the east Sib and Masqat map areas, above Sq1V, Saiq Sequence Sq1L is some 150 m thick and overlies metatuff and metatuffite of Sq1V (fromVilley et al., 1986; Le Métour et al., 1986a). It starts with limestone and dolomitic limestone that are commonly overlain by a discontinuous bed of black limestone containing foraminifera, bryozoans, bivalves, and solitary and colonial corals. Above this is a dolomitic, slightly sandy, commonly nodular limestone associated with lenses of carbonate conglomerate and breccia, whose clasts range in size from pebbles and blocks to olistoliths of several cubic meters in the Wadi al Hulw and Wadi Mayh sections. In the north Quryat map area, intense recumbent folding and thrusting allows only a rough estimate of the thickness, which is ca. 150 m, although in the east (Jabal Abu Daud and Sifat al Wasl) it probably attains more than 250 m. It is made up of strongly recrystallized, bedded limestone, dolomitic limestone and dolomite. The lowermost part of Saiq Member Sq1L was dated as Murgabian (Wordian) to Midian (Capitanian) (Le Métour et al., 1986a,b; 1993; D. Vachard, 2001, written communication).
The correlation in the Chart suggests that the Khuff SB is probably somewhat younger or approximately coeval to the oldest lava beds in Sequence Sq1V in Wadi Aday. Khuff Sequence KS6 is correlated in part to Saiq Sequence Sq1V (several 100 m thick) and the lower part of Sequence Sq1L (ca. 150–250 m thick).
Wadi Mayh, Oman:Weidlich and Bernecker (2003) divided the Saiq Formation into nine mainly carbonate cycles: WM Sq1 to WM Sq6 and WA Sq7 to WA Sq9 (WM and WA for Wadi Mayh and Wadi Aday added here to emphasize they are not the same as those of Rabu et al., 1993). They assigned WM Sq1 to WM Sq4 to Wordian – Capitanian Supersequence P2 (exceeds 400 m in thickness; Chart). They also identified the two volcanic units of Rabu et al. (1993) and placed the older Sq1V in the Roadian below their oldest WM Sq1. They positioned Sq2V (completely igneous, 40 m thick) above WM Sq4. By stratigraphic position Sequence KS6 may correlate to Sq1V (upper part), WM Sq1 and WM Sq2 (together greater than 200 m thick).
Orbital Calibration of Khuff SB and Sequence KS6
In the Al Huqf section (Figures 1, 3 and 4) Khuff Unit C is taken to represent the start of the Khuff Transgression, and the Khuff SB is correlated to orbital SB 19C (ca. 268.9 Ma) near the Roadian/Wordian Boundary (268.0 ± 0.7 Ma in GTS, Chart). Sequence KS6 (Figure 6, Chart) is correlated to Dozon 19C (ca. 268.9–264.0 Ma) corresponding by age to latest Roadian?, Wordian and Early Capitanian in GTS. Subsequences LK1 to LK12 in Sayyala-29 are correlated to stratons 664–653 (Figure 6). Notable correlations between the model’s MFIs and interpreted ones occur in three cases (Matthews and Al-Husseini, 2010, their table 2, see Generic Dozon: Model Sea Level in Chart).
MFI P17: This oldest Khuff MFI falls near the Roadian/Wordian Boundary of GTS, but is Wordian by biostratigraphy. It is represented in Subsequence LK2 (Figure 6) and Unit D of Khuff Member 2 in Al Huqf (Figures 3 and 4, Angiolini et al., 2004). Subsequence LK2 is the second-up subsequence of Dozon 19C (Straton 19C-2 = 663), which is predicted as a major MFI.
MFI P18: This MFI corresponds to the start of a second transgression in Subsequence LK7 near the Wordian/Capitanian Boundary of GTS and correlates to Straton 658 (Figure 6). It is one straton older than a predicted MFI (19C-8 = 657).
MFI P19: The Khuff Marker Limestone (KML) occurs in Subsequence LK10 and corresponds to the predicted MFI in Straton 655 (19C-10) in Capitanian of GTS (Figure 6).
These three MFIs are well-defined markers in many parts of subsurface Oman and are of orbital origin. Distinguishing them with biostratigraphic criteria may be possible because MFI P17 is Early Wordian (LK2) and the other two are Early Capitanian. Their correlation to other localities may be resolved if individual stratons can be separated.
KHUFF SEQUENCE KS5
Type Sections and Definition: Khuff Sequence KS5 was defined in Oman in the Saiq Plateau outcrop by Koehrer et al., 2010; Figures 1b and 8). In the subsurface it corresponds to the lower part of the Khuff Middle Member (up to top Median Anhydrite) as shown in Hasirah-1 (Figure 7; modified from Osterloff et al., 2004). The criteria for the surface-subsurface correlation are discussed below.
Lower Boundary: In the Saiq Plateau the lower boundary SB KS5 (KS6/KS5) is placed on top of a single 0.5-m-thick microbial laminite that may indicate restricted intertidal conditions (Koehrer et al., 2010, Microbial Marker 1 in Figure 8). This surface is believed to correlate to the boundary between the Lower and Middle Khuff members in subsurface (Figures 6 and 7).
Upper Boundary: In the Saiq Plateau the upper SB KS4 (KS5/KS4) is also placed within a microbial laminite unit (Microbial Marker 2 in Figure 8), which reflects subaerial exposure (Koehrer et al., 2010). This surface correlates to the top of the Median Khuff Anhydrite in subsurface (Figure 7; see Stage Assignment below).
Architecture: In the Saiq Plateau Sequence KS5 consists of 12 subsequences (cycle sets KCS 5.1 to KCS 5.12 top-down, ca. 18 m thick on average) with an MFS in KCS 5.8 (Koehrer et al., 2010). At this locality Sequence KS5 is 214 m thick compared to 147 m for KS6. Sequences KS6 and KS5 are 152 and 160 m thick in Sayyala-1 (near Sayyala-29; Osterloff et al., 2004; see their table 1, p. 144), and 192 and 188 m thick in Hasirah-1 (Figure 7). The maximum thicknesses for KS6 and KS5 in Oman occur in Lekhwair-70 attaining 325 and 300 m, respectively.
Khuff Sequence KS5 was divided by Osterloff et al. (2004) into three depositional sequences (DS P20, P23 and P27). They showed the position of MFS P20 as picked in Hasirah-1 by Sharland et al. (2001) occurs in the Middle Khuff Member and above Khuff Sequence KS6 (Figure 7). The higher-order subsequences of KS5 are not clearly distinguished in Hasirah-1. However, although the gamma-ray log is not as helpful as the density log in picking cycles, a suggested correlation seems possible with markers X to Z in Figures 7 and 8.
Stage Assignment: On fossil evidence, Koehrer et al. (2010) assigned Sequence KS5 to the Capitanian Stage. They reported that a burst of new faunal elements, including Shanita amosi and Paraglobivalvulina mira appears in uppermost KS5 beds. They stated that the same elements are recognized just below the Median Khuff Anhydrite in Oman boreholes and in the Capitanian Nar Member of the Dalan Formation in Iran (Insalaco et al., 2006).
Type Section of Sequences KS6 and KS5: The subsurface (Lower Khuff Member in Sayyala-29 and Hasirah-1) and Saiq outcrop KS5 type sequences are believed to be precise correlatives for several reasons. Firstly, on fossil evidence SB KS4 at outcrop correlates to top Median Khuff Anhydrite of subsurface and the Capitanian/Wuchiapingian Boundary. Secondly the thickness of KS5 at outcrop (214 m) is comparable to that in subsurface (in cited wells 160–188 m, maximum 300 m); this is also the case for the underlying KS6 at outcrop (147 m) and subsurface (in cited wells 152–192 m, maximum 325 m). Thirdly, the subsurface and surface KS5 sections are interpreted as Capitanian. In summary, the switch from Sayyala-29 and Hasirah-1 for KS6 type sections to Saiq Plateau for KS5 type section seems well supported but worthy of further confirmation.
Regional Correlation of Khuff Sequence KS5
In the Chart, Sequence KS5 is represented in most localities. It can be regionally correlated by its stratigraphic position below the Median Khuff Anhydrite, where present, and its Capitanian age (Al-Jallal, 1994, 1995; Sharland et al., 2001, 2004; Alsharhan, 2006). With more data and analysis it may become apparent that Sequence KS5 correlates closely to the Nar Member in Iran (Szabo and Kheradpir, 1978), which is dominated by anhydrite beds and has a comparable thickness of ca. 200 m in the Fars-Zagros region. Its upper part may correlate to the Satina Member of the Chia Zairi Formation in Iraq (van Bellen et al., 1959–2005; Al-Hadidy, 2007).
Wadi Sahtan, Oman (Figure 1b, Chart): At outcrop in Oman the Median Khuff Anhydrite bed is not present; it may have been dissolved and correspond to a brecciated interval. Sequence KS5 probably correlates to Unit A4 of Saiq Cycle A (150 m thick) of Baud et al. (2001). Unit A4 consists of gray dolomite and is characterized by thick intervals of breccia indicating a very restricted environment. The proposed correlation is based on comparable thickness of type-Sequence KS5 (214 m) and Unit A4 (150 m). The restricted setting is similar to that of the Nar Member of Iran and Khuff D Member of Saudi Arabia, which culminated with the deposition of the regional Median Khuff Anhydrite.
Wadis Aday and Mayh, Oman (Figures 1b and 2, Chart): In Wadi Aday, Rabu et al. (1993) interpreted Sq2V as a T-R sequence and its upper boundary as an exposure surface. It consists of lavas (andesitic, basaltic and rhyodacitic), accompanied either by their explosive counterparts (tuffs and brecciated pillow lavas) or by intrusive bodies (plugs of microangular rhyodacite, dolerite sills). In the extreme east of the Sib area and in the Quryat map area, dolomite is intercalated with the Sq2V volcanic rocks. In the Sib map area the dolomite beds are discontinuous but can be mapped as a separate interval. In the Fanjah and Sib areas, Villey et al. (1986), based on algal debris and benthic foraminifera interpreted a Murgabian (Wordian) to Midian (Capitanian) age for the carbonates at the footwall of Saiq Sq2V. The carbonates above the volcanic rocks are dated as Murgabian to Djulfian (Wordian – Wuchiapingian (Le Métour, 1987; Rabu et al., 1993; J. Le Métour, written communication, 2001).
In Wadi Mayh, Weidlich and Bernecker (2003) correlated the top of the basalt unit Saiq Sq2V (40 m thick) to the top of the Median Khuff Anhydrite, suggesting Sq2V together with underlying WM Sq3 (ca. 100 m thick) and WM Sq4 (ca. 105 m thick) probably correlate to Khuff Sequence KS5 (Chart). If the exposure surface at the top Sq2V approximately correlates to the top Median Khuff Anhydrite then the upper part of Sq1L of Rabu et al. (1993) correspond to Sequence KS5.
Saudi Arabia: The first Khuff transgressive deposits along the Arabian Shield and Central Arabian Arch (Proterozoic Basement) in Saudi Arabia are represented by the Ash Shiqqah Member above the Sub-Khuff Unconformity (Figures 1, 9 and 10; Vaslet et al., 2005; D. Vaslet and Y.-M. Le Nindre, written communication, 2003). The Ash Shiqqah was correlated to the subsurface Khuff D Member in Saudi Arabia (Vaslet et al., 2005), which is capped by the Khuff-D Anhydrite (Median Khuff Anhydrite). The subsurface Khuff-D Member together with the Basal Khuff Clastics (BKC) are assigned to the mid-Wordian and Capitanian OSPZ6 (Oman-Saudi Arabia Palynozone 6) of Stephenson (2006). The Ash Shiqqah Member, BKC and Khuff-D members are probably coeval with Sequence KS5.
Orbital Calibration of Khuff Sequence KS5
Sequence KS5 is correlated to Dozon 18A (264.0–259.1 Ma) implying a Capitanian age in GTS (Chart). Its top boundary SB KS4 correlates to SB 18 (ca. 259.1 Ma) near the Capitanian/Wuchiapingian (Mid/Late Permian, Guadalupian/Lopingian) Boundary at 260.4 ± 0.7 Ma in GTS. The 12 subsequences KCS 5.1–KCS 5.12 cycle sets of Koehrer et al. (2010) are correlated to stratons 652–641.
MFS P20: This MFS in Hasirah-1 has a position in the Middle Khuff Member and Sequence KS5 (Figure 7). It probably corresponds to stratons 651 or 648 and by age to Capitanian rather than Wordian as suggested by Sharland et al. (2001, 2004). These considerations disqualify correlating Saiq Plateau’s MFS KCS 6.3 to MFS P20 as proposed by Koehrer et al. (2010). Instead MFI P19 (Khuff Marker Limestone) is believed to encompass the MFS in KCS 6.3 of Koehrer et al. (2010). Koehrer et al. (2010) picked only one MFS (KCS 5.8, Figure 8) in Sequence KS5, which correlates to Straton 648 but is not a predicted MFI.
KHUFF SEQUENCE KS4
Type Sections and Definition: Khuff Sequence KS4 was defined in the Upper Dalan Member in subsurface and at outcrop in the Fars-Zagros region of Iran by Insalaco et al. (2006; Figure 11), and in Oman in the Saiq Plateau outcrop by Koehrer et al., 2010; Figure 8). In Iran its stratigraphy is characterized as layer cake over this vast region of ca. 250 x 300 km (Figure 1). On biostratigraphic evidence its lower boundary SB KS4 was precisely correlated to top Capitanian in Oman (Koehrer et al., 2010) and Iran (Insalaco et al., 2006). This correlation allows switching from type-Sequence KS5 in Oman to type-Sequence KS4 in Iran. As discussed below, Sequence KS4 can also be tied between Oman and Iran not only by biostratigraphy but also its higher-resolution architecture.
Boundaries (fromInsalaco et al., 2006): The lower boundary (SB KS4) is a major exposure surface at the top of the massive anhydrite of the Nar Member (Dalan Formation, Figure 11). As noted above, the top Nar surface has been correlated across most of the Arabian Plate to the top of Median Khuff Anhydrite and coeval evaporitic units (e.g. Khuff D Anhydrite, Middle Khuff Anhydrite; Al-Jallal, 1994, 1995; Sharland et al., 2001; Osterloff et al., 2004; Alsharhan, 2006; Chart). The upper boundary (SB KS3) is a major exposure surface in Iran (Figure 11) corresponding to Microbial Marker 3 in Oman (Figure 8).
Architecture (fromInsalaco et al., 2006): Sequence KS4, ca. 159 m thick, is divided into 12 conformable parasequence cycle sets (subsequences) that are not separated by exposure surfaces (Figure 11). It consists of an older group of five subsequences (KS4a1–KS4a5, ca. 48 m), and two younger groups consisting of seven subsequences in total (KS4b1–KS4b3, ca. 53 m; and KS4c1–KS4c4, ca. 58 m). The average thickness of subsequences is ca. 13 m and comparable to those of sequences KS6 (ca. 12 m in Sayyala-29) and KS5 (ca. 18 m in Saiq Plateau). Some subsequences are thin (ca. 10 m) and dominated by restricted facies; others are thicker (ca. 20 m) and represent maximum accommodation space (MAZ, MFI) containing MFSs in KS4a3, KS4b3 and KS4c2.
Stage Assignment:Insalaco et al. (2006), on fossil evidence, assigned Sequence KS4 precisely to the Wuchiapingian Stage. Strohmenger et al. (2002) suggested a latest Guadalupian – earliest Lopingian (ca. Capitanian/Wuchiapingian or Mid/Late Permian Boundary) age of ca. 260.5 Ma for the Median Khuff Anhydrite based on strontium isotope data taken from a core cut in a well in Abu Dhabi.
Regional Correlation of Khuff Sequence KS4
Sequence KS4 can be regionally correlated by its stratigraphic position above the Median Khuff Anhydrite, where present, and its Wuchiapingian age (Al-Jallal, 1994, 1995; Sharland et al., 2001; Alsharhan, 2006). The Chart shows possible KS4 correlatives and a few are discussed below.
Oman: In the Saiq Plateau (Figures 1a and 8) Koehrer et al. (2010) correlated their Sequence KS4 (170 m) to the Wuchiapingian Stage and to Iran’s Sequence KS4 of Insalaco et al. (2006; Figure 11). They divided Oman’s KS4 into 11 cycle sets (KCS 4.1 to KCS 4.11 top-down).
Other probable KS4 correlatives in Oman may be Saiq Cycle B (190 m thick) at outcrop in Wadi Sahtan (Baud et al., 2001). In Wadi Aday Saiq Sequence Sq2a (170 m, Rabu et al., 1993; Figure 2) and in Wadi Mayh Saiq WM Sq5 (154 m, Weidlich and Bernecker, 2003) may correlate to Khuff Sequence KS4. In subsurface Oman Sequence KS4 and KS3 correlate to the Middle Khuff Member above the Median Khuff Anhydrite and below the Upper Khuff Member. They are approximately separated in Figure 7.
Saudi Arabia: In the proximal setting in Central Saudi Arabia, Sequence KS4 correlates to the Wuchiapingian Huqayl Member of the Khuff Formation (54 m thick in Borehole SHD-1, Figures 1, 9 and 10, Vaslet et al., 2005, Vachard et al., 2005). The Huqayl Member consists of two cycles and is unconformably overlain by the Duhaysan Member. Al-Jallal (1995) correlated the Huqayl and overlying Duhaysan members to the Khuff C Member in subsurface Saudi Arabia.
Unknown Well: The sequence stratigraphy of a Middle East borehole with unknown location was interpreted in great detail by Knaust (2009, Chart). His analysis starts below a massive anhydrite bed (45 m thick) where he positioned the base of his Sequence KS4. The top of the anhydrite bed probably corresponds to top Median Khuff Anhydrite and top Nar Member. He interpreted high-frequency sequences HFS1 to HFS7, which range in thickness from 10–55 m, in his Sequence KS4. In the Chart, the seven HFS are repositioned so as to align the top of the massive anhydrite bed to the top Median Anhydrite. The resulting thickness of KS4 (minus anhydrite bed) is ca. 170 m and comparable to those at other localities (Chart).
Orbital Calibration of Khuff Sequence KS4
Sequence KS4 in Iran was precisely assigned to the Wuchiapingian Stage on fossil evidence (Insalaco et al., 2006). It is bounded by exposure surfaces and consists of 12 conformable subsequences (Figure 11). It is therefore correlated to Dozon 18B (ca. 259.1–254.2 Ma; Chart), which closely matches Wuchiapingian time (260.4 ± 0.7 to 253.8 ± 0.7 Ma in GTS). The correlative sequence boundaries at top Nar Member in Iran and SB KS4 in Oman are the Capitanian/Wuchiapingian Boundary (260.4 ± 0.7 Ma) and are correlated to SB 18B (ca. 259.1 Ma). The age of SB 18B meets the constraint of being slightly younger than the Median Khuff Anhydrite (uppermost part of KS5) with a strontium-isotope age of ca. 260.5 Ma (Strohmenger et al., 2002). The 12 subsequences in Iran’s KS4 are correlated to stratons 640–629 (Figure 11).
Sequence KS4 is capped by an exposure surface (Insalaco et al., 2006) corresponding to SB 18C above Straton 629. In Wadi Aday (Figure 2), Straton 629 may correlate to the short inter- to supra-tidal cycle above the Upper Saiq Sq2a Sequence (Rabu et al., 1993). In proximal Central Saudi Arabia SB 18C may be a significant hiatus represented by the unconformity between the Huqayl and Duhaysan members (Figure 8; Vaslet et al., 2005). In the Chart’s Chrono-Sequence column SB KS3 is highlighted as the Duhaysan SB.
MFS P30: Sequence KS4 (Dozon 18B) contains three MFSs in the Fars-Zagros region (Figure 11, Insalaco et al., 2006), which correlate closely to MFIs in stratons 639, 633 and 631 (Chart). Insalaco et al. (2006) correlated their MFS in KS4c2 to MFS P30 of Sharland et al. (2001, 2004). It falls in the sea-level peak of Straton 631.
In contrast to Iran’s 12 subsequences forming type-Sequence KS4, Oman’s KS4 in the Saiq Plateau consists of apparently only 11 (Koehrer et al., 2010; Figure 8). This suggests that in Oman either one straton is missing or one of the cycle sets consists of two stratons.
In the unknown Middle East well (Knaust, 2009), high-frequency HFS1 to HFS7 cycles are tentatively positioned on the basis of stratigraphic position and thickness (Chart). Some of the thicker ones are believed to consist of several stratons; for example HFS6 (55 m thick) may be three. This interpretation is generally consistent with Khuff stratons having typical thicknesses of ca. 5–25 m (Koehrer et al., 2010).
KHUFF SEQUENCE KS3
Type Section and Definition (Figures 1 and 12): Sequence KS3 was defined in the Upper Dalan Member in subsurface and at outcrop in the Fars-Zagros region of Iran by Insalaco et al. (2006). It is adopted here but with minor adjustments to represent its average thickness. Sequence KS3, as well as KS2 and KS1, were depicted in a chrono-stratigraphic manner in the figures of Insalaco et al. (2006, see their figures 9, 15, 16, 17 and 22) but with thickness scales that differ across their figures. In Figure 12, their original figures 16, 17 and 19 are shown with average thicknesses based on comparing all their figures and the reference borehole Kuh-e Siah-1 (Szabo and Kheradpir, 1978).
Boundaries (fromInsalaco et al., 2006): The lower boundary (SB KS3) is a major exposure surface at top of Sequence KS4. Sequence KS3 is capped by an epikarst-exposure surface (SB KS2) above which Sequence KS2 represents a platform-wide flooding event (Figure 12).
Architecture (fromInsalaco et al. (2006): Sequence KS3, ca. 115 m thick, consists of an older group of four subsequences (KS3a1–KS3a4, ca. 55 m) and a younger group consisting of five subsequences (KS3b0–KS3b4, ca. 60 m). Maximum accommodation space (MAZ) containing MFSs occur in KS3a3 and KS3b3 (their figure 15). The average thickness of the nine subsequences is ca. 9 m.
Sequence KS3 can be regionally correlated by its stratigraphic position below the PTrB, and its latest Permian age (Al-Jallal, 1994, 1995; Alsharhan, 2006). The Chart shows its possible correlatives with some of these discussed below.
Musandam Peninsula, Ras Al-Khaimah, United Arab Emirates:Maurer et al. (2009) interpreted the sequence stratigraphy of the Bih Formation in the Musandam Peninsula (Figure 1a). On fossil evidence they assigned their Sequence KS3 (131 m thick) to the Changhsingian Stage and correlated it to KS3 of Insalaco et al. (2006) (Chart).
Saiq Plateau, Oman: The biostratigraphy and sequence stratigraphy of the Changhsingian Stage and Lower Triassic in the Saiq Plateau are poorly constrained in the study of Koehrer et al. (2010). The PTrB is not identified in their section. Moreover, low-angle Upper Cretaceous thrust faults are suspected to cut the section near the boundary of their Sequence KS3 (uppermost Saiq) and overlying Sequence KS2 (lower Mahil Formation) by J. Mattner (written communication, 2009). Their section may therefore not be complete; KS3 is 68 m thick compared to the type section in Iran (105 m) and Musandam Peninsula (131 m) again suggesting missing section. It probably correlates to the lower part of KS3a although no biostratigraphic constraints are available.
Wadi Sahtan, Oman: At this locality the KS3 correlative sequence may be Saiq Cycle C (90 m thick, Baud et al., 2001).
Subsurface Oman: KS3 correlates to the upper part of the Middle Khuff Member in subsurface and is tentatively shown as a separate unit in Figure 7.
Wadi Aday and Wadi Mayh, Oman: In Wadi Aday Sequence KS3 may correlate to Saiq Sequence Sq2b and Sq3 (together 135 m thick; Rabu et al., 1993; Figure 2).
Weidlich and Bernecker (2003) interpreted Saiq Sequence WM Sq6 (195 m thick) in Wadi Mayh and then jump correlated about 8 km to Wadi Aday to continue with Sequence WA Sq7 to Sq9 (121 m, their Supersequence P4). By stratigraphic position Saiq WM Sq6 and WA Sq7 to Sq9 would have to correlate to Sequence KS3. However, taken together, their thickness (316 m) is nearly three times greater than other KS3 measured sections. It seems unlikely that this much greater thickness as reported by them represents greater syn-depositional subsidence. It is more probable that Sequence WM Sq6 duplicates WA Sq7 to Sq9, in part or completely, for several reasons: (1) WM Sq6 is ca. 8 km away from WA Sq7; (2) none of the sequences are controlled by biostratigraphy and the PTrB is not pinned down in Wadi Mayh; (3) Upper Cretaceous low-angle thrust faults cut the entire area such that sections may be repeated; and (4) WM Sq6 (195 m) and WA Sq7 to Sq9 (121 m) have more comparable thicknesses, not only to one another, but also to Wadi Aday’s Sequence Sq2b and Sq3 (135 m thick) of Rabu et al. (1993).
Saudi Arabia: In Central Saudi Arabia KS3 most likely correlates to the Late Wuchiapingian? – Changhsingian Duhaysan Member and Changhsingian Midhnab Member of the Khuff Formation (together 129 m thick in SHD-1; Vaslet et al., 2005, Figures 9 and 10). The Duhaysan unconformably overlies the Huqayl Member and the uppermost part of the Midhnab is cut by channels filled with terrestrial clastics. Vaslet et al. (2005) grouped the Duhaysan and Midhnab into Sequence DS PKm bounded by two unconformites. The Duhaysan/Midhnab Boundary is characterized by reworking and may represent a third-order sequence boundary.
Unknown Well:Knaust (2009) interpreted high-frequency sequences HFS8 to HFS12 (96 m, Chart), which range in thickness from 11–55 m, in their Khuff Sequence KS3. In the Chart the HFSs are tentatively positioned so as to align with higher-resolution calibrations in type KS3.
Orbital Calibration of Khuff Sequence 3
In the Chart Iran’s nine subsequences in KS3 (Figure 12) are assigned to Dozon 18C (254.2–249.4 Ma). As explained in the section dealing with the correlation to the Changhsingian Stage and PTrB in China (Chart), three ca. 405 Ky hiatuses are interpreted for stratons 628, 623 and 617. The three hiatuses are tentatively carried to other localities not only for consistency but also, in some localities, to tie significant sequence boundaries. For example, in Central Saudi Arabia, Straton 617 (immediately below SB 17 at 249.4 Ma) is correlated to the major unconformity (channeling) at the top of the Changhsingian Midhnab Member of the Khuff Formation (Figure 9, Vaslet et al., 2005). In this proximal locality the hiatus represents erosion and may be much longer than 405 Ky. In the Chart’s Chrono-Sequence column this position is emphasized as the Khartam SB.
In Al Jabal al-Akhdar, SB 17 is correlated to a major exposure surface at the top of the Saiq Formation (Baud et al., 2001). This surface, however, is not likely the PTrB, which probably occurs in the lowermost part of the Mahil Formation. It is possible that besides Upper Cretaceous faulting near the Saiq/Mahil Boundary, the section in the Saiq Plateau of Koehrer et al. (2010) may have remained exposed or partly eroded during this interpreted hiatus (SB 17).
MFS P40: The Changhsingian Stage breaks out into two third-order sequences in Iran (sequences KS3a and KS3b) and ambiguities occur when authors attempt to pick one to contain Changhsingian MFS P40 (Sharland et al., 2001, 2004).
KHUFF SEQUENCE KS2
Type Section and Definition (Figures 1 and 12): Sequence KS2 was defined in subsurface and at outcrop in the Fars-Zagros region of Iran by Insalaco et al. (2006). It straddles the Dalan and Kangan formations and is adopted here as the type section.
Boundaries (fromInsalaco et al. (2006, Figure 12): The lower boundary (SB KS2) is a karst-exposure surface at the top of Sequence KS3. Sequence KS2 is capped by a major exposure surface (SB KS1) above which Sequence KS1 represents a flooding interval.
Architecture (fromInsalaco et al. (2006, Figure 12): Sequence KS2, ca. 57 m thick, consists of four subsequences and represents the final Permian flooding over the Fars-Zagros platform. The four subsequences are ca. 15 m thick on average with the major flooding zone (MAZ) in KS2c. The youngest Subsequence KS2d is a lowstand characterized by carbonates and anhydrite beds.
Stage Assignment (fromInsalaco et al., 2006): Sequence KS2 contains the PTrB in Subsequence KS2b. In its lower part it includes the Permian Faunal Extinction (PFE) horizon, followed by an azoic zone (ca. 1–3 m thick) above which the Triassic faunal recovery zone is associated with the Early Triassic Thrombolite microbial event (Figure 12, Chart). Insalaco et al. (2006) stressed that the PTrB is not a sequence boundary or unconformity as suggested by other authors (e.g. Szabo and Kheradpir, 1978; Sharland et al., 2001).
Regional Correlation of Khuff Sequence KS2
Sequence KS2 can be regionally correlated by its stratigraphic position across the PTrB. The PTrB is reported to occur in Khuff B Member in subsurface Saudi Arabia and in the Upper Khuff Member in the UAE (Al-Jallal, 1994, 1995; Alsharhan, 2006). A few correlations are summarized below.
Ras Al-Khaimah, UAE:Maurer et al. (2009) interpreted their Sequence KS2 (64 m thick) in the Bih Formation and positioned the PTrB transition (Thrombolite equivalent zone) in it (Chart). They correlated it to type Sequence KS2 of Insalaco et al. (2006).
Oman Outcrops: In Al Jabal al-Akhdar Sequence KS2 probably correlates to Mahil Cycle D (30 m thick; Baud et al. 2001). The PTrB is unresolved in this outcrop and probably occurs in the lowermost Mahil Formation, but above the major exposure surface that defines the Saiq/Mahil Boundary. In Wadi Aday, Sequence KS2 also correlates to the lower part of the Mahil Formation. Sequence KS2 of Koehrer (2010) probably correlates to type-KS2 although no biostratigraphic constraints are available.
Subsurface Oman: Sequences KS2 and KS1 correlate to the Upper Member of the Khuff Formation as suggested in Hasirah-1 (ca. 120 m, Figure 7). Adopting the gamma-ray signature established for the PTrB in other localities (Insalaco et al., 2006; Alshrahan, 2006; Maurer et al., 2009), the PTrB is picked at the top of the decreasing GR trend some 5–15 m above the base of the Upper Khuff Member. The base of Sequence KS2 is tentatively taken at the top of the Shale Bed that separates the Middle and Upper Khuff members.
Saudi Arabia:Vaslet et al. (2005, Figures 9 and 10) assigned the lower part of the Lower Khartam Member of the Khuff Formation to the Late Changhsingian and the upper part of the Upper Khartam Member to the Early Triassic. They tentatively positioned the PTrB at the Lower/Upper Khartam Boundary, which they interpreted as a sequence boundary. The correlation of the PTrB to a sequence boundary is inconsistent with the biostratigraphic evidence and the arguements given by Insalaco et al. (2006). In the Chart the Khartam Member, in part, is correlated to Sequence KS2 such that the PTrB probably occurs within the Lower Khartam Member.
Unknown Well:Knaust (2009) interpreted high-frequency HFS13 to HFS15 in their KS2 and these are tentatively repositioned in the Chart. The Thrombolite interval (above the PTrB) is located in uppermost HFS13 implying this position correlates to within Subsequence KS2b of Insalaco et al. (2006). HFS15 was assigned to Sequence KS2 by Knaust (2009) but by stratigraphic position and thickness it may be the lower part of Sequence KS1 of other authors (Insalaco et al., 2006; Maurer et al., 2009, see Chart).
The orbital calibration of Khuff sequences KS2 and KS1 are discussed after Sequence KS1 is defined below.
KHUFF SEQUENCE KS1
Type Section and Definition (Figures 1 and 12): Insalaco et al. (2006) defined sequences KS1a to KS1c in the upper part of the Kangan Formation in subsurface and at outcrop in the Fars-Zagros region of Iran. The three sequences are adopted here and their average thickness is estimated from all their figures and the Kuh-e Siah-1 well (Szabo and Kheradpir, 1978).
Boundaries (fromInsalaco et al. (2006, Figure 12): The lower boundary (SB KS1) is an exposure surfaces at top of Subsequence KS2d. The upper boundary of the Kangan Formation is interpreted as a sequence boundary at the base of the Aghar Shale of the Dashtak Formation. The upper boundary of the Kangan and possible correlative surfaces are discussed in the section Sudair Sequence Boundary.
Architecture (fromInsalaco et al., 2006): Sequences KS1a to KS1c are together ca. 107 m thick. KS1a (ca. 45 m) is divided into four subsequence (KS1a1–KSa4) and KS1b (ca. 32 m) into three (KS1b1–KSb3). Sequence KS1c (ca. 30 m) was divided into lower Subsequence KS1c1 and an upper unit containing its MFS and overlying anhydrite beds (here denoted KS1c2). The nine subsequences have an average thickness of 12 m.
Stage Assignment (fromInsalaco et al., 2006Figure 12): Above the PTrB, the Kangan Formation and the overlying Aghar Shale of the Dashtak Formation are not dated by biostratigraphy and assigned to Lower Triassic (Scythian) by stratigraphic position.
Sequence KS1 is correlated on the basis of its uppermost position in the Khuff Formation below the Sudair Formation (Al-Jallal, 1994, 1995; Alsharhan, 2006; Chart). In Iraq, the Early Triassic Mirga Mir Formation correlates in part to Sequence KS1. Most of the correlations of KS1 to outcrops in Oman are not discussed here because the Mahil Formation is poorly constrained by biostratigraphy and sequence stratigraphy. In Jabal Sahtan, however, by stratigraphic position and thickness (ca. 70 m) Sequence KS1 may correspond to Mahil Cycle E of Baud et al. (2001).
Saudi Arabia: In Borehole SHD-1 the unnamed member consisting of evaporites and carbonates above the Khartam Member of the Khuff Formation is correlated, in part or completely to Sequence KS1 (Vaslet et al., 2005, Figure 10).
Unknown Well: The high-frequency cycles of Knaust (2009) HFS15 (their KS2), HFS16 and HFS 17 (their KS1) are ca. 100 m thick and probably correlate to type Sequence KS1.
ORBITAL CALIBRATION OF KHUFF SEQUENCES 2 AND 1
In the Chart, Iran’s Sequence KS2, and KS1a to KS1c1 are assigned to Dozon 17A (ca. 249.4–244.5 Ma). Together they contain 12 subsequences, which correspond to stratons 616–605. The uppermost Subsequence KS1c2 is characterized by channeling and evaporite beds and is correlated to the sea-level lowstand of Straton 604 above SB 17B (ca. 244.5 Ma). The correlation implies KS2 through at least KS1b fall in Early Triassic (Induan and Olenekian, i.e. Scythian) in GTS (Early Triassic: 251.0 ± 0.4 to 245.0 ± 1.5 Ma).
Early Triassic MFSs:Insalaco et al. (2006) positioned the Lower Triassic MFSs of Sharland et al. (2001) as follows: (1) MFS Tr10 in KS2c, (2) MFS Tr20 in KS1b2, and (3) MFS Tr30 in the Aghar Shale. The orbital calibration suggests that they correspond, respectively, to the Kangan MFSs in subsequences KS2c, KS1a3 and KSb3. Anisian MFS Tr40 would then fall above the Aghar Shale, probably in Straton 603.
SUDAIR SEQUENCE BOUNDARY (SUDAIR SB)
Type Section, Saudi Arabia
The boundary that defines the top of the Khuff and equivalent formations is taken at the Khuff/Sudair contact (e.g. Al-Jallal, 1994, 1995). It ends the so-called Khuff Supersequence (e.g. Strohmenger et al., 2002; Alshrahan, 2006). The Sudair Formation represents a major new depositional regime characterized by the progradation of fine clastics probably provenanced, in part, from western and southern Arabia (Powers, 1968). Sharland et al. (2001; see their figure 3.24) noted that another provenance may have been remnant Lower Triassic paleohighs located along the rift shoulders of the Neo-Tethys Ocean in the Zagros region (Figure 1a; see also Szabo and Kheradpir, 1978).
In Central Saudi Arabia, the Khuff/Sudair transition completely differs between outcrop and subsurface (compare Figures 9 and 10): units 1 and 2 above the Khartam Member in borehole SHD-1 (unnamed member of Khuff Formation) are absent in the outcrop belt. To clarify the position of the Sudair Sequence Boundary (Sudair SB) its type locality is here proposed in SHD-1 (Figure 10), wherein a 311-m-thick section was encountered between the Khartam Member of the Khuff Formation and Middle Triassic Jilh Formation; from base-up it consists of:
Unit 1: 18 m of interbedded gray gypsiferous clay, sulfated dolomite, saliferous gypsum, and massive halite in beds 1 m thick;
Unit 2: 57 m of essentially dolomitic limestone with sandy layers containing bioclasts and inclusions of spheroidal anhydrite;
Unit 3: 236 m of variegated green, or brick-red and purplish, silty claystone and gypsum, with common gypsiferous intercalations filling fissures or occurring as globular bodies, and thin bioclastic carbonate layers with a dolomicrosparitic cement occurring in places. Unit 3 corresponds to the massive “shale” of the Sudair Shale Formation at outcrop directly above the Khartam Member.
Al-Jallal (1995, his figure 6) correlated units 1 and 2 to the uppermost part of the subsurface Khuff Formation (Khuff B Anhydrite or Black Anhydrite, and overlying Khuff A Reservoir/Member). The evaporites of Unit 1 (with massive halite) offer a good correlative to the Khuff Black Anhydrite. These two units, previously assigned by Manivit et al. (1983) to the Sudair Formation, are here reassigned to the Khuff Formation with the Sudair SB taken at the base of Unit 3.
Sudair Unit 3 was dated as Early Triassic by palynological analysis of samples from SHD-1. Samples yielded abundant Lower Triassic Veryhachium sp. Units 1 and 2, for which there are no palynologic data, are also assigned to Lower Triassic by stratigraphic position above the Khartam Member and below Unit 3. The biostratigraphic interpretation places the Sudair SB in Lower Triassic.
Sudair Formation in Oman
In subsurface Oman the Khuff/Sudair contact is sharp in most localities. The base Sudair Formation is picked on lithostratigraphic criteria at the base of a continuous red, gray-green shale bed that occurs in its lowermost part (Figure 7). In some wells in Oman, Osterloff et al. (2004, see Zauliyah-3 in their figure 37, and Baqlah-1 in their figure 39) mapped the Khuff/Sudair Boundary as an unconformity that cuts into the uppermost Khuff.
Forbes et al. (2010) reported that the Sudair Formation in Oman is characterized by Palynozone 2351 (Densoisporites nejburgii with Endosporites papillatus) and, in the middle to (more typically) lower claystone part, by an influx of small marine acritarchs, which defines Subzone 1095 (Veryhachium spp.). They stated that the Veryhachium-Micrhystridium acritarch bloom, together with the associated miospores, appears to be of worldwide significance, occurring in Lower Olenekian – Induan rocks. The zone ranges into the Upper Khuff of interpreted Induan age.
The age of base Sudair in Oman may also be approximately inferred if it correlates to the base of the Sandstone and Shale Member of the Zulla Formation (Blechschmidt et al., 2004, see their figure 19). The Zulla Formation forms the lowermost unit of the allochtonous Hamrat Duru Group that was obducted onto the eastern margin of Oman in Late Cretaceous. The Sandstone and Shale Member is Late Olenekian to Mid- to Late Anisian based on conodont data in the underlying Zulla Limestone and Shale Member, and radiolarian data in the overlying Zulla Radiolarian Chert Member. If the Zulla Sandstone and Shale Member indeed correlates to the Sudair, then base Sudair may be Late Olenekian rather than Induan – Early Olenekian as interpreted by Forbes et al. (2010).
Aghar Shale of Dashtak Formation, Iran
In the Fars-Zagros region above the Kangan Formation, the Aghar Shale (ca. 5–10 m thick) defines the lowest part of the Triassic Dashtak Formation. Sharland et al. (2001, their figure 4.33) drew an unconformity at base Aghar and correlated it to near the top of the Sudair Formation in Abu Dhabi and Oman. They tentatively positioned Early Triassic MFS Tr30 in uppermost Kangan Formation but did not indicate a position for the next-younger Anisian MFS Tr40 in Iran. Instead they positioned Ladinian MFS Tr50 just above the Aghar Shale. These interpretations imply the Aghar Shale is Ladinian and the base Aghar unconformity represents an Anisian hiatus of more than 5.0 My.
Insalaco et al. (2006) interpreted the contact between the Kangan Formation and Aghar Shale as a sequence boundary (Figure 12), and correlated it to base Sudair. They placed MFS Tr20 in Khuff Subsequence KS1b3 (Kangan Formation) and MFS Tr30 in the Aghar Shale. In their interpretation, the Aghar Shale is Early Triassic in age, rather than Ladinian (Sharland et al., 2001). Their interpretations argue against the Kangan/Dashtak (base Aghar Shale) contact representing an Anisian hiatus in Iran.
Orbital Calibration of Sudair SB
The base Sudair is here interpreted as an unconformity (Sub-Sudair Unconformity) based on the absence of uppermost Khuff units 1 and 2 (unnamed member) at outcrop in Central Saudi Arabia (compare Figures 9 and 10) and truncation of the uppermost Khuff in some boreholes in subsurface Oman (Osterloff et al., 2004). The correlative conformity, the Sudair SB as defined in SHD-1 is Early Triassic in age by biostratigraphy. In the Chart, the Sudair SB is correlated to the base of Straton 603 (ca. 244.1 Ma just above SB 17B at ca. 244.5 Ma), which marks the start of the transgression of Dozon 17B. The age of the Sudair SB falls within the standard deviation for Early/Middle Triassic Boundary (245.0 ± 1.5 Ma). In Iran, the base Aghar and in Oman the base Sudair are also taken at this position and correlated to the Sudair SB.
THE PERMIAN/TRIASSIC BOUNDARY (PTrB) IN THE ARABIAN PLATE AND TYPE SECTION IN CHINA
Changhsingian Stage and Early Triassic in China
The Permian/Triassic Boundary is defined in the Meishan type section in South China (Jin et al., 2000; Hongfu et al., 2001; Wardlaw et al., 2004, and Ogg, 2004, inGradstein et al., 2004). The Changhsingian Stage in type section contains T-R sequences SQ1 and SQ2, each consisting of six subsequences (here named SQ1.1–1.6 and SQ2.1–2.6), and Subsequence SQ3.1 of Sequence SQ3 (Figure 13 and Chart). SQ1 and SQ2 contain MFIs (condensed section) in their fourth-up subsequences (SQ1.4 and SQ2.4). The PTrB is positioned at the top of SQ3.1, which forms the LST of Sequence SQ3. The Changhsingian Stage has an age between ca. 255.0–251.0 Ma according to Hongfu et al. (2001; Figure 13, Table 1), whereas in GTS base Changhsingian is estimated as 253.8 ± 0.7 Ma (Chart). Sequence SQ3 is interpreted in terms of seven subsequences and shown to end at 247.5 Ma (Hongfu et al., 2001; Figure 13).
Orbital Position and Age of the PTrB
The China-Iran correlation shown in the Chart starts at the sequence boundary taken by Insalaco et al. (2006) and Hongfu et al. (2001) as a precise correlative to base Changhsingian (orbital SB 18C). China’s sequences SQ1 and SQ2 are correlated to Dozon 18C (254.2–249.4 Ma), and their 12 subsequences to stratons 628–617. Iran’s Khuff Sequence KS3 is correlated to Dozon 18C but as noted above three hiatuses of 405 Ky are tentatively interpreted in it (Figure 12). These time gaps are required in order to not only correlate third-order sequence architectures (MFS to condensed sections, SBs) but more importantly to align the position of the PTrB surface. China’s Sequence SQ3 consisting of seven subsequences corresponds to the lower part of Dozon 17A between 249.4–246.5 Ma; its upper orbital age is 1.0 My younger than the 247.5 Ma cited by Hongfu et al. (2001, Figure 13). China’s SQ3 is correlated to Khuff Sequence KS2 and in part to KS1a. In the resulting correlation the PTrB is correlated to base Straton 615 at 249.0 Ma.
Age of the Permian/Triassic Boundary (PTrB)
The absolute age of the PTrB is not radiometrically determined in the Arabian Plate but its biostratigraphic position is pinned down in Iran (Insalaco et al., 2006; Figure 12 and Chart). Its estimated age has varied considerably over the past three decades of GTS vintages, ranging between 245.0 and 253.0 Ma (Table 1). In China’s type section it is dated by ash clay Bed 25, which occurs 13 cm below it, between 249.0 and > 254.0 Ma (Table 1fromOgg, 2004; Wardlaw et al., 2004)
The orbital calibration of the PTrB at 249.0 Ma is 2.0 My younger than the 251.0 ± 0.4 Ma cited in GTS 2004-2009, and 0.8 My older than that in GTS 1996 (Gradstein and Ogg, 1996). It’s age precisely matches the 249.0 ± 0.8 Ma of Mundil et al. (2001) considered by its authors as “clearly too young”. The PTrB’s orbital age correlates within 1.0 My to the ca. 250.0 Ma (40Ar/39Ar) ages obtained by Renne et al. (1995) for the Siberian Traps, which are regarded as synchronous with the PTrB (Ogg, 2004).
Do Orbital Clocks Work in the Permian – Triassic?
Resolving the absolute numerical accuracy of the orbital clocks in comparison to radiometric data bounding the PTrB is not possible. This is because the targeted radiometric PTrB age varies by 4.0 My. Nevertheless, considering the independence of the two approaches the result is considered significant. A different question one might ask is whether the orbital chronometer (405 Ky, Straton) as calibrated by the AROS time scale may be contaminated by shorter-period signals (short eccentricity 95–131 Ky, tilt/precession 20–40 Ky)? This seems unlikely for four reasons.
Firstly, the Mid-Permian to Early Triassic subsequences cover nearly 25 My and adequately distribute biostratigraphic stage assignments of Arabian Plate and Chinese subsequences as calibrated in GTS or by radiometric data. This is an important result because the Permian time scale is among the least internally constrained in the Phanerozoic Era (Wardlaw et al., 2004).
Secondly, consider the 59 subsequences spanning the Wordian to Early Triassic (268 ± 0.7 to 245.0 ± 1.5 Ma) of GTS: (1) 24 in Oman’s KS6 and KS5, (2) 12 in Iran’s KS4, (3) 13 in China’s Changhsingian SQ1.1 to SQ3.1, and (4) 10 in the Lower Triassic of Iran’s KS2b to KS1b3. Dividing 23.0 ± 2.2 by 59 yields 352–427 Ky with an average of 390 Ky. The result is sufficiently close to the 405 Ky chronometer of Laskar et al. (2004) to rule out contamination by shorter-period signals.
Thirdly, the Khuff subsequences are on average about 15 m thick. The range varies between 5–30 m depending on the position of the straton within long-period sequences and the platform. Shorter-period orbital sequences in these representative subsequences would be in the several to one meter-thickness range as recognized by authors as “parasequences”.
Finally, the regular manifestation of 12 subsequences groupings between prominent sequence boundaries in at least one type locality (including China) is the predicted number of stratons per dozon, but is completely unrelated to higher-order cyclo-stratigraphy (Matthews and Al-Husseini, 2010). Taken together, the results argue that the subsequences are indeed stratons, and when converted with the 405 Ky chronometer as tied to the AROS time scale, match the data of the interval bounding the PTrB.
Stage and Sequence Boundaries
Haq and Schutter (2008) correlate Permian stage boundaries precisely to sequence boundaries. Similarly, both Hongfu et al. (2001) and Insalaco et al. (2006) correlated base Wuchiapingian and base Changhsingian precisely to third-order SBs. However, a notable outcome of the correlation between the GTS and AROS time scales is that stage boundaries do not coincide in age with SBs. This is not surprising because the GTS age calibrations are based mostly on graphic correlation interpolations of a few radiometric data points (see figure 16.9 in Wardlaw et al., 2004).
Moreover it seems unlikely that stage and sequence boundaries should be correlative surfaces by construction. Stage boundaries are intentionally positioned so as to avoid unconformities and stratigraphic gaps (disconformities), which are in themselves sequence boundaries. It therefore seems improbable that biostratigraphers would intentionally select SBs to define stage boundaries. Indeed Hongfu et al. (2001), Insalaco et al. (2006) and Haq and Schutter (2008) showed that the PTrB is not an SB but rather it occurs in the early TST (Chart). Similar orbital-GTS age relationships occur near two more boundaries:
(1) Wuchiapingian/Changhsingian average age (253.8 Ma) is 400 Ky (Straton 628) younger than SB 18C (254.2 Ma).
(2) Roadian/Wordian average age (268.0 Ma) is 900 Ky (stratons 664 and 663) younger than SB 18B (268.9 Ma).
In all the above cases the stage boundary appears to represent the early TST following a major SB. This pattern may not be surprising. Major SBs represent polar glaciations, sea-level drops and changes in climate: they probably reflect the extinction phase ending one stage, whereas early TSTs herald the faunal recovery in the next stage.
The opposite pattern, however, occurs for the Capitanian/Wuchiapingian Boundary (Chart): its age is about 1.5 My older than SB 18B. It was taken by Insalaco et al. (2006) to precisely correlate to SB KS4. Such a correlation suggests the stage boundary may occur in the early TST at base Straton 639 (ca. 258.7 Ma), but the age would exceed the standard deviation cited in GTS (260.4 ± 0.7 Ma).
The position of the Wordian/Capitanian Boundary is not resolved in the Arabian Plate. Haq and Schutter (2008) show several high-frequency cycles in the Wordian, which may correspond to stratons 664–668 in lower Sequence KS6.
The orbital age of the Sudair Formation places it entirely in Mid-Triassic (Anisian) of GTS, completely contrary to its interpreted biostratigraphic age of Early Triassic. To keep the Sudair Formation in Early Triassic would require a major shift to a younger age for the Early/Middle Triassic Boundary (244.5 ± 1.5 My) in GTS. The age of this boundary is not adequately resolved in GTS 2004 with radiometric data only pinning the PTrB and the Middle/Upper Anisian Boundary, with the latter at 241.0 ± 2.0 Ma or as young as 233.0 ± 5.0 Ma. Indeed Ogg (2004) stated: “the discrepancy among ages in the Upper Anisian – Lower Ladinian interval obtained by different U-Pb studies and the 40Ar/39Ar versus U-Pb results is not resolved.”
The estimated age for Induan/Olenekian in GTS (249.7 ± 0.7 Ma) is much too old according to the orbital calibration of the PTrB. In the Chart the column between GTS and AROS speculates how some of these discrepancies might be resolved if stage boundaries are correlated to early TSTs pursuant to major SBs.
The age calibration of the Khuff sequences provides some insights for the interplay of sea level and plate tectonics. A simple measure is used here to estimate accommodation space (subsidence): the thickness-to-time ratio of sequences (uncompensated for compaction). The six Khuff sequences are taken to approximately represent Mid-Permian (KS1 and KS2 in 9.7 My), Late Permian (KS3 and KS4 in 9.7 My) and Early Triassic (KS2 and KS1 in 4.9 My) time intervals. In Table 2, the thickness-to-time ratios for these three epochs are shown for localities where the sequences attain the greatest thickness.
The ratios in Table 2 show a dramatic decrease from Mid-Permian (46–72 m/My) to Early Triassic (21–31 m/My). An explanation for this trend may be related to the rate of crustal subsidence of the Arabian Plate rather than global eustasy. Mid-Permian sequences KS6 and KS5 correlate closely to the interval between Saiq Volcanic Units Sq1V and Sq2V (Figure 2 and Chart), which reflect extensional tectonism along the newly forming Neo-Tethys Ocean. This suggests that during the Mid-Permian syn-rift phase the Arabian Plate not only subsided rapidly but may have also tilted to the east as implied by the westwards increase of the hiatus between the Sub-Khuff Unconformity and Khuff SB towards Central Saudi Arabia (Chart).
A tuned orbital-forcing glacio-eustatic model (Matthews and Al-Husseini, 2010) was used to calibrate the ages of Arabian Plate transgressive-regressive (T-R) type sequences in a ca. 25 million year interval spanning the Mid-Permian to Early Triassic. The calibration offers new criteria for how to correlate the important Khuff, Dalan and Kangan gas reservoirs of the Middle East Gulf region. Three orbital clocks were used with durations of 405 Ky (Straton), 4.86 My (Dozon) and 14.58 My (Orbiton) to date six Khuff long-period sequences and their subsequences. The orbital calibration adequately tracks major sequence boundaries, MFIs, and biostratigraphic assignments of the Khuff sequences as dated in GTS. The PTrB was dated at 249.0 Ma, an age considered comparable to radiometric estimates of 249.0–253.0 Ma at its type section in China, 248.2 Ma (GTS 1996) and 251.0 Ma (GTS 2004-2009).
A regional correlation between far-apart regions in the Arabian Plate is tentatively shown in the enclosed Middle East Geologic Time Scale Chart. Where detailed sequence-stratigraphic architecture for recognizing stratons is unavailable, the correlation used dozons (and their SBs) to position long-period T-R sequences in their likely spatio-temporal positions. The Chart is viewed as a work-in-progress that may assist in the better characterization and correlation of the Khuff reservoirs across the Arabian Plate at a resolution of ca. 10–20 m or better. More robust correlations may be possible in the future but only by the identification of stratons.
The Khuff sequences that are discussed in this paper are commercially important and have therefore been the subject of many sequence stratigraphic studies. Other intervals in the Arabian Plate also provide a natural laboratory for documenting and understanding T-R sequences. This is because the Plate was regularly flooded starting in the Late Ediacaran and throughout Phanerozoic times by the Tethyan Oceans and only affected by tectonism during relatively brief periods. It spanned the continental to ocean-margin realms and its stratigraphy is sampled at many localities. Identifying these sequences, however, requires complimenting lithostratigraphy and biostratigraphy with higher-order sequence stratigraphy and, as proposed here, calibrating the interpretations with orbital stratigraphy. The tuned orbital-forcing, glacio-eustatic model offers specific and testable rules for how to dissect and calibrate the architecture of sequences in a more rigorous manner than empirical sequence stratigraphy. The Arabian Orbital Stratigraphy (AROS) time scale and the proposed nomenclature may assist in unifying the collections of not only Arabian Plate MFS/MFIs and sequences, but also worldwide empirical sequences, into one physics model (Matthews and Al-Husseini, 2010).
According to Nobel laureate Ernest Rutherford (1871-1937): “All science is either physics or stamp collecting” (in “Rutherford at Manchester” by J.B. Birks, 1962). In Rutherford’s binary classification, stratigraphy falls in the stamp-collecting category. By analogy, a major endeavor of stratigraphy involves collecting and documenting multi-disciplinary geoscience data and interpretations towards, in part, building a unified geologic time scale. This is undoubtedly an important scientific phase but perhaps in need of a new paradigm. Orbital stratigraphy could catalyze the paradigm by placing the stratigraphic dataset into a physics model.
The authors thank G. Forbes and A. Immenhauser for their helpful comments and GeoArabia’s Arnold Egdane for designing the paper.
ABOUT THE AUTHORS
Moujahed I. Al-Husseini founded Gulf PetroLink in 1993 in Manama, Bahrain. Gulf PetroLink is a consultancy aimed at transferring technology to the Middle East petroleum industry. Moujahed received his BSc in Engineering Science from King Fahd University of Petroleum and Minerals in Dhahran (1971), MSc in Operations Research from Stanford University, California (1972), PhD in Earth Sciences from Brown University, Rhode Island (1975) and Program for Management Development from Harvard University, Boston (1987). Moujahed joined Saudi Aramco in 1976 and was the Exploration Manager from 1989 to 1992. In 1996, Gulf PetroLink launched the journal of Middle East Petroleum Geosciences, GeoArabia, for which Moujahed is Editor-in-Chief. Moujahed also represented the GEO Conference Secretariat, Gulf PetroLink-GeoArabia in Bahrain from 1999-2004. He has published about 50 papers covering seismology, exploration and the regional geology of the Middle East, and is a member of the EAGE and the Geological Society of London.
Robley K. Matthews is Professor of Geological Sciences at Brown University, Rhode Island, USA, and is general partner of RKM & Associates. Since the start of his career in the mid 1960s, he has had experience in carbonate sedimentation and diagenesis and their application to petroleum exploration and reservoir charcterization. Rob’s current interests center around the use of computer-based dynamic models in stratigraphic simulation.