ABSTRACT

The Middle Permian (Guadalupian), Upper Permian (Lopingian) and Lower Triassic Khuff and correlative formations in the Arabian Plate consist of six “third-order” sequences, from oldest to youngest KS6 to KS1, and at least 45 “fourth-order” sequences. They are here dated using biostratigraphic constraints and correlated to two independent sequence-stratigraphic time scales: (1) global sequences calibrated in the Geological Time Scale GTS 2012; and (2) orbital-forcing glacio-eustatic sequences that track the 0.405 million year (Myr) orbital eccentricity signal in the M&H-2010 scale (Matthews and Al-Husseini, 2010). The chronostratigraphic calibration of the Khuff sequences provides a reference section and common nomenclature that can be used for regional and global correlations. It permits positioning the hydrocarbon reservoirs of the Khuff and equivalent formations in a sequence-stratigraphic framework that can be used in exploration and reservoir characterization.

The lower sequence boundary of the Khuff Formation (Khuff SB) is correlated to global Wordian SB Wor1 near the Roadian/Wordian Boundary at 268.8 ± 0.5 Ma, and correlative SB 19C at 268.9 Ma in the M&H-2010 scale. The upper sequence boundary of the Khuff Formation with the overlying Sudair Formation (Sudair SB) is correlated to Olenekian SB Ole1 near the Induan/Olenekian Boundary at 250.0 ± 0.5 Ma, and correlative SB 17 at 249.5 Ma in the M&H-2010 scale. These calibrations imply the Khuff was deposited in about 19.4 Myr, and consists of 48 “stratons”; i.e. transgressive-regressive (T-R) depositional subsequences with an average duration of 0.405 Myr corresponding to long-eccentricity orbital cycles 664 to 617. The 48 stratons are predicted to form four “dozons” (19C, 18A, 18B and 18C), each consisting of 12 stratons. Individual dozons lasted 4.86 Myr and are separated by regional sequence boundaries (SB 19C to SB 17A).

In Oman, Khuff Sequence KS6 on the Saiq Plateau is correlated to the subsurface Lower Khuff Member, and both are interpreted to consist of 12 subsequences that are correlated to stratons 664–653 forming Dozon 19C between 268.9–264.0 Ma. KS6 is correlated to the four global sequences Wordian Wor1 to Capitanian Cap1 dated between 268.8–264.0 Ma in GTS 2012. Khuff Sequence KS5 corresponds to the Middle Khuff Member up to the top of Middle Khuff Anhydrite in subsurface Oman. On the Saiq Plateau, KS5 potentially consists of 12 cycle sets (Koehrer et al., 2010) that are correlated to stratons 652–641 of Dozon 18A, between 264.0–259.2 Ma. It is correlated to global sequences Capitanian Cap2 and Cap3 dated between 264.0–259.8 Ma in GTS 2012. Khuff Sequence KS4 consists of 11 cycle sets on the Saiq Plateau and other localities in Al Jabal al-Akhdar in Oman (Koehrer et al., 2010, 2012). It is assumed that one cycle set remains unidentified in KS4, completing its correlation to stratons 640–629 of Dozon 18B between 259.2–254.3 Ma. KS4 correlates to the global sequences Wuchiapingian Wuc1 and Wuc2 dated between 259.8–254.2 in GTS 2012. Khuff sequences KS3, KS2 and KS1 combined consist of 10 cycle sets in Al Jabal al-Akhdar (Koehrer et al., 2010, 2012), and two are presumed unidentified so as to correlate to the 12 stratons 628–617 of Dozon 18C between 254.3–249.5 Ma. Sequence KS3 correlates to Changhsingian global sequences Cha1 and Cha2 dated between 254.2–252.5, and KS2 and KS1 to latest Permian–Early Triassic global sequences Cha 3 and Induan–Olenekian Ind1 dated between 252.5–249.9 Ma in GTS 2012. The Permian/Triassic Boundary (PTB), dated at 252.2 ± 0.5 Ma in GTS 2012, occurs in lowermost Khuff Sequence KS2, in cycle set KCS 2.3, and based on the orbital calibration of the Upper Permian (Lopingian) Series in South China, it occurs in Straton 623 between 252.3 and 251.9 Ma.

INTRODUCTION

The Middle Permian–Lower Triassic Khuff and correlative formations in the Arabian Plate consist mainly of carbonates and evaporites that attain a thickness of more than 1,000 m (Al-Jallal, 1995; Sharland et al., 2001). The formations overlie terrestrial clastics typified by the Gharif Formation of Oman, and their lower boundaries represent the start of the regional transgression of the “Fusulinid Sea” over many parts of the Arabian Plate (Montenat et al., 1976; Le Métour, 1987; Baud et al., 2001a, b; Osterloff et al., 2004; Richoz et al., 2005; Baud and Bernecker, 2010, see references therein, Figure 1). The formations are overlain by the basal shales of the Lower Triassic Sudair Formation in Saudi Arabia (Manivit et al., 1983) and Oman (Osterloff et al., 2004; Forbes et al., 2010), and correlative rock units elsewhere in the Arabian Plate (Figure 2). The Khuff Formation has been interpreted in several regions as one “second-order” super-sequence consisting of six “third-order”, transgressive-regressive (T-R) sequences (Figures 1 and 2; Insalaco et al., 2006; Maurer et al., 2009; Koehrer et al., 2010, 2012):

  • Mid-Permian Khuff sequences KS6 and KS5,

  • Late Permian Khuff sequences KS4 and KS3,

  • latest Permian–Early Triassic Khuff Sequence KS2, and

  • Early Triassic Khuff Sequence KS1.

The reservoirs in sequences KS4 to KS1 contain some of the world’s greatest reserves of non-associated gas in fields in Bahrain, Iran, Saudi Arabia, United Arab Emirates and Qatar, as well as oil in the Yibal Field in Oman (Figure 1a). The age calibration and correlation of these and other T-R sequences across the Arabian Plate is a work-in-progress launched in 2008 as the Middle East Geological Time Scale (Al-Husseini, 2008).

This paper presents a revision to the age calibrations of the Khuff T-R sequences that was attempted in 2010 (Al-Husseini and Matthews, 2010). It starts by predicting the ages of the regional sequences of the Mid-Permian to Early Triassic based on a model of orbital-forcing glacio-eustasy, referred to as the M&H-2010 scale (after Matthews and Al-Husseini, 2010; Figure 2, Tables 1 and 2). Next the model sequences are correlated to the empirical global sequences compiled by Snedden and Liu (2011) from the charts of Hardenbol et al. (1998) and Haq and Schutter (2008) as calibrated in the Geological Time Scale GTS 2012 (Gradstein et al., 2012; Henderson et al., 2012; Ogg, 2012; see www.stratigraphy.org; C. Huang and J. Ogg, written communications, 2012, 2013). The model and global scales are shown to share five Mid-Permian–Early Triassic sequence boundaries that are used to date the six Khuff sequences. The paper also compares the dating of the Upper Permian in South China using radiometric data and the M&H-2010 scale. The results are encouraging, and support using orbital periods to tune empirical scales to as far back as the Mid-Permian.

SEQUENCE-STRATIGRAPHIC TIME SCALES

M&H-2010 Orbital Time Scale

The dominant periods of the Fourier representation of the eccentricity of the Earth’s orbit around the Sun are ca. 0.1, 0.405 and 2.4 Myr (Laskar et al., 2004, 2011). These periods are predicted by the glacio-eustatic model of Matthews and Frohlich (2002) to be manifested as T-R sequences that have average durations of 0.1, 0.405, and 2.0, 2.4 and 2.8 Myr. They recommended using the term “fourth-order” for the 0.405 Myr sequence, and “third-order” for the 2.0, 2.4 and 2.8 Myr sequences. The application of these recommendations proved impractical because they conflict with the existing definitions of “orders” used in empirical sequence stratigraphy (e.g. Emery and Myers, 1996; Tinker, 1998; Table 3).

To circumvent confusion regarding the usage of sequence-stratigraphic terms, Matthews and Al-Husseini (2010) introduced the term “straton” to mean the T-R sequence that has an average duration of 0.405 Myr (Laskar et al., 2004, 2011). In the M&H-2010 scale, stratons are identified by integer numbers with Straton 1 starting at ca. 372,000 years ago (0.372 Ma) and continuing today. The age for the start of any straton (e.g. Straton M) is:

 

Age for start of Straton M = (M-1) × 0.405 + 0.372 Ma

For example, Straton 623 (M = 623) is important because it may contain the Permian/Triassic Boundary (PTB) dated in GTS 2012 as 252.2 Ma (Table 1), and between 252.28 ± 0.08 and 252.10 ± 0.06 Ma (Shen et al., 2011). The age for the start of Straton 623 is estimated as:

 

(6231)  ×  0.405 + 0.372 = 252.281 = ca. 252.3 Ma

The M&H-2010 scale predicts that the Middle Permian to Lower Triassic contains five regional sequence boundaries (SB) that are separated by intervals of 4.86 Myr (12 x 0.405 Myr) and that bound groups of 12 stratons. From youngest to oldest the five regional boundaries are named SB 17A, SB 18C, SB 18B, SB 18A and SB 19C (Figure 2; Tables 1 and 2). The 36 stratons that occur between sequence boundaries denoted by the letter “A” form an “orbiton” (e.g. Orbiton 18 between SB 18A and SB 17A, Figure 2). The sequence boundaries denoted by the letter “A” may, in some cases, be more prominent and the age of their lower sequence boundary (SB without an “A”) is calculated as follows:

 

Age of SB N = 1.586 + N × 14.58 Ma

For example, for N = 17, the age of SB 17 (Sudair SB in Figure 2; Tables 1 and 2) is:

 

Age of SB 17 = 1.586 + 17 × 14.58 = 249.446 = ca. 249.5 Ma.

Global Sequence Boundaries in GTS 2012

Table 2 shows the age estimates for the stage boundaries of the Mid-Permian (Guadalupian), Late Permian (Lopingian) and Early Triassic epochs in GTS 2004 (Ogg, 2004; Wardlaw et al., 2004; Gradstein et al., 2004) and their revisions in GTS 2012 (Ogg, 2012; Henderson et al., 2012; Gradstein et al., 2012). The stage boundaries in GTS 2012 carry an estimated uncertainty of between ± 0.3 and ± 0.5 Myr) (Figure 2).

The global sequences in Figure 2 are compiled in Snedden and Liu (2011) from Hardenbol et al. (1998) for the Cenozoic–Mesozoic (Early Triassic in Figure 2), and Haq and Schutter (2008) for the Paleozoic (Middle and Late Permian in Figure 2). The sequences are named after the first three letters of the stage in which their lower sequence boundary (SB) occurs, followed by an integer. For example, Capitanian Sequence Cap2 occurs between SB Cap2 at 264.0 Ma and base Wuchiapingian SB Wuc1 at 259.8 Ma (Figure 2).

Three age estimates for each sequence boundary are given in Table 2. The first is from GTS 2004 (Sneddon and Liu, 2011), and the second is its recalibration in GTS 2012 by linear interpolation between stage boundaries. The third estimate was made by C. Huang and J. Ogg (written communications, 2012, 2013), and differs in some cases from the linear interpolation. For example, the linear interpolation from GTS 2004 to GTS 2012 implies an age of 268.2 Ma for Wordian SB Wor1. In contrast, C. Huang and J. Ogg correlate SB Wor1 to base Wordian at 268.8 ± 0.5 Ma. A significant difference occurs for the age of Capitanian SB Cap2 with an age of 262.4 Ma by linear interpolation versus 264.0 Ma taken at the base of Jinogondolella altudaensis Zone in GTS 2012 by C. Huang and J. Ogg (written communications, 2012, 2013).

The chronostratigraphic constraints for the global sequences (Hardenbol et al., 1998; Haq and Schutter, 2008), as well as those for Saudi Arabia (Haq and Al-Qahtani, 2005), have not been documented in the public domain. It is therefore not possible to assess their accuracy except possibly where they are shown to correlate to stage boundaries as in the case of the Middle and Upper Permian. As discussed below three stage boundaries are correlated to Khuff sequence boundaries (Figure 2). In contrast to the Permian stage boundaries, the Triassic stage boundaries and the Permian/Triassic Boundary (PTB) are not correlated to global sequence boundaries.

Correlation of Five Key Sequence Boundaries in the GTS 2012 and M&H-2010 Scales

The five Middle Permian–Lower Triassic sequence boundaries that separate dozons in the M&H-2010 scale can be correlated by numerical age to within ± 0.6 Myr to five global sequence boundaries (Figure 2; Tables 1 and 2) as calibrated by C. Huang and J. Ogg (written communications, 2012, 2013):

  • Orbital SB 17 at 249.5 Ma is about 0.4 Myr younger than SB Ole1 at 249.9 Ma, and is within the ± 0.5 Myr uncertainty for estimates in the Early Triassic.

  • Orbital SB 18C at 254.3 Ma correlates to SB Cha1 at 254.2 Ma.

  • Orbital SB 18B at 259.2 Ma is about 0.6 Myr younger than SB Wuc1 corresponding to the Middle/Lower Permian Boundary (Guadalupian/Lopingian Boundary, GLB) at 259.8 ± 0.4 Ma. The correlation is adopted because the alternative global correlatives, SB Cap3 or SB Wuc2, result in greater mis-ties. The age of the Middle/Lower Permian Boundary is not well constrained. It was revised from 260.4 ± 0.7 in GTS 2004 to 259.8 ± 0.4 Ma in GTS 2012 (Table 2), and other estimates range between 262.3 Ma (Menning et al., 2008) and 259.0 Ma (Shen et al., 2010). As discussed on pages 112–113 and in Figure 3, astronomical tuning of the Upper Permian Series in South China, suggests the age of the boundary is close to the orbitally predicted 259.2 Ma.

  • Orbital SB 18 correlates to early Capitanian SB Cap2 at 264.0 Ma.

  • Orbital SB 19C at 268.9 Ma correlates to SB Wor1 and the Roadian/Wordian Boundary dated at 268.8 ± 0.5 Ma.

KHUFF SEQUENCES KS6 TO KS1

This section reviews the sequence and biostratigraphic constraints of the six Khuff sequences KS6 to KS1, mainly in subsurface Oman (Figures 4 and 5, Osterloff et al., 2004), the outcrops in Al Jabal al-Akhdar in Oman (Enclosure, Figures 6 and 8, Koehrer et al., 2010, 2012), as well as Khuff Sequences KS4 to KS1 in the South Fars region in subsurface Iran (Figures 7 and 9, Insalaco et al., 2006). It starts with the lower boundary of the oldest Khuff Sequence KS6, the Khuff Sequence Boundary (Khuff SB), proceeds upwards through the six Khuff sequences KS6 to KS1, ending with the upper boundary of Khuff Sequence KS1, the Sudair Sequence Boundary (Sudair SB).

Khuff Sequence Boundary (Khuff SB): 268.9 Ma

The Khuff SB or correlative Sub-Khuff Unconformity in subsurface Oman separates the terrestrial clastics of the Gharif Formation from the overlying carbonates or mixed clastics-carbonates in the lower part of the Khuff Formation (Figures 4 and 5, Osterloff et al., 2004).

On the Saiq Plateau, Permian terrestrial clastics, about 15 m thick, unconformably overlie lower Paleozoic or Proterozoic formations (Enclosure and Figure 6; Koehrer et al., 2010; Bendias et al., 2013). The unconformity is sometimes referred to as “Hercynian unconformity”, “mid-Carboniferous unconformity” or “pre-Khuff unconformity” (e.g. Sharland et al., 2001). It is here referred to as the “Sub-Permian Unconformity”. The terrestrial clastics are known as the “lower Saiq member”, “Saiq unit A1”, “basal Saiq clastics”, or “basal Khuff clastics”. The latter term is believed to be misleading because by stratigraphic position, lithology and fluvial-estuarine depositional setting the terrestrial clastics correlate to the subsurface Gharif Formation (Figures 4 and 5; Al-Husseini and Matthews, 2010). The Khuff SB on the Saiq Plateau is picked at the boundary between the “lower Saiq member” and the overlying mixed clastics and carbonates of the Khuff-equivalent part of the Saiq Formation (Enclosure and Figure 6). Over paleohighs in Al Jabal al-Akhdar, the terrestrial clastics are absent and the Sub-Khuff Unconformity (Khuff SB) merges with the Sub-Permian Unconformity.

The Khuff SB can be traced from the Arabian Peninsula into the Zagros Mountains where it separates the carbonates and evaporites of the Dalan Formation from the underlying terrestrial clastics of the Faraghan Formation or older rocks (Szabo and Kheradpir, 1978; Al-Jallal, 1995; Insalaco et al., 2006). In the Alborz Mountains in North Iran it passes to the base of the Wordian–Capitanian Ruteh Limestone Formation, which overlies the terrestrial clastics of Shah Zeid Formation or Dorud ironstone (Gaetani et al., 2009; Crippa and Angiolini, 2012).

Age: The lowermost part of the Khuff Formation is assigned to the Neoschwagerina schuberti Zone implying the start of the transgression that deposited the Khuff Formation and equivalents is mid-Murghabian (Montenat et al., 1976; Rabu et al., 1986). However, the relationship between the Murgabian Stage of the Pamirs Realm and the North American Roadian and Wordian stages in GTS 2012 is not precisely established (Henderson et al., 2012; Davydov and Arefifard, 2013).

Most biostratigraphic studies assign the Khuff SB to the Wordian Stage. In the Al Huqf outcrops in Oman (Figure 1), brachiopod fauna found in Khuff Member 3, which occurs about 15 m above the Khuff SB, indicate a Wordian age (Angiolini et al., 1998, 2003, 2004). Palynological studies of the Gharif and Khuff formations in subsurface Oman and Saudi Arabia indicate the Khuff SB occurs at the base of Oman-Saudi Arabia Palynological Zone OSPZ6 in the Wordian (Stephenson et al., 2003; Stephenson, 2006). Baud and Bernecker (2010) confirmed the Wordian age for the basal part of the Khuff-equivalent Maqam Formation in Wadi Maqam (Figure 1) by citing the finding of conodonts Hindeodus excavatus and Hindeodus wordensis by C. Henderson and A. Nicora in 2009. Based on the foraminiferal assemblage found in Al Jabal al-Akhdar (Presumatrina and primitive Afghanella species), H. Forke (written communication, 2012) considered the onset of carbonate deposition to occur between the base of the Murgabian (late Roadian?–Wordian) to the lower part of middle Murgabian (Wordian).

Dating of the Khuff Formation using strontium-isotope measurements has been attempted but was inconclusive. Stephenson et al. (2012) measured 87Sr/86Sr ratios from several brachiopods taken from Khuff Member 3 in Al Huqf. They obtained a mean value of 0.7027, which implies a Roadian or even Kungurian age (see their figure 4, and figure 24.9 in GTS 2012; Henderson et al., 2012). They considered the global strontium-isotope dataset to be insufficiently sampled in the Wordian–Roadian interval and retained a Wordian age for the Khuff SB.

Global and Orbital Correlations: A Wordian age for the Khuff SB implies it is younger than early Wordian SB Wor1 with an estimated age of 268.2 Ma by linear interpolation or 268.8 Ma as estimated by C. Huang and J. Ogg (written communications, 2012, 2013). It may be as young as mid-Wordian SB Wor2 at 267.5 Ma, or late Wordian SB Wor3 at 266.4 Ma. In the M&H-2010 scale the Khuff SB is correlated to SB 18C at 268.9 Ma, close to the Roadian/Wordian Boundary (268.8 ± 0.5 Ma in GTS 2012) and SB Wor1 (Figure 2).

UPPER PERMIAN (LOPINGIAN) SERIES IN SOUTH CHINA

The chronostratigraphy and sequence stratigraphy of the Upper Permian Series in South China provide a good dataset for testing the accuracy of the M&H-2010 scale. Several sections that represent this series in South China contain radiometrically dated volcanic rocks at various stratigraphic levels, as well as the three Global Boundary Stratotype and Section Points (GSSP, Golden Spike) of the Upper Permian:

In many localities in South China the Upper Permian Series immediately overlies the Emeishan Basalt dated 259.0 ± 3.0 Ma (Shen et al., 2010), and which also correlates to global Wuchiapingian SB Wuc1 (Figures 2 and 3). One section that is particularly appropriate for testing the M&H-2010 scale is located at Zhuzang in the Zhijin coalfield, western Guizhou Province of South China (Figure 3 and Table 1; Wang et al., 2011). In this section the Upper Permian Series overlies the Emeishan Basalt, consists of 16 “fourth-order” sequences without evidence of significant hiatuses, and is capped by the Permian/Triassic Boundary at the top of Sequence 16 (Wang et al., 2011).

With the exception of Sequence 11, the thickness of the sequences ranges between 10 and 30 m and is 21 m on average (Figure 3). Sequence 11 is exceptionally thicker at 55 m and has a mire (bog) sedimentary facies near its interpreted maximum flooding surface, which in several other sequences is interpreted as a sequence boundary. It is therefore likely that Sequence 11 contains two stratons (2 cycles of 0.405 Myr) and that the Upper Permian in Zhuzang consists of the 17 stratons 640 to 624 in Figure 3. Converting the 17 stratons to time at 0.405 Myr/straton gives 6.9 Myr, and subtracting this interval from the age of model SB 18B at 259.2 Ma (Figure 2), implies the PTB at the top of Sequence 16 has an age of 252.3 Ma, just 0.1 Myr older than the estimate in GTS 2012.

Khuff Sequence KS6: 268.9–264.0 Ma

The Lower Khuff Member in subsurface Oman overlies the Gharif Formation and is overlain by the massive carbonates of the Middle Khuff Member (Figures 4 and 5). The thickness of the Lower Khuff Member varies from about 30 m in South Oman to a maximum of 330 m in the Lekhwair-70 Well (Figure 1a, Osterloff et al., 2004). A. Al-Harthy (2000, PDO unpublished report in Osterloff et al., 2004) interpreted the Lower Khuff Member in terms of three regional sequences, denoted P17, P18 and P19, the latter containing the regionally correlative “Khuff Marker Limestone” (KML, Figures 4 and 5), here interpreted to correspond to Wordian MFS P20 (Sharland et al., 2001, 2004). Al-Husseini and Matthews (2010) correlated the Lower Khuff Member to Khuff Sequence KS6 on the basis of stratigraphic position in the lowermost part of the formation. In the Sayyala-29 Well the Lower Khuff Member is 144 m thick, and based on the motif of the density log and carbonate-shale cyclicity they interpreted it as 12 subsequences, denoted LK1 to LK12 (LK for Lower Khuff) in ascending order (Figure 5 and Table 2).

On the Saiq Plateau in the Oman Mountains, Koehrer et al. (2010) interpreted Khuff Sequence KS6 (167 m thick) between the top of the “lower Saiq member” (possibly Gharif Formation below the Khuff SB) and Microbial Marker 1 (SB KS5) (Enclosure and Figure 6). They divided the sequence into 12 “cycles”, 5–30 m thick, and considered them as “fifth-order”. Bendias et al. (2013) interpreted the same Saiq Plateau KS6 section in terms of 14 “fourth-order” cycle sets of which two occur in the “lower Saiq member”. They interpreted 43 “fifth-order” cycles in Khuff Sequence KS6 (Enclosure). It is here suggested that the 12 “cycles” of Koehrer et al. (2010) are “cycle sets” and may correlate, one-for-one, to the 12 subsequences LK1 to LK12 in the Sayyala-29 Well (Enclosure, Figures 5 and 6).

Age: By stratigraphic position above the Khuff SB, the age of the lower part of KS6 is Wordian. The age of its upper part is not constrained, and may be Wordian or early Capitanian (Koehrer et al., 2010, 2012; Forke et al., 2012).

Wordian Maximum Flooding Surface MFS P20: Wordian MFS P20 of Sharland et al. (2001, 2004) is here interpreted to correspond to the MFS of Khuff Sequence KS6 (Figures 4 and 5). In Al Jabal al-Akhdar, Koehrer et al. (2010) interpreted one MFS in Sequence KS6 in a 15 m-thick unit named the “Muddy Marker” in cycle set KCS 6.4 (Enclosure). It contains thick stacks of massive dark-blue, bioturbated mudstone. They considered it the lowest energy zone, mainly characterized by suspension setting, starvation and background sedimentation below storm wave base. They suggested its dark blue color may be due to less oxygenated waters in a deeper-water setting. The “Muddy Marker” may correlate to the Khuff Marker Limestone (KML), which is a regionally correlative pure carbonate unit in subsurface Oman (Osterloff et al., 2004; compare Figures 4 to 6, Enclosure). A possible position for MFS P20 may be in Straton 655 (ca. 265.2 Ma) and the late Wordian MFS Wor3 at 265.8 Ma in global Sequence Wor3 (Figure 2).

Global and Orbital Correlations: If Sequence KS6 consists of 12 stratons and the Khuff SB occurs near the base Wordian, then KS6 would correlate to Dozon 19C between 268.9–264.0 Ma, and SB KS5 would correlate to SB 18 at 264.0 Ma (Figure 2). In this scenario, Sequence KS6 would correlate to the four global sequences Wor1 to Wor 3 and Cap1 between 268.8–264.0 Ma (Figure 2).

Khuff Sequence KS5: 264.0–259.2 Ma

Osterloff et al. (2004) interpreted three sequences, denoted P20, P23 and P27, between the top of the Lower Khuff Member (Sequence KS6) and the top of the “Khuff Middle Anhydrite” (Figure 4). The top of the Khuff Middle Anhydrite is a regional sequence boundary that is readily recognized in many regions of the Middle East: top “Khuff Median Anhydrite” in the United Arab Emirates and Qatar, top “Khuff-D Anhydrite” in Saudi Arabia, top Nar Member of Dalan Formation in Iran, and top Satina Member in Iraq (Al-Jallal, 1995; Sharland et al., 2001). Al-Husseini and Matthews (2010) correlated Sequence KS5 to sequences P20, P23 and P27 (Figure 4).

Koehrer et al. (2010) interpreted Khuff Sequence KS5, 214 m thick, on the Saiq Plateau between “Microbial Marker 1” (SB KS5) and “Microbial Marker 2” (SB KS4), and divided it into the 12 “cycle sets” KCS 5.1 to KCS 5.12 in descending order (Enclosure and Figure 6). In contrast, Walz et al. (2013) interpreted four “high-frequency” sequences (HFS KS 5-I to HFS KS 5-IV) and 19 “cycle sets” in the same section. The difference between 12 and 19 “cycle sets” in the same Saiq Plateau KS5 section indicates that more rigorous criteria and additional (chrono-) stratigraphic calibration data are required to interpret sequences in terms of sequence hierarchy and order. In the Wadi Hedek Section (272 m thick, Figure 1b), Walz et al. (2013) interpreted 21 medium-scale, “fourth-order” cycle sets and 66 small-scale “fifth-order” cycles.

Age: The age of Sequence KS5 is Capitanian based on the presence of Sphairionia sikuoides (Forke et al., 2012). In the uppermost part of the sequence the occurrence of miliolid foraminifer Shanita amosi and several other genera (e.g. Paraglobivalvulina, Rectostipulina) indicate a latest Capitanian age (Insalaco et al., 2006; Forbes et al., 2010; Koehrer et al., 2010, 2012; Forke et al., 2012). These genera occur immediately below the Middle Khuff Anhydrite in the subsurface (Insalaco et al., 2006; Forbes et al., 2010; Koehrer et al., 2010, 2012; Forke et al., 2012).

Capitanian Maximum Flooding Surface MFS P25 (?):Sharland et al. (2001, 2004) positioned Wordian MFS P20 in the lower part of the subsurface Middle Khuff Member, which is here interpreted as Capitanian (Figures 4 and 5). It is recommended to re-position MFS P20 into the Lower Khuff Member and use Capitanian MFS P25 (?) instead, corresponding to the “Chert Marker” in outcrops of the Al Jabal al-Akhdar (Koehrer et al., 2010; Walz et al., 2013).

Global and Orbital Correlations: As noted above biostratigraphic evidence indicates the upper part of Sequence KS5 is latest Capitanian implying SB KS4 correlates in the global scheme to basal Wuchiapingian SB Wuc1 at 259.8 Ma in GTS 2012 (i.e. Capitanian/Wuchiapingian Boundary, Guadalupian/Lopingian Boundary, GLB, Middle/Upper Permian Boundary) (Figure 2). In the orbital model Sequence KS5 is correlated to Dozon 18A between 264.0 and 259.2 Ma, and the 12 cycle sets KCS 5.1 to KCS 5.12 on the Saiq Plateau to stratons 652–641 (Figure 6). Based on the proposed orbital correlation, Sequence KS5 might correlate to two global Capitanian sequences Cap2 and Cap3 between 264.0–259.8 Ma (Figure 2).

Khuff Sequence KS4: 259.2–254.3 Ma

In subsurface Oman (Figure 4), Sequence KS4 corresponds to sequences P30 and P35 and possibly the lower part of P40 of Osterloff et al. (2004). Koehrer et al. (2010, 2012) interpreted Khuff Sequence KS4 on the Saiq Plateau (Enclosure and Figure 6) and correlated it across the outcrops in Al Jabal al-Akhdar in Oman (Figure 1b), where it ranges in thickness from 152 m to 171 m. They considered it a “third-order” sequence and divided it into two “high-frequency” sequences (HFS), HFS KS4b and HFS KS4a (here renamed as HFS KS4-I and HFS KS4-II to avoid confusion with terms used in South Fars by Insalaco et al., 2006; Enclosure), 11 “fourth-order” cycle sets (KCS 4.1–KCS 4.11, in descending order), and 66 “fifth-order” cycles.

Insalaco et al. (2006) interpreted Khuff Sequence KS4 above the Nar Member of the Dalan Formation in Iran (Figures 1a and 7). It is ca. 159 m thick in subsurface South Fars, 114 m in Kuh-e Surmeh and 117 m in Kuh-e Dena. They considered it a “third-order” composite sequence, and divided it into three “fourth-order” T-R sequences (KS4a, KS4b, KS4c in ascending order) and 12 “parasequence sets” (KS4a1 to KS4a5, KS4b1 to KS4b3, KS4c1 to KS4c4).

Age: Khuff Sequence KS4 is interpreted in both Iran and Al Jabal al-Akhdar as representing the Wuchiapingian Stage (Insalaco et al., 2006; Koehrer et al., 2010, 2012).

Wuchiapingian Maximum Flooding Surface MFS P30: In Al Jabal al-Akhdar, Koehrer et al. (2012) interpreted the main MFS in Sequence KS4 in cycle set KCS 4.6 (Figure 6 and Enclosure) at the maximum thickness of grainstones interbedded with muddy foreshoal textures, indicating the most open-marine conditions. Insalaco et al. (2006) interpreted three MFSs in Sequence KS4 in South Fars and considered the one in KS4c to be the dominant one (Figure 7). Both papers correlated their MFSs to Wuchiapingian MFS P30 of Sharland et al. (2001, 2004).

Global and Orbital Correlations: Khuff Sequence KS4 correlates to global Wuchiapingian sequences Wuc1 and Wuc2 between 259.8–254.2 Ma, which in turn may correlate to HFS KS4-I and HFS KS4-II (HFS4a and HFS KS4b of Koehrer et al., 2012). In the M&H-2010 scale KS4 correlates to Dozon 18B between 259.2–254.3 Ma (Figure 2). The 12 “parasequence sets” in Iran and 11 “cycle sets” in Oman (with one presumed unidentified) are correlated to stratons 640–629 and to “fourth-order” sequences 1 to 11 in the Zhuzang Section of South China (Table 1).

In the orbital model Straton 633 contains the MFS of HFS KS4-II (KS4a of Koehrer et al., 2012) and the MFS of KS4b of Insalaco et al. (2006) (Figures 6 and 7). In the Zhuzang Section of South China the base of Straton 633 correlates to the surface separating the transgressive and regressive sequence sets (TSS and HSS) corresponding to the MFS of “composite sequence” CSII (Wang et al., 2011). These correlations imply Straton 633 contains the MFS of global sequence Wuc1 and correlative MFS P30 of the Arabian Plate, with an age of ca. 256.1 Ma.

Khuff Sequence KS3: 254.3–252.3 Ma

In Al Jabal al-Akhdar in Oman, Khuff Sequence KS3 varies in thickness between 62 to 69 m, and consists of the four “cycle sets” KCS 3.4 to KCS 3.1 (Figures 1b and 8, Enclosure; Koehrer et al., 2010, 2012). It corresponds in part or completely to Late Permian sequence P40 of Osterloff et al. (2004). Insalaco et al. (2006) reported that Khuff Sequence KS3 is ca. 115 m thick in subsurface South Fars (Figures 1a and 9), or about twice as thick as in Kuh-e Surmeh (47 m) and Kuh-e Dena (60 m). They divided it into lower “third-order” Sequence KS3a, which consists of the four “parasequence sets” KS3a1 to KS3a4, and overlying “fourth-order” Sequence KS3b, which consists of the five “depositional units” KS3b0 to KS3b4.

Age: On biostratigraphic evidence the lower boundary of Sequence KS3, SB KS3, is correlated to the Wuchiapingian/Changhsingian Boundary (WCB) and its upper boundary, SB KS2, occurs below the Permian/Triassic Boundary (PTB) (Figure 2; Insalaco et al., 2006; Maurer et al., 2009; Koehrer et al., 2010, 2012). In subsurface Oman, the PTB occurs in the transition between the Middle and Upper Khuff members (Figure 4; Osterloff et al., 2004; Vachard and Forbes, 2009; Forke, 2009, unpublished PDO report inForbes et al., 2010), implying the uppermost Permian SB KS2 occurs near the top of the Middle Khuff Member, possibly the top of P40 of Osterloff et al. (2004).

Changhsingian Maximum Flooding Surface MFS P40: In Al Jabal al-Akhdar, Koehrer et al. (2010, 2012) interpreted the MFS of Sequence KS3 in cycle set KCS 3.2 (Figure 8 and Enclosure). It occurs in a distinctive one meter-thick coral-rich floatstone bed informally named the “Coral Marker”. Insalaco et al. (2006) interpreted the MFS of Sequence KS3 in South Fars in KS3a3 (Figure 9). Both studies correlate their MFSs to Late Permian MFS P40 of Sharland et al. (2001, 2004), and both occur in Straton 626 with an orbital age of ca. 253.3 Ma (Figure 2). These correlations imply Changsinghian MFS P40 correlates to the MFS Cha1 of global sequence Cha1, which has an age of ca. 253.5 Ma in GTS 2012. In the Zhuzang Section of South China the MFS of “composite sequence” CSIII is older if it is correlated to the base of Straton 627 at 253.9 Ma.

Global and Orbital Correlations: The lower boundary of Khuff Sequence KS3 is correlated to the Wuchiapingian/Changhsingian Boundary (WCB) and therefore to SB Cha1 at 254.2 Ma in GTS 2012 (Figure 2). The upper boundary of KS3 occurs below the Permian/Triassic Boundary and would therefore correlate to SB Cha3 at 252.5 Ma in GTS 2012 (Figure 2). Sequence KS3 is correlated to the five stratons 628–624 in the lower part of Dozon 18C (Figure 2 and Table 1). The five stratons 628–624 may correspond to the KS3a1 to KS3a4 “parasequence sets” and the “fourth-order” sequence KS3b in South Fars (Figure 9), to “fourth-order” sequences 12–16 in the Zhuzang Section (Figure 3), and to “cycle sets” KCS 3.4 to KCS 3.1 with one remaining unidentified in the Oman Mountains (Figure 8 and Table 1).

E. Insalaco (written communication, 2013) noted that the correlation of KS3b in South Fars to Straton 624 implies it consists of the five “depositional units” KS3b0 to KS3b4, and is much thicker than other stratons. He speculated that this anomaly might be related to the higher aggradation and possibly better preservation associated with this depositional package, but that this interpretation requires further clarification.

Khuff Sequences KS2 and KS1: 252.3–249.5 Ma

In Al Jabal al-Akhdar in Oman, Khuff Sequence KS2 ranges in thickness from 55 to 61 m and consists of the three “cycle sets” KCS 2.3 to KCS 2.1 (Koehrer et al., 2010, 2012, Figure 8 and Enclosure). Sequence KS1 attains a maximum thickness of 84 m in Wadi Hedek, where it consists of three “cycle sets” KCS 1.2 to KCS 1.0. Sequences KS2 and KS1, together, correspond to Triassic sequences Tr10 and Tr20 of Osterloff et al. (2004, Figure 4).

Insalaco et al. (2006) reported that Sequence KS2 ranges in thickness from 50 m at Kuh-e Dena to 57 m in subsurface South Fars, and interpreted it as a “fourth-order” sequence, which consists of four “parasequence sets” KS2a to KS2d, Figure 9). Khuff Sequence KS1 is ca. 100 m thick in South Fars and reaches 129 m in Kuh-e Dena. It is interpreted as a “third-order” sequence and divided into three “fourth-order” sequences KS1a, KS1b and KS1c, and 10 “depositional units” (KS1a1 to KS1a4, KS1b1 to KS1b3, and KS1c1 to KS1c3, Figure 9).

Age: Sequence KS2 straddles the Changsinghian and Induan stages with the Permian/Triassic Boundary picked in cycle set KCS 2.3 on the Saiq Plateau in Oman and KS2b in Iran (Enclosure, Figures 8 and 9; Table 1). Khuff Sequence KS1 is interpreted as Induan–?early Olenekian by stratigraphic position above the PTB and below the Sudair SB, which occurs near the Induan/Olenekian Boundary (see below).

Induan Maximum Flooding Surfaces MFS Tr10 and Tr20:Sharland et al. (2001) interpreted two maximum flooding surfaces, MFS Tr10 and MFS Tr20, in the Early Triassic (Scythian) and positioned them in the uppermost part of the Khuff Formation. In their revised study, Sharland et al. (2004) interpreted MFS Tr10 as Induan but did not discuss the age or position of MFS Tr20. The youngest two MFSs of the Khuff Formation occur in cycle sets KCS 2.2 and KCS 1.2 on the Saiq Plateau (Koehrer et al., 2010), and they are correlated to stratons 622 and 620, respectively, in the Induan. So it is proposed that MFS Tr10 (ca. 251.7 Ma) and MFS Tr20 (250.9 Ma) be taken as the flooding surfaces of KS2 and KS1, respectively, in global sequences Cha3 and Ind1 (Figure 2 and Table 1).

Global and Orbital Correlations: Sequences KS2 and KS1 correlate to global sequences Cha3 and probably Ind1 between SB Cha3 at 252.5 Ma and SB Ole1 at 249.9 Ma (Figure 2). They correlate to the seven stratons 623–617 in the upper part of Dozon 18C (Figure 2 and Table 1). In Oman “cycle sets” KCS 2.3, 2.2 and 2.1 are tentatively correlated to stratons 623, 622 and 621, and KCS 1.2, KCS 1.1 and KCS 1.0 to stratons 620, 619 and 618, with Straton 617 missing, possibly due to erosion and post-depositional tectonics during the Late Cretaceous (Koehrer et al., 2012).

“Cycle set” KCS 2.3 contains the Permian/Triassic Boundary (252.2 ± 0.5 Ma) as consistent with the age of Straton 623 between 252.3 and 251.9 Ma. In South Fars the four stratons 623 to 620 may correlate to the four “parasequence sets” KS2a, KS2b, KS2c and KS2d, and the three stratons 619 to 617 to the three “fourth-order” sequences KS1a, KS1b and KS1c (Figure 9 and Table 1).

Sudair Sequence Boundary

In the Arabian Peninsula, the Khuff Formation is overlain by the Sudair Formation and equivalent rock units, and the intervening boundary is referred to as the Sudair Sequence Boundary (Sudair SB, Figures 4 and 8, Enclosure). In South Fars in Iran, the Sudair SB passes to the boundary between the Kangan Formation and overlying Aghar Shale Member of the Dashtak Formation (Figure 9, Insalaco et al., 2006).

In Al Jabal al-Akhdar, Rabu et al. (1986) described the lowermost part of the Sudair-equivalent as: “commonly includes decimeter-thick beds of dolomite with quartz and intra-formational breccia with a sandstone-dolomite cement (Wadi Mu’aydin), beds of maroon siltstone (Wadi Misin), and a close succession of hardgrounds separating the dolomite beds and reflecting periodic emergence.” The Sudair SB represents a regional regression that was dated in the Oman Mountains by chemostratigraphy as late Induan (between the top of the Griesbachian to middle Dienerian) (Richoz, 2006; Baud and Richoz, 2013).

Forbes et al. (2010) characterized the Triassic Upper Khuff Member and Sudair Formation in subsurface Oman by Palynozone 2351 of Petroleum Development Oman (PDO) (Densoisporites nejburgii with Endosporites papillatus). They reported that in the lower shale of the Sudair Formation small marine acritarchs, characterized by Veryhachium spp., define PDO Palyno-subzone 1095. They added that the Veryhachium-Micrhystridium acritarch bloom, together with the associated miospores, appears to be an Induan-early Olenekian worldwide event. Palynological analysis of samples taken from the basal shale of the Sudair Formation in the SHD-1 Well in Central Saudi Arabia (Figure 1a) also contain abundant Veryhachium spp. of Early Triassic age (Manivit et al., 1983).

Forbes et al. (2010) also reported that a single specimen of Hemigordiellina tenuifistula, noted by Vachard (2007), suggests an Olenekian age at the base of the Sudair Formation outcrop-equivalent, and that Vachard (2007) recognized an Induan biozone (sporadic Hemigordiellina sinensis) in uppermost Khuff-equivalent outcrops in Al Jabal al-Akhdar.

In the Musandam Peninsula outcrops (Figure 1), Maurer et al. (2009) reported that a succession that is most likely equivalent to the Sudair Formation yields an association of Hoyenella sinensis and Meandrospira pusilla. They added that morphotypes and association of these foraminifera indicates a late Induan–Olenekian age. They considered the age of the youngest Khuff Sequence KS1 to be Induan. In the Al Jabal al-Akhdar outcrops, Oman (Figure 1), Koehrer et al. (2010) reported that foraminiferal fauna with sporadic occurrences of Hoyenella sinensis and H. tenuifistula occur in the lower part of the Sudair Formation outcrop equivalent on the Saiq Plateau (“Middle Mahil Member”). They considered the Sudair Formation as Olenekian, and Khuff Sequence KS1 as Induan.

Pöppelreiter et al. (2011) reported that in the Al Jabal al-Akhdar outcrops Cornuspira mahajeri occurs with shell fragments in the “Claraia beds” in the lowermost Sudair Formation outcrop equivalent. They took the occurrence of this Lower Triassic “disaster fauna” together with a prominent positive δ13CCarb isotope shift to indicate a late Induan–early Olenekian (late Dienerian–early Smithian) age for the lower part of the Sudair Formation. They positioned Olenekian MFS Tr30 in the Sudair Formation, corresponding to the Aghar Shale Member of the Dashtak Formation in South Fars (Insalaco et al., 2006).

The above-cited biostratigraphic studies place the Sudair SB near the Induan/Olenekian Boundary dated at 250.0 ± 0.5 Ma in GTS 2012, implying it correlates to either latest Induan SB Ind1 at 250.6 Ma, or earliest Olenekian SB Ole1 at 249.9 Ma (Figure 2 and Table 2). In the M&H-2010 scale the Sudair SB correlates to orbital SB 17 at 249.5 Ma (base Straton 616).

DISCUSSION

Putting Sequences in Order

The terms used in this paper to categorize sequences are quoted from the original papers. Most authors adopt the term “order” to categorize sequences; for example, a “third-order” sequence consists of several “fourth-order” sequences, which in turn are composed by “fifth-order” sequences (Table 3). The “fifth-order” sequence is generally synonymous with “cycle” and “parasequence”, and the “fourth-order” sequence with “parasequence set” and “cycle set”. Some authors use additional terms to subdivide “fourth-order” sequences into longer “high-frequency” sequences (HFS) and shorter “cycle sets” or “parasequence sets” (Tinker, 1998).

The calibrations that are made in this paper imply that Khuff sequences KS6, KS5 and KS4 have durations of ca. 5.0 Myr, while the briefest KS1 lasted only ca. 1.2 Myr. The durations of the 12 global sequences that span the same time interval in GTS 2012 vary from about 4.0 Myr for Wuc1 to 0.6 Myr for Cha2 (Figure 2). Thus the durations attributed to empirical “third-order” sequences range from 0.6–5.0 Myr, and do not match the prediction by the model of Matthews and Frohlich (2002) that “third-order” sequences lasted 2.0, 2.4 or 2.8 Myr.

Here, the four dozons 19C, 18A, 18B and 18A are interpreted to correspond to the four “composite” sequences KS6, KS5, KS4 and KS3–KS1 (Table 4). The global sequences used in the GTS 2012 (Wor1 to Ind1) are interpreted to represent “high-frequency” sequences rather than classical “third-order” composite sequences (Tables 3 and 4).

To avoid confusion regarding the sequence-stratigraphic hierarchy of the Khuff Formation across the Arabian Plate, we recommend using the M&H-2010 orbital scale to subdivide the Khuff Formation into 48 “stratons” and 4 “dozons” (Table 4). The four “dozons” 19C to 18C correspond to the four “composite” sequences KS6, KS5, KS4 and combined KS3 to KS1.

Converting Stratons to Time

The “cycle sets” of Koehrer et al. (2010, 2012) and “parasequence sets” of Insalaco et al. (2006) are good candidates for “stratons”. Several pitfalls, however, can occur when converting these sets or “fourth-order” sequences to time. The most common is assuming that the section is without hiatuses. On the Saiq Plateau, this pitfall is evident at the Sudair SB because cycle set KSC1.0, seen in Wadi Hedek, is missing on the Saiq Plateau (Figure 8). Two more stratons are predicted to be missing in Al Jabal al-Akhdar if the count of 12 is to be completed for Dozon 18C in combined “composite” sequence KS3 to KS1 (Table 4).

A strategy for recognizing significant hiatuses is to correlate sequence boundaries, maximum flooding surfaces and stratons between several localities. In this paper KS4 to KS1 are shown in Al Jabal al-Akhdar and South Fars, located about 500 km apart and each encompassing a study area of more than 100 square km (Figures 1, 6 to 9). A total of 24 stratons are predicted in these sequences, which appears to be the case in South Fars, but not so in Al Jabal al-Akhdar where only 22 are recognized. In the Ghawar Field of Saudi Arabia (Figure 1), 20 “high-frequency” sequences (HFS) have been recognized in KS4 to KS1 (Al-Eid and Tawil, 2011; Al-Dukhayyil et al., 2012; A. Al-Tawil, personal communication, 2012). It is possible that these HFS are rather “stratons” with four stratons missing over this structure that was located in a more proximal setting than South Fars.

Another pitfall is suspected to occur in the Zhuzang Section in South China (Figure 3; Wang et al., 2011), where “fourth-order” Sequence 11 may consist of two stratons implying it represents ca. 0.8 Myr. The opposite pitfall is suspected in South Fars where the five “depositional units” in Sequence KS3b may be “fifth-order” sequences with durations of about 0.1 Myr rather than 0.405 Myr (Figure 9). Some of these pitfalls may be avoided by collecting additional stratigraphic data to better constrain the ages of sequence boundaries and maximum flooding surfaces/intervals in terms of stratons.

F. Maurer (written communication, 2013) emphasized that the lack of detailed biostratigraphy in many Khuff sections makes a detailed correlation of sequences difficult. He suggested that the use of other proxies, such as carbon isotopes, may provide an alternative tool for correlations. Clarkson et al. (2012, their figure 4) show a correlation of the Triassic part of the Khuff Formation between Musandam and Wadi Sahtan in Al Jabal al-Akhdar. It illustrates that the Musandam Section has a much higher sedimentation rate, implying that in Al Jabal al-Akhdar region not all stratons might be recorded. He thinks it might be beneficial to use the carbonate-isotope signature for a section with high-resolution sequences, such as the Zhuzang Section, and compare it with the Arabian Plate for a better fine-tuning of the straton-to-straton correlation.

Regional, Global and Orbital Isochronous Surfaces

Three major sequence boundaries are recognized in Al Jabal al-Akhdar (Koehrer et al., 2010, 2012): (1) Khuff SB marking the start of the marine transgression, (2) SB KS4 at the top of the Middle Anhydrite or its equivalent, and (3) Sudair SB at the breccia level at the top of the Khuff Formation. In the global scheme only two Mid-Permian–Early Triassic sequence boundaries, SB Ole2 and SB Wuc1, are interpreted as “major” in the compilation of Snedden and Liu (2011). Therefore only one sequence boundary is recognized as “major” in both the Arabian and global frameworks: correlative SB KS4 and SB Wuc1.

In the orbital M&H-2010 scale, five sequence boundaries are predicted to be regional and these are recognized in both the Arabian and global schemes (Figure 2). Of the five, SB 17A (Sudair SB) and SB 18A (SB KS5) are predicted to be more prominent than the other three (Figure 2). This prediction is true for SB 17A corresponding to the major regression at the Sudair SB, characterized by the influx of terrestrial clastics and exposure above carbonates and evaporites.

It is less clear whether SB 18A is a more prominent sequence boundary in comparison to SB KS4 (top Middle Anhydrite) or the Khuff SB. The significance of SB 18A may be that its correlative SB KS5, at the base of Sequence KS5, represents the start of a second transgression of much greater regional extent than KS6. This scenario is suggested by regional correlations from Iran and Oman (Tethys-side) to Saudi Arabia (Al-Jallal, 1995), and the interpretation that the Khuff transgression first reached as far as the Arabian Shield during the Capitanian (Vaslet et al., 2005). This scenario implies that the extent of coastal onlap onto the Arabian Plate of Capitanian Sequence KS5 was far greater than that of Wordian–?early Capitanian KS6, or opposite to that depicted in the global onlap curve (Figure 2).

CONCLUSIONS

This study uses biostratigraphic and sequence-stratigraphic constraints to position the Khuff Formation between global Wordian sequence boundary SB Wor1 at ca. 268.8 ± 0.5 Ma and Olenekian sequence boundary SB Ole1 at ca. 249.9 ± 0.5 Ma as calibrated in GTS 2012 (Figure 2; Tables 1 and 2). These two boundaries correlate by numerical age to two model-predicted, regional sequence boundaries at 268.9 and 249.5 Ma in the M&H-2010 orbital scale. The model scale predicts that the Khuff Formation contains 48 T-R sequences with an average duration of 0.405 Myr, corresponding to the long-eccentricity orbital cycle and referred to as “stratons”. Moreover the 48 stratons are predicted to group into four “dozons” each lasting 4.86 Myr (Table 4). The chronostratigraphic framework presented in this paper is believed to have an accuracy of less than one million years, and perhaps approaching 0.405 Myr – the duration of a straton.

The Khuff Formation on the Saiq Plateau in Al Jabal al-Akhdar provides a comprehensively documented, reference section at outcrop for the Mid-Permian (Wordian) to Early Triassic (Induan–?early Olenekian) sequence stratigraphy (Enclosure). A total of 45 transgressive-regressive (T-R) “fourth-order” cycle sets are considered good candidates to represent all but three of the 48 stratons, with three remaining to be identified or absent due to faulting (Enclosure, Figures 6 and 8; Table 1). In the Al Jabal al Akhdar outcrops in Oman, the four dozons correlate closely to composite Khuff sequences KS6, KS5, KS4 and combined KS3 to KS1 (Figure 2; Tables 1, 2 and 4).

The M&H-2010 scale was also used to calibrate the Upper Permian (Lopingian) Zhuzang Section in South China (Figure 3 and Table 1). The section is bounded below by the Emeishan Basalt dated at 259.0 ± 3.0 Ma and corresponding to global basal Wuchiapingian sequence boundary SB Wuc1 dated at 259.8 Ma in GTS 2012. It is bounded above by the Permian/Triassic Boundary dated at 252.2 ± 0.5 Ma. The application of the M&H-2010 scale dates the Zhuzang Section between 259.2 and 252.3 Ma.

The results from Oman and South China are believed to support the prediction by Laskar et al. (2004, 2011) that the 0.405-Myr clock is stable as far back as the Permian/Triassic Boundary, and that the M&H-2010 scale can be used to estimate the ages of Mid-Permian–Early Triassic T-R sequences.

Geoscientists working on the Khuff reservoir in the Middle East are encouraged to use the M&H-2010 orbital scale as a sequence-stratigraphic template, instead of empirical cycle definitions and orders. In the M&H-2010 orbital scale, Khuff reservoir zones can be better positioned in a regional chrono- and sequence-stratigraphic framework using time-calibrated “stratons” (0.405 Myr cycles) and “dozons” (4.86 Myr cycles). This may allow correlating the Khuff and its correlative formations in a consistent way from field to field across the Arabian Plate.

ACKNOWLEDGEMENTS

The authors thank Jim Ogg and Chunju Huang for sharing their estimates for the ages of the global sequences calibrated in the Geologic Time Scale GTS 2012. Enzo Insalaco, Chunju Huang, Florian Maurer, Mike Stephenson and Daniel Bendias are thanked for their comments and suggestions that have improved the manuscript. Bastian Koehrer would especially like to thank Tom Aigner, Michael Pöppelreiter and his former colleagues of the Sedimentary Geology working group of the University of Tübingen. GeoArabia’s Assistant Editor Kathy Breining is thanked for proofreading the manuscript, and GeoArabia’s Production Co-manager, Arnold Egdane, for designing the paper for press.

ABOUT THE AUTHORS

Moujahed I. Al-Husseini founded Gulf PetroLink in 1993 in Manama, Bahrain. Gulf PetroLink is a consultancy aimed at promoting technology in 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.

geoarabi@batelco.com.bh

Bastian Koehrer is a Development Geologist in Wintershall’s German business unit, working on mature oil field and tight gas sands development in the German North Sea and Lower Saxony. He has more than five years of E&P project experience in Germany, Oman, Qatar and the UAE with a professional track record in both carbonate and clastic reservoirs. Bastian obtained a PhD degree (2011) in Carbonate Sedimentology (Khuff Formation, Oman) from the University of Tübingen (Germany) in research collaboration with Shell (Qatar) and Petroleum Development Oman. Bastian is a member of the EAGE, AAPG and DGMK and has published several papers on carbonate sequence stratigraphy and reservoir outcrop analogs. His work interests include integration of rock, log and test data to improve reservoir characterization and 3-D reservoir modeling during the development and production stages of the field life cycle.

bastian.koehrer@wintershall.com

Lithostratigraphy

The measured section starts in the Proterozoic–Paleozoic Mu’aydin Formation below the Sub-Permian Unconformity (sometimes referred to as the “Hercynian unconformity”, “middle Carboniferous unconformity”, “pre-Khuff unconformity” or “pre-Saiq unconformity”). Above the Sub-Permian Unconformity, the 15 meter-thick terrestrial unit is sometimes referred to as the “Saiq unit A1”, “lower Saiq member”, “basal Khuff clastics (BKC)” or “basal Saiq clastics”. The unit may be the Lower to Middle Permian Gharif Formation of probable Roadian–Wordian age by stratigraphic position. The Khuff Sequence Boundary (Khuff SB) is taken below the lowermost mixed carbonates and clastics. The overlying section, up to the Permian/Triassic Boundary (PTB), corresponds to the Saiq Formation. The upper part of the Khuff Formation between the PTB and the Sudair Sequence Boundary (Sudair SB) is confusingly named by some authors as the “Saiq unit C” or by others as the “lower Mahil member” (see Baud and Richoz, 2013).

Lithology, textures, facies and lithofacies associations

(Koehrer et al., 2010, 2012; Bendias et al., 2013; Walz et al. (2013).

Logs

Gamma-ray, Uranium, Potassium, Carbon 13 isotope and Oxygen 18 isotope curves are reproduced from Koehrer et al. (2010).

Sequence Stratigraphy

The hierarchy of sequences ranges from “cycle” (fifth-order), to “cycle set” (fourth-order), to “high-frequency sequence” (HFS) and to “third-order”. Khuff Sequence KS6 is amended from Koehrer et al. (2010) to show Khuff cycle sets KCS 6.1 to KCS 6.12, previously considered as cycles. The small-scale cycles of KS6 are taken from Bendias et al. (2013) and shown in the fifth-order column. Khuff Sequence KS5 is shown from Koehrer et al. (2010) with HFS KS5-I to HFS KS5-IV from Walz et al. (2013). Khuff Sequence KS4 is from Koehrer et al. (2010, 2012). Khuff Sequences KS3, KS2 and KS1 are individual high-frequency sequences (HFS), previously considered as third-order sequences. Together they stack to form one third-order sequence with its MFS being equivalent to that of Sequence KS2.

Chronostratigraphy

Following Koehrer et al. (2010, 2012) Khuff Sequences KS6 and KS5 span the Mid-Permian, Wordian and Capitanian based on biostratigraphic evidence; however, the stratigraphic position of the Wordian/Capitanian Boundary is unconstrained. On biostratigraphic evidence Sequence KS4 is dated as Wuchiapingian and KS3 as Changhsingian. Khuff Sequence KS2 spans the Permian/Triassic Boundary, and KS1 is Early Triassic, Induan–?earliest Olenekian on biostratigraphic and chemostratigraphic evidence. The lower Saiq member (Gharif Formation) is not dated and probably correlates to the Upper Gharif Member of subsurface Oman of ?Roadian–early Wordian age. The Sudair Formation is dated as Olenekian by biostratigraphy and chemostratigraphy.