Chrono- and sequence-stratigraphy of the Mid-Permian to Early Triassic Khuff sequences of the Arabian Plate

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.