Abstract
ABSTRACT
This Note presents a formal update to the Middle East Geologic Time Scale 2008 (ME GTS) for the Late Triassic and Jurassic rock units of Saudi Arabia. It reviews their lithostratigraphic nomenclature, ranks and stage assignments, and proposes names for third-order chrono-sequences as compiled and/or interpreted from the published literature. The review starts with the Late Triassic (Late Norian – Rhaetian) Minjur Sandstone of the Buraydah Group, with the Triassic – Jurassic (TrJ) boundary positioned at its top. The Minjur Sandstone consists of two units, here ranked as members, which are interpreted as the Late Norian – ?Early Rhaetian Lower Minjur Sequence (Lower Minjur Member) and ?Late Norian – Rhaetian Upper Minjur Sequence (Upper Minjur Member). The Early Jurassic Unconformity - Hiatus (Hettangian, Sinemurian and Pliensbachian stages) separates the Minjur Sandstone from the Toarcian Marrat Formation of the Jurassic Shaqra’ Group. The Shaqra’ Group consists of seven formations, from base-up: (1) Marrat Formation consisting of the Lower, Middle and Upper units, here ranked as members. (2) Dhruma Formation consisting of eight units (D1–D5, Wadi ad Dawasir “delta”, D6 and D7). The Dhruma units have been named in the literature as Balum Member (D1 and lower part of D2 units), Dhibi Limestone Member (upper part of D2 unit), Uwaynid Member (D3), Barrah Member (D4), Mishraq Member (D5), ‘Atash and Hisyan members (D7); the D6 and Wadi ad Dawasir “delta” units are not formally named as members. (3) Tuwaiq Mountain Limestone consisting of the Baladiyah (T1 unit), Mysiyah (T2 unit) and Daddiyah (T3 unit) members. (4) Hanifa Formation consisting of the Hawtah and Ulayyah members. (5) Jubaila Limestone with J1 and J2 units. (6) Arab Formation consisting of D to A members. (7) Hith Anhydrite consisting of Main Hith Anhydrite Member and Manifa Reservoir/Member.
The seven formations of the Jurassic Shaqra’ Group are interpreted as 11 third-order chrono-sequences: (1) Early Toarcian Marrat Sequence B (Lower and Middle Marrat members) and Mid- to ?Late Toarcian Marrat Sequence A (Upper Marrat Member), the latter containing Arabian Plate maximum flooding surface MFS J10. (2) Bajocian Lower Dhruma Sequence (Balum Member and Dhibi Limestone), formed by the Balum and Dhibi subsequences, the former containing MFS J20. (3) Late Bajocian – Mid-Bathonian Dhruma Sequence B (Uwaynid, Barrah, Mishraq members, and Wadi ad Dawasir “delta” unit), with the Mishraq containing MFS J30. (4) Late Bathonian – early Mid-Callovian Dhruma Sequence A (unit D6, ‘Atash and Hisyan members), with the Hisyan containing MFS J40. (5) Mid- to Late Callovian Tuwaiq Sequence (Tuwaiq Mountain Limestone) containing an undesignated MFS at the base the Daddiyah Member (T3 unit). (6) Early and Mid-Oxfordian Hawtah Sequence (Hawtah Member of Hanifa Formation) containing MFS J50. (7) Late Oxfordian – ?Early Kimmeridgian Ulayyah Sequence (Ulayyah Member of Hanifa Formation) containing MFS J60. (8) Kimmeridgian Jubaila Sequence (Jubaila Limestone) containing MFS J70. (9) Arab-D Sequence (Arab-D Member inclusive of the Arab-D Anhydrite) containing an undesignated MFS in the Arab-D carbonate. (10) ?Kimmeridgian – ?Tithonian Arab-C and B Sequence (Arab C and B members) containing fourth-order MFS J80 and J90 in the lower carbonates of the members; and (11) ?Kimmeridgian – Tithonian Arab-A - Main Hith Sequence (Arab-A Member and Main Hith Anhydrite below the Manifa Reservoir/Member) containing fourth-order MFS J100 in the Arab-A Member. The Tithonian Manifa Member (upper part of the Hith Anhydrite Formation) is interpreted as heralding a transgression (fourth-order MFS J110), which deposited the Late Jurassic – Early Cretaceous Sulaiy Formation of the Thamama Group. The Jurassic – Cretaceous (JK) boundary is placed in the Sulaiy Formation by stratigraphic position. Based on the Geologic Time Scale GTS 2004 and Arabian Orbital Stratigraphy (AROS), the ages in million years before present of the sequence boundaries and maximum flooding surfaces are estimated for these chrono-sequences.