Sahtan Gr

The Sahtan Group was defined by Glennie et al. (1974) in the Al Hajar Mountains outcrops to cover the shallow-marine carbonate Jurassic rocks lying between the Triassic dolomites of the Mahil Fm (Akhdar Gr) and the overlying fine-grained limestones and marls forming the lower part of the Kahmah Gr. See also Hughes Clarke (1988). Type section: Wadi as Sahtan, north flank of Al Jabal Al Akhdar. For additional subsurface reference sections see individual formations. Column: Oman Subsurface. In the subsurface sections in western Oman five formations are recognized in the Sahtan Group (in descending order): Jubaila Fm, Hanifa Fm, Tuwaiq Mountain Fm (Tuwaiq Mountain Limestone Fm), Dhruma Fm and Mafraq Fm (clastic/carbonate), the first four being entirely carbonate units originally defined in Saudi Arabia. In one western well only an uppermost thin evaporitic unit is present (Hughes Clarke, 1988). This unit may represent the feather-edge of the evaporitic Arab Fm and Hith Fm of Saudi Arabia.

Synonym:

The Sahtan Group evolved from a lower clastic succession into a shallow-marine carbonate shelf succession with grainstones, aphanitic mudstones and wackestones with dolomites developed locally. However, in some areas, for example, in the northwest of the Fahud Salt Basin, the dolomites predominate

"Rousseau et al. (2006) identified three major depositional episodes:

(1) a Pliensbachian – Toarcian coastal encroachment in a southward direction, represented by the dominantly clastic deposition of the Lower Mafraq Formation upon the Permian – Triassic carbonates, and as identified herein, clastics of the Late Triassic, Minjur Fm;

(2) a general Late Bajocian marine flooding diachronously overstepping the Lower Mafraq Fm, comprising a hybrid facies of marginal-marine environments of the Upper Mafraq Fm, followed through the Bathonian – Callovian by the carbonate Dhruma Fm -Tuwaiq Mountain Limestone Fm. This evolved through time from a low-angle, homoclinal ramp dipping in a (north) westwards direction, to a purely aggradational, flat-topped platform; and

(3) a Kimmeridgian – Tithonian onlap in an eastwards direction of fine-grained limestones (Hanifa Fm- Jubaila Fm) upon a post-Tuwaiq Mountain unconformity. They describe a significant post-Tuwaiq Mountain unconformity, with possibly up to 200 m of erosion in the east, which in time can be associated with a widespread Middle – Upper Jurassic unconformity, that has been recognized all over the Arabian Peninsula (Murris, 1981).

Sequence Stratigraphy: The Sahtan Group represents the upper AP6 (Lower Mafraq Fm only) and AP7 Megasequences of Sharland et al. (2001). Sharland et al. (2001) correlate their MFS J70 to J10 surfaces into the Group (see formational discussions)."

"The Jubaila Fm / Hanifa Fm and the major part of the Tuwaiq Mountain Limestone Fm belong to Biozone F47 (Kurnubia jurassica associated with Crassicollaria spp.). The lower part of the Tuwaiq Mountain Limestone Fm belongs to Biozone F45 (Pfenderina salernitana, P. trochoidea), as do the Dhruma Fm and Upper Mafraq Fm Member. The Lower Mafraq Fm Member comprises Biozone F30 (Pseudocyclammina liassica, Orbitopsella praecusor). However, the zonation remains relatively crude in places, e.g., the Hanifa Fm is poorly characterized, with little additional insight since Sikkema (1992). . . . The advances in understanding resulting from recent publications and in-house PDO palynological work significantly challenge the ages assigned to the historical Biozones of Sikkema (1992). This is further justification of the need for a comprehensive review and upgrade of the bio- and chronostratigraphy of this Group."

" The stable carbonate platform that was established on the Arabian Peninsula during Late Permian – Triassic times became notably influenced by regional scale tectonics during the Late Triassic. Even prior to this, doming of the Arabian Platform led to part emergence along the Al Huqf High. The greater part of the Arabian Platform became exposed at the end of the Triassic. Late Triassic structural movements took place throughout Oman along old north-south trending basement faults, especially in the north. These movements may have resulted from changes in the spreading of the Neo- Tethys that had developed during the separation of Gondwana from Permian times onwards. Spreading of the Neo-Tethys along its previous axis (the Hawasina Basin) ceased, and active spreading shifted eastwards (Glennie, 1995). Halokinesis of the Ara salts continued in the northern salt basins, with some growing diapirs piercing the surface (Peters et al., 2003), and the associated development of rim synclines. Substantial salt-withdrawal basins formed in the Ghaba Salt Basin, with up to 2,000 m thick Triassic sections preserved. The siliciclastics of the basal Sahtan Group (Mafraq Formation) were deposited over the shelf margin as well as over the foreland interior. From then on, shallow-marine carbonates were to dominate the shelf area and source the carbonate turbidites that characterise much of the Jurassic and Cretaceous platform-margin and basinal Hawasina deposition of Neo- Tethys (Glennie et al., 1974). This ime interval, corresponding to the Dhruma to Jubaila carbonates, comprises successive shallowing-up, regressive cycles of mudstones grading to grainstones. The mudstones in adjacent UAE developed into source rocks under anoxic conditions and have been tied to one of the petroleum systems recognised in the northeast of Oman (the Tuwaiq Petroleum System; Terken et al., 2001). These overall condensed and strongly regressive sequences, deposited when globally sea level was rising, indicate continued continental-margin uplift(s). This may be responsible for the post-Tuwaiq Mountain and post-Sahtan unconformities, where particularly the Upper Jurassic uplift and erosion seems to be observed over the entire eastern edge of the Arabian Plate (Rousseau et al., 2006)."