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A geological excursion to Jebels Rawdah, Buhays and Faiyah.

This one-day excursion involves off-road driving and a moderate amount of clambering over rough terrain. It concentrates mainly on the Maastrichtian (latest Cretaceous) Qahlah and Simsima Formations and the overlying Palaeocene Muthaymimah Formation at Jebels Rawdah, Buhays and Faiyah (Figs. 1 and 2). The Simsima Formation provides the main oil reservoir of the Shah Field which is located in the Liwa region of the UAE.

[FIGURE 1 OMITTED]

Whilst Jebels Buhays and Faiyah are located within the UAE, Jebel Rawdah is located in Oman and it is necessary for individuals to display their passports (not copies of) at the Omani border post along the road to Hatta from Al Madam although no visa is required. Four-wheel drive vehicles are recommended for such an excursion.

All three jebels are eroded remnants of anticlinal structures but Jebel Rawdah also includes a syncline. The folding occurred during the Oligo-Miocene.

Jebel Rawdah

This jebel is located on the northern side of the highway from Al Madam to Hatta, 15 kms east of Al Madam. Structurally, the jebel represents an eroded anticline-syncline couplet with a WNW-ESE trend. Viewed from the highway, light brown Simsima Formation overlies palaeo-hills mainly composed of black Semail ophiolite (former oceanic crust / upper mantle) and Hawasina Group sediments which were tectonically emplaced by obduction during the Campanian (Late Cretaceous).

Conglomeratic Qahlah Formation only locally separates the allochthonous strata from the orange-yellow coloured Simsima Formation as it mainly infills palaeovalleys but this formation is not easily accessible during this excursion. It will be examined at Jebel Buhays. Creamy white coloured Palaeocene Muthaymimah Formation overlies the Simsima Formation which appears to wedge out eastwards. The top of the Simsima Formation is reddened possibly due to exposure prior to Muthaymimah deposition or to lateritic outwash from contemporaneously exposed lateritic palaeosols at top ophiolite (Fig. 3a). In the extreme east, the Muthaymimah Formation directly overlies ophiolite but appears to progressively onlap the Simsima Fms. westwards although faulted contacts may also occur (Fig.3b).

From the Hatta road, the jebel is best approached by driving north along dirt tracks across the core of the eroded anticline towards the lowest point of the ridge (Fig. 4) defining the northern limb of the anticline / southern limb of the syncline. Here one can ascend to the crest of the ridge by crawling upwards along an extensive dipping fracture plane within the Simsima Formation. The shallow marine limestones of the Simsima Formation are dominated by diagenetic calcite nodules which are also extremely common in the Shah Field reservoir. The intensity of these nodules varies throughout the formation and they often impinge on each other to the exclusion of original sedimentary texture. Near the base of the ridge, slightly to the west of the lowest point, one can observe in situ rudist fossils (extinct bivalves; Fig. 5a).

[FIGURE 2 OMITTED]

The Simsima Formation was exposed and karstified prior to Muthaymimah being deposited with strong angular unconformity. Karst breccias fill fissures in the Simsima Formation. From the ridge crest one gains an impressive panoramic view of the Muthaymimah Formation outcropping in the core of the syncline north of the ridge.

Traversing the northern side of the ridge crest for about 100 metres, one can examine the unconformable contact. The lowermost Muthaymimah Formation is massively bedded and includes large angular exotic clasts of both carbonates and reddish 'basement' lithologies which were emplaced as submarine debris flowed along a submarine slope channel (Fig. 6a-c and 7). They are interbedded with, and pass up into, thin bedded muddy limestones containing marine planktonic foraminiferids indicating their accumulation as deep water oozes. They also contain dark brown chert nodules probably formed from remobilised silica of sponge spicules which are known to have occurred within the formation. Some of these thinner Muthaymimah beds, especially in the core of the syncline, display Bouma sequences indicative of calciturbidites. The deep water origin of the Muthaymimah Formation contrasts markedly with the relatively shallow water Simsima Formation. The sharp change in depositional setting was possibly due to contemporaneous faulting associated with rifting around the Cretaceous-Tertiary boundary. The slight change in dip and strike across the Simsima-Muthaymimah boundary is evidence of tectonic disturbance at the end of the Cretaceous.

[FIGURE 5 OMITTED]

One can either retreat to the base of the ridge by retracing steps along the route already taken or one can clamber relatively easily down a gully at the deepest part of the erosional channel (about 50 metres west of the ascent route). Some of these gullies expose the karstic breccias.

The northern limb of the syncline is accessible by vehicles along a dirt track around the western side of the jebel and has been a fruitful site for the collection of echinoid specimens by palaeontologists of the Natural History Museum, London.

Jebel Buhays

This represents the southern plunge of an eroded north-south trending anticline located on the western side of the highway from Al Madam to Dhaid, 12 kms north of Al Madam. Jebels Thanais and Aqabah constitute remnants of the northern plunge. Excellent rock outcrops can be approached by vehicle along a dirt track which passes a very tall radio mast on the northern flank of Jebel Buhays. The Simsima Formation is conspicuous with its heavy superficial coating of 'dripstone' caliche but about one kilometre from the main road is a prominent vertical wall of 'clean' exposure displaying good sedimentary layering worthy of close inspection (Fig. 8a).

A relatively easy climb to the base of this Simsima exposure begins with a traverse across dark serpentinised ophiolite. The ophiolite is overlain by a conglomeratic interval assigned to the Maastrichtian Qahlah Formation which comprises well rounded ophiolite pebbles and is interpreted as a beach deposit although with fluviatile basal sediments (Fig. 8b). The Qahlah Formation is overlain abruptly by the bedded Simsima Formation which again reveals abundant calcareous concretions. However, here it is obviously far more fossiliferous and displaying, for instance, abundant benthonic orbitoidal foraminiferids, red algal rhodoliths, large rudists (Dictyoptychus and Durania) and large gastropods (Campanile and Acteonella) (Figs. 8c, d and 9).

Half a kilometre further west along the dirt track is exposed a far greater thickness of the Qahlah Formation overlying the ophiolites.

The Muthaymimah Formation is well exposed in all the valleys around the southern side of Jebel Buhays. It is dominated by mass gravity flow deposits (Fig. 10) including calciturbidites and olistostromes (mega debris flows) containing olistoliths (clasts) as large as automobiles.

[FIGURE 6 OMITTED]

Jebel Faiyah

This jebel is a ridge located immediately north of the Jebel Buhays anticline again on the western side of the Al Madam - Dhaid highway, 17 kms north of Al Madam. It forms the western limb of a north-south trending anticline. Jebel Rumaylah on the opposite, eastern side of the highway is the eastern limb of the same anticline. Unfortunately, a metal fence has been erected around much of Jebel Faiyah and therefore limits access. The most rewarding approach is via a municipal rubbish tip in front of the lowest point of the north-south ridge. Permission should be sought from the on-site officials in charge of the tip before driving to the foot of the ridge. A steep dirt track passes over the ridge but it is not advisable to drive along it. Walking is recommended. Alternative access may be possible by driving around the southern tip of the jebel and along the western side.

Again the patchy Qahlah and/or shallow marine Simsima Formation overlie ophiolites and the Simsima Formation is again extremely fossiliferous. The Muthaymimah Formation again comprises calciturbidites and olistostromes. Although the Muthaymimah Formation has an unconformable base throughout the region, there is actually a tectonic contact between the Simsima and Muthaymimah Formations at this locality (Fig. 11). A steep walk up the ridge at its lowest point behind the municipal tip provides a stunning view of a thrust plane between these two formations. Calciturbidites of the Muthaymimah Formation reveal drag curvature immediately above the thrust plane, thus indicating easterly thrusting.

Footnote: Noweir et al. (1998) identified Maastrichtian foraminiferids within the Muthaymimah Formation and so renamed the the Muthaymimah as the Upper Simsima Member. They stated that the 'Muthaymimah' Formation was conformable on the Simsima Formation in the Faiyah Range area whereas Nolan et al. (1998) and Skelton et al. (1998) described at least a low angle unconformity to strong erosional contact. It is likely that Maastrichtian foraminiferids were reworked into Palaeocene resedimented carbonates that now comprise the Muthaymimah Formation. Nolan et al. (1998) described such reworking .

Abou Sayed and Mersal (1998) adopted the reassignment of 'Muthaymimah' as Upper Simsima at Jebel Rawdah without discussion but there is extremely strong lithostratigraphic evidence to separate out the Simsima and Muthamimah Formations at that locality.

[FIGURE 1 OMITTED]

References

Abou Sayed, M.G.S. and Mersal, M.A., 1998. Surface Geology of Jebel Rawdah, Oman Mountains. GeoArabia, 3, 401-414.

Nolan, S.C., Skelton, P.W., Clissold, B.P. and Smewing, J.D., 1990. Maastrichtian to Early Tertiary Stratigraphy and Palaeogeography of the Central and Northern Oman Mountains. In: A.H.F. Robertson, M.P. Searle and A.C. Ries (Eds.), The Geology and Tectonics of the Oman Region. Geological Society of London Special Publication, 49, 495-519.

Noweir, M.A., Alsharhan, A.S. and Boukhary, M.A., 1998. Structural and Stratigraphic Setting of the Faiyah Range, Northwestern Oman Mountain Front, United Arab Emirates. GeoArabia, 3, 387-398.

Skelton, P.W., Nolan, S.C. and Scott, R.W., 1990. The Maastichtian transgression onto the northwestern flank of the Proto-Oman Mountains: sequences of rudist-bearing beach to open shelf facies. In: A.H.F. Robertson, M.P. Searle and A.C. Ries (Eds.), The Geology and Tectonics of the Oman Region. Geological Society of London Special Publication, 49, 521-547.

by Graham Evans and Anthony Kirkham

Anthony Kirkham

Email: kirkhama@compuserve.com

Graham Evans

Email: grahamandrosemary@googlemail.com
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Author:Evans, Graham; Kirkham, Anthony
Publication:Tribulus
Geographic Code:7UNIT
Date:Jan 1, 2016
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