Geology of Oman
a potted guide

Oman | General | Travel | Visas | Locations | Maps, books | Geology | Snapshots
Bandar Khayran | Jabal Akhdar | Masirah | Mountains | Musandam | Salalah | Seifa |  Wadi Mistal | Wahibah

A geological laboratory
I have tried to follow the stratigraphic sequence from PreCambrian to Recent, but this is not intended to be a strictly academic work.  More of a picture board story which unfolds.  

Sketch-map of the geology of Oman	Click on the pictures to see larger versions in new windows.  You may also like to open the satellite image of Oman to refer to locations. This map has been taken from the Field Guide to the Geology of Oman by Samir S Hanna published by the Historical Association of Oman in 1995.  It will take approximately 30 seconds to download at 28.8 kpbs.
The track to Sharfat al Alamayn on Al Hajar mountains.  July 1998	When K W Glennie started mapping the northern Oman mountains for PDO (Petroleum Development of Oman) in the early 1970s, there were virtually no roads and he had to complete his mission by helicopter.  Nowadays, access is excellent although this track leading to Sharfat al Alamayn was an exception. Glennie's work remains a standard.  See below in the Bibliography.

Late Proterozoic to Cambrian rocks outcrop in Al Huqf on the eastern side of the region of Al Wusta just south of the turning to Masirah Island, and on Saih Hatat south of Muscat.  They have a largely terrigeneous and volcanic origin typical of continental shelf conditions with a transgression to a marine environment in later times.

Diamictite, Mistal Formation,  Ghubrah Bowl	Volcano-sedimentary rocks outcrop extensively in the centre of the Ghubrah Bowl on Al Jabal al Akhdar.  Small pebbles of both igneous and sedimentary rocks are embedded in a tuffitic or sandstone matrix.  The volcanics are succeeded by pillow lavas and greywackes.  The sequence is thought to represent a series of submarine debris flows dating from the end of the Precambrian era.
Glacial tillite,  Wadi bani Kharus	Recent work suggests that this rock found at the end of Wadi bani Kharus is a glacial tillite dating from approximately 750Ma at the end of the Precambrian.
Salt dome in the  Fuhud oil-field	The complex geology exposed on the mountains continues southwards beneath the desert sands.  Salt domes dating from the Cambrian era punctuate the desert sands south of the northern Oman mountains.  As a result, Oman has relatively small reserves of oil sited in structural traps, some of which are adjacent to the salt domes.  The oil can be difficult to extract and PDO has pioneered methods of horizontal drilling to reach it.
Polygonal structures in ancient evaporites	The relatively low density of the salt dome rocks has resulted in their diapiric rise to the surface.  Gypsum, anhydrite and some rarer evaporite minerals outcrop as well as halite. Polygonal structures on the surface of these ancient rocks are similar to those seen on present day evaporite deposits in hot dry deserts.
Unconformity between Permian and pre-Permian rocks,  Wadi bani Kharus	Stromatolite-bearing limestones were deposited in a shallow sea that existed at the end of the Precambrian era.  These limestones belonging to the Kharus Formation are a source of hydrocarbons found in rocks in the south of Oman.  Here in Wadi bani Kharus in the northern Oman mountains, they dip steeply northwards beneath the overlying Saiq Formation, largely dolomites, of Permian age. The age gap of 300 My represents a period of uplift before marine sedimentation began again in the Permian, continuing for 190 My well into the Cretaceous.
Palaeozoic-Mesozoic boundary, Wadi bani Kharus	The Open University Geological Society toured northern Oman in early 1998 following Hanna's guidebook extensively.  We weren't altogether sure we'd found the Palaeozoic-Mesozoic boundary in Wadi bani Kharus, but it does mark a major geological extinction.  You should find brachiopods and crinoids in the underlying Permian dolomite in contrast to the overlying fossil-poor Triassic mudstones and limestones.
	Here's the OUGS field trip party from 1998.  Wadi bani Kharus.
The Ghubrah Bowl, Wadi Mistal	The sedimentary rocks on Al Jabal al Akhdar have been weathered and exposed to reveal the succession from the Precambrian to the Cretaceous.  The Ghubrah Bowl and the many wadis flowing both north and south from the watershed offer invaluable opportunities to examine the sequences.
View northwards over  Al Jabal al Akhdar  from Sharfat al Alamayn	The region of Al Jabal al Akhdar is an anticline plunging gently westwards.  Its sedimentary and volcanic rocks formed an autochthonous platform over which contemporary and younger deep ocean sediments were thrust along with the overlying mantle ophiolites during the Cretaceous era.
Rocks of Al Hajar supergroup outcrop behind the American-British Academy in Muscat	Palaeozoic sediments of the Al Hajar supergroup form the northern flank of both the Al Jabal al Akhdar and the Seih Hatat anticlines.  They dip steeply to the north. This picture also shows a small wedge of ophiolite on the extreme left hand side.  The contact is marked by a wide crush zone in which the ophiolite has been serpentinized.  Fragments of ophiolite found on top of Al Jabal al Akhdar show that the mantle sequence once covered the sedimentary platform.
Shelf carbonates of the Natih Formation on Al Hajar dipping south from Sharfat al Alamayn	These are some of the upper sedimentary units found on Al Jabal al Akhdar.  Thinly bedded limestones with varying clay content and black shale and oyster layers form the upper unit of the Natih Formation, dating from the Middle Cretaceous.  These deposits were laid down in a partly euxinic (reducing) environment.  The Natih Formation is a widespread oil-bearing horizon in Oman
Shelly fossils in Natih Formation	Massive bioclastic limestones form a lower unit.  The presence of these rudist fossils in rocks over 2,000 m high testify to the degree of uplift.  The fossils are the remains of animals that lived in shallow marine and reef conditions on the continental shelf.

Since I wrote this, the University of Montpelier has produced an excellent virtual field trip through the Oman Ophiolite. Highly recommended.

Thrust zone, Wadi al Kabir leading to Al Bustan, Muscat

The northern Oman mountains form an arc extending for 700 km from Musandam in the north to the east coast at Ras al Hadd.  The mountains define an obduction zone  where the mid-oceanic rocks and deep ocean sediments of the ancient Tethys ocean were thrust upwards and over the continental shelf and slope rocks of the Arabian platform.   Although not clearly evident in this picture, Tertiary limestones overlie ophiolite on the left hand side of the road.  Al Hajar supergroup sediments dip almost vertically on the right hand side of the road.

The ophiolite and deep-ocean sediment cover is called allochthonous, that is, moved from another place. 

The deep-ocean sediments are revealed in the Hawasinah nappe, while the ophiolites comprise the Samail nappe.  The contact between the two nappes is marked by a thrust zone named the Samail thrust.

View from our room at the Sohar Beach hotel.

We travelled northwest along the Batinah coast to the Sohar area to look at the allochthonous rocks in Wadis Jizi and al Hawasinah. Being cheats, we didn't camp but made day excursions from the Sohar Beach hotel, which is to be highly recommended for a quiet break.

Mantle ophiolite in Wadi al Hawasinah

An ophiolite sequence is an uplifted section through rocks which form oceanic crust.  Ancient ophiolites provide very good evidence for what is happening at tectonic plate margins.  The Oman ophiolite is the largest and most studied in the world. Geologists disagree as to whether the Oman ophiolite was formed at a mid-ocean ridge or in a supra-subduction zone setting.  All authors agree that it formed at an ocean spreading centre in the ancient Tethys ocean. See Cox et al (1999) for discussion and references.

Standing on dunite looking north towards the recharge dam in Wadi al Jizi.	At the top of the igneous sequence are pillow lavas, which are succeeded by sheeted dolerite dykes, layered and massive gabbro, and finally, layered and massive mantle ophiolites very rich in iron and magnesium minerals such as pyroxenes and spinels.  Mantle ophiolites in Oman are generally classified as tectonized harzburgites.  Chromite occurs in layers and pods within the mantle rocks and has been mined near Nakhl.
Dave Rothery	Interestingly, analysis of satellite imagery reveals many mineralogical varieties of mantle ophiolite which would otherwise be almost impossible to map on the ground. Dave Rothery of the Open University, who led the 1998 OUGS field trip, is a specialist on this subject.
The Geo-Times pillow lavas in Wadi al Jizi	Nowhere else in the world can you see such a classic exposure of pillow lavas as on the road to Al Buraymi along Wadi al Jizi.  30 km on the southern side of the road.  Len Hutton is the scale here.  Can you find him?
Sheeted dykes  Wadi al Jizi	Just two km back along the road towards the east and on the northern side of the road, you will see this magnificent exposure of sheeted dykes. The ophiolites have been faulted since emplacement, so contacts are often structural rather than progressive.
Hydrothermal chimney in pillow lavas, Wadi al Jizi	George standing in a hydrothermal chimney, pillow lavas in Wadi al Jizi, 30 km from junction of coast road with road to Al Buraymi.  East side of road. Late magmatic fluids have altered the rock (chlorite metamorphism) and deposited copper minerals which can be seen as blue and green oxides in hand specimen.  There has been copper mining in the area, but generally, it isn’t economically viable now.

Go to the American Museum of Natural History Expeditions web site for more information about black smokers.

Strongly tilted radiolarian cherts, Wadi al Jizi	Radiolarian cherts, tilted strongly to the north in Wadi al Jizi,  were once deep ocean sediments which formed from deposition of the siliceous skeletons of the radiolaria plankton in the ocean waters.  Such sediments flank ocean ridges.
Metamorphic sole, 30 km from main road, Wadi al Hawasinah	Len Hutton at the metamorphic sole where the Samail Thrust has carried the ophiolites over the Hawasinah deep sea sediments and volcanics.  A very crumpled and altered  rock. We found one outcrop where a volcanic member had been metamorphosed to amphibolite (relatively high-temperature, high pressure metamorphism) but didn’t find any garnet which is a marker mineral for metamorphic grade
Overturned sediments in the Hawasinah group lie beneath alluvium, 40 km from main road, Wadi al Hawasinah	The Hawasinah basin stretched from the mid-ocean ridge of Tethys to the continental slope of the Arabian platform.  The deep ocean sediments shown here formed near the slope since they are largely turbidites with some volcanics and shales. The Hawasinah sediments were also thrust over the Arabian platform and lie structurally beneath the ophiolites, hence the high degree of folding and the evidence for overturning deduced from examination of the graded bedding.
White Exotic limestones, Wadi al Jizi.	The Oman exotics are white limestones which do not extend laterally.  Isolated outcrops are underlain by volcanic rocks. They are calcirudites, meaning that they’re made up of limestone rubble which accumulated on top of sea mounts in the ocean.  Sea mounts (extinct underwater volcanoes) underlie coral atolls in the Pacific today.
Sea arch near Bandar Jissah, Muscat.   Tertiary limestones.	Following the geological turbulence of the closing of the Tethys Ocean and the uplift of the Eo-Alpine chain in the early Tertiary, much of the land area again became submerged in shallow seas in which fossiliferous limestones were deposited.
Close-up of sea arch near Bandar Jissah	The Tertiary limestones lie sub-horizontally or are only gently tilted.  They outcrop widely round the coast from Bimah to Sur and beyond, and in the southern Dhofar mountains behind the Salalah plain dipping gently northward beneath the Najd desert.

Recent landscapes

Al Batinah plain looking to the foothills of Al Hajar	South and north of the northern Oman mountains, the plains are typically covered with a coarse grey gravel.  This alluvium, as it is collectively known, is generally very poorly sorted with sizes ranging from boulders to silt.
A blue pool on the southern side of Al Hajar al Sharqi, approx. 25 km east of Bid Bid	Blue pools are common on both sides of the northern Oman mountains where alkaline water flowing through and emerging from the fractured ophiolites reacts with the carbon dioxide in the atmosphere.  Calcium carbonate is precipitated forming fine white deposits at the bottom of the pools.   This white layer scatters the sunlight to reflect back blue light, in the same way that water vapour in the atmosphere causes the sky to appear blue.
The Intercon beach from the Crowne Plaza hotel, Muscat	Much of Oman's 1700 km coastline is defined by fine beaches such as this one which stretches from Muscat to Sohar and beyond.
Sand dune behind Al Khuwair 33, Muscat	Sand dunes stretch behind parts of the coast.  Housing is encroaching, requiring strong foundations. For information about the Ramlat al Wahybah sand sea, go to Wahibah Sands.
Desert landscape and sabkha	And of course, most of the country is covered by desert sand with extensive sabkha or salt flats.   The largest is called Umm as Samim (Mother of Poisons) and lies on the edge of The Empty Quarter close to the border with Saudi Arabia.
Outwash plain,  Wadi Andam, 100 km south of Al Mudaybi	Relics of wetter climes remain.  This is the outwash plain of Wadi Andam which flows south from the northern Oman mountains into the Interior. It's doubtful that water ever flows on the surface now, or extremely rarely.  Vegetation is probably supported by groundwater in the wadi aquifer.

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Bibliography

Abrams, M J, Rothery, D.A. and Pontual, A.  1988  Mapping in the Oman ophiolite using enhanced Landsat Thematic Mapper images, Tectonophysics, 151, 387-401
Bechennec, F et al  1993  Geological Map of the Sultanate of Oman, 1:1,000,000, with explanatory notes, Bureau de Recherches Geologiques et Minieres, France
Glennie, K W  1974  Geology of the Oman Mountains, Verhandelingen van het Koninklijk Nederlands geologisch mijnbouwkundig Genootschap, 31
Cox, J, Searle, M and Pedersen, R  1999   The petrogenic origin of leucogranitic dykes intruding the northern Semail ophiolite, United Arab Emirates: field relationships, geochemistry and Sr/Nd isotope systematics, Contributions to  Mineralogy and  Petrology 137, 267-287
Hanna, S S  1995  Field Guide to the Geology of Oman, Historical Association of Oman
Lippard, S J, Shelton, A W and Gass, I G   1986  The Ophiolite of Northern Oman, memoir no 11, published for The Geological Society by Blackwell Scientific Publications,  ISBN 0-632-01587-X
Robertson, A H F, Searle, M P and Ries, A C (eds)  1990  The Geology and Tectonics of the Oman Region, special publication no 49, The Geological Society,  
ISBN 0-903317-46-X
Searle, M, Waters, D, Martin, H and Rex, D   1994   Structure and metamorphism of blueschist-eclogite facies rocks from the northeastern Oman mountains,  Journal of the Geological Society of London, 151, 555-576

I do refer to the local geology on other pages on this site .

Sultan Qaboos University hosted an international conference on the geology of Oman from 12th to 16th January, 2001.

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Please note, all photographs on this website are copyright of Sue Hutton, unless otherwise stated.