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Oman
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Bandar Khayran | Jabal
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| 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
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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.
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The track to Sharfat al Alamayn on Al Hajar
mountains. July 1998
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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.
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The oldest rocks in Oman outcrop
most extensively in Dhofar, east of Marbat and on the Hallaniyat
islands. They are crystalline basement rocks of the granite clan,
gneisses and mica schists. Granodiorite also outcrops in a small
area near Jabal Ja'alan at the southern edge of the Wahibah Sands.
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.
Largely sediments
Diamictite, Mistal Formation,
Ghubrah Bowl
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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
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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.
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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. |
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Here's the OUGS field trip party from 1998. Wadi
bani Kharus. |
The Ghubrah Bowl, Wadi Mistal
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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. |
Mostly ophiolites
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.
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Ophiolites of the Tethyan suture zone stretch through
Iran, Turkey, Cyprus and southern Europe.
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.
|
The eclogite facies metamorphism of the Palaozoic rocks at Seifa
and radiometric dates of 96 + 2 Ma suggests
that the Arabian Platform was being subducted beneath the Oman ophiolite at
the same time that it and the metamorphic sole were being formed (Searle
et al, 1994).
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.
|
Similar
“chimneys”, conduits which transport mineral and sulphide rich
fluids, are found at ocean ridges today – known as “black
smokers”.
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.
To
top If you're interested in contacting
companies who can arrange specialist trips, try my list
of tour operators.
Please do use the amazon.com search facility to look for books about geology and
Oman.
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Please note, all photographs on this website are
copyright of Sue Hutton, unless otherwise stated.
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