Rocks of Oman
Ophiolites around Muttrah
Shelf limestones at Jebel Shams
Tertiary limestones at Barr al Jissah
Fossils within limestone formed in a shelf carbonate platform - shallow and warm sea water environment.
Did you ever imagine that the mountain tops of Jebel Shams and Jebel Akhdar were once under the sea? Danilo Cifola, a geologist from the sultanate, breaks down the geological history of Oman to give an understanding of its fascinating formation.
The geological history of Oman reflects the geology of the Tethys ocean which started to open 270 Ma (Ma = million years ago) and continued to open for 190 My (My = million years period). The Gulf of Oman represents the leftovers of this ocean. “Oman's geology has been studied since many decades. As stated by many authors, the rocks of Oman are strictly linked with the geological evolution of the Tethys ocean,” says Cifola.
Before the opening of the Tethys (first phase), Oman was part of a supercontinent called Gondwana, made up of a sequence of rocks older than 700 My. At the base of this sequence can be found dark grey to pink igneous rocks (result of the solidification of a molten magma) and reddish-greyish metamorphic rocks (formed when a rock is subjected to high temperatures and/or high pressures that modify the original rock creating a new one).
These igneous and metamorphic rocks form the 'Cristalline Basement. Above the Cristalline basement is the so-called 'Sedimentary Basement' made up of green and black sedimentary rocks (mostly limestones of shallow sea). “All of them were highly deformed and lifted above sea level for at least 300 My,” says Cifola.
The igneous and metamorphic rocks belonging to the Cristalline Basement are the oldest rocks in Oman. They do not contain fossils and crop out extensively at the Zalawt Plain at the base of Jebel Samhan and at Mirbat near Salalah. The metamorphic rocks are mainly represented by gneisses aged 770-600 Ma.
At about 270 Ma, after having experienced two Ice Ages (a first one at 600 Ma and a second one at 300Ma), Gondwana began to fragment into separate continents. With this fragmentation starts a new phase of the geological history of Oman’s rocks. “This phase is related to the opening of the Tethys ocean with the associated creation of new crust (called 'oceanic' because it is related to the formation and enlargement of a new ocean) and the deposition of almost 3km of shallow marine limestones on the landside (above the so-called 'continental' crust). As the continents drifted apart, new material started coming from the mantle (originally located at about 8 - 10km depth at temperatures ranging from 500 to 1200°C), raised up and provided material for the new oceanic crust,” says Cifola.
During this stage (where new oceanic crust is created) the Arabic peninsula (i.e the land side) was hosting a carbonate platform (land flooded by shallow - less the 100m - warm sea, and where fossil-rich limestones could form). An example of these fossils typical of a shallow warm sea are the Rudists. They can be easily observed in the Sallut area in Jebel Akdhar.
“A good example of the oceanic crust is the chocolate-brown rugged hills around the Muttrah area of Muscat and in many places within the Hajar Mountains. The mentioned mountains are made by a typical sequence of rocks. The oceanic crust is actually a sequence of rock that comprises pieces of mantle rocks at the base (called 'Harzburgite') and it is followed by different rocks (from Gabbro to Pillow lavas) originated by a magma that has solidified at different depth and at different rate. This sequence of rocks is known as 'ophiolite complex'. It starts with the Harzburgite at the base and close up with the pillow lavas at the top,” he says.
The chocolate coloured rocks in Muttrah contain many lenses or strata of a material that is white in colour. Cifola says that this whitish material is a mineral called Magnesite, very common in the ophiolites. The magnesite form when the ophiolite (mainly Harzburgite, very rich in magnesium) is exposed to weathering (carbon dioxide rich water). The magnesite tends to fill the fractures, and then becomes visible on the rocks' surface. Magnesite is widely used by gymnasts and mountain climbers for its anti-transpirant properties.
About 90 Ma (third phase), the Atlantic Ocean start to open in the west. The opening of the Atlantic ocean activated the convergence of the African plate with the Asian landmass. This fact caused an inversion of the movement. If before the Tethys ocean was enlarging, now with the opening of the Atlantic, it started to close and the oceanic crust started to be pushed over the land side. The closure of the Tethys has been partial because the Gulf of Oman that we have today represents the witness of ancient ocean. During this phase, the Hajar Mountains start to form and the process is still active - they are still rising by 1cm per year on an average, says Cifola.
At the beginning of the Tertiary (almost 65 Ma) and hence after the creation of the Hajar mountains by closure of the Tethys, is the last phase that again is characterised by a shallow and warm sea where limestones could form. These Tertiary limestones are mostly yellow in colour and contain fossils typical of shallow and warm sea environment, like snails, bivalves (shells), corals and many more.
“These Tertiary limestones can be observed in Wadi Bani Khalid, Rusayl Fanjah, Al-Khod, around Bandar Jissah and Wadi Al Kabir,” says Cifola.
Cifola explains the presence of massive solitary sharp pieces of boulders in areas like Fizayah Beach in Dhofar, as perhaps the result of some major earthquake that could have taken place hundreds of years ago, and that could have caused the disengaging of rocky masses already bound by fractures. “Those large masses of boulders most likely broke off and rolled down from the tops of mountains in the vicinity.”. Cifola says.
Lastly, Cifola talks about the geological meaning of Jebel Misht. He says that, as stated by many authors, Jebel Misht is a mountain made by a huge block of limestone. “It is a huge block of limestone that rolled down from the seamount (where it formed) into the surrounding ophiolite during the first stages of the closure of the Tethys ocean. Jebel Kawr and Jebel Misfa have the same geological meaning of Jebel Misht. When these huge blocks of limestone enter into contact with the hot ophiolite, they can get partially 'cooked' and gives rise to marble. In geological terms, these isolated white blocks within the Hajar mountains are called 'exotics' because they are found far from the place where they originally formed.”