Oman 2021: Ocean Crust and Mountains of Mantle

In December 2021, we ran our geological tour of Oman, which had been due to run in March 2021. Despite the ongoing pandemic and frequently changing travel requirements, our group were able to travel to Arabia to enjoy a full 9-day itinerary which took in some of the many geological marvels of Oman.

GeoWorld Travel’s ‘Oman: Ocean Crust and Mountains of Mantle’ route map

Day one: Arrival in Muscat

Tour participants arrived in Muscat throughout the day, with (almost!) the whole group gathering together for a first night meal with a chance to discuss the coming days touring.

Day two: Muscat to Sohar

Our first full day of touring started at Wadi al Abyad (EP58), the best place in the world to see the Moho – the boundary between the earth’s crust and mantle. Here we saw a sharp contrast between the mantle rocks, consisting of a type of peridotite called hartzburgite, and the ocean crust rocks made of layered gabbro. So how is it possible to see such rocks at the surface of the earth and on land? This because in the late Cretaceous these rocks, which form part of the Semail Ophiolite, were obducted onto Arabia as an ocean basin was closed. The Semail Ophiolite is the best exposed ophiolite anywhere in the world. We then moved on to Wadi Haslan where we examined Snowball Earth deposits (EP62). These deposits formed at a time when it is thought that the whole Earth was glaciated. The deposits we saw were 712ma tillites and glacial lake deposits with dropstones (rocks that fall out of icebergs) immediately overlain by tropical dolomites. These tropical dolomites are known as the cap carbonates and can be found all over the world on top of Snowball Earth deposits, showing that the climate warmed very rapidly at the end of the Snowball Earth glacial episode. Finally we saw an angular unconformity between Ediacaran-aged and late Permian rocks (EP61). The Precambrian rocks contained stromatolites while the Permian rocks showed a marine transgression from a beach to limestone. We then drove to the city of Sohar to spend two nights in the 5-star Crowne Plaza Hotel.

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Day three – Sohar to Sohar

Today we focused on the rocks of the upper part of the Semail Ophiolite. Our first stop was was to look at the Sheeted Dyke Complex (EP87). At the this site the outcrop was 100% dykes and these dykes formed as basalt melt left the gabbro magma chamber beneath and erupted out at the mid ocean spreading ridge to form pillow basalts; this is characteristic of the classic ophiolite sequence. Our next stop was a site where we saw basalt that seemed to be a hybrid of pillows basalts and hexagonal cooling structures (EP86). We then moved on to a fossilised Black Smoker (EP91) which was once on the ocean floor at a mid-ocean spreading ridge. The smoker could be seen from several miles away as a prominent red hill; the red colour is due to haematite. The structure was virtually all intact, and you could imagine diving down in a submarine to see it. We saw lots of malachite, azure copper mineralisation and epidote too. It was possible to see tube-like structures where the hydrothermal acidic fluids would have forced their way through the rock. The site may have been lightly mined over the last 4000 years and the chimneys of the smoker were no longer present. Recent mining surveys have shown that a copper ore body 20m x 250m x 100m lies directly under the structure. We then had our picnic lunch at a site where we could see metalliferous umber sediments (EP92). These sediments formed around the black smoker and are rich in metal sulphides that precipitated out from the smoker. We then relocated to one of Oman’s most famous geosites, the incredible “Geotimes” Pillow Basalts at Wadi al Jizzi (EP89). These are considered to be the best pillow basalt outcrops in the world. These basalts were erupted on to the seabed, and formed pillow structures as they were cooled by the sea water. Examining the pillows we were able to see their glassy outer skin, and two dykes that cut through them to feed younger basalts. We finished the day with a visit to a copper mine (EP90) that had been mined for most of the last 4,000 years only stopping a few decades ago, leaving a great hole in the ground. This would once have been a black smoker like the one we saw earlier in the day. A few hundred metres from the mine is what may be the only ziggurat in Arabia. A ziggurat is a stepped pyramid, and it would have been built by early copper miners who travelled to Oman from Mesopotamia. We then returned to the Crowne Plaza Hotel for a second night.

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Day four: Sohar to Nizwa

Our first stop of the day was a fossilised white smoker (EP93). This differed from the black smoker we saw yesterday because it was created by cooler fluids (150°C, as opposed to 300°C in the black smoker), and thus contains much less metal mineralisation, but contains lots of opaline silicate. It was possible to see hydrothermally altered pillow basalts and former feeder tubes that were completely scaled up with silicates. We then moved on to a site where we could see pillow basalts, feeder dykes and sills with columnar cooling structures all in the same cliff face (EP94). It was amazing to see dykes simply coming to an end as they reached the then surface and emitting the pillow lava. This site was also the site of copper smelting 1000 years ago.
Next we stopped at the hospital in Wadi Hawasina. Here there is a fantastic view – mantle peridotites of the Semail ophiolite nappe behind us, and in front of us heavily folded marine sediments of the Hawasina nappe. The Hawasina nappe had first thrust over the Arabian continent and the Semail ophiolite nappe then thrust over both of them. The boundary between the two nappes is now thought to have been a former subduction zone, with the Semail ophiolite forming at the spreading ridge of a back arc basin. A few hundred metres from the hospital we stopped in Wadi ad Dil (EP96). Here we saw granulite facies rocks known as the “metamorphic sole”. These rocks would have formed in the subduction zone. Furthermore, at this contact was a hill made of marble. This would once have been an atoll which got jammed into the subduction zone! Also at the site we saw radiolarian cherts that would have formed in a deep ocean basin. Soon after this stop we examined some beautiful chevron folds in the Hawasina sediments, that were deposited by turbidites. We then had our picnic lunch near a multi-coloured outcrop of Hawasina sediments in a road cut (EP99), before taking a spectacular mountain road through Wadi Murri (EP83) where we were able to see Cretaceous rocks of the Arabian continent plunging beneath both the Hawasina and Semail ophiolite nappes. Our final stop of the day was the Bronze aged Aly Ayn beehive tombs UNESCO World Hertiatge Site (EP82). These tombs sit on turbidite sediments of the Hawasaina nappe, and have a spectacular back drop: Jebel Misht (EP81), a mountain made up of Triassic-aged limestone and dolomite sitting on top of volcanic rock. It is known as an ‘Oman exotic’ because it was actually a submarine volcano with an atoll on top of it, which was scraped on to the continent as part of the Hawasina nappe! We then travelled to the city of Nizwa for two nights in the Nizwa Heritage Inn which is situated right in the walled old town near to the famous Nizwa Fort and souk.

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Day five – Nizwa to Nizwa

Our first stop today was the Al Hoota Cave in the Hajar Mountains. The cave is the largest show cave in Arabia and sits in mid-Cretaceous (Albian) aged limestones and dolomites. The cave has been formed by water flowing through tectonic fractures in the rock, and contains beautiful stalactites and stalagmites. Our next stop was a view over the ruined town of Ghul (EP75), which lies on Cretaceous limestone. The next site was a viewpoint of Jebel Shams, Oman’s highest peak (3,028m) and of Wadi al Nakhr , which is also known as the “Grand Canyon of Arabia” (EP76). The cliffs of the canyon are 1,500m high in places and from our vantage point we were able to view the entire autochthonous Cretaceous and Jurassic sequence in the walls of the canyon. After a picnic lunch near Jebel Shams, we visited Hasat Bin Salt also known as “Coleman’s Rock” (EP69). This site with several 4000-year-old carved human figures is thought to be the most important rock art monument in south-eastern Arabia. The final stop of the day was another UNESCO World Heritage Site, “The Aflaj Irrigation System of Oman” (EP70). This irrigation system is up to 4,500 years old and is seen as one of the driving forces behind the formation of Oman as a nation as it provided the reason for nomadic societies to settle down. We then spent a second night in the Nizwa Heritage Inn.

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Day six: Nizwa to Wahiba Sands

The day began with some free time in the city of Nizwa allowing people to visit its famous fort and souk. Our first geological stop was to see an outcrop of plagiogranite (EP36). This rock which is also known as tonalite or trondhjemite formed at the transition zone of the gabbro magma chamber and sheeted dyke complex by extreme fractional crystallisation of the magma chamber melt. Our next stop was a view the Moho (EP33); here the bottom three quarters of a mountainside was a different colour to the top quarter. This is because the bottom portion of the mountain was composed of peridotite rocks that formed in the mantle while the top of the mountain was composed of gabbro which formed in the crust. Our next stop was near to this site and was a chromite pit (EP34). Here we were able to find chromite and serpentinized minerals. This was followed by a late lunch at a nearby stop where we able to examine an outcrop of white reef limestone (EP35). This limestone is an ‘Oman Exotic’; it is late Permian to Triassic in age and was scraped onto Arabia as part of an atoll or submarine volcano. The outcrop has additionally been weathered so it resembles a skull. We then had an exhilarating high-speed drive into the Wahiba Sand dunes (EP32) to reach a desert camp where we spent the night. Before sunset we drove to the top of a nearby dune and watched the sun go down. Here we also discussed dune formation. Our hosts at the camp then provided us with a lesson in traditional Bedouin Dance, dinner and a talk on Bedouin life and culture by a campfire.

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Day seven: Wahiba Sands to Sur

We awoke in our desert camp in the Wahiba Sands, and had to first navigate the dunes to reach a road again. The first stop was to see rocks of the Precambrian crystalline basement (800-1,000 million years old) (EP31). Here we saw granites that formed as different terranes were accreted together, these were then cut by huge gabbro dykes, which were intruded as the basement went through a later period of extension. All of these rocks were themselves intruded by granitic pegmatite (containing large crystals) dykes. Our next stop was to see an ultramafic rock outcrop on the beach (EP30), which contained volcanic fragments (lapilli) that formed due to the ascent of magma. Some authors have called the rock a kimberlite, which is the deepest mantle derived volcanic rock of all and sometimes contains diamonds, although this outcrop contains no diamonds. Others authors argue this outcrop is actually a carbonatite which is also derived from deep in the mantle but less deep than kimberlite.
After our picnic lunch, we then visited one of Oman’s most famous outcrops, informally known as the ‘mother of all outcrops’ (EP24). Here red radiolarian chert is interbedded with white porcellanite. These sediments were deposited in water 4,000-4,500 metres deep, which is too deep for limestones to form, but rocks can form from radiolaria plankton because their skeletons are made of silica. Our group discussed at length why the outcrop consisted of alternating white and red layers – was it a secondary diagenetic process or was it directly due to cycles of sedimentation? However the sediments were deposited, they were later folded and faulted as they were thrust onto Arabia as part of the Batain Nappe (along with Oman’s other ophiolite, the Masirah Ophiolite), 15-20 ma after the emplacement of the main Semail Ophiolite. At our next stop, we saw a manganese pit (EP25), where pyrolusite (MnO2) was being mined from manganese-enriched layers within radiolarian cherts. The next stop of the day was an outcrop of carbonite volcanic rock (EP23). The rock appeared to be a tuff (volcanic ash) rather than lava, at the base of the outcrop and a dyke at the top of the outcrop. The rock also contained beautiful veins of amethyst. The final stop of the day (EP22) was a notch cut by molluscs when the sea level was 3.7m higher during the Eemian interglacial 127,000 – 106,000 years ago, when global temperatures were several degrees warmer than today (and, for example, hippos lived in the River Thames in the UK). We have already passed the CO2 atmospheric concentration of back then, so this record of sea level change is a stark warning of things to come!

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Day eight: Sur to Muscat

Today was the penultimate day of our tour. The first stop was a series of Miocene wave-cut platform terraces (EP19) which are overlain by Quaternary fossil corals. The terraces have formed due to tectonic uplift. Next we arrived at Wadi Shab (EP17), a magnificent canyon cut into Eocene limestone, considered by many to be Oman’s most beautiful Wadi. We spent three hours hiking and swimming in the wadi and enjoying the wonderful scenery, before having our picnic lunch here. We then visited a fascinating site with a tsunami deposit (EP16). Here huge boulders could be seen on top of a cliff. Some of the boulders were even stacked up, or ‘imbricated’, showing that they had been washed into place by a giant wave! This was followed by a visit to the Bimmah Sinkhole (EP10), which is actually a doline. The surface rock is not soluble but the layer beneath is and, when a sinkhole formed in it, the surface rock crashed through leaving a doline. Our final stop of the day, was at Wadi al Mayh (EP06) to see the world’s largest sheath fold, which also contains many smaller isoclinal folds. Sheath folds are associated with shear zones, in this case the the plate boundary along which the Semail ophiolite obducted. Normally they are only centimetre to metre in scale but this fold hinges along a 15 km long profile. Sheath folds are distinctive curvilinear folds in which the hinge actually wraps around on itself. In three-dimensions, sheath folds look much like their name implies, a sheath that might holster a sword. When eroded, the tubular-shape of a sheath fold displays a characteristic eye-shape in cross section, and that is what can be seen on the slopes above Wadi Mayh. We then arrived back in Oman’s capital Muscat to spend two nights at the Ramada Hotel.

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Day nine: Muscat to Muscat
Today was the last day of our geology of Oman tour and we visited sites near to the capital city Muscat. The first stop was in Muscat Harbour (EP01) here we climbed a steep trail to view ultramafic rocks of the Semail ophiolitie, including bands of harzburgites and lighter coloured dunites. We also saw holes in the peridotites were pods of olivine had eroded away as well as veins of magnesite formed from the alteration of serpentinised olivine. This hike also gave us wonderful views over Muttrah Harbour. Our next stop was a view of drowned valleys (EP03) showing that while in some places the Oman mountains are uplifting, in other places they are subsiding. Our next stop was a new road cutting where Permian-aged metasediments metamorphosed to blueschist facies (low temperature/high pressure) were seen. These sediments became metamorphosed as the Arabian continent subducted beneath the Semail Ophioliote. We then reached As Sifah (EP05); here there was a beautiful beach and an optional hike over difficult terrain to reach an outcrop of eclogite. The eclogite is the deepest derived rock in Oman and formed in a suduction zone. The eclogite was rich in garnets and the green mineral omphacite. The final stop of the day, and of the tour, was at a viewpoint over the Wadi al Kabir area of Muscat (EP02). Here we saw the city occupying a valley. The rocks on the far side of the valley were peridotites of the Semail Ophiolite, while the rocks we were standing on were Permain sediments of the Arabia continent. The Semail Ophiolite had been thrust right over the area we were standing on and subsequently eroded away, with the modern city then built on the fault line. Also at this site it was possible to view an impressive unconformity where Eocene marl overlies the peridotite of the Semail Ophiolite. We then returned to our hotel for a second and final night in Muscat (although some participants chose to extend their stay and take a city tour the following day).

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Day ten: Departure or optional city tour
Our tour ended after breakfast today. Two of the UK-based participants flew out in the early hours of the morning while the rest choose to stay on a join a city tour of Muscat.

A wonderful time was had by all in Oman – it really is an absolute paradise for geologists. Our thanks to Mansoor and Salim of Golden Highlands for logistical assistance, thanks to Gösta and Valeska (University of Bonn) for their outstanding knowledge of Omani geology and, of course, thank you to the GeoWorld Travel group for great company, lots of laughs and lively geological discussions!

We will be running this tour again in March 2022 – see our website for details: https://www.geoworldtravel.com/Oman.php

We would also like to thank Feedspot for including us in their Top 200 Travel blogs list and their Top 30 Geology blogs list

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