Oman 2019: Ocean Crust and Mountains of Mantle

Our GeoWorld Travel group at the "Mother of All Outcrops" - beds of red radiolarian cherts and white porcellanite

In February 2019 we completed our first ever geological tour of Oman. The tour was led by GeoWorld Travel’s Director, James Cresswell, and supported by local company, Golden Highlands. The 9-day tour was a great success and visited geological highlights in the north of the country.

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

Day 1

Today was our arrival day. We had seven participants on the tour: six from the UK and one from the USA. We all met at a hotel in Muscat and ran through introductions and an overview of the tour at a group welcome meeting.

Day 2

Our first full day of touring started with at Wadi al Abyad, the best place in the world to see the Moho – the boundary between the earth’s crust and mantle. The Moho is visible here because a spreading ridge plate boundary (a place where the Moho is near the surface) was obducted onto the Arabian continent as part of the Semail Ophiolite. The Semail Ophiolite is the best exposed ophiolite anywhere in the world. We then moved on to Wadi Haslan where we had a sumptuous picnic lunch and examined Snowball Earth deposits. 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 Snowball Earth deposits, showing when the climate very rapidly warmed at the end of Snowball Earth. Our final site of the day was an unconformity between Precambrian rocks and Permian rocks. The Precambrian rocks contained stromatolites while the Permian rocks showed a marine transgression from a beach to limestone.

Day 3

Today was all about seeing rocks of the mid-ocean spreading ridge that have now been obducted on the surface as part of the Semail Ophiolite. We started the day by visiting a fossilised Black Smoker that was once on the ocean floor at a mid-ocean ridge plate boundary. At the smoker we viewed copper mineralisation and interesting umber sediments which are rich in metals. At our next stop, we saw the sheeted dyke complex, hexagonal cooling structures and lava feeder tubes. These formed in the subsurface layers of the ophiolite, as magma fed up to the surface.

Next we visited one of Oman’s most famous geosites, the incredible “Geotimes” Pillow Basalts at Wadi al Jizzi. These are considered to be the best pillow basalt outcrops in the world. These basalts were erupted out on to the seabed, and formed pillow structures as they were cooled by the sea water.

We finished the day with a visit to a copper mine that had been mined for most of the last 4,000 years and saw what may be the only ziggurat in Arabia, built by early copper miners from Mesopotamia.

Day 4

Day 4 of the tour focused also focused on the Semail Ophiolite. In the morning, we visited a fossil white smoker – distinguished from the black smoker (seen yesterday) by the large amount of opal found at the site. We then moved on to the site of a 1000-year-old copper mine that displayed pillow basalts, dykes and columnar cooling joints. We then crossed off the Semail Ophiolite nappe and onto the Hawasina nappe. The Hawasina nappe is made up of oceanic sediments that were also thrust up and obducted as part of the Semail Ophiolite. Where the hot Semail nappe thrust over the Hawasina nappe, an ‘ironing’ effect occurred, metamorphosing the rocks along the thrust fault; this is called the metamorphic sole. We stopped to examine rocks in the metamorphic sole and then moved into the spectacularly folded Hawasina sediments. These sediments were laid down in deep-water conditions but were still partially made of turbidite flows. The highlight of the sediments were the amazing mullions. Mullions form as sediment layers are pulled apart by tension-forming boudins, and are then later squeezed back together, forming mullions.

Day 5

Today we were joined by two geologists from the German University of Technology in Muscat, including Gösta Hoffmann who wrote the excellent book ‘A Field Guide to the Geology of Northeastern Oman’. Our first stop was the Al Ayn beehive tombs World Heritage Site. Here we not only observed the early Bronze Age tombs, but also the Hawasina sediments on which they stood. Our next stop was a viewpoint of Jebel Misht – an ‘Oman exotic’. The whole mountain represents 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 viewed a large chevron fold in the Hawasina sediments, before climbing to one of the highest points in Oman to view the ‘Grand Canyon of Arabia’. Here the entire Cretaceous and Jurassic autochthonous sequences could be seen. After a picnic lunch we observed some rudist fossils before visiting the abandoned old town of Al Hamra. Our final stop of the day was another archaeological one at ‘Coleman’s Rock’, to observe important ancient rock engravings.

Day 6

Our tour today started with a visit to the Bahla Fort World Heritage Site, which sits on ophiolitic rocks. We then took a train ride into Al Hoota cave, which is 5km long (photography was not allowed inside the cave). We then visited Falaj Daris World Heritage Site in Nizwa to see the ancient irrigation system. After a traditional Omani lunch in Nizwa, we drove to a site where plagiogranite could be seen. This forms by partial melting of the gabbro within the ophiolite. We then had a distant view of the Moho on a mountainside, with the bulk of the mountain made of peridotite but with the top capped by gabbro. Finally, we reached the Wahiba Sands where we drove into the dunes for a desert experience.

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Driving in the Wahiba Sands

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Day 7

Today was the penultimate day of our tour. Our first stop was to see rocks of the Precambrian crystalline basement (800-1,000 million years old). At this site, granites were intruded by basalts which were themselves cut by pegmatite veins. These were the oldest rocks seen on our trip. Our next stop was to see a rare volcanic rock called kimberlite, which brings rock up from very deep in the mantle and sometimes contains diamonds (although not in this case). We then visited the ‘mother of all outcrops’, shown in the pictures below. Here, red radiolarian chert is interbedded with white porcellanite. These sediments were deposited in water 4,000-4,500 metres deep. They are folded and faulted and were thrust onto Arabia as the Batain Nappe 15-20ma after the emplacement of the Semail Ophiolite. We then had our picnic lunch before heading to a Manganese Pit which is located in the same sediments. Here we were able to observe manganese ore. Our final stop was Ras al Hadd where there is a Bronze Age settlement and a turtle nesting beach.

Day 8

Our final touring day started with a chance to inspect an unusual lava called carbonatite, which contains veins of amethyst. We then stopped at a notch cut into rock by molluscs when the sea level was higher in the last interglacial. This stop gave us warning about what sea levels may rise to under the current trend of global warming. Our next stop was a Miocene wave-cut platform which was overlain by Quaternary corals. We then took a boat across a river and headed into Wadi Shab, which is considered by many to be Oman’s most beautiful Wadi. After a picnic lunch at the Wadi, we visited a fascinating site with a tsunami deposit. Here huge boulders could be seen on top of a cliff. Some of the boulders were even stacked up (or ‘imbricated’), showing they had been washed into place by a giant wave! This was followed by a visit to the Bimmah Sinkhole, 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, and of the tour, was the the world’s largest sheath fold, which contains many smaller isoclinal folds. After this, we returned to Muscat for the final night, before people departed or continued touring the region the next day.

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