Oman 2023: Ocean Crust & Mountains of Mantle

January 2023 saw GeoWorld Travel return to Oman for the fifth time to run our tour of this wonderful country’s geological highlights. We welcomed a truly global group of participants, with guests from the USA, Canada, Brazil, the UK, Australia, The Netherlands and Italy! The tour started our 2023 programme – and what a way to start! Oman, yet again, did not fail to wow our guests with its variety of exceptional geological sites – from the stunning pillow basalts to the fossilised smokers, the world-class ophiolite and the jaw-dropping ‘Mother of All Outcrops’.

Route map of GeoWorld Travel’s ‘Ocean Crust & Mountains of Mantle’ tour

If you would like to visit Oman with us, the next tour dates and prices will be published here:

Previous trips have been described in more detail in earlier blogs (Oman 2022, Oman 2021, Oman 2020 and Oman 2019), so this time around, we have selected our favourite 40 photos of the trip (click on the images to enlarge) to give you a flavour of the sites we visited and the things that we saw…

Above, from L-R: The GeoWorld Travel group at the Moho in Wadi Al Abyad. At this viewpoint, we observed that the rocks at the base of the river were a different colour to the rocks higher up. There was a clear line that was offset by various faults. This line was the MOHO. The MOHO, short for “Mohorovičić discontinuity” is the boundary between rocks of the crust and rocks of the mantle. At the base of the cliff, we looked at peridotites that were once in the mantle. Above it were layered rocks called “gabbro” which are part of the Earth’s oceanic crust.

A 712 ma dropstone that had fallen from glacial ice through water into soft sediments beneath, during a period when the whole planet was glaciated in an episode known as Snowball Earth.

Tropical dolomite capping 712ma “Snowball Earth” tillites. This shows an incredible story when the Earth went from ice-house conditions to green-house conditions in a virtual geological instant.

Above, from L-R: An angular unconformity with tilted Ediacaran aged rocks beneath Permian rocks.

Our group standing on the same an angular unconformity shown in the previous photo. The Ediacaran aged rocks contain fossil stromatolites, Steve is touching these in the foreground. The Permian deposits are beach deposits at their base changing up the cliff to continental shelf deposits. These Permian deposits are being examined by Siem and Dave.

Copper mineralisation where fluids have forced their way through rocks beneath a Cretaceous aged black smoker, that once stood on the ocean floor at the spreading ridge.

Above, from L-R: A former ocean floor with metal-rich umber sediments in the foreground, and a fossilised black smoker in the background. The metal in the umber sediments came from the precipitates emitted by the black smoker.

The sheeted dyke complex occurs between the gabbro magma chamber and the pillow basalts in ophiolite sequences. Here Lee and Ellie point to a unusually thick dyke in within the sheeted dyke complex. The rocks either side of this dyke are also all dykes themselves.

Teresa rests on the famous ‘Geotimes’ pillow basalts. These are arguably the world’s finest outcrop of pillow basalts. The basalts, part of the ophiolite sequence, erupted on the sea floor and were fed by dykes rising from the sheeted dyke complex below. One such dyke is seen under her head cutting through the pillows to produce a younger generation of pillows above.

Above, from L-R: A former copper mine which had been mined for 4,000 years. Mining only stopped a few decades ago and all that remains is this large hole with a lake at the bottom.

James and Patrizia in front of a fossilised white smoker. It was formed by cooler temperatures than the black smoker we saw the day before (100°C rather than 300°C). These cooler fluids precipitated fewer metal sulphides. Most of the deposits are opaline silicates.

The photograph is taken at the site of a former plate boundary, a subduction zone. The rocks in the foreground are part of the “metamorphic sole” of the Semail ophiolite. They were dragged down the subduction zone to great pressures and metamorphosed to amphibolite. The rocks in the mid-ground were once an underwater volcano crowned with a coral reef top. They have been trapped in the subduction zone hinge. The distant rocks to the far right of the photo are rocks that were once part of the mantle.

Above, from L-R: The group observing spectacular folds in the Hawasina Nappe. These sediments originated as turbidites on the ocean floor and were thrust over the Arabian continental foreland in a nappe during the emplacement of the Semail ophiolite. In the process the sedimentary layers became dramatically folded.

Nivaldo examines a fault in turbidite sediments of the Hawasina Nappe.

A dramatic scene where the rock on the left, Cretaceous aged limestones of the Arabian foreland, is plunging beneath both rocks of the Hawasina Nappe and rocks of the Semail Ophiolite Nappe. The nappes were thrust over the Arabian Foreland during the emplacement of the Semail ophiolite.

Above, from L-R: The al-Ayn beehive tombs on the ridge in front of the Jebel Misht mountain, which is a fossilised atoll trapped in the subduction zone.

Nivaldo points out en-echelon views within Cretaceous limestone in a man-made railway tunnel that leads to the Al Hoota Cave.

The Al Hoota Caves. These caves are the largest show caves in Arabia. They are in Cretaceous-aged limestones and have been formed by water enriched with dissolved carbon, which makes carbonic acid, that has passed through the rock dissolving it to form the caves. However, the caves are quite dry now and most of the cave formation would have happened in the last Ice Age when it was much wetter and the water would have filled the cave to the roof.

Above, from L-R: Looking down into the Grand Canyon of Arabia, giving a view of the entire sequence of Jurassic rocks of the Arabian foreland. In the top left corner of the photo is Jebel Shams which, at just over 3000m, is Oman’s highest mountain.

A mountainside near Jebel Shams. The top of the mountain is limestone of the Arabian Foreland, beneath it is a sliver of Hawasina Nappe and beneath that is a sliver of volcanic rock and radiolarian chert from the Haybi nappe.

Coleman’s Rock, on which the most important rock art in north-east Arabia is inscribed.

Above, from L-R: The group examines an outcrop of plagiogranite that formed at the boundary of the gabbro magma chamber and sheeted dyke complex due to extreme fractional crystallisation.

A layer of serpentinised minerals lying on top of mantle rock, rich in chromite, seen in a former chromite pit.

James points to the Moho. It is visible as the colour change towards the top of the mountain; the upper band is gabbro from the crust and the lower portion is peridotite from the mantle.

Above, from L-R: Our group waits for sunset in the Wahiba Sands (also known as Sharqiya Sands).

The oldest rocks in Oman: Precambrian gabbro dykes cut through Precambrian granites. These rocks formed during the accretion of terranes that later had a period of extension.

A close-up view of the kimberlite outcrop, showing rounded lapilli that formed as the magma raced up towards the surface. Kimberlite derives from a very deep mantle source and contains exotic minerals. It is named after the town of Kimberley in South Africa which has one of the most famous diamond mines in the world. Diamonds are not present in this Omani Kimberlite.

Above, from L-R: Our group at the “Mother of All Outcrops”. This beautiful rock sequence of interbedded red radiolarian chert and white porcellanite is folded and faulted. These sediments were deposited in an ocean maybe 4,000-5,000m deep and were emplaced with the Masirah Ophiolite some 15 to 20 million years after the emplacement of the more famous Semail Ophiolite.

The group examines layers of red radiolarian chert, white porcellanite and black “pyrolusite”. Pyrolusite is a manganese ore and this site was a pyrolusite open cast mine.

The group examines an outcrop of the exotic carbon containing lava: carbonite. This outcrop was cut by amethysts veins.

Above, from L-R: Nivaldo examines an erosional notch cut into Eocene limestone in the Eemian interglacial. The holes were caused by marine molluscs that were boring holes into the rock, which demonstrates that this notch must have been created by the highest sea level at the time. The notch has been dated to around 120,000 years old which is the same age as the Eemian interglacial. This was a period in the last Ice Age, and at this site in Oman we can see that the sea level was 3.7m higher than the present day. What is concerning is that we already have a higher CO2 concentration today than in the Eemian, which is well known for being several degrees warmer in temperature. There were hippos, for example, living in the River Thames in England during this interglacial. Because our CO2 level is already higher than it was then, this is a warning of the type of temperature and sea level rises that we may expect to have in the not-too-distant future.

Wave cut platforms in Eocene and Quaternary aged rocks. The modern shore is at the base, then one step up there is a wave-cut platform of Eocene limestone, and another step up are the Quaternary coral fossils, and again another step up is a recent conglomerate from rocks that were deposited in fluvial conditions. This stepped shoreline shows us the different sea levels of the past. The terraces presumably formed due to the uplift of the area after the emplacement of the Semail Ophiolite (which caused the land to rise) and there were lots of interesting fossils to be seen.

Wadi Shab is a canyon cut through Eocene aged limestone and is regarded by many as Oman’s most beautiful wadi.

Above, from L-R: James points to huge boulders that have been stacked up or imbricated by the huge force of a tsunami wave.

The Bimmah Sinkhole. A sinkhole forms when a cave collapses into an open hole. This hole here actually has brackish water because the water comes from submarine caves which are connected to the open sea. However, the Bimmah Sinkhole is not actually a sinkhole, but a doline, which is a feature that occurs when there is a hole in soluble rock (like limestone) and the overlain insoluble rock layers sink into the hole in the soluble rock. In this case there is the Eocene limestone beneath, and on top of it is a thick conglomerate.

The group stands in front of a large fold in Wadi Mayh. This fold is part of a much larger mega-sheath fold complex.

Above, from L-R: The ‘eyes’ or lenses seen here in the cliff face are part of the Wadi al Mayh sheath fold. This is the largest sheath fold in the world. What is a sheath fold? A sheath fold is something quite complicated. Imagine taking a tablecloth, pinching the middle and pulling it up so it forms a fold in three dimensions. Here it occurs because the Arabian foreland was being dragged down the subduction zone as the ophiolite went over it. Normally, sheath folds are only centimetres to a metre in length, but this sheath fold is 15 km long! If you were to cut a sheath in half, it would look like concentric rings or ovals. Here, where it has been cut in section by the erosion of the Wadi, this phenomenon is visible.

The view down on to Mutrah Harbour with the Sultan’s yatchts moored in the port. The banded rock is peridotite, rock that was originally part of the mantle. The darker bands are harzburgite which contains olivine and pyroxene, while the lighter bands are dunite, which is nearly 100 % olivine. The holes form where dunite has been weathered away forming the mineral magnesite.

Above, from L-R: Our group in Wadi Mayh with a huge fold that is also part of the sheath fold complex.

The famous outcrop of eclogite on a beach near As Sifah, which we reached by fishing boat. These rocks had once been basalt and were part of the Arabian plate which had subducted the furthest underneath the ophiolite. The eclogite contained a green mineral called omphacite (a type of pyroxene), and some really well-formed brown-red coloured garnet crystals.

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