Thursday 27th June 2024
Rob Crossley from GeoMôn gave this catchily-titled talk to Mid Wales Geology Club members in Newtown and elsewhere via Zoom on 19th June 2024. Rob started off by telling us of his international geological experience which started off with a PhD thesis in the East African Rift Valley. He then moved on to what we know about geological events in several other moons and planets of our solar system. Obviously we don’t know much, but we have seen evidence of lava flows and water channels which took place in the distant past. There does not, however, seem to be anything like the range of geological appearance we see here on Earth.
Rob moved on to the diversity of the geology within Ynys Môn, and how it had been captured on previous maps, starting with Williams Smith’s 1815 map. Then on to John Henslow’s map published in 1822, in which he managed a very detailed and accurate map after just one summer of exploration. Finally we had the Greenly’s map of 1919, which is virtually the same as current BGS maps. Edward Greenly introduced the term mélange to geology with his description of the Gwna mélange.
In the latter part of his talk, Rob illustrated the wide range of geodiversity within the island starting with the last Ice Age and then working back through various geological periods to the late Precambrian. At the end of the talk, the audience were happy to answer the question in the title with a resounding YES.
Part of the Gwna melange seen near the Wylfa nuclear power station. This exact location had been accurately sketched and annotated by John Henslow in 1821 with the large block of pale quartzite centre left and the dyke intruding near the bottom right.
The northern side of the Menai Straits at Lleiniog. The best example of a sub-glacial tunnel valley in the UK
A closer view of the above shows small fragments of coal trapped within the material.
The following two photos depict pillow lavas on Llanddwyn Island. Speaker used for scale in first photo.
Closer view of the pillow lavas
Text and photos by Dr. Chris Simpson MWGC
Saturday 15th June 2024
The Geology of Oman by James Cresswell based on one of James’ tours.
James gave a really interesting talk about the fascinating geology to be found in Oman. The talk concentrated on the geology of the northern part of the country with the tour commencing at Muscat, travelling north to Sohar, then to Nizwa, on to Sharqiya Sands (Wahiba Sands), and then over to the coast at Sur before returning to Muscat.
A view over the capital city of Muscat. In the background, the mountains are made of peridotite which was once part of the mantle, whilst the city sits on the Kabir Fault, which was once a former subduction zone.photo by James Cresswell
Oman sits on the north-eastern portion of the Arabian plate at the convergence zone between the Arabian and Eurasian plates. At this point the oceanic crust of the Oman Sea is subducting under the lithosphere of the Eurasian Plate.To the north the Arabian plate is rifting away from the African plate at a divergent margin.
The main focus of the geo-tour was the Semail Ophiolite which is the world’s largest and best preserved ophiolite. The ophiolite is a vast thrust sheet of Cretaceous ocean crust and upper mantle emplaced onto the Arabian continental margin during late Cretaceous times. It is 500km long 50-150km wide and 15 km thick. New findings suggest that it was formed above an active NE dipping subduction zone
James then went on to describe the structure of the ophiolite along with the locations at which the various components may be found.
- The metamorphic sole amphiboles beneath the Semail thrust plane that carries the entire ophiolite above.
The rocks of the metamorphic sole were visited at Wadi Hawasina.The Metamorphic sole is suggested to have been formed as material moving down the subduction zone underwent metamorphism and finally being obducted onto the surface.The degree of metamorphism producing amphibolite and greenshist.
Beneath the metamorphic sole of the ophiolite is a series of thrust sheets (nappes) including the Haybi Complex comprising distal sedimentary and volcanic rocks, Late Triassic alkali basalt and limestone seamounts and mélanges. Below the Haybi Complex is the Hawasina complex, a series of 4–7 thrust sheets of basin to shelf slope sediments.
- The mantle sequence peridotites and the Moho Transition Zone with dark wehrlites and pale coloured gabbros intruding harzburgites.
These rocks including pale coloured dunites and brown harzburgites, can be found at Wadi al Abyad. Mantle,rock (peridotite) can be found at the base of a cliff with a clear line separating them from layered gabbros above. The clear line separating the two is the Moho ( boundary between the mantle and the earth’s crust). At Wadi Hawasina mantle peridotites of the Semail ophiolite may also be found along with heavily folded marine sediments of the Hawasina nappe. Peridotites of the Semail Ophiolite may also be found around Muscat.
The Moho at Wadi al Abyad. This is the best place in the world to see the Moho (The Mohorovičić discontinuity) – the boundary between the earth’s crust and mantle.Photo by James Cresswell
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Layered lower crustal gabbros inter-banded with dark coloured mantle peridotites could be seen at Wadi al Abyad. The layering is due to differential crystallisation in the magma chamber
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At Sohar sheeted dykes which form part of the classic ophiolite sequence can be seen. The complex consists of swarms of basaltic dykes which are the feeder dykes for the overlying pillow basalts.
Also at Sohar there is a fossilised Black Smoker that formed on the ocean floor as part of the ophiolite complex. This was a vent for superheated water loaded with dissolved sulphides and a high concentration of copper, zinc and manganese. This is a now a large ore body that is likely to be exploited. It is actually placed within the Geotimes Unit but is partially covered by the Tholeiitic Unit. Close to this site is another site known as the Umber sediments which are rich in iron and manganese and were formed in association with the Black Smoker. The metal enrichment is due to precipitants from the Black Smoker settling on the mud of the ocean floor. They are un-metamorphosed and cover the pillow basalts of the ophiolite.
At a second site close to Al Ghizayn is a fossilised White Smoker. Similar to the Black Smoker but has much lower temperatures and therefore less metal enrichment but contains lots of opaline silicate.
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The Geotimes unit lower V1 pillow lavas at Wadi Jizzi are considered to be the best outcrop of this kind in the world.The basalts were erupted onto the seabed and formed the pillow structures as they were cooled by the surrounding sea water. They are fed by dykes rising from the sheeted dyke complex. The pillows have a glassy outer skin ( basaltic glass). These particular pillow basalts are elongate in shape suggesting they were oozing outwards and downwards onto a slope and have a diameter of 0.5-1m.
Geochemical analysis of immobile trace elements like titanium have shown that volcanics found in hotspots are geochemically very distinct from those erupted at mid-ocean ridges and both of these are very distinct from andesitic lavas found in island arcs. Using this technique it was shown that the lavas of the Semail ophiolite were erupted in two different phases. The Geotimes unit was erupted at a spreading ridge but the later lavas included a rock known as boninite which is always found above subduction zones. Therefore it is suggested that to begin with the Geotimes basalt would have formed due to spreading but as subduction proceeded and increased in depth the chemistry of the melt changed producing the different lavas as seen. These other lavas have been mapped as the Tholeiitic Alley and Boninitic Alley Pillow lavas.
The tour not only focused on the ophiolite but explored many other aspects of the wonderful geology present. One of these sites, near Sur,is named "The Mother of all Outcrops”. Here, folded and faulted, red radiolarian chert is interbedded with white porcellanite ( an impure chert containing clay and carbonates). These sediments were deposited in water 4,000-4,500 metres deep, and 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.
The GeoWorld Travel group at the so-called ‘Mother of All Outcrops’ which are tightly-folded, alternating layers of radiolarian chert and porcellanite. These were folded with the obduction of Oman’s other ophiolite, the Masirah Ophiolite. photo by James Cresswell
Another site near to Sur is of Precambrian crystalline basement (800-1,000 million years old). Here there are granites that formed as terranes, were accreted together and then cut by huge gabbro dykes that were intruded as the basement went through a later period of extension. All these rocks are themselves intruded by granitic pegmatite dykes
Another fascinating geological structure is the Wadi al Mayh mega sheath fold. It is the best exposed sheath fold in the world and is 15km long. The rocks record progressive burial and exhumation of the thinned and stretched leading margin of the Arabian continental crust down a subduction zone to depths of up to 80-100km.
A large isoclinal fold, Part of the mega sheath fold at Wadi al Mayh photo by James Cresswell
In addition to all the above, the tour covered areas related to periods of “Snowball Earth” at Wadi Haslan; 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” and a visit to Wadi Shab, a magnificent canyon cut into Eocene limestone, which is considered by many to be Oman’s most beautiful Wadi. There were also some archaeological sites that the tour visited for example south eastern Arabia's most important Neolithic rock engravings, at Hasat Bin Salt also known as Coleman’s Rock and “The Aflaj Irrigation System of Oman”. 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. Both of these are World heritage sites.
This was a super talk about a country with fascinating geology.
If you would like to know about James Creswell’s tours his website is as follows:
[http://www.geoworldtravel.com]
If you would like to know more about the geology of Oman then the following book will be of interest:
Field Guide to the Geology of Northeastern Oman edited by Gosta Hoffman, Martin Meschede, Anne Zack and Mohammed Al Kindi. ISBN:978-3-443-15099-0
Wednesday 24th April 2024
The club was treated to a detailed summary of what Northern Ireland has to offer to anyone with even a passing interest in geology by karen Parks Geology Teacher at Methodist College Belfast. Karen imagined a visitor arriving in Belfast and their journey round the Antrim coast ending up in Port Rush, and showed us the wide variety of interesting sites they would pass through.
Several of the terrains and major fault systems in Scotland carry on over the Irish Sea and re-emerge in Northern Ireland. These rocks that are known to those of us familiar with UK mainland geology are supplemented by large areas of volcanic rocks forming part of the North Atlantic Igneous Province and the fascinating NE Antrim inlier – a block of Carboniferous age rocks uplifted to the level of the lower tertiary basalt. The Iapetus suture, which marks the site of closure of the previous Iapetus Ocean, traverses the island of Ireland.
Karen showed us the website of the Geological Survey of Northern Ireland, which includes an interactive geological map where details of bedrock geology anywhere in Northern Ireland can be viewed at the click of a mouse.
https://mapapps2.bgs.ac.uk/gsni_geoindex/home.html
The following pictures are examples of the geology shown to us by Karen.
Just North of Carnlough. A characteristic example of a rotational slide. The chalk is lying on lias clay. Water can penetrate the clay and lead to it becoming, in effect, a lubricant to allow the overlying chalk and basalt to slump down with rotation.
Cushendall. Dalradian schists, circa 550Ma, at the top with cross-cutting Devonian dykes composed of microgranite, circa 390Ma underneath. The microgranite may be seen in the image below
close-up of the microgranite
The town of Ballycastle is built on the Antrim lava plateau
Carrick-a-Rede volcano – reached by a rope bridge walkway. The photo shows a close-up view of the vent material, including large volcanic bombs (the bridge at top right gives the scale).
Smaller vents further along the coast. Here the dark volcanic material is coming up through chalk. Note the brecciated appearance of the chalk.
A small offshore island which is part of a basalt lava flow which passed over the chalk visible at the left. The vertical columnar jointing is clear in the closer view of the photo below.
The same lava flow as seen in the Antrim coast south of the island
The Giants Causeway. It is not all regular hexagons! Look at the irregular mass of cooled basalt on the right of this photo. The site is extensive, and requires several hours to do it justice.
There were multiple layers of basalt lava – at least 8. Some cooled slowly and show excellent columnar jointing as in the organ pipes seen in the centre. The lava flow above is much more irregular, indicating more rapid cooling.
If you look carefully, you will notice several dykes cross-cutting the basalt lava flows. The horizontal columnar jointing and darker colour show the true nature of this dyke – but they are easily missed!
A Wonderful tour of Northern Island geology.
Text and photos by Dr. Chris Simpson MWGC
Thursday 11th April 2024
At the last Meeting Dr.Joe Botting brought us all up to date with the new and exciting discovery of Burgess Shale type fauna in a quarry in mid-Wales. As often occurs, an exciting and important discovery comes just as work in an area is coming to an end. So it was with Joe and his wife Dr. Lucy Muir. They had been undertaking work in this area for 8 years but just as their work in this quarry was almost done they decided to take a last look at a very thin section they had not looked at previously. It was in this section that they made their highly important discovery.
Joe went on to explain that the middle Cambrian Burgess Shales as being one of the most important sites in the world in which many soft bodied animals are preserved, as carbon films, in very fine detail. In some cases not only is the soft tissue preserved but also internal structures, for example, neural tissue.This area and some other sites around the world, for example, The Chengjiang Biota have shown by how much life had evolved by the middle Cambrian. Not only do these sites show the fine details of the fauna but have also shown how markedly similar the fauna from all the sites actually is. It is from the Burgess Shale type formations that have shed light onto the Cambrian Explosion and the diversification of life. The end of the Cambrian sees little further diversification but is followed by the Great Ordovician Biodiversification Event leading to a large increase in biodiversity at the Family,Genus and Species level.
Fossil preserved as carbon film from Castle Bank By Joe Botting
He further explained that there are Ordovician Exceptional Fossil sites around the world but they are quite limited in one way or another when compared to the Burgess Shales, although there are two Ordovician Burgess Shale type areas, one at Afon Gam in north Wales and another at Fezouata in Morocco both from the early Ordovician. In contrast the Castle Bank fauna are from the middle Ordovician.
Castle Bank is situated within the northern part of the Builth Inlier. The stratigraphy here ranges from the Camnant mudstones through the Builth Volcanic Group to the Llanfawr Mudstones Formation. With fossils being found throughout.The area around Castle Bank is already well known for the fossil trilobites that can be found with the first trilobites being described from the area. Castle Bank itself is like any other farm quarry that can be found in the area with graptolitic shales and a few volcanic ash beds. But one section in this quarry was the one in which the soft bodied fauna was found. Like other fossils of the Burgess Shale type the fossils are preserved as carbon films and are very small!! The finds range from algae, hemichordates, sponges and many soft bodied animals that are still being identified. Joe and Lucy are working collaboratively with groups from other institutions on this task.
So in conclusion how good is castle Bank? Well, Joe suggests the following:
- At least 20 phyla (probably) - similar to Chengjiang.
- Nearly 200 species so far - similar to the Burgess Shales.
- Preservation of entire range of benthic organisms down to larger meiofauna.
- Some internal organ preservation and a lot of very soft and very fine tissue.
- Almost everything is new unlike the Cambrian Burgess Shale Type faunas which are similar to each other.
- Several major groups are the only fossil examples.
A fascinating talk on an equally fascinating discovery.
Sunday 10th March 2024
At the last meeting Chris Darmon and Colin Schofield gave a very informative talk on the North Atlantic Igneous Province (NAIP).
Chris explained that the NAIP is one of a number of what are known as Large Igneous Provinces ( LIP) which may be defined as “an intraplate magmatic event, with volumes of magma greater than 100,000 cubic kilometres". A couple of examples of other LIP’s are the Siberian Traps in Russia and the Deccan Traps in India. The NAIP is one of the larger LIP”s and is almost comparable with the Siberian Traps. It is centred on Iceland and occurs onshore in Greenland, the Faeroe Islands, the UK and Ireland and offshore surrounding these areas as well as the west coast of Norway.
Edge of a lava flow along the West coast of Skye.photo By Chris Simpson MWGC
The NAIP is made up of flood basalts,sills and dykes which were produced by intense igneous activity occurring during the Palaeocene and early Eocene. The cause of this activity being continental separation during the opening of the Atlantic Ocean. Isotopic dating has been undertaken which indicates that the most active magmatic phase of the NAIP was between ca.60.5 and ca. 54.5 Ma (mid-Palaeocene to early Eocene). The volcanism lasted for about 11 million years although each individual eruption was for a much shorter duration of about 2-3 million years.
The edge of a lava flow in NE Skye.There has been a landslip with separation of the areas of rock at the right of the photo from the rest of the lava flow. Note how these separate areas have rotated anti-clockwise as they slipped down slope. The orientation of the successive flows is clearly dipping West compared to the roughly horizontal direction of the main flows on the left of the photo photo by Chris Simpson MWGC
The cause of this continental breakup and volcanism has been explained by the presence of a mantle plume which today is centred under Iceland but there is a lot of controversy in this area and other models have been suggested. There is a lot of ongoing research into this area so that a complete model should eventually be found.
The bulk of the talk actually centred on the British portion of the NAIP which is known as the British Palaeogene Igneous Province (BPIP). The area covered by the BPIP include the Isles of Skye; Rum, Mull; Eigg and Arran, the Ardnamurchan peninsula, outer Hebrides along with Lundy off the coast of Wales and the Mourne Mountains in Northern Ireland.
Prior to the opening of the Atlantic NW Scotland was joined to Greenland and the Canadian Shield but as the Atlantic opened NW Europe moved away from North America and this separation continues to the present day due to sea-floor spreading along the mid-Atlantic Ridge.
Kilt Rock - a sill displaying well-developed columns photo by Chris Simpson MWGC
As North America and Europe moved apart huge volumes of basaltic lava erupted from long narrow fissures on what is now the west coast of Scotland. In addition there were extensive dyke swarms extending to the Outer Hebrides, southern Scotland, north Yorkshire and parts of north Wales. There were intrusions of granite, gabbro, peridotite and other rock types. The basaltic lavas produced the stepped or “trap” landscape that may be seen on Skye. Lavas also produced the Giant’s Causway in Northern Ireland and Fingal’s cave on the Isle of Staffa.Gabbro and peridotite have given rise to the rugged mountain scenery of St Kilda,Skye (Cuillins) and Rhum. The less rugged, mountains of northern Arran and the Skye Red Hills are composed of granite.
The Black Cuillins. This photo comes from about two-thirds of the way up. In the background are the tops of the Cuillins, black in colour and jagged in outline. In the foreground is smooth rock due to glaciation. It shows how high the glaciation came. Photo by Chris Simpson MWGC
Finally Chris highlighted the idea that the Palaeocene- Eocene Thermal Maximum (PETM), an event where there was a global increase of temperature between 5-8C can be linked to the NAIP producing changes in the earth’s carbon cycle and the increase in temperature. Work is still ongoing in this area.
In all a very instructive talk.
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