Becca Hedges
Geo 3510
Dr. Kaiser
5 Dec. 2016
​
The Welsh Basin
During the 19th century many advances were made in the field of geology through the country of Wales. Many geological events over time have shaped the Welsh landscape. Earthquake, swamp, desert, and glacial evidence can be found throughout all of Wales. This area has experienced different longitudes and latitudes through the movement of the plates. It has also converged with other plates and been stretched by other geologic events. One of these, among many took place along the northern region of Wales. This is where geologists first named the Cambrian, Ordovician, and the Silurian periods in the early Paleozoic Era. The names came from local ancient tribes which lived there (Talbot, 2-3). There were two major orogenies during the Paleozoic which created two very important basins. The Caledonian orogeny occurred first during the mid-Paleozoic with Laurentia and Gondwana. This orogeny created the Welsh Basin which is located in the central part of the country. The Caledonian orogeny occurred due to three separate events. The Caledonian mountain-building event first began in the early Cambrian when a chain of island arcs began colliding. This is called the Grampian Event. The second part took place around late Ordovician and early Silurian when Baltica collided. This is called the Scandian Event. The third and final part occurred when Eastern Avalonia began subducting under Laurentia, this was the Caledonian orogeny event. Throughout history various minerals have been mined from and around the Welsh Basin. Many metalliferous minerals are found in this area and is known as the Central Wales Mining Field. Copper was mined during the Bronze Age. The Romans were fond of the gold located here. The mid-19th century up to the 1930’s lead, silver and zinc were heavily mined (Schofield, 3). This is but a condensed list, we will explore more in further detail later on. The Variscan orogeny was the second major mountain building event and occurred during the late Paleozoic during the formation of Pangea. This is called the Variscan orogeny and created the Variscan Basin which is located on the southern part of the country. The series of events which lead to the Caledonian orogeny are still part of the story for the Variscan Basin. The southern part of Wales is buildup of sediment deposits. Though this paper will focus on the Welsh Basin, the Variscan Basin is worth a mention because of the large portion of the sediments which fill the Variscan Basin is a large portion of what has also filled the Welsh Basin. These deposits come from the Snowdonia Mountains and marine deposits. All Precambrian through Jurassic periods are represented by outcrops on the geologic time scale. The younger aged deposits are represented by sedimentary rocks located in the southern part of Wales. During the ice age in the early Quaternary period the country side was shaped with glaciers to the way it is seen today (Schofield, 10).
As you can see, Wales has a very long and important history in the study of geology. As David Schofield says in his article What’s in the Welsh Basin? “It would take a book to describe the geology of the Welsh Basin”. The formation of the Welsh basin began early on in earth’s history. This basin is filled with sediment that began in the early Paleozoic which has gone thru many changes based on tectonic movements.
The Precambrian and Cambrian
During the Precambrian and the Cambrian times this continent was tiny. It began to grow due to island arc systems near the equator. These deposits are a basalt. There is an outcrop that is well known as the Cliffs of Anglesey which showcase these basalt deposits. The composition that was formed during this time are an ultra-mafic to mafic composition. They are described as tholeiitic basalts. These types of basalts have a very low sodium content. They are high in silica, magnesium, and iron (Schofield, 5). We could expect to see some of the minerals in the lherzolite group here. Olivine, orthopyroxenes and clinopyroxenes like calcium rich pigionite, hypersthene, and spodumene would be present. The more plagioclase minerals like anorthite and some labradorite are also likely to be seen. With the suturing of additional arcs, higher temperatures and pressures could form garnet and spinel. The more felsic or especially sodium rich minerals like the k-feldspars, biotite, muscovite, and Quartz would be less common. At this time the country of Wales was located on a small continent named Avalonia near Gondwana. Towards the end of the Precambrian, Avalonia began to drift north away from Gondwana as the Iapetus Ocean closed (Schofield, 6). Volcanism occurring in the area produced another well-known outcrop called The Padarn Tuffs. During this time Avalonia was stretched by the subducting ocean floor under Laurentia thus creating the Welsh Basin. This basin was located on the interior of the Avalonia plate and became a deposit ground for sediments until the end of the Silurian (Talbot, 3). The Welsh Basin was most likely not having much material deposited during the time of its creation. The material that would have entering in the basin at this time would be from the volcanic deposits which created the continent.
Ordovician
During the Ordovician volcanic events were still very common and still predominantly producing tholeiitic basalts. This still aided in the creation of the minerals discussed in the Precambrian Era. Minor volumes of andesite resulted from low-pressure fractional crystallization are also found (Scholfield, 6). These deposits helped form the highest mountain in Wales, Snowdonia. Andesite’s are of an intermediate composition and would contain more sodium rich plagioclase and pyroxene. We could also see some amphiboles and orthoclase minerals. These basalts would be more likely to produce albite or augite. When there is volcanic activity, mineralization will occur. This is due to the fact that volcanism equals heat. With heat comes heated water, this heated water circulates through the faults and other weaknesses in the rocks. Because of this hydrothermal and supergene processes beginning to occur, this would result in the formation of iron, copper, lead and zinc sulphides. The resulted deposits of these sulphides were worked for copper during the Bronze Age which by the late 18th Century was Europe's biggest copper-mine. The speed of the plate movement increased during this time still closing in the Iapetus Ocean. The Welsh Basin was considered a marine basin as well as receiving erosional materials from Snowdonia. This resulted in fine sand, and silt deposits at a rapid rate as well as mud full of organic material (Talbot, 4-5). Subsidence and marine transgressions dominated the basin (Schofield, 6). In the late Ordovician there were deposition of carbonaceous sediments. This created the Caradoc Black Shales which are heavily mined to produce housing materials (Roberts, 637). With the marine transgressions we would likely see some evaporite sequences within the basin. These would likely be gypsum, sylvite, magnesite, aragonite, and dolomites. Given that much of the material being deposited into this basin from the surrounding formations lacking sodium content, halite, ulexite, kernite and borax would be least common. Understanding that some of these evaporite minerals are pretty rare in general it would be very uncommon for them to be present. With the sea water entering into the system future minerals with sodium could start to occur. Other rich carbonate deposits would be common as well. Towards the end of the Ordovician we see the beginning of the Caledonian orogeny.
Silurian
​
The early Silurian saw the Avalonia plate rapidly subduct beneath Laurentia. Volcanic events were becoming less frequent. Deposits still continued to accumulate in the Welsh Basin. By the end of this period the Iapetus Ocean had closed in and volcanic activity had completely stopped (Schofield, 7). This subduction zone would undoubtedly have seen major metamorphic facies formations occurring. This would include the creation of prehnite-pumpellyite, greenschist, amphibolite, granulite, and blueschist, eclogite, among others. The convergence of the area occurred very quickly and created extensive folding and faulting of the pre- existing rock sequences. These deformations are particularly common within the Welsh Basin itself. These deformations get more and more intense the farther north you go. The Cambrian and Ordovician slates of North Wales were likely created due to this. This event actually caused the Welsh basin to slightly invert (Roberts, 638). The material eroded and deposited from the Silurian period are some of the most important of the Welch foundations to the south (Schofield, 7). A large percentage of this country’s land area is highly enriched with rocks of this age more than any other. The deposits found above this foundation are rich in sandstones, mudstones, and conglomerates from the Devonian period. By the end of the Silurian the Welsh Basin was full (Schofield 8). Though the basin was full future geologic events altered the sediment.
Lower Paleozoic Summary
The Cambrian, Ordovician, and Silurian times are known as the Lower Paleozoic group and all played a very important role in the formation of Welsh mineral deposits. Due to the shallow seas many chemical precipitates accumulated here. Sometimes these were mixed with mud and other sediment. In some locations thick deposits of manganese carbonates and silicates are found (Talbot 4). Later in Ordovician times, similar chemical sediments like the ones in the Cambrian were deposited, but during the Ordovician time iron was the most abundant. The ores were oolitic ironstones that became metamorphosed and deformed changing into new minerals like magnetite (Schofield, 6).
​
The volcanic activity during the Cambrian-Ordovician boundary resulted in the formation of one of the oldest porphyry-copper deposits in the world. A large mass of intrusive diorite created copper and iron sulphides. Around the same area contains some the best metalliferous deposits in Wales. This is the location where the Welsh Royalty gathers the gold used for Royal wedding rings. The gold occurs in very localized but extremely rich pockets of quartz veins also containing sulphides of iron, copper, lead, zinc, arsenic, cobalt, bismuth, lead and silver. Mineral exploration in the area still continues (Roberts 636).
​
From Mid-Ordovician to late Silurian times, along the margins of the Welsh Basin, sedimentary debris of sand, gravel and mud gathered along the banks, forming unstable masses above the slopes leading down into the deeper water. At its peak, the submarine debris-flows, or turbidity currents formed strata called turbidities. As they slowed down, the particles of sediment began to settle out, the more coarse and heavy material settling out first. On the southern end of the Welsh basin there is a unique turbidite sequence which is associated with gold-arsenic deposits. The ore consists of pyrite and arsenopyrite in which extremely fine-grained gold is disseminated. The Romans worked near the surface of the deposits. Here the gold was easier to recover due to the sulphides having experienced oxidation reactions (Talbot, 5).
Late Paleozoic through the Quaternary
Other geologic process have not necessarily contributed to the basin deposits but they did have an effect on the alteration of the material located within the basin. By the beginning of the Devonian period, the sea was retreating from the Welsh Basin and the land was thrust up by the collision of the continents forming Pangea. This created a new range of mountains, the Welsh Caledonides. The strata was compressed and deformed in many places. The clay minerals recrystallized and developing a grain that allowed parallel cleavage, making it easy to split the rocks into thin flat sheets of stone known as slate (Talbot, 11). This was largely caused by very intense pressure. This new mountain range included the Welsh Basin itself. The Devonian and Permian periods introduced many hydrothermal systems altering pervious deposits within the Caledonide Mountains. There were also more episodes of subsidence to the south allowing for more area to be filled in by the sediments that have been eroded throughout the country. During the middle of the Devonian period, the climate was arid. The Caledonide Mountains are eroding away and generating new sediments in small isolated areas (Talbot, 12). Rivers carried debris away towards the Midlands and South Wales. When the sands consolidated they formed the famous Old Red Sandstones. These sandstones give the characteristic red color to the soils in parts of South and East Wales (Talbot, 12).
​
The Quaternary period which is really present day, includes periods of ice sheets and valley glaciers which developed during a series of ice ages which have significantly altered a landscape. One ice sheet which at its maximum covered virtually all of Wales. This left both erosional and depositional landforms. The glacial cirques of Snowdonia, to a lesser extent of the Cambrian Mountains, and the Brecon Beacons are well known. Many pre-existing valleys were further deepened by glacial ice. Cirque moraines in the mountains, terminal and recessional moraines in the major valleys of the south were the most affected (Schofield 9).
Minerology of the Welsh Basin
Due to the age of the Welsh Basin it is likely displays very distinct layers showing time itself. If it were possible to cut a slice out of the earth allowing us to look at the actual cross section we should see these very distinct times of deposition and deformations that occurred.
Beginning in the Precambrian and the Cambrian times no large amounts of deposition would be occurring. What we would see in the very bottom layers of this cross section of the Welsh basin would most likely be sedimentary fragments with igneous compositions. Here could be some olivine, possibly garnets, calcium rich orthopyroxenes, clinopyroxenes, and plagioclase minerals. Again we would see few minerals with sodium due to the tholeiitic basalts found.
​
In the Ordovician time the same sediments that were in the beginning of the basin are still continuing, now we start to see marine deposits entering the mix of our cross section. These deposits are adding in sodium that was dissolved into the ocean water, and organisms that would also be needed to form carbonate minerals. With the marine transgressions and regressions that took place we should see repeated patterns of evaporite deposits. Minerals like calcite, gypsum, halite, dolomite, and aragonites. Other rarer evaporite minerals could also be here but not in any economic value quantities. Interestingly enough limestones are rare in this basin, but are quite abundant in the Variscan Basin (Minerology of Wales, Web). Hydroxide deposits could also be present. During this time the porphyry-copper deposits are forming. With this we would see major minerals like pyrite, chalcopyrite, bornite, and molybdenite. These are usually mixed with quartz and sericite. Minor minerals include bornite chalcocite, magnetite, hematite, covellite, native copper and ilmenite. Other minerals that could be forming and eroding into the Welsh basin during this time could include other silver, zinc, copper, calcium, nickel, arsenic and manganese oxides and sulfides. Pumpellyite is found in the Lower Paleozoic igneous and sedimentary rocks of the Welsh Basin were metamorphosed largely under prehnite-pumpellyite facies conditions. Prehnite is widely developed in Wales, typically occurring as an alteration mineral in basic lavas and intrusions, particularly those of Ordovician age. There are coal seams found within the basin, but not near the quantity as are found in the Variscan Basin to the south known as the South Wales Coalfield (Minerology of Wales).
​
During the Silurian, the basin is becoming filled. With the introduction of more andesitic magmas in the Ordovician time feldspars breaking down would likely be creating clay minerals like Kaolinite and Smectite within the basin. Illite might not be common as this particular area would be around the top layers of the basin so it is a more low pressure system. Volcanic events have stopped so no new volcanic materials would be deposited however, the erosion of the out laying areas would still be contributing to the basin. There was little to no marine deposits during this time. This basin was located at a higher elevation due to continued uplift. The Silurian was mainly a time of more erosion from the Snowdonia Mountains that settled into the basin.
​
From the Devonian time till the present the Welsh Basin really only underwent deformations due to high pressure occurrence based off of further plate movements. Hydrothermal activity peaked during the Devonian further altering the basin fill. This would produce more alteration minerals like goethite, azurite, malachite, rhodochrosite, barite among other carbonates. Sulfides, sulfates and oxides that are high in copper, zinc, and arsenic bearing minerals. Glacial activity softened the countryside allowing the material to be eroded more easily. Wales is not currently considered an active seismic location but who’s to say it will not be in the future
Economic Value of Minerals in the Welsh Basin
As mentioned above mining in this area has been a common practice for a very long time. The people of the Bronze Age and the roman times were among the first. The Welch Basin is located within the Central Wales Orefield and is focused on the deposits created during the Ordovician and Silurian times (British Geological Survey). The deposits mined were used for weapons, to jewelry, and decorations. Out of aproxamently 4,900 recognized mineral species known around the world roughly 430 occur in Wales and not all of these are found within the Welch Basin (Minerology of Wales). The major metals that have been searched for and taken from the Welsh Basin include copper and gold, nickel, cobalt and sulfides. Silver and lead have been mined to a lesser extent. The early veins were of particular importance as a source of silver in Civil War times. Later veins were richer in lead and zinc. Most of the mining for these materials died off after the Second World War. (British Geological Survey).
Wales has a lot of hard rocks throughout the country especially towards the northern region. These are very useful for building various structures. It is known that the bluestones of the Ordovician dolerites is where the lintels of Stonehenge came from. Wales is also very well known for its slate, one mineral product that is still actively mined today. Slate was the economic foundation of many of North Wales' larger villages and towns. It owes its existence to the Cambrian rocks in the Snowdonia area which saw greater tectonic pressures and metamorphic conditions which created purple and green colored slates. The slate deposits in the Upper Ordovician and Silurian times are useful and commonly mined but not as desired as the Cambrian slates. The slate deposits are largely used for roofing, tiles and other building materials (British Geological Survey).
While there are many other mines around Wales, this wraps up the Welsh Basin discussion. Through time this land mass has been through some pretty dramatic changes within the crust as well as the surface. Its age has preserved time which has been helpful to the study of Geology. It has aided Europe in ways we can understand and possibly in ways we cannot. As time continues it will likely continue to see it as well, further capturing what is happening as our world shifts.
Bibliography
British Geological Survey http://earthwise.bgs.ac.uk. Web. 2 Dec. 2016.
“Minerology of Wales” https://museum.wales/mineralogy-of-wales/database/?mineral=341. Web. 2 Dec. 2016.
Roberts, B., Merriman, R.J. and Pratt, W. ‘The influence of strain, lithology and stratigraphical depth on white mica (illite) crystallinity in mudrocks from the vicinity of the Corris Slate Belt, Wales: implications for the timing of metamorphism in the Welsh Basin’, Geological Magazine, (1991). pp. 633–645. Academic Search Premiere. Web. 30 Nov. 2016.
Schofield, David. “What’s in the Welsh Basin?; Insites into the evolution of Central Wales and the Welsh Boarderlands during the lower Paleozoic”. British Geological Survey, (2009). pp. 1-11. Academic Search Premiere. Web. 29 Nov. 2016.
Talbot, James L and J. W Cosgrove. The Roadside Geology of Wales. 1st ed. London: Geologists' Association, 2011. Print. Google Scholar. Web. 27 Nov. 2016.