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January 10, 2018 at 8:18 pm #9398Anonymous
The Geology Group met at 10.30 am on Wednesday 10 January 2018 at Merlin's bridge Village hall. The topic this month was BRITISH CAMBRIAN GEOLOGY
During Cambrian times (544-510 Ma) England and Wales formed part of the area known as Avalonia around 60°S on the margins of the continent of Gondwana. The Iapetus Ocean was widening throughout the Cambrian separating Avalonia from Laurentia where Scotland was located around 25°S. The oceanic plates were diverging as basaltic lavas were being extruded from the mid ocean ridge. It is estimated that the ocean was around 5000 kms wide. Most Cambrian rocks in Britain are derived from sediments deposited on the margins of the Iapetus Ocean.
The Welsh Basin existed on the eastern margins of the Iapetus Ocean hence marine conditions existed over much of present day Wales. Turbidite mudstones were deposited on the continental slopes of the subsiding Welsh Basin, as seen in the Harlech Dome, whilst in North Pembrokeshire, in the shallow shelf seas on the margins of the basin, conglomerates and flaggy sandstones were laid down such as the Lingula Flags at Solva and the purple Caerbwdy Sandstone used in the building of St David’s Cathedral. Also mudstones rich in the fossilised remains of trilobites and brachiopods give an indication of the abundance of marine invertebrate life that developed during the Cambrian. The well-known trilobite Paradoxides davidis has been found in the Middle Cambrian shales at Porth y Rhaw near Nine Wells. The best specimens of this large trilobite are displayed in the National Museum of Wales.
Rocks of Cambrian age in North Wales are represented by a thick sequence of marine sandstones and shales (most of which are now metamorphosed into slates). The Llanberis Slate outcrops on the NW side of Snowdon where the formation is over 1000 metres thick. The slates were formed as a result of pressure exerted on the Cambrian mudstones during the Caledonian earth building movements at the end of Silurian times. The platy minerals like mica and chlorite in the mudstones were re orientated by pressure that created cleavage planes often perpendicular to the original bedding planes. The slates cleave into thin layers that make excellent roofing material. In the late 19th C at the peak of production, the slate quarrying industry exported Welsh slate across the world. A more modern use of the disused Dinorwic quarry is the pumped storage hydro electric scheme that was opened in 1984. The turbines are hidden in the old slate caverns and they are powered by water released from a high level reservoir which is refilled at night by pumping water up from the lower lake using cheap off peak electricity.
The Harlech Dome is essentially an inlier of Cambrian rocks uplifted as a result of the Caledonian orogeny. The oldest rocks in the centre of the eroded dome are surrounded by a rim of the Rhinog Grits which are topographically higher (Rhinog Fawr 720m). One of the best walks in the Rhinog National Nature Reserve starts at Roman Steps near Cwm Bychan. Manganese and iron pyrites occur in the shales above the Rhinog Grits and gold was mined in the Lower Cambrian rocks between Barmouth and Dolgellau during the 19th C. Note that the oldest rocks are in the centre of the dome and the youngest rocks (that are stratigraphically above the older rocks) outcrop on the periphery of the dome. The Welsh Basin existed on the eastern margins of the Iapetus Ocean hence marine conditions existed over much of present day Wales. Turbidite mudstones were deposited on the continental slopes of the subsiding Welsh Basin, as seen in the Harlech Dome, whilst in North Pembrokeshire, in the shallow shelf seas on the margins of the basin, conglomerates and flaggy sandstones were laid down such as the Lingula Flags at Solva and the purple Caerbwdy Sandstone used in the building of St David’s Cathedral. Also mudstones rich in the fossilised remains of trilobites and brachiopods give an indication of the abundance of marine invertebrate life that developed during the Cambrian. The well-known trilobite Paradoxides davidis has been found in the Middle Cambrian shales at Porth y Rhaw near Nine Wells. The best specimens of this large trilobite are displayed in the National Museum of Wales.
An inlier of Cambrian rocks occurs along the line of the Church Stretton fault in Shropshire. Here the basal Cambrian sandstone is known as the Wrekin Quartzite, a pure white sandstone that has a conglomerate at its base. This contains beach pebbles derived from the local Precambrian Uriconian volcanic hills which must have stood as islands in the Cambrian seas. Ripple bedding within the quartzite is indicative of shallow marine conditions on the margins of the Welsh Basin. The Cambrian sequence in Shropshire is completed with a greenish sandstone rich in glauconite and some thin limestones which have yielded some of the earliest trilobite fossils in Britain. Callavia, found in the Comley Quarry near Church Stretton, belongs to a separate faunal province from those trilobites of similar age found in NW Scotland which lived on the opposite side of the Iapetus Ocean.
In the North West Highlands of Scotland the Cambrian sea laid down the white Eriboll Quartzite unconformably on the eroded Torridonian Sandstone surface. The quartzite can be seen in the road cutting at Skiag Bridge where there is also the distinctive ‘pipe rock’ Here the beds are full of vertical tubes made by burrowing worms (Skolithus), one of the earliest forms of life in the Cambrian. The trilobite Olenellus lapworthi occurs in the sands and siltstones above the Pipe Rock and this fossil has affinities with North American trilobites because of Scotland’s position in Laurentia during Cambrian times. The overlying Durness Limestone is a shallow water deposit containing beds of dolomite (CaMg (CO3)2. The formation of dolomite is due to the later alteration of limestone by magnesium rich groundwater. The limestone is best seen around the cave of Smoo in the cliffs below Durness village.
THE CAMBRIAN EXPLOSION OF LIFE
The theory of the Cambrian explosion postulates that some 545 Ma ago an explosion of diversity led to the appearance of large numbers of multi celled organisms within a relatively short period of time (5-10 million years). Most of the major animal groups alive today derive from the Cambrian explosion. Natural selection favoured larger size and the need for hard skeletons to provide structural support; hence the trilobites developed an exoskeleton and shelled creatures such as brachiopods developed. But was the Cambrian explosion as sudden and spontaneous as it appears in the fossil record? Although very few fossil remains have been found in the Precambrian, recent research suggests that organisms were evolving continuously at the molecular level during the Precambrian.
The first indication of a possible explosion of life in the Cambrian came in 1909 when Charles Doolittle Walcott (the original Dr Doolittle) discovered the Burgess Shale in the Rocky Mountains of British Columbia. He unearthed a wide variety of arthropods (trilobites, crustaceans, etc) which were perfectly preserved including soft body parts. The organisms had apparently suffered catastrophic burial being overwhelmed by mudslides at the base of a massive escarpment and buried instantaneously as they sank into the adjacent deep sea basin. The fossils occur as black carbon films on shale and are thought to have been preserved in the stagnant muds where the anoxic environment prevented decay through lack of oxygen. Walcott was head of the Smithsonian Institute in Washington where thousands of his specimens are housed. He was a traditionalist who believed that all the Burgess shale fossils could be assigned to modern day phyla. Since he was a highly respected palaeontologist no one challenged his views at the time. However, in recent decades Walcott’s specimens have been re-examined and re-interpreted by researchers and many of the organisms such as the 5 eyed Opabinia and the strange Hallucigenia for example, simply do not belong to any known group of living creatures. In the 1970s Stephen Jay Gould published his book ‘Wonderful Life’ in which he put forward the theory of punctuated equilibrium which views evolution as long periods of statis punctuated by short bursts of rapid evolution as in the Cambrian explosion. The story of the last 500 years is one of restriction and extinction followed by periods of intense proliferation of a few favoured groups NOT general expansion in range and complexity as implied by the cone of increasing diversity (the tree of life).
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