Geology group Diary (28)

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    The Geology Group met at 10.30 am on 14 February 2018 at Merlin's bridge Village Hall. 19 members present. The topic this month was BRITISH ORDOVICIAN GEOLOGY.
    The Ordovician Period (510-439 Ma) was defined by Prof.Charles Lapworth  to resolve a dispute between Adam  Sedgwick (Cambridge Univ.) and Roderick Murchison (Geological Survey). During the 1830s Sedgwick was surveying Cambrian outcrops in North Wales, whilst Murchison was working on the Silurian in the Welsh Borders. The boundary between the two groups of rocks was ill defined and at the time produced great controversy in scientific circles. Finally, in 1879 Lapworth was able to show that there was a distinct faunal content in the strata between the Cambrian and Silurian sequences, hence he defined his new Ordovician system on reliable palaeontological grounds.
    During Ordovician times the Iapetus Ocean was closing as the ocean crust was subducted beneath Laurentia (N.America+Scotland) and Avalonia (England&Wales) At the same time the Rheic Ocean between Avalonia and Gondwana was opening. Above the subduction zones island arc vulcanicity was taking place erupting vast quantities of lava and ash in the Lake District and North Wales.
    North Wales The Snowdon mountain range is composed largely of Ordovician mudstones and volcanic rocks. In early Ordovician  times (Tremadoc age) marine conditions  existed and great thicknesses of muds were deposited in a geosynclinal basin. These muds were later converted by regional metamorphism into the Tremadoc slates. It is interesting to note that many of the trilobites found in these slates have been distorted by pressure during metamorphism. Island arc volcanoes erupted throughout Arenig and Llanvirn times including Rhobell Fawr and Cadair Idris. The most intensive period of volcanic activity produced some highly explosive eruptions that ejected masses of felsic pyroclastic materials and resulted in the creation of a huge caldera of subsidence in Central Snowdonia. The pyroclastics are of two main types: Airfall materials represent the products of nuée ardente eruptions where a glowing volcanic cloud deposited fine ash into the surrounding sea where it was reworked as a sediment known as air fall tuff. By contrast, ash flow materials roll down the volcanic slopes as incandescent avalanches of gas, steam and lava. They became welded together to form ash flow tuffs or ignimbrites. These often contain glass shards and flattened pumice fragments known as fiamme.
    South West Wales. The submarine pillow lavas of Strumble Head provide evidence of island arc vulcanicity on the margins of the Welsh Basin where marine sedimentation produced hundreds of metres of mudstone and volcanic ash bands. The mudstones contain the remains of marine organisms including the well-known ‘tuning fork’ graptolite Didymograptus murchisoni that is found in the Caerhys shales (Llanvirn stage) at Abereiddi. Inland on the Mynydd Preseli, the Ordovician shales are cut by numerous dolerite intrusions that create rugged tors from which the bluestones of Stonehenge are derived.
    Shropshire. The Shelve inlier of Ordovician rocks lies immediately west of the Longmynd and the Pontesford- Linley Fault. The oldest formation is the Stiperstones Quartzite that dips steeply westwards and forms a spectacular ridge of frost shattered tors developed under periglacial conditions at the end of Pleistocene times. The quartzite is a shallow water deposit that is an exceptionally pure white sandstone with a conglomeritic base. The succeeding Mytton Flags are over 900 metres thick; they are hard, blue-grey flaggy siltstones that display conspicuous rectilinear jointing. These joints formed as the sediments dried out. Geologists consider that during late Devonian times a deep seated granite pluton was the source of hydrothermal fluids that were injected into the joints of the Mytton Flags precipitating minerals such as galena (PbS), sphalerite (ZnS) and barytes (BaSO4). Snailbeach mine was a major producer of lead ore from1845 to 1913.
    Lake District. The oldest strata in the central Lake District belong to the Skiddaw Group of Lower Ordovician age. The Skiddaw slates, are a sequence of turbidites that have been deformed, folded and cleaved during the Caledonian orogeny.  The graptolitic fauna of these rocks also clearly indicates the deep water marine conditions which existed at this time. However, as the Iapetus Ocean began to close, subduction was taking place below the Solway trench and magmas were generated in the subducted oceanic crust. These magmas fed the granitic intrusions which in turn produced the Eycott and Borrowdale volcanoes that built up some 4000 metres of andesitic lavas, tuffs and agglomerates belonging to the Borrowdale Volcanic Group of Caradoc age. Such volcanic rocks are resistant to erosion and form the highest and most rugged mountain scenery in the Lake District.

    South West Scotland is geologically important since it lies on the northern side of the subduction zone below what is now the Solway Firth. As the Iapetus ocean crust was forced down beneath the Laurentia (Scotland) an accretionary prism developed along the leading edge of the continent. This is a sequence of major rock slices cut by reverse faults that can be seen in the steeply dipping Ordovician and Silurian graptolitic shales and greywackes of the Southern Uplands. The strata are also isoclinally folded with the dip consistently to the NW so that it was only through careful mapping that Lapworth first identified the fold structure using graptolites to show that a single shale band is repeated many times by folding. The gorge of Dob’s Linn near Moffat is the classic locality where Lapworth worked to unravel the structural geology of the region. At Ballantrae on the coast of Ayrshire, a fragment of  oceanic crust (ophiolite complex) appears above the subduction zone. It is a melange of igneous rocks including gabbro, agglomerate, basaltic pillow lava and serpentinite; typical constituents of dense mafic ocean crust which was pushed up during late Ordovician earth movements.

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