Geology Group Diary (26)

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    The Geology Group met at 10.30 am on Wednesday 13 December 2017 at Merlin's Bridge Village Hall. This month's topic was 'BRITISH PRECAMBRIAN GEOLOGY''.
    Drifting Continents. During the late Precambrian, approximately 700–550 million years ago, the northern regions of the British Isles (Scotland and Northern Ireland) together with Greenland and North America were located on the passive margin of the continent of Laurentia. The southern part of Britain lay on the margins of the sub continent of Avalonia, which was initially part of the super continent of Gondwana.  Avalonia was separated from Laurentia by the Iapetus Ocean that was actively opening and was probably several thousand kilometres wide. However, by the end of the Cambrian Period (510 Ma) the ocean began to close and by early Ordovician times the southern margin of the ocean floor began to be subducted below Avalonia, triggering considerable island arc volcanicity. The Iapetus Ocean continued to close until by the end of the Silurian period (418 Ma) the continents of Laurentia and Avalonia (together with Baltica) collided and the two parts of Britain were united along the Iapetus suture line. The result of this collision was the formation of the Caledonian mountains as the ocean sediments were compressed, uplifted and folded. From the late Precambrian to the end of the Silurian Period, Avalonia had moved from around 60º S of the equator to 20º S; a testimony to the power of plate movements.
    NW HIGHLANDS.  This region of Scotland has some of the most spectacular geology and scenery in the whole of the British Isles. It also contains some of Britain’s oldest rocks; the Lewisian gneisses that have been radiometrically dated to around 3300 Ma. This compares with 4000 Ma for the oldest Precambrian rocks in the Canadian Shield. Two geological surveyors, Ben Peach and John Horne, conducted detailed mapping in the NW Highlands in the late 19th century that first revealed the large scale thrust structure of the rocks that were emplaced on the Precambrian foreland.
    Lewisian Gneiss. The highly deformed and metamorphosed rocks are exposed along the coast from the SE corner of Skye to Cape Wrath and in the Outer Hebridean islands of Lewis and Harris. It is considered that these rocks were originally a sequence of magmatic intrusions and that sometime after 3000 Ma they were subjected to intense heat and pressure and converted into coarsely banded gneiss rich in quartz, feldspar and mica. Later around 2200 Ma the rocks were intruded by a series of mafic dykes that are particularly well exposed near the village of Scourie. About 2 kms north of Laxford Bridge on the A838 there are road cuttings where the Lewisian gneiss is cut by sheets of pink pegmatite that pinch and swell indicating that the gneiss was hot and semi molten at the time of the intrusion. Torridonian Sandstone. The gneisses were uplifted and folded over millions of years and by 1000 Ma a deeply eroded Lewisian landscape had emerged. Then in the late Precambrian the Torridonian sandstone was deposited, resting with a marked unconformity on the eroded Lewisian rocks. The coarse grained red aeolian sandstones are horizontally bedded and have gradually been stripped off  much of the Lewisian basement leaving an exhumed topography with isolated Torridonian mountains such as  Canisp, Suilven, Quinag and Stac Pollaidh. The ice scoured surface of the Lewisian gneiss in coastal Sutherland produces a ‘knock and lochan’ topography (Cnoc is gaelic for small hill) where numerous small lakes occupy glacial hollows that are interspersed with ice moulded hillocks and erratics of Torridonian sandstone.
    Cambrian Sediments. Around 544 Ma the region was inundated by the Cambrian sea which 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 Durness Limestone overlies the quartzites. It is interesting to note that the trilobite Olenellus lapworthi occurs in the sands and siltstones above the Pipe Rock and this fossil has affinities with North American trilobites. The plate tectonic evidence suggests that NW Scotland was part of Laurentia (NE North America) in Cambrian times and was separated from the rest of Britain by the Iapetus Ocean.
    The Moine Thrust Zone. The coastal region of NW Scotland is separated geologically from the rest of the Highlands by a zone of low angle faults known as the Moine Thrust Zone which was formed during the Caledonian orogeny in late Ordovician times. However, in the 19th C geologists were not aware of this structure and so the rock sequence caused major problems because it appeared that the unaltered Cambrian sediments passed conformably upwards into high grade schists.  The Law of Superposition indicates that the schists must be the youngest rocks in the sequence, but it was hard to explain how unaltered Cambrian sandstones and limestones could be overlain by highly metamorphosed schists. There was much controversy as to how this sequence could have formed. In 1883 Archibald Geikie, Director of the Geological Survey sent two experienced surveyors (on horseback) to this wild remote area of Scotland to carefully map the rock outcrops. As a result Peach was able to demonstrate that the eastern Moine schists (late Precambrian in age) had been thrust westwards over the rigid foreland block of Lewisian gneiss, Torridonian sandstone and Cambrian sediments by a series of large scale low angle thrust faults.(Moine, Glencoul and Sole thrusts). The thrust planes are separated by smaller imbricate faults and duplex structures. Thus we see how the older schists were moved many miles by major thrusts and placed on top of younger Cambrian rocks! A visit to the nature reserve at Knockan Cliff shows the position of the Moine Thrust which is marked there by a fine grained flinty rock called mylonite, produced by rock grinding along the thrust plane.
    CHARNWOOD FOREST.  The craggy hills of Charnwood to the north west of Leicester are formed of a late Precambrian inlier composed of volcanics, conglomerates, sandstones and slates. In the 1950s a schoolboy discovered one of the earliest known fossils in layers of volcanic ash dated c.560 Ma. Charnia masoni is a sea pen almost identical to those found in the Edicarna fauna of Australia.
    MALVERNS. These hills form a N-S ridge of resistant metamorphosed igneous rocks that separate the Triassic lowlands of Worcester to the east from the Vale of Severn to the west. Gullet Quarry exposes the highly deformed diorites which are in turn cut by numerous dykes and mineral veins. There is an unconformable junction between the late Precambrian Malvernian metamorphic igneous complex and the overlying Silurian sediments
    SHROPSHIRE. The oldest rocks in Shropshire are of late Precambrian age. The Uriconian volcanics (566 Ma) form a line of hills extending from the Wrekin south westwards to Caer Caradoc.  These volcanic andesitic lavas were formed in an island arc complex (similar to the Caribbean islands today) above a subduction zone where oceanic crust was forced down at a plate margin. Also rhyolitic ash flows were produced by explosive eruptions from volcanoes lying along the line of the present Church Stretton Fault. Adjacent to the Uriconian volcanics are the Precambrian Longmyndian sediments comprising up to 8000 metres of sandstones and shales forming a deep syncline with an inverted western limb. The higher parts of the sequence contain clasts of Uriconian lavas so these sediments must have been deposited in the shallow seas surrounding the volcanic islands. There is a major unconformity between the eastern (older) and western (younger) Longmyndian sediments followed by a period of intense folding and faulting during the late Precambrian. A good locality on the eastern side of the Longmynd is the Carding Mill Valley near Church Stretton where you can examine the steeply dipping mudstones and sandstones that contain various sedimentary structures including ripple bedding, graded bedding and also bands of volcanic ash. These were deposited under shallow marine conditions where volcanic ash would be erupted from time to time during the later phases of Uriconian vulcanicity. Some of the shales contain fossilised rain spots which were probably preserved as the mud flats dried out along the sea shore.
    The Ercall is a small hill on the north east side of the Wrekin. It is significant in that the unconformable junction between the Precambrian igneous rocks and the basal Cambrian sediments is well exposed in the old quarries.  About 560 Ma the Uriconian lavas of the Ercall were intruded by a large mass of granophyre; this is a type of pink granite formed mainly of quartz and orthoclase feldspar. However, banked up against the granophyre are the basal Cambrian sediments known as the Wrekin Quartzite. Since the Cambrian period began around 545 Ma the unconformity represents a time gap of at least some 15 million years. Quartzite is the term used for a metamorphosed sandstone but here the Wrekin quartzite is simply a quartz rich sandstone with a conglomerate at its base. It shows extensive ripple bedding indicative of shallow marine conditions and many of the pebbles in the basal conglomerate are of locally derived rhyolite, tuff and granophyre. The Cambrian shoreline must have surrounded the Precambrian volcanic island arc
    ANGLESEY.  The oldest rocks in North West Wales are found in Anglesey and the Lleyn peninsula. These rocks of late Precambrian age are known as the Monian Group (at least 615 Ma) and they are well displayed at South Stack and Rhoscolyn on Holy Island off the west coast of Anglesey. The Monian Group contains turbidite sandstones, volcanic ash and pillow lavas and a remarkable unit called the Gwna melange that is made up of a chaotic mixture of blocks and boulders some of which are several hundred metres across. This represents an enormous submarine slide down the continental slope known as an olistostrome. Pillow lavas are well exposed at Newborough on the south east side of Anglesey along the Menai Strait. Lava flows of this type represent submarine extrusions of basaltic lava that formed large globules or ‘pillows’ as they react with sea water. Much of the Monian sequence has been strongly metamorphosed into schist and gneiss and intensely folded during orogenesis at the end of Precambrian times when a subduction zone existed in NW Wales.
    ST.DAVID’S PENINSULA Using radiometric dating methods the oldest rocks in Pembrokeshire have been dated approximately 643 Ma in the late Precambrian; these occur as a faulted slice of an igneous intrusion that extends E-W through Johnston to the southern side of St. Bride’s Bay. In the St. David’s peninsula, the volcanic sequence is somewhat younger, around 610-575 Ma and this is intruded by the St. David’s granophyre. Roch Castle, Plumstead Rocks and Maiden Castle are outcrops of rhyolite lava that has long been considered to be of late Precambrian age, but now it is assigned by the BGS to the Lower Ordovician.

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