Geology Group Diary (36)

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    The Geology group met at 10.30am at Merlin's Bridge village hall on Wednesday 14 November 2018. The monthly topic was TERTIARY GEOLOGY IN BRITAIN.

    The Tertiary period began around 65 Ma and continued up to 2 Ma.  During the break up of the super continent Pangaea, the North American plate and the Eurasian plate rifted apart to form the North Atlantic Ocean. In Palaeocene times (65-56 Ma) Britain was situated above a mantle plume (sometimes referred to as a hot spot) where the earth’s crust was under stress from plate rifting. As a result fissures opened up along a line from Ireland to the Hebrides from which basaltic lavas were extruded and igneous centres developed, surrounded by ring dykes and cone sheets and radiating dyke swarms. A vast lava field (1.8 million km2) called the Thulean Plateau extended from West Scotland through the Faroes, to Iceland and Greenland.
    The Plateau Basalts
    Up to 2000 metres thickness of basalt lavas were erupted in Antrim, Mull and Skye during the Palaeocene. Even today after extensive erosion, the lavas are 1800 metres thick on Mull. The individual lava flows give rise to stepped topography which is caused by the weathering back of the softer vesicular slag at the top of each flow. Between the lava flows there are often found red lateritic soils. These iron rich deposits contain plant remains, particularly in the leaf beds of SW Mull where a temperate flora of ginkgo, plane, hazel and oak occur. The leaves are well preserved and are thought to have fallen into a shallow lake. In 1819 a magnificent specimen of a tree trunk was discovered embedded in lavas in the cliffs of western Mull; it is known as MacCulloch’s tree!
    The basalt lavas of the Giant’s Causeway and the Isle of Staffa are world famous for their hexagonal columnar jointing. The columns result from the cooling and contraction of the lavas under perfectly stable conditions and they form perpendicular to the base and surface of the flow. The top of the lava flow would be filled with escaping gases which produced a vesicular slaggy texture. On the island of Eigg the lava cooled so quickly that it produced a glassy rock called pitchstone (similar to obsidian) that forms the Sgurr of Eigg.
    The Igneous Complexes
    There are several major plutonic centres extending northwards from the Mourne Mts in Northern Ireland through the Western Isles of Arran, Mull, Ardnamurchan, Rum and Skye. These centres represent the remains of Tertiary volcanoes and their underlying magma chambers.
    On Skye the magma was emplaced in a sequence of Precambrian rocks (Lewisian gneiss and Torridonian sandstone overlain by basalt lavas. The magma was rich in mafic minerals (plagioclase, pyroxene and olivine). Layered gabbros were produced by the settling out of denser minerals and basaltic volcanoes erupted at the surface. Today we see the Cuillin Hills formed from the deeply eroded gabbronic magma chamber. However, the nearby Red Hills are formed of granitic magma produced within the same igneous complex. The granites were probably formed by the melting of the Precambrian basement rocks. In the Cuillins many minor intrusions occur as cone sheets rising out of the volcanic conduit and sloping outwards in a circle around the volcanic centre.
    The island of Mull is almost entirely formed of an extinct volcano and its eroded magma chamber with three major eruptive centres within the complex. Numerous cone sheets form a distinctive arcuate pattern around central Mull. However, the Mull and Ardnamurchan volcanic centres are also characterised by the presence of ring dykes. These are cylindrical intrusions produced by magma being forced up through ring faults surrounding a central area of subsidence. As magma is removed from the magma chamber the centre of the complex sinks creating a caldera of subsidence. Think of a full wine bottle as a magma chamber and the cork representing the overlying rocks. Force the cork downwards and wine will be forced upwards around the cork creating a ‘ring dyke’.
    Dyke swarms occur throughout the Tertiary volcanic province and they all demonstrate a NW-SE trend. They are thought to be associated with shearing stresses produced by the opening of the North Atlantic during the early Palaeocene. Some 300 dykes can be seen on the south coast of Arran and these represent a7% crustal extension in a 20 km section. Perhaps the most famous Tertiary dyke is the Cleveland dyke in NE England. This can be traced for 400 kms from Mull to North Yorkshire and is thought to have been emplaced as a single pulse within a few days!
    Tertiary sediments in Southern England
    Whilst volcanoes were erupting in the north west of Britain, sedimentation was taking place in southern England. During the Palaeocene and Eocene epochs the London and Hampshire basins were subject to alternate marine transgression and regression.
    London Basin. Initially the sea advanced from the east over the eroded chalk surface and laid down the Thanet Sands which are best exposed along the North Kent coast at Herne Bay.  Later, as the sea regressed the Reading Beds were deposited by meandering rivers flowing eastwards across mudflats . These continental type beds consist mainly of sands and clays but locally where silica cementation occurs, the sands form sarcen stones. These massive stones are often found on the chalk surface after the unconsolidated Reading beds have been removed by erosion. Sarcens can be seen on the Marlborough Downs on the western margin of the London Basin and they were used to build Stonehenge and other prehistoric monuments. Cemented pebble beds also occur such as the Hertfordshire Puddingstone; a superb example of a natural concrete! The succeeding cycle of sedimentation begins with a marine transgression that produced the London Clay up to 150 metres in thickness.  There are beds of calcareous concretions including septarian nodules at various levels within the London Clay. Numerous bivalves, brachiopods and gastropods provide evidence of the shallow marine environment. Sharks teeth are commonly found as fossils. .However, over 500 plant species have been recorded including mangroves, palms, laurel and magnolia which suggests a tropical climate existed in Eocene times. The plants and seeds were probably brought down by rivers and washed out to sea to be preserved in the silty marine sediments. A return to continental type conditions occurred as the sea shallowed and regressed and the Bagshot Sands were deposited in an estuarine environment. These beds are well developed in the western part of the London Basin giving rise to heathlands much used by the military around Aldershot and Bagshot.
    In the Hampshire Basin (which includes the IOW) several cycles of sedimentation can be distinguished in the Hampshire Basin which roughly correspond to those in the London basin.  One of the best places to examine the Palaeogene succession is Alum Bay on the NW side of the IOW. Here the cliffs are formed mainly of multicoloured sands and clays (Bracklesham Beds) that are stained red, brown and green by iron compounds derived from pyrite (FeS2) which is oxidised above high tide level. There are also brown coloured beds of lignite and several plant beds providing evidence of a subtropical climate. The overlying Barton Beds are best examined on the Hampshire coast at Barton-on-Sea, east of Christchurch. Here the Barton Clay yields numerous bivalves (eg.Cardita) and gastropods (eg.Turritella; Volutospina) and sharks’teeth.
    Much of the eastern part of East Anglia contains marine shelly sands known as Crag deposits. The Coralline Crag is of Pliocene age has an abundance of fossil bivalves, gastropods and brachiopods. The overlying Red Crag, Norwich Crag and Weybourne Crag form the lower part of the Pleistocene sequence. The molluscan fauna indicate a gradual climatic cooling from subtropical species in the Coralline Crag to boreal species in the Weybourne Crag, heralding the onset of the Pleistocene Ice Age.
    The Alpine Orogeny.
    The Alpine fold mountains were formed when Africa collided with Eurasia and the Tethys ocean crust was subducted northwards beneath the Eurasian plate. The Pyrenees, Alps, Apennines, Carpathians and Caucasus ranges were all formed by the Alpine orogeny, although some like the Pyrenees originated as Variscan structures. These earth movements occurred throughout the Tertiary but reached a peak during the Miocene when the Mediterranean was formed as a large saline basin on the site of the vanished Tethys Ocean. In southern Britain the ripple effect of Alpine crustal movements produced structures such as the Wealden anticline, the London and Hampshire synclinal basins and the Purbeck and IOW monocline.

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