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July 13, 2017 at 5:12 pm #9197Anonymous
The Geology Group met on Wednesday 12 July 2017 at 10.30am at Merlin's Bridge Community Hall. The topic was 'THE GEOLOGY OF PEMBROKESHIRE (2)
The conformable junction between the Gray Sandstone Group and the basal beds of the Old Red Sandstone can be seen just south of the sea stacks at Red Cliff, Marloes Sands [SM790068]. This locality is highly significant since the change in lithology marks a major environmental change from marine to terrestrial conditions of deposition. The palaeogeography of south-west Wales changed from being a shelf sea environment to one where alluvial plains were crossed by meandering rivers in an arid climate. The carbonate evaporite deposits known as calcretes, found in the red mudstones on Caldey Island and at Freshwater West, suggest an environment of high evaporation and seasonal rainfall. Thick conglomerates and breccias also indicate a rapid deposition by torrential streams under flash flood conditions. The Ridgeway Conglomerate at Sawdern Point in Angle Bay is a good example of this type of deposit. The occurrence of several bands of airfall tuffs in the early Old Red Sandstone of Pembrokeshire provide important marker horizons that can be traced across the Anglo Welsh basin. The Rook’s cave tuff is marked by a deep trench on the headland at Manorbier and the Townsend Tuff is well exposed on Little Castle Head to the west of Milford Haven. It is worth noting that the term, Old Red Sandstone, refers to the overall lithology and is not a geological period. In fact the ORS was laid down in a continental environment that began to be deposited in late Silurian times and continued throughout the Devonian period [418-354 Ma].
By the Lower Carboniferous [354-327 Ma] South Wales was covered by a shallow shelf sea with a tropical climate capable of supporting an abundant and varied fauna including corals, crinoids, and brachiopods. The thick marine limestones of South Pembrokeshire form majestic cliffs and the rocks often show features of karstic weathering such as limestone pavements and swallow holes. Collapsed caverns and fault zones sometimes contain gash breccia deposits formed of orange coloured sands and clays containing limestone clasts. The gash breccias are clearly seen in Flimston Bay and Bullslaughter Bay. The Carboniferous Limestone is well exposed along the coast from St Govan’s to the Green Bridge of Wales. Here there are sea stacks (Elegug Rocks), natural arches and deep fault guided inlets known as Geos ( Huntsman’s Leap). The Variscan earth movements are responsible for folds and faults within the limestone; for example, the Stackpole syncline, the vertical ‘organ pipes’ around Lydstep Hole and the Flimston wrench fault.
During the Upper Carboniferous [325-290 Ma] the seas were invaded by vast deltas as rivers advanced southwards from the Wales–Brabant landmass. Thick sandstones interbedded with shales were laid down in repeated cycles of sedimentation. Where swamp conditions prevailed on the delta plains, tropical forests provided the raw material for coal seams. The Pembrokeshire coalfield with its E–W axial trend is a westerly extension of the main south Wales coalfield. At Ragwen Point to the east of Amroth the Carboniferous Limestone is overlain by Marros Beds of Namurian age consisting of thick cross stratified deltaic sandstones. The upper part of the sequence known as the Telpyn Point sandstone passes upwards into the Lower Coal Measures that are well exposed around Amroth. Excellent sections through deltaic distributary channels can be seen in the cliffs near to the inn at Wiseman’s Bridge and further west at Saundersfoot is the famous Ladies’ anticline; a Variscan structure within the coal measures. In Waterwynch Bay near Tenby there are some splendid examples of soft sediment deformation within the coal measures. Here fractured sandstone blocks appear to have slumped down a depositional slope into liquefied sediment formed under high water table conditions. On the west side of the coalfield at Broad Haven there are several notable Variscan structures such as the much photographed thrust tip fold at Dens Door, Broad Haven. At Rickets Head multiple cyclothems can be observed illustrating the repeated deposition of a sequence of mudstones, cross stratified sandstones and coal seams. Each cyclothem represents a succession of sediments laid down as the delta builds out or progrades seawards. The delta plain would have been covered by swamp forests and crossed by numerous distributary channels. Eventually the delta plain would become flooded as sea level rose and the next cycle of sedimentation would begin as the sea deposited mud and sand that buried the vegetation which would slowly become compressed into coal.
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