Geology Group Diary (21)

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    The Geology Group met at Merlin's Bridge Community Centre at 10.30 am on Wednesday 14 June 2017. The topic considered was:


    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 (diorite) that extends E-W through Johnston to the southern side of St. Bride’s Bay. The diorite is well exposed in Bolton Hill Quarry near Tiers Cross where it provides an excellent roadstone. In the St. David’s peninsula, the volcanic sequence ( lavas and tuffs)  is somewhat younger, around 610-575 Ma and this is intruded by the St. David’s granophyre which forms the rugged coast around Carreg Fran. Roch Castle, Plumstead Rocks, Poll Carn and Maiden Castle are outcrops of brecciated rhyolite lava that stand as isolated tors representing the remnants of explosive volcanic outbursts during  late Precambrian times.

    During Cambrian times [543-490 Ma] marine conditions existed over much of 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.

    By the Ordovician period [490-443 Ma] igneous activity was widespread across Wales as shown by the volcanic rocks of Rhobell Fawr and Cadair Idris in North Wales and the spectacular coastal exposures of the Strumble Head pillow lavas in north Pembrokeshire. These lavas were erupted on to the sea floor as shown by their outer skins that formed by quenching under water. The ‘spotted’ dolerites of Mynydd Preseli as seen on Carn Menyn, were probably the source of some of the Stonehenge bluestones, and the massive layered gabbros of St. David’s Head are examples of igneous intrusions. Many early Ordovician volcanoes were associated with island arc development on the edge of the Welsh Basin and vast quantities of pyroclastic ash and volcanic debris were deposited in the surrounding seas. At the same time, marine sedimentation was continuing to lay down thick muds that became sandwiched between ash layers on the sea bed. The mudstones contain the remains of marine organisms including the well-known ‘tuning fork’ graptolite Didymograptus murchisoni that is found in the Caerhys shales at Abereiddi.

    In central Wales – around Aberystwyth, for example – there are great thicknesses of turbidite sandstones and graptolitic shales that were deposited within the deep waters of the Welsh Basin during the early part of the Silurian [443-418 Ma]. However, Pembrokeshire was located on the southern margins of the basin, and here the Skomer Volcanic Group represents the products of volcanism that occurred in shallow shelf seas. Some of the best exposures of lava flows can be seen at Martin’s Haven, Marloes Sands and Skomer Island. The lavas are often interbedded with sandstones and calcareous mudstones that contain a variety of brachiopods and corals. This classic Silurian sequence is well-exposed at Marloes Sands where in the 1960s pioneering research was carried out on brachiopod communities to demonstrate how they could be used as marine depth indicators. Lingula communities generally occupy shallow littoral environments; Eocoelia communities are found in water about 20 metres deep and Costistricklandia communities inhabit depths of up to 50 metres off shore. Thus the changes in brachiopod species within the strata outcropping between the Three Chimneys and about the first 20 metres of the Coralliferous Group suggest marine transgression with deepening water conditions.

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