Geology Group Diary (29)

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    The geology group met at 10.30 am on Wednesday 14 March 2018 at merlin's Bridge Community hall. 18 members present. This month's topic was BRITISH SILURIAN GEOLOGY.

    During the Silurian Period (439-409 Ma) Avalonia and Baltica moved towards Laurentia as the Iapetus Ocean closed. Then by the end of Silurian times continental collision occurred as the ocean crust was finally subducted below the Iapetus suture. Scotland became joined to southern Britain which had moved from around 60°S to 20°S since the early Ordovician. A movement of 4500 kilometres over 100 Ma represents a rate of about 4.5 cm per year. However, the most important consequence of the continental collision was to initiate the Caledonian Orogeny. This mountain building movement resulted in the upheaval of a zone of fold mountains stretching from New England through the Scottish Highlands to Greenland and Scandinavia.

    The base of the Silurian was defined by Lapworth in the Southern Uplands at Dob’s Linn where the graptolitic shales of the Upper  Ordovician (persculptus zone) pass conformably into the of the basal Silurian shales (acuminatus zone). Why is an unconformity not a good place to draw the boundary between two systems? Answer: erosion has removed strata from beneath the unconformity. The top of the Silurian can be recognized most clearly in the Czech Republic around Pridoli where there is a conformable sequence of marine graptolitic shales extending up into the Devonian which begins at the base of the Monograptus uniformis zone. In Britain marine conditions in the Silurian were terminated by the development of the continental deposits known as the Old Red Sandstone. This change to terrestrial conditions took place in the mid Silurian (430 Ma) in Scotland but occurred later in Wales and SW England.  At Red Cliff near Marloes Sands the transition from marine Gray Sandstone Group to continental Red Cliff Formation took place around 424 Ma (beginning of the Ludlow Epoch). The base of the ORS is therefore diachronous, meaning that it was deposited at different times in different areas.

    The Welsh Basin that was well established during the Ordovician, continued to exist up to late Silurian times. By studying the fossil content of the sediments laid down it can be shown that the sea advanced eastwards over the Midland Platform to the edge of the Malverns. Shallow water brachiopods such as Lingula and Eocoelia are found in the shelf sediments whereas in the west around Welshpool water depths of up to 100 m are indicated by the presence of Stricklandia and Clorina. Further west around Aberystwyth (Rheidol Gorge) graptolites are present in mudstones suggesting deep water basin conditions.
    Pembrokeshire was located on the southern margins of the Welsh Basin, and here the Skomer Volcanic Group represents the products of volcanism that occurred in shallow shelf seas during the early Silurian. 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 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.
    In Shropshire the Church Stretton Fault follows the edge of the Midland Platform formed of basement Precambrian rocks across which the shallow shelf sea encroached in mid Silurian times (Wenlock & Ludlow epochs). Here a shelly facies of thinly bedded shales and limestones was deposited and these can be seen around Wenlock Edge. This is an area of classic scarp and vale topography where resistant limestone alternates with softer shale. You will see on the cross section that the Wenlock Limestone forms a scarp overlooking Ape Dale (Wenlock Shale) and the Aymestry Limestone forms a scarp overlooking Hope Dale (Lower Ludlow Shale). The limestone is full of coral reefs that were formed in warm shallow seas around 25°S. The bedded limestone wraps round the reefs which are mainly built of corals, sponges and bryozoans, but there are also numerous brachiopods, trilobites and crinoids in the limestone. Some of the well known Silurian fossils include the trilobites Calymene (Dudley bug) and Dalmanites, the colonial chain coral Halysites and the large brachiopod Conchidium.

    Silurian strata extends across the Southern Uplands in a belt trending ENE-WSW from the Berwickshire coast (Siccar Point) to Galloway; roughly parallel to the Ordovician outcrop on its northern flank. This is the area made famous by Charles Lapworth (1878) when he used graptolites to demonstrate the structure of the Southern Uplands. He showed that graptolite zone fossils could be used to divide up the shales and greywackes into relatively small units and then deduce that the rocks must be isoclinally folded,  proving that a single condensed graptolitic band was repeated by intense folding  (and reverse faulting). Previously it had been thought that there were numerous shale bands all dipping NW, suggesting that the shale/greywacke sequence was hundreds of metres thick. The Ettrick Valley Fault is a major reverse fault within the accretionary prism of the southern uplands. Remember that these faults relate to the subducting of the oceanic plate beneath the Solway Firth as the Iapetus Ocean finally closed.

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