Features in the field: ignimbrites of the Yr Arddu syncline (Caradoc, Upper Ordovician)

In July 1979, I was one of more than 20 undergraduate students at the Department of Geology, University of Manchester, to undertake their final year mapping project in the Snowdonia National Park in North Wales. My mapping area was the Yr Arddu Syncline, about 4km southeast of Beddgelert in Gwynedd. The rock succession is comprised of slates and sandstones, overlain by acid volcanic rocks, with a range of intrusions (mainly acidic), such as microgranite, but also including dolerite. A feature of this succession was the range of features beautifully exposed in the volcanics and intrusions (Figs. 1 to 4).

I took a detailed suite of photographs of these features and this article is presented as a photographic field guide to them. A short glossary is provided at the end, which deals with some of the more specialist terminology, each of which appears in bold when first mentioned in the text. For an up-to-date introduction to volcanic rocks and igneous processes, see Lopes (2010). In the field, use the 1:50,000 Landranger Map 115 (‘Snowdon/Yr Wyddfa’) or a larger scale sheet. The brief papers of Rast et al. (1958) and Fitch (1971), although long-in-the-tooth, are still entertaining introductions to the acidic volcanism of North Wales. Modern treatments include Brenchley (1992), Howells & Smith (1997) and Rushton & Howells (1998).

Geology

The Yr Arddu region lies within the Snowdon region of northwest Wales, which has a broad northeast-to-southwest structural trend. Mount Snowdon lies at the centre. The rocks of the Harlech Dome outcrop to the south; and to the north and northwest, respectively, are Anglesey and the Lleyn Peninsula. The rocks of central Snowdonia are principally pyroclastics and rhyolites, extruded during the Caradoc (Upper Ordovician). The whole region was a late stage Caledonian basin, and underwent deformation in the Late Silurian and Early Devonian.

Caradoc volcanicity was widespread and violent (Brenchley, 1969). The Iapetus Ocean, which separated the continental blocks of Laurentia (that is, much of modern day North America) and Avalonia (southern Europe), was progressively closed. This brought together what would become Scotland and Northern Ireland (Laurentian), with England and Wales (Avalonian). The collision of the two tectonic plates would give rise to the Caledonian Orogeny, producing mountains possibly higher than the Alps, as Iapetus Ocean crust was subducted.

It was principally ignimbrite volcanism in an island arc environment; ignimbrites were laid down on irregular topographic surfaces. The ignimbrites vary from fine-grained, non-welded dusts (sillar) to welded agglomerates showing eutaxitic texture. Bedding is generally absent, indicating sub-aerial deposition. Acidic deposits commonly contain ‘nodular rhyolites’, which include spherical bodies of medium-grained welded tuff in a fine-grained, welded, tuff matrix. These Caradoc acidic volcanics were probably derived from numerous sub-aerial vents, which were repeatedly active until extrusion halted abruptly in the late Caradoc. The main volcanic zone was parallel to the principal Caledonian structures.


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