Features in the field: Ignimbrites of the Yr Arddu syncline

Stephen K Donovan (The Netherlands)

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.

Summary of the succession

The rocks of the Yr Arddu syncline fall into three groups. Note that this classification is rather coarser than the currently recognised scheme (Rushton & Howells, 1998):

1. The lowest beds, the Glanrafon Group, are slates and sandstones of probable marine origin.
2. These are overlain, probably unconformably, by pyroclastic deposits of the Snowdon Volcanic Group, which were laid down subaerially. The rocks of this group consist principally of moderately coarse-grained acid tuffs, grading up into welded tuffs (ignimbrites).
3. All of these rock types have been intruded by a variety of igneous rocks.

The whole area has been synclinally folded and it appears to have folded in two phases (Rast, 1961). The Glanrafon Group are comprised of unfossiliferous slates and sandstones, which are about 500m in thickness in the study area. The slates of the Glanrafon Group are uniform dark grey in colour and show little evidence of bedding, but cleavage is well developed and exposures tend to lie parallel to its strike. The slates may show honeycomb weathering, probably due to the presence of calcareous bands or nodules. The Glanrafon Group are interpreted as being metamorphosed mudrocks and greywackes of a deeper-water origin, whereas the overlying pyroclastic rocks were deposited in a sub-aerial environment. There was presumably a period of uplift and erosion between the end of sedimentary deposition and the start of acid volcanicity.

In the area around Yr Arddu, the Snowdon Volcanic Group is comprised of the Lower Pitts Head Tuff Formation, and the Upper Pitts Head Tuff Formation and Lower Rhyolite Tuff Formation. The Lower Pitts Head Tuff Formation lies at the base of this thick sequence of acid volcanics. It is similar to other pyroclastic deposits of Yr Arddu, but differs from rocks higher in the sequence by enclosing fragments of country rock. These clasts are dark grey in colour and were presumably derived from the Glanrafon Group, being removed from the side of the vent during extrusion and/or being ripped up from the surface of the unconformity.

These fragments are elongated and this fabric was partly determined by the bedded mudrocks, but probably further attenuated during folding, orientating fragments parallel to cleavage. This cleavage is poorly developed (although superior to that of younger pyroclastics), probably due to the presence of incompetent sedimentary rock fragments, making the acidic tuff more susceptible to deformation. The field identification of the Lower Pitts Head Tuff Formation is based mainly on slate content to distinguish it from the Rhyolite Tuff.

Apart from slaty clasts, the Lower Pitts Head Tuff Formation contains large acidic fragments, which may be bombs derived from solidification in the vent, reworking from the vent sides or rip-up clasts. The large illustrated fragment (Fig. 1A) is 60cm in maximum dimension. The matrix is pale grey, and contains phenocrysts of plagioclase and quartz up to 1mm in diameter. The weathering surface is commonly grey to rust brown. Evidence of bedding is rare (Fig. 1F) and the lack of other sedimentary structures may be ‘negative evidence’ for sub-aerial deposition.

Clusters of tension fissures – parallel arrangements of fissures now infilled by quartz – are common in the Lower Pitts Head Tuff Formation (Fig. 2B); note, in particular, the displacement of joints (Fig. 2A). Such fissures are opened in competent beds, as a result of strain produced during folding.

I have grouped the Upper Pitts Head Tuff Formation and Lower Rhyolite Tuff Formation together under the informal name Rhyolite Tuff, because they share many features and are difficult to separate on field evidence alone. They differ from the Lower Pitts Head Tuff Formation in lacking enclosed clasts of sedimentary rock. This part of the succession is composed solely of acidic tuffs and numerous ignimbrites (see the table accompanying this article). These rocks are very competent, showing little cleavage compared to the Lower Pitts Head Tuff Formation. The fine-grained, pale grey groundmass weathers to grey-white, preferentially along bands of unwelded tuff.

Table: Features used in the field identification of ignimbrites (adapted after Fitch, 1967, p. 206):

1) Pyroclastic composition – largely acid crystal/vitric tuff.

2) Commonly acid and uniform through great thicknesses. More rarely coarse-grained welded tuff breccias at base. Rare rude, sub-aerial bedding structures at top of units, but never showing true bedding.

3) Commonly extensive development of primary columnar joints.

4) Commonly well-developed eutaxitic structure in welded tuffs, such as flow banding and flow folds, but true fluidal structures are absent.

5) Nodular structures, tuff-veins and explosive dykes.

6) Wide lateral distribution of individual ignimbrite sheets.

7) Often associated with minor unconformities.

The absence of flow banding and flow folds enables this rock to be distinguished from the intrusive rhyolites. Fig. 3 illustrates part of a section through a succession of welded tuff flows. At the base of a flow is a thin, relatively incompetent layer known as the basal ash zone. This is overlain by the massive welded tuff flow. Fig. 1C-E illustrates the contrast in structure of the Rhyolite Tuff at different localities. Fig. 1E shows large, apparently sub-parallel sheets with columnar jointing developed in the thick upper flow. The ‘beds’ in Fig. 1D (see also Fig. 2E) appear similar to a theoretically perfect example, apart from the presence of large, rounded blocks, which give the rock a rubbly, nodular appearance (see below). Nodular tuff in Fig. 1C is interbedded with slightly ‘disconformable’ inter-layer surfaces by finer-grained tuffs.

There are two principal ignimbritic textures displayed in the Rhyolite Tuff. ‘Nodular rhyolite’ (Fig. 2C, D) is ignimbrite containing blocky or rounded acidic (pumiceous) fragments. The degree of rounding may be a consequence of distance travelled and/or speed of transport. Eutaxitic texture is produced by hot pumice fragments being flattened to a lenticular shape while still hot. These tend to be picked out by weathering (Fig. 2F).

Rare boulders (Fig. 4C) preserve evidence of a conglomerate within the Rhyolite Tuff, containing rhyolitic pebbles and indicating a break in acid volcanicity, during which time erosion occurred. However, I did not find this lithology in situ.

Intrusive igneous rocks in this area all appear to be sheet-like (sills and dykes), and include rhyolite, dolerite, microgranite and a composite intrusion. The main outcrop of the rhyolitic intrusion is to the west and east of the lake near the summit of the hill, Llyn Yr Arddu. Composition of intrusive rhyolites and the rhyolite tuffs is similar; they both weather to grey-white and are difficult to differentiate. The most obvious difference is the good flow banding commonly shown by the intrusive rhyolites, but absent from the rhyolite tuffs. The banding in intrusive rhyolite commonly shows good, sedimentary-like structures such as slumping and collapse structures (Fig. 4E, F), apparent in both fresh and weathered surfaces. Banding may be constant in rock thicknesses of several tens of metres. Fresh intrusive rhyolite is medium grey in colour, commonly speckled by larger crystals of quartz (up to 1mm), plagioclase or alkali feldspar, while large, enclosed rock fragments are rare. Jointing is generally poor and cleavage is not developed. The form of the intrusion appears to be a sill.

At [NGR SH 6290 4554], microgranite crops out at the base of a rhyolitic exposure and the latter may represent a chilled margin. Contacts are sinuous (Fig. 4D). The rock is massive, medium-grained and medium grey in colour, with quartz phenocrysts up to 0.5mm long.

The composite intrusion is so-called because it shows considerable variation along both the strike and dip of the outcrop. It is quite acid and fine-grained, when it is close to contacts with the country rock (mainly with slates; Fig. 1B), but is fine- to medium-grained elsewhere. Its geometry appears to be thin dykes and thicker sills, the later with poor columnar jointing (Fig. 4A, B). Larger crystals vary from well-shaped to rounded, composed principally of plagioclase or zeolites. At [NGR SH 6344 4603], the intrusion shows flow banding in its northwest face.

Dolerite is the only basic igneous rock found in the Yr Arddu area and outcrops in three small areas near Llyn Yr Arddu. The rock is a dark grey-green colour when fresh, with numerous phenocrysts of plagioclase up to 10mm long by 10mm wide, weathering to a rusty brown colour. At [NGR SH 6290 4677], the contact with Rhyolite Tuffs is sill-like. Vertical columnar jointing at [NGR SH 6290 4667] presumably formed perpendicular to the cooling surfaces. 

Glossary of terms

Acidic tuffs: a volcanic tuff of rhyolitic composition, such as an unwelded or a welded tuff (see below). Any acidic igneous rock has more than 60% SiO2.

Agglomerates: a rock formed from large, irregular, pyroclastic fragments in a finer matrix or groundmass. This term is used mainly in connection with volcanic agglomerates.

Caledonian Orogeny: the great mid-Palaeozoic episode of mountain building in northern Europe, which built the Caledonian mountains of Scotland.

Eutaxitic texture: “Said of banded structure in certain extrusive rocks, resulting from the parallel arrangement and [alternations] of layers of different textures, mineral composition, or colour. Commonly applied to banded structures … in welded tuff” (Neuendorf et al., 2005, p. 220).

Flow banding: flow structures commonly seen in rhyolitic rocks (such as Fig. 4E, F).

Greywacke: a ‘dirty’, unsorted sandstone, deposited probably no great distance from the source rock, and including a wealth of grains of contrasting mineralogy, size and degrees of roundness.

Ignimbrite: commonly extrusive, pyroclastic igneous rocks, such as sillars and welded tuffs (see below). Such eruptions are invariably explosive and form from nuée ardentes or glowing cloud eruptions, such as the one that destroyed St Pierre in Martinique in 1902, killing 27,000 people in just a few minutes.

Phenocryst: large crystals found in igneous rocks.

Pyroclastic: fragmentary rocks produced by explosive volcanism, such as an acidic tuff (see above).

Rhyolite: a fine-grained, extrusive igneous rock of acidic composition, similar to that of a granite.

Sillar: an ignimbrite (see above) that lithified “… after deposition by recrystallisation due to the activities of escaping hot gases and fluids” (Wyatt, 1986, p. 161). An unwelded tuff (see below).

Tension fissure: “A fracture that is the result of tensional stress in a rock” (Neuendorf et al., 2005, p. 661). They are commonly associated with the displacement of structures like joints (Fig. 2A). Also known as tension joints or tension fractures.

Unwelded tuff (=sillar): a pyroclastic rock (=tuff), in which the grains have not been welded together by its included hot gases and the weight of overburden. Contrast with welded tuff.

Welded tuff: “A glass-rich pyroclastic rock that has been indurated by the welding together of its glass shards under the combined action of the heat retained by particles, the weight of the overlying material, and hot gasses” (Neuendorf et al., 2005, p. 719).

References

Brenchley, P.J. 1969. The relationship between Caradocian volcanicity and sedimentation in North Wales. In Wood, A. (ed.), The Pre-Cambrian and Lower Palaeozoic Rocks of Wales: Report of a Symposium held at the University College of Wales, Aberystwyth, in honour of O.T. Jones and Sir William Pugh. University of Wales Press, Cardiff, 181-202.

Brenchley, P.J. 1992. Caradoc. In: Cope, J.C.W., Ingham, J.K. & Rawson, P.F. (eds), Atlas of Palaeogeography and Lithofacies. Geological Society Memoir, 13: 28, 30-33.

Fitch, F.J. 1967. Ignimbrite volcanism in North Wales. Bulletin Volcanologique (series 2), 30: 199-220.

Fitch, F.J. 1971. Volcanic Studies Group field meeting in Snowdonia. Journal of the Geological Society, 127: 533-534.

Howells, M.F. & Smith, M. 1997. The Geology of the Country around Snowdon. Sheet 119 (England and Wales). Memoir of the British Geological Survey.

Lopes, R. 2010. Volcanoes: A Beginner’s Guide. Oneworld Publications, Oxford, xi+150 pp.

Neuendorf, K.K.E., Mehl, J.P., Jr. & Jackson, J.A. 2005. Glossary of Geology. Fifth edition. American Geological Institute, Alexandria, Virginia, xii+779 pp.

Rast, N. 1961. Mid-Ordovician structures in south-western Snowdonia. Liverpool & Manchester Geological Journal, 2: 645-652.

Rast, N., Beavon, R.V. & Fitch, F.J. 1958. Sub-aerial volcanicity in Snowdonia. Nature, 181: 508.

Ruston, A.W.A. & Howells, M.F. 1998. Stratigraphical Framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Technical Report, RR 98/1.

Wyatt, A. (ed.). 1986. Challinor’s Dictionary of Geology. University of Wales Press, Cardiff, xviii+374 pp.