Snowdon Volcanic Group - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75
|From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)
- 1 Introduction to Crafnant and Snowdon volcanic groups
- 2 Snowdon Volcanic Group
- 3 References
- 4 Glossary
Snowdon Volcanic Group
The Snowdon Volcanic Group crops out in the Dolwyddelan Syncline and in two synclinal outliers on the southern flank of Moel Siâbod. The sequence is complete in the Dolwyddelan Syncline where the group, 270 m thick, is overlain by black slates (Williams and Bulman, 1931), but in the outliers on Moel Siâbod only incomplete sections of the Lower Rhyolitic Tuff Formation are preserved (Howells and others, 1973).
Lower Rhyolitic Tuff Formation
Three units, termed A, B and C from the base upward are recognised within the formation at Dolwyddelan, though because of lateral impersistence the only locality at which all three are represented is Tan-y-Castell, on the northern limb of the syncline. Only two units (A and B) are present at Blaenau Dolwyddelan and two (B and C) at Adwyr Dwr, while on the southern limb there is only one (B). Two tuff units (B and part of C) occur in the mod Siâbod outliers. Units B and C are correlated respectively with Nos. 1 and 2 units of the Lower Crafnant Volcanic Formation. Unit A of Dolwyddelan does not extend into the Crafnant country, nor is No. 3 Unit of Crafnant represented at Dolwyddelan (Figure 18).
At Dolwyddelan this unit thins from 43 m at Blaenau Dolwyddelan to 10 m at Tan-y-Castell (Figure 16). Farther east it passes into grey slates with lenses of tuff before dying out. Characteristically well bedded, it contains lithic clasts, which are prominent in the west and considerably less common to the east. Sodic feldspar, in clusters and individual crystals, forms a high proportion of the rock. Delicate shards occur in a matrix of fine-grained chlorite, sericite and quartz. At Blaenau Dolwyddelan the top of the unit is composed of recrystallised acidic clasts.
At Moel Siâbod the unit varies in thickness from 39 m in the western outlier to 29 m in the eastern. It conformably overlies siltstones and fine sandstones and is generally well bedded. In the west it comprises a lower well-bedded crystal-rich vitric tuff overlain by flaggy-bedded silty tuffites which are graded and pass upwards by progressive enrichment in crystals into crystal tuffs. They contain a few randomly distributed rounded rhyolitic clasts and form the top few metres of the unit. In the east the central part of the unit is composed of tuffaceous siltstones, and some of the crystal-rich tuffs show crude grading and diffuse lamination.
Siltstone clasts are common within the tuffs. Crystals, up to 8 mm in length (E39299), are predominantly of sodic plagioclase; many have a distinctive interrupted twinning and they are commonly sieved with chlorite. The siltstones and plagioclase crystals are distinctive features of correlative value in establishing that the unit is restricted to the south-western part of the district (Figure 18).
Strata between Units A and B
In the Dolwyddelan Syncline these two units are separated by cleaved siltstone and mudstone ranging in thickness from 7 m at Blaenau Dolwyddelan to 16 m at Tan-y-Castell. On Moel Siâbod the strata are up to 10 m thick and are well exposed in the western outlier. They consist of siltstone, fossiliferous and tuffaceous in parts, with a 1-m band of highly altered hyaloclastite (E39309).
At Dolwyddelan this unit varies from 30 m at Minffordd to approximately 65 m at Blaenau Dolwyddelan. Where seen, as at Adwyr Dwr, the contact with the underlying mudstone is sharp, and flames of mudstone, up to 9 m high, intrude the base of the tuff.
In places the basal 2.5 m are evenly banded. Even, thick flaggy bedding is characteristic throughout the unit although parts are massive. Clasts of mudstone and sandstone are numerous at the base and angular pumice fragments are common throughout. The unit is typically crystal-rich with common euhedra of sodic feldspar, often fragmented, and less common subrounded quartz crystals.
Shards show a wide range of size and shape. At the base of the unit, where the matrix is dominantly chlorite and sericite, the recrystallisation of the shard material is often micaceous. At higher levels both shards and matrix are generally more siliceous.
In both outliers on Moel Siâbod the unit is incompletely preserved. In the west the base is irregular and is locally formed of agglomerate. The unit is composed of well-bedded vitric tuff which becomes progressively more muddy towards the top. The lower parts are generally massive with evidence of crude upward grading and faint cross-bedded silty laminations. The upper parts consist of distinctive, flaggy alternations of cross-bedded vitric tuffites with channel features, and thin evenly bedded air-fall vitric tuffs.
The tuffs are typically shard-rich with minor proportions of sodic feldspar crystals and lithic clasts. In the east, crystals and mudstone clasts are concentrated at the base of the unit in association with carbonate concretions, up to 25 cm long.
Strata between Units B and C
Apart from a section at Adwyr Dwr, these strata are poorly exposed. They consist of pale grey siltstone in beds up to 2 m thick, with bands of crystals and pumice near the base. Locally, they are richly fossiliferous.
This unit thins westwards from 24 m at Adwyr Dwr to 15 m at Tan-y-Castell. At the latter locality the inverted base shows shallow channel casts. Bedding varies from poor to well defined with the fining of grade. Dispersed ooliths have been recorded from the basal parts of the unit. Lithic clasts are generally scarce.
Shards vary in size, shape, recrystallisation and concentration. At the base at Adwyr Dwr the fabric has been strongly overprinted by the recrystallisation of chlorite and sericite, and here the shards show strong mutual interference. At the top of the Tan-y-Castell section the rock is finely banded and composed of small fragmented shards with worm casts showing internal structure.
On the basis of established correlation (Howells and others, 1973) the Lower Rhyolitic Tuff Formation and Lower Crafnant Volcanic Formation are here treated together. The combined formation comprises four units of which Unit A is restricted to Dolwyddelan and Moel Siâbod, units B and C of Dolwyddelan and the respective equivalents Nos. 1 and 2 units of Crafnant occur throughout the district, and No. 3 Unit is restricted to the north-east (Figure 18).
The distribution and thickness variation of the lowest unit indicates a westerly source. The poor sorting at Blaenau Dolwyddelan implies deposition from a series of slurries, whereas the reversed grading at Moel Siâbod suggests deposition from suspension flows. The concentration of pebbles at the top of the unit at Moel Siâbod indicates derivation from a local shallow-water environment.
Units B and C at Dolwyddelan and their correlatives, Nos. 1 and 2 units of the Lower Crafnant Volcanic Formation, show little thickness variation across the district. This, together with their broad lateral extent, ill-defined sorting and overall upward grading are characters of flow and rapid emplacement. The lack of induration of included clasts, the preservation of fossil fragments and the absence of welding, further indicate that the flows lost most of their initial heat during transport. The bedding, which suggests water suspension, probably results from a loss of energy within the main flow, giving rise to the formation of sub-flows.
Fiske and Matsuda (1964) described accumulations of unwelded material in a subaqueous environment in the South Fossa Magna, Japan, and ascribed them mainly to slurries and turbidity currents originating from unstable accumulations on the flanks of a submarine volcano. In contrast Unit B/No. 1 and Unit C/No. 2 are single pyroclastic flows with no indication over the area of a composite nature. Because such massive units are not likely to have accrued from the flowage of earlier accumulated material, it is necessary to postulate eruption as the source of the flow. At Penllyn Quarry a cleaved banded micaceous basal zone is interpreted as a base surge deposit which has been cut into by the overlying massive tuff. At Moel Siâbod thin beds of graded laminated air-fall tuffs associated with reworked volcanoclastic deposits indicate an emergence of eruptive source and a shallowing of the sea.
The source area for these two widespread flows has been postulated to lie in Central Snowdonia (Howells and others, 1973). Of the continuous volcanic emissions in that area they are the only major flows which seem to have escaped eastwards.
The characters of the No. 3 Unit are more suggestive of composite flows emplaced as slurries, fluxoturbidites or the proximal parts of turbid flows. The unit differs from the underlying flows in lacking crystals. Moreover, it does not extend south of Afon Llugwy. It thus has a different source–one which probably lay fairly near and to the north.
Bedded Pyroclastic Formation
This formation is best exposed on the northern limb of the Dolwyddelan Syncline, where its thickness varies from 180 m at Blaenau Dolwyddelan to 60 m in the vicinity of the castle. It consists of well-bedded calcareous tuffs, tuffites and siltstones which contrast in lithology with the Lower Rhyolitic Tuff Formation, which they conformably overlie.
The best section [SH 7219 5203] is south-west of Dolwyddelan Castle (Figure 19) where individual beds of basic tuffs and tuffites, 0.1 to 1 m thick, are composed of alternations of coarse and fine fragments with varying amounts of carbonate. Grading and cross-lamination are common. Subangular to subrounded lapilli, up to 15 cm in diameter, tend to be concentrated in bands. They consist mainly of devitrified basaltic glass and basalt with less common fragments of chloritised tuffaceous siltstone. The matrix varies from a fine indistinct aggregate of carbonate, chlorite, iron ore and feldspar (E38728) to a mosaic of platy carbonate (E38730) (Figure 20). Lapilli of vesiculated basaltic glass are replaced by green chlorite and carbonate and the basalt fragments are almost entirely carbonated. The finer tuffite bands show some resemblance to the acid tuffs, though they generally contain sufficient chlorite in the matrix (E38731) to indicate their relationship to the coarser, basic tuffs.
Interbedded with the basic tuffs in Afon Ystumiau [SH 7312 5282] are thin flaggy beds of fine-grained acid tuff (E38682). They contain dispersed shardic fragments in a fine siliceous matrix and because they also include even laminae of dark grey mudstone, they probably accumulated by air-fall.
Near Dolwyddelan Castle the formation can be crudely subdivided into a lower division composed mainly of tuffs and an upper division composed mainly of tuffaceous siltstones. The latter are overlain by a fine-grained acid tuff which can be traced eastwards along the north limb of the syncline. This tuff is separated from the Upper Rhyolitic Tuff Formation by approximately 6 m of black slates in which shards and euhedral feldspar crystals are scattered throughout. A similar slate separates the Middle and Upper Crafnant Volcanic formations in the Sarnau area.
A transgressive junction between tuff and sediment is apparent on the north bank of the Mon Lledr, south of Dolwyddelan. Veins of tuff are intruded into adjacent siltstones, and blocks of indurated siltstone are incorporated in the tuff. The tuff, poorly bedded, coarse and agglomeratic, passes westwards into fine bedded tuffs with agglomeratic bands. Here the pyroclastics form the upper part of the formation. Eastwards from Dolwyddelan the formation is predominantly composed of grey siltstones with impersistent layers of basic tuff and tuffaceous sandstone. Small pods of altered basaltic rock, restricted to this horizon, occur around the eastern part of the syncline.
The agglomerates and tuff intrusions about the Afon Lledr are indications of local volcanic activity. From the marked westward thickening of the tuffs on the northern limb, however, it may be inferred that the bulk of the basic tuffs were derived from 'a major eruptive centre west of Dolwyddelan. The generally even bedding and the abrupt grading caused by the layers of coarse pumice fragments at the bases of otherwise uniformly fine-grained tuffs suggests that the material was transported and deposited as ash fall, though cross-lamination in some beds suggests reworking in a marine environment. The associated. evenly bedded, locally fossiliferous sandy siltstones and fine sandstones are consistent with shelf deposition.
Upper Rhyolitic Tuff Formation
Outcrops of the Upper Rhyolitic Tuff Formation, 54 to 75 m thick, on the northern and southern limbs of the tightly folded Dolwyddelan Syncline lie no more than 150 m apart between Ty'n-y-Ddol Quarry and Pont Tan-y-Castell. Dolwyddelan Castle is sited on the northern crop of the formation. East of the castle, between Ty Ucha'r Ffordd and Pen-y-Gelli, faulting and plunge variations around the easterly closure of the structure have greatly enlarged the width of outcrop.
The rocks are poorly bedded to massive, and are typically bluish grey. Lithologies are distinctly heterogeneous and range laterally from clean vitric tuffs to tuffaceous mudstones. Some of the mudstones are disturbed, others contain sporadic pods of pyroclastic material, and isolated crystals of feldspar can be seen in a fine muddy matrix. In general the formation tends to be more muddy towards the eastern closure of the structure. Bedding is laterally impersistent. Cleavage is generally well developed throughout the formation, reflecting the high muddy fraction.
Thin section examination shows that the heterogeneous admixtures of pyroclastic and fine argillaceous material, observed at outcrop extend to the finest elements. Specimens range from predominantly argillaceous (E38723) through dominantly pyroclastic (E38724) to clean vitric tuff (E38727). Shards range in size and shape from fine and elongate, less than 0.1 mm, to angular cusp-edged bubble walls, up to 2.5 mm. Rare crystals of albite-oligoclase, up to 3.8 mm (E38514), are invariably much altered and resorbed. Clasts of tubular pumice, many with indistinct margins, occur throughout the formation (E38514).
Cleavage is most prominent around the eastern closure of the syncline, where the planes are accentuated by iron oxide, and possibly opaque carbonaceous material and there is a strong alignment of flakes of secondary mica adjacent to the planes (E38684).
Williams and Bulman (1931) assumed that the irregular admixture of muddy and pyroclastic material resulted from 'rapid re-assorting and mixing in shallow water', but the lack of bedding does not support such a conclusion. On the contrary, the broad lateral extent of the formation together with the complete lack of sorting suggests deposition from high density turbid flows following the remobilisation of ash previously emplaced on unstable slopes of unlithified mud.