Crafnant Volcanic Group - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75

From Earthwise
Jump to navigation Jump to search
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.)

Map: Sheet SH 75 Capel Curig and Betws-y-Coed. 1:25 000 series - Classical areas of British geology]

Under construction

Figure 9 View westwards up the Llugwy Valley with Moel Siabod in the distance.
Figure 10 Sketch of crag of massive tuff in the Lower Crafnant Volcanic Formation.
Figure 11 Sketch of typical .ower and Middle Crafnant scenery around Llyn Bodgynydd.
Figure 12 Forestry road section in tuffs, tuffites and black mudstones showing even bedding and low dips typical of the Middle Crafnant Volcanic Formation in the Sarnau area.
Figure 13 Depositional and post-depositional structures in the Middle Crafnant Volcanic Formation, Sarnau area. 1 Turbidite sandstone band with pyrite-rich basal layer infilling scour structures in underlying black mudstone. Scour structures show evidence of deformation on right-hand margin of specimen. 2 Bedded tuff, tuffite and mudstone showing thixotropic yielding in central band with adjacent bands undeformed. 3 Bedded tuff, tuffite and mudstone showing cross-lamination in lower unit and extreme deformation of the base of a fine tuff layer with upward injection of mudstone near the top.
Figure 14 Field sketches of post-depositional structures in the tuffites of the Middle Crafnant Volcanic Formation.
Figure 15 Photomicrograph of muddy crystal tuff with squat cuspate shards and oligoclase crystals set in a matrix of sericite, chlorite and irregular smears of iron ore. From the Upper Crafnant Volcanic Formation. (E35261), x 45.
Figure 39 Fractured feldspar crystal.
Figure 40 Quartz xenocryst.
Figure 41 Vitroclastic texture in tuff.
Figure 42 Perlitic texture in rhyolite.
Figure 43 Vitroclastic texture.
Figure 44 Pumice fragment.
Map 3 Map showing the extent of the Crafnant and Snowdon Volcanic groups.

Introduction to Crafnant and Snowdon Volcanic groups[edit]

Crafnant Volcanic Group[edit]

The Crafnant Volcanic Group crops out in the high ground south of the Mon Llugwy, between Clogwyn-mawr in the west and Llyn-y-Parc in the east. Its subdivision into Lower, Middle and Upper formations is based on distinctive volcanic lithologies. The Lower Crafnant Volcanic Formation, up to 210 m thick, is composed of three units of ash-flow tuffs, up to 60 m thick, separated by slates, siltstones, a few thin limestones and local hyaloclastites and basic tuffs (Howells and others, 1973). The Middle Crafnant Volcanic Formation, 30 to 120 m thick, comprises alternating thin acid tuffs, tuffites, siltstones and mudstones which in places are highly admixed and disturbed. The Upper Crafnant Volcanic Formation, 60 to 150 m thick, is an ill-sorted pyroclastic deposit which is characterised by much included argillaceous debris.

Lower Crafnant Volcanic Formation[edit]

The following account summarises the detailed description of the formation already published by Howells and others (1973). The tuffs of the component units, numbered 1 to 3 from the base upwards, form scarp features (Figure 10) which extend westwards from Clogwyn Cyrau, in the forest near Betws-y-Coed, to Capel Curig, where they strike northwards along the ridges of Clogwyn-mawr, Clogwyn Cigfran and Crimpiau. They are predominantly massive, poorly cleaved grey rocks, weathering white. At some localities the basal rocks of units are mica-rich, bluish grey and well cleaved. The tops of Nos. 1 and 2 units are uniformly flinty.

In thin section these rocks are seen to be acid vitric tuffs with a variable content of crystals and lithic clasts. Shards are completely devitrified and their shape, independent of size, varies from broad and massive to delicate and spiky: they are commonly tectonically deformed, but are not welded. Crystals, consisting dominantly of sodic plagioclase, with subordinate amounts of fragmented euhedral quartz, are found only in units 1 and 2. Some of the plagioclase crystals are fragmented, others are marginally resorbed. Lithic clasts are common in all units and include fossil fragments, siltstones, acid tuff, pumice, basic tuff and dolerite. The matrix is a fine-grained admixture of quartz, feldspar, sericite and chlorite probably representing an original fine vitric dust. Carbonate concretions are randomly distributed throughout all three units, but siliceous concretions are peculiar to the top of No. 2 Unit.

No. 1 Unit[edit]

This unit varies in thickness from 21 m at Aberllyn [SH 7941 5730] to 56 m at Capel Curig [SH 7270 5760]. The regional setting suggests conformity with the underlying sediments, though the base is exposed only at Cae-mawr [SH 7567 5716] where the tuff locally cuts down into underlying siltitone to a depth of 1 m.

Through most sections west of Cae-mawr a general upward fining is apparent and subdivision can be made into a lower sub-unit with crystals and sedimentary clasts, a central subunit with small pumice clasts and few crystals, and an upper sub-unit of finer debris. Bedding is generally a distinctive feature of the unit away from the area of Allt Goch [SH 7443 5760] and Capel Curig. To the north of Capel Curig, thick regular beds, separated by silty bands with well-developed cleavage, occur from 7 to 21 m above the base.

Lithic clasts are generally prominent in the lower sub-unit, especially at Pen-yr-allt-uchaf [SH 7864 5728], where blocks up to 1.3 m across occur. Carbonate concretions up to 0.7 m are common and in places, as at Cae-mawr and Towers [SH 7578 5758], show a tendency to be concentrated near the top of the lowest sub-unit and also to coalesce.

Strata between nos. 1 And 2 Units[edit]

These strata generally consist of siltstone and mudstone less than 30 m thick although up to 70 m of strata, including a thick mass of hyaloclastite, are present north of Crimpiau.

Exposures of the sediments are mainly small and isolated. They include, near the base of the sequence, an argillaceous limestone rather less than 1 m thick, seen in a forestry road section [SH 7445 5788] at Allt Goch, to the south of Cae-mawr [SH 7526 5666] and in the small synclinal outlier in Coed Maesnewyddion [SH 7740 5691]. Longvillian faunas have been determined from the first two of these exposures.

The well-exposed hyaloclastite north of Crimpiau includes blocks of albitised and chloritised basalt (E38376) in a matrix of carbonate and chlorite, with fragments of vesicular basaltic glass, replaced by chlorite and ilmenite with sphene still discernible ( E38377) .

No. 2 Unit[edit]

The thickness of No. 2 unit ranges from 18 m at Cae-mawr to 73 m at Pencraig [SH 7662 5784]. At Pencraig a sharp contact with the underlying mudstones is exposed.

In the west the unit is generally uniform and massive, whereas eastwards bedding and ribbing are more apparent and pumice clasts, up to 20 cm across, occur. In the lowest 3 m of the unit brachiopod casts are common at Cae-gwyn [SH 7599 5820] and ooliths at Aber-llyn.

Crystals of sodic feldspar and quartz are present throughout the unit, but are concentrated in the basal part in most sections. Siliceous concretions are common and at the western extremity of outcrop the unit is characterised by an overall fine flinty siliceous character.

Strata between Nos. 2 and 3 Units[edit]

These strata consist of slates and siltstones rarely exceeding 30 m, but not completely exposed at any locality. Above the scarp on the north side of Afon Llugwy, near Pen-yr-allt-uchaf, small exposures show the lower beds to be distinctly tuffaceous, with rich Longvillian faunas. These beds are overlain by brittle blue cleaved slates up to the base of No. 3 Unit.

Farther west, near Waenhir [SH 7370 5860], a basic tuff with layers of mudstone pellets, up to 2 cm in diameter, can be traced in small crags below the No. 3 Unit. Similar thin layers of basic tuffs are interbedded with convoluted siltstones below the thick dolerite sill on Creigiau Geuallt [SH 7302 5894].

No. 3 Unit[edit]

This unit ranges in thickness from 40 m on the ridge north-north-east of Capel Curig to approximately 20 m at Cae-gwyn. It is generally heterogeneous, being composed of vitric tuff, often rich in clasts, with tuffites and silty intercalations.

At Capel Curig bedding is seen only 21 m above the base of the unit in a thin agglomeratic band composed of rounded clasts with interstitial lithic material. Northwards and eastwards from Capel Curig thin discontinuous crenulated ribs, accentuated by chlorite smears parallel to the bedding, are a feature of the unit. They are best displayed on the weathered lower surfaces at Allt Goch.

Except in the finer grained layers, lithic clasts are common and include andesite, hyaloclastite, basic tuff, vitric acid tuff, pumice and siltstone. The unit is also distinguished from Nos. 1 and 2 units by the absence of xenocrysts and the remarkable admixture and range of shard sizes.

Interpretation[edit]

The interpretation of the emplacement of the Lower Crafnant Volcanic Formation is given at the end of the section dealing with the equivalent Lower Rhyolitic Tuff Formation (p. 36).

Middle Crafnant Volcanic Formation[edit]

The Middle Crafnant Volcanic Formation comprises alternations of tuffs, tuffites and sediments. The range of lithologies is well seen in the type area, between Llyn Bodgynydd and Coed-mawr [SH 7854 5840], where the formation, characteristically even bedded and flaggy (Figure 11) and (Figure 12), includes bluish black mudstones, mudstones with scattered idiomorphic feldspar crystals, tuffites, muddy tuffs and pale grey vitro-elastic tuffs with lithic blocks.

Sediments[edit]

The sediments consist predominantly of mudstone and siltstone with infrequent narrow ribs of coarse sandstone. The siltstones (E35197) contain varying amounts of iron ore and a few feldspar fragments, up to 0.5 mm, in a fine matrix of sericite, chlorite and an aggregate of quartz with some feldspar. Occasionally the siltstones contain a distinct chlorite fraction (E37180), both finely disseminated and re-crystallised in vermicular growth.

Sole markings characteristic of turbidites are recognised at the bases of the few sandstone layers. The sandstones fill scour structures in the underlying mudstones and these show evidence of subsequent deformation with, in places, lobes of sandstone almost completely isolated by the 'flames' of the underlying mudstone. The sandstone shows normal grading with the coarsest fraction and a distinctive concentration of pyrite in the scoured hollows (Sanders, 1965, after Natland and Kuenen, 1951) ((Figure 13).1).

Tuffites[edit]

The tuffites are typically fine-grained, banded rocks that differ macroscopically from the associated mudstones in their pale grey to white colour, which is particularly apparent on weathered surfaces. Gradations can be seen from dark grey mudstones to fine-grained, pale grey tuffites. The finer tuffites vary in grade from chert-like to fine sandstone. Small pale patches which commonly break the homogeneity are composed of well-formed cuspate shards. Matrices consist of fine quartzo-feldspathic aggregates with sericite shreds and finely disseminated chlorite. The dark grey bands, which give the rocks a striped character, result from concentrations of chlorite, an opaque carbonaceous fraction and iron ore.

Coarser tuffites, which generally grade into tuffs, consist of small rounded albite crystals, up to 1.2 mm, in a dark grey base consisting of quartz, feldspar, shreds of chlorite, ragged fragments of carbonaceous material and iron ore. Shard-rich areas indicate the close admixture of epiclastic and pyroclastic material.

Post-depositional structures are a characteristic feature of the tuffite sequences. Alongside a forestry road [SH 7698 5926], near Bryn-y-fawnog, a massive tuff overlies a tuffite which is severely deformed at the base by upward flame-like incursions of the underlying siltstones ((Figure 14).1, 2); fine, even bedding in the siltstone is contorted into the flames. Here the upward transgressions of the siltstone are not all sharply pointed; broad upwarping also occurs.

Convolute laminations in the tuffites can be examined in the scarp features [SH 7755 5884] south of Sarnau (Figure 13) and (Figure 14).4. The contortions are confined to the cherty tuffitic layers within evenly banded tuffs and tuffites. In some instances [SH 7814 5848] the convolution is associated with small penecontemporaneous faults which clearly dislocate the evenly bedded layers below, but cannot be traced upwards through the zone of convolution.

Tuffs[edit]

These rocks range from tuffs to tuff-breccias in grade (Fisher, 1966) and from well sorted to ill sorted. Perfect cuspate shards in a good vitroclastic fabric (E35192) are typical, and included fragments of crinoid columnals and a graptolite fragment have been recorded.

The coarsest tuffs form massive beds, up to 1.3 m thick, in distinctive scarp features south-west of Bryn-y-fawnog. They contain isolated blocks, up to 0.5 m in diameter, of dark grey indurated mudstone which show no preferred orientation or position within the beds. Small vesicle-like cavities are distinctive on the exposed surfaces and are formed by the weathering of ill-defined plates of carbonate. A good vitroclastic texture (E37181) is slightly obscured by a fine recrystallised quartzo-feldspathic mosaic. Blocks of mudstone and tuffite, up to 0.2 m in diameter, occurring in coarse tuffs west-south-west of Sarnau, show faint internal planar structures which closely reflect an indented periphery and indicate that the blocks were unlithified at the time of incorporation ((Figure 14).3).

As in the sediments and tuffites, loading structures are a common feature of the tuffs. A specimen (Figure 13) collected from a roadside exposure [SH 7814 5848] shows three units—a lower finely laminated tuff, a central tuffite with planar laminations and an upper banded tuff-tuffite unit. The base of the upper unit is grossly distorted by loading and the main down-warping lobes are almost detached by the upward intruding flames of tuffite.

The evenly bedded, alternating lithologies of the type area can be traced in a steeply dipping succession through the forests to a fault striking south-south-west from Llyn Bodgynydd. North-east of Capel Curig the lowest beds of the formation are well exposed across a broad anticlinal structure west of Afon Abrach. Here, near the base of the formation is a thinly bedded, basic tuff exposed intermittently as far west as the margin of the thick dolerite sill south-east of Creigiau Geuallt.

Higher in the formation, in this area north-east of Capel Curig, the alternations of tuff, tuffite and siltstone are more difficult to distinguish. This is not entirely a function of exposure. On the heather-covered slopes south-west of Clogwyn Manod and on Ffrith Newydd, for instance, the numerous small exposures show such a lack of lateral continuity in lithology as to indicate some breakdown in the bedding. Similarly, in exposures on the dip-slopes about Nant Geuallt, south-west of Clogwyn Manod, there is a broad and randomly distributed range of tuff types from coarse agglomerates (E38868) with rounded clasts of spilitic basalt, altered basaltic glass, perlitic fractured rhyolites and chloritised shale, to fine, cream-coloured, cleaved vitroclastic tuffs. Siltstone intercalations, which are distinctly tuffaceous, are also generally irnpersistent.

The highest beds of the formation in the Sarnau area are bluish black graptolitic slates (p. 70) which crop out around the lake and in the road cutting north of Sarnau Cottage. They can be traced below the scarp features of the Upper Crafnant Volcanic Formation in Coed Bwlch-yr-Haiarn and Craig y Fuches-lâs overlooking the Nant Gwydir valley.

Interpretation[edit]

The Middle Crafnant Volcanic Formation accumulated in deeper water than did the lower and middle parts of the Carneddau Group. There is a predominance of mudstones and fine siltstones in the sequence and the few sandstones that occur show the characters of turbidites transported from a distant source.

The characters of the volcanic rocks are quite different from those of the Bedded Pyroclastic Formation of Dolwyddelan and Central Snowdonia, suggesting eruptions from different centres. The pyroclastic material of the Middle Crafnant Volcanic Formation probably emanated from a centre to the north. It may represent continued activity at the source of the highest ash flow of the Lower Crafnant Volcanic Formation (Howells and others, 1973). The transition from well-bedded to slumped sequences as the formation is traced from the Sarnau area to the ground north-east of Capel Curig may reflect passage across the margin of an unstable volcanic pile.

Around Sarnau the coarse tuffs are very different in grade from the associated sediments. The blocks of originally unlithified mudstone and tuffite, the high clay content in parts of the vitroclastic fabric, and the marine fossils indicate emplacement within a marine environment. The tuffs presumably originated in shallower parts of this environment as small ash flows or secondary slurries of unstable pyroclastics, and flowed into the deeper parts, incorporating sediment during transport.

The tuffites are essentially fine-grained, pale grey and evenly banded. The banding suggests the settling of fine material through a water column to a level below the wave-base. Where these deposits are thin and directly overlie coarser tuffs, it is reasonable to assume that they accumulated from dust clouds released into water during the transport and emplacement of the tuffs. The thicker deposits are possibly accumulations of fine shardic dust-rich eruptions which settled through the water after air transport from the source area.

The convolute bedding in narrow bands can be explained by the rapid emplacement of tuff on water-saturated, partly lithified tuffite which would have yielded thixotropically. The mechanism producing the convolution in thicker bands is not always apparent. As the internal folds clearly occur in beds that are not themselves folded it is suggested that the load-failure mechanism could have been initiated by earthquake shocks characteristic of volcanic environments.

Upper Crafnant Volcanic Formation[edit]

This formation is equivalent to the Upper Tuff Bed of Davies (1936) in the Crafnant country, to the north of Capel Curig. In the district described here it is well exposed in the high forest ground west of the Llyn-y-Parc Fault and across Mynydd Bwlch-yr-Haiarn, north of Sarnau. It can be traced into thefault-bounded synclinal structure around Llyn Goddionduon and on to the ridge of Clogwyn Manod and Ffrith Uchaf, north-east of Capel Curig where, however, it is difficult to delimit from the Middle Crafnant Volcanic Formation. The upper part of the formation is exposed only in the syncline about Llyn Goddionduon.

Typically the formation is massive and almost entirely without internal bedding structures, so that it forms only crude scarp features. It consists of tuffite composed of a heterogeneous admixture of pyroclastic and epiclastic material. The pyroclastic material consists of cuspate shards of highly variable form, from coarse bubble walls to small fragmented rods and spikes, together with fragmented, rounded and resorbed crystals of sodic feldspar and pumice fragments and blocks. The predominant epiclastic material is muddy in character and consists of a fine aggregate of chlorite, sericite, dusky carbonaceous material and iron ore. Lithic clasts are generally patchy in distribution, as north-east of Llyn Bychan [SH 7537 5958] and include siltstone, acid tuffs, dolerite and spilitic basalt (E35261) (see (Figure 15)).

The proportions of the various constituents is highly variable. In places the rocks are tuffaceous mudstones with a high proportion of the muddy fraction and few small fragmented shards and crystals, as seen (E35217) low in the formation south of Llyn Goddionduon. At about the same horizon, on the north side of the dam at Llyn Bodgynydd, a disturbed sequence of tuffaceous mudstone, tuffite and muddy tuff with blocks of vitric tuff is exposed. In the tuffite (E38918) the bedding is clearly distorted and the high proportion of mudstone in the sequence gives rise to a strong cleavage. Impersistent ribs of mudstone within a heterogeneous admixture (E35195) give crude indications of bedding. Smears of fine mudstone are often accentuated by a concentration of iron ore as are many of the cross-cutting cleavage planes (E37163). Clearer indications of bedding are entirely absent.

Carbonate nodules have been recorded in a few localities. In a forestry road cutting near the southern end of Llyn Bychan [SH 75175921] a strongly cleaved muddy crystal tuffite contains carbonate nodules, up to 0.7 m in longest dimension, which overprint and slightly deflect the cleavage planes. These nodules are almost entirely decomposed into a soft friable ferruginous mush, the hard cores of which are seen in thin section (E35216) to consist almost entirely of carbonate obscuring the original fabric.

Interpretation[edit]

The pyroclastic component of the litho-logical admixtures in the formation consists of coarse shards and crystals which are typical products of ash flows. The complete lack of sorting and minimal internal bedding features suggest deposition by a high-density turbid flow produced by the remobilisation of pyroclastic material that had been previously rapidly emplaced on unlithified muds.

References[edit]

Glossary[edit]