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

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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 31 Resorbed sodic plagioclase.
Figure 32 Cross-bedding.
Figure 33 Eutaxitic structure in welded tuff.
Figure 34 Accretionary lapillus.
Figure 35 Perlitic texture.
Figure 36 Parataxitic structure in welded tuff.
Figure 37 Cross-bedding.
Figure 38 Brachiopod Plaesiomys multifida.
Figure 4 Photomicrograph of accretionary lapillus from the Garth Tuff. (E35163), x40.
Figure 5 Photomicrograph of welded tuff with eutaxitic fabric. Shards deformed around albiteoligoclase crystal. Garth Tuff. (E39682), x40.
Figure 6 Photomicrograph of vitroclastic tuff with cuspate shards, bubble walls and fragmented feldspar crystals. Top of the Lledr Valley Tuffs. (E39162), x40.
Figure 7 Sketch of the top of Curig Hill from the south, showing bedded basic tuffs dipping steeply eastwards.
Figure 8 Map and section of the basic tuffs at Curig Hill.
Map 2 Map showing the extent of the Carneddau Group.

Chapter 2 Carneddau Group[edit]

The literature dealing with the Ordovician strata below the Snowdon and Crafnant Volcanic groups contains inconsistencies in nomenclature, particularly in the various usages of 'Glanrafon'. The term was first used in Central Snowdonia by H. Williams (1927) who divided the beds between the Llanvirn (Maesgwm) Slates' and the Snowdon Volcanic Group into 'Llandeilo (Glanrafon) Slates' below and 'Gwastadnant Grits' above. These lithological divisions remain valid only as far north as Nant Ffrancon (D. Williams, 1930). To the southwest and south-east they give way to a laterally variable sequence of slates and sandstones with subordinate tuffs to which Williams and Bulman (1931) and Beavon (1963) applied the term 'Glanrafon Beds'. As H. Williams (1927), D. Williams (1930) and Shackleton (1959) recorded a lithological, as well as a faunal change at the top of the Maesgwm Slates, their 'Glanrafon Beds' meet the modern specifications of a lithostratigraphic division and is used in that sense (modified to 'Glanrafon Group') on the published 1: 25 000 geological map of Central Snowdonia. It is clear from that map, however, that neither the Maesgwm Slates nor any equivalent Llanvirn strata have been recognised south of Snowdonia. Similarly, and more relevant to this account, no Maesgwm/Llanvirn line can be drawn farther north into the country between the Carneddau and the Conway Valley, where Davies (1936) and Stevenson (1971) have used 'Glanrafon Beds' in a more restricted sense than that applied in Central Snowdonia. Thus for this northern tract, of which the Capel Curig–Betws-y-Coed district forms part, the term Carneddau Group is proposed. The group is here defined (p. 1) to include all strata between the base of the Ordovician and the base of the Snowdon and Crafnant Volcanic groups.

Penanmen Tuffs and underlying strata[edit]

Carneddau Group beds underlying the Penamnen Tuffs crop out mainly in the vicinity of Penmachno. They consist of up to 125 m ofsiltstone and slate, with ribs of false-bedded sandstone forming small exposures throughout the steep forest slope north of Afon Glasgwm.

The type section of the Penamnen Tuffs is located at the head of Cwm Penamnen, just outside the southern boundary of the district, where two units of acid tuffs, each up to 9 m thick, are separated by a similar thickness of siltstones. The outcrop extends into the district south-east of Pen-y-Benar, on the steep western side of Cwm Penamnen [SH 7340 5001] where part of the higher tuff unit is seen in a small exposure. The tuff has a strong vitroclastic fabric of devitrified shards within a fine aggregate of chlorite and sericite, with crystals of sodic feldspar, a few crystals of quartz and isolated rounded lithic clasts, mainly of iron-rich siltstone.

Outcrops of tuff farther east, near Penmachno, are correlated with the Penamnen Tuffs. One can be traced for nearly 1 km through thick forest 1 km WNW of the village. The field relationships of two further isolated exposures around Bryn Bedyddfaen, to the north-east of the village, are uncertain. There is, however, a close lithological similarity between the rocks exposed around Penmachno and those of the type area. All are rich in shards, which range widely in shape and size and are moderately tightly packed. Crystals of sodic feldspar showing unusual resorbed peripheries are numerous (E37352), quartz crystals are fewer and generally smaller. Lithic clasts, mainly of pumice and siltstone, are invariably present.

Strata between the Penamnen Tuffs and the Capel Curig Volcanic Formation[edit]

South of Dolwyddelan the Penamnen Tuffs are overlain by about 360 m of sediments composed predominantly of siltstone which are best exposed in the crags south of Clogwyn-y-Benar. They show an overall upward coarsening from mudstones at the base to siltstones with thin beds of sandstone showing ripple-drift and cross lamination at the top. The top of the sequence is ill defined in this area, for the Capel Curig Volcanic Formation consists of isolated masses of tuff, closely associated with coarse- to medium-grained sandstones. Farther east, however, the 350 m of strata between the Penamnen Tuffs at Penmachno and the more persistent Lledr Valley (Capel Curig) Tuffs of Rolwyd and Iwerddon consist of interbedded sandstones and siltstones together with one thin local tuff [SH 776 515]. In the tight anticlinal structure in the Lledr Valley, the equivalent beds consist mainly of siltstone, but sandstone, generally fine-grained, occupies the core of the fold and is well exposed on the southern limb of the structure, upstream from Gethin's Bridge.

Only the upper part of the sequence is seen in the Capel Curig Anticline where 100 to 135 m of sandstones with rare impersistent intercalations of siltstone form the core of the structure. The sandstones, exposed principally on the steep slopes of Creigiau'r Garth, are greenish, cross-bedded and medium- to coarse-grained, with local pebbly lenses. They owe their colour to a high content of finely divided chloritic material (E36831) which probably represents altered volcanic detritus.

Capel Curig Volcanic Formation[edit]

The formation crops out in four areas within the district. In three of these, namely in the type area around Llynnau Mymbyr, in the Lledr Valley and in isolated faulted ground north and west of Penmachno, it consists of two or three tuffs separated by sediments. In the fourth area, south of Dolwyddelan, the formation is represented by scattered isolated bodies of tuff which have ovoid outcrops. Correlation of individual tuffs from one area to another is uncertain, but the broad correlation is supported by their stratigraphy, their distinctive petrography and most importantly by their unusual relationships with subjacent sediments. At their lower contacts some of the tuffs locally transgress the underlying sediments at angles up to 900, with minor apophyses resembling magmatic intrusions. Some of the apophyses are detached from the main mass of the tuffs so as to form discrete bodies simulating tuff-pipes at outcrop. The tuffs are generally welded and in most places the eutaxitic foliation remains parallel to the regional dip, though locally it is parallel to adjacent margins. Moreover, the sediments adjacent to the transgressive undersurfaces are disturbed and reconstituted. To explain these features, Francis and Howells (1973) postulated deposition of ash flows in a subaqueous environment, whereby large-scale downsags analogous to load-casts were formed by liquefaction and yielding of the sediments during and after the emplacement of the tuffs. The orientation of the eutaxitic foliation is assumed to be related to the time of collapse relative to cooling.

The tuffs are cream to pale grey in colour, and are invariably well jointed, though they only locally show good cleavage. They consist mainly of shards, with few crystals, minor lithic debris and matrix. Vitroclastic texture ranges from non-welded through eutaxitic to parataxitic (Beavon and others, 1961).

Capel Curig[edit]

South-west of Capel Curig the formation, 120 to 220 m thick, is folded into a broad anticline and comprises three tuff members which are separated by sediments and which are named Garth, Racks and Dyffryn Mymbyr tuffs in upward succession.

The Garth Tuff is characterised by a remarkably discordant base, with apophyses and pipe-like load-casts, up to 100 x 250 m in plan, detached in the underlying sediments. The lower and middle parts of the unit are welded, but there is an upward gradation to evenly bedded, reworked tuff which is concordant with, and locally passes gradually into, the overlying sandstones. Layers of accretionary lapilli are recorded from the bedded upper part of the tuff on the southern limb of the anticline (Figure 4) and (Figure 5).

The sediments between the Garth and Racks tuffs consist mainly of fine-grained pale green sandstones, up to 60 m thick, which locally yield Soudleyan faunas (see p. 70; also Diggens and Romano, 1968). At the top is an impersistent band of mudstone which is thickest, 18 m, towards the north-eastern closure of the anticline, where it is locally disturbed beneath the irregular base of the Racks Tuff.

The Racks Tuff, up to 50 m thick, varies laterally in litho-logy from well bedded and unwelded in the west to massive and welded elsewhere. Traced north-eastwards along the north limb of the structure, the tuff, locally welded, becomes discontinuous, first forming isolated pods then vertical and horizontal stringers retaining a welded fabric. Clasts of welded tuff, devitrified glass with perlitic fractures and patches of siliceous nodules are common throughout the outcrop.

The Dyffryn Mymbyr Tuff is recognised only on the northwestern limb of the anticline where it is separated from the Racks Tuff by 28 m of cleaved grey siltstones and mudstones. It thins north-eastwards and passes from a coarse-grained lithic tuff, south of Cwm Clorad Isaf, to a fine muddy tuffite, north of Plas-y-Brenin. The lower part of the unit is muddy and contains accretionary lapilli. The content of mud and the sedimentary structures suggest that the Dyffryn Mymbyr Tuff is water-laid.

Lledr Valley[edit]

The Lledr Valley Tuffs (the local equivalent of the Capel Curig Volcanic Formation) form a faulted outcrop on the flanks of a tight anticline. The lowest unit is impersistent and consists of coarse elastic lithic tuff seen in small outcrops in Gallt Tan-yr-Allt.

The main and most persistent unit, up to 75 m thick, crops out between Lledr Cottage and the River Conway in the steep forest slopes on the north side of Mon Lledr. Correlation along the faulted crop is difficult, for it is apparent that in some places the tuff wedges out and in others its base cuts down into disturbed sediments. The general lithology is closely comparable to that of the Garth Tuff of Capel Curig; the lower and middle parts are massive and welded while the upper part contains undeformed shards, often fragmented, associated with complete bubble forms, clasts of pumice, spherulitic re-crystallised tuff and devitrified glass with perlitic fractures. Siliceous nodules, in places occupying the whole fabric, are a distinctive feature. Zones of brecciation, in which angular silicified tuff blocks are associated with patches of siliceous nodules, occur in the gorge of the Mon Lledr, 0.5 km E of Lledr Cottage.

Tuffs of similar lithology to the main unit crop out northeast of Lledr Cottage in three small bodies, oval in plan, transgressing bedded silty sandstones, at a horizon about 30 m above the main unit. In places the bodies have nodular peripheral zones in which welding can be seen to persist almost to the outer margins. The bodies may be the lobe-like remnants of a once continuous ash flow which was partly removed by intraformational erosion. Supporting evidence is provided farther east where an impersistent band of crystal-lithic tuffite crops out at the same horizon.

North and West of Penmachno[edit]

South of the Lledr Valley the Capel Curig Volcanic Formation crops out south-west of Ty-Mawr, on the steep slope near Bwlch-y-Maen, and in an outlier north of Iwerddon. Tuffs can be determined at up to three different horizons; because they are faulted and discontinuous they cannot be individually correlated from one area to another.

South-west of Ty-Mawr the massive, white-weathered and strongly welded lowest tuff is folded into a broad anticlinal structure. A similar tuff crops out in the steep valley side, west of Bwlch-y-Maen, and in the shallow synclinal structure north of Iwerddon. About Pigyn Esgob, south-west of Ty-Mawr, at a slightly lower stratigraphical horizon, similar tuff is seen in small oval-shaped outcrops which cross-cut the associated sandstones (Francis and Howells, 1973).

North of Iwerddon two welded tuff units are separated by siltstones and sandstones with an impersistent bed of crystallithic tuffite.

South of Dolwyddelan[edit]

Discrete bodies of tuff, similar to those described around Pigyn Esgob, and at approximately the same horizon, are the sole representatives of the formation to the south of Dolwyddelan. They occur near the base of the dominantly sandstone sequence that underlies the Snowdon Volcanic Group in this area. Outcrops are irregularly oval in plan and up to 170 m in their longest dimension. Margins are steep and clearly transgress the subjacent sediments. Above Afon Penamnen, nine such bodies have been mapped in a relatively small area. One exposed at Carreg Alltrem [SH 7395 5068] appears to be conformable at the base, where the lowest 2 m of tuff are green, cleaved and crystal-rich, resembling the basal part of the Rolwyd body figured by Francis and Howells (1973, fig. 6). The rest of the tuff is white and massive with prominent columnar jointing and a strong welded fabric (E38686), generally coincident with the regional bedding.

Nodules occur throughout the smaller bodies and near the margins of the large. The nodules are siliceous, with quartz and chlorite filling septarian fractures. Where observed, the fabric within the nodules is strongly welded parallel to the peripheries of the bodies. Similar nodules occur within the adjacent, often highly disturbed sediments.

Because these bodies resemble the pipe-like bodies of Capel Curig, the Lledr Valley and Rolwyd, it is assumed that they were formed similarly, as detached lobes of a submarine ash flow. As in the Lledr Valley, the parent sheet may have been removed by intraformational erosion and, if so, it is represented now only by a nearly conformable line of lenses of cleaved tuff in Cwm Penamnen.

Strata between the Capel Curig Volcanic Formation and the Crafnant and Snowdon Volcanic groups[edit]

Although considerable lateral variation in lithology is apparent in the strata between the Capel Curig and the Crafnant and Snowdon volcanics, they can be subdivided as follows:

3 Siltstones and sandstones up to 150 m
2 Sandstones with subordinate beds of acid tuffite, basic tuff and siltstone up to 225 m
1 Mudstones and siltstones with subordinate sandstones up to 600 m

This subdivision is convenient for purposes of description, though it should be noted that it is difficult to uphold in two areas. One is around Dolwyddelan, where the whole sequence consists of alternating thick siltstones and sandstones in the south, but of little more than siltstones in the north, where only the thin impersistent tuffite beds of 2 serve for correlation. The other is the ground between the Lledr and Llugwy valleys where both tuffites and sandstones in 2 are so thin or impersistent that they are not continuously mappable units.

1 The Capel Curig tuffs are nearly everywhere overlain by sandstone of variable thickness. On the northern flank of the Capel Curig Anticline it is thin and wedges out eastwards: on the southern flank, where the Dyffryn Mymbyr Tuff is absent or is represented only by volcanic detritus, it merges eastwards with the underlying sandstone. In the Lledr Valley up to 120 m of sandstone directly overlies the upper tuff unit east of Pen-aeldroch but it wedges out on the northern side of the closure of the anticline; it becomes more silty when traced eastwards.

The succeeding cleaved, commonly pyritic mudstones and siltstones are thickest (at least 450 m) on the flanks of the Capel Curig Anticline. There, between Cefn-y-Capel and Creigiau'r Gelli to the north, and across the northern and north-western slopes of Moel Siâbod to the south, they form craggy ground on which erosion has distinctively picked out bedding, cleavage and fault lineaments. They have been worked extensively for slates at Rhos Quarry [SH 7285 5635] and, on a smaller scale at two other quarries [SH 7155 5695]; [SH 7170 5550]. No macrofossils have been found in this area, nor has treatment of several samples yielded any organic-walled microfossils. To the east, however, a quarry [SH 7837 5501] in siltstone at the side of a forestry road east of Llyn Elsi yielded a predominantly shallow-water molluscan fauna. Other faunas obtained nearby [SH 7865 5596]; [SH 7845 5593] are of late Soudleyan to early Longvillian age (p. 70) and resemble the assemblages at the top of the Allt Ddu Mudstones and Lower Gelli-grin Calcareous Ashes of the Bala District (Bassett and others, 1966). In the southern part of the area, between Penmachno and Dolwyddelan, dominant mudstones give way to a sequence of silty mudstones with up to three thick sandstones.

2 Sandstones with subordinate acid tuffites, basic tuffs and siltstones collectively form a distinctive lithological sequence. In the ground to the north of the district, broadly equivalent strata were termed the Llyn Cowlyd and Bwlch Cowlyd Sandstone formations by Diggens and Romano (1968). The sequence can be traced southwards to Capel Curig, across the Clogwyn-yr-Eryr Syncline and the Pont Cyfyng Anticline to flank the synclines of the southern slopes of Moel Siâbod and of Dolwyddelan.

Because they weather white, the acid tuffites are the most distinctive members of the sequence. They are up to 10 m thick, though commonly 4 m or less, and occupy at least four horizons. At most localities, however, only two – not necessarily the same two – can be seen, so that long-range correlation of individual units is impossible. All are fine-grained rocks, and although a few of the specimens collected approach the composition of tuff, with perfect cuspate shards of about 0.1 mm (E38722), most are composed of fragmented shards set in a fine matrix which probably represents original volcanic dust with or without a mudstone fraction. The matrix consists either of a fine quartzose aggregate which has made the rock suitable for use as a honestone, or of a recrystallised micaceous aggregate (E38179) showing cleavage orientation. Small subangular fragments of quartz and feldspar are characteristic of some tuffites (E35155), and irregular plates of carbonate scattered throughout are a feature of others (E38176). Slender skeletal rod-like aggregates of ragged chloritic flakes showing no preferred orientation are a persistent microscopic feature throughout the district from north of Capel Curig (E35156) to Dolwyddelan (E38179). Cross-bedding is commonly seen, particularly at the tops of units: in places it is disturbed, as at an outcrop [SH 7258 5811] north of the Capel Curig Youth Hostel. The absence of well-defined sorting, the evidence of current-bedding and disturbance at the tops of units and the local impersistence all indicate that the tuffites represent the air-fall of fine ash into shallow water where it was subjected to reworking and to local penecontemporaneous erosion. The almost invariable presence of sandstone above the tuffites further suggests that the sudden arrival of some thickness of ash into shallow water attracted coarse sediment by shoaling until such time as subsidence re-established the deposition of silt.

Basic tuffs occur at two horizons, the main outcrops being north of Capel Curig. The lower horizon, which lies within the sequence of acid tuffites and sandstones, gives rise to a wide outcrop forming Curig Hill but thins rapidly to north and south (Figure 7) and (Figure 8). The prominently cleaved tuffs are chloritised, spilitised and are mainly vitroclastic. They are bedded throughout, although poorly sorted and rarely graded, and are apparently conformable at the top and at the base. Detailed measurements made in the course of 25-in mapping by Dr J. W. Baldock include a semi-quantitative size analysis of lapilli and bombs at 18 localities–all indicating a gradual increase in size towards the centre, near the top of the hill, where there is a marked change to a zone of slumped agglomerate with blocks measuring up to 0.5 x 1.0 m in diameter over an area some 10 m across. Even more significant are differences which he noted in structure on each side of a plane of discordance which crops out as a median north–south line within the basic tuffs. To the west of it dips are steep and centroclinal, decreasing from 80° at the margin to about 50° near the coarse slumped central agglomerate; this pattern is modified in the south by a small (penecontemporaneous slump?) fault. East of (that is above) the plane of discordance the dip is uniformly eastward, generally at 25° to 30°, concordant with that of the overlying sediments. This eastern sector includes a lens offine-grained, aqueously reworked, tuffs and tuffaceous sediments.

Bedded tuffs with marginally steep centroclinal dips are characteristic infillings of the upper levels of funnel-shaped necks or pipes, particularly those which once fed small, short-lived submarine or other maar-type volcanoes (bibliographies and mechanics of subsidence into such pipe-structures are given by Francis, 1970 and Lorenz, 1973). The bedded tuffs which lie above the discordance, passing upwards without a break into younger sediments of the normal succession, are similarly characteristic in marking the final destruction of ash rings and ash cones by post-eruption erosion and sedimentation at the sites of such volcanoes (Francis and others, 1968, pp. 404 405). On this evidence there can be little remaining doubt that Curig Hill marks the site of a former depression infilled by basic vitroclastic material derived from a small underlying volcanic vent.

The upper of the two basic tuffs lies near the top of the uppermost sandstone of the sequence and crops out in the same area north of Capel Curig. It forms a small crag near Bryn Llŷs [SH 7275 5790] and can be traced northwards along the western slopes of Clogwyn-mawr beyond the northern margin of the district. The unit is formed of two components, a lower mudflow and an upper basic tuff, both rather less than 2 m thick, separated by a parting of siltstone. The mudflow consists of angular clasts of sediments and acid tuffs rudely aligned parallel to the local bedding; it is probably the same bed as the lahar mapped discontinuously in the ground around Llyn Cowlyd farther north (Diggens and Romano, 1968). The basic tuff is bedded and contains clasts of basalt and basaltic pumice, both highly chloritised and carbonated, in a fine-grained matrix containing conspicuous amounts of chlorite and ore minerals (E38373), (E38374). As the unit cannot be traced south of the River Llugwy, a 2- to 3-m basic tuff which is impersistently exposed [SH 7100 5384] at about the same horizon 4.5 km to the south on the slopes of Moel Siâbod, is assumed to represent a deposit from a different volcanic centre.

The sandstones of the sequence are generally greyish green and well bedded, and are locally rich in shelly faunas. The bed overlying the lowest acid tuffite on the north side of Curig Hill is rich in Plaesiomys multifada Salter ( = Dinorthis multiplicata Bancroft) and is possibly the equivalent of the Multiplicata Sandstone mapped farther north around Llyn Cowlyd (Diggens and Romano, 1968), though there is evidence to show that the fossil is not restricted to one horizon. Towards the top of the sandstone sequence, north of Coed Bryn-y-Gefeiliau, a calcareous sandstone [SH 7411 5691] yielded a fauna of probable Longvillian age. The Soudleyan–Longvillian boundary thus lies within the sequence — probably nearer the base than the top on the evidence of forms obtained from Dolwyddelan (p. 70; Romano and Diggens, 1969).

In the ground south-west of Betws-y-Coed the sequence is ill defined. Three sandstones are recognised north-east of Mynydd Cribau, but they pass laterally into a sequence dominated by siltstones and slates farther east. Slates underlying the lowest local sandstone have been quarried at Hafod-las, near Betws-y-Coed, where they include a tuffaceous band, less than 30 cm thick, composed of talcose white mica and highly altered crystals of feldspar (E36235). On the west side of Mynydd Cribau the highest sandstone is underlain by a white-weathering band of tuffite (E37201) which does not appear to persist along the strike.

3 To the north of the district, around Llyn Cowlyd, Diggens and Romano (1968) divided the argillaceous strata at the top of the 'Glanrafon Beds' into a lower Pen Llithrig-y-wrach Siltstone Formation and an upper Marian Mawr Mudstone Formation.' They state that the Crafnant Volcanic Group unconformably overlies these beds, progressively cutting out the mudstone so as to rest on the lowest 45 m of siltstone at the northern margin of the present district. This unconformity has not been confirmed by the recent survey, which has shown both divisions (not shown separately on the published map) to persist, without reduction in thickness of the upper mudstone division, throughout the ground around Capel Curig. Moreover, although these argillaceous beds are much thinner in the southern part of the district, a siltstone/mudstone boundary can also be traced around the Dolwyddelan Syncline, where the mudstones have been worked for slate in the Pen Llyn and Rhiw Goch quarries.

The twofold division is less apparent around Betws-y-Coed, where a thick wedge of rhyolite crops out low on the slopes of Coed Aber-llyn, west of Hafanedd. Although the rhyolite is highly silicified, with the original fabric largely obliterated by a fine quartzose mosaic, flow-banding can still be distinguished, particularly in the northern exposures. To the south the rock is highly brecciated and the angular fragments of rhyolite show distinctive spherulitic recrystallisation of quartz and chlorite controlled by the flow-banding (E35629).