Journal of the Geological Society, London, Vol. 143, 1986, pp. 411-423, 9 figs. Printed in Northern Ireland

The submarine eruption and emplacement of the Lower Rhyolitic Tuff Formation (Ordovician) , N

M. F. HOWELLS,A. J. REEDMAN, & S. D. G. CAMPBELL British Geological Survey (NERC), Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed, Wales SY23 4BY

Abstrad: The Lower Rhyolitic Tuff Formation (up to 600 m thick) represents an eruptive cycle of acidicash-flow tuff whichis stratigraphically associated with marine sediments and subaqueously emplacedbasalt lavas. The formation comprises volcaniclastic and pyroclastic megabreccias and breccias, massive welded and non-welded acidic ash-flow tuffs, reworked tuffs and tuffities, siltstones, rhyolite intrusions and extrusions. Its basal contacts vary from conformable, to disconformable and unconformable.The inter-relationships of thesevariations to pre-, syn- and post-emplacement structuresdefine a submarine, asymmetric downsag caldera. The main eruptive centre, coincident with the thickest accumulation of intracaldera tuffs, lies close to its north margin, on the north side of the Massif. To the SW, the intracaldera tuffs thin progressively and much of the formation comprises tuffs reworked in the vicinity of a Caradocian shoreline. To the NE and E, outflow tuffs escapedinto a deeper marine basin. Many of thefeatures of thecaldera are similar to those of subaerial calderas, and it is concluded that the enveloping sediments and lavas, and the character of thereworked tuffs, hold the key to the recognition of its submarinedevelopment. Subsequent resurgence resulted in only local and short-lived emergence of the intracaldera tuffs.

The LowerRhyolitic Tuff Formation (LRTF) is the wder volcanotectonic context. It was proposed that Wales lowermost formation of the Snowdon Volcanic Group was the site of aback-arc basin and the volcanism was (Howells et al. 1983, 1985b). It crops out (Fig. 1) about the related to extension. The British Geological Survey are and hasbeen correlated (Howells et al. currently investigating the geochemistry of the Caradoc 1973, 1978, 19856) with part of the LowerCrafnant volcanic rocks of .This paper describes the Volcanic Formation in E and NE Snowdonia (Fig. 2). More variationin the physical characters of theLRTF and its recently, Campbell (1983) has recognized the formation to associated strataand discusses the environmentsand the SE of Betws y Coed. processes of its eruption and emplacement. The formation in central Snowdonia has figured largely in the descriptions andinterpretations of Ordovician volcanism in N Wales. Ramsay (1881) referred to the main Subjacent strata and basal contacts lithology as‘feldspathic porphyry’. Greenly (in Dakyns &L The LRTF rests mainly upon marine siliclastic sedimentary Greenly 1905) drew comparison between the ’felsitic slates’ rocks of the Cwm Eigiau Formation (Howells et al. 1983). with the deposits fromthe PelCan eruptionsat Soufrikre Its relationship tosubstrate varies fromconformable, to described by Anderson & Flett (1903). Williams (1927) disconformable and unconformable. Thecontact, where described facies variations within the formation about the conformable, lies above the Soudleyan-Longvillian stage Snowdon massif and supported Greenly’s observations.It boundary (Howells et al. 1978; Campbell 1983), which was not until the work of Oliver (1954) and particularly of coincides approximately with the Pitts Head Tuff (Fig. 2), Rast et al. (1958) thatthe dominant lithology of the an acidic ash-flow tuff. Basaltsoccurring within the formation was recognized as acidic ash-flow tuff. As it was sedimentary rocks consistently show evidence of subaqueous considered that the eruption and emplacement of ash-flows accumulation with the local development of pillows, pillow could only occur subaerially, the Caradoc palaeogeography breccias and hyaloclastites. was reappraised. The proposal by Shackleton (in discussion The broad facies variations of the subjacent strata are of Beavon 1963) of asubaerial volcanotectonic structure outlined in Fig. 3a. Massive bedded, coarse to fine-grained, about Snowdon was developed by Rast (1969) and Bromley cross and parallel-laminatedsandstones enclose anarea (1969) into a volcano with a central caldera, complete with wheresiltstones with impersistentsandstones in thin rim syncline and caldera fault, although this model was lenticular beds weredeposited. The sandstones contain contested by Roberts (1979). Beavon (1980) further shelly faunas which indicate (after Pickerill & Brenchley proposed a resurgent subaerial cauldron, with three major 1979) shallow marine environments,although most com- eruptive cycles, one of which is the LRTF. Although many monly they aretransported assemblages-crowding low- of the observations in this paperare similar to those of angled foresets, as atLlanberis Pass, and occurring as Beavon (1980), the overwhelmingevidence of the scatteredfragments within massive sandstones.Common sedimentary and basic extrusive rocks, associated with the dewatering structures in the coarse sandstones reflect rapid acidic volcanic rocks, indicates that the caldera developed in deposition, probably from mass flows. The sandstones also a subaqueousenvironment, and that its evolutionrelates include fine-grained tuff-turbidite beds, as in Cwm Idwal only to the eruption of the LRTF. (Fig. 1) and such beds underlie the LRTF in the Lanberis Kokelaar et al. (1984) have recently reviewed the Pass (Fig. 1). The variations in the sandstone facies suggest Ordovician volcanism and sedimentation in Wales and its high energy regimes with both wave and current processes

41 1

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CAPEL CURlG

BLAENAU Fig. 1. The outcrop of the Lower FFESTINIOG Rhyolitic Tuff Formation (LRTF) with reference to localities men- tioned in the text.

involved. Deposition in tidal and shallow subtidal On the NW limb of the Idwal Syncline (Fig. 1) the base environments, with periodic stormevents, is envisaged. of the LRTF is disconformable. To the SW, disconformity There is evidence, however, both in the NW, at Cwm Idwal increases and for most of theoutcrop between and Bwlch y ddeufaen, and in the SE, on the south side of Pass and Rhyd Ddu the base of the LRTF lies close to, or the Dolwyddelan Syncline (Fig. l), that the supply of coarse on top of the Pitts Head Tuff. South of the terrigenes was progressively depleted immediately prior to LRTF rests unconformably on an erosion surface of the deposition of the LRTF. upper Pitts Head Tuff flow. On the SE side of the Idwal Apart from local ridges, as at Y Braich and east of Cape1 Syncline in the Llanberis Pass the base of theLRTF Curig (Fig. l), the main area of siltstone deposition is not transgresses down through the Cwm Eigiau Formation to well exposed. In these two areas, thin flaggy beds of crystal below the Pitts Head Tuff (Howells et al. 1981b). The anddust tuff-turbidites occur within the siltstones. transgression from disconformity to marked unconformity is Intercalated sandstones are uncommon, of greywacke type, extremely local and is marked by a series of small faults and wedge out laterallyalong strike. The siltstones may which areinterpreted as having been active beforeand represent intensely bioturbatedand homogenized fine- during the emplacement of the LRTF. grainedsand and mud. Evidence of wave and current Onthe SW side of the Snowdon massif, angular processes is limited. A generally lower energy regime than discordance between the LRTF and the subjacent strata is for thesandstone facies is suggested-probably a deeper apparentover a wide area (Fig. 3a). Passage from subtidal environment largely beneath storm wave-base. concordancein the west to gross discordance in the east

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LU SNOWDON. MOEL HEBOG 8 DOLWYDDEIAN N E SNOWDONIA BLACK SLATES LIANRHYCHWYN SLATES UPPER RHYOLlTlC TUFF FORMATION i MIDDLE AND UPPER BEDDED PYROCLASTIC b CRAFNANT VOLCANIC FORMATION FORMATIONS ______\ \ \ \ \ LOWER CRAFNANT \ VOLCANIC FORMATION

LOWER RHYOLlTlC 'L_ TUFF FORMATION

1 PIIT'S HEAD TUFF CWM ElGlAU ' FORMATION CWM ElGlAU FORMATION ----_ CAPEL CURlG VOLCANIC FORMATION LLEWEL?~------VOLCANIC CAPEL CURlG GROUP VOLCANIC FORMATION 0 -1l m

L200 m

Fig. 2. Generalized vertical sections showing the stratigraphic relationship of the LRTF.

a b mWelded facles Discordance at base

Rhyollte mPltts Head Tuff mDlscordance i

Fig. 3. (a) Lithologies of strata subjacent to the LRTF, the basal discordance and isopachsof the ash-flow tuff of the LRTF. (b) The distribution of the welded faciesof the LRTF, the basal discordance, and associated rhyolite intrusions. Both 3a and 3b are palinspastic reconstructions assuming concentric folding.

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takesplace across a narrow NE-trending zone extending corrected for strain. Current studies indicate that over most from the west side of Moel Ddu to Cwm Llan (Fig. l) and of the area the vertical extensionin the tuffs varies from correspondingapproximately tothe BeddgelertFault of 0.95 to 1.30 (I. Wilkinson, pers. comm. 1985); values Rast (1961, 1969) and Beavon (1963, 1980). The base of the insufficient to invalidatetheinterpretation of major LRTF transgresses SE across a series of tilted fault blocks, thickness changes made below. The thickest sequence, and locally restsunconformably on steeply dipping >500m, occurs in the Snowdon massif, to the south of the sediments of the Cwm Eigiau Formation. The evidence at Llanberis Pass. Away from this area the LRTF thins to 125 Cwm Llanindicates that pillowed basaltswere extruded m in less than 1 km, across the Llanberis Pass. To the SW it after faulting commenced,and at Moel Ddu, faulting thins gradually and wedges out on Moel Hebog. Further E accompanied by disruption of the sequence by gravity and NEthe LRTF is represented by a single complete sliding was initiated immediately after the deposition of the ash-flow tuff, <70 m thick. Pitts Head Tuff and persisted during the early stages of the It is estimated thatthe central area, lying within the LRTF deposition. 200 m isopach (Fig. 3) contains a minimum of 34 km3 of ash-flow tuff while a minimum of 20 km3 lies outside this Thickness and volume of ash-flow tuffs area. The thickness of the primary ash-flow tuff component of the From the evidence of thickness variations of the primary LRTF variesconsiderably across its outcropand the ash-flow tuffs, the variations in their basal contacts,their variations are broadlycoincident with internal facies internal facies andthe distribution of associated rhyolites changes. The isopach map (Fig. 3) has been restored to its (Fig. 3b), it is considered that the central area represents a pre-folding configuration,but thicknesses have notbeen caldera about the eruptive centres of the formation.

B n

Fig. 4. Generalized vertical sec- tions of the LRTF related to its outcrop and conjectural position of the zaldera margin. Vertical bars to the left of the sections indicate thickness of the primary ash-flow tuffs.

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Lithological variations locally at, and near the base (B, Fig. 4), with lenses of the The LRTF comprises sedimentary and pyroclastic breccias, breccia up to 30m thick, which thin and pass laterally into acid ash-flow tuffs, reworked and remobilized tuffs, rhyolites trains of isolated blocks within an ash-flow tuff matrix. The and intercalated sediments (Fig. 4). natureand restricted outcrop of the pyroclastic breccias suggests that they are co-ignimbrite lag breccias similar to those described on Santorini by Druitt & Sparks (1982). Breccias Onthe southside of Snowdon, such basal pyroclastic Megabreccias. To the SE of the Gwynant valley (G, Fig. breccias are less persistent. On Moel Ddu, a similar breccia 4) a wedge of sedimentary breccia crops out at the base of is locally developedabove theupper discontinuity. Here, the LRTF. It includes very large blocks and rafts of bedded the blocks are predominantly of Pitts Head Tuff, up to 2.5 m sandstone and siltstonein amatrix that is dominantly long atthe base where they are locally sufficiently epiclastic, but with aminor pyroclastic component.The concentrated tobe self-supported.Generally these blocks rafts, up to 50m in length, and blocks, and their contained decrease in size and abundance upwards. faunas,are closely comparable with theadjacent undis- turbedstrata and the breccia is interpreted as a slide Ash-j70~ breccia. This breccia passes laterally and vertically into a The LRTF is dominated by acid ash-flow tuffs which can be volcaniclastic breccia composed of rafts,up to 50 m in divided into intracaldera and outflow facies, (Fig. 6). length, and blocks, of acid tuff, sandstone and siltstone in a crystal, vitric tuff matrix which locally contains abundant Intracaldera tugs epiclastic debris. The blocks of acid tuff, which are (a) Massive non-welded tuffs. Inthe area about lithologically similar tothe overlying tuffs within the Snowdon theLRTF, >500 m thick is almostentirely formation, were lithified prior to incorporation in the composed of unbedded, non-welded ash-flow tuff. Typically breccia,and represent disruption of,and mass-gravity the tuff consists of avariable admixture of shardsand sliding of some of the earliest erupted material of the LRTF. feldspar crystals in a matrix of sericite and chlorite (Fig. 7a). As a result of the small outcrops the division between the Locally, where the shards are tightly packed, thereare twomegabreccia types is difficult to define accurately. indications of mutualinterference. Wherethe matrix However, they are intimately associated, and reflect active component is greater, it is commonly well cleaved and in faulting which preceded and accompanied the earliest theseinstances the shards are tectonically distorted. The eruptive phase of the LRTF. Both megabreccias form part shards are generally less than 0.2 mm, occurring mainly as of the Llyn Dinas Breccias of Beavon (1963, 1980) which he fragmentedrods and spikes although multicuspate and interpreted as the product of collapse caused by tumescence completebubble forms arecommon. Locally a distinctly related to a subjacent magma chamber. bimodal shard population can be distinguished (Fig. 7a) with Similar megabreccias which developed by sliding from larger,thicker cuspate andtabular fragmentsscattered in the caldera wall duringthe earlystages of caldera the densely packed finer shards of the matrix. The shards development occur on Moel Ddu (Fig. 5).Here, two are devitrified and recrystallized, consisting of a fine quartz unconformities have been distinguished, one at the base of mosaic, which contrasts sharply with the matrix comprised and the other within the LRTF, which merge along strike. of a fine-grained aggregate of sericite andchlorite Between the unconformities, large rafts of Pitts Head Tuff representing devitrified and recrystallized vitric dust. have been incorporatedboth within ash-flow tuff and an Siliceous recrystallization of the matrix is less common intercalated sandstone lens. The confused relationships althoughwhere it occurs it tendsto obscure the original between these two elements result from tectonic disruption shardfabric and is associated with segregations of green and mass flow which occurred before, during and after the biotite flakes. LRTFemplacement. On Moel Hebog, megabreccias The crystal fraction is represented by albite-oligoclase comprise rafts of the Pitts Head Tuff >l00 m in length, and feldsparand much less abundantquartz, although locally smaller blocks of the local welded basal LRTF in a vitric tuff both are of rare occurrence. Typically the crystals are of matrix. The occurrence of this breccia within the sequence indentedsubhedral and hollow form, reflecting irregular of reworked LRTF tuffs suggests it was emplacedas a crystallization. subaqueous slide fromthe faulted margin of the caldera Above the basal zone, clasts are almost entirely of small following the main eruptive phase. Both these occurrences tubular pumice (Fig. 7b), up to 4mm, with extremely of megabreccia are similar to thosedescribed by ragged terminations. Compositionally the pumice clasts are Busby-Spera (1984) in the submarine caldera of the Mineral identical to the fragmented material of the host and can only King area of California. be determined as a result of the perfect preservation of the fabric by recrystallization. Pyroclastic breccia. Onthe N and SW sides of the Snowdon massif, pyroclastic breccias (Fig. 8a) locally form a (b) Welded tuffs. The intracaldera tuffs are locally distinctive facies at the base of the LRTF (Fig. 4). In the welded. To the SE of the Gwynant valley, SW of Moel y Llanberis Pass, a coarse lithic breccia, up to 40 m thick, is Dyniewyd (Fig. l),the lowest tuffs are welded (Fig. 7c) and clast-supported at its basebut grades up througha crowded with siliceous nodules. Above this basal zone the matrix-supported zone into ash-flow tuff. The blocks, up to predominant lithology is of very fine-grained silicic tuff, 0.7 m long, are mainly ot basalt, acid tuff and rhyolite, with locally intensely jointed, with a distinctive even foliation. In rare sandstone and siltstone. The matrix comprises lithic, thin section the fabric is seen to comprise a very fine shardic, crystal tuff with little epiclastic debris. Northwards aggregate of quartz, sericite and chlorite in varying along the outcrop into Cwm Idwal, a similar facies persists proportions.Fine impersistent segregations of slightly

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Main eruptive phase; Acid ash- mainly non-welded. flow tuffs Base unconformable. of the LRTF

:.:.. v I11 I *.*: *.*: .~:~:~:+:v1 st eruptive phase; """""1" ""1"" ,:* ,:* mainly welded, silicified. kbrecciation

:..., .. . .': Megabreccia; Pitts Head Tuff rafts in sandstone matrix.

Pitts Head Tuff; welded, brecciated, siliceous nodules.

Sandstone, siltstone, conglomerate U. . . . . and tuffite.

.@~~OO

m+++++++ Rhyolite; shallow-level intrusion.

Low-angle slides A

A28 Bedding, dip in degrees

Welding foliation, 200 4% 5 dip in degrees E 100

l...... I

Fig. 5. Detailed map and cross section of Moel Ddu, locality 14 on Fig. 1.

coarser quartzose recrystallization accentuate the foliation. In the SW, from Cwm Llan to Moel Hebog, the basal In most of these rocks the only indications of the original welded tuffs form a prominent feature. On the SE limb of fabric arethe isolated sericitized and chloritized feldspar the syncline, NE of Rhyd Ddu,the distinctively crystals. However, in rare instances there is clear evidence white-weathered tuffs contrast sharply with the overlying thatthe foliation reflects a welded shard fabric that is ochreous-weathered, locally clast-rich, non-welded tuffs. overprinted by areas, or 'clouds', of recrystallization (Fig. The welding foliation is accentuated by fine quartz 7c). The contacts between the two fractions are gradational. segregations and locally the tuffis columnar-jointed. On

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Reworked or remobilizedtuff m .' '.v; a: AL..,&! Volcanicrnegabreccla m003 Sedimentary megabreccia

m m Pitts head tuffblocks

b

0- 5 km

SW NE

Fig. 6. (a) Diagrammatic section through the LRTF showing the distributionof lithologies and relationship to the Pitts Head Tuff. (b) Section to scaleof the LRTF primaryash-flow tuffs, palinspastically restored to indicate the form of the LRTF caldera.

Moel Hebog, the top of the welded tuff was eroded prior to Crafnant Volcanic Formation (Howells et al. 1973, 1978, deposition of the overlying tuffs which wedge out rapidly to 1981a). Thisformation comprises three discrete ash-flow the south. On Yr Arddu, the sequence is at least 200 m thick tuffs interbedded with siltstones. Howells et al. (1973, 1978, and comprises 12 discrete flow units of welded ash-flow tuff. 1981a) considered the lowest two ash-flows were the Fossiliferous tuffites indicatemarine reworking between correlatives of the ash-flow of the LRTF. However, it is now flows low in the sequence. recognized that in this area the lowest ash-flow is the sole Where the welded facies clearly forms the basal zone of representative of the LRTF and thesecond flow was erupted the ash-flow tuffs, the welding is regarded as a normal post from a separate volcanic centre to the NE. emplacement compactional feature. However, to the NW of The LRTF outflow tuff NE of Cape1 Curig, is up to 70 m the Gwynant valley, irregular, silicified bodies of similar thick (L, Fig. 4) and lies conformably in the siltstone lithology occur within the non-welded vitroclastic tuffs. sequence.It comprises a crystal- and clast-rich base,a Even though no original fabric has been clearly determined massive uniform centralzone and a fine-grained top. The in thesebodies the similarity of the recrystallization and main body of the tuffis petrographically similar, in shard foliation, enhanced by silicification, is so close to those of types,feldspar crystal content and variable proportions of the basal welded tuffs that they are considered to represent matrix, to the massive facies of the intracaldera tuff. Locally pods of laterally impersistent, silicified, welded ash-flow tuff. the shards in the basal zone are replaced by sericite and in In these instances, as a result of their irregular form, it is these instances the cleavage is well developed. Clasts in the difficult to ascribe the welding to normal and uniform basal zone include irregular blebs of siltstone, which were post-emplacementcompaction. It is suggested thatthe incorporated while unlithified (Fig. 7d), and well-preserved welding and silicification is related to localized post- brachiopods and trilobites, which indicate that the flow was emplacement high-level heat-flow and volatile streaming, emplaced in a marine environment. within the intracaldera tuff. Upwardsthrough the tuff there is a slight decrease in abundance and size of the crystal fraction (Howells et al. Outflow tuffs To the E and NE of the Snowdon massif, 1973). The fine-grained top, varying between 1 and 6m, theLRTF has beencorrelated with part of the Lower comprises devitrified and recrystallized vitric dust which

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Fig. 7. (a) Photomicrograph of non-welded tuff. Devitrified cuspateshards, rods and spikes in a fine micaceous matrix; (b) Photomicrograph of pumice fragment (P) in shardic matrix;(c) Photomicrograph of welded tuff fabric, overprinted (lowerright) by cloud of fine siliceous recrystallization; (a) Photomicrograph of siltstone clast invitroclastic tuff. Irregularclast shapeand included shards atits margins indicate unliihified stateon incorporation into the ash-flow. settled from the water column following emplacement of the 1973). This foliation is accentuated by thin chloritic laminae body of the flow. It probably represents fine material although there is no variation in the ash-flowtuff elutriated from the flow head. constituents on either side. If defines intra-flow units within Siliceous nodules are locally common in the outflow tuff the outflow tuff and is progressively better developed to the and carbonate nodules, characteristically weathered out, are east with low-angle internal cross-lamination at Moel sparsely distributed. Welding is extremely localized; it has Siabod. The foliation reflects the development of laminar been recognized atthe west end of the Dolwyddelan shear horizons within the flow. A similar phenomenon is Syncline and near Pen y Castell (Fig. 1) in NE Snowdonia discussed by Busby-Spera (1986) in subaqueous ash-flows (Howells et al. 1981~). In bothinstances it is considered that of the Mineral King area, California. Further E, Campbell welding developed as a result of local insulation of the (1983) described 14 m of massive and weakly-bedded ash-flow following its emplacement. ash-flow tuff overlying c. 2m of flaggy-bedded, Weakly developed bedding is a characteristic feature of cross-laminated, tuff-turbidite. Thislower unit may repre- the outflow tuffs. In Cwm Idwal, about 3 km north of the sent flow-transformation (Fisher 1983, 1984) dueto the caldera margin, the lower part of the LRTF comprises beds ingestion of water at theflow head and thewaning energy of of primary ash-flow tuff up to 1.5 m thick. There are no the flow distally. intercalated sediments. The composition and fabric of these tuffs show no variation either between or within the beds and they may represent repeated pulses of ash-flows from a Reworked and remobilized tuffs single eruptive phase. Additionally, further from thecaldera Reworked and remobilized tuffs occur both within and margin, the single outflow of E and NE Snowdonia displays outside the caldera. Shallow marine reworking is mainly a crude internal bedding foliation (Fig. 8b) (Howells et al. confined to the top of the LRTF about the Snowdon massif

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Fig. 8. (a) Pyroclastic breccia. Large blocksof basalt in acid ash-flow tuff matrix at the baseof the LRTF intracalderatuffs. SW of Snowdon summit. (b) Outflow tuff with weakbedding (B). NE of Cape1 Curig.

and to most of the formation on Moel Hebog. Both these flaggy to massive beds of pyroclastic debris, which most localities include somebeds of tuff remobilized as mass probably represent mass flows generated from previously gravity flows. Inthe formation to the SE and N of the emplaced pyroclastic material. These beds are overlain by Snowdon massif, outside the caldera, remobilized tuffs are thin flaggy beds of reworked tuff and volcaniclastic the dominant lithology. sandstone (Fig. sa). About Snowdon, the top of the non-welded massive tuff Similarly, tothe north of Llanberis Pass, theLRTF is reworked and comprises up to 20 m of green-grey tuff shows a progressive increase upwards in the influence of the beds, 0.1-1 m thick. Some beds are poorly sorted with no backgroundsedimentation. Abovethe outflow tuff, the internal laminations while others show parallel and sequenceincludes remobilized ash-flow tuff, mixed with cross-lamination. Some bedsinclude much fine-grained varying proportions of epiclastic debris with intercalated epiclastic debris and towards the top of the sequence thin marinesiltstones and thinsandstones becoming dominant conglomerate beds, composed of rhyolite and some acid tuff higher in the sequence. The siltstones include fine-grained clasts form a distinctive component. Such conglomerates are tuff bands, display soft sediment deformation structures and not thick or extensive but clearly indicate uplift and local are locally bioturbated. Towards the top of the sequence littoral reworking of the LRTF. remobilized tuffs again become prominent. On Moel Hebog, above the basal welded tuff and a thin wedge of non-welded tuff, the formation is dominated by the reworked facies. In the lower parts of this sequence, impersistentconglomerates, with pebbles and cobbles of Suprajacent strata welded tuff and rhyolite are common. These are overlain by Thestrata overlying theLRTF reflect twocontrasting flaggy-bedded, cross-laminated,coarse- and medium- depositional environments with differing volcanic activity; a grained tuffs, tuffites and volcaniclastic sandstones which shallow waterenvironment with basic volcanism and a locally contain a rich, low-diversity shelly fauna. deeperwater environment with coeval acid and basic Remobilizedprimary tuffs are progressively more volcanism. Inthe vicinity of the caldera theLRTF is prominent to the SE of the Snowdon massif, on the SE side overlain by volcaniclastic sediments, tuffites, basic tuffs and of the Gwynant valley and in the outliers at Dolwyddelan basalt lavas of theBedded Pyroclastic Formation. The and MoelSiabod (Fig. 1) (Howells et al. 1973). Also the contactbetween the two formations is well exposed, epiclastic fraction increases in this direction. In particular in generally conformablealthough locally unconformable. these areas the lowest elements of the formation comprise Pillowed basalt lavas and basaltic hyaloclastite are of massive, coarse-grained, clast-rich tuffs and feldspar-rich frequent occurrencethroughout the formationand the tuffs interbedded with siltstones. The tuffs suggest sediments include much basaltic debris. The lowermost beds emplacement by mass and debris flowof previously represent a prograding pile of lithic breccias produced by emplaced pyroclastic debris. The predominance of indented the erosion of subaerial volanic centres (B. P. Kokelaar ash-flow tuff clasts in the massive tuffs indicates they were pers.comm. 1985). The activity at thesecentres was unlithified when incorporatedand these beds possibly typically Strombolian, with explosive eruptions interspersed represent the sloughed lateral equivalents of the volcaniclas- with quiescent basalt effusion. The succeeding strata are of tic megabreccia, at the base of the formation, northof Moel turbiditicsediments with intercalated basaltic pillow lavas Dyniewyd (Fig. 1). Towards theeast, the sedimentary and hyaloclastites which reflect generalsubsidence and a component increases progressively and the tuffaceous beds deeper water environment. In the Dolwyddelan Syncline the wedge out. epiclastic influence in theBedded Pyroclastic Formation Also at the outflow tuff is overlain by thick increases progressively from W to E where beds of black

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Fig. 9. (a) Reworked tuffs and tuffaceous sandstone with low anglecros ,S laminations and small channels. Secondary silicifications accentuates the tuff beds (T). Moel Siabod. (b) Rhyolite with contortedflow bands. Llanberis Pass

mudstone are interbedded with siltstones and impersistent extrusive domes of rhyolite are common within the sandstones. intracaldera tuff of central Snowdonia (Fig. 3b) and the field NE of the Snowdon massif, the outflow tuff is overlain evidenceindicates that they were emplaced immediately by siltstones and mudstones interbedded with acid ash-flow prior to, during and after the deposition of the LRTF. The tuffs, tuffites and tuff-turbidites (Howells et al. 1978, 1981~). rhyolites are typically flow-banded, fine-grained and sparsely This sequence of tuffs and sediments shows much evidence porphyritic. The flow-banding is characteristically contorted of secondaryslumping andreflects deep awater (Fig. 9b) and autoclastic brecciation is common. In outcrop accumulation with no indication of shallow-marine erosion the rhyolite may be intensely jointed with columns locally ordeposition. This interpretation is supported by the well developed. Perliticfractures, which in places can be palaeoecology of the associated faunas with shallow-marine, determined in hand specimen, indicate the originally glassy brachiopod-trilobiteassociations in the vicinity of the character of the rhyolites. In thinsection this fabric is Snowdon massif being replaced laterally tothe NW by devitrified and recrystallized into a fine quartzo-feldspathic graptolitic faunas. A subaqueous acidic volcanic centre has aggregate with sericiteand chlorite shreds. The flow- been defined (Howells et al. 1978, 1981a; Kokelaar et al. banding is accentuated by segregation of coarser quartzose 1984) in the area of SW of Dolgarrog. recrystallization, varying proportions of included sericite Somefine-grained acid tuff bandsoccur in the basic andchlorite and spherulitic recrystallization in adjacent sequence of centralSnowdonia and scarce thin turbiditic bands. sandstones, rich in iron oxides, areinterbedded with the On the south side of Llanberis Pass the large intrusions mudstones overlying the LRTF in E Snowdonia. However, of rhyolitein the LRTF extend into extrusions which in the main the contrasting contemporaneous depositional predatedthe emplacement of theBedded Pyroclastic environmentsand differing volcanic activity of thestrata Formation.The lowermost beds of thelatter include overlying the LRTF in central and E Snowdonia show little conglomerates with rounded clasts of rhyolite in the vicinity evidence of mutualinterference. It has been suggested of small stacks of flow-banded rhyolite(B. P. Kokelaar pers. (Howells 1977; Kokelaar et al. 1984) thatthe two comm. 1985). environments, shallow marine with basic volcanism in NW of the Gwynant valley, on the south side of the centralSnowdonia and deep marine with predominantly Snowdon massif, thecontemporaneity of local rhyolite acidic volcanism in the east, were separated by a positive extrusion and the Bedded Pyroclastic Formation emplace- topographic feature, possibly a ridge or a fault scarp. ment is displayed in small flow-banded rhyolite domes with In the extreme NE of the area, just SW of COnwy, a marginal breccias in juxtaposition withmass-flow basaltic basic volcanic centre was active before, duringand after breccias of the Bedded Pyroclastic Formation. Similarly, on eruptionand emplacement of theLRTF. This activity is Moel Ddu and Moel Hebog, the concentration of rhyolite reflected in the Tal y Fan Volcanic Formation (Howells et clastsin the bedded tuffs suggests reworking of extrusive al. 198%). The second ash-flow tuff of the Lower Crafnant rhyolite domes or flows. Volcanic Formation can be tracedthrough the basic tuffs Thedistribution of rhyolites within theLRTF is and hyaloclastites of theTal y Fan Volcanic Formation concentrated (Fig. 3b) mainly within or close to the thickest which indicatesthat the emplacement of thelatter was expression of the ash-flowtuff in thearea between the accompanied by local subsidence. Llanberis Pass andthe Gwynant valley. The rhyolites intrudingthe upper part of theBedded Pyroclastic Formationand the Upper Rhyolitic Tuff Formation Associated rhyolite intrusions and extrusions immediately to the east of Snowdon indicate the continuing The acidic ash-flow tuffs of the LRTF were producedby the availability ofacid magma well after the main episode of explosive eruption of rhyoliticmagma. Intrusive and ash-flow tuff emplacement.

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Mineralization tectonism triggered subaqueous mass gravity sliding towards In centralSnowdonia, Cu,Pb and Zn mineralization is the west with the production of sedimentary megabreccias. largely confined tothe base of theBedded Pyroclastic The earliestphase of LRTF volcanism involved the Formationabove the intracaldera tuff of theLRTF eruption of acid ash-flow tuffs some of which were (Reedman et al. 1985). Also there is a close coincidence of subsequentlydisrupted by continued mass gravity sliding the mineralization with the high-level intrusive rhyolites and and incorporated as rafts and blocks of lithified tuff in the irregularpods of silicified welded tuff, particularly in the volcanic megabreccias. This early volcanic activity, a zone of pre- and post-caldera faulting extending NE from precursor to caldera formation, probablyresulted from Moel Ddu to east of Cwm Llan (Fig. 1) and identified by fissure eruptions within the zone of faulting and gravity Beavon (1980) as the site of an apical graben.The sliding. Palaeontological and geochemical studies in progress mineralization (Reedman et al. 1985) generally occurs as suggest that the earliest ash-flow tuff eruptions were in the sub-vertical veins, usually 5-30 cm wide (max. 5 m), narrow vicinity of Yr Arddu (F, Fig. 4) where up to twelve flow stockworks and disseminations of quartzand sulphide. unitsaccumulated. Rhyolites geochemically identical with Pyrite is the earliestmineral, succeeded by pyrite with the Yr Arddu tuffs cropout ina narrow zone extending chalcopyrite, sphalerite and lastly galena. 6 km to the NEof Yr Arddu, probably reflecting pre-caldera faulting. There is evidencein the disposition of later rhyolitic intrusions and sulphide mineralization (Reedman et al. 1985) thatthe pre-calderaNE-trending fault zone Evolution of the LRTF extending from Moel Dduthrough and Cwm The LRTF represents a major period of acidic ash-flow tuff Llan, continued to be influential during and after the main eruption.The formation reflects one main eruptive cycle. caldera-forming eruption. The latter event may itself have Variations across outcrop of the ash-flow tuffs can be related been initiated by mass gravity sliding from the active fault to differentsites of emplacement with respect to distance zone (Beavon 1980). from the eruptive centre and local palaeogeographic setting. On Moel Ddu close tothe southern margin of the Based mainly on the evidence of lateral thickness changes of caldera,and at thesouthern end of the pre-caldera fault the primary pyroclastic flows, acaldera has been defined. zone, slumped and disrupted ash-flowtuffs of theLRTF Here it is necessary to considermodela forthe containing largerafts and blocks of the underlying Pitts palaeogeographic settingand development of the caldera. Head Tuff, are discordantly overlain by massive, mainly rhis model has to account for the following points: non-welded ash-flow tuffs representing the main pulse of the (1) the presence of marinesedimentary rocks or LRTF eruptive cycle. Similarly at Moel Hebog, at the SW subaqueously emplaced basaltic lava immediately below the margin of the caldera, welded tuff forming the base of the LRTF, with the exception of a restricted area in the SW; LRTF sequence was locally eroded prior to the emplace- (2) the lack of major erosional breaks within the thickest ment of non-welded ash-flow tuff, while in the SE, on Yr part of the intracaldera tuff sequence; Arddu (F, Fig. 4), the lowest ash-flow units have reworked (3) the restricteddistribution of the reworked tuffs, topscontaining a marine shelly fauna. Incontrast, away predominantly atthe top of the intracaldera tuffs, and from the southern margin of the caldera, the intracaldera thickest in the SW about Moel Hebog; tuffs form a single sequence lacking any laterally persistent (4) the local distribution of remobilized tuffs close to the erosional breaks. In the central area this sequence comprises north and east margins of the caldera; welded tuff in the lower part overlain by massive (5) the presence of a single outflow tuff unit in the north non-welded tuffs, and in the north over 500m of massive, and east, concordantly enveloped by marine sediments and non-welded tuff which lack clear indication of individual lacking evidence of shallow marine reworking of its top over flow units though current geochemical studies suggest that most of its outcrop; two cycles of magma withdrawal were involved. The caldera (6) the contrastingenvironments of the suprajacent developed in two main stages;during the initial stagea sequence, varying from shallow-marine volcaniclastic sedi- relatively small volume (

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commencement of the secondphase of caldera co-ignimbrite lag breccias, in the caldera to bedded distal development. outflow tuff, can be matched in many subaerial centres. A ring fracture defining the caldera structure has been Busby-Spera (1984) has described a Triassic to early recognized only at Moel Ddu. In the north,in the vicinity of Jurassic submarine caldera complex exposed in the Mineral the Llanberis Pass the position of the calderamargin, as King area of the SierraNevada, California, and Ohmoto defined by an abrupt change in thickness and facies of the (1978) has interpreted the Miocene Hokuroko Basin in the ash-flow tuffs (Fig. 3a), is either occupied by late-stage Green Tuff region of Japan as a submarine caldera complex. rhyolite intrusions or obscured by drift deposits. Elsewhere Marinesediments in the Mineral King areaindicate that the evidence indicates that the margin of the caldera was a water depths of 10-500 m, a range similar to that envisaged downsag with onlyminor faulting of the immediately for the LRTF, but much less than the 4000 m proposed for subjacent sediments. Deeper-seated ring fractures which did the Hokuroko calderas (Ohmoto 1978). According to Fisher not propagate to surface may be reflected by the arcuate (1984), however, the limiting depth for the extensive distribution of some of the late-stagerhyolite intrusions vesiculation of silicic magma, the PressureCompensation (Fig. 3b). Level, is 500 m. Even this depth of water might be expected The top of the intracaldera tuff sequence was subjected partially to suppress the eruption column and might explain to shallow-marinereworking. The upwardtransition from the absence of any significant Plinian fall-out tuffs beneath primary to reworked tuff is gradational suggesting gradual the ash-flow deposits of theLRTF. A similar lack of a encroachment above wave-base either by excess of Plinian phase at the subaerial Cerro Galan caldera in NW deposition over subsidence or by subsequent uplift possibly Argentina, however, was ascribed by Sparks et al. (1985) to representing resurgence of the caldera. The greatest uplift, the extremely rapid development of the eruption to massive with the local development of an unconformity between the proportions with no initial period of sufficientlylow LRTF and the suprajacent strata, was in the vicinity of the discharge rate when a convecting column could be northern margin of the caldera. Here the intracaldera tuff sustained. It is difficult therefore to find any feature of the sequence is thickest and late-stagerhyolite intrusions and primary products of subaqueous ash-flow eruptions that can extrusive domes are particularly abundant. Uplift may also be uniquely ascribed to theirsubaqueous setting. It is have been responsible forthe sloughing away of un- envisaged thatthe hot pyroclastic flows were largely consolidated tuffs into deeper water to the north and east protected from direct contact with water by a carapace of to form theremobilized tuff facies. There is no evidence that steam (Sparks et al. 1980; Kokelaar et al. 1984, 1985; uplift causedextensive emergence of the intracaldera Howells et al. 198%) and only the outflow became admixed sequence as, exceptin thenorth, the LRTF is overlain with substantial amounts of seawater in its most distal conformably by shallow-marine basaltic volcaniclastic expression where flow transformation is postulated. sandstones, tuffs, hyaloclastites and pillow lavas. In contrast tothe primary tuffs, the reworkedand remobilized tuffs together with the subjacent and suprajac- ent sediments and lavas clearly reveal theirsubmarine Discussion setting. It is apparenttherefore that the recognition of ancient submarine caldera-forming eruptions depends on a Large volume, subaerial, caldera-forming ash-flow eruptions careful examination of the character of sequences developed have been extensively discussed in the literature but there immediatelyprior to, andafter the caldera-forming are few descriptions of their subaqueous counterparts. Do eruption,rather than eitherthe character of the ash-flow significant differences exist between the products of tuffs themselves or of the caldera structure. subaerial andsubaqueous calderaforming ash-flow eruptions? This paper is published by permission ofthe Director, British Most of the features of the LRTF caldera and primary Geological Survey (NERC). It results from work undertaken as part ash-flow tuffs are common to subaerialcalderas of of the Snowdonia Regional Geological Survey. We acknowledge the comparable size. The control of various features of assistance received from all our current colleagues and particularly caldera-related volcanism by pre-existing faultzones, for the contribution of P. Kokelaar in discussions ofthe model example, has been describedinnumbera of caldera presented. Also we are grateful to B. Leveridge and C. Evans for complexes(e.g., Valles Caldera, Nielson & Hulen 1984; their contribution during the early phase of BGS workin Questa Caldera, Lipman 1983). Asymmetric, trapdoor-type Snowdonia. Finally we wish to dedicate this paper to the memory of subsidence of the calderafloor, followed by asymmetric Professor Howel Williams whose career, with its massive uplift, greatest in the area of thickest tuff accumulation, is contribution to the understandingof ash-flow tuff volcanism and seen in the Platoro Caldera (Lipman 1975) and the Grizzly caldera development, began in the Snowdon area. Peak Caldera (Fridrich 1983). Walker (1984) haspointed outthat manysubaerial calderas lack clear ring fractures andare bestdescribed as downsags. Lipman (1984) has References suggested that pyroclastic eruptions of less than 50-100 km3 may often cause incomplete, hinged caldera subsidence or ANDERSON,T. & FLETT,J. S. 1903. Report on the eruption of the Soufrikre in structural sags. Incremental caldera growth has been noted St. Vincent in 1902 and on a visit to Montagne Pel& in Martinique. Philosophical Transactions of the Royal Society of London, Series A., by Walker (1984), and Lipman (1984) has stated that most Zoo, 353-553. large calderas collapse concurrently with ash-flow eruption, BEAVON, R. V.1963. The succession and structure east of the Glaslyn river, resulting in the accumulation of very thick intracaldera fill North Wales. Quarterly Journal of the Geological Society of London, often interleaved with marginal collapse breccias. Facies 119, 479-512. - 1980. A resurgent cauldron in the early Palaeozoic of Wales, UK. variations within the primary ash-flow tuffs of the LRTF, Journal of Volcanology and Geothermal Research, l, 151-74. varying from massive welded and non-welded, locally with BROMLEY,A. V. 1969. Acid plutonic igenous activityin the Ordovician of

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NorthWales. In: WOOD, A.(ed.) The Precambrianand Lower andAssociated Sedimentary and Tectonic Processes in Modern and Palaeozoic rocks of Wales. 387-408. University of Wales Press, Cardiff. Ancient Marginal Basins. Special Publication of the Geological Society, BUSBY-SPERA,C. J. 1984.Large-volume rhyolite ash-flow eruptions and London, 16, 245-69. submarinecaldera collapse in theLower Mesozoic Sierra Nevada, LIPMAN,P. W. 1975. Evolution of the Platoro caldera complex and related California. Journal of Geophysical Research, 89, 8417-28. volcanicrocks, southeastern San Juan Mountains, Colorado. United -, 1986. Depositional features of rhyolitic and andesitic volcanic rocks of States Geological Survey Professional Paper, 852. theMineral King SubmarineCaldera Complex, Sierra Nevada, -. 1983. The Questa caldera, northern New Mexico-relation to batholith California. Journal of Volcanology and Geothemal Research, 27, 43-76. emplacementand associated molybdenum mineralization. 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Received 1 July 1985; revised typescript accepted 17 December 1985

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