Submarine Silicic Volcanism and Associated Sedimentary and Tectonic Processes, Ramsey Island, SW Wales

1. geol. Soc. London, Vol. 142, 1985, pp. 591-613, 7 figs, 3 tables. Printed in Northern Ireland

Submarine silicic volcanism and associated sedimentary and tectonic processes, Ramsey Island, SW Wales

B. P. Kokelaar, R. E. Bevins” & R. A. Roach?

Department of EnvironmentalStudies, University of Ulster at Jordanstown, Newtownabbey, Co. Antrim BT37 OQB, UK; *Department of Geology, National Museum of Wales, CathaysPark, Cardiff CF13NP, UK; ?Department of Geology, The University, Keele, Staffordshire ST5 5BG, UK

CONTENTS General geological setting 592 The geological history of Ramsey Island 593 Pre-Arenig rocks 593 Tnvyn Llundain Formation (a) 593 Ogof Velvet Formation (b) 593 Development of marine conditions through the Arenig 593 Ogof HCn Formation (c & d) 594 Road Uchaf Formation (e) 594 Late Arenig to early Llanvirn ‘distal’ volcanic episodes 594 Aber Mawr Formation (f-i) 595 Interpretations of ‘distal’ volcanism 596 Early Llanvirn ‘proximal’ volcanic episode 596 Porth Llauog Formation (j) 596 Penecontemporaneous sliding of sediments 600 Interpretation of volcanotectonism 600 Carn Llundain Formation (k-p) 60 1 Ogof Colomenod Conglomerate Member (k) 60 1 Pwll Bendro Member (1) 602 Cader Rhwydog Member (m+) 603 Allt Felin Fawr Member (p) 606 Intrusions 609 Porphyritic rhyolite 609 Microtonalite 609 Rhyolite 610 Microdiorite 610 Interpretation of a submarine rhyolitic centre 610 Conclusions 611 Acknowledgements 612 References 612

SUMMARY: LowerOrdovician marine strata, formed in an ensialicmarginal basin environment,record early episodes of distant explosive silicic volcanism andlatera volcanotectonicepisode whichwas aprecursor tothe development of a widevariety of near-to-source submarine silicic volcanic phenomena. The early episodes are represented by well-bedded turbiditic tuffs. Theseand older strata, atleast 770 m thickin normal stratigraphical succession, areallochthonous, and large-scale wet-sediment slidingwith reworking of strata in cohesive debris flows reflects major disturbance of the sea floor during the emplacement of silicic magmas. Volcanotectonic uplift along a N-S fault exceeded 1km and led to the emergence and erosion of a rhyolitic volcanic island. This was followed closely by submergence,and in waterdepths perhaps of about 500 m thereaccumulated welded and non-welded rhyolitic ash-flow tuffs, rhyolitic turbiditic tuffs, rhyolite lavas and a wide variety of debris flow deposits. Shallow intrusions of ‘low’-viscosity rhyolite were emplaced contemporaneously. The more or less simultaneous emplacement of rhyolitic tuff with both ‘low’-viscosity and high-viscosity rhyolites is attributed to variable water content related perhaps to volatile fluxingin a magma body. The location of thenear-to-source extrusiverocks and consanguineous intrusions along the contemporary N-S fault is considered to reflect channelling of magma, initially along a deep crustal fracture.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 592 B. P. Kokelaar, R. E. Bevins & R. A. Roach Subaqueous volcanic processes are poorly understood. dingly the island was re-mapped at 1 :2500, and a new This is particularly the casefor subaqueous silicic lithostratigraphy erected. Figure 2 (facing p. 610) is a volcanism because there are few modern examples and simplified version of the map and shows the revised they are not easily studied. However, relative to their stratigraphy. Letter codes a-p have been given to subaerial counterparts,subaqueous silicic volcanic formations and members. Palaeontological evidence, rocks are common in the geological record because of assessed by R. M.Owens and R. A. Fortey, has theirgreater potential for preservation.Theoretical facilitated biostratigraphicalcorrelations withwell- and experimentalconsiderations of magma-water known mainland successions. Representative collec- interactions(e.g. Sheridan & Wohletz (1983) and tions of both rock and fossil material utilized in this references therein) and of the influences of hydrostatic study have been deposited at the National Museum of pressure(see Fisher & Schmincke (1984) and refer- Wales (accession number NMW 85.1G). ences therein) have yielded valuable insights into RamseyIsland is privately owned and permission subaqueous volcanic processes but have tendedto for access must be sought from the owner. concentrateon basaltic compositions. Inthe determination of processes of subaqueous silicic volcanism, ancient rocks afford the best means of complementing General geological setting these studies. On Ramsey Island, off NPembrokeshire in SW N Pembrokeshire(now in the county of Dyfed) is Wales(Fig. l), rocks of mid Cambrianthrough to dominated by sedimentary and volcanic rocks of Llanvirn(early Ordovician) age are well exposed in Cambrian and Ordovician age which unconforrnably continuouscoastal exposures. The Ordovician strata overliea late Precambrianbasement composed of include a wide variety of both intrusive and submarine andesitic to rhyolitic lavas and tuffs intruded by extrusive rhyolitic igneousrocks. In thisstudy these granites and granophyres(see British Geological rocks and associated sedimentary sequences have been Survey Sheet SM72 & Parts of SM62, 73 (St David’s), examined todetermine processes of eruption and 1973). distribution of ashes, processes of emplacement of The Cambrian and Ordovician strataare clastic lavas and shallow intrusions, and the natureof directly sedimentary rocks with submarine volcanic rocks relatedsedimentation and tectonism.A preliminary occurring in those of mid Arenig to late Llanvirn age. examination of the geology showed that previous The extrusive rocks are associated with coeval high- interpretations and maps were inadequate, and accor- level intrusions. These igneous rocks constitute an acidic and basic bimodalassemblage, with rare intermediate rocks, and are interpreted as having been emplaced in a marginal basin. References to detailed 40 W work,and a discussion of the Ordovician marginal basin of Wales, are given by Kokelaar et al. (1984b). Also, a field guide to Ramsey Island andadjacent areas of volcanological interest is given by Kokelaar et al. (1984~).Bevins et al. (1984) present petrological and geochemical evidence of a volcanic arc tomarginal basin transition in Wales from early through to mid Ordovician times. Following uplift and erosion in late Tremadoctimes, the lower Arenigsedimentary succession inN Pem- brokeshirerecords a progression with timefrom onshore to offshore conditions with some deepening of the depositionalenvironment. By the middle of the early Arenig, sedimentation was dominated by the accumulation of black muds in anarea apparently starved of terrigenous detritus, situationa which persistedthrough most of earlyOrdovician times. Against this background of black mudstones, the major volcanic influence in this area is readily recognized. FIG. 1. Location of Ramsey Island, showing major lineaments and margins (stipple) of the Ordovician The NPembrokeshire area was subjected to end- marginal basin of Wales(from Kokelaar et al. Silurian deformation which produced ENE-WSW 1984u, fig. 1). MS, MenaiStraits Fault System; trending, open, uprightfolds. This orientation prob- P-L,Pontesford-Linley Fault; CC-CS, Carreg ably reflects tectonica grain in the Precambrian Cennen & Church Stretton Faults. basement(see Kokelaar et al. 1984b). The argillites

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have a slaty cleavage andthe morecompetent Trwyn Llundain Formation (a) volcanic rocks show eithera spaced cleavage orno cleavage at all. Low-grade regional metamorphism in Green, buff, redand purple, medium- to fine- the prehnite-pumpellyiteand pumpellyite-actinolite grained, locally micaceous, quartzosesandstones are facies hasbeen determined (Bevins 1978; Bevins & exposed between Trwyn Llundainand the Ramsey Rowbotham 1983) and a similar grade is indicated in Fault. They are mostly moderately well sortedand illite crystallinity studies(Robinson et al. 1980). All poorly to well bedded. Although generally lacking original volcanic glass has suffered early devitrification sedimentary structures, normal grading does occur in and later recrystallization to quartzo-feldspathic inter- coarserbeds and fine parallel lamination in the growths (commonly spherulitic) with sericite and thinner, finer beds. Rare beds of pebbly sandstone chlorite in the acidic rocks, and to chlorite, typically occur, with some of the pebbles derived locally from with epidote, albite and sphene, in the basic rocks. the Precambrian basement. Also occurring rarely are dark grey to black,laminated siltstones and silty mudstones. The age of this formation is uncertain. Pringle (1930) collected a lingulid brachiopodfauna The geological history of which he considered tobe diagnostic of a Llanvirn Ramsey Island age,but, because of close lithological similarity to rocks of the Solva Group (of early Middle Cambrian The geological evolution of Ramsey Island has been age) exposed nearby on the mainland (see Cox et al. the subject of considerableinterest since the first (1930~))and the absence of similar rocks in the known descriptions by Kidd in 1814. Fordetails see Hicks Ordovician sequence of N Pembrokeshire, we corre- (1866,1873, 1875,1877), Hopkinson (1872, 1873), late this formation with the Middle Cambrian, so that Green (1908, 1911), Pringle (1914, 1915, 1930), Cox et it occurs atthe base of the island’s stratigraphic al. (1930a, 1930b), and the more recent work by Bates succession. (1969) and Jenkins (1979). Twostratigraphical columns are shown in Fig. 2, from east and west of the major N-S striking Ramsey Ogof Velvet Formation (b) Fault. This fault was active during Llanvirn times and strongly influenced thenature of the successions on Rocks of the Ogof Velvet Formation, which equate either side. The complex structure of the island is in with the Lingula Flags of Pringle (1930) and are contrast to the moresimple deformation style on theN restricted to the east of the Ramsey Fault, crop out Pembrokeshire mainland, and early, large-scale, soft- aroundthe northern end of the island, onthe sediment slide movements on Ramsey Island are easternmost stacks of The Bitches, and as a thin sliver mainly responsible. These slides are described briefly in asoft-sediment slide north of Road Uchaf (see below. below). The maximum preserved thickness (-87 m) is The island’s geological evolution is described here in exposed in the vicinity of Ogof Velvet and consists terms of the pre-Arenighistory, the pre-volcanic mainly of alternating, thinly bedded, greenish-grey development of marineconditions in Arenig times, micaceous siltstones and fine sandstones, with parallel late Arenig to early Llanvirn ‘distal’ volcanic episodes, to wavy lamination and lingulid brachiopods. Discon- and an earlyLlanvirn ‘proximal’ volcanic episode. tinuous siliceous sandstones (‘ringers’) show cross- and Description of the pre-volcanic strata is necessary convolutelamination. These sediments are lithologi- here, first to appreciate the evolution of the general cally similar to the Lingula Flags of the mainland. On environment and, secondly, to facilitate recognition of both palaeontological and sedimentological grounds the later-developed complex volcanotectonic proces- Turner (1977) concluded that the Lingula Flags of N ses. Most of the contemporaneous intrusions are dealt Pembrokeshire represent intertidaland subtidal de- with separately. The volcanic, sedimentary and tecto- posits. Around the north coast of Ramsey Island beds nic processes are described and discussed, as appropri- belonging to the Ogof Velvet Formation are complex- ate, ineach section. Finally, general conclusions ly folded. In the west, on Trwyn Drain-du, fold axes concerning the processes of intrusive and extrusive areoriented approximately NE-SW, but on Trwyn submarine silicic magmatism are summarized. ShBn Owen the folding is irregular and some overturn- ing of the beds is present. Sediment sliding is mainly responsible (see below). Pre-Arenig rocks Pringle (1930) identified only one sequence of rocks Development of marine conditions which he considered to be of pre-Arenig age; from through the Arenig lithological similarities heequated these with the Lingula Flags of the mainland. In this account we The lower Arenig rocks of SW Wales recorda identify two pre-Arenig formations. progressive transgression and deepening of the deposi-

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 5 94 B. P. Kokelaar, R. E. Bevins & R. A. Roach tional environment following uplift and erosion late in Road Uchaf Formation (e) theTremadoc (Fortey & Owens 1978). On Ramsey Island evidence for this transgressive episode is seen in The term Road Uchaf Formation is applied here to the Ogof HCn Formation. themonotonous, black, strongly cleaved mudstones exposed in the northern part of the island, to the east of the Ramsey Fault. The formation is at least 280 m thick and corresponds, in part, to strata that Pringle Ogof H& Formation (c & d) (1930) termed the Tetragraptus Shales. The succession The Ogof HCn Formation, restricted to the east of is fossiliferous, particularly atRoad Uchaf, where the Ramsey Fault, crops out at the northern endof the Didymograptus (Expansograptus) sparsus Hopkinson, island, on the inner stacks of The Bitches, and in the Zsograptus caduceus (Salter), Pseudisograptus stellus slide complex exposed atRoad Uchaf. The most (Hopkinson), Dendrograptus recurvus Hopkinson, Pti- complete (and autochthonous) sequence, 190 m thick, lograptus cristula Hopkinson, Callograptus radicans is exposed onthe eastern side of Bay Ogof HCn, Hopkinson,and Desmograptus cancellatus Hopkinson . whereit overlies the Ogof Velvet Formation with have been collected (seeJenkins (1979, 1982)). This slight angularunconformity. The position of the assemblage suggests that the formation belongs to the unconformity is that initially determined by Cox et al. I. gibberulus Biozone (i.e. is of early upper Arenig age) (1930b) and differs fromthat reported by Bates and it is important to note that thereis a stratigraphic- (1969). As defined here,the Ogof HCn Formation a1 gapbetween the Ogof HCn and Road Uchaf comprises two members (Fig. 2), the lower Sandstone formations. On the nearby mainland intervening strata Member (c) corresponding to the Abercastle Beds of of mid Arenig age are exposed at Pwlluog (Bevins & Pringle (1930), and the upper Mudstone Member (d) Roach 1982); theirabsence on Ramsey Island is including the Porth Gain Bedsof Pringle (1930) which thought to be due to the effects of penecontempor- Bates (1969) renamedthe Ogof HCn Formation. aneous (Llanvirn) wet-sediment sliding. Because of lithological and fauna1 similarity to beds The formation represents the establishment of the below, strata above the top of Pringle’s Porth Gain deeper water, mainly oxygen-deficient, offshore condi- Beds (his Tetragraptus Shales in part), up to the Road tions, which persisted generally through the subse- Uchaf Slide, are included in the Mudstone Member. quent episodes of volcanism with only local, volca- At the base of the lower member a discontinuous, notectonically induced, shallowing and emergence. thin (up to 20cm), pebbly sandstone contains whole The development of successively deeper water andfragmented phosphaticoncoliths (formerly the conditions in the Arenig and dominance of black mud supposedbryozoan Boloporaundosa: Hofmann sedimentation is mostly attributable to the extensional (1975)) and is overlain by 40 m of laminated, well- tectonism and formation of fault-bounded basins sorted, fine sandstones and silty mudstones with within the Ordovician marginal basin of Wales cross-laminations, vertical burrows and trilobite trails. (Kokelaar et al. 1984b), althoughprobably a slight Bates (1969) misidentified these strata as Lingula Flags eustatic rise insea level was also involved (see and reported a disconformity with ‘Bolopora’ at their Harland et al. (1982)). Abyssal depthsare not contact with the overlying beds. An horizon of envisaged. weathered-out carbonate nodules occurs at this level but we could find no ‘Bolopora’and the lower member grades rapidly into blue-grey silty mudstones, 150 m Late Arenig to early Llanvirn ‘distal’ thick, with minor dark grey to black mudstones. These volcanic episodes lack theabundant bioturbation and well-defined lamination of the lower memberbut are richly In contrast to the early Arenig interval when there fossiliferous, characterized by a shelly fauna diagnostic was apparently no volcanic activity in this part of SW of the lower part of the lower Arenig in SW Wales, Wales, the middle Arenig (missing on Ramsey Island) which here includes Merlinia murchisoniae (Murchi- and upper Arenig to lower Llanvirn strata record son), Neseuretus ramseyensis Hicks, Monorthis mena- sporadic silicic volcanism. There is no indication of the piemis (Davidson), Paralenorthis [Orthambonites] ala- source of these rocks which are thusreferred to ta (Sowerby) and Ramseyocrinus cambriensis (Hicks). loosely as ‘distal’. Sporadic ‘distal’ activity persisted in This fining-upwards sequence is considered to repre- N Pembrokeshire into late Llanvirn times (Cox 1915; sent a deepening of the depositional environment, Bevins & Roach 1979), although in Llanvirn strata relatedto transgression,although nogreat depth silicic igneous rocks close to source (‘proximal’), and below wave-base is implied. The exposedcontacts basic to intermediate rocks, are also present. Problems with the overlying Road Uchaf Formation are never related to the distance of sources (vents) are discussed conformable and are interpreted here as wet-sediment below. On RamseyIsland ‘distal’ silicic volcanism is slide planes (see below). well represented at Aber Mawr.

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Aber Mawr Formation (j-i) onthe east side of the island, onthe southside of Road Isaf, where the tuff member is cut out rapidly This formation (see Table l), 215 m thick and with andthe adjacent strataare intensely convoluted or its type section at Aber Mawr, comprises in part the Aber Mawr Beds of Pringle (1930), but it does not disrupted. South of the slide plane the Lower Aber Mawr Tuff accord completely with the Aber Mawr Shale Forma- Member, 83 m thick, consists mainly of turbiditic tuffs. tion of Jenkins (1979) and Hughes et al. (1982), which In thelowest 20 m these are mostly fine-grained, thinly has never been formalized and was correlated with the bedded or laminated, and intercalated with mud- D. bifdus Beds of Cox (1915). As outlined below, we stones. Inthe upper 63 m the turbiditic tuffs are consider the Aber Mawr Formation to include strata medium- or fine-grained andthicker bedded,and older than those of the D. ‘bifdus’ Biozone and of the mudstone partings are rare. The tuffs chiefly comprise Aber Mawr ShaleFormation of Jenkins (1979) and shards and theirfragments, vitric dust, pumice Hughes et al. (1982). fragments, and whole and broken quartz and feldspar crystals. Thorough recrystallization is common, into TABLE1: Lithostratigraphical subdi- diffuse segregations of quartzo-feldspathic aggregates visions of the Aber Mawr Formation with more micaceous areas. Carbonate nodules are Dark Mudstone Member (i) abundant locally. Three main lithofacies are present, Upper Aber Mawr Tuff Member (h) namely: Pencil Slate Member (g) (i) Massive beds, 1.5-5 m thick and most common Lower Aber Mawr Tuff Member (f) in the upper part of the member, which show normalcoarse-tail grading and have parallel laminations attheir top. Fine tuff andmud- Detailed mapping, combined with palaeontological stone intraclasts and flute casts are present. evidence, has shown that the geology of Aber Mawr is These beds are mainly suspensiondeposits morecomplicated than described by Pringle (1930). from high-density turbidity currents (the S3 di- Atthe northern end of the bay a thin sequence of vision of Lowe (1982) or T, of Bouma (1962)), mudstonesprobably belonging tothe Road Uchaf with deposition by traction sedimentation from Formation (spotted as a result of contact metamorph- low-density currents andlor reworking by such ismby the intrusion of Carn Ysgubor) is in fault currents of the top of the S3 division, resulting contact with dark, faintly laminatedmudstones con- in the production of laminations (Tb). taining abundantpendent didymograptidgraptolites (ii) Beds of finely laminated tuffs, up to 2m but and ascribed to the Dark Mudstone Member (i; see mostly much thinner, which lack grading, below). South of these dark mudstones, rhyolitic although slump-folding and convolute lamina- turbiditictuffs, here named the Lower Aber Mawr tion are commonly present. Thesebeds are Tuff Member (Q,are overlain conformably by 50 m of associated in places with others dominated by strongly cleaved sooty-black mudstones, the Pencil ripple cross-lamination and current ripples and Slate Member (g), which contain abundant Didymog- the deposits are interpreted as mainly traction raptus(Expansograptus)spp., rarependent Didymog- sediments(Tbac), with some tractionhspen- raptus, and the trilobites Ormathops nicholsoni (Sal- sion sediments (Td), of low-density turbidity ter), Degamella sp. nov., Ellipsotaphrus monophthal- currents, perhaps related byflow transforma- mus(K1ouEek) and Placoparia cambriensis Hicks,in- tion (Fisher 1984) to immediately underlying dicative of alate Arenig (D. hirundo Biozone)age. andlor more proximal S3 units. These are overlainin turnto the south by further (iii) Thin beds of massive, in places laminated, very rhyolitic turbiditic tuffs, named the Upper Aber Mawr fine tuffs, which probably mostly represent Tuff Member (h), and then by faintly laminated dark settling out of current-suspended ash (Td), mudstones which are 35 mthick, contain pendent althoughsome may be distal ash-fall deposits didymograptidsand constitute theDark Mudstone and may include pelagic detritus (Te). Member(i). All the tuffaceous beds young tothe To the south of the Pencil Slate Member, the Upper south. As a repetition of the Dark Mudstone Member Aber Mawr Tuff Member, 47m thick, comprises two occurs, we interpret the contact between this member turbiditic tuff sequences, with lithofacies virtually and the Lower Aber Mawr Tuff Member,at the identical to those of the lower tuff member, and an northernend of the bay, as being a major wet- intervening black mudstonehorizon. The youngest sediment slide plane, of the kind exposed elsewhere beds of the Aber Mawr Formation (Dark Mudstone onthe island (see below). AtAber Mawr the dark Member) are variably cleaved, faintly laminated black mudstonesbeneath the slide arecontorted and mudstones which contain pendent didymograptid discordant to the strata above and below, indicating graptolites. They generally dip steeply southward but deformation during movement, and theslide is evident arecontorted andoverturned in places. Thefauna

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indicates that thesemudstones belong tothe D. beneaththe Carn LlundainFormation onthe east ‘bifidus’ Biozone andthus the formation is of late side. It will beargued that the Porth Llauog Arenig to early Llanvirn age. Formation reflects tectonism resulting from theem- placement of silicic magmas which was a precursor to the development of a major rhyolitic eruptive centre Interpretations of ‘distal’ volcanism represented by the Carn Llundain Formation. The turbiditic tuffs accumulated in moderately deep water. The Pencil Slate Member is lithologically similar and equivalent in age tothe Pontyfenni Porth Llauog Formation (j) Formation of Fortey & Owens (in press), which they regardfrom fauna1 evidenceinferredand The boundary between the Aber Mawr Formation palaeogeography as having accumulated in tranquil and the Porth Llauog Formation is placed at the base water at depths of at least 300m. From the presently of the first debris flow incursion (=2 m thick; exagger- available datathere appears to be no means of ated in Fig. 2) into theblack mudstones exposed at the determining with certainty whetherone or more southernend of Aber Mawr. Deposits of cohesive eruptive centres were involved or whether the erup- debris flows (Lowe 1979) characterize thePorth tionswere subaerial orsubmarine. However, the Llauog Formation.Exposure of the formation is consistently very poorly sortednature of the high- discontinuous(Fig. 2) and the precise thickness and densityturbidity current deposits suggests that ash- sequence are unknown as the beds have been severely flow eruptions predominated. Also, the ratheruniform disrupted by intrusions of porphyriticrhyolite. An occurrence of juvenile constituents,shards, pumice anticline is interpreted from opposing dips north and clasts, and crystals and the absence of other material northeast of Ogof Thomas Williams, and slumping with suggest thatthe high-density volcaniclastic-gravity intense convolution of strata is also evident in places. flows were derived from submarine eruptions, orfrom Background sedimentation during deposition of the subaerial eruptions with little orno intermediate Porth LlauogFormation is represented by black storage of deposits on the subaerial or littoral flanks of mudstones which are commonly pyritous and can be avolcano. Relative to the lower part(20m) of the highly fossiliferous, as at Aber Myharanwhere lower tuff member, the upper part, with its dominant Protolloydolithusramsayi (Hicks), Platycalymene cf S3 divisions, paucity of mudstone partings, and coarser tasgarensis Shirley, Ogygiocaris seavilli Whittard, and grade, perhaps reflects an increase in the intensity and abundant pendent didymograptids occur. These indi- frequency of eruptions.The upper tuff member cate a D. ‘bijidus’ Biozone age for the Porth Llauog represents two further episodes of activity. The Formation. Although the total thickness of such strata distance to the vent(s) cannot be determined,although is uncertain, their presence indicates protracted dura- clearly the source is not nearby and the deposits are tion of the instability recorded by the formation. not ‘proximal’ like those of the younger Carn Llundain In the cliffs south of Aber Mawr vitroclastic crystal- Formation (seebelow). Since the dispersal was and pumice-bearing turbiditic tuffs with mudstone dominantly of gravity flow type, the distance could be intraclasts testify tothe continuation of explosive great, given a sufficient slope, and the thicknesses silicic volcanism like that of Aber Mawr Formation could reflect ponding on an uneven sea-floor topogra- times. Graded, thinbeds and laminae of tuff,and phy. Palaeocurrent indicators are too sparse to derive laminated siltstones, occur here as well as in exposures the dominant flow direction, but with topographically along the eastcoast where beds generally young controlleddispersal and withoutknowledge of the southwards. overallform of the depositsthey would notbe Debris flow deposits, however, are most abundant. meaningful anyway. Also,the entire sequence is Most have a mudstone or silty mudstone matrix and allochthonous (see below). angular to rounded clasts, in one case up to lOm in diameter. The presence of several flow units in thicker sequences is indicated by slight differences in clast Early Llanvirn‘proximal’ volcanic episode contentand proportion.The clasts show a wide Two distinct lithostratigraphical units, younger than lithological variation. Those of volcanic material are the Aber Mawr Formation, have been mapped. The particularly commonand include various rhyolitic lower unit,the Porth Llauog Formation, crops out vitroclastic tuffs, some of which can be matched with only to the east of the Ramsey Fault. The upper unit, deposits in the Aber Mawr Formation, volcanogenic theCarn Llundain Formation, occurs onboth sides sandstones including examples containingmudstone and can readily be correlated across the fault, but on intraclasts, thinly bedded granuleconglomerates, the west it rests unconformably on the Trwyn Llundain pumice, and rhyolite. Crystals of quartz and feldspar Formation.The Porth Llauog Formation provides commonly ‘pepper’ thematrix, and abundant well- evidence for the processes responsible for the absence, rounded pebbles of rhyolite, some flow banded, are a to the west of the fault, of the 1.3km of strata present characteristic feature of the deposits at most localities

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except those on the west coast, which wereformed crenulatedmargins, in a matrix of fine, angular, earlier on. An important group of clasts is purely of rhyolitic fragments and silicic mudstone. sedimentary origin. Laminated siltstones andmuddy The bedded sequence on top of the rhyolite, up to debris flow deposits similar to those of other horizons 9 m thick, comprises grey silty mudstone overlain by within the formation occur as clasts, and of especial massive, rhyolitic pebbly and granule-bearingsand- importance are clasts of black mudstone which are stones intercalated with thinly bedded and laminated virtually identical to the matrix of the deposits and are rhyolitic sandstones and siltstones. The massive, thus easily overlooked. In the cliffs west of Foe1 Fawr poorly sorted sandstones are composed of angular to clasts of this nature reach 10 m in diameter. At Porth sub-roundedpebbles and granules of rhyolite and Llauog Didymograptus artus Elles & Wood, D. mudstonelenticles, in a sandy matrix of rhyolite acutidens Elles & Wood, D. stabilis Elles & Wood, fragments, quartz and plagioclase crystals. The beds, Protolloydolithusramsayi (Hicks) and Ogygiocaris up to 75 cm thick, mostly show normalcoarse-tail seavilli Whittard occur in both mudstone blocks and grading, although some are inversely graded close to matrix indicating a D. ‘bifidus’ age. Pringle (1930, their base (Fig. 4A). Erosion and, locally, convolution p. 19) also recorded the typically Arenig Tetragraptus of the underlying beds of laminated fine sandstone or bigsbyi (Hall) from the deposit and suggested that it siltstone is present. These laminatedsandstones in- must havea greater range than previously assumed. clude beds with currentripple cross-lamination and Randomly oriented, ellipsoidal Fe-Mn concretions up beddingplanes with straight to sinuous asymmetric- to 0.8 m in diameter are common. ripplepatterns. These strataare interpreted as In the cliffs on the southeast side of Porth Llauog turbidites. The massive, poorlysorted beds are (Fig. 3), NW-dipping beds of the formation have been predominantly suspension deposits (S3) of high-density disturbed by penecontemporaneous shallow intrusion turbidity currents with traction carpet (Sz) deposits at of rhyolite. The brecciated rhyolite has gently domed some bases (Fig. 4A).The fine sandand silt units and locally transgressed the bedded sequence, parts of show features of deposition from low-density currents which have slipped to the northwest and overlie their and are traction(Tbkc) and traction/suspension (Td) originally lateralequivalents. The breccia in the deposits. The sequence reflects slumping and genera- rhyolite contactzone is peperitic, comprising mostly tion of volcaniclastic-gravity flows of rhyolitic hyaloc- angular and irregularly shaped fragments, some with lastite formed during submarine extrusion of rhyolite.

porphyriticrhyolite

microdiorite

~ (ODI brecciated rhyolite ...... mudstone 1.- # muddydebris flow deposits pebblyrhyolite mass m flowdeposits hyaloclastites coast(high 6 lowwater)

I*-, clifftop * bestexposures \ geologicalboundary

52\’ dip of strata

FIG. 3. Detail of geology at Porth Llauog, SW Ramsey Island.

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FIG. 4. Volcanogenic mass-gravity flows fromRamsey Island (annotatedsketches opposite): A, Inversely to normally graded turbiditic (S&) rhyolite hyaloclastite (r) with base locally erosive into turbiditic (Tb,c&d)fine sandstones (t). (Porth Llauog Formation, Porth Llauog; handlensis 4 cm long.) B, Tuffaceous mudstone debrisflow deposit showing clasts of tuff and a large rounded clast (top left of centre) of debris flow within debris flow. (Porth Llauog Formation, west of Foe1 Fawr; hammer is 40cm long.) C, Poorly sorted rhyolite pebble (r) and granule conglomerate,interpreted as a density-modified coarse-grain flow deposit.(Carn Llundain Formation, Ogof

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Colomenod; hammer is 40 cm long.) D, Massive rhyolite lapilli tuffs and parallel-laminated finer tuffs, interpreted as suspension deposits (S,) from high-density turbidity currents and mainly traction deposits (Tb) from low-density turbidity currents, respectively. (Carn Llundain Formation, Pwll Bendro; hammer is 40 cm long.) E, Graded and laminated fine turbiditic tuffs (tt) compacted and deformed over brecciated rhyolite lava (br). The basal deposit does not mantle the protruding rhyolite clast. (Carn Llundain Formation, Allt Felin Fawr; lens cover is 5.3 cm in diameter.)

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A rhyoliteextruded at *ea floor I B (no scaleslumping of unstablehyaloclastite implied) turbiditicrhyolite hyaloclastites r_-.- 7 pushedaside

turbiditicrhyolite hyaloclastites

rhyoliteintrusion v v muddydebris

flowdepositsYv v ~ vv \'. .g rhyoliye in:rusionv vv( V

FIG. 5. Diagrammatic interpretation of the origin (A) and disturbance (B) of the turbiditic rhyolite hyaloclastite exposed at Porth Llauog. The dashed enclosure in B indicates the present cliff exposure (see Fig. 3).

The shallow intrusion which disturbed the turbiditic lithological changes wherestrata are known to be hyaloclastites was probablycoeval and emplaced missing or repeated. At Road Uchaf,for example, the shortly after their deposition (Fig. 5). Cambrian-Ordovician unconformity with pebbly sand- The bedded sequence is overlain sharply at a locally stone containing 'Bolopora', as seen near Ogof Velvet erosivecontact by a massive cohesive debris flow (see above), occurs ina slice of strata, 11 mthick, depositapproximately 20 m thick.This is faulted sandwiched between mudstones of the later Arenig. In against similar deposits that dip southeastward,includ- the north and west there are also angular discordances ing a distinctive pale-brown and yellow mass flow across such planes, at up to 90". Some slices of strata deposit with abundant rounded (although tectonically show complex internaldeformation with irregular deformed) rhyolitic pebbles, angular clasts of rhyolite folds restricted to the slice and cut by cleavage of more and pumice, and a fine rhyolitic matrix. consistent,unrelated orientation.The rocks in the Onthe headland immediately west of Foel Fawr, area of 'no exposure' east of Carn Ysgubor (Fig. 2) are close to the top of the Porth Llauog Formation, the almost certainly complexly sliced and deformed. The debris flow deposits (Fig. 4B) contain a preponderance youngest strata certainly involved in the sliding are of rhyolite,pumice, and tuffaceous mudstone clasts those of the Dark Mudstone Member at the top of the with quartz and plagioclase crystals in a fine tuffaceous Aber Mawr Formation, which, provided that sliding mudstone matrix. Rounded boulders of this material did not occur in more than one episode, suggests that occur in a matrix of similar composition. A rhyolite the sliding occurred at the earliest during deposition of granule conglomerate bed is also present. thePorth Llauog Formation, i.e. in early Llanvirn In the cliffs south of Foel Fawr themajor debris times. There is no evidence of brittle-rock behaviour flow deposits are succeeded by black mudstone, 1m at the inferred slide planes. The Ogof Velvet Forma- thick, and2.5m of interbedded black mudstones, tion atthe base of the thin slice at Road Uchaf pebbly mudstones, volcanogenic sandstones and tuff- displays ductile deformation of silt layers and liquefac- aceous debris flow deposits. This sequence is overlain tion of the sands, showing that at least in the early by bedded tuffs (1) which are correlated with those of Llanvirn theUpper Cambrian strata were still not the Carn Llundain Formation cropping out west of the completely lithified. At Bay Ogof H&n thesole slide is Ramsey Fault at Pwll Bendro. defined by pieces of Ogof Velvet Formation beds in a matrix of black mudstone, showing that Ogof Velvet Penecontemporaneous sliding of sediments Formation strata slid overthat position but did not come to rest there. It is argued below that the sliding From Aber Mawr around the northern and eastern took placeduring deposition of thePorth Llauog parts of RamseyIsland toThe Bitches,planes of Formation. sliding cut through all strata ranging in age from late Cambrian to early Llanvirn (Dark Mudstone Member Interpretation of volcanotectonkm of the Aber Mawr Formation). The sole-slide (Road Uchaf Slide; Fig. 2) strikes N-S into Road Uchaf and During deposition of the Porth Llauog Formation through The Bitches, with allochthonous sequences to the tranquilaccumulation of black muds was inter- the west. The slide planes are defined mostly by sharp rupted periodically by incursions of large amounts of

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both volcanic and non-volcanic materials, mostly in of theCarn LlundainFormation (Fig. 2). Although cohesive debris flows. These flows included material sometranscurrent movement may have taken place derived from earlier formations. The pre-Porth Llauog this cannot have been great because of the correlation Formation Ordovician and part of the Cambrian to the of Carn Llundain strata and thematch of intrusions on east of the Ramsey Fault and west of the sole slide either side of the fault. (RUS), in normal succession totalling morethan 770m, are allochthonous. These strata and the Porth LlauogFormation are missing tothe west of the Carn Llundain Formation (k-p) Ramsey Fault, whereas post-Porth Llauog Formation The uplift and erosion of strata west of the Ramsey strata correlate across the fault. The wide distribution Fault clearly was coincident with, and almost certainly and uniform nature of pre-Porth Llauog mudstones in largely due to, the emplacement of rhyolitic magma. SW Wales suggests that they must haveextended The Carn Llundain Formation rests unconformably on originally west of the Ramsey Fault. the uplifted block and extends across the Ramsey Wecontend thatthe Porth LlauogFormation Fault. Together with associated intrusions,members reflects contemporaneous tectonism andthat the of the formation display considerable evidence of contrastingsequences indicate that the N-S Ramsey near-to-source subaqueous silicic volcanism which, in Fault was active during deposition of the formation, contrast tothe activity recorded in theAber Mawr with substantial uplift occurring to the west. The scale Formation, can beregarded as ‘proximal’. The of sliding (which we believe affects the nearby formation comprises four members (see Table 2), in mainland succession) indicates that similar faults and upward succession, the Ogof Colomenod Conglomer- uplift occurred further west. Initial uplift probably ateMember (k), the Pwll BendroMember (l), the caused minor flexuring andlor tilting of the sea floor Cader Rhwydog Member which includes theCader and reworking of only the youngest (Aber Mawr Rhwydog Tuff (m), the Trwyn yr Allt Tuff (n) and the Formation) strata, but since rounded pebbles, indica- Ogof Glyma Tuff (o), andthe Allt Felin Fawr tive of emergence, ‘soon appear’ in the debris flow Member(p). These are described and discussed in deposits(e.g. Ynys Hwrddod)the major sliding order of succession along with a sillwhich was probablyoccurred atan earlystage of deposition. emplacedduring deposition of the Allt Felin Fawr Later debris flows reflect further redistribution of Member. The uppermost bed of the latter member, at slipped strata. The development of rhyolitic hyaloclas- thesouthern end of the sequence west of Porth tite(turbidites) and coeval shallow intrusion of Llauog, is the youngest stratum on the island. Because rhyolite in theupper part of thePorth Llauog immediatelypreceding volcanotectonism occurred Formation are evidence thatthe tectonism was during early Llanvirn times (background sediments in coincident with silicic magmatism. This is suggested thePorth LlauogFormation indicate a D. ‘bifidus’ further by theabundance of silicic volcanic detritus Biozone age) we favour such an age forthe Carn other than that attributable to reworking of the Aber LlundainFormation. However, a later Llanvirn age Mawr Formation.Thus the almost ubiquitous well- cannotbe ruled out.There is in this case no clear rounded pebbles of rhyolite are considered to reflect correlation with the mainland. shore-line erosion of an emergent block where rhyolite lavas or intrusions had beenemplaced. The occurrence near the top of the formation of tuffaceous mass Ogof ColomenodConglomerate Member (k). The flow deposits with abundant rhyolite pebblesand conglomerate (Fig. 4C) is the basal member of the granules, as at Porth Llauog and west of Foe1 Fawr, Carn LlundainFormation and is absenteast of the reflects the eventual domination of sedimentation by Ramsey Fault. It is 35 m thick and rests with extreme input from an actively eroded, emergent rhyolitic angular unconformity (up to 907 on Middle Cambrian volcanic island. The unconformity which cuts the sandstones of the Trwyn Llundain Formation and on a TnvynLlundain Formation and rhyolite intrusion, rhyolite intrusion. It is poorlysorted and comprises west of the Ramsey Fault at Ogof Colomenod (Fig. 2; well-rounded cobbles andpebbles of rhyolite,sup- see below), indicates such a situation, although most ported in a matrix of rounded small pebblesand of the missing strata wereprobably removed by granules which are mostly of rhyolite but also, towards submarine sliding or slumping prior to emergence. In the base, of fine sandstone. Distinct bedding is absent, the extensional,marginal basin setting that is envis- although slight variations in matrix proportion occur aged (Kokelaar et al. 1984b), compressional causes of throughoutand indicate the presence of sub-units. the uplift arenot likely, while the evidence of Sorting improves slightly, and mean clast size general- contemporaneous silicic magmatism,and the subse- ly decreases, upwards. In places,large clasts have quent development of a rhyolitic volcanic centre in the been tectonically moulded against oneanother and immediate vicinity, argue strongly for uplift due to the small clasts squeezed into the interstices. At its top the rise of silicic magma. conglomerategrades over 34m into granulecon- The Ramsey Fault was reactivated after deposition glomerates and very coarse sandstones, which,

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Thusthe deposits may reflect atype of contents reflect juvenile magmatic explosivity and the density-modified coarse grain flow (Lowe 1979). The lapilli of rhyolite may have been derived from a coeval flows probably originated in a littoral or supralittoral non- or poorly vesicular magma, or a pre-existing lava environmentwhere the clasts became rounded and or plug, fragmented by the magmatic explosion(s) and their deposition in at least moderately deep water is one or both of steam-associated explosivity and inferredfrom theinterbedded and succeeding tuffs. quench granulation. Such flows could have been initiated by a tsunami or Although this sequence appears to be more proxim- directly by volcanotectonism. The rhyolitegranule al than those of the Aber Mawr Formation, the thin conglomerate bed near the top of the Porth Llauog beds and lack of very coarse material suggest that the Formation is probably of a similar origin andan source was not very close (cf. later tuffs). The absence actively erodedemergent rhyolitic volcanic island is of reworking implies deposition in at least moderately required as source,but submergencepreceding de- deep, tranquil water, certainly below wave-base, position of the conglomerate is also required here. possibly in deeper, open water. Positive identification of the style of eruption, or of The Pwll Bendro Member (I). This member, approx- subaeriala or submarine source, doesnot seem imately 165 m thick, comprises massive rhyolitic lapilli possible. However, because of the absence of accre- tuffs interbedded with thinly beddedand laminated tionary lapilli and of well-graded (watersettled) finer tuffs (Fig. 4D). The lapilli tuff beds range from a ash-fall beds we prefer theinterpretation of suba- few centimetres to 20 m thick but are most commonly queous eruptionswhere eruptive columns were sup- between 1 and 2m. They are composed of abundant pressed, major fall-out was restricted to the vicinity of lapilli of rhyolite and randomly oriented tube pumice the vent (fine ash may havebeen carried awayin clasts (1 cm), sparse rhyolite granules and pebbles (up aqueoussuspension and closed-vesicle pumice may to7cm) and whole and fragmental quartzand have floated),and generation of ash-flowswas plagioclase crystals in an intensely recrystallized, fine favoured.Considerable water depth is thus implied. vitroclastic matrix.Shard fabrics are preserved in Fiske & Matsuda (1964) determined that a massive, places andthe deposits are non-welded.Sorting is graded lapilli ash-flow tuff succeeded by a fining-up poor and some beds containfewer lithics and more sequence of graded turbiditic tuffs could be ascribed to pumice towards their top. Normal coarse-tail grading, a single eruption,where the ash-flowtuff was with rhyolitepebbles and sparse argillite fragments generated directly from subaqueous column collapse concentrated towards the bases of beds, can be and the succeeding beds by repeated slumping of the distinguished near the bottom of the sequence. Most successively finer tuff settling out of suspension. Less bed bases are flat, although locally the coarse tuff cuts simple interbedded sequences could equally be due to down obliquely or by steps into the underlying tuffs by post-eruption slumping of tuff or rapidly repeated as much as 30 cm. With decreasing lithic content these eruptions. Doubly graded sequences are scarce and -tuffs grade into thin beds of pumice-rich medium and imperfectly developed in the Pwll Bendro Member so fine vitroclastic tuff, some of which show normal although some massive lapilli beds may have origin- grading, with crystal concentrationtowards their ated as discrete ash-flows, derived directly from some bases. These massive tuffs commonly become thinly form of subaqueous column collapse, the majority are bedded or laminated towards their tops, fining rapidly more probably due to slumping of tuff penecontem- into parallel-laminated tuffs (Fig. 4D) composed poraneous with subaqueous eruption.In the former mainly of crystal fragments and recrystallized shards. case the flows must havebeen deflated, cold and Ripples and ripplecross-laminated beds are present waterlogged at the time of deposition. As in the Aber but uncommon. Locally the thin beds and laminae Mawr turbiditic tuffs, the low-density turbidity current havebeen disturbed by dewatering or soft-sediment depositsprobably represent fractions of the im- faulting. A thick (20 m) lapilli tuff displays a rotational mediately underlying andior more proximal S3 units. slump-with-scar, involving its base and the underlying Approximately 15 m of bedded tuffs of the Pwll laminated tuffs which are also disrupted by de- Bendro Member crop out to the east of the Ramsey watering. Fault, in the mostly inaccessible cliff south ofFoe1 The massive tuffs are suspension deposits (S3) from Fawr. The correlation of the Carn Llundain Formation cold, locally erosive, high-density turbidity currents, across the fault is largely based on the similarity of the

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West of the Ramsey Fault East of the Ramsey Fault Allt Felin Fawr (p) Rhyolitic lava (p') Member Ogof Glyma Tuff (0) Ogof Glyma Tuff (0) Cader Rhwydog Laminated turbiditic tuffs (n') Member unconformity Trwyn yr Allt Tuff (n) Trwyn yr Allt Tuff (n) unconformity Cader Rhwydog Tuff(m) Cader Rhwydog Tuff (m) unconformity unconformity Pwll Bendro (1) MemberPwll Bendro (1) Member Ogof Colomenod I(k) Member

succeeding ash-flow tuffs of theCader Rhwydog connection with the underlying deposits other than the Member and the Pwll Bendro tuffs, although charac- exposed narrowdyke is suggested by the clast size. teristically well bedded on thisside of thefault, Early dykes are cut by later examples which also cut are lithologically dissimilar to those onthe other FeMn veinlets. Towards the western end of the cliff side. This is in part due to the intense silicification of section a protrusion from the muddy sequence, 1-2 m the western tuffs, an effect which is absent to the east. wide, cuts upwards through approximately 7 m of the The lowest bed south of Foe1 Fawr is an ash-flow bedded tuffs beforespreading laterally fora short tuff which is 3.5 m thick, massive, and poorly sorted, distance. and containsangular clasts of rhyolite, lenticles of Evidently the muddy deposits underlying the tuffs mudstone, abundant pumices up to 5 mm, and crys- were water-rich so that dewatering and loading tals, in a fine vitroclastic matrix. In places the pumices eventually led to mobilization. Since the muddy are flattened and define a locally swirling fabric which dykes cut FeMn veinlets (which are demonstrably is deflected around lithics. This tuff is overlain by 2 m epigenetic), it seems that their emplacement may have of thinly bedded and laminated tuffs, in places with occurred some considerable time after the deposition cross-lamination in sets 5 cm thick. Locally at the top of thebedded tuffs, perhapsduring loading by the these tuffs are cut by channels up to 25 cm deep and succeeding major ash-flow tuffs. filled with coarserhyolite and pumice clasts with a sparse muddy matrix. Channelled density currents are probably responsible. Another ash-flowtuff overlies Cader RhwydogMember (m-o). This member in- this and the remaining thickness appears to consist of a cludes three rhyolitic ash-flow tuffs, each of which is similar (inaccessible) alternation of massive and lamin- overlain by thinly bedded andlaminated turbiditic ated or bedded tuffs. Accumulation of these beds is tuffs (mostnot shown on Fig. 2).Above the interpreted as having been largely similar to that of uppermost ash-flow tuff these bedded tuffs constitute theirwestern counterparts, except thatthe massive the dominant element of the Allt Felin Fawr Member. units are more clearly the direct products of eruption Thus, thin turbiditic tuff beds arethe background and some were still hot on emplacement. deposits in the upper members of the formation. In several places the lowest ash-flow tuff is cut by West of the Ramsey Fault the lowest ash-flow tuff, dykes, mostly 2-5 cm wide, of granule-bearing mud- theCader Rhwydog Tuff (m), is 186m thick and stone that commonly shows flow alignment of clasts consists of 161 m of uniformly coarse lapilli tuff parallel tothe dyke margins andlorvariation in overlain by lapilli tuff grading into extremely fine vitric content between the dyke margins and centre. These tuff at thetop. The tuff rests unconformably on a originate in the underlying muddy mixed sequence surface which slightly cuts down into the underlying (see above) and in places terminate in upright bulbous Pwll Bendro tuffs tothe eastand infills adeeply or horizontallenticular bodies, mostly within the incised gully close to the cliff top (see Fig. 2). Locally, upperpart of the ash-flow tuff. One such bulbous near the base, the tuff includes rounded pebbles and pocket is approximately 1.5 m high and 0.5 m wide and granules of rhyolite,angular clasts of perlitic and contains rounded pebbles of rhyolite up to 25mm in flow-banded rhyolite, and lenticularfragments of diameter, with other clasts in a muddy matrix. A black mudstone. Atthe gully this mixture extends

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FIG. 6. Various physical forms of rhyolites from Ramsey Island (annotated sketches opposite):A, Massive, coarse lapilli tuffof the lowerpart of theCader Rhwydog Tuff, showing strongly flattenedtube-pumice clasts (p) commonly with less-flattened (‘frayed’) terminations, and sparse lithic rhyoliteclasts (l), in a strongly recrystallized vitroclastic matrix (stippled) (Cader Rhwydog; pencil for scale). B, Plan view of top contact zone of porphyritic rhyolite sill. Irregular and rounded protrusions of rhyolite (r) mostly display sharp chilled contacts but in places are peperiticand admixed with the fine-grained tuffaceous host (t).(Allt Felin Fawr;hammer is 40cm long.) C, Brecciated rhyolitelava. The exposureis close to the baseof the flow and shows intersticescompletely pervaded by tuffaceous mudstone (t), rhyolite clasts (r) locally ‘fitting together’ and one clast pressed into another, which are indicative of in situ autobrecciation. Note: dark areas are lichen. (Carn Llundain Formation, Allt Felin Fawr; lens cover is 5.3cmin diameter.) D, Surfaces of intrusions of porphyriticrhyolite. The cliff face isan uncovered intrusion surface and shows extensive patterns of parallel wrinkles (nearly horizontal across middle of cliff, larger scale folds (also across middle of cliff, labelled Is), and lobate protrusion (sea level on right, labelled Ip). The sea stack also displays intrusion surfaces. (West cliff Ogof Thomas Williams; height approximately 45 m.) E, Bulbous and irregular protrusions,pillows and peperite developedalong one side of an irregular sheetof porphyritic rhyolite (r) intruded into muddy debris flow deposits (m). The sheet dips away and to the right such that the offshoots were initially orientated vertically downwards beneath it. (125 m west of Foe1 Fawr; hammer is 40 cm long.)

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irregularly upwards,approximately 12 m intothe volatiles streamingthrough ephemeral pipes also overlying tuff. The main body of the tuff (SiOz occur. Crude columnar joints extend from the base to = 75%) consists of ragged clasts, up to 32 cm long, of about 100 m into the tuff, and are further evidence of tube pumice, with variable proportions of rhyolite heat retention. clasts and crystals of quartz and plagioclase in a Although the relative proportions and grades of the strongly recrystallized vitroclastic matrix (Fig. 6A). main tuff constituents are variable, no sub-units or Although welded shard fabrics are notpreserved, overall grading can be distinguished here in the lower moderate to strong flattening of the pumice, roughly 161 m of the unit. However, above this there is a parallel tothe sheet dip, is widely (although not gradation into an unbedded fining-up sequence, 25 m ubiquitously) developed, including at places along the thick,through crystal pumice vitroclastic tuff with base. Pumices moulded around lithic clasts,and strongly flattened pumice clasts, into extremely fine- strongly flattened, ragged fragments of porphyritic grained vitroclastic tuff. Fine laminations in the lowest rhyolite are common. Such flattening fabrics are as metre of conformably overlying, extremely fine tuffs significant as welded shard fabrics in indicating heat show intense convolution. Close to the Ramsey Fault retention. Small pockets of crystals winnowed by these contain large (20 cm) siliceous nodules.

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The Cader Rhwydog Tuff is interpreted as a closely The unconformity beneath the Cader Rhwydog Tuff proximal,subaqueously emplaced, hot ash-flow with probablydeveloped mainly by volcanotectonically associated ash-fall, produced by column collapse and induced sliding and slumping, with gullies cut by subsequent deposition from suspensionin a single, sediment gravity flows. Apparently the unconformity entirely submarineeruption. Emplacement under surface and gully close to Pwll Bendro were overlain water is implied by the gradationfrom the main by a muddy debris flow deposit which became admixed massive tuff to the finely laminated tuff on top with with the ash-flow, perhaps facilitated by explosive clearevidence of wet-sedimentdisturbance. The evolution of steam from trapped water. The rhyolite absence of penecontemporaneous slumping or major pebbleswere probably incorporated during eruption incursions of remobilized tuff indicates thatthe throughdeposits like those of the Ogof Colomenod depositionalsurface here,and perhaps widely, was Conglomerate Member. virtually horizontal. Proximity to the vent is suggested The laminated tuffs on top of the Cader Rhwydog by the great thickness of the tuff and the thickness of Tuff are conformably succeeded by a massive rhyolitic the associated fall which gradesdirectly fromthe lapilli ash-flow tuff, the Trwyn yr Allt Tuff (n), coarse grade of the flow. This proximity, together with approximately 22 m thick and comprising pumice the apparent lack of slope, indicates that the vent must clasts with quartzand plagioclase crystals set in a havebeen submarine. This is also supported by the recrystallized vitroclastic matrix. Nearthe Ramsey flattening fabrics inthe gradedfall deposit which could Fault the lower part of the tuff contains intraclasts of not have developed if the tuff had settled from air into the underlyinglaminated tuffs and the contact is water and then through a water column. Similarly, the irregular. At the top there is a thin sequence of thinly graded fall could not represent a cloud suspended in bedded, fine turbiditic tuffs showing normal-graded water by the ash-flow, butmust have been rapidly and laminated divisions, and wet-sediment disturbance deposited from suspension in the eruption column. It is evident beneaththe base of the succeeding Ogof is envisaged that, compared with asubaerial plinian Glyma Tuff (0). The latteris approximately 15 m thick event, the explosive violence of the submarine erup- and is lithologically similar tothe underlying tuff, tion would havebeen suppressed andthe column although it containsflattened pumices in astreaky height substantially reduced. Once initiated, much of matrix which indicates welding. East of the Ramsey the eruption would have proceeded within a cupola of Fault (north of Foel Fawr), the Trwyn yr Allt Tuff has steam and magmatic volatiles, resulting in less chilling a nearly vertical and unconformable contact with the than might otherwise be expectedin a subaqueous Cader Rhwydog Tuff but is itself cut out towards the eruption. As theeruption waned much of the northeast where the succeeding thinly bedded and suspendedcolumn rapidly settled through the steam laminated turbiditic tuffs, 14 m thick (n‘), rest directly and finally the remaining finer suspension settled more on the Cader Rhwydog Tuff. These bedded tuffs are slowly through water to produce the fine-grained succeeded by the Ogof Glyma Tuff which here con- vitroclastic tuff. Hot clasts enveloped by steam were tains clasts of flow-banded rhyolite upto 5 cm in readily flattened during burial. If this interpretation is diameter, irregular pumices, and quartz and feldspar correct,the Cader Rhwydog Tuff includes the first- crystals in a fine vitroclastic matrix. recorded subaqueously welded ash-fall deposits. De- The associated turbiditic tuffs show that the Trwyn tails of the grading and textures are to be presented yr Allt and Ogof Glyma Tuffs were emplaced elsewhere (Kokelaar & Bevins, in press). subaequeously. Theextreme complexity of the tuff East of the Ramsey Fault the Cader Rhwydog Tuff inter-relationships north of FoelFawr, where the rests unconformably on Pwll Bendro tuffs and, in the Cader Rhwydog Tuff is deeply ‘cut’ by the Trwyn yr cove north of Foe1 Fawr,on sparsely pebblymud- Allt Tuff and this in turn ‘cut out’ by succeeding stones of the Porth Llauog Formation. At the latter anomalously thick turbiditic deposits, almost certainly locality the base of the tuff extends as large bulbous results from volcanotectonically induced sliding of protrusions,upto 10mdeep, into the pebbly thick tuff sequences and subsequent infilling of irregu- mudstones beneath. Flattened pumice clasts occur at lar sea-floor topography. the contacts and throughout the tuff, and the protrusions probably formed by fluidization of the subjacent wet sediment in a manner analogous to that proposed Allt FelinFawr Member (p). Northeast of Ogof by Kokelaar (1982) to account forthe transgressive Glyma the Ogof Glyma Tuff is succeeded by several bases of subaqueouslyemplaced welded tuffs in N metres of thinly beddedand laminated tuffs. These Wales. Nearby rhyolite pebble-rich horizons in the tuff deposits (not shown in Fig. 2), which are cut by the indicate localized pulsing or multiple flow unitbe- slightly discordantbase of a 35 m thick porphyritic haviour. The fining-up top of the tuff is partly rhyolite sill, are the lowermost strata of the Allt Felin preserved just north of Foel Fawr, but further north it Fawr Member. Theythicken to approximately 25 m is absent dueto downcutting of the base of the towards the southwest, beneath the sill on Allt Felin succeeding tuff. Fawr, and on top of the sill southeast of the headland

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 Ramsey Island, SW Wales 607 they are up to 8m thick. Here, however,they are and form a jig-saw fabric, suggesting that they are at repeated by sliding. The bedded tuffs (similar to those least in part derived by in situ autobrecciation. in Fig. 4E) comprise crystals of plagioclase and quartz Homogeneous fine tuffaceous material fills interstices with lithic rhyolite and pumice clasts ina very fine here (Fig. 6C). silicic vitroclastic matrix. Thebeds, most commonly At the base of the lower tuffaceous wedge (Fig. 7, 1-2 cm thick, are normally graded, contain intraclasts columns 5 & 6) there is a thin sequence of turbiditic of fine tuff, and haveloading structures along their fine tuffs which covers the rhyolite breccia in the bases. Parallel-laminated fine and very fine tuffs southwest butdoes not persist tothe northeast, consist of minute fragments of quartz and plagioclase indicating topographicrestriction of the turbidity crystals and pumice, and vitroclastic ‘dust’. Ripple currents. The succeeding (lower) ash-flow tuff in the cross-lamination is presentplaces,in and soft- northeast (columns 3 & 4) consists of a basal zone of sedimentfaults and convolutelamination are com- coarse lapilli tuff, which grades up into a poorly sorted mon. Theseare the deposits of minorturbidity lapilli tuff showing normal grading of rhyolite clasts currents. On the small headlandimmediately south- and inversegrading of pumices. Lapilli in the basal east of AlltFelin Fawr these deposits are cut by a zone are of subangular to subrounded perlitic rhyolite channel 20 mwide, up to 6m deep, and filledwith with irregular tube pumices upto 20cm, and the normally graded beds (10-20 cm) of medium-grained matrix comprises medium- to fine-grained vitroclastic tuffs with abundant pumice, crystals, and shards. tuff with quartz and plagioclase crystals and fragments These are S3 turbidites. The channel too is repeated by of fine tuff. The top gradessharply into indistinctly sliding (see below). Above this sequence a heterolithic bedded fine tuff (0.5 m). Towards the southwest debris flow deposit, 3 mthick, contains clasts of (columns 5 & 6) the tuff shows astrong flattening pumice,porphyritic rhyolite, rhyolite, fine tuff,and fabric and contains randomly oriented, contorted slabs crystals ina fine tuff matrix.This is overlain by of fine, in places laminated, crystal- and pumice- conglomerate, OSm, a further debris flow deposit, bearing tuffs. These slabs are up to 1 m thick and 10 m 1.5 m, and further laminatedand thinly bedded long and are associated in places with large blocks of turbiditic tuffs up to 2 m thick. These are succeeded by pumice. In addition, a block, 2.1 m in diameter with brecciated rhyolite lava (p’). lobate and peperitic protrusions of porphyritic rhyolite The above-mentionedporphyritic rhyolite sill dis- in laminatedturbiditic tuffs (column 5), lies at the plays extremely irregular bulbous and peperiticprotru- base.This block was derivedfrom thetop contact sions along its upper and lower contacts and clearly zone of the closely subjacent sill (see above). shows evidence of having been intruded in amod- This lower ash-flow tuff is overlain by a debris flow, erately fluid state when the host deposits were wet and up to 0.5 mthick, composed of rhyolite clasts and unlithified (Fig. 6B).Kokelaar (1982) described in crystals in a tuffaceous mudstone matrix. Towards the detail the contact features and processes of emplace- northeast this bed is overlapped by the upper ash-flow ment of the sill, with particular reference to exposures tuff, which is lithologically similar to the lower tuff. In along thetop contact on and close tothe small thenortheast (column 2) normal grading iswell promontory southeast of Allt Felin Fawr. In summary, developed in this upper ash-flow tuff but tothe it is clear from this locality that emplacement of the sill southwest the grading breaks down. Coarse and fine was attended by fluidization of the host deposits along tuffs in ill-defined patchesand large, chaotically itscontacts. This resulted in a loss of cohesion contorted slabs of laminated fine tuff occur throughout between the sill and the overlying strata which allowed the unit. In places the tuff grades upinto bedded a large slab to slide off and thus expose the intrusion sandy tuffs (columns 4 & 5) and atthe base the at the sea floor while causing the repetition of strata underlying debris flow deposit is disturbed, with across the slide, as seen at this locality. Fragments of flame-like protrusionspenetrating upwards for 2m the sill’s distinctive contact zone occur in strata that (column 5). Further southwest the,tuff occurs as large, form a wedge within rhyolite lavas above the sill (see rounded andirregular sack-like bodies separated by Fig. 7). The lavas cannot have been involved in sliding thin silicified veneers of the debris flow deposit so that the maximum depth of intrusion beneath the (column 6).A contorted slab of pelagic black sea floor was approximately 13 m. Clasts of porphyritic mudstone is included.This remarkable deposit also rhyolite in debris flows above the sill suggest that the contains intrusions, up to 8 m in diameter, of porphyri- depth may have been as little as 6 m. tic rhyolite with peperitic margins. The fine turbiditic The rhyolite lava (p’; SiOz 2- 75%) is sparsely tuffs that generally overlie this deposit are commonly quartz- and plagioclase-phyric, flow-banded,and en- contorted, soft-sediment faulted, or ploughed into by tirely brecciatedat outcrop.To the east the lava is the succeeding rhyolite breccia. apparently a single unit approximately 35 m thick, but Theupper and lower tuffs of the wedge are two tuffaceous wedges in the west show it tobe interpreted as slumped, poorly sorted ash-flow tuffs. compound (Fig. 7). In places along the base the The contorted slabs are interpreted as turbiditic and breccias have sunk into and disturbed turbiditic tuffs water-settled ash deposits, originally overlying and

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 608 B. P. Kokdaar, R. E. Bevins & R. A. Roach

r 5 I l \

contorted slabs of fine tuff

...-..-.-.-.~.,-.-....-*U-".*-.., ~..~.~-".*-~*-.-~*-.~

FIG.7. Diagrammatic cross-section and columnsshowing details of rhyolite lavas with intercalated slumped ash-flow tuffs, debris flow deposits and turbiditic tuffs at Allt Felin Fawr, SW Ramsey Island.

genetically associated with the primary ash-flows, but The flattening fabricsin the lower slumped tuff subsequently incorporated during slumping and mass- indicate that slumping occurred very soonafter gravity flow. Thelateral variations of the deposits primary deposition. Penecontemporaneous emplace- reflect telescoping of various facies. The sections are ment of the sill, lava,and primary ash-flows is thus interpreted as deposits of the sides of the flows, where indicated. thematerial at the edge (northeast) came to rest The upper wedge in the rhyolite pile comprises before that towards the centre (to thesouthwest). The turbiditic medium- and fine-grained tuffs, and approx- upper slumped ash-flow ploughed intothe muddy imately 20m of these tuffs overlie the rhyolite (Fig. debris flow deposit and, for reasons that presently are 4E). A debris flow deposit, 1-2 m thick, is also unclear, bodies of the tuff maintained their integrity present. As the turbiditic tuffs wedge out against the and balled-up in the mud. rhyolite there must have been topographic restriction Much of the lithic rhyolite content of the slumped of the depositing currentsand interpretation of the ash-flow tuffs was probably incorporated during erup- fine tuffs as being due to ash-fall is clearly precluded tion through and/or flow acrosslava similar tothat by this relationship.Sparse large pumice clasts which is presently spatially associated with the tuffs. represent sinking of water-logged material from float- The block of the closely subjacent sill's contact zone ing pumice-rafts. The coarsertuffs, with clasts of indicates that this zone was exposed so as to be pumice, rhyolite, quartz, and plagioclase crystals, in a incorporated in the lower slumped tuff. Sill intrusion commonly well-preserved vitroclastic matrix, show closely preceded the emplacement of the rhyolite lava. normal grading, erosive bases, and loading structures

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 Ramsey ManId, SW Wales 609

which are locally truncated and isolated by erosion of the sill and fromflattened, ragged fragments of theparent bed.Cross-, parallel- and convoluted porphyritic rhyolite which occur in the Cader Rhwy- laminations, and slumping are all locally well de- dog Tuff (m), show that the rock represents magma of veloped. the proximal eruptive centre. The angles between the upper surface of the rhyolite These rhyolites are petrographically and chemically breccias and the laminations of the turbiditic tuff are rather uniform (Table 1) and are composed of approximately 6”. Such shallow slopes and the pre- abundant euhedral to anhedral quartz and plagioclase sence of jig-saw fitting breccias indicate thatthe phenocrysts in a fine, quartzo-feldspathic groundmass rhyolites are in situ lava flow-units andnot talus which is locally spherulitic. Small plagioclase crystals, (reposeslope) deposits peripheral to asteep-sided apatites,zircons, Fe-oxides and chloritic pseudo- dome. morphs (possibly after hornblende or biotite) occur sparsely. Chilled margins cannotbe distinguished, although perlitic fractures occur close to some con- Intrusions tacts. Hornfelsing of host rock is absent, but thin There are four types of intrusive rock on Ramsey silicified mudstoneveneers adhereto somecontact Island (Fig. 2); indecreasing order of outcroparea surfaces. At Ynys Hwrddod the normal lithology these are porphyritic rhyolite, microtonalite, rhyolite, grades eastwards into flinty flow-banded rhyolite. Flow andmicrodiorite. Chemical analyses of the first two banding occurs locally elsewhere. types are presented in Table 3. On theeast and west cliffs of Ogof Thomas WiIIiams dearly displayed intrusion surfaces are broadlyun- dulose with distinct and extensively developed wrink- TABLE3: Representative major element analyses porphyritic rhyolite and rnicrotonalite les, commonly closkly spaced and parallel over tens of (XRFJ of metres (Fig. 6D). In addition, the surfaces display l 2 3 4 larger scale protrusions, tucks and folds which locally preserve veneers of the host. Such patterned surfaces 74.58 73.94 70.53 62.85 are common and may indicate slight withdrawal of 0.40 0.48 0.68 1.16 magma soon after development of maximum volume, 11.75 12.06 13.15 13.34 so that the still-plastic margin ‘shrivelled’. Alternative- 3.36 4.07 4.19 6.14 ly, or in addition, the slightly denser chilled (but still 0.11 0.12 0.07 0.13 0.53 0.60 1.37 2.00 plastic) margins may have slid gravitationally or may 1.02 0.61 0.53 3.30 havebeen dragged by magma flow. In any case, 4.74 4.72 4.70 4.58 cohesion with the wet hostsediments was probably 1.77 1.83 2.46 2.05 lost by fluidization along the contacts (see Kokelaar 0.06 0.08 0.13 0.27 (1982)). 1.21 1.53 1.37 3.50 One hundred and twenty-five metres west of Foe1 99.53 100.04 99.18 99.32 Fawr, minorintrusions of porphyritic rhyolite occur within black mudstones of cohesive debris flow de- Samples l and 2 are porphyritic rhyolites from the posits. Here thecontacts are distinctively peperitic, with dome south of Aber Mawr and the sill at Allt Felin angularand irregular clasts of rhyolite supported in Fawr, respectively. Samples 3 and 4 are microtonalite mudstone, commonly close to the site of fragrnenta- from Carn Ysgubor. Sample sites are shown in Fig. 2. tion. Also, extremely irregular fingers, bulbous protru- Analyses by D. E. James (Edinburgh University). sions and isolated pillows of rhyolite occur within the host to one side of an irregular sheet (Fig. 6E). These features clearly indicate emplacement of the rhyolites Porphyritic rhyolite into the debris flow deposits while they were still wet and unlithified, and the asymmetry of the off-shoots of South of Aber Mawr the island is dominated by the sheet reflects gravitationally induced budding and numerous steep-sided, rounded, and irregular domes sinking of protrusions from its under surface, aided by of porphyritic rhyolite. (These also form the small fluidization-displacement of the sediment (see Koke- islands south of Ramsey Island which are not shown laar (1982) and Lorenz (1984, fig. 5)). on Fig. 2.) Mostly they occur with the dominantly The uncharacteristically ‘low’-viscosity behaviour of black argillites of the Porth Llauog Formation pinched these rhyolites is discussed below. between them, and are associated with numerous minor irregular sheets, but west of the Ramsey Fault Microtonalite thesame rock forms the shallowly emplaced sill described above (Allt Felin Fawr) which is itself cut by Microtonalite crops out only atCarn Ysgubor. a discordanta intrusion of similar lithology. The Petrographically and chemically the rock is rather concentration of the intrusions, and the evidence from heterogeneous (Table 3), although no sharp internal

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 610 B. P. Kokelaar, R. E. Bevins & R. A. Roach contacts have been found. Unlike other intrusions on matrix. Some post-brecciation movement is indicated the island, this has a contact metamorphic aureole up inzones whereadjacent clasts do not fit together. to 10 mwide, within which the hostmudstones are Late-consolidationexsolution of volatiles fromthe spotted. Contacts indicate that crudely the exposure is rhyolites possibly accountsfor most of the (crackle) a section through an inverted cone or cylinder which brecciation. Movement may have resulted from explo- plunges northwards at between 30" and 50". Chemical- sive release of volatiles, volatile streaming, or, ly, the microtonalite shows affinities with the porphyri- perhaps, further intrusion.Streaming volatiles, or ticrhyolites (Table 3), with a trend towards more later percolating fluids, corroded the clasts and intermediate compositions. The coarser grain and produced the altered matrix. The explosive release of marked contact effects may in part reflect the deeper magmatic volatiles probably caused fragmentation of level of exposure and perhaps repeated emplacement the host sediments,although explosive boiling of of magma. connate water trapped in lithified sediments would produce similar results. Rhyolite On RamseyIsland thereare numerous minor Microdiorite intrusions of rhyolite. They are extremely fine grained A microdiorite dyke (Si02 = 58%), up to 4 m wide and consist of quartzo-feldspathic aggregates with or and with vesicular margins, was emplaced along parts without very sparsephenocrysts of quartz andlor of the fault plane which strikes northeastwards from plagioclase. Many intrudestrata beneath the Aber Pwll Bendro (Fig. 2). Nearby a thin sheet discordantly Mawr Formation but others cut strata above, and, as cuts the Pwll Bendro tuffs and a similar sheet occurs in the Aber Mawr tuffs reflect only distal activity, the Porth Llauog (Fig. 3). Elsewhere on the island, rock of majority of the intrusions are probably related to the thiscomposition is absent andthe context of the later magmatism. By its content of abundant rounded microdiorite intrusions is not known. pebbles of rhyolite and the emplacement of rhyolite and associatedhyaloclastites atPorth Llauog, the Interpretation of a submarine rhyolitic Porth Llauog Formation has been interpreted (above) as indicatingvolcanotectonism dueto the emplace- centre ment of rhyolitic magma with the eventual emergence Although there is considerable evidence of uplift and erosion of a rhyolitic volcanic island. Thusthe and emergence of a rhyolitic volcanic island west of rhyolite intrusions are attributed to an early proximal, the RamseyFault during Porth Llauog Formation magmatic phase, a precursor to accumulation of the times, the Carn Llundain Formation records at least Carn Llundain Formation. However, thepresence of a moderately deep watervolcanism, so that followinguplift rhyolite sheet cutting the Cader Rhwydog Tuff (Fig. 2) there musthave been considerable subsidence. The andthe brecciated lavas of the Allt Felin Fawr depth of water will be considered later. Memberattest to the continued availability of the Field, petrographicaland petrochemical (Table 3 crystal-poor magma at the eruptive centre. and unpublished analyses) evidence shows thatthe The irregularly shaped intrusions intothe Trwyn rhyolitic tuffs and lavas of the Carn Llundain Forma- Llundain Formation aregood examples of the magma- tion,together with the intrusions of porphyritic tic phase which preceded the Carn Llundain activity; rhyolite and microtonalite, represent magmas which the largest is cut by the unconformityat Ogof were simultaneously available andconsanguineous. Colomenod. Mostly the intrusivecontacts aresharp The rhyolites emplaced during Porth Llauog Forma- and discordant with finger-like apophyses of rhyolite tion times are almost certainly genetically related to penetrating thesediments. Rhyolitemargins, locally the later rocks. flow-banded parallel to the contacts, are commonly While it seems likely that there were other active brecciated and in places theadjacent sediments are faults in the vicinity, the Ramsey Fault played a major also brecciated up to several metres from the contact. rolein the emplacement of magmas by acting asa Some rhyolitemargins are peperitic, with trains of channelway. The larger intrusions are localized along rhyolite fragments within the sediments. Such contact the fault, the rhyolites east and west of Porth Llauog relationshipsindicate thatthe rhyoliteswere fluid wereemplaced close tothe fault, and the Cader when intruded into a locally unlithified, wet host. Rhwydog Tuff requires asource in the immediate Within the largest intrusion, exposedbetween vicinity. Thus RamseyIsland marks the site of an Trwyn Llundain and Ogof Colomenod, the flinty and important (Llanvirn) submarine rhyolitic centre. locally flow-banded rhyolitecontains extensive and N-S trendingfaults which profoundly influenced irregularzones of breccia. Massive rhyolitegrades volcanism and associated sedimentation during the through rhyolitewith hairline fracturesand jig-saw Ordovician are widely recognized in N Wales, where breccias, into breccias comprising angular to sub- theywere active during theTremadoc arc volcanic rounded rhyolitic clastsin a relatively chlorite-rich episode and through the Arenig to Caradoc evolution

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 I 5"20'W TrwynSh6n Owen EAST OF THE RAMSEY FAULT*

ash-flow tuff

*L.q*_ laminatedturbiditic tuffs Velvet

turbiditic tuffs (proximal) bedded hyaloclastites

debris flow deposits

mudstones with minor siltstones a turbiditic tuffs (distal)

darkmudstones turbiditic tuffs (distal) darkmudstones (pencilslates)

turbiditic tuffs (distal)

blackmuds tones

Bitches

U S LMBRIAN

WEST OF THE RAMSEY FAULT"

laminated turbiditic tuffs a I I mass flow deposits wiith brecciated rhyolite lavas(') ash-flow tuff ash-flow tuff

ash-flow tuff with overlying 0 25m of associated P, 7 ash-fall tuff S -l- C S p. P,-. cunconformity S -n " 1> ' 3 S

3 conglomerate

l- "=--l 2 MID.CA ; MBRIAN ? _. a S

*All igneousrocks shown in th e columns ar*e rhyolitic

FIG.2. Geological map and stratigraphic columns of Ramsey Island. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 Ramsey Island, SW Wales 611 of the marginal basin (Kokelaar 1979; Kokelaar et al. pumice clasts do occur. There seems to be no way of 19846). It is possible, therefore, that although uplift distinguishing lithologically betweenlaminae dueto west of the RamseyFault is attributableto the ash-fall and those dueto low-density turbiditycur- emplacement of magma, the orientation of the fault rents. The ‘background’ turbidites of the upper part of was tectonically controlled. Indeedthe post- the fQrmation show topographical restriction and emergence subsidence may have been purely tectonic mantling has not been found. These deposits represent in origin. The source of the rhyolites is presently the continualdisturbance of the volcanic pile and it is subject of investigation (Kokelaar & Bevins, in press), quite possible that the turbidity currents could rework but, whethercrustal melts or derivatives of a more fine water-settled ash. basic parent, there seems a likelihood that a deep N-S Except where ponding occurred, the confinement of crustal fracture facilitated magma ascent. eruptions by surrounding water promoted the generation of volcaniclastic-gravity flows by causing material to bedumped rapidly close tothe ventwhere it formed unstable slopes and sloughed away. Depend- Conclusions ing mainly onthe rate of eruption,the flows so generated could be hot or cold, volatile-rich or Concerning the ‘distal’ volcanism of the Aber Mawr water-rich. High eruption rates favoured subaqueous Formation, the problems of determination of the en- ash-flows with potential for forming welded deposits. vironment, processes and distances of the source(s) Distally such flows would transform into cold high- might bebetter solved by detailed sedimentological density turbidity currents. and petrographical logging of the sections. This would Several features are relevant with regard to deter- facilitate comparison with ash layers, or models, de- mination of water depth. The turbiditic tuffs require rived from possible modern analogues; e.g. the deep- moderately deep, open water,as they show no signs of water ashes aroundthe Lesser Antilles or in the reworking by wave or tractioncurrent activity. The marginal basins of the W Pacific (see Carey & Sigurd- Cader Rhwydog Tuff requires sufficient depthto sson (1984)). However, the intense alteration will be a suppress and contain an explosive eruption column, major problem in the determination of primary fabrics butnot so much as to inhibit explosivity. The and particle-size distributions, as well as in determin- fluidization along the margins of the Allt Felin Fawr ing by composition whether one or more sources were sill (and other such intrusions)requires shallowness involved (cf. LesserAntilles; Carey & Sigurdsson sufficient to allow substantial expansion of water to 1984). steam. Of course the water depth may have changed ThePorth LlauogFormation and associated wet- dramatically during volcanism, particularly during sediment slides graphically record major sea-floor eruption of theCader Rhwydog Tuff when partial disturbancesprior to volcanotectonicemergence of evacuation of a magma chamber might have led to a rhyolitic volcanic island. The slides, of great area1 subsidence. extentand thickness, must have generateddevastat- The critical pressure of saline water corresponds to a ing tsunamis. The uplift onthe RamseyFault was depth of about 3 km but, because of the fluidization, a probably considerably in excess of l km (lower Mid- depth of more than2 km seems unlikely (see Kokelaar dle Cambrian rocks exposed) andthe recent uplift (1982)). The maximum depthat which rhyolitic on the island of Ischia, northwest of Naples, of780 magmatic explosions can occur is unknown. Although m at a minimum rate of approximately 0.25 cm per the magmatic vapour is largely water,the depth at year (Capaldi et al. 1976) seems anappropriate which highly vesiculated pumice can be produced is analogue. almost certainly considerably shallower than 2 km. The subsequent ‘proximal’ activity, after submerg- The water depth required to contain theCader ence, shows a remarkable variety of closely successive Rhwydog eruption column imposes a minimum value, or contemporaneously developed phenomena with no which, speculatively, may be of the order of 500 m. known modern analogues. The close temporal and spatial association of In the Carn Llundain Formation, with the exception rhyolitic pumice tuff, ‘low’-viscosity rhyolites and of the special case of part of the Cader Rhwydog Tuff, high-viscosity (brecciated)rhyolites (e.g. Allt Felin there is a notable absence of well-graded beds Fawr Member) is interesting. It is notable thatthe representingwater-settled ashes, and volcaniclastic- ‘low’-viscosity rhyolites are always markedly porphyri- gravity flow deposits are dominant.This almost tic and the breccias almostaphyric. TheCader certainly reflects the occurrence of mainly subaqueous Rhwydog Tuff contains abandant crystals and ragged eruptions where dispersal of ash by fall-out was fragments of the ‘low’-viscosity rhyolite. Such varia- severely restricted by the surrounding water, as tions in magma behaviour, and the relationships, can indicated by the Cader Rhwydog Tuff. Some laminae be explained interms of water content.Thus the of fine tuff probably aredue to settling out of viscous rhyolites were relatively dry,the porphyritic water-suspended ash, and sparse sunken water-logged rhyolites contained dissolved water which reduced

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/591/4888435/gsjgs.142.4.0591.pdf by guest on 02 October 2021 612 B. P. Kokelaar, R. E. Bevins & R. A. Roach viscosity and (on ascent of the magma) promoted ACKNOWLEDGEMENTS.Theauthors are greatlyindebted to crystallization, but were undersaturated at the ambient Robin Pratt, owner of Ramsey Island during the period of pressures (depths),and the explosive rhyolites re- field work, for providing considerable logistical support and sulted from magmas that were crystallizing, became excellent accommodation without which the work could not havebeen completed. Benefit has beenderived from oversaturated,and rapidly exsolved volatiles. Such discussions in the field with Dick Fisher, Malcolm Howells, variations in a consanguineous suite could be attribut- andSteve Sparks, and the content of thepaper has been able to volatile fluxing in a body of magma, although improved as a result of comments from Mike Bassett, Dick variableinteraction with connate waters in separate Fisher, Richard Fortey, Bob Owens and Cathy Busby-Spera. bodies could also apply. Dodie James is thanked for the chemical analyses in Table 3 This case study on Rarnsey Island serves to and Graham Lees for other (unpublished) analyses of rocks emphasize that there is a great deal still to be learnt from RamseyIsland. DaleEvans skilfully producedthe about submarine silicic volcanism. annotated sketches of Figs 4 and 6.

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Received 21 December 1984; revised typescript accepted 1 March 1985.

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