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The Submarine Eruption and Emplacement of the Lower Rhyolitic Tuff Formation (Ordovician) , N Wales

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The Submarine Eruption and Emplacement of the Lower Rhyolitic Tuff Formation (Ordovician) , N Wales 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 Wales 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 Snowdon 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 Snowdon massif and hasbeen correlated (Howells et al. currently investigating the geochemistry of the Caradoc 1973, 1978, 19856) with part of the LowerCrafnant volcanic rocks of Snowdonia.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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/411/4893220/gsjgs.143.3.0411.pdf by guest on 27 September 2021 412 M. F. HOWELLS, A. J. REEDMAN & S. D. G. CAMPBELL 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 Llanberis 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 Moel Hebog 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/411/4893220/gsjgs.143.3.0411.pdf by guest on 27 September 2021 LOWER RHYLITIC TUFF FORMATION, N WALES 413 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/411/4893220/gsjgs.143.3.0411.pdf by guest on 27 September 2021 414 M. F. HOWELLS, A. J. REEDMAN & S. D. G. CAMPBELL 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 extension in the tuffs varies from correspondingapproximately tothe BeddgelertFault
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