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 contempor- aneously. 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 deter- mination 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 Arenig sedimentary succession in N 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 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 593 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 to be 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,

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