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The significance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey

Published online by Cambridge University Press:  20 December 2012

JACK E. TREAGUS ,
SUSAN H. TREAGUS  and
NIGEL H. WOODCOCK
JACK E. TREAGUS
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, England
SUSAN H. TREAGUS*
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, England
NIGEL H. WOODCOCK
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England
*
†Author for correspondence: [email protected]
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Abstract

The boundary between the Rhoscolyn and New Harbour formations on Holy Island, Anglesey, has been described as a high strain zone or as a thrust. The boundary is here described at four localities, with reference to the contrasting sedimentary and deformational character of the two formations. At one of these localities, Borth Wen, sandstones and conglomerates at the top of the Rhoscolyn Formation are followed, without any break, by tuffs and then mudstones of the New Harbour Formation. It is concluded that there is clear evidence of depositional continuity across the boundary here, and that both formations subsequently shared a common two-phase deformation. The first (D1) was manifestly different in intensity and scale in the two formations, whereas the second (D2) produced very similar structures in both. The other three localities provide continuity of sedimentary and tectonic features at this boundary in a traverse along the length of Holy Island, leading us to identify two previously unrecognized major D1 folds in addition to the Rhoscolyn Anticline. At one of these localities (Holyhead), we confirm the presence of Skolithos just below the boundary, supporting radiometric evidence for a lower Cambrian or later age for the Rhoscolyn Formation. A turbidite interpretation for both the Rhoscolyn and New Harbour formations best fits the available evidence. A deep-water depositional environment is still compatible with the sporadic presence of Skolithos burrows, but less so with reported observations of hummocky and swaley cross-stratification lower down the South Stack Group.

Keywords

North WalesMonian SupergroupRhoscolyn FormationNew Harbour Formationstructurefolds
Type
Original Articles
Information
Geological Magazine , Volume 150 , Issue 3 , May 2013 , pp. 519 - 535
Copyright
Copyright © Cambridge University Press 2012

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References

Barber, A. J. & Max, M. D. 1979. A new look at the Mona Complex (Anglesey, North Wales). Journal of the Geological Society, London 136, 407–32.CrossRefGoogle Scholar
Bouma, A. H. 1962. Sedimentology of Some Flysch Deposits. A Graphic Approach to Facies Analysis. Amsterdam: Elsevier.Google Scholar
British Geological Survey . 1996. Tectonic Map of Britain, Ireland and Adjacent Areas. Keyworth, Nottingham: British Geological Survey.Google Scholar
Cobbold, P. R., Cosgrove, J. W. & Summers, J. M. 1971. Development of internal structures in deformed anisotropic rocks. Tectonophysics 12, 23–53.Google Scholar
Collins, A. S. & Buchan, C. 2004. Provenance and age constraints of the South Stack Group, Anglesey, UK: U-Pb SIMS detrital zircon data. Journal of the Geological Society, London 161, 743–6.Google Scholar
Cooper, A. H., Rushton, A. W. A., Molyneux, S. G., Hughes, R. A., Moore, R. M. & Webb, B. C. 1995. The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District. Geological Magazine 132, 185–211.CrossRefGoogle Scholar
Cosgrove, J. W. 1980. The tectonic implications of some small-scale structures in the Mona Complex of Holy Isle, North Wales. Journal of Structural Geology 2, 383–96.Google Scholar
Davies, J. R., Fletcher, C. J. N., Waters, R. A., Wilson, D., Woodhall, D. G. & Zalasiewicz, J. A. 1997. Geology of the Country around Llanilar and Rhayader. British Geological Survey Memoir. Keyworth, Nottingham: Memoir, British Geological Survey, Sheets 178 & 179 (England & Wales).Google Scholar
Föllmi, K. B. & Grimm, K. A. 1990. Doomed pioneers: gravity-flow deposition and bioturbation in marine oxygen-deficient environments. Geology 18, 1069–72.Google Scholar
Frey, R. W., Pemberton, S. G. & Saunders, T. D. A. 1990. Ichnofacies and bathymetry: a passive relationship. Journal of Paleontology 64, 155–8.Google Scholar
Gibbons, W. 1983. Stratigraphy, subduction and strike-slip faulting in the Mona Complex of North Wales – a review. Proceedings of the Geologists' Association 94, 147–63.CrossRefGoogle Scholar
Gibbons, W. & Ball, M. J. 1991. A discussion of Monian Supergroup stratigraphy in northwest Wales. Journal of the Geological Society, London 148, 5–8.CrossRefGoogle Scholar
Gibbons, W. & Horak, J. M. 1996. The evolution of the Neoproterozoic Avalonian subduction system: evidence from the British Isles. Geological Society of America Special Papers 304, 269–80.Google Scholar
Gibbons, W., Tietzsch-Tyler, D., Horák, J. & Murphy, F. C. 1994. Precambrian rocks in Anglesey, southwest Llŷn and southeast Ireland. In A Revised Correlation of Precambrian Rocks in the British Isles: Special Report 25 (eds Gibbons, W. & Harris, A. L.), pp.75–84. London: Geological Society.Google Scholar
Greenly, E. 1919. The Geology of Anglesey. Geological Survey of Great Britain Memoir. London: HMSO.Google Scholar
Greenly, E. 1923. The succession and metatmorphism in the Mona Complex. Quarterly Journal of the Geological Society 79, 334–51.Google Scholar
Hassani, H., Covey-Crump, S. J. & Rutter, E. H. 2004. On the structural age of the Rhoscolyn antiform, Anglesey, North Wales. Geological Journal 39, 141–56.Google Scholar
Holdsworth, R. E., Woodcock, N. H. & Strachan, R. A. 2012. Geological framework of Britain and Ireland. In Geological History of Britain and Ireland (2nd ed.) (eds Woodcock, N. H. & Strachan, R. A.), pp. 19–39. Chichester: Wiley-Blackwell.CrossRefGoogle Scholar
Horák, J. M. & Evans, J. A. 2011. Early Neoproterozoic limestones from the Gwna Group, Anglesey. Geological Magazine 148, 78–88.CrossRefGoogle Scholar
Hudson, N. F. C. & Stowell, J. F. W. 1997. On the deformation sequence in the New Harbour Group of Holy Island, Anglesey, North Wales. Geological Journal 32, 119–29.Google Scholar
Kawai, T., Windley, B. F., Terabayashi, M., Yamamoto, H., Maruyama, S. & Isozaki, Y. 2006. Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex. Journal of Metamorphic Geology 24, 591–602.Google Scholar
Kawai, T., Windley, B. F., Terabayashi, M., Yamamoto, H., Maruyama, S., Omori, S., Shibuya, T., Sawaki, Y. & Isozaki, Y. 2007. Geotectonic framework of the Blueschist Unit on Anglesey-Lleyn, UK, and its role in the development of a Neoproterozoic accretionary orogen. Precambrian Research 153, 11–28.Google Scholar
Lowe, D. R. 1979. Sediment gravity flows: their classification and some problems of application to natural flows and deposits. Special Publication of the Society of Economic Paleontologists and Mineralogists 27, 75–82.Google Scholar
McConnell, B. J., Morris, J. H. & Kennan, P. S. 1999. A comparison of the Ribband Group (southeastern Ireland) to the Manx Group (Isle of Man) and Skiddaw Group (northwestern England). In In Sight of the Suture: The Palaeozoic Geology of the Isle of Man in its Iapetus Ocean Context (eds Woodcock, N. H., Quirk, D. G., Fitches, W. R. & Barnes, R. P.), pp. 337–43. Geological Society of London, Special Publication no. 160.Google Scholar
McIlroy, D. & Horák, J. 2006. Neoproterozoic: the late Precambrian terranes that formed Eastern Avalonia. In The Geology of England and Wales (eds Brenchley, P. J. & Rawson, P. F.), pp. 9–23. London: The Geological Society.CrossRefGoogle Scholar
Passchier, C. W. 2007. Photograph of the month. Journal of Structural Geology 29, 1281.Google Scholar
Pemberton, S. G., MacEachern, J. A. & Frey, R. W. 1992. Trace fossil facies models: environmental and allostratigraphic significance. In Facies Models: Response to Sea Level Change (eds Walker, R. G. & James, N. P.), pp. 47–72. St Johns, Newfoundland: Geological Association of Canada.Google Scholar
Phillips, E. 1991 a. The lithostratigraphy, sedimentology and tectonic setting of the Monian Supergroup, western Anglesey, North Wales. Journal of the Geological Society, London 148, 1079–90.Google Scholar
Phillips, E. 1991 b. Progressive deformation of the South Stack and New Harbour Groups, Holy Island, western Anglesey, North Wales. Journal of the Geological Society, London 148, 1091–100.Google Scholar
Roper, H. 1992. Superposed structures in the Mona Complex at Rhoscolyn, Ynys Gybi, North Wales. Geological Magazine 129, 475–90.Google Scholar
Shackleton, R. M. 1954. The structure and succession of Anglesey and the Lleyn Peninsula. Advancement of Science 11, 106–8.Google Scholar
Shackleton, R. M. 1969. The Pre-Cambrian of North Wales. In The Pre-Cambrian and Lower Palaeozoic Rocks of Wales (ed. Wood, A.), pp. 1–22. Cardiff: University of Wales Press.Google Scholar
Talling, P. J., Amy, L. A., Wynn, R. B., Peakall, J. & Robinson, M. 2004. Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments. Sedimentology 51, 163–94.Google Scholar
Treagus, S. H. 1999. Are viscosity ratios of rocks measurable from cleavage refraction? Journal of Structural Geology 21, 895–901.CrossRefGoogle Scholar
Treagus, S. H. 2003. Viscous anisotropy of two-phase composites, and applications to rocks and structures. Tectonophysics 372, 121–33.Google Scholar
Treagus, S. H., Treagus, J. E. & Droop, G. T. R. 2003. Superposed deformations and their hybrid effects: the Rhoscolyn Anticline unravelled. Journal of the Geological Society, London 160, 117–36.Google Scholar
Walker, R. G. 1976. Facies models. 2, Turbidites and associated coarse clastic deposits. Geosciences Canada 3, 25–36.Google Scholar
Wood, D. S. 1974. Ophiolites, melanges, blueschists and ignimbrites: early Caledonian subduction in Wales. In Modern and Ancient Geosynclinal Sedimentation (eds Dott, R. H. & Shaver, R. H.), pp. 334–44.Google Scholar
Woodcock, N. H. & Morris, J. H. 1999. Debris flows on the Ordovician margin of Avalonia: Lady Port Formation, Manx Group, Isle of Man. In In Sight of the Suture: The Palaeozoic Geology of the Isle of Man in its Iapetus Ocean Context (eds Woodcock, N. H., Quirk, D. G., Fitches, W. R. & Barnes, R. P.), pp. 121–38. Geological Society of London, Special Publication no. 160.Google Scholar
Woodcock, N. H., Morris, J. H., Quirk, D. G., Barnes, R. P., Burnett, D., Fitches, W. R., Kennan, P. S. & Power, G. M. 1999. Revised lithostratigraphy of the Manx Group, Isle of Man. In In Sight of the Suture: The Palaeozoic Geology of the Isle of Man in its Iapetus Ocean Context (eds Woodcock, N. H., Quirk, D. G., Fitches, W. R. & Barnes, R. P.), pp. 45–68. Geological Society of London, Special Publication no. 160.Google Scholar