Hostname: page-component-cd9895bd7-hc48f Total loading time: 0 Render date: 2024-12-23T16:13:23.639Z Has data issue: false hasContentIssue false
  • English
  • Français

Rootless vents in welded ash-flow tuffs from northern Snowdonia, North Wales, indicating deposition in a shallow water environment

Published online by Cambridge University Press:  01 May 2009

J. V. Wright  and
M. P. Coward
J. V. Wright
Affiliation:
Department of Geology, Royal School of Mines, Imperial College, London, SW7 2BP
M. P. Coward
Affiliation:
Department of Earth Sciences, University of Leeds, Leeds, LS2 9JT
Get access Rights & Permissions [Opens in a new window]

Summary

In northern Snowdonia welded ash-flow tuffs, correlated with the Pitt's Head Tuff Formation occur intercalated with Caradoc shallow marine shelf sediments. Within the tuffs there are vent-like structures which have a diapiric form where fiamme are drawn in towards a central core. The cores may have a constrictional linear fabric of streaked out fiamme, or may consist of vitrous tuff surrounded by a zone of buckled planar fiamme. There are associated zones of quartz nodules which are thought to represent quartz filled vesicles. The diapiric structures are believed to be rootless vents. Steam generated by the heat of the flow in contact with shallow water is thought to have caused explosive disruption of the tuffs as consolidation and welding occurred. These structures are thus themselves indicators of deposition of ash-flow tuffs in a shallow water environment.

Type
Articles
Information
Geological Magazine , Volume 114 , Issue 2 , March 1977 , pp. 133 - 140
Copyright
Copyright © Cambridge University Press 1977

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Allen, J. R. L. 1963. Henry Clifton Sorby and the sedimentary structures of sands and sandstones in relation to flow condition. Geologie Minjb. 42, 223–8.Google Scholar
Allen, J. R. L. 1967. Depth indicators of clastic sequences. Marine Geol. 5, 429–46.CrossRefGoogle Scholar
Beavon, R. V. 1974. Discussion of Ordovician transgressive ash-flow tuffs of Snowdonia, N. Wales. J. geol. Soc. Lond. 130, 479—80.Google Scholar
Beavon, R. V., Fitch, F. L. & Rast, N. 1961. Nomenclature and diagnostic character of ignimbrites, with special reference to Snowdonia. Lpool Manchr geol. J. 2, 600–11.CrossRefGoogle Scholar
Clifton, H. E., Hunter, R. E. & Phillips, R. L. 1971. Depositional structures and process in the non-barred high energy near shore. J. sedim. Petrol. 41, 651—70.Google Scholar
Dzulynski, S. & Walton, E. L. 1965. Sedimentary features of flysch and greywackes. Developments in Sedimentology, 7, Amsterdam: Elsevier.Google Scholar
Fitch, F. J. 1967. Ignimbrite volcanism in North Wales. Bull. Volcan. 30, 199–219.CrossRefGoogle Scholar
Folk, R. L. 1968. Petrology of sedimentary rocks. Austin Tex. Hemphills.Google Scholar
Francis, E. H. & Howells, M. F. 1973. Transgressive welded ash-flow tuffs among Ordovician sediments of N.W. Snowdonia, N. Wales. J. geol. Soc. Lond. 129, 612–41.Google Scholar
Gadow, S. & Reineck, H. E. 1969. Albandiger Sandtransport bei Sturmfluten. Stenckenbergiana marit, 1, 63–76.Google Scholar
Howells, M. F., Leveridge, B. E. & Evans, C. D. R. 1971. The Lower Crafnant Volcanic Group, eastern Snowdonia. Proc. geol. Soc. Lond. 1664, 284–5.Google Scholar
Howells, M. F., Leveridge, B. E. & Evans, C. D. R. 1973. Correlation and genesis of Ordovician ash-flow tuffs in eastern Snowdonia. Inst. Geol. Sd. Rep. No. 73/3.Google Scholar
Macdonald, G. A. 1967. Forms and structures of extrusive basalt rocks. In Basalts: the Poldervaart treatise on rocks of basaltic composition. vol. 1. (eds. Hass, H. H. & Poldervaart, A.) New York: Interscience.Google Scholar
McLeish, A. J. 1971. Strain analysis of deformed pipe rock in the Moine Thrust zone, Northwest Scotland. Tectonophysics 12, 469–503.CrossRefGoogle Scholar
Newton, R. S. 1968. Internal Structure of wave-formed ripple marks in the nearshore zone. Sedimentology, 11, 275–92.CrossRefGoogle Scholar
Potter, P. E. & Pettijohn, F. J. 1963. Palaeocurrents and Basin Analysis. Berlin, Gottingen, Heidelberg: Springer-Verlag.Google Scholar
Rast, N. 1962. Textural evidence for the origin of ignimbrites. Lpool Manchr geol.J. 3, 97–108.Google Scholar
Reineck, H. E., Dorjes, J., Gadow, S. & Hertweck, G. 1968. Sedimentologie, Faunenzonierung und Faziesabfolge vor der Ostkuste der inneren Deutschen Bucht. Senckenbergiana lethaea. 49, 261–309.Google Scholar
Roberts, B. & Siddans, A. W. B. 1971. Fabric studies in the Llwyd Mawr Ignimbrite, Caernarvonshire, North Wales. Tectonophysics 12, 283–306.CrossRefGoogle Scholar
Simons, D. B. & Richardson, E. V. 1961. Forms of bed roughness in alluvial channels. Proc. Amer. Soc. Civil Engineers, 87, No. HY3, 87.Google Scholar
Thorarinsson, S. 1953. The Crater Groups in Iceland. Bull. Volcan. 14, 3–44Google Scholar
Waters, A. C. 1960. Determining direction of flow in basalts. Am. J. Sci. 258-A, 350–66.Google Scholar
Williams, D. 1930. The geology of the country between Nant Paris and Nant Ffrancon. Q. J. geol. Soc. Lond. 83, 191–233.Google Scholar
Williams, H. 1927. The geology of Snowdon (North Wales). Q. Jl geol. Soc. Lond. 83, 346–431.Google Scholar