Hostname: page-component-cd9895bd7-gbm5v Total loading time: 0 Render date: 2024-12-23T06:40:07.033Z Has data issue: false hasContentIssue false
  • English
  • Français

Tectonic development and stratigraphy at the western margin of the Caledonides: Islay and Colonsay, Scotland

Published online by Cambridge University Press:  03 November 2011

W. R. Fitches  and
A. J. Maltman
W. R. Fitches
Affiliation:
Department of Geology, University College of Wales, Aberystwyth, Wales SY23 3DB.
A. J. Maltman
Affiliation:
Department of Geology, University College of Wales, Aberystwyth, Wales SY23 3DB.
Get access Rights & Permissions [Opens in a new window]

Abstract

Islay and Colonsay occupy a crucial position in the Caledonian frontal zone S of the Great Glen fault in western Scotland. However, some of the stratigraphical units which comprise these islands are of doubtful age as they have tectonic boundaries and contain no fossils or distinctive lithological components. New information is presented here on the deformational histories and styles of the rocks and this has prompted a reassessment of their stratigraphical position and a major reinterpretation of t0he tectonism of this frontal zone.

In contrast to the conventional view, all the stratigraphical units have been deformed together during a regional shearing event. The Colonsay Group contains intrafolial and sheath folds and has a pronounced linear fabric which is parallel to grain elongations in parts of the Bowmore and Dalradian sequences. Most junctions between the Colonsay Group and basement of the Lewisian complex are highly sheared. The junction between the Bowmore Group and Dalradian Supergroup is also tectonised and is normally referred to as the Loch Skerrols thrust but here as a shear zone. This structure is shown not to be a profound discontinuity, alone representing the Caledonian front S of the Great Glen fault, but one element in a major flat-lying NW-directed ductile shear zone. The Caledonian front therefore lies to the W of the islands.

The Bowmore Group is suggested to be a lateral equivalent of the Crinan Grits in the Dalradian Supergroup, lying in the overturned sheared limb of the Islay anticline. The age of the Colonsay Group remains enigmatic; the identification of previously unrecognised early structures which may precede shearing in the Grampian episodes of the Caledonian orogenic cycle allows the possibility of a pre-Caledonian deformation of the Group.

Keywords

Bowmore GroupCaledonian frontcleavageColonsay GroupDalradian Supergroupdeformational phasegrain elongationintrafolial foldIslay anticlineLewisian complexlineationLoch Skerrols shear zonemylonitesheath foldthrust
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 75 , Issue 4 , 1984 , pp. 365 - 382
Copyright
Copyright © Royal Society of Edinburgh 1984

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

Amos, B. J. 1960. The geology of the Bowmore district, Islay. Unpublished Ph.D. Thesis, London University.Google Scholar
Anderton, R. 1975. Tidal flat and shallow marine sediments from the Craignish Phyllites, Middle Dalradian, Argyll, Scotland. GEOL MAG 112, 337–48.CrossRefGoogle Scholar
Anderton, R. 1982. Dalradian deposition and the late Precambrian-Cambrian history of the N. Atlantic region: a review of the early evolution of the Iapetus Ocean. J GEOL SOC LONDON 139, 421–31.CrossRefGoogle Scholar
Anderton, R., Bridges, P. H., Leeder, M. R. & Sellwood, B. W. 1979. A Dynamic Stratigraphy of the British Isles. London: George Allen & Unwin.Google Scholar
Bailey, E. B. 1917. The Islay Anticline, Inner Hebrides. Q J GEOL SOC LONDON 72, 132–64.CrossRefGoogle Scholar
Basahel, A. N. 1971. The Dalradian stratigraphy and structure of South Islay, Argyll. Unpublished Ph.D. Thesis, Liverpool University.Google Scholar
Borradaile, G. J. 1979. Strain study of the Caledonides in the Islay region, S.W. Scotland: implications for strain histories and deformation mechanisms in greenschists. J GEOL SOC LONDON 136, 77–88.CrossRefGoogle Scholar
Borradaile, G. J. 1980. Pre-tectonic reconstruction of the Islay Anticline: implications for the depositional history of the Dalradian rocks in the S.W. Highlands. In Harris, A. L., Holland, C. H. & Leake, B. E. (eds) The Caledonides of the British Isles—Reviewed, 229–38. SPEC PUBL GEOL SOC LONDON 8.Google Scholar
Bowes, D. R. 1978. Shield formation in early Precambrian times: the Lewisian. In Bowes, D. R. & Leake, B. E. (eds) Crustal evolution in northwestern Britain and adjacent regions, 39–80. GEOL J SPEC ISSUE 10.Google Scholar
Brewer, J. A., Matthews, D. H., Warner, M. R., Hall, J., Smythe, D. K. & Whittington, R. J. 1983. BIRPS deep seismic reflection studies of the British Caledonides. NATURE 305, 206–10.CrossRefGoogle Scholar
Brewer, M. S., Brook, M. & Powell, D. 1979. Dating of the tectono-metamorphic history of the south-western Moine, Scotland. In Harris, A. L., Holland, C. M. & Leake, B. E. (eds) The Caledonides of the British Isles—Reviewed, 129–37. SPEC PUBL GEOL SOC LONDON 8.Google Scholar
Coward, M. P. 1983. The thrust and shear zones of the Moine thrust zone and the N.W. Scottish Caledonides. J GEOL SOC LONDON 140, 795–811.CrossRefGoogle Scholar
Cunningham, Craig E. M., Wright, W. B. & Bailey, E. B. 1911. The geology of Colonsay and Oronsay, with part of the Ross of Mull. MEM GEOL SURV SCOTLAND.Google Scholar
Dunning, F. W. 1966. Tectonic Map of Great Britain and Northern Ireland. London: Institute of Geological Sciences.Google Scholar
Evans, D., Kenolty, N., Dobson, M. R. & Whittington, R. J. 1979. Geology of the Malin Sea. REP INST GEOL SCI 79/15.Google Scholar
Fairchild, I. J. 1980. The structure of N.E. Islay. SCOTT J GEOL 16, 189–97.CrossRefGoogle Scholar
Green, J. F. N. 1924. The structure of the Bowmore-Portsakaig District of Islay. Q J GEOL SOC LONDON 80, 72–105.CrossRefGoogle Scholar
Harris, A. L., Baldwin, C. T., Bradbury, H. J., Johnson, H. D. & Smith, R. A. 1978. Ensialic basin sedimentation: the Dalradian Supergroup. In Bowes, D. R. & Leake, B. E. (eds) Crustal evolution in northwestern Britain and adjacent regions, 115–38. GEOL J SPEC ISSUE 10.Google Scholar
Harris, A. L., Rathbone, P. A. & Watson, J. 1981. The Pre-Caledonian Evolution of the British Isles. In Cogné, J. & Slandry, M. (eds), Geology of Europe, 22–8. MEM BUR RECH GEOL MIN 108.Google Scholar
Johnson, M. R. W., Sanderson, D. J. & Soper, N. J. 1979. Deformation in the Caledonides of England, Ireland and Scotland. In Harris, A. L., Holland, C. H. & Leake, B. E. (eds) The Caledonides of the British Isles—Reviewed, 165–8. SPEC PUBL GEOL SOC LONDON 8.Google Scholar
Johnstone, G. S. 1966. The Grampaign Highlands. Edinburgh: Her Majesty's Stationery Office.Google Scholar
Johnstone, G. S., Smith, D. I. & Harris, A. L. 1969. The Moinian assemblage of Scotland. In Kay, M. (ed). North Atlantic—Geology and Continental Drift, a symposium, 159–80. MEM AM ASSOC PET GEOL 12.Google Scholar
Maltman, A. J. 1981. Primary bedding-parallel fabrics in structural geology. J GEOL SOC LONDON 138, 475–83.CrossRefGoogle Scholar
Peach, B. N. & Horne, J. 1930. Chapters on the Geology of Scotland. London: Oxford University Press.Google Scholar
Piasecki, M. A. J., van Breeman, O. & Wright, A. E. 1981. Late Precambrian geology of Scotland, England and Wales. In Kerr, J. W. & Fergusson, A. J. (eds) Geology of the North Atlantic Borderlands, 57–94. MEM CAN SOC PET GEOL 7.Google Scholar
Piasecki, M. A. J. & van Breemen, O. 1983. Field and isotopie evidence for a c. 750 ma tectonothermal event in Moine rocks in the Central Highland region of the Scottish Caledonides. TRANS R SOC EDINBURGH EARTH SCI 73, 119–34.CrossRefGoogle Scholar
Roberts, J. L. 1974. The structure of the Dalradian rocks in the SW Highlands of Scotland. J GEOL SOC LONDON 130, 93–124.CrossRefGoogle Scholar
Smith, D. I. & Watson, J. 1983. Scale and timing of movements on the Great Glen Fault, Scotland. GEOLOGY 11, 523–6.2.0.CO;2>CrossRefGoogle Scholar
Soper, N. J. & Anderton, R. 1984. Did the Dalradian slides originate as extension faults? NATURE 307, 357–60.CrossRefGoogle Scholar
Stewart, A. D. 1960. On the sedimentary and metamorphic history of the Torridonian, and the later igneous intrusions of Colonsay and Oronsay. Unpublished Ph.D. thesis, Liverpool University.Google Scholar
Stewart, A. D. 1962a. On the Torridonian sediments of Colonsay and their relationship to the main outcrop in north-west Scotland. LIVERPOOL MANCHESTER GEOL J 3, 121–55.CrossRefGoogle Scholar
Stewart, A. D. 1926b. Greywacke sedimentation in the Torridonian of Colonsay and Oronsay. GEOL MAG 99, 399–419.CrossRefGoogle Scholar
Stewart, A. D. 1969. Torridonian rocks of Scotland reviewed. In Kay, M. (ed) North Atlantic—Geology and Continental Drift, a symposium. 595–608. MEM AM ASSOC PET GEOL 12.Google Scholar
Stewart, A. D. 1975. “Torridonian” rocks of western Scotland. In Harris, A. L., Shackleton, R. M., Watson, J., Downie, C., Harland, W. B. & Moorbath, S. (eds) A Correlation of the Precambrian rocks in the British Isles, 43–51. GEOL SOC LONDON SPEC REP 6.Google Scholar
Stewart, A. D. 1982. Late Proterozoic rifting in NW Scotland: the genesis of the ‘Torridonian’. J GEOL SOC LONDON 139, 413–20.CrossRefGoogle Scholar
Stewart, A. D. & Hackman, B. D. 1973. Precambrian sediments of western Islay. SCOTT J GEOL 9, 185–201.CrossRefGoogle Scholar
Watson, J. 1984. The ending of the Caledonian orogeny in Scotland. J GEOL SOC LONDON 141, 193–214.CrossRefGoogle Scholar
Westbrook, G. K. & Borradaile, G. J. 1978. The geological significance of magnetic anomalies in the region of Islay. SCOTT J GEOL 14, 213–24.CrossRefGoogle Scholar
Wilkinson, S. B. 1907. The geology of Islay, including Oronsay and portions of Colonsay and Jura. MEM GEOL SURV SCOTLAND.Google Scholar
Wright, W. B. 1908. The two earth-movements of Colonsay. Q J GEOL SOC LONDON 64, 297–312.CrossRefGoogle Scholar