The Dalradian Succession in Islay and Jura

Abstract

I. Intoduction This paper is concerned only with rocks of accepted Dalradian age. I have, therefore, not taken into account formations lying south-west of the Loch Skerrols thrust, which include the problematical Bowmore Sandstone. The succession, which I think may be considered as proved, is identical with that put forward by Prof. E. B. Bailey in 1917; but it has been established in the present instance mainly by consideration of current-bedding and graded bedding in several members of the sequence. The significance of certain conglomerates on Beannan Dubh, however, has again been investigated. In the following pages it is intended to retain the stratigraphical nomenclature used by Prof. Bailey, and also to discuss formations put forward by other authors in terms of this nomenclature. The Dalradian succession in Islay and Jura, according to my determination, is as follows, the oldest rocks being at the bottom of the table :— 11. Ardmore and Laphroaig Quartzites. 10. Port Ellen Phyllites. 9. Scarba Conglomeratic Group. 8. Jura Slates. 7. Main Quartzite. 6. Dolomitic Group of North Islay. 5. Power Fine-Grained Quartzite of North Islay. 4. Portaskaig Conglomerate. 3. Islay Limestone. 2. Mull of Oa Phyllites. 1. Maol an Fhithich Quartzite. Since opinions on the geological structure of this area are many and varied, it does not seem out of place here to review and analyse very briefly

The Dalradian rocks of the south-west Grampian Highlands of Scotland

“…In one of the first studies to use sedimentary ‗way-up' structures in metamorphic rocks in the British Isles, Allison (1933) confirmed Bailey's interpretation that the ‗Dolomitic Group' lies stratigraphically between the older, Port Askaig Tillite, and the Jura Quartzite above. Spencer and Spencer (1972) later combined groups 3-5 of Wilkinson (1907) to erect the Bonahaven Dolomite Formation, which is made up of four members and has a maximum total thickness of some 350 m (Fairchild, 1991, fig.…”

“…Allison (1933) confirmed Bailey's (1917) stratigraphical sequence by the use of way-up structures, and published the first detailed sketch map of part of the site. Spencer (1972) established the current informal subdivision of the Bonahaven Dolomite into 4 units and prepared a detailed map of the whole section.…”

“…As a result, some parts of the stratigraphical sequence had to be revised when way-up techniques were first applied (Vogt, 1930;Bailey, 1930;Allison, 1933).…”

“…There was an early dispute over whether this fold is a syncline or an anticline, and this was not resolved conclusively until the work of Allison (1933). The Islay Anticline is poorly exposed; for such a large structure, there is surprisingly little published information on its geometry, and few structural symbols on the BGS 1:50 000 sheets for the area.…”

“…(1911) and later by Allison (1933). More recent work includes that of Anderton (1976Anderton ( , 1977Anderton ( , 1979Anderton ( , 1980, which is mainly concerned with the sedimentology of the Islay and Easdale subgroups of the Argyll Group.…”