Colonsay Group

Colonsay Group
Colonsay Group
Stratigraphic range: Neoproterozoic
	Smaull Greywacke Formation, near Rubha Lamanais, Rinns of Islay
Type	Group
Lithology
Primary	Sandstone
Other	Shale, siltstone
Location
Region	Inner Hebrides
Type section
Named for	Colonsay

The Colonsay Group is an estimated 5,000 m thick sequence of mildly

Oronsay and the surrounding seabed. They have been correlated with the Grampian Group, the oldest part of the Dalradian Supergroup

.

Stratigraphy

The sequences on Islay and Colonsay/Oronsay are not identical and correlation between them is uncertain. Both sequences are divided up into a series of formations. The Islay sequence lies unconformably on gneisses of the Rhinns complex and is thought to be mostly older than the Colonsay/Oronsay sequence. A possible correlation has been made between sandstones of the Oronsay formation which form the base of the preserved sequence on Colonsay/Oronsay and those of the Smaull and Sanaig Greywacke formations towards the top of the Islay sequence.^[1] Within Colonsay, the following sequence is recognised (in stratigraphical order with topmost/youngest first):^[2]

Staosnaig Phyllite Formation
Kiloran Flags Formation
Milbuie Arkose Formation
Kilchattan Formation
Machrins Arkose Formation
Dun Gallain Grit Formation
Oronsay Greywacke Formation

Age

The upper bound to the age of the sequence is given by the presence of detrital

Early Silurian).^[1]

Correlation

The Colonsay Group has at various times been correlated with the

Moine Supergroup and the Appin Group and Grampian Group of the lower Dalradian Supergroup. Analysis of the provenance area for the Colonsay Group compared to these other sequences, suggests that the best match in terms of provenance is with the Grampian Group, the lowermost part of the Dalradian sequence.^[1] The relationship of the Colonsay Group to the Bowmore Sandstone Group

, with which they are in tectonic contact across the Loch Gruinart fault, is uncertain, although both have been correlated with the Grampian Group.

References

^
doi:10.1016/j.precamres.2010.05.013
. Retrieved 16 September 2012.

^ British Geological Survey 1:50,000 scale provisional geological map sheet (Scotland) 35 Colonsay 1996

56°4′N 6°13′W / 56.067°N 6.217°W / 56.067; -6.217

[McAteer-1] 
doi:10.1016/j.precamres.2010.05.013
. Retrieved 16 September 2012.

[2] British Geological Survey 1:50,000 scale provisional geological map sheet (Scotland) 35 Colonsay 1996

[1]

[2]

v t e Islay
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Geographical features	Ardnave Point Aros Bay Beinn Bheigeir Bowmore Sandstone Group Claggain Bay Colonsay Group Eilean Mhic Coinnich Gruinart Flats Lagavulin Bay Loch an t-Sailein Loch Gruinart Loch Indaal Machir Bay The Oa Rhinns complex Rinns of Islay River Laggan Sound of Islay
History and architecture	Battle of Benbigrie Battle of Traigh Ghruinneart Battle of Epiphany Cenél nÓengusa Clan MacDonald of Dunnyveg Clark Cottage Dun Bhruichlinn Dun Guaidhre Dun Nosebridge Dunlossit House Dunyvaig Castle Finlaggan Islay Charter Islay House Islay LIMPET Islay Woollen Mill Kildalton Castle Kilarrow Parish Church RAF Kilchiaran Kildalton Cross Loch Gorm Castle Margadale Mulindry House Ramsay Hall, Port Ellen St John's Church, Port Ellen
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