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

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...

s that outcrop on the islands of

Colonsay Colonsay (; gd, Colbhasa; sco, Colonsay) is an island in the Inner Hebrides of Scotland, located north of Islay and south of Mull. The ancestral home of Clan Macfie and the Colonsay branch of Clan MacNeil, it is in the council area of Arg ...

, Islay and

Oronsay This is a list of islands called Oronsay (Scottish Gaelic: '), which provides an index for islands in Scotland with this and similar names. It is one of the more common names for Scottish islands. The names come from ''Örfirisey'' which transla ...

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

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

es of the

Rhinns complex The Rhinns complex is a deformed Palaeoproterozoic igneous complex that is considered to form the basement to the Colonsay Group of metasedimentary rocks. The largest outcrop of the complex is on the Rhinns of Islay, from where the complex g ...

and is thought to be mostly older than the Colonsay/Oronsay sequence. A possible correlation has been made between

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s of the Oronsay formation which form the base of the preserved sequence on Colonsay/Oronsay and those of the Smaull and Sanaig

Greywacke Greywacke or graywacke ( German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or li ...

formations towards the top of the Islay sequence. Within Colonsay, the following sequence is recognised (in stratigraphical order with topmost/youngest first):British Geological Survey 1:50,000 scale provisional geological map sheet (Scotland) 35 ''Colonsay'' 1996 * 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

titanite Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals in ...

grains that give an age of about 942 Ma and detrital zircon grains giving an age of about 1025 Ma, suggesting that the Colonsay Group is Neoproterozoic. The minimum age is given by dating of an intrusion that cuts the sequence at Kiloran Bay, which is dated to about 440 Ma ( Early Silurian).

Correlation

The Colonsay Group has at various times been correlated with the

Torridonian In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The st ...

, 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. 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

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