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 last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s that outcrop on the islands of

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

Islay Islay ( ; , ) is the southernmost island of the Inner Hebrides of Scotland. Known as "The Queen of the Hebrides", it lies in Argyll and Bute just south west of Jura, Scotland, Jura and around north of the Northern Irish coast. The island's cap ...

and 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

gneiss Gneiss (pronounced ) 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. This rock is formed under p ...

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

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

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 ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...

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 (), 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 including cerium and yttrium; calcium may be partly rep ...

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, the

Moine Supergroup The Moinian or just the Moine, formerly the Moine Supergroup, is a sequence of Neoproterozoic metasediments that outcrop in the Northwest Highlands of Scotland between the Moine Thrust Belt to the northwest and the Great Glen Fault to the s ...

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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