Colonsay Group
From Wikipedia, the free encyclopedia
| 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 |
| Country | Scotland |
| Type section | |
| Named for | Colonsay |
The Colonsay Group is an estimated 5,000 m thick sequence of mildly metamorphosed Neoproterozoic sedimentary rocks that outcrop on the islands of Colonsay, Islay and Oronsay and the surrounding seabed. They have been correlated with the Grampian Group, the oldest part of the Dalradian Supergroup.
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
