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.

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

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.