The BGS Lexicon of Named Rock Units — Result Details

Iona Group

Computer Code: IONA Preferred Map Code: SYMB
Status Code: Full
Age range: Tonian Period (AT) — Tonian Period (AT)
Lithological Description: Lower Unit: very coarse cobble-boulder conglomerate, interbedded with, and grading upwards a pebble conglomerate. Followed by a pink to grey coarse, arkosic sandstone, interbedded with thin mudstone. Upper Unit: interbedded fine-grained grey sandstone and thin dark mudstone beds with ripple laminations.
Definition of Lower Boundary: Unconformable upon Lewisian Gneiss inlier (L); unconformity locally sheared.
Definition of Upper Boundary: Not exposed - eroded.
Thickness: c. 700 m: Lower unit: c. 0-165 m; Upper Unit c. 500 m (minimum).
Geographical Limits: none recorded or not applicable
Parent Unit: Wester Ross Supergroup (WROS)
Previous Name(s): none recorded or not applicable
Alternative Name(s): Colonsay Group
Torridonian [Obsolete: use T]
Stratotypes:
Reference Section  Coastal section on NE coast of Iona [NM 292 262 - NM 295 260]. 
Reference(s):
Krabbendam, M. 2021. A stratigraphic framework for the early Neoproterozoic successions of the Northern Highlands of Scotland. UK Stratigraphic Framework Series. British Geological Survey Open Report, OR/21/072. 86pp. 
Gibbons, W and Harris, A L. 1994. A revised correlation of Precambrian rocks in the British Isles. Geological Society of London Special Report, No.22. 
Stewart, A D. 2002. The later Proterozoic Torridonian rocks of Scotland: their sedimentology, geochemistry and origin. Geological Society Memoir. Vol. 24. (The Geological Society.) 
McAteer, C A, Daly, J S, Flowerdew, M J, Whitehouse, M J, and Monaghan, N M. 2014. Sedimentary provenance, age and possible correlation of the Iona Group SW Scotland. Scottish Journal of Geology, Vol. 50, 143-158. 
Stewart, A D. 1962. On the Torridonian sediments of Colonsay and their relationship to the main outcrop in north-west Scotland. Geological Journal, Vol. 3, 121-156. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S043