The BGS Lexicon of Named Rock Units — Result Details

Islay Subgroup

Computer Code: DBIS Preferred Map Code: Isly
Status Code: Full
Age range: Neoproterozoic Era (AZ) — Neoproterozoic Era (AZ)
Lithological Description: Diamictite ('Boulder Bed') with limestone clasts in its lower part and quartzite and granite clasts in upper part attains maximum thickness in Port Askaig area and on Garvellachs; overlying quartzite reaches 5km in thickness on Jura but is more usually 0.5 - 1km thick; dolomitic beds are locally developed near base of quartzite. In Ladder Hills area graded psammites and semipelites intervene between diamictite and quartzite.
Definition of Lower Boundary: Generally transitional over a few metres at base of tillite of Port Askaig (Schiehallion) Boulder Bed.
Definition of Upper Boundary: Abrupt transition from quartzite to graphitic pelite (Easdale) or mixed psammite, semipelite and pelite (Perthshire).
Thickness: Up to 5km
Geographical Limits: Connemara; North Mayo; Donegal; Islay, Jura, from Loch Rannoch to Glen Tilt, from near Spittal of Glen Shee via Upper Deeside, Donside and Ladder Hills to the coast between Sandend and Portsoy.
Parent Unit: Argyll Group (ARGY)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  North-east Islay and Jura. (Spencer, A M, 1971. Anderton, R, 1976). 
Reference(s):
Harris, A L and Pitcher, W S, 1975. The Dalradian Supergroup. 52-75 in Harris, A L, Shackleton, R M, Watson, J, Downie, C, Harland, W B and Moorbath, S. A correlation of the Precambrian rocks in the British Isles. Special Report of the British Geological Society, London, No.6. 
Anderton, R. 1977. The Dalradian rocks of Jura, Scottish Journal of Geology, Vol.13, 135-142. 
Harris, A L, Haselock, P J, Kennedy, M J and Mendum, J R, 1994. The Dalradian Supergroup in Scotland, Shetland and Ireland. 33-53 in Gibbons, W E and Harris, A L (editors). A revised correlation of Precambrian rocks in the British Isles. Special Report of the Geological Society, London, No.22. 
Spencer, A M, 1971. Late Precambrian glaciation in Scotland. Memoir of the Geological Society of London, No.6. 
Stephenson, D and Gould, D , 1995. British Regional Geology: the Grampian Highlands (4th edition). (London: HMSO for the British Geological Survey), 67-68. 
Treagus, J E. 2001. Solid geology of the Schiehallion district. Memoir of the British Geological Survey, Sheet 55W (Scotland). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S019 S027 S028 S036 S055 S055 S056 S065 S065 S075 S075 S085 S086 S096 S028 S045 S045 S086 S096