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): Tillite (Banffshire) [Obsolete Name And Code: See DBIS, DBST, DBPT] (DBBT)
Alternative Name(s): none recorded or not applicable
Type Area  North-east Islay and Jura. (Spencer, A M, 1971. Anderton, R, 1976). 
Harris, A L and Pitcher, W S, 1975. The Dalradian Supergroup. "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. Geological Society Special Report, No.6. p.52-75. Scottish Academic Press, Edinburgh. 
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. 
Stephenson, D and Gould, D , 1995. British Regional Geology: the Grampian Highlands (4th edition). (London: HMSO for the British Geological Survey), 67-68. 
Spencer, A M, 1971. Late Precambrian glaciation in Scotland. Memoir of the Geological Society of London, No.6. 
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