The BGS Lexicon of Named Rock Units — Result Details

Beinn Dorain Semipelite Formation

Computer Code: DORA Preferred Map Code: DORA
Status Code: Full
Age range: Cryogenian Period (AC) — Cryogenian Period (AC)
Lithological Description: Massive, poorly-bedded garnet mica semipelite and muscovite-quartz semipelite, poor in biotite but with abundant quartz sweats. Typically weathers darker grey/brown to brown than the older Coire Daingean Formation; semipelites of the Beinn Dorain Formation are commonly steely grey on fresh surfaces and may take on a "knotted" appearance where conspicuous garnet is wrapped by phyllosilicates. Garnets are typically 3 to 4mm in diameter and always appear fresh (dark red) compared to the more commonly chloritised garnets in the Coire Daingean Formation. Magnetic susceptibility is twice that of the Coire Daingean semipelite (<0.5 cf. <0.2 mu).
Definition of Lower Boundary: Abrupt change from the alternating muscovite-rich semipelite, psammite and quartzite of the Beinn an Dothaidh Formation to the more uniform garnetiferous semipelite rocks of the Beinn Dorain Semipelite Formation. The uppermost parts of the Beinn an Dothaidh Formation are dominantly quartzite in the area around Beinn a' Chuirn (Allt a'Chuirn Quartzite Member).
Definition of Upper Boundary: Not seen.
Thickness: Minimum c.130-150m.
Geographical Limits: Formation occurs on Beinn Dorain, Beinn a'Chaisteil and Beinn an Dothaidh, southwest Grampians.
Parent Unit: Lochaber Subgroup (DALO)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Summit area of Beinn Dorain (and Beinn a'Chaisteil, Beinn an Dothaidh). Details will be included in Sheet Description for Sheet 46W (Crainlarich). Presently in draft BGS internal report: Leslie et al., Progress Report on the Geology of the Tyndrum area, 2003. 1:50k Sheet 46W (Crianlarich). 
Reference(s):
Will be included in Sheet Description for Sheet 46W (Crianlarich) and 46E (Killin). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable