The BGS Lexicon of Named Rock Units — Result Details

Altnaharra Psammite Formation

Computer Code: ALPS Preferred Map Code: Qa
Status Code: Full
Age range: Tonian Period (AT) — Tonian Period (AT)
Lithological Description: Siliceous to feldspathic psammite with micaceous psammite, grey to buff; locally with micaceous layers. Gritty psammite occurs in the lower part; higher up in the unit, thin semipelitic beds become more common. Generally, strongly deformed and flaggy ('bed' thickness 5 - 30 cm). In low strain zones, the psammite shows thick beds (0.5 - 5 m): sedimentary structures include channels with pebble lags, planar and trough cross-bedding, slump-like folds, dewatering pipes and oversteepened cross-beds (Krabbendam et al., 2008). Palaeo-currents are to east or NE.
Definition of Lower Boundary: Unconformable upon Lewisian Gneiss inlier (L); unconformity highly sheared; locally conformable (?) on Meadie Schist Formation. In many places, base is cut and deformed by the Moine Thrust.
Definition of Upper Boundary: The upper part of the Altnharra Formation is the Glen Achall Psammite and Semipelite Member (GACH). The upper boundary of this boundary is marked by a return to psammite of the Glascarnoch Psammite Formation. In western parts, the Altnaharra Formation is directly overlain by the Vaich Pelite Formation (Bonsor et al. 2010). In the north, the upper boundary is commonly marked by thrusts.
Thickness: 3 to 5 km. Accurate thickness is difficult to estimate due to subsequent deformation, see discussion in Krabbendam et al. (2014).
Geographical Limits: Northern Highlands: Central Rosshire, Sutherland. Occurs in various thrust sheets.
Parent Unit: Morar Group (MORR)
Previous Name(s): A'Mhoine Psammite Formation (AMPS)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Type section: Glen Cassley: River Cassley to Carn nan Bo Maola [NC 436 089 - NC 462 098]. 
Reference Section  Reference area: Beinn Direach to Ben Hee: [NC 406 380 - NC 445 342]. 
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. 
Holdsworth, R E, Strachan, R and Alsop, G I. 2001. Solid geology of the Tongue district; Memoir for 1:50 000 Geological Sheet 114 E (Scotland),British Geological Survey, Keyworth, Nottingham. 
Krabbendam, M, Leslie, A, and Goodenough, K. 2014. Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions. Scottish Journal of Geology, Vol. 50, 125-142. 
Krabbendam, M, Strachan, R A, Leslie, A G, Goodenough, K M and Bonsor, H C. 2011. The internal structure of the Moine Nappe Complex and the stratigraphy of the Morar Group in the Fannichs-Beinn Dearg area, NW Highlands. Scottish Journal of Geology, 47, 1-20. 
Krabbendam, M, Prave, A P and Cheer, D. 2008. A fluvial origin for the Neoproterozoic Morar Group, NW Scotland; implications for Torridon - Morar group correlation and the Grenville Orogen Foreland Basin. Journal of the Geological Society of London, 165, 379-394. 
Bonsor, H C, Strachan, R A, Prave, A and Krabbendam, M. 2010. Fluvial braidplain to shallow marine transition in the early Neoproterozoic Morar Group, Fannich Mountains, northern Scotland. Precambrian Research 183, 791-804. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S114 S094 S094 S108 S102 S102 S103 S108