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 psammite and micaceous psammite; generally grey to buff. Medium-fine grained, but locally gritty or pebbly (clast size up to 30 mm) layers, especially lower down in the stratigraphy. Locally pelite or semipelite layers up to a few metres thick; these increase in abundance upwards in the formation. Commonly deformed, then with flaggy structure. Where undeformed, locally well preserved sedimentary structures that include planar cross-bedding, trough cross-bedding, channels and abundant soft-sediment deformation structures, eg. water escape structures and slump folds. Bed thickness where undeformed ranges from 0.2 to 4 m thick.
Definition of Lower Boundary: The Altnaharra Formation is thought to be deposited unconformably upon the Lewisianoid Gneiss Complex, but the contact is generally highly sheared. Locally, as at Achness Falls, a semipeltic layers occus just above the unconformity. In most places the exposed base of the formation is formed by the Moine thrust or the Achness Thrust (Krabbendam et al. 2011).
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: The Altaharra Formation occurs in the NW Highlands, in central Sutherland and central Ross and Cromarty. The outcrop is bounded in the west by the Moine Thrust, in the north by the coast, and in the NE by the Naver Thrust. In the SE, the limit is stratigraphic (see Kabbendam et al. 2011). The boundary and continuation to the south of the Fannich Mountains is as yet unclear.
Parent Unit: Morar Group (MORR)
Previous Name(s): A'Mhoine Psammite Formation (AMPS)
Alternative Name(s): none recorded or not applicable
Stratotypes:
none recorded or not applicable
Reference(s):
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, 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. 
Friend, C R L, Strachan, R A, Kinny, P D and Watt, G R. 2003. Provenance of the Moine Supergroup of NW Scotland; evidence from geochronology of detrital and inherited zircons from (meta)sedimentary rocks, granites and migmatites. Journal of the Geological Society of London, 160, 247-257. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S114 S094 S094 S108 S102 S102 S103 S108