30 August 2016

Northwest Highlands unconformities (3 of 3): Sub-Ardvreck

Posted by Callan Bentley

It’s time to cover the third and final unconformity I observed this summer in the North-West Highlands of Scotland: the unconformity between the Neoproterozoic Torridonian Group below and the Cambrian Ardvreck Group above.

Where I saw it, south of Loch Assynt on the mountain called Beinn Garbh (north of Canisp), it actually is displayed alongside the sub-Torridonian unconformity. The mountain hosts a “double unconformity”! Here is a view, looking south:


The bottom of the mountain – its “plinth” is on Lewisian gneiss. Atop that is horizontal Torridonian. The left “flank” of the mountain is Ardvreck that dips at about 15° to the east. So there’s a nonconformity and an angular unconformity in this view.

Here, I’ve highlighted the three units and two unconformities in this annotated version:


The weird thing about thisangular unconformity is that the Torridonian (older) is horizontal, and the Ardvreck (younger) is tilted. What this implies is kind of extraordinary: The Torridonian was deposited horizontally, then tilted (at about 15° to the west) and then eroded. The Ardvreck was deposited atop this erosional surface (horizontally) and then lithified. Then the whole mess was tilted back to the east, again by about 15°!

Here is a GigaPan of Beinn Garbh, from a position slightly further to the west along Loch Assynt’s north shore:

Link GigaPan by Callan Bentley