16 August 2016

Northwest Highlands unconformities (1 of 3): Sub-Stoer Group

Posted by Callan Bentley

There are three major unconformities in the North-West Highlands of Scotland. I’ll be profiling each over the coming weeks.

Schematically, this is their relationship in cross-section:


The oldest of them is a nonconformity between the ~3.0 Ga Lewisian gneiss basement complex and the ~1.2 Ga Stoer Group sedimentary package (the number “1” on the diagram above). On the coast at Clachtoll, I made two GigaPans of this “gap in the geologic record.” Here’s the first of them:

Link GigaPan by Callan Bentley

That shows a paleo-valley, cut more than 1.2 billion years ago into the Lewisian gneiss, then filled with a jumble of boulders and cobbles. These, later, were cemented into a breccia.



Here’s a view from the valley across the cove to the spot where the GigaPan was shot from:



Some clean looks at the breccia filling this paleo-valley:






Here’s another exposure of the unconformity surface, which dips at a steep angle, almost parallel to the outcrop surface:

Link GigaPan by Callan Bentley

See if you can find pre-erosional structures in the gneiss, as well as recent paleomagnetic research drill holes in the breccia!

It’s interesting to visit because the unconformity surface shows significant relief – in other words, it preserves the three-dimensional nature of the 1.2 Ga land surface, with hills and valleys all smothered by the Stoer Group red beds.