Permian sandstones (New Red Sandstone) outcrop along Corrie shore just North of Brodick on the Isle of Arran (Figure 1). These well sorted aeolian sandstones are host to networks of deformation bands which drastically reduce the porosity and permeability of the host sandstones, causing challenges for hydrocarbon production. Underhill & Woodcock, (1987) relate their occurrence to the intrusion of the northern granite diaper which reactivated basin controlling faults - which then propagated upwards into the aeolian sandstones into the stretched layers above the intrusion.
Figure 1. Isle of Arran Location. Red arrows point to Arran and approximate location of outcrop. Source: Google Maps
On Figure 2, a compound zone of deformation bands (see previous blog - deformation bands in high porosity sandstones) splays towards the camera. Crossing deformation bands create rhombs and squares of pristine host rock surrounded by bands. Intersecting bands create raised almost tepee like structures due to relative resistance to erosion (Figure 3).
Some compartments are filled with nodular cements - possibly barite (desert rose). It is interesting that along individual bands these thicken towards the band in some areas but in other areas the band is barren of cements (Figure 4). Nevertheless, the cemented compartments create raised bumps on the sandstone wave-cut platforms.
Figure 2. Deformation bands which splay from a compound zone in the New Red Sandstone, Corrie shore, Arran.
Figure 3. Intersecting deformation bands create raised structures due to relative resistance to erosion e.g., just to the left of the car keys.
Figure 4. Cemented (barite ?) compartments. Note how the cement thickens up close to individual deformation bands in some areas but in others the band is barren of cements.
I couldn’t see much evidence of relative motion on the deformation band sets, although Figure 5 shows a left lateral component in one area.
Figure 5. Left-lateral component of displacement between 2 deformation band sets.
Along the coastline are some igneous dykes (dolerite ?) that have intruded into the sandstone (Figure 6) during the Tertiary ? Also of geological interest are glacial erratics which are strewn on the wave-cut platforms (Figure 7). These boulders are rock types that do not belong here - brought in by glaciers during the Ice-age.
Figure 6. Dolerite ? dyke intrusion. Baked margin along the yellow moss line.
Figure 7. Glacial erratics on a wave-cut platform of New Red Sandstone. A compound zone of deformation bands forms a knife-edge ridge in the background - due to relative resistance to erosion.
Reference
Underhill, J.R & Woodcock, N,H (1987). Faulting mechanisms in high-porosity sandstones; New Red Sandstone, Arran, Scotland. From, Jones, M.E, Preston, R.M.F (eds), Deformation of Sediments and Sedimentary Rocks, Geological Society Special Publication, No. 29, p 91 - 105.