Deformation bands are fault like structures that drastically reduce the porosity and permeability of otherwise good quality sandstones. Examples here are in Permian sandstones from the Hopeman Sandstone in the NE of Scotland and Isle of Arran in SW Scotland (Fig. 1 & 2). In both areas, dunes of sand were deposited by wind in the hot and dry Permian period. The Arran sandstones are red due to an iron oxide coating on the grains (https://www.arran-geopark.org.uk/geological-timeline/).

Fig. 1. Location of outcrops in NE Scotland and Isle of Arran, shown by blue markers.

Fig. 2. Compound zone of deformation bands in Permian sandstones (a) Hopeman Sandstone, Moray Firth, UK and (b) Corrie Shore, Isle of Arran, UK.

The sands are relatively clay free and the main process relevant to fault seal is grain size reduction or cataclasis. It is easier for new bands to form than continued development of an existing band; a process known as strain hardening. Deformation bands will coalesce to create compound zones of deformation bands with well developed slip surfaces (Fig. 3).

Fig. 3. Sequential development of zones of deformation bands (a) single band (b) zone of deformation bands (c) compound zone of deformation bands with well developed slip surface.

Often the slip surfaces have polished surfaces and slickenlines (Fig. 4) which indicate the direction of shear.

Fig. 4. Polished slip surfaces with slickenlines in the Hopeman Sandstone. Some slickenlines shown with white dashed lines.

Key takeaways

Compound zones of deformation bands are fault like structures that form sequentially from the growth and amalgamation of deformation bands.
Their porosity and permeability can be significantly less than the host sandstone
This has implications for hydrocarbon field development although they can be efficient in sealing large columns of hydrocarbons.