Unconformity maps

Siccar Point

James Hutton was the first geologist to understand the concept of unconformity (1788), using a boat to study the Acadian unconformity at Siccar Point on the Berwickshire coast, and having traced it to other onshore localities.

The BGS has produced, and is currently updating, maps of the three main discontinuities in the stratigraphical succession which occur widely in the UK both on- and offshore. These are the late Cimmerian unconformity (non-orogenic) of Cretaceous age, the Variscan unconformity of late Carboniferous age and the Acadian (late Caledonian) unconformity of mid Devonian age.

The original hard copy maps (subcrop and depth) were compiled for regional hydrocarbon studies, using borehole and seismic data available at the time; these have been scanned and form the backdrop of the GIS data. New maps of the supercrop (formations above the unconformities) have also now been compiled.

Ongoing surface mapping

Unconformity diagram

Continuing work on current surface mapping areas has revealed more useful data and these are now accessible via borehole scans. This and other borehole data acquired since the production of these maps has been entered into BGS databases; in this case the Stratigraphical Surfaces Database (SSD) and links have been made to a GIS.

Linked database tables record the supercrop formation, depth and subcrop formation. Borehole provings are colour-coded according to formation or age and depth, giving a rapid view of any inaccuracies in or refinements required to the legacy maps. As a result, finer subdivision of stratigraphical units, compared to the legacy maps, is now possible (particularly in Carboniferous strata —see Figure 1).

These maps have proved useful for research into:

  • aquifers
  • coal maps
  • concealed minerals
  • conventional hydrocarbon exploration
  • in the future: shale gas and underground mining

Further discoveries

The GIS also reveal concealed uplifts, caused by tectonic inversion, and provide insights into the deeper concealed geology of the UK. They help explain the reason for the difference in hydrocarbon exploration success in the East Irish Sea (Millstone Grit source beneath Ormskirk reservoir). And the why failure in the Cheshire Basin wells the intervening, younger Carboniferous strata present in the latter.

Work is ongoing on updating the maps with new borehole information. There is therefore substantial scope for adding to the range as well as the quantity of information contained to enhance subsurface maps and predictability of the subcrop.

Contacts

Chief Geologist England: John Powell

Geology and Landscape England staff contacts