The structural evolution of the Devonian rocks and associated basement in Foula and Shetland: An analogue for the Clair Basin

The Clair Field, west of Shetland, represents the largest remaining hydrocarbon resource in the UKCS. It comprises fractured Devonian-Carboniferous sandstones overlying a fault bounded ridge of Precambrian metamorphic basement. Foula, an island situated 25km SW of the Shetland Isles may represent the nearest and best onshore analogue to the Clair Field, but is relatively little studied. 1600m of gently folded Middle Devonian sandstones correlated with the Lower Clair Group offshore, are exposed in continuous, kilometre-long cliff sections up to 376m high. These rocks unconformably overlie basement gneisses and sheeted granites. On Shetland, the Devonian sandstones of the Walls Peninsula and South-East Shetland again, unconformably overlie metamorphic basement.Here we present an ongoing reappraisal of the structure, stratigraphy and tectonic evolution of the three Devonian basins in Shetland (Foula, Walls, SE Shetland). Detailed land and aerial (drone) based studies of exposed basement-cover contacts, and the structure and broad stratigraphic architecture of the overlying sandstone dominated sequences are being carried out.Field data and structural analysis are supplemented by the use of photogrammetry to capture the geology in inaccessible coastal sections. Initial findings show that faulting and folding were contemporaneous with sedimentation during the earliest stages of Mid-Devonian basin development.
The geometries of faults and folding on Foula and Shetland are consistent with models of constrictional strain related to regional sinistral transtension along the Walls Boundary Fault during the Mid-Devonian. Our findings suggest that the transtensional folded Devonian basins of Shetland may represent a better surface analogue for the Clair Basin.