Strike-slip influenced stratigraphic and structural development of the Foula Sandstone Group, Shetland: implications for offshore Devonian basin development on the northern UK continental shelf

The island of Foula, located 25 km SW of Shetland, preserves a gently folded, 1.6 km thick sequence of Middle Devonian sandstones spectacularly exposed in kilometre-long cliff sections >350 m high. These rocks unconformably overlie likely Precambrian-age amphibolite facies basement rocks that are intruded by sheeted granites. The onshore succession is similar in age to the nearby Lower Clair Group offshore to the west. New mapping, incorporating the use of drone imagery in the inaccessible cliff sections, uses down-plunge projections to show that growth folding and faulting on Foula were contemporaneous with sedimentation during basin filling. The large-scale structural geometry is consistent with the regional constrictional strain due to the sinistral transtension associated with movements along the Walls Boundary-Great Glen fault zone system during the Mid-Devonian. Detrital zircon provenance studies indicate that the Devonian sequences of Foula (and nearby Melby in western Shetland) show similarities with the Clair Group and Orkney successions. We suggest that NE-SW transtensional fold development contemporaneous with regional subsidence in the Devonian basins of Scotland may be more widespread than previously realized. Large, kilometre-scale folds previously interpreted to be related to Permo-Carboniferous inversion may therefore have initiated earlier in the basin evolution sequence than previously realized.

Onshore ‘analogue’ outcrops of strata are commonly studied during hydrocarbon exploration in offshore settings to better understand the stratigraphic, sedimentological and structural character of potential or actual reservoir units in the subsurface (e.g. Tamas et al. 2022). These rocks are typically deemed to be equivalent based on similarities in their age, proximity or geological character. The Clair Field, located 75 km west of Shetland, is the largest known hydrocarbon resource on the UK continental shelf, with a closure of c. 250 km2 and an estimated 7-8 billion barrels of oil equivalent in place (Witt et al. 2010; Ogilvie et al. 2015). It comprises naturally fractured Devonian-Carboniferous sandstones of the Clair Group, which unconformably overlie an up-faulted ridge of fractured Precambrian (Neoarchean) metamorphic basement (Coney et al. 1993; Holdsworth et al. 2019a). The Devonian sequences that crop out in mainland Scotland and Orkney are widely used as analogues for the Clair Group, but lie >200 km to the south.

This paper details the stratigraphy, provenance, structure and tectonic evolution of little-studied Devonian rocks that unconformably overlie a ridge of Precambrian metamorphic basement on the island of Foula. These outcrops lie <70 km SE of the Clair Field and, given their proximity and the close similarities in their scale, geology and structural setting, we explore whether these outcrops might be used as an onshore analogue for the Devonian rocks in the Clair Field. We also show that the structural development of the Foula basin-fill shares common features with the transtensional Devonian basins of Shetland and western Norway. This implies that existing tectonic models for the development of offshore Devonian basins around Scotland may require revision and reappraisal.