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

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 (Fig. 1a, c), 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) (Fig. 1c). 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 (see Fig. 1a, inset), 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.

The Orcadian and Clair basins belong to a series of Devonian continental sedimentary depocentres that formed in the North Atlantic region (Friend 1981; Friend et al. 2000) following the collision of Laurentia, Baltica and Avalonia and the subsequent collapse of the Caledonian mountain belt (Fig. 1b). Several kilometres of siliciclastic sediments and smaller volumes of volcanic rocks were deposited in a series of rift basins from the late Silurian to early Carboniferous in eastern Greenland, northern Scotland and western Norway (Friend et al. 2000).

The Orcadian Basin occurs both onshore and offshore in the Shetland, Orkney, Caithness and Moray Firth regions of NE Scotland (Fig. 1a). In mainland Scotland, Lower Devonian synrift alluvial fan and fluvial–lacustrine deposits are mostly restricted to the fringes of the Moray Firth region (Rogers et al. 1989; Tamas et al. 2021) and parts of Caithness (Holliday et al.1994), occurring in a number of small fault-bounded basins. These are partially unconformably overlain by Middle Devonian synrift alluvial, fluvial, lacustrine and, locally, marine sequences that dominate the onshore sequences exposed in Caithness and Orkney (Marshall and Hewett 2003). Upper Devonian post-rift fluvial and marginal aeolian sedimentary rocks (Friend et al. 2000) are only found as small fault-bounded outliers in Caithness and Orkney.

The north–south-orientated Shetland Islands, part of the extensive Shetland Platform (Fig. 1a), predominantly comprise highly deformed and metamorphosed Precambrian rocks (Mykura et al. 1976a). These were intruded by a suite of granitic plutons and associated volcanics during the Ordovician–early Devonian Caledonian Orogeny and were later flanked and overlain by Mid-Devonian (Mykura et al. 1976a) and later Mesozoic sedimentary basins, which now lie predominantly offshore. The Shetland archipelago is cut by large, subvertical strike-slip faults initiated during the Caledonian Orogeny, including the Walls Boundary Fault (WBF), the proposed northward continuation of the Great Glen Fault (Fig. 1a; Flinn 1979, 1992; McGeary 1989; Ritchie and Hitchen 1993; McBride 1994; Watts et al. 2007). The exact amount of strike-slip motion along the WBF and associated structures has been a topic of debate, but is generally accepted as being in the region of several tens or hundreds of kilometres of sinistral motion during the Silurian–Devonian, followed by subsequent dextral motion of 20–30 km during the Carboniferous and Permian and a further 15 km in the Mesozoic (Watts et al. 2007; Armitage et al. 2021). To the west of the Melby Fault, Devonian sedimentary and volcanic rocks are exposed at Melby, Eshaness and on the islands of Foula and Papa Stour (Fig. 1a). They are juxtaposed to the east against thermally mature and more deeply buried and folded Devonian sedimentary and volcanic rocks that form a large part of the Walls Peninsula (Melvin 1985), into which granites are emplaced, bounded to the east by the WBF. East of the Nesting Fault, in the SE Shetland Basin, Devonian sedimentary rocks are exposed in a strip along the eastern coast from Lerwick to Sumburgh Head, on nearby offshore islands and, further to the south, on Fair Isle (Fig. 1a).

The Clair Basin lies on the eastern margin of the Mesozoic–Cenozoic Faroe–Shetland Basin, forming a large half-graben flanking an up-faulted NE–SW-trending ridge of Precambrian basement: the Rona Ridge (Fig. 1a; Ritchie et al. 2011; Holdsworth et al. 2019b). Blackbourn (1987) termed the Devonian–Carboniferous basin-fill the Clair Group. This succession is both faulted against, and unconformably overlies, the Precambrian basement and both of these are overlain by Cretaceous marine mudstones of the Shetland Group, which form a regional seal (Fig. 1b). The Devonian–Carboniferous fill records a general upwards increase in sediment maturity from basal alluvial fans, through mainly braided streams in the Lower Clair Group (Mid- to Upper Devonian) and up into the higher sinuosity rivers and lake deposits of the Upper Clair Group (Lower Carboniferous) (Allen and Mange-Rajetzky 1992; Schmidt et al. 2012).

The 13 km² island of Foula (Fig. 2) lies 25 km SW of Shetland and c. 70 km SE of the Clair Field (Fig. 1a). A 1.6 km thick sequence of gently folded and fractured continental clastic deposits unconformably overlies, and is locally faulted against, fractured metamorphic basement and granite, which are exposed on the low-lying eastern side of the island (Mykura et al. 1976a, b) (Figs 2, 3a, b). Foula lies at the southern end of the Foula Ridge, one of several NE–SW-trending basement structural highs in this region (BGS 1988). The Foula Ridge forms a positive gravitational anomaly and a positive topographic feature on geophysical and bathymetric surveys and in regional 2D seismic reflection data (Fig. 1d). It is bounded to the west by the Foula Fault, which may truncate or link to the Moine Thrust at depth (Figs 1d, 2). Andrews (1985) postulated that the northwards continuation of the Moine Thrust Zone subcrops to the west of Foula and continues north, passing to the east of Ve Skerries, where supposedly Lewisian-type basement is exposed. To the east of Foula, the Melby Fault forms a relatively steep structure with little vertical offset of reflectors within the West Fair Isle Basin, which is bounded to the east by the WBF (Fig. 1d).

Mykura et al. (1976a) suggested correlation of the sedimentary rocks of Foula with the Old Red Sandstone (Devonian) succession observed on mainland Shetland at Melby (Fig. 1a). The most recently published regional correlation confirms this linkage and assigns an Eifelian age to the Foula and Melby successions (Marshall and Hewett 2003). They are slightly older than the Devonian rocks of the Walls and SE Shetland basins (Marshall and Hewett 2003) and probably equivalent to the Lower Clair Group (Fig. 1a), which, on the basis of wells drilled in the Clair Field, accumulated during the Givetian to Frasnian or later (Allen and Mange-Rajetzky 1992). However, these authors report abundant sporomorphs of late Eifelian to early Givetian age within a Kimmeridgian conglomerate penetrated in well 206/5-1, drilled a little to the north of the Clair Field, indicating that deposition of the Devonian in the area had commenced by the Eifelian and so may overlap with that of Foula. Despite the slight age difference, it has been long recognized that the sandstones on Foula are a suitable analogue to those of the Clair Group based on their proximity and the similarity of the predominantly fluvial sedimentary sequences (Blackbourn 1981c). To date, no comprehensive account of the sedimentological, stratigraphic or structural evolution of Foula has been published.

Geological mapping of the island by Blackbourn (1981c) at a scale of 1:10 000, building on earlier work by Mykura et al. (1976b), has been revised by Blackbourn to create the new stratigraphy for the Foula Sandstone Group presented here (see Fig. 2c; Table 1). A new geological map and cross-sections (Fig. 2a, b) integrate this mapping and stratigraphy with the findings of new fieldwork (Utley 2020). As a result of poor inland outcrops and the inaccessible coastal nature of many of the exposed cliff sections, which are up to 376 m high, new fieldwork was supplemented by the use of aerial and drone-based imagery and photogrammetric 3D models interpreted in Virtual Reality Geological Studio to extract geological information and structural measurements (Burnham and Hodgetts 2019). Lineament analysis of aerial photographs (Fig. 2d) was carried out at a scale of 1:500 in ArcGIS. Lineament and structural data were analysed using Stereonet 9.5 (Allmendinger et al. 2011). Palaeocurrent measurements were made where possible, mostly from the dip direction of cross-bedding foresets, although the poor or inaccessible exposure, combined with an abundance of soft sediment deformation, impeded precise measurements in many locations. It was, however, often possible to determine the general foreset dip direction even where exact measurements could not be made. Sampling was undertaken for thin section, microstructural, heavy mineral provenance analysis and detrital zircon geochronology. Detailed methodologies for these techniques and the full results of our analysis can be found in Appendix A of the Supplementary Material.

Metamorphic basement rocks form a c. 1 km wide strip along the eastern side of the island (Fig. 2a), comprising metasediments and a suite of microgranite sills and dykes (Fig. 3a, b) (Mykura et al. 1976b). The basement is dominated by interbanded psammitic and semi-pelitic paragneisses (Fig. 3a), with subordinate metabasic intrusions. The basement is lithologically comparable with the early Neoproterozoic Yell Sound Group on mainland Shetland (deposited at c. 1019–941 Ma; Jahn et al. 2017). Flinn et al. (1979) reported K–Ar hornblende and biotite ages of 443 ± 14 to 426 ± 20 Ma, respectively, from Foula, indicating that the basement here was most recently metamorphosed during the Caledonian Orogeny. In NE Foula (Fig. 2a), a pink microgranite (the Ruscar Head Microgranite) forms part of a little deformed, laccolith-like complex of sills and feeder dykes with most contacts sub-parallel to the basement foliation (Fig. 3a, b). South of Ruscar Head, on the coast east of Da Swaa [HT 397486 1140048], a second thick sill (10–15 m) is exposed within inaccessible cliff sections (Fig. 2a). An age for these intrusions is undetermined, but it is likely to be pre-Mid-Devonian because lithologically identical granite clasts are preserved in sedimentary breccias in the basal part of the Foula Sandstone Group immediately overlying the basement.

The succession that outcrops through most of the island, apart from the northeastern coastal strip, is subdivided into six formations (Fig. 3d–h), comprising three major subarkosic sandstone units separated by more mixed shale-dominated units (Table 1). In total, it is c. 1.6 km thick and comprises fluvial, floodplain, lacustrine and possible alluvial fan sedimentary rocks.

The Da Ness Formation is at least 165 m thick in the north of Foula, where the base is not seen. It is subdivided into three members, with a 3–5 m thick localized basal breccia exposed on the coast at Shoabill in the SE [HT 397390 1138110] resting unconformably on an irregular basement surface. Overlying the basal breccia, the Sheepie Member (at least c. 50 m thick in the north) is only exposed in an inaccessible cliff section and sea stacks. It comprises massive yellow sandstones with rare, thin (centimetre to decimetre), finer grained beds (Fig. 3e). Above this, the Brough Member comprises c. 15 m of grey mudstones and sandstones. Unlike most of the sedimentary rocks on Foula, the Brough Member is both calcareous and mica-rich. Fine- to medium-grained sand bodies up to 3 m thick display cross-laminations and planar or trough cross-bedding. Scoured hollows up to 1 m deep and 6 m wide are draped by sandstone and mudstone; the thicker sandstones are enclosed in a sequence of grey, green and red siltstones and laminated fine- to very-fine-grained sandstones. Soft sediment deformation is widespread, with concentric laminated slump balls and convolute bedding, and sand-filled desiccation cracks are well developed in the mudstones (Fig. 3f). The overlying Logat Member, c. 100 m thick, is broadly similar to the Brough Member, but with sandstone beds up to 5 m thick that are less micaceous and dominated by trough cross-bedding.

The Da Ness Formation is interpreted as comprising mainly fluvial channel deposits, with finer grained material representing fluvial overbank and floodplain deposits that underwent prolonged periods of desiccation. Figure 4a is a sedimentological log of a well-exposed and accessible section of the upper Brough Member and lower Logat Member. Palaeocurrent data based on the dips of cross-bedding foresets demonstrate an abrupt switch in current direction from flow towards the westerly quadrant in the Brough Member to a southeasterly quadrant in the Logat Member (Fig. 4b). The relatively immature, micaceous nature of the Brough Member is thought to indicate a local sediment source lying to the east. Tabular cross-bedded sets, most abundant in the Brough Member, may represent sheetflood deposits, possibly formed on the toes of alluvial fans. The Sheepie Member may represent less well-organized alluvial fan sands, although this is speculative owing to their inaccessibility. The more compositionally mature, thicker sandstones of the Logat Member may have been deposited in a larger scale fluvial system sourced from the NW.

The Soberlie Formation forms a uniform succession of buff to yellow, medium- to fine-grained sandstones with occasional scattered pebbles, reaching 400 m thick in the west of Foula (Fig. 3c, d). The pebbles are up to 7 cm in size, but most are 1–2 cm, well-rounded and consist of quartz, quartzite, pink granite and granitic gneiss. The dominant sedimentary structures are trough cross-bedding and convolute bedding. Towards the top of the sequence, tabular beds up to 15–20 cm thick with pebbly lags are more common. Palaeocurrents measured across most of the island flowed consistently towards the southeastern quadrant (Fig. 4b). The Soberlie Formation is poorly exposed and much thinner in the centre and SE of the island, reducing to <200 m west of the Foula Fault in the SE (Fig. 2b i), and only c. 35 m east of the fault, where it comprises fine- to coarse-grained sandstones with rare pebbles and intraclasts of siltstone and mudstone. Occasional cross-bedding foresets here indicate generally SW-directed palaeocurrents (Fig. 4b). The depositional environment of the Soberlie Formation is similar to that of the Da Ness Formation, but it is distinguished by the presence of thicker sandstones and by less frequent and thinner floodplain and fluvial overbank facies.

The Blobersburn Formation is up to c. 85 m thick immediately to the west of the Foula Fault in the SE of Foula, and possibly a little thinner to the east of the Foula Fault, where is it poorly exposed or inaccessible. It is clearly distinguished from the underlying Soberlie sandstone by its distinctive darker colour and relative susceptibility to erosion. It is well-exposed, but inaccessible, in the cliffs on the west of the island, although it can be seen to have reduced in thickness here to c. 45 m (Fig. 3d). Poor exposures in the stream bed at Blobersburn [HT 395540 1140636] show that it is composed of interbedded fine-grained sandstones, dark grey micaceous siltstones and thin calcareous siltstones. It provides a key marker horizon due to its distinctive lithology and it exhibits facies types similar to those reported in the lacustrine successions of SE Shetland (e.g. Allen 1979, 1981a, 1982; Allen and Marshall 1981; Table 1).

The total organic carbon values are generally <0.5%, but this is the most organic-rich unit on the island and the only unit to contain amorphous organic matter within the kerogens (Marshall et al. 1985). The preservation of this form of organic material, derived from algal matter, indicates that anoxic conditions prevailed for a time, but did not become established for long periods as in the fish-bearing units of Caithness and Shetland (Marshall et al. 1985). A single probable Asmussia (Estheria), a small freshwater branchiopod, has been discovered in the Blobersburn Formation on Foula (Donovan et al. 1978).

The Blobersburn Formation is interpreted to have been deposited in a shallow freshwater lake with variable water depth, probably no greater than 10 m. Well-developed sedimentary cyclicity, as seen in Caithness (Andrews et al. 2016), is not evident, but overall it is similar to other lacustrine units in the Orcadian Basin, such as the Achnarras Horizon in Caithness and Orkney, and the Melby Formation and Exnaboe Fish Beds in Shetland.

The Sneug Formation has a maximum thickness of c. 650 m in the west of the island, where it forms the greater part of the cliffs (Fig. 3d, g). It comprises red- to buff-coloured, fine- to coarse-grained, locally pebbly sandstones, which thin considerably to 220 m in the east. This thinning, much like that seen in the Soberlie Formation, can be attributed to deposition in a developing half-graben bounded by the Foula Fault. Sedimentary structures include trough cross-bedding (sets averaging c. 50 cm) and symmetrical ripples with predominant SSW-directed palaeocurrents (Fig. 4b). Floodplain and fluvial overbank deposits are preserved as thin red mudstone beds, which contain some convolute bedding and sand-filled desiccation cracks. This depositional environment of the Sneug Formation is interpreted to be similar to that of the Soberlie Formation, representing a fluvial system with axial flow through the basin.

The Daal Formation, with a fairly uniform thickness of c. 220 m, is largely composed of medium-grained sandstones with scattered pebbles (Fig. 3h) and laterally extensive interbeds of buff yellow dolomitic siltstones and grey green mudstones. Sedimentary structures include well-defined planar beds, with less common trough cross-bedding. The interbeds are usually only a few centimetres thick, although some reach 0.5 m and include horizontal and ripple cross-lamination, with some soft sediment deformation structures (Fig. 3h). Plant material is also common. This unit is interpreted as having been deposited on an extensive floodplain, possibly largely submerged, crossed by fluvial channels with fine-grained overbank and crevasse-splay deposits, and with possible sheetflood deposits. Relatively poor exposure and soft sediment deformation leads to sparse reliable palaeocurrent data, but there are indications of generally southwards-directed currents. The rate of deposition, and therefore possibly of subsidence, appears to have been slower than during the deposition of the under- and overlying formations.

The Noup Formation is the youngest formation outcropping on Foula (Fig. 3c). It has a minimum thickness of 240 m, with the top unseen. The lithology and sedimentary structures are like those of the underlying Soberlie and Sneug formations, especially the latter, which, together with a dominant southwards palaeocurrent direction (Fig. 4b), suggests a similar tectonic and sedimentary setting, with fluvial systems being channelled along the basin axis. Some discontinuous overbank silts and muds occur and yield occasional plant remains.

The structure of Foula is dominated by a regional-scale north–south- to NNW–SSE-striking, steeply west-dipping normal fault known as the Foula Fault (Fig. 2a). This splays into a series of fault branches, which can be observed in numerous steep-sided, narrow geos (local term for gullies) (Hansom 2003) along the coastline to the south. The underlying geology has a strong control on the present day topography. Landslips and subsidence/sinkholes pick out both north–south- and east–west-orientated faults and fracture zones, such as at Da Sneck o Da Smaalie [HT 395044 1138286]. Here, a relatively recent landslip on a seawards-dipping bedding plane has opened a narrow chasm almost down to sea-level. Recent glaciation and landslips are also thought to have taken advantage of these pre-existing weaknesses to generate geomorphological features, such as the corries at Ouvrandal, Da Fleck and Netherfandal (Fig. 2a) (Finlay et al. 1926; Mykura 1976; Mykura et al. 1976b; Flinn 1977).

The basement on Foula is strongly foliated and records multiple phases of ductile and brittle deformation. The gneissic to schistose basement foliation is folded into a large-scale open synform plunging SW sub-parallel to the metamorphic mineral lineations (Fig. 5a, e). The basement is highly fractured and dominated by north–south-orientated, east-dipping normal faults and lesser north–south-orientated fracture zones and brittle shear zones (Fig. 5a, b); this is reflected by a dominance of north–south lineaments in areal images (Figs 2d, 5b). In addition to these major structures, three well-defined joint/fracture sets trend NNE–SSW to NE–SW, east–west to ENE–WSW and NNW–SSE to north–south (Fig. 5e).

Most normal faults in the basement dip moderately to the east (Fig. 5c) and have measurable throws of a few tens of centimetres to several metres; their orientation appears to be significantly influenced by the similarly orientated basement foliation. They are associated with epidote, quartz feldspar, base metal sulfide mineralization and are significantly iron-stained as a result of weathering. The mineralization and associated alteration appear to predate the emplacement of the Ruscar Head Microgranite and deposition of the Foula Sandstone Group. Some faults have normal to sinistral-normal slickenlines preserved, with local drag folds (Fig. 5d) and mineralized en echelon tension-gash arrays with a sinistral sense of shear. Some of the NE–SW faults exhibit faint slickenlines with dextral kinematics and appear to form antithetic structures to the major north–south faults with sinistral movement components. The fractures and faults within the granites are largely barren of mineralization.

The Foula Fault is exposed on the coastline at the northern end of the island in Rotten Geo, Wurrwick [HT 396749 1140994] as a west-dipping normal fault zone (Fig. 6a, b and i). Foliated basement with abundant granite sheets in the footwall are juxtaposed against interbedded sandstones and siltstones of the Da Ness Formation. A throw of c. 100 m is estimated on the basis of the stratigraphy omitted. On the principal slip surface, a thin (3–8 mm thick) layer of blue grey, clay-rich fault gouge is developed, underlain by 20–30 cm of mullioned, chaotic microbreccia (Fig. 6b) that has been folded by a series of centimetre-scale open, south- to SSW-plunging fold hinges (Fig. 6c). In the hanging wall, the Da Ness Formation is folded and cut by sub-parallel conjugate curviplanar faults and fracture corridors (Fig. 6a, d, i). Some shale-rich units are locally folded into tight chevron folds plunging SW (Fig. 6f). Further to the west are several shallow, subhorizontal, roughly bedding-parallel faults that appear to have acted as slip zones to accommodate differential displacement between the bedded units and subordinate fracture corridors and faults (Fig. 6g). The beta axes of the folded bedding, foliation and curviplanar faults all plunge south to SW (Fig. 6h).

On the southeastern coast of Foula at Shoabill [HT 397390 1138110], a 3–5 m zone of sheared basal conglomerate is in contact with a SW-dipping undulating, erosional basement unconformity surface, above which the bedding of the Da Ness Formation is sub-parallel (Fig. 7a–d). This sedimentary breccia covers an irregular unconformity surface. In the zone of shearing, the long axes of the clasts are orientated parallel to the SW-dipping fabric and exhibit well-developed asymmetrical boudinage with a top-to-the-SW sense of shear (Fig. 7b, c). The clasts comprise basement material set in a matrix of coarse- to medium-grained sand. Minor synsedimentary NE-dipping normal faults can be seen cutting through this zone and can be traced into the basement units below (Fig. 7c). Collectively, these indicate top-to-the-SW/SSW extension coeval with slip along this unconformity surface. Open brittle fractures of unknown age trending NW–SE and north–south occur locally in the basement, particularly within more granitic and pegmatite-rich units. Rare, small-scale millimetre- to centimetre-scale sediment-filled fractures are present, which are infilled with fine- to medium-grained sand- and clay-rich silt. These fractures are interpreted to have formed in proximity to the base-Devonian unconformity during deposition.

The entire Devonian sequence on the island is folded into a broad open structure (the Foula Syncline), which plunges c. 20° to the SSW (Figs 2a, b and 3c). Field data were supplemented by data derived from virtual outcrop models (Fig. 8a and b(i), b(ii)) and remote sensing to include orientation data from the largely inaccessible cliffs that follow the western limb of the fold. The basement foliation is folded in a similar manner (Figs 5e, 8b(iii)). A down-plunge projection of the Foula Syncline reveals an upwards-opening fold with interlimb angles increasing upwards from 129° to 168° (Fig. 8c). Major thickness changes are apparent within the Soberlie and Sneug sandstones, consistent with mapped changes in thickness in these units over the Foula Fault (Fig. 2a, b). These relationships are interpreted to show that the Foula Syncline was initiated and progressively tightened during sedimentation – that is, like the Foula Fault, it is interpreted to be a growth structure. Smaller scale, more localized folds, which also have south- to SSW-orientated fold hinges, are observed in the western cliff section (Fig. 9g).

The earliest brittle structures in the Devonian strata are NE–SW- to north–south-orientated deformation bands (granulation seams) and fractures (Fig. 9a, i) with rare quartz cement fills. These structures increase in number with proximity to major faults and are the likely precursor structures to the major north–south-orientated normal to sinistral oblique normal faults, which also increase in number closer to the Foula Fault in the east. North–south-orientated normal faults (Fig. 9a–c) are the dominant structures seen, and are widely eroded to form distinct geos (Fig. 2a). There is minor brecciation and the development of small (<5 mm) vuggy fractures in the footwalls of these faults. Unlike in the basement, these major north–south faults do not contain significant mineralization and form clean breaks that preserve rare sinistral oblique to sinistral strike-slip slickenlines and dextral oblique slickenlines on antithetic Reidel shears. Subvertical, en echelon north–south and east–west discrete fracture corridors (Fig. 9c, d) with limited normal offsets are common and decrease in frequency away from the Foula Fault. Viewed in cross-section, these left-stepping en echelon arrays have a flower structure-like geometry (Figs 6i, 9b, c).

In a small quarry near to Hamnabrek [HT 397101 1138331], a c. 15 m wide fault zone juxtaposes the sandstones and siltstones of the Soberlie Formation against the sandstones of the Da Ness Formation (Fig. 9e, f). The fault zone comprises steeply west-dipping north–south-orientated faults and antithetic east-dipping shear fractures with abundant tensile fractures, sigmoidal tension gashes with a sinistral sense of shear and small open tensile fractures with some rare quartz fills. Shale-rich units close to the faults have a well-developed cleavage and their brittle deformation has contributed to the development of 1–2 cm thick clay-rich fault gouge along the main fault plane. In the hanging wall, a crackle/crush breccia is developed with clasts of c. 0.5 mm.

Even in regions well away from larger faults, the Devonian sequences are heavily fractured and have an overall polymodal fracture pattern with a dominant north–south trend (Fig. 9c, i) and small normal to strike-slip offsets (Fig. 9a, b, h) with mutually cross-cutting and abutting relationships (Fig. 9c). Many of the smaller faults have a scissor-like style of offset, with faults tipping out over short distances. The fractures are commonly open with small apertures (<2 mm) with no fracture fill or mineralization. Some faults preserve minor quantities of fine angular fault breccia or green–red gouge <2 mm thick that is partially cemented with quartz.

The generally barren clean break nature of the faults means that relatively few faults preserve slickenlines. Fault-slip slickenline data measurements from 10 faults (Fig. 10a) were used to carry out a conventional stress inversion. The results of this analysis (Fig. 10b, c) show that the minimum principal stress (⁠$σ$₃) was orientated NE–SW (09/240). The shape ratio (ø = 0.4) (Fig. 10c, d) indicates that local faulting represents an extension-dominated transtension (Fig. 10e), consistent with regional sinistral transtension along the north–south basin-bounding structures, including the Foula Fault (Fig. 10f).

Based on the field observations and analysis of structures, a schematic basin development model for the Foula Sandstone Group (Fig. 11) illustrates how the depositional systems and changes in palaeocurrent (e.g. Fig. 4a, b) may have evolved in response to changes in tectonics and climate in this part of the regional Orcadian Basin. The local basin may have initiated as a half-graben, with sediments being shed off regional highs to the north and west and from more localized features, such as the Foula Ridge (Fig. 11a, b) to the NE. With progressive subsidence and periods of active faulting, the basin began to fill. During lacustrine highstands, a broad alluvial plain was flooded, forming units such as the Blobersburn Formation (Fig. 11c) and other more minor lacustrine intervals elsewhere in the succession. Very widespread soft sediment deformation structures and water-escape structures are preserved throughout the succession on Foula (e.g. Fig. 3e, h) and are most evident within the interbedded sandstones and siltstones of the stratigraphically lower units. These are indicative of high sedimentation rates, rapid burial (Leeder 1987), synsedimentary deformation and the expulsion of high-pressure pore fluids. Their extensive development, together with major changes in thickness across the Foula Fault, indicate an active tectonic environment with growth faulting and gentle folding during deposition. The upwards-opening of the Foula Syncline seen in the down-plunge cross-section (Fig. 8c) implies that low-angle erosional unconformities (discordances <5°) should exist within the succession (e.g. as shown schematically in Fig. 11f). However, the limited and often inaccessible coastal exposures on the island did not allow unequivocal identification of such features in the field.

Spore colours in the Foula Sandstone Group are mid- to dark brown with a vitrinite reflectance between 1.2 and 1.8% (Hillier and Marshall 1992), considerably lower than values from the Walls Group, eastern Shetland and Fair Isle. The values on Foula appear to be closer to those obtained from Melby, Orkney and Caithness (Marshall et al. 1985; Hillier and Marshall 1992), although they are higher than at Melby, which has vitrinite reflectance values of 0.8–1.0% (Hillier and Marshall 1992), suggesting slightly deeper burial. Thus the burial history of the Foula succession seems to have closer affinities to that of the main part of the Orcadian Basin in Orkney–Caithness rather than Shetland. This leads to broader questions concerning the age and affinities of the Foula Sandstone Group relative to the other Devonian sequences in northern Scotland, issues that we explored using new heavy mineral analyses and detrital zircon geochronology.

A Mid-Devonian Eifelian age has been proposed for the Foula Sandstone Group (Marshall and Hewett 2003). The distinctive lacustrine facies of the Blobersburn Formation have been correlated with the so-called Fish Bed horizons in western Shetland (Melby), Orkney (Sandwick) and Caithness (Achanarras) (see Supplementary Materials, Appendix B).

Palaeocurrents in the Foula Sandstone Group (Figs 4a, b and 11a–f) show material initially being shed from palaeohighs immediately to the north and east (the Da Ness and Soberlie formations), evolving to more distally derived material transported from the NW as the basin began to fill (the Sneug Formation and younger units). Heavy mineral analyses and detrital zircon geochronology of sandstones were undertaken (for methodology, see Supplementary Material, Appendices C and D) to learn more about the provenance of the Foula Sandstone Group and the nearby Melby Formation.

Heavy mineral data (Supplementary Material, Appendix C) show that samples from Foula and Melby overlap with those from the Lower Clair Group and the Late Devonian sequences in the Orcadian Basin, suggesting that these sandstones were sourced from similar source areas. The ratios of the key heavy minerals (Fig. 12) confirm the similarity in provenance between sandstones from Foula, Melby, Hoy and the Lower Clair Group. The garnet to zircon ratios are highly variable within samples from Foula (Fig. 12) and may reflect diagenesis due to garnet dissolution (Morton and Hallsworth 1999). Higher garnet to zircon ratios in the samples from Melby may indicate shallower burial and would be consistent with the lower vitrinite reflectance values reported by Hillier and Marshall (1992) for this succession (0.8–1.0) compared with Foula (1.0–1.8). The higher proportion of garnet in the Melby and Soberlie formations may also reflect proximity to local basement source rocks. The lower apatite to tourmaline ratios in some Foula samples are interpreted to be the result of weathering leading to apatite dissolution.

U–Pb laser ablation inductively coupled plasma mass spectrometry detrital zircon geochronology was carried out on five samples (four from Foula and one from Melby) to determine the age and provenance of sediment source regions and enable comparison with published detrital zircon data obtained from the Clair Group and other Devonian sequences of the Orcadian Basin, as well as the Neoproterozoic metasedimentary rocks of the East Mainland Succession that underlies much of Shetland (Fig. 13a–d). The results are displayed as kernel density estimation and multi-dimensional scaling plots (Fig. 13a–d). A total of 476 grains were considered to be reliable, with zircon ages falling in the range 366.1 ± 3.7 to 3483 ± 6.7 Ma, with zircons showing ±10% discordance being discarded. Data tables are presented in the Supplementary Materials, Appendix D.

The kernel density estimation plots suggest that the zircons in the Foula Sandstone Group and Melby Formation reflect a diversity of sediment sources, with the zircon ages falling into three or four main groups: Phanerozoic, Proterozoic (with broad age peaks at c. 1200–1000 and 1800–1500 Ma) and Neoarchean (Fig. 13a). These groups are also identifiable within the other Devonian samples from the Clair Group, Shetland and Orkney. The Proterozoic and Neoarchean groups are present within the East Mainland Succession (Fig. 13). The multi-dimensional scaling plots indicate a more variable provenance for the Clair–Foula–Shetland samples compared with the Orkney samples (Fig. 13d). The Foula Sandstone Group and the Melby Formation appear to be intermediate between the Clair Group and Orkney samples.

It is debatable whether the detrital zircon grains are first or second cycle (or more). Within the Foula and Shetland Devonian samples, the Phanerozoic grains are most likely relatively proximal first-cycle detritus because the grade of Caledonian (Ordovician) metamorphism on Shetland was sufficiently high to crystallize new zircon (Cutts et al. 2011) and the basement rocks there are intruded by Ordovician–Devonian granitic plutons (Lancaster et al. 2017). More problematic is whether the older zircon grains are first- or second- (or greater) cycle detritus. There are no exposed proximal basement sources that fall within the age ranges of c. 1200–1000 and 1700–1500 Ma within this sector of the North Atlantic. Given the probability that the Phanerozoic grains were proximally derived, it therefore seems likely that at least some of the Proterozoic grains are second-cycle (at least) material derived from reworking of the East Mainland Succession, although sources further afield in East Greenland could also have contributed detritus (Schmidt et al. 2012; Saßnowski 2015). By contrast, the Neoarchean grains are plausibly a mix of second-cycle detritus also derived from reworking of the East Mainland Succession and first-cycle material eroded from the basement of this age that underlies much of the Faroe–Shetland terrane west of Shetland (Holdsworth et al. 2019b) and crops out onshore in North Rona/Uyea (Kinny et al. 2019).

The youngest detrital zircons within the Foula Sandstone Group and the Melby Group conflict with the currently accepted Eifelian age for these successions (Marshall and Hewett 2003). Within the Foula Sandstone Group, an average of the three youngest zircons that overlap in age at 2$σ$ gives a maximum depositional age (Dickinson and Gehrels 2009) of c. 386 Ma, indicating a Frasnian or younger age. Moreover, the youngest detrital zircon from the Sneug Sandstone gives an Upper Devonian Fammenian age of 366.1 ± 3.7 Ma. The youngest detrital zircon within the Melby Formation yields a Mid-Devonian Givetian to Late Devonian Frasnian age of 382.2 ± 7.7 Ma. The significance of these ages is uncertain given the small sample size and the possibility that the analysed grains may have suffered post-depositional Pb loss: further work is therefore needed to assess their reliability.

The Foula Sandstone Group records a dynamic and evolving depositional environment (Table 1). Figure 14 presents a generalized Devonian palaeogeographical map of the Orcadian and Clair basins with post-Devonian, mainly dextral movements along regional strike-slip structures restored, including the Melby, Walls Boundary and Nestings faults. In line with the provenance data, the reconstruction places Foula and the Clair Basin in closer proximity to Orkney and mainland Scotland and separates these areas from the Walls, East Shetland and Fair Isle basins, which lay further to the north.

The opening-upwards, growth folding geometry of the Foula Syncline and the presence of sinistral-normal faults and fracture corridors, orientated sub-parallel and perpendicular to the hinge of the Foula Syncline, are consistent with syndepositional folding and faulting related to sinistral transtension (Figs 10f and 11a–f; Fossen et al. 2013) during the Devonian and the development of the Orcadian Basin. This is supported by the results of the stress inversion analysis of slickenline data on Foula (Fig. 10a–e) and is also consistent with observations made by Seranne (1992) from Shetland. It is also commensurate with recent models for the transtensional development of the Orcadian Basin as a whole due to sinistral shearing along the Great Glen Fault–WBF system and associated structures (Fig. 14; Dewey and Strachan 2003; Wilson et al. 2010; Dichiarante et al. 2020; Tamas et al. 2021). Similar fold and strike-slip structures are also observed in the Inner Moray Firth (Clarke and Parnell 1999), Greenland (Hartz 2000), the East Shetland Platform (Patruno and Reid 2017) and the Devonian basins of Norway (Seranne and Seguret 1987; Séguret et al. 1989; Krabbendam and Dewey 1998). Collectively, all are thought to be related to sinistral oblique transtension and collapse following the sinistral oblique collision of Laurentia and Baltica (Dewey and Strachan 2003; Fossen 2010; Dewey et al. 2015).

More generally, the folding seen on Foula is similar in geometry to the low-amplitude, long-wavelength fold structures common throughout Devonian sequences onshore and offshore around northern Scotland, many of which have previously been attributed to Carboniferous or younger inversion events (e.g. Coward et al. 1989). This raises the possibility that not all the fold structures in the Orcadian Basin are necessarily solely inversion-related, compressional structures. It is possible that, in at least some cases, Devonian-age transtensional folds have been reactivated and tightened, forming some of the more substantial fold structures, such as the Eday Syncline in Orkney (Mykura et al. 1976a) and folds associated with the Brough Fault in Caithness (Dichiarante et al. 2020). Further work is needed regionally to clarify this possibility.

The large-scale structural setting of Foula resembles that of the Clair Field in that it comprises Devonian sandstones that overlie a NE–SW-orientated up-faulted ridge of metamorphic basement. The stratigraphy of Foula has strong similarities to that of the Lower Clair Group due to the presence of thick sequences of fluvial and alluvial sediments (Blackbourn 1981c, 1987; Table 1). The physical size of Foula is comparable in scale to the Clair Field (Fig. 1a, inset) and may provide a particularly good analogue for the unconformable and fault relationships seen around the Clair Ridge (Fig. 1b). This raises the possibility that models of the Clair Basin require reassessment to determine whether there is evidence for regional sinistral transtension and growth folding during the Devonian, a significant departure from the more traditional extensional models that are generally applied to this basin (e.g. Barr et al. 2007; Ogilvie et al. 2015; Robertson et al. 2020). It is important to add that, on Foula, the absence of definitive post-Devonian deformation contrasts with the Clair Field, where most of the important structures are Permian or younger (Holdsworth et al. 2019a; Robertson et al. 2020). Thus the Devonian rocks of Caithness, where the effects of Permian faulting related to the development of the West Orkney Basin are widespread (see Dichiarante et al. 2016, 2020), probably continue to represent a better analogue for the Clair Field in terms of the fracture systems that it displays in the subsurface.

Folds and trapping geometries that developed early in the Devonian basins, and not during later reactivation and/or inversion, could have important implications for modelling the petroleum systems of the offshore West Orkney Basin and East Shetland Platform, together with basins SE of the Great Glen Fault in the Central North Sea and Moray Firth regions (e.g. Whitbread and Kearsey 2016). Potential growth-fold-related hydrocarbon-trapping geometries may have developed in Devonian reservoir sequences during basin formation and in proximity to potential Devonian lacustrine source rocks, which, in the Orcadian Basin, were mature during the Carboniferous (Astin 1985, 1990; Marshall et al. 1985; Hillier and Marshall 1992; Parnell et al. 1998; Vane et al. 2016). Potential scenarios of this kind have been recorded onshore in Caithness by Baba et al. (2018) and in parts of Scandinavia (Rønningen 2015). Offshore Devonian lacustrine source rocks are also known to contribute to the Beatrice, Oseberg, Judy and Embla oilfields (Stevens 1991). Fold traps may therefore have initiated across a wide region of the UK continental shelf prior to Permo-Carboniferous (or younger) basin inversion and exhumation and would have remained available for later reactivation and/or hydrocarbon charge.

The Devonian rocks of Foula (and nearby Melby) show sedimentological, heavy mineral and detrital zircon characteristics that are intermediate between those of the lower part of the Clair Basin fill to the NW and the Orcadian Basin to the SE. Faulting and folding on Foula were contemporaneous with sedimentation during basin filling and can be related to regional constrictional strain due to sinistral transtension associated with movements along the Walls Boundary–Great Glen fault zone system during the Mid-Devonian. Transtensional fold development contemporaneous with regional subsidence may be more widespread than previously realized in the onshore and offshore Devonian basins of Scotland, meaning that kilometre-scale folds previously interpreted to be related to later inversion may therefore have initiated much earlier in the evolution of the basin.

Andy Conway is thanked for his input during the initial fieldwork on Foula. We thank BP for the provision of, and permission to publish, imagery examples from their archives and Shell for seismic data. The British Geological Survey, Ordnance Survey and the North Sea Transition Authority (Oil and Gas Association) are acknowledged for providing geological, topographic and offshore data, respectively, which are available and reproduced under an Open Government Licence. Thorough reviews by Adrian Hartley and Louis Howell, together with discussions with John Marshall, are much appreciated and have helped to strengthen this article.

TAGU: data curation (lead), formal analysis (lead), investigation (lead), visualization (lead), writing – original draft (lead), writing – review & editing (supporting); REH: conceptualization (lead), funding acquisition (lead), investigation (supporting), project administration (lead), supervision (lead), writing – original draft (supporting), writing – review & editing (lead); GAB: investigation (supporting), supervision (supporting), writing – original draft (supporting), writing – review & editing (supporting); ED: data curation (equal), investigation (supporting), supervision (supporting), writing – review & editing (supporting); RAS: conceptualization (supporting), writing – original draft (supporting), writing – review & editing (supporting); KJWM: supervision (supporting), writing – review & editing (supporting); ACM: data curation (equal), formal analysis (supporting), investigation (supporting), writing – original draft (supporting), writing – review & editing (supporting); AFB: data curation (supporting), formal analysis (supporting), investigation (supporting), writing – review & editing (supporting); RRJ: data curation (supporting), methodology (supporting), supervision (supporting); AS: data curation (supporting), formal analysis (supporting), investigation (supporting); RJW: visualization (supporting), writing – review & editing (supporting).

The work reported in this paper includes work conducted during a PhD study undertaken as part of the Natural Environment Research Council Centre for Doctoral Training in Oil and Gas (Grant No. NEM00578X/1) and was funded by Durham University. We are grateful to the Clair Joint Venture Group for supporting this research and providing funding for sample preparation and analysis. Postdoctoral geological mapping of Foula by Graham Blackbourn was undertaken with assistance of the Geological Society's Gloyne Outdoor Geological Research Fund, which is gratefully acknowledged.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All data generated or analysed during this study are included in this published article (and if present, its supplementary information files).