Late Berriasian–Barremian

Figure 9a depicts the Faroe–Shetland region following Late Jurassic–earliest Cretaceous uplift and erosion, which instigated the formation of the BCU. During the late Berriasian–Barremian interval, proven active basin development is largely restricted to the East Solan and North Rona basins, which form part of the SE Marginal Basin domain, and the SW West Shetland Basin, though sporadic deposition is also recorded from the South Solan Basin and the NE West Shetland Basin (Fig. 4). The SW West Shetland Basin accumulated a thick sequence of paralic and sandy shallow-marine deposits of the Victory Formation adjacent to the Shetland Spine Fault (Ritchie et al. 1996; Harker 2002; Stoker & Ziska 2011) (Figs 4 and 6). In contrast, the North Rona and East Solan basins preserve a thinner record of mixed siliciclastic and carbonate shallow-marine deposition, assigned to the Valhall Formation (Ritchie et al. 1996), which is punctuated by intra-Valanginian and Hauterivian hiatuses. Maximum-drilled sediment thicknesses indicate that the SW West Shetland Basin accumulated at least five times more sediment than any of the adjacent basins (Table 4). Whereas the average sediment accumulation rate across all basins was 8.2 m Ma⁻¹ (Fig. 7), the specific rate for the SW West Shetland Basin was 51 m Ma⁻¹ (Table 4). This suggests that the Shetland Spine Fault was the most active of the faults at this time, with more intermittent (as indicated by the hiatuses), smaller-scale movements on the faults bounding the North Rona and East Solan basins.

These paralic to shallow-marine basins appear to have been relatively isolated within a largely exposed hinterland that covers much of the West Shetland region. In particular, the Orkney–Shetland High, Rona High, West Shetland High and North Shetland High might have acted collectively as a barrier (perhaps even a watershed) between the West Shetland Basin and SE Marginal Basins and the larger North Sea Basin to the east. Further west, the exposed area extends at least as far as the Judd High–Outer Hebrides High, which imparts a marked offset in the palaeogeography of the hinterland.

The southern and eastern flanks of the Faroe–Shetland Basin, including the Judd sub-Basin, the SE margin of the Flett and Foula sub-basins, and the Yell sub-Basin were also emergent, as were the intra-basinal Westray and Corona highs, possibly the Flett High, as well as much of the Erlend High (Larsen et al. 2010; Stoker & Ziska 2011). The likelihood of pre-Aptian rocks in the deeper axial parts of the Foula and Flett sub-basins cannot be discounted, though information on pre-Aptian Cretaceous rocks is lacking from these locations.

According to Ritchie et al. (1996), the mudstones of the Valhall Formation in the Faroe–Shetland region were deposited in a predominantly aerobic environment, which implies a relatively open-water circulation and, thus, a connection with adjacent areas. However, the degree of connectivity with the wider geographical realm remains unclear on the basis of the following observations: (1) to the NW, the conjugate SE Greenland margin, specifically the Kangerlussuaq–Blosseville Kyst region (Fig. 8), was exposed at this time (Larsen et al. 1999a, b; Stoker et al. in press); (2) to the SW, a hiatus in the North Lewis and North Minch basins suggests that there was no connection via the Hebridean region to the Erris or southern Rockall basins, which were open at this time (Stoker et al. in press), whereas the North Rockall and West Lewis basins were probably not active until the late Barremian/early Aptian (Musgrove & Mitchener 1996; Smith 2013) (Figs 1 and 8); (3) east of Shetland, shallow-marine clastics recovered from the Unst Basin (Stoker & Ziska 2011) are interpreted by Copestake et al. (2003) as indicative of an extensive mixed clastic and carbonate shelf, flanking the semi-emergent western margin of the Viking Graben (Fig. 8), whereas Harker (2002) proposed that the East Shetland High was exposed; and, (4) to the NE, the SW Møre Basin and the Magnus Basin (Figs 1 and 8) were not actively accumulating sediment until the late Hauterivian (Copestake et al. 2003; Stoker & Ziska 2011).

Aptian–Albian

The onset of a significant change in the basinal development of the Faroe–Shetland region is evident in this interval, with the rock record suggesting that the Faroe–Shetland Basin became a larger, more integrated depocentre (Fig. 9b). The Judd, Foula and Flett sub-basins accumulated predominantly marine mudstones of the Valhall, Carrack, Cruiser and Rødby formations, fringed by coarse clastic deposits, including basal conglomerate and mass-flow sandstones, of the Commodore, Royal Sovereign and Neptune formations (Ritchie et al. 1996; Harker 2002; Stoker & Ziska 2011) (Figs 5 and 6). The previously emergent Corona and Westray intra-basinal highs were drowned and buried beneath a cover of marine mudstone. The NE and SW ends of the Rona High also record a sediment cover at this time; however, the bulk of the high remained exposed. On the NE flank of the Faroe–Shetland Basin, the Yell sub-Basin and Muckle Basin were probably also instigated at this time (Larsen et al. 2010), along with increased development of the NE West Shetland Basin. To the SW, paralic to shallow-marine deposition persisted within the SW West Shetland Basin, and marine mudstone accumulated in the SE Marginal Basins, though the preserved record in the North Rona, West Solan, South Solan and East Solan basins is sporadic and commonly punctuated with hiatuses (Fig. 4). By way of contrast, the major hinterland areas of the Orkney–Shetland High, West Shetland High and North Shetland High persisted, as did the Judd High–Outer Hebrides High.

Maximum-drilled sediment thicknesses highlight this shift in basin development, with the Flett and Foula sub-basins, in particular, accumulating over 1 km of Aptian–Albian sediment and displaying sediment accumulation rates of 44 – 47 m Ma⁻¹ (Table 4). These thicknesses and accumulation rates strongly hint at major fault movement along the Rona Fault, and probably the Judd Fault at this time. The high accumulation rate in the Flett and Foula sub-basins contrasts with a lower average basinal accumulation rate of 13.3 m Ma⁻¹, though the latter does mark an overall increase across the Faroe–Shetland region (Fig. 7). Whereas the accumulation rate is much reduced in the SW West Shetland Basin compared to the pre-Aptian interval, the increased deposition in the NE West Shetland Basin implies that the Shetland Spine Fault might have been active along a greater proportion of its length. The relatively thin and punctuated sequences in the SE Marginal Basins imply that fault activity in this area remained intermittent and of a smaller-scale.

One area of uncertainty concerns the genetic interpretation of the basal coarse clastic rocks that comprise the Neptune, Royal Sovereign and Commodore formations, and which fringe the Judd, Flett and Foula sub-basins. For example, Ritchie et al. (1996) have assigned both shallow- and deep-marine environments to the Neptune Formation, solely on the basis of its gamma-ray signature. Whereas it is acknowledged that well-logs provide important information on sandbody geometry, it is unclear to the present author how water depth can be derived solely from such data. A comparable ongoing controversy concerns the interpretation of a basal coarse clastic unit within the Upper Jurassic Kimmeridge Clay Formation in the SE Faroe–Shetland Region, for which both deep-water fan (Haszeldine et al. 1987; Hitchen & Ritchie 1987) and subaerial–shallow-marine fan delta (Vestralen et al. 1995) depositional settings have been proposed.

The deposition of the Valhall, Carrack, Cruiser and Rødby formations occurred under fluctuating oxic/anoxic conditions (Ritchie et al. 1996), which suggests that marine connections between the Faroe–Shetland region and adjacent areas continued to be restricted to some degree. The basins in the SE part of the region might have been most restricted as there remained no obvious link through the Hebridean region to the open depocentres of the Erris and southern Rockall Basin (Harker 2002; Stoker et al. in press) (Fig. 8). Marine mudstone was deposited in the West Lewis and North/NE Rockall basins at this time (Musgrove & Mitchener 1996; Smith 2013), though these basins were separated from each other by the West Lewis High, and both were probably separated from the Faroe–Shetland Basin by the Judd High–Outer Hebrides High (Mudge & Rashid 1987) (Figs 1, 8 and 9b). The conflicting interpretations – as described above – regarding the degree of exposure of the western flank of the North Sea Basin, i.e. the East Shetland High (Harker 2002; Copestake et al. 2003) maintains ambiguity over any potential east–west linkage across the Orkney–Shetland hinterland. In contrast, a northern linkage to the SW Møre Basin might have been initiated (Brekke 2000), whilst on the conjugate SE Greenland margin, the Kangerlussuaq Basin (Fig. 8) was instigated and preserves a record of late Aptian–Albian paralic sedimentation and subsequent marine transgression (Larsen et al. 1999a, b; Nøhr-Hansen 2012; Stoker et al. in press).

Cenomanian–Turonian

On the SE flank of the Faroe–Shetland Basin, the Albian/Cenomanian boundary is marked by an erosional hiatus in the SE Marginal Basins, in the NE West Shetland Basin and on the Rona High (Figs 4 and 6), which implies widespread uplift and/or exposure of the Orkney–Shetland hinterland. In the Faroe–Shetland Basin, the northern part of the Westray High (Fig. 5) was also exposed at this time. Although sedimentation resumed in the North Rona and East Solan basins in the following Cenomanian–Turonian interval, much of the Rona High and parts of the NE West Shetland Basin remained exposed (Fig. 9c).

In the Late Cretaceous, the Faroe–Shetland region was located at the northern limit of deposition of the Chalk Group (Harker 2002). Whereas limestone of the Hidra and Herring formations has been reported from the SW West Shetland, North Rona and East Solan basins, the bulk of the Cenomanian–Turonian sequence comprises calcareous mudstone of the Svarte and Macbeth formations of the mudstone-dominated Shetland Group (Ritchie et al. 1996; Harker 2002; Stoker & Ziska 2011) (Figs 6 and 9c). Localized coarse clastic rocks are associated with the Haddock Sandstone unit (part of the Hidra Formation) adjacent to the Shetland Spine Fault, and the mass-flow deposits of the Commodore Formation instigated in the Albian continued to accumulate on the eastern flank of the Faroe–Shetland Basin. The Orkney–Shetland hinterland and Judd High–Outer Hebrides High remained expansive.

The clastic facies associated with the Commodore Formation and the Haddock Sandstone unit are most probably indicative of fault activity along the Judd, Rona and Shetland Spine faults. Maximum-drilled sediment thicknesses in the Judd, Flett and Foula sub-basins exceed 0.5 km for the Cenomanian–Turonian interval, and sediment accumulation rates ranging from 49 – 76 m Ma⁻¹ are measured from these sub-basins (Table 4). Lower accumulation rates are measured from the West Shetland Basin, though the rates of 9.4 m Ma⁻¹ in the NE to 23.4 m Ma⁻¹ in the SW both represent an increase on the Aptian–Albian accumulation rates. The average basinal accumulation rate across the region is 25.7 m Ma⁻¹, which is almost twice the average rate for the Aptian–Albian (Fig. 7).

Whereas the increased sediment accumulation rate might be indicative of an intensification of extensional fault activity, there is also evidence of contemporary compressional tectonics across the region (Fig. 9c), including: (1) Turonian folds in the Foula sub-Basin (Grant et al. 1999); (2) latest Cenomanian–Turonian inversion (flower structure) in the East Solan Basin (Booth et al. 1993); and (3) folding and erosion of Albian–Turonian sediments in the North Rona and West Solan basins (Fig. 3), though the timing of deformation is less precise (Turonian–early Campanian). Synformally-disposed surfaces at the Cenomanian–Turonian level are also observed in the West Shetland Basin and the Judd sub-Basin. This deformation is inextricably linked to the creation of the MCU, and might be a consequence of differential uplift and subsidence (sagging) during the proceeding phase of basin enlargement (see below).

In terms of the wider geographical area, a link between the SE Marginal Basins and the Inner Hebridean region during the Turonian has been suggested (Harker 2002). Tectonic activity in the Inner Hebrides region is regarded by Mortimore et al. (2001) and Emeleus & Bell (2005) as a precursor to the deposition of the Upper Cretaceous Inner Hebrides Group, comprising shallow-marine sandstones and carbonate rocks comparable with the preserved sequences in the SW West Shetland Basin and the SE Marginal Basins. Marine mudstone deposition in the Kangerlussuaq Basin of SE Greenland, and the West Lewis and North and NE Rockall basins might be indicative of increasing inter-basinal connectivity with the Faroe–Shetland Basin in this part of the developing NE Atlantic rift zone (Fig. 8), though this does not necessarily imply a single through-going rift basin (see below). The Orkney–Shetland hinterland probably remained a barrier to east–west connection with the North Sea Basin (Harker 2002).

Coniacian–Santonian

A major uncertainty during this interval is the extent of the hinterland (Fig. 9d). Various authors (e.g. Hancock & Rawson 1992; Harker 2002; Cope 2006) have suggested that regionally only remnants of the Scottish Highlands and Southern Uplands remained exposed during the Coniacian to Maastrichtian, and commonly show the Orkney–Shetland hinterland to be wholly submerged. This interpretation is largely predicated on the basis of a high eustatic sea-level throughout the Late Cretaceous (Fig. 7). However, this contradicts the evidence from wells in the area of the SE Marginal Basins and the Rona High (Stoker & Ziska 2011). In both the North Rona and West Solan basins, Albian to Turonian rocks were deformed and eroded prior to Campanian sedimentation; the SW West Shetland Basin was also partially exposed (Fig. 4). Whereas some parts of the Rona High were accumulating sediment, a large tract of the high remained exposed. Collectively, these data suggest that the Orkney–Shetland hinterland, and extending into the Judd High–Outer Hebrides High region, might have remained as a largely subaerial region, part of a larger exposed Scottish landmass (e.g. Roberts et al. 1999).

The Faroe–Shetland Basin was the main focus of sedimentation at this time and was characterized by the deposition of shallow-marine to basinal mudstone of the Kyrre Formation (Ritchie et al. 1996; Harker 2002; Stoker & Ziska 2011) (Fig. 9d). The Kyrre Formation is also recorded from the West Shetland Basin, including the NE part of the basin which showed renewed fault activity at this time. In this basin, as well as on the adjacent Rona High, the Dab Limestone and Whiting Sandstone units (of the Kyrre Formation) reflect a mixed clastic-carbonate inner shelf facies.

The predominance of the Faroe–Shetland Basin as the main depocentre is supported by the sediment accumulation rates for the Judd, Flett and Foula sub-basins, which range from 67 – 128 m Ma⁻¹ (Table 4). These rates contrast with an average basinal rate of 43.3 m Ma⁻¹ (Fig. 7). Whereas the average basinal rate is at its highest in the subsequent Campanian–Maastrichtian interval, the peak rates for the Faroe–Shetland Basin are in the Coniacian–Santonian (Table 4). The accumulation rates for the West Shetland Basin are also greater than in preceding intervals. On the basis of these data, it is suggested that a major phase of basin enlargement was instigated in the Coniacian with extension, deepening and high sediment accumulation rates evident from both the Faroe–Shetland and West Shetland basins (Fig. 2). By way of contrast, the SE Marginal Basins might have been largely exposed and part of the Orkney–Shetland hinterland. The formation and shaping of the MCU is one consequence of this process.

Sedimentation throughout the region occurred largely within an aerobic, open-marine environment (Ritchie et al. 1996; Harker 2002). However, despite the high rate of sediment accumulation, the provenance of the mainly fine-grained clastic material remains uncertain. An extensive hinterland to the south and east of the Faroe–Shetland Basin bordered by an inner shelf facies in the West Shetland Basin is depicted in Figure 9d, and adopts the view of Roberts et al. (1999) that a relatively large Scottish landmass existed at this time. This is consistent with the well data described above, and invokes a low relative sea-level in this region. As this scenario contrasts with the generally high eustatic sea-level that prevailed in the Late Cretaceous (Fig. 7), it strongly suggests that tectonic activity might have had a major bearing on the relatively low sea-level assumed in this reconstruction. Major fault displacements along the Rona and Shetland Spine faults, including footwall uplift, have been described by Dean et al. (1999) and Goodchild et al. (1999), whereas the uplift and erosion of the SE Marginal Basins prior to the Campanian implies activity on the fault network bounding these basins, including the Judd Fault.

From the wider geographical area, shallow-marine rocks in the inner Hebridean region, which are comparable to the West Shetland Basin, contain sporadic conglomerate beds that are interpreted as evidence of tectonic activity throughout the Late Cretaceous (Mortimore et al. 2001; Emeleus & Bell 2005). Further west, Coniacian–Santonian conglomerates adjacent to the West Lewis High are cited as evidence of tectonic activity along the high, whereas marine mudstone continued to accumulate in the adjacent West Lewis and NE Rockall basins (Smith 2013). In the Kangerlussuaq Basin of SE Greenland, shallow-marine mudstone deposition prevailed during the Coniacian; however, a major unconformity marks the Coniacian/Santonian boundary (Larsen et al. 1999a, b, 2005; Nøhr-Hansen 2012; Stoker et al. in press). Significantly, perhaps, the uplift and erosion of this basin during the Santonian might have provided a separate northwesterly provenance for sediment input into the Faroe–Shetland Basin at this time (Nøhr-Hansen 2012). Evidence for a northerly provenance is also forthcoming from the Møre Basin where an increasingly expansive basinal drape of Coniacian–Santonian rocks developed (Brekke 2000), and it seems probable – from the available well evidence – that a marine connection to the Faroe–Shetland Basin was fully established at this time (Stoker & Ziska 2011).

Campanian–Maastrichtian

The Campanian–Maastrichtian interval is characterized by the widespread deposition of marine mudstones of the Kyrre and Jorsalfare formations across the Faroe–Shetland and SE Marginal basins, as well as many of the adjacent highs, including the total submergence of the Rona High (Fig. 9e). Sedimentation prevailed under aerobic, open-marine conditions (Ritchie et al. 1996; Harker 2002) and most basins, including the SE Marginal Basins, accumulated their thickest Cretaceous sections during this interval (Table 4). Sediment thicknesses in the West Shetland and East Solan basins exceed 1 km and, in the South Solan Basin, Campanian–Maastrichtian rocks exceed 2.5 km in thickness. All of the SE Marginal Basins experienced dramatic increases in sediment accumulation rates, up to 142.5 m Ma⁻¹ in the South Solan Basin. Whereas the average basinal rate across the region is 65.9 m Ma-¹, which continues the general upward trend (Fig. 7), it is interesting to note that the rates in the SE Marginal Basins and the West Shetland Basin largely outstrip those of the Faroe–Shetland Basin, where rates peaked in the Coniacian–Santonian interval (Table 4).

The increased sediment thicknesses and accumulation rates are consistent with the process of progressive basin enlargement during the Late Cretaceous (Dean et al. 1999). However, as was argued for the preceding Coniacian–Santonian interval, the combination of thick marine mudstones and high eustatic sea-level does not necessarily imply total submergence of the Faroe–Shetland region in the Campanian–Maastrichtian. Intra-Campanian tectonic activity resulted in local basinal readjustments, such as within the NE West Shetland Basin where compression and folding created a late Campanian unconformity (Goodchild et al. 1999). The folding, uplift and erosion observed in the North Rona and West Solan basins might also have persisted into the Campanian, along with the continued exposure of the hinterland. The absence of Campanian–Maastrichtian rocks from the Westray and Corona highs (Fig. 5) might also reflect contemporary uplift of intra-basinal highs within the Faroe–Shetland Basin. Although latest Cretaceous/Early Paleocene uplift cannot be discounted, the sequence preserved on the Westray High implies intra-Campanian/Maastrichtian uplift and erosion.

As a reflection of the uncertain extent of the Campanian–Maastrichtian hinterland, Figure 9e depicts a potential land/shelf transition zone. Whereas a Scottish landmass remains a viable sediment provenance, the possibility of a greater degree of hinterland submergence begs the question: where else could the vast quantity of sediment deposited in this interval have been sourced from? It is notable that throughout most of the Cretaceous development of the Faroe–Shetland region, rifting was not accompanied by a significant thermal anomaly or an increase in heat flow (Dean et al. 1999). However, the early manifestations of break-up-related igneous activity, and their potential for thermally-induced uplift in areas immediately adjacent to the Faroe–Shetland region, were instigated in the Campanian–Maastrichtian. These include: (1) the Maastrichtian Anton Dohrn and Rosemary Bank volcanoes (seamounts) in the North Rockall Basin (Jones et al. 1974; Morton et al. 1995); and (2) the Campanian and latest Maastrichtian instigation of the intrusion of a regional suite of basic igneous sills in extending from the Møre Basin to the NE Rockall Basin (Ritchie et al. 1999; Archer et al. 2005; Passey & Hitchen 2011), which climaxed throughout the Faroe–Shetland–Rockall region in the Paleocene/Early Eocene. In the NE Rockall Basin, the intrusion of basic sills into Upper Cretaceous mudstones created a domal uplift, which might have had subaerial expression (Archer et al. 2005).

Elsewhere, much of the Campanian–Maastrichtian interval in the inner Hebridean region is marked by a hiatus (Mortimore et al. 2001). In contrast, the Kangerlussuaq Basin of SE Greenland was transgressed by shallow-marine mudstones (Larsen et al. 2005; Nøhr-Hansen 2012; Stoker et al. in press), and there was widespread deposition of Campanian–Maastrichtian rocks in the Møre Basin (Brekke 2000). It is conceivable that the thick clastic sequences might be a reflection of a general exhumation of parts of the NE Atlantic rift system close to the line of incipient break-up (Doré et al. 1999), and which included the Faroe–Shetland region (Fig. 8). Ultimately, this may have been a factor in the formation of the BTU which reflects widespread uplift, re-emergence and erosion of most of the area flanking the SE margin of the Faroe–Shetland Basin in latest Maastrichtian/earliest Danian time (Fig. 4). In the inner Hebridean region, Mortimore et al. (2001) describe palaeovalleys cut at the end of the Cretaceous in response to faulting, uplift and erosion prior to the onset of Paleocene volcanism. In SE Greenland, the Maastrichtian/Danian boundary is marked by a major erosional unconformity that is attributed to an abrupt fall in relative sea-level (Larsen et al. 2005; Nøhr-Hansen 2012). Similarly, on the eastern flank of the Orkney–Shetland hinterland, Maastrichtian and Danian units are separated by a break in sedimentation linked to a fall in relative sea-level; this resulted in a seaward shift of the shoreline towards the eastern edge of the East Shetland High (Knox 2002).