2.4.1 CENOMANIAN LIMESTONE
The top of a set o f high amplitude sub-parallel reflections, highlighted in turquoise in Figure 2.9 (p. 60), marks the location o f a Cenomanian limestone bed from the base o f the Svarte Formation (RITCHIE et a i, 1996). The deposition o f these limestone beds were preceded by an influx o f turbiditic sandstones, demarcating the suit o f high-amplitude reflections (GRANT et a i,
1999). These sandstones were also deposited during Cenomanian times, and are referred to as the Commodore Formation by RITCHIE et a l (1996). However, the age o f these limestone beds and sandstones is unclear: they are dated as Cenomanian - Turonian by GRANT et a l (1999). The sandstone and limestone beds are present throughout the Foula Sub-basin, abutting the Rona Ridge (on the Rona Fault), and pinching out towards the Flett Ridge (against the Flett Fault). Presence beyond the Flett Ridge is not proven, partly due to poorer seismic resolution with increasing depth (see Figs 2.9-2.10, p. 60-61). Towards the NE in the seismic sections along the strike o f the basin, the multiple set o f high amplitude reflections is reduced to one single high amplitude reflection representing the limestone beds o f the Svarte Formation. This suggests that deposition o f the Commodore Formation is confined to the Foula Sub-basin between the Clair and Victory Transfer Zones (GRANT et a l, 1999).
2.4.2 INTRA-CAMPANlAN UNCONFORMITY
Subsequently, a thick succession o f turbidite derived deep marine mudstones from the Kyrre and Jorsalfare Formations has accumulated in the basin. In the seismic sections this interval is transparent, apart from a single reflection, high-lighted in green in Figure 2.9 (p. 60). In the seismic sections along the basin axis (i.e. NE-SW strike), this horizon is most distinctive, represented by a single high amplitude reflection. By cross-examining with the seismic sections perpendicular to these lines (i.e. NW-SE strike), it appears that the horizon represents an angular unconformity. In the seismic section displayed in Figure 2.9 (p. 60), subsequent reflections are noted to downlap on this horizon.
In the available seismic sections, the reflection o f the unconformity can be traced in many places (Fig. 2.10, p. 61). It is clearly observed in the lines across wells 206/5-1 and 206/3-1, although in the area of well 205/10-2B the horizon becomes weak or is poorly developed, possibly obscured by the presence o f dolerite intrusions and an extensional fault. In the area of the Flett Ridge and further to the northwest, this reflection is not clearly observed anymore. When the unconformity is observed to be present in a well, the seismic depth (in seconds) can be converted to actual depth (in feet/meters). In well 206/3-1 the unconformity appears to be at a depth o f 7240 ft. In the wireline well logs, this level is accompanied by a small downhole
increase in velocity (Fig. 2.3, p. 41). For well 206/5-1, a depth of 6250 ft. is found, which is characterised by a small downhole increase in gamma ray and resistivity values (Fig. 2.4, p. 42). For well 205/10-2B it can be inferred that, if the unconformity is present, it should occur at or below the level of the dolerite intrusions (deeper than 10500 ft.).
From the literature, this unconformity is reported to be most clearly observed in the West Shetland Basin (RiDD, 1981; Hit c h e n & RITCHIE, 1987), although it is also reported from parts of the Foula Sub-basin (HiTCHEN & RITCHIE, 1987). The event is related to a pulse o f slight
uplift o f the Rona Ridge along the Rona Fault (RiDD, 1981; DEAN et a/., 1999). During this event, some erosion took place on the Rona Ridge, where subsequent Campanian - Maastrichtian mudstones can be seen to overstep lower Campanian and older sediments (RiDD,
1981; Hit c h e n & Rit c h ie, 1987). From the steep slope off the Rona Ridge into the Foula Sub basin, occasional slumps gave rise to thin turbidites in the basin (RiDD, 1981).
Although the local extent o f this uplift event is limited, it is likely related to a more regional tectonic phase in response to the opening o f the North Atlantic. Or it might be related to the so- called late early Campanian Peine tectonic compressional phase which is seen throughout north western Europe and the southern North Sea Basin in relation to the Afiican and European plate convergence (Mo r t im o r e & POMEROL, 1997).
In the Foula Sub-basin, the angular unconformity is not the result o f (active) erosion. It represents a period o f tectonically induced deepening of the basin, with little or no deposition taking place. After the tectonic event, the basin gradually filled in again with turbidite derived deep marine mudstones (coming from the south-east), downlapping on the basin floor.
By calibration of the depth of this unconformity in wells 206/3-1 and 206/5-1 with the biostratigraphic ft-amework (see Chapter 3.5, p. 139), it can be concluded that the event o f uplift o f the Rona Ridge took place during the Caudammina ovula - Caudammina ovuloides Zone. This implies a mid Campanian age o f the event.
2.4.3 BASE TERTIARY UNCONFORMITY
The next distinctive seismic reflection represents the base o f the Tertiary (high-lighted in dark blue in Fig. 2.9, p. 60). This horizon appears to reflect an unconformity, related to the Base Palaeocene erosional event. Throughout the Foula Sub-basin, this horizon is recognised.
During the Palaeocene to early Eocene, the area west o f the Shetland Islands was subjected to thermal doming, associated with the development o f the Icelandic mantle plume (Eb d o n et n/.,
2.4 Seismic Interpretations
regional extension in the Faeroe-Shetland Basin, The extension was accompanied by uplift and erosion on the West Shetland margin, with, simultaneously, deposition o f the erosional products as submarine fans in the accelerated subsiding Faeroe-Shetland Basin (EARLE eta /., 1989). The Rona Ridge acted as the shelf edge from where submarine fans prograded into deeper parts of the basin (KNOTT et a/., 1993). From early late Palaeocene times onward, marine sedimentation continued across almost the entire Faeroe-Shetland Basin. Occasionally, renewed uplift events took place on the Rona Ridge and in the West Shetland Basin during late Palaeocene, causing erosion (DEAN et a/., 1999). In the eastern part o f the Foula Sub-basin, which is closest to the Rona Ridge, one o f those events is reflected in the Base Moray Group unconformity. By the end of late Palaeocene times, marginal marine to shallow marine conditions prevailed into early Eocene. Sedimentation took place in a shallow marine environment with deposition of progradational and aggradational wedges o f coastal plain facies o f the Flett Formation (Colsay and Hildasay Members) into a relatively shallow sea (KNOX et a/., 1997).
The early Palaeocene uplift of the West Shetland Platform and Basin also led to erosion in large parts o f the Foula Sub-basin. Further to the NW, beyond the Flett Ridge, the basin was not affected by the uplift event and deep marine sedimentation still continued (MUDGE & RASHID,
1987). The area o f the Flett Ridge (well 205/10-2B) is furthest away from the uplifted Shetland Platform and is only affected by the early Palaeocene erosional event. Here, the hiatus encompasses upper Maastrichtian to lowermost Palaeocene sediments, as lower Palaeocene sediments o f the Sullom Formation (age from RiTCHiE et al., 1996) rest unconformably on late Maastrichtian sediments (Fig. 2.2, p. 40). The extent o f the erosional event into the Maastrichtian mudstones is indicated by the microfossil assemblages observed near the top of the Jorsalfare Formation (see Chapter 3.6, p. 148). In well 205/10-2B, the uppermost studied samples o f the Jorsalfare Formation are o f late Maastrichtian age (Globigerinelloides volutus -
Reussella szajnochae 2 Zone, see Chapter 3.6, p. 148). About the same succession o f sediments is also missing in well 208/22-1, where the Sullom Formation is resting unconformably on sediments o f the Jorsalfare Formation (Fig. 2.5, p. 43), although sediments o f the uppermost Maastrichtian Pseudotextularia elegans Zone are still present (see Chapter 3.6, p. 148). Additional erosional events removed sediments o f both the Sullom and Vaila Formations in well 206/3-1. The Lamba Formation appears to be complete with the occurrence o f the Kettla Member (see Fig. 2.6, p. 52). Some sediments o f the upper Maastrichtian Jorsalfare Formation are still present {Pseudotextularia elegans Zone, see Chapter 3.6, p. 148), and therefore, the hiatus in well 206/3-1 encompasses sediments o f the uppermost Maastrichtian to Selandian age. Also in well 206/5-1, the Sullom Formation is eroded, but sediments o f the Vaila Formation (V I) are still present (see Fig. 2.6, p. 52), which rest on late Maastrichtian sediments
2.4.4 TOP BALDER FORMATION
The topmost distinctive seismic marker is taken at the top o f a set o f high amplitude reflections (high-lighted in magenta in Fig. 2.9, p. 60). This is a widespread seismic horizon, recognised throughout large parts o f the NW European offshore area. The reflection is caused by the tuffaceous lithology o f the Balder Formation, dated early Eocene (Ea r l eet a l, 1989; MORTON & Kn o x, 1990). The tuffs are derived from the late phase o f the Thulean volcanism in the Greenland-Faeroe area (CHALMERS et a l, 1995). This horizon produces a non-diachronous distinctive seismic marker, indicating the configuration of the basin at the time that active rifting ceased in the area. This event is thought to be time-equivalent with the onset o f active sea-floor spreading between Greenland and Northwest Europe (RITCHIE et a l, 1999). Since then, the basin has been subjected to thermal subsidence.
2.4 Seism ic Interpretation