3.1. Prestfjall Formation

Observations on the thickness and facies of the Prestfjall Formation on Suðuroy are largely dependent on limited exposures (e.g., Fig. 2) and the measurements of Rasmussen & Noe-Nygaard (Reference Rasmussen and Noe-Nygaard1970; Supplementary Data S3). Many of their measured sections (see Fig. 7a) were established in now long-closed coal mine adits and coal prospecting excavations. Additional data from the KOLBH1-04 borehole, field observations, and from the photographic profile of the west Suðuroy cliff line are included here (Passey et al. Reference Passey, Vosgerau, Sørensen and Vautravers2016). With these additional data, the creation of isopach maps and integration of palynological data, a modified depositional environment model can be proposed.

Figure 7 Isopach maps for the Prestfjall Formation and named bed units on Suðuroy. Data presented here indicate that these beds are to varied extent, diachronous facies within the Prestfjall Formation. These maps were constructed from the profile data presented in Rasmussen & Noe Nygaard (Reference Rasmussen and Noe-Nygaard1970), supplemented by data from the boreholes and sections discussed here. These locations are shown using the profile numbers of Rasmussen & Noe Nygaard (Reference Rasmussen and Noe-Nygaard1970). Isopach maps are contoured in metres, and differential shading is used to highlight the contours with the most intense shading used for the thickest strata. Shading colour is changed to emphasise different lithologies. The data are presented at Supplementary Data S3.

An isopach map for the gross thickness of the Prestfjall Formation on Suðuroy (Fig. 7a), shows the thickest deposits to be centred on the west side of the Rokhagi valley, in the north and in the extreme east at Holið í Helli [61.547N, 6.746W]. Overall, the Prestfjall Formation thins abruptly across the centre and northeast of the island centred around Stapin ([61.583N, 6.829W] Figs 1, 7a). There is no evidence for the Prestfjall Formation having been deposited in situ in the Stapin area, although allochthonous rafts of Prestfjall Formation occur in thick sills intruding the Hvannhagi Formation (Fig. 4a). At Stapin, volcaniclastic Hvannhagi Formation conglomerates and shales overlie an older, Beinisvørð Formation volcanic vent (the ‘Stapin Vent’; Passey Reference Passey2004; Passey & Jolley Reference Passey and Jolley2009). Northwest of Stapin, thick Prestfjall Formation coals and shale beds are inferred from peperitised rocks in the base of the Hvannhagi Formation succession at Reyðibarmur ([61.610N, 6.924W] Figs 1, 7a). These exposures are dissimilar to others of the Prestfjall Formation in being overlain by a thick volcaniclastic Hvannhagi Formation succession (Fig. 7h).

Thin profiles of the Prestfjall Formation have been recorded over central Suðuroy, extending from Tjørnunes [61.598N, 6.890W] on the northeast coast to Fámjin [61.525N, 6.881W] on the central west coast. Although exposures are limited, a profile comprising basaltic lava flows and thin volcaniclastic sediments at Hvannagjógv [61.569N, 6.889W] indicates that a localised flow terrain may have influenced part of the western limit of the Prestfjall Formation lacustrine basin. Difficulties in differentiating intrusive from extrusive basalt units also appear to have resulted in profile 20 near Tjørnunes being underestimated or misinterpreted (Fig. 7a; Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970). These central Suðuroy extrusive volcanics and the intrusion of sills into the Prestfjall Formation are orientated from Reyðibarmur in the north, south southeast to profiles 29, 30 and 31 on Oyrnafjall ([61.547N, 6.849W] Fig. 7a, d, e, f; Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970). Here, basalt intrusions invade the Prestfjall Formation in a manner similar to that at Reyðibarmur. Deposition within the southern area of the lacustrine basin was thickest at Rangabotnur [61.551N, 6.890W] and to the west (Fig. 7a), while east of profile 36, pre-existing topography resulted in a thin sedimentary succession.

Isopach mapping of the gross thickness of coaly sediments of the Prestfjall Formation was undertaken by Ellis et al. (Reference Ellis, Jolley, Passey and Bell2009). In addition to mapping the gross thickness, isopach mapping of the five informal units of the Prestfjall Formation (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970) has highlighted incompatibilities between individual profiles, particularly with reference to the KOLBH1-04 borehole. Isopach mapping of the ‘underclay’ (Fig. 7b) shows that the borehole contains an anomalously thick unit in comparison to those only a few tens of metres away (e.g., Figs 4, 6). In contrast, the ‘lower coal’ (Fig. 6) is significantly thinner in the KOLBH1-04 borehole and in profiles 10–13 (Fig. 7c) immediately to the south, than in the Rokhagi profile (Fig. 4). The regular occurrence of coal lenses and flasers in the ‘underclay’ of KOLBH1-04 and profiles 10–13 indicate lateral facies change from these lacustrine proximal shales to lacustrine distal, durain-rich coals. Comparison of palynological data from the Rokhagi mine profile with that in the KOLBH1-04 borehole (Figs 4, 6) supports this interpretation of the lithological data. Based on palynological data, both the ‘upper’ and ‘lower’ coals in the KOLBH1-04 profile are age equivalent to the ‘upper coal’ in the Rokhagi mine profile. Similarly, the ‘lower’ coal of the Rokhagi mine profile is correlative with the ‘underclay’ of the KOLBH1-04 borehole.

Given that the carbonaceous content of the Prestfjall Formation is allochthonous, lateral variation in the concentration of drifted organic debris would have resulted from water current and wind effect on the lake basin. Prevailing wind directions from the northwest in the Selandian to lower Ypresian are indicated from tephra distribution in the North Sea and Faroe–Shetland basins (Knox & Morton Reference Knox, Morton, Morton and Parson1988). Amplified by fluvial input down the regional slope from the north northwest, this would have resulted in accumulations of woody debris along the distal, eastern margin of the lacustrine basin, against the Reyðibarmur–Hvannagjógv–Oyrnafjall zone (Fig. 7c). Although the highly carbonaceous profiles recorded in mines on the east side of the Rokhagi valley are not the thickest of the Prestfjall Formation successions (Fig. 7a, c), the thicker coal units in these locations would have been subjected to greater compaction (compaction factors of 6–10; Elliott, Reference Elliott1984) than areas of dominantly clastic sedimentary rocks (compaction factors of 1.2–2.0; Mondol et al. Reference Mondol, Bjørlykke, Jahren and Høeg2007; Bjørlykke et al. Reference Bjørlykke, Jahren, Mondol, Marcussen, Croize, Peltonen and Thyberg2009). Decompaction of these profiles suggests that the eastern margin of the Prestfjall Formation basin was at its deepest on the eastern side of Rokhagi.

Conglomerates and normally graded pebbly sandstones recorded at the top of the Prestfjall Formation in the northwest of the basin (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970) mark a period of uplift and erosion. These deposits have also been recorded on the west coast of Suðuroy near Grímsfjall ([61.598N, 6.987W] Fig. 2d). Here, a paleovalley (15–20 m in depth) eroded into the Prestfjall Formation was filled by pillow-based, columnar jointed lava flows of the Malinstindur Formation. The erosive, conglomerate and sandstone filled fluvial channels present in the northwest of the lacustrine basin are not observed to cut the red-brown claystones of the ‘takleir’ unit of the ‘roof clay’. Combination of conglomerate–sandstone filled channel thicknesses with those of the ‘taklier’ claystone (Fig. 7g) indicate that the claystones were overbank facies of the fluvial channel fill sandstones and conglomerates. This combined ‘taklier’ and conglomerate unit is confined to the northwest of the Prestfjall Formation lacustrine basin (Fig. 7g), coincident with the lava filled valley eroded into the top of the Prestfjall Formation at Grímsfjall (Fig. 2d). Together, these indicate incision of a limited fluvial drainage system into the carbonaceous Prestfjall Formation. Uplift and incision of this area could have been a response to localised thermal uplift of the area following the onset of Malinstindur Formation volcanism. However, this erosion occurred in the same period as uplift and incision of the eastern margin of the Faroe–Shetland Basin following relative sea level fall. Here, basin flank uplift resulted in the rejuvenation of shelf margin drainage systems to form the age equivalent Flett Unconformity (Shaw Champion et al. Reference Shaw Champion, White, Jones and Lovell2008; Hartley et al. Reference Hartley, Roberts, White and Richardson2011; Jolley et al. Reference Jolley, Millett, Schofield, Broadley and Hole2021).

3.2. Hvannhagi Formation

The gross thickness isopach for the Hvannhagi Formation (Fig. 7h) clearly identifies a depocentre in the northeast of Suðuroy. Composed of >25 m thickness of tuffs and volcaniclastic sedimentary rocks (Passey & Jolley Reference Passey and Jolley2009), these are at the thickest between Stapin and Hvannhagi proximal to the source. These rocks thin significantly to the south with distance from source, where ~3 m of tuffaceous rocks are recorded at Holið í Helli (Fig. 3). Mapping the western limit of these rocks is problematic, their exposure being obscured by thick dolerite sills. However, they appear to pass laterally into volcaniclastic sedimentary rocks and basaltic lava flows at Hvannagjógv. Overlying the intruded, fluidal peperitised Prestfjall Formation strata exposed at Reyðibarmur, the Hvannhagi Formation includes normally graded volcaniclastic sandstones, lapillistones, and tuffs. These are composed of palagonitised volcanic glass, larger fragments exhibiting cuspate edges. The absence of intrusions in these sedimentary rocks suggests that the peperitisation of the underlying Prestfjall Formation took place during shallow burial, prior to Hvannhagi Formation deposition.

Approximately 2 m above the base of the Hvannhagi Formation at Leirbakki ([61.581N, 6.831W] Fig. 1), clast supported conglomerates made up of 2 cm–8 cm pebbles occur in lensoid bodies up to 10 m across (Fig. 2a, c). Resting unconformably on the surface of the late Beinisvørð Formation Stapin Vent, the conglomerates include large blocks up to 50 cm wide, but are overlain by up to 1 m of finely bedded mudstones which include some thin, bright coals up to 2 cm thick (Fig. 2a). These shales and drifted coals yielded a lacustrine palynoflora dominated by taxa common in transported assemblages (Inaperturopollenites hiatus and Pityosporites spp.). Occurrences of Caryapollenites circulus and Cupuliferoipollenites cingulum subsp. oviformis in these shales indicate coeval deposition of lowermost Hvannhagi Formation volcaniclastic and upper Prestfjall Formation carbonaceous beds.

A second, different flora was recorded from the upper part of the Hvannhagi Formation succession in the Svalbaá stream [61.571N, 6.835W], near Trongisvágur (Fig. 1). Here, C. circulus occurs commonly with Caryapollenites inelegans and Caryapollenites veripites in a coal rip-up clast within the upper conglomerate beds. This clast along with others in the uppermost Hvannhagi Formation are clearly derived from the Prestfjall Formation coals, presumably from an area in the northwest where they have been largely eroded away.

4.1. Tindhólmur

The south facing cliffs of the island of Tindhólmur expose a section through the Prestfjall and Hvannhagi formations and the base of the Malinstindur Formation (Fig. 8). On Mykineshólmur (Fig. 1) and the on western end of Tindhólmur, thick Beinisvørð Formation simple lava flows dip to the southeast. On Tindhólmur, these are overlain by the Prestfjall Formation which is composed of horizontally bedded volcaniclastic coarse-grained to fine-grained sandstones in beds 10 cm–30 cm thick. These sandstones are ~5 m thick in the west, thickening abruptly over a Beinisvørð Formation lava flow termination to ~28 m over the centre and east of the island. There is no evidence of a significant erosive unconformity at the base of the Prestfjall Formation, the sedimentary rocks having been deposited at the eastern side of a series of Beinisvørð Formation simple lava flow terminations (Fig. 8).

Figure 8 Tindhólmur interpreted photogrammetry model. Figure 8a, view of Tindhólmur northwest from Vágar, with Mykines in the distance. The south face of Tindhólmur is shown in interpreted model Fig. 8b. Transparent shading is used to clarify geological relationships; Beinisvørð Formation, purple; invasive flows and sills, blue; Prestfjall Formation, red; Hvannhagi Formation, orange; Malinstindur Formation valley fill flows, red; and Malinstindur Formation, pale purple. Formation boundaries are picked out by white dashed lines, other intra formation boundaries by white dotted lines. F = fault, yellow dashed boxes define position of other figures. 8c plan view of Tindhólmur showing poor rock exposure on the northern face, although the Hvannhagi Formation forms a distinct spring line where it intersects the surface. Valley fill flows can be seen to extend to the north in the centre of the island, as does the lava tube exposed on the east of the island. The faults evident from the model show little vertical movement and are assumed to be predominantly strike-slip. Yellow dashed boxes show position of photographs in Fig. 9.

Figure 9 Interpreted field photographs of Tindhólmur taken from sea level: (a) central valley fill showing both pre-Hvannhagi Formation and post-Hvannhagi Formation valley fill Malinstindur Formation lava flows. The oldest set of valley fill flows pre-date the Hvannhagi Formation, which can be seen to overlie them at the margins. Later valley fill flows were emplaced into a shallow valley eroded into the Hvannhagi Formation as part of the overlying Malinstindur Formation succession. (b) Parallel bedded Prestfjall Formation volcaniclastic sandstones incised by the eastern valley, showing Malinstindur Formation compound flow fill; (c) Hvannhagi Formation exposure at the western end of island showing transition from hyperconcentrated flow deposits to matrix supported and cross-bedded conglomerates and sandstones deposited under fluvial flow; and (d) Hvannhagi Formation on the western flank of the central valley fill succession.

Numerous shallow invasive flows and sills are exposed at the boundary between the Prestfjall Formation and the overlying Hvannhagi Formation, obscuring the nature of the contact (Fig. 8b). In the centre of the island, a 260 m wide channel eroded into the underlying Prestfjall Formation was filled with crudely bedded lava flows, which overstep the edge of the channel structure onto the upper surface of the Prestfjall Formation (Figs 8, 9a). Orientated 160^o south southest, this channel occurs in the middle of the south face of Tindhólmur, its eastern margin being influenced by a north northeast–south southwest trending fault (Figs 8, 9, 10), interpreted as dominantly strike slip with some minor downthrow component to the west. The upper surface of these valley fill compound lava flows is reddened to a few centimetres (Fig. 9a), resulting from an eruptive hiatus and weathering of the lava flow surface.

Figure 10 Schematic Tindhólmur depositional history diagram. The fault on the eastern margin of the central valley is assumed to have been dominantly strike-slip. However, it also focussed erosion, developing accommodation space for subsequent valley fill lava emplacement.

A second steep sided valley fill is partly exposed at the eastern tip of the island (Figs 8b and 9b). The valley is >150 m wide, the eastern margin having been eroded away. The valley was incised through Prestfjall Formation sandstones, and was subsequently filled by compound lava flows of the Malinstindur Formation (Figs 8b, d, 9b). Overlying the valley fill of compound flows is a 4.8 m thick succession of mostly clast supported, coarse polymictic basalt clast conglomerates of the Hvannhagi Formation (Fig. 8b). These thicken into the centre of Tindhólmur, reaching 24 m–26 m thickness on either side of the central valley, thinning to 7.2 m in the west (Fig. 8b). In this unit, low angle, trough cross bedding is evident across the island, with evidence of incision and erosion of earlier Hvannhagi Formation sediments. A well-exposed section (Fig. 9c) in the west of the island shows that the upper Hvannhagi Formation transitioned upsection from the clast supported conglomerates to clast poor conglomerates and volcaniclastic sandstones, some with cross bedding.

The Hvannhagi Formation is interpreted to have been deposited as a volcanic lahar (Smith & Lowe Reference Smith, Lowe, Fisher and Smith1991). Clast rich, ungraded beds were deposited under hyperconcentrated flow. Clast poor, weakly horizontally bedded and cross bedded sandstones in the upper beds (Fig. 9c) reflect the transition from deposition by hyperconcentrated flow to streamflow. Shallow incisions of younger, clast rich conglomerate beds into older Hvannhagi Formation conglomerates are associated with wide channels and clinoforms, suggesting that these were deposited by a dilute lahar flow (cf. Pierson et al. Reference Pierson, Pringle and Cameron2011).

A subsequent period of erosion removed some of the previous valley fill flows and created accommodation space along the axis of the central valley (Figs 9a, 10). Rejuvenation of the Tindhólmur central valley and differential uplift of the east flank by 8 m with respect to the west flank, was calibrated from the photogrammetry model (Figs 8, 9a). Differential movement of one valley flank with respect to the other is attributed to further movement on the valley edge fault (Fig. 10). This irregular topography was onlapped and overstepped by the prograding Malinstindur Formation compound flow field (Figs 8, 9, 10). In addition to infilling the central valley structure, these flows covered the western flank of the central valley before the uplifted eastern flank was submerged (Fig. 10).

4.2. Vágar

The Gásadalur 1989-1 borehole (Fig. 1; [62.09471N, 7.40663W]) was drilled on the island of Vágar in 1989, approximately halfway between the villages of Gásadalur and Bøur, penetrating the Malinstindur, Hvannhagi and Prestfjall formations before terminating in the uppermost lava flows of the Beinisvørð Formation. The Prestfjall Formation rests on a basalt lava flow where a sandy paleosol developed (Fig. 11). Overlying this are approximately 11 m of volcaniclastic sandstones and conglomerates attributed to the Prestfjall Formation. These are, in turn, overlain by matrix supported, very poorly sorted, polymictic, granulite to pebble grade volcaniclastic conglomerates of the Hvannhagi Formation. Low-angle cross bedding occurs in the sandstone units of the Prestfjall Formation while the conglomerates of the Hvannhagi Formation appear as unstructured or normally graded beds.

Figure 11 Gásadalur 1989-1 borehole lithostratigraphy, lithology log, and selected palynology data expressed as square roots. Plots for weathering indexes are derived from X-ray fluorescence analysis (major and trace element ratios Al₂O_4:base, Ba:Sr and P₂O₄:Al₂O₄) The photographs of pieces of the 5 cm core show cross bedded sandstones of the Prestfjall Formation (33.32 m–33.60 m) and two cores of the immature, clast supported conglomerates of the Hvannhagi Formation.

Fifty-two samples from the borehole were analysed for palynology, although recovery was concentrated within the Prestfjall Formation. These palynofloras are similar in composition to those recorded in the Prestfjall Formation of the KOLBH1-04 borehole. Dominated by Pityosporites spp. and Inaperturopollenites hiatus (Figs 5, 11) these floras are typical of fluvially transported assemblages (Daly et al. Reference Daly, Jolley, Spicer and Ahlberg2011a). In comparison to Prestfjall Formation records from Suðuroy, Caryapollenites species, typical of transitional swamp floodplain communities, occur at lower frequency in the Gásadalur borehole. Palynofloras also decline in diversity and frequency in the upper beds of the Prestfjall Formation. Common Laevigatosporites haardtii and Deltoidospora adriennis fern spores were recorded in the upper beds of the Prestfjall Formation (Fig. 11). Laevigatosporites haardtii is derived from ferns which exploited disturbed substrates and newly exposed overbank areas (Collinson Reference Collinson2002; Jolley et al. Reference Jolley, Bell, Williamson and Prince2009), while the parent plant of D. adriennis was associated with colonisation of crevasse splays and inter channel sand and mud banks (Jolley et al. Reference Jolley, Bell, Williamson and Prince2009; Daly et al. Reference Daly, Jolley, Spicer and Ahlberg2011a). These beds also yielded common Triporopollenites coryloides (Myricaceae, Myrtle types) which indicate dryer, better-drained substrates associated with sandy floodplains or sandy channel margins.

In addition to these indications of a dynamic depositional environment, the dinocyst Impletosphaeridium ligospinosum, the prasinophycean algae Leiosphaeridia spp., and reworked Mesozoic palynomorphs occurred in the uppermost siltstones of the Prestfjall Formation (40.29 m–40.20 m; Fig. 11). Characteristic of low salinity marginal marine conditions, their occurrence indicates that the upper Prestfjall Formation at Gásadalur was deposited within tidally influenced fluvial channels. As with the uppermost bed of the Prestfjall Formation in the KOLBH1-04 borehole on Suðuroy, the presence of reworked Mesozoic palynomorphs (Fig. 11), indicates a drainage system integrated outside the lava field. The source area of these reworked palynomorphs is currently unknown, as with other occurrences of Mesozoic and some Carboniferous taxa in the upper Malinstindur Formation and Sneis Formation interbeds (Passey & Jolley Reference Passey and Jolley2009). Given the transition into a marine environment in the Faroe–Shetland Basin to the southeast (Jolley et al. Reference Jolley, Millett, Schofield, Broadley and Hole2021), a clastic source to the northwest is thought probable.

The pollen and spore record from the Prestfjall Formation in the Gásadalur borehole was derived from disturbed fluvial margin and flow top habitats, deposited in a fluvial environment. Because the overlying Hvannhagi Formation was deposited by a high-energy system derived from an active volcanic zone, the palynofloras are poor. Only low frequencies of fluvially transported taxa occur. However, they do indicate that the catchment area of the Hvannhagi Formation conglomerates was vegetated to some extent.

Samples used for palynological analysis were also subjected to XRF analysis of major and trace elements (Fig. 11, Supplementary data S2). The plot of MgO values against an average value for the Malinstindur Formation highlights the first evidence of volcanism in the uppermost few centimetres of the Prestfjall Formation prior to the comprehensive volcanism of the Malinstindur Formation. This is supported by the Ba:Sr and Al₂O_4:base ratios which drop significantly in this interval.

The architecture of the Prestfjall and Hvannhagi formations deposits around Vágar is obscured by the inaccessible nature of the exposures. Boat surveys have proven to be of limited value because of the low-angle of view, although Rasmussen & Noe-Nygaard (Reference Rasmussen and Noe-Nygaard1970) recorded complex disruption of the Prestfjall Formation from basaltic intrusions. Because of this lack of accessibility, a photographic survey was undertaken from a helicopter which provided the basis for a photogrammetry model (Passey et al. Reference Passey, Vosgerau, Sørensen and Vautravers2016). This model provided coverage of the Prestfjall and Hvannhagi formations over the islands of Tindhólmur and western Vágar.

The succession drilled by the Gásadalur 1989-1 borehole is exposed in inaccessible sea cliffs southeast and northwest of the village of Bøur on Vágar. Creation of a photogrammetry model from the stereo photographic survey of the western coastline of Vágar has allowed these sections to be placed in the wider context of the transition between the Beinisvørð and Malinstindur formations (Fig. 12).

Figure 12 Interpreted photogrammetry model panels for the coast of Vágar from Bøur to Ritunøva. Transparent shading is used to clarify geological relationships; Beinisvørð Formation, purple; sills, blue; Prestfjall Formation, red; and Hvannhagi Formation, orange. Formation boundaries are picked out by white dashed lines, other intra formation boundaries by white dotted lines. F = fault, yellow dashed boxes define position of other figures in the text. Photograph (a) prograding hyaloclastite foresets towards south southeast on the foreshore at Gasadlaur; (b) intra Beinisvørð Formation volcaniclastic sandstone beds near Gasadalur village; (c) exposure of Prestfjall Formation voclaniclastic sandstones in the cliffs north of Gasadalur. The common habit of sill intrusion into these volcaniclastic sediments is illustrated here; and (d) the coast at Barðið showing compound lava flows overlain by simple flows both of the Beinisvørð Formation. The Beinisvørð–Prestfjall Unconformity is illustrated by the onlap of the Malinstindur Formation compound flows.

Mapping of the Beinisvørð–Prestfjall Unconformity surface (Passey & Varming Reference Passey and Varming2010) identified an east southeast dip across Vágar. The intra Malinstindur Formation Kvívík Beds (Passey & Jolley Reference Passey and Jolley2009), also showed a dip to the east, indicating little overall structural change during the deposition of the Prestfjall and Hvannhagi formations. Examination of photogrammetry profiles of the west coast of Vágar showed that the interval between the Beinisvørð–Prestfjall Unconformity surface and the base of the Malinstindur Formation thins from Bøur [62.086N, 7.369W] in the south to Barðið [62.143N, 7.457W] in the north. From Barðið to Ritunøva [62.145N, 7.341W] on the north coast, the same interval thins to a few metres before it passes below sea level to the west of Viðvík ([62.124N, 7.356W]; Figs 1, 12).

Thinning of the Prestfjall–Hvannhagi formation interval to the north and east of Vágar is linked to the underlying topography at the cessation of Beinisvørð Formation eruptions. In north-western Vágar, the exposed upper Beinisvørð Formation is composed of a thick sequence of compound basalt flows onlapped by a succession of dipping simple lava flows and volcaniclastic rocks, which thicken to the south (Figs 12, 13). These compound lava flows are interpreted to have originated from a low-angle shield volcano point source (Passey & Bell Reference Passey and Bell2007), here termed the Mykines–Vágar low-angle shield volcano. An isopach map for the upper unit of the compound flow field demonstrates a source to the northwest (Fig. 13a). This is honoured by the onlap pattern of the subsequent Beinisvørð Formation simple lava flows, which overlie the compound flow field (Figs 12, 13b). A gravity anomaly, some 15 km north of Mykines was identified by Schrøder (Reference Schrøder1971); this probably reflects the location of the Mykines–Vágar low-angle shield.

Figure 13 (a) Isopach map of the upper unit of the compound lava flow succession belonging to the Beinisvørð Formation in north Vágar. This isopach map is contoured in metres, and differential shading is used to highlight the contours with the most intense shading used for the thickest lavas. This map includes the area seen in Fig. 12d; and (b) comparable compound lava flow facies overlain by simple lava flows of the same formation, Støpulin, eastern Mykines (see Fig. 1 for location).

In the cliffs north and south of Gásadalur village, the simple lava flows of the upper Beinisvørð Formation are interbedded with dark red volcaniclastic sandstones and siltstones which thicken to the south (Fig. 12b). Where exposed at Reyðastíggjatangi [62.104N, 7.436W], these are composed of blocky, fissile, red-brown volcaniclastic shales and poorly sorted, fine sandstones ~1.3 m thick. Low angle, trough bedding occurs in exposures of this unit tens of metres to the north. Two of these shale units are exposed, separated by a simple lava flow, the oldest shale resting on south southeast prograding hyaloclastite foresets (Fig. 12a). These hyaloclastites are correlative with a simple lava flow exposed to the north (Fig. 12b). Uppermost Beinisvørð Formation red volcaniclastic sedimentary interbeds persist and thicken to the south southeast towards Bøur before dipping below sea level (Figs 12, 14). Together, the change in volcanic facies and south southeast thickening sandstones reflect an increase in accommodation space south of Gásadalur village. This exploited topography is inherited from the southern margin of the Beinisvørð Formation, Mykines–Vágar low-angle shield volcano. This accommodation space was subsequently filled by the deposition of the Prestfjall and Hvannhagi formations.

Figure 14 (a) Interpreted photograph of the cliff exposure at Reyðastíggjatangi, south of Gásadalur; see Fig. 12a for location. Note the Malinstindur Formation lava flows have filled an eroded channel, the northern limit of which is defined by a strike slip fault; and (b) interpreted photograph of cliff exposures north of Bøur showing extensive intrusion of the volcaniclastic sedimentary rocks by basalt sills. The upper limit of the Hvannhagi Formation was clearly subjected to significant erosion before the emplacement of the overlying Malinstidur Formation flows. This is particularly notable to the left of the photograph, where thin bedded compound Malinstindur Formation flows fill a ‘mini basin’ in the top of the Hvannhagi Formation.

Exposure of the Prestfjall Formation on Vágar is limited, cropping out in inaccessible sea cliffs north and south of Gásadalur. A record of 90 cm of shales with coaly material included was made by Rasmussen & Noe-Nygaard (Reference Rasmussen and Noe-Nygaard1970) at Hellisgjógv ([62.144N, 7.449W] Fig. 1) in the northwest. No beds of carbonaceous sediment or disseminated woody debris were observed during fieldwork for this study.

Along the coastal cliffs from Gásadalur to Ritunøva, the Prestfjall and Hvannhagi formations are very poorly exposed and were regarded as one unit in the photogrammetry interpretation of the Gásadalur to Bøur section (Fig. 12). From Gásadalur to Bøur, there are isolated clear exposures of the Prestfjall Formation, but as noted by Rasmussen & Noe-Nygaard (Reference Rasmussen and Noe-Nygaard1970), the volcaniclastic sedimentary rocks are extensively intruded by sills and invasive lava flows. Near Reyðastíggjatangi (Fig. 14a), a ~18 m thick unit of the Prestfjall Formation crops out. Overlying a basal sandstone bed <2 m thick, red to orange, crudely bedded, poorly sorted, and fine-grained sandstones and mudstones occur. These sedimentary units have a sharp upper contact with the overlying clast-rich conglomerates of the Hvannhagi Formation.

The Hvannhagi Formation in the Gásadalur to Bøur section is thicker and more complex than any recorded previously (Figs 12, 14). Dominantly composed of clast supported, ungraded and normally graded, crudely trough bedded volcaniclastic conglomerates, and thin planar bedded sandstones, these beds are ~4 m thick (Figs 12, 14). Clasts are angular to sub-rounded and composed of basalt derived from flow top and inter flow facies from different lava flows. Examination in the field shows that the matrix of these beds is composed of altered volcanic glass and sand grade volcaniclastic material.

Two depositional phases of the Hvannhagi Formation have been interpreted from the photogrammetry model, but exhibit no obvious lithological difference. The oldest unit occurs along the whole of the Gásadalur to Bøur cliff exposure, except where the latter has been penecontemporaneously eroded (Fig 12, Gásadalur to Bøur section, 14b). In the north, this unit attains 53 m thickness, expanding to ~80 m in the south of the exposure. Overlying this unit is a second, younger conglomerate unit. Separated from the older Hvannhagi Formation conglomerates by shallow invasive flows and sills which follow the boundary, the upper volcaniclastic conglomerates reach a maximum thickness of 94 m (Fig. 14b). The edge of this upper Hvannhagi Formation unit appears to have been penetrated by the Gásadalur 1989-1 borehole. Separated from underlying conglomerates by an intrusive sheet (Fig. 11, 75.6–81.28 m), the upper conglomerate unit (81.28 m–92.62 m) is ~11 m in thickness. However, there is a significant angular unconformity at the boundary between the Hvannhagi Formation and the overlying Malinstindur Formation, with a topographical relief of up to 119 m over 1.2 km of outcrop (Fig. 12 Gásadalur to Bøur section). Due to this unconformity, the depositional architecture of the upper Hvannhagi Formation is difficult to reconstruct.

5.1. Mykines–Vágar area

Photogrammetry and lava facies evidence of a large compound flow field in northwest Vágar and north Mykines, have confirmed the existence of the Mykines–Vágar low-angle shield volcano. Thick successions of compound lava flows seen on the easternmost tip of Mykines (Fig. 13), indicate that the structure was to the north of Mykines and northwest of Vágar, corresponding to an earlier gravity anomaly interpretation (Schrøder Reference Schrøder1971). Subsequently, upper Beinisvørð Formation simple lava flows and volcaniclastic sedimentary rock units onlapped the low-angle shield volcano. These crudely stratified sandstones and shales are lacking in any palynoflora, supporting an origin as weathered and remobilised volcanic ash derived from fissures that erupted the late-stage simple lava flows. Ash, scoria, and weathered basalt clasts from the fissure zone would have been remobilised by rainfall and transported by hyperconcentrated flow as a muddy slurry (cf. Smith & Lowe Reference Smith, Lowe, Fisher and Smith1991; Zernack Reference Zernack, Roverato, Dufresne and Procter2021). These muddy lahars progressively infilled a pre-existing low, previously a zone of lava–water interaction south of Gásadalur village. The occurrence and thickness of these volcaniclastic deposits is clearly not related to the duration between eruptions (Millett et al. Reference Millett, Hole, Jolley, Passey and Rossetti2020). Their stratigraphical and paleogeographical location is instead a depositional system response to the presence of a substantial low-angle shield volcano to the northwest.

Despite the topographical annealing effect of the late-stage Beinisvørð Formation simple lava flows and interbedded lahar deposits, the geometry of the succeeding Prestfjall and Hvannhagi formations indicates persistence of a paleovalley in the Gásadalur to Bøur area. In the area of Tindhólmur, the Prestfjall Formation depositional system occupied a topographical low to the east of successive Beinisvørð Formation simple lava flow terminations. The finely bedded volcaniclastic sandstones of the Prestfjall Formation exposed on Tindhólmur (Fig. 9b) indicate deposition of volcaniclastic sediment in a localised lacustrine basin. This contrasts with the Prestfjall Formation fluvial sandstone/shale deposits drilled in the Gásadalur 1989-1 borehole and exposed within the Gásadalur to Bøur paleovalley, indicating a source of remobilised volcanic ash from a primary volcanic accumulation.

No lithological change is apparent in the Prestfjall Formation on Tindhólmur, but the blocky claystones at the top of the formation in the Gásadalur 1989-1 borehole contain evidence of a tidally influenced fluvial channel and a link to a source of extra-basinal clastic reworking. Several rises in relative sea level have been recorded within the FIBG lava pile (Jolley et al. Reference Jolley, Passey, Hole and Millett2012), potentially linked to drowning of the low-lying basalt plain in response to thermal subsidence at the cessation of Beinisvørð Formation eruption (Millett et al. Reference Millett, Hole, Jolley, Passey and Rossetti2020). This would have resulted in the formation of ria-like drowned valleys rather than sheet flooding of the basalt plain (Shaw Champion et al. Reference Shaw Champion, White, Jones and Lovell2008; Jolley et al. Reference Jolley, Passey, Hole and Millett2012, Reference Jolley, Millett, Schofield, Broadley and Hole2021).

The 150 m thick conglomeratic succession of the Hvannhagi Formation on Vágar is interpreted as a sequence of lahar deposits derived from remobilisation of a sector collapse volcanic debris avalanche (VDA) in the northwest of Vágar. Evidence of a VDA source for the Hvannhagi Formation on Vágar is provided by the high volume of very poorly sorted basalt clasts, derived from multiple lava flows. These VDA deposits were reworked as dilute lahars (cf. Zernack Reference Zernack, Roverato, Dufresne and Procter2021) down the pre-existing valley system (Figs 12, 14–16). Repeated beds of very poorly sorted, unstructured to faintly bedded, and normally graded conglomerates indicate deposition under hyperconcentrated flow. Horizontally bedded sandstone units deposited in transition to fluvial flow occur in some units, suggesting dilution of the hyperconcentrated flow front, waning lahar states, or as lateral overbank facies. Further down the drainage slope at Tindhólmur, lahars initially deposited debris flow conglomerates. Here, waning flow resulted in the deposition of cross-bedded sandstones and pebbly sandstones showing transition from hyperconcentrated to streamflow (Fig. 9).

Considered by previous authors to be a transitional contact between the volcaniclastic deposits and compound lava flow field (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970; Waagstein Reference Waagstein, Morton and Parson1988; Passey & Jolley Reference Passey and Jolley2009), new photogrammetry data of the Hvannhagi–Malinstindur Unconformity indicate a more nuanced relationship. This unconformity marked a significant relative uplift event associated with dominantly strike-slip faulting both on Vágar and Tindhólmur. Faulting and erosion occurred along the fault scarp associated with erosion of a significant proportion of the upper Hvannhagi Formation, leaving an irregularly eroded topography of 119 m on Vágar. On Tindhólmur this relationship is seen to be more complex. Erosion of valley features and their subsequent partial infilling by basalt lava flows, was followed by a hiatus reflected in weathering of the central valley fill lavas. Subsequent deposition of the Hvannhagi Formation conglomerates was, in turn, followed by a rejuvenation of the central valley, with Malinstindur Formation compound lavas filling the central valley and onlapping the surrounding topography (Figs 8, 10). This relationship confirms that eruption of lava flows characteristic of the Malinstindur Formation began following Prestfjall Formation deposition, prior to the VDA and associated lahar deposits of the Hvannhagi Formation in the north of the islands. The volcaniclastic deposits of the Hvannhagi Formation therefore represent dynamic, landscape scale events within the early Malinstindur Formation lava field.

Mapping of the basal Malinstindur Formation lava flow package (Fig. 15), illustrates that the flow field originated to the northwest, indicating continued eruptive activity sourced from the Mykines–Vágar shield. Subsequent basalt plains eruptions drowned the complex erosional topography of the Hvannhagi–Malinstindur Unconformity. The eroded surface of the upper Hvannhagi Formation created a series of ‘mini-basins’ which were the last to be overstepped by the compound flow field.

Figure 15 Both Fig. 15a and 15b have isopach maps contoured in metres, and differential shading is used to highlight the contours with the most intense shading used for the thickest strata. (a) Isopach map of the combined thickness of the Prestfjall and Hvannhagi formations in the Vágar and Tindhólmur area. The south southwest trending main thickness of the Hvannhagi lahar turns more to the south at Tindhólmur (shown by the white arrows), following the trend recorded in the photogrammetry model (Fig. 8). Sourced from a reworked volcanic debris avalanche, the lahars originated north of Vágar. Note that because of post-Hvannhagi Formation erosion evident in Figs 12 and 14, the depositional thickness of the upper beds of the Hvannhagi Formation was significantly reduced. (b) The isopach of the overlying basal lava flows of the Malinstindur Formation highlights the trend of the lahar system, even allowing for post-depositional erosion. This map indicates that the low-angle shield producing these compound lavas lay to the north northwest off the north coast of Vágar. The sector collapse that sourced the lahars probably subcrops beneath thick Malinstindur lava flows to the northwest of Vágar.

5.2. Suðuroy

A thick succession of eastwards dipping Beinisvørð Formation simple lava flows is exposed on the west coast of Suðuroy. These are regarded as having been sourced from an eruptive fissure system west of Suðuroy (Waagstein Reference Waagstein, Morton and Parson1988; Passey & Varming Reference Passey and Varming2010). Consequently, late Beinisvørð Formation lava flows and sedimentary rock interbeds vary in composition and thickness west to east across the island. Typical of this are the ponded columnar jointed (e.g., Kulagjógv Flow) and multi-tiered (e.g., Hov Flow) lava flows exposed in the interior of the island and north of Hov, respectively (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970). These flows are not present on the west coast, being ponded down the depositional slope to the east. Similarly, interbedded volcanic debris flow units (e.g., at Hov) and volcaniclastic shale units (e.g., at Froðba) are recorded between lava flows in the east of the island (Fig. 1), but not seen in the west.

Vent proximal volcanic and volcaniclastic deposits occur in the northeast of Suðuroy, sourced from the Stapin Vent (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970; Passey & Jolley Reference Passey and Jolley2009). The pyroclastic deposits of this vent form an annular structure, which elevated the area above the surrounding Beinisvørð Formation surface, a topography inherited during deposition of the Prestfjall Formation. Exposures inland are limited, but photogrammetry interpretation of the west coast has shown no evidence of significant erosion at the base of the Prestfjall Formation (Passey et al. Reference Passey, Vosgerau, Sørensen and Vautravers2016). As a consequence of this, deposition of the Suðuroy Prestfjall Formation lacustrine system was controlled by this landscape inheritance. Within the lacustrine Prestfjall Formation, repeated cycles in palynofloral ecology, geochemical ratios, and sedimentology, indicate that longer duration environmental forcing occurred (Fig. 6). There is no evidence that these cycles were related to lava field processes, but rather that they reflect orbital or solar forcing controlling lacustrine water depth via precipitation. Assuming 21 k obliquity or 40 ky eccentricity cycling (Berger Reference Berger1988), this would indicate that the lacustrine record is 105–205 ky in duration.

Lateral facies variation between the uppermost units of the Prestfjall Formation and the basal Hvannhagi Formation/Malinstindur Formation is suggested by palynological data from Stapin, fossil wood in the lowermost Hvannhagi tuffs at Holið í Helli, and raised residual MgO values in the upper Prestfjall Formation of the KOLBHI-04 borehole. Incised channels filled with normally graded conglomerates and sandstones are linked here to overbank reddened clay stones which indicate localised erosion following deposition of the Prestfjall Formation in northwest Suðuroy. Elsewhere, the Prestfjall Formation is overlain by apparently conformably bedded volcanic, or thin volcaniclastic rocks of the Hvannhagi and Malinstindur formations.

The syn-eruptive facies of the Hvannhagi Formation on Suðuroy include primary pyroclastic tuffs, conglomerates, sandstones, and shales. These, primary olivine-phyric and welded tuffs confirm the linkage between the Hvannhagi Formation and the olivine-phyric lavas of the basal Malinstindur Formation (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970; Hald & Waagstein Reference Hald and Waagstein1984; Waagstein Reference Waagstein, Morton and Parson1988; Larsen et al. Reference Larsen, Waagstein, Pedersen and Storey1999; Passey & Jolley Reference Passey and Jolley2009).

Exposures of the base of the Malinstindur Formation are infrequent on Suðuroy, and they are often poor or invaded by numerous dolerite sills. Previous authors (Rasmussen & Noe-Nygaard Reference Rasmussen and Noe-Nygaard1970; Waagstein Reference Waagstein, Morton and Parson1988; Passey & Jolley Reference Passey and Jolley2009) identified a transitional zone where lower Malinstindur Formation lava flow lobes are interbedded with volcaniclastic rocks attributed to the Hvannhagi Formation. This is supported by the succession recorded in the KOLBH1-04 borehole, where these volcaniclastic interbeds occur in the Malinstindur Formation above the Hvannhagi Formation.