3.a. Lava fields

In the NAIP, intense volcanic activity from fissure systems resulted in thick, laterally extensive sequences of sub-aerial flood basalt lavas, interbedded with rarer pyroclastic units. Equally significant was the development of hyaloclastite deltas where lavas entered water in adjacent sedimentary basins, for example, in central western Greenland, the Faroe–Shetland Basin and the Rockall Trough. An important feature of the volcanic sequences is the presence of interbedded sedimentary lithologies, both clastic and chemical, deposited in environments ranging from wholly terrestrial through to wholly marine (e.g. Saunders et al. Reference Saunders, Fitton, Kerr, Norry, Kent, Mahoney and Coffin1997; Jolley & Bell, Reference Jolley and Bell2002).

In the BPIP, fissure-fed lava fields filled Mesozoic basin structures (Thompson & Gibson, Reference Thompson and Gibson1991; Butler & Hutton, Reference Butler and Hutton1994) and are preserved on Eigg, Skye and Mull (Fig. 1). The lavas are typically sub-aerial alkali olivine basalts, although sub-aqueous examples are also present. Locally, the lavas are interbedded with sedimentary and volcaniclastic rocks, and palaeosols, although these units are typically laterally discontinuous and <10 m thick. These rocks are discussed in detail in Section 5 below.

3.b. Central complexes

Contemporaneous with these fissure-related lavas are laterally restricted volcanic sequences related to central volcanoes. Complex assemblages of shallow intrusive units, ranging in composition from peridotite through to granite, represent the solidified magma chambers of these volcanoes (Saunders et al. Reference Saunders, Fitton, Kerr, Norry, Kent, Mahoney and Coffin1997). These ‘central complexes’ include Skye and Rum in the Inner Hebrides of Scotland (see reviews, Bell & Williamson, Reference Bell, Williamson and Trewin2002; Emeleus & Bell, Reference Emeleus and Bell2005), Slieve Gullion, the Mournes and Carlingford in NE Ireland (see review, Mitchell, Reference Mitchell2004), and Skaergaard in east Greenland (Wager & Deer, Reference Wager and Deer1939; McBirney, Reference McBirney1975), and they offer an excellent opportunity to investigate crystallization and fractionation processes.

In the BPIP, central complexes are preserved onshore at Ardnamurchan, Arran, Mull, Rum and Skye, typically along, or near, the sites of pre-Paleocene faults (Figs 1, 2). The central complexes comprise a variety of shallow intrusions, including ‘nested’ plutons, laccoliths, lopoliths, stocks, ring-dykes, cone sheets and dykes. These intrusions cut the lava fields and/or country rocks, and have long been interpreted as the ‘roots’ of larger Paleocene volcanoes. However, at the present level of erosion these volcanic edifices are no longer preserved and thus little is known of their nature (caldera, stratovolcano, shield?), composition, geomorphology and physical volcanology.

Figure 2.

Simplified geological maps of the central complexes of the BPIP. (a) Ardnamurchan. (b) Mull. (c) Rum. (d) Northern Marginal Zone of Rum Central Complex. (e) Skye. (f) Arran.

Extrusive igneous rocks are much less common in the BPIP central complexes, but where present they typically occur as isolated ‘screens’ between intrusions or as more continuous outcrops. They are often surrounded by arcuate or ring-shaped intrusions and/or ring-faults, and so were interpreted as subsided caldera fills (e.g. Mull and Rum) (Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924; Emeleus, Reference Emeleus1997; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000). Recently, rocks previously considered to be intrusive ‘felsites’ have also been re-interpreted as intra-caldera silicic pyroclastic rocks (e.g. Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000; Holohan et al. Reference Holohan, Troll, Errington, Donaldson, Nicoll and Emeleus2009, this issue). The volcanic rocks preserved in the vicinity of the central complexes are also typically associated with various sedimentary units. These include coarse breccias, previously thought to represent explosive fragmental rocks associated with vents (e.g. Harker, Reference Harker1904; Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924; Richey & Thomas, Reference Richey and Thomas1930), but now interpreted as catastrophic mass wasting deposits associated with caldera and ‘sector’ collapse (e.g. Emeleus, Reference Emeleus1997; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000; Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007; Holohan et al. Reference Holohan, Troll, Errington, Donaldson, Nicoll and Emeleus2009, this issue). The evolution of thought on the nature and origin of these volcanic and sedimentary rocks is discussed in Sections 6 and 7.

3.c. Chronology

The majority of igneous activity in the BPIP occurred c. 60–55 Ma. The central complexes typically cut the lava fields but there is overlap between the different centres and lava fields. A simplified chronology of the BPIP is provided in Figure 3, based on cross-cutting relationships and radiometric dating.

Figure 3.

Summary of the magmatic evolution of the BPIP based on field relationships and radiometric dating. At the time of compilation, the radiometric and palynological data were not fully reconciled. 1 – L. M. Chambers, unpub. Ph.D. thesis, Univ. Edinburgh, 2000 (Ar–Ar); 2 – M. A. Hamilton, unpub. data in Emeleus & Bell, Reference Emeleus and Bell2005 (U–Pb,); 3 – Chambers, Pringle & Parrish, Reference Chambers, Pringle and Parrish2005 (Ar–Ar); 4 – Chambers & Pringle, Reference Chambers and Pringle2001 (Ar–Ar); 5 – Troll et al. Reference Troll, Nicoll, Emeleus and Donaldson2008 (Ar–Ar); 6 – Hamilton et al. Reference Hamilton, Pearson, Thompson, Kelley and Emeleus1998 (U–Pb); 7 – Jolley, Reference Jolley and Widdowson1997 (palynology).

3.d. Uplift

During Paleocene times, the NAIP underwent transient uplift, attributed to the emplacement of the proto-Icelandic plume. Large volumes of basaltic melt were added to the crust over relatively short periods of geological time, raising the surface, and resulting in large amounts of clastic sediment being shed into surrounding basins (White & Lovell, Reference White and Lovell1997; Maclennan & Lovell, Reference Maclennan and Lovell2002; Mudge & Jones, Reference Mudge and Jones2004; Rudge et al. Reference Rudge, Shaw Champion, White, McKenzie and Lovell2008). Such uplift would lower base levels of erosional systems, leading to deeper incision of the palaeo-landsurface and rapid, increased erosion rates. In addition to this regional trend, localized uplift in the BPIP has been attributed to emplacement of the central complex intrusions, with direct evidence including folding and tilting of country rocks in the vicinity of the complexes (Fig. 2) and the generation of localized unconformities with structurally uplifted basement formations. The presence of mass wasting deposits also indicates locally elevated land surfaces, increased slope angles and erosion rates, and possible tectonism (Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924; Richey & Thomas, Reference Richey and Thomas1930; Richey, Reference Richey, MacGregor and Anderson1961; LeBas, Reference Le Bas1971; Jolley, Reference Jolley and Widdowson1997; Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007). Thus, the BPIP represents a dynamic, rapidly uplifting and eroding landscape on both regional and local scales.

3.e. Collapse

A variety of deposits associated with collapse of a volcanic landscape are identified in the BPIP. In particular, a distinction has arisen between mass wasting deposits formed by caldera collapse and those formed marginal to a postulated volcanic edifice and triggered by intrusion-induced uplift (‘sector’ collapse). At some localities, differentiation between the two is restricted by erosion and exposure. In this study, ‘calderas’ have been identified on the basis of: (1) the presence of ring-dykes and/or ring-faults at their margins, (2) a collapse succession of breccias and/or ignimbrites and (3) evidence of subsidence (e.g. displacement of country rocks). Intrusion-induced mass wasting deposits are identified on the basis of: (1) their position at the margins of the central complexes, (2) an absence of pyroclastic rocks and (3) the absence of any ring-dyke/fault structures. The term ‘sector collapse’ has generally been avoided, due to the uncertainty of the geomorphology of the postulated volcanic edifices.

3.f. Depositional environments

The BPIP was subject to warm and wet conditions associated with the internationally recognized Paleocene–Eocene Thermal Maximum and the temperature increases preceding this event (Jolley & Widdowson, Reference Jolley and Widdowson2005). Palynomorphs collected from sedimentary rocks interbedded with the lava fields of the Province reveal a landscape of upland Pine forests, and lowlands with mixed Mesophytic forests and broad valleys filled by tree ferns, flowering plants and swamp-dwelling flora (Jolley, Reference Jolley and Widdowson1997). These data are confirmed by the presence of leaf macrofossils, of types that thrive in warm, wet environments, in inter-lava sedimentary units (e.g. Ardtun Leaf Beds, Mull: Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924; Boulter & Kvacek, Reference Boulter and Kvacek1989). Palaeosols interbedded with the lava fields suggest intense chemical (and physical) weathering during this period (Jolley & Widdowson, Reference Jolley and Widdowson2005). The sedimentary rocks themselves provide evidence of a wide range of depositional environments in the BPIP, including upland areas prone to mass wasting events, as well as alluvial fans, braided rivers, lakes and swamps. These rocks are discussed in detail in Sections 5–7 below. This climate, together with uplift on a regional and local scale, facilitated the erosion of the volcanic landscape.

5.a. Skye Lava Field

In west-central Skye, three main sedimentary sequences, the Minginish Conglomerate Formation, the Eynort Mudstone Formation and the Preshal Beg Conglomerate Formation, are recognized (Figs 1, 4: section A; Table 1). The Minginish Conglomerate Formation comprises three members, typically 10–15 m thick, of which the Allt Geodh’ a’ Ghamhna Member is representative (Fig. 5) (Williamson & Bell, Reference Williamson and Bell1994). This member comprises three cycles, each of which typically contains packages of massive conglomerates, lenticular sandstone bodies and localized coals/siltstones with plant remains. The conglomerates were deposited by high-energy debris flow processes, and locally fine up into high- to low-energy sheet and channel fill sandstone deposits. The lenticular sandstone bodies are interpreted as within-channel dune deposits, and the coals and siltstones as overbank and swamp pool deposits. The other members of the Minginish Conglomerate Formation include fine to coarse sandstones with trough to planar cross-bedding, indicative of channel dune/channel fill and scour, and bar deposition. Overall, the Minginish Conglomerate Formation conglomerate–sandstone–siltstone–coal sequences are interpreted as the alluvial–fluvial deposits of braided river systems (Williamson & Bell, Reference Williamson and Bell1994).

Figure 5.

Stratigraphical log for the Allt Geodh’ a’ Ghamhna Member [NG 3690 1970] of the Minginish Conglomerate Formation, Skye (after Williamson & Bell, Reference Williamson and Bell1994).

The 2–15 m thick Eynort Mudstone Formation (Fig. 4: section A) typically comprises claystones, siltstones, ironstones, carbonaceous shales, thin coals and numerous thick ‘laterites’ (palaeosols). These sequences indicate shallow (ephemeral?) lakes and swamp ponds, and intense periods of weathering (Williamson & Bell, Reference Williamson and Bell1994). The Preshal Beg Conglomerate Formation (Fig. 4: section A) is up to 20 m thick and comprises volcaniclastic conglomerate, breccia, sandstone, and laminated, rarely carbonaceous siltstone. These coarse units are typically poorly sorted, and are thought to represent talus and alluvial fan deposits. Coarse units with lobate geometries and chaotic assemblages have been interpreted as debris flow deposits, whereas the fine units are thought to represent transient lakes fed by small streams draining the alluvial fan-complex (Williamson & Bell, Reference Williamson and Bell1994).

In Northern Skye, similar alluvial–fluvial to lacustrine sequences, with rare plant macrofossils in the finer units (Anderson & Dunham, Reference Anderson and Dunham1966) and volcaniclastic deposits are preserved (Bell et al. Reference Bell, Williamson, Head and Jolley1996). At the base of the lava field, the up to 40 m thick Portree Hyaloclastite Formation comprises hyaloclastites, pillow lavas and various volcaniclastic sandstones and siltstones (Fig. 4: section B). The recognition of hyaloclastites and pillow lavas indicates the presence of small lakes and sub-aqueous environments (Anderson & Dunham, Reference Anderson and Dunham1966).

On Canna, Sanday and NW Rum, part of the Skye Lava Field, the Canna Lava Formation, is exposed. On Canna and Sanday the lavas are interbedded with conglomerate–sandstone–siltstone sequences, interpreted to be of fluviatile origin (Emeleus, Reference Emeleus1973, Reference Emeleus1985, Reference Emeleus1997) (Fig. 4: section C). On east Canna at Compass Hill (Fig. 1), the conglomerates are ~50 m thick, contain clasts up to 2 m across, and are poorly sorted, suggesting a possible debris flow origin (Bell & Williamson, Reference Bell, Williamson and Trewin2002). On Rum, the lavas are also interbedded with thick conglomerate–sandstone–siltstone sequences, and fill palaeo-valleys in bedrock granite and Torridonian sandstone (Fig. 4: section D). The fine-grained units preserve leaf impressions and carbonized logs. The conglomerates and sandstones are interpreted as fluviatile units deposited by fast-flowing streams and rivers in steep-sided valleys, and the siltstones as localized lacustrine deposits (Emeleus, Reference Emeleus1985, Reference Emeleus1997).

5.b. Mull Lava Field

The lowermost Staffa Formation of the Mull Lava Field contains several well-exposed sedimentary sequences (Fig. 4: section E). The base of the Staffa Formation is marked by the 1–6 m thick, laterally impersistent Gribun Mudstone Member (Fig. 1), which comprises reddish-orange mudstone or calcareous mudstone. This volcaniclastic mudstone is interpreted as an extremely weathered basaltic ash, developed during a major hiatus in volcanism (Bell & Williamson, Reference Bell, Williamson and Trewin2002). In the area around Malcolm's Point (Fig. 1), lavas near the base of the Staffa Formation overlie a heterogeneous sequence of conglomerates consisting of a basal deposit dominated by basalt clasts, overlain by coarse-grained, flint-dominated (derived from Cretaceous chalk deposits), clast-supported, trough-bedded conglomerates, capped by basalt-dominated, clast-supported conglomerates. Exposures of volcaniclastic sandstone, extremely poorly sorted basalt–flint conglomerate, basaltic sandstone and mudstone are preserved laterally (Fig. 6). Together, these units represent a complex sequence of alluvial fan, channel and debris flow deposits. The flint clasts in the conglomerates indicate erosion and transportation of material from ‘highlands’ of Cretaceous rock (Bell & Williamson, Reference Bell, Williamson and Trewin2002).

Figure 6.

Basalt-flint conglomerate–sandstone sequence, east of Malcolm's Point, Staffa Formation, Mull. Hammer is 30 cm in length.

Claystone–siltstone–coal sequences are also preserved in the Staffa Formation on southern Mull, in particular below the famous Macculloch's Tree Flow (MacCulloch, Reference MacCulloch1819), which contains the cast of a large conifer tree, and below the Fingal's Cave Flow on Staffa, which contains abundant woody debris (Bell & Williamson, Reference Bell, Williamson and Trewin2002) (Fig. 1). The Ardtun Conglomerate Member (Fig. 1) marks another major volcanic hiatus in the Staffa Formation and comprises a conglomerate–sandstone–siltstone sequence. The conglomerates occur in trough-bedded sets with sandstone lenses, while parallel and ripple laminae are present in some of the sandstones. These units represent alluvial fan and fluvial deposits (Bell & Williamson, Reference Bell, Williamson and Trewin2002). The siltstones overlie a thin coal and contain well-preserved leaves at the famous Ardtun Leaf Beds, and represent riparian overbank and lacustrine deposits (Boulter & Kvacek, Reference Boulter and Kvacek1989; Jolley, Reference Jolley and Widdowson1997).

The lavas of the Staffa Formation typically exhibit well-developed columnar joints, pillowed/hyaloclastite facies and laterally restricted lensoid geometries. These characteristics, together with the argillaceous nature of the sedimentary rocks, indicate that the lavas were erupted into lakes and swamps impounded within broad, well-vegetated valleys, which formed part of an actively subsiding graben (Bell & Williamson, Reference Bell, Williamson and Trewin2002; Jolley et al. Reference Jolley, Bell, Williamson and Prince2009). Syn-depositional movement on graben margin faults is indicated by thick alluvial sedimentary rocks and ponded lava flows on the downthrown sides (Bell & Williamson, Reference Bell, Williamson and Trewin2002; Jolley et al. Reference Jolley, Bell, Williamson and Prince2009).

Sedimentary units are relatively rare in the overlying Plateau Formation of the Mull Lava Field (including Morvern and Ardnamurchan) and typically consist of thin volcaniclastic sandstones and siltstones. Weathered flow tops and thin, discontinuous palaeosols are present in both the Staffa and Plateau formations, indicating intense weathering and perhaps the location of kipuka (‘islands’ of land surrounded by lava flows) during formation of the lava field.

5.c. Eigg Lava Field

The Eigg Lava Field is exposed onshore on Eigg and Muck and as fault-controlled slivers on SE Rum (Emeleus, Reference Emeleus1997). Sedimentary units are typically restricted to thin reddish-orange volcaniclastic sandstones and siltstones between the lavas, and represent reworked trachytic material (Emeleus et al. Reference Emeleus, Allwright, Kerr and Williamson1996). A breccia–sandstone unit comprising locally available Jurassic clasts is exposed at the base of the lava field on the south coast of Muck (Fig. 4: section F), and is of apparent mass flow origin. Three post-lava masses of conglomerate, up to 50 m thick, are exposed on the west coast of Eigg beneath the Sgurr of Eigg Pitchstone (Fig. 4: section G), and have been interpreted as fluviatile conglomerates filling a palaeo-valley (Emeleus, Reference Emeleus1997). However, the overall motif of these deposits is extremely chaotic, with clasts up to 2 m across and numerous examples of both angular and rounded blocks. The conglomerates are thus more likely to be valley-confined debris flow deposits locally interbedded with hyperconcentrated flow deposits.

5.d. Processes

Although formed independently in space and time, the three main lava fields of the BPIP display a similar range of palaeo-environments and sedimentary processes (Table 1). Field and palynological evidence indicate that the lavas were typically emplaced into a series of broad valleys comprising alluvial fans, braided rivers, lakes and swamps (Fig. 7). Palynological analysis of inter-lava lithologies on Skye and Mull demonstrates the presence of montane conifer forests, upland Taxodiaceae forests (similar to extant Redwood species) and mixed mesophytic forests (swamp plants, tree ferns) in lowland areas (Jolley, Reference Jolley and Widdowson1997; Bell & Jolley, Reference Bell and Jolley1997; Bell & Williamson, Reference Bell, Williamson and Trewin2002). Conglomerate–sandstone–siltstone–coal sedimentary assemblages were deposited on this landscape, and locally they are superimposed, indicating repeated waxing and waning of current activity.

Figure 7.

Schematic diagram illustrating a generalized interpretation of sedimentary processes during development of lava fields. This reconstruction depicts an amalgamation of features from throughout the BPIP and does not refer to any particular locality.

Coarse conglomerates or breccias are typically deposited by: (1) gravity (e.g. talus or alluvial fan accumulations on steep valley slopes), (2) debris flows (occurring on steep slopes, or in well-defined channels, valleys, or even canyons) and (3) braided rivers. Deposits characteristic of these three processes are present in the BPIP.

Talus/alluvial fan deposits interbedded with the lava fields in the BPIP are typically monomict, clast-supported, poorly sorted, conglomerates (and breccias), such as the Preshal Beg Conglomerate Formation, Skye, and at Malcolm's Point, Mull (Table 1). Their monomict nature indicates the existence of discrete ‘highlands’ or elevated areas of pre-Paleocene lithologies.

Debris flows are typically initiated in mountainous areas from failures of soil, regolith and/or weathered bedrock, commonly triggered by heavy rainfall, resulting in water–debris slurries (Johnson, Reference Johnson, Brunsden and Prior1984; Smith, Reference Smith1986; Pierson & Costa, Reference Pierson, Costa, Costa and Wieczorek1987; Smith & Lowe, Reference Smith, Lowe, Fisher and Smith1991). They typically form massive, polymict, poorly sorted conglomerates and breccias, as seen in the BPIP in Eigg, Muck, Rum, Canna, and the Minginish and Preshal Beg Conglomerate formations of Skye (Table 1). The warm and wet climate of the Paleocene (Jolley, Reference Jolley and Widdowson1997), and perhaps volcanic activity and tectonic instability, would have been contributory factors in debris flow initiation in the BPIP.

Debris flows may transform distally into hyperconcentrated and sediment-laden stream flows by either: (1) incorporation of over-run ambient streamwater and/or (2) deposition of particulate material (Pierson & Scott, Reference Pierson and Scott1985; Scott, Reference Scott1988; Best, Reference Best1992; Scott, Vallance & Pringle, Reference Scott, Vallance and Pringle1995). Debris flows are transported along, and deposit their sediments in, broad valleys with rivers, where they are subject to mixing with ambient water. The flows may become longitudinally segregated, comprising a preceding debris flow and a trailing dilute flow (Sohn, Rhee & Kim, Reference Sohn, Rhee and Kim1999). Both mixing and segregation can give rise to deposits of coarse conglomeratic facies overlain by finer, sandier facies. Following deposition, some of this clastic material can be reworked by more conventional sedimentary processes (e.g. fluvial, upon re-establishment of background sedimentation) to produce stratified sandstones. Such transformation and reworking processes may have produced the sandy conglomerates, pebbly sandstones and stratified sandstones of the type seen on Mull and Skye (Table 1).

Cross-bedded sandstones and conglomerates, both locally normally graded and/or well sorted, and lenticular sandstone bodies, are indicative of fluvial activity. Such units are preserved on Mull, Skye, Rum, Eigg and Canna (Table 1), and indicate that fast-flowing rivers and streams drained the Paleocene landscape. Trough and planar cross-bedded sandstones are indicative of minor channel switching and migration. In the BPIP, channels were formed on alluvial/debris fan surfaces (e.g. Allt Geodh’ a’ Ghamhna Member, Skye), or were carved into underlying rocks of Paleocene or older age (e.g. Canna Lava Formation, Rum). However, well-developed accretion surfaces such as point bars are absent, which suggests that mature, meander-belt fluvial sedimentation did not develop (Williamson & Bell, Reference Williamson and Bell1994). In high-flow regimes, overbank flood deposits can accumulate, and thin, massive sandstones in the Skye and Mull Lava fields record such events (Table 1).

Small lakes, swamps and overbank/quiescence ponds develop in valley floors/floodplains, and claystones and siltstones are deposited. In modern lava fields, overlapping, undulating and collapsed lava (e.g. lava tube collapse) can also form numerous depressions capable of holding small bodies of standing water (Kiernan, Wood & Middleton, Reference Kiernan, Wood and Middleton2003). Such water bodies become small-scale ephemeral depocentres. Laminated claystones, siltstones and fine sandstones, often with plant remains and woody debris, on Mull, Skye, Rum, Eigg and Canna (Table 1) (Emeleus, Reference Emeleus1985, Reference Emeleus1997; Williamson & Bell, Reference Williamson and Bell1994; Jolley, Reference Jolley and Widdowson1997; Bell & Williamson, Reference Bell, Williamson and Trewin2002) are indicative of similar, low-energy, depositional environments. Palynological data from the BPIP also confirm a well-vegetated landscape with extensive plant communities, whose decay culminated in the formation of thin coals. Riparian communities were common in channel areas (Jolley, Reference Jolley and Widdowson1997).

As discussed in Section 3.f, the BPIP was subject to warm and wet conditions. This climate promoted chemical weathering, indicated by the oxidation (reddening) of many sedimentary and palaeosol sequences interbedded with the lava fields, and the presence of thick weathering profiles on lava surfaces. In the BPIP, sedimentary units are more abundant at, or near, the bases of the lava fields, giving way to palaeosols up-sequence. As outlined above, the earlier lavas and sedimentary rocks were emplaced into broad valleys with steep slopes, which helped to generate sediments. However, with time the lavas filled this accommodation space and progressively reduced the topographical relief. As a result, later lavas were emplaced over a more topographically subdued landscape, such that erosion and deposition in these areas were gradually suppressed.

6.a. Ardnamurchan

Two large outcrops of volcaniclastic rocks occur on the Ardnamurchan Peninsula, around Ben Hiant (Ben Hiant Member) in the south, and the Achateny Valley (Achateny Member) in the north (Fig. 2a). Both rest unconformably on Paleocene basalt lavas and older country rocks. The outcrops are dominated by massive to poorly bedded, dark brown to black to grey, coarse-grained, poorly sorted, clast-supported conglomerates (and rarer breccias) with a matrix of comminuted sand-grade basaltic material (Table 1; Fig. 8a, b). Clasts range in size from particles a few millimetres across up to blocks metres across, and locally, megablocks up to 30 m across are present. Clasts are chaotically organized, sub-rounded to sub-angular, and typically 2 cm to 1 m across. In the Ben Hiant Member (Fig. 8), the conglomerates are stratified with well-developed, commonly undulating, erosive bases, and are interbedded with laterally discontinuous, poorly laminated siltstones (Fig. 8c) and sandstones, some of which fill topographical depressions, oriented N–S, on the underlying conglomerates (Fig. 8a). Clasts are predominantly basalt from the subjacent Mull Lava Field, although locally concentrations of Mesozoic country rock and other ‘exotic’ lithologies, including welded rhyolitic ignimbrite, are present. In places, the clasts are imbricated, indicating palaeoflow to the south and SW. Clast roundness, size and degree of clast-support, all increase towards the south. To the south of Ben Hiant a sequence (~8 m thick and 15 m across) of tuff, lapilli–tuff and breccia overlies the conglomerates and is the first in situ evidence of pyroclastic activity in the Ardnamurchan area (D. J. Brown, unpub. data).

Figure 8.

(a) Stratigraphical log located NE of MacLean's Nose, Ben Hiant, Ardnamurchan [NM 5370 6194], depicting strata and characteristics of the Ben Hiant Member. (b) Matrix-supported conglomerate/breccia from Conglmerate B of the Ben Hiant Member. (c) Muddy laminae in Siltstone ii of the Ben Hiant Member.

In the Achateny Member, the conglomerates are poorly stratified, although locally, units with distinctive clast assemblages form lobes, fill topographical depressions (~1–2 m relief) and display normal grading. Rare imbrication indicates palaeoflow was to the north and northwest. The dominant clast types are basalt and Mesozoic sedimentary rocks. Clast roundness, size and degree of clast-support all increase towards the north.

The Ben Hiant and Achateny volcaniclastic rocks were originally interpreted as explosive vent deposits, thought to represent pyroclastic ‘agglomerates’ and ‘tuffs’ (Richey & Thomas, Reference Richey and Thomas1930; Richey, Reference Richey1938). Richey's interpretation involved repeated, or ‘rhythmic’, explosive eruptions of trachytic magma. However, recent work (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007) has demonstrated the absence of any primary pyroclastic material in these deposits. The coarse units were re-interpreted as sedimentary conglomerates and/or breccias, whose chaotic nature, together with the magnitude of some clasts (up to 30 m), indicate that they were formed by extremely high-energy mass flow events (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007). Clast textures suggest dominant deposition by debris flow, although it is possible that landslide, talus accumulation and debris avalanche mechanisms were also involved. The interbedded siltstones and sandstones are interpreted as having been deposited in low-energy fluvio-lacustrine environments, during hiatuses in, or shortly after, debris flow deposition (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007).

Palynological analysis of the fine-grained units also provides information on the palaeo-geography and palaeo-botany of the Ardnamurchan landscape (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007), which comprised upland areas with a mature pine forest vegetation, and rivers and streams draining into lowland areas within broad valleys. Locally, small rivers fed swamps and lakes (filling depressions on lava flows?), which supported a dense vegetation of ferns and flowering plants. Clast–matrix fabric analyses, together with topographical depressions, interpreted as channel axes, and imbrication data, are used to identify a ‘sedimentary watershed’, or ‘drainage divide’, from which the conglomerates were deposited to the north and south. The debris flows were transported over several kilometres, from upland, mountainous areas into more topographically subdued lowlands and valley floors, where they were finally deposited. The interbedded siltstones and sandstones were deposited by rivers and lakes in these lowland areas (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007).

The Ardnamurchan conglomerates were deposited synchronous with intrusion as they both contain clasts of, and are cut by, petrographically distinctive dolerite cone sheets of the central complex (Richey & Thomas, Reference Richey and Thomas1930; Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007). The cumulative thickness of cone sheets intruded into the area around Ardnamurchan is over 1 km (Richey & Thomas, Reference Richey and Thomas1930), and although it is difficult to quantify their surface expression, broad structural uplift associated with their emplacement at shallow levels is probable. Similar kilometre-scale uplift induced by cone-sheet emplacement has been calculated for Gran Canaria (Schirnick, Bogaard & Schmincke, Reference Schirnick, van den Bogaard and Schmincke1999). Radially outward dip directions in the Mesozoic strata flanking the Ardnamurchan igneous centre (Fig. 2a) are further evidence for uplift and doming associated with its intrusion. This intrusion-induced uplift would have led to increased slope angles and tectonic instability around the Ardnamurchan volcanic centre, and thus provided a mechanism for the resultant, catastrophic, mass wasting events.

6.b. Mull

Coarse, fragmental deposits are located at Coire Mor and Barachandroman at the east and SE margin of Centre 1 of the Mull Central Complex (Fig. 2b, Table 1). Although relationships are obscured by a plethora of cone sheets, particularly at Coire Mor, these rocks comprise a thick succession of grey-brown, coarse, poorly sorted matrix-supported breccias, containing sub-angular to rounded clasts (Paleocene igneous lithologies and pre-Paleocene sedimentary rocks), ranging in size from 2 cm to 50 cm across, with rarer blocks up to 3 m. A series of annular folds (the ‘Coire Mor and Loch Spelve Synclines’), which Bailey et al. (Reference Bailey, Clough, Wright, Richey and Wilson1924) attributed to intrusion of the central complex, are located at the east and SE margins of the Mull Central Complex. Lavas in the vicinity of the complex are ‘domed’ and, in places, unconformably overlain by the breccias, which are not folded. Bailey et al. (Reference Bailey, Clough, Wright, Richey and Wilson1924) interpreted the Coire Mor and Barachandroman deposits as ‘surface accumulations’, and suggested that ‘the lavas were breaking up under sub-aerial decay at the time of breccia formation’, although Richey (Reference Richey, MacGregor and Anderson1961) argued they were explosion breccias formed by subsurface gas brecciation. New observations, including the discovery of small channels (<5 m across, <1 m deep) and graded beds (up to 5 m thick) confirm that these breccias are surface, sedimentary deposits of mass flow origin (D. J. Brown, unpub. Ph.D. thesis, Univ. Glasgow, 2003).

6.c. Rum

Upwelling silicic and mafic magmas caused folding, outward-tilting and ~1.5 km of uplift of overlying Torridonian and Lewisian country rocks on Rum. Much of this uplift (and later subsidence; see Section 7.a) was accommodated along the Main Ring Fault system, a set of steeply inclined arcuate faults, which delimit the Rum Central Complex (Bailey, Reference Bailey1945; Emeleus, Reference Emeleus1997; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000) (Fig. 2c). The sedimentary response to this uplift involved the rapid erosion and removal of up to 1 km of country rock overlying the developing magmatic system. This is evidenced within the Northern Marginal Zone and Southern Mountains Zone (Fig. 2c) by a marked unconformity inside the Main Ring Fault, between uplifted Torridonian and Lewisian rocks from the base of the pre-Paleocene stratigraphy, and directly overlying breccias, sandstones and ignimbrites of Paleocene age (Section 7.a) (Emeleus, Reference Emeleus1997; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000; Holohan et al. Reference Holohan, Troll, Errington, Donaldson, Nicoll and Emeleus2009, this issue). Moreover, stratigraphical and structural data indicate that the breccias and ignimbrites were, at least locally, deposited in palaeo-valleys orientated radially outwards from the centre (Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000; Holohan et al. Reference Holohan, Troll, Errington, Donaldson, Nicoll and Emeleus2009, this issue); these observations are compatible with erosion of, and deposition onto, an uplifted and domed palaeo-surface. This uplift process likely contributed to the genesis of some of the lower-level breccias currently preserved inside the Main Ring Fault (see Holohan et al. Reference Holohan, Troll, Errington, Donaldson, Nicoll and Emeleus2009, this issue), but the extent to which the whole sequence can be accounted for by this mechanism is unclear, as influences from later caldera collapse must also be considered (see Sections 7.a and 8, below).

6.d. Skye

The country rocks marginal to the Skye Central Complex (pre-Paleocene sedimentary rocks and Paleocene basalt lavas) show evidence for structural uplift, including circumferential-folding and outward-tilting (Butler & Hutton, Reference Butler and Hutton1994), similar to Ardnamurchan and Rum. Palynological analysis of sedimentary units from the Skye Lava Field also provides evidence for a major elevation change. Palynomorphs found within these sedimentary units indicate a landscape with montane conifer forests, upland Taxodiaceae forests, and mixed mesophytic forests in lowland areas (Jolley, Reference Jolley and Widdowson1997). Correlation of these units in west-central Skye has allowed the recognition of five distinct erosion surfaces (all of which were formed in a period of <0.24 Ma, at ~58 Ma; see Fig. 3), each of which can be linked to distinct geomorphological and palynological features (Jolley, Reference Jolley and Widdowson1997) (Fig. 4: section A). Between two of these erosion surfaces (E3 and E4), the palynofloras indicate a change in palaeo-elevations from 100–200 m to 1200 m, implying a major uplift of around 1 km during this period (about 58 Ma). This rapid elevation change has been attributed to emplacement of the Skye Central Complex and a major period of erosion has been suggested, although no direct sedimentary evidence is now preserved (Jolley, Reference Jolley and Widdowson1997).

6.e. Process

Shallow intrusion has been cited as a triggering mechanism for mass wasting events in other ancient flood basalt provinces (Mawson Formation debris avalanche deposits, Antarctica: Reubi, Ross & White, Reference Reubi, Ross and White2005), and in active volcanoes such as Hawai'i and certain of the Canary Islands (Moore, Normark & Holcomb, Reference Moore, Normark and Holcomb1994; Moore et al. Reference Moore, Bryan, Beeson and Normark1995; Masson et al. Reference Masson, Watts, Gee, Urgeles, Mitchell, Le Bas and Canals2002). In the Hawai'i and Canary Island examples, thick accumulations of lava are present, generating high relief, but slope angles are relatively low. This demonstrates that the large mass wasting events recorded do not necessarily require oversteepened slopes, of the type found at stratovolcanoes. Reubi, Ross & White (Reference Reubi, Ross and White2005) noted the presence of debris avalanche deposits containing megablocks up to 80 m across, in the Jurassic Ferrar Large Igneous Province, Antarctica. These deposits were generated early in the formation of this LIP and comprise sedimentary material derived from underlying units. The debris avalanche deposits also contain ovoid to spherical bodies of basalt, interpreted as hot, fluid intrusions of early Ferrar LIP material. Based on these relationships, Reubi, Ross & White (Reference Reubi, Ross and White2005) suggested that debris avalanches can accompany LIP volcanism, despite the common absence of large central volcanic edifices, and that a combination of uplift, normal faulting and contemporaneous emplacement of shallow intrusions is the most likely cause of collapse. The BPIP offers an important window into such LIP processes because the level of exposure allows us to directly see evidence for both the uplift associated with shallow intrusion, and the sedimentary response.

Doming, structural elevation and/or concentric folding of country rocks provide evidence of uplift and deformation during intrusion of the Ardnamurchan, Mull, Rum and Skye central complexes (Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924; Richey & Thomas, Reference Richey and Thomas1930; Bailey, Reference Bailey1945; Richey, Reference Richey, MacGregor and Anderson1961; LeBas, Reference Le Bas1971; Jolley, Reference Jolley and Widdowson1997). Abundant cone sheets also contributed substantially to uplift at several centres, in particular Ardnamurchan. Such intrusion-induced uplift would have led to tectonic instability and increased slope angles at the surface above and around the BPIP centres.

On Ardnamurchan, Mull and Rum, erosion and/or sedimentation can be directly linked to these volcano-tectonic uplift events. On Ardnamurchan, the conglomerates contain clasts of, and are cut by, dolerite cone sheets of the central complex, providing direct evidence for syn-intrusion mass wasting (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007). The Coire Mor and Barachandroman breccias on Mull post-date doming-related concentric folds in country rocks marginal to the central complex, and Paleocene lavas tilted by this doming event provide a significant component of their clast populations. Thus, these observations from Ardnamurchan and Mull provide evidence for intrusion-induced mass wasting. These events have no direct link to volcanic activity, as all volcanic clasts in the Ardnamurchan and Coire Mor/Barachandroman deposits are recycled materials. On Rum, the presence of coarse Paleocene sedimentary rocks overlying an intra-ring-fault unconformity carved into structurally uplifted and folded basement strata, is also convincing evidence for intrusion-induced uplift and associated mass wasting.

Although the lavas of the BPIP are generally regarded as ‘flood basalts’, parts of the Mull and Skye lava fields are thought to have formed on the low-angle flanks of shield volcanoes (Williamson & Bell, Reference Williamson and Bell1994; Kent et al. Reference Kent, Thomson, Skelhorn, Kerr, Norry and Walsh1998; Single & Jerram, Reference Single and Jerram2004), which may have been prone to collapse in the same way as Hawai'i and the Canary Islands (Moore, Normark & Holcomb, Reference Moore, Normark and Holcomb1994; Moore et al. Reference Moore, Bryan, Beeson and Normark1995; Masson et al. Reference Masson, Watts, Gee, Urgeles, Mitchell, Le Bas and Canals2002). None the less, hydrothermal mineral zonation patterns (Walker, Reference Walker, Murty and Rao1971) indicate the removal of at least 1 km of material from the Mull Lava Field, suggesting high elevations (and potential instability), whether in flood basalt or shield volcano form. The majority of this material is most likely basalt; however, on Ardnamurchan, the presence of clasts of rhyolitic ignimbrite within the Ben Hiant conglomerates, and the recognition of rarer pyroclastic units, indicates that other volcanic edifices (stratocones?) were undoubtedly being constructed. Although little is known about the nature and extent of the surface volcanic edifices fed by magma from the central complex intrusions, it can be surmised that edifice construction also contributed to raising elevations and increasing topographical relief.

Viable conditions and triggering mechanisms for the mass wasting events in the BPIP were thus provided by the combination of: (1) an already well-developed Palaeocene landscape with broad valleys and steep slopes, (2) a warm, wet climate, (3) thick, perhaps shield-like lava piles, and possible stratocones, (4) uplift due to shallow intrusion and (5) resultant high topography and volcano-tectonic faulting (cf. this study; Moore, Normark & Holcomb, Reference Moore, Normark and Holcomb1994; Moore et al. Reference Moore, Bryan, Beeson and Normark1995; Masson et al. Reference Masson, Watts, Gee, Urgeles, Mitchell, Le Bas and Canals2002; Reubi, Ross & White, Reference Reubi, Ross and White2005).

In mass wasting events, large blocks or ‘megablocks’ of material are detached and collapse or slide downslope, typically in large debris avalanches or slides (cf. Pierson & Costa, Reference Pierson, Costa, Costa and Wieczorek1987; Glicken, Reference Glicken, Fisher and Smith1991; Smith & Lowe, Reference Smith, Lowe, Fisher and Smith1991; Yarnold, Reference Yarnold1993; Schneider & Fisher, Reference Schneider and Fisher1998; Masson et al. Reference Masson, Watts, Gee, Urgeles, Mitchell, Le Bas and Canals2002). The megablocks of basalt lava and Mesozoic sedimentary rocks in the Ardnamurchan conglomerates and breccias record such collapse events. Detachment of megablocks is also enhanced at weathering-prone weak strata (van Wyk de Vries & Francis, Reference van Wyk de Vries and Francis1997; Hürlimann, Marti & Ledesma, Reference Hürlimann, Marti and Ledesma2004), and the numerous weathered palaeosols, sedimentary and volcaniclastic rocks interbedded with the lava fields in the BPIP (see Section 5) may have acted as such potential slip planes. Granulation of rock by hydrothermal activity (Frank, Reference Frank1995), common in the BPIP (Walker, Reference Walker, Murty and Rao1971), and tectonic instability (e.g. faulting and/or seismicity in response to intrusion-induced uplift), may also have been contributing factors.

In volcanic settings, loose debris (including soil, regolith and/or weathered bedrock) and collapsed blocks are mobilized downslope in debris flows, often in response to continuing uplift, tectonism and heavy rainfall (Palmer & Neall, Reference Palmer and Neall1991; Smith & Lowe, Reference Smith, Lowe, Fisher and Smith1991). As large blocks are detached, they may be transformed into mixed block and matrix facies downslope, producing a more typical debris flow (e.g. Kessler & Bedard, Reference Kessler and Bedard2000; Reubi & Hernandez, Reference Reubi and Hernandez2000). These combined mechanisms most likely generated the bulk of the debris in the Ardnamurchan and Mull conglomerates and breccias (D. J. Brown, unpub. Ph.D. thesis, Univ. Glasgow, 2003; Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007).

As debris flows move downslope, they can be ‘bulked up’ by entrained material from the land surface, with the largest clasts being pushed to the heads of flows (Takahashi, Reference Takahashi1978). Scott et al. (Reference Scott, Vallance, Kerle, Macías, Strauch and Devoli2005) note an initial rockslide–debris avalanche at Casita, Nicaragua, which evolved on the flank of the volcano to form a watery debris flood, with a sediment concentration <60% by volume, at the base of the volcano. However, 2.5 km from here the debris flood entrained enough sediment to transform entirely to a debris flow. Similar behaviour was noted in the Electron Mudflow event at Mount Rainier, Washington (Scott, Vallance & Pringle, Reference Scott, Vallance and Pringle1995). In the Ben Hiant and Achateny members of Ardnamurchan, increasingly heterogeneous and rounded clast populations are noted in distal areas (Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007), suggesting such incorporation of weathered, eroded debris into the flows. Locally in the Ben Hiant Member, deposits characteristic of hyperconcentated flow (see Section 6.a; Table 1), and ultimately stream-flow, indicate that some flows were diluted with water, as described in Section 5.d (Pierson & Scott, Reference Pierson and Scott1985; Scott, Reference Scott1988; Best, Reference Best1992; Scott, Vallance & Pringle, Reference Scott, Vallance and Pringle1995; Sohn, Rhee & Kim, Reference Sohn, Rhee and Kim1999; Brown & Bell, Reference Brown and Bell2006, Reference Brown and Bell2007). During periods of quiescence, rivers and streams may drain debris flow fans, and small lakes and/or swamps can develop (Palmer & Neall, Reference Palmer and Neall1991; White & Riggs, Reference White and Riggs2001). Fluvio-lacustrine siltstones and sandstones in the Ben Hiant Member of Ardnamurchan are indicative of such activity.

These events are collectively summarized in Figure 9. The pattern of high- to low-energy sedimentation is repeated several times in the Ardnamurchan deposits, and similar activity may have occurred on Mull. Flow transformations provide a plausible mechanism for the bulking and dilution of the debris involved in these deposits.

Figure 9.

Schematic diagram illustrating a generalized interpretation of early sedimentary response to intrusion in the BPIP. (a) Initial uplift; (b) continuing uplift.

7.a. Rum

As outlined in Section 6.c, upwelling of magma on Rum led to doming and uplift of country rocks inside a ring-fault system (Emeleus, Reference Emeleus1997). Work on the Rum caldera (Fig. 2c) has focused primarily on an area of rocks called the Northern Marginal Zone (Fig. 2d) located around Coire Dubh in the east of the island (see review, Donaldson, Troll & Emeleus, Reference Donaldson, Troll and Emeleus2001). A suite of similar rocks, called the Southern Mountains Zone (Fig. 2c), is located in the south (Hughes, Reference Hughes1960; Holohan et al. Reference Holohan, Troll, Errington, Donaldson, Nicoll and Emeleus2009, this issue), and this area provides structural evidence for a phase of caldera-forming subsidence along the Main Ring Fault system. At Beinn nan Stac (Fig. 2c), the Main Ring Fault displays inner and outer faults. Here, slivers of Mesozoic strata and basalt of Eigg Lava Field type, are found down-faulted, inside previously uplifted Lewisian gneiss (Smith, Reference Smith1985; Emeleus, Wadsworth & Smith, Reference Emeleus, Wadsworth and Smith1985; Emeleus, Reference Emeleus1997).

7.a.1. Northern Marginal Zone

The Northern Marginal Zone contains a ~30–120 m thick sequence of breccias (‘mesobreccias’), tuffs and pebbly sandstones that unconformably overlie uplifted and deformed Torridonian country rocks (Emeleus, Reference Emeleus1997; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000) (Table 1; Figs 2d, 10a–c). The breccias comprise angular to sub-rounded clasts, 2–55 cm across, of Torridonian country rock and rarer basalt, dolerite and Lewisian gneiss, set in a finely comminuted matrix of Torridonian material (Fig. 10c). The breccias change from clast- to matrix-support up-section, and in places can be subdivided into distinct normally graded packages (~5–15 m thick). Up to three packages of very thin (10–20 cm), laterally discontinuous (up to 100 m), lithic and crystal tuffs are interbedded with the breccias (Fig. 10a). The breccias directly above these units comprise more angular clasts and contain glass shards, lapilli, scoria and crystals reworked from the underlying tuffs. The uppermost breccias of the Northern Marginal Zone sequence grade into a pale-grey or cream sandstone (Fig. 10a, b), which is 1.5 to 6 m thick, and fills a palaeo-topography on the breccias. The breccia sequence is capped by 40–80 m thick sheet-like exposures of grey to black, feldspar–porphyritic rhyodacite (or ‘felsites’) with a well-developed eutaxitic texture defined by fiamme (Fig. 10a, d). The rhyodacite is locally interbedded with lithic tuff and breccia horizons (<2 m thick). Locally, steeply inclined rhyodacite feeder dykes are also present (Fig. 10b), and apparently grade into the sheets (Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000).

Figure 10.

(a) Representative log through the Coire Dubh area, Northern Marginal Zone (NMZ), Rum. Redrawn from Troll, Emeleus & Donaldson (Reference Troll, Emeleus and Donaldson2000). (b) Field relationships of Torridonian sandstone, breccia and rhyodacite in the NMZ, Coire Dubh. Redrawn from Troll et al. (Reference Troll, Nicoll, Emeleus and Donaldson2008). (c) Clast-supported breccias from the NMZ, Coire Dubh. Hammer is 30 cm in length. (d) Fiamme in rhyodacite, NMZ, Coire Dubh texture.

In the early part of the 20th century, the breccias and rhyodacites of the Northern Marginal Zone were generally regarded as subterranean explosion breccias (or ‘vent agglomerates’) and sills, respectively (see review, Donaldson, Troll & Emeleus, Reference Donaldson, Troll and Emeleus2001). By the early 1980s, increased knowledge of pyroclastic processes led to re-classification of the rhyodacites as welded ignimbrites, or ‘ash-flow tuffs’ (Williams, Reference Williams1985). Moreover, as structural evidence for subsidence was found in the Southern Mountains Zone, an intra-caldera setting for formation of the rhyodacite sheets and breccias was proposed (Smith, Reference Smith1985; Emeleus, Wadsworth & Smith, Reference Emeleus, Wadsworth and Smith1985; Bell & Emeleus, Reference Bell, Emeleus, Morton and Parson1988; Emeleus, Reference Emeleus1997; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000). The breccias were thus interpreted as the products of the collapse of unstable caldera walls, and inwards slumping and sliding of debris in large mass flow events (cf. Nelson et al. Reference Nelson, Bacon, Robinson, Adam, Platt Bradbury, Barber, Schwartz and Vagenas1994; Branney, Reference Branney1995; Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000; Bacon et al. Reference Bacon, Gardner, Mayer, Buktenica, Dartnell, Ramsay and Robinson2002). The thick pale-grey sandstone unit was deposited from washed-out fines from the underlying breccias and is thought to represent a time-gap between phases of caldera collapse (Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000).

7.b. Mull

The Mull Central Complex includes three successive, but partially overlapping, centres of activity: Centre 1 (Glen More), Centre 2 (Beinn Chaisgidle) and Centre 3 (Loch Ba) (Fig. 2b). Several features of Centres 1 and 3 have been related to calderas, and these have been termed the Early and Late calderas, respectively (Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924).

7.c. Skye

The Skye Central Complex includes four successive, but partially overlapping, centres of activity: (1) the Cuillin Centre, (2) the Srath na Creitheach Centre, (3) the Western Red Hills Centre and (4) the Eastern Red Hills Centre. Two large breccia outcrops are associated with these centres: the Srath na Creitheach breccias are intruded by the Srath na Creitheach Centre, and the Kilchrist breccias are intruded by the Eastern Red Hills Centre (Fig. 2e).

7.d. Arran

The Isle of Arran comprises Neoproterozoic metamorphic rocks and Palaeozoic sedimentary rocks that are intruded by the Paleocene North Arran Granite, the Central Ring Complex and various sills (Tyrell, Reference Tyrrell1928). How the North Arran Granite relates to the Central Ring Complex is unclear. The Central Ring Complex is ~5 km across and contains lavas, ‘felsites’ and breccias (‘agglomerates’) surrounded by arcuate ring-faults and ring-intrusions (granitic) (Fig. 2f), and has long been interpreted as a caldera (King, Reference King1954). The current level of erosion means that the Central Ring Complex arguably preserves the most complete picture of a caldera within the BPIP; however, poor exposure has limited its study.

Laterally discontinuous exposures of poorly sorted breccia, or ‘agglomerate’ (and rarer well-sorted conglomerate), up to 10 m thick and 50 m across, within the Central Ring Complex contain sub-angular to rounded fragments of various lithologies, including Paleocene igneous rocks, together with Permian and Mesozoic sedimentary rocks, all set in a matrix of similar comminuted material (Table 1). Bedding relationships are obscured. The breccias include blocks of local country rocks tens of metres, and rarely hundreds of metres, across. Locally, such masses of Permian sandstone within the Central Ring Complex are juxtaposed against Devonian Old Red Sandstone country rock (King, Reference King1954). Sheet-like bodies of ‘felsite’ up to 30 m thick are locally intercalated with the breccias and typically comprise ‘flow banded’ plagioclase–porphyritic rhyolite and dacite (King, Reference King1954). The breccias (‘agglomerates’) and felsites are cut by three small, distinct, centres comprising basaltic, andesitic and dacitic lavas and breccias.

The main breccias were originally interpreted as explosive ‘vent agglomerates’, although the option of other unspecified processes of prolonged attrition to explain their formation was also considered (King, Reference King1954). The juxtaposition of Permian sandstone within the caldera against Devonian country rock outside provides evidence for subsidence along the caldera-bounding ring-fault (King, Reference King1954). The presence of clasts and blocks of Jurassic and Cretaceous sedimentary rocks and Paleocene basalt lavas within the breccias also indicates the removal of a substantial country-rock cover now absent from Arran. Later, Bell & Emeleus (Reference Bell, Emeleus, Morton and Parson1988) argued that the breccias were formed from erosion and collapse of the unstable, topographically elevated caldera walls (e.g. Nelson et al. Reference Nelson, Bacon, Robinson, Adam, Platt Bradbury, Barber, Schwartz and Vagenas1994; Branney, Reference Branney1995; Bacon et al. Reference Bacon, Gardner, Mayer, Buktenica, Dartnell, Ramsay and Robinson2002). The felsites were not described in detail, although King (Reference King1954) suggested they were intrusive. The three smaller distinct centres are interpreted as late-stage or post-collapse cones, comprising lavas and breccias, which developed on the caldera floor (King, Reference King1954).

Recent observations in an ongoing study of the Central Ring Complex (D. J. Brown, K. J. Dobson & K. M. Goodenough, unpub. data) agree with Bell & Emeleus (Reference Bell, Emeleus, Morton and Parson1988) in suggesting that the majority of the breccias are formed by sedimentary processes. The presence of large blocks of country rock and clast-supported breccias are indicative of massive collapses of caldera wall and/or floor, together with talus accumulations, while poorly sorted, variably clast- to matrix-supported breccias are consistent with debris flow activity. The ‘felsites’ are re-interpreted here as ignimbrites and comprise tuffs, lapilli-tuffs and lithic breccia (D. J. Brown, K. J. Dobson & K. M. Goodenough, unpub. data).

7.e. Process

Large exposures of volcaniclastic rocks in the Mull and Arran central complexes have long been linked to caldera collapse events (e.g. Bailey et al. Reference Bailey, Clough, Wright, Richey and Wilson1924; King, Reference King1954). In other centres, such as Rum and Skye, recognition of the role of caldera collapse in the generation of volcaniclastic rocks has stemmed from more recent re-evaluations and discoveries (e.g. Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000). Although evidence of caldera/cauldron subsidence in the BPIP is incomplete at many localities, and new perspectives on ‘ring-dyke’ emplacement have been proposed at others (e.g. the re-interpretation of ring-dykes as lopoliths: O'Driscoll et al. Reference O'Driscoll, Troll, Reavy and Turner2006), such re-evaluations have led to increased awareness of the potential for caldera collapse to generate many of the enigmatic exposures of breccias and ‘felsites’ in the BPIP. Furthermore, caldera subsidence provides a structural mechanism for the juxtaposition of surface-level sedimentary and pyroclastic rocks against deep level intrusions, as seen in the BPIP.

Due to the current erosion and exposure levels in the BPIP, the exact nature of the proposed caldera structures is still uncertain. Possibilities include: (1) low-lying calderas formed by rapid, multiple collapse events and voluminous ignimbrite eruption (e.g. Taupo, New Zealand: Wilson et al. Reference Wilson, Houghton, McWilliams, Lanphere, Weaver and Briggs1995; La Garita, Colorado: Lipman, Dungan & Bachmann, Reference Lipman, Dungan and Bachmann1997; Long Valley, California: Wilson & Hildreth, Reference Wilson and Hildreth1997), (2) calderas at stratovolcanoes formed by collapse of domes and stratocones (e.g. Karakatau: Self & Rampino, Reference Self and Rampino1981; Mt. Mazama/Crater Lake: Bacon, Reference Bacon1983; Santorini: Druitt et al. Reference Druitt, Edwards, Mellors, Pyle, Sparks, Lanphere, Davies and Barriero1999) and (3) calderas at shield volcanoes formed by tumescence, possible flank eruptions and drawdown of magma (e.g. Hawai'i: Walker, Reference Walker1988), or basaltic explosive activity (e.g. Masaya, Nicaragua: Rymer et al. Reference Rymer, van Wyk de Vries, Stix and Williams-Jones1998).

In the BPIP, the postulated Arran, Mull and Rum calderas are all >5 km across, suggesting they were major collapse structures. On Arran, Kilchrist and Rum, the breccias are interbedded with silicic ignimbrite sheets, and within ring-dykes/ring-intrusions, indicating syn-eruptive caldera subsidence. The relationship of the breccias with the ignimbrites is variable, but the volumes of material involved and composition of the pyroclastic rocks suggest the possibility of the type 1 and 2 calderas discussed above. The breccias typically form discrete units from the ignimbrites, rather than tuffaceous lithic breccia zones within ignimbrites (cf. this study; Moore & Kokelaar, Reference Moore and Kokelaar1998; Branney & Kokelaar, Reference Branney and Kokelaar2002). The Early and Late calderas of Mull have not been described in detail but they contain breccias interbedded with ‘felsite’ sheets (probable ignimbrites) and lavas, within ring-dyke/ring-fault structures, consistent with syn-eruptive caldera subsidence.

The absence of pyroclastic activity at a caldera suggests withdrawal of magma and collapse without associated eruption, as observed, for example, at the basaltic Miyakejima caldera, Japan, in 2000 (Geshi et al. Reference Geshi, Shimano, Chiba and Nakada2002). In this example, collapse was triggered by lateral withdrawal of magma from the volcano, and no major eruption occurred from the summit caldera, which subsided by up to 1.6 to 2.1 km. A similar process occurred at the andesitic–rhyolitic Mount Katmai/Novarupta system, Alaska, in 1912 (Hildreth, Reference Hildreth1991). Pyroclastic material is absent from the breccias at Srath na Creitheach on Skye, and they form part of a smaller collapse structure (~2 km across). While erosion and/or intrusion may have removed part of the stratigraphy, these breccias may have formed in a similar way.

Intra-caldera megabreccia and mesobreccia sequences described by Lipman (Reference Lipman1976) are dominated by coarse, poorly sorted breccias, whose textures resemble debris flow, slide and avalanche units (Glicken, Reference Glicken, Fisher and Smith1991; Yarnold, Reference Yarnold1993). These mass flow events were most likely triggered by gravitational collapse and slumping of topographically elevated caldera/crater/vent walls, although contemporaneous volcanism, faulting, seismicity, intrusion and rainfall may also be implicated. Loose debris, including recently collapsed material, talus, and detritus on the caldera floor is then mobilized in debris flows (Miura & Tamai, Reference Miura and Tamai1998; Moore & Kokelaar, Reference Moore and Kokelaar1998; Bacon et al. Reference Bacon, Gardner, Mayer, Buktenica, Dartnell, Ramsay and Robinson2002). Such collapse mechanisms and products (e.g. coarse breccias, megablocks) can be recognized throughout the BPIP intra-caldera breccias (Fig. 11).

Figure 11.

Schematic diagram illustrating a generalized interpretation of sedimentary processes during caldera collapse events in the BPIP. Intra-caldera ignimbrites are omitted to elucidate the sedimentary events.

As caldera walls collapse, large blocks of country rock become heavily fractured and grade into clast-supported breccia, features typical of debris avalanche megablocks (Glicken, Reference Glicken, Fisher and Smith1991; Yarnold, Reference Yarnold1993; Kessler & Bedard, Reference Kessler and Bedard2000; Reubi & Hernandez, Reference Reubi and Hernandez2000), although some may simply represent pieces of subsided/segmented caldera floor (Miura & Tamai, Reference Miura and Tamai1998) (Fig. 11). Large blocks of country rock and Paleocene lava preserved in the BPIP breccias, for example, on Arran and Skye, display similar textures and geometries. The location of proposed collapse avalanches would have been controlled by the timing and position of the fault scarps (e.g. piecemeal caldera collapse: Lipman, Reference Lipman1997).

‘Quiescent’ periods following, and between pulses of, caldera collapse can be marked by fluvio-lacustrine sedimentation (e.g. Moore & Kokelaar, Reference Moore and Kokelaar1998; Kokelaar, Raine & Branney, Reference Kokelaar, Raine and Branney2007). On Rum, such fluvial sandstones are present, and packages of breccia become less angular up-section, indicating reworking of debris by background sedimentary processes (Troll, Emeleus & Donaldson, Reference Troll, Emeleus and Donaldson2000).

The caldera collapse events envisaged in the BPIP are collectively illustrated in Figure 11. In summary, the BPIP volcanoes appear to have been subject to multiple periods of syn-eruptive collapse, although in some cases, lateral withdrawal of magma may have resulted in collapse without eruption. Collapse involved the break-up of caldera walls and/or floor, and the transportation of material in debris avalanches, flows and slides, interspersed by fluvio-lacustrine sedimentation.