2.a.1. Lesser caledonoid faults in the upper Firth

Several northeasterly faults are present in and adjacent to south Bute and the Cumbraes, although records of their movements are generally less clear than those of the other two groups of faults that displaced them during repeated reactivation.

In terms of delineating the NW coast of Little Cumbrae, the submarine fault located a short distance off the S coast of south Bute and detected by various geophysical surveys (Deegan, Reference Deegan, McLean and Deegan1978, fig. 5.1; McLean & Walker, Reference McLean, Walker, McLean and Deegan1978, fig. 3.1; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1) is a strong contender. It bridges the NW-trending Ardrossan Harbour – Glencallum Bay and Sound of Bute faults (Figs 1, 8), where it defines the northward limit of a patch of igneous rocks responsible for a magnetic anomaly recorded by McLean & Wren (Reference McLean, Deegan, Wren, McLean and Deegan1978, fig. 2.5). This patch may well be a faulted repetition of the igneous rocks present on the adjacent south Bute coast (Fig. 5). Following slight offset resulting from net shear displacement on the Ardrossan Harbour – Glencallum Bay Fault (Young & Caldwell, Reference Young and Caldwell2012), it then extends northeastwards as the Little Cumbrae Fault in strict parallelism and close proximity to the NW coast of Little Cumbrae island. That the South Bute Fault identified by McLean and Deegan was not originally mapped to the NE of the Ardrossan Harbour – Glencallum Bay Fault is scarcely surprising: few geophysical observation sites were permitted across the main shipping channel of the upper Firth, and the contrast between juxtaposed rock types is lost NE of the Ardrossan Harbour – Glencallum Bay line (Deegan, Reference Deegan, McLean and Deegan1978, pp. 61, 63; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1).

The ENE submarine fault on the N side of Kilchattan Bay, south Bute (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1; Figs 1, 5) may continue WSW beneath raised-beach deposits of the Kilchattan–Stravannan lowlands (Geological Survey of Scotland, 1897; British Geological Survey, 2008). More probably, however, it is the northeasterly extension of the fault already traced across south Bute from the south side of Dunagoil Bay (Young & Caldwell, Reference Young and Caldwell2011c ; Fig. 5). The fault's role in determining the northward limit of the Clyde Plateau lavas in the various fault blocks of the promontory was probably owing to late- or post-Viséan rejuvenation, involving also the northwesterly faults that displaced it. The Kilchattan Bay Fault may extend northeastwards to cross Great Cumbrae from about Bell Bay to Ferry Hole (Fig. 4), where a fault is suspected on broad stratigraphical and topographical grounds but cannot be delineated within acceptable limits of accuracy.

Still farther N in Bute, the Ascog and Craigmore faults define the wedge-like Ascog Block of rocks ranging as high as the Clyde Plateau lavas (Fig. 7). Other caledonoid faults in mid-Bute include the Scalpsie Bay Fault, which in crossing the island with a more easterly orientation obliquely truncates the western end of the HBF sensu stricto (Young & Caldwell, Reference Young and Caldwell2011b , fig. 1).

The most prominent northeasterly fault on Great Cumbrae is the Craignan Fitheach Fault, which extends from the intrusion of that name on the W coast to Clashfarland Point on the E (Caldwell & Young, Reference Caldwell and Young2012, fig. 9; Fig. 4). This structure again illustrates later displacement by northwesterly faults and provides evidence of early, dextral, displacement on the Great Cumbrae Fault. A caledonoid fault that crosses Millport Bay is also repeatedly cut by northwesterly faults, as shown by displacement of the Clyde Street Sill (Caldwell & Young, Reference Caldwell and Young2012, section 1.2; Fig. 4).

2.a.2. Origin and age span

Although all three groups of faults began to move in response to proto-Variscan stresses in Late Devonian time, the NE-trending group was probably the first to do so as at least its major members developed on the sites of reactivated Caledonian structures (Bluck, Reference Bluck, Bowes and Leake1978, Reference Bluck, Reading and Balance1980, Reference Bluck, Friend and Williams2000; Francis, Reference Francis, Bowes and Leake1978a , Reference Francis, Ramberg and Neumann b ; Upton, Reference Upton and Sutherland1982; Read et al. Reference Read, Browne, Stephenson, Upton and Trewin2002; Goodenough, Stephenson & Loughlin, Reference Goodenough, Stephenson, Loughlin, Stephenson, Loughlin, Millward, Waters and Williamson2003). The same group may also have been the first to become inert under a cover of as much as 5 km of Carboniferous sedimentary and volcanic rocks (Guion, Gutteridge & Davies, Reference Guion, Gutteridge, Davies, Woodcock and Strachan2000). This applies to many of the lesser NE-trending faults detected in south Bute and the Cumbraes where only the Little Cumbrae Fault points to Late Carboniferous or post-Carboniferous movements in its displacement of the Great Cumbrae Fault. In contrast, some major faults in this category were reactivated repeatedly during the Carboniferous Period, as shown by the syndepositional record of the Dusk Water Fault (Richey, Anderson & MacGregor, Reference Richey, Anderson and MacGregor1930; Hall, Reference Hall1974; Monro, Reference Monro1999). Furthermore, if this fault extends across the entire Clyde Basin, partly in the guise of the Plateau Fault of McLean & Deegan (Reference Deegan, McLean and Deegan1978, fig. 8.1; see also McLean & Wren, Reference McLean, Deegan, Wren, McLean and Deegan1978, figs 2.7, 2.8; British Geological Survey, 1985), as suggested by Young & Caldwell (Reference Young and Caldwell2012), movements were perpetuated well into Mesozoic and even Cenozoic times because the Plateau Fault brings Permian sandstones into contact with Mesozoic (and possibly Cenozoic) rocks in the South-West Arran Trough.

2.b.1. Faults of the upper Firth

The SW coast of Little Cumbrae lies close and parallel to one of the most regionally important members of the NW-trending group of faults: the Ardrossan Harbour Fault, which on the basis of geophysical data extends NW to enter south Bute at Glencallum Bay (Fig. 1). It is one of several such faults that divide the south Bute promontory into a series of small, unnamed, tilted fault blocks in which the layered rocks dip to the WSW. These in turn have long been grouped into three larger, named blocks (Figs 5, 6). The small tilted fault blocks are capped by lavas assigned to the Strathgryfe Member of the Clyde Plateau Formation, which lie disconformably on the lower member of the Clyde Sandstone Formation (Figs 2, 6). The Inverclyde Group is much condensed relative to the sequence in adjacent Great Cumbrae, implying rapid thinning on the south Bute shoulder, as suggested by Young & Caldwell (Reference Young and Caldwell2012) (see also Caldwell & Young, Reference Caldwell and Young2011; Fig. 1, and cf. Fig. 6 to British Geological Survey, 2008, cross-section 3). The promontory of south Bute is the most instructive part of the upper Firth for study of the northwesterly fault group.

Figure 6.

Geological cross-section of the south Bute promontory along line of section A–B–C in Figure 5 to show the disposition of the half-graben that compose the promontory and their allocation to the three major ‘blocks’ (Lighthouse, St Blane's and Garroch Head).

Lesser faults in the group are present on both Cumbrae islands. On Little Cumbrae, they maintain the strict northwesterly trend of those in south Bute and differ only in their reversal of direction of downthrow (Figs 3, 5). On Great Cumbrae, they are more north-northwesterly in orientation and become progressively more so eastwards towards the Great Cumbrae Fault (Fig. 4). Faults of this group reveal a mix of net dextral and sinistral displacement (sinistral slightly more common). With the exception of the Finnie's Bog Fault (Fig. 3) and the Foul Port Fault (Fig. 4), the sense of movement on the northwesterly faults of the Cumbraes is difficult to decipher, and the presence of small shear components is commonly detected only by displacements of dykes and other faults. The strongest concentration of northwesterly faults on Great Cumbrae is around Millport Bay. Small splays radiate from the main NW-trending faults between Foul Port and Knox's Port and, in the centre of the bay, small faults of the same orientation are countered by similar northeasterly ones to produce a localized pinnate arrangement of fractures. This is apparently responsible for the position and shape of the Eileans and the other smaller islets of the bay (excepting the Clach), and the alignment of the Eileans with the sand-covered bedrock ridge of The Rigg, which links them to the shore (Lytteil, Reference Lytteil1886; UK National Hydrographic Service, 2007; Fig. 4).

The overall impression conveyed by the NW-trending faults of south Bute and the Cumbraes is of a system of fractures gradually changing in orientation eastwards from NW to NNW and concurrently weakening in magnitude of displacement, ultimately giving way to the Great Cumbrae – Largs system of N-trending faults that dominates the nexus between the mainland and the upper Firth.

2.b.2. Origin and age span

The origin of the NW- to NNW-trending faults in the western offshore MVS is not immediately evident because early movements have been masked or obliterated by rejuvenation that persisted into the Palaeogene Period. Until quite recently, the northwesterly faults in the coastal mainland were commonly thought of as originating during the later Carboniferous Period in response to culminating Variscan stresses. Undoubtedly many did, when older faults were rejuvenated with concurrent generation of new faults of similar trend, and this kind of proliferation evidently continued into Permian times (Mykura, Reference Mykura1967; Francis, Reference Francis, Ramberg and Neumann1978b ; Paterson, Hall & Stephenson, Reference Paterson, Hall and Stephenson1990, p. 38).

Better evidence of an original age for the northwesterly faults and how they behaved after initiation, however, comes from the Sound of Bute, where the cuneate North-East Arran Trough (McLean, Reference McLean, Bowes and Leake1978, fig. 1; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, pp. 108–9, fig. 8.1; Fig. 1) is bound northeastwards by the Sound of Bute Fault and southwestwards by the Brodick Bay – Laggan Fault. The trough narrows northwestwards and, prior to becoming distorted by emplacement of the North Arran Granite Pluton (Woodcock & Underhill, Reference Woodcock and Underhill1987; England, 1982), the two boundary faults converged on the entrance to Loch Fyne. The southeasterly end of the trough may have been truncated initially, or later, by seaward extension of the NE-trending Dusk Water Fault or the parallel faulted extension of the Paisley Ruck, as Young & Caldwell (Reference Young and Caldwell2012) favoured, or have been bevelled by extension of the N-trending Great Cumbrae Fault, as McLean & Deegan (Reference Deegan, McLean and Deegan1978, fig. 8.1) suggested. The controlling influence of the NW-trending boundary faults on accumulating sediments of the Inverclyde Group (Figs 1, 2) and possibly even of the Old Red Sandstone (Famennian) was discussed by Young & Caldwell (Reference Young and Caldwell2012). Such early influence is suggested also by movements of the Largs–Hunterston faults on the eastern shoulder of the Cumbraes Trough during deposition of the Kelly Burn sands of the Stratheden Group (Section 2.c.3 below).

McLean (Reference McLean, Bowes and Leake1978) and McLean & Deegan (Reference Deegan, McLean and Deegan1978, p. 108) linked the North-East Arran Trough to similarly aligned structures, such as the Stranraer, Mauchline, Sanquhar, Thornhill and Dumfries basins in the Southern Uplands, which trend towards the Vale of Eden (Hall, Reference Hall1974; Francis, Reference Francis, Bowes and Leake1978a , Reference Francis, Ramberg and Neumann b ). Some of these also subsided along northwesterly axes and faults in Late Carboniferous time, others more pronouncedly in Permian–Triassic (New Red Sandstone) time (Bott & Masson-Smith, Reference Bott and Masson-Smith1960, p. 330; McLean, Reference McLean1961, p. 126; Mykura, Reference Mykura1967, p. 87; Francis, Reference Francis, Bowes and Leake1978a , Reference Francis, Ramberg and Neumann b ).

The New Red Sandstone of the South-West Arran Trough and that of the North Channel Basin, S of the Mull of Kintyre, includes deposits of Triassic age (some in Upper Triassic Keuper facies) and, in places, overlying undifferentiated Mesozoic–Cenozoic deposits, all of which are cut by NW- to NNW-trending faults (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, pp. 108–11, fig. 8.1). In the upper Firth, NW-trending faults not only cut intrusive igneous rocks of early to middle Palaeogene age but present evidence of a number of prior movements. The Ardrossan Harbour – Glencallum Bay Fault illustrates this point. At Ardrossan Harbour, Falconer (Reference Falconer1907, p. 60l) interpreted it to have a dextral shear displacement of some 275 m and to be ‘probably of Tertiary age’. Above Glencallum Bay in south Bute, extension of this fault effects a weak, apparently sinistral displacement of an olivine-dolerite sheet of Palaeogene age (Buist, Ineson & Mitchell, Reference Buist, Ineson and Mitchell1979) and, about 2 km to the NW, a much stronger sinistral displacement of the Lighthouse fault block (Figs 5, 6). In contrast, the geophysical trace of the NE-trending fault off the S coast of the south Bute promontory (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1) has a dextral offset on the Ardrossan Harbour – Glencallum Bay Fault (Section 2.a.1 above, Fig. 8). These apparently conflicting displacements illustrate the complex reactivation history of the Ardrossan Harbour – Glencallum Bay Fault, involving early sinistral and vertical displacements and later dextral offsets.

In summary, NW-trending faults were active during much of Late Palaeozoic time. Some were initiated as early as the Late Devonian Epoch when they probably developed as conjugate structures to NE- trending faults inherited from the Caledonian basement. More were products of recrudescence, however, particularly in the Carboniferous Period, continuing into the Permian Period, and probably thereafter with reduced frequency. Many additions to this family of faults were made when older members were reactivated, especially as the Variscan climax was approached. New stresses in Mesozoic and early Cenozoic times caused further adjustments, some involving strike- or oblique-slip displacement.

2.c.1. The Great Cumbrae Fault

The Great Cumbrae Fault cannot be interpreted as either a simple normal fault (Gunn, Reference Gunn, Gunn, Geikie, Peach and Harker1903, p. 128) or a ruck (Patterson, Reference Patterson1952, pp. 431–3; McLean & Walker, Reference McLean, Walker, McLean and Deegan1978, pp. 36–8; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, p. 108; Cameron & Stephenson, Reference Cameron and Stephenson1985, p. 13). It is, rather, a structure of considerable complexity and longevity that has undergone repeated sinistral and dextral strike-slip displacements with net downthrow to the W. Long suspected of continuing northwards to determine the straight line of the Skelmorlie coast and, in particular, that of the stubby promontory at Wemyss Bay, the fault is shown by geophysical records to extend N of Gourock and effect sinistral displacement of the HBF system where it enters Loch Long (McLean & Walker, Reference McLean, Walker, McLean and Deegan1978, p. 36; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1; British Geological Survey, 1985; Fig. 1). Southwards, the surface trace of the fault is less certain. It may extend beyond the Ardrossan Harbour – Glencallum Bay Fault to reach the seaward extension of the Dusk Water Fault, off Ardrossan, but there is uncertainty about its precise path, and the degree to which its movements may have been masked by later ones on the boundary faults of the North-East Arran Trough (Sections 2.b.1 & 2.b.2; Fig. 8).

Patterson (Reference Patterson1952) first claimed the Great Cumbrae Fault to be a ruck, and contrary to the implication of McLean & Deegan's (Reference McLean, Deegan, McLean and Deegan1978, p. 108) statement, Caldwell (Reference Caldwell and Bluck1973, p. 143) did not concur. Patterson may have been unduly influenced by the presence of rucks among the Largs–Hunterston faults (Patterson Reference Patterson1949, figs 1, 3; Reference Patterson1952, fold-out map) and by a patch of steeply dipping Inverclyde rocks on the mainland shore at Auchengarth, 1.5 km S of Skelmorlie Castle (Caldwell & Young Reference Caldwell and Young2012, fig. 12), which he attributed to northerly extension of the Great Cumbrae Fault. This interpretation, however, does not accord with the submarine geophysical data of McLean & Deegan (Reference Deegan, McLean and Deegan1978, fig. 8.1), which point to the extended Great Cumbrae Fault continuing northwards into Loch Long (Fig. 1, NE inset). The steep westerly dips at Auchengarth are due to the Skelmorlie Ruck, which veers eastwards from Auchengarth across the Skelmorlie Water and Kelly Burn to Loch Thom and Greenock – a stretch in which it is severely fragmented by other faults (British Geological Survey, 1990).

Patterson's (Reference Patterson1952, pp. 430–1) evidence for the Great Cumbrae Fault being associated with a narrow, sharp, anticlinal or monoclinal fold is unconvincing. A few anomalous strikes and steep dips are developed locally in the Kelly Burn sandstones close to the fault on the E side of Inner Circle Road (Fig. 4), but as these dips are scattered and variable in direction, they are most readily explained as drag effects. In Kames Bay and White Bay, where the Great Cumbrae Fault runs out to sea, well-exposed sandstones show no signs of abnormal deformation.

The re-naming by the British Geological Survey (2008) of the Great Cumbrae Fault as the ‘Kames Bay Fault’ is ill-advised and unnecessary. The former name has precedence (Gunn, Reference Gunn, Gunn, Geikie, Peach and Harker1903, p.128), and other localities in the Firth of Clyde bear the name ‘Kames Bay’.

2.c.2. Defining the Cumbraes Trough

The North-East Arran Trough (Section 2.b.2 above) provides an analogue for what Young & Caldwell (Reference Young and Caldwell2012) identified as the Cumbraes Trough, which occupies most of the uppermost Firth (Fig. 8). Defined by the Great Cumbrae Fault to the E and, to the W, by the Kerrycroy Fault (named herein) and perhaps other major faults within the shipping channel of the upper Firth, the N–S Cumbraes Trough is taken to extend from the Ardrossan Harbour Fault to the HBF and to incorporate a number of small, subordinate basins. The Cumbrae islands, at the trough's southern end, display the most prominent of these with an anomalously thick succession of sedimentary and volcanic rocks (Fig. 2).

Uncertainty attaches to the submarine geology of the Cumbraes Trough (compare, for example, McLean & Deegan, Reference Deegan, McLean and Deegan1978, fig. 8.1 and British Geological Survey, 1985), particularly regarding the western boundary. Deegan (Reference Deegan, McLean and Deegan1978, p. 63) confirmed the presence of two main channels within the upper Firth (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 1.1). The eastern one coincides with northward prolongation of the Great Cumbrae Fault, but the western one – orientated c. N10°W, parallel to the E coast of mid-Bute, and lying a little more than 2 km offshore – is apparently without structural foundation, except for a short stretch S of Ascog in Bute, where a residual gravity anomaly points to the presence of a fracture now identified as the Kerrycroy Fault (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1). Surficial deposits on the trough floor are thicker in both channels. A plot of total magnetic intensity exposes two, parallel, linear series of elongated anomalies, c. 2 km apart, aligned with the Bute coast (McLean & Walker, Reference McLean, Walker, McLean and Deegan1978, pp. 39–41, fig. 3.1). The eastern line can be traced from just S of Toward Point to midway between Kerrytonlia Point, on the northern side of Kilchattan Bay, south Bute, and Fintray Bay, Great Cumbrae (Figs 4, 8). There it ends abruptly against another anomaly of ENE orientation believed to mark the trace of the Kilchattan Bay Fault on route across the main shipping channel towards northern Great Cumbrae (Section 2.a.1 above). The western line can be traced from Toward Quay to the southern promontory of Bute where, about 1 km SE of Glencallum Bay, it meets the Ardrossan Harbour Fault. From this point, it merges with the magnetic anomaly located some 800 m off the S coast of the south Bute promontory, possibly created by faulted repetition of the igneous rocks that dominate that coast (Section 2.a.1 above).

McLean & Walker (Reference McLean, Walker, McLean and Deegan1978, p. 39) interpreted the linear magnetic anomalies along the E coast of mid-Bute as expressions of large dykes in pre-existing faults. The N–S orientation of these dykes is consistent with that of many major Palaeogene dykes S of the HBF in mid-Bute (Geological Survey of Great Britain, Scotland, 1948; British Geological Survey, 1985, 2008), southern Cowal (Anderson, Reference Anderson1951, fig. 15), and eastern Arran (Tyrrell, Reference Tyrrell1928, pp. 241–7, fig. 34; Richey, Reference Richey1939, p. 420, fig. 6; British Geological Survey, 1987). The western margin of the Cumbraes Trough is regarded, therefore, as defined by a largely dyke-filled Kerrycroy Fault (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1), a fracture analogous to the Brodick Bay–Laggan Fault in the North-East Arran Trough on the western side of south Bute (Figs 1, 8).

Thus, although the boundary faults of the North-East Arran Trough and the Cumbraes Trough belong to differently orientated fault groups, they behaved similarly, their offshore sides undergoing relatively high rates of subsidence as Carboniferous sediments were deposited. To a considerable extent, the troughs complement and supplement one another in terms of their post-Devonian, Palaeozoic sedimentary records. The Cumbraes Trough apparently lacks rocks younger than the Early Carboniferous (Viséan) lavas of the Clyde Plateau Formation, whereas the North-East Arran Trough includes c. 1 km of Carboniferous sedimentary rocks above the top of these lavas, together with a further c. 0.5 km of New Red (mostly Permian) Sandstone. Both troughs display a marked reduction in thickness of the Lower Carboniferous sedimentary rocks below the Clyde Plateau lavas in their marginal zones and across their boundary faults (see Hall, Reference Hall, McLean and Deegan1978, p. 48; McLean & Wren, Reference McLean, Wren, McLean and Deegan1978, p. 19; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, p. 108, fig. 8.1; and Young & Caldwell, Reference Young and Caldwell2012 for details of the Brodick Bay – Laggan Fault). At the S end of the Cumbraes Trough, the Cumbraes Basin carries perhaps the thickest Inverclyde section of the upper Firth (Fig. 2).

Although not strictly part of the Cumbraes Trough, the most westerly member of the N-trending fault group in the upper Firth is that extending from the southern reaches of Loch Striven to define the E coast of Bute between Kames Bay and Rothesay Bay and entering mid-Bute at Rothesay. This fault, herein designated the Rothesay Fault, effects sinistral displacement of the HBF on the S side of Rothesay Bay, recalling similar movements on the Great Cumbrae Fault just S of the entrance to Loch Long, some 12 km to the NE (Figs 1, 8).

2.c.3. Origin and age span

The origin of the Great Cumbrae Fault and associated fractures in the Largs system is less than certain, but the available evidence points to them having been initiated in Early Devonian times or even earlier as splays off the HBF system, then reactivated and further multiplied. Monro (Reference Monro1999, p. 109) suggested that association of the Largs–Hunterston faults with the kind of detritus found in the wedge of Portencross Formation at the S end of the Hunterston promontory may indicate the faults’ existence by Early Devonian time. To this may now be added four other disparate lines of direct and circumstantial evidence for Late Devonian and subsequent reactivations that include kilometre-scale displacements of Carboniferous and Palaeogene rocks by the Great Cumbrae Fault.

(1) Relation to the HBF. Two possible connections to the HBF system have already been advanced. Paterson, Hall & Stephenson (Reference Stephenson, Paterson, Hall and Stephenson1990, pp. 36–7, fig. 1; see also Anderson, Reference Anderson1951, pp. 104–5; Patterson, Reference Patterson1952, p. 435) drew attention to a series of faults, trending close to the orientation of the Great Cumbrae Fault, that combine sinistral strike-slip and vertical displacements and may be related to the HBF system. These faults displace the axial zone of the Aberfoyle Anticline (leading edge of the Tay Nappe) in the Dalradian rocks immediately N of the highland line, where the axial zone strikes across the Roseneath peninsula and Loch Long towards Cowal and Bute (British Geological Survey, 1990). One such fault – the Rothesay Fault – has already been discussed (Section 2.c.2 above; Figs 1, 8).

Another link of the Great Cumbrae – Largs system to the HBF system is suggested by the manner in which some Largs–Hunterston faults veer increasingly northeastwards to run subparallel to, or ultimately merge with, components of the HBF (Patterson, Reference Patterson1952, fold-out map; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1; British Geological Survey, 1985; Fig. 8). Paterson, Hall & Stephenson (Reference Paterson, Hall and Stephenson1990, pp. 36–7, figs 1, 8) and Monro (Reference Monro1999, p. 109) saw them as ‘splays’ off the latter – an interpretation still further bolstered by the manner in which the N-trending faults continue through the Spango Gap to Greenock and thence across the Clyde estuary into the HBF zone SW of Loch Lomond (British Geological Survey, 1990; Fig. 8).

(2) Reactivation history. The reactivation history of the Great Cumbrae Fault – minimally Early Carboniferous to Early Palaeogene (Caldwell & Young, Reference Caldwell and Young2012) but more probably pre-Carboniferous to Early Palaeogene – shows that it and other members of its group are old established structures. This particular fault also provides the clearest record of strike-slip displacement and thus provides a standard against which to assess the movements of other faults in the group. The Great Cumbrae Fault underwent strike-slip displacement after the Craignan Fitheach Fault was initiated and after the Early Carboniferous (Viséan) dyke swarm was emplaced in later Holkerian time (Fig. 4). The latter fault, one of the lesser NE–SW faults in the upper Firth, probably originated shortly after the terminal phase of Caledonian earth movements. Thus, although individual members of the Clyde Plateau dyke swarm cannot be reliably matched across the Great Cumbrae Fault, displacement of the Great Cumbrae Trachyte Belt can be shown essentially to match that of the Craignan Fitheach Fault, and restoration to their original positions brings about a more-focused alignment of all major ‘trachytic’ intrusions with their inferred magmatic source: the Misty Law Trachytic Centre of the Renfrewshire Hills (Caldwell & Young, Reference Caldwell and Young2012, figs 3, 12).

The Great Cumbrae Fault moved again after the quartz-dolerite dykes were intruded during the Silesian (Westphalian D) ‘tholeiitic event’ (Caldwell & Young, Reference Caldwell and Young2012, tables 1, 5; Fig. 4), possibly as a late adjustment to the extensional forces that led to emplacement of that dyke swarm. The final, recorded, major transcurrent movement on the fault was in mid- to later Palaeogene or post-Palaeogene times, following emplacement of the dykes of feldspar-macrophyric hyaloandesite that Tyrrell (Reference Tyrrell1917, Reference Tyrrell1918) called ‘cumbraite’, when the fault's movement may have been a far-field response to rifting, crustal stretching and dyke emplacement in the Hebridean Igneous Province of the North Atlantic Superprovince (Emeleus & Gyopari, Reference Emeleus and Gyopari1992; Saunders et al. Reference Saunders, Fitton, Kerr, Norry, Kent, Mahoney and Coffin1997; Bell & Williamson, Reference Bell, Williamson and Trewin2002; Emeleus & Bell, Reference Emeleus and Bell2005; Macdonald & Fettes, Reference Macdonald and Fettes2007). These conspicuous displacements, manifest on a kilometre scale, may still record only part of the fault's history: they do not preclude movements of the same or lesser magnitudes that failed to imprint themselves on the surviving geological record.

(3) Marginal growth faults of the Cumbraes Basin. The Great Cumbrae Fault delineates the eastern edge of the small sedimentary basin that is a distinctive component of the larger Cumbraes Trough (Fig. 8). This basin contains an unusually thick succession of Dinantian rocks belonging mainly to the Tournaisian Inverclyde Group (Fig. 2). The succession is not found E of the fault and is not comparably developed elsewhere in the upper Firth, which suggests that the fault served as a growth margin to the basin. Other faults in the northerly trending group, including those beneath Fairlie Roads and the Largs Channel, may have shared this role, building a stepped eastern margin from the basin axis to the Largs–Hunterston shoulder (Fig. 1). Other pointers to the character of the eastern margin derive from local details but, unfortunately, tend to be moot. For example, if the Great Cumbrae Fault were the key or only controlling structure, lithofacies changes might be anticipated in the Clyde Sandstone Formation, which is exposed near the fault plane on the western side of Kames Bay (Fig. 4). No particular changes are evident. Elsewhere on Great Cumbrae, however, the Clyde Sandstone Formation contains olistostromal pockets which could well have been generated by tectonic movement on a local fault, in keeping with the interpretation of some syndepositional structures in sandstones of the Stratheden Group of the mainland (see below).

(4) Related faults and facies control. Other N-trending faults exerted some control on lithofacies and spatial distribution in the largely Famennian sandstones of the Stratheden Group in the coastal belt of the adjacent mainland. Differences across the Largs–Hunterston faults between Largs and West Kilbride allowed Bluck (Reference Bluck, Reading and Balance1980) and Monro (Reference Monro1999) to differentiate the successions nominally on the eastern and western sides of these structures. In addition, Monro (Reference Monro1999, p. 16) found the Kelly Burn Sandstone on Great Cumbrae to be ‘very much thicker’ than that at Hunterston on the mainland, and McLean & Walker (Reference McLean, Walker, McLean and Deegan1978, p. 37) noted that it increased in thickness by about one-third between the W side of the Hunterston promontory and the E side of Great Cumbrae. Monro also noted that, adjacent to the Largs–Hunterston faults, the Kelly Burn Sandstone in its Seamill facies carries syndepositional slump structures recording local tectonic instability.

The Great Cumbrae Fault and associated members of the Largs–Hunterston system, are interpreted, therefore, as ‘old’ structures dating at minimum to the earliest expressions of Variscan deformation that followed termination of the Caledonian Orogeny in the Late Devonian Epoch, about 370 million years ago, and probably to considerably earlier than that (Paterson, Hall & Stephenson, Reference Paterson, Hall and Stephenson1990, p. 37; Monro, Reference Monro1999, p. 109; Macdonald & Fettes, Reference Macdonald and Fettes2007, p. 260).

3.b.1. The North Ayrshire and Inverclyde mainland

Two discrete sets of folds involving Upper Devonian and Carboniferous rocks have been reported from the coastal mainland of the upper Firth.

First, Paterson, Hall & Stephenson (Reference Stephenson, Paterson, Hall and Stephenson1990, pp. 36–8, fig. 1) identified the Leap Moor Syncline, Loch Thom Anticline and adjacent folds in the Inverclyde–North Ayrshire district, NE of Skelmorlie – open folds that, in axial orientation and scale, are like the folds of the Cumbraes and similarly involve rocks of the Stratheden and Inverclyde groups (Fig. 8). As these workers have pointed out, formation of the folds may have been influenced by certain NE- and N-trending faults of the HBF and Largs–Hunterston systems.

Second, Monro (Reference Monro1999, p. 111, fig. 29) distinguished a group of open folds associated with the major caledonoid Dusk Water Fault, NE of Ardrossan (Fig. 8). Slightly smaller in scale than the Cumbrae and Skelmorlie folds, they also show more variation in axial orientation, some trending N30°E. A small, plunging fold couplet at Ardrossan has a northeasterly inclination.

3.b.2. The Cumbraes Trough

Apart from the Cumbraes Basin, there may be other small sedimentary basins within the Cumbraes Trough. One such structure may lie between the northern end of Great Cumbrae and Toward Point, where geophysical surveys revealed a patch of layered Carboniferous rocks resting on Upper Old Red Sandstone and cut by a NNW-trending dyke presumably of Palaeogene age (McLean & Walker, Reference McLean, Walker, McLean and Deegan1978, pp. 35–6, fig. 3.3; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1). Defined by a gravity anomaly, it is probably an erosional outlier of Inverclyde sedimentary rocks, possibly with a cap of Clyde Plateau lavas, but without additional data or samples, the stratigraphical assignment remains speculative. This outlier is not shown on the British Geological Survey's (1985) map of the Firth of Clyde.

The Old Red Sandstone and Carboniferous rocks of the Toward Point outlier at the S end of the Cowal Peninsula (Stark, Reference Stark1927) expose some of the most northerly folds in the Cumbraes Trough (Fig. 10). Within the outlier, the principal faults trend NE in conformity with the HBF, which defines the outlier's northern boundary, and a thin sequence of the Inverclyde Group is most completely preserved between two of them.

Similar offshore components of the HBF system or extensions of the faults that define the Ascog Block in mid-Bute may also have contributed to preserving Carboniferous rocks within the Cumbraes Trough as far N as Toward Point (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, fig. 8.1; Paterson, Hall & Stephenson, Reference Paterson, Hall and Stephenson1990, fig. 8; Fig. 8). Several small, open, complementary, southward-plunging, anticlinal and synclinal warps project offshore within an E–W distance of c. 2 km. The folds are much smaller than the Leap Moor and Loch Thom folds on the opposite side of the upper Firth or the primary folds of the Cumbraes, and their lateral extent beyond Toward Point is unknown. They may be locally developed and attributable to compression generated by transcurrent movement on the HBF.

3.b.3. Bute

A member of both the N–S and the E–W fold-sets is present on the E coast of the Isle of Bute, between Rothesay Bay and Kilchattan Bay, although neither is exposed to its full extent (Figs 7, 8).

Figure 7.

Geological map of the coast of northeastern mid-Bute, from Rothesay Bay to Ascog Bay, to show the truncated, plunging nose of the Craigmore Anticline around Bogany Point and the complexity of the coastal geology of the Ascog Block between the Craigmore and Ascog faults. Note that this is one of a few localities where argillaceous beds – the Birgidale Formation – are exposed beneath the Clyde Plateau lavas (cf. Fig. 2).

Figure 8.

Sketch map depicting the principal geological structures in the upper Firth of Clyde described in the paper. Abbreviations: Faults: AF – Ascog Fault; AHF – Ardrossan Harbour Fault (extended as GBF); BBF – Brodick Bay Fault (extended as Laggan Fault); CF – Corloch Fault (extension of HBF); CRF – Craigmore Fault; DWF – Dusk Water Fault; GBF – Glencallum Bay Fault (extension of AHF); GCF – Great Cumbrae Fault; HBF – Highland Boundary Fault (represented by a system of fractures on the N side of the upper Firth of Clyde. For details, see Qureshi, Reference Qureshi1970; McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978; British Geological Survey, 1990); KBF – Kilchattan Bay Fault; KF – Kerrycroy Fault; LF – Laggan Fault (extension of BBF); LCF – Little Cumbrae Fault; LFS – Largs Fault System (Largs–Hunterston Fault system of some pre-existing literature); MF – Muirshiel Fault; RF – Rothesay Fault; SBF – Sound of Bute Fault; ScBF – Scalpsie Bay Fault; SoBF – South Bute Fault. Other small faults are depicted in detailed maps of individual localities (see Figs 3, 4, 5, 7 & 10). Folds: BHA – Boat House Anticline; CA – Craigmore Anticline; EA – Elbow Anticline (Little Cumbrae); GCA – Great Cumbrae Anticline; GCS – Great Cumbrae Syncline; HNS – Hawk's Neb Syncline; LMS – Leap Moor Syncline; LTA – Loch Thom Anticline; MS – Montford Syncline. Complementary folds on the N side of the Dusk Water Fault and at Toward Point are neither all shown nor individually named (for details, see British Geological Survey, 2006; Fig. 10).

First, the Craigmore Anticline (Hill & Buist, Reference Hill and Buist1994, pp. 46–50, fig. 8; Reference Hill and Buist2006, pp. 44–8, fig. 10), with axial trace close to N20°E, is centred on Bogany Point and Canada Hill, 1.5 km E of Rothesay. The fold is truncated by northeasterly trending faults of the Ascog Block. Second, in the structurally complex shoreline exposures of the Ascog Block, Young & Caldwell (Reference Young and Caldwell2011c , pp. 151–2) described a small, partly fault-bound, E-plunging syncline containing rocks belonging to the upper part of the Clyde Sandstone (Ascog Member) and possibly the basal part of the Birgidale Formation, the latter overlying the Clyde Sandstone and known elsewhere to conformably underlie the Clyde Plateau Formation. The scale of the fold is unknown because of faulting and limited exposure (see Young & Caldwell, Reference Young and Caldwell2011c ).

Although the geology and geomorphology of south Bute are so thoroughly dominated by faults of the NE- and NW-trending groups (Sections 2.a.1 and 2.b.1 above; Fig. 5), remapping has revealed the presence of two open folds with gentle westerly plunge – the Boat House Anticline and the Hawk's Neb Syncline – referred rather arbitrarily to the E–W fold-set on the basis of their northeasterly axial traces (Figs 5, 8). The Hawk's Neb Syncline includes rocks as young as Clyde Plateau volcanic products correlative with the Misty Law Trachytic Member of the Renfrewshire Hills Block (Caldwell & Young, Reference Caldwell and Young2011, fig. 10; Young & Caldwell, Reference Young and Caldwell2011b , pp. 599–606, figs 2, 7).

3.b.4. Arran and beyond

In the first detailed description of the Carboniferous rocks of the Isle of Arran, Gunn (Reference Gunn, Gunn, Geikie, Peach and Harker1903, pp. 37–52) went to considerable lengths to contrast the section on the NE coast at Corrie and that further N between the Fallen Rocks at Corloch and Laggan (see British Geological Survey, 1987 and Fig. 8 for locations). In addition to marked differences in the stratigraphy, he pointed to structural differences: the Corloch–Laggan section, which dips to the NW, more disrupted by faults than the thinner section at Corrie, which is inclined to the SE and apparently more structurally intact. The sections seemed to lie on opposing limbs of a large, northeastward-plunging anticline. Gunn, however, may not have appreciated the contrasted facies of the two sections, the full effect of the small faults or the distortion brought about by subsequent emplacement of the North Arran Granite Pluton (Woodcock & Underhill, Reference Woodcock and Underhill1987; England, Reference England1992). Young & Caldwell (Reference Young and Caldwell2012) showed also that structural comparison of these sections must take into account their lying on opposite sides of the NW-trending Brodick Bay–Laggan Fault: the key line of structural separation between the syndepositional North-East Arran Trough (McLean & Deegan, Reference McLean, Deegan, McLean and Deegan1978, pp.108–9), in which the much thicker Corloch–Laggan sediments accumulated, and the flanking Corrie shoulder (Young & Caldwell, Reference Young and Caldwell2012, figs 1–5).

In the Kintyre Peninsula of the outer Firth of Clyde, Devonian and Carboniferous rocks are downthrown to the S against Dalradian metamorphic rocks by the NW-trending Kilchenzie Fault and are exposed in sizeable tracts of the Laggan of Kintyre from Machrihanish to Campbeltown (McCallien, Reference McCallien1927; McCallien & Anderson, Reference McCallien and Anderson1930; British Geological Survey, 1996b ). Some of the largest E–W folds in the entire Firth are developed between the Laggan and the southern extremity of the peninsula, near Southend (Fig. 11). The extent to which these folds and those of the upper Firth should be compared in detail, however, is questionable. They are widely separated geographically, fall into discrete geological terranes (Grampian and Midland Valley, respectively) and contain sedimentary and volcanic successions that in a number of respects are different from those of the upper Firth and closer to those around Ballycastle in Antrim (McCallien, Reference McCallien1927; McCallien & Anderson, Reference McCallien and Anderson1930; Wilson & Robbie, Reference Wilson and Robbie1966; Smedley, Reference Smedley1986, Reference Smedley1988). Southern Kintyre may have been subject to a discrete, stronger pulse of northward compression, producing not only the E–W folds but localized, small-scale thrusting of the Lower Carboniferous lavas (Clyde Plateau Formation equivalents), as at the Galdrings, on the W side of the peninsula, S of Machrihanish (McCallien, Reference McCallien1927, p. 96).

3.c.1. The Great Cumbrae Syncline and other N–S folds: site implications

Examination of the N–S folds reveals that deformation occurred in discrete phases, none of which can be dated with much precision, and more than one deformational mechanism may have been involved.

The youngest preserved lava flow of the Lower Strathgryfe Member of the Clyde Plateau Formation on Little Cumbrae establishes a maximum, relative age for the N–S folds of the Cumbraes, and Monaghan & Parrish (Reference Monaghan and Parrish2006) assigned that member a Dinantian (Viséan, middle to late Holkerian) relative age and a radiometric age of 335 ± 2 Ma in terms of the B time-scale of Menning et al. (Reference Menning, Weyer, Drozdzewski, Vanameron and Wendt2000).

Most similar in orientation and scale to the Cumbrae folds, the Leap Moor–Loch Thom folds, near Skelmorlie (Fig. 8), were related by Paterson, Hall & Stephenson (Reference Stephenson, Paterson, Hall and Stephenson1990, pp. 36–8) to Chadian–Holkerian uplift that preceded outpouring of the Clyde Plateau lavas and having their crests overstepped by lavas of the Upper Strathgryfe Member. The Cumbrae folds must be younger, therefore, because the Lower Strathgryfe Lavas, which they incorporate, lie well above the base of the Clyde Plateau Formation (Stephenson, Reference Stephenson, Paterson, Hall and Stephenson1990, pp. 19–26; Caldwell & Young, Reference Caldwell and Young2011; Section 3.a above). The Leap Moor–Loch Thom folds may then be middle Chadian – early Holkerian in age, the Cumbrae folds no older than middle to late Holkerian and possibly much younger.

It is also unlikely that the other folds of the North Ayrshire coastal tract (Monro, Reference Monro1999) have a common origin with those of the Cumbraes. The mainland folds are associated with the Dusk Water Fault, which underwent repeated reactivation in Carboniferous times (Section 2.a and 2.a.2 above), and they incorporate rocks as young as late Namurian and early Westphalian in the adjoining Dalry Basin (Monro, Reference Monro1985). Attributing the folds to transcurrent (or ‘reversed’) movement on the Dusk Water Fault Monro (Reference Monro1999, p. 111, fig. 29) finds support in a comparable parallelism of local fold-axes to the faults extending the Paisley Ruck immediately to the NE of the Dusk Water Fault (British Geological Survey, 2006). Monro's explanation, which implies a Late Carboniferous (at oldest) deformation, cannot be applied to the Cumbrae folds. The two groups of folds are separated by the southern end of the Great Cumbrae–Largs fault system and, despite being much further removed from the Dusk Water Fault, the Cumbrae folds are considerably more prominent (Fig. 8).

The ages of folds at localities N and W of the Cumbraes are also difficult to assess. At Toward Point (Section 3.b.2 above; Fig. 10), rocks as young as the lower Clyde Sandstone (Chadian) are deformed, and still younger rocks could be incorporated offshore. The character of the Toward structures is distinctive and, like Monro's (Reference Monro1999) folds adjacent to the Dusk Water Fault, they could have been generated by strike-slip on a major NE-trending fault – in the case of Toward, the Roseneath branch of the HBF system.

Finally, the Craigmore Anticline at Bogany Point in Bute (Section 3.b.3 above; Figs 7, 8), can be said only to post-date the Upper Old Red Sandstone Bute Conglomerate (Paterson & Hall, Reference Paterson and Hall1986, p. 9, figs 1, 3; Browne, Smith & Aitken, Reference Browne, Smith and Aitken2002, p. 47, fig. 8).

Thus, with the exception of the folds on the N side of the Dusk Water Fault, the N–S fold-set of the upper Firth involves only rocks of Late Devonian and Early Carboniferous (Dinantian) age, with the Lower Strathgryfe Lavas of the Clyde Plateau Formation (Viséan, Holkerian) the youngest involved. The N–S folds of the Cumbraes and other localities in the upper Firth are cut by E–W quartz-dolerite dykes of Westphalian D age (Monaghan & Parrish, Reference Monaghan and Parrish2006), which fail to show any effects of the folding. This suggests that the deformation is no younger than Carboniferous, although a cross-cutting relationship of this kind must be considered a somewhat equivocal guide to relative chronology insofar as, had the dykes been intruded prior to the folding, their attitude may have precluded their being noticeably deformed.

3.c.2. The Little Cumbrae Anticline and other E–W folds: site implications

The age of the E–W fold-set is also difficult to constrain, but these folds post-date the more prominent N–S structures. Arching of the western limb of the main syncline on Little Cumbrae involves the entire volcanic succession there (Figs 3, 9), which is probably Viséan (middle to late Holkerian) in age (Sections 3.a and 3.c.1 above). The same episode of E–W folding may well have been responsible for the small folds in the mid-Courceyan Foul Port Member of the Kinneswood Formation near Portachur Point on Great Cumbrae (Figs 2, 4).

Figure 9.

Lava flows belonging to the Lower Strathgryfe Member of the Clyde Plateau Formation at Cumbrae Elbow, Little Cumbrae, to show the anticlinal cross-fold superimposed on the western limb of the syncline that provides the principal structural control on the island (see Fig. 8 for location: view looking E from south Bute).

The Boat House Anticline and the Hawk's Neb Syncline of south Bute (Section 3.b.3 above; Fig. 5) call for a somewhat younger maximum age because they involve trachytic lavas believed to be correlative with the Misty Law Trachytic Member of the Clyde Plateau Formation in the Renfrewshire Hills, which separates the Lower and Upper Strathgryfe members (Stephenson, Reference Stephenson, Paterson, Hall and Stephenson1990, pp. 23–5). The folding in south Bute may post-date the remaining volcanic rocks there held to correlate to part of the Upper Strathgryfe lavas of the Renfrewshire Hills, but this cannot be demonstrated unequivocally in the field (Caldwell & Young, Reference Caldwell and Young2011, fig. 10; Young & Caldwell, Reference Young and Caldwell2011b , figs 2, 7).

In Kintyre, the Mull Anticline and Southend Syncline deform Old Red Sandstone and Carboniferous rocks including lavas similar to those of the Clyde Plateau Formation (Fig. 11). But differences between these lavas and those of the mainland plateau preclude stratigraphical correlation to the Renfrewshire Hills standard succession, and it is not known whether the deformed lavas of the Southend Syncline are younger or older than those of Little Cumbrae and south Bute.

Figure 10.

Geological map of the Toward Point shore section on the Cowal Peninsula to expose the stratigraphical succession and the unusual character of the N–S folds.

Figure 11.

Geological sketch map of the southern portion of the Kintyre Peninsula to illustrate the prominent E–W folds in the Upper Palaeozoic rocks between the Laggan and the S coast.

In summary, none of the E–W folds is known to be older than Viséan (middle to late Holkerian). They could be much younger, their age-range having no known upper limit, and they could be products of one or more compressional episodes. The simplest interpretation based on field relationships would attribute them to a single deformational episode probably no older than middle Namurian (Arnsbergian).