Geology of the Aberfoyle district: Neoproterozoic to Cambrian: Dalradian supergroup
|This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
The Dalradian Supergroup is a succession of sedimentary and basic volcanic rocks which underlies most of the Grampian Highlands of Scotland. The rocks were deposited on a continental margin during the Neoproterozoic and early Cambrian, sometime between about 800 Ma and 500 Ma ago (Harris et al., 1994; Stephenson and Gould, 1995). To date, there are no precise limits on the maximum and minimum ages of the Dalradian, but recent work on the timing of orogenisis in Moine Supergroup metasedimentary rocks of the Northern Highlands of Scotland (Strachan et al., 2004), and Sr isotope chemistry of Dalradian carbonate rocks (Thomas et al., 2004), suggests that deposition of the Dalradian began not much before about 700 Ma. Following deposition, the rocks were deformed and metamorphosed by Caledonian orogenic events that occurred mainly in the Ordovician, peaking between about 470 and 465 Ma (Friedrich et al., 1999; Soper et al., 1999).
The Dalradian is divided up into four groups, largely on the basis of lithological characteristics and associations. These are, in ascending stratigraphical order: Grampian Group, Appin Group, Argyll Group and Southern Highland Group. Only the uppermost two formations of the Argyll Group, and overlying Southern Highland Group occur in the Aberfoyle district (Figure 3 and Figure 4), and of these the Southern Highland Group occupies more than 90 per cent of the Dalradian outcrop.
Although the Grampian, Appin and Argyll Groups are divided up into Subgroups and Formations (e.g. Stephenson and Gould, 1995), the Southern Highland Group has traditionally been left undivided because of rapid vertical and lateral facies changes and the lack of regionally extensive stratigraphical marker horizons. However, by combining the lithostratigraphical data with structural observations, it has proved possible to subdivide the Southern Highland Group within the Aberfoyle district into several formations and to elucidate the relationships between them (Figure 3 and Figure 4).
The Argyll and Southern Highland groups consist of coarse clastic rocks, mudrocks and subordinate limestones, together with locally important mafic volcanic rocks, including pillow lavas. These sediments reflect an initially active rifting environment in which deposition occurred largely by turbidity currents and sediment-charged flows (e.g. Burt, 2002). Sedimentation occurred on the subsiding, passive continental margin of Laurentia as it responded to the opening of the Iapetus Ocean. The different lithostratigraphical units reflect different stages in the rifting history.
Early rifting of the initially shallow submarine slope is reflected in the mafic volcanism associated with the Loch Tay Limestone, but the sediment comprised carbonate and mature, quartz-rich clastic detritus (Ardnandave Sandstone). Deposition of the Aberfoyle Slate Formation is interpreted by Burt to suggest very rapid rifting in the region that includes the Aberfoyle district, outstripping sedimentation. Continued rifting, margin break-up and uplift in the hinterland resulted in the supply of immature sediment, including volcaniclastic detritus, via point sources along the margin. This led to the development of complex sedimentary architectures controlled by local fault scarps and basins (Loch Katrine Volcaniclastic, Creah Innich Sandstone and Ben Ledi Grit formations). As tectonic activity declined, a true, Atlantic-like passive margin sequence developed (Keltie Water Grit, Loch Ard Grit and Bofrishlie Slate formations). This tectonic regime persisted until the onset of tectonic events that resulted in the closure of the Iapetus Ocean.
Given that the Dalradian sedimentary rocks in the Aberfoyle district are just a few kilometres thick in total, that most of them must represent very rapid depositional processes, and that they were deposited over at least 80 Ma, they represent a very punctuated record of the geological history through that time. Thus, unconformities and or disconformities must exist within the sequence. Some of these are manifest as erosional bases to units like the Creag Innich Sandstone Formation. Others, such as break-up unconformities (Burt, 2002, and references therein) will simply not be discernible because of the complexity of the sedimentation and the lack, presently, of suitable direct dating techniques.
In the following account, much of the interpretation of the Sedimentary facies of the Dalradian is based on Burt (2002).
Lithostratigraphy of the Argyll and Southern Highland Groups
The lithostratigraphy of the Argyll and Southern Highland Groups within the Aberfoyle district is summarised in (Figure 3). Of significance with regard to this account is the explicit inclusion of the Keltie Water Grit Formation and the Bofrishlie Slate Formation within the Dalradian. The former is important because it contains the Cambrian Leny Limestone, dated at 515–520 Ma on the basis of its trilobite fauna. The assignment of the limestone and its host siliciclastic rocks to the Dalradian or the Ordovician Highland Border Complex (see Highland Border Complex) has long been debated, but there is now a general consensus that the Keltie Water Grit Formation is part of the Dalradian (e.g. Tanner, 1995).
The Bofrishlie Slate Formation is exposed in Loch Ard Forest, south-west of Aberfoyle. Traditionally, this formation has been included in the Highland Border Complex. However, previous studies (Harris, 1972; Henderson and Fortey, 1982) and recent work by Dr P W G Tanner (Glasgow University), indicates that the Bofrishlie Slate Formation should be considered as a part of the Dalradian Supergroup. The formation is lithologically similar to the dominant black pelites of the Leny Limestone and Slate Member (Keltie Water Grit Formation), which is now firmly regarded as Dalradian (Tanner, 1995). Various constraints on the age of the Bofrishlie Slate Formation are discussed further below, but the prevailing geological evidence favours its inclusion in the Dalradian.
Recently, Tanner and Sutherland (2007) proposed that the Keltie Water Grits, and related and succeeding units (including the Bofrishlie Slate Formation) along the Highland Border, be placed in a newly designated Trossachs Group, succeeding the Southern Highland Group. Although there is merit in this approach, the need for consistency with the published map means that it is not adapted herein.
Whilst a complete Southern Highland Group lithostratigraphy is observed to the west of the Loch Tay Fault in the Aberfoyle district, only the Ben Ledi Grit and Keltie Water Grit formations occur to the east of the fault (Figure 4).
Only uppermost Argyll Group rocks crop out within the Aberfoyle district. These are assigned to two formations: the Ben Lui Schist and the Loch Tay Limestone (Figure 3). Because of the regional structural inversion of strata in the Flat Belt (see Structural history), these formations occupy high ground in the north-east of the Aberfoyle district, between Ben Vane [NN 535 137], the upper part of Glen Casaig, Stuc Dubh [NN 552 125] and Loch Lubnaig, north of the Ben Ledi–Ardnandave Hill massif (Figure 4). The Ben Lui Schist Formation may also crop out on Meall Mor [NN 582 156] to the north-east of Loch Lubnaig, although the Loch Tay Limestone appears to be absent.
BEN LUI SCHIST FORMATION (SQC)
The Ben Lui Schist Formation comprises a largely monotonous sequence of mid to dark grey and green-grey, thinly bedded, schistose, semipelitic rocks with pelite and micaceous psammite. Subordinate, thickly bedded and sometimes gritty quartzose psammite and quartzite only occur locally, most notably in the Allt Stronyre section [NN 54 13], where these rocks are in normal stratigraphical contact with the Loch Tay Limestone. Some of the gritty units contain sufficient chlorite to impart a greenish colour, but lack the epidotic volcaniclastic mineralogy of the ‘Green Beds’ in the overlying Loch Katrine Volcaniclastic Formation (see below). Although graded bedding is seen in places (showing the rocks to be inverted) and bedding is generally well preserved, sedimentary structures are uncommon. Deformation is significant with D2 and D4 structures dominant (see Structural history).
LOCH TAY LIMESTONE FORMATION (LL T)
The Loch Tay Limestone Formation is a key Dalradian lithostratigraphical marker, defining the top of the Argyll Group. It is about 600 Ma old, based on the relationship between the lithostratigraphically equivalent Tayvallich Limestone and the Tayvallich Lavas, dated at 595 ± 4 Ma (Halliday et al., 1989) and 601 ± 4 Ma (Dempster et al., 2002) in the Tayvallich Peninsula, Argyll.
In the Aberfoyle district, thin beds of Loch Tay Limestone crop out discontinuously between the Ben Lui Schist and Ardnandave Sandstone formations, in a normal, but inverted lithostratigraphical succession. The lenticular outcrop is only well exposed in Allt Stronyre [NN 5419 1387]. Here, the metalimestone is about 12 m thick and consists of pale grey to cream-coloured, crystalline carbonate rock. The thinly banded appearance, due to colour variations, is parallel to abundant interbeds of pelite and psammite. Elsewhere, the location of the Loch Tay Limestone outcrop is indicated by loose, angular ‘float’ blocks which are locally associated with notably richer grassy vegetation. The outcrop of the Loch Tay Limestone is commonly associated with a particularly massive metadolerite sheet that preserves igneous textures.
The Loch Tay Limestone was deposited in a relatively shallow slope-apron environment, above storm wave base. Carbonate sediment precipitated in very shallow near-shore settings was redeposited by turbidity currents to the shelf slope and into basinal lows. The turbidity currents were probably triggered by local extensional tectonic activity and possibly by storm wave disturbance of unstable, unconsolidated sediment. Extensional activity also resulted in volcanism, manifest by contemporaneous intrusion of mafic sheets and the eruption of pillow lavas, as seen in other outcrops of the Loch Tay Limestone and its correlatives elsewhere in Scotland.
Southern Highland Group
Within the Aberfoyle district, the heterolithic assemblage of the Southern Highland Group has been divided into several formations (Figure 3). Elucidation of the lateral and vertical relationships between these formations is made difficult by the complex F1 major folds, and their later modification by D2 and D4 deformation. For example, it is herein proposed the Loch Katrine Volcaniclastic Formation and the Aberfoyle Slate Formation are approximately laterally equivalent. This correlation arises from interpretation of the early fold structures, since the two formations are separated across the regional-scale Ben Ledi Antiform. The original lateral separation of these units was probably of the order of several tens of kilometres, and they may have only limited [vertical] equivalence. Nevertheless, the vertical successions are broadly established by the abundant and widespread younging evidence preserved in the siliciclastic rocks.
ARDNANDAVE SANDSTONE FORMATION (QAR)
This formation marks the base of the Southern Highland Group within the Aberfoyle district. It crops out mainly on the northern and western flanks of Ardnandave Hill [NN 567 125] and on the northern flanks of Ben Vane [NN 535 136] (Figure 4).
The formation consists of white to pale grey and cream-coloured, and commonly gritty, quartz-rich metasandstone, interbedded with abundant semipelite, pelite and epidote-bearing volcaniclastic ‘green bed’ lithologies; the ‘green beds’ herald the incoming of a much more significant development of volcaniclastic sediment in the overlying Loch Katrine Volcaniclastic Formation (see below). Gritty metasandstones exhibit good graded bedding in places, as do finer-grained lithologies which develop micaceous tops to beds with coarse bases. Bed thicknesses generally increase up through the formation. Massive metasandstone units up to 1.5 m thick are recorded locally and micaceous metasandstones are commonly of the order of 30 cm in thickness.
Lenticular mid to dark grey semipelitic and pelitic units, intercalated with thin metasandstone beds, are developed in the upper and lower parts of the formation on the northern flanks of Ben Vane. These more pelitic lithologies are biotite rich and strongly schistose.
Deposition of the formation occurred via turbidity currents of variable density, volume and grain size. The siliciclastic (nonvolcanogenic) sediment is similar to that contained in thin siliciclastic units in the Loch Tay Limestone, and is considered to be relatively mature.
ABERFOYLE SLATE FORMATION (SAB)
The Aberfoyle Slate Formation is one of the key lithostratigraphical units in the district, as it is this unit that is interpreted to lie within the down-turned nose of the synformal Aberfoyle Anticline (see Structural history). The formation occupies a linear tract of ground, bounded to the east by the strike-slip Duke’s Pass Fault (Figure 4). Differences in rock types, lithological succession and structural style on either side of the Duke’s Pass indicate that the Aberfoyle Slate Formation does not extend east of the fault. This interpretation is reinforced by the strike of units in the vicinity of the Duke’s Pass area becoming markedly oblique to the trend of the HBFZ. This obliqueness results from major, dominantly strike slip on the Loch Tay–Duke’s Pass Fault system on the scale of many kilometres. Previous interpretations of Dalradian lithostratigraphy in the Highland border show the ‘Aberfoyle Slates’ (s.l.) extending along the Highlands border, roughly parallel to the Highland Boundary Fault Zone (HBFZ).
The outcrop width of the Aberfoyle Slate Formation, as currently defined, is approximately 1.5 km, but the true thickness of the Aberfoyle Slate itself must be less than half of this because of thickening by folding. Generally poor natural exposure of the Aberfoyle Slate has been compensated for by quarrying, particularly the extensive Aberfoyle Slate Quarries that provide the type locality for the formation and which are situated just west of the Dukes Pass at [NN 505 033] (see Concealed geology).
The slaty rocks exposed in the quarries comprise moderately well-cleaved semipelite and pelite, with locally developed more coarsely grained silty and fine sandy laminae and films. Pyrite is rare, but there are numerous fine quartz veins that are generally parallel to cleavage and that sometimes delimit colour banding (Richey and Anderson, 1944; Walsh, 2000). The lithologies are chiefly bluish grey, grey to grey-green in colour, with black, maroon or purple and green units; some are mottled purple and green. The colour bands appear to be parallel to the cleavage (Walsh, 2000). Bedding is often difficult to observe, but can be picked out by the more quartzose laminae and slight changes in grain size within the coarser material. Only a single penetrative cleavage is observed in the field and in thin section; this dips north to north to north-west at steep angles. Bedding showing both younging and bedding–cleavage relationships are very rarely observed in quarry faces, but where seen together, they show that the cleavage faces downwards (Shackleton, 1957) (see also Structural history).
Petrographically, the Aberfoyle Slate Formation is dominated by chlorite with subordinate quartz. Although the chlorite crystals now present in the formation are now almost entirely metamorphic in origin, Burt (2002) suggested that a small amount of the chlorite retains evidence for being detrital in origin. Some Fe-Ti oxides are present in thin heavy mineral laminae in places and there are rare albite grains. Geochemically, Burt showed that samples of Aberfoyle Slate Formation have compositional characteristics consistent with other Southern Highland Group lithologies, apart from the ‘Green Beds’ (see below).
Although the formation was worked for slate, principally in the Aberfoyle Slate Quarries 3 km north-west of Aberfoyle, the quality of the slate currently exposed is relatively poor because of the coarseness and variability of the cleavage. The coarseness of the grain size exacerbates the problem in folds, where incipient bedding and cleavage interfere to produce blocky material. The quarrymen worked particular seams of higher quality slate (Richey and Anderson, 1944), as discussed further in Concealed geology.
Burt (2002) interpreted the Aberfoyle Slate Formation as a relatively deep water deposit. Generally, deposition occurred in quiet, sediment-starved conditions below storm wave base, with the occasional influx of sandier sediment brought in by turbidity currents; deposition was chiefly from suspension. The Aberfoyle Slate Formation represents a phase of rapid subsidence of the submarine continental slope that outstripped sediment supply (Burt, 2002).
LOCH KATRINE VOLCANICLASTIC FORMATION (SQVLK)
Although the Loch Katrine Volcaniclastic Formation essentially consists of metasandstone, it is characterised by the presence of epidote- and chlorite-rich clastic rocks that have long been interpreted to be volcaniclastic in origin, and which have traditionally been called ‘Green Beds’ (Pickett et al., 2006). These distinctive rocks can be traced throughout much of the Dalradian outcrop of Ireland and Scotland, and are important lithostratigraphical marker horizons within the Southern Highland Group (Harris et al., 1994; Stephenson and Gould, 1995). Within the Aberfoyle district, the volcaniclastic units are interbedded with metasandstones and subsidiary semipelitic rocks with less abundant epidote and chlorite.
The formation crops out extensively in the north of the district (Figure 4). South of the Highland Border Downbend, it is well exposed to the north of Ben Venue [NN 474 065], where it crops out over some 2 km. To the north-east, across Loch Katrine, the formation crops out on the high plateau ground north of Ben An [NN 505 083], and also between Ardnandave Hill [NN 566 125] and Ben Ledi [NN 562 099]. To the north of the Highland Border Downbend, the formation crops out in a complex pattern arising from the combined effects of local stratigraphical variations, shallow dip, multiphase folding and later faulting.
The lower contact of the formation is only exposed in the ground occupied by the Ben Ledi–Ben Vane massif, where it structurally underlies the Ardnandave Sandstone Formation. The formation is stratigraphically overlain either by the Ben Ledi Grit Formation or, in the Ben Venue area, by the coarse, locally pebbly metasandstones of the Creag Innich Sandstone Formation. The contact with the latter is very sharp and probably largely erosive.
The metasandstones comprise medium- to coarse-grained arenites and wackes, with subordinate semipelitic lithologies. Gritty units and clean quartz arenites occur in places, but are rare. The formation as a whole commonly has a greenish grey colour. The units containing the most abundant volcaniclastic material are typically bottle green in colour when fresh, particularly where coarser grained. Such dark units are well exposed on the north shore road of Loch Katrine, east of grid line NN48 and north of grid line NN09. The finer-grained volcaniclastic units typically weather to a greenish or pinkish, sandy buff colour, with a sandpaper-like and commonly pitted surface texture.
Units are generally medium to thick bedded, but bed thicknesses range widely and vary laterally. The coarser volcaniclastic units are usually very thickly bedded and massive, and many probably represent amalgamated beds. Examples of such units crop out over Bealach nam Bo [NN 480 075], north of Ben Venue and Ardnandave Hill [NN 567 425].
Sedimentary structures are common in the formation and are typical of sediments deposited on continental margin systems by turbidity currents, mass flows and bottom currents (Burt, 2002). Graded bedding is widely developed, although fine silty and muddy tops are only rarely preserved, as, for example, in the Strone Burn [NN 453 106]. Slumping, loading and dewatering structures occur locally, as does parallel lamination and planar and herringbone cross-lamination. Semipelitic and pelitic rip-up clasts, up to 0.3 m across, are found in places.
The mineralogy of the Loch Katrine Volcaniclastic Formation reflects the mixing (in variable quantities) of two sedimentary components, one siliciclastic, the other volcanogenic. The volcanogenic component is basaltic in composition. The original mineralogy of the sedimentary rocks was replaced by greenschist facies assemblages during metamorphism in the Caledonian Orogeny (see Metamorphism).
In addition to quartz, feldspar and white mica, detrital minerals include epidote, sphene and ilmenite, together with zircon, monazite and tourmaline, all in variable amounts. Rare lithic clasts include mud clasts and fragments of polycrystalline quartz and quartz-plagioclase rock. The mud clasts are considered to be rip-up clasts from the fine tops of turbidite beds. The polycrystalline quartz and quartz-plagioclase rock suggest a granitic source for these components.
The chlorite responsible for the dark green colour in the ‘Green Beds’ is considered to be entirely recrystallised by metamorphism (Hyslop and Pickett, 1998). However, a significant proportion was probably detrital in origin, derived from the weathering of volcanogenic source rocks. Although there is detrital epidote and sphene, new metamorphic epidote, zoisite and sphene have developed, and tourmaline commonly has metamorphic overgrowths. Actinolite appears in the more mafic lithologies towards the northern margins of the district, where the metamorphic grade increases. Albite is also important locally, forming conspicuous and very abundant, inclusion-rich porphyroblasts, as seen elsewhere at this level in the Dalradian (cf. Watkins, 1983). Calcite is present locally in the matrix. It may be diagenetic in origin or possibly formed, at least in part, by retrogression. Pyrite is the most common opaque mineral. Its generally euhedral form and textural relationships to the dominant S2 fabric indicate that it is late, but it may well have recrystallised from earlier diagenetic or lower-grade metamorphic pyrite.
The nature of the volcanogenic material in the Loch Katrine Volcaniclastic Formation is important in elucidating the tectonic regime prior to and during deposition of the Southern Highland Group. The geochemistry of the formation has been investigated as part of a wider study of mafic volcanic rocks in the Argyll and Southern Highland groups (Pickett, 1997; Hyslop and Pickett, 1998; Picket et al., 2006; and Burt, 2002). Recent geochemical studies have been undertaken. The trace and rare earth elements suggest a continental source for the sedimentary detritus that is common to both the Loch Katrine Volcaniclastic Formation and the other formations within the Southern Highland Group. The basaltic volcanogenic component of the Loch Katrine Volcaniclastic Formation is compositionally very similar to the composition of the Loch Avich lavas, suggesting a common source arising from contemporaneous volcanism (Burt, 2002; Pickett et al., 2006).
The Loch Katrine Volcaniclastic Formation reflects deposition in channel distributory systems on a submarine ramp. Some metasandstones are considered to reflect deposition from the traction stage of sandy, high density turbidity currents and both normal and tail grading are observed. The semipelitic rocks and some thin-to medium-bedded metasandstones are interpreted to result from deposition by low density, low volume turbidity currents and to represent Bouma Tb and Tc sequences (Bouma, 1962). The semipelites contain parallel and small-scale cross lamination, with some fluidised beds, slumps and dewatering structures.
The Green Beds seen at Bealach nam Bo [NN 484 070] and in Allt Gleann nam Meann [NN 526 117] may represent channel-fill sequences in a mid to inner deep water submarine fan environment, deposited by high density turbidity currents. The thick-bedded, possibly amalgamated, massive and graded sandstones at Bealach nam Bo represent major channel sands deposited in an inner to mid fan setting by very high energy debris flows and high density turbidity currents. The presence of coarse sandstone lenses and fine grained rip-up clasts also suggests deposition from turbidity currents. The fine-grained sandstone intervals within the Bealach nam Bo succession may represent overbank facies and deposits in interchannel areas.
CREAG INNICH SANDSTONE FORMATION (ZQCI)
The Creag Innich Sandstone Formation is considered to overlie both the Loch Katrine Volcaniclastic and Aberfoyle Slate formations (Figure 4), and it is overlain in turn by units of the Ben Ledi Grit Formation. The Creag Innich Sandstone is only recognised in the western part of the Dalradian outcrop, west of the Loch Tay Fault, where it crops out on either side of the Aberfoyle Slate Formation, appearing to be thinner on the southern side. The formation is well exposed in a series of outcrops on the northern flanks of Ben Venue, particularly around [NN 480 070]. From here, it thins towards and beyond Beinn Bhreac [NN 457 058] and toward Ben A’An [NN 505 082], highlighting its apparent wedge-like geometry. A small area of outcrop north-west of the Loch Katrine Volcaniclastic Formation around Loch Lisher [NN 445 068], and quartzose metasandstones cropping out on the south side of the Ben Ledi massif that are structurally below the Loch Katrine Volcaniclastic Formation, are also assigned to the Creag Innich Sandstone.
The unit is thickest (1200 m) in its type area just east of Creag Innich [NN 492 035], but here the formations may be thickened by folding. Elsewhere the formation is generally less than 1 km thick and thins laterally where traced.
The Creag Innich Sandstone is characterised by quartz arenites and arkosic rocks. The lithologies are commonly coarse to very coarse grained, and include common microconglomerates with well-rounded quartz and feldspar clasts up to about 20 mm in diameter. The formation is matrix poor, particularly compared to adjacent lithologies.
Individual arenite units are commonly thick to very thick bedded, and include amalgamated beds. Although beds are commonly massive, bed structures are preserved locally. Graded bedding is developed in the upper, finer-grained parts of some beds, and coarse-tail grading is also developed in places. Metre-scale channelling is a relatively common feature seen in some outcrops. Gravel lags are developed in the bases of some channels, indicating winnowing of finer-grained sediment. Loading structures include ‘pillar and ball’ and poorly defined flames.
Quartz dominates the mineralogy, comprising between 40 and 90 per cent of the rock. Some units are particularly feldspathic, with 20 to 40 per cent alkali feldspar and up to 10 per cent of mainly albitic plagioclase in the more arkosic rocks. The largest clasts are commonly lithic. The most abundant clasts are polycrystalline quartz. Clasts of intergrown chlorite and feldspar probably represent altered basic igneous rocks, and clasts of mudstone and siltstone are also present. The matrix is dominated by fine-grained white mica with minor chlorite which form the cleavage developed in these rocks.
LITHOSTRATIGRAPHICAL CORRELATION OF UNITS BELOW THE BEN LEDI GRIT FORMATION
The assignment of lithologically very similar but apparently widely separated units to the Creag Innich Sandstone Formation is justified on structural grounds, as there is good evidence that the formation can be traced across the major early (specifically D1) synclinal Ben Ledi Antiform (see Chapter 10). The corollary of this interpretation is that the Aberfoyle Slate Formation and the Loch Katrine Volcaniclastic Formation are both within the lower part of the Southern Highland Group, though the detailed spatial and temporal relationship between the two is indeterminable. Thus, the Aberfoyle Slate Formation is possibly in a similar lithostratigraphical position to the Pitlochry Schist Formation (e.g. Harris et al., 1994). If this is the case, then there is a significant change in the lithostratigraphy across the breadth of the Southern Highland Group outcrop, since south of the Aberfoyle Anticline, the Loch Katrine Volcaniclastic Formation is not identified between the Aberfoyle Slate and the Ben Ledi Grit.
The overall lithological characteristics of the Creag Innich Sandstone indicates deposition in channels of varying scales, and it is probable that this unit as a whole represents the fill of a large-scale submarine channel system. The predominance of matrix-poor, siliceous and very coarse-grained deposits suggests deposition from gravel- and sand-charged debris flows and high density turbidites. Deposition was mainly from the traction load, with rapid deposition on underlying unlithified, water-charged sediments resulting in loading, pillar and ball and flame structures.
BEN LEDI GRIT FORMATION (QGSB)
The Ben Ledi Grit Formation comprises about half of the Dalradian outcrop in the Aberfoyle district (Figure 4). It occupies most of the ground between the Highland Boundary Fault and the Ben Ledi–Trossachs–Ben Venue area to the west of the Loch Tay Fault.
The formation overlies different units locally. In general, it overlies the Loch Katrine Volcaniclastic Formation in the north of its outcrop and the Aberfoyle Slate Formation in the south, except where the lenticular Creag Innich Sandstone Formation intervenes (Figure 4). The outcrop is widest east of the Loch Tay Fault, extending well north of the sheet boundary. To the west of the Loch Tay Fault, the outcrop tends to thin westwards north of the Aberfoyle Slate Formation, but to the south it forms a wedge-like outcrop due to the eastward swing in strike of units and the development/presence of early structures towards the Loch Tay and Highland Boundary Faults. The outcrop pattern and thickness variations are strongly influenced by the oblique slip faulting that is particularly strong in the ground between Loch Ard and Loch Venachar (see Structural history). In Loch Ard Forest and on the southern flanks of Ben Ledi, the Ben Ledi Grit Formation is also strongly folded. The deformation makes it difficult to estimate the thickness of the Ben Ledi Grit Formation, but it is considered unlikely to be much more than 1000–2000 m.
In many respects, the Ben Ledi Grit Formation can be regarded as the typical ‘background’ lithology of the Southern Highland Group. The Loch Katrine Volcaniclastic Formation is considered a variant distinguished by its volcaniclastic component. However, the Ben Ledi Grit is lithologically diverse. In many areas it mainly comprises coarse-grained metasandstones, but it also contains significant units of more semipelitic and pelitic composition. These finer-grained lithologies appear dominant in the ground to the west of the Loch Tay Fault. The most important of these is the mainly semipelitic Ledard Burn Member, which crops out to the north of the Creag Innich Sandstone, between Loch Ard and Ben Venue and in the Achray Forest, north of Creag Innich [NN 492 035].
Metasandstones in the Ben Ledi Grit Formation range from clean arenites and quartz-arenites to wackes, with the latter dominating. They are commonly coarse grained, though there is much local variation. Most lithologies are grey or greenish-grey in colour; in particular, the wackes commonly have a greenish tinge due to the chlorite content of the matrix. The clast component is dominated by quartz. Although most of this is monocrystalline, polycrystalline lithic quartz clasts are also present, probably derived from quartz veins or quartz-feldspar pegmatites. Feldspar clasts include twinned plagioclase, perthitic K-feldspar and albite. The feldspar is commonly altered to white mica in the form of sericite. Lithic clasts are the largest, and include basaltic material (largely altered to chlorite), felsitic lithologies and semipelitic and pelitic material. Large detrital muscovite flakes are typical of the formation, and are scattered throughout the matrix. They commonly show distortion due to early grain compaction. Volcaniclastic-rich lithologies contain a high proportion of chlorite, both in the matrix and, less commonly, as detrital flakes, and detrital epidote. The matrix in the metasandstones comprises chlorite, white mica and haematite with fine-grained quartz and feldspar; again the latter is commonly replaced by sericite. Accessory minerals are relatively abundant and include zircon, illmenite and tourmaline. Zircon is ubiquitous and occurs as typically fractured or broken ovoid to elongate grains that are typically zoned and have dark brown, metamict cores. Zircon also occurs as inclusions in large polycrystalline quartz clasts.
There are several petrographical differences between the Ben Ledi Grit Formation and the underlying Creag Innich Sandstone Formation. Chlorite is the dominant matrix component in the Ben Ledi Grit Formation, and plagioclase dominates the feldspar component. In contrast, while mica and perthitic K-feldspar are characteristics of the Creag Innich Sandstone Formation. In addition, the Ben Ledi Grit contains a greater abundance and variety of accessory minerals, most notably zircon, tourmaline and epidote. These differences reflect distinctly different source areas and physical processes of weathering and sedimentation between source and deposition.
Some lithologies are particularly distinctive. On the south-east side of the Loch Tay Fault at Meall Ear [NN 530 027], there is a coarse pebbly unit, characterised by clasts of white quartz and pink feldspar with pale grey-green, fine- grained lithic fragments in a chlorite-rich, green matrix. The pink feldspar is probably albite, following work by Tanner and Pringle (1999), and this unit is similar to those described by Tanner and Pringle from the Ben Ledi Grit Formation, immediately below transitional units at the boundary between the Ben Ledi Grit Formation and the Keltie Water Grit Formation (see below). In contrast, exposures on parts of the south-eastern flanks of Stuc Odhar [NN 551 088] include clean, coarse-grained quartz-arenites, similar to some of the lithologies in the Creag Innich Sandstone Formation.
A characteristic of some of the cleaner metasandstone of the Ben Ledi Grit Formation is the presence of dark brown-weathering, slightly calcareous lenses. Good examples are to be seen in the stream section some 500 m south-west of the Achray Hotel [NN 502 063]. The lewes are ellipsoidal in shape and up to ~0.5 to 0.75 m long. They are thought to be originally diagenetic in origin. The fabrics so clearly seen in the host rocks are generally poorly developed in the lenses.
Finer-grained lithologies, including fine-grained metasandstones, semipelitic and pelitic lithologies are both locally and regionally important. The most extensive developments of the finer-grained lithologies include the Ledard Burn Member and units east of Loch Lubnaig and the Loch Tay Fault. The Ledard Burn Member crops out west of the Creag Innich Fault, and is best exposed in the Ledard Burn, which flows south from Beinn Bhreac [NN 457 058]. It is dominated by laminated to medium-bedded, fine-grained metasandstone and semipelitic lithologies, with thin units of gritty metasandstone developed at the base of some upward-fining metasandstone–semipelite packets. Mudflake rip-up clasts are common locally.
Elsewhere, units of dominantly semipelitic and pelitic rocks, up to about 200 m thick, are developed in the ground south of Loch Ard, in Loch Ard Forest, and in the ground between Meall Ear [NN 530 028] and the Dukes Pass. They vary in colour from black, dark grey to green-grey, green and maroon and carry a ubiquitous slaty cleavage. Folding is common, and cherty laminae together with sulphide minerals occur in some exposures of black pelite. Bedding is usually indistinct, especially where folded, though small but consistent changes in grain size produce a subtle ribbing, and these grain-size variations are often accompanied by refraction of the slaty cleavage. The south-western margin of this outcrop is faulted, producing bleached and fractured pelite and semipelite. Where the pelitic rocks abut the Loch Tay Fault in the ground around Meall Ear [NN 530 028], the strike of the predominant cleavage is parallel to the fault zone.
Volcaniclastic-rich metasandstones, though uncommon, are developed locally in the Ben Ledi Grit Formation, cropping out in the Loch Ard Forest around [NN 460 995]. As with the volcaniclastic-rich sediments of the Loch Katrine Volcaniclastic Formation, these lithologies are rich in epidote and chlorite, imparting the characteristic dark green colour. Elsewhere, some finer grained units of the Ben Ledi Grit Formation are green and chloritic and probably contain some volcaniclastic detritus.
The mineralogy of the Ben Ledi Grit Formation stays relatively uniform across the outcrop as the metamorphic grade remains within the greenschist facies. Northwards the matrix chlorite and white mica recrystallise and align to form a cleavage, wrapping clasts. Albite clasts recrystallise, forming grains with broad, clear rims on cores rich in very fine-grained opaque inclusions. Tourmaline, which occurs as broken detrital grains south of Loch Ard, has undergone extensive metamorphic recrystallisation in pelitic units to form euhedral grains aligned in the cleavage (see also Metamorphism).
The effects of deformation are more apparent to the north, particularly with the onset of D2 (see Structural history). Quartz clasts have undergone pressure solution, and commonly reveal a strained state in their undulose extinction in the ground south of Loch Ard. To the north, quartz and feldspar show dissolution, and material has been reprecipitated in pressure shadows. The fabric defined initially in the matrix becomes strong and composite north of Loch Katrine, comprising a D2 crenulation to penetrative fabric superimposed on a S1 grain alignment and pressure solution cleavage.
In the first, medium to very thick bedded metasandstones represent sedimentation from debris flows and turbidites with a high to very high particulate density, and mostly represents Bouma divisions Ta to Tc. Where grading is shown, it is usually normal, although reverse grading is present locally, representing traction deposits from a high density turbidity current. Semipelitic beds within the unit represent deposition from finer-grained, low-density, low-volume turbidity currents. Laminated pelitic units are interpreted to represent hemipelagic sedimentation from suspension between flows. Where studied, sequences appear to coarsen up overall, indicating locally developed, progradational depositional systems within the Ben Ledi Grit (Burt, 2002).
The second facies is dominated by thin to medium bedded, semipelitic rocks, with normal grading and parallel lamination. They are typical of deposits from turbidity currents, and are interpreted as Bouma Tb and Td divisions, with some cross-laminated (Tc) beds. The Ta division is represented by beds with coarse-tail grading. The pelitic Te division is only rarely developed.
The unbroken nature of feldspar and many quartz clasts indicates a high particulate density within the transporting flow regime that limited the effects of contraflow abrasion, and confirms a proximal to medial setting within fan systems. Granule and gravel trails within finer grained units indicates deposition from surging turbidity currents and a non uniform depositional rate. The greater part of the Ben Ledi Grit Formation represents a sequence rapidly deposited during periods of high tectonic activity that produced a readily available source of siliciclastic material, and a constantly changing fault geometry within and around an actively subsiding basin. Overall, the Ben Ledi Grit Formation fines upwards. This may be due to variations in sediment supply and/or reduced basin subsidence. Either way, the data suggest waning tectonic activity.
KELTIE WATER GRIT FORMATION/LOCH ARD GRIT FORMATION (QGPK, QGLA)
The Ben Ledi Grit Formation is overlain by a heterolithic succession of pale grey, gritty metasandstones and pale grey to black pelites (within which occurs the fossiliferous Leny Limestone) (Figure 4). Some coarse, dark green chlorite- and pink feldspar-rich wacke sandstones are also present locally. The pink feldspar is albite, rather than K-feldspar as generally assumed (Tanner and Pringle, 1999).
The succession comprises two formations: the Keltie Water Grit Formation (Tanner, 1995; Tanner and Pringle, 1999), outcropping east of the Loch Tay Fault, and the Loch Ard Grit Formation (P W G Tanner, written communication, 1998), which crops out in the Loch Ard Forest south-west of the Loch Tay Fault and in ground immediately adjacent to the Highland Boundary Fault, south-east of Meall Ear [NN 530 020].
Although the two units are defined separately, it is very likely that they represent the same succession. Geographical separation, paucity of exposure and complications arising from faulting preclude direct correlation or assignment to a single formation. The lithostratigraphical status and affinity of these units with respect to the Dalradian and the Highland Border Complex successions has long been debated, and is discussed below.
KELTIE WATER GRIT (QgPk)
Detailed mapping by Tanner (1995; written communication, 1998) in the ground between the River Teith and the Keltie Water, just to the east of the district, has shown the following informal lithostratigraphy (Tanner and Pringle, 1999):
4 Upper Keltie Water Grits
3 Leny Limestone and Slate
2 Lower Keltie Water Grits
- (Including the Keltie Limestone and Slate)
1 Transition Group
- Ben Ledi Grit formation
Younging evidence and structural data show that the Leny Limestone and Slate (PL) is not a repetition of the Keltie Limestone and Slate unit, and that the Keltie Water Grit Formation is stratigraphically and structurally continuous with the underlying Ben Ledi Grit Formation (Tanner, 1995; Tanner and Pringle, 1999).
Paucity of exposure within the district precludes the subdivision recognised by Tanner in the Keltie Water, and only the transition ‘group’ (here termed the Transition Member (QSk)) and the Leny Limestone and Slate Member are identified discretely within the Keltie Water Grit Formation.
Transition Member (QSK)
This unit comprises grey-green gritty sandstones and pelitic rocks, passing south-eastwards into dark green and brown to pale grey to white, gritty sandstones, interbedded with grey, green and purple slates (Tanner, 1995; personal communication, 1998). The colour of the gritty sandstones, particularly the pale varieties, is only seen on fresh surfaces. When weathered they develop red-brown outer ‘skins’. Tanner (1995) defined the boundary between the Ben Ledi Grit Formation and the Keltie Water Grit Formation at the disappearance of ‘grey and grey-green grits with abundant chlorite and detrital pink feldspar, and quartz’ (Tanner, 1995, p.477; Tanner and Pringle, 1999). The boundary is not sharp and Ben Ledi Grit lithologies interdigitate with those of the Keltie Water Grit Formation. Rocks belonging to the Transition Member have been mapped in the River Teith at the Falls of Leny [NN 593 089].
Dominant Keltie Water Grit Lithologies (QSPF)
The Transition Member is overlain by the dominant pale grey and white gritty sandstones of the Keltie Water Grit Formation. Associated with these rocks are rare grey, black and purple to ochre coloured, slaty, pelitic rocks. Bedding in the gritty sandstones is up to 3 m thick or more, and such thickly bedded units probably represent amalgamated beds.
Tanner and Pringle (1999) showed that the lithology and petrology of the coarse metasandstones in the Keltie Water Grit Formation change only gradually up sequence, and that there is no clear break between this formation and the underlying Ben Ledi Grit Formation. This gradual variation is also seen in the geochemical compositions of the metasandstones: although, overall the rocks in the Keltie Water Grit Formation are different in composition, there is no marked change in composition across the Ben Ledi Grit–Keltie Water Grit boundary.
Tanner (1995) noted that these gritty lithologies, together with the dark slaty rocks, are very similar to those of the Margie Formation in the River North Esk in Angus, and other arenaceous lithologies found elsewhere along the Highland border. Hitherto these units have all been considered as part of the Highland Border Complex and therefore unconnected with the Dalradian (Bluck et al., 1992).
Leny Limestone and Slate Member (PL)
This unit crops out near the eastern margin of the district to the north of the River Teith, but with the exception of Leny Quarry [NN 615 098], is only poorly exposed. Here, the Leny Limestone and Slate Member comprises moderately to strongly cleaved, black, very graphitic pelite with limestone beds (LL where mappable). A grey, sandy limestone unit that was previously worked in the Leny Quarry is now unexposed, and is probably largely worked out, having been recorded by Clough as being no more than 1.5 m thick in 1896 (BGS unpublished data). However, several beds of black limestone that occur in black graphitic pelite on the south-east side of the quarry (the ‘Leny Limestone’) are some of the most important units in the Dalradian succession because they contain the typical Laurentian Pagetides trilobite assemblage, dating the limestone to late early–earliest mid Cambrian times (Rushton, 1999). The faunal assemblage consists chiefly of trilobites, but includes some brachiopods, tubular organisms and sponge spicules. The main grey limestone bed is unfossiliferous, apart from a few fragments of brachiopod shells. The significance of these limestones with regard to the upper age of Dalradian sedimentation is discussed below.
The geochemistry of the limestones from Leny Quarry has been studied by Tanner and Pringle (1999) and Thomas et al. (2004). Tanner and Pringle described the grey limestone as being similar in composition to more magnesian limestones from the Dalradian of north-east Scotland (Thomas, 1989). Two analyses of the graphitic limestones showed them to be impure, containing between 10 and 30 wt per cent SiO2, occurring mainly as quartz, and ~1.5 to 3 wt per cent MgO (BGS unpublished data; Thomas et al., 2004). The presence of up to 3 wt per cent MgO is consistent with the presence of small amounts of dolomite recorded petrographically by Tanner and Pringle (1999). Other oxide components, derived from siliciclastic contamination, are low. The black limestones have also been analysed for their 87Sr/86Sr, δ18O and δ13C compositions (Thomas et al., 2004). The 87Sr/86Sr values range from 0.711059 ± 30 to 0.712701 ± 17, and are the most radiogenic of any of the known values for Dalradian calcitic metalimestones. It is probable that 87Sr/86Sr ratios of the thin limestones have been affected by diagenetic fluid–rock interaction with the host pelites, yielding more radiogenic values in the limestones. This is borne out by δ18OSMOW values of 10.3–15.7‰, which indicate interaction between the limestones and fluids with low δ18O relative to the limestones, probably derived from the pelitic rocks. The strongly graphitic character of the limestones is manifest in the very low, negative δ13CPDB values (–4.6 to –5.0 ‰). These low δ13C values reflect the buffering of carbon isotopes in the carbonate by isotopically light carbon, relatively richer in C12, derived from graphite in the limestone and host pelites.
Pale metasandstones near Lime Craig Quarry [NN 5340 0107]
Buff-weathering, coarse, gritty metasandstones, interbedded with weathered, pale buff to grey pelitic rocks, crop out on a forestry track in Achray Forest where it leads to open ground at about [NN 533 019], near the entrance to Lime Craig Quarry [NN 5340 0170] (Figure 4). To date, these rocks have been considered to be Ordovician in age (Ashgill–Caradoc) based on chitinozoa (Burton and Curry, 1984). However, more recent work has shown that the putative chitinozoa figured by Burton and Curry are inorganic artefacts (Tanner and Sutherland, 2007), and thus these sandstones are of unknown age. Their lithological characteristics are very similar to the pale sandstones assigned to the Keltie Water Grit Formation in the Milton Glen Burn [NN 575 063], north of Loch Venachar and the sandstones in the type section of the Keltie Water Grit Formation. Given their similar lithostratigraphical setting, it is most likely that they are the same unit. The tectonostratigraphical relationship of the pale sandstones to undoubted Highland Border Complex rocks at Lime Craig Quarry is highly equivocal, the units being separated by faults and outcrop obscured by quarry spoil.
THE SIGNIFICANCE OF THE KELTIE WATER GRIT FORMATION IN DALRADIAN EVOLUTION
The lithostratigraphical affinity of the Leny Limestone and the host Keltie Water Grit Formation, and the concomitant implications for the upper age of the Dalradian, has long been a matter of debate. Detailed mapping by several authors has shown that, although the area is cut by faults locally (Harris, 1969), there is no strong evidence for stratigraphical, structural or petrological discontinuity between the Ben Ledi Grit Formation and the Keltie Water Grit Formation (see Tanner, 1995, and references therein). This, coupled with the faunal evidence that the Leny Limestone is Cambrian in age, indicates that deposition of the Dalradian continued at least up to 510–520 Ma. This conflicts with the conventional Precambrian interpretation for the depositional and tectonic history of the Dalradian, and the assignment of the Leny Limestone to the Highland Border Complex (e.g. Curry et al., 1982; Rogers et al., 1989; Bluck and Dempster, 1992; Bluck and Ingham, 1997; Bluck and Rogers, 1997).
The conventional view was based mainly on the interpretation of the structural setting of the 590 million year old Ben Vuirich Granite in Perthshire. This intrusion, once regarded as at least post D1 (Rogers et al., 1989; Tanner and Leslie, 1994), is now known to be entirely pre-tectonic (Tanner, 1996). Its emplacement probably relates to a phase of extensional tectonics which was responsible for the Tayvallich Volcanic Formation of the south-west Highlands, dated at 595 ± 4 Ma (Halliday et al., 1989) and 601 ± 4 Ma (Dempster et al., 2002), and the Keith–Portsoy granites, dated at 600 ± 1 Ma (BGS unpublished data; J R Mendum, personal communication 2001). Given the clear field evidence for the contiguity of the Keltie Water Grit Formation with the Ben Ledi Grit Formation (Tanner, 1995; Tanner and Pringle, 1999), the simplest interpretation is that Dalradian sedimentation continued into at least the lowermost mid Cambrian and that the Dalradian and the Highland Border Complex (s.s., as defined in Highland Border Complex) were juxtaposed during subsequent Caledonian orogenic events that occurred principally during the Ordovician.
In this account, all units above the Ben Ledi Grit Formation are included within the Southern Highland Group, as shown on the current edition of the 1:50 000 geological map of the Aberfoyle district. However, Tanner (2007) erected the Trossachs Group (Dalradian Supergroup), into which he places the Keltie Water and related units, including the Bofrishlie Slate Formation, to separate them from the Southern Highland Group. Although this approach is entirely reasonable, for consistency with the published map it has not been adopted herein. Since the Cambrian-dated Leny Limestone lies near the base of the succession included in Tanner’s Trossachs Group, sedimentation on the Laurentian margin is likely to have continued until the Early Ordovician at least, and, with black, cherty pelites, slump breccias and pillow lavas, became increasingly oceanic in character. Thus, deposition of the Southern Highland Group, as defined here, lasted for at least 100 Ma. Given that the entire Dalradian succession above the 600 Ma old Loch Tay Limestone is only perhaps 5000–6000 m thick at most, it is clear that there must be significant, but as yet unrecognised, unconformities and non-sequences within the Southern Highland Group.
The depositional development of the predominant sandstone-rich units is similar to the Loch Ard Grit Formation (below). The higher quartz content of this facies indicates increasing maturity and Burt (2002) noted that beds have more consistent thickness and sheet-like form. These features are taken to imply that sediment gravity flows were less restricted and that deposition was occurring in a deep-water slope-apron to slope-base setting with limited topography. Burt interpreted this as indicating transition from the active rifting phase to the tectonically less-active thermal subsidence phase of the margin.
Beds of sandstone within the formation are representative of Bouma Tab and Tbc divisions. The former have erosive bases, commonly with loading features, and normal grading. The latter have parallel and cross-bedding structures on the 1000–1000 mm scale. Silt rip-up clasts are considered to have been sourced from the background hemipelagic sedimentation that is represented by the pelitic beds. Whilst many of the sedimentary structures observed in this facies are indicative of deposition by high energy sediment gravity flows, the absence of slumping features and liquefied beds suggest lower energy environments compared to the underlying rocks of the Ben Ledi Grit Formation (Burt, 2002).
The Leny Limestone and Slate Member and other, similar, unnamed units within the Keltie Water Grit Formation, constitutes the establishment of a stable margin starved of siliciclastic input. The black, graphitic and locally sulphide-rich character of the pelitic rocks indicates deposition in relatively deep anoxic waters, preserving organic material. The relative depth of water is difficult to establish, but indications of anoxic conditions and the lack of siliciclastic detritus implies limited mixing with oxygenated surface waters and isolation from sources of siliciclastic sediment. The latter may result simply because of distance from sources or it may indicate a region starved of sediment because of topographical barriers. For example, the mud protolith may have been deposited on a topographical high the only received sediment deposited from suspension.
LOCH ARD GRIT FORMATION (QGLA)
Rocks assigned to this formation crop out along the south-eastern margin of the Dalradian in Loch Ard Forest, south-west of Aberfoyle (Figure 4). The formation comprises an heterolithic assemblage of green-grey to pale gritty, coarse metasandstones, interbedded with purple and grey slaty pelites. Also present are black gritty metasandstones with black pelites (P W G Tanner, written communication, 1998). Lithologically, this assemblage is very similar to the Transition Member of the Keltie Water Grit Formation and, although direct correlation is precluded because of paucity of exposure and geographical separation, the two units are probably lithostratigraphically equivalent. The boundary of both units with the underlying Ben Ledi Grit Formation is defined by the youngest of the coarse, pink feldspar and chlorite-bearing metasandstones in the latter unit (Tanner, 1995).
The depositional development of the Loch Ard Grit Formation is described above (see Keltie Water Grit Formation).
BOFRISHLIE SLATE FORMATION (grBBL)
The Bofrishlie Slate Formation crops out in Loch Ard Forest between the mafic meta-igneous components of the Highland Border Complex and the Dalradian (s.s.) (Figure 4) and, traditionally, has been regarded as part of the Highland Border Complex (see Highland Border Complex).
The formation is dominated by black, sulphidic and graphitic pelite, interbedded with grey cherts up to 15 cm thick, grey cherty pelites and semipelites, and some metasandstones. The metasandstones were assigned to the Margie Series by Jehu and Campbell (1917). Several lenses of spilite (basaltic lavas altered by interaction with sea-water) were recorded locally by Jehu and Campbell (1917). Tanner (written communication, 1998) found two of these lenses and reclassified one as hornblende schist; the remainder could not be located due to the impenetrable forest cover.
The Bofrishlie Slate Formation carries a slaty cleavage that is crenulated. Henderson and Fortey (1982, p.239) noted that the slaty cleavage is ‘comparable to that in the Dalradian greywackes’. Tanner (personal communication, 1998) reported that younging evidence shows the formation is overturned, and the slaty cleavage is steeper than bedding. The cleavage is therefore downward facing, as in the Ben Ledi Grit Formation.
The formation is notably fossiliferous, containing brachiopods and radiolaria (Jehu and Campbell, 1917; Summary of Progress of the Geological Survey of Great Britain, 1963, page 57). More recently, Curry et al. (1984) described chitinozoa retrieved from pelitic rocks sampled close to the original locality from which Jehu and Campbell extracted brachiopods and other fossils. However, as with the putative chitinozoa from the pale sandstones adjacent to Lime Craig Quarry, Tanner and Pringle (1999) maintained that these supposed microfossils are again inorganic artefacts, having re-examined the material using scanning electron microscopy. The brachiopods are poorly preserved and silicified, and have proved difficult to identify positively. The original material collected by Jehu and Campbell was re-examined in 1962, by Dr A J Rowell (University of Nottingham) on behalf of the Geological Survey. In a written communication (British Geological Survey archives) Dr Rowell asserts that ‘the only faunal element that is definitely present is the Acrotretacea (brachiopoda)’ and suggests an age range of late early Cambrian to early Ordovician, although the determination is not at all precise.
The nature of the contact between the Bofrishlie Slate Formation and the Dalradian (s.s.) has long been debated. C T Clough was very uncertain about the nature of the contact with the Ben Ledi Grit Formation (then termed Aberfoyle grits), as discussed in Jehu and Campbell (1917, pp.177 – 178) and the Annual Report of the Geological Survey for 1895 (pages 25–26). Clough was inclined to believe that there is a stratigraphical or structural break between the slates and the Dalradian, based on a postulated Silurian age (Peach and Horne tentatively correlated them with similar rocks in the Southern Uplands) and some possible discordance in strike between the slates and the grits. However, the Annual Report goes on to discuss the similarity in metamorphic grade between the Bofrishlie Slate and Ben Ledi Grit formations, and the presence in each of similar black and purple slate lithologies. Jehu and Campbell (1917) considered that the boundary between their Highland Border Complex suite of units and the Dalradian was marked by their Leny Fault. In more recent years, some authors have noted the structural continuity between the Dalradian (s.l.) (Harris, 1969; Tanner, 1995; Tanner and Sutherland, 2007) and at least the clastic sediments in the HBC, indicating that they share a common structural history (i.e. they were deformed together by Grampian orogenic events). These authors have questioned the interpretation that all of the HBC is separate from the Dalradian.
There are several good reasons for including the formation in the Dalradian:
- The boundary between the Bofrishlie Slate Formation and the Dalradian (s.s.) may be a normal sedimentary transition. However, given the proximity of the formation to the HBFZ, a faulted contact does not necessarily preclude its inclusion in the Dalradian.
- The top of the formation is strongly sheared and the slates adjacent to the hornblende schist are metamorphosed to a higher degree than those to the north. This suggests that the protolith to the hornblende schist was hot when it was emplaced against the slates. The corollary is that this contact represents the true boundary to the Highland Border Complex in the Aberfoyle district.
- The cleavage in the slates faces downwards, as it does in the Ben Ledi Grit Formation, indicating a common deformational history from D1 onwards.
- The fossils in the slates overlap in age with those in the Leny Limestone and Slate Member.
The sulphidic, black to grey, graphitic, locvally cherty pelites and rare metasandstones of the Bofrishlie Slate Formation constitute deep marine sediments, probably deposited on the outer margins of a stable shelf. The sandstones are likely to represent turbidity current deposits. As such, this formation can be considered as a continuation of the Keltie Water grit Formation, with which it appears to be in lithostratigraphical continuity.
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