Geology of the Aberfoyle district: Arbuthnot–Garvock Group (ATGK)
|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 oldest Lower Devonian strata in the district are assigned to the Arbuthnot–Garvock Group (Browne et al., 2001). This group crops out continuously in the ground immediately south-east of the Highland Boundary Fault and comprises coarse, commonly conglomeratic sedimentary and volcanic rocks. The lowermost beds are faulted against rocks of the Highland Border Complex, as seen at the old limestone quarry at Dounans, about 1.5 km north-east of Aberfoyle. The top of the group is marked by the incoming of poorly sorted mudrocks belonging to the Cromlix Formation (Strathmore Group; Table 1).
Lithologies within the Arbuthnot–Garvock Group are dominated by conglomerates, sandstones and andesitic lavas forming relatively minor components. Igneous clasts predominate in conglomerates in the north-east of the district, whereas metamorphosed quartzite clasts dominate in the south-west. The latter are remarkable for their large size and degree of rounding, indicating a polycyclic history. Phillips and Aitken (1998) identified two types of conglomerate in the group. The first is generally very massive and poorly bedded, but locally contains thin beds of planar or cross-bedded sandstone; the matrix is a litharenite with an abundance of devitrified glassy volcanic material. The second type is represented by matrix supported, poorly sorted but typically well-bedded rocks, with a wide range of clast sizes. Normal grading and trough and planar cross bedding are common.
Two formations are defined within the group: the Craig of Monivreckie Conglomerate Formation, overlain by the Ruchill Flagstone Formation.
Craig of Monievreckie Conglomerate Formation (CMVC)
The formation crops out in an almost continuous tract adjacent to the Highland Boundary Fault from north of Callander [NS 628 079], immediately east of the district, to ground around Maol Ruadh [NS 478 967], some 6 km south-west of Aberfoyle. It forms the high, rectilinear ridge of the Menteith Hills that reach 400 m in elevation and range over 6 km from Aberfoyle to Loch Venachar (P001225). Over much of their outcrop, the lowermost rocks seen in the formation are tectonically juxtaposed with rocks in the Highland Boundary Fault Zone. The upper boundary is strongly diachronous as seen in the ground south-west of Aberfoyle. Here, the formation is overlain at successively lower stratigraphical levels by the Ruchill Flagstone Formation, with which it interdigitates. The formation thickens again in the Maol Ruadh area before dying out in ground west of the district.
The formation comprises volcaniclastic conglomerates with some beds of very coarse grained lithic-rich volcaniclastic sandstone (P219918). Volcanic clasts are andesitic to dacitic in composition, with devitrification textures; the relative freshness of the clasts indicates only limited transport (Phillips and Aitken, 1998). The matrix of the conglomerates and the sand-grade material in the sandstones is mineralogically very diverse. Volcanic material dominates, but quartz, feldspar, metasedimentary and sedimentary rock fragments, garnet, micas, epidote and pseudomorphs after amphibole and pyroxene, are also present.
At Callander, the unit comprises volcaniclastic conglomerate containing vesicular andesitic and basaltic clasts in a volcaniclastic matrix. South of Loch Venachar, the volcanic clast component declines progressively as quartzite clasts increase in abundance. This change occurs along strike to the south-west and upwards through the formation. At Craig of Monievreckie [NN 5474 0200], the proportion of volcanic clasts is less than 50 per cent. From Aberfoyle south-westwards, the clast content is dominated by quartzite and psammite, and the thickness of the formation declines rapidly. In the Menteith Hills, the formation is about 1150 m thick, and probably exceeds 1200 m north of Loch Venachar.
North-east of Aberfoyle, the thickness of conglomerate indicates a series of coalesced fans. South-west of Aberfoyle, conglomerates thin rapidly, over about 4 km, before thickening again. The total thickness of conglomerate is less than 250 m in the ground between Deer Craig [NS 513 996] and Arndrum [NS 502 988]. However, one kilometre south-west of Arndrum, the conglomerate thickens rapidly again, and near Drum of Clashmore [NS 488 975] and Maol Ruadh, the mapped thickness of conglomerate is about 900 m, presumably corresponding to the apex of a fan.
The formation contains two distinct members: the Bofrishlie Burn Sandstone Member and the Balleich Lava Member.
BOFRISHLIE BURN SANDSTONE MEMBER (BBSA)
The Bofrishlie Burn Sandstone Member is the oldest Devonian unit within the district. It forms a 30–200 m thick, wedge-shaped unit that crops out over about 6 km adjacent to the Highland Boundary Fault to the south-west of Aberfoyle. It is composed chiefly of sandstone, bounded at its base (to the NORTH-WEST) by the Highland Boundary Fault, and capped by the Balleich Lava Member (see below). The unit is commonly offset by small dip-slip faults. Exposure is poor owing to the terrain and forest cover, but the principal lithology is reddish grey to reddish brown, fine- to coarse-grained sandstone. The sandstone is commonly poorly sorted, lithic, with igneous clasts in places, and is locally micaceous. Rare beds of pebbly sandstone and conglomerate have also been recorded. Sandstone beds at a similar stratigraphical level in the north-east of the district are included in the member, but a definite correlation has not been made.
BALLEICH LAVA MEMBER (BALH)
The Balleich Lava Member is a single lava flow, approximately 30 m thick, that can be traced intermittently for more than 5 km between Balleich and Drum of Clashmore. It is in apparent stratigraphical continuity with the underlying sandstones, as seen, for example, in the Bofrishlie Burn [NS 5102 9957]. The lava is locally a vesicular and amygdaloidal microporphyritic basalt (S98572, from Drum of Clashmore [NS 490 977]). Extensive hydrothermal alteration is common, resulting in a secondary carbonate + chlorite assemblage. The lava appears to be cut out against the Gualann Fault to the north of the River Forth and its outcrop is repeatedly displaced by many small dip-slip faults oblique to the Highland Boundary Fault Zone. The upper boundary is not visible, but is probably conformable.
Ruchill Flagstone Formation (RLF)
The outcrop of the Ruchill Flagstone Formation crosses the district from Callander to Conic Hill near Balmaha. The mean formation thickness is about 1400 m. The underlying boundary with the Craig of Monievreckie Conglomerate Formation is strongly diachronous, particularly south-west of Aberfoyle where the Ruchill Flagstone Formation reaches a maximum thickness of 175 m. The contact with the overlying Cromlix Mudstone Formation (Strathmore Group) appears to be a broadly synchronous transition. Around Gualann [NS 459 944], a marked swing in strike, coupled with a complex fault pattern, precludes an accurate assessment of the thickness of the unit.
The grey to reddish brown lithic sandstones that typify the Ruchill Flagstone Formation are massive, fine to medium and locally coarse grained. Compositionally, they are heterolithic to volcaniclastic. Volcanic material is andesitic to dacitic and, as in the Craig of Monivreckie Conglomerate, indicates relative little transport from its source. Metamorphic lithic clasts are also present and are more rounded than the volcanic material, possibly indicating a polycyclic history (Phillips and Aitken, 1998). The sandstones contain a very diverse mineralogy, reflecting metamorphic, volcanic and sedimentary sources. The matrix is dominated by material derived from degraded volcanic detritus. Units of parallel-bedded, reddish brown siltstone, up to 8 m thick, are common, and the sandstones often contain rip-up clasts of mudstone and siltstone. The Ruchill Flagstone Formation also contains three distinct conglomeratic members described below.
INCHMURRIN CONGLOMERATE MEMBER (IMC)
The Inchmurrin Conglomerate Member crops out as a series of prominent ridges between Conic Hill and Gualann. At Balmaha the beds strike slightly clockwise of the NE–SW trending Gualann Fault, but north-eastwards from Conic Hill, there is a progressive dextral swing in strike until, at Gualann, the strike is approximately east–west.
The Inchmurrin Conglomerate is typically poorly sorted, matrix supported and contains clasts ranging up to the size of small boulders. Although conglomerate predominates, pebbly sandstones are also common. The coarsest clasts are generally subangular to subrounded, although well rounded, polycyclic quartzite clasts also occur. Clast composition is varied: vein quartz is dominant in pebbly sandstones, but the conglomerates are more variable with clasts of vein quartz, quartzite, psammite and volcanic lithologies in differing proportions. The clasts in the lower beds are dominantly composed of quartzite. In higher beds the quartzite clasts are mixed, with clasts of acid and intermediate volcanic rock. Clasts in the sandstones are dominated by andesitic material, quartz and plagioclase, but metamorphic clasts are present in places. As in most of the other Devonian sandstones, the sand-grade material is mineralogically and lithologically very diverse, indicating immaturity and local derivation of much of the material.
CALLANDER CRAIG CONGLOMERATE MEMBER (CCCO)
The Callander Craig Conglomerate Member comprises a laterally discontinuous unit of conglomerate-rich rock. It forms a prominent two kilometre long ridge overlooking Callander. This ridge dies out south-westwards in the low ground of the alluvial plain of the River Teith. A conglomeratic unit that crops out in the Menteith Hills about 1 km west of Lochan Balloch [NN 592 042], lies at about the same stratigraphical level as that seen near Callander, and is correlated with it. This unit extends for 4 km, before dying out south-westwards. A thin conglomerate, forming the spine of the ridge known as Drum Wood, 400 m west of Crinigart [NS 528 985], can be mapped intermittently south-westwards as far as the Kelty Water.
The Callander Craig Conglomerate is generally 50 to 150 m thick, but can be as thin as 1 m. It is characterised by poorly sorted but well bedded, matrix-supported, vein quartz and quartzite-rich conglomerate, interbedded with subordinate beds of locally pebbly sandstone. Sandstone beds normally bound the conglomerate member, but between Drum Wood and the Kelty Water, the conglomerate is underlain by red silty mudstone.
GARTARTAN CONGLOMERATE MEMBER (GACO)
The Gartartan Conglomerate Member can be traced for over 12 km from the grounds of Gartmore House [NS 529 976] to Lochan Balloch [NN 592 042], and north-easterwards to the edge of the district. It is typically poorly sorted and matrix supported, with clasts of vein quartz, quartzite and igneous lithologies. The unit is 20–25 m thick at the type locality, increasing to about 100 m at Lochan Balloch. The Gartartan Conglomerate Member is underlain by purple and reddish grey lithic sandstones of the Ruchill Formation, and is overlain at the type locality by poorly sorted silty sandstones typical of the Cromlix Formation.
- Browne, M A E, Smith, R A, and Aitken, A M. 2001. A lithostratigraphical framework for the Devonian (Old Red Sandstone) rocks of Scotland south of a line from Fort William to Aberdeen. British Geological Survey Research Report, RR/01/04.
- Phillips, E R, and Aitken, A M. 1998. The petrology and composition of the Lower Old Red Sandstone exposed in the Aberfoyle area (Sheet 38E), central Scotland. British Geological Survey Technical Report, WG/98/10.