Geology of the Aberfoyle district: Inverclyde Group (INV)
|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).
Carboniferous strata within the district are assigned to the Kinnesswood, Ballagan and Clyde Sandstone formations within the Inverclyde Group (Browne et al., 1999).
Kinnesswood Formation (KNW)
The Kinnesswood Formation mainly occurs in the south-east of the district underlying Balgair Muir [NS 60 91] and the area around Ballikinrain Castle [NS 562 872] (Figure 6). The boundary with underlying Upper Devonian strata is not exposed within the Aberfoyle district, but is generally interpreted to be conformable, except in ground to the east-north-east of Edinbellie [NS 576 890], where strata dip moderately steeply south-east. The formation also occurs in a small area north of the Highland Boundary Fault Zone, around the headwaters of the Burn of Mar [NS 44 94]. Here, the formation lies unconformably on the Dalradian.
The Kinnesswood Formation consists mainly of poorly bedded, fine to medium grained sandstone, interbedded with paler coarse grained sandstones. Colours range from red to grey purple to pink or yellowish. The fine to medium grained sandstones contain carbonate concretions of limestone and dolostone that, in some instances, pass up into well-developed pedogenic limestone beds. The sandstones may represent overbank or channel deposits that have lost their original structures due to pedogensis after channel abandonment. The coarser sandstones have sharp, locally erosional bases, and are commonly cross-stratified and carbonate cemented. Small quartz pebbles are scattered throughout these beds in the lower part of the formation, and a few small clasts of Dalradian lithologies occur near the base of the formation. These sandstones are interpreted as channel fill deposits.
Ballagan Formation (BGN)
The succeeding Ballagan Formation is exposed in the south-east corner of the district in the tributaries of the River Endrick, draining Balgair Muir, the western flanks of the Gargunnock Hills and the northern flanks of the Campsie Fells. The base of the formation is transitional with the underlying Kinnesswood Formation, although the boundary is faulted out along the western edge of the outcrop by a north-north-east trending fault that downthrows strata on its eastern side. Where the contact is observed, sandstone beds are intercalated with mudstone with increasing frequency in a zone up to 20 m thick, as seen, for example, at [NS 597 901] and [NS 603 902] in small streams flowing south off Balgair Muir. Some reddening locally affects the mudstones and pedogenic dolostones, though it is mostly associated with the sandstones.
Strata within the Ballagan Formation typically consist of grey, silty, poorly laminated mudstones, with frequent thin nodular beds of ferroan dolostones. The mudstone is patchily calcareous in places, and may have desiccation cracks, (‘cementstone’), diagenetic brecciation (Browne et al., 1999) and pyrite and plant debris also occur locally in the mudstones. Though not abundant, thin, locally ripple-laminated or cross bedded, fine grained sandstones occur at intervals throughout the sequence. Although the majority are massive and carbonate cemented, a few are poorly cemented and very micaceous. The cross laminated sandstones represent minor channel deposits; those lacking cross-lamination probably represent deposition from sheet floods.
The rocks are cut locally by thin veins of fibrous gypsum, but these are less common and thinner than in adjacent districts. Likewise, halite pseudomorphs also occur in the mudstones, but are less common than in adjacent areas. Similar textural and mineralogical features in the Ballagan Formation of Berwickshire have been interpreted as evidence for a former sulphate evaporite facies (Scott, 1986). Scott considered that deposition of gypsum would increase the Mg/Ca ratio in the pore waters sufficiently for dolomite to form. Magnesium was probably supplied by marine flooding events in the coastal alluvial plains (Andrews and Nabi, 1998).
Though rarely exceeding 0.2 m in thickness, thin beds of pedogenic, nodular ferroan dolostone or horizons of discrete ellipsoidal nodules, individual beds may persist laterally for distances of over 2 km. Commonly the nodules have long axes parallel to the bedding. Good exposures of pedogenic dolostones and mudstones occur about 200 m north-west of Craigend at [NS 613 904] and about 600 m east of Harvieston [NS 604 894]. Most have no apparent internal structure but some are brecciated and a few are laminated. In the Aberfoyle district, lamination was only observed in the bedded dolostones. Septarian cracks are commonly in the dolostones and may be lined by gypsum or infilled with crystalline dolomite.
Clyde Sandstone Formation (CYD)
Within the Aberfoyle district, the Clyde Sandstone Formation conformably overlies the Ballagan Formation, cropping out in just two small geographically isolated areas: one on the northern flanks of the Campsie Fells [NS 622 868], just outside the district, and the other on the western flanks of the Gargunnock Hills [NS 619 890] (Figure 6). The successions in these areas contain creamy white channel sandstones interbedded with a few thin concretionary sandy limestones, probably of pedogenic origin. Just to the south of the district, at Little Corrie [NS 576 844], a 45 m thick succession comprises coarse sandstones with pebbles passing up into finer, commonly ripple cross-laminated, micaceous sandstones, with desiccation cracks, siltstones and mudstones. Some of the sandstones are concretionary with a carbonate matrix. The majority of the clasts comprise quartz with some quartzite.
On the western flanks of the Gargunnock Hills, just inside the district, a few poorly exposed sections reveal the lowest part of a 50 m thick sequence of the formation. The dominant lithology is white, fine- to coarse-grained, cross stratified sandstone. Some of the coarser sandstone beds have clay or limestone clasts near their bases. Calcareous and sandy limestones, of probable pedogenic origin, occur near the base of the formation, suggesting a transitional boundary with the underlying Ballagan Formation.
The distribution of clasts suggests that rivers transported sediment in a south or south-east direction. This is consistent with south south-easterly current directions determined by Read and Johnson (1967) for equivalent strata in the Stirling district.
- Browne, M A E, Dean, M T, Hall, I H S, McAdam, A D, Monro, S K, and Chisholm, J I. 1999. A lithostratigraphical framework for the Carboniferous rocks of the Midland Valley of Scotland. British Geological Survey Research Report, RR/99/07.
- Scott, W B. 1986. Nodular carbonates in the Lower Carboniferous, Cementstone Group of the Tweed Embayment, Berwickshire; evidence for a former sulphate evaporite facies. Scottish Journal of Geology, Vol. 22, 325–345.
- Andrews, J E, and Nabi, G. 1998. Palaeoclimatic significance of calcretes in the Dinantian of the Cockburnspath Outlier (East Lothian–North Berwickshire). Scottish Journal of Geology, Vol. 34, 153–164.
- Read, W A, and Johnson, S R H. 1967. The sedimentology of sandstone formations within the Upper Old Red Sandstone and lowest Calciferous Sandstone Measures west of Stirling, Scotland. Scottish Journal of Geology, Vol. 3, 242–267.