Geology of the Aberfoyle district: Carboniferous
|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).
Early Carboniferous strata belonging to the Inverclyde Group (Browne et al., 1999) crop out in the south-easternmost corner of the district, where they are conformable on Devonian strata. They also occur in a small outlier north of the Highland Boundary Fault, where they lie unconformably on Dalradian rocks (Figure 6). Browne et al. (1999) summarised the nature of sedimentation during the Carboniferous in Scotland, précised here with regard to the Carboniferous strata of the Aberfoyle district.
Basin development, initiated in the Devonian by progressive lithospheric stretching and rifting across central and northern Britain, continued though the early Carboniferous. In response to this rifting, the Midland Valley of Scotland developed as a graben, flanked by regions of positive relief to the north and south. This resulted in more or less continuous sedimentation across the Devonian–Carboniferous boundary in the Midland Valley. Sediment was deposited from rivers flowing axially along the Midland Valley from the south-west and from distributary systems on the valley sides, the main sediment source being the Caledonian highlands north of the Highland Boundary Fault.
At the end of the Devonian and during the early Carboniferous, Scotland lay a few degrees south of the equator, but was drifting northwards (Scotese and McKerrow, 1990). The sedimentological character of the Inverclyde Group strata reflects this geographical change as predominantly arid conditions, under which the red fluvial and aeolian sandstones of the Upper Devonian were deposited, changed to wetter tropical conditions under which the mainly grey fluviodeltaic and marine Carboniferous strata were laid down (Read et al., 2002). Lowermost Carboniferous strata typically contain pedogenic carbonate concretions (‘cornstones’) within alluvial sandstone, and thin dolomitic and calcitic limestone beds (‘cementstones’) in mudstones. These carbonate rocks were formed in soil profiles subject to variable water tables and dessication, under the hot, more humid, semi-arid climatic conditions that prevailed during the early part of the Carboniferous. Pedogenic limestone was well-enough developed to have been worked in numerous, now degraded and overgrown opencast pits on Balgair Muir [NS 61 91].
Basal sandstone (Kinesswood Formation) gives way to mudstone and siltstone with subordinate thin sandstone (Ballagan Formation). The formation also contains ferroan dolomite cementstone with gypsum, rare anhydrite and pseudomorphs after halite. These features, together with dessication cracks, syndiagenetic brecciation and reddening, provide further evidence for wet, but strongly evaporitic hot climatic conditions. Subsequently, more coarsely grained sandstone with pebbly and conglomeratic beds, deposited by higher energy fluvial systems, become more common (Clyde Sandstone Formation), though mudstone is still present. Pedogenic carbonate concretions and limestone persist in some areas, though they are less abundant than in the underlying units. The mineralogy of the pedogenic limestone and concretions is dominated by calcite rather than the dolomite that typifies the underlying formations, as shown in outcrops and analyses by Hall et al., (1998). The sandstone is pebbly in places with the clast content dominated by either quartz and Dalradian pebbles, or intraformational clasts of mudstone and limestone.
The chrono- and lithostratigraphy of Carboniferous strata within the Aberfoyle district are summarised in Table 2. The continuity in deposition and the gradual change in style of deposition makes determination of the basal lithostratigraphical boundary of the Carboniferous somewhat arbitrary, but it is taken at the first appearance of pedogenic, calcareous horizons (calcretes) within unfossiliferous fluvial and aeolian sedimentary rocks. Lowermost Carboniferous strata contain few fossils of zonal value. Beyond the district, disarticulated scales of fossil fish (Bothriolepis sp. and Holoptychius sp.) have been found in conglomerate at the base of the Kinnesswood Formation at Fairy Knowe Quarry, near Dumbarton [NS 369 789] (Aspen, 1974). These fish remains are Famennian in age, but may have been reworked from older deposits. In adjacent areas, grey mudstone in the Ballagan Formation contain a moderately abundant fauna of Modiolus sp., Estheria sp. and fish fragments. However, in the Aberfoyle district, only a sparse fauna including Modiolus latus, rhizodont scales and ostracoda has been recorded, as, for example, in outcrops about 300 m west of Harvieston [NS 594 895].
Palynological assemblages in siltstones at Gass Water, New Cumnock, indicate that the lowermost parts of the Kinesswood Formation probably straddle the Devonian–Carboniferous boundary (McNestry, 1993; Turner, 1994; Smith, 1996). The remainder of the Carboniferous succession is Tournasian in age (Table 2), as indicated by miospore assemblages recovered from argillaceous beds in the Ballagan and Clyde Sandstone formations in BGS boreholes at Barnhill [NS 4269 7571] and Loch Humphrey [NS 4592 7555], in the Glasgow district immediately to the south.
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- Scotese, C R, and McKerrow, W S. 1990. Revised World Maps and Introduction. 1–24 in Palaeozoic Palaeogeography and biogeography. Mckerrow, W S, and Scotese, C R (editors). Geological Society of London Memoir, No. 12.
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- Hall, I H S, Browne, M A E, and Forsyth, I H. 1998. Geology of the Glasgow district. Memoir of the British Geological Survey, Sheet 30E (Scotland).
- Aspen, P. 1974. Fish and trace fossils from the Upper Old Red Sandstone of Dunbartonshire. Proceedings of the Geological Society of Glasgow, Vol. 113, 4–7.
- McNestry, A. 1993. Palynology Report on 18 samples from 1:50k Sheet 15W: New Cumnock. British Geological Survey Technical Report, WH/93/335R.
- Turner, N. 1994. Palynology of Devonian and Carboniferous rocks of the Cumnock and Doon Valley District, Strathclyde. British Geological Survey Technical Report, WH/94/294R.
- Smith, R A. 1996. Geology of the Gass Water area. British Geological Survey Technical Report, WA/96/22.