Geology of the Aberfoyle district: Highland Border Complex: Introduction
|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 Highland Border Complex (HBC), as defined in this account, comprises a tectonostratigraphical sequence of clastic, carbonate and igneous rocks of oceanic affinity, including ophiolitic elements, that crop out within the Highland Boundary Fault Zone (Figure 5). The temporal and spatial relationship of HBC rocks with adjacent Dalradian strata has been the subject of considerable investigation and controversy over the last 100 years or so (Geikie, 1897; Jehu and Campbell, 1917; Curry et al., 1982; Henderson and Fortey, 1982; Henderson and Roberston, 1982; Curry et al., 1984; Robertson and Henderson, 1984; Tanner, 1995; Bluck, 2002; Oliver, 2002; Tanner and Sutherland, 2007). Paucity of exposure, faulting along the outcrop of the Highland Border Complex in the Aberfoyle district, and variable constraints on the ages of the component tectonostratigraphical units have, until recently, prevented clear elucidation of the relationship of the HBC with Dalradian and the Midland Valley of Scotland, resulting in diverse interpretations. A detailed review of these is outside the scope of this account, and the reader is referred to recent summaries by Bluck (2002), Oliver (2002) and Tanner (2007) for modern discussion, whilst Jehu and Campbell (1917) provided a thorough review of the debate up until the early part of the 20th century. Widely differing interpretations centre on whether the HBC is continuous with the Dalradian (in terms of sedimentation on the Laurentian margin) (Geikie, 1897; Johnson and Harris, 1967), or whether it is allochthonous (Bluck and Dempster, 1991), and the degree to which the components of the HBC, as seen in the Aberfoyle area, can be considered as an assemblage of temporally and spatially associated lithological units. Following review of available data, including recent mapping by the BGS and Dr G Tanner (Glasgow University), this account incorporates a significant revision of the tectonostratigraphy of the Highland Border Complex. This revision has been corroborated and widened in scope by Tanner (2007). Tanner’s work revises the Highland Border Complex stratigraphy along its entire strike length in Scotland and resolves apparent differences, with the Highland Border Complex in Scotland and the lithologically equivalent Clew Bay Complex in western Ireland (Chew, 2003).
The problems with lack of exposure encountered earlier waters have been exacerbated markedly by the growth of impenetrable coniferous plantations of the Loch Ard Forest. Today, many of the outcrops recorded by Jehu and Campbell (1917) are covered by vegetation and virtually impossible to find. Although a limited number of new exposures have been created by forestry roads and related quarries for road metalling, the ground is still very poorly exposed.
- Geikie, A. 1897. Annual report of the Geological Survey of the United Kingdom and of the Museum of Practical Geology for the year ending December 31, 1896. (London: HMSO.)
- Jehu, T J, and Campbell, R. 1917. The Highland Border rocks of the Aberfoyle district. Transactions of the Royal Society of Edinburgh, Earth Sciences, Vol. 52.
- Curry, G B, Ingham, B K, Bluck, B J, and Willams, A. 1982. The significance of a reliable Ordovician age for some Highland Border rocks in central Scotland. Journal of the Geological Society of London, Vol. 139, 451–454.
- Henderson, W G, and Fortey, N J. 1982. Highland Border rocks at Loch Lomond and Aberfoyle. Scottish Journal of Geology, Vol. 18, 227–245. Cite error: Invalid
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- Curry, G B, Bluck, B J, Burton, C J, Ingham, J K, Siveter, D J, and Williams, A. 1984. Age, evolution and tectonic history of the Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh, Earth Sciences, Vol. 75, 113–133.
- Robertson, A H F, and Henderson, W G. 1984. Geochemical evidence for the origins of igneous and sedimentary rocks of the Highland Border, Scotland. Transactions of the Royal Society of Edinburgh, Earth Sciences, Vol. 75, 135–150.
- Tanner, P W G. 1995. New evidence that the Lower Cambrian Leny Limestone at Callender, Perthshire, belongs to the Dalradian Supergroup, and a reassessment of the ‘exotic’ status of the Highland Border Complex. Geological Magazine, Vol. 132, 473–483.
- Bluck, B J. 2002. The Midland Valley Terrane. 149–166 in The Geology of Scotland (Fourth edition). Trewin, N H (editor). (London: The Geological Society.)
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- Tanner, P W G, and Sutherland, S. 2007. The Highland Border Complex, Scotland: a paradox resolved. Journal of the Geological Society of London, Vol. 164, 111–116.
- Johnson, M R W, and Harris, A L. 1967. Dalradian–?Arenig relations in parts of the Highland Border, Scotland, and their significance in the chronology of the Caledonian Orogeny. Scottish Journal of Geology, Vol. 3, 1–16.
- Bluck, B J, and Dempster, T J. 1991. Exotic metamorphic terranes in the Caledonides: tectonic history of the Dalradian block, Scotland. Geology, Vol. 19, 1133–1136.
- Chew, D M. 2003. Structural and stratigraphic relationships across the continuation of the Highland Boundary Fault in Western Ireland. Geological Magazine, Vol. 140, 73–85.