Geology of the Aberfoyle district: Devonian
|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).
Sedimentary and volcanic rocks of Devonian age within the Aberfoyle district belong to the Scottish ‘Old Red Sandstone’ lithofacies, and lie south-east of the Highland Boundary Fault (Figure 6). The succession of terrigenous sedimentary and volcanic rocks was deposited in intracontinental basins in the Scottish Midland Valley and adjacent areas. These basins developed in a sinistral strike-slip tectonic setting (Phillips et al., 1998) in response to Caledonian and Acadian orogenic events (Friend et al., 2000). Many of the characteristics of the Devonian rocks indicate that the prevailing climate was seasonally wet or dry and hot (Browne et al., 2001).
Clastic Devonian sediments were originally thought to represent molasse eroded off emergent Caledonian mountains to the north-west and south-east of the Midland Valley (Mykura (1991). This view has been challenged more recently by Haughton and Bluck (1988) and Bluck (2000). These authors cited the low relief of the Caledonian mountains by late Silurian times, and lack of abundant first-cycle detritus in the conglomerates, as indicating that the Grampian Highlands were not the primary source of Old Red Sandstone sediment in the northern Midland Valley. Consequently, Bluck (2000) suggested that the Scandian orogen, developed on the margin of Scandinavia during the late Silurian and Early Devonian, was possibly a more significant source of much Devonian sediment. However, petrographical work by Phillips and Aitken (1998) showed that the sand and finer clast fractions in the Lower Devonian of the Aberfoyle district is immature, with much weathered and labile sediment, indicating derivation from local sources and limited transport. Thus, even though the notably well-rounded cobbles and boulders of quartzite in the spectacular conglomerates, together with rounded metamorphic lithic clasts reported by Phillips and Aitken (1998), must be polycyclic, it is probable that the Grampian Highlands did contribute significantly to the clastic component of the Lower Devonian succession. It is likely that the Grampian Highlands were much reduced in relief in the about 20–30 Ma following the Grampian Orogeny (about 480–465 Ma). For example, Oliver (2001) argued that high-grade metamorphic material in the Southern Uplands terrain represents detritus derived from the rapidly uplifting and eroding Grampian Highlands, only a few million years after peak metamorphic conditions. Rejuvenation of the Highlands is likely to have occurred due to tectonism and granite intrusion during Scandian orogenesis (about 435–420 Ma), with vertical displacement possibly occurring across a developing Highland Border Downbend. Redistribution of post-Grampian alluvial sediment stored within the Highlands, and erosion of metamorphic bedrock (as indicated by the presence of staurolite and other high grade, but labile metamorphic detritus within lowermost (late Silurian) Old Red Sandstone sediments at Stonehaven (Phillips and Carroll, 1995), would have supplied sediment to the developing Lower Devonian basins of the northern Midland Valley.
The sedimentary rocks range from mudstones to coarse boulder conglomerates; those within the Upper Devonian are generally finer grained than those in the Lower Devonian. Basaltic to andesitic and dacitic volcanic rocks occur as discrete lava flows and as abundant volcanic clasts in the sedimentary rocks. Lower Devonian strata in the district are represented by the Arbuthnott–Garvock and Strathmore groups, and range in age from about mid Lochkovian (about 415 Ma) to latest Emsian (about 390 Ma).
Upper Devonian strata crop out in the south-east of the district (Figure 6) and are represented by the Stratheden Group, ranging in age from mid Frasnian (about 367 Ma) to latest Famennian (about 354 Ma). They were deposited unconformably on deformed and eroded Lower Devonian strata following uplift and erosion after the Acadian Orogeny (Browne et al., 2001; Soper and Woodcock, 2003). Although Upper Devonian strata are generally finer grained than those of the Lower Devonian, pebbly and conglomeratic rocks are present within the district. This is characteristic of Upper Devonian rocks within the northern and western part of the Midland Valley, where the major rivers were closer to the sediment supplied by alluvial fans in the west and north (Browne et al., 2001, and references therein). Sandstones with features characteristic of aeolian deposition are present within the upper parts of the Upper Devonian succession. The uppermost part of the Devonian succession (Stockiewood Sandstone Formation) is transitional into overlying Carboniferous strata of the Kinnesswood Formation.
The Devonian succession occurs within the north-western limb of the Strathmore Syncline, and occupies most of the ground south-east of the Highland Boundary Fault (Figure 6). The dip of the bedding steepens markedly towards the Highland Boundary Fault in the Hills of Menteith (P001225), to the extent that beds are vertical or locally overturned immediately adjacent to the fault. The litho- and chronostratigraphy of the Lower and Upper Devonian rocks in the Aberfoyle district is summarised in Table 1. The lithostratigraphical relationships between units are shown in more detail in Figure 7. In the following account, petrological details are taken from Phillips and Aitken (1998).
- Phillips, E R, Smith, R A, and Carroll, S. 1998. Strike-slip, terrane accretion and the pre-Carboniferous evolution of the Midland Valley of Scotland. Transactions of the Royal Society of Edinburgh, Earth Sciences, Vol. 89, 209–224. Cite error: Invalid
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- 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.
- Haughton, P D W, and Bluck, B J. 1988. Diverse alluvial sequences from the Lower old Red Sandstone of the Strathmore region, Scotlan — implications for the relationship between late Caledonian tectonics and sedimentation. 269–293 in Devonian of the World. Proceedings of the second International Symposium on the Devonian System, Calgary. Mcmillan, N J, and Embry, A F (editors). Memoir of the Canadian Society of Petroleum Geologists, No. 14.
- Bluck, B J. 2000. Old Red Sandstone basins and alluvial systems of Midland Scotland. 417–437 in New Perspectives on the Old Red Sandstone. Friend, P F, and Williams, B P J (editors). Geological Society of London Special Publication, No. 180.
- Oliver, G J H. 2001. Reconstruction of the Grampian episode in Scotland: its place in the Caledonian Orogeny. Tectonophysics, Vol. 332, 23–49.
- Phillips, E R, and Carroll, S. 1995. The petrology and provenance of the basal Lower Old Red Sandstone exposed between Ruthery Head and Dunnicaer, Stonehaven, Scotland. British Geological Survey Technical Report, WG/95/31.
- Soper, N J, and Woodcock, N H. 2003. The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension? Geological Magazine, Vol. 140, 627–647.