Geology of the Aberfoyle district: History of research
|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).
Primary survey of the Aberfoyle district was undertaken from 1871 to 1899. The first edition of Sheet 38 (solid only) was published at 1:63 360 scale in 1901. Although no memoir was published for this sheet, a draft was prepared by Cunningham-Craig in 1901 and is available as a BGS technical report (Cunningham-Craig, 2000). In addition, several references are made to aspects of Dalradian stratigraphy and structure and the problematical status of the Highland Border Complex in the Annual Reports of the Director of the Geological Survey of 1895 to 1899, copies of which are held in the BGS library in Murchison House and Keyworth.
Much research has concentrated on the structure and stratigraphy of the Dalradian in the Aberfoyle district. Nicol (1863) first recognised the Highland Boundary Fault as a major structure separating the Highlands from the Midland Valley of Scotland. Henderson (1938) determined from graded bedding and other evidence that the metasandstones young away from either side of the Aberfoyle Slate Formation, and thus identified the Aberfoyle Anticline. Anderson (1947) considered that this structure could be traced along the Highland Border from Edzell to Arran and he interpreted it as an anticline sensu stricto. Subsequently, Shackleton (1957) showed that the Aberfoyle Anticline was, in fact, a downward-facing (i.e. inverted), synformal anticline, and considered the fold to be the down-folded closure of the regional-scale, recumbent Tay Nappe. Stone (1957) mapped out lateral facies changes in the Dalradian near Callander, drawing attention to the apparent asymmetry of the Aberfoyle Anticline. He confirmed that the structure closed downwards and plunged west-south-west. Harris (1962), working in the SE of the district, suggested that the Aberfoyle Slate might be diachronous, and attributed the increase in outcrop width of slate to the east to facies variations. Harris and Fettes (1972) studied cleavage relationships in the Dalradian along the Highland Border, questioning the correlation of slates and grits in this area. Mendum and Fettes (1985) showed that the nose of the Tay Nappe in the Aberfoyle district consisted of three downward-facing F1 fold closures: the Ben Vane Synform, the synformal Aberfoyle Anticline and the Ben Ledi Antiform. Mendum and Fettes also determined the southern limit of D2 deformation, discussed the relationship of the Highland Border Downbend to D4, and recognised the stratigraphical significance of the Green Beds. Harte et al. (1984) discussed the nature of the Highland Border Downbend, attributing its formation to block uplift along basement lineaments.
The volcaniclastic sandstone that forms the ‘green beds’ had been recognised as a distinctive lithology by the original surveyors (Pickett et al., 2006). Phillips (1930) demonstrated that they contain a significant basic igneous component, and Roberts (1966) and Kamp (1970) recognised from geochemical and petrological data that the green beds comprise a mixture of siliclastic and volcaniclastic sediment. The relationship of the green beds to other expressions of basic magmatism within the Dalradian outcrop is the subject of recent and current research (Pickett, 1997; Hyslop and Pickett, 1998; Burt, 2002; Pickett et al., in press).
Work on the metamorphism of Dalradian rocks in the Aberfoyle district, has elucidated both the appearance of index minerals in metasedimentary rocks and the progressive changes in mineralogy of volcaniclastic and mafic rocks with increasing metamorphic grade. Tilley (1925) mapped what he considered to be the garnet isograd in pelitic rocks in the Aberfoyle area. Subsequently, Mather (1970) demonstrated that the incoming of garnet and biotite is dependent on bulk rock composition, and suggested that a more appropriate measure of metamorphic grade in low-grade metasediments is the incompatibility of chlorite and potassium feldspar. Phillips (1930) described mineralogical changes within the Green Beds, whilst Wiseman (1934) described progressive metamorphism in metabasic sheets, recognising both bulk compositional and retrograde effects. Van de Kamp (1970) noted that amphibole occurrence in Green Beds was related to bulk rock calcium content. Hyslop and Pickett (1997) postulated qualitative changes in metamorphic grade with changing mineralogy of the Green Beds across the Southern Highlands, with the lowest grades near Aberfoyle, but metamorphic conditions in this area are yet to be quantified.
Burt (2002) recently completed the first systematic study of the sedimentology of the Southern Highland Group. Although the investigation extends throughout the length of the Southern Highland Group outcrop, particular use is made of exposed sections in the Aberfoyle district. This work shows that sediment gravity flows (debris flows, turbidity currents) were the key depositional agents within a dominantly passive, rifted-margin setting. Different lithostratigraphical units mapped by the British Geological Survey in the late 1990s reflect deposition in different settings on the continental margin, including ramps, aprons and channel systems.
Rocks assigned to the Highland Border Complex (HBC) in the Aberfoyle district have long been the subject of study. During the original survey, Clough (reported in the Summary of Progress of the Geological Survey, 1896, 1897) considered that the black shales of the Bofrishlie Slate Formation at the Highland border were probably Silurian, but could not detect a structural break between these and the Dalradian metasandstones. Jehu and Campbell (1917) mapped the HBC outcrop in detail, identifying the presence of serpentinites, and compiled the first HBC lithostratigraphy.
The lithostratigraphical status of the Kelty Water Grit Formation, in which occurs the Leny Limestone, has long been controversial. Pringle (1940) determined that the Leny Limestone is of late early to early mid Cambrian age, based on trilobite fossils. Anderson (1947), Johnson and Harris (1976) and Harris (1969) considered there to be structural and stratigraphical continuity of at least part of the HBC with the Dalradian. In contrast, Curry et al. (1984) considered the Leny Limestone to be part of the Highland Border Complex, and exotic to the Dalradian. More recently, detailed mapping by Tanner (1995) has reaffirmed the conclusions of Anderson, Johnson and Harris, showing that the Keltie Water Grit Formation is in stratigraphical and structural continuity with Dalradian rocks. This indicates that Dalradian sedimentation continued into at least the early mid Cambrian.
Henderson and Robertson (1982) described conglomerate with gabbro detritus in the HBC near Aberfoyle. Ikin (1983) presented evidence for spilitisation of mafic rocks in the HBC, and suggested formation within a Cambro-Ordovician marginal basin. Ikin and Harmon (1984) supported the view of Henderson and Robertson (1982) and Robertson and Henderson (1984) that mafic and ultramafic rocks in the HBC formed in a marginal oceanic basin, and were juxtaposed against the Dalradian during or prior to Grampian D2 deformation. This tectonic interpretation is disputed by Curry et al. (1984), Bluck (1985; 1990) and Bluck and Ingham (1997), who suggested that the HBC is exotic to the Dalradian and was docked after the main (D2) Grampian deformation.
Curry et al. (1982) reported Ordovician (Arenig to possibly Caradoc/Ashgill) faunas from the HBC. Subsequently, Curry et al. (1984) divided the HBC into four lithostratigraphical units, ranging in age from early Cambrian to Late Ordovician. They related these units in part to an exotic island arc massif. Ingham et al. (1985) reported mid Arenig trilobites of North American provenance within the Dounans Limestone near Aberfoyle. Dempster and Bluck (1989) presented evidence for mid Proterozoic metasedimentary boulders of possibly exotic provenance within the HBC. Debate continues over the origin and tectonic relationship of the HBC and the age of the Grampian Orogeny. Bluck and Ingham (1997) interpret the HBC as a far-travelled exotic unit, emplaced against the Dalradian after the Grampian Orogeny, possibly in Devonian times. Conversely, Tanner et al. (1997) argue that although some of the complex is exotic, and of probable island arc origin, the HBC docked with the Dalradian in the Early Ordovician during the Grampian Orogeny.
Du Toit (1905) described the Lower Old Red Sandstone of the Balmaha–Aberfoyle area. Wilson (1971, 1980) suggested that the Lower Old Red Sandstone in the western Midland Valley was deposited in a rapidly subsiding semi-arid fluvial basin. Read and Johnson (1967) and Bluck (1978) suggested that Upper Devonian fluvial sedimentation in the Midland Valley occurred in an east-north-east trending basin, with eastward flowing rivers and Hall and Chisholm (1987) described Upper Devonian aeolian sedimentation along the axis of the main basin. Phillips and Aitken (1998) attributed sedimentary provenance of the [Old Red Sandstone] of the Aberfoyle district to both penecontemporaneous volcanics and the southern Dalradian. Jack and Etheridge (1877) described fossil plants from the Dalmary Sandstone Member near Callander, and Henderson (1932) described a similar flora from Thornhill. Owens and Richardson (1972) considered this flora to be of early Emsian age.
Read and Johnston (1967) discussed deposition of the lower Carboniferous Inverclyde Group in a range of fluviatile environments, with current directions to the south-south-east. Macro- and microfossils in the lower parts of the Kinnesswood Formation in Dumbartonshire indicate that the lowermost rocks of this formation are of Famennian (latest Devonian) age (Aspen, 1974; McNestry, 1993; Turner, 1994). The stratigraphy of the lower Carboniferous Inverclyde Group is described in Browne et al. (1996). General descriptions of the stratigraphy of Lower Devonian, Upper Devonian and lower Carboniferous rocks of the Midland Valley, which outcrop on Sheet 38E, are presented in the memoirs for the adjacent districts, Sheet 39 (Francis et al., 1970 and Sheet 30W (Paterson et al., 1990).
Simpson (1933) described moraines from the Highland border west of the Tay, which he related to a late glacial re-advance. Thompson (1972) described a terminal moraine relating to the Loch Lomond Stadial (11 000–10 000 years BP) near Callander, and Rose (1981) described the terminal moraine of the Loch Lomond Glacier of this stadial. Price (1983) recorded the shoreline elevation of periglacial Lake Blane, west of Balfron. Smith et al. (1978) related deglaciation features in the Teith valley around and south-east of Callander to both the Dimlington Stadial ice sheet (27 000–13 500 years BP) and the later Loch Lomond Stadial glaciation. Merritt et al. (1990) described organic silts from the upper Teith valley, near Callander, which relate to both the Windermere Interstadial (13 500–11 000 years BP) and the Loch Lomond Stadial. Sissons and Smith (1965) presented evidence for rapid sea level rise in this area during the Flandrian Interstadial (<10 000 years BP). The formation names for glacial deposits present in the area are given in Browne and McMillan (1989).
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