Stephenson, D., Leslie, A.G., Mendum, J.R., Tanner, P.W.G., Treagus, J.E. (Editors) 2013. The Dalradian of Scotland. "Accepted manuscript" version. Proceedings of the Geologists' Association Vol. 124 Issues 1–2
18 Tempar Burn
J.E. Treagus
Published in: The Dalradian rocks of the central Grampian Highlands of Scotland. PGA 124 (1–2) 2013 https://doi.org/10.1016/j.pgeola.2012.07.009. Also on NORA
18.1 Introduction
This GCR site, off the Kinloch Rannoch road on the south side of Strath Tummel, includes part of the Tempar Burn and exposures along an adjacent track that together comprise one of the classic sections through the Schiehallion Boulder Bed Formation (the local equivalent of the Port Askaig Tillite Formation), at the base of the Argyll Group (Table 1). The particular interest of this formation is that it is interpreted as having been deposited from a grounded ice-sheet, which carried material on to a shallow continental shelf. The tillite generally consists of a gritty matrix, in which are scattered clasts of various underlying rock types a few centimetres in length. Most spectacularly, in the upper half, are boulders of pink granite up to 30 cm across. The site also includes a section through the Schiehallion Quartzite (the equivalent of the Jura Quartzite), which originated as shelf sands above the tillite, and through a metaconglomerate of reworked tillite. The Schiehallion Boulder Bed Formation is possibly the most important ‘marker’ horizon that has enabled stratigraphical correlation within the Dalradian outcrop from the west of Ireland to Banffshire. It has also been a key horizon in the correlation of the Dalradian with Neoproterozoic successions in Greenland and Scandanavia.
Although this section through the formation was first described by Bailey (1917) and was correlated by him with the Port Askaig ‘Conglomerate’ of Islay, it was Anderson (1923) who first discussed the arguments for its glacial origin. The formation has also been described by Treagus (2000). Together with the classic localities of the Port Askaig Tillite (see the Caol Isla and Garvellach Isles GCR site reports), the site has attracted wide international interest in the ongoing investigations and debate on the evidence for and implications of Neoproterozoic glaciations.
18.2 Description
The Schiehallion Boulder Bed Formation, where most fully developed and exposed as in this section, may be divided into a sequence of lower calcareous metadiamictites with dominantly calcareous clasts and an upper sequence of quartz-rich metadiamictites with a mixture of clast-types, including distinctive pink granitic rocks. The two metadiamictite sequences are generally separated by discontinuous units of quartzite
The sequence of the lower calcareous metadiamictites, up to 50 m thick, is best seen 100 m east of the sheep dip
The matrix of these lower metadiamictites is a schistose calcareous (dolomitic) rock, locally rich in amphibole (radiating blades of white tremolite or pale green actinolite and large hornblende porphyroblasts) and generally rich in a phlogopitic biotite. The matrix, commonly weathering to a rusty-brown, characteristically shows a regular network of depressions, owing to small patches of carbonate minerals, which are possibly small clasts. According to Bailey and McCallien (1937) ‘lime-rich’ scapolite and diopside occur locally in the matrix. The dominant clasts in the lower metadiamictites are 1–20 cm diameter, disc-shaped fragments of schistose dolomitic limestone or limestone, elongate parallel to the schistosity. Although commonly completely weathered out, vestiges of the clasts can be seen at the rims of the holes. Biotite-muscovite semipelite clasts, which are less common, are very elongate and up to 20 cm in length, and the rarer quartzite and granitic clasts, are up to 25 cm long but less elongate (ratios of 2:1 or 3:1) and more angular; these clasts become more common upwards.
Two discontinuous units of quartzite, each up to 50 m thick and occurring at slightly different levels, are seen east of the track around
The younger part of the Schiehallion Boulder Bed Formation, up to 70 m thick, is best seen on both sides of the track above the small burn that crosses at
The contact with the Schiehallion Quartzite is not exposed, but evidence elsewhere in the district shows that it is sharp (Treagus, 2000). A thin bed of metadiamictite with reworked granitic clasts is present near the base of the quartzite in the East Tempar track exposures at
The metaconglomerates have great potential for both sedimentological and strain studies. Where measured, in the Tempar Burn section, the proportion of clasts to matrix is very variable over a few square metres, from 1% with isolated single clasts, usually granite, to approximately 70%, with crowded clasts of a mixture of rock types. The clasts are deformed into disc shapes within the dominant cleavage, which is very close to the bedding; observation of shape is difficult, but most clasts display approximately circular sections. Deformation appears to be more pronounced than in the metadiamictites; measured on surfaces perpendicular to the bedding, granite clast length:width ratios, range between 10.4:1 and 2.1:1, but are commonly about 5:1 and longest dimensions ranged from 2 to 47 cm. Semipelite clast ratios are from 5 to 13:1, and sizes range from 3 to 18 cm.
Downstream, after a further few metres of quartzite, the burn and banks provide excellent exposure of the Tempar Dolomitic Member, the probable equivalent of the Bonahaven Dolomite of Islay.
18.3 Interpretation
Subsequent to Bailey’s (1917) correlation of the Schiehallion Boulder Bed with the Port Askaig ‘Conglomerate’, Anderson (1923) discussed the arguments for its glacial origin. He pointed out that the uniform, unbedded nature of the matrix and the haphazard arrangement of the clasts strongly suggest an origin as a till. He noted the similarity of the carbonate-rock and the quartzite clasts to the underlying ‘Pale Limestone’ and ‘Banded Group’ formations respectively (now the Drumchastle Pale Limestone and Cnoc an Fhithich Banded Semipelite formations of Treagus, 2000), which are exposed to the east of the GCR site. He used this correlation to demonstrate the order of superposition. He further pointed out that the granitic (‘nordmarkite’) clasts have no likely local source in Britain and must be far travelled. He noted that the base of the Schiehallion Quartite closely approaches the ‘Blair Atholl grey limestone’ (now the upper calcitic metalimestone of the Blair Atholl Dark Schist and Dark Limestone Formation) in the north of the district, but accepted the possibility that this might be due to either erosion of the sub-quartzite surface, or to later tectonic movements. He noted that the presence of amphibolite clasts suggests a period of igneous activity that pre-dated the diamictites. The present investigation, however, has failed to reveal clasts of true amphibolite, as against green-amphibole (actinolite)-bearing mica-schist.
More-recent discussion concerning the depositional environment of the diamictites in general has concentrated on evidence from the superior exposures of the Garvellach Islands and Islay, e.g. Kilburn et al. (1965), Spencer (1971, 1981, 1985), Eyles and Eyles (1983), Eyles (1988). These authors considered that the diamictites represent a major glacial event of global extent and that they may be correlated with the Varanger tillites of Scandanavia and East Greenland (but see Stephenson et al., 2013a for more-recently suggested correlations with other global glacial events). It is generally accepted that the geographically extensive, though discontinuous, distribution of the deposits in the British Isles favours a glacial origin, as opposed to sedimentary slide deposits of a more-local character. The general evidence, throughout their outcrop in the Dalradian, of the mixture of locally derived and exotic clasts, the random distribution and size of the clasts, the lack of bedding within the ill-sorted matrix, the record of dropstone structure and the particular evidence of other glacially-related structures in the South-west Grampian Highlands, makes the glacial origin indisputable.
In the Schiehallion district, the massive nature of the diamictites, their inclusion of stratified, sorted sands, and the subrounded shape of some clasts, supports the interpretation of a marginal glaciomarine environment (Edwards, in Reading, 1986). The local thinning and probable absence of the formation from some areas and the inclusion of clasts of at least two of the preceding formations in the lower diamictites, strongly suggests an unconformable base. The distinctive sedimentary structures attributed to tillites and present in some exposures of the Port Askaig Tillite (see the Caol Isla and Garvellach Isles GCR site reports) have not been observed at this GCR site. Possible dropstone structures have been recorded, but it is difficult to exclude the effect of the strong tectonic deformation. The clasts show no evidence of having been sorted or of having had their distribution controlled by water movement.
There are two beds of metaconglomerate, within the Schiehallion Quartzite, that have clasts of identical lithologies to those in the metadiamictites (Anderson, 1923; Treagus, 2000). The significant difference between the metaconglomerates and the metadiamictites is that there appears to be a more strongly bimodal size distribution in the metaconglomerates, such that the intermediate 1–5 cm sizes are missing; in addition the clasts are less dispersed, although rarely in contact, and they are significantly more rounded. This supports the interpretation of Anderson (1923) that these rocks represent reworked diamictite material derived by fluviatile or marine erosion.
18.4 Conclusions
The Tempar Burn GCR site is of international importance for its clear exposures of a Neoproterozoic glacial deposit. The ‘boulder beds’ (metadiamictites) that make up the Schiehallion Boulder Bed Formation are composed of blocks and pebbles up to almost a metre across, sitting isolated in a matrix that was originally sand and silt. The blocks and pebbles can only have been dropped from floating ice. They include a wide range of rock types, including some derived locally from underlying formations, but particularly significant, are the commonly occurring ‘stones’ of a type of granite that does not crop out in Britain and therefore might have been carried a great distance across an ocean. The GCR site is thus of great importance for the light it sheds upon climate and sedimentary processes in the late Precambrian, but it is also celebrated for its significance in the correlation of Dalradian formations across the British Isles, as well as with possibly equivalent deposits in Greenland and Scandinavia. Studies of the deformed shapes of the ‘stones’ are important for estimating the strains that the Dalradian rocks were subjected to during the Caledonian Orogeny.