Journal of the Geological Society, London, Vol. 154, 1997, pp. 917–919, 1 fig. Printed in Great Britain
Discussion on a late Vendian age for the Kinlochlaggan Boulder Bed (Dalradian)
Journal, Vol. 153, 1996, pp. 823–826
J. E. Treagus writes: I am puzzled by the title of the paper by lower Appin Group elsewhere in the Dalradian of Scotland, Evans & Tanner (1996), in which they propose to correlate although I have pointed out (Treagus 1969, 1981) that there the Kinlochlaggan Boulder Bed with the Port Askaig Tillite are gravelly beds with pink (quartz and microcline) pebbles in Formation. According to Harland (1990), the Vendian is the Glen Orchy, Loch Leven and on Islay at the horizon I take period preceding the Cambrian and divided into two epochs, to be equivalent to the KBB. However, I consider it is exactly the earlier Varanger and the later Ediacara. It is with this the isolated nature of the occurrences of the KBB that early Vendian Varanger epoch, that the Port Askaig Tillite distinguishes it from the Port Askaig Tillite which, wherever Formation of the Dalradian of Scotland and Ireland can be there are continuous sections below the Islay (=Schiehallion) broadly correlated. As far as I am aware, the late Vendian Quartzite, can usually be identified from scattered boulders or Ediacara epoch contains no glaciogenic materials. from its characteristic gritty matrix. It is significant that over The real subject matter of the paper, however, is the authors’ the 50 km length of outcrop of the Kinlochlaggan succession, proposal of the stratigraphical and structural position for the there are many complete sections in which the KBB can be Kinlochlaggan Boulder Bed (KBB). Contrary to my original shown not to be present, but only two where it is now suggestion (Treagus 1969) that the KBB lay in an upward- recognized. facing succession which could be correlated with the lower part In contrast to the title of the authors’ paper, I would of the Appin Group, they propose that it lies within an speculate that the Kinlochlaggan Boulder Bed (KBB) may well inverted succession which can be correlated with the upper part mark the base of the Vendian period. The Port Askaig Tillite is of the Appin and lower part of the Argyll Group. I considered usually correlated with the upper (Mortensnes) of the two the latter correlation (Treagus 1969), but rejected it on the Varangian tillites at its type locality in Finnmark, which can grounds of dissimilarities of the lithologies involved, and be correlated in detail through northern Norway, Spitzbergen the greater similarities with the succession near the base of the and east Greenland (Hambrey 1983). In common with the Appin Group. Mortensnes, the Port Askaig Tillite is overlain by a lag Without repeating the broad similarities of the two conglomerate and incorporates carbonate material at its base successions in the alternative correlations, I would like to make (Edwards & Foyn 1981). The lower tillite (Smalfjord) has not the following points, in favour of my original interpretation. been recognized in Scotland, but both its dolomite ‘cap-rock’ (1) The succession of pelites and psammites above the KBB, and its unconformable relations to a thick sequence of silici- frequently contain microcline perthite clasts. Microcline grains clastic beds are similar to the situation of the KBB. Glover are common in the Grampian Group below the Appin Group, (1993) has recently recognized an unconformity, at Spean and often present in the lower Appin Group (Lochaber and Bridge 20 km to the west of Kinlochlaggan, where although Ballachulish subgroups) with which I have correlated the the Boulder Bed is absent, the lower Appin Group carbonates Kinlochlaggan succession. However, microcline is rare in the and pelite, equivalent to those above the KBB, show on-lap upper part of the Appin Group (e.g. the pelites and psammites relations to the quartzite and psammites of the Grampian of the Blair Atholl subgroup in the nearby Schiehallion area), Group below. with which Evans & Tanner (1996) propose to correlate the The consequences of the Evans & Tanner (1996) strati- succession. graphic correlation for the structure of the Highlands is (2) Of the same pelites, the authors remark that they are considerable, implying a major inversion and discontinuity characterized by abundant large kyanite prisms, whereas with the underlying Grampian Group, followed by the later garnet is scarce. Again, in the Schiehallion area (Treagus Kinlochlaggan Syncline (Synform). It is regrettable that their 1987), it is the graphitic pelites of the lower Appin Group paper should have no structural data to support such a radical (equivalent to the type Ballachulish Slate) which are charac- hypothesis, apart from one reference to refolding of minor terized by large kyanite prisms and a relative paucity of garnet; isoclinal folds. As Evans & Tanner conclude themselves, the Blair Atholl graphitic pelites of the upper Appin Group the solution to the problems of the area awaits a detailed contain only small kyanite and staurolite porphyroblasts and structural study. ubiquitous garnet. 8 January 1996 (3) The Kinlochlaggan Limestone has a lower dolomitic edge (Treagus 1969), which is reminiscent of the Ballachulish R. H. S. Evans & P. W. G. Tanner reply: We thank Treagus Limestone in the type area, where the succeeding grey calcitic for his comments on our note, which was intended primarily limestones are similarly interbedded with dark pelites. The to document the discovery of new exposures of the Blair Atholl limestone sequence of the Central Highlands, on Kinlochlaggan Boulder Bed in a remote part of the Central the other hand, is generally not dolomitic and is followed Highlands. As the deposit is at least 20 m thick and contains by psammites and semipelites with a dolostone immediately extrabasinal boulders up to 45 cm across, we explored the beneath and gradational into the lower calcareous tillite; this is possibility that it could be correlated with the Varangian a very different sequence from that seen at Kinlochlaggan. diamictites (i.e. Port Askaig Tillite) which provide an (4) It is true, as Evans & Tanner (1996) state, that tillite-like unique chronological marker in rocks of Neoproterozoic age deposits have not been found in the extensive outcrop of the throughout the North Atlantic region.
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Regarding lithostratigraphical correlations, we pointed out that the structural succession found above the top of the Grampian Group in the Kinlochlaggan Synform differs signifi- cantly from the Lochaber–Appin stratigraphical successions seen in the area between Glen Orchy and Spean Bridge, and at Schiehallion. Whereas lateral variations in the Lochaber stratigraphy may be explained by the deposition of these rocks in separate, rift-controlled, basins (Anderton 1985; Glover et al. 1995), the carbonate-dominated sequences of the Ballachulish and Blair Atholl subgroups are considered to have been laid down during a widespread post-rift subsidence phase; to be laterally continuous; and to maintain their chemical characteristics over large areas (Thomas 1989). Viewed in this context, the petrographic comparisons made by Treagus (points1&2)with the Blair Atholl rocks provide strong evidence which runs counter to the correlation which we have proposed, and clearly some quantitative means of testing for lithostratigraphic correlations between the Kinlochlaggan sequence and rocks of the Appin Group is needed. Encouraged by earlier geochemical work on the Appin carbonate rocks (Rock 1986; Wright 1988), and with refer- ence to Treagus’s point 3, we have carried out a pilot study of the petrography and trace element geochemistry of some of the carbonate horizons at Kinlochlaggan and Schiehallion. Only 13 samples have been analysed so far for trace elements, and major element data are not yet available, but Fig. 1. (a) Relative stratigraphical ages of the Varanger tillites in the the preliminary results show that the Coire na Coichille and North Altantic region (adapted from Fairchild & Hambrey 1995). Kinlochlaggan limestones of the Kinlochlaggan area (Evans 1East Greenland; 2North Greenland; 3NE Svalbard; 4North Norway; 5 & Tanner 1996) have a similar trace element pattern (and Scotland. (b) Simplified stratigraphical column for the Dalradian characteristically high Sr values of 1500–2800 ppm) to the rocks of Scotland. For A and B, see text. Pale and Dark Limestones at Schiehallion (equated with the Islay and Lismore Limestones respectively), and to the Lismore and Blair Atholl limestones analysed by Rock (in Our reason for assigning the Kinlochlaggan Boulder Bed to Wright 1988, table 15.6). Although these results give some the ‘late Vendian’ is as follows. The Varangian tillites consist support for our hypothesis, a comprehensive geochemical of a lower tillite with locally-derived dolomitic stones, and an study of these rocks (possibly including the associated non- upper tillite characterized by an abundance of extrabasinal calcareous clastic metasediments: Winchester & Max 1996), stones (Fig. 1a). The Port Askaig Tillite at the type locality on and backed by rigorous statistical analysis of the data the Island of Islay, western Scotland, has a total thickness (Thomas 1989), is required before any firm conclusions can which is over three times that of either of these tillite horizons; be drawn. as it consists of a lower part having the features of the lower Regarding Treagus’ point 4, the sporadic development of tillite, and an upper part very similar in character to the upper tillite in the Kinlochlaggan area mirrors the situation in tillite, it appears that both tillites are represented by this single Connemara, where the post-glacial ‘cap dolomite’ normally deposit (Spencer 1975; Harland et al. 1993). The interglacial seen in the North Atlantic region (Fairchild & Hambrey 1995) horizon which normally occurs between the lower and is also missing. Sporadic preservation of the tillite may be due upper tillites is absent here, as it is in other parts of the British to the fact that erosion has removed the ‘cap dolomite’ and Isles. part of the tillite sequence prior to deposition of the overlying On Islay, the Port Askaig Tillite is separated from the Quartzite. As the Dalradian rocks at Kinlochlaggan and in overlying Bonahaven Dolomitic Formation (Fig. 1a) by Connemara have both been subjected to intense, repeated c. 100 m of shallow-marine or intertidal sediments (Lower deformation and high grade metamorphism it would be diffi- Fine-grained Quartzite Formation: Klein 1970; Kessler & cult to distinguish this unconformity from tectonic excision, Gollop 1988), which some authors consider to be part of the which latter has undoubtably taken place along the limbs of Tillite sequence. The Dolomitic Formation, and in particular major folds in Connemara. unit 3, is correlated with the Late Vendian (Ediacara) Spiral Treagus’ speculation that the Kinlochlaggan Boulder Bed Creek Formation in East Greenland (Fairchild & Herrington occurs in rocks which may be correlated with those found 1989). As there is probably only a small time gap, represented elsewhere in Scotland at the base of the Appin Group (‘A’ on by a sedimentary transition, between the Tillite and the Fig. 1b), and is equivalent to the lower (Smalfjord) tillite in Dolomitic Formation in western Scotland, the Kinlochlaggan Finnmark, would require that the upper and lower Varangian Boulder Bed (equated with the upper part of the Port Askaig tillites in Scotland are separated by the complete thickness of Tillite) may therefore be considered as being ‘late’ rather than the Appin Group (estimated at 4 km, see Gibbons & Harris ‘early’ Vendian in age. From the proposed correlations shown 1994), less that of the Kinlochlaggan Quartzite. This is in stark on Fig. 1a it would be more correct to call it ‘mid-Vendian’, contrast with thicknesses of 150–330 m recorded for inter- but in view of the uncertainty over the position of the base of glacial beds found between the two tillites elsewhere in the the Vendian, this would seem needlessly pedantic. North Atlantic region, away from the British Isles (Hambrey
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1983). As the Smalfjord tillite is also dissimilar to the F, I.J. & H, M.J. 1995. Vendian basin evolution in East Kinlochlaggan Boulder Bed, and contains mainly sedimentary Greenland and NE Svalbard. Precambrian Research, 73, 217–233. stones (Hambrey 1983), this proposal would seem to have little ——&H, P.M. 1989. A tempestite-stromatolite-evaporite association (late Vendian, East Greenland): a shoreface-lagoon model. Precambrian to recommend it. Research, 43, 101–127. If one is seeking an alternative correlative for the G,W.&H, A.L. (eds) 1994. A revised correlation of the Kinlochlaggan Boulder Bed, we consider that the scattered Precambrian rocks in the British Isles. Geological Society, London, Special dropstones (up to 1.5 cm in diameter) reported by Glover Report No. 22. (1993) from mudstones at the top of the Grampian Group (‘B’ G, B.W. 1993. The sedimentology of the Neoproterozoic Grampian Group on Fig. 1b) may provide the clue, and give evidence of a and the significance of the Fort William Slide between Spean Bridge and Rubha Cuil-cheanna, Inverness-shire. Scottish Journal of Geology, 29, pre-Varangian glacial event. If this correlation were confirmed, 29–43. the Kinlochlaggan Boulder Bed could have formed from ——, K, R.M., M, F., C, G.C., P,E.R.&C, B.C. ice-rafted material carried to low lattudes during a restricted 1995. A Neoproterozoic multi-rift sequence: the Grampian and Appin glacial period in Late Riphean times (uppermost Grampian groups of the southwestern Monadhliath Mountains of Scotland. Journal Group to lowermost Appin Group), which was not linked with of the Geological Society, London, 152, 391–406. the Varangian glaciations. H, M.J. 1983. Correlation of Late Proterozoic tillites in the North Regarding our structural interpretation, the Dalradian rocks Atlantic region and Europe. Geological Magazine, 120, 209–320. H, W.B. 1990. A geologic time scale 1989. Cambridge University Press, of the Kinlochlaggan area have been isoclinally folded and are Cambridge, 1–263. ff a ected by many slides (ductile faults) and, from mapping ——, H,M.J.&W, P. 1993. Vendian geology of Svalbard. around Aonach Beag, R.H.S.E. has been able to demonstrate Norsk Polarinstitutt Skrifter, No. 193. that several of these mesoscopic isoclinal folds can be traced K,L.G.&G, I.G. 1988. Inner shelf/shoreface—intertidal transition, around the closure of the Kinlochlaggan Synform, so that the Upper Precambrian, Port Askaig Tillite, Isle of Islay, Argyll, Scotland. In: latter is at least of D2 age. However, the location and D B, P.L. . (eds) Tide-influenced Sedimentary Environments and interpretation of the tectonic contact between the inverted Facies. D. Reidel, 341–358. Appin Group rocks and the Grampian Group is made difficult K,G. V. 1970. Tidal origin of a Precambrian quartzite – the Lower Fine-grained Quartzite (Middle Dalradian) of Islay, Scotland. Journal of by poor exposure in the critical area, and by the fact that Sedimentary Petrology, 40, 973–985. subsequent recrystallization during upper amphibolite-facies R, N.M.S. 1986. Chemistry of the Dalradian (Vendian-Cambrian) metalime- metamorphism has largely obliterated stretching lineations and stones, British Isles. Chemical Geology, 56, 289–311. shear sense indicators in the early shear zones. S, A.M. 1975. Late Precambrian glaciation in the North Atlantic region. In conclusion, in the absence of a stratigraphical section In:W,A.E.&M, F. (eds) Ice Ages: Ancient and Modern. across the Grampian Group/Dalradian contact which pre- Geological Journal, Special Issues, 6. Seel House Press, Liverpool, 217– 240. serves clear younging and facing evidence, correlation of the T, C.W. 1989. Application of geochemistry to the stratigraphic correlation Kinlochlaggan succession with any of the ‘standard’ Dalradian of Appin and Argyll Group carbonate rocks from the Dalradian of sequences will rely upon extensive geochemical fingerprinting northeast Scotland. Journal of the Geological Society, London, 146, 631– of the Kinlochlaggan rocks, especially the carbonates, perhaps 647. combined with a more detailed structural analysis of the area T, J.E. 1969. The Kinlochlaggan Boulder Bed. Proceedings of the than we were able to carry out. Geological Society of London, 1654, 55–60. —— 1987. The structural evolution of the Dalradian of the Central Highlands of 4 April 1997 Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 78, 1–15. —— 1981. The Lower Dalradian Kinlochlaggan Boulder Bed, central Scotland. References In:H,M.J.&H W.B. (eds) Earth’s pre-Pleistocene glacial A, R. 1985. Sedimentation and tectonics in the Scottish Dalradian. record. Cambridge University Press, Cambridge, 637–639. Scottish Journal of Geology, 21, 407–436. W,J.A.&M, M.D. 1996. Chemostratigraphic correlation, structure E,M.B.&F, S. 1981. Late Precambrian tillites in Finnmark, North and sedimentary environments in the Dalradian of the NW Co. Mayo Norway. In:H,M.J.&H W.B. (eds) Earth’s pre-Pleistocene inlier, NW Ireland. Journal of the Geological Society, London, 153, 779– glacial record. Cambridge University Press, Cambridge, 606–610. 801. E, R.H.S. & T, P.W.G. 1996. A late Vendian age for the W, A.E. 1988. The Appin Group. In:W, J.A. (ed.) Later Kinlochlaggan Boulder Bed (Dalradian)? Journal of the Geological Society, Proterozoic stratigraphy of the Northern Atlantic regions. Blackie, Glasgow, London, 153, 823–826. 177–199.
J E. T, Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK (e-mail: [email protected]). R. H. S. E, Shell P.D.C. (Nigeria) Ltd, PO Box 263, Port Harcourt, Nigeria P. W. G. T, Department of Geology & Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK (e-mail: [email protected])
Scientific editing by Gordon Taylor.
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