Letters to the Editors

CORRELATION AND FACIES CHANGES IN THE CAMBRIAN/DALRADIAN

ROCKS OF THE SOUTH-WEST HIGHLANDS OF SCOTLAND

SIRS In a recent paper in this journal, Harris and Fettes (1972) have reinterpreted the structure of the Dalradian rocks of the Callander area and shown that the Leny Lime­stone, formerly thought to lie on the southern limb of a major primary fold, the downward-facing Aberfoyle Anticline (Stone 1957), is actually situated to the north of that structure, on the southern limb of a secondary downward-facing anticline. This brings the Leny Limestone structurally closer to the Loch Tay Limestone and prompts me to re-open the discussion about the possible correlation between these two lime­stones (Roberts 1966; Harris 1966; Cummins 1966).

The Loch Tay Limestone reaches southeastwards across the Loch Tay Flat Belt and the Cowal Antiform, to within ten miles of the Highland Boundary Fault, in two areas; one south of Loch Earn and the other in southern Kintyre (Shackleton 1958, Pi. xxx). In both areas, the limestone has been quarried or would be workable (Robert­son et ah 1949, pp. 50, 156) and there can be little doubt about its stratigraphic position. In both areas, the Leny Limestone is exposed not many miles away across the strike, towards the Highland Boundary Fault. These then are the areas where the correlation might be tested.

South of Lochearnhead, the horizon of the Loch Tay Limestone is intermittently exposed for a few miles in stream sections in the forested slopes, east of the main road (A 84) to Callander, between Balquhidder station and Strathyre. In this area, the strike swings from NE.-SW. in the north to N.-S. in the south, and the rocks dip at moderate angles to the east and southeast. Within a distance of about two miles, the Loch Tay Limestone changes from a thick massive limestone, with subordinate black slate layers, in the north, to soft contorted black slates, almost devoid of limestone beds, in the south; from typical Loch Tay Limestone facies to typical Leny Limestone facies, within the 'black slate affiliation', as envisaged by Roberts (1966, p. 218). The rocks adjacent to the Loch Tay Limestone in this area, both above and below, are grits and pelites, lithologically similar to the Ben Ledi Grits and Slates. Good graded bedding is common in the grits and consistently demonstrates that the stratigraphic succession is inverted. The cleavage in the grits shows that the primary structures face gently down to the south. Minor secondary folds have axial planes dipping rather more steeply towards the north. Structurally, the rocks are situated in an open synform, north of the Ben Ledi Antiform (Shackleton 1958).

Scott. J. Geol. 9, (4), 311-324, 1974

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312 LETTERS TO THE EDITORS

In southern Kintyre, the outcrop of the Loch Tay Limestone, as it swings eastwards round the nose of the plunging Cowal Antiform, is cut off by the sea. The underlying grits and pelites are inverted and, along the eastern shore of the peninsula, dip eastwards and south-eastwards (Shackleton 1958, p. 378). The outcrop of the Loch Tay Limestone, if continued beneath the sea, ought to strike NNE. along Kilbrannan Sound, between Kintyre and the Isle of Arran. In north-west Arran, east of Kilbrannan Sound, the Leny Limestone (black shales with thin limestones) is exposed along the inner limb of the Catacol Synform, the rim synform around the North Arran Granite (Shackleton 1958, fig. 10). Graded bedding in the associated grits shows that the succession is inverted. It is quite clear from the structure of north-western Arran, that the Leny Limestone should reappear on the outer limb of the Catacol Synform, beneath Kil­brannan Sound (Anderson 1947, fig. 16) dipping to the southeast.

Structurally and stratigraphically, there seems good reason to suppose, not only that the Loch Tay and Leny Limestones are lateral equivalents, but also that their outcrops may actually be physically united beneath Kilbrannan Sound. This correlation carries with it all the stratigraphic implications mentioned by Harris (1966), as well as shortening the Dalradian succession by some 3 km (Downie et al. 1971, table 3).

The Upper and Lower Ben Ledi (Leny) Grits must be equivalent, at least in part, to the Pitlochry and Ben Lui Schists respectively. This is the result of lateral facies change across the strike, over a distance of about ten miles. North of Loch Tay, the Loch Tay Limestone is overlain by typical Ben Lui Schists and underlain by Pitlochry Schists. South of Loch Earn, the Loch Tay Limestone, itself changing in facies as described above, is overlain and underlain by grits and pelites of Ben Ledi type. The possible correlation of the older Aberfoyle Slates with the Ben Lawers Schists would not involve any serious facies problems.

The importance of this correlation, as pointed out in the earlier discussion (Cummins 1966), is that it carries with it the late Lower Cambrian age of the Leny Limestone (Stubblefield 1956, p. 29). This would imply that much of the Middle Dalradian (Knill 1963, table 1) should be of Lower Cambrian age. The cross-strike correlation chart (Fig. 1) shows the base of the Cambrian just above the Portaskaig Boulder Bed and the Cambrian period beginning with a post-glacial marine transgression. This is consistent with the palynological evidence for dating the Dalradian (Downie et al. 1971, table 3) and with a Lower Cambrian age for the Dolomitic Group, suggested long ago by Peach and Home (1930, pp. 213, 214). The date of the base of the Cambrian is not well established (Cowie 1964) and one can only say that the recent Rb/Sr isochron date of 572 ± 20 m.y. for the Dolomitic Group (Leggo and Pidgeon 1970) does not conflict with the correlation proposed here.

The Middle Dalradian of the Islay-Loch Awe region is about thirty times the thickness of the Lower Cambrian succession of northwest Scotland. Much of the extra thickness is contained in three clastic formations, the Jura Quartzite, the Ardrishaig Phyllites and the Crinan Grits. The Crinan Grits thin north-westwards across the Loch Awe area, from over 1500 m to about 100 m, in a distance of little over 15 km (Knill

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LETTERS TO THE EDITORS 313

North-west South-east

Lower Cambrian of North-west Dalradian of Dalradian of Dalradian of Scotland Islay-Loch Highland Loch Tay

Awe Border Flat Belt

FIGURE I . Cross-strike correlation chart of the Cambrian-Dalradian rocks of the west of Scotland. Not to scale. The Lower Cambrian succession of the Northwest, which is only about one thirtieth the thickness of the Islay-Loch Awe Dalradian succession, is shown on the same scale as the Dalradian in the left hand column and on a suitably enlarged scale in the second column. The Durness Limestone part of the Lower Cambrian sequence (units 5 and 6) is drawn at twice the scale of the underlying beds, and the thinner stratigraphic units in all columns have been relatively enlarged for clarity of illustration. The numbered formations shown on this chart are identified below.

The Lower Cambrian succession of northwest Scotland: 1. Basal Quartzite, 2. Pipe Rock, 3. Fucoid Beds, 4. Serpulite Grit, 5. Ghrudaidh Group, 6. Eilean Dubh Group, 7. Sailmhor Group.

The Dalradian succession of the Islay-Loch Awe region: 1. Islay Limestone, 2. Portaskaig Boulder Bed, 3. Fine Quartzite, 4. Dolomitic Group, 5. Jura Quartzite, 6. Scarba Conglomerate and Transition Group, 7. Easdale Slates, 8. Socach Limestone, 9. Ardrishaig Phyllites 10. Shira Limestone, 11. Crinan Grits, 12. Tayvallich Limestone, 13. Tayvallich Lavas, 14. Loch Avich Grits.

The Dalradian succession of the Highland Border: 1. Aberfoyle Slates, 2, 3, 4. Lower Ben Ledi Grits and Slates, 5. Leny Limestone, 6. Upper Ben Ledi Grits, with Green Beds (north of Ben Ledi Antiform).

The Dalradian succession of the Loch Tay Flat Belt, shown in its palaeographical position, southeast of the Aberfoyle Anticline-Highland Border area: 1. Cam Mairg Quartzite, 2. Ben Eagach Schists, 3. Ben Lawers Schists, 4. Ben Lui Schists, 5. Loch Tay Limestone, 6. Pitlochry Schists, with Green Beds.

Q—quartzite facies; L—Limestone facies; T—turbidite facies; P—pelite facies.

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314 LETTERS TO THE EDITORS

1963, p. 109). If the other clastic formations change in thickness at the same rate, then the great disparity in thickness is no barrier to the proposed correlation.

The correlation chart (Fig. 1) illustrates the relationships between four main facies. The lower part of the succession is mainly of a shallow water quartzite facies (Q). The upper half of the succession is mainly limestone (L) in the northwest and clastic in the southeast. The clastic formations of this part of the succession can be assigned to two broad facies, one dominated by turbidites (T) and the other by pelites (P). These facies change laterally across the strike and interfinger with one another. The interfingering of limestones in the clastic formations can be demonstrated in the Loch Awe area. The Ardrishaig Phyllites rest on the Socach Limestone, east of Dalmally, and are overlain by the Shira Limestone, both limestones being confined to this area. All the clastic for­mations in this area, from the Scarba Conglomerate up to the Tayvallich Lavas, contain beds of limestone, limestone boulders, or limestone breccia (Knill 1963), especially in the northwest of the area. The lateral passage from turbidite to pelite facies above and below the Loch Tay and Leny Limestones has been discussed above.

ACKNOWLEDGEMENTS

I visited the area south of Loch Earn with Dr. R. Beeson, Mr. D. Cresswell, Mr. R. J. Hawkins, Dr. D. M. Taylor and Mr. A. J. Woodward, in April 1968, while they were final year undergraduates. Subsequently, Mr. M. A. Wilson did some independent mapping in the area, as a final year under-graduate, in the summer of 1971.

REFERENCES

ANDERSON, J. G. C. 1947. The geology of the Highland Border: Stonehaven to Arran. Trans. R. Soc. Bdinb. 61 , 479-515.

COWIE, J. W . 1964. The Cambrian Period. In Harland, W . B., Smith, A. G. and Wilcock, B. (eds.). The Phanerozoic Time-scale. Q.Jl Geol. Soc. Lond. 120s, 255-259.

CUMMINS, W . A. 1966. Stratigraphic correlation of the Dalradian rocks in the south-western Highlands of Scotland. Scott. J. Geol. 2, 316-317.

D O W N I E , C , LISTER, T. R., HARRIS, A. L. and FETTES, D. J. 1971. A palynological in­vestigation of the Dalradian rocks of Scotland. Rep. 71/9. Inst. geol. Sci.

HARRIS, A. L. 1966. Stratigraphic correlation of the Dalradian rocks in the south-western Highlands of Scotland. Scott. J. Geol. 2, 314-316.

HARRIS, A. L. and FETTES, D . J. 1972. Stratigraphy and structure of Upper Dalradian rocks at the Highland Border. Scott. J. Geol. 8, 253-264.

KNILL, J. L. 1963. A sedimentary history of the Dalradian Series. In Johnson, M. R. W . and Stewart, F. H. (eds.). The British Caledonides. Edinburgh.

LEGGO, P. J. and PIDGEON, R. T. 1970. Geochronological investigations of Caledonian history in western Ireland. Eclog. geol. Helv., 63, 207-212.

PEACH, B. N . and H O R N E , J. 1930. Chapters on the geology of Scotland. Oxford. ROBERTS, J. L. 1966. Sedimentary affiliations and stratigraphic correlation of the Dalradian rocks

of the south-west Highlands of Scotland. Scott. J. Geol. 2, 200-223. ROBERTSON, T., SIMPSON, J. B. and ANDERSON, J. G. C. 1949. The limestones of Scotland.

Mem. Geol. Surv., Sp. Rept. Min. Res. G.B., XXXV.

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LETTERS TO THE EDITORS 315 SHACKLETON, R. M. 1958. Downward-facing structures of the Highland Border. Q. Jl geol.

Soc. Lond. 113, 361-392. STONE, M. 1957. The Aberfoyle Anticline, Callander, Perthshire. Geol. Mag. 94, 265-276. STUBBLEFIELD, C. J. 1956. Cambrian palaeogeography in Britain. In Rogers, J. El Sistema

Cambrio, su palaeogeografia y elproblema de su base. Int. Geol. Congr., I, 1-43.

W. A. CUMMINS Department of Geology University of Nottingham

MS received $th February 1973

SIRS Dr Cummins' letter has served further to highlight the problems inherent in lithostratigraphic correlation where this is supplemented only by limited palaeontology and radiometric data. The lithostratigraphic succession in the Dalradian below the Loch Tay Limestone is reasonably well established, largely on the basis of strike sections. Across the strike the problems are posed by possible facies change of which we have very little direct knowledge. In spite of Dr. Cummins' persuasive and interest­ing arguments it is likely that the cross-strike correlations are not yet finalised.

One of the suggestions put forward in our recent paper (1972) is that the intermittent outcrops of black pelite with limestone found at widely separated localities in the Southern Highlands are not necessarily equivalent one to another or to that one (the Leny Limestone at Callander) which yielded the trilobite fauna to Dr Pringle. Con­sequently, the proven correlation of any of these outcrops with the Loch Tay/Tay-vallich Limestone would not justify the drawing of a time-line of more than local significance. Dr Cummins has remarked that our structural model for the Callander district has brought the two limestones closer together. Nevertheless the absence of any record of black slates with limestone on the northern limb of the secondary fold at Callander prevents any confident correlation between the Leny Quarry and the Lochearnhead limestone localities.

In the Central Highlands the stratigraphy above the Loch Tay Limestone is poorly known and the difficulties are increased by lack of detailed structural work on the inverted limb of the Tay Nappe in this sector. With our present knowledge the possible correlation of supposed Upper Dalradian with formations from the Middle Dalradian, already hinted at by Roberts (1966) can neither be verified nor dismissed.

Although related to the limestone correlation problem Dr Cummins' stratigraphic correlation with the NW. Highland foreland succession draws on a much wider body of evidence. Downie et ah (1971) suggested that the Loch Tay Limestone is probably lower in the Lower Cambrian than the Leny Limestone placed by Stubblefield in Brown et ah (1965) high in the Lower Cambrian. This proposal was partly based on the Cambrian fauna from the limestone of the Loch Nant district of Argyll which is somewhat younger than the Easdale Slate of Benderloch, the latter having yielded a fauna comparable with that from Brioverian rocks. If Dr Cummins' suggested corre­lation with the NW. Highland Cambrian is valid the Easdale Slate and its fauna must

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3i6 LETTERS TO THE EDITORS

be Cambrian in age, a possibility within the range drawn for the base of the Cambrian

in Downie et at (1971, table 3), see also Dunning (1972, p. 181). The placing of the

Dolomitic Beds on Islay into the late Precambrian seems more likely in the light of the

radiometric date of 660 m.y. obtained by Pringle (in Dunning 1972) from shales of the

Varanger Till ( = Portaskaig Boulder Bed) and according to Dr Downie (personal

communication) the microfossils are comparable with those in the Middle Brioverian.

W e are grateful for Dr Downie's comments on the manuscript of this letter.

REFERENCES

BROWN, P. E., MILLER, J. A., SOPER, N. J. and YORK, D. 1965. Potassium-argon age pattern of the British Caledonides. Proc. Yorks.geol. Soc. 35, 103-138.

DOWNIE, C , LISTER, T. R., HARRIS, A. L. and FETTES, D. J. 1971. A palynological investi­gation of the Dalradian rocks of Scotland. Rep. Inst. geol. Sci.

DUNNING, F. W, 1972. Dating events in the Metamorphic Caledonides: impressions of the symposium held in Edinburgh, September, 1971. Scott. J. Geol. 8, 179-191.

HARRIS, A. L. and FETTES, D. J. 1972. Stratigraphy and structure of Upper Dalradian rocks at the Highland Border. Scott. J. Geol. 8, 253-264.

ROBERTS, J. L. 1966. Sedimentary affiliations and stratigraphic correlation of the Dalradian rocks in the south-west Highlands of Scotland. Scott. J. Geol. 2, 200-223.

A. L. HARRIS Department of Geology, University of Liverpool.

D. J. FETTBS Institute of Geological Sciences, Edinburgh.

MS received i$th May 1973

MLCROCLINE PORPHYROBLASTS IN THE MOINIAN ROCKS OF

THE WESTERN HIGHLANDS

SIRS 1 would suggest that there is a different interpretation of the observations by

Drs Smith and Harris (1972) regarding microcline porphyroblasts. Their plate 1 seems

to be a very good example of graded bedding. 'Porphyroblasts' of microcline, ortho-

clase-perthite and oligoclase are common in semipelitic and psammitic rocks of the

Moine series in the Ross of Mull (Haynes 1969) and frequently occur at the base of

well-graded beds, which contain current-bedding in their upper parts. They also occur

in pebbly psammitic horizons within an essentially pelitic sequence. These clastic grains

of feldspar have not grown during metamorphism but have often recrystallized.

The validity of this argument is supported where metamorphic grade is considered.

Smith and Harris' statement that "it is probable therefore that garnet growth and

microcline growth were coeval during the post-F2, pre-F3 period and likely that both

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