Late Pre-Cambrian Glaciation in Scotland

MEMOIRS OF T H E G E O L O G I C A L S O C I E T Y OF L O N D O N no. 6

LATE P R E - C A M B R I A N G L A C I A T I O N IN S C O T L A N D

A. M. S P E N C E R

Price s

MEMOIRS

OF THE

GEOLOGICAL

SOCIETY

OF LONDON

no. 6

LATE

PRE-CAMBRIAN IN

GLACIATION

SCOTLAND

FRONTISPIECE Air photograph mosaic of the Garvellachs. Scale approximately 1 9 25 000. Ministry of Defence (Air Force Department) photographs, Crown Copyright reserved.

M E M O I R S OF T H E G E O L O G I C A L

SOCIETY OF L O N D O N

no. 6

LATE P R E - C A M B R I A N G L A C I A T I O N IN S C O T L A N D

ANTHONY

MANSELL

SPENCER

Research Student and Fellow, 1963-7, University of Liverpool

Published by the GEOLOGICAL

SOCIETY

OF LONDON

B U R L I N G T O N H O U S E 9 L O N D O N . W1V 0JU

1971

Submitted 21 February 1968; revised typescript received 18 July 1969 This Memoir was presented and discussed at the Geological Society meeting on 20 November 1968; the discussion at this meeting was published in Proc. geol. Soc. Lond. no. 1657, pp. 177-98. Xerox copies of a field excursion guide to the Garvellachs and the Port Askaig region may be obtained by writing to the Librarian of the Society. Further information may be obtained from the author, whose present address is: The British Petroleum Co. Ltd., Geological Division, Britannic House, Moor Lane, London E.C.2.

Published by the Geological Society of London Burlington House, London W1V 0JU March 1971 Printed by John Wright & Sons Limited, Bristol 9 Geological Society of London 1971

ABSTRACT This field study attempts to explain, in as much detail as possible, the depositional processes, environment and history of the Port Askaig Tillite in the Dalradian succession. In the 750 m thick Tillite sequence, abundant and presumably far-travelled granite stones (up to 1-5m in diameter) and sedimentary fragments (the largest of which measures 320 x 64 x 45m) are contained in 47 mixtites (till-like beds with thicknesses from 50cm to 65m), which are separated by siltstone, sandstone, conglomerate and dolomite interbeds (ranging from a few centimetres to 200m in thickness). The Tillite lies at the same horizon, between formations which are rich in carbonates and contain stromatolites, for 700kin from north-east Scotland to western Ireland. Successions at five outcrops (GarveUachs, Port Askaig, Mull of Oa, Fanad, Schichallion) are described and a type section of the Tillite, containing five members, is erected in the area round Port Askaig. These members are correlated between the outcrops described and certain individual mixtites are correlated for 160km between the Garvellachs, Islay and Fanad. Several sedimentary features are described: (of the mixtites)

their sharp lower contacts, internal bedding, soft sediment folds, sandstone downfolds and the tectonic nature of the pebble fabric; (of the interbeds) their very variable palaeocurrents, the beach conglomerates, wave-cut erosion surfaces, varves and the outsize stones and drop-in structures produced by ice-rafting. Polygonal sandstone wedges---inferred to be of periglacial origin--are described for the first time in a pre-Pleistocene formation and are contrasted with postcompactional sandstone dykes. After discussing and rejecting mass-flow (mudflow) and ice-rafting origins, evidence is presented that the mixtites were deposited by grounded ice sheets. Many interbeds and sandstone wedge horizons record ice-free conditions and at least 17 glacial advances and meltings are thus recognized. Many are recorded by the cycle: (base) marine (?) sediments deposited during a rise of sea-level, glacial mixtite, subaerial permafrost conditions (sandstone wedges), beach conditions (recording a transgression), marine sediments, etc. (top). Late Pre-Cambrian topography in the area was very flat and lowlying. This, plus continued subsidence, produced the thick, extensive tillite sequence.

CONTENTS ABSTRACT

9

V

9

1

9

5

9

5

1 INTRODUCTION (A) History of research on mixtites. (B) Previous research on the Port Askaig formation in Scotland (c) Method of treatment 9

2 STRATIGRAPHICAL

~

.

.

OUTLINE

(A) Position of the Port Askaig formation in the Dalradian succession (B) Lithology and sequence (c) The type succession. (D) Correlations 3 SPECIAL

5 6 8

FEATURES

OF

THE

SEDIMENTS

.

11

(*) The mixtites . . . . (i) The lower contacts of individual mixtites (ii) Internal bedding in mixtites (iii) Soft sediment fold structures (iv) Sandstone downfold structures . (v) Pebble fabric in the mixtites (a) The interbeds . . . (i) Palaeocurrent systems (ii) The origin of the conglomerates (iii) Erosion surfaces (iv) Beds of dolomite (v) Outsize stones in bedded sediments (vi) Varves (vii) Deflection and penetration of laminae about ice-rafted stones (c) Other structures (i) Sandstone wedges (ii) Sandstone dykes (iii) The periglacial origin of the sandstone wedges (iv) Irregular sandstone veins . (v) Stellate structures--possibly crystal pseudomorphs 9

4 PETROGRAPHY (A) (a) (c) (O)

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.

9

9

9

9

.

9

9

54

The stones . The sediments The upward change in stone content and litholog~y within the formation Conclusions

5 PETROGENESIS

9

OF

11 11 12 16 18 23 25 25 29 31 32 34 36 38 39 40 45 49 53 53

THE

9

MIXTITES

~

54 54 55 56

~

.

56

(A) The possible origins of mixtites (a) Deposition from downslope movements powered by gravity (i) Slumping, sliding or gliding (ii) Plastic mass flow (iii) Pulsatory turbulent flow (C) The evidence against a mudflow origin for ihe mixtites . . . (D) The evidence in support of a glaciaI origin for the mixtites . (i) Criteria used as evidence of former glaciations . . . (ii) Glacial criteria applied to the Port Askaig formation (E) The mechanism of glacial deposition. (i) The opinions of workers on other tillites (ii) Discussion of the evidence used by workers on other tillites to infer deposition from floating ice (iii) Deposition of the Port Askaig tillite from a grounded ice-sheet . . . . . ~

9

~

9

9

vi

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,

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.

9

9

9

9

9

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9

56 57 57 58 59 61 63 63 63 65 65 66 67

6 THE

GLACIAL

7 DESCRIPTION

RECORD

OF THE

PORT

ASKAIG

TILLITE

69

OF STRATA

71

(A) Introduction

71 72 72 75 86 87 87 91 91 92 93 94 94

(8) The Garvellachs

(c)

(D)

(E) (F) (G) (I-I)

(i) The Islay Limestone. (ii) The Port Askaig TiUite The Port Askaig area, Islay (i) The Islay Limestone. (ii) The Port Askaig TiUite (iii) The Upper Dolomitic Formation The Mull of Oa, Islay Fanad, Co. Donegal Schichallion Braemar Banffshire

8 ACKNOWLEDGEMENTS.

94

9 REFERENCES

95

.

PLATES Frontispiece

Plates 1-8 Plate 9 Plate 10 Plate 11

Air photograph mosaic of the Garvellachs (following p. 98) Photographs and photomicrographs Geological map of the Port Askaig area Stratigraphical columns Horizontal section chart and geological map of the Garvellachs

TABLES Table Table Table Table Table Table Table Table Table Table Table Table Table

The position of the Port Askaig TiUite in the Dalradian sequence . The type succession of the Port Askaig Tillite The effects produced during the Caledonian orogeny at the various outcrops "of the formation, from central Scotland to western Ireland The correlation and thicknesses of the members the Port Askaig Formation The horizons and the characters of the sandstone downfold structures Cross-stratification vector mean azimuths at individual horizons in the Port Askaig Formation Locations of well-exposed current structures List of the more important conglomerates . Stratigraphic horizons of erosion surfaces Chemical analyses of bedded dolomites and dolomite stones . The sandstone wedge horizons in the Port Askaig Formation Faulted sandstone dykes . . . . . . . . . Criteria for identifying glacial and cold climate deposits applied to the Port Askaig Formation 9

4 5 6 7 8 9 10 11 12 13

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vii

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8 10 19 28 28 30 32 33 43 47 64

A. M. SPENCER The author has examined the outcrops in the Schichallion district briefly but the degree of metamorphism and deformation is such that they yield little sedimentological information. The lithologies given by the above authors are those of most of the principal members of the formation and a little detailed mapping should confirm that the sequence is--(base) a calcareous mixtite with only carbonate fragments, a more arenaceous mixtite with carbonate and granite fragments, an arenaceous mixtite with only granite fragments and containing a quartzite intercalation and, at the top, a quartzite with a granite conglomerate horizon. (G) B R A E M A R Outcrops of the Port Askaig Formation in the intensely folded and metamorphosed rocks of Aberdeenshire, at distances of four and seven miles south of Braemar, are recorded by Barrow & Cunningham Craig (1912, 31-2). Few details of the succession are given but the following quotations show the identity of these metasediments with the Port Askaig Formation. "Two outcrops of a thin bed of conglomerate containing wellrounded pebbles of granitic rocks occur in this area; the bed in both cases lying between the Main Limestone and the quartzite . . . . The matrix here (at the first outcrop) is a very fine-grained, grey, siliceous rock, composed mainly of fine quartz grains and minute crystals of brown mica, the latter often arranged criss-eross fashion. The little granite pebbles are about two inches in diameter, or less, and consist of two types--one pink and the other grey . . . . In addition to the granite pebbles of foreign origin, there are a number of fragments of altered sedimentary material, the origin of which is mostly doubtful. There are also a few clearly of local origin, and of these the most important is a fragment of the tremolite rock, which suggests some local erosion at the base of the conglomerate". At the second outcrop the matrix, "contains a fair number of very small pebbles of the typical granites. Here the matrix is dark grey and schistose, but following the rock downhill to the north-east, towards the limestone, it becomes bluish and very like a chloritic epidiorite. Pebbles are now rare and very small, and only isolated patches of the rock can be seen". The outcrops have not been visited by the author but the above details indicate the presence of some of the higher members of the Port Askaig Formation, and possibly of member 1. (H) B A N F F S H I R E The most north-easterly outcrop of the Port Askaig Formation in Scotland is at Muckle Fergie Burn, near Tomintoul. In addition, the presence of numerous loose blocks of mixtite near Fordyce in the north of Banffshire confirms the occurrence of the formation, even though unexposed, in the Dalradian succession of the Banffshire coast. Both outcrops are described in Spencer & Pitcher (1968). They yield little sedimentological information, although the succession at Muckle Fergie Burn demonstrates the presence at least of member 2 of the formation.

8. A C K N O W L E D G E M E N T S I wish to thank Professor W. S. Pitcher for suggesting the research topic and for his continual encouragement and interest during the progress of the work, which was carried out in the Department of Geology, University of Liverpool. Assistance and discussion--mostly in the field--with Dr J. D. Taylor, Dr G. Warrington, Dr R. J. Howarth, Mrs M. O. Spencer, Mr D. H. Harrison, Dr I. M. Platten, Dr K. Bjodykke, Dr N. Rast and Professor J. Sutton, and the able servicesof Lachlan MacLachlan of CuUipool, Luing (boatman), helped the progress of the research; the criticisms of Dr K. Bj~rlykke and Mr W. B. Harland improved the manuscript. The awards of a D.S.I.R. Research Studentship, followed by a N.E.R.C. Research Fellowship, are gratefully acknowledged. The cost of printing this Memoir has been supported in part by an allocation from the Parliamentary Grant-in-Aid of Scientific Publications administered by The Royal Society. 94

Mem. geoL Soc. Lond. no. 6

CONTENTS ABSTRACT

9

V

9

1

9

5

9

5

1 INTRODUCTION (A) History of research on mixtites. (B) Previous research on the Port Askaig formation in Scotland (c) Method of treatment 9

2 STRATIGRAPHICAL

~

.

.

OUTLINE

(A) Position of the Port Askaig formation in the Dalradian succession (B) Lithology and sequence (c) The type succession. (D) Correlations 3 SPECIAL

5 6 8

FEATURES

OF

THE

SEDIMENTS

.

11

(*) The mixtites . . . . (i) The lower contacts of individual mixtites (ii) Internal bedding in mixtites (iii) Soft sediment fold structures (iv) Sandstone downfold structures . (v) Pebble fabric in the mixtites (a) The interbeds . . . (i) Palaeocurrent systems (ii) The origin of the conglomerates (iii) Erosion surfaces (iv) Beds of dolomite (v) Outsize stones in bedded sediments (vi) Varves (vii) Deflection and penetration of laminae about ice-rafted stones (c) Other structures (i) Sandstone wedges (ii) Sandstone dykes (iii) The periglacial origin of the sandstone wedges (iv) Irregular sandstone veins . (v) Stellate structures--possibly crystal pseudomorphs 9

4 PETROGRAPHY (A) (a) (c) (O)

.

.

9

9

9

9

.

9

9

54

The stones . The sediments The upward change in stone content and litholog~y within the formation Conclusions

5 PETROGENESIS

9

OF

11 11 12 16 18 23 25 25 29 31 32 34 36 38 39 40 45 49 53 53

THE

9

MIXTITES

~

54 54 55 56

~

.

56

(A) The possible origins of mixtites (a) Deposition from downslope movements powered by gravity (i) Slumping, sliding or gliding (ii) Plastic mass flow (iii) Pulsatory turbulent flow (C) The evidence against a mudflow origin for ihe mixtites . . . (D) The evidence in support of a glaciaI origin for the mixtites . (i) Criteria used as evidence of former glaciations . . . (ii) Glacial criteria applied to the Port Askaig formation (E) The mechanism of glacial deposition. (i) The opinions of workers on other tillites (ii) Discussion of the evidence used by workers on other tillites to infer deposition from floating ice (iii) Deposition of the Port Askaig tillite from a grounded ice-sheet . . . . . ~

9

~

9

9

vi

9

,

.

.

9

9

9

9

9

.

9

56 57 57 58 59 61 63 63 63 65 65 66 67

6 THE

GLACIAL

7 DESCRIPTION

RECORD

OF THE

PORT

ASKAIG

TILLITE

69

OF STRATA

71

(A) Introduction

71 72 72 75 86 87 87 91 91 92 93 94 94

(8) The Garvellachs

(c)

(D)

(E) (F) (G) (I-I)

(i) The Islay Limestone. (ii) The Port Askaig TiUite The Port Askaig area, Islay (i) The Islay Limestone. (ii) The Port Askaig TiUite (iii) The Upper Dolomitic Formation The Mull of Oa, Islay Fanad, Co. Donegal Schichallion Braemar Banffshire

8 ACKNOWLEDGEMENTS.

94

9 REFERENCES

95

.

PLATES Frontispiece

Plates 1-8 Plate 9 Plate 10 Plate 11

Air photograph mosaic of the Garvellachs (following p. 98) Photographs and photomicrographs Geological map of the Port Askaig area Stratigraphical columns Horizontal section chart and geological map of the Garvellachs

TABLES Table Table Table Table Table Table Table Table Table Table Table Table Table

The position of the Port Askaig TiUite in the Dalradian sequence . The type succession of the Port Askaig Tillite The effects produced during the Caledonian orogeny at the various outcrops "of the formation, from central Scotland to western Ireland The correlation and thicknesses of the members the Port Askaig Formation The horizons and the characters of the sandstone downfold structures Cross-stratification vector mean azimuths at individual horizons in the Port Askaig Formation Locations of well-exposed current structures List of the more important conglomerates . Stratigraphic horizons of erosion surfaces Chemical analyses of bedded dolomites and dolomite stones . The sandstone wedge horizons in the Port Askaig Formation Faulted sandstone dykes . . . . . . . . . Criteria for identifying glacial and cold climate deposits applied to the Port Askaig Formation 9

4 5 6 7 8 9 10 11 12 13

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O f .

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vii

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8 10 19 28 28 30 32 33 43 47 64

1. I N T R O D U C T I O N THIS WORK presents the results of the first exhaustive study of a very thick (750m) late Pre-Cambrian mixtite formation. The work was undertaken in order to amplify the findings of Kilburn, Pitcher & Shackleton (1965) who examined the two possible origins of the Port Askaig Formation, by mass movement (mudflows) and by glaciation and concluded that the latter was amply proved. In addition, they suggested that the mixtites were deposited not by iceberg rafting, an hypothesis favoured by many recent tillite workers (section 5.E (i)), but by ice-sheets resting on the surface of the sediments. In the present work a large amount of new evidence, gained by detailed mapping in the beautifully exposed Garvellachs and around Port Askaig, Islay (Fig. 1), is used to confirm both conclusions. The principal importance of this new evidence, however, ;~ ~

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FIG. 1. Outcrops of the Port Askaig Tillite in Scotland and Ireland (large dots). The outcrop of the Iltay Dalradian is ruled. Mem. geoL Soc. Lond. no. 6

1

A. M. SPENCER is that it provides for the first time a very detailed and clear picture of the nature and geological history of a late Pre-Cambrian glaciation. Nomenclature. Many of the terms employed in descriptions of tills and tillites have been listed and defined by Harland, Herod & Krinsley (1966, p. 228); few special terms are used here. Pebble, cobble and boulder are everywhere used formally to refer to fragments falling into the appropriate class of the Wentworth grade scale. Extra-basinal (or exotic), a term used by Schermerhorn & Stanton (1963, p. 214), is applied to stones derived from outside the basin of sedimentation in which the tillite was deposited, whilst intra-basinal stones have lithologies identical to the sediments which underlie the tillite within the basin of sedimentation. Mixtite, a non-genetic term proposed and defined by Scherrnerhorn (1966, p. 834) for rocks with "a wide range of grain sizes and characterized by a sparse to subordinate coarse fraction (phenoclasts of all shapes and sizes) englobed in a groundmass composed of varying proportions of sand, silt and clay", is preferred to another term proposed to cover such rocks, diamictite (Flint, Sanders & Rodgers 1960), because the latter includes greywackes and similar arenites (Schermerhorn 1966, p. 833). Mixtite is used here in preference to tillite, in order not to prejudge the arguments in the conclusions. (A) H I S T O R Y O F R E S E A R C H O N M I X T I T E S Harland et al. (1966, p. 227) noted that "Tillites differ from tills in that they generally belong to thicker successions and tend increasingly with age to be limited to marine and/or geosynclinal sequences". Few thick Pleistocene till sequences are exposed for study (see Charlesworth 1957, p. 222) and very few are of marine glacial origin (Armstrong, Crandell, Easterbrook & Noble 1965) and are likely to be entombed in the sedimentary record. The study of tillites may thus be expected to yield certain information on glaciations which is often lacking in the Pleistocene. Because they occur in thick sedimentary sequences, however, tillites are quite likely to be confused with mixtites formed by such non-glacial processes as sliding and mudflow. In addition, all types of glacial deposits, ranging from ice-rafted boulders to tillites formed on land, may be expected to occur in ancient sediments and, finally, all these glacial deposits may have been subjected to later mass movements. The following history of research illustrates these difficulties of interpretation. Ramsey (1855), dealing with the Permian breccias of Shropshire, England, was the first to suggest (mistakenly) a glacial origin (by ice rafting) for a pre-Pleistocene sediment. Evidence for a late Pre-Cambrian glaciation was first recognized at Port Askaig by Thomson (1871), although the geological age of the formation was then uncertain. By 1875 Croll was able to occupy 19 pages of his book Climate and Time with accounts of "Former Glacial Epochs; Geological Evidence of". The trend for interpreting mixtites as glacial deposits has continued until recently; Harland (1964A, p. 119) noted that, "For a time it was fashionable to interpret boulder beds in this way (e.g. Ramsey 1880; David 1907A); by 1926 (Coleman) evidence of ice ages in many periods and continents seemed well established. However, some of these on closer inspection proved to be non-glacial in origin, and after the impressive evidence for tillites in Vol. 6 of the report of the 1937 I.G.C., Moscow (1940), the pendulum swung to the present position, in which doubt about some tillites seems to have inhibited thought on the implications of others". Bailey, Collet & Field (1928) were amongst the first to re-interpret mixtites of allegedly glacial origin, the 'Conglomerates' at Quebec and Levis, as submarine landslips formed in the earlier stages of the development and deformation of a geosyncline. More recently, following the emergence of the turbidity current hypothesis, Crowell (1957, 1964) has proposed that the majority of 'pebbly mudstones' can be explained as submarine slump or mudflow deposits, associated with and grading into turbidity current deposits. Newell (1957), Dott (1961), Winterer (1964) and Condie (1967) have convincingly re-interpreted, along the lines suggested by CroweU, deposits which were previously supposed to be glacial. The deposits described by them are relatively local and isolated in their occurrence and are not (with the possible exception of the Granville 2

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND deposit described by Winterer) members of very extensive, penecontemporaneous mixtite horizons. In this respect Schermerhorn & Stanton's (1963) interpretation of the late Pre-Cambrian mixtites of Equatorial West Africa as submarine mudflow deposits is a potentially more serious challenge to the glacial hypothesis, for the mixtites described outcrop at the same stratigraphical horizons over an area in excess of 800km by 150km. The interpretation proposed by them is critically discussed in section 5.B ('lii). Many authors have concluded in favour of glacial origins for mixtites occurring in thick sedimentary sequences and the general problem of the origin of such mixtites has been highlighted by two recent conferences (Nairn 1964 and Geol. Rdsch. 1964, 1-522). Recent papers on the late Palaeozoic glaciation include an important work by Carey & Ahmed (1961), who discuss the theory of sedimentation from ice shelves and icebergs, giving an outline of the application of their theories to the Tasmanian Permian. Hiibner (1965), Whetten (1965) and Rattigan (1967) describe laminated sediments of late Palaeozoic age which show evidence of iceberg rafting. Frakes, Matthews, Neder & Crowell (1966), Frakes & Crowell (1967), Martin (1964A), Long (1964), Teichert (1967) and David (1907B) all describe thick tillite sequences of late Palaeozoic age many of which contain tills with preferred pebble fabrics and/or striated pavements (sometimes occurring within the tillite sequences), presumably deposited by grounded ice-sheets. Recent reviews of some late Palaeozoic glacial deposits and of the late Palaeozoic glaciation are given by Hamilton & Krinsley (1967) and Frakes & Crowell (1968), respectively. Beuf, Biju-Duval, Stevaux & Kulbicki (1966) and Rognon, Charpal, Biju-Duval & Gariel (1968) have summarized the evidence which indicates a Silurian glaciation in the Sahara. The relatively thin tillite sequence contains more than one striated pavement horizon and appears to have been deposited by a continental ice-sheet or ice-sheets. In the Pre-Cambrian, the Eocambrian glacial deposits of Norway have recently been reviewed by Spjeldnaes (1964) and described by Reading & Walker (1966) and Bjorlykke (1966, 1967). They mostly conclude in favour of ice rafting mechanisms. Dow (1965), Perry & Roberts (1968) and Schenk (1965) describe striated pavements associated with relatively thick Pre-Cambrian tillite sequences; the first two are from Australia and the last from Canada. Martin (1965) described a thick late Pre-Cambrian tillite and inferred (p. 24) a largely glacio-marine origin. Biju-Duval & Gariel (1969) have recently described a late Pre-Cambrian tillite from the Sahara. From the above summary it is clear that the glacial origin of many mixtite sequences is now accepted and in many cases a particular type of glacial mechanism (i.e. ice rafting, sedimentation from floating or grounded ice shelves, deposition from continental ice-sheets) is suggested. In most cases, however, adequate criteria to discriminate between these possibilities are not presented. In the present work such criteria are discussed in full and the resulting conclusions are then used in extending the analysis to demonstrate the glacial history of the Tillite Formation.

(B) P R E V I O U S R E S E A R C H O N T H E P O R T A S K A I G FORMATION

IN SCOTLAND

Because it contains large granite boulders of unknown source, the Port Askaig Formation has long been known as a distinctive marker horizon (the 'Boulder Bed') within the metamorphosed sequence of the Scottish and Irish Dalradian. It is present at many localities from north-east Scotland to the west coast of Ireland (Fig. 1). MacCulloch (1819, pp. 158, 249) discovered the outcrops at Port Askaig on Islay and in the Garvellachs, suggested they lay at the same horizon as the Schichallion 'Boulder Bed' and described the upwards sequence from dolomitic to more arenaceous rocks in the Garvellachs. Thomson (1871) suggested a glacial origin (by ice rafting), for the formation. B. N. Peach visited the Garvellachs for H.M. Geological Survey and was surprised by the little altered state of the rocks. He recognized the existence of mixtites separated by sandstones and dolomites but interpreted the fold in the raft in the Great Breccia of Eileach an Mem. geoL Soc. Lond. no. 6

3

A. M. SPENCER Naoimh (P1. 1) as tectonic and consequently failed to appreciate the true thickness or sequence in the formation (Peach, Kynaston & Maufe 1909). On Islay, S. B. Wilkinson (1907) of H.M. Geological Survey recognized the presence of some bedded horizons within the outcrop of the Port Askaig Formation, as did Bailey (1916) and Allison (1933). Both in the GarveUachs and Islay however, the great thickness and complex sequence of the formation were first realized and measured by Pitcher and Shackleton (in Kilburn et al. 1965). They recorded several new outcrops of the formation in Ireland, proposed an outline correlation of the fillite sequence and described the lithology of the Garvellachs sequence in detail. (c) M E T H O D

OF

TREATMENT

A stratigraphical outline of the formation is given (section 2). This includes brief accounts of the lithology and sequence of the formation, an account of the five members into which the formation can be divided (these are formally defined and provide a type-sequence for the formation around its title locality of Port Askaig) and a section on the correlation of the successions seen at several outcrops of the formation. The bed by bed descriptions of the successions at Schichallion, the Garvellachs, Port Askaig, the Mull of Oa and Fanad, although important, are probably not of wide interest; they are therefore placed at the end of the text as section 7. Next, because of their genetic importance, particular structures and features of the mixtites and interbeds, as well as certain other structures, are described in detail (section 3); they provide much of the evidence on the origin of the mixtites. The petrography of the sediments is described (section 4). The mechanism of origin of the mixtites is discussed in detail (section 5). Firstly, although the conclusions of Kilburn, Pitcher & Shackleton are not in question, the recent theories of formation of mixtites by downslope movements powered by gravity are critically examined and, in the case of the Port Askaig mixtites, the evidence against a mudflow origin and in support of a glacial origin is given; all three sections are of general applicability. Secondly, the evidence, outlined by Kilburn et al. (1965, p. 356), which indicates the nature of the glacial origin of the mixtites is given in full. The conclusion that the mixtites were deposited from grounded whitesondstone I~!!i~i{ii!iiilt

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dolomite

u

FIG. 2. Key to the ornaments used in the text-figures. ice is confirmed. A consequence of this conclusion is that the formation records a complex sequence of glacial advances and retreats and so, finally (section 6), the glacial record of the formation is examined and the basic glacial advance-retreat cycle is recognized. British National Grid References are given in the form [1431 6668]; references in the form {465 058} are to the grid on P1. 11. 4

Mem. geol. Sac. Land.

no.

6

LATE PRE-CAMBRIAN

GLACIATION

2. S T R A T I G R A P H I C A L

IN SCOTLAND

OUTLINE

(A) P O S I T I O N O F T H E P O R T A S K A I G F O R M A T I O N DALRADIAN SUCCESSION

IN THE

This was not established until Allison (1933) confirmed the sequence on Islay proposed by Bailey (1916). The formation everywhere lies at the same horizon, between a limestone formation beneath and a dolomitic formation, overlain by a quartzite, above. The formation is presumed to be late Pre-Cambrian in age, for it lies low down in the thick Dalradian sequence (Table 1), probably some 10000 m (Rast 1963, p. 126) beneath the Leny Limestone, which has yielded a late Lower Cambrian fauna (Pringle 1940; Rayner 1965; Stubblefield 1956, p. 29).

TABLE 1" The position of the Port Askaig Tillite in the Dalradian sequence

Approximate thicknesses (Rast 1963, p. 126)

Central Highlands (slightly modified from Knill 1963, p. 100, using Kilburn et al. 1965, p. 347)

lslay

4~0m

Top not seen Ben Ledi Grits*** Green Beds Pitlochry Schist Loch Tay Limestone

Middle Dalradian

5000m

Ben Lui Schist Farragon Group Ben Lawers Schist Ben Eagach Schist Carn Mairg Quartzite (Succession continues above Killiecrankie Schist this level) Schichallion Quartzite = Jura Quartzite Upper Dolomitic Formation = Upper Dolomitic Formation Port Askaig Tillite = Port Askaig Tillite

Lower Dalradian

1000m to 2000m

Upper Dalradian

Blair AthoU 'Series' (excluding the tillite) Base not seen

= Islay Limestone Mull of Oa Phyllites Maol an Fhithich Quartzite Base not seen

***,Indicates a stratigraphical level equivalent (Stone 1957) to that of the Leny Limestone, which has yielded late Lower Cambrian trilobites.

(13) L I T H O L O G Y

AND SEQUENCE

The Port Askaig Formation consists of mixtites (unsorted and usually unstratified till-like beds) separated by stratified siltstone, sandstone, conglomerate and dolomite interbeds. The mixtites contain abundant rock fragments scattered randomly through them (Pls. 2, 3). Granite stones of up to 1.5 m in diameter are present and must presumably be far-travelled, having been brought from outside the basin of deposition (the Caledonian geosyncline). The largest intra-basinal fragment measures 320 x 64 x 45 m. Adjacent mixtite beds often have subtly different lithologies and are more easily distinguished in some cases by those differences Mem. geol. Soc. Lond. no. 6

5

A. M. SPENCER than using the thin, and sometimes discontinuous, stratified interbeds. Shackleton and Pitcher (Kilburn et al. 1965, fig. 3) distinguished 38 successive mixtites in the sequence on the east and south coasts of Garbh Eileach in the Garvellachs, numbering them from the base upwards (P1. 10). Despite difficulties in applying these numbers to the type sequence at Port Askaig, the introduction of a new numbering system was not thought to be justified and the same numbers are retained. They are used extensively here when referring to particular mixtite beds, e.g. mixtite 17. It must be stressed, however, that not all the interbeds--and consequently not all the mixtites--should be regarded as of equal status (see section 6). Certain of the interbeds thin to zero laterally and where absent the adjacent mixtites may merge inseparably. This is no disadvantage to the numbering scheme and the possibility is covered by referring to the appropriate group of mixtites thus: mixtites 27-29 (meaning mixtites 27, 28 and 29). In the Port Askaig district of Islay the formation measures 750m in thickness and contains about 22 separate mixtite beds (P1. 10e-h). Because of the unconformity beneath the formation there, the lowest beds are much thinner than in the outcrop on the Garvellachs (P1. 10a, b), where there is little evidence of a break in sedimentation beneath the formation. The complete, but composite, sequence obtained by adding the lowest part of the formation seen in the Garvellachs to the sequence seen at Port Askaig produces a maximum thickness for the formation of approximately 870m. Forty-seven mixtite beds occur in this combined sequence. The mixtites in the formation vary from as little as 50cm in thickness up to a maximum of 65m and usually maintain an even thickness when traced laterally. The interbeds range from only a few centimetres in thickness to a sandstone measuring 200m. There is a marked over-all change in lithology upwards in the formation. Both mixtites and interbeds are dolomite-rich at the base of the formation and change to quartzofeldspathic at the top, whilst bedded dolomites are present only in the lower half of the formation. The stone content of the mixtites also changes in a gradational manner; the lowest mixtites contain only intra-basinal stones, whilst extra-basinals become more abundant upwards and are overwhelmingly dominant in most of the highest mixtites. This upwards sequence from dolomitic to more arenaceous rocks was used by Pitcher and Shackleton to subdivide the formation (Kilburn et aL 1965, p. 347). Their three-fold division of the formation is here further refined and five members are proposed (see below). More important, they took as their standard the sequence seen in the Garvellachs. Their correlation of this sequence (op. cit., table 1) is now known to be incorrect, for the highest two members in formation at Port Askaig must lie unexposed beneath the sea in the Garvellachs (except for the outcrop on the isolated skerry of Dubh Fheith). Because of this it is proposed to select the complete, but less well exposed sequence of the Port Askaig area as the type and to formally define the members of the formation at outcrops there. (C) T H E T Y P E S U C C E S S I O N The formation. The name Port Askaig Tillite is designated as the formal name of the formation and the

Port Askaig district is chosen as the type area. No single section in this area provides a complete succession through the formation (see section 7.C) and the latter has therefore been built by combining, by means of detailed field mapping, the successions in the five members at the localities where the members are designated. The distinguishing characters, dimensions and correlations of the formation are the subject of this work and need not be elaborated here. The base of the formation is drawn beneath the lowest mixtite bed and the top is drawn above the level of the highest horizon with extra-basinal stones, which at Port Askaig is a conglomerate. The underlying and overlying formations, the Islay Limestone and Upper Dolomitic Formation, have not been formally defined and this is not attempted here. Nevertheless, the dolomitic beds above the Tillite are not here included in the Tillite ( e l Kilburn et al. 1965) for they contain quite different sediments and, at least on Islay, are sufficiently thick and extensive to rank as a separate formation. The members. The formal names are taken from the type localities of the members (see P1. 9) and are shown in Table 2; for the sake of brevity the members will be referred to by number rather than name. The 6

Mem. geoL Soc. Lond. no. 6

A. M. SPENCER than using the thin, and sometimes discontinuous, stratified interbeds. Shackleton and Pitcher (Kilburn et al. 1965, fig. 3) distinguished 38 successive mixtites in the sequence on the east and south coasts of Garbh Eileach in the Garvellachs, numbering them from the base upwards (P1. 10). Despite difficulties in applying these numbers to the type sequence at Port Askaig, the introduction of a new numbering system was not thought to be justified and the same numbers are retained. They are used extensively here when referring to particular mixtite beds, e.g. mixtite 17. It must be stressed, however, that not all the interbeds--and consequently not all the mixtites--should be regarded as of equal status (see section 6). Certain of the interbeds thin to zero laterally and where absent the adjacent mixtites may merge inseparably. This is no disadvantage to the numbering scheme and the possibility is covered by referring to the appropriate group of mixtites thus: mixtites 27-29 (meaning mixtites 27, 28 and 29). In the Port Askaig district of Islay the formation measures 750m in thickness and contains about 22 separate mixtite beds (P1. 10e-h). Because of the unconformity beneath the formation there, the lowest beds are much thinner than in the outcrop on the Garvellachs (P1. 10a, b), where there is little evidence of a break in sedimentation beneath the formation. The complete, but composite, sequence obtained by adding the lowest part of the formation seen in the Garvellachs to the sequence seen at Port Askaig produces a maximum thickness for the formation of approximately 870m. Forty-seven mixtite beds occur in this combined sequence. The mixtites in the formation vary from as little as 50cm in thickness up to a maximum of 65m and usually maintain an even thickness when traced laterally. The interbeds range from only a few centimetres in thickness to a sandstone measuring 200m. There is a marked over-all change in lithology upwards in the formation. Both mixtites and interbeds are dolomite-rich at the base of the formation and change to quartzofeldspathic at the top, whilst bedded dolomites are present only in the lower half of the formation. The stone content of the mixtites also changes in a gradational manner; the lowest mixtites contain only intra-basinal stones, whilst extra-basinals become more abundant upwards and are overwhelmingly dominant in most of the highest mixtites. This upwards sequence from dolomitic to more arenaceous rocks was used by Pitcher and Shackleton to subdivide the formation (Kilburn et aL 1965, p. 347). Their three-fold division of the formation is here further refined and five members are proposed (see below). More important, they took as their standard the sequence seen in the Garvellachs. Their correlation of this sequence (op. cit., table 1) is now known to be incorrect, for the highest two members in formation at Port Askaig must lie unexposed beneath the sea in the Garvellachs (except for the outcrop on the isolated skerry of Dubh Fheith). Because of this it is proposed to select the complete, but less well exposed sequence of the Port Askaig area as the type and to formally define the members of the formation at outcrops there. (C) T H E T Y P E S U C C E S S I O N The formation. The name Port Askaig Tillite is designated as the formal name of the formation and the

Port Askaig district is chosen as the type area. No single section in this area provides a complete succession through the formation (see section 7.C) and the latter has therefore been built by combining, by means of detailed field mapping, the successions in the five members at the localities where the members are designated. The distinguishing characters, dimensions and correlations of the formation are the subject of this work and need not be elaborated here. The base of the formation is drawn beneath the lowest mixtite bed and the top is drawn above the level of the highest horizon with extra-basinal stones, which at Port Askaig is a conglomerate. The underlying and overlying formations, the Islay Limestone and Upper Dolomitic Formation, have not been formally defined and this is not attempted here. Nevertheless, the dolomitic beds above the Tillite are not here included in the Tillite ( e l Kilburn et al. 1965) for they contain quite different sediments and, at least on Islay, are sufficiently thick and extensive to rank as a separate formation. The members. The formal names are taken from the type localities of the members (see P1. 9) and are shown in Table 2; for the sake of brevity the members will be referred to by number rather than name. The 6

Mem. geoL Soc. Lond. no. 6

LATE P R E - C A M B R I A N G L A C I A T I O N IN S C O T L A N D division of the formation into five members is not arbitrary. Each of the members has a distinctive nature and certain of the characters of the members change sharply at their contacts, and these boundaries can therefore be drawn with accuracy and consistency. F o r example, m e m b e r 2 contains only thin, brown sandstone

TABLE2: The type succession of the Port Askaig Tillite

Lithology Member names

Mixtite bed numbers

Thickness

5. Con Tom

47 46 45

325m

Thin granite conglomerates

4. Ruadh-phort Beag

44 43 42 41 40 39

200m

Dark, silty sandstones

3. Creagan Loisgte

37-38 ?36 33-35

94m

Light-grey silty sandstones

> 75

< 95 ~o

Thick, white sandstones

2. An Tamhanachd

32" 32' ?24 19-22

82m

Grey dolomitic sandstones

> 25

< 75 ~o

Several thin, dolornitic sandstones

1. Beannan Buidhe

718 ?17 ?16 Disrupted beds 13

47m

Yellow dolomitic siltstones

Matrix of mixtites (approximately)

Proportion of extra-basinal stones in mixtites

Interbeds

Upper dolomitic formation > 50

< 90 ~o

> 95 ~o

Thick, white sandstones Few, thin sandstones

PORT ASKAIG TILLITE

<25~o

Dolomitic sandstones, dolomite conglomerates and dolomites

Islay limestone

interbeds and the base of member 3 is therefore placed at the base sequence. The contacts of members 3, 4 and 5 are of a similarly Members 1 and 2 are easily distinguished in the Garvellachs but are and Fanad. In all three localities, however, a thin bed which is m e m b e r 2 and confirms the positioning of the contact. Mem. geol. Soc. Lond. no. 6

of the first thick white sandstone in the striking and easily recognizable nature. more difficult to separate at Port Askaig rich in iron ore occurs at the base of

7

A. M. S P E N C E R (D) C O R R E L A T I O N S

(i) THE FORMATION The identical stratigraphical position of the formation at many outcrops in the Scottish and Irish Dalradian has been demonstrated previously (Kilburn et al. 1965). On a larger scale still, the author agrees with Kilburn et al. (I 965, p. 354) that the many resemblances between the Port Askaig TilIite and the sequence in which it is enclosed and the Tillites and late Pre-Cambrian sequences of East Greenland (Schaub 1950), Spitsbergen (KulIing 1934) and Norway (Bjorlykke 1967, fig. 15) suggest a wider correlation. The latter tillites lie (respectively) 400m, 740m and 200 to 400m beneath the lowest horizons containing Lower Cambrian fossils, so final acceptance of this correlation must await discovery of Cambrian fossils relatively close above the Port Askaig Formation, perhaps in the extensive pelitic formations above the Jura Quartzite. Within the Port Askaig Formation the correlations proposed by Kilburn et al. (1965, p. 348) are, apart from the mistaken position of the GarvelIachs succession, generally supported and amplified; correlations at member and then bed level are proposed below. Before detailing these correlations, however, it will be useful to outline the varying metamorphic and structural changes suffered by the sediments during the Caledonian orogeny. This should serve to emphasize the remarkable similarity of the sequences seen at these often isolated outcrops. TAnLE 3" The effects produced during the Caledonian orogeny at the various outcrops of the formation, from central Scotland to western Ireland

Results of deformation on

CONNEMARA (/Ilion, Lissoughter)

SOUTHERNDONEGAL (Glencolumbkille)

Sediments

Schistosities and minor folds

Schistosities, few minor folds

Cleavages

2 cleavages

1 main cleavage Schistosities and minor folds

Dolomite stones

Flattened and re-folded

Maximum elongation 16:1

Maximum elongation 5:1

Somewhat deformed

Maximum Considerably elongation of elongated oolites 2 : 1

Granitic stones

Considerably stretched

Somewhat stretched

Stratigraphical thicknesses

Difficult to Modified measure and probably considerably modified

Sediments

Completely recrystaUized

Results of metamorphism on

FANAD

ISLAY

GARVELLACHS

SCHICHALLION

Probably undeformed

Undeformed

Somewhat deformed

? Probably only slightly modified

Only slightly modified

Difficult to measure and probably considerably modified

Silt and clay grade material recrystallized, sand grade material little altered (original outlines preserved)

Completely recrystallized

Dolomite stones

Converted to green marble

Granoblastic dolomite

Still fine grained yellow dolomites

Granoblastic dolomite

Textures of granitic stones

Considerably modified

Somewhat modified

Little changed

Modified

Unchanged

Structure and metamorphism. Table 3 gives, in outline, a guide to the changes which the rocks at various

outcrops of the formation have undergone during the Caledonian orogeny. In the Schichallion district, the 8

Mem. geol. Soc. Lend. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

south of Donegal and in Connemara these changes have been very considerable; the rocks show evidence of several phases of deformation, metamorphic recrystallization is severe and the dolomite and sometimes the granitic stones have changed in shape. In the Garvellachs-Port Askaig-Fanad area the rocks show much less alteration; minor folding is slight, only one or two prominent cleavages are present and metamorphism has modified, but not obliterated, the sedimentary textures. For this reason most examples of sedimentary structures described in section 3 are from the Garvellachs and correlation of individual mixtite beds is attempted with confidence only in the Garvellachs-Port Askaig-Fanad area; correlations with the sequences at Schichallion, Glencolumbkille and in Connemara are effected at member level only. The stratal thicknesses now seen are undoubtedly somewhat modified (probably reduced), but it is difficult to estimate the amount of alteration accurately especially as it probably varies with lithology. Granitic stones (which are undeformed) in the varved siltstones seen in the Garvellachs (see Fig. 19) suggest that deformation there has produced only a slight reduction in stratal thicknesses and this is also probably true in the Port Askaig and Fanad areas. Nevertheless it must be stressed that all the rocks described in this work are deformed and the thicknesses now measured are certainly not identical to those originally deposited. (ii) THE MEMBERS Table 4 shows the proposed correlation of the members in the formation, using the characters outlined in Table 2, for a total distance of 500 km between seven outcrops. In the Garvellachs, Port Askaig, Mull of Oa and Fanad outcrops the members are easily distinguished and all are correlated with great certainty. In the other outcrops the contacts between members cannot be distinguished with such certainty (e.g. member 3 has not yet been distinguished at Schichallion and Cleggan) but the members can usually be recognized and are always present in the same order. At Glencolumbkille using the succession units erected by Howarth, Kilburn & Leake (1966, 131-3), the author proposes the following correlation: member 1--units 1 and 2; member 2--unit 3; member 3--units 4 to l0 (probably); member 4---units 11 to 26 (probably); member 5-unit 27. The thick sandstones in member 4 here make the boundary between members 3 and 4 difficult to place. At Knockateen, another locality in Donegal, the faulted lower contact of the sequence (Kilburn et al. 1965, p. 348) is confirmed; members 4 and 5 only are present.

(iii)

INDIVIDUAL BEDS

Certain individual beds can be correlated between the outcrops of the formation in the Garvellachs, at Port Askaig and at Fanad (P1. 10). From the base upwards these beds are: Member 1 : representatives ofmixtites 1-12 of the Garvellachs occur at Fanad; the Great Breccia and Disrupted Beds occur both at Port Askaig and the Garvellachs; mixtites 14-18 of the Garvellachs are represented at Port Askaig. Member 2: a mixtite containing green fragments and rich in magnetite occurs at the base of the member in all three localities; mixtites 19-22 of the Garvellachs and the sandstone wedges at their top also occur at Port Askaig; varved siltstones occur close to the top of the member both in the Garvellachs and at Port Askaig. Member 3: mixtites 33-35 of the Garvellachs are present both at Port Askaig and (probably) at Fanad. Member 4: the particularly arenaceous mixtite, number 44, at its top at Port Askaig is also present at the top at Fanad; in both localities this mixtite rests on a sandstone which overlies a sandstone wedge horizon. Member 5: three (possibly four) mixtites are present in similar positions in the member both at Port Askaig and Fanad. None of the sequences is identical however, and the following is a list of the more important differences. Member 1 is quite variable in thickness: mixtites 1-11 (those containing almost no exotic stones) are present only at Schichallion, the Garvellachs and Fanad. Elsewhere these beds are probably cut out along the disconformity which is seen at the base of the formation at Port Askaig and the Mull of Oa. This disconformity is slight, however, for the formation everywhere rests on a thin dolomitic member which lies at the top of the Islay Limestone (Kilburn et al. 1965, p. 347). Other examples of beds which must thin out laterally are the Mem. geol. Soc. Lond. no. 6 41

A. M. SPENCER

Great Breccia and the Disrupted Beds, both of which are absent at Fanad. Member 2 has a less complex sequence at Port Askaig and Fanad than in the Garvellachs. The sequences at Glencolumbkille and Cleggan show two important differences with the other successions; member 3 is difficult to distinguish and conglomerate beds are absent from the highest member.

(iv) COMMENTS ON CORRELATIONS

When considered at member level, the same sequence is present in the Port Askaig Formation from Schichallion to Connemara, a distance of over 500 km. In addition, several individual beds are present in the successions at three outcrops, spaced a total of 160km apart, but none of the successions is identical. The formation is thickest in the (probably Schichallion) Garvellachs-Port Askaig-Fanad area; the small thicknesses seen in the Glencolumbkille and Cleggan outcrops are unlikely to be due entirely to tectonic deformation.

TABLE 4: The correlation and thicknesses of the members of the Port Askaig Formation

Outcrops and the distances between outcrops in km

CLEGGAN, 1

GLENCOLUMBKILLE2

CONNEMARA 160

FANAD

80

100

Jura quartzite formation

White quartzite

White quartzite

White quartzite

Upper dolomitic formation

Semipelites and quartzite 35m

Pelitic and calcareous schist 15m

Dolomitic sandstone, pelite and dolomite 40m

o

~9

P O R T ASKAIG

32 White quartzite

Unexposed (faulted)

GARVELLACHS

48

SCHICHALLION

110

White quartzite

White quartzite

Dolomites, pelites and sandstones 80m +

Pelites, dolomites ar magnesian limestones. Tens of metres

325 m

Unexposed (sea)

Hundreds of metres

5. Con Tom member

75m

13m

180m

4. Ruadh-phort Beag member

40 m

? 54 m

175 m

200m

3. Creagan Loisgte member

?45m

? 30m

25m

94m

215m

?

2. An Tamhanachd member

35m

6m

30m

30m

82m

141 m

Tens of metres

1. Beannan Buidhe member

? Absent

8m

25 m

8m

47m

222m

Tens of metres

Dolomites, pelites and limestones

Dolomites, pelites and limestones

Dolomites and limestones

White limestone (magnesian) and pelites

748 m

578 m (incomplete)

Several hundred metres

o

"~ < 1:: o ~.,

M U L L OF OA

Islay limestone formation

Total thickness of Port Askaig Formation

White marble

195 m

Dolomites, pelites and limestones

111 m

Limestone and pelites

435 m

x Approximate thicknesses measured by the author (latitude and longitude of outcrop--53~ a Approximate thicknesses from Howarth, Kilburn & Leake (1966, 131-3).

10

Hundreds of metres

' N, 10~189, W).

Mem. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND

3. S P E C I A L

FEATURES

OF T H E

SEDIMENTS

Under this heading various structures which are new and/or of particular importance in interpreting the origin of the formation are described and discussed. They are dealt with in three groups: structures present in the mixtites, or structures closely associated with them; structures and features of the interbeds; and, lastly, four structures which do not fit into the previous groups. The majority of the examples described are from the Garvellachs and the rest are mostly from the Port Askaig area. In most cases the importance of the structure is indicated, the structures are described and then their origin is discussed. (A) T H E M I X T I T E S (i) TI-IE LOWER CONTACTS OF INDIVIDUAL MIXTITES From the point of view of the mechanism of mixtite formation, the lower contact of such a bed is especially important because across this surface the mudflow or ice-sheet which deposited the mixtite must have flowed or, alternatively, on this surface deposition of material from floating icebergs must have commenced. Description. Almost all the mixtites in the Garvellachs have conformable lower contacts along the whole of their outcrop length (P1. 11); only two examples of disconformable relationships have been seen (beneath mixtite 1, see Fig. 33; beneath mixtite 26 on A'Chuli), the larger of which, the latter, involves the mixtite in transgressing only 6m of bedded sandstone. With the exception of the disconformities present in the lower part of the Port Askaig Formation at Loch Lossit and the Mull of Oa, the mixtites seen on Islay have similar conformable bases. In detail, the lower contacts of mixtites are most commonly knife sharp, the change-over from bedded, sorted sediment to mixtite occurring in a thickness of a few millimetres (P1. 4a). Less commonly the contacts are slightly gradational through thicknesses of 1 m or less. Bedding may be less strongly developed than normal or even absent in the uppermost 50cm of the sediment beneath the mixtite and exotic cobbles or pebbles sometimes occur at this horizon. (Both these features are well shown beneath mixtite 36 at the landing jetty on Garbh Eileach.) The lowermost 30cm or so of the mixtite may be more pelitic than normal or may contain bedded sandstone or siltstone lenses more commonly than the overlying mixtite (Fig. 3). This type of gradation is never perfectly developed, however, and a sharp contact between mixtite and bedded sediment can usually still be recognized. The lower contacts of mixtites are 0*

o

~250"

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FIG. 3. Field sketches of the base of mixtite 37 at its easternmost outcrop, Garbh Eileach. The strike and dip of the surface of the exposure are indicated. Mem. geol. Soc. Lond. no. Ci

11

A . M. S P E N C E R

rarely absolutely planar but the amplitude of any irregularities is small, usually less than 20cm. As a corollary to the conformable and almost planar nature of the lower contacts it is only very rarely that there is any sign of disturbed bedding around the contact; in the few cases in which this is present it is on a small scale (see Fig. 6). Finally, bottom structures, such as flute casts or load casts, are not present on the lower contacts of mixtites and striations have not been seen on these surfaces. Interpretation. The very sharp lower contacts of mixtite beds are difficult to explain by iceberg rafting. Firstly, the continuous sedimentation associated with the gradual increase in the numbers of icebergs to glacial maximum might typically be expected to produce a gradational upward sequence from normal bedded sediments to unbedded mixtites--perhaps occupying a thickness of several centimetres to several metres, depending upon the over-all rate of sedimentation. Such gradational lower contacts to mixtites are absent here. Secondly, sedimentation at the base of the Port Askaig mixtites was not continuous, for unbedded mixtites often rest on, for example, sandstones with large scale cross-stratification. Discontinuous sedimentation could result in sharp lower contacts to ice-rafted mixtites, if the mechanism of sedimentation of the interbeds came to a halt and was replaced--after some interval--by iceberg sedimentation alone. Such an arrangement cannot be ruled out, but it is difficult to think of any reason--to account for the ubiquitous presence of sharp contacts--why normal sedimentation should in every case halt before iceberg sedimentation commenced. Sharp contacts might be expected beneath mixtites deposited by continental ice-sheets, however, and are present beneath some Pleistocene land tills. The completely conformable nature of most lower contacts, even when traced over long distances, is nevertheless remarkable. There are very few signs of disturbance of the underlying bedded sediments by the movements of 'over-riding' ice-sheets (a few examples are given in section 3.A (iii)). (ii) INTERNAL BEDDING IN MIXTITES Certain of the mixtites within the Port Askaig Formation (numbers 2 and 19-22 in the Garvellachs, all those in member 4 at Port Askaig) are nearly homogeneous, containing very few bedded horizons. Although such homogeneity and lack of bedding is one of the most important and striking characters of mixtites, bedded horizons do occur within mixtites and are present in approximately 10 per cent of the cumulative thickness of all the mixtites in the Port Askaig Formation. The criterion which shows that the bedded horizons lie within mixtites (and do not just separate mixtites formed by two successive events) is that the horizons thin laterally to zero and where they are absent the mixtite is completely homogeneous and undivided. This criterion, together with the fact that the bedded horizons are not transported blocks of material, must mean that the bedded horizons formed whilst the mixtite material around them was also being deposited. For this reason the discontinuous bedded horizons have a much greater importance than their small frequency of occurrence might suggest, for they provide the only direct evidence of the mode of deposition of the mixtites. These discontinuous bedded horizons are described below, starting with examples of simple, isolated lenses and considering progressively more complicated arrangements. Description. The best examples of discontinuous bedded horizons seen in the Garvellachs are the siltstones (1 m thick) which lie 7m above the base and 1m beneath the top of mixtite 9 in its completely exposed outcrop on Eileach an Naoimh. Both horizons thin to zero laterally and where they are absent the mixtite is homogeneous and undivided (P1. 11). A smaller but equally clear example is provided by the granite conglomerate horizon within mixtite 30 on the east coast of Garbh Eileach. This reaches a maximum thickness of 60cm but for a distance of a few metres along the outcrop it is absent and the mixtite appears to be unbroken in its succession. A similar example is the dolomite conglomerate which separates mixtites 2 and 3 and which thins laterally to zero in outcrops on the north coast of Garbh Eileach and on Dun ChonnuiI; where the conglomerate is absent mixtites 2 and 3 merge inseparably (Fig. 4a). A great many other examples are present in the Garvellachs and although in most cases it is not possible, because of poor exposure, to trace the horizon to the point at which it actually thins to zero, this must occur because the horizons are 12

Mem. geol. Soc. Lond. no. 6

A . M. S P E N C E R

rarely absolutely planar but the amplitude of any irregularities is small, usually less than 20cm. As a corollary to the conformable and almost planar nature of the lower contacts it is only very rarely that there is any sign of disturbed bedding around the contact; in the few cases in which this is present it is on a small scale (see Fig. 6). Finally, bottom structures, such as flute casts or load casts, are not present on the lower contacts of mixtites and striations have not been seen on these surfaces. Interpretation. The very sharp lower contacts of mixtite beds are difficult to explain by iceberg rafting. Firstly, the continuous sedimentation associated with the gradual increase in the numbers of icebergs to glacial maximum might typically be expected to produce a gradational upward sequence from normal bedded sediments to unbedded mixtites--perhaps occupying a thickness of several centimetres to several metres, depending upon the over-all rate of sedimentation. Such gradational lower contacts to mixtites are absent here. Secondly, sedimentation at the base of the Port Askaig mixtites was not continuous, for unbedded mixtites often rest on, for example, sandstones with large scale cross-stratification. Discontinuous sedimentation could result in sharp lower contacts to ice-rafted mixtites, if the mechanism of sedimentation of the interbeds came to a halt and was replaced--after some interval--by iceberg sedimentation alone. Such an arrangement cannot be ruled out, but it is difficult to think of any reason--to account for the ubiquitous presence of sharp contacts--why normal sedimentation should in every case halt before iceberg sedimentation commenced. Sharp contacts might be expected beneath mixtites deposited by continental ice-sheets, however, and are present beneath some Pleistocene land tills. The completely conformable nature of most lower contacts, even when traced over long distances, is nevertheless remarkable. There are very few signs of disturbance of the underlying bedded sediments by the movements of 'over-riding' ice-sheets (a few examples are given in section 3.A (iii)). (ii) INTERNAL BEDDING IN MIXTITES Certain of the mixtites within the Port Askaig Formation (numbers 2 and 19-22 in the Garvellachs, all those in member 4 at Port Askaig) are nearly homogeneous, containing very few bedded horizons. Although such homogeneity and lack of bedding is one of the most important and striking characters of mixtites, bedded horizons do occur within mixtites and are present in approximately 10 per cent of the cumulative thickness of all the mixtites in the Port Askaig Formation. The criterion which shows that the bedded horizons lie within mixtites (and do not just separate mixtites formed by two successive events) is that the horizons thin laterally to zero and where they are absent the mixtite is completely homogeneous and undivided. This criterion, together with the fact that the bedded horizons are not transported blocks of material, must mean that the bedded horizons formed whilst the mixtite material around them was also being deposited. For this reason the discontinuous bedded horizons have a much greater importance than their small frequency of occurrence might suggest, for they provide the only direct evidence of the mode of deposition of the mixtites. These discontinuous bedded horizons are described below, starting with examples of simple, isolated lenses and considering progressively more complicated arrangements. Description. The best examples of discontinuous bedded horizons seen in the Garvellachs are the siltstones (1 m thick) which lie 7m above the base and 1m beneath the top of mixtite 9 in its completely exposed outcrop on Eileach an Naoimh. Both horizons thin to zero laterally and where they are absent the mixtite is homogeneous and undivided (P1. 11). A smaller but equally clear example is provided by the granite conglomerate horizon within mixtite 30 on the east coast of Garbh Eileach. This reaches a maximum thickness of 60cm but for a distance of a few metres along the outcrop it is absent and the mixtite appears to be unbroken in its succession. A similar example is the dolomite conglomerate which separates mixtites 2 and 3 and which thins laterally to zero in outcrops on the north coast of Garbh Eileach and on Dun ChonnuiI; where the conglomerate is absent mixtites 2 and 3 merge inseparably (Fig. 4a). A great many other examples are present in the Garvellachs and although in most cases it is not possible, because of poor exposure, to trace the horizon to the point at which it actually thins to zero, this must occur because the horizons are 12

Mem. geol. Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

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A. M. SPENCER absent in the next coastal outcrop (P1. 11). Examples are the sandstones which separate mixtites 6 and 7 (undivided on the north coast of Garbh Eileach), mixtites 20, 21 and 22 and mixtites 28 and 29 (all undivided on the west coast of Garbh Eileach). The discontinuous horizons described above are not flat plates of material which have been transported to the positions in which they are now seen. Because of their strict parallelism to the general bedding, their great lateral continuity and the presence of undisturbed bedding within them it is certain that they actually accumulated in the positions in which they are now seen. The author believes that this conclusion is also true for the smaller, less continuous bedded lenses present within many mixtites. The lowermost 7m of mixtites 5 and 6 (Fig. 4d), for example, contain large numbers of lenses of (in order of abundance) sandstone, dolomite conglomerate and siltstone. In cross-section certain of these sandstones are pod-shaped and might be boulders transported to their present positions en m a s s e . But because of the complete gradation in cross-sectional shapes, from pod shaped to very elongate, because the lenses all lie parallel to the base of the mixtite and because lenses of particular lithologies are sometimes concentrated at certain horizons, it seems certain that they mostly accumulated in the positions in which they are now seen, rather than all being transported blocks of material. Similar but larger scale lenses of bedded siltstone and conglomerate occur in the lower part of mixtite 12 and exhibit undisturbed internal bedding and pronounced lateral discontinuity

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Fro. 5. Field sketches of internal bedding in mixtites in the Garvellachs. (a), (c) and (f) Mixtite ll at {475 025}, {504 023} and {540 021} respectively. (b) Mixtite 26' at {475 062}. (d) The top of mixtite 8 at {147 042}. (e) and (g) The base of mixtite 9 at {437 024} and {433 023} respectively. Siltstone beds in (a) and (f) are shown in black. 14

Mem. geol. Soc. Lond.

no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND (Fig. 4b) and further examples occur in mixtite 20 on A'Chuli and mixtites 28-29 on Eileach an Naoimh. A good parallel stratifcation, due to thin sandstone laminae, is developed in one part of mixtites 28-29 (P1. 4b). Lenses of bedded sediment similar to those just described and which are as much as a few decimetres thick and a few metres in length are present occasionally in almost all mixtites. Those just described were chosen because they are the best exposed examples. In addition to the bedded horizons described above, which thin gradually to zero when traced laterally, certain bedded horizons within mixtites end laterally at steep contacts (Figs. 4a, 5c, d). Complicated structures occur in the lowest 6 m of mixtite 9 on Eileach an Naoimh (Fig. 5e, g) where the siltstones which normally underlie the mixtite in places interdigitate with the mixtite and form structures resembling large scale crossstratification. The most complicated arrangement of internal bedded horizons seen, however, occurs in mixtite 11 on Eileach an Naoimh. In certain parts of its outcrop the latter contains abundant siltstone laminae, which are sometimes arranged in structures resembling channel infillings (Fig. 4c), whilst in other parts of its outcrop siltstone laminae are almost absent (P1. 11). Less organized arrangements also occur (Fig. 5a, f). The sandstones and conglomerates which separate mixtites 33, 34 and 35 and mixtites 37 and 38 (see P1. 11) and which occur within mixtite 36 (Fig. 41) are discontinuous laterally and because of this they cannot be regarded as of the same status as the thick sandstones which separate mixtites 35, 36 and 37. It seems that mixtites 33-35 and 37-38 form two groups which, like mixtite 36, have a central discontinuous sandstone horizon. Similar internal bedding is present in certain of the mixtites in the Port Askaig Formation on Islay. Closely spaced sandy and more dolomitic laminae, lying parallel to the base of the bed, occur in the highest mixtite in member 1 at Beannan Dubh. Sandy laminae, forming quite complicated structures, are well shown in the highest mixtite in member 2 at Creagan Loisgte, and a central, discontinuous sandstone horizon is present in the lowest mixtite in member 3 at Creagan Loisgte. Above this level in the succession, however, internal bedding in mixtites is relatively uncommon. In almost all the above examples the bedded horizons have sharp contacts with the mixtite matrix which surrounds them and the latter is unbedded and homogeneous. A distinctly different type of structure occurs in a few mixtites in the Garvellachs within which a part or all of the matrix itself is crudely bedded or even well laminated. A relatively good planar bedding occurs in a small part of mixtite 1, for example, and a similar structure occurs throughout the whole thickness of mixtite 26 on Sgeir leth a'Chuain. A much stronger and better developed planar lamination occurs throughout mixtites 19, 23 and 25 and a similar structure occurs in the laminated siltstone beneath mixtite 30. These four beds are quite distinct from the majority of the mixtites, however, because as well as the strong lamination structure present in them they contain only small numbers of stones and these are all of cobble grade or smaller. Because of these characters the beds occupy a position intermediate between true mixtites and varved siltstones (section 3.B (vi)). Interpretation. The presence of planar bedded, discontinuous, horizons of sorted sediment within otherwise structureless mixtites cannot be explained if the mixtites are the deposits of far-travelled mudflows (see section 5. C, The internal structure of rnudflows); such deposits would be expected to be completely homogeneous. Internal bedding also provides evidence against the deposition of the mixtites by iceberg rafting (see section 5. E (iii)). The abrupt lateral margins of certain lenticular sandstone and conglomerate horizons within the mixtites imply that the powerful water currents forming such beds must have been restricted to particular small areas, often only tens of metres in cross-section. This follows from the fact that the mixtite on either side of the lenses is completely unbedded and shows no sign of erosion by powerful currents. Marine currents acting on the bottom sediments of a sea in which icebergs were floating would be much more extensive in distribution. Similarly marine bottom currents acting on iceberg sediments could not produce the complicated three-dimensional arrangement of bedded horizons present within some of the mixtites described above. All the internal bedding described above could have been produced by meltwater currents, Mem. geol. Soc. Lond. no. 6

15

A. M. S P E N C E R

perhaps flowing in distinct channels and acting on till forming either at the base of or within a grounded ice-sheet. Similar structures are common in Pleistocene tills deposited by grounded ice-sheets (Carruthers 1940; Simpson 1961). (iii) SOFT SEDIMENT FOLD STRUCTURES Twenty-one examples of soft sediment fold structures affecting bedded sediments, which yield either the strike of their fold axis or the general direction of overturning or both, have been seen in the Garvellachs. Their importance lies in the information they provide on the sense of movement occurring during the very last stages of the transport of the mixtites. The folds are not related in any way to the tectonic cleavage present in the Garvellachs (hence the name soft sediment fold), despite the fact that a large number of the folds have axes lying parallel to the strike of the cleavage. Two sets of folds, for example, are demonstrably penecontemporaneous in date (Fig. 6d, f, g) and those affecting the dolomite conglomerate beneath mixtite 1 on the north coast of Garbh Eileach are cross-cut by a planar sandstone dyke; the latter is cleaved and the folds must therefore pre-date both the sandstone dyke and the cleavage. Similarly, the folds and thrusts present within certain rafts of sediment in the Great Breccia are truncated at the margins of the rafts and must also be penecontemporaneous in date. Such positive evidence is lacking in the other examples but the considerable range in the sizes and styles of the folds and their restricted distribution (they are only found within or immediately beneath mixtite beds) makes it certain that they have not been produced in association with the cleavage. (N.B. Tectonic folds are almost completely absent from the rocks of the Garvellachs, only a single fold structure related to the cleavage has been found, see Fig. 6a.) Description. Thirteen of the examples of soft sediment folds come from horizons at the base of individual mixtites and the rest from lenticular bedded horizons within mixtites. In addition, six large intra-basinal stones, which occur within mixtites, show measurable fold axes. The folds affecting the bedded sediments vary in amplitude from only a few centimetres (Fig. 6c) up to a few metres (Fig. 6h). All the folds are asymmetrical and the majority are overturned (Fig. 6h) or are even recumbent (Fig. 6c and mixtite 11 on Eileach an Naoimh in P1. 11). Amongst the intra-basinal clasts a similar variation in fold sizes and styles is present (Fig. 6d; P1. 1) and the largest soft sediment fold structure seen in the Port Askaig Formation, measuring 40m across the limbs, occurs within a huge dolomite raft in the Great Breccia (P1. 1). In the centre of the latter the axis of the fold has broken and been thrust and similar thrusts affect the large and magnificently exposed folded raft in the Great Breccia on A'Chuli. It is difficult to determine whether the folds seen in the blocks of intra-basinal material within the mixtites pre-date the formation of the blocks, or were impressed on the blocks during transport. It should be possible to distinguish between these two cases and the distinction may be important for, to judge from the work of Crowell (1957), intra-basinal fragments which have been folded during transport are characteristic of mudflow deposits, whilst the present author believes that glacial tills may well contain both types of fragment. The sediment in the intra-basinal fragment shown in Fig. 6d may have been folded before the fragment gained its present outlines. Few other fold structures are present. Examples occur in the Disrupted Beds (see Fig. 35) and in the sandstones and dolomites at the top of the varved siltstone group above mixtite 32 on the east coast of G a r b h Eileach, which are affected by very complicated disturbed bedding, the only example of its type in the GarveUachs. Origin. The folds described above have not originated by slumping on an extensive palaeoslope. They are much too restricted in distribution for this; recumbently folded and undisturbed stratified horizons within the same mixtite bed can occur only a short distance from each other. In addition, the uniform direction of overturning coupled with the range in size of the folds, from amplitudes of a few decimetres to several tens of metres, is difficult to explain by slumping or mudflowage on a very local scale. Martin (1964A) and Banham & Ransom (1965) have described folds from, respectively, the Upper Palaeozoic tillites of Brazil and Pleistocene 16

Mem. geoL Soc. Lond. no. 6

A. M. S P E N C E R

perhaps flowing in distinct channels and acting on till forming either at the base of or within a grounded ice-sheet. Similar structures are common in Pleistocene tills deposited by grounded ice-sheets (Carruthers 1940; Simpson 1961). (iii) SOFT SEDIMENT FOLD STRUCTURES Twenty-one examples of soft sediment fold structures affecting bedded sediments, which yield either the strike of their fold axis or the general direction of overturning or both, have been seen in the Garvellachs. Their importance lies in the information they provide on the sense of movement occurring during the very last stages of the transport of the mixtites. The folds are not related in any way to the tectonic cleavage present in the Garvellachs (hence the name soft sediment fold), despite the fact that a large number of the folds have axes lying parallel to the strike of the cleavage. Two sets of folds, for example, are demonstrably penecontemporaneous in date (Fig. 6d, f, g) and those affecting the dolomite conglomerate beneath mixtite 1 on the north coast of Garbh Eileach are cross-cut by a planar sandstone dyke; the latter is cleaved and the folds must therefore pre-date both the sandstone dyke and the cleavage. Similarly, the folds and thrusts present within certain rafts of sediment in the Great Breccia are truncated at the margins of the rafts and must also be penecontemporaneous in date. Such positive evidence is lacking in the other examples but the considerable range in the sizes and styles of the folds and their restricted distribution (they are only found within or immediately beneath mixtite beds) makes it certain that they have not been produced in association with the cleavage. (N.B. Tectonic folds are almost completely absent from the rocks of the Garvellachs, only a single fold structure related to the cleavage has been found, see Fig. 6a.) Description. Thirteen of the examples of soft sediment folds come from horizons at the base of individual mixtites and the rest from lenticular bedded horizons within mixtites. In addition, six large intra-basinal stones, which occur within mixtites, show measurable fold axes. The folds affecting the bedded sediments vary in amplitude from only a few centimetres (Fig. 6c) up to a few metres (Fig. 6h). All the folds are asymmetrical and the majority are overturned (Fig. 6h) or are even recumbent (Fig. 6c and mixtite 11 on Eileach an Naoimh in P1. 11). Amongst the intra-basinal clasts a similar variation in fold sizes and styles is present (Fig. 6d; P1. 1) and the largest soft sediment fold structure seen in the Port Askaig Formation, measuring 40m across the limbs, occurs within a huge dolomite raft in the Great Breccia (P1. 1). In the centre of the latter the axis of the fold has broken and been thrust and similar thrusts affect the large and magnificently exposed folded raft in the Great Breccia on A'Chuli. It is difficult to determine whether the folds seen in the blocks of intra-basinal material within the mixtites pre-date the formation of the blocks, or were impressed on the blocks during transport. It should be possible to distinguish between these two cases and the distinction may be important for, to judge from the work of Crowell (1957), intra-basinal fragments which have been folded during transport are characteristic of mudflow deposits, whilst the present author believes that glacial tills may well contain both types of fragment. The sediment in the intra-basinal fragment shown in Fig. 6d may have been folded before the fragment gained its present outlines. Few other fold structures are present. Examples occur in the Disrupted Beds (see Fig. 35) and in the sandstones and dolomites at the top of the varved siltstone group above mixtite 32 on the east coast of G a r b h Eileach, which are affected by very complicated disturbed bedding, the only example of its type in the GarveUachs. Origin. The folds described above have not originated by slumping on an extensive palaeoslope. They are much too restricted in distribution for this; recumbently folded and undisturbed stratified horizons within the same mixtite bed can occur only a short distance from each other. In addition, the uniform direction of overturning coupled with the range in size of the folds, from amplitudes of a few decimetres to several tens of metres, is difficult to explain by slumping or mudflowage on a very local scale. Martin (1964A) and Banham & Ransom (1965) have described folds from, respectively, the Upper Palaeozoic tillites of Brazil and Pleistocene 16

Mem. geoL Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

tills of Norfolk, England and used them to infer the movement direction of the ice-sheets. The smaller folds described here are similar to certain examples figured by Banham & Ransom, and the folds and thrusts "315"

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FIG. 6. Field sketches of soft sediment fold structures in the Garvellachs. (a) Tectonic folds related to cleavage, in laminated siltstone at {123 132} (shown for comparison). (b) Folded and thrust sandstone beneath mixtite 18 at {282 051}. (c) Recumbently folded siltstone beneath mixtite 1 at {067 102}. (d) Folded sandstone, siltstone and mixtite beds contained within a fragment within mixtites 6-8 at {143 042}. (e) Folded sandstone and dolomite conglomerate beneath mixtite 12 at {551 021}. (f) and (g) Siltstones showing folds truncated beneath mixtite 9 at {452 022} and {454 022} respectively. (h)Siltstones in mixtite 11 at {507 022}. Mem. geol. Soc. Lond. no. 6

17

A. M. SPENCER affecting the huge blocks in the Great Breccia are exactly analogous to the structures which affect huge chalk rafts present in the Norfolk tills. In conclusion, it is suggested that the folds indicate localized and probably N

Strike of ford axis and direction of ford overturning

1r Approximate (*-/,5") \

direction of ford overturning \ where axis not measurabte

Fro. 7. The strikes and directions of overturning of soft sediment fold structures in the Garvellachs. very late ice movement directions for the mixtites in which they occur (mostly in member 1). The similar strikes and directions of overturning found indicate that this movement was from east to west or SE-NW (Fig. 7).

(iv)

SANDSTONE D O W N F O L D STRUCTURES

Within the Port Askaig Formation, basin-shaped and trough-shaped sandstone structures occur at some 15 horizons, lying either in the tops of mixtite beds or within bedded sandstones (Table 5). One important suggestion regarding their origin (Kilburn et al. 1965, p. 353), is that the structures at two horizons may be involutions formed in a periglacial environment. Most of the structures, however, have probably resulted from large-scale load-casting, or quicksand movement. Description. Where sufficiently well exposed, most of the structures can be seen to be basin-shaped in three dimensions (Figs. 8, 9) and to have amplitudes of up to 3.5m. Internal bedding is present in some and, with only a few exceptions (Fig. 10b), runs parallel to the walls of the structures, suggesting that they have originated by a 'downfolding' of a pre-existing planar bedded sandstone. In the majority of cases the upturned bedding is overlain disconformably by planar bedded sediments, showing that the downfolding is of penecontemporaneous date (Figs. 8, 9). In other cases, however, the sandstone basins are continuous with the overlying sandstone (Fig. 10b). Most downfolds lie above homogeneous mixtites and the relations at their bases are unknown. In the case of the horizons within bedded sediments, however, there is a rapid downward gradation into undisturbed sediments (Fig. 9b, d, f ) . Although at most horizons the sandstone basins have been seen in only a single locality, at two horizons sandstone basins are present along outcrops which are several kilometres in length (Table 5). In these cases the downfolded basins are very closely spaced (Fig. 9a, c , f ) and, when seen in cross-section, are separated by sharp, narrow anticlines; in two areas the downfolds above mixtite 26 can be seen to be approximately equidimensional in plan view (Fig. 8c, d). The structures at these two horizons resemble convolute lamination, although their scale is much larger (commonly 1 to 2m amplitude) for "most beds containing convoluted lamination are one to ten inches (2.5 to 18

Mem. geol. Sac. Land. no. 6

A. M. SPENCER affecting the huge blocks in the Great Breccia are exactly analogous to the structures which affect huge chalk rafts present in the Norfolk tills. In conclusion, it is suggested that the folds indicate localized and probably N

Strike of ford axis and direction of ford overturning

1r Approximate (*-/,5") \

direction of ford overturning \ where axis not measurabte

Fro. 7. The strikes and directions of overturning of soft sediment fold structures in the Garvellachs. very late ice movement directions for the mixtites in which they occur (mostly in member 1). The similar strikes and directions of overturning found indicate that this movement was from east to west or SE-NW (Fig. 7).

(iv)

SANDSTONE D O W N F O L D STRUCTURES

Within the Port Askaig Formation, basin-shaped and trough-shaped sandstone structures occur at some 15 horizons, lying either in the tops of mixtite beds or within bedded sandstones (Table 5). One important suggestion regarding their origin (Kilburn et al. 1965, p. 353), is that the structures at two horizons may be involutions formed in a periglacial environment. Most of the structures, however, have probably resulted from large-scale load-casting, or quicksand movement. Description. Where sufficiently well exposed, most of the structures can be seen to be basin-shaped in three dimensions (Figs. 8, 9) and to have amplitudes of up to 3.5m. Internal bedding is present in some and, with only a few exceptions (Fig. 10b), runs parallel to the walls of the structures, suggesting that they have originated by a 'downfolding' of a pre-existing planar bedded sandstone. In the majority of cases the upturned bedding is overlain disconformably by planar bedded sediments, showing that the downfolding is of penecontemporaneous date (Figs. 8, 9). In other cases, however, the sandstone basins are continuous with the overlying sandstone (Fig. 10b). Most downfolds lie above homogeneous mixtites and the relations at their bases are unknown. In the case of the horizons within bedded sediments, however, there is a rapid downward gradation into undisturbed sediments (Fig. 9b, d, f ) . Although at most horizons the sandstone basins have been seen in only a single locality, at two horizons sandstone basins are present along outcrops which are several kilometres in length (Table 5). In these cases the downfolded basins are very closely spaced (Fig. 9a, c , f ) and, when seen in cross-section, are separated by sharp, narrow anticlines; in two areas the downfolds above mixtite 26 can be seen to be approximately equidimensional in plan view (Fig. 8c, d). The structures at these two horizons resemble convolute lamination, although their scale is much larger (commonly 1 to 2m amplitude) for "most beds containing convoluted lamination are one to ten inches (2.5 to 18

Mem. geol. Sac. Land. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

TABLE 5" The horizons and the characters of the sandstone downfoM structures

Horizon, given by mixtite number

Two, possibly three horizons in sandstones between 31 and 30 As many as three horizons in sandstone between 27 and 26 /26

Within 24 /22 /17 /16

/4

Maximum amplitude of downfolds in metres

Internal structure

PA G G PA G

? .9 B and some C ? B

1.5 2 3.5 3 1

h

PA

?

G

Port Askaig (PA)

/(37-38) /37 /34 In 33-35 In sandstone just beneath 33 Two horizons in sandstone just beneath 33 /31

/25

Threedimensional shape

Garvellachs

(G) or

Relations at the top of the downfolds

Grid reference of one locality at which structure occurs, or lateral extent of horizon

Position and suggested environment of formation

f and h h f

E ? E .9 E

[1423 6663] {268 088} 0-5kin [1423 6663] 3.5km

** *** *** *** *

1

f

?

[1423 6663]

*

B+C

1

f

E

G

B and small wedge shaped structures

1

i (1 example)

At least one connects

0.6km {126 133} {125 133} {261 057} {516 063}

G

?

1

f

E + .9

{245 049} ~316 062} {368 043}

*

G G

B ?

2 3

f ?

E Connect

** ***

G G G G G

Irregular B ? ? .9 ?

0-5 0-5 0.2 1.7 1"2

f f ? H h

E E Connect E Connect

5km 0.5km {367 041} {545 052} {105 132} {100 127} {099 127} {068 107}

*** ** *** *** ***

'/17' = 'penetrating the top of mixtite 17'; B, basin-shaped; C, semi-cylindrical in shape; h, homogeneous; f, laminae folded parallel to the walls; i, infdl bedding; E, truncated at an erosion surface; 'Connect' = 'joins with the overlying sandstone'. *** Overlie mixtite but formed subglacially, ** overlie mixtite but formed when ice-sheets were absent from the Argyll area, * within sandstone interbeds which formed when ice sheets were absent from the Argyll area.

25 cm) thick" (Potter & Pettijohn 1963, p. 153). At most of the other horizons the basins are isolated from each other (Figs. 8a, b; 9e). Other notable features of individual downfolds are" the sandstone downfolds beneath mixtite 33 overlie siltstones which contain penecontemporaneous disturbed bedding (Figs. 9i, 10c); structures which may be small sandstone downfolds are associated with sandstone wedges in the top of the varved siltstones above mixtite 32; folds which affect the bedding of the sandstone above mixtite 2 are associated with narrow tongues of siltstone which appear to have been injected up into the sandstone (Fig. 10a). Origin. The following discussion depends largely upon the conclusions that the mixtites formed as deposits from grounded ice-sheets (section 5. E (iii)) and that the interbeds are not all of the same status; some interbeds can be interpreted as the products of sub-glacial meltwater streams, whilst others accumulated Mem. geol. Soc. Lond. no. 6

19

A.

M.

SPENCER T

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P l a n views a n d cross-sections o f s a n d s t o n e d o w n f o l d s t r u c t u r e s in t h e G a r v e l l a c h s . (a) T h e t o p o f mixtite 31 at {127 133}. (b) T h e t o p o f m i x t i t e 34 at {170 117}9 ( c ) T h e p e b b l y s a n d s t o n e a b o v e mixtite 26 ( n o t s h o w n ) at {112 132}. ( d ) P l a n view o f d o w n f o l d s in t h e p e b b l y s a n d s t o n e a b o v e m i x t i t e 26 at {366 042}9 (e) P l a n view o f folds affecting p e b b l y s a n d s t o n e , o v e r l a i n by u n d i s t u r b e d siltstones, all c o n t a i n e d w i t h i n mixtite 24 at {544 051}.

F I G. 8.

during periods when ice-sheets were absent f r o m the Argyll area (section 6). The sandstone downfold structures described above all have similar characters but occur in three quite different positions with respect to the interbeds a n d mixtites in the formation. (i) Sandstone downfolds are present within sandstone interbeds deposited during periods w h e n ice-sheets were absent f r o m the Argyll area (Table 5*). (ii) Other sandstone 20

Mem.

geol.

Soc.

Lond.

no. 6

LATE

PRE-CAMBRIAN

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.... -;...': ".-- ~ ..~.~...~.~.~.r..<~---. .: :-.:-.-- :...~...,.-...~. ?_.:.~:.=-. 9.:.~ ,..:;.:......... 9~..~.-c FIG. 9. Field sketches o f cross-sections of sandstone downfold structures from the Garvellachs. (a), (c), ( f ) a n d (h) The pebbly sandstone which overlies mixtite 26 at {110 133}, {112 132}, {297 064} and {364 043} respectively. (b) and ( d ) W i t h i n the sandstone between mixtites 26 a n d 27 at {247 050} and {562 054} respectively. ( e ) T h e top of mixtite 22 at {105 132}. ( g ) T h e top of mixtite 16 at {099 127}. (i) The sandstone just beneath mixtite 33 at {375 073}.

downfolds occur in the tops of mixtites which are overlain by interbeds deposited when ice-sheets were absent from the Argyll area (Table 5**). (iii) Sandstone downfolds also occur where sandstones deposited by sub-glacial melt-water streams overlie mixtites (Table 5***). Three origins may be suggested for the sandstone downfold structures; they may have formed as quicksand structures, or as large load casts or by cryoturbation (repeated freezing and thawing in a periglacial climate). Mem.

geol.

Soc.

Lond.

no. 6

21

A.

M. SPENCER

Before discussing these possibilities, it should be stated that the structures are not the result of lateral mass movement (slumping) for although the structures are frequently overturned towards the north-west in the Garvellachs, suggesting slumping, this effect is entirely related to the predominant cleavage. In addition, if the extensive horizons of downfolds had formed by slumping the folds would be underlain by a decollement surface. This is not the case; downfolds overlying bedded sediments always have a gradational, conformable lower contact (Figs. 9, 10).

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F x o . 10. Field sketches o f sandstone downfold, load cast and disturbed bedding structures in the Garvellachs. (a) Sandstone d o w n f o l d and 'injection' structures in mixtites 3 and 4 at {068 108}. (b) In the siltstones above mixtite 30 at {123 132}. (c) S l u m p folds and fragments in siltstones at {365 073}, which lie just beneath the horizon with sandstone downfolds shown in Fig. 9L (d) The top o f mixtite 17 at {101 125}. (e) The top o f mixtite 11 at {552 021}. ( f ) Within the laminated siltstones o f mixtite 25 at {475 059}.

Returning to the three origins suggested above. The sandstone downfold horizons most likely to be of cryoturbation origin are those of group (ii). At two of these horizons the sandstone wedges which penetrate the underlying mixtite are truncated at the lower contact of the downfolded sandstone (Fig. 9a, e). In these cases it might be suggested that the sequence records the following history of climatic amelioration: deposition of mixtite from a melting ice-sheet, establishment of permafrost conditions (sandstone wedges), deposition of gravel beds which were subjected to alternate freezing and thawing and, lastly, a period of marine erosion. The sandstone downfolds themselves yield no incontrovertible evidence of a cryoturbation 22

Mem.

geol.

Soc.

Lond.

no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND origin, however; vertically oriented stones, perhaps the only diagnostic character of 'involutions' of cryoturbation origin, are not obvious. In the absence of such evidence it is most likely that all the downfolds described here originated by quicksand movement or load-casting. The sandstone downfolds of group (i) for example, are similar to structures in the Torridonian sandstones of north-west Scotland described by Selley, Shearman, Sutton & Watson (1963, text figs. 2B, 6D) and Stewart (1963, text fig. 11). Selley et aL (1963, p. 239) suggested an origin by the shifting and re-arrangement of quicksands due to "small differences of loading or to minor seismic disturbances" which "record the re-arrangement of the fine material to produce a more stable form of packing". The sandstone downfolds of group (iii) are perhaps the most remarkable of all the structures. They must record the penecontemporaneous, sub-glacial load-casting, often on a large scale, of bedded sandstones downwards into mixtite and imply that the mixtites were water saturated and thus very mobile. (V) PEBBLE FABRIC IN THE MIXTITES

The preferred orientation of the long axes of pebbles in Pleistocene land tills often shows a parallel or transverse alignment, when considered on a regional scale, with the inferred direction of ice movement (Krumbein 1939; Holmes 1941; Harrison 1965). Several authors have made ~ebble fabric determinations on ancient tillites (Pre-Cambrian tillites: Gaertner 1943; Pettijohn 1962; Lungersgausen 1963, p. 574; Bjorlykke 1966. Devonian to Permian tillites: H~ilbich 1964; Martin 1964A, 28-9; Frakes & Crowell 1967; Bigarella, Salamuni & Fuck 1967) and in some cases have thus deduced the ice movement direction. As well as giving evidence of the possible movement direction of the ice, however, strong preferred pebble orientations imply that the till must have been released from an ice sheet resting on the surface of the sediments. By contrast, elongated pebbles dropped from floating ice might be expected to show a distribution of their long axes which was radial and showed no strong horizontal preferred orientation, an argument suggested by several authors (Carey & Ahmed 1961, p. 893; Spjeldnaes 1964, p. 39; Bjorlykke 1966, p. 31). Thus the presence or absence of a preferred pebble orientation in glacial mixtites may provide an important criterion for distinguishing ground moraines and ice rafted tills. It was hoped to measure the pebble fabrics of successive mixtites in the Garvellachs succession and, using the above criterion, distinguish those formed as ground moraines from those of ice rafting origin. This has not been possible, for the tectonic cleavage has affected the pebbles sufficiently for it to be very difficult to recognize whether the pebbles were originally orientated in a sedimentary fabric or not. The measurements. The preferred orientation of the stones in the mixtites of the Garvellachs and Islay is not obvious from a casual inspection, for the majority (75 per cent) of the dolomite stones and the great majority (90 per cent) of the extra-basinal stones are not strongly elongated. It was therefore measured in the following way. The stones cannot be cleanly extracted from the matrix and so the azimuths of their apparent long axes were measured. A surface lying within a mixtite and approximately parallel to the base of the mixtite was chosen. On this surface the azimuths of the long axes of stones larger than I cm, which had a ratio of apparent long axis to apparent intermediate axis greater than 1.5 or than 2, were measured. It was found important to select a surface on which 60 to 100 such pebbles were exposed (usually a few square metres) and cover this systematically. This is because in measuring a preferred orientation one is measuring the relative frequency with which pebbles of certain orientations occur; measuring ten pebbles here and ten there may introduce a bias. The results were plotted as rose diagrams and have not been reoriented to allow for the tectonic dip (this is always less than 35 ~ and its removal has a maximum effect of less than 6~ Results. All the mixtites which have been measured show strong preferred orientations of their elongate stones (Fig. 11), with vector mean azimuths in the range 055 ~ to 087 ~ and standard deviations of 22 ~ to 41 ~ In view of the following facts, however, it seems unlikely that these preferred orientations are entirely original. Firstly, the elongate pebbles in conglomerates also show the same preferred orientation direction (Fig. 11), despite the fact that in one case the particular cross-stratification current direction in the conglomerate does Mem. geoL Soc. Lond. no. 6

23

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND origin, however; vertically oriented stones, perhaps the only diagnostic character of 'involutions' of cryoturbation origin, are not obvious. In the absence of such evidence it is most likely that all the downfolds described here originated by quicksand movement or load-casting. The sandstone downfolds of group (i) for example, are similar to structures in the Torridonian sandstones of north-west Scotland described by Selley, Shearman, Sutton & Watson (1963, text figs. 2B, 6D) and Stewart (1963, text fig. 11). Selley et aL (1963, p. 239) suggested an origin by the shifting and re-arrangement of quicksands due to "small differences of loading or to minor seismic disturbances" which "record the re-arrangement of the fine material to produce a more stable form of packing". The sandstone downfolds of group (iii) are perhaps the most remarkable of all the structures. They must record the penecontemporaneous, sub-glacial load-casting, often on a large scale, of bedded sandstones downwards into mixtite and imply that the mixtites were water saturated and thus very mobile. (V) PEBBLE FABRIC IN THE MIXTITES

The preferred orientation of the long axes of pebbles in Pleistocene land tills often shows a parallel or transverse alignment, when considered on a regional scale, with the inferred direction of ice movement (Krumbein 1939; Holmes 1941; Harrison 1965). Several authors have made ~ebble fabric determinations on ancient tillites (Pre-Cambrian tillites: Gaertner 1943; Pettijohn 1962; Lungersgausen 1963, p. 574; Bjorlykke 1966. Devonian to Permian tillites: H~ilbich 1964; Martin 1964A, 28-9; Frakes & Crowell 1967; Bigarella, Salamuni & Fuck 1967) and in some cases have thus deduced the ice movement direction. As well as giving evidence of the possible movement direction of the ice, however, strong preferred pebble orientations imply that the till must have been released from an ice sheet resting on the surface of the sediments. By contrast, elongated pebbles dropped from floating ice might be expected to show a distribution of their long axes which was radial and showed no strong horizontal preferred orientation, an argument suggested by several authors (Carey & Ahmed 1961, p. 893; Spjeldnaes 1964, p. 39; Bjorlykke 1966, p. 31). Thus the presence or absence of a preferred pebble orientation in glacial mixtites may provide an important criterion for distinguishing ground moraines and ice rafted tills. It was hoped to measure the pebble fabrics of successive mixtites in the Garvellachs succession and, using the above criterion, distinguish those formed as ground moraines from those of ice rafting origin. This has not been possible, for the tectonic cleavage has affected the pebbles sufficiently for it to be very difficult to recognize whether the pebbles were originally orientated in a sedimentary fabric or not. The measurements. The preferred orientation of the stones in the mixtites of the Garvellachs and Islay is not obvious from a casual inspection, for the majority (75 per cent) of the dolomite stones and the great majority (90 per cent) of the extra-basinal stones are not strongly elongated. It was therefore measured in the following way. The stones cannot be cleanly extracted from the matrix and so the azimuths of their apparent long axes were measured. A surface lying within a mixtite and approximately parallel to the base of the mixtite was chosen. On this surface the azimuths of the long axes of stones larger than I cm, which had a ratio of apparent long axis to apparent intermediate axis greater than 1.5 or than 2, were measured. It was found important to select a surface on which 60 to 100 such pebbles were exposed (usually a few square metres) and cover this systematically. This is because in measuring a preferred orientation one is measuring the relative frequency with which pebbles of certain orientations occur; measuring ten pebbles here and ten there may introduce a bias. The results were plotted as rose diagrams and have not been reoriented to allow for the tectonic dip (this is always less than 35 ~ and its removal has a maximum effect of less than 6~ Results. All the mixtites which have been measured show strong preferred orientations of their elongate stones (Fig. 11), with vector mean azimuths in the range 055 ~ to 087 ~ and standard deviations of 22 ~ to 41 ~ In view of the following facts, however, it seems unlikely that these preferred orientations are entirely original. Firstly, the elongate pebbles in conglomerates also show the same preferred orientation direction (Fig. 11), despite the fact that in one case the particular cross-stratification current direction in the conglomerate does Mem. geoL Soc. Lond. no. 6

23

A. M. SPENCER not lie at right-angles to this direction. Secondly, at two horizons (bed 2 of the Disrupted Beds and the varves beneath mixtite 31, west Garbh Eileach) the other available evidence indicates that the pebbles have probably been ice rafted and a random long axis distribution would therefore be expected. The rose diagrams from these horizons (Fig. 11) are not circular but mostly show good preferred orientations (vector means range from 022 ~ to 082 ~) which are, admittedly, slightly more variable than the fabrics discussed above.

Ice-rafted sediments

Mixtites

j

Congtomerates N

i

h

F]G. 11. Rose diagrams of pebble fabric in mixtites, laminated siltstones (ice rafted sediments) and conglomerates in the Garvellachs. The number of stones measured is indicated. (a) to (e) Mixtites 1, 15, 20, 35 and 38 respectively. (f), (g) and (h) Bed 1 of the Disrupted Beds at {133 057} and {090 125} (g and h). (i) Conglomerate overlying mixtite 15 at {233037}. (j) and (k) Bed 2 of the Disrupted Beds at {223 036} and, (1) and (m), at {442 047} and {610 030} respectively. (n)Varved siltstone above mixtite 30 at {260 060}. Crystalline and dolomite stones were both measured except in the following cases: (h) and (j), dolomites only; (g), (k), (1) and (n), crystallines only. Thirdly, pebbles of a dolomite pisolite occur very sparsely in the mixtites and despite the large size of the ooids (up to 5 m m in diameter), the beautifully concentric internal structure of the pisolite grains makes it certain that they originated as oolites. Consequently an analysis of their deformation was undertaken. Thirteen hand specimens, spaced along the strike and up the succession in the Garvellachs, were collected and the orientations of the Z Y plane and the Z-axis of the deformation ellipsoid for each were determined by sawing and visual inspection in the laboratory. The axial ratios ( Z : Y: X) of the deformed oolites were determined approximately; the ratios from the most deformed specimen was 2 : 1.5 : 1 and that from the least deformed 1.4 : 1.2 : 1. The Z Y planes are nearly parallel with the planes of the main cleavage, whilst the Z-axis plunges towards the north-east (Fig. 12). Thus the azimuths of the longest axes of the deformation 24

Mem. geoL Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

ellipsoids of these pisolite pebbles agrees very closely with the vector means of the pebble fabric determinations. Origin of the fabric. The above arguments suggest that the preferred orientation of pebbles in the mixtites of the Garvellachs is at least in part tectonic. It is difficult to determine whether the mixtites had a sedimentary pebble fabric which was oriented or random before deformation. The only slight evidence on this

\

N \

•

g

g

g 9

9 .. e ~ o 9 o 9 ~9 9 9 1 7 6

9" :

o9

9

.

9

-..-

9

o

o

X

o

+

o

FIG. 12. Stereogram showing the relationship between cleavage, oolite deformation and pebble fabric in the Garvellachs. Circles: Z-axes of the strain ellipsoids of deformed dolomite oolite pebbles. Crosses: poles to the Z Y planes of the ellipsoids. Dots: poles to the cleavage planes in mixtites. The vector means of the pebble fabric in several mixtites are shown at the centre of the arcs outside the stereogram; half of each arc represents one standard deviation. Equal area, lower hemisphere, stereogram.

point is that the exotic stones from the horizons where ice rafting is indicated are not so strongly oriented (standard deviations 40 ~ to 48 ~ as the fabrics of the other mixtites where only granite pebbles have been measured (standard deviations 33 ~ to 41~ which perhaps suggests the presence of an initial orientation in the latter and its absence in the former. (B) T H E I N T E R B E D S (i) PALAEOCURRENT SYSTEMS

Collection and treatment of data. Every ripple mark system or set of cross-strata seen was measured, one measurement being taken per outcrop. Outcrops in the Garvellachs are mostly coastal and are inland on Mem. geol. Soc. Lond. no. 6

25

LATE

PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

ellipsoids of these pisolite pebbles agrees very closely with the vector means of the pebble fabric determinations. Origin of the fabric. The above arguments suggest that the preferred orientation of pebbles in the mixtites of the Garvellachs is at least in part tectonic. It is difficult to determine whether the mixtites had a sedimentary pebble fabric which was oriented or random before deformation. The only slight evidence on this

\

N \

•

g

g

g 9

9 .. e ~ o 9 o 9 ~9 9 9 1 7 6

9" :

o9

9

.

9

-..-

9

o

o

X

o

+

o

FIG. 12. Stereogram showing the relationship between cleavage, oolite deformation and pebble fabric in the Garvellachs. Circles: Z-axes of the strain ellipsoids of deformed dolomite oolite pebbles. Crosses: poles to the Z Y planes of the ellipsoids. Dots: poles to the cleavage planes in mixtites. The vector means of the pebble fabric in several mixtites are shown at the centre of the arcs outside the stereogram; half of each arc represents one standard deviation. Equal area, lower hemisphere, stereogram.

point is that the exotic stones from the horizons where ice rafting is indicated are not so strongly oriented (standard deviations 40 ~ to 48 ~ as the fabrics of the other mixtites where only granite pebbles have been measured (standard deviations 33 ~ to 41~ which perhaps suggests the presence of an initial orientation in the latter and its absence in the former. (B) T H E I N T E R B E D S (i) PALAEOCURRENT SYSTEMS

Collection and treatment of data. Every ripple mark system or set of cross-strata seen was measured, one measurement being taken per outcrop. Outcrops in the Garvellachs are mostly coastal and are inland on Mem. geol. Soc. Lond. no. 6

25

A. M. S P E N C E R Islay. In b o t h areas adjacent outcrops o f any horizon are usually more t h a n 200m apart. A total o f 188 crossstratification and 90 ripple m a r k measurements (155 a n d 69 respectively f r o m the Garvellachs) were collected at a total o f 77 outcrops f r o m 29 horizons within the Port Askaig F o r m a t i o n .

2O10-

L

oF

.... L k Gr--

l--- I ~

J

0" " ~ 5 '

N

1 2 3 Z, 5 > 5 m

f.g

r-"l

0

I

d 50

ff

"0:5"

'1 2 . ~

4 5.>~m

o§ FIG. 13. Palaeocurrent structures at horizons within the Port Askaig Formation. Between the circles: palaeocurrent directions from individual cross-strata. Outside the circles: ripple marks, with the direction of sediment transport, where known. Histograms of set thicknesses are shown on the left; the vertical scale gives the number of structures seen. (a) Between mixtites 1-12. (b) In the Disrupted Beds. (c)Between mixtites 22-25. (d)Between mixtites 26 and 27. (e)Above mixtite 30 and beneath member 3. (f) and (i) Between the top of member 2 and mixtite 33. (g) Between mixtites 35 and 36. (h) Between mixtites 36 and 37. (i) Is from the Port Askaig area, all others are from the Garvellachs. Set thicknesses of cross-beds from all horizons are shown for the Port Askaig area (j) and the Garvellachs (k). 26

Mem. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND For the cross-strata, both from Port Askaig and the Garvellachs, the present tectonic dip has been removed by rotation about the strike. This pre-supposes that the axes of the folds producing the tectonic dip do not plunge, a fact which is not known and for which no allowance can be made. The ripple marks, both in Islay and the Garvellachs, have not been reoriented; the tectonic dip of less than 35 ~ makes less than 6 ~ difference to the azimuth (Potter & Pettijohn 1963, fig. 10-7). The resulting palaeocurrent data for each horizon is shown in Fig. 13. For each horizon with a sufficient number of cross-stratification measurements the vector mean azimuth and its percentage magnitude were calculated and the statistical significance (Raleigh Test) of the vector mean was determined (Table 6) following Curray (1956, fig. 4). Vectors significant at the 5 per cent level are shown in Fig. 15a. The sample variance around the vector mean was calculated; the distributions at two individual horizons and that formed by all the measurements from the GarveUachs, can be shown to be random at the 5 per cent level using the F test (Potter & Pettijohn 1963, p. 365). Histograms of cross-stratification set thicknesses for the whole succession and for the particular horizons (Fig. 13) and a diagram showing the original angle of cross-bedding inclination in the Garvellachs have been compiled (Fig. 14a). The angles in the latter have been partly modified by tectonic deformation (Fig. 14b).

(a)

,,

20

26

so"

9

2r

e,

20 ~

28" -

,0 ~

20%'

=

=

1 2/;*

i

,

*

21"

10"/,"

17"

30*

O'

10"

20"

30*

/;0"

50* 17"

16"

FIG. 14. (a) Histogram of maximum angles of inclination (with respect to the regional bedding) of foreset strata in cross-beds in the Garvellachs. (b) The angles of foreset inclination for the 148 cross-strata of the GarveUachs depicted in Fig. 15b. Outer circle of figures: average maximum inclinations (between four and nine individual measurements occur in each 15~ segment). Inner circle: maximum observed inclination. The systematic variation in values (high in the north and north-west, low in the south and south-east) probably demonstrates the amount of change in shape resulting from the deformation which produced the regional cleavage. Nature o f the structures (Table 7). Large-scale cross-stratification sets occur only in the sandstones and

conglomerates of the formation (P1. 4f) and are mostly isolated although at some horizons they are grouped (Allen 1963). The sets are most commonly from 25 to 75cm thick, with a secondary mode in the 1 to 3m class (Fig. 13). Their lower bounding surfaces are usually planar and either erosional or non-erosional and the cross-strata in the set commonly have a discordant relationship to the lower bounding surface of the set. Almost all cross-strata are lithologically homogeneous throughout a particular set. On Allen's classification the solitary sets belong to the alpha and beta types and the grouped sets are the omicron type. In the case of the grouped sets with thicknesses of several metres exposures are not sufficiently good to be certain that the sets are not pi-cross-stratification, with scoops measuring tens or hundreds of metres across. Mem. geol. Soc. Lond. no. 6

27

A. M. S P E N C E R

Ripple marks occur predominantly in sandy siltstones rather than sandstones, have straight crests and may be either symmetrical or asymmetrical (P1.4e). Ripple heights are all less than 3 cm and ripple lengths predominantly under 15cm. Ripple-drift bedding (lambda-cross-stratification, Allen 1963) occurs in the siltstone between mixtites 8 and 9 and above mixtite 30 in the Garvellachs. Ripple marks are usually not associated with the large-scale cross-stratification described above.

TABLE 6: Cross-stratification vector mean azimuths at individual horizons in the Port Askaig Formation

Garvellachs or Port Askaig

Cross-stratification vector mean azimuth

Vector magnitude per cent

Variance

Number of observations

Significance of Raleigh test

37/36 36/35 33/32 Beneath 33 32/30 Beneath 28 27/26 26/25 Conglomerate at base of Disrupted Beds

G G G PA G G G G G

330 191 182 226 226 353 240 031 118

66-2 69.9 66.6 34.9 30.9 68-3 16-7 54.9 67.8

3012 2745 3066 6950 7779 2825 10013 5177 2672

15 7 20 17 31 6 17 29 7

< 0-01 > 10-3 < 0.04 > 0.03 < 10-3> 10-4 >0.1<0.2" < 0.05 > 0.04 < 0.05 > 0.04 >0.6<0.7* < 10-8 > 10-4 < 0.04 > 0.03

Total all measurements

G

066

4-1

10528

155

> 0.95 < 0.99*

Horizon, given by mixtite number

Garvellachs or Port Askaig

Grid or Brit. Nat. Grid

G PA

{190 105} [1421 6660]

Horizon, given by mixtite number

* Probability value not significant (5 per cent level accepted). '19/18' = 'within the bedded sediments between mixtites 19 and 18'.

TABLE 7: Locations o f well-exposed current structures

Structure

Set thickness in metres

Lithology

Omicron Alpha and omicron X-stratification

3 av. 0.6

Sandstone Sandstone

36/35 Beneath 33

Alpha Omicron Ripple-drift bedding

av. 0.5

Sandstone

Above 30

G

{515 064}

av. 0.7

Sandstone Dolomitic siltstone Dolomite conglomerate

26/25

G

{365 042}

19/18

G

{182 049}

Base of Disrupted Beds

G

{583 032}

Sandy siltstone

9/8

G

{150 040}

Dolomite conglomerate Sandstone

3/2

G

{067 107}

Beneath 1

G

{068 099}

Alpha

6

Ripple-drift bedding Alpha Ripple marks

2

'19/18' ---- 'within the bedded sediments between mixtites 19 and 18'; 'av.' -----'average'. 28

Mem. geol. Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

Interpretation. The outstanding feature of the results is that although quite strong preferred palaeocurrent directions (cross-stratification) are present at certain horizons, when the vector means for these are plotted on one diagram there is no over-all preferred orientation (Fig. 15a). Furthermore, if9all the individual crossstratification measurements are plotted together a completely radial distribution is produced (Fig. 15b), which, tested statistically, does not differ significantly from a uniform one at the 0-05 level of significance

~ 6"/,

3 70"/~

%

/:

-~

~~~68,/, i ;6./,

(a)

(b)

FIG. 15. (a)Palaeocurrent vector mean azimuths and their strengths at horizons given in Table 6. (b)All palaeocurrent measurements for the Port Askaig area (inner circle, cross-strata only) and the Garvellachs (middle and outer circles, crossstrata and ripple marks respectively). (F test). Similarly two of the cross-stratification palaeocurrent distributions for individual horizons are almost uniform (Table 6). The absence of a uniform preferred cross-stratification palaeocurrent direction and the presence of different preferred palaeocurrent directions at different horizons suggests that a single palaeoslope of regional extent was absent throughout the deposition of the whole formation. A possible explanation of the different preferred palaeocurrent directions at different horizons is that the palaeocurrents were of tidal origin and their directions were determined by different local conditions, e.g. differing positions of areas of emergence or 9 at each horizon. By contrast with the cross-stratification the ripple marks do show a preferred orientation (Fig. 15b), predominantly NE-SW, perhaps controlled by wind induced waves. (ii) THE ORIGIN OF THE CONGLOMERATES Beds of conglomerate occur at about 30 horizons within the formation and are important because they provide evidence of the action of powerful water currents and, at certain horizons, of the existence of beach or very shallow water environments. Description. The conglomerate beds range in thickness from only a few centimetres to 14m and in lateral extent from only a few metres up to more than 5-5km. They usually consist of a framework of pebble- or Mem. geol. Soc. Lond. no. 6

29

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

Interpretation. The outstanding feature of the results is that although quite strong preferred palaeocurrent directions (cross-stratification) are present at certain horizons, when the vector means for these are plotted on one diagram there is no over-all preferred orientation (Fig. 15a). Furthermore, if9all the individual crossstratification measurements are plotted together a completely radial distribution is produced (Fig. 15b), which, tested statistically, does not differ significantly from a uniform one at the 0-05 level of significance

~ 6"/,

3 70"/~

%

/:

-~

~~~68,/, i ;6./,

(a)

(b)

FIG. 15. (a)Palaeocurrent vector mean azimuths and their strengths at horizons given in Table 6. (b)All palaeocurrent measurements for the Port Askaig area (inner circle, cross-strata only) and the Garvellachs (middle and outer circles, crossstrata and ripple marks respectively). (F test). Similarly two of the cross-stratification palaeocurrent distributions for individual horizons are almost uniform (Table 6). The absence of a uniform preferred cross-stratification palaeocurrent direction and the presence of different preferred palaeocurrent directions at different horizons suggests that a single palaeoslope of regional extent was absent throughout the deposition of the whole formation. A possible explanation of the different preferred palaeocurrent directions at different horizons is that the palaeocurrents were of tidal origin and their directions were determined by different local conditions, e.g. differing positions of areas of emergence or 9 at each horizon. By contrast with the cross-stratification the ripple marks do show a preferred orientation (Fig. 15b), predominantly NE-SW, perhaps controlled by wind induced waves. (ii) THE ORIGIN OF THE CONGLOMERATES Beds of conglomerate occur at about 30 horizons within the formation and are important because they provide evidence of the action of powerful water currents and, at certain horizons, of the existence of beach or very shallow water environments. Description. The conglomerate beds range in thickness from only a few centimetres to 14m and in lateral extent from only a few metres up to more than 5-5km. They usually consist of a framework of pebble- or Mem. geol. Soc. Lond. no. 6

29

A. M. S P E N C E R sometimes cobble-sized fragments, often with smaller amounts of interstitial sand. The pebbles and cobbles are subangular to r o u n d e d in shape. U p to the level of mixtite 15 the conglomerate stones are predominantly dolomites; in m e m b e r 2 granites and dolomites occur in almost equal numbers (P1.4c), whilst in the conglomerates of members 3, 4 and 5 granite stones predominate.

TABLE8: List of the more important conglomerates

Horizon, given by mixtite number

Garvellachs or Port Askaig

Maximum thickness (metres)

Lateral continuity (km)

47 45 /40 /39 S.I.a.C. /38 /36 In 36 /35 /35 32/30 /29 /26' 27/26 27/26 /22 [15 Base of Disrupted Beds In 12/1 (many horizons)

PA PA PA PA G G G G G PA G G G G G G G G

2"5 2 1 0-1 9 0.1 0.1 2 0.3 7 7 2 0.3 4 0.2 0-1 0.1 14

Many Many 1 0-2 .9 e.d. e.d. 0-02 e.d. 5 0.1 0.35 0.7 1-7 0.25 e.d. e.d. 5.5

G

<4

e.d. to 0.5

Overlie sandstone wedges in mixtite ?yes yes

yes

yes yes yes yes yes

Environment of deposition

Average sediment diameter (cm)

Diameter of largest fragment (cm)

*** *** *** * *** ** ** * ** *** *** *** ** *** *** ** ** ?

4

18 80 x 40 38 15

3

28 50 32x 18 x 10 20 28 x 28 x 25 80 x 42 x 29

all *

2 2-2O <15

3

10

1-3

< 10-20

/19, overlying mixtite; In 19, within mixtite; 19/18, within the bedded sediments between mixtites; e.d., extremely discontinuous; S.I.a.C., Sgeir leth a'Chuain. * sub-ice; ** winnowed beach; *** transported beach.

The conglomerates can be grouped into three types. At some 14 horizons (Table 8*) conglomerates occur which lie within an individual mixtite bed (the conglomerates thin laterally to zero). These conglomerates are analogous to the other discontinuous bedded horizons within mixtites and are the products of sub-glacial water currents which must have been strong, because the average size of the stones is a r o u n d 2 to 3 cm and stones with diameters of 10cm or more are quite common. At six horizons (Table 8"*), thin (less than 30cm), discontinuous conglomerates overlie mixtites and appear to have been produced in place by water currents winnowing sand and silt grade material away. This mechanism is suggested because in certain cases the stones at these horizons form almost complete pavements, one or two fragments thick, which are composed of boulders, cobbles and pebbles in the relative proportions in which they occur in the underlying mixtite. A conglomerate produced by transport for even a short distance should show considerably better sorting than this. The majority of these winnowed conglomerates have been subjected to at least some lateral transport, however, and have built up into a thin, 30

Mem. geoL Soc. Lond. no, 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND sorted pebble bed, so that conglomerates belonging to this group grade naturally into those of the following group. At a further eleven horizons occur thicker (0.2 to 14m) and more extensive beds of conglomerate (Table 8***). Half of these overlie mixtites and half occur within bedded sediments and may represent mixtites which have been completely reworked. Stones average middle pebble grade, but cobble grade fragments are more common than in the conglomerates of the first group (P1. 4c). These conglomerates cannot be winnowed deposits, for the winnowing process must stop when an armoured layer one or two stones thick has been formed on top of a mixtite. Nor are they gravels left on the sea-floor because the fines from material released year after year from melting icebergs have been removed by water currents, for such deposits should form beds which extend with uniform thickness over many kilometres. These conglomerates are neither so extensive nor so uniform in thickness. Therefore the conclusion that they have been transported by water currents to the positions in which they are now found seem inescapable and is supported by the presence of cross-stratification within a few of the conglomerates. Interpretation. There is little evidence available to suggest the environment of deposition of the conglomerates of the second and third groups. The conglomerate at the base of the Disrupted Beds in the Garvellachs does contain large exotic boulders which are most likely to have been ice-rafted and must therefore have been deposited in water deep enough for bergs capable of carrying 1 m boulders to float. On the other hand four of these conglomerates and four of the winnowed conglomerates overlie mixtites penetrated by sandstone wedges; these particular conglomerates rest on surfaces which had been emergent above water level. Water currents capable of transporting fragments of cobble or even small boulder grade, such as occur in the majority of these conglomerates, are most likely to occur above wave base in a subaqueous environment or in a beach or fluvial environment. It is difficult to be certain in which of these the conglomerates are most likely to have formed. The great majority of the conglomerates are extremely well sorted, which is a characteristic of beach gravels (Emery 1955), and do not have the bimodal size frequency distribution characteristic of a fluviatile gravel (Pettijohn 1957, pp. 44, 245-8); a matrix of sand grade material is absent. In addition, the conglomerates are homogeneous; bedded sandstones, which might be expected within a river gravel, very rarely occur in them. The palaeocurrent distributions in the sandstones associated with certain conglomerates (Fig. 13) are rather variable to have been produced in a fluvial environment. Similarly, the swift currents needed to transport some of the large cobbles in the conglomerates, imply, in a fluvial environment, the existence of a fairly steep palaeoslope, for which there is no evidence in the palaeocurrent distributions. Thus the conglomerates of all but the first group seem most likely to have formed in very shallow water, above wave base, or on beaches. (iii) EROSION SURFACES Although the lower contacts of mixtites are usually conformable with the underlying sediments (except at the base of the formation at Port Askaig, see section 7. C (ii)), the upper contacts are quite commonly erosion surfaces. Many of the conglomerates described above record periods of erosion and re-working of the tops of mixtites. In addition several good examples of much larger-scale erosion surfaces can be identified because progressively lower underlying beds (often mixtites) are cut out when traced laterally (Table 9). The conglomerates and erosion surfaces are both products of the same process, the re-working and transport of material by powerful currents in very shallow water. Description. The Upper Dolomite rests on progressively lower horizons, completely cutting out mixtite 18, as it is followed westwards across Garbh Eileach (P1. 11). Other examples of erosion surfaces are provided by mixtite 26 on A'Chuli and by the surface beneath the lowest sandstone in member 3 on Garbh Eileach. In all three examples the lateral thinning to zero is not due to a gradational facies change, for all the sedimentary contacts involved are sharp. Nor can the relations be explained as due to the non-deposition of the beds in certain areas, for in two of the above cases the erosion surfaces cut down through a sequence of beds. Mem. geoL Soc. Lond. no. 6

31

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND sorted pebble bed, so that conglomerates belonging to this group grade naturally into those of the following group. At a further eleven horizons occur thicker (0.2 to 14m) and more extensive beds of conglomerate (Table 8***). Half of these overlie mixtites and half occur within bedded sediments and may represent mixtites which have been completely reworked. Stones average middle pebble grade, but cobble grade fragments are more common than in the conglomerates of the first group (P1. 4c). These conglomerates cannot be winnowed deposits, for the winnowing process must stop when an armoured layer one or two stones thick has been formed on top of a mixtite. Nor are they gravels left on the sea-floor because the fines from material released year after year from melting icebergs have been removed by water currents, for such deposits should form beds which extend with uniform thickness over many kilometres. These conglomerates are neither so extensive nor so uniform in thickness. Therefore the conclusion that they have been transported by water currents to the positions in which they are now found seem inescapable and is supported by the presence of cross-stratification within a few of the conglomerates. Interpretation. There is little evidence available to suggest the environment of deposition of the conglomerates of the second and third groups. The conglomerate at the base of the Disrupted Beds in the Garvellachs does contain large exotic boulders which are most likely to have been ice-rafted and must therefore have been deposited in water deep enough for bergs capable of carrying 1 m boulders to float. On the other hand four of these conglomerates and four of the winnowed conglomerates overlie mixtites penetrated by sandstone wedges; these particular conglomerates rest on surfaces which had been emergent above water level. Water currents capable of transporting fragments of cobble or even small boulder grade, such as occur in the majority of these conglomerates, are most likely to occur above wave base in a subaqueous environment or in a beach or fluvial environment. It is difficult to be certain in which of these the conglomerates are most likely to have formed. The great majority of the conglomerates are extremely well sorted, which is a characteristic of beach gravels (Emery 1955), and do not have the bimodal size frequency distribution characteristic of a fluviatile gravel (Pettijohn 1957, pp. 44, 245-8); a matrix of sand grade material is absent. In addition, the conglomerates are homogeneous; bedded sandstones, which might be expected within a river gravel, very rarely occur in them. The palaeocurrent distributions in the sandstones associated with certain conglomerates (Fig. 13) are rather variable to have been produced in a fluvial environment. Similarly, the swift currents needed to transport some of the large cobbles in the conglomerates, imply, in a fluvial environment, the existence of a fairly steep palaeoslope, for which there is no evidence in the palaeocurrent distributions. Thus the conglomerates of all but the first group seem most likely to have formed in very shallow water, above wave base, or on beaches. (iii) EROSION SURFACES Although the lower contacts of mixtites are usually conformable with the underlying sediments (except at the base of the formation at Port Askaig, see section 7. C (ii)), the upper contacts are quite commonly erosion surfaces. Many of the conglomerates described above record periods of erosion and re-working of the tops of mixtites. In addition several good examples of much larger-scale erosion surfaces can be identified because progressively lower underlying beds (often mixtites) are cut out when traced laterally (Table 9). The conglomerates and erosion surfaces are both products of the same process, the re-working and transport of material by powerful currents in very shallow water. Description. The Upper Dolomite rests on progressively lower horizons, completely cutting out mixtite 18, as it is followed westwards across Garbh Eileach (P1. 11). Other examples of erosion surfaces are provided by mixtite 26 on A'Chuli and by the surface beneath the lowest sandstone in member 3 on Garbh Eileach. In all three examples the lateral thinning to zero is not due to a gradational facies change, for all the sedimentary contacts involved are sharp. Nor can the relations be explained as due to the non-deposition of the beds in certain areas, for in two of the above cases the erosion surfaces cut down through a sequence of beds. Mem. geoL Soc. Lond. no. 6

31

A. M. SPENCER Also, mixtite 26 shows no change in lithology as it thins to zero; if this were either the original end moraine position of a till sheet or the end of a mudflow considerable changes in lithology might be expected. Using similar reasoning, it is suggested that erosion surfaces must exist beneath mixtite 25 and above 29 in the Garvellachs and beneath the base of the lowest sandstone in member 3 at Port Askaig (Table 9). In addition, a non-sequence must exist beneath the base of the Lower Dolomite, for at Bealach an Tarabairt the dolomite disconformably overlies a raft in the Great Breccia.

TABLE9: Stratigraphical horizons of erosion surfaces

Horizon, given by mixtite number

Garvellachs or Port Askaig

Maximumthickness (m) and lithology of beds cut out

Overlie sandstone wedges

Exposed(E) or inferred(i)

Critical outcrops

Base of thick sandstone beneath 33 Base of thick sandstone beneath 33 /32 Base 31

PA

20

Mixtite,sandstone

E

[1422 6672]

G

10

Varves,sandstone

E

Garbh Eileach

G G

2 4

Mixtite Varves,sandstone

E i

/29 /26 beneath 25 /18 /13

G G G G G

20 13 3 20 ?

{127 133} Western Garbh Eileach Sgeir leth a'Chuain A'Chuli {470 059} Garbh Eileach {173 044}

Mixtites Mixtite Siltstone,sandstone Mixtites,siltstone Raft within breccia

yes yes yes

i E i E E

'/19' = 'overlying mixtite 19'.

Interpretation. A very shallow-water environment is indicated by these erosion surfaces, for the water currents which cut the surfaces must have been capable of transporting the cobble and boulder grade material present in the eroded mixtites. By analogy with the conglomerates, transport of such material is most likely to occur above wave base in very shallow water or on beaches. In most cases it is possible to gain some idea of the thickness of sediment which has been removed at the erosion surface (Table 9). This is relatively small, which is in keeping with the conformable appearance of the formation as a whole and with the great lateral continuity of the beds in the formation. To judge from the density of stones in mixtites, for example, the winnowed conglomerates on their tops involve the removal of only a few metres of sediment. This type of evidence suggests an important conclusion, namely that the overall sequence of mixtites deposited in the Garvellachs-Islay area were little affected by penecontemporaneous erosion and therefore the sequence now preserved represents closely the sequence of depositional events.

(iv) BEDS OF DOLOMITE Beds of dolomite, as much as 26m in thickness, occur at four horizons within the formation. Dolomites are also present at the top of the underlying formation, the Islay Limestone, and occur in the overlying formation at several localities (Table 4). In view of the current theory which suggests that the formation of dolomite requires high temperatures, the presence of bedded dolomites might be difficult to explain if the Port Askaig Formation is of glacial origin. After describing the dolomites, this problem is discussed and shown to be common to several late Pre-Cambrian tillites. 32

Mem. geol. Soc. Lond. no. a

A. M. SPENCER Also, mixtite 26 shows no change in lithology as it thins to zero; if this were either the original end moraine position of a till sheet or the end of a mudflow considerable changes in lithology might be expected. Using similar reasoning, it is suggested that erosion surfaces must exist beneath mixtite 25 and above 29 in the Garvellachs and beneath the base of the lowest sandstone in member 3 at Port Askaig (Table 9). In addition, a non-sequence must exist beneath the base of the Lower Dolomite, for at Bealach an Tarabairt the dolomite disconformably overlies a raft in the Great Breccia.

TABLE9: Stratigraphical horizons of erosion surfaces

Horizon, given by mixtite number

Garvellachs or Port Askaig

Maximumthickness (m) and lithology of beds cut out

Overlie sandstone wedges

Exposed(E) or inferred(i)

Critical outcrops

Base of thick sandstone beneath 33 Base of thick sandstone beneath 33 /32 Base 31

PA

20

Mixtite,sandstone

E

[1422 6672]

G

10

Varves,sandstone

E

Garbh Eileach

G G

2 4

Mixtite Varves,sandstone

E i

/29 /26 beneath 25 /18 /13

G G G G G

20 13 3 20 ?

{127 133} Western Garbh Eileach Sgeir leth a'Chuain A'Chuli {470 059} Garbh Eileach {173 044}

Mixtites Mixtite Siltstone,sandstone Mixtites,siltstone Raft within breccia

yes yes yes

i E i E E

'/19' = 'overlying mixtite 19'.

Interpretation. A very shallow-water environment is indicated by these erosion surfaces, for the water currents which cut the surfaces must have been capable of transporting the cobble and boulder grade material present in the eroded mixtites. By analogy with the conglomerates, transport of such material is most likely to occur above wave base in very shallow water or on beaches. In most cases it is possible to gain some idea of the thickness of sediment which has been removed at the erosion surface (Table 9). This is relatively small, which is in keeping with the conformable appearance of the formation as a whole and with the great lateral continuity of the beds in the formation. To judge from the density of stones in mixtites, for example, the winnowed conglomerates on their tops involve the removal of only a few metres of sediment. This type of evidence suggests an important conclusion, namely that the overall sequence of mixtites deposited in the Garvellachs-Islay area were little affected by penecontemporaneous erosion and therefore the sequence now preserved represents closely the sequence of depositional events.

(iv) BEDS OF DOLOMITE Beds of dolomite, as much as 26m in thickness, occur at four horizons within the formation. Dolomites are also present at the top of the underlying formation, the Islay Limestone, and occur in the overlying formation at several localities (Table 4). In view of the current theory which suggests that the formation of dolomite requires high temperatures, the presence of bedded dolomites might be difficult to explain if the Port Askaig Formation is of glacial origin. After describing the dolomites, this problem is discussed and shown to be common to several late Pre-Cambrian tillites. 32

Mem. geol. Soc. Lond. no. a

LATE P R E - C A M B R I A N G L A C I A T I O N IN S C O T L A N D N a t u r e o f the dolomite beds. The dolomite beds are described in detail, and in their stratigraphical context,

in section 7, so a short summary of their characters will suffice here. Beds of dolomite occur above mixtite 13 (11 to 26m thick, Garvellachs), within the Disrupted Beds (several beds each under 1 m in thickness), above the Disrupted Beds (3 m, Port Askaig), above mixtite 18 (11 m, Garvellachs), above mixtite 30 (several beds each under l m in thickness, Garvellachs) and in the thick sandstone between mixtites 33 and 32 (2m, Garvellachs). In addition irregular beds of dolomite occur within mixtite 12 in the Garvellachs and the upper 50cm of the varved siltstones above mixtite 32 in the Garvellachs is very rich in dolomite. These dolomites are all fine-grained but, because of slight metamorphism, original textures have been destroyed and only irregular polygonal mosaics can now be seen in thin section. A few of the dolomites contain recognizable detrital fragments of small pebble grade. With the exception of the dolomite bands in the Disrupted Beds, extra-basinal fragments are absent from the dolomites. Table 10, shows the compositions of dolomites from horizons beneath, within and above the Port Askaig Formation.

TABLE 10: Chemical analyses ~ of bedded dolomites and dolomite stones

1

2

3

4

5

6

7

8

6m beneath I

1m beneath 1

Stone in 11

Stone in Disrupted Beds

above 13

above 18

above 30

Upper Dolomitic Formation

G

Beannan Dubh, Islay

G

G

G

G

G

Bagh an da Dhorius, Islay

36"48 43.43 CALCITE - - C a C O 3 7"39 --Fe~O~ 2.2 RESIDUE --SiO2 3"5 --Insoluble z 0-5 --Soluble 3 4"1

43"83 52.18 2"23 0.8 0"7 0-6 0.5

36"09 42.96 12"91 2.0 2"0 1.9 0.7

39"85 47.43 3"60 2.1 2"6 1"6 2-1

38-66 46.02 2"66 0.7 7"1 1-8 3.3

40"86 48.63 4"21 1.5 0"6 0-2 2"5

30"00 35.71 6"22 3-9 16"5 7"0 --

31"88 37.95 14"06 2.6 5"7 2-9 3.8

TOTAL

100"9

98"6

99"1

98"6

99"8

98"9

Horizon, given by mixtite number Locality Garvellachs (G) DOLOMITE--MgCO3 --CaCO3

97"5

100"4

1 Analyst, M. Brotherton, University of Liverpool. ~ Residue after HF. 3 Amount dissolved in 1 : 1 HCI not accounted for.

Origin. Firstly, the dolomites may possibly be of detrital origin and have formed by the accumulation of

silt and fine sand grade fragments, which are now obscured due to metamorphism. Such a mechanism is unlikely because of the lack of a suitable source. The only relatively pure source for detrital dolomites, the top of the Islay Limestone, lies too far down in the succession. In addition, the mixtites all have sufficiently high quartz and mica contents to rule them out as sources for pure detrital dolomites. Lastly, detrital dolomite beds (dolomite conglomerates) are present in the formation, but contain abundant pebble grade fragments and can easily be distinguished from the dolomites described here. Secondly, the dolomites are not the result of dolomitization long subsequent to the deposition of the beds. Their contacts with over- and underlying beds poor in dolomite are very sharp. In addition, the bands and pebbles of dolomite in the Disrupted Beds are enclosed within blue siltstones which are very poor in carbonate; such beds and pebbles cannot have been dolomitized subsequently to their deposition in such Mem. geol. Soc. Lond. no. 6

33

A. M. SPENCER positions. Thus it seems likely that the original bedded dolomites formed by direct precipitation, or by penecontemporaneous or early dolomitization at some time before their erosion into the stones. It is not possible to decide between these possibilities and, in any case, they imply that conditions around the time of deposition of the mixtites were suitable for the formation of dolomites. There is a consensus of opinion that the formation of primary or penecontemporaneous dolomite requires relatively high temperatures (e.g. Alderman 1965; Bissell & Chilingar 1962). Halla, Chilingar & Bissell (1962) and Sass (1965) suggest that temperatures of 35 ~ to 55 ~ and over 22~ respectively, are preferred for the formation of dolomite. It is not certain, however, that such high temperatures were necessary for the formation of dolomite in late Pre-Cambrian times but if they were, then extreme variations in climate, between periods of formation of glacial mixtites and of dolomite interbeds, are implied. (One interesting field relationship is seen on the east coast of Garbh Eileach. An 11 m thick dolomite overlies mixtite 18 where the latter is penetrated by sandstone wedges; this sequence is here interpreted as recording deposition from a grounded ice-sheet, followed by permafrost conditions, followed by the formation of dolomite.) Such climatic variations are probably more extreme than any which have occurred at any locality on the Earth during the Pleistocene. The magnitude of these implied climatic variations might be used as evidence against the mixtites being of glacial origin, especially against their being the deposits of grounded ice-sheets. Such considerable climatic variations are not impossible, however, but would, by comparison with Pleistocene events, require long periods of time for their accomplishment (many tens of thousands of years). Considerable erosion might be expected to occur during such periods, but the conformable sequence in the formation gives little evidence of this (section 3. B (iii)). Even this does not rule out the possibility of such extreme climatic variations, however, but it does imply that the contacts between mixtites and certain non-glacial interbeds must represent long periods with little deposition or erosion. Dolomites in other tillite sequences. The problem of the presence of dolomites is common to several late Pre-Cambrian tillites. For example dolomites occur immediately above a tillite in East Greenland (Berthelsen & Noe-Nygaard 1965, p. 247), below and above a tillite in Spitsbergen (Wilson & Harland 1964), below a tillite in parts of Finnmark, Norway (Spjeldnaes 1964), immediately above both 'Tilloid' formations in Angola (dolomitic limestones only, Schermerhorn & Stanton 1963), immediately above and beneath tillites in South West Africa (Martin 1965), immediately above a tillite in north-western Australia (Dow 1965) and immediately beneath a tillite in South Australia (Mawson 1949). There are few records of dolomite beds within tillite formations (Fr/inkl 1953A, p. 70, 1953B, p. 25, Sommer 1957, fig. 8; Wilson & Harland 1964), such as occur in the Port Askaig Formation, but the occurrence of dolomites immediately above or beneath tiUite formations is too widespread to be coincidental. It is premature, however, to suggest an actual connection between tillite deposition and the formation of dolomites. (V) OUTSIZE STONES IN BEDDED SEDIMENTS Extra-basinal stones occur only very rarely in the bedded sediments within the formation and are almost exclusively confined to the conglomerates described above. In most other lithologies of bedded sediment extra-basinal stones are absent (only one example known, see below), with one lithology excepted: scattered extra-basinal stones occur within thin laminated siltstones at some eight horizons within the formation on the Garvellachs (Fig. 16). The stones have diameters greater than the thickness of the enclosing laminae, because of which Harland et al. (1967, p. 244) stated that such "stones could not have been carried laterally into place contemporaneously with the sediment" and they must therefore have been rafted. Such outsize stones provide certain evidence of the former presence of floating ice for, in one bed (see below), they occur in such large numbers that rafting by Pre-Cambrian seaweeds can be ruled out. In addition, the association of rafted stones with thin laminated, varve-like siltstones is just that which would be expected from ice-rafting. Information from these horizons is shown on Fig. 16, which relates the diameter of the larger stones to the stratification thickness. 34

Mem. geol. Soc. Lond. no. 6

A. M. SPENCER positions. Thus it seems likely that the original bedded dolomites formed by direct precipitation, or by penecontemporaneous or early dolomitization at some time before their erosion into the stones. It is not possible to decide between these possibilities and, in any case, they imply that conditions around the time of deposition of the mixtites were suitable for the formation of dolomites. There is a consensus of opinion that the formation of primary or penecontemporaneous dolomite requires relatively high temperatures (e.g. Alderman 1965; Bissell & Chilingar 1962). Halla, Chilingar & Bissell (1962) and Sass (1965) suggest that temperatures of 35 ~ to 55 ~ and over 22~ respectively, are preferred for the formation of dolomite. It is not certain, however, that such high temperatures were necessary for the formation of dolomite in late Pre-Cambrian times but if they were, then extreme variations in climate, between periods of formation of glacial mixtites and of dolomite interbeds, are implied. (One interesting field relationship is seen on the east coast of Garbh Eileach. An 11 m thick dolomite overlies mixtite 18 where the latter is penetrated by sandstone wedges; this sequence is here interpreted as recording deposition from a grounded ice-sheet, followed by permafrost conditions, followed by the formation of dolomite.) Such climatic variations are probably more extreme than any which have occurred at any locality on the Earth during the Pleistocene. The magnitude of these implied climatic variations might be used as evidence against the mixtites being of glacial origin, especially against their being the deposits of grounded ice-sheets. Such considerable climatic variations are not impossible, however, but would, by comparison with Pleistocene events, require long periods of time for their accomplishment (many tens of thousands of years). Considerable erosion might be expected to occur during such periods, but the conformable sequence in the formation gives little evidence of this (section 3. B (iii)). Even this does not rule out the possibility of such extreme climatic variations, however, but it does imply that the contacts between mixtites and certain non-glacial interbeds must represent long periods with little deposition or erosion. Dolomites in other tillite sequences. The problem of the presence of dolomites is common to several late Pre-Cambrian tillites. For example dolomites occur immediately above a tillite in East Greenland (Berthelsen & Noe-Nygaard 1965, p. 247), below and above a tillite in Spitsbergen (Wilson & Harland 1964), below a tillite in parts of Finnmark, Norway (Spjeldnaes 1964), immediately above both 'Tilloid' formations in Angola (dolomitic limestones only, Schermerhorn & Stanton 1963), immediately above and beneath tillites in South West Africa (Martin 1965), immediately above a tillite in north-western Australia (Dow 1965) and immediately beneath a tillite in South Australia (Mawson 1949). There are few records of dolomite beds within tillite formations (Fr/inkl 1953A, p. 70, 1953B, p. 25, Sommer 1957, fig. 8; Wilson & Harland 1964), such as occur in the Port Askaig Formation, but the occurrence of dolomites immediately above or beneath tiUite formations is too widespread to be coincidental. It is premature, however, to suggest an actual connection between tillite deposition and the formation of dolomites. (V) OUTSIZE STONES IN BEDDED SEDIMENTS Extra-basinal stones occur only very rarely in the bedded sediments within the formation and are almost exclusively confined to the conglomerates described above. In most other lithologies of bedded sediment extra-basinal stones are absent (only one example known, see below), with one lithology excepted: scattered extra-basinal stones occur within thin laminated siltstones at some eight horizons within the formation on the Garvellachs (Fig. 16). The stones have diameters greater than the thickness of the enclosing laminae, because of which Harland et al. (1967, p. 244) stated that such "stones could not have been carried laterally into place contemporaneously with the sediment" and they must therefore have been rafted. Such outsize stones provide certain evidence of the former presence of floating ice for, in one bed (see below), they occur in such large numbers that rafting by Pre-Cambrian seaweeds can be ruled out. In addition, the association of rafted stones with thin laminated, varve-like siltstones is just that which would be expected from ice-rafting. Information from these horizons is shown on Fig. 16, which relates the diameter of the larger stones to the stratification thickness. 34

Mem. geol. Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

Description. At seven horizons stones of from pebble to small boulder grade occur in siltstones which are finely and regularly laminated (laminae with thicknesses of 0.1 to 10mm). Of these horizons, stones occur in the largest numbers in the 3.5m thick varved siltstones on the west coast of Garbh Eileach (P1. 5a, d). There one stone with one diameter greater than 1 cm occurs, on average, in every 1.5 m ~ of outcrop face. The smallest number of stones--one every 60m ~ of outcrop---occurs in the 6m thick varved siltstones above mixtite 32. (I) n," LU I-LU ~" <

lm-

C3 ILl .._! L.) I--n,"

X

X

100-

n<

10-

Ill,lit1

lmm-

I I I I I I Ii i

i

i i:,

0.1~

0.01-

0"001-

i

I~

lOO

io

1~

10

lOOm

STRATIFICATION THICKNESS F I G . 16. Outsize stones (crosses) related to the stratification thickness and grain-size (arrowed lines) of the beds in which they occur; all from the Garvellachs. The diagonal ruling shows the field occupied by the mixtites in the Port Askaig Formation. Diagram modified from Harland e t al. (1966, fig. 3).

Densities of stones at the other laminated siltstone horizons are closer to the second value than the first. In all the seven beds stones occur scattered through the whole thickness of the bed and are rarely concentrated at any particular horizons. Another horizon with extra-basinal stones (less than eight seen) in siltstones lies immediately above mixtite 30; it differs from those just described in that the stones are not confined to a particular bed but are scattered through a few metres of siltstones and dolomites. Two other occurrences of outsize granite stones are known. In the sandstone above mixtite 36 granite fragments occur associated with a thin, discontinuous lens of mixtite (Fig. 17). Secondly, in the conglomerate at the base of the Disrupted Beds the average particle diameter is 3 crn and fragments larger than 10cm occur very rarely. Scattered boulders

....:-...........'..:-..'.'...-.........::...

._

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~

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.'"

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i: ;i!

.

.

;::":":.:." ; .-'.~ :!.! L'...:?. .

i!

•

:i

~"-iiz!:::- 3E::-:.?i-!!i/:?~:):"::--:i;:;i-?::~!.;.:!!?:...:-.7 ~:::s~

: :

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. . . . -=

,

F I o . 17. Field sketch of outsize stones associated with a thin mixtite in the sandstone above mixtite 36, west coast of Garbh

Eileach. M e m . geol. Soc. Lond. no. 6

35

A. M. S P E N C E R

are present, however, and occur in the ratio of about one or two per 1000m ~ of outcrop (e.g. dolomite boulder measuring 128 x 72cm and an exotic measuring 86 x 56 x 20cm). Despite the thick bedded nature of this conglomerate, the great difference in size between these and the average particle, together with the extra-basinal nature of many of the stones, suggests that the large stones may have been transported by icebergs. (vi) VARVES

The seven siltstone beds described above all have a regular, even lamination structure which is very continuous laterally. At two horizons, beneath mixtite 31 in the west and above mixtite 32 in the east of Garbh Eileach, the individual laminae are beautifully graded (P1. 5b) and can be identified as varves. The laminae in the siltstones above mixtite 30 and in the Disrupted Beds are less well graded, but are also probably varves. The other siltstones are not graded and are varve-like only because of their even lamination and the presence in them of rafted stones. As noted in section 3. A (ii), these siltstones occupy a position which is intermediate between the varves described above and true, unlaminated mixtites. Description. The varved siltstone beneath mixtite 31 (4m) and that above 32 (6m) both have transitional contacts with the underlying sandstones. In both the lamination structure is very clear and almost all the individual laminae (varves) consist of two layers, a lower of fine sand grade and an upper of silt grade. The contact between these layers is almost always gradational, the grain size of the whole laminae decreasing from its coarse base to its fine top (P1.5b). The base of each varve is very sharp and planar. In a few varves as many as five laminae are present within the varve and are identified as internal laminae (and not as individual varves) because they are much thinner and much less coarse grained than the average varve (P1. 5f). Individual varves are very uniform in thickness throughout the outcrop in which they have been studied. (A general view of the varves above mixtite 32 is given by Kilburn et al. (1965, pl. 19B).) Because of their simple graded structure and constant lateral thickness, it has been possible to measure a section through the central part of the upper varves. These measurements, which were kindly made directly on to tracing paper in the field by Dr G. Warrington, are shown in Fig. 18. Of the 2587 laminae measured over 98 per cent are between 1mm and 5 mm in thickness; individual laminae from the other varve horizons in the Garvellachs also have thicknesses of this order. Both the varve horizons described above have sandstone wedges penetrating downwards from their tops. An even more remarkable feature of the upper varves is that in their topmost 50cm they are very dolomitic (P1. 5f); in thin section they contain almost 50 per cent dolomite. At Port Askaig a 9 m thick bed of varved siltstones occurs in the identical stratigraphical position to the varves just described, lying at the top of Member 2, beneath the erosional base of the lowest thick sandstone of Member 3. The individual laminae (varves) in this bed have all the characteristics described above but are much thicker, varves of up to 10mm thickness occurring quite commonly. Origin. After studying Pleistocene examples, De Geer (1912) suggested that varves are annual deposits formed from bottom-flowing meltwater currents in glacial lakes. Kuenen (1951) amplified this explanation maintaining that varves are the deposits of very protracted turbidity currents. The varves described here are extremely similar to Pleistocene examples, particularly in their regular lamination and the double structure of each laminae. They are not identical to Pleistocene varves, however, and have three prominent differences with the latter. The varves here have an average thickness (2 to 3 mm) smaller than many Pleistocene varves (e.g. Lajtai 1967, p. 634, 6 to 50mm; Antevs 1951, pp. 1228-40, commonly 10 to 20mm; Sauramo 1929, p. 41, 2 to 30mm). Secondly, Lajtai (1967), from a review of the literature, suggested that Pleistocene varves are not well graded and consist of a lower coarse grained band separated by a relatively sharp contact from an upper fine-grained band. Thirdly, despite being very thin, all the varves here contain sand grade material at their bases, whilst most Pleistocene varves are composed of silt and clay only, although Antevs (1951, p. 1250) noted that the proximal part of a varve may contain much sand. Despite these differences, the varves described here seem most likely to have originated in a manner analogous to Pleistocene varves for they are not 36

Mem. geoL Soc. Lond. no. 6

A. M. S P E N C E R

are present, however, and occur in the ratio of about one or two per 1000m ~ of outcrop (e.g. dolomite boulder measuring 128 x 72cm and an exotic measuring 86 x 56 x 20cm). Despite the thick bedded nature of this conglomerate, the great difference in size between these and the average particle, together with the extra-basinal nature of many of the stones, suggests that the large stones may have been transported by icebergs. (vi) VARVES

The seven siltstone beds described above all have a regular, even lamination structure which is very continuous laterally. At two horizons, beneath mixtite 31 in the west and above mixtite 32 in the east of Garbh Eileach, the individual laminae are beautifully graded (P1. 5b) and can be identified as varves. The laminae in the siltstones above mixtite 30 and in the Disrupted Beds are less well graded, but are also probably varves. The other siltstones are not graded and are varve-like only because of their even lamination and the presence in them of rafted stones. As noted in section 3. A (ii), these siltstones occupy a position which is intermediate between the varves described above and true, unlaminated mixtites. Description. The varved siltstone beneath mixtite 31 (4m) and that above 32 (6m) both have transitional contacts with the underlying sandstones. In both the lamination structure is very clear and almost all the individual laminae (varves) consist of two layers, a lower of fine sand grade and an upper of silt grade. The contact between these layers is almost always gradational, the grain size of the whole laminae decreasing from its coarse base to its fine top (P1.5b). The base of each varve is very sharp and planar. In a few varves as many as five laminae are present within the varve and are identified as internal laminae (and not as individual varves) because they are much thinner and much less coarse grained than the average varve (P1. 5f). Individual varves are very uniform in thickness throughout the outcrop in which they have been studied. (A general view of the varves above mixtite 32 is given by Kilburn et al. (1965, pl. 19B).) Because of their simple graded structure and constant lateral thickness, it has been possible to measure a section through the central part of the upper varves. These measurements, which were kindly made directly on to tracing paper in the field by Dr G. Warrington, are shown in Fig. 18. Of the 2587 laminae measured over 98 per cent are between 1mm and 5 mm in thickness; individual laminae from the other varve horizons in the Garvellachs also have thicknesses of this order. Both the varve horizons described above have sandstone wedges penetrating downwards from their tops. An even more remarkable feature of the upper varves is that in their topmost 50cm they are very dolomitic (P1. 5f); in thin section they contain almost 50 per cent dolomite. At Port Askaig a 9 m thick bed of varved siltstones occurs in the identical stratigraphical position to the varves just described, lying at the top of Member 2, beneath the erosional base of the lowest thick sandstone of Member 3. The individual laminae (varves) in this bed have all the characteristics described above but are much thicker, varves of up to 10mm thickness occurring quite commonly. Origin. After studying Pleistocene examples, De Geer (1912) suggested that varves are annual deposits formed from bottom-flowing meltwater currents in glacial lakes. Kuenen (1951) amplified this explanation maintaining that varves are the deposits of very protracted turbidity currents. The varves described here are extremely similar to Pleistocene examples, particularly in their regular lamination and the double structure of each laminae. They are not identical to Pleistocene varves, however, and have three prominent differences with the latter. The varves here have an average thickness (2 to 3 mm) smaller than many Pleistocene varves (e.g. Lajtai 1967, p. 634, 6 to 50mm; Antevs 1951, pp. 1228-40, commonly 10 to 20mm; Sauramo 1929, p. 41, 2 to 30mm). Secondly, Lajtai (1967), from a review of the literature, suggested that Pleistocene varves are not well graded and consist of a lower coarse grained band separated by a relatively sharp contact from an upper fine-grained band. Thirdly, despite being very thin, all the varves here contain sand grade material at their bases, whilst most Pleistocene varves are composed of silt and clay only, although Antevs (1951, p. 1250) noted that the proximal part of a varve may contain much sand. Despite these differences, the varves described here seem most likely to have originated in a manner analogous to Pleistocene varves for they are not 36

Mem. geoL Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

200 9

p m ==,= m

m

m

i

9

m D

9

100"

m

-Q

TOP.

Q

~

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=

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.-.

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BASE FIG. 18. T h i c k n e s s variation graph o f 5 . 5 m o f the varved siltstones a b o v e m i x t i t e 32 o n the east c o a s t o f G a r b h Eileach. T h e length o f e a c h line gives the true thickness o f the graded lamina, 2587 o f w h i c h are s h o w n . F i e l d m e a s u r e m e n t s kindly m a d e by D r G. W a r r i n g t o n .

Mem. geol. Soc. Lond. no. 6

37

A. M. S P E N C E R

geosynclinal turbidite deposits. They are thinner and of a more uniform thickness up the succession than even a very distal turbidite deposit and they contain rafted stones. In addition, all three beds of varves are discontinuous laterally, a feature which would not be expected in distal turbidites but which is typical of varves deposited either in lakes or in shallow seas and later subjected to erosion. It would be interesting to know whether the sub-aqueous environment of deposition of the varves was fresh-water or marine. Sauramo (1923) produced evidence from Pleistocene varves in Finland suggesting that under more saline or marine conditions the graded structure of varves is suppressed because deposition is considerably accelerated due to the flocculating effect of the salt. Kuenen (1951, p. 175) and Schwarzbach (1963, p. 85) supported this idea but Pettijohn (1957, p. 177), arguing by analogy with geosynclinal turbidites, doubted the efficiency of the mechanism. If this suggestion is correct, (and if the mechanism will work for these relatively coarser grained varves) the simple graded structure of the varves described here perhaps suggests that they are of fresh-water origin--a conclusion supported by the relatively localized distribution of the three beds. A further point, but one which needs stressing even though its origin is not known, is the remarkably dolomitic nature of part of one of the varved beds (above mixtite 32, Garbh Eileach, P1. 5f). Cyclicity. The thickness variation graph (Fig. 18) has been investigated for significant cyclic fluctuations in varve thickness by Dr T. V. Loudon, who gives the following account of the method: " I f regular cyclicity occurred in the sequence, so that the thickness of each bed had a tendency to resemble that of the bed at an interval of i beds below it, then this might be apparent in statistics which summarized, for the sequence as a whole, the correlation between beds at an interval i beds apart. It was suggested to me that repetition at an interval of about 10 beds could exist and would be of interest. Correlation coefficients between beds at an interval i apart, for i from 1 to 18, were therefore computed for the section as a whole and also for a selected part of the section. On examination of the correlation coefficients and of a multiple linear regression analysis based on the coefficients, I found no evidence to contradict the hypothesis that such cyclicity was not a statistically significant feature of the sequence".

Similar negative results have been reported by Jackson (1965), who analysed a total of 310 varves from the Pre-Cambrian Gowganda Formation of Ontario and by Anderson & Koopmans (1963, fig.9) for 126 limestone 'varves' from the Nama System of South-West Africa. (vii) DEFLECTION AND PENETRATION OF LAMINAE ABOUT ICE-RAFTED STONES

Harland et al. (1966, p. 243) suggested that when a stone is found in finely laminated sediments "Penetration and distortion of the stratification beneath, with unconformable covering on top, may be decisive evidence for rafting . . . . Distortion of laminae round a stone may, however, be due to compaction of clay, which is generally likely to be found in some degree; therefore some asymmetry between the underlying and overlying distortion should also be observed". Compaction and cleavage (in this case) should produce an equal and opposite bending of the laminae on either side of the stone, whilst the original impact of the rafted stone may have produced more distortion beneath the stone than was developed by the overlapping of laminae above the stone. (There is some evidence, however, that stones falling into varved clays do not penetrate far; Hardy & Legget (1960) report an 8in diameter stone which penetrated only 2in of sediment). In the Garvellachs the best examples of ice-rafted stones in laminated siltstones occur in the varves beneath mixtite 31 and the siltstones of the Disrupted Beds. Deflection of the laminae beneath a stone can be proved to be due to the impact of the falling stone in only one case, where the downbent layers beneath the pebble are overlain unconformably by the first layer deposited after the pebble fell (Fig. 19a). In addition, in several cases, the deflection of laminae beneath the pebble is much greater than that above (Fig. 19b, c), which strongly suggests that the deflection beneath is a primary character due to the impact of the stone. In almost as many other cases, however, there is little significant difference between the deflection of the laminae beneath and above the pebble (Fig. 19a, d, f). Thus the relationship of outsize stones to the enclosing lamination is not always conclusive and is often difficult to see in the field (most of the specimens illustrated here 38

Mem. geoL Soc. Lond. no. 6

A. M. S P E N C E R

geosynclinal turbidite deposits. They are thinner and of a more uniform thickness up the succession than even a very distal turbidite deposit and they contain rafted stones. In addition, all three beds of varves are discontinuous laterally, a feature which would not be expected in distal turbidites but which is typical of varves deposited either in lakes or in shallow seas and later subjected to erosion. It would be interesting to know whether the sub-aqueous environment of deposition of the varves was fresh-water or marine. Sauramo (1923) produced evidence from Pleistocene varves in Finland suggesting that under more saline or marine conditions the graded structure of varves is suppressed because deposition is considerably accelerated due to the flocculating effect of the salt. Kuenen (1951, p. 175) and Schwarzbach (1963, p. 85) supported this idea but Pettijohn (1957, p. 177), arguing by analogy with geosynclinal turbidites, doubted the efficiency of the mechanism. If this suggestion is correct, (and if the mechanism will work for these relatively coarser grained varves) the simple graded structure of the varves described here perhaps suggests that they are of fresh-water origin--a conclusion supported by the relatively localized distribution of the three beds. A further point, but one which needs stressing even though its origin is not known, is the remarkably dolomitic nature of part of one of the varved beds (above mixtite 32, Garbh Eileach, P1. 5f). Cyclicity. The thickness variation graph (Fig. 18) has been investigated for significant cyclic fluctuations in varve thickness by Dr T. V. Loudon, who gives the following account of the method: " I f regular cyclicity occurred in the sequence, so that the thickness of each bed had a tendency to resemble that of the bed at an interval of i beds below it, then this might be apparent in statistics which summarized, for the sequence as a whole, the correlation between beds at an interval i beds apart. It was suggested to me that repetition at an interval of about 10 beds could exist and would be of interest. Correlation coefficients between beds at an interval i apart, for i from 1 to 18, were therefore computed for the section as a whole and also for a selected part of the section. On examination of the correlation coefficients and of a multiple linear regression analysis based on the coefficients, I found no evidence to contradict the hypothesis that such cyclicity was not a statistically significant feature of the sequence".

Similar negative results have been reported by Jackson (1965), who analysed a total of 310 varves from the Pre-Cambrian Gowganda Formation of Ontario and by Anderson & Koopmans (1963, fig.9) for 126 limestone 'varves' from the Nama System of South-West Africa. (vii) DEFLECTION AND PENETRATION OF LAMINAE ABOUT ICE-RAFTED STONES

Harland et al. (1966, p. 243) suggested that when a stone is found in finely laminated sediments "Penetration and distortion of the stratification beneath, with unconformable covering on top, may be decisive evidence for rafting . . . . Distortion of laminae round a stone may, however, be due to compaction of clay, which is generally likely to be found in some degree; therefore some asymmetry between the underlying and overlying distortion should also be observed". Compaction and cleavage (in this case) should produce an equal and opposite bending of the laminae on either side of the stone, whilst the original impact of the rafted stone may have produced more distortion beneath the stone than was developed by the overlapping of laminae above the stone. (There is some evidence, however, that stones falling into varved clays do not penetrate far; Hardy & Legget (1960) report an 8in diameter stone which penetrated only 2in of sediment). In the Garvellachs the best examples of ice-rafted stones in laminated siltstones occur in the varves beneath mixtite 31 and the siltstones of the Disrupted Beds. Deflection of the laminae beneath a stone can be proved to be due to the impact of the falling stone in only one case, where the downbent layers beneath the pebble are overlain unconformably by the first layer deposited after the pebble fell (Fig. 19a). In addition, in several cases, the deflection of laminae beneath the pebble is much greater than that above (Fig. 19b, c), which strongly suggests that the deflection beneath is a primary character due to the impact of the stone. In almost as many other cases, however, there is little significant difference between the deflection of the laminae beneath and above the pebble (Fig. 19a, d, f). Thus the relationship of outsize stones to the enclosing lamination is not always conclusive and is often difficult to see in the field (most of the specimens illustrated here 38

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

had to be broken from the outcrop, re-assembled in the laboratory and then sawn). It is suggested that a more immediately obvious criterion of rafting is the presence of stones lying with their long axes at rightangles to the stratification, for they could not easily have slid sideways into that position (P1.5c and e.g. Martin 1964B, fig. 2).

(a) o ~ 1 7 6 1 7. 6. .1. 7 6

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loom .._.-

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(f)

FIG. 19. Field and hand-specimen sketches of outsize stones in fine laminated siltstones from the Garvellachs. (a) to (c) and (e) Granitic stones from the varved siltstones above mixtite 30, western Garbh Eileach. (d), ( f ) and (g) Dolomite fragments and mixtite lenses in siltstones from the Disrupted Beds. Plate 5(a) is a photograph of the stone shown in (a) above.

(c) O T H E R S T R U C T U R E S In this section four special structures are described. The most important from the point of view of the origin of the Port Askaig Formation are the sandstone wedges. They are described first but their origin is dealt with after the sandstone dykes have been discussed, for they are best explained by contrast with the latter. Mem. geol. Soc. Lond. no. 6

39

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

had to be broken from the outcrop, re-assembled in the laboratory and then sawn). It is suggested that a more immediately obvious criterion of rafting is the presence of stones lying with their long axes at rightangles to the stratification, for they could not easily have slid sideways into that position (P1.5c and e.g. Martin 1964B, fig. 2).

(a) o ~ 1 7 6 1 7. 6. .1. 7 6

,=

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(f)

FIG. 19. Field and hand-specimen sketches of outsize stones in fine laminated siltstones from the Garvellachs. (a) to (c) and (e) Granitic stones from the varved siltstones above mixtite 30, western Garbh Eileach. (d), ( f ) and (g) Dolomite fragments and mixtite lenses in siltstones from the Disrupted Beds. Plate 5(a) is a photograph of the stone shown in (a) above.

(c) O T H E R S T R U C T U R E S In this section four special structures are described. The most important from the point of view of the origin of the Port Askaig Formation are the sandstone wedges. They are described first but their origin is dealt with after the sandstone dykes have been discussed, for they are best explained by contrast with the latter. Mem. geol. Soc. Lond. no. 6

39

A.

(i)

M.

SPENCER

SANDSTONE

WEDGES

Within the Port Askaig Formation more than one thousand examples o f sandstone wedge structures have been discovered. They are V-shaped in cross-section (Fig. 20a) and always narrow to zero downwards, because o f which they have been termed sandstone wedges. Such structures penetrate downwards from a 9 . " ' ' "

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FIG. 20. Field sketches o f s a n d s t o n e wedges in cross-section. (g) is f r o m Port Askaig; all others are f r o m the Garvellachs. Stratigraphical h o r i z o n s (given by the n u m b e r o f the mixtite bed the wedges penetrate) and localities: (a) 15, {232 038}. (b) 26', {476 061}. (c) 2, {028 121}. (d) a n d ( f ) 22, {365 037}. (e) 6 0 c m below the top o f mixtite 26, {294 064}. (g) 22, [1431 6668]. (h), ( j ) a n d (k) I n the varved siltstones above mixtite 30, {260 060}. (i) In the siltstones above mixtite 30, {512 065}. 40

Mem.

geol,

Soc.

Lond.

no.

6

A.

(i)

M.

SPENCER

SANDSTONE

WEDGES

Within the Port Askaig Formation more than one thousand examples o f sandstone wedge structures have been discovered. They are V-shaped in cross-section (Fig. 20a) and always narrow to zero downwards, because o f which they have been termed sandstone wedges. Such structures penetrate downwards from a 9 . " ' ' "

9

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FIG. 20. Field sketches o f s a n d s t o n e wedges in cross-section. (g) is f r o m Port Askaig; all others are f r o m the Garvellachs. Stratigraphical h o r i z o n s (given by the n u m b e r o f the mixtite bed the wedges penetrate) and localities: (a) 15, {232 038}. (b) 26', {476 061}. (c) 2, {028 121}. (d) a n d ( f ) 22, {365 037}. (e) 6 0 c m below the top o f mixtite 26, {294 064}. (g) 22, [1431 6668]. (h), ( j ) a n d (k) I n the varved siltstones above mixtite 30, {260 060}. (i) In the siltstones above mixtite 30, {512 065}. 40

Mem.

geol,

Soc.

Lond.

no.

6

LATE P R E - C A M B R I A N G L A C I A T I O N IN S C O T L A N D total of 27 distinct stratigraphical horizons (Table 11) and, when seen in plan view (Fig. 21), they often form a branching or polygonal network. In section 3. C (iii), after considering tension crack, desiccation crack and sandstone dyke hypotheses, it is argued that they have been produced by contraction cracking

FIG. 21. The patterns formed by sandstone wedges in plan view. (a) to (e) Drawn from photographs. (f) and (h) to (k) Field sketches. (g) Rose diagram of the orientation of 120 wedges from the polygonal system in the top of mixtite 22 at {565 048}; white and black parts measured where polygons are, respectively, poorly and well developed. Stratigraphical horizons (given by the number of the mixtite bed the wedges penetrate) and localities: (a), (b) and (h) 22, {565 048}. (c) 22, {464 057}. (d) 15, {097 126}. (e) 29, {121 132}. (f) 35, {187 105}. (j) 24, {464 059}. (k) 40, Port Askaig, [1430 6675]. All sketches have the same geographical orientation. In (a) to (f) and (i) the strike and dip direction of the surface on which the wedges are exposed is indicated. Horizontal ruling: bedded strata overlying the sandstone wedges. Mem. geol. Soc. Lond. no. 6

41

A. M. SPENCER under permafrost conditions. In addition to the support it provides for the glacial origin of the mixtites, this conclusion has several important implications, which are also outlined in section 3. C (iii). Lithology, shape and size. The wedges are almost always infilled with a medium grained pebbly sandstone, the only exceptions being six wedges infilled with dolomitic siltstone. Most wedges are less than 10cm in width but wedges up to 30cm occur commonly (Fig. 22a). A few wedges with widths of 70 to 150era are 2"

2 0 84

FIG. 22. Histograms of the width (a), length (b) and penetration (c) of sandstone wedges in the Garvellachs. The vertical axis gives the number of measurements. present at five horizons within the succession. The length of individual sandstone wedges, measured parallel to the stratification, ranges from a few centimetres to 30m; most are a few metres in length (Fig. 22b). Few wedges are linear in plan view, the majority are slightly sinuous in detail. The third measurable dimension is the penetration perpendicular to the stratification. This ranges from a few centimetres to 12 metres (Fig. 22c); the polygonal sandstone wedges rarely penetrate more than 3 m. The longest wedges very often penetrate the greatest distances, but mostly have widths under 30cm; the group of wedges with widths of 70 to 150cm, although often over 10m in length, penetrate less than 2 to 3 m through the succession. Stratigraphical and horizontal distribution. Sandstone wedge structures occur at 27 horizons, ranging in position from just beneath mixtite 1 in the Garvellachs to the top of mixtite 44 on Islay (Table 11). They are closely restricted to the Port Askaig Formation; only a few have been seen in the top of the underlying formation and none in the overlying formation. At eleven horizons, when seen in plan view, the wedges form connecting networks covering the bedding planes from which they originate; at five of these horizons the networks are polygonal, whilst at the other six individual wedges connect with each other only occasionally and no polygonal pattern is visible. (Because of the tectonic dip in the Garvellachs, bedding planes are exposed for less than 10m in the dip direction; polygonal patterns with larger diameters than this would be difficult to recognize. Bedding planes are equally poorly exposed on Islay.) These eleven horizons contain the great majority of all the sandstone wedges seen in the formation. A total of less than 80 individual sandstone wedge structures have been seen from the other 16 horizons and at four of these only a single wedge structure has been found. The horizons at which large numbers of wedges are present are also the most continuous when traced laterally. For example, the polygonal wedge structures in the tops of mixtites 22 and 29 in the Garvellachs occur at several localities along a strike outcrop of over 5 km. The sandstone wedges in the tops of mixtites 26 and 35 in the Garvellachs are also present along outcrops several kilometres long. In addition, the sandstone wedge horizon in member 2 at Port Askaig [1431 6668] may be equivalent to the well-developed wedge horizon above mixtite 22 in member 2 of the Garvellachs. Similarly, the sandstone wedges seen on Islay [1431 6698] may be equivalent to those seen at Croaghan Hill, Fanad (Fig. 46b) for both lie in the same stratigraphical position, at the top of mixtite 43 and beneath the very siliceous uppermost mixtite of the succession. Detailed stratigraphieal relations. Of the 27 horizons at which wedges occur, 17 lie in the tops of individual mixtites. The latter include all the horizons with polygonal sandstone wedges and all but one of the horizons 42

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

with large numbers of sandstone wedges. A total of only about 50 individual wedge structures have been seen from the other ten horizons, at which they penetrate down into bedded sediments. The great majority of the sandstone wedge structures can be shown to be penecontemporaneous in age; they are truncated at their top by an erosion surface above which lies a pebble bed or bedded horizon whose

TABLE 11 : The sandstone wedge horizons in the Port Askaig Formation

Arrangement of sandstone wedges in plan

In polygons

Relatively large numbers of wedges, some of which connect with each other

Horizons with less than ten wedges, none of which can be seen to connect with each other

Horizons with some wedges of greater than 70cm width

Horizons, given by mixtite number

Number of individual wedges seen

Locality G, Garvellachs PA, Port Askaig Fa, Fanad

Nature of upper contacts of wedges

/22 /24 /29 /35 /40

Hundreds Tens Tens Hundreds Tens

G + PA [1431 6668] G {465 058} G polygonal at {121 132} G PA [1431 6696], polygonal at [1430 6675]

E E E E .9

/15

Tens 20 Tens

G G G

E E E

/18 /26 Base of thick sand- / stone beneath 33 J /32

15

G {130 130} {260 060}

/43

7 8 12

G {127 132} {516 063} PA [1431 6697] [1416 6712] Fa, Croaghan Hill*

E+c ? ?

E

Beneath 1 /3 /6 /9 Disrupted Beds 1 m above 22 2 m above 24 8 m above 25 /26' /28 31/30 31/30

15 4 1 3 1 2 10 1 3 1 4 8

? ?+c ? ? c ? c ? E ? ? ?+c

/38 /44 /45

4 10 3 2

G {067 097} G {070 105} {024 122} G {090 111} G {500 021} G {298 043} G {355 041} G {473 058} G {560 054} G {476 061} {522 057} G {116 135} G {516 063} G {125 133} {261 057} {516 063} G {265 093} PA [1431 6693] PA [1431 6699] PA [1425 6723 ]

/18 3 m above 24 /29

4 3 4

G e.g. {283 052} G {472 058} G e.g. {513 058}

? E Seen in loose block E ? E

'/19' = penetrating into the top of mixtite 19'; '19/18'--'within the bedded sediments between mixtites 19 and 18'; E, truncated at an erosion surface; ? unknown; c, connected with the overlying bedded sediments; * three localities with wedges are given by the following distances and true bearings from the summit of Croaghan Hill, 60m at 054 ~ 210m at 011 ~ and 500m at 295 ~. Mem. geol. Soc. Lond. no. 6

43

A. M. SPENCER lithology is different from that of the sandstone which in fills the wedges (Fig. 20). This type of erosional contact is present in at least 11 of the 27 sandstone wedge horizons, including most of the horizons with polygonal wedges and with large numbers of wedges (Table 11). Sandstone wedges which connect with the base of an overlying bedded sandstone occur exclusively at only one horizon (2m above mixtite 24), apart from which only four other individual examples have been seen. At the remaining 15 sandstone wedge horizons it has not been possible to determine whether the wedges are truncated or connect with the overlying sandstone. Internal structure. The material which infills the wedges is usually structureless and in these cases it is impossible to determine how the wedges were infilled. Only three wedges with internal stratification which is parallel to the regional bedding have been seen (Fig. 20c); in these cases the wedges must have been filled from the base upwards by sand pouring into an open crack. In many more wedges, however, a faint structure aligned parallel to the walls can be seen weathering out (Fig. 20a; P1. 8c). In some cases this is caused by pebble-sized fragments arranged in distinct bands aligned parallel to the walls of the wedge. Orientation. Most wedges, although slightly sinuous, do have an over-all trend, which has been measured in two areas of a well-developed polygonal sandstone wedge system (Fig. 2 lg). No strong over-all preferred orientation is present, although the distributions show similar peaks. The polygonal sandstone wedges. The best developed polygonal sandstone wedges, those in the top of mixtite 22 on Eileach an Naoimh (Fig. 21a-c; P1. 8a, b, d), will be described as an example nearly typical of all polygonal wedges. Polygons with four or five sides are the commonest and occur in almost equal numbers (Fig. 23b). The lengths of the sides of these polygons have a mode of 75 to 100cm (Fig. 23a) and in 20

(a) I

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9

9

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.

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.

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ooe .

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ell

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.

.

.

oo.

1;~

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(d)

F1G. 23. Data on the best-developed sandstone wedge polygons in the Garvellachs--those in the top of mixtite 22 at {565048}. The histograms show the number of measurements plotted against (a) the polygon side length, (b) the number of sides to any polygon and (c) the internal angle between adjacent sides. (d) Graph of the maximum diameter of a polygon against the diameter measured at right-angles. plan view the polygons tend to be elongated and commonly measure 150 by 100cm (Fig. 23d). The internal angle subtended between adjacent sides of a polygon has a modal value of just over 90 ~ (Fig. 23c). Over 90 per cent of the junctions at the corners of polygons are three-fold (three wedges intersect) and the others are all four-fold. The thickness distribution is similar to that for sandstone wedges as a whole (Fig. 22a) and the majority of the wedges penetrate only 1 m down into the mixtite; one wedge penetrates over 4m. The widths of the individual wedges forming any particular polygon are very often quite similar, although within a few polygons a series of thinner sandstone wedges are present (Fig. 21h). The horizon also outcrops on the west coast of A'Chuli, where the wedges do not form a dense polygonal pattern but penetrate as deep as 10m. Sandstone wedges next occur in large numbers in the outcrop of the top of mixtite 22 on the east coast of Garbh Eileach. In the intervening area, despite good exposures, only a single sandstone wedge 44

Mere. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND has been seen. Their scarcity is unlikely to be due to penecontemporaneous erosion, for mixtites 19-22 are as thick there as elsewhere. The other possible explanation of their absence is that they did not form. Polygonal sandstone wedges from other stratigraphical horizons are rarely so perfectly developed and occasionally show slightly different properties. In the top of mixtite 24, for example, wedges occur at only one locality, where they are arranged in polygons as small as 30cm in diameter (Fig. 21j). Similarly, in the top of mixtite 40 at Port na Seilich, Islay, the sandstone wedges are arranged in four-sided polygons which are often only 50cm in diameter (Fig. 21k), even though the wedges penetrate 3 to 4m down into the mixtite. Other horizons with sandstone wedges. Large numbers of sandstone wedges, some of which connect with each other, but which are not arranged in the type of polygons just described, occur at six horizons (Fig. 21d, f, i). The existence of polygons with diameters of many metres or tens of metres can only be speculated upon; none have been seen but, as noted above, present day outcrops are insufficient for them to be exposed. At least four of the 15 horizons at which < 10 sandstone wedges have been seen show few wedges because they are poorly exposed (e.g./26',/38,/44 and/45). In most other cases only two or three wedges have been seen despite excellent exposures. Wedges at the four poorly exposed horizons are similar to those described above. Those at other horizons are mostly small--often less than 4cm in width and 1 m in penetration--and penetrate downwards from only minor bedding planes within mixtites. (ii) SANDSTONE DYKES Lithology, shape and size. Almost 100 sandstone dykes have been seen in the Garvellachs (P1. 8e, f ) . The dykes are infilled with a structureless, medium to coarse grained, well sorted sandstone in which pebblesized fragments are usually absent. The dykes commonly occur in parallel swarms, containing from three to ten or more dykes. These outcrop in a restricted area, within which the individual dykes often replace each other en echelon. Most dykes are narrow, straight and parallel sided. When seen in three dimensions they are planar and have dips which usually exceed 60~ the present horizontal and vertical extent of each sandstone dyke has been measured wherever possible (Fig. 24). These measurements should be regarded as

FIG. 24. Histograms of the width (a), length (b) and present vertical dimension (c) of sandstone dykes in the Garvellachs. The vertical axis gives the number of measurements. minima, for the sandstone dykes are rarely completely exposed, especially in the vertical plane. The largest sandstone dyke seen is 32cm wide and is exposed in two outcrops which are 230m apart. Stratigraphical and geographical distribution and orientation. The sandstone dykes are not concentrated along distinct stratigraphical horizons and only five individual dykes--four of which are very small-connect with a bedded sandstone (see Fig. 27e). The dykes do, however, show a marked preference for penetrating thick, homogeneous mixtites and occur much less frequently penetrating bedded sediments (Fig. 25a). On a larger scale still, the dykes are concentrated in three groups, trending ENE,NEand NW (Fig. 26), each of which shows a restricted geographical distribution. The largest number of the dykes belong to the ENE-trending group, which is concentrated on the north coast of Garbh Eileach. ww-trending dykes occur on Mem. geoL Soc. Lond. no. 6

45

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND has been seen. Their scarcity is unlikely to be due to penecontemporaneous erosion, for mixtites 19-22 are as thick there as elsewhere. The other possible explanation of their absence is that they did not form. Polygonal sandstone wedges from other stratigraphical horizons are rarely so perfectly developed and occasionally show slightly different properties. In the top of mixtite 24, for example, wedges occur at only one locality, where they are arranged in polygons as small as 30cm in diameter (Fig. 21j). Similarly, in the top of mixtite 40 at Port na Seilich, Islay, the sandstone wedges are arranged in four-sided polygons which are often only 50cm in diameter (Fig. 21k), even though the wedges penetrate 3 to 4m down into the mixtite. Other horizons with sandstone wedges. Large numbers of sandstone wedges, some of which connect with each other, but which are not arranged in the type of polygons just described, occur at six horizons (Fig. 21d, f, i). The existence of polygons with diameters of many metres or tens of metres can only be speculated upon; none have been seen but, as noted above, present day outcrops are insufficient for them to be exposed. At least four of the 15 horizons at which < 10 sandstone wedges have been seen show few wedges because they are poorly exposed (e.g./26',/38,/44 and/45). In most other cases only two or three wedges have been seen despite excellent exposures. Wedges at the four poorly exposed horizons are similar to those described above. Those at other horizons are mostly small--often less than 4cm in width and 1 m in penetration--and penetrate downwards from only minor bedding planes within mixtites. (ii) SANDSTONE DYKES Lithology, shape and size. Almost 100 sandstone dykes have been seen in the Garvellachs (P1. 8e, f ) . The dykes are infilled with a structureless, medium to coarse grained, well sorted sandstone in which pebblesized fragments are usually absent. The dykes commonly occur in parallel swarms, containing from three to ten or more dykes. These outcrop in a restricted area, within which the individual dykes often replace each other en echelon. Most dykes are narrow, straight and parallel sided. When seen in three dimensions they are planar and have dips which usually exceed 60~ the present horizontal and vertical extent of each sandstone dyke has been measured wherever possible (Fig. 24). These measurements should be regarded as

FIG. 24. Histograms of the width (a), length (b) and present vertical dimension (c) of sandstone dykes in the Garvellachs. The vertical axis gives the number of measurements. minima, for the sandstone dykes are rarely completely exposed, especially in the vertical plane. The largest sandstone dyke seen is 32cm wide and is exposed in two outcrops which are 230m apart. Stratigraphical and geographical distribution and orientation. The sandstone dykes are not concentrated along distinct stratigraphical horizons and only five individual dykes--four of which are very small-connect with a bedded sandstone (see Fig. 27e). The dykes do, however, show a marked preference for penetrating thick, homogeneous mixtites and occur much less frequently penetrating bedded sediments (Fig. 25a). On a larger scale still, the dykes are concentrated in three groups, trending ENE,NEand NW (Fig. 26), each of which shows a restricted geographical distribution. The largest number of the dykes belong to the ENE-trending group, which is concentrated on the north coast of Garbh Eileach. ww-trending dykes occur on Mem. geoL Soc. Lond. no. 6

45

A. M. S P E N C E R

the east coast of Eileach an Naoimh and NE-trending dykes occur on the east coast of A'Chuli and the west coast of Garbh Eileach. The relationship of sandstone dykes and faults. Eight sandstone dykes show an offset of the bedding in the sediments on either side of the dyke, i.e. the dyke has been the locus for a fault movement (Fig. 27a-c). None of the dykes show any sign of brecciation and the bedded sediments through which they penetrate are not bent at their contacts with the dykes. Details of these dykes are given in Table 12.

- I %,

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FIG. 25. (a) The stratigraphical distribution of individual sandstone dykes in the Garvellachs. (b) Equal area, lower hemisphere stereogram showing the poles (dots) to the planes of present day elongation of 54 sandstone dykes in the Garvellachs. Dashed line: outcrop of the average bedding plane in the Garvellachs. Crosses: poles of faults associated with dykes. Arrows indicate the downthrown side of faults and faulted sandstone dykes.

The age of the sandstone dykes. Several characters allow the age of intrusion of the sandstone dykes to be estimated in relation to the depositional and post-depositional history of the sediments. The dykes, unlike the sandstone wedges, are not truncated at erosion planes; none can be shown to be of penecontemporaneous date. The sandstone dykes have been injected along planar joints and fractures, implying that the bedded sediments must have been sufficiently rigid at the time of injection to break along planar surfaces. (Most sediments have fractured cleanly, but in one case the brecciation of a dolomite bed is recorded by the presence of little transported, angular dolomite fragments within the dyke (Fig. 27d).) Despite the fact that the sandstone dykes penetrate through the succession for considerable distances at high angles to the bedding, they are planar (P1. 8e, f). The dykes must post-date the compaction which the sediments have undergone. Examples of the effects of this compaction on other structures can be seen in Figs. 19; 20a, b; 32b. 46

Mem. geoL Soc. Lond. no. 6

LATE P R E - C A M B R I A N G L A C I A T I O N IN S C O T L A N D At the outcrop of the top of mixtites 19-22 on the north-east coast of Eileach an N a o i h m occur three examples of sandstone dykes which penetrate through granite pebbles enclosed in mixtite (P1. 8g). The sediments enclosing these pebbles must have been sufficiently rigid and lithified, at the time of dyke injection, for the fracture to penetrate through matrix and pebble alike.

FXG. 26. Map of sandstone dykes in the Garvellachs. The smaller dykes are shown cliagrammatically.

TABLE 12: Faulted sandstone dykes

Example

Locality

Dyke width (cm)

Dyke length (m)

Fault displacement (cm)

Nature of fault displacement, normal (N) or reverse (R)

1 2 3 4 to 8

{066 105} {115 056} {122 053} {465 058}

<6 4 2 <7 < 1.5 3 thin dykes

30 + 38+ 16 18 ? ?

210 + 60 150 <6 22 60

N N R N N N

Dyke 3 is a direct continuation of a fault with the noted displacement

All but three of the 66 sandstone dykes shown in Fig. 25b dip at angles to the present day horizontal of 60 ~ or greater. If the sandstone dykes are re-oriented to allow for the tectonic dip of the bedded sediments then after re-orientation 41 of the dykes dip at angles less than 60 ~. The three sandstone dykes with normal fault displacements which occur in the ~NE-trending group have dips, after re-orientation, of 27 ~ 37 ~ and 57 ~ Mem. geol. Soe. Lond. no. 6

47

A.

M.

SPENCER

The present-day distribution of angles of dip of sandstone dykes seems more normal for a system of fractures with injected sandstone than the distribution produced after re-orientation. The dips, after re-orientation, of the dykes with normal fault displacements are in two cases much lower than would be expected, for E. M. Anderson (1951, p. 17) stated that "normal faults usually dip at more than 45 ~ It is therefore suggested that the intrusion of the sandstone dykes may have post-dated the regional tilting of the beds. This suggestion cannot be regarded as proved, for the present orientation of the dykes may have been acquired during the development of the regional cleavage which had a marked effect on cross-bed inclination (Fig. 15b). The sandstone wedges, for example, appear to have been tectonically rotated to a present-day vertical position, for it is most likely that they originally penetrated at right angles to the stratification. r

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FIG. 27. Sketches of sandstone dykes in the Garvellachs. (a) Semi-diagrammaticsketch, and (b) close-up view, of sandstone dykes (continuous lines) and faults (dash-dot lines) cutting mixtites 22-24 at {465058} (see PI. 7h for a view of one of these dykes). (c) Faulted sandstone dyke, {066 105}. (d) Dyke, with included dolomite fragments, penetrating bedded dolomites (blank) and siltstones at {169 043). (e) Small dykes projecting upwards from the base of mixtite 28, {475062}. (c) and (d) drawn from photographs, others are all field sketches. The last piece of evidence concerning the age of the sandstone dykes is that they are cleaved. From the six points listed above it is clear that the sandstone dykes were not injected until the sediments were partly lithified and until after compaction had occurred. The dykes may even post-date the regional tilting of the rocks of the Garvellachs, but certainly pre-date the development of the cleavage. Such time relations are not uncommonly found in sandstone dykes (Potter & Pettijohn 1963, p. 163; Peterson 1966). Because of these age relationships there is no reason to suppose that the origin of the sandstone dykes throws any light on the origin of the mixtites in the formation. Their presence cannot be used (Kilburn et aL 1965, p. 356) in support of a mudflow origin for the mixtites. The origin of the sandstone dykes. Sandstone dykes are thought to form as a result of earthquake shocks which cause momentary liquefaction of water-saturated sand followed by injection into fissures opened by 48

Mem. geoL Soc. Lond. no. a

LATE

PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

the shock; the driving force causing the injection is assumed to be pressure of the overlying strata. This mechanism explains the Garvellachs sandstone dykes well. In addition, the dykes are intimately connected with faults or are themselves faulted in several localities. This, together with the absence of frictional drag in the sediments adjacent to faulted sandstone dykes, suggests that the sand was injected along the fracture whilst fault movement was in progress and had the effect of lubricating the sides of the fault. Such a connection between sandstone dyke emplacement and the development of fracture patterns due to regional structural deformation has recently also been suggested by Peterson (1966). The preference which the sandstone dykes show for thick mixtites must be due to the ease with which fractures can propagate themselves through such a homogeneous medium; the dykes are much less regular and continuous in certain bedded sediments. A noteworthy feature is the narrowness of several dykes, some of which are less than lmm wide. This must reflect the extreme mobility of the fluidized sand, for frictional drag in a crack which is little wider than the sand grains themselves must be very great. Only three sandstone dykes can be traced back to a source bed. All the dykes occur within the Port Askaig Formation (Fig. 25a). As the formation is over- and underlain by carbonate formations poor in sandstone, it seems certain that the dykes have been supplied from bedded sandstones within the formation. Thus although most of the sediments in the Garvellachs may have been well lithified at the time of dyke injection, certain of the bedded sandstones within the succession must have been unconsolidated and contained large amounts of connate water. The clear distinction between sandstone dykes and sandstone wedges. The sandstone dykes, unlike the sandstone wedges, are narrow, straight and parallel sided and when they do branch they do so at very acute angles. The sandstone dykes are not associated with particular stratigraphical horizons and are not of penecontemporaneous date; they probably formed a considerable time after the sediments were deposited. In origin they seem to be connected with minor faults (Fig. 26) and thus have a structural rather than a sedimentological meaning. (iii) THE PERIGLACIAL ORIGIN OF THE SANDSTONE WEDGES All the sandstone wedges have similar characters and seem likely to have been produced by the same geological process or processes. Furthermore, as the wedges from at least 10 of the horizons within the formation are of penecontemporaneous date, this process must have been repeated at least ten times during the deposition of the succession. During the production of each of the horizons with sandstone wedges the following sequence of events must have occurred: (1) after the deposition of a particular bed, wedge-shaped cracks or channels must have formed, extending downwards from the top surface of the bed. (2) At some time after the initiation of these cracks or channels, sand grade material must have entered and filled them from the top. (3) In the great majority of cases, after the cessation of crack or channel infilling, either, (a) the source bed of sand overlying the crack or channel was eroded away and a lag conglomerate formed or (b) the transport of the sand to the area of the crack ceased. (4) A period of time probably elapsed. (5) Bedded sediments were deposited disconformably on the wedge structures. In the sequence of events which lead to the production of wedge structures, it is the formation of (1) the wedge-shaped cracks or channels and of (3) the erosion surfaces or disconformities overlying the wedge structures which require explanation; sand to fill the wedges was readily available, as witnessed by the frequency with which bedded sandstones occur within the formation. The possible origins of the cracks or channels will be discussed first. Wedge-shaped cracks or wedge-shaped channels ? The sandstone wedge structures have not been produced by the infilling of narrow channels produced by erosion. The wedges are much too deep and narrow ever to have been eroded by water currents and their sinuous and even polygonal arrangement in plan would be impossible to produce by erosion. Solution along polygonal joints, analogous to that producing rectangular Mem. geoL Soc. Lond. no. 6

49

LATE

PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

the shock; the driving force causing the injection is assumed to be pressure of the overlying strata. This mechanism explains the Garvellachs sandstone dykes well. In addition, the dykes are intimately connected with faults or are themselves faulted in several localities. This, together with the absence of frictional drag in the sediments adjacent to faulted sandstone dykes, suggests that the sand was injected along the fracture whilst fault movement was in progress and had the effect of lubricating the sides of the fault. Such a connection between sandstone dyke emplacement and the development of fracture patterns due to regional structural deformation has recently also been suggested by Peterson (1966). The preference which the sandstone dykes show for thick mixtites must be due to the ease with which fractures can propagate themselves through such a homogeneous medium; the dykes are much less regular and continuous in certain bedded sediments. A noteworthy feature is the narrowness of several dykes, some of which are less than lmm wide. This must reflect the extreme mobility of the fluidized sand, for frictional drag in a crack which is little wider than the sand grains themselves must be very great. Only three sandstone dykes can be traced back to a source bed. All the dykes occur within the Port Askaig Formation (Fig. 25a). As the formation is over- and underlain by carbonate formations poor in sandstone, it seems certain that the dykes have been supplied from bedded sandstones within the formation. Thus although most of the sediments in the Garvellachs may have been well lithified at the time of dyke injection, certain of the bedded sandstones within the succession must have been unconsolidated and contained large amounts of connate water. The clear distinction between sandstone dykes and sandstone wedges. The sandstone dykes, unlike the sandstone wedges, are narrow, straight and parallel sided and when they do branch they do so at very acute angles. The sandstone dykes are not associated with particular stratigraphical horizons and are not of penecontemporaneous date; they probably formed a considerable time after the sediments were deposited. In origin they seem to be connected with minor faults (Fig. 26) and thus have a structural rather than a sedimentological meaning. (iii) THE PERIGLACIAL ORIGIN OF THE SANDSTONE WEDGES All the sandstone wedges have similar characters and seem likely to have been produced by the same geological process or processes. Furthermore, as the wedges from at least 10 of the horizons within the formation are of penecontemporaneous date, this process must have been repeated at least ten times during the deposition of the succession. During the production of each of the horizons with sandstone wedges the following sequence of events must have occurred: (1) after the deposition of a particular bed, wedge-shaped cracks or channels must have formed, extending downwards from the top surface of the bed. (2) At some time after the initiation of these cracks or channels, sand grade material must have entered and filled them from the top. (3) In the great majority of cases, after the cessation of crack or channel infilling, either, (a) the source bed of sand overlying the crack or channel was eroded away and a lag conglomerate formed or (b) the transport of the sand to the area of the crack ceased. (4) A period of time probably elapsed. (5) Bedded sediments were deposited disconformably on the wedge structures. In the sequence of events which lead to the production of wedge structures, it is the formation of (1) the wedge-shaped cracks or channels and of (3) the erosion surfaces or disconformities overlying the wedge structures which require explanation; sand to fill the wedges was readily available, as witnessed by the frequency with which bedded sandstones occur within the formation. The possible origins of the cracks or channels will be discussed first. Wedge-shaped cracks or wedge-shaped channels ? The sandstone wedge structures have not been produced by the infilling of narrow channels produced by erosion. The wedges are much too deep and narrow ever to have been eroded by water currents and their sinuous and even polygonal arrangement in plan would be impossible to produce by erosion. Solution along polygonal joints, analogous to that producing rectangular Mem. geoL Soc. Lond. no. 6

49

A. M. SPENCER grikes in limestone, cannot be responsible for the wedges for they penetrate into arenaceous mixtites and siltstones, all composed dominantly of quartz, feldspar and layer lattice minerals. Polygonal cracks or polygonal loading structures ? Dzulynski (1963) has produced structures experimentally which resemble those described here. By administering a shock to a layer of relatively heavy plaster-of-Paris overlying waterlogged fine clay, intrusions of the clay were produced which rose to the surface and spread until the plaster-of-Paris was arranged in a polygonal pattern, with diameters of lcm, beneath which it tapered in rounded U-shaped wedges. The polygonal sandstone wedge structures described here are very different in shape and size; wedges which penetrate 2m are often less than 10cm in width at their tops and in extreme cases, wedges which penetrate 10m have widths at their tops of less than 30cm. It would be extremely difficult to produce such narrow, deep and sharp wedge-shaped structures by a loading mechanism. The production of polygonal wedge-shaped cracks. Cracks in clay with maximum dimensions of 15cm width, 2m depth and 50m length have been described by Monroe (in Shrock 1948, p. 219) and attributed by him to subsoil creep. Cracks which form as a result of downslope mass movement might be expected to have a strong preferred orientation parallel to the slope. The sandstone wedges described here show no strong preferred orientations (Fig. 21g) and are arranged in polygonal patterns which are only slightly elongated. The networks of sandstone wedges in the Garvellachs, especially the polygonal networks, resemble contraction cracks in their arrangement. Sub-aerial desiccation cracks are the commonest type of contraction crack in soft sediments but are rarely, if ever, of the size of the wedges described here. Such an origin can, however, be ruled out. Firstly, the mixtites into which most of the wedges penetrate contain such low amounts of layer lattice minerals (less than 15 per cent in mixtite 22, which contains the best developed polygonal structures) that mud-cracking, especially on the scale required, would be very unlikely. More important, in one case, above mixtite 40 [1430 6676], sandstone wedges penetrate a 2m thick granite conglomerate, which cannot have contraction-cracked by desiccation. Thirdly, horizontal bedding, which would be expected in the sand infill of a desiccation crack, is not present. Internal lamination aligned parallel to the wedge walls is sometimes present however (see below). Two possible mechanisms remain. The wedges may have been produced as sand wedges or ice-wedge casts, formed by contraction cracking under permafrost conditions. If so, their presence would provide considerable support for the glacial hypothesis of the formation of the mixtites. Alternatively they could be downward-injected sandstone dykes of penecontemporaneous date. An origin by frost contraction cracking. Throughout large areas of the Arctic, in periglacial regions free from a permanent ice cover, the ground surface is covered by polygonal patterns of markings. These are the surface expressions of ice-wedge polygons which penetrate down into the permafrost. The wedges are assumed to develop, over periods of hundreds or thousands of years, by the repeated cracking, resulting from tensions due to thermal contraction during wiriter, of the upper part of the permafrost. During each spring the cracks are supposed to become infilled with meltwater which freezes and prevents re-expansion of the permafrost in the summer from closing the cracks. Repetition of this annual cycle of cracking and infilling with ice causes the wedges to grow. The polygonal arrangement of the wedges is thought to be a natural consequence of a contraction-crack origin. In the Antarctic, investigations in the McMurdo Sound region (Prw6 1959) have shown that polygonal contraction cracks are forming which, because of the arid climate (annual precipitation 2 to 6in, in the form of snow), are not filled with ice. Instead wind-blown sand fills the cracks and, over a period of many years, a structure is produced which resembles the ice-wedges of the Arctic but is composed of sand and has been termed 'sand-wedge polygons'. The dimensions of modern ice- and sand-wedge polygons is very variable. Lachenbruch (1962) stated that ice-wedge polygons can range from 10ft (3 m) to 100ft (30m) in diameter and that the ice-wedges can be up to several feet wide and 20ft (6m) or more deep. Prw6 (1959) gives the following dimensions for the Antarctic 50

Mem. geoL Soc. Lond. no. a

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND sand-wedge polygons: diameter of polygons, 20 to 40ft (6 to 12m); width of wedges at the top, 0.25 to 4ft (7.5 to 120cm); depth of wedges, 1 to 10ft (0.3 to 3m). Modern ice- and sand-wedges have been found in most types of unconsolidated sediments. Prw6 (1959) reported sand-wedges in sand, gravel, till and even in stagnant ice. A feature which is commonly associated with both ice- and sand-wedges is the upturning of the bedding in the sediment adjacent to the top of the wedge. This is thought to be due to the compressional forces set up in the sediments surrounding the wedge during their expansion in summer. Fossil structures which are interpreted as replacements of ice-wedge polygons, but which may be sandwedge polygons, are common in Pleistocene deposits of large areas of North America and northern EuropeIn a study of wedge structures formed in southern Sweden since the last glaciation, Johnsson (1959) has used the following criteria to distinguish, in cross-section, true ice-wedges from wedge structures formed in other ways (e.g. mineral funnels, collapses due to melting ice lenses, ice tectonic phenomena): (1) the filling material of the wedge has come from above; (2) in cross-section the structure is wedge shaped; (3) any stones present in the wedges stand vertically; (4) the wedge must be widest at the top; (5) the wedge structures should stand vertically. To this list must be added: (6) the wedge structures should be arranged in polygonal patterns and (7) deformation structures may often be present in the sediments adjacent to the wedges. Criteria (1), (2), (4), (5) and (6) are all present in the sandstone wedge structures in the Port Askaig Tillite. In addition, the wedge structures in the tillite have similar widths and penetrations to modern ice- and sandwedge structures. The sandstone wedge polygons here (1.5m in diameter), although of the same order of size, are considerably smaller than the modern polygonal structures described above. Criteria (3) has not been seen but may have been overlooked for pebbles are rare within the sandstone wedges. Criteria (7) may or may not be present in modern and Pleistocene wedges. Although upturning of the bedding in the sediments flanking wedge structures seems to be characteristic of modern wedges in the Arctic and Antarctic, examples from the Pleistocene of Sweden (Johnsson 1959, p. 29) show downward pitching of the bedding in the adjacent sediments. Sandstone wedges occur within bedded sediments at only four horizons within the formation. At two of these the sediments flanking the tops of the wedges are upturned. At the other two horizons no such structures can be seen. The upturning in the two former examples might, however, be a post-depositional effect caused by compaction. The wedge structures in the formation show considerable resemblance to modern and Pleistocene sandand ice-wedge structures and there seems no obvious reason why they should not be of the same origin as the latter. An origin as downward-injected, penecontemporaneous sandstone dykes. Recently a group of Polish sedimentologists have re-examined various Pleistocene periglacial structures and, after comparing them with structures in flysch and other formations, concluded that "The 'periglacial structures' are obviously softsediment deformations inherent to all water-saturated soils, bottom sediments of seas, lakes and rivers" (Butrym, Cegla, Dzulynski & Nakonieczny 1964, p. 1). Commenting on structures in the Pleistocene which have been interpreted as ice-wedge casts, the same authors (p. 18) conclude that "V-shaped fissures as noted originate in more than one way and many of the non-periglacial wedges are arranged in polygonal pattern. This applies e.g. to some downward injected clastic dykes". The authors give no examples of such downward injected polygonal sandstone dykes, however, and the occurrence of such structures must be rare, for they are not reported in Shrock (1948, Art. 128). Two downward-injected sandstone dykes, which may be of penecontemporaneous date, are illustrated by Shrock (1948, fig. 175) and although generally similar to the wedges described here are much more irregular in width. The evidence for the frost contraction origin of the wedges. A feature present occasionally is the development of a narrower series of sandstone wedges within a polygonal sandstone wedge (Fig. 21h). There seems no reason why such an arrangement should develop in the case of downward-injected penecontemporaneous sandstone dykes. Such an arrangement, of wedges of different generations, does occur in frost

Mem. geol. Soc. Lond. no. 6

51

A. M. SPENCER contraction-crack polygons, however, and is described by Black & Berg (1964, p. 113) as a natural consequence of the growth of polygonal structures. Two other structures point to a frost contraction origin for the sandstone wedges. Firstly, bedding aligned parallel to the walls of the wedges is present and would be expected if the structures had originated as sandwedge polygons. P6w6 (1959, fig. 3) showed that sand-wedges in the Antarctic are infilled each winter by sand trickling down a narrow (under 7 mm) central, vertical crack. If the grain size, or composition of the material being supplied to this crack varied from time to time, a bedding structure would be produced aligned parallel to the walls of the wedge. It seems unlikely that such regular bedding (Fig. 20a) would be produced or preserved inside the fluidized mass of sand in a wedge-shaped sand dyke. Bedding occasionally occurs within parallel sandstone dykes (Peterson 1966, p. 837) but no such structure has been observed in the sandstone dykes in the Garvellachs. Secondly, in the cases where the sand-wedges connect up with the base of a bedded sandstone, if the wedges had originated by downward injection into cracks produced by earthquakes, it might be expected that the bedded sandstone would show highly irregular stratification, quicksand structures and convolute lamination. This is not the case. In addition, where the wedges penetrate into bedded sediments, it might again be expected that the intensive development of penecontemporaneous earthquake cracks would cause some disturbance of the bedding, especially in such sediments as varved siltstones. Again, this is not the case (Fig. 20). On the other hand, Pleistocene contraction cracks characteristically occur in undisturbed, bedded sediments which have not been affected by frost heaving (see, for example, Watson 1965). Because of the above lines of evidence, because they much resemble Pleistocene and modern periglacial structures and because polygonal sandstone wedges of earthquake origin are apparently very rare, the author considers that the sandstone wedge structures in the Port Askaig Formation are of frost contraction-crack origin. Some implications of the periglacial origin of the wedges. Frost contraction-cracks are of sub-aerial origin. They cannot develop beneath deep bodies of water or thick ice covers because of the much decreased annual temperature variations caused by the blanketing effect of such bodies (Berg & Black 1966, p. 66). Lachenbruch (1962, p. 60) suggested that ice-wedge polygons could possibly form and grow under shallow lakes, although none have yet been found in such positions. The interpretation of the sandstone wedge structures as frost contraction-crack infillings necessarily implies that they are sub-aerial in origin and that at may times during the deposition of the Port Askaig Formation the surface of the sediments was above sea-level. Independent evidence of sub-aerial conditions, such as desiccation cracks and mudflake breccias, associated with each of the sandstone wedge horizons would provide considerable supporting evidence for an origin of the wedges by frost contraction cracking. Such direct evidence is not present, but many features of the sediments in the formation as a whole suggest very shallow or emergent conditions (section 5. C), however, which does not conflict with and is in quite good agreement with a sub-aerial origin for each of the sandstone wedge horizons. It was previously noted that the production of erosion surfaces overlying the sandstone wedge structures required explanation. On the downward-injected sand dyke hypothesis, the wedges must have been supplied from a bed of sand lying on the sea floor which must then have been removed by erosion. There seems no obvious reason why this should happen in almost all cases. On the frost contraction hypothesis of the origin of the sandstone wedges the erosion surfaces fit into a natural sequence of events. Erosion surfaces above the sandstone wedges would have been produced in the transgression which must have preceded the deposition of the overlying sub-aqueous sediments. A frost contraction origin also precludes the sandstone wedges from forming subglacially. This leads to the conclusion that each horizon of sandstone wedges must represent a period during which ice-sheets were absent. At most of the 13 horizons at which wedge structures lie in the tops of mixtites this conclusion is not contradicted by the stratigraphical relations; the mixtites are overlain by relatively thick, bedded sediments which could have been deposited during interstadial or interglacial periods. 52

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND In conclusion, the author would like to emphasize the remarkable abundance of sandstone wedges and polygonal sandstone wedges in the Port Askaig Formation. Probably because of poor exposure, Pleistocene sand and ice wedge structures are found only rarely; Johnsson (1959, p. 15) stated that only 70 individual examples of such structures, formed since the last ice retreat, had been found in southern Sweden.*

(iv) IRREGULAR SANDSTONE VEINS A minor sedimentary feature present within certain of the mixtites in member 2 of the Garvellachs (e.g. mixtites 19-22 and 26 on the east coast of A'Chuli) are structures which are here termed sandstone veins. These are parallel-sided veins of well-sorted sand and are up to a few centimetres in width and a few decimetres in length. Few lie parallel to the over-all bedding direction and in this respect they are quite distinct from the lenses of bedded sand present within many mixtites. They often follow very irregular courses and are therefore more analogous with the sandstone wedges than the sandstone dykes. Despite excellent outcrops, however, none have been seen to connect either with a sandstone wedge or a sandstone dyke. They must represent the infillings of cracks by water-borne sand but their great irregularity makes it unlikely that they are connected with the sandstone dykes in origin. It seems more likely that they are the deeper representatives of sandstone wedges, whilst perhaps the most likely possibility is that they are the fillings of very early joints connected with the deposition of the mixtite. Their origin, then, remains problematical.

(V) STELLATE STRUCTURESwPOSSIBLY CRYSTAL PSEUDOMORPHS

Description. Quartz-dolomite segregations, which are arranged in distinct blade shapes measuring a few

centimetres in length, occur at various levels in beds 28 to 38 (Fig. 31) in the sediments beneath mixtite 1 in the Garvellachs. Occasionally the individual blade-shaped structures are arranged in radiating clusters (P1.6f) but more commonly they occur densely packed in thin horizons (P1. 6e) in which although few well formed radiating clusters can be seen, there is no preferred orientation of the blades. The principal example of the latter type of concentration occurs in a bed which lies one metre beneath the top of an horizon of sandstone wedges. The quartz-dolomite mixture which now infills the blade-shaped structures is ubiquitous as an infilling material of the tectonic veins in the sediments both beneath and within the lowest mixtites and is of secondary origin. The blade-shaped structures themselves may be penecontemporaneous in date; they are not tectonic tension gashes for they show no distinct preferred orientation. Interpretation. Because of their shapes, the stellate structures are unlikely to be ice-crystal impressions. It is more likely that they are pseudomorphs of some mineral crystal with a low symmetry (triclinic). Radiating crystal clusters (glendonites) occur in mudstones associated with Permo-Carboniferous marine glacial sediments in New South Wales (Brown 1925). The clusters are much fatter than those in the Garvellachs, however, and the latter are perhaps more likely to have originated in the environment outlined by Gibson (1962). He described frost polygons which cut mineral horizons produced by the precipitation of salts (Na and Ca sulphates, nitrates and iodides), at the base of the 60cm thick seasonal melting layer in Victoria Valley, Antarctica. In the Garvellach examples no definite proof of such an origin, such as the identification of the original crystal shape or the composition of the crystalline substance which has been pseudomorphed, has been found. Further difficulties to the acceptance of this suggestion are the presence of undoubted quartzdolomite tectonic veins and the possible confusion of the sandstone wedges with nearby sandstone dykes (Fig. 31 and P1. 8g). Nevertheless an association of possible frost cracks and mineral salt horizons in what are probably supratidal sediments (mudcracks in bed 26, Fig. 31) is one which should be looked for in other tillite successions. * See note added in proof on p. 98. Mem. geoL Soc. Lond. no. 6

53

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND In conclusion, the author would like to emphasize the remarkable abundance of sandstone wedges and polygonal sandstone wedges in the Port Askaig Formation. Probably because of poor exposure, Pleistocene sand and ice wedge structures are found only rarely; Johnsson (1959, p. 15) stated that only 70 individual examples of such structures, formed since the last ice retreat, had been found in southern Sweden.*

(iv) IRREGULAR SANDSTONE VEINS A minor sedimentary feature present within certain of the mixtites in member 2 of the Garvellachs (e.g. mixtites 19-22 and 26 on the east coast of A'Chuli) are structures which are here termed sandstone veins. These are parallel-sided veins of well-sorted sand and are up to a few centimetres in width and a few decimetres in length. Few lie parallel to the over-all bedding direction and in this respect they are quite distinct from the lenses of bedded sand present within many mixtites. They often follow very irregular courses and are therefore more analogous with the sandstone wedges than the sandstone dykes. Despite excellent outcrops, however, none have been seen to connect either with a sandstone wedge or a sandstone dyke. They must represent the infillings of cracks by water-borne sand but their great irregularity makes it unlikely that they are connected with the sandstone dykes in origin. It seems more likely that they are the deeper representatives of sandstone wedges, whilst perhaps the most likely possibility is that they are the fillings of very early joints connected with the deposition of the mixtite. Their origin, then, remains problematical.

(V) STELLATE STRUCTURESwPOSSIBLY CRYSTAL PSEUDOMORPHS

Description. Quartz-dolomite segregations, which are arranged in distinct blade shapes measuring a few

centimetres in length, occur at various levels in beds 28 to 38 (Fig. 31) in the sediments beneath mixtite 1 in the Garvellachs. Occasionally the individual blade-shaped structures are arranged in radiating clusters (P1.6f) but more commonly they occur densely packed in thin horizons (P1. 6e) in which although few well formed radiating clusters can be seen, there is no preferred orientation of the blades. The principal example of the latter type of concentration occurs in a bed which lies one metre beneath the top of an horizon of sandstone wedges. The quartz-dolomite mixture which now infills the blade-shaped structures is ubiquitous as an infilling material of the tectonic veins in the sediments both beneath and within the lowest mixtites and is of secondary origin. The blade-shaped structures themselves may be penecontemporaneous in date; they are not tectonic tension gashes for they show no distinct preferred orientation. Interpretation. Because of their shapes, the stellate structures are unlikely to be ice-crystal impressions. It is more likely that they are pseudomorphs of some mineral crystal with a low symmetry (triclinic). Radiating crystal clusters (glendonites) occur in mudstones associated with Permo-Carboniferous marine glacial sediments in New South Wales (Brown 1925). The clusters are much fatter than those in the Garvellachs, however, and the latter are perhaps more likely to have originated in the environment outlined by Gibson (1962). He described frost polygons which cut mineral horizons produced by the precipitation of salts (Na and Ca sulphates, nitrates and iodides), at the base of the 60cm thick seasonal melting layer in Victoria Valley, Antarctica. In the Garvellach examples no definite proof of such an origin, such as the identification of the original crystal shape or the composition of the crystalline substance which has been pseudomorphed, has been found. Further difficulties to the acceptance of this suggestion are the presence of undoubted quartzdolomite tectonic veins and the possible confusion of the sandstone wedges with nearby sandstone dykes (Fig. 31 and P1. 8g). Nevertheless an association of possible frost cracks and mineral salt horizons in what are probably supratidal sediments (mudcracks in bed 26, Fig. 31) is one which should be looked for in other tillite successions. * See note added in proof on p. 98. Mem. geoL Soc. Lond. no. 6

53

A. M. SPENCER

4. P E T R O G R A P H Y The petrography of the sediments and stones in the Port Askaig Formation has been stualecl in detail only in specimens from the Garvellachs and from Port Askaig. At most other outcrops metamorphism has completely obscured many features of the sediment petrography (see Table 3) and so the data given below relate mostly to vertical changes within the formation and the conclusions illustrate the varied history of deposition shown by the formation. It is most important to realize that extra-basinal stones--predominantly granites--are present in vast numbers. Their source region is unknown but may be localized after systematic petrographical studies of the stones from several of the late Pre-Cambrian tillites of the North Atlantic region have been undertaken. This study is in progress and so only brief details of the petrography of the Port Askaig stones, sufficient to emphasize their exotic nature and hence the implication of long-distance transport, the important conclusion from the point of view of the origin of the Port Askaig Formation, are included here. The microscopic petrography of the sediments yields information on their sources, which are suggested, and is treated more fully. (A) T H E S T O N E S The fragments of pebble to boulder grade in the mixtites of the formation are mainly sub-angular to subrounded; a few granites are well rounded, whilst the large sedimentary rafts in the Great Breccia are extremely angular. Extra-basinal fragments are most commonly of cobble grade but range up to 2m in length. Kilburn et al. (1965, p. 351) outline the type of fragments present noting that unfoliated granitic rocks are much more common than granite gneisses and that this distinctive feature can be found in most British outcrops of the formation. There are fewer types of intra-basinal stones. Fine-grained yellow dolomites outnumber all other lithologies, which include sandy dolomites, dolomitic sandstones, dolomitic siltstones, dolomitic conglomerates, stromatolitic dolomites and dolomite pisolites. Limestone fragments are extremely rare. Analyses of two dolomite stones are given in Table 10. The ratio of granite to dolomite stones (Table 2) and the abundances of stones in a mixtite (P1. 10) were measured by counting all fragments with one diameter greater than 1cm seen in a one foot square marked on a vertical outcrop surface. The stones present were mostly of small pebble grade. Such fragments are homogeneously distributed through the matrix for the abundances and ratios were nearly constant for any particular mixtite bed. The dimensions of the largest stone seen in each mixtite have been measured. They show no systematic variation either up the succession (the largest stones of one lithology do not tend to occur where stones of the lithology are most abundant), or within individual mixtites (no grading present), or in relation to the thickness of the enclosing mixtite (with the exception of the Great Breccia, the largest stones are not concentrated in the thickest mixtites). (13) T H E S E D I M E N T S The mineralogy of the formation sediments has been studied in a total of 150 thin sections (Pls. 2, 3, 5, 6). All the sediments examined consist of certain of the following: quartz, feldspar (chess-board albite, albiteoligoclase, untwinned plagioclase, microcline), muscovite, biotite, dolomite, ores (magnetite, pyrite) and accessory minerals (apatite, green tourmaline, colourless zircon, red rutile, brown rutile). The majority of the textures seen in arenaceous rocks are of primary origin, although quartz and feldspar grains of less than 0.1 mm diameter no longer show their original outlines because of grain growth and pressure solution due to the slight Caledonian metamorphism (P1.6d). Such effects are particularly noticeable in the carbonate rocks where the present polygonal mosaics show only faint traces of the original (? diagenetic) textures. Few new minerals have been produced by the metamorphism, principally the layer lattice minerals which are aligned in the main cleavage and, in the mixtites, produce a good microscopic schistosity. 54

Mem. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

Modal analyses (500 points) show the ranges in compositions of the mixtites (Fig. 28b) and the sandstones (Fig. 28a) within the formation. Fewer than 10 per cent of the sandstones contain appreciable amounts (more than 3 per cent) of mica and this has been ignored in plotting Fig. 28a. The sandstones may all be classified as either feldspathic dolomitic sandstones, or as calcareous sub-arkoses (Pettijohn 1957, p. 291). C

(a)

C

r 1. 1~

1e

1.t

'~1.

2~

e2

e2 2.

,3

2 o

3"

S.o 3".__

Q

~

3

F

2,_

Q.

,

.

F

2"

2

02. .,o

1. 3.2

~

M

FIo. 28. Modal analyses of (a) sandstones and (b) mixtites from the Port Askaig Formation, showing carbonate (C), quartz (Q), feldspar (F) and mica (M). The numbers give the member of the formation from which each specimen comes. Solid dots: Garvellachs specimens. Circles: specimens from Port Askaig.

Using the same classification, it is more difficult to label the mixtites. Because of their high original clay mineral and detrital mica content (now represented by micas) some could be termed greywackes, although many contain more carbonate than mica (Fig. 28b). Many of the dolomitic rocks within the formation are quite pure (Table 10), although some sandy dolomites are present. Certain of the detrital minerals are identical to those occurring in the crystalline stones and are probably from the same source as the granites. Examples are: quartz containing acicular rutile needles, chess-board albite (P1.5e), albite-oligoclase, colourless zircon, rutile and apatite. Others are not. Green tourmaline has not been seen in any of the crystalline stones sectioned, nor has microline. Microcline is not present in the mixtites and is confined to the sandstones between mixtites 33/32, 36/35 and 37/36 suggesting that part of the material of these sandstones was derived from a source quite distinct from that of the mixtites. Cubes of pyrite are present in very small amounts in many of the mixtites. Detrital magnetite is more concentrated, occurring in amounts from 1 to 5 per cent in mixtites 19-22 (P1. 3b), 23 and 25 and in amounts up to 100 per cent in some thin bedded layers in the Disrupted Beds and the base of mixtite 19. (c) T H E U P W A R D

CHANGE IN STONE CONTENT WITHIN THE FORMATION

AND LITHOLOGY

The over-all upward change in the stone content of the mixtites is not gradual (P1. 10). Mixtites 1-13 contain over 99-98 per cent of intra-basinals, a few extra-basinal stones are present. Ten to 15 per cent of the stones Mem. geol. Soc. Lond. no. 6

55

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

Modal analyses (500 points) show the ranges in compositions of the mixtites (Fig. 28b) and the sandstones (Fig. 28a) within the formation. Fewer than 10 per cent of the sandstones contain appreciable amounts (more than 3 per cent) of mica and this has been ignored in plotting Fig. 28a. The sandstones may all be classified as either feldspathic dolomitic sandstones, or as calcareous sub-arkoses (Pettijohn 1957, p. 291). C

(a)

C

r 1. 1~

1e

1.t

'~1.

2~

e2

e2 2.

,3

2 o

3"

S.o 3".__

Q

~

3

F

2,_

Q.

,

.

F

2"

2

02. .,o

1. 3.2

~

M

FIo. 28. Modal analyses of (a) sandstones and (b) mixtites from the Port Askaig Formation, showing carbonate (C), quartz (Q), feldspar (F) and mica (M). The numbers give the member of the formation from which each specimen comes. Solid dots: Garvellachs specimens. Circles: specimens from Port Askaig.

Using the same classification, it is more difficult to label the mixtites. Because of their high original clay mineral and detrital mica content (now represented by micas) some could be termed greywackes, although many contain more carbonate than mica (Fig. 28b). Many of the dolomitic rocks within the formation are quite pure (Table 10), although some sandy dolomites are present. Certain of the detrital minerals are identical to those occurring in the crystalline stones and are probably from the same source as the granites. Examples are: quartz containing acicular rutile needles, chess-board albite (P1.5e), albite-oligoclase, colourless zircon, rutile and apatite. Others are not. Green tourmaline has not been seen in any of the crystalline stones sectioned, nor has microline. Microcline is not present in the mixtites and is confined to the sandstones between mixtites 33/32, 36/35 and 37/36 suggesting that part of the material of these sandstones was derived from a source quite distinct from that of the mixtites. Cubes of pyrite are present in very small amounts in many of the mixtites. Detrital magnetite is more concentrated, occurring in amounts from 1 to 5 per cent in mixtites 19-22 (P1. 3b), 23 and 25 and in amounts up to 100 per cent in some thin bedded layers in the Disrupted Beds and the base of mixtite 19. (c) T H E U P W A R D

CHANGE IN STONE CONTENT WITHIN THE FORMATION

AND LITHOLOGY

The over-all upward change in the stone content of the mixtites is not gradual (P1. 10). Mixtites 1-13 contain over 99-98 per cent of intra-basinals, a few extra-basinal stones are present. Ten to 15 per cent of the stones Mem. geol. Soc. Lond. no. 6

55

A. M. SPENCER in the Disrupted Beds are extra-basinal, very commonly metamorphic gneisses. In the higher mixtites the proportion of extra-basinal stones (mostly unfoliated granites) in the fragment suite is 20 per cent in mixtites 14-22, 40 per cent in 27-29, 45 per cent in 30, 50 per cent in 31-32, 80 per cent in 33-38, 98 per cent in 39-44, 50 per cent in 45-46 and 90 per cent in 47. The change in the matrix lithology of the mixtites generally parallels the stone ratio, but does not do so exactly. For example, although mixtites 14-18 have a stone content identical to that of mixtites 19-22, their matrix is identical to that of mixtites 1-13. Similarly, mixtites 26-29 contain a higher proportion of extrabasinals than 19-22 but their matrix is much more dolomitic (over 40 per cent and under 25 per cent dolomite respectively). The sandstones within the formation also change in lithology, becoming less dolomitic and slightly more feldspathic upwards (Fig. 28a). There is little upward change in the accessory minerals of either the sandstones or the mixtites and most of those found in the Port Askaig Formation are also present in thin sections of rocks from the top of the Islay Limestone. Thus there is no marked influx of new accessory minerals in the lowest mixtite and, more surprisingly, the first influx of large numbers of extra-basinal stones in the formation (in mixtite 14) produces no corresponding increase in the quantity or types of accessory minerals present. (D) C O N C L U S I O N S There is no marked break in provenance at the base of the formation; the matrix and stones in the lowest mixtites have been derived by re-working the dolomitic sediments of the underlying formation. The dolomite and granitic stones are from geologically distinct sources. The decreasing abundance of dolomite stones upwards in the formation parallels the decrease in the dolomite content of the mixtites and sandstones and it seems reasonable to suggest a single origin for both. Kilburn et al. (1965, p. 357) suggested that these changes "may be explained by a progressive blanketing of the source rocks by glacial deposits". An alternative is that the source rocks were stripped off progressively larger areas to reveal granitic and metamorphic rocks lying unconformably beneath them. (The granitic rocks which produced the stones are unlikely to post-date the intra-basinal sediments for no pebbles of contact metamorphosed dolomite have been found.) A third possibility is that the granitic and dolomite stones may have been derived from geographically separate areas. Much of the quartz-feldspar-heavy mineral fraction in the matrix of the higher mixtites is probably derived from the same source as the granite stones. Re-working of material previously deposited within the formation occurred at one horizon (the Great Breccia). In addition, most of the sandstone and conglomerate interbeds probably formed by the re-working of previously deposited mixtite beds, which would account for the parallel change in lithology of mixtites and interbeds upwards in the formation. There must also have been a change in the conditions under which the interbeds formed, however, for dolomite interbeds only occur in the lower half of the formation, up to the level of mixtite 33. In only a few eases was material (microcline) from a source distinct from that of either the granitic or dolomite stones supplied to certain sandstone interbeds.

5. P E T R O G E N E S I S

OF THE

MIXTITES

(A) T H E P O S S I B L E O R I G I N S O F M I X T I T E S Harland et aL (1966, 234--40) have listed, with examples, the possible origins of mixtites as follows: (1) Rafting: (a)ice rafting, (b)ice sliding, (c) biological rafting. (2)Glacial transport and deposition. (3) Cryoturbation and ice solifluxion. (4)Cryo-tectonic transport. (5)Tectonic transport. (6)Tectonic deformation. (7)Mass flow phenomena: (a)sub-aqueous rock fall, (b)sub-aqueous slumping, sliding or gliding, (c)plastic mass flow. (8)Turbulent fluid flow: (a)puisatory turbulent flow, (b)secular turbulent flow. (9) Selective weathering or alteration of conglomerates in place. 56

Mem. geol. Soc. Lond. no. 6

A. M. SPENCER in the Disrupted Beds are extra-basinal, very commonly metamorphic gneisses. In the higher mixtites the proportion of extra-basinal stones (mostly unfoliated granites) in the fragment suite is 20 per cent in mixtites 14-22, 40 per cent in 27-29, 45 per cent in 30, 50 per cent in 31-32, 80 per cent in 33-38, 98 per cent in 39-44, 50 per cent in 45-46 and 90 per cent in 47. The change in the matrix lithology of the mixtites generally parallels the stone ratio, but does not do so exactly. For example, although mixtites 14-18 have a stone content identical to that of mixtites 19-22, their matrix is identical to that of mixtites 1-13. Similarly, mixtites 26-29 contain a higher proportion of extrabasinals than 19-22 but their matrix is much more dolomitic (over 40 per cent and under 25 per cent dolomite respectively). The sandstones within the formation also change in lithology, becoming less dolomitic and slightly more feldspathic upwards (Fig. 28a). There is little upward change in the accessory minerals of either the sandstones or the mixtites and most of those found in the Port Askaig Formation are also present in thin sections of rocks from the top of the Islay Limestone. Thus there is no marked influx of new accessory minerals in the lowest mixtite and, more surprisingly, the first influx of large numbers of extra-basinal stones in the formation (in mixtite 14) produces no corresponding increase in the quantity or types of accessory minerals present. (D) C O N C L U S I O N S There is no marked break in provenance at the base of the formation; the matrix and stones in the lowest mixtites have been derived by re-working the dolomitic sediments of the underlying formation. The dolomite and granitic stones are from geologically distinct sources. The decreasing abundance of dolomite stones upwards in the formation parallels the decrease in the dolomite content of the mixtites and sandstones and it seems reasonable to suggest a single origin for both. Kilburn et al. (1965, p. 357) suggested that these changes "may be explained by a progressive blanketing of the source rocks by glacial deposits". An alternative is that the source rocks were stripped off progressively larger areas to reveal granitic and metamorphic rocks lying unconformably beneath them. (The granitic rocks which produced the stones are unlikely to post-date the intra-basinal sediments for no pebbles of contact metamorphosed dolomite have been found.) A third possibility is that the granitic and dolomite stones may have been derived from geographically separate areas. Much of the quartz-feldspar-heavy mineral fraction in the matrix of the higher mixtites is probably derived from the same source as the granite stones. Re-working of material previously deposited within the formation occurred at one horizon (the Great Breccia). In addition, most of the sandstone and conglomerate interbeds probably formed by the re-working of previously deposited mixtite beds, which would account for the parallel change in lithology of mixtites and interbeds upwards in the formation. There must also have been a change in the conditions under which the interbeds formed, however, for dolomite interbeds only occur in the lower half of the formation, up to the level of mixtite 33. In only a few eases was material (microcline) from a source distinct from that of either the granitic or dolomite stones supplied to certain sandstone interbeds.

5. P E T R O G E N E S I S

OF THE

MIXTITES

(A) T H E P O S S I B L E O R I G I N S O F M I X T I T E S Harland et aL (1966, 234--40) have listed, with examples, the possible origins of mixtites as follows: (1) Rafting: (a)ice rafting, (b)ice sliding, (c) biological rafting. (2)Glacial transport and deposition. (3) Cryoturbation and ice solifluxion. (4)Cryo-tectonic transport. (5)Tectonic transport. (6)Tectonic deformation. (7)Mass flow phenomena: (a)sub-aqueous rock fall, (b)sub-aqueous slumping, sliding or gliding, (c)plastic mass flow. (8)Turbulent fluid flow: (a)puisatory turbulent flow, (b)secular turbulent flow. (9) Selective weathering or alteration of conglomerates in place. 56

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IN SCOTLAND

In the case of the origin of the mixtites in the Port Askaig Formation several of these processes can be eliminated from consideration immediately. Ice sliding is an extreme case of ice rafting, in which the water is only a few centimetres deep. Such a process is incapable of producing thick mixtites which contain boulders and extend tens or hundreds of kilometres laterally. Biological rafting (by Pre-Cambrian seaweeds .9) is wholly inadequate to account for the millions of cobbles and boulders present in the Port Askaig Formation. Cryoturbation and ice solifluxion may modify pre-existing sediments but are incapable of producing the thick sequence of extensive mixtites seen. Cryo-tectonic transport should really be grouped with process 5. The Port Askaig Formation is neither a tectonic slide breccia nor a post depositional tectonic breccia, for the rocks are very little deformed, show all the features of little altered sediments and lie in a succession which can be traced laterally for several hundred kilometres. Mechanism 7(a) is inadequate to produce the 750m thick succession, consisting of tens of mixtites (containing rounded granite boulders) interbedded with normal sediments, especially as the same over-all succession can be traced laterally from Schichallion to Connemara (600km). Secular turbulent flow "includes normal transport by turbulent currents in streams, tides, ocean currents, winds etc" (op. cir., p. 239), and although it may rework tills to produce boulder sands and conglomerates, it is incapable of forming mixtites. Nor have the mixtites been produced by the type of selective weathering which, acting on a sub-aerial surface, results in conglomerates; such a mechanism is inadequate to produce the thick sequence of mixtites interbedded with normal sediments. The remaining hypotheses numbers 1, 2, 7 and 8 can be grouped as follows: (a) Deposition from ice: (i) ice-sheet on land, (ii) ice-sheet resting on the sea-bed, (iii) ice-sheet floating on the sea, (iv) icebergs or sea ice. (b) Deposition from downslope movements powered by gravity: (i) slumping, sliding or gliding, (ii) plastic mass flow (i.e. sub-aerial or sub-marine mudflows), (iii) pulsatory turbulent flow (principally turbidity currents). (B) D E P O S I T I O N

FROM

DOWNSLOPE

MOVEMENTS

POWERED

BY G R A V I T Y

(i) SLUMPING, SLIDING OR GLIDING

Harland et aL (1966, p. 237) have suggested that "Sub-aerial and subaqueous downslope movements of various kinds belong to a continuous series, ranging from brittle fracture through degrees of plastic deformation, to fluid flow". Sub-aqueous slumping, sliding or gliding occurring in the earlier stages of this series should "be restricted to mass movements of either rigid or semi-consolidated masses along discrete shear planes with relatively minor internal flow" (Dott 1963, p. 111). Because of this, such a mechanism could only account for homogeneous, even textured mixtites and their scattered, included, pebble- to boulder-sized stones if the slumping or sliding affected sediments already possessing such characters. There is no evidence that the mixtites in the Port Askaig Formation have been transported, even the last few kilometres to the positions in which they are now seen, by such a mechanism. There are relatively few soft sediment folds which could be interpreted as slump folds and there is no independent evidence of a palaeoslope, certainly not one sufficiently steep for the huge fragments in the Great Breccia to have slid down. There is no evidence of the existence of gliding planes or drcollements along the bases of individual mixtites. The great majority of such contacts are conformable and some are even gradational; only a single example of fragments which appear to be caught in the process of being ripped up from the underlying surface is known and this occurs beneath the Great Breccia where it forms the base of the formation at Port Askaig. Thus the mixtites in the Port Askaig Formation have neither been formed solely by a slumping or sliding mechanism, nor are they deposits which, although formed principally by another mechanism, have been modified by very late slumping or sliding. Mem. geoL Soc. Lond. no. 6

57

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IN SCOTLAND

In the case of the origin of the mixtites in the Port Askaig Formation several of these processes can be eliminated from consideration immediately. Ice sliding is an extreme case of ice rafting, in which the water is only a few centimetres deep. Such a process is incapable of producing thick mixtites which contain boulders and extend tens or hundreds of kilometres laterally. Biological rafting (by Pre-Cambrian seaweeds .9) is wholly inadequate to account for the millions of cobbles and boulders present in the Port Askaig Formation. Cryoturbation and ice solifluxion may modify pre-existing sediments but are incapable of producing the thick sequence of extensive mixtites seen. Cryo-tectonic transport should really be grouped with process 5. The Port Askaig Formation is neither a tectonic slide breccia nor a post depositional tectonic breccia, for the rocks are very little deformed, show all the features of little altered sediments and lie in a succession which can be traced laterally for several hundred kilometres. Mechanism 7(a) is inadequate to produce the 750m thick succession, consisting of tens of mixtites (containing rounded granite boulders) interbedded with normal sediments, especially as the same over-all succession can be traced laterally from Schichallion to Connemara (600km). Secular turbulent flow "includes normal transport by turbulent currents in streams, tides, ocean currents, winds etc" (op. cir., p. 239), and although it may rework tills to produce boulder sands and conglomerates, it is incapable of forming mixtites. Nor have the mixtites been produced by the type of selective weathering which, acting on a sub-aerial surface, results in conglomerates; such a mechanism is inadequate to produce the thick sequence of mixtites interbedded with normal sediments. The remaining hypotheses numbers 1, 2, 7 and 8 can be grouped as follows: (a) Deposition from ice: (i) ice-sheet on land, (ii) ice-sheet resting on the sea-bed, (iii) ice-sheet floating on the sea, (iv) icebergs or sea ice. (b) Deposition from downslope movements powered by gravity: (i) slumping, sliding or gliding, (ii) plastic mass flow (i.e. sub-aerial or sub-marine mudflows), (iii) pulsatory turbulent flow (principally turbidity currents). (B) D E P O S I T I O N

FROM

DOWNSLOPE

MOVEMENTS

POWERED

BY G R A V I T Y

(i) SLUMPING, SLIDING OR GLIDING

Harland et aL (1966, p. 237) have suggested that "Sub-aerial and subaqueous downslope movements of various kinds belong to a continuous series, ranging from brittle fracture through degrees of plastic deformation, to fluid flow". Sub-aqueous slumping, sliding or gliding occurring in the earlier stages of this series should "be restricted to mass movements of either rigid or semi-consolidated masses along discrete shear planes with relatively minor internal flow" (Dott 1963, p. 111). Because of this, such a mechanism could only account for homogeneous, even textured mixtites and their scattered, included, pebble- to boulder-sized stones if the slumping or sliding affected sediments already possessing such characters. There is no evidence that the mixtites in the Port Askaig Formation have been transported, even the last few kilometres to the positions in which they are now seen, by such a mechanism. There are relatively few soft sediment folds which could be interpreted as slump folds and there is no independent evidence of a palaeoslope, certainly not one sufficiently steep for the huge fragments in the Great Breccia to have slid down. There is no evidence of the existence of gliding planes or drcollements along the bases of individual mixtites. The great majority of such contacts are conformable and some are even gradational; only a single example of fragments which appear to be caught in the process of being ripped up from the underlying surface is known and this occurs beneath the Great Breccia where it forms the base of the formation at Port Askaig. Thus the mixtites in the Port Askaig Formation have neither been formed solely by a slumping or sliding mechanism, nor are they deposits which, although formed principally by another mechanism, have been modified by very late slumping or sliding. Mem. geoL Soc. Lond. no. 6

57

A. M. S P E N C E R

The principal characters of the mixtites, their homogeneous unbedded lithology and the presence of scattered extra-basinal stones of up to 1-5 m in diameter, imply that any mass movement mechanism forming them must have produced (i) a complete mixing of all grain sizes and (ii) relatively long distance transport (iii) of all grain sizes up to large boulders. Only two mechanisms which explain all three features have been proposed, they are the plastic mass flow and pulsatory turbulent flow hypotheses. (As sub-aqueous sediments are often interbedded with the mixtites in thick 'tillite' sequences most authors who have proposed a mass movement origin have assumed that the mass movements are of sub-aqueous origin. It is possible, however, that continental mudflows and wadi fan deposits could accumulate in a thick sequence formed sufficiently close to sea-level for marine sediments to become interbedded in the sequence. Dr H. R. Grunau, Dr P. G. Llewellyn and Mr D. J. Shearman have suggested to the author that such a modern environment lies along part of the southern shore of the Persian Gulf. Such an analogy explains the dolomites interbedded in the Port Askaig Formation very well. But the author believes this hypothesis can be eliminated because it is possible to be certain the mixtites are neither sub-aerial nor sub-marine mudflows.) (ii) PLASTIC MASS FLOW

Dott (1963) has considered the theoretical aspects of sub-aqueous mass movements and suggests that slumping and sliding is an elastic deformation which may, if the 'yield' limit of the sediment is exceeded, develop into plastic mass flow. Such flow may, in turn, if the liquid ('plastic') limit is exceeded produce a viscous fluid flow, or turbidity current. Dott (op. cit., p. 114) explained plastic mass flow as follows, "Mass flow deposits which have retained most of their initial stratification, even though much contorted, have behaved essentially plastically and their liquid limit was never exceeded . . . . If the rate of shear and turbulence increase sufficiently to destroy bulk cohesion and if dilution by in-mixing of water increases the void ratio sufficiently, then viscous fluid flow ensues and a sub-aqueous mudflow or 'mudstream' may be formed. In these cases the material passes beyond a wholly plastic state and becomes sufficiently fluid for turbulence to destroy internal stratification and mix together all size fractions in a wholly unsorted fashion . . . . Such masses just begin to behave according to fluid flow, but are arrested before all of the material can form true turbidity currents . . . . Some pebbly mudstones contain masses that still retain some contorted stratification, thus were still partially plastic when movement ceased". Discussing the nature of the source area of such mass flows, Dott (op. cit., p. 116) suggested that, "The Squantum 'tillite'... does appear to have formed only where layers of gravel and sand were carried unusually far offshore and became interstratified with fine muds and silts". A somewhat similar nature for the source sediments of sub-aqueous mudflows is suggested by Crowell (1957, p. 1004), except that the initial gravels may have been transported far from shore by turbidity currents which "from time to time deposited gravel on new sediment. If this sediment were sandy and already somewhat indurated, the gravel was laid down on top in a stable layer. On the other hand, if the sediment were soft and water saturated, gravel sank as great lobes into the s u b s t r a t u m . . , and set off down-slope movement of gravel and mud together. As this mass gathered speed it churned, mixed and dispersed pebbles throughout the mud. Perhaps the mass moved basinward as a discrete flow with an incipient internal turbidity, if it were relatively fluid, and so is found today interbedded within strata as a uniform layer of pebbly mudstone". One of the most serious difficulties for this hypothesis is providing the boulder-grade material at the source of the sub-aqueous mudflow (sub-aerial mudflows can easily pick up such material). The suggestion of transport by turbidity currents for such material, made by Crowell, cannot be accepted for various reasons (Heezen & Hollister 1964, p. 102), including the fact that turbidity currents are not thought capable of transporting clasts with diameters greater than 8 to 10 cm (Natland & Kuenen 1951). It seems most reasonable that such beds of gravel should, as Dott implies (op. cit., p. 116, see also Schermerhorn & Stanton 1963, p. 228), have relatively near shore origins. (It is interesting to note that 600ft of a 2700ft succession chosen by CroweU (op. cit., fig. 3) to illustrate the occurrence of pebbly mudstones is formed of conglomerates 58

Mem. geol. Soc. Lond. no. 6

A. M. S P E N C E R

The principal characters of the mixtites, their homogeneous unbedded lithology and the presence of scattered extra-basinal stones of up to 1-5 m in diameter, imply that any mass movement mechanism forming them must have produced (i) a complete mixing of all grain sizes and (ii) relatively long distance transport (iii) of all grain sizes up to large boulders. Only two mechanisms which explain all three features have been proposed, they are the plastic mass flow and pulsatory turbulent flow hypotheses. (As sub-aqueous sediments are often interbedded with the mixtites in thick 'tillite' sequences most authors who have proposed a mass movement origin have assumed that the mass movements are of sub-aqueous origin. It is possible, however, that continental mudflows and wadi fan deposits could accumulate in a thick sequence formed sufficiently close to sea-level for marine sediments to become interbedded in the sequence. Dr H. R. Grunau, Dr P. G. Llewellyn and Mr D. J. Shearman have suggested to the author that such a modern environment lies along part of the southern shore of the Persian Gulf. Such an analogy explains the dolomites interbedded in the Port Askaig Formation very well. But the author believes this hypothesis can be eliminated because it is possible to be certain the mixtites are neither sub-aerial nor sub-marine mudflows.) (ii) PLASTIC MASS FLOW

Dott (1963) has considered the theoretical aspects of sub-aqueous mass movements and suggests that slumping and sliding is an elastic deformation which may, if the 'yield' limit of the sediment is exceeded, develop into plastic mass flow. Such flow may, in turn, if the liquid ('plastic') limit is exceeded produce a viscous fluid flow, or turbidity current. Dott (op. cit., p. 114) explained plastic mass flow as follows, "Mass flow deposits which have retained most of their initial stratification, even though much contorted, have behaved essentially plastically and their liquid limit was never exceeded . . . . If the rate of shear and turbulence increase sufficiently to destroy bulk cohesion and if dilution by in-mixing of water increases the void ratio sufficiently, then viscous fluid flow ensues and a sub-aqueous mudflow or 'mudstream' may be formed. In these cases the material passes beyond a wholly plastic state and becomes sufficiently fluid for turbulence to destroy internal stratification and mix together all size fractions in a wholly unsorted fashion . . . . Such masses just begin to behave according to fluid flow, but are arrested before all of the material can form true turbidity currents . . . . Some pebbly mudstones contain masses that still retain some contorted stratification, thus were still partially plastic when movement ceased". Discussing the nature of the source area of such mass flows, Dott (op. cit., p. 116) suggested that, "The Squantum 'tillite'... does appear to have formed only where layers of gravel and sand were carried unusually far offshore and became interstratified with fine muds and silts". A somewhat similar nature for the source sediments of sub-aqueous mudflows is suggested by Crowell (1957, p. 1004), except that the initial gravels may have been transported far from shore by turbidity currents which "from time to time deposited gravel on new sediment. If this sediment were sandy and already somewhat indurated, the gravel was laid down on top in a stable layer. On the other hand, if the sediment were soft and water saturated, gravel sank as great lobes into the s u b s t r a t u m . . , and set off down-slope movement of gravel and mud together. As this mass gathered speed it churned, mixed and dispersed pebbles throughout the mud. Perhaps the mass moved basinward as a discrete flow with an incipient internal turbidity, if it were relatively fluid, and so is found today interbedded within strata as a uniform layer of pebbly mudstone". One of the most serious difficulties for this hypothesis is providing the boulder-grade material at the source of the sub-aqueous mudflow (sub-aerial mudflows can easily pick up such material). The suggestion of transport by turbidity currents for such material, made by Crowell, cannot be accepted for various reasons (Heezen & Hollister 1964, p. 102), including the fact that turbidity currents are not thought capable of transporting clasts with diameters greater than 8 to 10 cm (Natland & Kuenen 1951). It seems most reasonable that such beds of gravel should, as Dott implies (op. cit., p. 116, see also Schermerhorn & Stanton 1963, p. 228), have relatively near shore origins. (It is interesting to note that 600ft of a 2700ft succession chosen by CroweU (op. cit., fig. 3) to illustrate the occurrence of pebbly mudstones is formed of conglomerates 58

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GLACIATION

IN SCOTLAND

containing stones of up to 12in in diameter, a rock type which is difficult to form in deep water. On the other hand, the mixtites described by Crowell (1957, figs. 2, 3, 6) do not contain such large fragments as those described here; stones in the pebbly mudstones are usually less than 189 in diameter, although cobbles of up to 9in do occur at one locality.) What distinctive features may be looked for in the deposits of sub-aqueous mudflows (or sub-aerial ones) which will distinguish them from glacial deposits? Frakes & Crowell (1967, 46-7) recently considered this problem and concluded that, "The obvious lack of characteristic features in both till and mudflow deposits therefore makes an attempt to distinguish origin in massive diamictites almost hopeless". The author suggests that there are at least two at-an-outcrop features which, whilst not absolutely certain indicators of mudflows, may strongly suggest a mudflow origin. Firstly, intra-basinal fragments within mudflows may often be very contorted and have very ragged or even gradational outlines. Fragments of similar lithologies in tills may be folded, but their outlines tend to be smooth and they do not give the appearance, like those in mudflows, of being caught in the process of disintegration. The number of such fragments should decrease in proportion to the distance the mudflow travels, however, and in far-travelled mudflows they may be very uncommon. A second feature which may be present in mudflows is coarse grading, caused by the largest fragments lying at the base of the flow (Crandell & Waldron 1956). Glacial tills are not usually graded. Thirdly, elongate stones in a till exhibit preferred alignment which is in most cases parallel, or less commonly perpendicular, to the direction of ice flow. Harland et al. (1966, p. 245) discussed the production of such a fabric and concluded that "the most plausible mechanisms depend on movement of competent stones in an incompetent matrix, a situation common to both glacial and non-glacial transport. Therefore stone orientation, while giving evidence of movement, probably does not identify glacial movement as such unless the orientation remains constant over very large areas, thus precluding mudflows". Few studies of the fabric of mudflows are available (G/Srler & Reutter 1968, fig. 9; Lindsay 1968; Sestini 1968) and more are needed before it is certain that they do have an identical fabric to tills. Fourthly, large numbers of contorted slump folds, as well as turbidite features might be more likely to occur in association with submarine mudflows than with marine tillites. Neither of these two criteria can be conclusive in their own right; both could occur in marine glacial sediments. Lastly on a regional scale, it should be possible to distinguish mudflow and tillite deposits by their lateral continuity and uniformity of thickness. Individual till sheets may extend with uniform thickness (several metres or tens of metres) over areas measuring hundreds or thousands of kilometres square (e.g. White 1969) and although a single mudflow might possibly cover such an area to a uniform thickness, such a deposit would surely be exceptional.

(iii)

PULSATORY TURBULENT FLOW

Harland et al. (1967, p. 238) suggested that pulsatory turbulent flow in a density current may produce a "sedimentary unit difficult to distinguish from an ortho-till in so far as both may be largely unsorted". In other respects, however, turbidity current deposits and (autochthonous) tills can easily be distinguished (Heezen & Hollister 1964); turbidity currents cannot transport the cobble- and boulder-grade stones which occur in mixtites. The mechanism proposed by Schermerhorn & Stanton (1963) also falls into the pulsatory turbulent flow class. Its importance lies in the fact that it may explain both the features (great areal extent of the tilloid formations and the far-travelled nature of the extra-basinal stones) whose combined presence is difficult to account for on either a simple mudflow or a simple turbidity current hypothesis. They suggested (op. cit., p. 226) that, "the extensive and regularly bedded tilloid formations of the West Congo geosyncline are composed of well-mixed tiUoids which have travelled for great distances as highly mobile mudstreams. We propose to distinguish chaotic, immature slide tilloids from far-travelled well-mixed flow tilloids. In the sequence leading from slumping on a slope to deposition in a basin slide tilloids represent the mature endstage". Mem. geol. Soe. Lond. no. 6

59

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containing stones of up to 12in in diameter, a rock type which is difficult to form in deep water. On the other hand, the mixtites described by Crowell (1957, figs. 2, 3, 6) do not contain such large fragments as those described here; stones in the pebbly mudstones are usually less than 189 in diameter, although cobbles of up to 9in do occur at one locality.) What distinctive features may be looked for in the deposits of sub-aqueous mudflows (or sub-aerial ones) which will distinguish them from glacial deposits? Frakes & Crowell (1967, 46-7) recently considered this problem and concluded that, "The obvious lack of characteristic features in both till and mudflow deposits therefore makes an attempt to distinguish origin in massive diamictites almost hopeless". The author suggests that there are at least two at-an-outcrop features which, whilst not absolutely certain indicators of mudflows, may strongly suggest a mudflow origin. Firstly, intra-basinal fragments within mudflows may often be very contorted and have very ragged or even gradational outlines. Fragments of similar lithologies in tills may be folded, but their outlines tend to be smooth and they do not give the appearance, like those in mudflows, of being caught in the process of disintegration. The number of such fragments should decrease in proportion to the distance the mudflow travels, however, and in far-travelled mudflows they may be very uncommon. A second feature which may be present in mudflows is coarse grading, caused by the largest fragments lying at the base of the flow (Crandell & Waldron 1956). Glacial tills are not usually graded. Thirdly, elongate stones in a till exhibit preferred alignment which is in most cases parallel, or less commonly perpendicular, to the direction of ice flow. Harland et al. (1966, p. 245) discussed the production of such a fabric and concluded that "the most plausible mechanisms depend on movement of competent stones in an incompetent matrix, a situation common to both glacial and non-glacial transport. Therefore stone orientation, while giving evidence of movement, probably does not identify glacial movement as such unless the orientation remains constant over very large areas, thus precluding mudflows". Few studies of the fabric of mudflows are available (G/Srler & Reutter 1968, fig. 9; Lindsay 1968; Sestini 1968) and more are needed before it is certain that they do have an identical fabric to tills. Fourthly, large numbers of contorted slump folds, as well as turbidite features might be more likely to occur in association with submarine mudflows than with marine tillites. Neither of these two criteria can be conclusive in their own right; both could occur in marine glacial sediments. Lastly on a regional scale, it should be possible to distinguish mudflow and tillite deposits by their lateral continuity and uniformity of thickness. Individual till sheets may extend with uniform thickness (several metres or tens of metres) over areas measuring hundreds or thousands of kilometres square (e.g. White 1969) and although a single mudflow might possibly cover such an area to a uniform thickness, such a deposit would surely be exceptional.

(iii)

PULSATORY TURBULENT FLOW

Harland et al. (1967, p. 238) suggested that pulsatory turbulent flow in a density current may produce a "sedimentary unit difficult to distinguish from an ortho-till in so far as both may be largely unsorted". In other respects, however, turbidity current deposits and (autochthonous) tills can easily be distinguished (Heezen & Hollister 1964); turbidity currents cannot transport the cobble- and boulder-grade stones which occur in mixtites. The mechanism proposed by Schermerhorn & Stanton (1963) also falls into the pulsatory turbulent flow class. Its importance lies in the fact that it may explain both the features (great areal extent of the tilloid formations and the far-travelled nature of the extra-basinal stones) whose combined presence is difficult to account for on either a simple mudflow or a simple turbidity current hypothesis. They suggested (op. cit., p. 226) that, "the extensive and regularly bedded tilloid formations of the West Congo geosyncline are composed of well-mixed tiUoids which have travelled for great distances as highly mobile mudstreams. We propose to distinguish chaotic, immature slide tilloids from far-travelled well-mixed flow tilloids. In the sequence leading from slumping on a slope to deposition in a basin slide tilloids represent the mature endstage". Mem. geol. Soe. Lond. no. 6

59

A. M. SPENCER By stressing the gradation which may exist between mudflows and turbidity currents (op. cit., pp. 226-7), by the use of the term mudstreams, by suggesting that "the internal stratification and lamination sometimes observed in finer grained tilloid beds may be explained by deposition from the diluted and slower-moving tails of mudflows" (op. cit., p. 228), and that the momentum of the fows is sufficient to carry them a long way across the basin floor, Schermerhorn & Stanton imply that their flow-tilloids have properties intermediate between mudflows and turbidity currents. Flow-tiUoids therefore combine the capacity of a turbidity current for very long distance transport with the ability of mudflows to transport boulders and produce thick beds with a homogeneous matrix (e.g. op. cit., fig. 9, the southern limit of tilloids rich in spilite pebbles is shown as at least 150km from the source area of the pebbles). Such a mechanism has not been proposed before and if it does indeed work it would provide a thoroughly convincing explanation of the mixtites and associated graded beds described by Schermerhorn & Stanton. Some of the evidence given by Schermerhorn & Stanton does not support their proposed mechanism. Most important, submarine mudflows and turbidity current deposits originate from one or more source areas towards which they must thin out. Isopachytes of the Angola Tilloid Formations are not given, but if fig. 1 (op. cit.), which shows the over-aU thickness of the Lower Tilloid Formation, is compared with fig. 9 (op. cir.), which shows the suggested source areas of the same formation, it can be seen that there is no overall thinning towards either the south-western or the south-south-eastern source. Uniform thickness over very extensive areas is a character which might be expected in glacial tills. In addition, one of the central pieces of evidence in support of Schermerhorn & Stanton's hypothesis is the discovery by them of graded bedding at numerous levels in the succession and within the Tilloid Formations. These graded beds are interpreted as deep-water turbidites, but it is noted that, "With the exception of some doubtful examples from M'Pioka greywackes, sole marks other than load-casts have not been found. Bedding plane surfaces are, however, rarely well exposed" (op. cir., p. 206). The presence of bottom structures, such as flute casts, groovemarks, prod marks etc., which usually occur abundantly in turbidites, would considerably strengthen the interpretation of the graded beds as deep-water turbidites. The absence of such structures in a succession which is 15 000m thick and extends over 100000 sq. km, even though poorly exposed, is suprising and suggests that the graded beds may not be true turbidites. Schermerhorn & Stanton have also noted the presence of internal bedding within certain of the tilloid beds. The present author suggests that far from being evidence in support of a mudflow origin, as they suggest, such internal bedding is difficult to explain if the tilloids are mudflows but may be readily explained if they are autochthonous tills (section 3. A (ii)). In addition, the evidence presented by Schermerhorn & Stanton is not all compatible with the tilloids being deep-water deposits. Both tilloid formations rest on unconformities which the authors suggest were produced by sub-aerial erosion. Both tilloids are overlain by thin dolomitic limestones; that above the Upper Tilloid Formation shows (op. cit., p. 230) "astonishing extent and regularity although averaging only 10m thick" and was supposed by other authors to "be a lagoonal deposit marking the transition from continental Co tillite to marine C2". Dolomitic limestones of the lithology and extent indicated are most likely to form in shallow water. Schermerhorn & Stanton, referring to the limestone just described, used the following circular argument to infer a deep-water origin, "its stratigraphical position between two deep-water formations indicates that it, too, was probably deposited in deep water. Locally there occurs a similar dolomitic limestone at the top of the calcareous facies of Mo (Lower Tilloid Formation)... again between two deep-water formations" (op. cit., p. 230). Similarly, when referring to the quartzites which occur within the Tilloid Formations, Schermerhorn & Stanton declared that (op. cit., p. 206) "Although it is not usual to postulate a turbidite origin for clean quartzites, their lithology and associations make it necessary here". The presence of graded bedding within the Angola Tilloid Formations, both in the bedded horizons and in the matrix of the Tilloids, is remarkable. No such graded bedding occurs within the Port Askaig Formation. The non-glacial submarine mudflow origin which Schermerhorn & Stanton suggest for this feature may be the correct explanation, an alternative explanation, suggested by Carey & Ahmed (1961, 884-5), is that 60

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LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND turbidity currents and mudflows may occur on a steep foreset slope of till, beneath the margin of the grounded area of an ice-shelf. One further type of mass flow mechanism--pseudo-laminar flow of granular material (grain flow, Stauffer 1967) or the fluxoturbidites of Dzulynski, Ksiazkiewkz & Kuenen (1959)--may produce mixtites. The mechanism is tittle known but the resulting deposits are relatively free of mud, and seem unlikely to be confused with the mixtites described here.

(c) T H E E V I D E N C E

AGAINST

A MUDFLOW

ORIGIN

FOR THE MIXTITES

This evidence will be listed under six headings, each of which is based on a separate character of the sediments in the formation. The six criteria are not, however, equally significant in deciding against a mudflow origin. Certain of these lines of evidence have been stated before (Kilburn et al. 1965, p. 356) and are amplified below. The lithology of the bedded sediments. The Port Askaig Formation is underlain by limestones, dolomites and dolomitic siltstones, whilst within the formation the bedded sediments consist of (in stratigraphical order) dolomitic siltstones, dolomite conglomerates, dolomitic sandstones, dolomites, granite conglomerates and sandstones. At Port Askaig the overlying formation contains dolomites and dolomitic siltstones and is in turn succeeded by the very thick Jura Quartzite. From the list above it can be seen that the bedded sediments associated with the Port Askaig Formation, and especially those interbedded with the individual mixtites, do not belong to a poorly sorted greywacke suite; the sandstones, although having a high feldspar content, do not have the detrital micaceous matrix which occurs in greywackes. In addition, no graded bedding or turbidite bottom structures have been seen in the formation. A particular search for bottom structures at the base of mixtites and for grading within mixtites has been made; neither has been found and their absence cannot be attributed to lack of exposure. Similarly, only a few soft sediment folds have been found, all of which are capable of explanation by mechanisms other than slumping (see section 3. A (iii)). Lastly, despite excellent exposures, none of the many siltstone fragments seen appear to be caught in the process of disintegration. The depth of water during the deposition of the formation. There is considerable support from several tines of evidence, for the idea that the bedded sediments associated with the Port Askaig Formation were deposited in very shallow water or even under conditions of emergence. For example, the calcareous and dolomitic sediments beneath the lowest mixtite, both in the Garvellachs and at Port Askaig, are of very shallow water lithology and contain respectively, algal stromatolites, limestone flake breccias and desiccation cracks and algal stromatolites, limestone flake breccias and oolites. Close to the top of the member 1 in the Garvellachs are two dolomites, both of which are more likely to have been deposited in relatively shallow rather than deep water. In addition at several horizons within the formation are coarse granite conglomerates which may well have formed in an intertidal or fluvial environment (see section 3. B (ii)). The Dolomitic Formation, above the highest mixtite on Islay, contains mudcracks and abundant algal stromatolites and must have been deposited in a very shallow, or even intertidal environment. In addition to the considerable firm evidence just listed, the nature of the cross-stratification palaeocurrent distributions in the sandstones within the formation indicates that they are most likely to have formed in a shallow, tidal sea environment. Also, sandstone wedges occur at a large number of horizons throughout the formation and are thought to have formed under permafrost conditions (see section 3. C (iii)). Such conditions can only develop on emergent surfaces so that many of the horizons with wedges provide evidence of sub-aerial conditions. The great lateral continuity of some individual mixtites and the presence of major erosion surfaces. In section 2 it was shown that the same five members can be recognized within the formation for a distance of over 500km, from Schichallion to Connemara. In addition, several individual mixtite beds can be correlated a distance of 160km between three outcrops. In the discussion after their paper Schermerhorn & Stanton

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LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND turbidity currents and mudflows may occur on a steep foreset slope of till, beneath the margin of the grounded area of an ice-shelf. One further type of mass flow mechanism--pseudo-laminar flow of granular material (grain flow, Stauffer 1967) or the fluxoturbidites of Dzulynski, Ksiazkiewkz & Kuenen (1959)--may produce mixtites. The mechanism is tittle known but the resulting deposits are relatively free of mud, and seem unlikely to be confused with the mixtites described here.

(c) T H E E V I D E N C E

AGAINST

A MUDFLOW

ORIGIN

FOR THE MIXTITES

This evidence will be listed under six headings, each of which is based on a separate character of the sediments in the formation. The six criteria are not, however, equally significant in deciding against a mudflow origin. Certain of these lines of evidence have been stated before (Kilburn et al. 1965, p. 356) and are amplified below. The lithology of the bedded sediments. The Port Askaig Formation is underlain by limestones, dolomites and dolomitic siltstones, whilst within the formation the bedded sediments consist of (in stratigraphical order) dolomitic siltstones, dolomite conglomerates, dolomitic sandstones, dolomites, granite conglomerates and sandstones. At Port Askaig the overlying formation contains dolomites and dolomitic siltstones and is in turn succeeded by the very thick Jura Quartzite. From the list above it can be seen that the bedded sediments associated with the Port Askaig Formation, and especially those interbedded with the individual mixtites, do not belong to a poorly sorted greywacke suite; the sandstones, although having a high feldspar content, do not have the detrital micaceous matrix which occurs in greywackes. In addition, no graded bedding or turbidite bottom structures have been seen in the formation. A particular search for bottom structures at the base of mixtites and for grading within mixtites has been made; neither has been found and their absence cannot be attributed to lack of exposure. Similarly, only a few soft sediment folds have been found, all of which are capable of explanation by mechanisms other than slumping (see section 3. A (iii)). Lastly, despite excellent exposures, none of the many siltstone fragments seen appear to be caught in the process of disintegration. The depth of water during the deposition of the formation. There is considerable support from several tines of evidence, for the idea that the bedded sediments associated with the Port Askaig Formation were deposited in very shallow water or even under conditions of emergence. For example, the calcareous and dolomitic sediments beneath the lowest mixtite, both in the Garvellachs and at Port Askaig, are of very shallow water lithology and contain respectively, algal stromatolites, limestone flake breccias and desiccation cracks and algal stromatolites, limestone flake breccias and oolites. Close to the top of the member 1 in the Garvellachs are two dolomites, both of which are more likely to have been deposited in relatively shallow rather than deep water. In addition at several horizons within the formation are coarse granite conglomerates which may well have formed in an intertidal or fluvial environment (see section 3. B (ii)). The Dolomitic Formation, above the highest mixtite on Islay, contains mudcracks and abundant algal stromatolites and must have been deposited in a very shallow, or even intertidal environment. In addition to the considerable firm evidence just listed, the nature of the cross-stratification palaeocurrent distributions in the sandstones within the formation indicates that they are most likely to have formed in a shallow, tidal sea environment. Also, sandstone wedges occur at a large number of horizons throughout the formation and are thought to have formed under permafrost conditions (see section 3. C (iii)). Such conditions can only develop on emergent surfaces so that many of the horizons with wedges provide evidence of sub-aerial conditions. The great lateral continuity of some individual mixtites and the presence of major erosion surfaces. In section 2 it was shown that the same five members can be recognized within the formation for a distance of over 500km, from Schichallion to Connemara. In addition, several individual mixtite beds can be correlated a distance of 160km between three outcrops. In the discussion after their paper Schermerhorn & Stanton

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A. M. SPENCER stated (1963, p. 237) that, "If the individual boulder-beds in the Dalradian were continuous and without facies changes over such great distances they would be hard to interpret as mudflows". Whilst certain of the individual mixtites are continuous for tens of kilometres, others die out abruptly because of the presence of considerable erosion surfaces (section 3. B (iii)). As noted above, the great lateral continuity of the formation and its five members as a whole, and of certain individual mixtites in detail, is very unlikely to have been produced if the mixtites are of mudflow origin. The erosion surfaces present in the succession, however, could not have developed in the deep, geosynclinal basin in which such continuous sub-aqueous mudflows must accumulate. The absence of a regional palaeoslope. Potter & Pettijohn (1963, p. 82) stated that "The relation between cross-bedding or any other directional structure and the initial sedimentary dip or slope at the time of deposition is critical to regional interpretation. This slope is called the palaeoslope". The absence of one overall preferred cross-stratification palaeocurrent direction throughout the whole of the Port Askaig Formation (Fig. 14b) and the presence of different preferred palaeocurrent directions at different horizons (Fig. 13) suggests that a single palaeoslope, of regional extent, was absent throughout the deposition of the whole formation. The size of material which can be carried by a mudflow. One of the mixtites in the Garvellachs, the Great Breccia, contains blocks of intra-basinal sediment which are up to several hundred metres in length and several tens of metres in width and thickness (P1. 11). These blocks must have been transported at least a few kilometres to account for (a) the complete size mixing and homogeneity of the matrix of the Breccia, (b) the fact that in the Garvellachs the Breccia is separated from the source of many of the huge fragments, the underlying bedded sediments of the Islay limestone, by 100m of mixtites and (c) that the Breccia can also be found on Islay, 50km distant from the Garvellachs, where it rests directly on the Islay Limestone. The huge dolomite fragments must have been transported from just such an area, across a sedimentary basin containing previously deposited mixtites (some of which were eroded, producing the huge mixtite fragments) and finally deposited with a conformable contact on a thick group of mixtites; such a lateral sequence implies transport of several kilometres. But blocks of this size could only be transported by a mudflow fowing down a very steep slope. The existence of such a steep slope, over the distances just given is ruled out by (i) the absence of any reflection of such a palaeoslope on the palaeocurrent directions (see section 3. B (i)) and (ii) the absence of abundant slump folds. It therefore seems very unlikely that the Breccia could have originated as a mudfow. On the other hand an exact glacial analogy of the Great Breccia, the Cromer Till, occurs in the Pleistocene of north Norfolk (Reid 1882). The internal structure o f mudflows. There is very little published information on this subject so that the short discussion which follows must necessarily be speculative and the conclusions regarding the internal structure of mudfows will need checking in the field. Schermerhorn & Stanton (1963, p. 226) stated that "The generally even texture of the Co and Mo tilloids indicates a thorough mixing of all size-fractions by turbulence in a medium of high density without much sorting during transport. (Kuenen (1956) refers to 'a kind of slow boiling agitation' taking place in moving mudfows)". Sharp & Nobles (1953) record that a 15-mile long, sub-aerial mudflow, travelling with an average velocity of 10ft/sec, buried and infilled a log cabin to the eaves without in any way damaging it. The mudflows which may have formed very continuous mixtites must have possessed such great mobility. It therefore seems very unlikely that within such mudflows internal bedding, other than a coarse grading due to the sinking of large stones, could either have formed or have been preserved. Internal bedding within mixtites, produced by well sorted, lenticular, siltstone, sandstone and pebble beds, is very common on all scales and at many horizons within the Port Askaig Formation (see section 3. A (ii)). Such internal bedding, although unlikely to develop in very mobile mudfows, is, however, commonly present in Pleistocene glacial tills. Conclusions. Each of the lines of evidence listed above provides evidence against a mudflow origin for the mixtites in the Port Askaig Formation. Several authors (Crowell 1957; Dott 1963; Schermerhorn & Stanton 62

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IN SCOTLAND

1963; Winterer 1964) have stressed the connection and association between submarine mudflow deposits and turbidites. The bedded sediments beneath, within and above the Port Askaig Formation are all typically of 'shelf' facies, however, and show no evidence of the action of turbidity currents. Moreover, there are many indicators that the water during the periods of deposition of the bedded sediments was very shallow or even emergent. In addition, the great lateral continuity of certain individual mixtite beds (implying a large and even basin on the mudflow hypothesis) is incompatible with the presence of erosion surfaces of considerable magnitude; the latter could not have formed in a deep marine basin. These three criteria seem to rule out the formation of the mixtites by submarine mudflows. In addition, the measured cross-stratification directions provide no evidence of an over-all, well-defined palaeoslope down which mudflows of either sub-aerial or submarine origin could have flowed. This lack of evidence of a palaeoslope is especially important in the case of the Great Breccia, for the enormous blocks within the latter could only have been transported by a mudflow moving down a steep palaeoslope. Lastly, the bedded lenses of sorted sediment which occur within individual mixtites are unlikely to form within mudflows. The last three criteria rule out the formation of the mixtites by either submarine or sub-aerial mudflows. (D) T H E E V I D E N C E

IN SUPPORT

OF A GLACIAL

ORIGIN

FOR THE

MIXTITES

(i) CRITERIA USED AS EVIDENCE OF FORMER GLACIATIONS Several authors have recently considered the evidence by which glacial and cold climate deposits may be recognized (Flint 1961; Harland 1964A, B; Schwarzbach 1964). The most comprehensive list is given by Flint (1961, 142-53), whose criteria are used in Table 13. Harland et al. (1966, p. 240) have recently reviewed these criteria in order to determine those which are 'critical in distinguishing tillites from other rocks, and in recognizing the glacial agent in their formation'. These features are also given in Table 13 and are graded according to the certainty with which they indicate a glacial origin. (ii) GLACIAL CRITERIA APPLIED TO THE PORT ASKAIG FORMATION

Table 13 sets out the criteria of glaciation and their relative significance and shows which are present, absent, not identified or found and not expected in the Port Askaig Formation. The following discussion will make the table clear. Abraded bedrock surfaces and streamline topography, both products of glacial erosion, would probably not be expected in a continuous sedimentary sequence. The shape of the stones in the mixtites has not been studied systematically, but some are certainly faceted. Striated stones have not been found; none of the crystalline stones which have been extracted cleanly show striations (but crystalline stones in British Pleistocene tills rarely show striations). The stones which should be striated, the dolomites, are almost impossible to extract cleanly due to the slight metamorphism and their original surfaces cannot be studied. In lithology the mixtites contain abundant extra-basinal stones but their matrix is of relatively local origin and is not fresh and unweathered. The thickness of the formation and its great lateral extent could both occur in glacial tills. It is unlikely that evidence of the presence of drumlins and moraines would be found, as the upper surfaces of individual mixtites are only exposed in cross-section and, more important, the majority of the surfaces are erosion planes and the erosion would have destroyed features of this type. Outwash sediments have not been identified; Flint (1961 p. 147) stated that "The sites of outwash are valleys extremely vulnerable to erosion, as is testified to by the dissected state of outwash bodies of even quite recent origin. Ancient tillites are therefore unlikely to be accompanied by much sedimentary rock of outwash origin". (Outwash sediments may sometimes be present;e.g. Spjeldnaes (1964, p. 32) suggested that certain of the interbedded sediments in the Finnmark tillites may be of 'glaci-fluvial' origin.) There are structures within the mixtites which can be interpreted as having been produced by glacial movement (see section 3. A (iii)). Flint (1961, p. 152) noted that solifluction sediments may be difficult to distinguish from other mass movement deposits and they have not been recognized here. There are structures in the formation for Mem. geoL Soc. Lond. no. 6

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IN SCOTLAND

1963; Winterer 1964) have stressed the connection and association between submarine mudflow deposits and turbidites. The bedded sediments beneath, within and above the Port Askaig Formation are all typically of 'shelf' facies, however, and show no evidence of the action of turbidity currents. Moreover, there are many indicators that the water during the periods of deposition of the bedded sediments was very shallow or even emergent. In addition, the great lateral continuity of certain individual mixtite beds (implying a large and even basin on the mudflow hypothesis) is incompatible with the presence of erosion surfaces of considerable magnitude; the latter could not have formed in a deep marine basin. These three criteria seem to rule out the formation of the mixtites by submarine mudflows. In addition, the measured cross-stratification directions provide no evidence of an over-all, well-defined palaeoslope down which mudflows of either sub-aerial or submarine origin could have flowed. This lack of evidence of a palaeoslope is especially important in the case of the Great Breccia, for the enormous blocks within the latter could only have been transported by a mudflow moving down a steep palaeoslope. Lastly, the bedded lenses of sorted sediment which occur within individual mixtites are unlikely to form within mudflows. The last three criteria rule out the formation of the mixtites by either submarine or sub-aerial mudflows. (D) T H E E V I D E N C E

IN SUPPORT

OF A GLACIAL

ORIGIN

FOR THE

MIXTITES

(i) CRITERIA USED AS EVIDENCE OF FORMER GLACIATIONS Several authors have recently considered the evidence by which glacial and cold climate deposits may be recognized (Flint 1961; Harland 1964A, B; Schwarzbach 1964). The most comprehensive list is given by Flint (1961, 142-53), whose criteria are used in Table 13. Harland et al. (1966, p. 240) have recently reviewed these criteria in order to determine those which are 'critical in distinguishing tillites from other rocks, and in recognizing the glacial agent in their formation'. These features are also given in Table 13 and are graded according to the certainty with which they indicate a glacial origin. (ii) GLACIAL CRITERIA APPLIED TO THE PORT ASKAIG FORMATION

Table 13 sets out the criteria of glaciation and their relative significance and shows which are present, absent, not identified or found and not expected in the Port Askaig Formation. The following discussion will make the table clear. Abraded bedrock surfaces and streamline topography, both products of glacial erosion, would probably not be expected in a continuous sedimentary sequence. The shape of the stones in the mixtites has not been studied systematically, but some are certainly faceted. Striated stones have not been found; none of the crystalline stones which have been extracted cleanly show striations (but crystalline stones in British Pleistocene tills rarely show striations). The stones which should be striated, the dolomites, are almost impossible to extract cleanly due to the slight metamorphism and their original surfaces cannot be studied. In lithology the mixtites contain abundant extra-basinal stones but their matrix is of relatively local origin and is not fresh and unweathered. The thickness of the formation and its great lateral extent could both occur in glacial tills. It is unlikely that evidence of the presence of drumlins and moraines would be found, as the upper surfaces of individual mixtites are only exposed in cross-section and, more important, the majority of the surfaces are erosion planes and the erosion would have destroyed features of this type. Outwash sediments have not been identified; Flint (1961 p. 147) stated that "The sites of outwash are valleys extremely vulnerable to erosion, as is testified to by the dissected state of outwash bodies of even quite recent origin. Ancient tillites are therefore unlikely to be accompanied by much sedimentary rock of outwash origin". (Outwash sediments may sometimes be present;e.g. Spjeldnaes (1964, p. 32) suggested that certain of the interbedded sediments in the Finnmark tillites may be of 'glaci-fluvial' origin.) There are structures within the mixtites which can be interpreted as having been produced by glacial movement (see section 3. A (iii)). Flint (1961, p. 152) noted that solifluction sediments may be difficult to distinguish from other mass movement deposits and they have not been recognized here. There are structures in the formation for Mem. geoL Soc. Lond. no. 6

63

A. M. S P E N C E R w h i c h a n i n v o l u t i o n o r i g i n c a n n o t b e r u l e d o u t (section 3. A (iv)), b u t t h e y a r e at least as l i k e l y to h a v e f o r m e d as large l o a d casts. T w o f u r t h e r criteria a r e g i v e n b y H a r l a n d et al. (1966, 240, 242); t h e p r e s e n c e o f

TABLE 13: Criteria for identifying glacial and cold climate deposits applied to the Port Askaig Formation

Criteria

A. Evidence of former glaciation

Critical features which determine glacial origin rather than by any other mechanism (according to Harland et aL 1966, pp. 240-51)

1. Abraded bedrock surfaces 2. Streamline topographic forms 3. Till and tillite a. Great range of grain sizes b. Lack of size sorting c. Shapes of stones (faceting) d. Striated stones

Not expected Not expected P P P ?P Not found, obscured by metamorphism Obliterated by tectonic deformation P (extra-basinal stones)

e. Till fabric f. Lithology (far travelled stones and undecomposed minerals) g. Boulder pavements (bevelled, striated conglomerate layer in till) h. Thickness (up to hundreds of metres) and lateral extent (up to hundreds of km) i. Underlying floor

Not expected (same as criteria 1 and 2)

j. Form of upper surface (e.g. drumlins, morainic ridges) 5. Stratified glacial drift a. Outwash sediments b. 'Varved clays' etc. 6. Structures made by glacial movement 7. Altitude of glacial features

? (Not identified) P P Not a criterion

Features made by freezing and thawing of the ground a. Solifluction sediments

B. Non-glacial evidence of cold climate

Not found (probably not expected) ?P P P Not a criterion

b. Involutions c. Ice-wedge pseudomorphs Rafted cobbles and boulders in alluvium 10. Colours of sediments .

11. Presence of rock flour 12. Large blocks of varied extra-basinal suite over very extensive area

Present (P) or absent (--) in the Port Askaig Formation

** **

Not identified P

Harland et al. (1966, p. 251)noted: *** this is the "one criterion for glacial origin which can be used alone and considered decisive", ** these "can be explained in more than one way. However, in sufficient quantity or quality, these factors may justify a claim for glacial origin"; * the results of the very extensive areas over which tills may be present. This factor "may be overwhelmingly convincing but is not in itself decisive".

64

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rock flour with a chemical composition similar to that of the average crustal rock indicates glacial grinding and the 'occurrence of large and varied blocks over a wide area suggests ice transportation'. Of the 24 criteria given in Table 13, ten are present, two may be present (?P), six are absent (but are either unlikely to be found, or are difficult to recognize in a dipping succession of ancient tillites; 1, 2, 3i, 5a, 8a) and two are not criteria at all. Only four criteria which might be expected to occur (3d, 3e, 3g, 1l) are absent. In interpreting the mixtites as glacial deposits certain of the criteria carry more weight than others. For example criteria 3, 3a and 3b occur both in tills and mudflows. Criteria 3f, 3h and 12 are less likely to occur in mudflow deposits, but their presence does not rule out a mudflow origin. It is the presence of criteria 9 and 5b in combination (see sections 3. B (v) and 3. B (vi)) which provides certain evidence of the former presence of floating ice. In addition the presence of criteria 8c provides direct evidence of a cold climate. Flint (1961, p. 153) noted that "Although ice-wedge pseudomorphs are capable of being preserved, none of pre-Pleistocene age has been reported". As the sandstone wedges from the Port Askaig Formation appear to be the first recorded pre-Pleistocene occurrence of such features* considerable care must be taken to ensure that the structures are not clastic dykes which have been misidentified. The evidence that the structures are of frost contraction origin is presented in section 3. C (iii) and is considered to be conclusive in its own right. In conclusion then, many of the features to be expected in glacial deposits are present in the Port Askaig Formation but the particular features which make a glacial origin certain are, (i) the great lateral extent of the formation, its members and certain individual mixtites, (ii) the large blocks of far-travelled crystalline rock present over a very extensive area, (iii) permafrost features and, most important of all, (iv) the varved siltstones with ice-rafted stones. (E) T H E M E C H A N I S M

OF GLACIAL

DEPOSITION

(i) THE OPINIONS OF WORKERS ON OTHER TILLITES Thomson (1877, p. 211), commenting on the origin of the Port Askaig Tillite, stated that "The entire absence of stratification in one part of the section, which in another shows signs of regular deposition, and the occurrence of far-transported rocks of the character already stated, indicate that the mass has been transported and dropped from melting ice in a shallow, tranquil sea, the bottom consisting of mud and sand. The absence of stratification in one part of the section may be accounted for by the disturbing action of icebergs which moved to and fro, grounding as they went before they were finally stranded". Reusch (1891) on the other hand, conluded that the Bigganjargga Tillite in the Eocambrian of northern Norway had originated as a moraine and Coleman (1926, 216, 228) believed that most tillites, even those interstratified with marine sedimentary rocks, were continental in origin. More recently the majority of researchers have postulated that the late Pre-Cambrian tillites in the North Atlantic region are deposits dropped from floating ice. Kulling (1934, p. 224), describing the Sveanor Tillite of Spitsbergen, first concluded that the deposits were probably marine. "The first question set here is, if this district has been glaciated, i.e. if it has been markedly raised above sea-level. An immediate negative answer seems indicated. It has anyway not been part of any centre of glaciation, which is proved by the circumstance that on the whole the tillites are conformable to the underlying Rysso Series . . . . The tillites conform not only to the underlying, but also to the overlying series". Then he continued, "The Sveanor Tillite is a marine tillite, and therefore contains no arkose-like moraine-beds... (the Sveanor Tillite)is a two-component sediment, composed of one more or less local component assorted by currents and waves on its way to the sedimentation area, and another long-transport component, which has joined the former by dropping to the bottom of the sea from melting ice-bergs". Holtedahl (quoted in Kulling 1934, p. 239), discussing the upper tillite at Varanger Fiord, northern Norway, said that "No mass of ice can have pushed forward on land over so large areas without disturbing the underlying strata. Nor would a mass of ice on land have left behind it * See note added in proof on p. 98. Mem. geol. Soc. Lond. no. 6

65

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

rock flour with a chemical composition similar to that of the average crustal rock indicates glacial grinding and the 'occurrence of large and varied blocks over a wide area suggests ice transportation'. Of the 24 criteria given in Table 13, ten are present, two may be present (?P), six are absent (but are either unlikely to be found, or are difficult to recognize in a dipping succession of ancient tillites; 1, 2, 3i, 5a, 8a) and two are not criteria at all. Only four criteria which might be expected to occur (3d, 3e, 3g, 1l) are absent. In interpreting the mixtites as glacial deposits certain of the criteria carry more weight than others. For example criteria 3, 3a and 3b occur both in tills and mudflows. Criteria 3f, 3h and 12 are less likely to occur in mudflow deposits, but their presence does not rule out a mudflow origin. It is the presence of criteria 9 and 5b in combination (see sections 3. B (v) and 3. B (vi)) which provides certain evidence of the former presence of floating ice. In addition the presence of criteria 8c provides direct evidence of a cold climate. Flint (1961, p. 153) noted that "Although ice-wedge pseudomorphs are capable of being preserved, none of pre-Pleistocene age has been reported". As the sandstone wedges from the Port Askaig Formation appear to be the first recorded pre-Pleistocene occurrence of such features* considerable care must be taken to ensure that the structures are not clastic dykes which have been misidentified. The evidence that the structures are of frost contraction origin is presented in section 3. C (iii) and is considered to be conclusive in its own right. In conclusion then, many of the features to be expected in glacial deposits are present in the Port Askaig Formation but the particular features which make a glacial origin certain are, (i) the great lateral extent of the formation, its members and certain individual mixtites, (ii) the large blocks of far-travelled crystalline rock present over a very extensive area, (iii) permafrost features and, most important of all, (iv) the varved siltstones with ice-rafted stones. (E) T H E M E C H A N I S M

OF GLACIAL

DEPOSITION

(i) THE OPINIONS OF WORKERS ON OTHER TILLITES Thomson (1877, p. 211), commenting on the origin of the Port Askaig Tillite, stated that "The entire absence of stratification in one part of the section, which in another shows signs of regular deposition, and the occurrence of far-transported rocks of the character already stated, indicate that the mass has been transported and dropped from melting ice in a shallow, tranquil sea, the bottom consisting of mud and sand. The absence of stratification in one part of the section may be accounted for by the disturbing action of icebergs which moved to and fro, grounding as they went before they were finally stranded". Reusch (1891) on the other hand, conluded that the Bigganjargga Tillite in the Eocambrian of northern Norway had originated as a moraine and Coleman (1926, 216, 228) believed that most tillites, even those interstratified with marine sedimentary rocks, were continental in origin. More recently the majority of researchers have postulated that the late Pre-Cambrian tillites in the North Atlantic region are deposits dropped from floating ice. Kulling (1934, p. 224), describing the Sveanor Tillite of Spitsbergen, first concluded that the deposits were probably marine. "The first question set here is, if this district has been glaciated, i.e. if it has been markedly raised above sea-level. An immediate negative answer seems indicated. It has anyway not been part of any centre of glaciation, which is proved by the circumstance that on the whole the tillites are conformable to the underlying Rysso Series . . . . The tillites conform not only to the underlying, but also to the overlying series". Then he continued, "The Sveanor Tillite is a marine tillite, and therefore contains no arkose-like moraine-beds... (the Sveanor Tillite)is a two-component sediment, composed of one more or less local component assorted by currents and waves on its way to the sedimentation area, and another long-transport component, which has joined the former by dropping to the bottom of the sea from melting ice-bergs". Holtedahl (quoted in Kulling 1934, p. 239), discussing the upper tillite at Varanger Fiord, northern Norway, said that "No mass of ice can have pushed forward on land over so large areas without disturbing the underlying strata. Nor would a mass of ice on land have left behind it * See note added in proof on p. 98. Mem. geol. Soc. Lond. no. 6

65

A. M. S P E N C E R

such an even thickness of moraine. This even thickness is certainly primary, and not due to subsequent levelling by waves, for there is no sign of such scouring. I cannot conceive of any other explanation than that this is of glacial origin, deposited in water. In view of the fact that a tillite zone quite similar to that at the head of Varanger Fiord is also met with in several places on the Tana Fiord--let alone in the Alten district-it is also likely that we have here deposits from loose ice floes, and not from a glacier resting on land". Spjeldnaes (1964), discussing the Eocambrian glacial deposits of the whole of Norway, favoured an icerafting origin for the tillites in conformable sequences. Reading & Walker (1966) suggested an origin by rafting from various types of floating ice for tillites in Finnmark, Norway and, apart from the Bigganjargga Tillite, Bjorlykke (1967, p. 37) also considered the Norwegian tillites to be mostly of iceberg origin. (ii) DISCUSSION OF THE EVIDENCE USED BY WORKERS ON OTHER TILLITES TO INFER DEPOSITION FROM FLOATING ICE

The principal lines of evidence used to suggest that many late Pre-Cambrian tillites were deposited by rafting from floating ice are listed and discussed below. The tillites occur within marine sedimentary successions and themselves contain horizons of bedded, subaqueous (?marine) sediments. Although this is the arrangement expected of ice-rafted tillites, it does not rule out the possibility of deposition from an ice-sheet resting either on the sea floor, or on a low-lying land surface. Sedimentation from ice-shelves resting on the sea floor is considered, from a theoretical standpoint, by Carey & Ahmed 0 9 6 0 ; such a mechanism is surely capable of forming a sequence of tills interbedded in a marine succession. Alternatively, eustatic lowering of sea-level may have been sufficient, even after isostatic down-warping due to the weight of ice, to produce extensive areas slightly above sea-level on which ground moraines could be deposited. The converse of the eustatic and isostatic movements during de-glaciation would result in the tills being overlain by marine sediments. Repetition of this cycle, together with subsidence, would resuIt in a sequence containing ground moraines interbedded with marine sediments. The continuous development of the tillite formations over large areas. Again this is a feature which would probably be expected of ice-rafted tills but which could be produced by deposition from grounded ice. Arguments suggesting an ice-rafting origin for extensive tiIlites are often based on analogies with the Pleistocene. It is argued that the majority of Pleistocene deposits from grounded ice are on land and have little chance of being entombed in a succession of marine sediments (Crowell 1964). The argument continues that the area covered by deposits from floating ice (all sub-marine, all with the potential to be preserved) is very much greater than the area covered by those deposits of grounded ice-sheets which have a similar chance of entombment in the sedimentary record. This argument is not as strong as it seems. Firstly, few of the icerafted tills deposited in the ocean basins are likely to be preserved in sedimentary successions; there is little evidence to suggest that ocean bottom deposits are incorporated in sedimentary sequences which eventually come to be exposed on the continents. More important, the reason for the relatively small area covered by ground moraines resting on the sea floor is that the Continental Shelf in areas of Pleistocene glaciation is relatively narrow (usually less than 100km wide); few larger areas of shelf have even been subjected to glaciation (the North Sea, Canadian Arctic Islands, Ross Sea). This need not have been the case in the late Pre-Cambrian for glaciated areas which were also parts of very extensive continental shelves or low-lying lands may have been very much more common then. Evidence to support this idea comes from the very widespread occurrence of thick, shallow water (oolitic and stromatolitic) carbonates immediately beneath the late Pre-Cambrain tillites in several parts of the world (East Greenland, Spitsbergen, Norway, British Isles, Congo). The uniform thickness of the tillite formations over large areas. Again, this would be expected of ice-rafted tills, icebergs should scatter debris relatively evenly over large areas. A grounded ice-sheet which contained till material homogeneously distributed through the ice and which moved over a very flat substratum, could also deposit an extensive and even layer of till if ice recession was continuous. Again, the widespread presence 66

Mere. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

of extremely shallow water carbonates beneath the late Pre-Cambrain tillites of the North Atlantic area suggests that the surface of the sediments was everywhere at about the same level; thus the surface over which ice-sheets may have spread must have been very flat-lying. Although icebergs deposit material over a wide area there is relatively little information available on the volume of material they carry or on the volume of the resulting deposits. Hough (1950) recorded the lithology and thickness of bottom sediments in three cores, taken in the Ross Sea, about 1000km from the Ross Ice Shelf. The recovered cores, which were 2-4m long, were supposed to span the whole of the Pleistocene. Deposition of the mixtites (totalling 500m in thickness) in the Port Askaig Formation at an equivalent rate would have taken 200 million years, a figure which seems far too large (by a factor of ten) to fit in with the over-all time-scale of the Torridonian-Moine and Dalradian of Scotland. It would be interesting to know whether Pleistocene iceberg sediments are generally so thin, if so it would be difficult to account for the great thickness of ancient tillites by an iceberg-rafting hypothesis. The undisturbed, conformable lower contacts of the tillites. Ice-rafted tillites should have conformable lower contacts (sedimentation was probably continuous from pre-glacial to glacial times), whilst the sole of an advancing continental ice-sheet might well disturb the underlying sediments. The lower contact of the Port Askaig Tillite is disconformable at Port Askaig but most of the lower contacts of individual mixtites are conformable (section 3. A (i)). An ice-sheet can advance over unconsolidated sediments without disturbing them. For example, Simpson & West (1958) described a 2m thick Pleistocene till overlying undisturbed sands which contain an interglacial peat bed; the till is not of solifluxion origin and the ice-sheet which deposited it must have advanced across the sands without in any way disturbing them. It is not known whether an ice-sheet can advance for long distances over such sediments and leave them undisturbed. Advancing ice must possess this ability, however, if the individual mixtites of the Port Askaig Formation are to be explained as ground moraine deposits, for the mixtites almost always have conformable, littledisturbed lower contacts (P1. 11). Again it may be suggested that ice-sheets are likely to cause the least disturbance if the surface over which they advanced was very fiat. The presence of some bedding within individual tillites. Theoretically, all gradations between normal bedded sediments containing few rafted pebbles and unbedded mixtites formed solely by ice-rafting are possible. Pleistocene land tills may also contain a great deal of bedding (see Reid 1882; Carruthers 1953) and so careful examination of the bedding should be undertaken in order to decide whether the till in which it occurs is of ice-rafted or ground moraine origin (sections 3. A (ii) and 5. E (iii)). Tillites are two component sediments. This is not true in the Port Askaig Formation, for the majority of the mixtites do not obviously consist of two components, a long transport one supplied by ice-rafting and a local one brought by sub-aqueous currents. The range of grain-sizes of the extra-basinal fragments almost completely overlaps that of the intra-basinal dolomites in most mixtites. In addition, the majority of the mixtites are unbedded; the matrix material has usually not been sorted by sub-aqueous currents. At only a few horizons is it obvious that certain material can only have been carried by icebergs, whilst most was brought by sub-aqueous currents (see section 3. B (v)). The first four of the above pieces of evidence, although they point to an iceberg-rafting origin for tillites when compared with features in Pleistocene glacial deposits, are explicable even if the tillites are deposits from grounded ice. If it can be shown that the topography of the area glaciated in late Pre-Cambrian times was different from most of the regions of Pleistocene glaciation in that it included very much larger areas which were relatively flat and close to sea-level, many of the features found in certain late Pre-Cambrian tillites are capable of explanation on a grounded ice-sheet hypothesis.

(iii)

DEPOSITION OF THE PORT ASKAIG TILLITE FROM A GROUNDED ICE-SHEET

In section 3. A (ii) (Internal bedding within mixtites), it was noted that the discontinuous bedded horizons provide the only direct evidence of the mode of deposition of the mixtites. This evidence is critical to the Mem. geoL Soc. Lond. no. 6

67

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

of extremely shallow water carbonates beneath the late Pre-Cambrain tillites of the North Atlantic area suggests that the surface of the sediments was everywhere at about the same level; thus the surface over which ice-sheets may have spread must have been very flat-lying. Although icebergs deposit material over a wide area there is relatively little information available on the volume of material they carry or on the volume of the resulting deposits. Hough (1950) recorded the lithology and thickness of bottom sediments in three cores, taken in the Ross Sea, about 1000km from the Ross Ice Shelf. The recovered cores, which were 2-4m long, were supposed to span the whole of the Pleistocene. Deposition of the mixtites (totalling 500m in thickness) in the Port Askaig Formation at an equivalent rate would have taken 200 million years, a figure which seems far too large (by a factor of ten) to fit in with the over-all time-scale of the Torridonian-Moine and Dalradian of Scotland. It would be interesting to know whether Pleistocene iceberg sediments are generally so thin, if so it would be difficult to account for the great thickness of ancient tillites by an iceberg-rafting hypothesis. The undisturbed, conformable lower contacts of the tillites. Ice-rafted tillites should have conformable lower contacts (sedimentation was probably continuous from pre-glacial to glacial times), whilst the sole of an advancing continental ice-sheet might well disturb the underlying sediments. The lower contact of the Port Askaig Tillite is disconformable at Port Askaig but most of the lower contacts of individual mixtites are conformable (section 3. A (i)). An ice-sheet can advance over unconsolidated sediments without disturbing them. For example, Simpson & West (1958) described a 2m thick Pleistocene till overlying undisturbed sands which contain an interglacial peat bed; the till is not of solifluxion origin and the ice-sheet which deposited it must have advanced across the sands without in any way disturbing them. It is not known whether an ice-sheet can advance for long distances over such sediments and leave them undisturbed. Advancing ice must possess this ability, however, if the individual mixtites of the Port Askaig Formation are to be explained as ground moraine deposits, for the mixtites almost always have conformable, littledisturbed lower contacts (P1. 11). Again it may be suggested that ice-sheets are likely to cause the least disturbance if the surface over which they advanced was very fiat. The presence of some bedding within individual tillites. Theoretically, all gradations between normal bedded sediments containing few rafted pebbles and unbedded mixtites formed solely by ice-rafting are possible. Pleistocene land tills may also contain a great deal of bedding (see Reid 1882; Carruthers 1953) and so careful examination of the bedding should be undertaken in order to decide whether the till in which it occurs is of ice-rafted or ground moraine origin (sections 3. A (ii) and 5. E (iii)). Tillites are two component sediments. This is not true in the Port Askaig Formation, for the majority of the mixtites do not obviously consist of two components, a long transport one supplied by ice-rafting and a local one brought by sub-aqueous currents. The range of grain-sizes of the extra-basinal fragments almost completely overlaps that of the intra-basinal dolomites in most mixtites. In addition, the majority of the mixtites are unbedded; the matrix material has usually not been sorted by sub-aqueous currents. At only a few horizons is it obvious that certain material can only have been carried by icebergs, whilst most was brought by sub-aqueous currents (see section 3. B (v)). The first four of the above pieces of evidence, although they point to an iceberg-rafting origin for tillites when compared with features in Pleistocene glacial deposits, are explicable even if the tillites are deposits from grounded ice. If it can be shown that the topography of the area glaciated in late Pre-Cambrian times was different from most of the regions of Pleistocene glaciation in that it included very much larger areas which were relatively flat and close to sea-level, many of the features found in certain late Pre-Cambrian tillites are capable of explanation on a grounded ice-sheet hypothesis.

(iii)

DEPOSITION OF THE PORT ASKAIG TILLITE FROM A GROUNDED ICE-SHEET

In section 3. A (ii) (Internal bedding within mixtites), it was noted that the discontinuous bedded horizons provide the only direct evidence of the mode of deposition of the mixtites. This evidence is critical to the Mem. geoL Soc. Lond. no. 6

67

A. M. SPENCER interpretation of the mixtites as deposits from grounded ice. The author believes that the mixtites have originated in the manner suggested by Carruthers (1940, p. 326) who stated that, "In the (Pleistocene) drifts of northern England both the tills and the interbedded sediments have their origin in a melt which made its way upwards through stagnant ice". By this means a till, whose internal structures are a replica of those present in the former ice-sheet, is deposited. All the bedded horizons within mixtites represent the sorted material deposited by sub-glacial and en-glaeial melt waters (section 3. A (ii)). For example the sharp margin of the dolomite conglomerate which ends laterally against a poorly bedded mixtite (Fig. 4a) is explicable only if the mixtite material was held rigid within ice at the time the margin of the conglomerate formed. For the following reasons such internal bedding is unlikely to result from sub-marine currents acting on the matrix of an ice-rafted tillite. Firstly, bedded horizons within the mixtites are often discontinuous laterally and where they are absent the mixtite is almost always homogeneous and undivided. This is the arrangement expected of deposits formed by discontinuous meltwater streams at the base of, or within, a sediment choked ice-sheet. Marine currents acting on ice-rafted till material on the sea floor would be expected to produce more continuous bedded horizons; they could not produce bedded lenses of sandstone and conglomerate with abrupt lateral margins. Secondly, some of the bedded horizons within the mixtites are arranged in large-scale sedimentary structures (e.g. Fig. 4c). Such arrangements are unlikely to result from submarine currents acting on the top surface of a till lying beneath the sea-floor, but could be produced by sub- or en-glacial meltwater streams confined to particular areas at the base of the ice. Thirdly, although, because of its genetic importance, the presence of internal bedding in mixtites has been stressed, it is rarely abundant and is not ubiquitous in its occurrence. In the Garvellachs, mixtites 2 (8 to 14m thick) and 26 (12 to 20m thick) are perfectly homogeneous and mixtites 19-22 (23.5 to 42m thick) are homogeneous in most outcrops. At Port Askaig many of the mixtites in the uppermost three members are several tens of metres thick and are quite homogeneous. It is the opinion of the author that such mixtites are unlikely to be produced by iceberg rafting. Their homogeneity implies the absence, for long periods of time, of submarine currents capable of sorting even fine-grained material. Such thick, homogeneous tills occur in the Pleistocene and can be laid down from grounded ice. Further evidence that the mixtites in the Port Askaig Tillite were deposited from grounded ice-sheets is available. Firstly, all the lower contacts of the mixtites are either knife sharp or gradational through less than 50cm; such is also the case in many Pleistocene land tills. In the case of iceberg sedimentation the normal course of events should be the gradual incursion of icebergs into a sea area and their gradual increase in numbers to glacial maximum. Even if the rate of sedimentation were very slow this would be unlikely to produce a sharp contact between bedded sediments which contain no ice-rafted stones and the overlying mixtite which is unbedded and tens of metres thick. If large numbers of icebergs were suddenly to enter a sea area a mixtite with a sharp lower contact might be laid down. Although this would explain the sudden arrival and deposition of mixtite material it does not account for the complete cessation of interbed sedimentation-i.e, the relative absence of bedded horizons within the mixtite bed. Secondly, there is considerable evidence that much of the succession was either deposited just beneath sea-level--and the water was perhaps too shallow for icebergs to float--or that the sediments were even emergent above sea-level (section 5. C, The depth of water during the deposition of the formation). Thirdly, it is difficult to see how the Great Breccia could form by ice rafting. Icebergs can carry huge blocks but it would be remarkable if they melted to produce a deposit in which such large rafts of material occurred so closely concentrated (see P1. 11). A grounded ice-sheet can certainly produce such a deposit, e.g. the Pleistocene tills of north Norfolk (section 5. C). Fourthly, iceberg rafting of stones can be recognized at several distinct horizons within the formation, where outsize stones are present in thinly laminated sediments (section 3. B (v)). These lithologies are easily distinguished from the unbedded mixtites of the formation, however, for they do not grade laterally or 68

Mem. geoL Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

vertically into unbedded mixtites. Thus at the few horizons at which ice rafting can be demonstrated there is no apparent connection with the unbedded mixtites. One certain criterion--the presence of till fabric--which might be used to discriminate between tills deposited by grounded ice (giving an horizontal preferred pebble orientation) and by ice rafting (giving a radial pebble orientation in the horizontal plane) (Fig. 11) cannot be used conclusively. There is now only a slight difference between the fabrics of the mixtites and of those horizons where ice rafting is suspected; any major difference has been obliterated by tectonic deformation (section 3. A (v)). Even in the absence of conclusive till fabric evidence, the arguments listed above indicate an origin for the majority of the individual mixtites by deposition from grounded ice-sheets lying at around sea-level. (iv) ARE OTHER THICK, CONFORMABLE TILLITE SEQUENCES ALSO OF GROUNDED ICE-SHEET ORIGIN 9. The late Pre-Cambrian tillites of East Greenland (Schaub 1950), Spitsbergen (Kulling 1934), the Congo (Schermerhorn & Stanton 1963), South West Africa (Martin 1965) and Australia (for example, Mawson 1949)--to mention only a few--greatly resemble the Port Askaig Tillite. If these tillites were also deposited by grounded ice-sheets, rather than icebergs, enormous palaeoclimatic implications result. Harland (1964A, 8) has summarized the evidence for world-wide glaciation in the late Pre-Cambrian. In addition, palaeomagnetic data suggests that many of the glaciated areas may have been close to the equator (Girdler 1964). If it were demonstrated that other low latitude tillites were deposits from grounded ice-sheets, the implication would be that ice-sheets encompassed all the land areas of the earth.

6. T H E G L A C I A L

RECORD

OF T H E P O R T A S K A I G

TILLITE

The Port Askaig Tillite totals as much as 750m in thickness and contains as many as 47 mixtite beds separated by sandstone, siltstone, dolomite and conglomerate interbeds, which range from a few centimetres to over 100m in thickness. In the previous section it was concluded that the mixtites were deposited from grounded ice-sheets resting on flat surfaces at around sea-level. In section 3. A (ii) it was shown that certain of the bedded horizons in the formation actually lie within mixtites and are discontinuous laterally; they formed within melting ice-sheets. Many of the interbeds, however, because of their lithology (e.g. dolomites), sedimentary structures and palaeocurrents and their lateral extent, must have been deposited in an extensive (? marine) sub-aqueous environment. During their deposition even icebergs were absent from the Argyll area, for rafted stones occur in few interbeds. At other times the area was not only ice free, but also emergent (sections 3. B (ii) and 3. C (i)). The succession therefore records a sequence of glacial and non-glacial conditions. Discounting those interbeds formed within ice-sheets, the total number of glacial advances and meltings in the Argyll area was at least 17 (Fig. 29). (The sequence now preserved seems unlikely to be absolutely complete; erosion surfaces have been shown to cut out small parts of the succession.) In detail, the sequence--(base) interbed, mixtite, sandstone wedges, erosion surface, (beach) conglomerate, interbed (top)--seems to be the fundamental unit from which the formation is built. The suggested cycle of environments represented by this unit is shown in Fig. 30. The special conditions which it is suggested produced this cycle are (i) a very flat topography, (ii) lying at around sea level, (iii) continual subsidence and (iv) alternate advance and melting of grounded ice-sheets. The time spans involved in the deposition of the whole formation, of individual mixtites and of interbeds are unknown. The formation is thick by Pleistocene standards. Charlesworth (1957, p. 222) gave examples of exceptionally thick Pleistocene drift sequences; few of those from the North German Plain (containing, presumably, grounded ice deposits) exceed 300m. The interbeds may represent only local retreats of the ice (interstadials) or de-glaciations (interglacials). Their thicknesses cannot be used to estimate their duration, for the complicated sequences present in some thin interbeds (e.g. between mixtite 30 and the base of member 3 Mem. geol. Soc. Lond. no. 6

69

A. M. S P E N C E R

Ice present UGrounded ice sheet

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FIG. 29. The glacial record of the Port Askaig Tillite. Left column: the Garvellachs succession. Right column: members 4 and 5, Port Askaig (P1. 10, G and H). 70

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

in the Garvellachs) suggest quite long time spans. The most useful time indicators are, perhaps, the sandstone wedges. Berg & Black (1966) have measured the rate of increase in width of modern wedges in Antarctica as from 0.5 to 3.8mm per year. If the much smaller wedges described here grew at similar rates, time spans of the order of hundreds of years are implied for the development of many of the horizons of wedges. It is suggested then, that the Port Askaig Tillite because it records 17 successive ice advances, as many as 27 periods of periglacial conditions (sandstone wedges) and is considerably thicker than analogous Pleistocene sequences represents a glacial period of comparable, or even greater magnitude than that of the Pleistocene Ice Age. GENERALIZED

]

ROCK SUCCESSION

DETAILED

INTERPRETATION

INTERPRETATION (as in Fig. 29)

INTERBED (e. g.

cross-bedded sandstones)

D e p o s i t e d in shallow ( ? m a r i n e ) water. I c e b e r g s u s u a l l y absent

SPECULATION on the c a u s e s of m o v e m e n t s of s e a - l e v e l with r e s p e c t to the surface of the s e d i m e n t s

Beneath s e a - l e v e l

/\ Eustatic r i s e of s e a - l e v e : g r e a t e r than i s o s t a t i c r i s e of land

Marine t r a n s g r e s s i o n p r o d u c e s e r o s i o n surface with lag conglomerate and beach g r a v e l . P e r m a f r o s t condi!ions

Till deposited by the melting of a grounded i c e sheet

Above s e a - l e v e l

E u s t a t i c fall of s e a - l e v e l g r e a t e r than i s o s t a t i c d e p r e s s i o n of land

Surface over which a grounded(q)

Approx. to 10m

scale

Underlying strata r a r e l y folded

ice sheet advanced

INTERBED (e.g. cross-bedded

D e p o s i t e d in shallow ( ? m a r i n e ) water. I c e b e r g s u s u a l l y absent

sandstones)

Beneath s e a - l e v e l

i

FIo. 30. The hypothetical glacial advance-retreat cycle characteristic of the Port Askaig Tillite.

7. D E S C R I P T I O N

OF S T R A T A

(A) I N T R O D U C T I O N This section contains detailed descriptions of the successions in the Port Askaig Formation, and certain relevant information on the over- and underlying formations, at four main outcrops--the Garvellachs, the Port Askaig area, the Mull of Oa and Fanad, Co. Donegal (Fig. 1). A brief outline of the successions at Schichallion, at Braemar and in Banffshire are also given. Several outcrops on Islay (e.g. around Tallant Farm [336 587]; on the northern slope of Beinn Bhan [400 574]; around Ballivicar Farm [342 469] are not described; they yield very little stratigraphical or sedimentological information. Each of the successions is described, bed by bed, from base to top. In a few cases the origin of features of particular interest is also described. An outline of the structure and metamorphism of the rocks at the various outcrops has been given previously (Table 3); details are presented in the introduction to each section. Mem. geol. Soc. Lond. no. 6

71

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

in the Garvellachs) suggest quite long time spans. The most useful time indicators are, perhaps, the sandstone wedges. Berg & Black (1966) have measured the rate of increase in width of modern wedges in Antarctica as from 0.5 to 3.8mm per year. If the much smaller wedges described here grew at similar rates, time spans of the order of hundreds of years are implied for the development of many of the horizons of wedges. It is suggested then, that the Port Askaig Tillite because it records 17 successive ice advances, as many as 27 periods of periglacial conditions (sandstone wedges) and is considerably thicker than analogous Pleistocene sequences represents a glacial period of comparable, or even greater magnitude than that of the Pleistocene Ice Age. GENERALIZED

]

ROCK SUCCESSION

DETAILED

INTERPRETATION

INTERPRETATION (as in Fig. 29)

INTERBED (e. g.

cross-bedded sandstones)

D e p o s i t e d in shallow ( ? m a r i n e ) water. I c e b e r g s u s u a l l y absent

SPECULATION on the c a u s e s of m o v e m e n t s of s e a - l e v e l with r e s p e c t to the surface of the s e d i m e n t s

Beneath s e a - l e v e l

/\ Eustatic r i s e of s e a - l e v e : g r e a t e r than i s o s t a t i c r i s e of land

Marine t r a n s g r e s s i o n p r o d u c e s e r o s i o n surface with lag conglomerate and beach g r a v e l . P e r m a f r o s t condi!ions

Till deposited by the melting of a grounded i c e sheet

Above s e a - l e v e l

E u s t a t i c fall of s e a - l e v e l g r e a t e r than i s o s t a t i c d e p r e s s i o n of land

Surface over which a grounded(q)

Approx. to 10m

scale

Underlying strata r a r e l y folded

ice sheet advanced

INTERBED (e.g. cross-bedded

D e p o s i t e d in shallow ( ? m a r i n e ) water. I c e b e r g s u s u a l l y absent

sandstones)

Beneath s e a - l e v e l

i

FIo. 30. The hypothetical glacial advance-retreat cycle characteristic of the Port Askaig Tillite.

7. D E S C R I P T I O N

OF S T R A T A

(A) I N T R O D U C T I O N This section contains detailed descriptions of the successions in the Port Askaig Formation, and certain relevant information on the over- and underlying formations, at four main outcrops--the Garvellachs, the Port Askaig area, the Mull of Oa and Fanad, Co. Donegal (Fig. 1). A brief outline of the successions at Schichallion, at Braemar and in Banffshire are also given. Several outcrops on Islay (e.g. around Tallant Farm [336 587]; on the northern slope of Beinn Bhan [400 574]; around Ballivicar Farm [342 469] are not described; they yield very little stratigraphical or sedimentological information. Each of the successions is described, bed by bed, from base to top. In a few cases the origin of features of particular interest is also described. An outline of the structure and metamorphism of the rocks at the various outcrops has been given previously (Table 3); details are presented in the introduction to each section. Mem. geol. Soc. Lond. no. 6

71

A. M. S P E N C E R

(B) T H E G A R V E L L A C H S The Garvellachs are a chain of four small islands and numerous skerries, whose Gaelic name means 'the Isles of the Sea'. The 25ft late-Pleistocene raised beach which surrounds most of the islands provides beautiful exposures of the rocks and although the strata are offset by NNw-trending faults which lie in the sea between the islands, it is possible to trace the same succession through all the islands (P1. 11). Lateral variations in the sequence and thicknesses of the beds are shown on a horizontal succession chart (P1. 1 la). Structure: The beds all dip towards the south and south-east at a very uniform angle of around 35 ~ They are unfolded and show one main cleavage, best seen in the mixtites, which dips towards the south-east at angles of around 65 ~ (Fig. 12). A later cleavage, only seen in pelites beneath mixtite 1, dips at low angles towards the north-west and a third structure, a series of kink bands trending N-S affects the main cleavage along the south coast of Garbh Eileach. Deformation associated with the development of the main cleavage has affected many of the sedimentary structures; the pebble fabric in the mixtites is tectonic (section 3. A (v)). Most sandstone wedges dip parallel to the main cleavage and have, presumably, been rotated from an orientation originally perpendicular to the bedding and the outlines of certain stromatolites suggest deformation in the main cleavage, as does the oblique dip of the polygonal cracks in banded limestones (P1. 7d). (i) THE ISLAY LIMESTONE This outcrops at the north-east corner of Garbh Eileach, where 72m of thin-bedded limestones, dolomites and pelites are exposed between sea level and the base of the lowest mixtite. The succession shows very little change along the strike and can be divided into a lower calcareous part, which is dominantly fine grained and

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Mem. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND contains three horizons of calcareous algal stromatolites, and an upper part composed of dolomites, dolomitic pelites and sandstones (Fig. 31). The lower calcareous member (36m +). The calcareous algal stromatolites are ovoid structures commonly measuring 2m in length, by 1 m in width, by 30cm in height, although the largest structure seen measures 12m in outcrop length and the smallest is only 15cm in diameter. The lowest of the three horizons is the best exposed and contains 25 such stromatolites irregularly spaced along an outcrop 250m in length; 12 stromatolites occur at the second, and only one at the upper horizon. The contact of the stromatolites with the surrounding bedded sediments is sharp (Fig. 32b), and the latter overlap the upper sides of the stromatolites. The stromatolites from the lower horizon have a relatively simple, concentric, internal lamination structure (P1. 7a, c), in which the layers range in thickness from 0.5 to 3mm each and consist of alternating bands, often ill defined, of coarser (0.3 mm) and finer (less than 0.1 mm) grained carbonate mosaic. Staining (Dickson 1965) has shown that the grey-weathering, coarser grained layers, which commonly have ragged quartz grains (up to 0.3 mm) present in amounts up to 20 per cent, are of calcite and the yellow-weathering, finer grained layers are of dolomite. Stromatolites from the two higher horizons have a much less regular concentric structure. In some of the ovoid stromatolites more than one centre of concentric lamination is present, whilst in others minor up-domings are superimposed on the general concentric structure (Fig. 32a). In certain cases the up-domings affect continuous layers and in others the laminae are discontinuous, so that the up-domings are separated by channels or pits which are often filled with flake debris (Fig. 32a). Logan, Rezak & Ginsburg (1964, p. 82) stated that, 'modern stromatolitic sediments and structures are features of the intertidal or near intertidal zones'. That this is also the case here is shown by the presence of pockets of flake breccia within the stromatolite masses. Prolonged desiccation is needed in order to dry out and flake the mucilaginous algal sheath which surrounds modern stromatolites. So this association of laminated algal stromatolites with flake breccias of their own debris, now sealed up within the stromatolites, points to the existence of inter- and supra-tidal conditions at three times during the deposition of this member. Persistent limestone flake breccia beds, of 0.5 to 1.5m in thickness, occur beneath the lower and middle stromatolite horizons (Fig. 31 ; P1.7b), whilst the upper stromatolite horizon consists almost entirely of such flake breccias. In the two lower breccias grey limestone flakes occur very tightly packed together in a yellowweathering dolomitic siltstone matrix. The flakes are planar and undeformed and commonly measure 10mm by 1mm in cross-section. The upper breccia contains, in addition to fragments of grey limestone, flakes of red-weathering hematitic limestone which lie closely spaced through a grey limestone matrix. Certain of these hematitic flakes appear to have been bent prior to their incorporation in the breccia. The flakes in these breccia beds, like those within the stromatolites, are interpreted as having been produced by desiccation and concentrated by wind or water action. Few mudcracks have been seen, however. Regular banding, due to both compositional and grain size differences, is extremely well developed at certain horizons; beds 3, 4, 9, 11 and 17 show sharply bounded alternating limestone and dolomite bands (Fig. 31). In bed 3, for example, bands of red-weathering dolomite alternate with white limestone, each averaging 1 cm in thickness. In bed 4 thin dolomite laminae occur regularly at 1 cm intervals in a limestone. The most regular banding is in beds 9 and 11, where blue limestones alternate with yellow-weathering, slightly pyritic dolomites (P1. 7d). A pair of bands averages 0.7cm in thickness and varies from 2cm to 0-5cm. Cracks, filled with yellow-weathering dolomite, penetrate down from the base of the underlying limestone band. In plan, the cracks are arranged in polygons with diameters of 1 to 5 cm (P1. 7e), whilst in cross-section they maintain a constant width of under 0.5mm (P1. 7d). Almost all the cracks link up from one dolomite band, through the underlying blue limestone, to the dolomite band beneath but none have been seen to penetrate through this and into a lower blue limestone band. Except for the lowest 50cm, these cracks are present in all 600 bands in bed 11. Graded bedding occurs in the top of bed 9 and in bed 17 (the grain size of the carbonate mosaics decreases from 0.1 mm to less than 0-01 mm and 0.05mm respectively, from base to top of each band) but is far less regular than in the varved siltstones above mixtite 32. Mem. geoL Soc. Lond. no. 6

73

A.

M.

SPENCER

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Mem. geoL Soc. Lond. n o . 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND The origin of the cracking is problematical. In the case of the polygonal cracks a penecontemporaneous origin cannot be proved but it is difficult to suggest a plausible post-depositional mechanism. Sub-aerial desiccation is unlikely as an explanation for there are no associated flake breccias and the cracks never penetrate more than one limestone layer. White (1961) has produced sub-aqueous contraction cracks with similar characters, including uniform width downwards and lack of curling at the surface, by varying the salinity of a liquid overlying soft sediments. The banded and cracked limestone described here may perhaps have formed in a similar way. The upper dolomitic member (36m). This member consists of clastic sediments, including dolomitic sandstones, pebbly dolomites and mudflake breccia and pebble bands, although towards its top several yellow dolomite bands occur. In the pelite at the base of the member well-preserved mudcracks, with associated mudflake breccias, occur at several horizons. The mudflakes measure up to 36mm by 2mm in cross-section. There are several planar bedded gritty dolomites, which are very similar to mixtites 1-12 in lithology. The yellow dolomites are laminated on a 1mm scale and sometimes have a quartzose banding recurring every 10cm; they may be of algal stromatolitic origin. No distinctive stromatolitic growth forms have been seen, but both the lamination and banding are gently undulatory. The dolomitic sandstones, dolomites and pebbly dolomites of bed 47 form a continuous sheet except on the north coast of Garbh Eileach (Fig. 33), where the bed is represented by an horizon of ovoid masses, containing the same succession of lithologies. These are not channel infill structures and may have formed by the sediment trapping capacities of discontinuous algal sheets.

(ii) THE PORTASKAIGTILLITE (i) M e m b e r 1: mixtites 1 to 18 (188 to 241m) M i x t i t e s 1-12 (100m). These have a very dolomitic matrix and contain stones which are almost exclusively intra-basinal dolomites (P1. 3c); exotic stones only appear commonly in mixtite 12. Mixtites 1 and 2 are exposed in only three outcrops. On Dun Chonnuil and on the east coast of Garbh Eileach, mixtite 1 has an apparently conformable base and is conformably overlain by bedded dolomites and dolomitic siltstones, above which lies mixtite 2. On the north coast of Garbh Eileach (Fig. 33), mixtite 1 has only a thin, lower representative, which in one place has an erosional contact with the underlying bedded sediments. This mixtite thins to zero towards the west and is replaced laterally by dolomites and bedded dolomitic sandstones. In addition, the upper part of mixtite 1 is represented here by a group of unbedded dolomitic siltstones, which contain a single exotic cobble (Fig. 33) and which are in turn overlain by the bedded siltstones beneath mixtite 2. Mixtites 1-12 can be distinguished on the east coast of Garbh Eileach using the thickest and most continuous of the bedded horizons present there (Kilburn et al. 1965, p. 349). Elsewhere in the Garvellachs (P1. 11) many of these horizons are absent and the superimposed mixtites merge inseparably. Examples are the absence of bedded sediments separating mixtites 3/2, 6/5, 7/6, 8/7 on the north coast of Garbh Eileach and the absence, in the east of Dun Chonnuil, of the persistent siltstones which elsewhere separate mixtites 9/8. The only bedded horizons which are continuous along their whole outcrops are (outcrop lengths in brackets), the dolomitic siltstones between mixtites 2/1 (2.3km), the dolomitic siltstones and conglomerates between mixtites 4/3, and 5/4 (2.5km) and the thin siltstones and dolomite conglomerates between mixtites 10/9 and 11/10 (5km). Mixtites 1-12 vary in their degree of internal homogeneity. Mixtite 2 is almost completely homogeneous and lacking in internal bedded horizons whilst, at the other extreme, parts of mixtite 11 on Eileach an Naoimh contain a great number of siltstone laminae (Fig. 4c; P1. 11). These bedded horizons and those between the mixtites are most commonly either dolomitic siltstones or dolomite conglomerates; dolomitic sandstones are less common and fine-grained dolomites are present only between mixtites 2/1 and within mixtite 12. These bedded horizons are well sorted by comparison with the mixtites, for material of pebble grade or larger is absent from the siltstones and sandstones. The fragments in the dolomite conglomerates Mem. geol. Soc. Lond. no. 6

75

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND The origin of the cracking is problematical. In the case of the polygonal cracks a penecontemporaneous origin cannot be proved but it is difficult to suggest a plausible post-depositional mechanism. Sub-aerial desiccation is unlikely as an explanation for there are no associated flake breccias and the cracks never penetrate more than one limestone layer. White (1961) has produced sub-aqueous contraction cracks with similar characters, including uniform width downwards and lack of curling at the surface, by varying the salinity of a liquid overlying soft sediments. The banded and cracked limestone described here may perhaps have formed in a similar way. The upper dolomitic member (36m). This member consists of clastic sediments, including dolomitic sandstones, pebbly dolomites and mudflake breccia and pebble bands, although towards its top several yellow dolomite bands occur. In the pelite at the base of the member well-preserved mudcracks, with associated mudflake breccias, occur at several horizons. The mudflakes measure up to 36mm by 2mm in cross-section. There are several planar bedded gritty dolomites, which are very similar to mixtites 1-12 in lithology. The yellow dolomites are laminated on a 1mm scale and sometimes have a quartzose banding recurring every 10cm; they may be of algal stromatolitic origin. No distinctive stromatolitic growth forms have been seen, but both the lamination and banding are gently undulatory. The dolomitic sandstones, dolomites and pebbly dolomites of bed 47 form a continuous sheet except on the north coast of Garbh Eileach (Fig. 33), where the bed is represented by an horizon of ovoid masses, containing the same succession of lithologies. These are not channel infill structures and may have formed by the sediment trapping capacities of discontinuous algal sheets.

(ii) THE PORTASKAIGTILLITE (i) M e m b e r 1: mixtites 1 to 18 (188 to 241m) M i x t i t e s 1-12 (100m). These have a very dolomitic matrix and contain stones which are almost exclusively intra-basinal dolomites (P1. 3c); exotic stones only appear commonly in mixtite 12. Mixtites 1 and 2 are exposed in only three outcrops. On Dun Chonnuil and on the east coast of Garbh Eileach, mixtite 1 has an apparently conformable base and is conformably overlain by bedded dolomites and dolomitic siltstones, above which lies mixtite 2. On the north coast of Garbh Eileach (Fig. 33), mixtite 1 has only a thin, lower representative, which in one place has an erosional contact with the underlying bedded sediments. This mixtite thins to zero towards the west and is replaced laterally by dolomites and bedded dolomitic sandstones. In addition, the upper part of mixtite 1 is represented here by a group of unbedded dolomitic siltstones, which contain a single exotic cobble (Fig. 33) and which are in turn overlain by the bedded siltstones beneath mixtite 2. Mixtites 1-12 can be distinguished on the east coast of Garbh Eileach using the thickest and most continuous of the bedded horizons present there (Kilburn et al. 1965, p. 349). Elsewhere in the Garvellachs (P1. 11) many of these horizons are absent and the superimposed mixtites merge inseparably. Examples are the absence of bedded sediments separating mixtites 3/2, 6/5, 7/6, 8/7 on the north coast of Garbh Eileach and the absence, in the east of Dun Chonnuil, of the persistent siltstones which elsewhere separate mixtites 9/8. The only bedded horizons which are continuous along their whole outcrops are (outcrop lengths in brackets), the dolomitic siltstones between mixtites 2/1 (2.3km), the dolomitic siltstones and conglomerates between mixtites 4/3, and 5/4 (2.5km) and the thin siltstones and dolomite conglomerates between mixtites 10/9 and 11/10 (5km). Mixtites 1-12 vary in their degree of internal homogeneity. Mixtite 2 is almost completely homogeneous and lacking in internal bedded horizons whilst, at the other extreme, parts of mixtite 11 on Eileach an Naoimh contain a great number of siltstone laminae (Fig. 4c; P1. 11). These bedded horizons and those between the mixtites are most commonly either dolomitic siltstones or dolomite conglomerates; dolomitic sandstones are less common and fine-grained dolomites are present only between mixtites 2/1 and within mixtite 12. These bedded horizons are well sorted by comparison with the mixtites, for material of pebble grade or larger is absent from the siltstones and sandstones. The fragments in the dolomite conglomerates Mem. geol. Soc. Lond. no. 6

75

A. M. SPENCER are predominantly of pebble grade, but material of sand and cobble grade is also present. The pebbles are sub-rounded to rounded and are of a uniform cream dolomite lithology. Cross-stratification is present in most of the thicker horizons (Fig. 13) and ripple drift bedding occurs in the siltstones between mixtites 8/9. The Great Breccia (about 40m). The Great Breccia is the thickest mixtite bed in the Garvellachs and contains the largest fragments seen in any mixtite, the largest of which measures 320 • 64 x 45m. At the base of the bed is a tripartite bedded horizon, measuring up to 11 m in thickness, which contains sandstones, bedded dolomite conglomerates and a central thin mixtite. The three divisions within this horizon are not continuous along the whole of their outcrop (P1. 11). The central mixtite is represented by lenses within conglomerates on the east coast of Garbh Eileach, and the upper conglomerate is arranged in lenses on the north coast of Garbh Eileach and thins to zero laterally in the south-western outcrop on Eileach an Naoimh. The conglomerates contain subangular to sub-rounded dolomite fragments of pebble to small cobble grade, are well bedded and lack cross-stratification. The Great Breccia is remarkable for the size of the sedimentary fragments it contains, which are commonly up to tens of metres in diameter. For convenience, the fragments will be described in two groups, those measuring 1 to 20m in diameter and those greater than 20m in diameter. All fragments of 1 to 20m in diameter, numbering 68 in all, exposed in a representative area of approximately 2000m 2 on the north coast of A'Chuli have been noted. In order of abundance the lithologies are: dolomite (32 fragments), dolomitic mixtites (17), siltstone (6), sandstone (4), interbedded siltstones and sandstones (4), interbedded siltstones and dolomites (4), dolomite conglomerate (1). The fragments of dolomitic mixtite are identical in lithology to mixtites 1-12. There are at least 13 rafts exposed along the whole outcrop of the Great Breccia which have one diameter greater than 20m (P1. 11). They fall into three lithological groups: mixtite (8) dolomite (7) and interbedded lithologies (2). The mixtite rafts are dolomitic, free of exotic stones and are mostly quite thick and homogeneous, i.e. like the smaller fragments, they are very similar in lithology to mixtites 1-12. None of the successions within these rafts can be accurately matched, however, with any part of mixtites 1-12 of the Garvellachs. The largest of the dolomite rafts contains alternating bands of dolomitic siltstones and thick white dolomites and within the raft the sequence lies in a recumbent fold which closes towards the northwest (P1. 1). The best exposed raft, that seen on the western coast of A'Chuli (P1. 11), shows a succession complicated by folds and thrusts but comprising interbedded dolomites, dolomitic siltstones and sandstones overlain by a dolomitic mixtite. The majority of these large rafts are elongated parallel to the base of the Breccia and in one case (on the north-east coast of Eileach an Naoimh) the raft has the appearance of resting conformably on the underlying bedded sediments. This suite of huge erratics must have been derived from a succession similar to that now seen in the Garvellachs, i.e. composed of a member containing beds of dolomite and dolomitic siltstone overlain by a member containing thick, homogeneous dolomitic mixtites; the contact between these two members may be that seen in the raft exposed on A'Chuli. At the top of the Great Breccia is a distinctive dolomite 'conglomerate' which contains closely spaced rounded dolomite boulders, which are commonly up to 1 m in diameter, and lie, without touching, in a gritty dolomitic siltstone matrix. What little evidence there is seems to show that the fragments were not soft at the time of their deposition. None of them appear to push into their neighbours and the sedimentary banding present within several fragments is undeformed and sharply truncated at their surfaces. The conglomerate has a gradational base and may be regarded as a level within the Great Breccia at which dolomite fragments of a particular size and lithology are concentrated. The top of the breccia often appears to be gradational with the overlying dolomite, but this is misleading. In the south-west of Eileach an Naoimh the two are separated by a dolomitic siltstone bed and in the centre of Garbh Eileach the dolomite rests disconformably across the bedding within one of the rafts and the conglomerate is absent (P1. 11). An erosion surface must in fact separate the two. In section 5. C (The size of material which can be carried by a mudflow) it is concluded that the Great 76

Mere. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND Breccia is a normal till, analogous to the Pleistocene tills of north Norfolk (Reid 1882). The origin of the upper dolomite 'conglomerate' is more puzzling. The mechanism by which rounded dolomite boulders have become concentrated in a siltstone matrix is problematical for, because of its fine-grained matrix, the conglomerate cannot be explained as a beach deposit. Nor can it have been produced in a manner analogous to pseudonodules--by the load-casting of the base of the overlying dolomite--for at various places along the outcrop the conglomerate and dolomite are separated by a thin mixtite. The conglomerate has a gradational base and contains some large rafts of sediment, both of which suggest that it should have the same origin as the Great Breccia. Even so, the origin of the conglomerate remains a problem; the author knows of no comparable glacial (or other) deposit. The lower dolomite (11 to 26 m). This dolomite is of a cream- to yellow-weathering, fine-grained lithology, usually with an absence of well marked bedding planes. It often appears homogeneous and even on A'Chuli, where subangular to rounded dolomite fragments of 1 mm to 2cm size are present in about half of the bed, stratification is only poorly developed. Elsewhere the dolomite contains some fine white laminae and quartzose laminae, both lying parallel to the bedding and spaced at 0.5 to 1cm intervals. No algal stromatolites or oolites have been seen, despite a careful search. Most significant of all, no fragments of larger than 2cm or of extra-basinal lithology have been seen despite the excellent outcrops of the dolomite. The Disrupted Beds (29 to 40m). These are composed of dark-blue siltstones with dolomite stones, and of dolomite conglomerates and dolomites, all of which contain scattered exotic stones. It is possible to divide the Disrupted Beds into five units (Fig. 34), in two of which (beds 2 and 4) the blue siltstones contain semicontinuous dolomite bands. Kilburn et al. (1965) interpreted these dolomites as having been pulled apart and hence named the division the Disrupted Beds. Bed 1 (Fig. 34), consists of cream-coloured dolomite conglomerates, containing tightly packed, subrounded to rounded pebbles, which average 3 cm in diameter and are identical in lithology to the underlying dolomite. A few pebbles of red sandstone are also present, whilst exotic stones, commonly of around 10cm diameter, occur sparsely (several per 100m ~ of outcrop). Large boulders, both of dolomite and of exotic material, are not uncommon (e.g. an exotic measuring 86 x 56 x 2 0 c m at (118 069}1; a dolomite boulder measuring 100 x 90 x 90cm at (601 033}). Large-scale cross-stratification occurs within the conglomerate at several localitites (Fig. 13) and the largest unit seen has a ripple height of 5.5m. The upper part of bed 1 contains dolomite conglomerates with a black sandy siltstone matrix and dark, laminated sandy siltstones. Some of the latter are ripple-marked and others contain dolomite cobbles (Fig. 19f). A distinctive ore band (probably hematite), which lies parallel to the bedding and is up to 15cm thick, occurs around the top of this bed and a second band occurs in the upper part of the bed in two localities (Fig. 34). Bed 2 is composed of dolomite and dolomite conglomerate bands, both up to 2m in thickness, lying in unbedded grey-blue siltstones which contain abundant dolomite stones. None of the dolomite bands is continuous laterally for more than 100m or so and many are very discontinuous and irregular in thickness (Fig. 35). Although many of the dolomite bands are gently undulatory, very few are sharply folded and the majority lie nearly parallel to the over-all bedding. A few well-stratified siltstones and sandstones are present within this otherwise poorly stratified bed. The dolomite pebbles in the blue siltstones usually have smooth rounded surfaces and are angular to sub-rounded in shape. They appear to have been rigid at the time of deposition for where pebbles are in contact they are not deformed and bedding present within them is sharply truncated at their surfaces. Pebbles of dolomite conglomerate and pebbly dolomite are also present. Bed 3 is usually well stratified and consists of bedded siltstones, conglomerates and conglomeratic mixtites, all of which have a dark-coloured matrix. The siltstones are planar laminated and often contain rafted pebbles (Fig. 19g; P1. 5c). In the east of A'Chuli, for example, siltstones with laminae of 1 to 3cm in thickness commonly contain dolomite pebbles of up to 6cm in diameter (Fig. 19d). On the north coast of

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Mem. geol. Soe. Lond.

no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND Garbh Eileach, in a distance of 40m, bedded sandstone grades laterally through bedded dolomite conglomerates into dark unbedded siltstones with dolomite boulders; a dolomite boulder measuring 2.3 • 1.8 • 1-0m occurs in the centre of the bedded conglomerates (Fig. 36). ,ler .............

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FIG. 35. Field sketches or irregular dolomite bands (black) in members 2 and 4 of the Disrupted Beds. Bed 4 is similar to bed 2 but is thinner and contains a less well-developed series of disrupted white dolomites than the latter. Bed 5 contains red-brown to yellow-coloured dolomites, dolomite conglomerates and sandstones, all of which are bedded and contain no exotic stones.

FIG. 36. Field sketch of relationships within bed 3 of the Disrupted Beds on the north-west coast of Garbh Eileach.

Origin of the Disrupted Beds. There are many horizons of planar-bedded, we1I-sorted sediment within the Disrupted Beds and certain of these contain undisturbed cross-stratification and ripple marks. The Disrupted Beds are unique, however, because of beds 2 and 4. The dolomite bands in these pebbly, blue siltstones, although they mostly lie parallel to the general bedding, appear to have been disturbed since their deposition. They are irregular in thickness, show penecontemporaneous folds and end abruptly as though they had been disrupted or pulled apart (PI. 6a). (The Disrupted Beds also occur on Islay (P1. 6b) but the dolomites within blue siltstones there are not discontinuous and do not appear to have suffered disruption.) The evidence of disruption having taken place is the fact that all the dolomite bands are discontinuous. There is every gradation between continuous dolomite bands, dolomite conglomerates and dolomite pebbles and boulders in blue siltstones, although few ragged ends of dolomite bands which fit into the ends of adjacent bands can be seen. The dolomites are unlikely to have been deposited in discontinuous beds as concretions. There are no concentric structures within the dolomites and the dolomite/siltstone contacts are always sharp, not gradational. The disrupted structures are unconnected with the Caledonian deformation and must pre-date the period of sandstone dykes intrusion, for in their outcrops just west of Bealach an Tarabairt the Disrupted Beds are cross-cut by planar sandstone dykes which are cleaved. The most likely Mem. geol. Soc. Lond. no. 6

79

A. M . S P E N C E R

mechanism seems to be that of slight lateral mass movement of a few tens or hundreds of metres resulting in the disruption of the bands by tensional stresses. Lateral mass movement of this type must have occurred on two occasions, for beds 2 and 4 are separated by undisturbed bedded sediments. Jarring due to earthquake shocks may have triggered the lateral mass movement, but cannot itself have produced the discontinuous structure, for many of the dolomites end laterally and are absent completely or for many tens of metres, yet all lie approximately parallel to the bedding. Disturbance by grounding icebergs cannot account for discontinuous dolomite bands lying parallel to the general bedding direction. The Disrupted Beds show considerable evidence of the action of floating ice. Banded siltstones with rafted pebbles and cobbles occur in parts of beds 1 and 3 as do large boulders in conglomerates; the outsize stones in both lithologies have probably been rafted by icebergs (section 3. B (v)). Certain features of the Disrupted Beds, their dark siltstone and sandstone matrix, the presence of rafted stones, of carbonate beds and of ore bands and disseminated ore in blue siltstones, also occur in parts of the Permian System of Tasmania. Carey & Ahmed (1961, p. 887) interpreted the latter as deposited beneath a dry-base ice shelf. M i x t i t e s 14-18 (3.5 to 24m). These mixtites have a dolomitic siltstone matrix similar to that of mixtites 1-12, but unlike the latter as many as 25 per cent of the stones are exotics. The base of mixtite 14 rests with a sharp contact, often marked by a thin pebble bed, on the underlying dolomite. Mixtites 14 and 15 have gritty dolomitic siltstone matrices and contain large numbers of stones of all sizes up to boulder grade. Mixtites 16-18 have a finer grained dolomitic siltstone matrix and contain much smaller numbers of stones, the diameter of which is usually less than 5cm. The mixtites have sharp contacts with the bedded sediments between them and the latter do not contain exotic stones and show normal current structures. Sandstone wedges penetrate downwards into mixtites 15 (Figs. 20a, 21d) and 18 on the east coast of Garbh Eileach and, together with the lithological differences between mixtites 14-15 and 16-18, they make it possible to distinguish the two groups of mixtites throughout the Garvellachs. Mixtites 14 and 15 are continuous along the whole outcrop, although they do vary considerably in thickness. By comparison mixtites 16-18 are much less continuous, vary even more in thickness and are absent in one outcrop. Much of this variation in thickness is probably due to erosion prior to the deposition of the overlying dolomite, for mixtites 18 and 17 can be seen to be cut out beneath the dolomite when traced south-westwards across Garbh Eileach (P1. 11). The upper dolomite (4.5 to 11 m). This bed undergoes a marked lateral facies change in the centre of Garbh Eileach. To the east it is a poorly bedded, cream-coloured dolomite of up to 11 m in thickness, whilst to the west it is less than 4.5m thick and contains well bedded rusty-yellow dolomites and beds of dolomite conglomerate. The fragments in the latter are identical in lithology to the surrounding dolomite and, although mostly of pebble grade, are occasionally of boulder grade. The eastern dolomite lacks current structures, whilst in the west it is current ripple bedded. In addition, a single structure which may possibly be of algal stromatolitic origin occurs in the western dolomite (Fig. 37). The dolomite is everywhere overlain by a bed of laminated green siltstones (1.5 to 6.5m thick) and, despite their excellent outcrops, exotic stones have been found in neither. (ii) M e m b e r 2, m i x t i t e s 19-32 (145m) The lithology and sedimentary structures of the mixtites and interbeds belonging to this member are described in order. Mixtites 19-22, 23-26, 27-29, and 30 fall into four natural groups and will be so described. The sandstone bed between mixtites 27 and 26, and the varved siltstone division above mixtite 30, because they contain interesting and important structures, are also described separately. M i x t i t e s 19-22 (23.5 to 42m). This unit has at its base a 2m thick bed very rich in iron ore and its top is marked by a prominent sandstone wedge horizon (Figs. 9e; 20d, f ; 21a-c). The three highest mixtites are identical in lithology and all have a very arenaceous matrix which is blue-grey when fresh. Stones, mostly of pebble grade or less, are abundant. Dolomites predominate and granite stones form only 20 per cent of the 80

Mem. geoL Soc. Lond. no. 6

LATE

PRE-CAMBRIAN

GLACIATION

IN

SCOTLAND

assemblage. Mixtite 19 is more pelitic than the others and is distinctive because it contains green fragments. These four mixtites can only be separated from each other on the east coast of Garbh Eileach. Elsewhere the thin sandstone interbeds are absent and the lithological difference of mixtite 19 cannot be seen so that the unit appears to be homogeneous. Lenticular bedded horizons within the mixtite are uncommon; only a few thin sandstone laminae have been seen, but two, dark coloured siltstone lenses, which are rich in ore, are present.

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FIG. 37. Sketch of a sawn hand specimen of laminated dolomite from the sediments between mixtites 18 and 19 at {578 039} The fold is not tectonic and is probably of algal stromatolitic origin. The area dotted is completely recrystallized. Mixtites 23-25 (15 to 27m). Mixtites 23 and 25 both have a regularly laminated matrix and in both stones occur very sparsely and are up to cobble grade only in size (the largest fragments seen in each measure 9 x 6cm and 22 x 13cm in cross-section, respectively). The lamination in these beds is, except for a few horizons with disturbed bedding, usually planar but is never so regularly developed as in the varved siltstones above mixtite 32. In mixtite 25 the laminae vary from 0.25mm to 1 cm in thickness and do not always maintain an even thickness when traced laterally. Both mixtites are rich in iron ore, containing up to 5 per cent (majority magnetite, some pyrite). Mixtite 24 is, by contrast, very similar in lithology to mixtites 20-22. Its lower and upper contacts are almost everywhere quite sharp, although in one place the laminated siltstones of mixtite 23 pass upwards by intergradation into mixtite 24 and on the east coast of Eileach an Naoimh mixtites 25/24 are separated by a bedded division which contains two horizons of sandstone wedges (Figs. 21j, 27a). M i x t i t e 26 (up to 20m). This overlies mixtite 25 with a sharp, conformable contact and is itself overlain by a pebbly sandstone horizon, which contains distinctive involution structures (Fig. 8d; 9a, c,f, h). It is very similar in lithology to mixtites 20-22, although granites form 40 per cent of the fragment suite. As many as three thin sandstone, siltstone or conglomerate lenses are present at various levels within mixtite 26; the thickest is a 2m bed of laminated siltstone seen in the east of A'Chuli. Mixtite 26 is remarkable because in the centre of A'Chuli in a distance of 400m, it thins from 21 m to zero whilst still maintaining its normal lithology (P1. 11). Part of the thinning is compensated for by increase in thickness of mixtite 25 and the appearance of a cross-stratified sandstone between mixtites 26/25. To the west of A'Chuli only two possible representatives of mixtite 26 are seen; a 2m thick, laminated mixtite occurs beneath the involuted sandstone Mem. geol. Soc. Lond. no. 6

81

A.

M.

SPENCER

horizon on Sgeir leth a'Chuain and a similar, 1.5m thick mixtite, containing no stones larger than pebble grade, outcrops in the south-west of Eileach an Naoimh. A sandstone division (9 to 15m) overlies mixtite 26. Its base is marked by the pebbly sandstone with disturbed bedding referred to above (Fig. 38). Disturbed bedding or involution structures (Kilburn et aL 1965, p. 353) occur at several levels within the sandstone but are best developed in the horizon at its base, where SGEIR LETH A'CHUAIN ! i

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they are of very similar thickness and character along a 4.5km long outcrop (Fig. 9a, c,f, h). Within the sandstone, sandy siltstones, often with ripple-drift bedding, and sandstones with planar cross-stratification predominate. Despite the extensive outcrops of this division, no extra-basinal stones have been found. Above this sandstone division on Garbh Eileach and A'Chuli rest mixtites 27-29, but on Sgeir leth a'Chuain and Eileach an Naoimh a tripartite succession, comprising a granite conglomerate, a 3m thick mixtite (number 26') and a sandstone, separates the sandstone division from mixtites 28-29. The granite conglomerate is well seen on Sgeir leth a'Chuain where it rests in a broad, erosional channel in the centre of which it is 4m thick, thinning to 50cm on the flanks. The suggested relationships of mixtite 26' can be seen in P1. 11 ; it does not appear to equate with mixtite 27 for unlike the latter it contains large numbers of stones of all sizes up to boulder grade and, when traced towards the north-east on Sgeir leth a'Chuain, it thins to zero and is replaced laterally by a granite conglomerate. M i x t i t e s 27-29 (10 to 31 m). This unit overlies the sandstone just described and is overlain by a sandstone wedge horizon (Fig. 21e). The three mixtites are not identical; mixtite 27 has a homogeneous siltstone matrix and contains stones, which are predominantly of pebble grade, in relatively small numbers. Mixtites 28 82

Mere. geol. Soc. Lond.

no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND and 29 both have an arenaceous matrix and contain stones of pebble to boulder grade in larger numbers than any of the underlying mixtites. They are also the lowest mixtites in the succession in which extrabasinal fragments outnumber dolomites. Mixtite 27 outcrops on Garbh Eileach and A'Chuli but is absent to the south-west of this (P1. 11), for on Eileach an Naoimh a mixtite with the same lithology as numbers 28-29 rests with a sharp contact on the underlying sandstone. In addition, on Sgeir leth a'Chuain mixtites 27-29 are all absent and their place is taken by a bedded sandstone (P1. 11). The thin, bedded horizons between mixtites 27, 28 and 29 are only present on the east coast of Garbh Eileach, elsewhere the three mixtites are inseparable. Mixtites 28 and 29 contain few lenticular bedded horizons except in their outcrops on Eileach an Naoimh, where siltstone and sandstone laminae are in places abundant (P1. 4b). Besides these there is at the north-east end of the A'Chuli outcrop a conglomerate patch within the mixtites which contains extra-basinal and dolomite fragments in much larger numbers than the surrounding mixtite. This conglomerate is unbedded, does not lie parallel to the base of the mixtite and has gradational margins; it is neither a lenticular bedded conglomerate nor does it appear to be a large transported fragment. A sandstone lens near the north-east end of the Eileach an Naoimh outcrop, which is very rich in iron ore, has similar characters. A third problem of similar nature is posed by certain large patches in mixtite 28-29 on Eileach an Naoimh within which the matrix is of a very dolomitic nature and dolomite stones outnumber granites. Certain of these dolomitic areas have sharp (Fig. 39a), and others gradational contacts with the surrounding mixtite. The larger patches

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FIG. 39. (a)Field sketch of more dolomitic mixtite (denser ornament) and sandstone lenses in mixtite 28-29 at {483 063}. (b) Strongly folded sedimentary fragment lying at the base of mixtite 30 at {257 055} (black: dolomite; dotted: sandstone; blank: siltstone). are almost equidimensional in cross-section, whilst other dolomitic levels are elongate parallel to the base of the mixtite (P1. 11). Yet others form complicated structures, some of which have been folded before the formation of the sandstone wedges which penetrate downwards from the top of the mixtite (P1. 11). Mixtites 28 and 29 and the sandstone wedges which penetrate them are overlain by a conglomerate which, although usually only a few centimetres thick, reaches 2m in central Garbh Eileach. On Garbh Eileach a 2m thick sandstone overlies this conglomerate, whilst to the south-west of this a bed of regularly laminated siltstones of 7 to 11 m in thickness occurs in the same position. Mixtite 30 (6 t o 11 m) is very similar in lithology to mixtites 27-29, except that it contains many fewer boulder-sized stones than the latter. Lenticular bedded horizons are present in this mixtite; a conglomerate band of up to 60cm in thickness occurs on the east coast of Garbh Eileach and Fig. 39b shows an included, strongly folded sedimentary fragment. Mem. geoL Soc. Lond.

no. 6

83

A.

M.

SPENCER

A bedded division (24 to 35m) (Fig. 40b) which contains dolomites, sandstones, siltstones, two mixtites

(numbers 31 and 32) and two horizons of varved siltstones overlies mixtite 30 with a sharp contact. Three metres of varve-like but ungraded siltstones, which contain three granite pebbles, occur just above its base on the east coast of Garbh Eileach. Within the division the sandstones are commonly cross-stratified, whilst the sandy siltstones are ripple drift bedded. Several other types of sedimentary structure are present; loading structures occur at the base of two sandstones where they overlie sandy siltstones; isolated sandstone wedges GARBH EILEACH f

EILEACH AN NAIOMH i i

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occur in similar positions at three other horizons (Fig. 40b) and the dolomite at the top of the division on Eileach an Naoimh has a very disturbed structure. Mixtites 31 and 32 have a silty sandstone matrix and contain extra-basinal stones in larger numbers than dolomites. Both beds are discontinuous laterally (P1. 11). Mixtite 31 thins to zero beneath the base of the overlying sandstone in the south-west of Garbh Eileach, whilst mixtite 32 is banked against poorly bedded sandstone (Fig. 40a) and thins from 1-5m to zero in a lateral distance of 2m. Large, penecontemporaneous sandstone downfolds occur in mixtite 31 at three localities (Fig. 8a). The two horizons of varved siltstones mentioned above are, respectively, a 4m thick bed which underlies mixtite 31 in the south-west of Garbh Eileach and a 6m thick bed which overlies the 84

Mem. geol. Soe. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

sandstone above mixtite 32in the east of Garbh Eileach. The top of the latter is penetrated by narrow sandstone wedges (Fig. 21i). The varved laminae in both maintain a very even thickness laterally, are of remarkably uniform thickness up the succession (Fig. 18) and are all beautifully graded (P1. 5b). (iii) Member 3, mixtites 33-38 (215m) The granite-rich, arenaceous mixtites of this member occur interbedded with thick white sandstones. Three groups of mixtites, 33-35, 36 and 37-38, outcrop in the Garvellachs, but the contact with the overlying member, which is seen on Islay, probably lies just above the top of the exposed succession here. For comparative purposes the sandstones are described first. The four sandstones are of 51 to 94m (lowest), 17.5m, 10.5 to 14m and 22m (highest) in thickness respectively. All have a clean, white appearance and have sharp bases. The lowest sandstone has a disconformable lower contact (P1. 11), whilst each of the three higher sandstones has a thin, discontinuous winnowed conglomerate at its base. Both the lower two sandstones are composed, throughout most of their thicknesses, of sets of cross-strata. The average set height is 3 m, but both sandstones contain one set measuring 9m in height and both show predominantly north-derived palaeocurrents (Fig.13). Cross-strata occur much less frequently in the sandstone above mixtite 36, and the sets, which indicate south derived palaeocurrents, average only 45cm in height. The sandstone above mixtite 38 is finer grained (fine sand grade) than those below and contains ripple marks and ripple-drift bedding more commonly than cross-stratification. Discontinuous dolomitic horizons, containing dolomitic sandstones and sandy dolomites, occur at two levels within the lowest sandstone. In the second sandstone near the harbour on Garbh Eileach is a 1.5m thick unbedded sandstone which contains five pebble- and cobble-sized granite stones in an outcrop 15 m in length. In the third sandstone lies a 15cm thick 'mixtite', which contains several cobble-sized granite stones in a poorly bedded silty sandstone (Fig. 17). This bed can be traced for 20m before thinning to zero. Mixtites 33-35 (about 26 m thick) are separated from the underlying white sandstone by a brown-weathering sandstone (3 to 13 m thick) which contains an horizon of involution structures, at a level just beneath the base of the mixtite, in all localities from the west of Garbh Eileach to the most south-westerly outcrop (Fig. 9i). The contact of mixtite 33 with this sandstone is usually sharp, although in some localities a poorly bedded silty sandstone containing scattered small pebbles separates the two. The sandstones which separate mixtites 33, 34 and 35 are discontinuous (P1. 11), but the mixtites are lithologically distinct. Mixtites 33 and 35 have well-cleaved argillaceous matrices, whilst 34 has a massive arenaceous matrix; mixtite 35 contains larger numbers of boulders than 33, whilst mixtite 34 is very poor in stones larger than pebble grade. In all three dolomite fragments form less than 20 per cent of the fragment suite. Discontinuous bedded sandstones occur at two other levels in mixtites 33 and 34 besides that which commonly separates the two mixtites (P1. 11). The bedded siltstones, sandstones and breccia which separate mixtites 34 and 35 vary in thickness from zero to 7.5m and show several interesting features. The lowest sandstone lies in large penecontemporaneous basins (Fig. 8b) and the breccia which overlies this sandstone includes angular fragments of mixtite which range up to large boulders in size. Around the harbour on Garbh Eileach, mixtite 35 contains polygonal sandstone wedges which are truncated by a thin conglomerate overlying the mixtite (Fig. 21f). Mixtite 36 (4-5 to 6m thick) is very similar in lithology to mixtite 35, although it contains less boulder-sized fragments than the latter. Its base is well shown on the landing jetty on Garbh Eileach; the underlying sandstone there is less pure and less well stratified than normal in its uppermost 1 m and contains, in its uppermost 50cm, twelve granite pebbles in a 20m long outcrop. The actual contact of the sandstone with the mixtite, although not planar when traced laterally, is knife sharp. Within the mixtite, two persistent bedded horizons, which appear to vary in level when traced laterally, are present (P1. 11). On the west coast of Garbh Eileach, the lower of these consists of alternating cobble conglomerates and sandstone beds arranged in a crossstratification unit which is 2m thick; this unit lies completely within the mixtite and thins to zero towards both the east and the west (Fig. 41). Mem. geol. Soc. Lond. no. 6

85

A.

M.

SPENCER

37-38 (27m), like mixtites 33, 35 and 36, have a silty sandstone matrix and contain abundant granite fragments of all sizes up to 1.5 m in diameter; dolomites form under 10 per cent of the fragment suite. The contact of mixtite 37 with the underlying sandstone is knife sharp (Fig. 3; P1. 4a), although sandstone laminae frequently occur in the lowest 1m of the mixtite. Subdivisions are present within mixtite 37 in the west of Garbh Eileach but cannot be traced through to the east coast. The sandstone above mixtite 37, analogous, especially in the presence of a very irregular base, to that above mixtite 34, is up to 3.6m thick and shows some trough cross-stratification. Mixtite 38 overlies this sandstone with a sharp contact and again Mixtites

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SEA

q-gn F I G . 41. H o r i z o n t a l

section showing

the lenticular granite conglomerate

w i t h i n m i x t i t e 36 o n t h e w e s t c o a s t o f G a r b h

The strike and range of dips of the outcrop

Eileach.

surface are given.

contains a fine lamination structure in its lowest 50cm, but is otherwise remarkably homogeneous and free of internal bedding. A 9 m granite conglomerate outcrops on Sgeir poll an Dhobrain. At low tide it can be seen to overlie the sandstone above mixtite 38 and is thus the highest horizon exposed in the Garvellachs. (iv) M e m b e r 4, D u b h F h e i t h The relationship of the mixtites seen on these two isolated skerries, which lie 3km north-east of the Garvellachs, to the succession in the latter is not known. Both skerries are formed of a massive, arenaceous mixtite which is uncleaved and contains fragments, predominantly of pebble and cobble grade, only sparsely (the largest fragment seen measures 62 x 52 x 25cm); granite fragments greatly outnumber dolomites. The mixtite is extremely homogeneous; four thin, lenticular, bedded horizons, which are present on the southern skerry, dip at approximately 30 ~ towards the east and indicate that the mixtite there is at least 29m thick. The stone content, lithology and thickness of this mixtite are most comparable with mixtite 44 of Islay, thus confirming the presence of the highest mixtites of Islay in the area of the Garvellachs. (c) T H E P O R T A S K A I G A R E A , I S L A Y The Port Askaig Formation outcrops in a strip about 2km wide, centred on Port Askaig, along the east coast of the island of Islay. Coastal exposures around Port Askaig show the lithology of the mixtites well, but the sequence in the Port Askaig Formation (P1. 10) has been built up by detailed mapping of the inland outcrops (Table 2 and Plate 9). Structure. The area lies along the axis and on the eastern limb of the N~-trending Islay Anticline (Bailey 1916). Two synclines affect the rocks in the area but the outcrop pattern is largely controlled by a system of r~E-trending faults. Dips are generally low (under 40 ~ and minor folds are uncommon. Two cleavages are present in the mixtites; an earlier, widespread cleavage dips towards the north-west at angles between 32 ~ and 72 ~ A later cleavage, restricted to pelites and pelitic mixtites, dips towards the south-east at angles 86

Mem. geoL Soc. Lond.

no. 6

A.

M.

SPENCER

37-38 (27m), like mixtites 33, 35 and 36, have a silty sandstone matrix and contain abundant granite fragments of all sizes up to 1.5 m in diameter; dolomites form under 10 per cent of the fragment suite. The contact of mixtite 37 with the underlying sandstone is knife sharp (Fig. 3; P1. 4a), although sandstone laminae frequently occur in the lowest 1m of the mixtite. Subdivisions are present within mixtite 37 in the west of Garbh Eileach but cannot be traced through to the east coast. The sandstone above mixtite 37, analogous, especially in the presence of a very irregular base, to that above mixtite 34, is up to 3.6m thick and shows some trough cross-stratification. Mixtite 38 overlies this sandstone with a sharp contact and again Mixtites

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SEA

q-gn F I G . 41. H o r i z o n t a l

section showing

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w i t h i n m i x t i t e 36 o n t h e w e s t c o a s t o f G a r b h

The strike and range of dips of the outcrop

Eileach.

surface are given.

contains a fine lamination structure in its lowest 50cm, but is otherwise remarkably homogeneous and free of internal bedding. A 9 m granite conglomerate outcrops on Sgeir poll an Dhobrain. At low tide it can be seen to overlie the sandstone above mixtite 38 and is thus the highest horizon exposed in the Garvellachs. (iv) M e m b e r 4, D u b h F h e i t h The relationship of the mixtites seen on these two isolated skerries, which lie 3km north-east of the Garvellachs, to the succession in the latter is not known. Both skerries are formed of a massive, arenaceous mixtite which is uncleaved and contains fragments, predominantly of pebble and cobble grade, only sparsely (the largest fragment seen measures 62 x 52 x 25cm); granite fragments greatly outnumber dolomites. The mixtite is extremely homogeneous; four thin, lenticular, bedded horizons, which are present on the southern skerry, dip at approximately 30 ~ towards the east and indicate that the mixtite there is at least 29m thick. The stone content, lithology and thickness of this mixtite are most comparable with mixtite 44 of Islay, thus confirming the presence of the highest mixtites of Islay in the area of the Garvellachs. (c) T H E P O R T A S K A I G A R E A , I S L A Y The Port Askaig Formation outcrops in a strip about 2km wide, centred on Port Askaig, along the east coast of the island of Islay. Coastal exposures around Port Askaig show the lithology of the mixtites well, but the sequence in the Port Askaig Formation (P1. 10) has been built up by detailed mapping of the inland outcrops (Table 2 and Plate 9). Structure. The area lies along the axis and on the eastern limb of the N~-trending Islay Anticline (Bailey 1916). Two synclines affect the rocks in the area but the outcrop pattern is largely controlled by a system of r~E-trending faults. Dips are generally low (under 40 ~ and minor folds are uncommon. Two cleavages are present in the mixtites; an earlier, widespread cleavage dips towards the north-west at angles between 32 ~ and 72 ~ A later cleavage, restricted to pelites and pelitic mixtites, dips towards the south-east at angles 86

Mem. geoL Soc. Lond.

no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

between 30 ~ and 70 ~. The first cleavage is folded about the second in an outcrop at [1431 6673]. Deformation, associated with the development of the first cleavage, has considerably modified the fabrics of conglomerates, whilst pebbles in the mixtites are aligned (probably due to deformation) in a generally E-W direction. (i) THE ISLAY LIMESTONE

This formation outcrops over large areas between Bridgend and Port Askaig and is composed of white limestones and thin bedded, dark, sandy pelites. The following description applies to the uppermost 100m of the formation at Beannan Dubh (P1. 9). The pelites, which are similar to many of the pelites in the Islay Limestone, consist of alternating beds of sandstone and dark, finely laminated pelite, each with thicknesses of a few centimetres to a few decimetres. The sandstone beds, which are variable in thickness when traced laterally, show small loading structures and ptygmatic sand dykelets at their bases (Fig. 42a). The lowest 5 m of the limstone is finely laminated in parts and contains structures which, although not as distinctive as those in the Garvellachs, are probably of algal stromatolitic origin (Fig. 42b). Limestone flake breccia bands

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(b) FIG. 42. (a) Field sketch of load casts and ptygmatic sandstone dykelets in thinly interbedded sandstones and siltstones near the top of the Islay Limestone, 250m east of the summit of Dun Boraraic. (b) Field sketches of possible algal stromatolites, approximately 12m beneath the top of the Islay Limestone at Beannan Buidhe.

overlie one of the stromatolite levels. Much of the limestone is regularly bedded in 20 to 50cm thick units, and contains few fragments. The uppermost 10m of the limestone, however, consists largely of flake breccias containing plate-shaped limestone fragments as large as 15cm by 2cm in cross-section. The 4m thick white dolomite at the very top of the succession shows finely laminated concentric structures, possibly of algal stromatolitic origin. The above succession is not present everywhere in the area mapped but the lithologies are usually similar to those just described, the only addition being the presence of oolitic limestones which were found by H.M. Geological Survey at several localities (Wilkinson 1907, 33-35). (ii) TrlE PORT ASKAIG TILLITE (i) Member 1 : the Great Breccia, the Disrupted Beds and mixtites ?16, ?17 and ?18 (53m) The Great Breccia (4m). In the area to the east and north-east of Loch Lossit the lowest mixtite is a 4m thick breccia which lies disconformably on the top of the Islay Limestone, resting in some places on a dolomite and in others on the underlying pelite (Fig. 43). The breccia includes many boulders of the underlying dolomite, which can be identified with certainty because of the presence of a fine, concentric lamination structure. Stones occur sparsely in certain areas and are very closely spaced in others. The breccia is sometimes completely replaced by a planar bedded brown sandstone, which is usually pebble free. Bailey (1916, p. 143) first recognized the disconformable relationship of this breccia, using it to establish that the succession was Mem. geoL Soc. Lond. no. 6

87

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

between 30 ~ and 70 ~. The first cleavage is folded about the second in an outcrop at [1431 6673]. Deformation, associated with the development of the first cleavage, has considerably modified the fabrics of conglomerates, whilst pebbles in the mixtites are aligned (probably due to deformation) in a generally E-W direction. (i) THE ISLAY LIMESTONE

This formation outcrops over large areas between Bridgend and Port Askaig and is composed of white limestones and thin bedded, dark, sandy pelites. The following description applies to the uppermost 100m of the formation at Beannan Dubh (P1. 9). The pelites, which are similar to many of the pelites in the Islay Limestone, consist of alternating beds of sandstone and dark, finely laminated pelite, each with thicknesses of a few centimetres to a few decimetres. The sandstone beds, which are variable in thickness when traced laterally, show small loading structures and ptygmatic sand dykelets at their bases (Fig. 42a). The lowest 5 m of the limstone is finely laminated in parts and contains structures which, although not as distinctive as those in the Garvellachs, are probably of algal stromatolitic origin (Fig. 42b). Limestone flake breccia bands

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(b) FIG. 42. (a) Field sketch of load casts and ptygmatic sandstone dykelets in thinly interbedded sandstones and siltstones near the top of the Islay Limestone, 250m east of the summit of Dun Boraraic. (b) Field sketches of possible algal stromatolites, approximately 12m beneath the top of the Islay Limestone at Beannan Buidhe.

overlie one of the stromatolite levels. Much of the limestone is regularly bedded in 20 to 50cm thick units, and contains few fragments. The uppermost 10m of the limestone, however, consists largely of flake breccias containing plate-shaped limestone fragments as large as 15cm by 2cm in cross-section. The 4m thick white dolomite at the very top of the succession shows finely laminated concentric structures, possibly of algal stromatolitic origin. The above succession is not present everywhere in the area mapped but the lithologies are usually similar to those just described, the only addition being the presence of oolitic limestones which were found by H.M. Geological Survey at several localities (Wilkinson 1907, 33-35). (ii) TrlE PORT ASKAIG TILLITE (i) Member 1 : the Great Breccia, the Disrupted Beds and mixtites ?16, ?17 and ?18 (53m) The Great Breccia (4m). In the area to the east and north-east of Loch Lossit the lowest mixtite is a 4m thick breccia which lies disconformably on the top of the Islay Limestone, resting in some places on a dolomite and in others on the underlying pelite (Fig. 43). The breccia includes many boulders of the underlying dolomite, which can be identified with certainty because of the presence of a fine, concentric lamination structure. Stones occur sparsely in certain areas and are very closely spaced in others. The breccia is sometimes completely replaced by a planar bedded brown sandstone, which is usually pebble free. Bailey (1916, p. 143) first recognized the disconformable relationship of this breccia, using it to establish that the succession was Mem. geoL Soc. Lond. no. 6

87

A.

M.

SPENCER

uninverted. But, although the top surface of the underlying dolomite is irregular on a small scale, the example figured by Allison (1933, fig. 4) as a crevice in the dolomite seems to represent the contact of an allochthonous block with dolomite which is still in its original position (Fig. 43). WSW A .

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The Disrupted Beds (19m). These consist of pebbly dolomites and blue siltstones, both of similar lithologics to those in the Disrupted Beds of the Garvellachs, overlain by a poorly bedded dolomite breccia. At the base of the Disrupted Beds, in the outcrops south of Beannan Dubh, lies a 2cm thick bedded ore band, whilst 100m due north of the summit Dun Boraraic a 40cm thick bedded magnetite band occurs some way above the base. Above the 2cm ore band, is a 1 m thick, finely laminated, siltstone (laminae 0-05 to 0.01 mm thick), which contains scattered dolomite pebbles of up to 5 cm in diameter. The rest of the lower half of the member there consists of irregularly bedded, discontinuous dolomite bands interbedded with thin blue siltstone beds (P1. 6b). As in the Garvellachs both these lithologies contain abundant dolomite pebbles. Few exotic pebbles have been found here, which contrasts with their abundance in the Disrupted Beds of the Garvellachs. The sediments just described pass upwards by gradation into a poorly bedded dolomite breccia which contains angular dolomite fragments of up to small boulders in size and which is comparable to the dolomite conglomerate horizon at the top of the Disrupted Beds in the Garvellachs. A dolomite breccia, which is probably equivalent to that just described, rests directly on the petites at the top of the Islay Limestone in an outcrop 250m east of Dun Boraraic, the intervening sediments having been cut out. Dolomitic mixtites (30m). Above the breccia just described and beneath member 2 is a sequence containing three mixtites, all of which have a dolomitic sandstone or sandy siltstone matrix. The mixtites contain large numbers of dolomite stones, mostly of pebble grade, and few exotics. Only a single exotic pebble has been seen in each of the lower two mixtites and exotics form less than 25 per cent of the fragment suite in the uppermost mixtite. The lower two mixtites outcrop only in the crags south of Beannan Dubh and the second mixtite can be seen to thin laterally to zero in the exposure at [412 648] where it overlies bedded dolomites and dolomite conglomerates with an irregular contact (Fig. 44). Cross-stratification, with set thicknesses of as much as 1 m, is present in these conglomerates. The upper mixtite has a crude bedded structure, produced by horizons which are more dolomitic or sandy than normal. It is much more extensive than the two lower mixtites for it also outcrops in the area south of Loch nam Ban, 4-5 km north of Dun Boraraic. The diseonformities at the base of, and in the lower part o f the formation. The magnitude of the disconformity at the base of the formation is considerable. In the GarveUachs the Great Breccia is underlain by mixtites 1-12 88

Mem. geol. Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND (100m thick), beneath which lies the upper dolomitic member (35m thick) of the Islay Limestone (P1. 10). At Beannan Dubh, the former are absent and the latter is probably represented by only a 4m thick dolomite. A second disconformity is present beneath the lowest of the three mixtites and this results in all the underlying beds of the formation (some 35m thick) being cut out towards the south-west corner of Beannan Dubh (P1. 9). A small erosion surface occurs beneath the second mixtite (Fig. 44) and the base of the highest mixtite is also disconformable. On the shore of Loch Lossit only 4m of dolomite separates the latter from the top of the Islay Limestone. In addition, in the northern area, from Port nam Borrachaig to Torrabolls, this mixtite often rests directly on the pelites and dolomites which there form the top of the Islay Limestone.

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FIo. 44. Field sketches of small discontinuitiesbeneath the first (a) and second (b) mixtite beds in member 1 at Beannan Buidhe. Bedded dolomite shown in black in (b). (ii) Member 2: mixtites 19-22, ?24, 32', 32" (85m) Mixtites which, because of their stone content and the lithology of their matrix, are thought to be equivalent to member 2 of the Garvellachs, outcrop on Beannan Dubh, around An Tamhanachd, at Am Meall and from Eileanan Gainmhich to Cnoc an t'Sleibh. Of these localities, it is only at An Tamhanachd and Cnoc an t'Sleibh that a continuous section of the member is exposed (P1. 10); the section at Am Meall (P1. 10), is better exposed but incomplete. In all the above localities the highest of all the dolomitic mixtites is overlain by a 2 m thick siltstone which contains green fragments. The latter are mostly under 1cm in diameter and are identical to those present in the lowest mixtite (number 19) in member 2 in the GarvelIachs. At Am Meall, 35 m of homogeneous mixtite overlies this horizon and is capped by a thin sandstone beneath which, in an 8 m long outcrop, seven examples of sandstone wedges have been seen (Fig. 20g). Other wedges, probably at the same stratigraphical horizon, occur in outcrops at An Tamhanachd (P1. 9) and at Eileanan Gainmhich. Above this sandstone wedge horizon at Am Meall only 11 m of mixtites are exposed, comprising a 4m thick mixtite succeeded by 5m of poorly bedded siltstones with sparse stones, overlain by a 2 m thick conglomeratic mixtite. South-west of Loch nam Ban, the sandstone which probably overlies the sandstone wedge horizon just described is succeeded by a very much thicker sequence of mixtites than at An Meall or An Tamhanachd. The top of member 2 both at Loch nam Ban and Am Tamhanachd is, however, formed of a similar arenaceous mixtite in which stones occur very sparsely and dolomites are almost absent. A poorly developed, rather irregular bedding structure, caused by more sandy or silty laminae, is common within this mixtite. In outcrops south of the west shore of Loch Allan higher beds occur within member 2; the arenaceous mixtite is overlain by 9m of regularly laminated siltstones, succeeded by a 15m thick mixtite containing dolomite and exotic stones in equal numbers. Both siltstones and mixtites are absent in outcrops away from Loch Allan. (iii) Member 3: mixtites 33-35, ?36 and 37-38 (95m) Three mixtites interbedded with sandstones outcrop at Creagan Loisgte, in a succession which overlies that of member 2 of An Tamhanachd. The sandstone lying above the latter is 30 m thick and maintains a generally similar thickness (30 to 60m) in other outcrops in the Port Askaig area. Planar cross-stratification sets with Mem. geol. Soc. Lond. no. 6

89

A. M. S P E N C E R

heights of up to 1.5m are present in certain of its outcrops, whilst at Chacha Buidge a lenticular dolomite conglomerate, up to 10m in thickness, occurs within the sandstone. The upper 5m of the sandstone at Creagan Loisgte is pelitic and contains two horizons of penecontemporaneous disturbed bedding structures, similar to those in the pelites beneath mixtite 33 in the Garvellachs. Above this sandstone lies a 17m thick mixtite in which the fragment suite is 20 per cent dolomite and 80 per cent exotic. This bed can be subdivided into a lower more pelitic mixtite containing fewer stones than the upper or more arenaceous mixtite. The division plane is marked by a discontinuous sandstone horizon, beneath which a few large sandstone downfold structures occur; the latter are similar to those in the tops of mixtite 34 in the Garvellachs. The upper mixtite is overlain by a granite conglomerate containing much cobble and boulder grade material. This persistent bed is a good marker horizon and has been extremely useful in determining the structure of the Port Askaig area. It is thicker in the north of the mapped area, reaching a maximum of 7m around Loch nam Ban, and in this area the underlying mixtite is either very thin or is completely absent. At Creagan Loisgte the conglomerate is overlain by a 22m thick sandstone, which is usually planar bedded and which is capped by two mixtites interbedded with sandstones. The lower of these is very thin (1-2m) and is not represented in outcrops away from Creagan Loisgte. (iv) Member 4: mixtites 39--44 (215 to 235m) The thick, relatively homogeneous mixtites of this member outcrop from Creagan Loisgte to Port Askaig, along the coast between Port Askaig and Caol Ila and south of Ardnahoe Loch. The thickness of the beds in this member have been calculated from their widths of outcrop between Beinn na Cille and the sea, between Port Askaig and Caol Ila and at Cnoc Reamhar (P1. 10). These successions are thought to be equivalent; they are very similar in over-all thickness and lithology and they all rest on a mixtite which overlies the sandstone with the persistent conglomerate at its base. The successions are not identical; for example, two of the bedded horizons present on Beinn na Cille have no equivalents in the succession to the north of Port Askaig. The latter is the best exposed but is complicated by small, retch-trending faults, for which it is difficult to make allowance. The lowest mixtite in member 4 has an arenaceous matrix, is usually well cleaved and contains abundant stones; dolomites form less than 5 per cent of the stone suite. Some 60 to 115 m above the base of the member is a persistent 7 to 11 m thick sandstone and conglomerate. At Ruadh-phort Beag and Port na Seilich this has beneath it a well-developed system of sandstone wedges (Fig. 21k). The mixtite (50 to 60m thick) above this sandstone (P1. 10) is the thickest in the Port Askaig Formation and is very similar in lithology to the underlying mixtite. On the coast north of Port Askaig it is succeeded by two mixtites, each about 10m thick the lower of which is more arenaceous and less well cleaved than the upper. To the west of Ardnahoe Loch, however, the probable equivalents of these mixtites are separated by a bedded sandstone. Sandstone wedges penetrate downwards into the upper of the two mixtites both in the outcrop west of Ardnahoe Loch and on the coast north of Port Askaig. The highest mixtite in the member has the same distinctive lithology both at Caol Ila and to the west of Ardnahoe Loch. It is much more arenaceous, less well cleaved and contains stones in smaller numbers than the other mixtites in the member. At the highest stratigraphical level seen at Caol Ila (80m east of the fault by the distillery) a bedded sandstone overlies this mixtite and truncates the sandstone wedges which penetrate it. (v) Member 5: beds 45 to 47 (approximately 325m) The presence of a granite conglomerate in this quartzite was first recognized by H.M. Geological Survey (Wilkinson 1907, p. 43) and the name 'Lower Fine-Grained Quartzite' given to the member by Bailey (1916, p. 147). The upper part of the member contains three thin mixtites and is well exposed in the cliff at Am Miadar; its thickness has been calculated from the dip and width of outcrop there and must be very nearly correct. The inland outcrop of the member is not well exposed and so the total thickness of the member, 90

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND which must be calculated there, is only approximate. Fallen blocks of conglomerate occur at one place along the coastal outcrop of the lower half of the quartzite (P1. 9) and may have come from a fourth conglomerate horizon not seen in place. The three conglomeratic mixtites are similar in thickness and lithology. The lower two consist of coarse granite conglomerates and true mixtites; the conglomerates present at the lowest horizon contain boulders as large as 50cm in length. The uppermost horizon is a granite conglomerate containing fragments up to only 17cm in diameter. The mixtites at the two lower horizons contain angular yellow dolomite fragments and granites in almost equal numbers; large fragments of dolomite breccia are also present within these mixtites. By contrast, dolomite stones form less than 10 per cent of the stone suite in the conglomerates of all three horizons. (iii) THE UPPER DOLOMITIC FORMATION (80m+) This formation was first recognized and described by H.M. Geological Survey (see Wilkinson 1907, 42-4) and was mapped by them over large areas of northern Islay. Bailey (1916, 147-50)first determined that the formation, although lying stratigraphically above member 5 (the Lower Fine-Grained Quartzite) was overlain by the whole of the Jura Quartzite and Allison (1933, p. 135) suggested a total thickness for the formation of 270ft (80m). Peach & Horne (1930, p. 214) were the first to refer to the presence of algal stromatolites in the formation and these structures were subsequently figured by F. W. Anderson (1951, fig. 7) and by Hackman & Knill (1962), who described two types of stromatolite from the outcrops at Rubha a'Mhill. A tripartite succession was erected within the formation by H.M. Geological survey, comprising two dolomitic members separated by a quartzite; the dolomitic members contain shales, dolomitic shales and yellow dolomites and commonly show ripple marks, mudcracks and worm tubes. (D) T H E M U L L O F OA, I S L A Y The Port Askaig Formation outcrops on the south coast of Islay, at Port nan Gallan, about 1 km east of the Mull of Oa. A continuous sequence from the Mull of Oa Phyllites through the Islay Limestone and up to the Port Askaig Formation is exposed there (P1. 10) but the rocks show small-scale folds and are cleaved. This section was described by H.M. Geological Survey (Wilkinson 1907, 39-40), who demonstrated that the lowest mixtite rests unconformably on the Islay Limestone and described the sequence of mixtites in some detail. Not to be confused with the latter is the large stack of ?Triassic cliff breccia which lies a few tens of metres to the west of the outcrop of the Port Askaig Formation. The Mull of Oa Phyllites are dark, finely striped sandy siltstones which show two cleavages; an earlier, steeply dipping cleavage which is axial plane to minor folds, and a later cleavage, which dips gently towards the reNW and rarely produces more than a pucker lineation on the earlier cleavage planes. The measured coastal section shows only that part of the sequence (from a level probably just above the base of the Islay Limestone) which is relatively little folded; the pelites in this may have changed considerably in thickness due to the folding but the overall order of the sequence is probably correct. The [slay Limestone is represented by a lower member containing thin bedded dark-blue limestones, dark calcareous pelites and dolomites, overlain by a dark-grey pelite member, which is capped by a member containing dolomitic pelites and dolomites; the thick bedded grey limestones found in the Islay Limestone around Port Askaig are absent here. Certain of the lenticular dolomites in the uppermost member of the formation may be of algal stromatolitic origin, although no distinctive small-scale algal growth structures have been seen. Four sandstone dykes or wedges can be seen cutting the dolomitic pelites 15 m beneath the base of the lowest mixtite at H.W.M. Less than 40m of sediment belonging to the Port Askaig Formation are exposed in either the inland cliff or the foreshore at Port nan Gallan. The following description applies to the foreshore (Fig. 45). The two lowest mixtites both have a dolomitic siltstone matrix and contain only dolomite pebbles. The next five Mem. geol. Soc. Lond. no. 6

91

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND which must be calculated there, is only approximate. Fallen blocks of conglomerate occur at one place along the coastal outcrop of the lower half of the quartzite (P1. 9) and may have come from a fourth conglomerate horizon not seen in place. The three conglomeratic mixtites are similar in thickness and lithology. The lower two consist of coarse granite conglomerates and true mixtites; the conglomerates present at the lowest horizon contain boulders as large as 50cm in length. The uppermost horizon is a granite conglomerate containing fragments up to only 17cm in diameter. The mixtites at the two lower horizons contain angular yellow dolomite fragments and granites in almost equal numbers; large fragments of dolomite breccia are also present within these mixtites. By contrast, dolomite stones form less than 10 per cent of the stone suite in the conglomerates of all three horizons. (iii) THE UPPER DOLOMITIC FORMATION (80m+) This formation was first recognized and described by H.M. Geological Survey (see Wilkinson 1907, 42-4) and was mapped by them over large areas of northern Islay. Bailey (1916, 147-50)first determined that the formation, although lying stratigraphically above member 5 (the Lower Fine-Grained Quartzite) was overlain by the whole of the Jura Quartzite and Allison (1933, p. 135) suggested a total thickness for the formation of 270ft (80m). Peach & Horne (1930, p. 214) were the first to refer to the presence of algal stromatolites in the formation and these structures were subsequently figured by F. W. Anderson (1951, fig. 7) and by Hackman & Knill (1962), who described two types of stromatolite from the outcrops at Rubha a'Mhill. A tripartite succession was erected within the formation by H.M. Geological survey, comprising two dolomitic members separated by a quartzite; the dolomitic members contain shales, dolomitic shales and yellow dolomites and commonly show ripple marks, mudcracks and worm tubes. (D) T H E M U L L O F OA, I S L A Y The Port Askaig Formation outcrops on the south coast of Islay, at Port nan Gallan, about 1 km east of the Mull of Oa. A continuous sequence from the Mull of Oa Phyllites through the Islay Limestone and up to the Port Askaig Formation is exposed there (P1. 10) but the rocks show small-scale folds and are cleaved. This section was described by H.M. Geological Survey (Wilkinson 1907, 39-40), who demonstrated that the lowest mixtite rests unconformably on the Islay Limestone and described the sequence of mixtites in some detail. Not to be confused with the latter is the large stack of ?Triassic cliff breccia which lies a few tens of metres to the west of the outcrop of the Port Askaig Formation. The Mull of Oa Phyllites are dark, finely striped sandy siltstones which show two cleavages; an earlier, steeply dipping cleavage which is axial plane to minor folds, and a later cleavage, which dips gently towards the reNW and rarely produces more than a pucker lineation on the earlier cleavage planes. The measured coastal section shows only that part of the sequence (from a level probably just above the base of the Islay Limestone) which is relatively little folded; the pelites in this may have changed considerably in thickness due to the folding but the overall order of the sequence is probably correct. The [slay Limestone is represented by a lower member containing thin bedded dark-blue limestones, dark calcareous pelites and dolomites, overlain by a dark-grey pelite member, which is capped by a member containing dolomitic pelites and dolomites; the thick bedded grey limestones found in the Islay Limestone around Port Askaig are absent here. Certain of the lenticular dolomites in the uppermost member of the formation may be of algal stromatolitic origin, although no distinctive small-scale algal growth structures have been seen. Four sandstone dykes or wedges can be seen cutting the dolomitic pelites 15 m beneath the base of the lowest mixtite at H.W.M. Less than 40m of sediment belonging to the Port Askaig Formation are exposed in either the inland cliff or the foreshore at Port nan Gallan. The following description applies to the foreshore (Fig. 45). The two lowest mixtites both have a dolomitic siltstone matrix and contain only dolomite pebbles. The next five Mem. geol. Soc. Lond. no. 6

91

A. M. SPENCER mixtites contain exotics in larger numbers than dolomites and have a more arenaceous matrix. It seems likely that the two lowest mixtites represent mixtites 14-18 of the Garvellachs and the higher mixtites belong to member 2, although no green fragment mixtite has been seen here at the base of the latter. The top of the measured section is faulted (P1. 10), and by far the greatest part of the formation is missing; the mixtites do not pass up insensibly into the overlying quartzite (Wilkinson 1907, p. 40). ..: . . . . . . . - . . . . - . . .

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CO. D O N E G A L

The principal outcrops of the Port Askaig Formation in this peninsula, are at Ballyhork and Croaghan Hill, on the west and east of Mulroy Bay respectively. At Ballyhork only the lower part of the formation is present, outcropping as a fiat-lying outlier (Fig. 46a). At Croaghan the lower part of the formation is intruded by dolerite sills and is poorly exposed whilst the upper part is well exposed and passes upwards into the Jura Quartzite (Fig. 4 6 b ) . The thickness of the sections from both localities have been produced by combining field measurements with calculations from the dip and width of outcrop. The two sections overlap stratigraphically, both having the same successions, matrix lithologies and stone contents in the over-lapping parts. Combined, they provide a section through the whole formation (P1. 10). The rocks in both areas are quite strongly cleaved but pebble deformation is only obvious in the dolomitic mixtites. The Islay Limestone is composed of thick bedded grey limestones, containing some flake breccia bands. Stromatolites are absent. The formation is well seen along the shore of Mulroy Bay at Ballyhork but its contact with the lowest mixtites is not well exposed. The latter are strongly cleaved, dolomitic siltstones containing exclusively dolomite stones, some of which have been considerably deformed in the cleavage and show axial ratios as great as 5 to 1. In the mixtites of the overlying member some 25 per cent or more of the stones are granites and the matrix is a dolomitic sandstone. The lowest 3 m of this member is a mixtite with a dark-green laminated matrix, which contains few stones and is rich in magnetite in parts (Fig. 46). Above this second member, and interbedded between two sandstones, is an arenaceous mixtite which contains granites as 75 per cent of its stone suite. The top of the hill at Ballyhork is capped by a thick arenaceous mixtite which contains granites of up to 1 m in diameter, almost exclusively. This fourth member also forms 92

M e m . geol. Soc. Lond.

no. 6

A. M. SPENCER mixtites contain exotics in larger numbers than dolomites and have a more arenaceous matrix. It seems likely that the two lowest mixtites represent mixtites 14-18 of the Garvellachs and the higher mixtites belong to member 2, although no green fragment mixtite has been seen here at the base of the latter. The top of the measured section is faulted (P1. 10), and by far the greatest part of the formation is missing; the mixtites do not pass up insensibly into the overlying quartzite (Wilkinson 1907, p. 40). ..: . . . . . . . - . . . . - . . .

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FI6. 45. Geological sketch map of the outcrop of the Port Askaig Formation at Port nan Gallan, Mull of Oa. (E) F A N A D ,

CO. D O N E G A L

The principal outcrops of the Port Askaig Formation in this peninsula, are at Ballyhork and Croaghan Hill, on the west and east of Mulroy Bay respectively. At Ballyhork only the lower part of the formation is present, outcropping as a fiat-lying outlier (Fig. 46a). At Croaghan the lower part of the formation is intruded by dolerite sills and is poorly exposed whilst the upper part is well exposed and passes upwards into the Jura Quartzite (Fig. 4 6 b ) . The thickness of the sections from both localities have been produced by combining field measurements with calculations from the dip and width of outcrop. The two sections overlap stratigraphically, both having the same successions, matrix lithologies and stone contents in the over-lapping parts. Combined, they provide a section through the whole formation (P1. 10). The rocks in both areas are quite strongly cleaved but pebble deformation is only obvious in the dolomitic mixtites. The Islay Limestone is composed of thick bedded grey limestones, containing some flake breccia bands. Stromatolites are absent. The formation is well seen along the shore of Mulroy Bay at Ballyhork but its contact with the lowest mixtites is not well exposed. The latter are strongly cleaved, dolomitic siltstones containing exclusively dolomite stones, some of which have been considerably deformed in the cleavage and show axial ratios as great as 5 to 1. In the mixtites of the overlying member some 25 per cent or more of the stones are granites and the matrix is a dolomitic sandstone. The lowest 3 m of this member is a mixtite with a dark-green laminated matrix, which contains few stones and is rich in magnetite in parts (Fig. 46). Above this second member, and interbedded between two sandstones, is an arenaceous mixtite which contains granites as 75 per cent of its stone suite. The top of the hill at Ballyhork is capped by a thick arenaceous mixtite which contains granites of up to 1 m in diameter, almost exclusively. This fourth member also forms 92

M e m . geol. Soc. Lond.

no. 6

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND Croaghan Hill. The base of the member, and the underlying sandstones and mixtites, can be seen in outcrops lying beneath the road and 250m north-west of the summit9 Three thin interbeds occur within this thick mixtite member on Croaghan Hill. Beneath the uppermost of these, sandstone wedges have been seen in four /

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FIG. 46. Geological maps of the outcrops of the Port Askaig Formation at (a) Ballyhork and (b) Croaghan Hill, Fanad. The outcrops of members 1 to 5 are numbered and the localities where sandstone wedges have been seen (Croaghan Hill only) are marked with a wedge-shaped symbol. Cross-hatched areas are metadolerites. Roads are shown as solid lines. localities. The fifth member is a planar bedded white sandstone merate horizons, an horizon of scattered granite stones and an latter, and beneath the very thick white Fanad Quartzite, lies an dolomites, perhaps representing the Upper Dolomitic Formation

containing, in order, two granite congloextremely arenaceous mixtite. Above the horizon of dolomitic sandstones and thin of Islay.

(F) S C H I C H A L L I O N In the highly metamorphosed and deformed rocks (Rast 1958, 1959) of central Perthshire the existence of a boulder-bed between the Blair Atholl ( = Islay) Limestone and the Schichallion ( = Jura) Quartzite has long been known. H.M. Geological Survey (Barrow, Grant Wilson & Cunningham Craig 1905, 60--1) recognized three divisions in this formation, a lower calcareous mica-schist containing only carbonate fragments (the 'honeycomb' rock shown in Bailey & McCallien 1937, pl. 1, fig. 3), a middle, biotite-rich, schist containing granite fragments and an upper quartzite division, containing a granite conglomerate and overlain by a dolomitic member. Anderson (1923, p. 430) recognized that carbonate fragments are not confined to the calcareous mixtites, but occur in the more arenaceous parts alongside granite stones. In addition, Bailey & McCallien (1937, p. 86 and fig. 6) noted an important intercalation of quartzite within the arenaceous mixtites. Mem. geol. Soc. Lond. no. 6

93

LATE PRE-CAMBRIAN GLACIATION IN SCOTLAND Croaghan Hill. The base of the member, and the underlying sandstones and mixtites, can be seen in outcrops lying beneath the road and 250m north-west of the summit9 Three thin interbeds occur within this thick mixtite member on Croaghan Hill. Beneath the uppermost of these, sandstone wedges have been seen in four /

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FIG. 46. Geological maps of the outcrops of the Port Askaig Formation at (a) Ballyhork and (b) Croaghan Hill, Fanad. The outcrops of members 1 to 5 are numbered and the localities where sandstone wedges have been seen (Croaghan Hill only) are marked with a wedge-shaped symbol. Cross-hatched areas are metadolerites. Roads are shown as solid lines. localities. The fifth member is a planar bedded white sandstone merate horizons, an horizon of scattered granite stones and an latter, and beneath the very thick white Fanad Quartzite, lies an dolomites, perhaps representing the Upper Dolomitic Formation

containing, in order, two granite congloextremely arenaceous mixtite. Above the horizon of dolomitic sandstones and thin of Islay.

(F) S C H I C H A L L I O N In the highly metamorphosed and deformed rocks (Rast 1958, 1959) of central Perthshire the existence of a boulder-bed between the Blair Atholl ( = Islay) Limestone and the Schichallion ( = Jura) Quartzite has long been known. H.M. Geological Survey (Barrow, Grant Wilson & Cunningham Craig 1905, 60--1) recognized three divisions in this formation, a lower calcareous mica-schist containing only carbonate fragments (the 'honeycomb' rock shown in Bailey & McCallien 1937, pl. 1, fig. 3), a middle, biotite-rich, schist containing granite fragments and an upper quartzite division, containing a granite conglomerate and overlain by a dolomitic member. Anderson (1923, p. 430) recognized that carbonate fragments are not confined to the calcareous mixtites, but occur in the more arenaceous parts alongside granite stones. In addition, Bailey & McCallien (1937, p. 86 and fig. 6) noted an important intercalation of quartzite within the arenaceous mixtites. Mem. geol. Soc. Lond. no. 6

93

A. M. SPENCER The author has examined the outcrops in the Schichallion district briefly but the degree of metamorphism and deformation is such that they yield little sedimentological information. The lithologies given by the above authors are those of most of the principal members of the formation and a little detailed mapping should confirm that the sequence is--(base) a calcareous mixtite with only carbonate fragments, a more arenaceous mixtite with carbonate and granite fragments, an arenaceous mixtite with only granite fragments and containing a quartzite intercalation and, at the top, a quartzite with a granite conglomerate horizon. (G) B R A E M A R Outcrops of the Port Askaig Formation in the intensely folded and metamorphosed rocks of Aberdeenshire, at distances of four and seven miles south of Braemar, are recorded by Barrow & Cunningham Craig (1912, 31-2). Few details of the succession are given but the following quotations show the identity of these metasediments with the Port Askaig Formation. "Two outcrops of a thin bed of conglomerate containing wellrounded pebbles of granitic rocks occur in this area; the bed in both cases lying between the Main Limestone and the quartzite . . . . The matrix here (at the first outcrop) is a very fine-grained, grey, siliceous rock, composed mainly of fine quartz grains and minute crystals of brown mica, the latter often arranged criss-eross fashion. The little granite pebbles are about two inches in diameter, or less, and consist of two types--one pink and the other grey . . . . In addition to the granite pebbles of foreign origin, there are a number of fragments of altered sedimentary material, the origin of which is mostly doubtful. There are also a few clearly of local origin, and of these the most important is a fragment of the tremolite rock, which suggests some local erosion at the base of the conglomerate". At the second outcrop the matrix, "contains a fair number of very small pebbles of the typical granites. Here the matrix is dark grey and schistose, but following the rock downhill to the north-east, towards the limestone, it becomes bluish and very like a chloritic epidiorite. Pebbles are now rare and very small, and only isolated patches of the rock can be seen". The outcrops have not been visited by the author but the above details indicate the presence of some of the higher members of the Port Askaig Formation, and possibly of member 1. (H) B A N F F S H I R E The most north-easterly outcrop of the Port Askaig Formation in Scotland is at Muckle Fergie Burn, near Tomintoul. In addition, the presence of numerous loose blocks of mixtite near Fordyce in the north of Banffshire confirms the occurrence of the formation, even though unexposed, in the Dalradian succession of the Banffshire coast. Both outcrops are described in Spencer & Pitcher (1968). They yield little sedimentological information, although the succession at Muckle Fergie Burn demonstrates the presence at least of member 2 of the formation.

8. A C K N O W L E D G E M E N T S I wish to thank Professor W. S. Pitcher for suggesting the research topic and for his continual encouragement and interest during the progress of the work, which was carried out in the Department of Geology, University of Liverpool. Assistance and discussion--mostly in the field--with Dr J. D. Taylor, Dr G. Warrington, Dr R. J. Howarth, Mrs M. O. Spencer, Mr D. H. Harrison, Dr I. M. Platten, Dr K. Bjodykke, Dr N. Rast and Professor J. Sutton, and the able servicesof Lachlan MacLachlan of CuUipool, Luing (boatman), helped the progress of the research; the criticisms of Dr K. Bj~rlykke and Mr W. B. Harland improved the manuscript. The awards of a D.S.I.R. Research Studentship, followed by a N.E.R.C. Research Fellowship, are gratefully acknowledged. The cost of printing this Memoir has been supported in part by an allocation from the Parliamentary Grant-in-Aid of Scientific Publications administered by The Royal Society. 94

Mem. geoL Soc. Lond. no. 6

A. M. SPENCER The author has examined the outcrops in the Schichallion district briefly but the degree of metamorphism and deformation is such that they yield little sedimentological information. The lithologies given by the above authors are those of most of the principal members of the formation and a little detailed mapping should confirm that the sequence is--(base) a calcareous mixtite with only carbonate fragments, a more arenaceous mixtite with carbonate and granite fragments, an arenaceous mixtite with only granite fragments and containing a quartzite intercalation and, at the top, a quartzite with a granite conglomerate horizon. (G) B R A E M A R Outcrops of the Port Askaig Formation in the intensely folded and metamorphosed rocks of Aberdeenshire, at distances of four and seven miles south of Braemar, are recorded by Barrow & Cunningham Craig (1912, 31-2). Few details of the succession are given but the following quotations show the identity of these metasediments with the Port Askaig Formation. "Two outcrops of a thin bed of conglomerate containing wellrounded pebbles of granitic rocks occur in this area; the bed in both cases lying between the Main Limestone and the quartzite . . . . The matrix here (at the first outcrop) is a very fine-grained, grey, siliceous rock, composed mainly of fine quartz grains and minute crystals of brown mica, the latter often arranged criss-eross fashion. The little granite pebbles are about two inches in diameter, or less, and consist of two types--one pink and the other grey . . . . In addition to the granite pebbles of foreign origin, there are a number of fragments of altered sedimentary material, the origin of which is mostly doubtful. There are also a few clearly of local origin, and of these the most important is a fragment of the tremolite rock, which suggests some local erosion at the base of the conglomerate". At the second outcrop the matrix, "contains a fair number of very small pebbles of the typical granites. Here the matrix is dark grey and schistose, but following the rock downhill to the north-east, towards the limestone, it becomes bluish and very like a chloritic epidiorite. Pebbles are now rare and very small, and only isolated patches of the rock can be seen". The outcrops have not been visited by the author but the above details indicate the presence of some of the higher members of the Port Askaig Formation, and possibly of member 1. (H) B A N F F S H I R E The most north-easterly outcrop of the Port Askaig Formation in Scotland is at Muckle Fergie Burn, near Tomintoul. In addition, the presence of numerous loose blocks of mixtite near Fordyce in the north of Banffshire confirms the occurrence of the formation, even though unexposed, in the Dalradian succession of the Banffshire coast. Both outcrops are described in Spencer & Pitcher (1968). They yield little sedimentological information, although the succession at Muckle Fergie Burn demonstrates the presence at least of member 2 of the formation.

8. A C K N O W L E D G E M E N T S I wish to thank Professor W. S. Pitcher for suggesting the research topic and for his continual encouragement and interest during the progress of the work, which was carried out in the Department of Geology, University of Liverpool. Assistance and discussion--mostly in the field--with Dr J. D. Taylor, Dr G. Warrington, Dr R. J. Howarth, Mrs M. O. Spencer, Mr D. H. Harrison, Dr I. M. Platten, Dr K. Bjodykke, Dr N. Rast and Professor J. Sutton, and the able servicesof Lachlan MacLachlan of CuUipool, Luing (boatman), helped the progress of the research; the criticisms of Dr K. Bj~rlykke and Mr W. B. Harland improved the manuscript. The awards of a D.S.I.R. Research Studentship, followed by a N.E.R.C. Research Fellowship, are gratefully acknowledged. The cost of printing this Memoir has been supported in part by an allocation from the Parliamentary Grant-in-Aid of Scientific Publications administered by The Royal Society. 94

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN S C O T L A N D

9. R E F E R E N C E S ALDERMAN,A. R. 1965. Dolomitic sediments and their environment in the south-east of South Australia. Geochim. cosmochim. Acta 29, 1355-65. ALLEN, J. R. L. 1963. The classification of cross-stratified units, with notes on their origin. Sedimentology 2, 93-114. ALLISON, A. 1933. The Dalradian succession in Islay and Jura. Q. Jl. geol. Soc. Lond. 89, 125-44. ANDERSON, E. M. 1923. The geology of the schists of the Schichallion district. Q. Jl. geol. Soc. Lond. 79, 423-45. 1951. The dynamics of faulting and dyke formation with applications to Britain (2nd edn.). Edinburgh (Oliver & Boyd). ANDERSON, F. W. 1951. Some reef-building calcareous algae from the Carboniferous rocks of northern England and southern Scotland. Proc. Yorks. geol. Soc. 28, 5-28. ANDERSON, R. Y. & KOOPMANS, L. H. 1963. Harmonic analysis of varve time series. J. geophys. Res. 68, 877-93. ANTEVS, E. 1951. Glacial clays in Steep Rock Lake, Ontario, Canada. Bull. geoL Soc. Am. 62, 1223-62. ARMSTRONG, J. E., CRANDELL,D. R., EASTERBROOK,D. J. & NOULE, J. B. 1965. Late Pleistocene stratigraphy and chronology in southwestern British Columbia and northwestern Washington. Bull. geol. Soc. Am. 76, 321-39. BAILEY, E. B. 1916. The Islay anticline (Inner Hebrides). Q. Jl. geol. Soc. Lond. 72, 132-64. , COLLET, L. W. & FIELD, R. M. 1928. Palaeozoic submarine landslips near Quebec City. J. geoL 36, 577-614. --, & MCCALLIEN, W. J. 1937. Perthshire tectonics: Schichallion to Glen Lyon. Trans. R. Soc. Edinb. 59, 79-118. BAI,,a-IAM, P. H. & RANSON, C. E. 1965. Structural study of the contorted drift and disturbed chalk at Weybourne, north Norfolk. GeoL Mag. 102, 164-74. BARROW, G. & CUNNINGrIAM CRAIG, E. H. 1912. Geology of Braemar, Ballater and Glen Clova. Mem. GeoL Surv. Scotland. --, GRANTWmSON, J. S. & CUNNINGHAMCRAIG, E. H. 1905. The geology of the country around Blair Atholl, Pitlochry and Aberfeldy. Mem. GeoL Surv. Scotland. BERG, T. E. & BLACK, R. F. 1966. Preliminary measurements of growth of nonsorted polygons, Victoria Land, Antarctica. In TEDROW, J. C. F. (Ed.) Antarctic soils and soil forming processes. Vol. 8. Antarctic Research Series. American Geophysical Union, pp. 61-108. BERTHELSEN,A. & NOE-NYGAARD,A. 1965. The Precambrian of Greenland. In RANKAMA,K. (Ed.) The Precambrian Vol. 2. New York (Interscience), pp. 113-263. BEUF, S., BIJU-DUVAL, B., STEVAUX,J. & KULBICKI, G. 1966. Ampleur des glaciations "Siluriennes" au Sahara: leurs influences et leurs consequences sur la sedimentation. Revue Inst. fr. Petrole 21, no. 3, 363-81. BIGARELLA,J. J., SALAMUNI,R. • FUCK, R. A. 1967. Striated surfaces and related features developed by the Gondwana ice sheets (State of Parana, Brazil). Palaeogeogr. Palaeoclim. PalaeoecoL 3, 265-76. BIJU-DLrVAL, B. & GARIEL, O. 1969. Nouvelles observations sur les ph6nom~nes glaciaires "]~ocambriens" de la bordure Nord de la Syn6clise de Taoudeni, entre le Hank et le Tanezrouft, Sahara occidental. Palaeogeogr. Palaeoclim. Palaeoecol. 6, 283-315. BISSELL, H. J. & CHILINGAR, G. V. 1962. Evaporite type dolomite in salt flats of western Utah. Sedimentology 1,200-10. BJORLYKKE,K. 1966. Sedimentary petrology of the sparagmites of the Rena District, S. Norway. Norg. geol. Unders. no. 238, 5-53. 1967. The Eocambrian "Reusch Moraine" at Bigganjargga and the geology around Varangerfjord; northern Norway. Norg. geol. Unders. no. 251, 18-44. BLACK, R. F. & BERG, T. E. 1964. Glacier fluctuations recorded by patterned ground, Victoria Land. In ADIE, R. J. (Ed.) Antarctic geology. Amsterdam (North-Holland), pp. 107-22. BROWN, I. A. 1925. Notes on the occurrence of glendonites and glacial erratics in the Upper Marine Beds of Ulladulla, N.S.W. Proc. Linn. Soc. N.S.W. 50, 25-31. BUTRYM, J., CEGLA,J., DZULYNSKI, S. & NAKONIECZNY,S. 1964. New interpretation of "periglacial structures". Folia Quaternaria no. 17. CAREV, S. W. & AHMED, N. 1961. Glacial Marine sedimentation. In RAASCH,G. O. (Ed.) Geology of the Arctic, vol. 2, University of Toronto Press, pp. 865-95. CARRUTHERS, R. G. 1940. On Northern glacial drifts; some peculiarities and their significance. Q. Jl. geol. Soc. Lond. 95, 299-333. -1953. Glacial drifts and the undermelt theory. Newcastle upon Tyne (Harold Hill). CHARLESWOR'rH, J. K. 1957. The Quaternary Era (2 vols.). London (Arnold). Cn-OMAI~OV, N. M. 1968. [Late Precambrian glaciation of Spitsbergen.] Dokl. Akad. Nauk S S S R 180, 1446-9 (in Russian). COLEMAN, A. P. 1926. Ice ages recent and ancient. London (Macmillan). CONDIE, K. C. 1967. Petrology of the Late Precambrian Tillite(?) association in northern Utah. Bull. geol. Soc. Am. 78, 1317-44. CRANDELL,D. R. & WALDRON,H. H. 1956. A recent volcanic mudflow of exceptional dimensions from Mt Rainier, Washington. Am. J. ScL 254, 349-62. CROLL, J. 1875. Climate and Time in their geological relations. London (Daldy, Isbister & Co.). CROWELL, J. C. 1957. Origin of pebbly mudstones. Bull. geol. Soc. Am. 68, 993-1009. 1964. Climatic significance of sedimentary deposits containing dispersed-megaclasts. In NAIRN, A. E. M. (Ed.) Problems in Palaeoclimatology. London (lnterscience), pp. 86-99.

Mem. geol. Soc. Lond. no. 6

95

A. M. S P E N C E R

CURRAY, J. R. 1956. The analysis of two-dimensional orientation data. J. Geol. 64, 117-31. DAVID, T. W. E. 1907A. Conditions of climate at different geological epochs, with special reference to glacial epochs. Int. geol. Congr. 10 (1), 437-65. 1907B. Some problems of Australian glaciation. Rept. I lth meeting Austral. Assoc. Adv. Sci. 457-65. DE GEER, G. 1912. A geochronology of the last 12,000 years. Int. geol. Congr. 11, 241-53. DICKSON, J. A. D. 1965. A modified staining technique for carbonates in thin section. Nature, Lond. 205, 587 only. Do't-r, R. H. JR. 1961. Squantum "Tillite" Mass.--Evidence of glaciation or mass movement ? Bull. geol. Soc. Am. 72, 1289-1306. 1963. Dynamics of subaqueous gravity depositional processes. Bull. Am. Ass. Petrol. Geol. 47, 104-128. Dow, D. B. 1965. Evidence of a late Pre-Cambrian glaciation in the Kimberly Region of Western Australia. Geol. Mag. 102, 407-14. DZULYNSKI, S. 1963. Polygonal structures in experiments and their bearing upon some periglacial phenomena. Bull. Acad. pol. Sci. Sdr. Sci. gdoL gdogr. 9, 145-50. , KSAZKIEWKZ,M. & KUENEN, Ph. H. 1959. Turbidites in flysch of the Polish Carpathian Mountains. Bull. geol. Soc. Am. 70, 1089-1118. EMERY, K. O. 1955. Grain size of marine beach gravel. J. Geol. 63, 39--49. FLINT, R. F. 1961. Geological evidence of cold climate. In NAIRN, A. E. M. (Ed.) Descriptive Palaeoclimatology. New York (Interscience), pp. 140-55. , SANDERS, J. E. & RODGERS, J. 1960. Diamictite, a substitute term for symmictite. Bull. geoL Soc. Am. 71, p. 1809 only. FRAKES, L. A. ~; CROXVELL,J. C. 1967. Facies and palaeogeography of Late Palaeozoic diamictite, Falkland Islands. Bull. geol. Soc. Am. 78, 37-58. &~ 1968. Late Palaeozoic glacial facies and the origin of the South Atlantic Basin. Nature, Lond. 217, 837-8. FRAKES, L. A., MATI'HEWS,J. L., NEDER, I. R. & CROWELL,J. C. 1966. Movement directions in Late Palaeozoic glacial rocks of the Horlick and Pensacola Mountains, Antarctica. Science, N. Y. 153, 746-9. FRAiNKL, E. 1953A. Geologische untersuchungen in Ost-Andr6es Land (NE-Gronland). Meddr Gronland 113(4). 1953B. Die geologische karte von Nord-Scoresby Land (NE-Grenland). Meddr. Gronland 113(6). GAERTNER, H. R. VON. 1943. Bemerkungen uber den Tillit von Bigganjarga am Varangerfjord. Geol. Rdsch. 34, 226-31. GIBSON, G. W. 1962. Geological investigations in southern Victoria Land, Antarctica. Part 8. Evaporite salts in the Victoria Valley region. N.Z. Jl. Geol. Geophys. 5, 361-74. GIRDLER, R. W. 1964. The palaeomagnetic latitudes of possible ancient glaciations. In NAIRN, A. E. M. (Ed.) Problems in palaeoclimatology. London (Interscience), pp. 115-8. G~RLER, K. dk R E ~ R , K.-J. 1968. Entstehung und merkmale der Olisthostrome. Geol. Rdsch. 57, 484-514. HACKMAN, B. D. & KNILL, J. L. 1962. Calcareous algae from the Dalradian of Islay. Palaeontology 5, 268-71. H.KLBICH, I. W. 1964. Observations on primary features in the Fish River Series and the Dwyka Series in South West Africa. Trans. geol. Soc. S. Aft. 67, 95-109. HALLA, F., CHILINGAR, G. V. dk BISSELL, H. J. 1962. Thermodynamic studies on dolomite formation and their geological implications, an interim report. Sedimentology 1, 296-303. HAMILTON, W. & KRINSLEY, D. 1967. Upper Palaeozoic glacial deposits of Soutla Africa and Southern Australia. Bull. geol. Soc. Am. 78, 783-800. HARDY, R. M. & LEGGET, R. F. 1960. Boulder in varved clay at Steep Rock Lake, Ontario, Canada. Bull. geol. Soc. Am. 71, 93-4. HARLAND,W. B. 1964A. Evidence of Late Pre-cambrian glaciation and its significance. In NAIRN, A. E. M. (Ed.) Problems in palaeoclimatology. London (Interscience), pp. 119-49. 1964B. Critical evidence for a great infra-Cambrian glaciation. Geol. Rdsch. 54, 45-61. , HEROD, K. N. dk KRINSLEY, D. H. 1966. The definition and identification of tills and tillites. Earth-Sci. Rev. 2, 225-56. HARRISON, P. W. 1956. A clay-till fabric; its character and origin. J. GeoL 65, 275-307. HEEZEN, B. C. & HOLLISTER,C. 1964. Turbidity currents and glaciation. In NAIRN, A. E. M. (Ed.) Problems inpalaeoclimatology. London (Interscience), pp. 99-108. HOLMES, C. D. 1941. Till fabric. Bull. geol. Soc. Am. 52, 1299-1354. HOUGH, J. L. 1950. Pleistocene lithology of Antarctic ocean-bottom sediments. J. Geol. 58, 254-60. HOWARTH, R. J., KILBURN, C. & LEAKE, B. E. 1966. The Boulder Bed succession at Glencolumkille, Co. Donegal. Proc. R. It. Acad. (a) 65, 117-56. HOBNER, H. 1965. Permokarbonische glazigene und periglaziale ablagerungen aus dem zentralen teil des Kongobeckens. Stockh. Contr. Geol. 13(5), 41-61. JACKSON, T. A. 1965. Power spectrum analysis of two "varved" argillites in the Huronian Cobalt Series (Precambrian) of Canada. J. sedim. Petrol. 35, 877-86. JOHNSSON, G. 1959. True and false ice-wedges in southern Sweden. Geogr. Annlr. 41, 15-33. KILBURN, C., PITCHER,W. S. • SHACKLETON,R. M. 1965. The stratigraphy and origin of the Port Askaig Boulder Bed Series (Dalradian). GeoL Jl 4, 343-60. KRUMaEIN, W. C. 1939. Preferred orientation of pebbles in sedimentary deposits. J. Geol. 47, 673-706. 96

Mem. geoL Soc. Lond. no. 6

LATE PRE-CAMBRIAN

GLACIATION

IN SCOTLAND

KUENEN, Ph. H. 1951. Mechanics of varve formation and the action on turbidity currents. Geol. For. Stockh. Forh. 73, 69-84. 1956. The difference between sliding and turbidity flow. Deep-Sea Res. 3, 134-9. KULLING, O. 1934. Scientific results of the Swedish-Norwegian Arctic Expedition in the summer of 1931, part XI, the "Hecla Hoek Formation" around Hinlopenstredet. Geogr. Annlr 16, 161-253. LACHENBRUCH, A. H. 1962. Mechanics of thermal contraction cracks and ice wedge polygons in permafrost. Spec. pap. geol. Soc. Am. 70. LAJTAI, E. Z. 1967. The origin of some varves in Toronto, Canada. Can. J. Earth Sci. 4, 633-9. LOGAN, B. W., REZAK, R. & GINSBURG, R. N. 1964. Classification and environmental significance of algal stromatolites. J. Geol. 72, 68-83. LONG, W. E. 1964. The stratigraphy of the Horlick Mountains. In ADIE, R. J. (Ed.) Antarctic Geology. Amsterdam (NorthHolland), pp. 352-63. LUNGERSGAUSEN, G. F. 1963. [Traces of glaciations in Late Pre-Cambrian of South Siberia and Urals and their stratigraphic significance.] Int. geol. Congr. 21. Doklady Sov. Geol. Probl. No. 8, Stratigraphy of the Late Pre-Cambrian and Cambrian, pp. 97-108 [in Russian]. LINDSAY, J. F. 1968. The development of clast fabric in mudflows. J. sedim. Petrol. 38, 1242-53. MACCULLOCH, J. 1819. A description of the Western Islands of Scotland including the lsle of Man (3 vols.). London. MARTIN, H. 1964A. The directions of flow of the Itarare Ice Sheets in the Parana Basin, Brazil. Bol. Paranaense de Geografia. 25-76. -1964B. Beobachtungen zum problem der jung-Prakambrischen Glazialen Ablagerungen in Sudwestafrika. Geol. Rdsch. 54, 115-27. 1965. The Precambrian geology of Southwest Africa and Namaqualand. Capetown (Precambrian Research Unit). MAWSON, D. 1949. Sturtian tillite of Mount Jacob and Mount Warren Hastings North Flinders Ranges. Trans. R. Soc. S. Aust. 72, 244-51. NAIRN, A. E. M. (Ed.) 1964. Problems in Palaeoclimatology. London (Interscience). NATLAND, M. L. & KUENEN, Ph. H. 1951. Sedimentary history of the Ventura Basin, California, and the action of turbidity currents. Spec. Pubis. Soc. econ. Palaeont. Miner. No. 2. NEWELL, N. D. 1957. Supposed Permian tillites in northern Mexico are submarine slide deposits. Bull. geol. Soc. Am. 68, 1569-76. PEACH, B. N., KYNASTON, H. & MAUFE, H. B. 1909. Geology of the seaboard of mid-Argyll. Mem. geol. Surv. Scotland. & HORNE, J. 1930. Chapters on the geology of Scotland. London (Oxford University Press). PERRY, W. J. & ROBERTS, H. G. 1968. Late Precambrian glaciated pavements in the Kimberley region, Western Australia. J. geol. Soc. Aust. 15, 51-6. PETERSON, G. L. 1966. Structural interpretation of sandstone dykes, NW Sacramento Valley, California. Bull. geol. Soc. Am. 77, 833--42. PETTIJOHN, F. J. 1957. Sedimentary rocks (2nd edn). New York (Harper). 1962. Dimensional fabric and ice flow, Precambrian (Huronian) glaciation. Science, N. Y. 135, p. 442 only. P~w~, T. L. 1959. Sand wedge polygons (tesselations) in the McMurdo Sound area, Antarctica; a progress report. Am. J. Sci. 257, 545-52. POTTER, P. E. & PETTIJOHN, E. J. 1963. Palaeocurrents and basin analysis. Berlin (Springer-Verlag). PRINGLE, J. 1940. The discovery of Cambrian trilobites in the Highland Border rocks near Callander, Perthshire. Advmt Sci., Lond. 1, p. 252 only. RAMSEY, A. C. 1855. On the occurrence of angular, subangular, polished and striated fragments and boulders in the Permian breccia of Shropshire. Q. Jl. geol. Soc. Lond. 11, 185-205. 1880. On the recurrence of certain phenomena in geological time. Nature, Lond. 22, 383. RAST, N. 1958. The tectonics of the Schichallion complex. Q. Jl. geol. Soc. Lond. 114, 25-46. 1959. Metamorphic history of the Schichallion complex (Perthshire). Trans. R. Soc. Edinb. 63, 413-32. 1963. Structure and metamorphism of the Dalradian rocks of Scotland. In JOHNSON, M. R. W. & STEWART, F. H. (Eds.) The British Caledonides. Edinburgh (Oliver & Boyd), pp. 123-42. RATTIGAN, J. H. 1967. Depositional soft sediment and post-consolidation structures in a Palaeozoic aqueoglacial sequence. J. geol. Soc. Aust. 14, 5-18. RAYNER, D. H. 1965. In discussion of BROWN, P. E., MILLER, J. A., SOPER, N. J. & YORK, D. Potassium-argon age pattern of the British Caledonides. Proc. Yorks. geol. Soc. 35, p. 133 only. READING, H. G. & WALKER, R. G. 1966. Sedimentation of Eocambrian tillites and associated sediments in Finnmark, Northern Norway. Palaeogeogr. Paleoclim. Palaeoecol. 2, 177-212. REID, C. 1882. The geology of the country around Cromer. Mem. geol. Surv. U.K. REUSCH, B. H. 1891. Skuringsmaerker og moraenegruns eftervist i Finmarken fra en periode meget neldre end "istiden". Norg. geol. Unders. Aarbog for 1891, 78-85. ROGNON, P., de CHARPAL, O., BIJU-DUVAL, B. & GARIEL, O. 1968. Les glaciations "Siluriennes" dans l'Ahnet et ie Mouydir (Sahara central). Bull. Publ. Serv. g~ol. Alg~rie, (Nile sdrie). 38, 53-81. -

-

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97

A. M. S P E N C E R SAGS, E. 1965. Dolomite--calcite relationships in sea-water. Theoretical considerations and preliminary experimental results. J. sedim. Petrol. 35, 339-47. SAURAMO,M. 1923. Studies on the Quaternary varve sediments in southern Finland. Bull. commn, geol. Finl. no. 60. 1929. The Quaternary geology of Finland. Bull. commn, geol. Finl. no. 86. SCHAUB, H. P. 1950. On the Pre-Cambrian to Cambrian sedimentation in NE-Greenland. Meddr. Gronland. 114 (10). SCHErqK, P. E. 1965. Precambrian glaciated surface beneath the Gowganda Formation, Lake Timagani, Ontario. Science, N. Y. 159, 309-18. SCHERMERHORN,L. J. G. 1966. Terminology of mixed coarse-fine sediments. J. sedim. Petrol. 36, 831-5. & STANTON, W. I. 1963. Tilloids in the West Congo geosyncline. Q. Jl. geol. Soc. Lond. 119, 201-41. SCHWARZaACI-I, M. 1963. Climates of the Past. London (Van Nostrand). 1964. Criteria for the recognition of ancient glaciations. In NAIRN, A. E. M. (Ed.) Problems in palaeoclimatology. London (Interscience), pp. 81-5. SELLEY,R. C., SHEARMAN,D. J., SUTTON, J. & WATSON,J. 1963. Some underwater disturbances in the Torridonian of Skye and Raasay, Geol. MaR. 10tl, 224-43. SESTINI, G. 1968. Notes on the internal structure of the major Macigno Olistostrome (Oligocene, Modena and Tuscany Apennines). Boll. Soc. geol. ital. 87, 51-63. SHARP, R. P. • NOBLES,L. H. 1953. Mudflow of 1941 at Wrighton, southern California. Bull. geol. Soc. Am. 64, 547-60. SHROCK, R. R. 1948. Sequence in layered rocks. New York (McGraw-Hill). SIMPSON, E. S. 1961. A ground-water mechanism for the deposition of glacial till. Int. geol. Congr. 21(26), 103-7. SIMPSON, I. M. & WEST, R. G. 1958. On the stratigraphy and palaeobotany of a Late-Pleistocene organic deposit at Chelford, Cheshire. New Phytol. 57, 239-50. SOMMER,M. 1957. Geologie von Lyells Land (NE-Gr6nland). Meddr. Gronland. 155(2). SPENCER, A. M. & PITCHER, W. S. 1968. Occurrence of the Port Askaig Tillite in north-east Scotland. Proc. geol. Soc. Lond. no. 1650, 195-8. SPJELDNAES,N. 1964. The Eocambrian glaciation in Norway. Geol. Rdsch. 54, 24-45. STAUFFER, P. H. 1967. Grain-flow deposits and their implications, Santa Ynez Mountains, California. J. sedim. Petrol. 37, 487-508. STEWART,A. D. 1963. On certain slump structures in the Torridonian sandstones of Applecross. Geol. MaR. 100, 205-18. STONE, M. 1957. The Aberfoyle Anticline, Callander, Perthshire. Geol. MaR. 94, 265-76. STUBBLEFIELD,C. J. 1956. Cambrian palaeogeography in Britain. Int. geol. Congr. 20, El Sistema Cambrico, 1, 1-43. TEICHERT, C. 1967. Nature of Permian glacial record, Salt Range and Khisor Range, West Pakistan. N. Jb. geol. palaeont. Abh. 129, 167-84. THOMSON, J. 1871. On the occurrence of pebbles and boulders of granite in schistose rocks in Islay, Scotland. 40th meeting Brit. Assoc. Liverpool Trans. p. 88 only. 1877. On the geology of the Island of Islay. Trans. geol. Soc. Glasg. 5, 200-22. WATSON, E. 1965. Periglacial structures in the Aberystwyth region of central Wales. Proc. Geol. Ass. 76, 443-62. WHETTEN, J. T. 1965. Carboniferous glacial rocks from the Werrie Basin, New South Wales, Australia. Bull. geol. Soc. Am. 76, 43-56. WHITE, G. W. 1969. Pleistocene deposits of the north-western Allegheny Plateau, U.S.A.Q. Jl geol. Soc. Lond. 124, 131-54. WHITE, W. A. 1961. Colloid phenomena in the sedimentation of Argilaceous rocks. J. sedim. Petrol. 31,560-70. WILKINSON,S. B. 1907. The Geology of Islay. Mere. geol. Surv. Scotland. WILSON, C. B. & HARLAND,W. B. 1964. The Polarisbreen Series and other evidence of late Pre-Cambrian ice ages in Spitsbergen. GeoL MaR. 101, 198-219. WINTERER,E. L. 1964. Late Precambrian pebbly mudstone in Normandy, France; Tillite or Tilloid ? In NAIRN, A. E. M. (Ed.) Problems in palaeoclimatology. London (Interscience), pp. 159-78.

( N o t e a d d e d in p r o o f (see p. 53)" I n the a b s t r a c t to this M e m o i r it is stated t h a t s a n d s t o n e w e d g e s are h e r e d e s c r i b e d " f o r the first t i m e in a p r e - P l e i s t o c e n e f o r m a t i o n " . Since t h e latter w a s written, N . M . C h u m a k o v (1968, Fig. l d ) h a s figured w e d g e s f r o m the late P r e - C a m b r i a n tillite o f S p i t s b e r g e n . S a n d s t o n e wedges s h o u l d b e l o o k e d for in all tillite sequences. T h e i r c o m m o n p r e s e n c e c o u l d b o t h give f u r t h e r s u p p o r t to a p e r m a f r o s t o r i g i n for t h e m a n d , i f a p e r m a f r o s t o r i g i n is c o n c l u d e d , h a v e c o n s i d e r a b l e c l i m a t i c a n d h i s t o r i c a l i m p l i c a t i o n s in e a c h case.)

98

Mere. geoL Soc. Lond. no. 6

EXPLANATION

OF P L A T E S 1-11

Unless otherwise stated, all photomicrographs are in plane polarized light and all rulers and hammers are 50 cm in length. PLATE 1

View looking north-eastwards at a folded fragment in the Great Breccia. The cliff is 40m high and the fragment is composed of white dolomites, surrounded by siltstones. Bedding, apart from that within the fragment, cannot be seen but the regional dip here is 35 ~ towards the south-east (towards the right). Institute of Geological Sciences photograph (Crown Copyright reserved). PLATES 2 and 3 Mixtites in members 1 (3c, d), 2 (3a, b), 3 (2c, d) and 4 (2a, b) of the Port Askaig Tillite. With the exception of Pls 3a and 3b, which are from the Port Askaig area, all the photographs are from the Garvellachs. PLATE 2

(a) Mixtite 39, which contains only granitic stones in a strongly cleaved matrix. (b) Photomicrograph of mixtite 41. Grains of quartz and a little feldspar are set in a fine-grained groundmass composed predominantly of mica; very little carbonate is present. (c) Mixtite 38, containing granitic stones. (d) Photomicrograph ofmixtite 35. The mineralogy is similar to (b), but a little more carbonate is present. PLATE 3

(a) Mixtite 29, with two granitic boulders. The holes result from the weathering of dolomite stones. (b) Photomicrograph ofmixtite 19. The clear rounded grains are quartz and the grey, rounded grains dolomite. Small, black magnetite grains are abundant. (c) Mixtite 15, with dolomite pebbles only. The ruler is 50cm long. (d) Photomicrograph ofmixtite 8. The clear grains are quartz and the rest is mostly dolomite, with a little mica. PLATE 4

(a) The basal contact of mixtite 37 with the underlying sandstone. Garbh Eileach. (b) Internal bedding in mixtite 28-29, Eileach an Naoimh, produced by fine sandstone laminae. (c) A conglomerate above mixtite 25, Sgeir leth a'Chuain. Granitic fragments predominate; dolomites are relatively small and weather into holes. The ruler is 1 m long. (d) Sandstone downfold structures in the top of mixtite 26, Garbh Eileach. The ruler is 1 m long. (e) Ripple marks in the sandstone above mixtite 25, Sgeir leth a'Chuain. The ruler is 60cm long. (f) Large-scale cross-stratification in the sandstone at the base of member 3, Garbh Eileach. The scale is given by the man in the centre of the photograph. PLATE 5

(a) Rafted granitic pebble in the varved siltstones above mixtite 30, on the west coast of Garbh Eileach. The ruler is 30cm long. (b) Graded laminae seen in a photomicrograph of the varved siltstones above mixtite 32, on the east coast of Garbh Eileach. (c) Rafted dolomite pebbles in laminated siltstones in bed 3 of the Disrupted Beds, on the north coast of Garbh Eileach. The pebble at the centre of the photograph which is in shadow and standing vertically is 8 cm long. (d) Rafted granitic cobble within the varved siltstones above mixtite 31, western Garbh Eileach. (e) Chess-board albite. Photomicrograph of a detrital grain in mixtite 28. ( f ) Complex varves. Hand specimen showing complicated grading, from the dolomitic top of the varved siltstones above mixtite 32. East coast of Garbh Eileach. PLATE 6 (a) and (b) The Disrupted Beds. (a) Central Garbh Eileach. The beds shown are 12m thick. (b) Beannan Dubh, Port Askaig. (c) and (d) Photomicrographs of sandstones from the Garvellachs. (c) Dolomitic sandstone in member 1, between mixtites 16 and 17. (d) Feldspathic sandstone in member 3, between mixtites 36 and 37 (crossed polars). (e) and (f) Stellate structures from the top of the Islay Limestone, Garbh Eileach. (e) View onto a bedding plane in dolomitic siltstones. The hammer head is 10cm long. (f) Hand specimen showing an individual stellate structure in pebbly dolomitic siltstones. PLATE 7 Sediments from the Islay Limestone, Garbh Eileach. (a) Calcareous algal stromatolite. A concentrically laminated structure lying isolated in dolomitic siltstones. The ruler is 50 cm long. (b) Limestone flake breccia. Limestone flakes, which weather as hollows, are enclosed in a more prominent, light coloured, dolomitic siltstone. (c) One quarter of an algal stromatolite, to show the concentric lamination structure. From the same horizon as (a).

Mem. geol. Soc. Lond. no. 6

99

EXPLANATION

OF PLATES

(d) and (e) Small-scale polygonal cracks. Cross-sectional and plan views of polygonal cracks, infilled with dolomitic siltstone, which occur in a rock containing thinly interbedded limestones and dolomitic siltstones. In (d) the cracks can be seen to cut obliquely through the limestone bands and not to penetrate the lighter coloured, more prominent, dolomitic siltstone bands. PLATE 8 Sandstone wedges and sandstone dykes from the Garvellachs. (a) Polygonal sandstone wedges seen in plan view on the top surface of mixtite 22, Eileach an Naoimh. The ruler is 1 m long. (b) Sandstone wedge seen in cross-section penetrating downwards from the top of mixtite 15, west coast of Garbh Eileach. The wedge is truncated at its top by a pebble bed, overlain by bedded siltstones; it is of penecontemporaneous date. Notice also the faint structure within the wedge aligned parallel to its walls. The ruler is 25 cm long. (c) Polygonal sandstone wedges. A close-up view of the area just beneath the ruler in (a). The ruler here is 50cm long. (d) Sandstone dyke cutting through mixtite 24 and the siltstones of mixtite 23, north-eastern Eileach an Naoimh. The ruler is 1 m long. (e) An individual polygon from just to the right of (a). The ruler is 50cm long. ( f ) Granite stone cross-cut by sandstone dyke. In the top of mixtites 19-22 at the same locality as (e). The granite stone is 7 cm across. (g) Sandstone dyke cutting the siltstone of the Islay Limestone (bed 27, Fig. 31). PLATE 9 Geological map of the outcrops of the Port Askaig Tillite around Port Askaig. The I km lines of the British National Grid are marked at the map margins. The beds shown solid black near Loch Allan are regularly laminated (? varved) stiltstones. P L A T E l0 Stratigraphical columns in the Port Askaig Tillite at the following localities: A - - E a s t and south coast of Garbh Eileach. B - - C e n t r a l Eileach an Naoimh. C--South-eastern slopes of Beannan Dubh. D - - B y high water mark at Am Meall. E - - F r o m An T a m h a n a c h d to Creagan Loisgte. F - - F r o m Beinn na Cille to the Sound of Islay. G - - C o a s t a l section from Port Askaig to Caol Ila. H - - C o a s t a l section from Am Miadar to Con Tom. I - - F r o m Cnoc an t-Sleibh, via Cnoc Reamhar and the west end of Ardnahoe Loch, to [142 724]. J p C o a s t a l section from the area shown in Fig. 45 westward. K - - T h e south-eastern slope at Ballyhork Hill (see Fig. 46a). L - - T h e complete succession above the metadolerite at Croaghan Hill (see Fig. 46b). The mixtite bed numbers are given on the right side of each column. The numbers on the left side of each column, which are shown thus: 7/14, refer to the ratio of dolomite to granitic stones, respectively. They give the average number of stones larger than 1 cm present in an area 1 ft 2 measured on a vertical outcrop face. Solely to reduce the height of the diagram, parts of beds in columns H and I have been omitted; these beds are present and exposed in the field. P L A T E 11 (a) Horizontal section chart of the Port Askaig Tillite in the Garvellachs. The chart is semi-diagrammatic and shows the present exposures of the beds as if they were seen in the wall of an enormous trench. The horizontal scale is the same as for the map (b). Stratigraphical thicknesses of the beds are represented in the vertical dimension of the chart. These have only been measured in coastal outcrops and in a few inland localities. The variations in thickness shown for individual beds inland may not be real. They are commonly the result of the 'joining up' of the coastal thicknesses. (b) Geological map of the Garvellach Islands. The scale is given by the grid-lines, which are drawn at 200m intervals. Throughout the text localities in the Garvellachs are identified by means of references to this grid shown thus: {220 110} which is the reference for Sgeir poll an Dobhrain. For the sake of clarity, the localities where sandstone wedges occur are omitted from this map; they may be deduced from PI. 1 la.

100

Mem. geoL Soc. Lond. no. 6

Mem. geol. Soc. Lond. no. 6

~i8484

PLATE 1

i84!~i~ i

Folded dolomite fragment in the Great Breccia. [Full explanation on p. 99]

Mem. geol. Soc. Lond. no. 6

PLATE 2

(a)

(b)

(c)

(d) Mixtites in members 4 (a, b) and 3 (c, d). [Full explanation on p. 99]

Mem. geol. Soc. Lond. no. 6

PLATE 3

(b)

(d)

Mixtites in members 2 (a, b) and 1 (c, d). [Full explanation on p. 99]

Mem. geol. Soe. Lond. no. 6

PLATE 4

(a)

(5)

(c)

(,1)

(e)

(j) Features of the sediments [Full explanation on p. 99]

Mem. geol. Soc. Lond. no. 6

P L AT E 5

th)

Varved siltstones ( a - & . / ) a n d chess-board albite (e) [Full e x p l a n a t i o n on p. 99]

Mem. geol. Soc. Lond. no. 6

PLATE 6

(~,)

(b)

(c)

{d)

Ie)

(f) The Disrupted Beds (a, b), sandstones (c, d) and stellate structures (e, f). [Full explanation on p. 99]

Mem. geol. Soc. Lond. no. 6

P L AT E 7

(b)

J

(e)

Structures in the lslay Limestone, G a r b h Eileach. [Full explanation on p. 99]

Mere. geol. Soc. Lond. no. 6

t, L A1"1~ 8

(b)

(a)

~"" 7D,, "

:'+'

(d)

{<'t

(.f) (,c,) Sandstone wedges and sandstone dykes. [Full explanation on p. 100]

MEMOIRS OF THE GEOLOGICAL SOCIETY OF L O N D O N 1. Ring-complexes in the Younger Granite province of Northern Nigeria, by R. R. E. JACOBSON,W. N. MACLEOD & R. BLACK (1958) (£1.25) 2. Geological results of petroleum exploration in Britain 1945-1957, by N. L. FALCON & P. E. KENT (1960) (£1.25) 3. The Barr and Lower Ardmillan Series (Caradoc) of the Girvan district, south-west Ayrshire, with descriptions of the Brachiopoda, by ALWYN WILLIAMS(1962) (£5)

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