Mechanisms and environments of deposition of late precambrian geosynclinal tillites: Scotland and East Greenland

Palaeogeography, Palaeoclimatology, Palaeoecology, 51 (1985): 143--157

143

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

MECHANISMS A N D ENVIRONMENTS OF DEPOSITION OF LATE PRECAMBRIAN GEOSYNCLINAL TILLITES: SCOTLAND AND EAST GREENLAND

A. M. SPENCER

Statoil, Forus, Postboks 300, N-4001 Stavanger (Norway) (Received November 15, 1984; revised version accepted December 10, 1984) ABSTRACT Spencer, A. M., 1985. Mechanisms and environments of deposition o f late Precambrian geosynclinal tillites: Scotland and East Greenland. Palaeogeogr., Palaeoclimatol., Palaeoecol., 51: 143--157. The late Precambrian tillites in Scotland and East Greenland lie in marine, geosynclinal sequences and extend for hundreds o f kilometers. Both tillite formations, are hundreds o f meters thick and contain largely conformable internal stratigraphies with many individual mixtite beds separated by sandstone, siltstone, conglomerate and dolomite interbeds. Direct evidence for deposition of the mixtites from grounded ice sheets is provided by the angular unconformities beneath the tillite formations, by the huge rafts of deformed sediment in the Great Breccia in Scotland, and by the bedded horizons within mixtites, which are often lenticular as a result of deposition in sub- or englacial tunnels. Indirect evidence that the surface o f deposition was mostly too shallow for icebergs, or was even above sea level comes f r o m : algal stromatolites, mudcracks, tidal palaeocurrent patterns and coarse (beach?) conglomerates; from the several internal, marine erosion surfaces in Scotland; and from the sandstone wedges, inferred to be o f subaerlal permafrost origin. Both tillite formations thus record complex glacial histories, with 17 main glacial advances in Scotland and 2 in Greenland. The tillites are compared with the thick, but little known Quaternary sequences o f the North Sea Basin: they may have accumulated on continental shelves that were more gentle and extensive than the latter.

INTRODUCTION

The thick tillite formations in the late Precambrian sequences of Scotland and East Greenland are reviewed to assess their deposition. Both tillite formations are m a n y hundreds of meters thick and lie in marine geosynclinal sequences which are 10--20 km thick. Both show complicated internal stratigraphies, with boulder-rich tillite beds separated b y stone-free horizons. Both are geographically very extensive and b o t h show the intimate association of dolomite beds -- beneath, within and above the formations -- which is c o m m o n p l a c e in late Precambrian tfllites. The interpretation given to the deposition of the sequences are thus important. The non-glacial hypotheses for the origin of the tillites have been thoroughly debated before (Spencer, 1971, 1975; Schermerhorn, 1974, 1975) and so in this article only the glacial 0031-0182/85/$03.30

© Elsevier Science Publishers B.V.

144 hypotheses will be discussed. The most far reaching consequences result if the deposition of the tillites took place from ice sheets, for then repeated glacial advances and retreats are implied and large-scale, continental glaciations. In this article, therefore, two main topics will be discussed. First and most important, the criteria used to infer the mechanism of glacial deposition of the tillites will be reviewed: the criteria selected could be relevant in other thick tillite sequences. Secondly, several distinctive structures and facies/lithologies give clear indications of the environments of deposition, allowing suggestions to be made about the overall nature of late Precambrian topography. MECHANISMS OF TILL DEPOSITION The understanding of till deposition mechanisms is inherently difficult. It is perhaps the most hidden mode of deposition to which to apply uniformitarian methods. Depositional processes beneath grounded ice sheets can be predicted theoretically or studied by indirect means (e.g. geophysically), but they cannot be observed directly. Thus only the terminal melting zones at the peripheries of ice sheets and glaciers are available for direct observation today. Even there, observations are difficult -- the stratigraphy of the modern till deposits is rarely well seen -- and the relevance of the local observations and detailed mechanisms to interpreting widespread ancient glacial tills and tillites is doubtful. N. Eyles et al. (1983) have reviewed the possible mechanisms of deposition from grounded ice sheets. The ice sheets themselves were classified according to their basal thermal regimes, from which depositional mechanisms were inferred. The main features of the possible mechanisms and characters of the deposits suggested by N. Eyles et al. (1983) were these. Dry-based, polar, ice sheets (e.g. parts of Antarctica) remain frozen to and generally protect the substrate; during recession englacial debris is lowered onto the substrate; the resulting tills are crudely stratified reflecting the distribution of debris within the original glacier body. Wet-based, temperate glaciers ~e.g. Iceland) slide over the substrate, developing a basal debrisrich layer; shearing structures are to be expected there and in the main body of the till; lenticular fluvial stratified sediments form in subglacial channels. All intermediate stages between dry and wet-based types are possible. In addition, till, once deposited, is prone to re-sedimentation: d o w n , l o p e movements as mudflows are most common. Where glacial deposition takes place in open water from floating ice shelves or ice bergs, till-like deposits are highly likely to be accompanied by stratified, sorted sediments deposited by traction currents. Ice-rafted stones should be present. The till-like deposits are likely to be re-sedimented by winnowing or downslope mass movements (Miall, 1983). Interpreting the mode of deposition of the late Precambrian tillites studied here can also be undertaken by analogy with Pleistocene tills. In that case

145

the validity of any interpretation of Precambrian tillite deposition relies on the correctness of the interpretation of the mode of deposition of the Pleistocene till. SCOTLAND

The Port Askaig Tillite occurs in isolated outcrops for 700 km from northeast Scotland to western Ireland (Fig.l) and everywhere !ies at the same stratigraphic level within the Dalradian sequence, between formations

CHARCOT LAND : i

ii 9:

GAASELAND~

i

EAST GREENLAND

O

Fig.1. L o c a t i o n m a p of the late Precambrian tillites. Dots s h o w tillites within thick sequences; crosses are tillites resting u n c o n f o r m a b l y on crystalline basement rocks. The Caledonian orogenic belt is stippled. Contours s h o w thicknesses of Pleistocene in the North Sea in meters (Caston, 1979a).

146 which are rich in carbonates and contain st~omatolites. The Tillite is late Precambrian in age, probably pre-dating the beginning of the Cambrian by a few tens of millions of years. It is best exposed and least tectonically altered in the Garvellach Islands and at Port Askaig, where it is 750 m in thickness and contains 47 individual mixtites (0.5--65 m thick), separated by siltstone, sandstone, conglomerate and dolomite interbeds (0.1--200 m thick). The stones present range from almost exclusively sedimentary (e.g. dolomites, resembling lithologies in the underlying formation) in the lowest mixtites to mainly granitic with some regional metamorphic types in the highest mixtites. The deposition of the TiUite has been thoroughly studied only once (Spencer, 1971), subsequent authors' re-interpretations being based solely on that description (Schermerhom, 1974, 1975) or on minor additional fieldwork (C. H. Eyles and N. Eyles, 1983). In 1971 the reasons for rejecting a mass flow origin for the mixtites were given in full. Schermerhorn (1974, pp. 749--751) disputed that rejection, but in my view the rejection still seems valid and will not be debated again here. C. H. Eyles and N. Eyles (1983) have suggested an origin for the mixtites by glaciomarine deposition below floating ice. They based their suggestion on the internal bedding within the mixtites (evidence of traction currents in a lake or sea); the relative rarity of glaciotectonic structures/glacial erosion/glacial striation (no direct signs of the movement of grounded ice); and the overall conformable nature of the sequence, which was held to be strikingly unlike any Pleistocene or modern till sequence deposited directly by glacier ice. In 1971 I suggested 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". It is timely to review the lines of evidence on whether the mixtites are the deposits of grounded or of floating ice. With hindsight, it seems clear that insufficient importance was placed on some aspects of the information presented in 1971.

Interpreting mechanisms of tillite deposition The movement of grounded ice can leave a record directly in the form of abraded surfaces and glaciotectonic structures. In 1971 the emphasis in the account of the lower contacts of the mixtites was to stress that they were sharp, but conformable; that the gradational contacts to be expected if the tills were ice-rafted were not seen. This is true of the individual mixtites within the Formation, but the lower contact of the Formation itself is clearly a major unconformity (Spencer, 1975, p. 232). At Port Askaig, boulders of a 2 m thick dolomite can be seen almost in process of being broken off and incorporated into the lowest mixtite (Spencer, 1971, fig.43). This bed, the "Great Breccia", is again present at the Garvellachs 50 km away, but there it lies 100 m up in the Tillite (Fig.2, mixtite 13). In addition

147

FEATURES OF TOP

GLACIAL

SURFACE

INTERPRETATION

OF MIXTITE S

Ii_':i:/iii}! co

37 . ' , : ~ , . * .

EROSION ! SANOSTONE SANOSTONECONGLOMSURFACES WEOGES )OWNFOLD~ ERATES

t 0.~.1 oo

36

UJ

~ 34#o¢~" " 0

35

~',]

32

9

rr"

I,Ll

~'~

"

o.

;. ~'ol ~

u

1~

iooooooooOl

2C

~-G- ? :f,':! 16

--z-x-Ta-rzT-~ •-

~.,,o,¢

ee ,,,

,,... .

, --_-_. o

•',::~-::,,

,

o~

o =

.I

1 . . . . . . . . . . .

i

Si £,ZlC] -"

~ ='-"

° S.omato.t.s Stromatolites

_~

m~

granitic stones

Fig.2. The glacial record of Members 1--3 of the Port Askaig Tillite in the Garvellachs, Scotland.

148

to this gentle overlap, this basal unconformity also shows overstep, for the 72+m of thin bedded limestones, dolomites and pelites in the Garvellachs are cut out at Port Askaig. This fits with the dolomite stones in the mixtites, which have been eroded from such a sequence. The Great Breccia itself contains huge blocks of dolomite, of mixtite and more rarely of siltstone and sandstone, the largest measuring 320 X 64 X 45 m. Numerous of these fragments are folded and internally thrust (Spencer, 1971, plate 1). These features are glaciotectonic, and analogous to the huge folded chalk rafts in the Pleistocene tills of north Norfolk (Reid, 1882). Thus this particularly coarse mixtite shows several features typical of deposits from grounded ice: the pronounced glacial erosion of the underlying formation, the glacial erosion and recycling of the earlier mixtites; and glaciotectonic deformation. All these features are related to a regionally very gentle sub-tillite unconformity. Internal bedding within mixtites provides direct evidence of their mechanism of deposition. Most of the mixtites in the Port Askaig Tillite are homogeneous and lack internal stratification, but in perhaps 20% of the mixtites beds of sandstone, conglomerate and siltstone are present. Where present they are laterally discontinuous, often stopping at abrupt lateral contacts (Spencer, 1971, figs.4, 5, 39 and 41). How should these discontinuous lenses be interpreted? The clean sandstones and the coarse conglomerates require strong traction currents for transport and yet where these stratified lenses are absent there is no sign of a stratal break in the mixtite. Deposition of the lenses in sub. or englacial tunnels could explain this configuration; the winnowing action of marine currents acting on a sea floor composed of ice rafted mixtites would not. A third, but indirect, line of evidence that most mixtites are not icerafted is this. Within the Formation dropstones are clearly seen at seven or more horizons (Fig.2) in fine, regularly laminated siltstones, perhaps varves. The rafted stones penetrate and deflect the laminae. These horizons at which ice-rafting can be demonstrated are clearly distinct from the homogeneous or lenticularly stratified mixtites deposited by grounded ice sheets. Also, these ice-rafted horizons form only a minor part of the Formation, most of the interbeds between the mixtites are completely free of rafted stones. There are several other indirect lines of evidence that suggest the mixtites are deposits of grounded ice. All of them also throw light on the geographical/ topographical environment at the time of deposition.

Environments of deposition The bedded sediments beneath, in and above the Port Askaig Tillite were deposited in shallow waters, probably in a shelf sea: algal stromatolites, mudcracks, tidal palaeocurrent patterns, coarse (beach?) conglomerates and the overall "clean" lithologies all suggest this.

149

Throughout the Formation, minor disconformities and erosion surfaces are present. For example, the erosion surface above mixtite 26 (Fig.2) cuts down c. 20 m in a horizontal distance of 1 km (Spencer, 1971, plate 11): the mixtite, its stones and boulders have all been removed. Powerful erosion is implied -- probably due to the transgressing action of a sea. Sandstone wedges which are interpreted to be of subaerial, permafrost origin are present at 27 stratigraphic horizons (Fig.2). They are not load structures: they are too narrow and deep (Fig.3), in one case they penetrate a granite conglomerate, and they contain vertical bedding and verticallyaligned pebbles. The most decisive feature demonstrating they are not load structures is their stratigraphic relationship, however, for they are very often overlain and truncated by an erosion surface. Since these wedges were described in 1971 other examples have been found in tillites around the world (e.g. Chumakov, 1968; Hambrey and Harland, 1981, pp. 1 0 4 , 4 6 0 , 6 2 2 , 6 4 8 , 785,814, 847; Deynoux, 1982). Sandstone downfold structures are basin-shaped structures which occur at 15 horizons, many at the tops of mixtites (Fig.2). They may be cryoturbation structures, but load casting is an equally likely explanation. Often they are stratigraphically associated with sandstone wedges, conglomerates and an erosion surface. The relationships at the top of mixtite 26 (Fig.4) show the time sequence of this association: (1) Mixtite deposition (ice sheet); (2) Sandstone wedge formation (permafrost); (3) Erosion (marine transgression); (4) Sandstone downfold formation (load casting); ( 5 ) " N o r m a l " sedimentation. These environmental indicators, taken together, imply that the geographical area was frequently exposed subaerially during the deposition of the Formation. This, plus the origin of the mixtites as deposits of grounded []

'Normal'

Sediments

Bedded

--

' ~Lag

•

"

" ~ ; f ~ a ~Ji[~~ .

• •

.

'

•

;

.;~:i

' .

.

.

..:"~;

•

•

"

.~

'

.

•

. .

. .

"o

' ,,

o

.

¢~

.t~i~(" ~ :""

O ".

O ,

-

~

-

.

~-~

•

. o

"

•t ~ " . ~ .

"

o o

"

"i!."

'

surface

o

:i;i!(i'

o"

conglomerate

b "~'Er0son .

::,!.:

•

•

"

.~

o

'

"¢>

• -.= "

ti'!.~ .~-...~:.:.-/..

° " qb "

~-t.-

'o'

.

-

~

".

. .

.

, • ~. " - ~ •

O

"

.

.;c,,..°

~

.

~' m

t~ " Y " . o

[]

Mixtite deposition

,

p

.

P--,-t

Widths

< 300mm

Fig.3. Sandstone wedges (2) in the Port Askaig Tillite. The diagram shows that wedges form after mixtite deposition ( / ) and before erosion and lag conglomerate formation (3). Vertical lamination (a) and vertically aligned pebbles (b) are present in a few wedges.

150

!

1 kilometre

i [10 metres

Fig.4. Diagrammatic profile to show the stratigraphic relationships of mixtite 26 in the Garve]lachs. Mixtite deposition (1) was followed by sandstone wedge formation (2); then the mixtite and wedges were completely eroded (3) and finally the transgressive pebbly sandstone was deformed into sandstone downfolds (4). Redrawn based on Spencer (1971, plate 11).

icesheets, leads to the glacial history shown in Fig.2. There were repeated cycles of glacial advance, melting, permafrost conditions, erosion (by marine transgression) and shallow water sedimentation. EAST GREENLAND

The Upper and Lower Tillites of Central East Greenland occur in beautifully exposed outcrops in the fjord zone between 72°N and 74°N, a distance of over 300 km. T h e y lie in the 900 m thick Tillite Group (Haller, 1971; Higgins, 1981) above the many kilometers thick Eleonore Bay Group and beneath the 2--3 km thick Cambro~:)rdovician sequence (Fig.5). The Lower TiUite (up to 100 m thick) contains stones of limestone and dolomite and, at its top, crystalline fragments. In the Upper Tillite (up to 200 m thick) crystalline stones predominate (e.g. red granite, porphyry, gneiss) and the matrix is usually red in colour. Lower Cambrian body fossils are first recorded in the Shell Limestones of the Upper Bastion Fro. (Fig.5) (Cowie and Adams, 1957). The sandstones in the Lower Bastion Fro. have yielded trace fossils (Cowie and Spencer, 1971), including probable arthropod scratch marks. The base of the Cambrian has conventionally been taken at the base of the Kl~bftelv Fro. (Haller, 1971), but -- on the basis of regional correlations (Cowie and Spencer, 1971) -could be older and lie within the Spiral Creek or Canyon Fins. Acritarchs have been found in the Canyon Fm., the Inter Tillite Beds and the Lower Tillite (Vidal, 1976, 1977, 1979) and indicate a Vendian age. In the underlying Limestone--Dolomite Series, acritarchs suggest a Riphean age (Vidal, 1976, 1977, 1979). Earlier studies of stromatolites from the Limestone-Dolomite series, however, suggested a Vendian age (Bertrand-Saffati and Caby, 1974).

151

~ 2200m exposed (to MOrdovlcian) CAMBRIAN BASTION FM

fossils

1000

KL~FTELV FM

gentle unconformify SPIRAL CREEK FM

CANYON FM

500

TILLITE GROUP

UPPER TILLITE

INTER TILLITE BEDS

LOWER TILLITE

:onformffy Bed group 20

UPPER ELEONORE BAY GROUP

Bed group 19

Bed group 18 ~800m exposed

Fig.5. Outline stratigraphic column of the Tillite Group in East Greenland.

In interpreting the mechanisms of tillite deposition and the environments of deposition many characters are similar to Scotland and the same overall conclusions can be reached. The Lower Tillite is the first sandy formation in the stratigraphic sequence for over 1 0 0 0 m: its lower contact is everywhere knife sharp. This basal contact is, again, a very gentle angular unconformity.

152 The contact oversteps from Bed Group 20 down onto Bed Group 19 towards the south (see Fig.7). No outcrops which expose the contact in plan view (to allow the presence of striations to be observed) have been seen. However, stones derived from the black limestones of Bed Group 20 are commonly striated (Poulsen, 1930, fig.17; Schaub, 1955, fig.2). This abrupt, erosional contact would not be expected if the Tillite were of ice-rafting origin: it could be developed by the abrasion of a grounded ice sheet. Glaciotectonic structures have not been seen, but the contact itself is rarely well exposed; such structures should be looked for in the extensive outcrops of the contact in Suess Land, to the north of Ella Island. Internal bedding is present within both Tillites. These are beds of sandstone and of dolomite or granitic conglomerate, which mostly lie parallel to the overall stratification, themselves show planar or cross-bedding intemaUy and are frequently seen to be lenticular. This lenticularity is best seen in the Upper Tillite on Ella Island. For example, the detailed internal stratigraphy of the Upper Tillite shown in Fig.5 is the stratigraphic column measured at the Storeelv there; the lowest conglomerate bed shown (marked c) dies out 200 m to the east and the lower two mixtites merge inseparably. Similarly, the thicker bedded horizons within, particularly, the Upper TiUite cannot be correlated from outcrop to outcrop within the fjord zone, probably due to the lenticularity of the horizons. These lenticular bedded horizons are evidence for a ground moraine origin. Such lenticularity would not be developed by marine bottom currents acting on ice-rafted till, but could be produced by meltwater flowing in subglacial channels. As in Scotland, a third but indirect line of evidence that most mixtites are not ice-rafted is that a few ice-rafted horizons are clearly recognizable. There are three horizons, 9-m, 8-m and 6-m thick, of fine laminated siltstones (laminae 1--10 mm thick) which contain stones as large as 1.25 × 0.55 × 0.4 m. Again, these ice-rafted horizons form only minor beds in the two Tillite Formations, whilst all of the Inter Tillite Beds are completely free of rafted stones. Sandstone wedges and sandstone downfolds are both present, but often in association. They are best developed on the top surface of the Lower Tillite, where they form polygons with diameters of 3--5 m (Fig.6a); in cross-section they are mostly rounded basins of pebbly sandstone penetrating 1.5 m into the tillite. At Syltoppene sandstone downfolds are associated with narrow wedges (Fig.6b) and are penecontemporaneous, for they are truncated by overlying shales. The clearest, narrow, V-shaped wedges were seen at Brogetdal penetrating 3 m through a dolomite conglomerate; they are truncated by the base of the overlying sandstone bed. The origins of the sandstone wedge and downfold structures must be linked. Both are often demonstrably penecontemporaneous, and both frequently lie at the tops of mixtite beds but occasionally penetrate bedded sediments. Narrow, V-shaped sandstone wedges are much less frequent here than in the Port Askalg Tillite but very similar in character: they are probably periglacial

153

- ~

..~

:,::"'-., .

"

....

0 I

:~,:i:~

•

-

~" ........

/

-...',:; ~ . .

:C

/

....

.~.

...... i

i

"./22 C~,~,.'~-

~:

:'~-

~

' :.~

i

i

•

. . . . . .

~;~':";'. ~<::.';

........

~

I

:,"o

...........

:

6~0 m

red s h ~ e

.

.

"

-

- . . ~ . . "

:--v...-

~31:..

:.

'."-

. . . . .

SOUTH

-

80mj NORTH

Ifoutt

1fault

. . ~ ~. :. : . . , . .. . . .. .. . . . .:. :. ~ .--. #-.,., .:-, , ~_..... !~.:-<_-~-...° .. . . . ,o

Ifctutt

.

. ..,

Ib)

,

*

Fig.6. Field sketches of sandstone wedges and downfolds from East Greenland. (a) Plan view o f polygonal pattern o f sandstone wedges/downfolds in t o p o f Lower Tillite, south of Ulves~, Ella Island. (b) View in profile of sandstone wedges/downfolds at t o p o f 15-m mixtite bed within Lower Tillite at Sylltopene.

contraction~rack infillings. The origin of the sandstone downfolds could not be decided in the Port Askalg Tillite. Here, however, t h e y are arranged in polygons and are associated with wedges: t h e y are thus likely to be glacial involution structures produced b y cryoturbation. The sequence above and below the Tillites shows many signs of shallow or emergent conditions (pisolites, stromatolites, m u d cracks, salt crystal imprints). Thus, just as in Scotland, the environmental indicators support the features which directly suggest grounded ice sheet action (basal unconformity; lenticular internal bedding). The L o w e r and Upper TiUite are thus interpreted as representing two separate glaciations when grounded ice sheets covered the area. Some of the internal bedded horizons, particularly in the U p p e r TiUite, m a y represent minor ice retreat episodes, as do the three horizons at which ice-rafting was recognized. DISCUSSION

Which of the criteria are most useful in allowing the mechanisms of deposition of the tfllites to be interpreted? Firstly, the unconformable basal contact of the Tillite formations is characteristic of the action of an invading, grounded ice sheet. Such a contact would n o t be expected if the mixtites originated b y ice-rafting into an extensive sea. Secondly, although glaciotectonic structures have n o t been seen affecting the t o p of the preglacial formations, appropriate exposures are few or n o t y e t studied. In Scotland, however, the signs of glaciotectonic action are preserved in the

154 folded and thrust boulders and rafts present in the Great Breccia. Third, the abrupt lenticularity of the bedded siltstone, sandstone and conglomerate lenses within the mixtites is direct evidence of the depositional mechanism: by sub- or englacial meltwaters in a grounded ice sheet. Fourth, in the minor beds where ice-rafting can be recognized, the laminated siltstones with dropstones are clearly distinct from the massive to lenticularly stratified mixtites. Fifth, all of the evidence on the geographical environments at the time of deposition points to very shallow water to emergent conditions: permafrost features axe frequently developed on the tops of mixtite beds. Taken together, these criteria demonstrate that both in Scotland and Greenland the great majority of the mixtites were deposited by the melting of grounded ice sheets. This, plus the ice-free conditions implied by the "normal" sediments, leads in the case of Scotland, for example, to the glacial history shown in Fig.2. Except at the bases of the formations, no signs of disturbances due to the repeated ice advances have been seen. Why is this? Perhaps the explanation is that the analogous Pleistocene deposits are n o t the well known, thin, irregular tills preserved on land in, for example, England, but the very much thicker sequences in, for example, the North Sea depositional basin (Fig.7). Do the latter show conformable or gradational contacts? Do they show many or few signs of glaciotectonics? How great is the strati~aphic variability SW YORKSHIREI O

NE

I DENMARK

4OO 6OO

8O METRES.

A

SW GLENCOLUMBKILLE

PORT NE ASKAIG GARVELLACHB SCHICHALLION

FANAD

200 4OO

VERTICAL EXAGGERATION xlO0

6OO 80

B

lo METRES

0'~ ~00 METR~

~; I

~'~::; ~

V ...... ,~ . . . . ~w.-Uppe¢Tillite i ...... Inter-TiIlile Beds Lower Tilllt e 0

Fig.7. Profiles o f the Quaternary sequence in the North Sea (A), the Port Askaig Tillite (B) and the TiUites in East Greenland (C). See Fig.1 for localities. In the Port Askaig Tillite members 1--5 are shown, but only 1--4 are rich in mixtites and have therefore been stippled.

155 km. 8•

Average height ~

2~

o,

0-

Continental shelves

~_ -

-~

....

I__

~

~ ~

I

5o

I

too

I

~5o

~'

I

2oo

I

25o

~

~

~o

35o

-2 e,~,. Average depth of Sea

i

Milli~ Square Kitometres i I I 4oo

0

I

4~o

I--10 km II

soo

Fig.8. Hypsographic curve of present day Earth (from Holmes, 1965)•

there? These questions remain to be answered even though, in recent years, many new data have become available as a result of the exploration for and exploitation of hydrocarbons (Caston, 1979b). The stratigraphic completehess and wealth of detail available from the beautifully exposed late Precambrian tillites of Scotland and East Greenland is not available at present for the comparable, thick, Pleistocene sequences of the North Sea basin. Perhaps the former should serve as a model for the latter? Was the overall topography o f the areas subjected to glaciation in late Precambrian times comparable to that of the regions glaciated in Pleistocene times? The late Precambrian tfllites are often associated with very shallow water facies (stromatolites), are predominantly conformable in their stratigraphic relations, are of great lateral extent and often show similar, simple "unroofing" sequences (intrabasinal, dolomitic stones in lower tillites; extrabasinal, granitic stones in higher ones). These features appear anomalous because of their simplicity. Pleistocene grounded ice sheet deposits appear much more complex. Hence it has been suggested that the Precambrian sequences cannot be the deposits of grounded ice sheets, but must have been formed by ice-rafting. However, these anomalous features can perhaps be explained if the glaciated regions possessed very gentle, flat topographies, close to sea level. If late Precambrian continental shelves were much more extensive than those of today (Fig.8) many of these apparent "anomalies" of the late Precambrian tillites might be expected. ACKNOWLEDGEMENTS

The data on Greenland were collected when the author was kindly allowed to accompany the 1968 Cambridge expedition under the leadership of Dr. P. F. Friend. Mr. M. Parr is thanked for assistance in the field. Dr. A. K. Yeats is thanked for commenting on the manuscript.

156 REFERENCES Bertrand-Sarfati, J. and Caby, R., 1974. Pr6cisions sur l'~ge pr6cambrien terminal (vendien) de la s6rie carbonat6e ~ stromatolites du groupe d'Eleonore Bay (Groenland Oriental). C.R. Acad. Sci. Paris, 278: 2267--70. Caston, V. N. D., 1979a. A new isopachyte map of the Quaternary of the North Sea. In: E. Oele, R. T. E. Schuttenhjem and A. J. Wiggers (Editors), The Quaternary History of the North Sea. Acta Univ. Ups. Syrup. Annu. Quingentesimum Celebrantis, 2: 23--28. Caston, V. N. D., 1979b. The Quaternary sediments of the North Sea. In: F. T. Banner, M. B. Collins and K. S. Massie (Editors), The North-West European Shelf Seas: the Sea Bed and the Sea in Motion. Vol. L Geology and Sedimentology. Elsevier, Amsterdam, pp. 195--270. Chumakov, N. M., 1968. Late Precambrian glaciation of Spitsbergen. Dokl. Akad. Nauk S.S.S.R., 180:1446--1449 (in Russian). (Transl. Am. Geol. Inst., 180: 115--118). Cowie, J. W. and Adams, P. J., 1957. The Geology of the Cambro-Ordovician rocks of Central East Greenland. Part I. Stratigraphy and Structure. Medd. Gr~bnl., 153(1). Cowie, J. W. and Spencer, A. M., 1971. Trace fossils from the Late Precarnbrian/Lower Cambrian of East Greenland. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils. Seel House Press, Liverpool, pp. 91--100. Deynoux, M., 1982. Periglacial polygonal structures and sand wedges in the late Precambrian glacial formations of the Taoud~ni Basin in Adrar of Mauritania (West Africa). Palaeogeogr., Palaeoclimatol., Palaeoecol., 39: 55--70. Eyles, C. H. and Eyles, N., 1983. Glaciomarine model for upper Precambrian diamictites of the Port Askaig Formation, Scotland. Geology, 11: 692--696. Eyles, N., Eyles, C. H. and Miall, A. D., 1983. Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences. Sedimentology, 30: 393--410. Hailer, J., 1971. Geology of the East Greenland Caledonides. Interscience, New York, N.Y., 413 pp. Hambrey, M. J. and Harland, W. B. (Editors), 1981. Earth's Pre-Pleistoeene Glacial Record. Cambridge Univ. Press, Cambridge, 1004 pp. Higgins, A. K., 1981. The late Precambrian TiUite Group of the Kong Oscars Fjord and Kejser Franz Josefs Fjord region of East Greenland. In: M. J. Hambrey and W. B. Harland (Editors), Earth's Pre-Pleistocene Glacial Record. Cambridge Univ. Press, Cambridge, pp. 778--781. Holmes, A., 1965. Principles of Physical Geology. Nelson, London. Miall, A. D., 1983. Glaciomarine sedimentation in the Gowganda Formation (Huronian), Northern Ontario. J. Sediment. Petrol., 53: 477--492. Poulsen, C., 1930. Contributions to the stratigraphy of the Cambro-Ordovician of East Greenland. Medd. Gr~bnl., 74: 297--316. Reid, C., 1882. The geology of the country around Cromer. Mere. Geol. Surv. G.B. Schaub, H. P., 1955. Tectonics and morphology of Kap Oswald (NE-Greenland). Medd. Gr~nl., 103, 33 pp. Schermerhorn, L. J. G., 1974. Late Precambrian mixtites: glacial and/or non-glacial? Am. J. Sci., 274: 673--824. Schermerhorn, L. J. G., 1975. Tectonic framework of Late Precambrian supposed glacials. In: A. E. Wright and F. Moseley (Editors), Ice Ages: Ancient and Modern. Geol. J., 6: 241--274. Spencer, A. M., 1971. Late Precambrian glaciation in Scotland. Mere. Geol. Soc. London, 6,100 pp. Spencer, A. M., 1975. Late Precambrian glaciation in the North Atlantic region. In: A. E. Wright and F. Moseley (Editors), Ice Ages: Ancient and Modern. Geol. J. Spec. Iss., 6: 217--240.

157 Vidal, G., 1976. Late Preeambrian acritarchs from the Eleonore Bay Group anti Tillite Group in East Greenland. Rapp. Gr~bnl. Geol. Unders., 78, pp. 1--17. Vidal, G., 1977. Late Precambrian microfossils. Geol. Mag., 114: 393--394. Vidal, G., 1979. Acritarchs from the Upper Proterozoic and Lower Cambrian of East Greenland. Bull. Gr~bnl. Geol. Unders., 134: 1--~55.