Till lithology and glacial transport in Kuhmo, eastern Finland HANNU PELTONIEMI

Peltoniemi, Hannu 1985 03 01: Till lithology and glacial transport in Kuhmo. eastern Finland. B o r m . V ~ I . 14, pp. 67-74. 0s10. ISSN 0300-9483. BOREAS Till lithology and transport distance were studied along five transects running in the direction of ice flow and intersecting the N-S-oriented Kuhmo Greenstone Belt, which is somc 5 km in width. A total of 531 stone counts were performed on three fractions (>20 cm, 6-20 cm and 2-6 cm) in 162 pits dug with a mcchanical excavator. An experimental model is developed for predicting the transport distances of clasts in basal tills. It shows the traditional method of expressing transport in terms of half-disrance (i.c. the distance at which the proportion of a given rock type in the till has been halved from what it was at the distal contact of a given rock type in the bedrock) to be dependent upon the width of the source unit in the bedrock, varying in the present case from 0 km to 16 km as the width of the source belt increases from 0 km to infinity. The Kuhmo Greenstone Belt being 5 km broad, the mean half-distance for the transport of stones and boulders in the till is 2 km, the boulders having been movcd somcwhat shorter distances and the pebbles longer distances. It is rccommendcd that transport distances for till material should he exprcswd in terms of thc renewal distance (i.c. the distance over which the proportion of a ncw rock typc increases from 0 % to 50 %). In the Kuhmo area this distance is 16 km.

Hannu Peltoniemi, University of Oulu, Department of Geology, SF-90570 Oulu. Finland; March. 1984.

The purpose of this work was to determine the lithological composition of the boulder and stone fractions and the transport distances of these fractions in till in the northwestern part of the commune of Kuhmo in central eastern Finland (Fig. 1). The area is crossed in a north-south direction by the Kuhmo Archaean Greenstone Belt, with a mean breadth of 5 km and contain- ing predominantly ultrabasic and basic rocks which differ markedly from the surrounding granitoids. The principal directions of movement of the continental ice sheet run virtually at right- angles to this Greenstone Belt, i.e. WNW-ESE, and consequently the area offers ample scope for research of the present kind.

The fieldwork was carried out in the summers of 1979 and 1980 as part of the Kuhmo Ore Project (Saarnisto et af. 1980, 1981), and the results published in more extensive form in Finn- ish at that stage (Peltoniemi 1981). The present article is a summary of the paper and its results.

The area studied The area in question is of relatively flat topogra- phy, with absolute altitudes ranging 160-260 m above sea level. Typical features of the landscape are small hills and elongated ridges separated by innumerable lakes and mire areas.

Fig. I . Index map of the area studied in Kainuu, castcrn Finland.

hX Hunnu Peltonierrii BOKEAS 14 (1985)

1 9

29.40

)ig. 2. Glacial landfornms in the area studied. The outer limlts of the Kuhmo Greenstone Belt and the transects used for sampling are also indicated

General bedrock geology The bedrock of Kuhmo forms part of the exten- sive Archaean Basement Gneiss Complex of eastern Finland (Simonen 1980), and is crossed by the long but relatively narrow N-S-oriented Kuhmo Greenstone Belt, which is characteristi- cally composed of mafic, ultramafic and felsic vulcanites, and to a lesser extent of iron forma- tions and sedimentary rocks (see Taipale et al. 1983). Proterozoic ultramafic and mafic intru- sives aged around 2100 Ma are fairly common in this part of the schist belt (cf. Hanski 1983). The

bedrock on both sides of the Greenstone Belt is composed of tonalites, granodiorites, granites and granite-gneisses.

General glacial geology The glacial geology of the area has been studied by Kilpi (1937) and Virkkala (1948a, 1948b) as part of their more extensive surveys, while its glacial stratigraphy has been described recently in more detail by Saarnisto etal. (1980, 1981) and Saarnisto & Peltoniemi (1984), who recognize three till units, all of which were deposited by ice

BOREAS 14 (1985) Till lithology and glacial transport 69

flows running in nearly the same direction, WNW. The lowest of these is a rather sporadic dense, greenish till containing large amounts of bedrock weathering products. This is overlain by the ubiquitous till in the area, a massive basal till, while above this is a loose, non-oriented, or only poorly oriented till which has been interpreted as having been deposited in part subglacially in a semi-stagnant marginal zone to the glacier and in part as a genuine supra-glacial ablation till (Saar- nisto & Peltoniemi 1984).

This ablation till forms extensive fields of hum- mocky moraines, especially in the central and northwestern parts of the area, these in turn frequently constituting elongated chains running parallel to the eskers. The second type of till landform commonly encountered in the area is represented by the drumlins (Fig. 2 ) , which were formed of the basal till during the Weichselian deglaciation about 10,000 B.P., when the ice margin was located close to the Kuittijarvi for- mations in Soviet Karelia. The absolute age of the lowermost till is not known, but it is separat- ed from the uppermost till in places by sorted deposits which could be indicative of a deglacia- tion (Saarnisto & Peltoniemi 1984).

Methods The fieldwork included the digging of 162 pits with a mechanical excavator along five transects crossing the schist belt in a direction parallel to the movement of the ice sheet (Fig. 2). These pits were located at a mean distance 0.5 km apart in the area overlying the schist belt and close to it on either side, the intervals becoming progres- sively longer further away from the distal contact, up to a maximum distance between two consecutive pits of the same transect of 2.5 km. The southernmost transect was the longest, mea- suring 45 km in all and extending 35 km from the distal contact of the schist belt. The shortest transect was 11 km in length.

A total of 531 stone counts were made from the pits, employing three fractions: >20 cm, b 20 cm and 2-6 cm, involving the identification of 100-200 stones to rock type in each case. The >20 cm fraction was determined from surface boulders found in the immediate vicinity of the pit, while the counts for the smaller fractions were based on the till units identifiable in the pit, at least one count being made for each size class in each unit. A geological hammer, sheathknife,

magnet and stereo microscope (8 X ) were used as aids in identifying the rock types.

As the stone counts advanced the following classes of rocks emerged to which the stones encountered in the tills could be assigned:

A. mafic and ultramafic vulcanite and serpen-

B. felsic schists (incl. felsic vulcanites, mica

C. gabbroic amphibolite and diabase, and D. granitoids.

tinite,

schists and quartzite).

Results Till lithology The results of the stone counts were analysed in terms of a set of six zones drawn at right-angles to the direction of ice flow: the proximal zone (west of the Greenstone Belt), the belt itself, and 0 4 km, 5-10 km, 10-20 km and 20-35 km from the distal contact (east of the belt). The mean lithological composition for each of the till frac- tions was calculated for each zone separately from the stone counts for the two uppermost till units, which proved to be very similar in compo- sition (Fig. 3). The results for the lowest till, which occurred only sporadically and appeared to have been transported over shorter distances. were discounted. The counts included in the ana- lysis represent for the most part basal till which had been transported subglacially or englacially, although some supraglacial ablation till results are also used. The material is therefore some- what heterogeneous in origin.

Where more than one stone count had been made from each fraction in a given pit, the mean lithological compositions for the various fractions were calculated first and these results then used to obtain the mean compositions for the fractions in each zone. Since these mean values were de- rived from pits located on five separate transects they may be said to be unaffected by the local fluctuations in till composition which can be caused by topographical factors or varying thick- nesses of the till bed (cf. Perttunen 1977; Pelton- iemi 1981), and to reveal more clearly the full extent of the effect of the Greenstone Belt upon the till.

The first feature to stand out from Fig. 3 is the fact that the proportion of rock types in class A, mafic and ultramafic vulcanite and serpentinite,

7( ) Nun 11 u Pelton i c w i I BOREAS 14 (1985)

% > 20 cm %

0 100

20 80

40 60

60 40

80 20

100 0 THE KUHMO 0 5 10

ESE k m

WNW GREENSTONE BELT

% 2-6 cm %

100 0 THE KUHMO 0 5 10

ESE I

WNW BELT km

.J.q 3. Mean lithological compos~t~on ol three t i l l fractions h) m n c s along transects running in the direction of ice flow. A. Mafic m d ultramafic ~ u l c a n i t c and serpcntlnitc. I3 felsic schists. D. gabhrolc mphibol i te and d~ahase. and D. granitoids.

BOREAS 14 (1985) Till lithology and glacial transport 71

% 20

18

16

14

12

10

8

6

4

2

THE KUHMO 0 5 10 GREENSTONE 1

km ESE

BELT WNW

Fig. 4. Proportions of material derived from the Kuhmo Greenstone Belt in the till overlying the belt and in distal zones arranged in the dircction of ice flow (means for the thrcc fractions comhined). A . Mafic and ultramafic vulcanite and scrpentinite. B . fclsic schists, and C. gabbroic amphibolite and diahase

which accounts for the majority of the rocks of the Greenstone Belt in this area, is highest in the boulder fraction at the sites on the belt itself and lowest in the pebble fraction. Equally clear, too, is the fact that this class of rocks declines in importance on the distal side of the Greenstone Belt most rapidly in the boulder fraction and least so in the pebble fraction. This longer trans- port distance for the smaller stone sizes, which has been noted in numerous earlier works (see Hellaakoski 1930; Dreimanis & Vagners 1969; Virkkala 1971; Perttunen 1977), may be best explained in terms of the ‘milling’ effect of the ice, which produces ever greater amounts of the smaller fractions at the expense of the larger ones, an effect which holds good to the extent that the smaller the fraction one considers the greater the range of larger fractions from which it can receive additional material. A further conse- quence of this is that the reduction in the propor- tion of the rock type studied in the fractions which gain material in the course of glacial trans- port does not take place in an exponential man- ner on the distal side of the source rock unit. Hence the negative exponential function of Krumbein (1937), as employed by authors such as Gillberg (1965, 1967a, 1967b) and Perttunen (1977), is not capable of describing the transport

distance of the till material, with the possible exception of the largest, boulder fraction.

A second obvious feature to emerge from Fig. 3 is the influence of the Kainuu Schist Belt on the results for rock class B, the felsic schists, in the area west of the Greenstone Belt, especially in the stone fractions. This some 20-30 km broad Schist Belt is composed mainly of rocks of sedi- mentary origin and located some 40 km away to the west (see Fig. 1). This class continues to increase in proportion in the area of the belt itself, but declines on the distal side until it be- comes so insignificant by the outermost zone (20-35 km) that the granitoids, class D, achieve higher percentages in all the fractions than they did to the west of the Greenstone Belt.

A third observation regarding Fig. 3 is the fairly even representation of rock class C, gab- broic amphibolite and diabase, from one zone to another in all the fractions studied.

Since it is not possible to estimate directly on the basis of Fig. 3 the proportions of the boulders and stones in the various zones which are derived from the Kuhmo Greenstone Belt, these are pre- sented separately in Fig. 4, calculated as means for the three fractions studied. As far as rock class A is concerned, this diagram relies on the assumption that all the stones and boulders of

72 Hannu Peltonienii BOREAS 14 (1985)

l 6

l o o T -

- 5 i o 1’5 20 2 5 30 35 i o 45 so 55 e’o 6 5 70 75 ao 65 00 05 loo km

Fig. 3. Expermental model t o r the tran$port distances of till claats in the Kuhmo area (for explmation. see text)

this kind found on top of the Greenstone Belt o r on its distal side have originated from the belt. Similarly. the principle has been followed with class B that the proportions of these rocks in the various fractions in the area west of the Green- stone Belt have been subtracted from the corre- sponding proportions at the sites on the belt

contact, 10 % at distances of 5-10 km, 6.4 % at 10-20 km and 5.5 % at 20-35 km. It can also be seen that the proportions of the rocks of the Greenstone Belt as they appear in the till corre- late well with the proportions observed in that bedrock unit itself (see Taipale et al. 1983).

&elf. after which the mean difference for the fractions combined has been calculated. This fi-

Mode[ for the transport distances of till clasts

gure is then taken as the mean proportion of rocks of class B derived from the Greenstone Belt to be found in the till overlying the belt, the proportion being assumed to decline with dis- tance on the distal side at the same rate as the proportion of rocks in class A . Finally, since the proportion of rocks of type C remains fairly con- stant in all zones and all fractions, at around 7 . 5 % . this figure is taken in the diagram to represent the mean proportion of boulders and stones of this rock class derived from the Green- stone Belt to be found in the overlying till, from which point it is again assumed to decrease on the distal side at the same rate as the rocks of class A.

The estimates constructed according to the above principles allow us to conclude from Fig. 4 that an average of approx. 20 % of the boulders and stones in the till in the area of the Kuhmo Greenstone Belt is derived from the belt itself, the proportion then decreasing to 15 ‘4- on aver- age in sites less than 5 krn away from the distal

Extrapolating from these data obtained from the 5 km wide Kuhrno Greenstone Belt, one may go on to estimate the extent to which rock units of widths 10, 15, 20, . . . km might affect the litholo- gical composition of the corresponding tills. For this purpose the model depicted in Fig. 5 has been constructed. in the following manner. The proportions of material derived from the known 5 km wide unit found in the till overlying that unit and at given distances from its distal contact, as indicated in Fig. 4, are depicted in the shaded columns. It is then assumed that every 5 km increase in the width of the source belt will lead to an increase in the proportion of derived mate- rial in the overlying and distal tills equal in mag- nitude to that contributed by the first 5 km. The thick solid curve in the diagram thus denotes the increasing proportion of material in the till as a function of the width of the source unit, setting out from 0 % , when the breadth of the source unit is 0 km. The increase is seen to be rapid at first, but to become progressively less marked as

BOREAS 14 (1985) Till lithology and glacial transport 73

the source unit becomes wider, reaching 100% only at infinite width. The thinner solid lines falling away from this curve then denote the corresponding decreases in the proportions of derived material with increasing distance from the distal contact. These are replaced by broken lines once the proportion has decreased to a half of its initial value, i.e. that which it had at the distal contact. Thus by projecting the beginning and end points of one of these solid lines onto the distance scale and measuring the interval concerned, one can determine the ‘half-distance’ value of the material from a lithological unit of a given breadth, i.e. in the traditional sense, the distance at which the proportion of a given rock type is halved from what it was at the distal contact of a given rock type in the bedrock. This is evidently the most commonly used measure of the transport distance of till material (see Hirvas et al. 1977; Perttunen 1977).

The half distance for the rocks of the 5 km wide Kuhmo Greenstone Belt is 2 km, whereas the longest possible half-distance for the Kuhmo area predicted by this model would be 16 km. This represents the distance over which a new rock type can increase from 0 % to 50% of the material content of the till, during which time the other rock types would have to decline corre- spondingly from 100 % to 50 %.

Conclusions The effect of the width of the source-rock unit on the transport distance of stones contained in till, as depicted above, has passed unnoticed in earli- er work on this topic. In the author’s opinion, however, this needs to be taken into considera- tion, in the sense that the traditional interpreta- tion of the concept of half-distance, especially when normally defined on the basis of only one transect, should be abandoned in favour of a definition in terms of the distance required for a given rock type to increase its proportion in till from 0 % to 50%. This distance, which is the same thing as the longest possible half-distance, could well be referred to as the renewal distance. In the Kuhmo area this distance is thus 16 km. One requirement for this would be that the field- work would have to be carried out using a num- ber of transects, as in the present case, or using an extensive network of sites for the stone counts, as was done by Gillberg (1965, 1967a,

er consistency to the transport distances quoted in different pieces of research, which at present can differ widely even in closely adjacent areas.

It is only when we are able to take proper account of the battery of ‘secondary’ factors, such as the width of the source unit, the topogra- phy, fluctuations in the depth of the till beds and the differing resistance of rock types to erosion during glacial transport, that it will prove possi- ble to compare the results of research into trans- port distances in till areas of differing types. And it is only then that we will be able to reach the crux of the matter, namely the influence of the manner of glacial transport and deposition upon transport distance.

Acknowledgements. -This work was carried out at the Univer- sity of Oulu as part of the Kuhmo Ore Project financed by the Ministry of Trade and Industry. I am deeply grateful to the leader of the project, Professor Tauno Piirainen and to my teacher and leader of the till group, Professor Matti Saarnisto. for their continuous interest in my work and for all the encour- agement they have given me. I express my warmest thanks to my colleagues Keijo Uusikartano. Cand. Phil. and Raimo Ne- valainen, Cand. Phil. and to the bedrock group, Dr. Kalle Taipale, Eero Hanski, Cand. Phil., Hannu Kairakari. Cand. Phil. and Ilkka Tuokko, Cand. Phil. for many useful discus- sions. I also thank Malcolm Hicks, M. A. and Dr. Sheila Hicks for translating the manuscript into English. Pirkko Pytkonen for drawing the figures, and Riitta Nurkkala for typing the manuscript.

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