General Characteristics of Glacigenic Sedimentation

General characteristics of glacigenic sedimentation

19

GENERAL CHARACTERISTICS OF GLACIGENIC SEDIMENTATION What is so far the most complete monograph on the Quaternary was published in 1957 by Charlesworth, who analysed the development of knowledge regarding glacigenic sediments: Although the clays, sands and gravels belong t o the youngest and most accessible formation, their apparently chaotic state and seeming lack of interest made them the last to be investigated: they were for long a synonym for confusion, and except for their fossil shells and bones seemed unattractive and unimportant. The 'extraneous rubbish' was a troublesome hindrance in examining the 'solid' geometry. Long after Agassiz had revived the glacial theory, official state surveys ignored them. Thus the British drifts were passed over almost without scrutiny until most of Southern England had been examined. They were first mapped in Norfolk by J. Trimmer. Their mapping was only undertaken when, somewhat belatedly, their connection with agriculture, drainage, dwelling sites and engineering problems had been recognized". This view from the time of Charlesworth is now, while only a few decades old, a thing of the past. Glacial geology now receives much more attention and new research methods continue t o be developed. One of the characteristic differences between Charlesworth's and our time is the present emphasis on facies analysis. In spite of the rather recent tendency towards facies analysis and sedimentary models, some early reports on glacigenic deposits showed a fairly modern sedimentological approach. Such reports received, however, less attention from glacial geologists than they deserved. Some of these early reports concern glaciolacustrine sediments (A. Smith, 1832; Hitchcock, 1841; Jamieson, 1863); other works concerned sandur plains and glacigenic deltaic sediments (Gilbert, 1885, 1890; Davis, 1890; Salisbury, 1896) and the sedimentology of glacial diamicts (Agassiz, 1840; A. Geikie, 1863; Jamieson, 1865; Goodchild, 1874; J. Geikie, 1877, 1894; Torell, 1877; Chamberlin, 1894; Crosby, 1896). These reports might even be considered as the predecessors of the more recent publications that devote much attention t o facies associations and sedimentological patterns (e.g., Potter and Pettijohn, 1963; Broussard, 1975; Bull, 1977; Collinson, 1978; Friedman and Sanders, 1978; Reading, 1978a; Reineck and Singh, 1980; Leeder, 1982; Miall, 1984; Gradziiiski et al., 1986). 'I...

20

General characteristics of glacigenic sedimentation

Recent requirements for studies on glacial sedimentology include the reconstruction of the palaeogeographic development of the ice-covered area (e.g., Bouchard and Martineau, 1985). DEPOSITIONAL PROCESSES IN THE GLACIGENIC ENVIRONMENTS The t w o glacigenic environments show distinct variations in the predominance of the depositional processes. Material may be transported by ice, water, wind or due to gravitation. Deposition may take place from active or passive ice, in running or stagnant water, by large-scale or local winds, and along steep or barely inclined, subaqueous or subaerial slopes. This results in a complex pattern that changes rapidly in both time and space. The frequent facies changes depend heavily on, for instance, the dynamics of the ice sheets and on their sediment supply. Sedimentation by ice The feature most characteristic of the glacial environment is, from a sedimentological point of view, the deposition of debris supplied by the ice mass (Fig. 9). The most common depositional process is the settling of material from melting ice. This process often leaves poorly sorted sediments (diamicts) in which the larger clasts may still show a preferred orientation that, although commonly vague, is in accordance t o their position within the ice mass. Deposits thus formed are commonly indicated by the (genetic) term 'till'; a special type are the ice-raft deposits and related types of sediments, which contain clasts derived from a melting ice cover on top of a water body. Melting of ice takes place during both active (forward moving) and passive (gradually retreating due t o ablation) stages of the ice. The resulting sediments show somewhat different characteristics, mainly due to differences in the original flow lines of the ice, the rate of melting, the character of ablation, etc. More common characteristics stem from the precise place of deposition, the local topography, climatic factors and the occurrence of endogenic processes. The combination of all these parameters gives rise to sediments (tills) that generally have a diamict character (tills were previously often called 'boulder clays', or something similar in several countries).

Depositional processes in the glacigenic environments

21

ice movement

climatic conlrol

Fig. 9. Relationships between the main agents that influence sedimentation by glacial ice. Dashed arrows indicate main relationships, black arrows indicate intermediate ones, and white arrows indicate minor relationships.

Subaqueous sedimentation Glacigenic areas are commonly characterised by poorly permeable soils. This is due, particularly directly in front of ice caps, t o the permafrost and to the occurrence of sediments such as diamicts or loesses with low permeability. Undulations in the topography therefore easily lead t o lakes. Another lake-forming process is the irregular movement of ice lobes, resulting in dammed-off meltwater streams. Whatever is the origin of a lake, one of the main characteristics is the (almost) stagnant water in which even the finest sediment particles may settle. The water in glacial lakes is due only for a minor part to local melting of ice. Most of the water is supplied by meltwater streams originating a t a more or less remote place. Such meltwater streams tend to have a braided character, indicative of changes in water supply and thus of stream

22

General characteristics of glacigenic sedimentation

velocities and channel depth. These circumstances result in deposits much more irregular than those formed in lakes. Sedimentation from running water

Considerable quantities of meltwater may be formed in the ablation zone of a n ice sheet if the climatic conditions are favourable. The meltwater streams can be found on top of the ice, in tunnels within the ice and underneath the ice. They finally leave the ice mass and flow, with often large amounts of debris, into the foreland of the ice mass, where the material is deposited sooner or later. The dynamics and the transport capacity of the meltwater streams are fairly variable in time and space (Ostrem, 1975), being determined by the ablation rate, local topography, type of material transported, etc. The deposits formed from such streams are all designated by t h e general (genetic) term 'glaciofluvial deposits' (see, e.g., German et al., 1979; Williams and Wild, 1984); synonyms used less frequently are 'glaciofluvial deposits' (Paul and Evans, 1974), 'fluvioglacial deposits' (Augustinus and Riezebos, 1971), 'glacifluvial deposits', 'meltwater deposits' (Ehlers and Grube, 1983) and 'melt-water deposits' (Pessl and Frederick, 1981). Glaciofluvial deposits (Fig. 10) generally constitute the major part of all glacigenic sediments and they show most of the same characteristics as fluvial deposits of non-glacial origin. Most glaciofluvial deposits a r e relatively coarse-grained because the flow rate is temporarily too high for settling of the finest particles, but also because the fine-grained material is trapped in pools and lakes. The final characteristics are mainly determined by a limited number of parameters (Leopold et al., 1964; Allen, 1982; Gradzinski et al., 1986): bed geometry, amount of water, flow velocity, water depth and type of substratum. These parameters show interrelations and are largely influenced by the ablation conditions of the ice. Sedimentation in stagnant water

An irregular topography may, as well as ice lobes, damm off meltwater streams (cf. R. Gilbert, 1971) and thus form pools and lakes. Most of such glacial lakes are rather small (up t o a few kilometres in diameter, at most) and of short duration, but very large lakes may occur and survive for several thousands of years. The sediments formed in such lakes are most commonly called 'glaciolacustrine deposits', although the terms 'glacilacustrine' and 'glaci(o)

Depositional processes in the glacigenic environments

23

Fig. 10. Glaciofluvial channel fill (Balderhaar, Federal Republic of Germany; exposure known as 'wall of the angry farmer'). Note the channel lag with angular pieces of unconsolidated sand. These sand pebbles were transported in frozen form.

limnic' are also used. Sediments in glacial lakes may be derived from melting ice along the lake margin, from meltwater streams embouching in the lake or from dust-bearing winds. Most glaciolacustrine sediments are relatively fine-grained because the water is stagnant or has a low flow velocity, so that even the finest particles may settle. Factors responsible for the final depositional process are: settling out of suspension (wind-blown material, surficial currents), bottom currents, wave action (either or not in combination with tides) and mass movements along the slopes. The lithological characteristics depend on the prevailing process(es), but one commonly finds relatively coarse lake-margin deposits and fine-grained bottomsets; the latter frequently show varves (Fig. 11): graded layers that may originate from seasonal settling when a n ice cover melts in the spring, but may also be due to turbidity currents.

24

General characteristics of glacigenic sedimentation

Fig. 11. Varves in a glaciolacustrine succession of Drenthian age (overburden of the BekhaMw browncoal mine, central Poland).

Aeolian sedimentation The presence of large ice caps has a considerable influence upon the atmospheric circulation. Thermal inversion occurs frequently and cold air masses from above the ice cap meet the warmer air from the foreland a t the ice margin. These conditions are favourable for the production of intensive winds. Wind action in the area in front of the ice (Kida, 19851,where braided streams flow between subaerially exposed fluvioglacial sediments, results in wind erosion which happens even more easily since no or almost no vegetation is present. Snow storms may even erode particles larger than sand size, thus giving rise to relatively coarse niveo-aeolian deposits (cf. Baranowski and Pekala, 1982).The eroded material may be blown away over extremely large distances (dust that has originated now from the African Sahara can be traced in Western Europe and in the United States), but commonly results in a zone of coversands (Fig. 12)followed by a zone of the finer-grained (silty) loesses. The final depositional extent and

Depositional processes in the glacigenic environments

25

Fig. 12. Coversands of Vistulian (Weichselian) age (14,000-10,000 years BP) exposed i n a browncoal mine in central Poland (Kleszczow graben, near Lodi). Note the frequent alternations of coarse and fine laminae, resulting from phases with higher and lower wind velocities respectively. Photograph: J. Gokdzik.

grain-size distribution of the coversands and loesses (Smalley and Leach, 1978) depend on the wind velocity, prevailing wind direction, nature of the eroded material, topography of the area (both coversands and loesses tend t o level off height differences in the depositional area), vegetation, etc. (Catt, 1977). Aeolian deposits become quite commonly reworked (Mucher and De Ploey, 1977), either by new wind activity or by surficial currents (rain water). Some more or less classical loess areas (e.g., in southern Poland) even turned out to have few original loesses but mainly glaciolacustrine sediments that were almost entirely derived from loess.

Deposition from mass movements Each slope, either subaerial or subaqueous, easily induces mass transport. Inclinations of less than one degree may be sufficient for processes like subaqueous slumping but other forms of mass transport may require steeper slopes.

26

General characteristics of glacigenic sedimentation

Subaqueous mass movements Rivers may induce rock fall by undercutting the walls but the smaller or larger blocks thus formed in the river bed have almost no preservational potential if consisting of unlithified material. Well preserved mass-movement deposits are much more common in glaciolacustrine facies (Fig. 13) where the supply of sediment from meltwater streams may build up unstable slopes. Slumps, slides, mudflows and turbidity currents will then result. A special type of sediment is formed by material that enters the lake more or less directly from the ice, commonly by plastic flowage (flow till). Subaerial mass movements Subaerial mass movements are quite common all over the glacigenic environments, though they are not evenly distributed. All types of sediments (glacial, fluvioglacial, glaciolacustrine and aeolian) may undergo

Fig. 13.The irregular surface of sediments on top of dead-ice bodies, due to collapse after melting of buried ice, triggers subaerial mass-transport processes (Hornsund area, Svalbard).Photograph:J. Cegka.

Characteristics of glacigenic sedimentation

27

such reworking, particularly if sedimentation or erosion has created differences in height (a slope of a few degrees is enough) and when the soil is wet, e.g., after rainfall or when any other process has reduced the mechanical strength of the sediment. The intensity of the mass-movement process determines i n how far the original sedimentary characteristics of the reworked material will be preserved. The deposits that have undergone subaerial reworking have been named 'slope deposits', but the reworking has commonly been so slight t h a t there seems t o be no reason t o consider them as a separate group of sediments; one might even consider some slight subaerial reworking as part of the more general pedological processes. CHARACTERISTICS O F GLACIGENIC SEDIMENTATION The general characteristics of the glacigenic facies depend largely on the nature of the material supplied. Lack of specific grain sizes, for instance, will result in the absence of specific sedimentary structures. The source areas therefore influence the glacigenic facies, but other factors (transgressive or regressive tendencies, tectonic activity, isostatic movements, climate, intrabasinal processes such as reworking, compaction, etc.) also play a role. Knowledge of glacigenic sedimentation has greatly profited from the current interest in the environmental conservation of relatively undisturbed regions. This has resulted in more frequent earth-science research in areas such as Antarctica (see, e.g., Jacobs et al., 1970; Hughes, 1975, 1982; Moyan, 1976; Macharet, 1981; Lennon et al., 1982; Lindner et al., 1982; McKelvey, 1982; Kristensen, 1983; Rabassa, 1983; Domack, 1985) and Spitsbergen - often called 'Svalbard' in the literature - (Gripp, 1929; Klimaszewski, 1960; Kozarski, 1982; Szczypek, 1982; Kida, 1985).

The source of glacigenic sediments There are three main sources for the debris transported by glaciers and ice caps, i.e. material eroded from the substratum (and if present, valley walls: Larsen and Mangerud, 1981; Rastas and Seppala, 1981), detritus falling from nunataks (due to, e.g., frost weathering: Fig. 14; see also Brockie, 1973; Reheis, 1975; Latridou and Ozouf, 1982) on the ice surface, and particles that were supplied by the wind. The last type of debris is commonly of minor importance, while i t is t h e first type t h a t predominates (Fig. 15).

28

General characteristics of glacigenic sedimentation

Fig. 14.Irregular rock shapes due to frost weathering at an altitude of some 3300 m in the Zillertaler Alps (S. Austria).

Fig. 15. Sources of mineral particles in the glacigenic system, and main interrelationships of the factors influencing the characteristics of the glacial debris.

Characteristics of glacigenic sedimentation

29

Material from nunataks and wind-blown particles start their glacial transport on the ice surface. They may become incorporated in the ice when transported by supraglacial meltwater streams disappearing in englacial crevasses and tunnels, but also when fresh snow forms new covers. The material eroded from the substratum may be transported at the ice base, but may also become incorporated in the ice due to shearing that takes place in the ice mass. These processes imply that all debris transported by the ice become more or less mixed, which is one of the reasons for the diamict character of most tills (the breakage and pulverisation of clasts during transport are another reason: Hallet, 1981; Nahon and Trompette, 1982). The only glacial deposits that commonly show rather specific (non-mixed) characteristics are tills formed in the ablation zone by melting of ice with debris that has been eroded shortly before and that had no time to be mixed with other material or to be pulverised; such tills can show characteristics that resemble local pre-glacial surface deposits. Grain size of glacigenic sediments

The mixing of detritus during transport by ice results in poor sorting. This, however, does not imply that all glacigenic sediments have equal characteristics. Differences may occur due to, for instance, variations in time of source area, the prevailing transport mechanism and the position of clasts within the ice. Even though the glacigenic facies may thus vary, they commonly show debris of all grain sizes, particularly if the ice cap has eroded continental lowlands. A characteristic diamict is formed if ice containing debris of all these fractions should melt. It should be kept in mind, however, that meltwater streams may wash out such deposits; since the clay fraction and the boulders are most difficult to erode, it is quite common that a typical 'boulder clay' is left and that most sand and silt is washed away and deposited elsewhere in a fluvioglacial facies. If the meltwater streams are strong enough, no till will be formed or previously formed tills will be eroded and material comprising all grain sizes will be deposited in the fluvioglacial facies that commonly shows alternating layers of coarser and finer material, representing flows with more and less energy respectively. Transgressive and regressive tendencies

An ice sheet or glacier constitutes an energy system. The development and disappearance of such a system are lengthy processes. Growth of the ice

General characteristics of glacigenic sedimentation

30

mass and, consequently, a n increase in energy are, on the long term, mainly determined by climatic developments. An increased accumulation of snow, gradually converted into ice, is commonly due to a lowering of the temperature and a n increased precipitation rate. Only if a certain threshold has been passed, does the mere existence of the ice body itself influence climatic development (less precipitation through dry atmospheric conditions, high albedo): the climate becomes colder and dryer. The resulting decrease in precipitation implies that ablation may start predominating over accumulation, and that the energy level is distinctly lowered, Thus, transgression changes into stabilisation or even regression. This development is complicated by t h e time l a g between t h e occurrence of specific processes and the final effects that they induce. In fact, a wetter and cooler climate existed for a long time before a n ice mass shows a real transgressive behaviour; on the other hand, the transgression can continue if the climatic conditions already favour a regression. This example - many more are available - indicates that all dynamics of glacigenic processes, evidently including glacigenic sedimentation, depend on complex mass-balance relations. An additional complication is that the processes that determine the sedimentary pattern are different during the transgressive, stable and regressive phases (Fig. 16). Transgressive phases are characterised by prevailing erosion, with incorporation i n the ice body of much rock detritus eroded from the substratum. The relatively low level of energy output (mainly in the form of meltwater) means that sedimentation plays

[ L

-3 7

- ~ _ _ _ glacial retreat

&JI

preservational potential preservat2::l

potential

P

Tie,: o climatic change

,

~ glacial advance

little deposition

much erosion

+2-,

preservational potential

Fig. 16. Main factors controlling the preservational potential of glacigenic sediments in relation to ice advance and retreat.

p

Characteristics of glacigenic sedimentation

31

a comparatively minor role: fairly few sediments are formed and their preservational potential is limited. On the contrary, deposition prevails during regression of the ice when melting of debris-laden ice increases due to more intense ablation or t o lack of 'fresh' ice as a result of decreased snowfall in the accumulation areas. The deposits formed during retreats of the ice have a fairly good preservational potential, although they may soon afterwards become eroded during a recessional re-advance of the ice (Schliichter, 1983). The large net deposition during regression of the ice mass is only a small part of the total energy output of the glacial system under these conditions; much more energy is lost in the form of meltwater. Influence of ice dynamics and extent upon sedimentation Debris transported by ice can be found far beyond the outer limit of the farthest ice extent because meltwater streams and winds take over the transport activity. Truly glacial deposits, however, can only be found in the areas covered by the ice. The ice cap is not simply moving towards a final point then again retreating during one ice age: there are many oscillations with extending (transgressive) ice masses, separated from each other by recessions (regressions). Although much is known about the physics of glaciers (see, among others, Paterson, 1981), the fluctuations in ice extent are still a matter of speculation (Mickelson et al., 1981). Both regional uplift or subsidence of the Earth's crust and sea-level changes (which themselves are partly a result from glaciation and deglaciation; see, e.g., Walcott, 1970; Andersen, 1979; Vorren and Elvsborg, 1979; Sollid and Reite, 1983) may play a role (Edwards, 1978; Miall, 1984). It is likely, however, that autocyclic and allocyclic large-scale climatic changes are much more important. The mechanism behind these changes is still under discussion, although Milankovitch's (1920, 1930, 1936, 1938) views concerning astronomical factors now seem fully justified; the main problem is that other factors must also play a role, but these are not yet well enough known to be included in clear and detailed models. The climatic fluctuations contribute much to the characteristics of the glacigenic facies because they induce sea-level changes and isostatic movements, influencing both the erodibility of the source area and the characteristics of the depositional basins. A direct relationship between climate and, for instance, glaciofluvial deposits is nevertheless not really traceable. An important role is most probably also played by weather fluctuations (difference between day and night, and seasonal changes), but this role is even much more difficult to specify.

32

General characteristics of glacigenic sedimentation

The character of the sediment input Accumulation of glacial deposits may take place gradually if the dynamics of the ice remain more or less stable and if there are no major changes in climate. A much more abrupt type of deposition may occur if debris concentrated in englacial crevasses is suddenly set free, for instance by rapid melting of a last remnant of ice underneath the crevasse, a process which can be triggered by complex factors such as ice characteristics, ablation conditions and local topography (Fig. 17). Such 'triggered' sedimentation is relatively common in ice lobes that extend considerably in front of the main ice mass. It will be obvious from the data presented above that transgressive conditions are characterised by a more or less uninterrupted sediment input, whereas an input of this type occurs in pulses during periods of regression.

Fig. 17. Character of sediment input in the glacigenic system, and major interrelationships between the factors that control the input.

Characteristics of glacigenic sedimentation

33

THE INFLUENCE OF CLIMATE ON GLACIGENIC SEDIMENTATION Both the accumulation of snow in the firn basin and the melting of ice in the ablation zone are largely controlled by the climate. The dynamics of the ice mass depend on the energy balance that results from snow accumulation and ice melting (Fig. 18),which implies that the alternation of ice advances and retreats during a glacierisation also depends on this factor. Climate and weather thus influence the possibilities and character of glacial deposition and lead to differences between the various glacigenic facies. Temperature, precipitation and wind activity are considered the most important meteorological factors. Role of temperature Changes in the air temperature affect the ablation rate immediately, not only in the frontal area but over the entire supraglacial area that thus becomes covered with scattered detritus or even with a more or less continuous layer of debris (Sugden and John, 1976).

thermal regime of ice

net energy balance of

Fig. 18. Interrelationships between the main factors that determine the dynamics of an ice sheet or glacier.

34

General characteristics of glacigenic sedimentation

A much more complex aspect is the influence of air-temperature changes, together with other meteorological and climatic elements, upon the thermal regime of the ice (Fig. 19). This regime must be considered as a complex function of the energy balance at the ice surface (Fig. 20). A cold or moderate regime influences the type of deposits formed (by influencing the depositional processes), whereas rapidly varying regimes (a common feature: Baranowski, 1977; Brodzikowski, 1987) dominate the dynamics and the changes in the depositional processes in the entire glacial environment. The ice dynamics are also strongly influenced by the thermal regime (Boulton, 1972a, 1979; Embleton and King, 1977). The four most characteristic ice-regime situations are presented in Figure 21, which is based on studies in recently glaciated areas and on studies carried out in the European Lowlands where Pleistocene glaciations left their imprints.

conditions of snow and ice accumulation

3

k

net energy balance of ice body

m

thermal regime of the ice body

depositional conditions

Fig. 19. Most important factors controlling the thermal regime of a n ice body.

The influence of climate on glacigenic sedimentation

of water v a p o u r

I/

i

35

friction in i c e

V L

heat i n p u t

1

heat loss t o atmosphere

net energy balance of ice surface

freezing of water

outflow Of ‘ w a r m ’ water

ablation

A

I1

vertical m o v e m e n t of i c e m a s s e s

equilibrium

zone

lht3 I

100 krn

cold thermal regime

I

Pielstocene glaclatton of N Asia and Canada

polar continental (high latitude)

B

100 km

cold

equilibrium

thermal

IhW

u

illtie preclpltatlon

polar continental (middle latitude)

Plei~toceneqiaciation of Europe

Pleistocene glaciation of Middel Europe I Southern Canada

D

zone 01 50 km

surgtng

L_

/ / / / / / / / / / / / / / / / / / / / , , / , / I // / / / / ,

’,/

I / ,

,

Pleistocene glaciation of mountains and their forelands

Fig. 21. Hydrological and thermal regimes of large ice bodies (after Baranowski, 1977). The four possible (main) possibilities (A-D) are presented in simplified form.

36

General characteristics of glacigenic sedimentation

Role of precipitation The glacigenic environments are characterised by subpolar, polar and Arctic climates. It is most important whether a cyclonal or an anticyclonal circulation prevails (Fig. 221, since this factor influences strongly most of the meteorological parameters, particularly the rate and type of precipitation, which parameters determine the type and the intensity of the ablation process.

Fig. 22. Reconstruction of two phases of ice extent in northern Europe, with emphasis on the pattern of cyclonal circulation. Black arrows indicate prevailing routes of the cyclones; dashed areas indicate the ice covers. Above figure: Karelo-Barentz ice sheet has grown together with the Scandinavian ice sheet; new centres of glaciation are developing in Ireland. A subarctic climate prevails in middle Europe; cyclonal activity becomes minimal and thermal continentalism increases. The position of the Karelo-Barentz anticyclone area is very stable. Snow accumulation decreases distinctly.

The influence of climate on glacigenic sedimentation

37

Cyclonal atmospheric circulation tends to result in a high precipitation rate. This means more snow in the accumulation area and more rain in the ablation zone. Increasing precipitation rates have a complex effect on the energy gradients of meltwater streams (Sugden and John, 1976; Baranowski, 1977; Embleton and King, 1977) and therefore also on the characteristics of the glaciofluvial deposits. Detailed palaeoclimatological reconstructions (regarding the palaeocirculation in particular) have shown that, during the Pleistocene, some

Fig. 22 (continued). Above: Phase of maximum ice extent in Europe. The Karelo-Barentz ice sheet has grown considerably, but has also split up locally. The maximum gradients in atmospheric pressure are situated between the centre and the margin of the ice sheet. The precipitation on the ice-covered area decreases again; the cyclonal circulation in middle Europe increases.

38

General characteristics of glacigenic sedimentation

middle European areas were under the influence of prevailing cyclonal circulation for more than half of the year. This type of circulation produced meteorological conditions (Fig. 23) that caused very specific palaeoglaciological circumstances (Fig. 24), for instance wet and dynamic conditions over the entire extended ablation zone of the ice (which had a temperate thermal regime). These conditions resulted in a fairly constant and high accumulation rate.

Fig. 23. Palaeoclimatic reconstruction of the most common weather conditions i n middle Europe during the optimum of the Drenthian ( = maximum Pleistocene) ice extent, based on a palaeosynoptic model. A, B, C, D: precipitation zones. ACA = arctic cold air; PTA = polar temperate air.

The influence of climate on glacigenic sedimentation

A

C

L external zone 01

I

model o f ablation area

I

ice sheet

39

I

I

palaeocirculation

A

--

1 - zone 01 polar cyclons

--I

Fig. 24. Palaeoclimatological reconstruction, based on palaeosynoptic models, of the ice-marginal zone in the DDR and the Sudetic Mountains during Elsterian a n d Drenthian times (maximum Pleistocene ice extent: Dnieprovian). A: palaeoglaciological model of the ice-marginal zone. B: ablation and accumulation. C: palaeosynoptic model. PTA = polar temperate air; PCA = polar cold air; WF = warm front; CF = cold front; AF = arctic front; As = altostratus; Cb = cumulonimbus.

There is much less precipitation if anticyclonal circulation prevails. In combination with low temperatures, such circumstances induce a significant increase of ice sublimation in the ablation zone. This process affects the position of clasts in the upper layers of the ice and, if supraglacial deposits are finally formed, could lead t o particular lithofacies characteristics (Sugden and John, 1976; Shaw, 1977a). Such conditions prevailed in Eastern Europe during the Pleistocene periods of maximum ice extent. It is most probable that the zone where the Dniepr lobe was situated (the Dnieprovian is comparable to the Western European Drenthian) in particular witnessed a dominant anticyclonal circulation throughout the year (Fig. 22). The climate was therefore dry, cold and sunny (Fig. 25) and the ice sheet was characterised by a continuously cold thermal regime (Fig. 26). The intensity and the dynamics of the depositional processes were much lower than those in Middle Europe.

40

General characteristics of glacigenic sedimentation

Fig. 25. Palaeoclimatological reconstruction of the most common weather conditions in Eastern Europe during the maximum Pleistocene ice extent (Dnieprovian), based on a palaeosynoptic model. See Figure 23 for explanations.

Role of winds Winds are primarily a result of atmospheric circulation. It should be emphasised, however, that the transitional zones between ice-covered and ice-free regions affect the wind pattern and the wind intensity. The ablation zones are often characterised by much wind activity, especially in the zone of cyclonal circulation (Fig. 21). Winds are not only responsible for the formation of regional or local aeolian deposits, but also change the surficial humidity in the sedimentary basins by vaporisation

41

The influence of endogenic forces

L external zone of ice sheet A

L -

C

Lm

-

-150-250 km

palaeocirculation

45%N

0

~

L L

low-pressure

co

model of ablation area

oess

polar tropopause

co 50e N

Fig. 26. Palaeoclimatological model of the ice-marginal zone of the extremely continental Dniepr lobe (Soviet Union). See Figure 22 for explanations.

of surface waters (and of glacial ice as well if the temperature is low and sufficient insolation takes place). If surface waters are rare or absent, the dry winds may easily carry away the finest particles from the sediment cover in front of the ice. This may result in dust clouds that can be transported over hundreds of kilometres. Winds therefore greatly influence the depositional pattern in large parts of the periglacial environment (Jahn, 1950,1970; Cegla, 1972; Rozycki, 1979).

THE INFLUENCE OF ENDOGENIC FACTORS Extending ice sheets do not discriminate between tectonically active and more stable regions. Consequently, the ice caps may cover a rising or subsiding substratum, accompanied or not by earthquakes. The upheaval or subsidence of the substratum is of special importance in this context because it influences the depositional pattern under the ice, whereas it also determines to a large degree the preservational potential of the glacial deposits.

42

General characteristics of glacigenic sedimentation

Another endogenic factor, of even more importance for the behaviour of the ice, is the Earth's heat flux, which greatly influences the energy balance of the ice. Since ice is a good heat insulator, much of the heat coming from the Earth's interior is absorbed by the ice, sometimes giving rise to melting of considerable masses and thus to subglacial streams and spaces where sedimentation can take place. It can be stated that, in general, endogenic processes affect not only the physiography of glacigenic sedimentary basins, but also the intensity of local thermal - subglacial - subrosion, the volume of the sediment output, the ratio between sedimentation and erosion, the character of redeposition processes and the frequency of facies changes in space and time. Vertical movements of the Earth's c r u s t Vertical movements have three important aspects in the framework of glacigenic sedimentation: they influence the energy input into the system (Fig. 27), they influence the preservational potential of the deposits underneath the ice cover, and they (may) influence the lateral extent of the ice masses. Crustal movements tend to influence the borders of sedimentary basins in general. The same holds for glacial sedimentary basins. The location of the movements (under the ice sheet or in front of it) is obviously of the greatest importance. If upheaval takes place underneath the ice, erosion of the substratum will increase, thus enriching the subglacial zone in detritus. The eroded particles may later become part of the englacial subenvironment (by transport along shear planes). In general, deposition in the periglacial environment will profit from these circumstances. In contrast, subsidence of the substratum underneath the ice will diminish erosion, finally possibly resulting in a reduced sediment supply to the periglacial environment. The same subsidence provides better depositional circumstances, however, within the subglacial environment, thus increasing the preservational potential of the subglacial deposits. If crustal movements take place in front of the ice (e.g., because of isostatic compensation), the depositional pattern in the periglacial environment may be affected. Subsidence in front of the ice results in basins where meltwater deposits may accumulate, but a t the same time such a subsidence may accelerate the ice advance, thus resulting in a n overriding and possibly in erosion of the sediments deposited earlier. Upheaval in front of the ice may result in stagnant ice because the barrier thus formed cannot be passed by the ice until the barrier can be overriden.

The influence of endogenic forces

from the glacial system

43

output of meltwater from the glacial System

Fig. 27. Energy input in the glacigenic depositional system by vertical tectonic movements of the substratum.

Earthquakes Earthquakes affect the glacial environment in two ways. First, they represent sudden movements (faults) of the Earth's crust, forming o r reactivating zones of weakness where a n increased heat flow from the Earth's interior t o the ice mass may take place. This accelerates melting of the ice in the ablation zone or may induce melting where this process would not otherwise have taken place. Earthquakes may also disturb the equilibrium within the ice or the sedimentary cover; the latter (Fig. 28) may result in distinct structural changes (Brodzikowski et al., 1987b,d).The ice movement may thus undergo a sudden pulse a t the beginning of a changing thermal regime. The pulse may become visible because of a relatively fast advance of the ice front over several kilometres. Disturbance of the equilibrium within

44

General characteristics of glacigenic sedimentation

IV

r l ~

0

rnax

P o

d

b

D

o

rnax

Fig. 28. Deformation horizons (D) within Elsterian and Saalian sediments due to earthquakes in the Kleszcz6w graben (central Poland). 1 = glacial till; 2 = fluvioglacial sediment; 3 = glaciolacustrine sediment; 4 = relative scale for intensity of the endogenic activity; 5 = endogenic activity; 6 = earthquake-induced deformation horizons; 7 = distinct changes in sedimentary conditions; 8 = relative scale for abruptness of facies transitions; 9 = facies transitions; 10 = sedimentary cycle; 11 = sedimentary subcycle; 12 = horizon with large-scale deformations.

the sediment may be expressed by mass movements from topographic heights or by destruction of barriers responsible for the existence of glacial lakes. A sudden outflow of lake waters may not only result in specific (‘catastrophic’)deposits but may also affect the depositional pattern in the terminoglacial and proglacial environments.

The influence of endogenic forces

45

The geothermal heat flux The geothermal heat flux may be relatively high in zones with active faulting, but it may also be high in zones with other endogenic activity (6ermak and Rybach, 1979). Such zones tend to be of limited extent. Other areas, however, do not have a uniform (lower) heat flux, but show regional or even local differences. This has as a result that the values for the heat flux form a kind of mosaic (Fig. 29). Consequently, the heat flux has a diverse influence on the permafrost, the subglacial ablation, the quantity of waters in the subglacial zone and the dynamics of the ice masses. Extreme situations can result in an increased regional advance of the ice and in high flow velocities. The ice dynamics and the ablation rate are generally dependent on the heat flux, thus influencing the depositional conditions (Fig. 30).

2 Fig. 29. Geothermal heat pattern (in mW.m- ) in Europe (modified after Eermak and Rybach, 1982).

46

General characteristics of glacigenic sedimentation

The heat flux may induce collapse of the basal ice masses if part of the ice has melted away (Rubulis, 1983). This process may be restricted to a few centimetres, but in extreme cases some tens of metres may be involved. If collapse structures are found, i t is difficult - were it possible to determine the mechanism responsible because other processes can result in similar structures (Eissmann, 1975, 1981). It is obvious, however, that collapsing in the subglacial subenvironment will greatly influence the depositional conditions.

Fig. 30. Influence of the geothermal heat flux upon glacigenic depositional conditions.

The sedimentary facies

47

THE SEDIMENTARY FACIES The history of the term 'facies' reaches back a long way. According to Walker (1984),it was first used in geology in 1669 by Nicolaus Steno but it received its modern meaning from Gressly (1838). Much more precise elaborations appeared later (by, e.g., Walther, 1894; Teichert, 1958; Weller, 1958; Krumbein and Sloss, 1963); Middleton (1978), Reading (1978b) and Walker (1984)have provided the most recent definitions, with comments and discussions. In spite of the clear definitions available, the term has been applied (and misapplied) in several ways. It should, however, always indicate a geological unit or a number of geological units with specific features in common. Lithofacies, biofacies and geochemical facies are examples definable by means of parameters that can be determined unambiguously. The type of facies that should be used depends on the purpose of the research involved. The sedimentologist Middleton (1978) states that it is understood that (the facies) will ultimately be given an environmental interpretation". The present authors use the term 'lithofacies' (cf. N. Eyles et al., 1984b, 198813; Shaw, 1987a) where the rock type is considered (mineralogical composition, grain-size distribution) and the term 'sedimentary facies' where an environmental analysis is involved. The sedimentary facies, the main type of facies dealt with in this book, is not entirely unambiguous, for it is determined by the prevailing depositional process(es) and thus requires interpretation. As will be seen, this use leads t o specific facies types, e.g., 'melting-ice facies' and 'proglacial Iacustrine facies', being distinguished. This approach thus requires more than mere description of specific characteristics that can be observed directly. Specific characteristics may help in determining the correct type of sedimentary facies, but more frequently the lateral and vertical transitions of facies must be studied t o ensure a reliable interpretation (Brodzikowskiand Van Loon, 1983,1987). The term 'sedimentary facies' is thus intermediate between 'environment' and 'lithofacies'. This bridge function is essential in sedimentological analyses, because a (sub)environment may include a wide variety of lithofacies (e.g., the subglacial subenvironment with poorly sorted diamicts, varved glaciolacustrine deposits, etc.), whereas specific lithofacies (e.g., poorly sorted diamicts) may occur in a wide variety of (sub)environments (supraglacial, englacial, subglacial and terminoglacial). It is thus necessary t o provide details about environmental conditions, depositional mechanisms and lithological characteristics; this is achieved by means of the sedimentary facies. 'I...

48

General characteristics of glacigenic sedimentation

Facies analysis Lithofacies are represented by actual deposits. Specific lithofacies have names that are usually, although not always, sufficiently clear. It is obvious, for instance, that a deposit consisting mostly of quartz grains that have a grain-size distribution almost entirely in the 64-2000 micron range can be called a sandstone. It is less generally known that a matrixsupported, massive, sheared sediment with a wide grain-size range can be called a diamict. On the other hand, it is far from obvious from the lithofacies data presented above to which sedimentary facies the sandstone belongs, while the other sediment is obviously a lodgement till, thus belonging to what is termed here the 'subglacial melting-ice facies'. Sedimentary facies are thus described and analysed in order t o establish which parameters determine the regularities and variations within the sediments. The depositional basin is therefore investigated as regards its environmental characteristics, including the palaeogeographic reconstruction and interpretation of the depositional mechanisms (Potter and Pettijohn, 1963; Reading, 1978b; Miall, 1984). Although a sedimentary basin is in many respects an entity, a wide variety of sediment types may be deposited. This is due to physiographic differentiation and to changes in prevailing processes in time and/or space. The various facies also are not stable in time once they have been formed: complete lithological units may become eroded and the boundaries between adjacent facies types may shift (e.g., due to the gradual growth of a delta). Sedimentary facies differ from each other by their lithology, extent, structures, energy vectors, etc. (Allen, 1970b, 1982; Friedman and Sanders, 1978; Leeder, 1982; Miall, 1984). All these parameters are related to processes or combinations of processes that may have changed, either gradually or abruptly, simultaneously or one by one, either t o reach a new stable value or t o continue changing. Important parameters that may be changed are the amount and nature of sediment supplied, the climate, the height of the sea level and the stability or instability of the substratum. Facies analysis as understood by Selley (1970), Miall (1973) and Walker (1984) is based largely on statistical methods. Selley (1970) aims a t the presentation of facies associations and sequences in a clear, objective, graphic manner characterising both facies interrelationships and facies patterns. This can be done by tabulating the numbers of specific transitions observed, converting these numbers into relative frequencies, calculating a matrix with the assumption of the null hypothesis (i.e. that such transitions are random and that they depend only on the relative

The sedimentary facies

49

abundance of the facies that are being studied) and, finally, by establishing random probabilities t o produce a matrix emphasising any differences from random which are at large (Walker, 1984). A detailed analysis of local or regional facies changes could be hampered by a lack of outcrops. Lowland areas (where most Pleistocene glacigenic sediments have been studied) particularly tend to be poorly exposed. This makes i t all the more necessary to have plentiful information about facies associations and sequences. If there are too few exposures and if borings cannot provide the data required, it might be useful t o first study comparable facies i n a better exposed area or i n hard-rock equivalents that have already been investigated in detail (cf. Vanney and Dangeard, 1976). Descriptions of the characteristics of lithified glacigenic rocks concern glacigenic conditions from many ages. Facies data from Precambrian glacigenic sediments have been provided by, among others, Coleman (19071, Bjorlykke (1969), Lindsey (1969, 1971), Roscoe (19691, Young (1970, 1973, 1974, 1978, 1981), Aalto (1971, 1981), Spencer (1975a,b, 1981), Deynoux and Trompette (1976, 19811, Edwards (19761, Nystuen (1976), Sumartojo and Gostin (1976), Nystuen and Seather (1979), Anderton (1980, 1982), Gravenor (1980), Boulton and Deynoux (19811, Chumakow (19811, Edwards and Foin (1981), Legun (1981), Link and Gostin (19811, Donaldson and Munro (1982),Hambrey (1982),Stupavski et al. (1982),Anderson (1983),Christie-Blick (1983),C.H. Eyles and N. Eyles (1983b, 19851, Fairchild (1983,1985),Miall (1983a, 19851, Gravenor et al. (19841, Dowdeswell et al. (1985) and Fralick (1985). Similar data on Cambrian and/or Ordovician glacigenic rocks were presented by, among others, Spjeldnaess (1973), Tucker and Reid (19731, Davies and Walker (1974), Deynoux (1980), Hein and Walker (1982) and Fortuin (1984). Facies data on Carboniferous and/or Permian glacigenic deposits a r e numerous; to mention only a few: Rattigan (1967), Frakes and Crowell (1969), Le Blanc Smith and Eriksson (1979), Bull et al. (1980), Davis and Mallett (1981), W.K. Harris (1981), Jackson and Van de Graaff (19811, Rogerson and Kadybka (1981), Casshyap and Tewari (1982),Visser (1982, 1983b), Visser and Kingsley (1982), Coretelezzi and Solis, 1983; Cuerda (1983), Gonzalez (1983), Gravenor and Rocha-Campos (1983), Stauffer and Peng (1984), Visser et al. (1984, 1986, 1987), Visser and Hall (1985), Visser and Loock (1987), and S.Y. Johnson (1989). Glacigenic facies of Tertiary age have been described by Plafker and Addicott (19761, Dalland (1977), Plafker (1981), Barett and Powell (19821, McKelvey (19821, Minicucci and Clark (1983) and C.H. Eyles (1985). An overview of the chronology of glaciations has been provided by Harland (1981).

50

General characteristics of glacigenic sedimentation

FACIES INTERPRETATION The analysis and interpretation of facies require both careful sampling of the data, and the development (or application) of a model which must fit the various data. This, of course, also holds for glacigenic sediments. The data that should be collected in the field and the laboratory comprise the lithology (including grain-size analysis, mineralogy and petrography), inventary of sedimentary structures and of early-diagenetic deformations, geometry and size of the various units, signs of erosional surfaces, type of contacts between the various units, and palaeocurrent or ice-movement directions (Hill and Prior, 1968). These data must first be interpreted in terms of depositional (and erosional) processes. Once the interpretation is completed, a logical framework must be found to explain the vertical and horizontal transitions. This means that much attention must be directed to the rclative abundance or scarcity of specific features, their associations and other interrelationships.

Lithological characteristics The lithology of sedimentary units must be determined because it can facilitate the correlation between various outcrops. Determination of the extent and of the lateral and vertical transitions is important for a n environmental reconstruction. Grain size, mineralogical composition, sedimentary structures, deformations and palaeocurrent indicators are also helpful tools if the depositional history is t o be reconstructed. Size and geometry of the units The size of lithological units (thickness and areal extent) depends on the size of the depositional basin and on the basin development, the depositional rate (net sedimentation rate), the duration of the depositional process(es) and the possible erosion afterwards. There is generally insufficient information about these parameters to estimate their relative contribution. Nevertheless, it seems worth paying more attention to these aspects as our insight into the depositional process of glacial sedimentation might thereby be much improved. Why, for instance, are most Pleistocene tills only a few metres to maximally some tens of metres thick (admittedly, there are Wisconsinan till sequences with a thickness of several hundred metres) whereas Precambrian tillites often seem to reach much greater thicknesses? Also how did some Miocene glaciomarine

Facies interpretation

51

tillites accumulate to several thousands of metres? Much research must still be done t o find answers t o questions such as these. The geometry of a deposit depends on the shape of the depositional basin, the depositional mechanism, the interrelationship with adjacent depositional areas, and erosion. The same problems as mentioned for the size of the deposits however still arise. In spite of these uncertainties, size and geometry together can give rather reliable information, particularly if trends in grain-size distribution are also taken into account. Contact characteristics The characteristics of the contacts between adjacent lithofacies are important for facies analysis. Aspects that should be investigated in particular are the geometry of the contact plane, the type of contact (erosive or non-erosive), deformation of the contact plane, etc. Erosive contacts - Erosion is part of almost all depositional processes. Sedimentary breaks are therefore very common. It is most important, however, t o recognise the erosional contacts that point to a process other than sole alternation of sedimentation and erosion as an ongoing process. The importance of 'real' erosional contacts had already been emphasised by Walther (1894). Nevertheless, the interpretation of this phenomenon still receives insufficient attention. Erosion in glacigenic sediments is a most important feature because glacial erosion may indicate various stages of ice (re)advance whereas, in various other types of deposits, erosion may indicate subaerial exposure and mass wasting along a slope. Non-erosive contacts - Non-erosive contacts can be either sharp or gradual (sometimes also called 'progressive'). The nature of the contact may be an indication of transgressive or regressive development: transgressive phases give commonly rise t o relatively many sharp contacts whereas regressive phases tend to lead t o more gradual contacts. The nature of the contact cannot be taken as a criterion, however, because the underlying processes (commonly changes in hydrodynamic properties) occur in a rather unpredictable way. Alternations of sharp and gradual contacts in fine-grained sediments may give more indications, as in the case of varves. Obviously, non-erosive contacts may show structures that point t o at least a small break in sedimentation. Such structures include outwash phenomena, sole marks (groove casts, prod marks, etc.), raindrop

52

General characteristics of glacigenic sedimentation

imprints, etc. Other changes in the sediment, e.g., in colour, consistency or cementation, may also indicate sedimentary breaks. This implies that the analysis of the sedimentary history requires more than a rough impression of erosional or non-erosional contacts (cf. Reading, 1978a; Reineck and Singh, 1980; Allen, 1982; Gradziiiski et al., 1986). This problem has been dealt with in more detail by Twenhofel (19391, Shrock (19481, Kuenen and Menard (1952), Sanders (1960), Ksiqikiewicz (19611, Diulyiiski and Sanders (1962), Diutyliski (1963b, 1965), Diu€ydski and Walton (1965) and several others in more recent years.

Grain size A grain-size analysis may reveal possible sedimentation mechanisms (or, a t least, exclude some mechanisms) Wisher, 1969). The grain-size data, although rarely unambiguous, may thus be used for hydraulic interpretation (Glaiser et al., 1974). Experiments in this context seldom yield reproducible data (Harms and Fahnestack, 1965),partly because granulometry depends on various parameters such as bed form and local flow regime. The distance from the source also plays a role (Teisseyre, 1975),so that, if other data are lacking, granulometric data may also be used t o reconstruct a palaeocurrent direction (Middleton, 1965; Reineck and Singh, 1980; Gradzifiski et al., 1986), because the coarsest particles will generally remain closest to the source. Much less is known about the relationship between grain-size distribution and glacigenic melt-out or subaerial mass movements (although an increasing number of detailed studies into glacigenic diamicts have been published in the last few years); some sedimentary structures found in deposits formed under glacigenic conditions can therefore not be explained properly. Analysis of the grain size is also important because of the influence on the geotechnicaUengineering characteristics of the sediment (see, among others, Boulton, 1976a; Lee and Focht, 1976; Brand and Brenner, 1981; Browzin, 1981).

Mineralogy

A mineralogical analysis of the sediments (or a petrological analysis if coarse clasts are concerned) is useful for the determination of the source area (Zandstra, 1983). Recognition of the source area is important for palaeogeographical reconstructions because it allows transport routes to be found (Di Labio and Shilts, 1979). It should be kept in mind, however,

Facies interpretation

53

that the mineralogical composition of a glacigenic sediment is almost always the resultant of erosion in the source area, erosion during ice movement, and erosion in the neighbourhood of the final depositional site, thus giving a mixture of assemblages, each of which must be recognised as such. Mineralogical analyses are commonly restricted to heavy minerals (which give rather reliable and easily obtainable results). Much more time-consuming and specialised equipment, requiring trace-element or trace-mineral analysis, can however provide much more precise data. The petrological characteristics of clasts especially may change from bottom to top within one lithological unit, either because of mixing ice masses from different sources, or due t o different processes occurring within one ice mass, dependent on the location of the clast (sub-, en- or supraglacial, embedded in a relatively rigid ice mass or located in a shear zone, etc.) (Haldorsen, 1977; Hallet et al., 1978; Slatt and Eyles, 1981; Houmark-Nielsen, 1983a). Nevertheless, the petrography may give indications about the source area and thus about the ice movement (Meyer, 1983a; Schuddebeurs and Zandstra, 1983).

Sedimentary structures Sedimentary structures (see, e.g., Collinson and Thompson, 1982) should be inventoried because they give valuable information about both the depositional process(es) and the palaeocurrent directions. This is particularly true for aeolian sediments and deposits formed in current water. Much less is known about the significance of the various, often vague and rather irregular, structures that can be found in the most characteristic glacigenic deposits: the diamicts. It is not unlikely, however, that the lack of generally accepted interpretation of such structures has lessened the interest of researchers who are not primarily interested in this specific problem. The authors are of the opinion, based on their own field investigations, that a much more systematic inventory of structures in diamicts might contribute greatly to a better understanding of the genesis of these sediments.

Deformation structures Deformation structures are fairly common in glacigenic sediments. They range from simple undulations to complex multi-phase discontinuities and may be formed by a process or a number of processes that can be grouped (cf. Van Loon, 1990) into the following ten categories: bioturbations, cryoturbations, glaciturbations, thermoturbations, graviturbations,

54

General characteristics of glacigenic sedimentation

hydroturbations, chemoturbations, atmoturbations, endoturbations and astroturbations. A detailed analysis of the penecontemporaneous and postdepositional early-diagenetic deformations (Fig. 3 l),including determination of their relative age and frequency, could give an insight into the dynamics of the environment during (or shortly after) deposition, or into the processes affecting the sediments afterwards. This body of data could form a n elegant though not always reliable basis for determining the genesis of the sediment in a particular (sub)environment or facies when there are insufficient other data. As an example, some deformational structures, while they are not diagnostic, can be characteristic of specific circumstances. This holds, e.g., for the joints formed in subglacial diamicts due t o loading and subsequent unloading by the overlying ice cover; the type of discontinuities and their spatial distribution may help englacial and subglacial diamicts to be distinguished. Some types of structures are signs of 'en masse' reworking before final deposition of the sediments.

Fig. 31. Deformations in a sand quarry near Ossendrecht (The Netherlands), possibly due to a combination of load casting and cryoturbation.

Facies interpretation

55

Palaeocurrents Palaeocurrent indicators are most important for reconstruction of the palaeogeography. One should keep in mind, however, that traces left by palaeocurrents can vary widely: meandering streams, for instance, have current directions that may be opposite, even a t relatively small distances from each other (Teisseyre, 1977, 1978a,b, 1980, 1984). It is therefore essential t o measure as many palaeocurrent indications as possible if a reliable regional picture with prevailing directions is to be obtained. Palaeocurrent directions may be reconstructed in various ways, mainly depending on the depositional and/or erosional processes that took place. This implies, for instance, that the approach in melt-out facies must be different from that in glaciofluvial, glaciolacustrine or aeolian facies. Consequently, a vertical section may require different analytical methods for the various units (cf. Gradzifiski et al., 1986). A proper analysis of the palaeocurrent data should not only yield information about prevailing transport directions (and thus about the direction of the source area) but also should show the relative frequency of changes in the hydrodynamic regime, in the morphology of the substratum and in the dynamics of the depositional process. There are large numbers of traces from which palaeocurrent directions may be deduced. These include: the orientation of the foresets (Fig. 32) in current ripples or wind ripples (Momin, 1968; Kumar and Bhandari, 1973), gradual horizontal changes in the average and/or maximum grain size (Agterberg et al., 1967; Miall, 1974), orientation of objects (imbrication of pebbles, orientation of shells: Van Loon, 19721, depositional 'shades', sole marks such as flute casts (Pelletier, 1965), etc. .4 large number of structures may, however, only show the axial direction of the transport, such as glacial striae (Fig. 33) (Von Brunn and Marshall, 1989; Visser, 1990), prod marks, sole marks such a s groove casts, parallel orientation of plant debris, etc., thus requiring additional data if a definite conclusion is to be drawn. It is most probable that some soft-sediment folds may be induced or a t least influenced by palaeocurrents (Johansson, 1965; Griffiths, 1967; Parkash and Middleton, 1970; Teisseyre, 1975; Potter and Pettijohn, 1963,1977). The morphology in the glacial and periglacial environments is, in general, rather complicated (Embleton and King, 1975; Sugden and John, 1976). This results in a complicated pattern of palaeocurrent directions, which can be interpreted correctly only if sufficient data are available. Use of a n unduly small amount of palaeocurrent data could hide rather than unravel the palaeogeography.

56

General characteristics of glacigenic sedimentation

Fig. 32. Climbing ripples (ripple-drift cross-lamination in glaciofluvial sands (quarry Eggestedt Nord, 20 km north of Bremen, Federal Republic of Germany). The ripples are good palaeocurrent indicators (current from left to right).

Fig. 33. Glacial striae made by a Pleistocene mountain glacier in the wall of a valley near Tabescih (central Pyrenees, Spain).

Facies interpretation

57

Textural characteristics Textural characteristics of the sediments include the nature of the surfaces of the grains, their shape, their roundness and their orientation (fabric) within the sediment. These characteristics can be studied in the field as far as clasts are concerned, but grains of sand size or smaller need to be studied with a binocular, hormall microscope or even with a SEM (scanning electron microscope) (Bull, 1981). Such textural studies are not specific for glacigenic sediments and will therefore be dealt with briefly. In general, texture may give indications about the processes that the sedimentary particles have undergone. Surface analysis, for instance, may provide indications of aeolian transport whereas the roundness may provide information about the transport of the particle by currents or waves. An aspect that is quite typical of glacial sediments and therefore deserves special attention is the degree of weathering of large clasts. The occurrence of strongly weathered clasts (often granitic boulders) that crumble as soon as they are isolated from the deposit (Fig. 34),is a fairly

Fig. 34. Completely weathered erratic in a Weichselian moraine near Wartenberge (Federal Republic of Germany).

58

General characteristics of glacigenic sedimentation

strong argument favouring transport of the clast while embedded in ice (Embleton and King, 1977; Embleton and Thorns, 1979). The fabric of diamicts, and of other glacigenic sediments also, is important because it allows the prevailing stress conditions during sedimentation to be reconstructed (Richter, 1930,1932; Seifert, 1954; Lawson, 1979; Prange, 1983; Dowdeswell and Sharp, 1986). One should, however, do this with care because postdepositional processes (ice pushes, compaction, etc.) may affect the original fabric.

Occurrence Palaeogeographical reconstructions of glacigenic areas require that the spatial ( = lateral and vertical) relationships of a specific unit with other deposits be established. The preservational potential (units may have disappeared completely by erosion) is most important in this context.

Preservational potential Deposits formed under different conditions tend to have varying preservational potentials (Reading, 1978a; Gradziiiski et al., 1986).Few sediments are preserved without being affected by erosion (or at least abrasion). Various glacigenic sediments tend to have a rather small chance of surviving erosion (N. Eyles, 1983~). Energy changes are a factor of prime importance as far as the preservational potential of a sediment is concerned. Moving ice masses represent a giant amount of energy, which implies that it is the sediments that are directly or indirectly influenced by active ice that tend t o undergo erosion. Deposition may prevail locally in the 'shadow' of a barrier, if the substratum is subsiding or if the erosional base is changed (e.g., by a eustatic sea-level rise). This is of more importance than momentary climatic or meteorological conditions for determining the preservational potential. A limited preservational potential will usually be expressed by a relatively large number of erosional phases. The ratio between erosive and non-erosive contacts might therefore be a measure of the preservational potential but insufficient data are available to rank the various types of glacigenic deposits according t o this parameter.

Horizontal and vertical facies associations The various facies and their deposits in the glacial and periglacial environments commonly show well recognisable relationships touching

Facies interpretation

59

their horizontal and vertical transitions into each other. This is, of course, due t o the gradually changing boundaries between the facies resulting from the logical succession of depositional and erosional processes. Walther's facies law already recognised this in the 19th century. Glacigenic facies face yet another changing parameter: climate. Even relatively small fluctuations in the average temperature or precipitation may induce significant changes in depositional patterns and should therefore be considered as an important factor influencing the distribution of facies in space and time (Boulton, 1972a; Sugden and John, 1976). The normal sedimentary processes and the climatic fluctuations are the main reasons for the common occurrence of closely interrelated facies, both vertically and horizontally. Such groups of facies that apparently have a number of elements in common, are called 'facies associations'. A well known example is the association of proglacial deltaic and lacustrine facies with scattered erratics supplied by melting ice masses in the lake. Such deposits from associated facies may, in turn, become included in the tills of an advancing glacier (N. Eyles, 1983a) and become part of diamicts. Sequences - The term 'sequence' is commonly used when facies associations form a vertical succession. A sequence consists of a succession of lithological units with gradual, sharp or erosive contacts, formed by an uninterrupted, more or less predictable series of depositional processes which occurred at a specific place due to a set of depositional conditions that changed according t o a logical depositional model. Characteristic examples of sequences are the coarsening upward sediments of deltas (Oomkens, 1967; Van Loon, 1972) and the fining upward fluvial sequence (Allen, 1965; Kessler and Cooper, 1970; Leeder, 1973; Harms et al., 1975, 1982; Cant and Walker, 1978; Bluck, 1980) (Fig. 35). Sequences need not be characterised by changing grain sizes: the sedimentary structures can also change as to frequency, nature or direction, or fossil assemblages may appear or disappear, etc. Whether such changes occur gradually or suddenly, and whether they take place frequently or rarely, is an indication of the underlying processes and therefore often provides a clue for the interpretations of environmental changes. In general, the wide variety of facies associations can only give clues for detailed interpretation if additional data are gathered. This is also true for glacial sequences (Crowell, 1978; Schwan et al., 1980; Beard et al., 1982), although the interpretation may raise severe discussions (Dreimanis, 1984b; N. Eyles et al., 1984a; Karrow, 1984a; Kennis and Hallberg,

General characteristics of glacigenic sedimentation

60

I

f--

1

rooilet zone

coliche nodules

overbank floodplain deposits

E 0 Ni IC)

point- bar deposits

1

channel

cross-bedde d sondstonss

conq lome r a t ! c sandstone w i t h introclasts srosionol baae

Fig. 35. Idealised fining-upward fluvial sequence, as commonly found in the various glaciofluvial facies. Adapted from Pettijohn (1975).

1984). However, if the sequences iesult from a distinct and logical succession of depositional processes, they will usually be a key to the genetic interpretation.

Depositional mechanisms The glacigenic conditions are so diverging that a wide variety of depositional processes play a role. Each subenvironment and each facies is characterised by a specific combination of prevailing depositional processes, but the local conditions are so important that it is not possible to base a facies interpretation on the mere relative importance of the various processes that are presumed t o have formed the pertinent deposits. It should also be kept in mind that periods of 'normal' sedimentation may alternate with phases of 'catastrophic' processes. There is no general relationship between the relative frequency or duration of these different situations and the impact that they have on the final sediment. On the

Facies interpretation

61

other hand, 'rare' deposits may represent 'common' depositional conditions (with a low net sedimentation rate) and vice versa.

Normal and catastrophic sedimentation 'Normal' sediments are the net result of the depositional and erosional processes that prevail a t a given location under regular conditions. Such sediments increase in thickness at a rate that corresponds with the prevailing rate of net deposition, which factor depends on the general energy level. There may, however, occur short-term, incidental processes with greatly different energy levels, resulting in what are commonly called 'catastrophic' sediments (Reading, 1978a; Gradzifiski et al., 1986). The glacial melt-out process and the englacial and subglacial deposition of diamicts are examples of 'normal' processes, whereas subaerial slumps or subaqueous suspension currents are examples of the 'catastrophic' category. Drumlins may be associated with catastrophic subglacial floods (Menzies, 1989; Shaw et al., 1989). Vertical cross-sections through glacigenic deposits commonly show both types, suggesting that 'normal' and 'catastrophic' processes alternate more or less regularly. This is not the case, however, because long periods with 'normal' sedimentation and erosion can easily result in a much thinner succession than one momentary 'catastrophic' event. The relative abundance of 'catastrophic' sediments is therefore no indication of the frequency of such events but indicates only the energy and the transport capacity involved under these extreme conditions, and the preservational potential of both categories of deposits. This, however, does not exclude the possibility that 'catastrophic' events occur frequently; subaqueous slope sediments, for instance, may become reworked, redeposited, again reworked, etc. Flow tills may be composed of a number of lithological units that have undergone a n increasing number of reworking phases with increasing age; consequently, the oldest sediments in such a flow till may show a much more irregular character t h a n the youngest sediments involved, even if the most recent flowage process had not affected them in different ways.

Exceptional conditions - 'Catastrophic' sedimentation is commonly but not necessarily due to exceptional conditions; on the other hand, some exceptional situations may be difficult to reconstruct because they leave no traces or because what traces are left cannot easily be recognised as such. Nevertheless, recognition of exceptional conditions may be most important if the development of a glacigenic area is to be reconstructed.

62

General characteristics of glacigenic sedimentation

Exceptional situations differ from catastrophic situations in that the latter may distinctly interrupt the normal depositional process but nevertheless be a part of the regular development. It is t o be expected and is therefore not exceptional that, e.g., areas with almost no vegetation may undergo fairly catastrophic sheet flooding at more or less regular intervals. Exceptional situations will arise, for example, if well developed vegetation arises locally in a sheltered area nearby the ice cover, resulting in organic-rich deposits. Such exceptional traces may have a strong influence on the reconstruction of a glacigenic development; one could even state that, in general, the more exceptional a find is, the more attention should be given t o the fitting of such data into the general model. It should always be kept in mind, however, that exceptional (or catastrophic) events may result in deposits that are not or that are hardly to be distinguished from 'normal' deposits. On the other hand, an exceptional combination of 'normal' factors may result in apparently exceptional deposits. There is no unambiguous method available to distinguish with 100%certainty 'catastrophic' deposits from 'normal' deposits. Common and rare types of deposits It is clear from the literature on glacigenic sediments that the various types of deposits involved occur with strongly varying relative frequency and extent. Local and regional differences are common but obviously there are also some general trends as regards the probability of finding a specific type of deposit. The two main reasons for this are well known: the frequency and extent of the various types of deposits may differ, and their preservational potential may be different. Original differences in frequency may represent differences in the dynamics of the prevailing processes, especially differences in energy gradients. Rapid alternations of high-energy and low-energy processes tend to lead to much erosion, resulting in a sediment of restricted thickness and extent - if any sediment is left (Zielinski, 1982b). The sediments formed along the margin of an ice cap are exposed to this set of conditions. Consequently, the remaining sedimentary pattern is often quite chaotic and difficult t o interpret. This implies that it may be helpful t o reconstruct the dynamics on the basis of the relative frequency of deposit types.

63

Facies interpretation

Sedimentary cycles Sedimentary cycles are due to repetitions of sedimentary conditions and therefore of sedimentary sequences. The sediments that form part of such cycles have been given various names, e.g., 'cyclites' and 'rhythmites' (e.g., Duff and Walton, 1962; Duff et al., 1967; Reineck and Singh, 1980; Gradzifiski et al., 1986).The sediments of one specific cycle are commonly called 'cyclothems'. Cyclic sedimentation has been described from several facies, among others from fluvial facies (Allen, 1964, 1970a; Beerbower, 19641, deltaic facies (Moore, 1959; Oomkens, 1967),lacustrine facies (Lambert and Hsu, 'nl

C

tm

0

5

lo I

10

E

9 3

10

6

12

Fig. 36. Characteristic examples of glaciodeltaic and glaciolacustrine cycles in the Jaroszow Zone (SW Poland). 1 = structureless coarse and medium sands; 2 = idem with cross-bedding; 3 = idem, with trough sets; 4 = structureless fine sands; 5 = idem with horizontal lamination; 6 = idem with climbing ripples; 7 = fine and medium sands with small-scale cross-bedding; 8 = silts with horizontal lamination; 9 = silts and clays with wave ripples; 10 = silts with wavy lamination; 11 = clay; 12 = varved clay; 13 = small-scale deformations. After: Brodzikowski and Van Loon (1983).

64

General characteristics of glacigenic sedimentation

1979a,b), aeolian facies (Hunter and Rubin, 1983) and submarine fans (Maldonado and Stanley, 1976,1979). The characteristics found for these cycles under non-glacigenic conditions apply in principle also t o similar deposits formed in glacigenic areas (Fig. 36). Although sedimentary cycles occur relatively frequently - in glacigenic (Miller et al., 1977; Crowell, 1978; Beard et al., 1982) and glaciomarine (Mode et al., 1983) deposits also - precise interpretation often seems difficult and the controversies are evident from the literature. These controversies stem often from the differences in opinion regarding the position of the base in each cycle and thus the real cycle of processes. Some authors have thus suggested that the term 'cyclic sediments' be replaced by 'repeating sediments'. Attempts to approach the cyclicity problem on a more methodological basis (Zeller, 1964) have thus far not found much support. Instead of this, the main trend in sedimentology during the past twenty years has been the application of mathematical (statistical) procedures such as Markov chain analysis, factor analysis and probabilistic calculations. Cycles in glacigenic sediments can be found on a macro-, a meso- and a microscale, which implies that cycles may show their own subcycles. This is quite plausible since many characteristic glacigenic deposits are formed in the neighbourhood of the ice front, and the position of this front is subject to a large number of both smaller and larger fluctuations. Each fluctuation may result in a cycle (in fact a sequence) and each cycle may include deposits with their own cycles (e.g., varves in glaciolacustrine sediments). GLACIGENIC FACIES MODELS Sedimentological field work, and basin analysis in particular, requires that models be established, verified in the field and finally rejected, or accepted as useful for further research. A generally accepted facies model constitutes, in its widest sense, a summary of a specific depositional environment (or subenvironment) or a closely related group of (sub)environments (Walker, 1984). Numerous facies models have been developed by sedimentologists throughout the world. Comparison of large numbers of such models shows that there exist models that seem t o be well applicable for most situations within a specific sedimentary environment. These descriptive models can be used as basis for more detailed and perhaps speculative models for a particular area.

Terminology and use of symbols

65

Models must combine all the information that can be derived from field data such as lateral and vertical facies transitions, the occurrence of sequences and/or cyclothems, energy gradients, erosional phases and sediment supply. This implies that glacigenic models must deal not only with the area (and the processes taking place) in front of a n ice cap, but also with the area on top of, within and underneath the ice. There do not yet exist good methods to study sedimentary processes within or underneath a n ice cap. Glacigenic models then, of necessity, include uncertainties, perhaps even more uncertainties than the models from any other sedimentary environment. Fortunately, our insight into glacigenic processes has increased considerably in the last few years and even though some details a r e impossible to verify, existing models appear to be sufficiently accurate to have a fairly good predictive value when regional studies are initiated. Laboratory experiments have been of great help for understanding the processes and the resulting sedimentary characteristics, although i t must be emphasised that such experiments are commonly carried out on a small scale (Rozycki, 1958); there are several indications that extrapolation of the experimental results t o full-scale conditions is not always feasible. The same holds for experiments and observations in 'natural laboratories' such as waste-dumping areas, alluvial fans in sand pits, tailings, etc., although observations made in such 'laboratories' under polar or subpolar conditions can indeed give reliable information about relatively smallscale processes. Experiments, field observations and theoretical analyses all have contributed t o the models of glacigenic facies. Such models obviously become less accurate as they become more detailed. The models t o be presented in this book will therefore be of two types: rather general models that can apply superficially to each situation dealt with, and much more detailed models that have as primary aim to show actual situations on a smaller scale. TERMINOLOGY AND USAGE O F SYMBOLS Descriptions of glacigenic lithofacies by different authors are difficult to compare because each author tends to develop a terminology t h a t is most suitable for (1)his specific research interest and (2) the region of his work. Lithofacies are most commonly designated by letters and/or numbers: lithofacies A, B, C or 1, 2 , 3 or (with subdivisions) A-1 etc., when referred to in literature.

66

General characteristics of glacigenic sedimentation

Several attemps have been made to improve communication between researchers by devising a generally applicable terminology. Miall (1977) and Rust (1978) designated lithofacies by a two-letter code characterising the lithology and the structure. These proposals made it possible to carry out relatively simple comparative studies and the concept was developed further by Miall (1978,1983a, 1985) and Eyles (1983,1985).

Classificationsystem used in this book A much more detailed classification system was elaborated in some steps by the present authors (Brodzikowski and Van Loon, 1980, 1983, 1987). This classification involved (1)environments and subenvironments (based on the spatial relation with the ice cap), (2) the depositional facies (based on the depositional conditions, in particular the depositional processes), and (3) the glacigenic deposits (based on the depositional mechanism). This classification proposal raised important discussions with fellow researchers, most of whom considered the approach very consistent and easily applicable in practice. There were, however, useful suggestions for adaptations. The authors therefore decided t o follow the same approach in the classification scheme in the present book, although with a number of adaptations. Four-level subdivision

The classification proposed by Brodzikowski and Van Loon (1987) comprises four levels, indicated by Roman numbers, capital letters, Arabic numbers and lower-case letters, respectively. The reader is referred t o following sections of this book for details. Only some schematic explanations will be provided in this subsection. The first level distinguishes between the glacial (I) and the periglacial (11) environments. The glacial environment is roughly the area with a continuous ice cover. The periglacial environment is not covered by ice (or is covered in a discontinuous way), but is still under the influence of the ice regime (meltwater streams, loess deposition or comparable features); the continental periglacial environment is the region characterised by a permafrosted soil. The second level (subenvironments) distinguishes parts of the two environments on the basis of their spatial relation t o the ice cap. The (continental) glacial environment, for instance, includes a supraglacial (I-A), an englacial (I-B) and a subglacial (I-C) subenvironment, situated on top of, within and underneath the ice cap, respectively.

Terminology and use of symbols

67

The third level refers to the facies on the basis of the most characteristic depositional conditions (processes). Some adaptations of the earlier proposal (Brodzikowski and Van Loon, 1987) were made at this level: the suggestion that each specific facies type be indicated by the same Arabic number, irrespective of the subenvironment in which it occurs, was followed. Not all facies are present in all subenvironments, so that the consequence of this adaptation is the existence of 'empty' places in the scheme. The following facies were distinguished: melting-ice facies (Arabic number 1; the supraglacial (continental) melting-ice facies is therefore denoted as 1-A-l),fluvial facies (2), deltaic facies (3), lacustrine facies (41,aeolian facies (5) and mass-transport facies (6). The fourth level indicates with a lower-case letter the deposits formed by a specific mechanism within a particular facies. For example, three types of deposits can be distinguished in the (continental) terminoglacial fluvial facies, viz. terminoglacial tunnel-mouth deposits (II-A-2-c), terminoglacial stream deposits (II-A-2-d) and terminoglacial sheet- and streamflood deposits (II-A-2-e). Moreover, a terminoglacial fluvial complex (II-A-2-a) is introduced for those cases where a mixture of the just mentioned fluvial deposits exists, or where it is impossible t o determine for a specific fluvial deposit t o which type it belongs. Further subdivision

That it may be useful t o handle the sedimentary characteristics of a deposit in an equally systematic way has become apparent from the work of various authors, in particular of Miall (1977, 1978, 1983b, 1985), Rust (1978) and N. Eyles (1983b, 1985, 1987). This implies that additional codes must be used. It should be emphasised that such a n approach implies that one is leaving the sedimentary facies and entering the lithofacies. The additional codes t o be mentioned here at a 'lower-than-fourth' level therefore do not inform about the sedimentary facies as such, but may be helpful in the inventorising of lithofacies data for the various sedimentary facies. The lithofacies codes applied by the above mentioned authors are simple and easy t o work with but all show inconsistencies that make later comparisons with other lithofacies ambiguous. The present authors have therefore developed a lithofacies code scheme that is definitely based on previous work, in particular on that by N. Eyles (1985), but with adaptations that not only make the coding itself more consistent but also render it consistent with the approach followed in the coding of the sedimentary facies.

68

General characteristics of glacigenic sedimentation

It seems most appropriate to place additional codes as superscripts and subscripts behind the code for the last level. According t o N. Eyles (1985) one could code: (1)the grain size, (2) the composition, (3) the sedimentary structures and (4) the bedding characteristics. Eyles also provides a code for the supposed genesis of the deposit, but such an additional code is superfluous in our classification because the genesis is already clear from the main (4-level)coding. It is much less feasible t o base a subdivision upon a systematic grouping at these sublevels than at the main four levels. The authors thus found it useful t o follow Eyles' suggestion for coding by means of 'recognisable' letters, in principle the first letter of the word that characterises the property involved. It is expected that there will usually be no need t o indicate all subcodes simultaneously and the following notation might therefore be applied: grain size with a capital superscript, composition with a lower-case superscript, sedimentary structures with a capital subscript and bedding characteristics with a lower-case superscript.

Codes for grain size - A rough distinction can be made between deposits consisting mainly of boulders, gravel, sand and 'fines' (silt and clay). There may, of course, also exist mixed deposits (in practice these are even the most common). The same distinction (and the same codes) should be used for lithified counterparts. The superscript B should be used for sediments that appear to consist mainly of boulders (Fig. 37-A). One problem is that truly coarse deposits are not suited for reliable grain-size analyses; it therefore seems acceptable from a practical point of view to apply this code in cases where material coarser than sand (over 2 mm) dominates and where boulders seem t o constitute the greater part of the coarse particles. There will commonly be a fine-grained matrix, so that most of the sediments of this category may be called 'diamicts'. The superscript G should be used for sediments that consist mainly of gravel, although scattered boulders may be present. A gravelly supraglacial ablation till would thus be indicated by the code I-A-1-bG. The superscript S is applied for sandy deposits (Fig. 37-B). Larger clasts, as well as finer particles, may be present but the sand fraction should account for at least 50% (if possible, as determined in the laboratory). It is important t o mention in this context that a deposit tends t o have a sandy appearance in the field only if the fraction of silt and clay is low (generally less than about 25%);this implies that laboratory analysis of grain-size should be used to check the field data if one is not experienced in estimating the grain size of a sediment.

Terminology and use of symbols

69

I

I.

I, . :.

,L

Fig. 37. Various typical types of glacigenic deposits with different grain sizes. A: densely packed boulders and cobbles. B: glaciofluvial sands. C: horizontally laminated silts and clays (lacustrine bottomsets). D: typical diamict.

70

General characteristics of glacigenic sedimentation

In practice, mainly silty material (a rare phenomenon, but loesses may belong to this group) is difficult t o distinguish from mainly clayey material, particularly when there is some admixture of sand. It was therefore decided, as suggested by other investigators, to group silt- and clay-sized deposits (Fig. 37-C) within one category, indicated by the superscript F (fines). Glacigenic sediments, and tills in particular, are commonly characterised by extremely bad sorting: particles ranging from clay to boulder size may be present. Such badly sorted material (Fig. 37-D) - if fines, sand and coarser particles are all present in significant quantities - should be designated by the superscript D (diamict). Diamicts may result from a direct depositional process, or from postdepositional processes. It is also possible that deposits are relatively well sorted, but with an average grain size more or less a t the boundary between two fractions, or they may be composed of material belonging to two grain-size classes. A combination of the code letters could be used in this case, e.g., superscript SG for a sandy-gravelly deposit. Codes for composition - Most sediments in the glacial and periglacial environments are siliciclastic. Other types of sediments may occur as well, however, and their presence can provide interesting information about the geological (climatological) development. It therefore seems useful to use a specific code for such sediments. As mentioned before, a lower-case superscript will be used for the purpose. Organic material may be designated by the superscript 0. Sediments with such a composition tend t o be of rather limited extent, both horizontally and vertically. They are most commonly peaty levels; such peat may be either in situ o r reworked (Petersen, 1983) in, for instance, the proglacial or extraglacial subenvironment (Fig. 38). Sediments of chemical origin are denoted by the superscript c. Such sediments are rather rare in the glacigenic area; if present, they have often been formed diagenetically, e.g., by transport in solution and subsequent precipitation of iron in the form of oxides and hydroxides. Such precipitates may form crusts, especially in the contact zone with a layer of low permeability. Diagenetically formed carbonate layers may also occur, especially if surrounding sediments contain limestone clasts or calcareous shells (Fig. 39). It does not seem justified t o attribute the code for chemical sediments t o veins that have been formed and filled inside glacigenic sediments because such veins do not form part of the sedimentary succession in a strict sense. Layers that consist mainly of concretions, however, might be denoted with a superscript c.

Terminology and use of symbols

71

Fig. 38. Peat horizon within an aolian deposit (terrace of Kopanica river, Poland). Such organic deposits are indicated with superscript '0'.Photograph: J. Burdukiewicz.

Fig. 39. Limonite horizon (dark lower band) formed due to precipitation of iron-rich percolation water on top of an impermeable, fine-grained layer. Such chemical units are indicated with subscript 'c'. Photograph: J. Burdukiewicz.

72

General characteristics of glacigenic sedimentation

Palaeosoils or comparable pedogenic levels, though not necessarily bedparallel, are important types of levels. They are most important for the reconstruction of the palaeogeographic development of an area and should therefore be indicated in stratigraphic sections. These levels are often made up of specific sedimentary layers that show characteristic colours due t o leaching and concentration of specific elements as a result of the pedogenesis. It is useful, in such a case, to give the superscript p to the layers that represent a soil horizon (Fig. 40).

3:

Fig. 40. Example of a soil horizon (to be indicated with subscript 'p') within fluvial deposits of Holocene age. Photograph: J. Burdukiewicz.

Codes for sedimentary structures - Sedimentary structures are one of the main keys for unravelling the sedimentary mechanism and the lateral and vertical changes in the depositional processes. In our opinion it is not practical to give codes for all types of sedimentary structures but the most meaningful structures do deserve such notation as a capital subscript. Current- or wind-induced cross-bedding (see, e.g., Jopling, 1965; J.R.L. Allen, 1968, 1973a,b, 1980a,b; Boersma et al., 1968; N.D. Smith, 1972; Banks, 1973b; Hunter, 1977) is a most important structure because it allows the direction of the palaeocurrent to be measured. Cross-bedding (Fig. 41),designated by the subscript C, can be found, for instance, in drift sands. Trough-shaped cross-bedding can be found in sandy dune stratifi-

Terminology and use of symbols

73

Fig. 41. Regular cross-bedding (indicated by subscript 'C') in glaciofluvial sands. Photograph: A. Hahszczak.

cation deposited under a low flow regime. Planar cross-bedding may be found in fluvial outwash deposits of sand size and in gravelly or sandy deltaic material. Low-angle cross-bedding (less than 10") is often formed under upper flow-regime conditions. Cross-bedding resulting from scour-and-fill processes, thus indicative of alternating erosional and depositional phases, is designated by the subscript S. The same symbol can be used for the inclined lamination that can be found in channel infillings (Picard and High, 19731, as well known from supraglacial stream deposits (Fig. 42). Subscript R is attributed to ripple-drift cross-lamination, also called climbing ripples, because of the specific depositional circumstances. Such structures (Jopling and Walker, 1968; Allen, 1970c, 1971; Hunter, 1977) are commonly found in proglacial lake-margin deposits (Fig. 43) and wherever currents and settling from suspension occur simultaneously. Wave ripples (Davidson-Arnott and Greenwood, 1974; Piper et al., 1983) may be designated by subscript W. They are found in, e.g., terminoglacial lacustrine deposits (Fig. 44). Graded bedding is designated by the subscript G. This structure may occur as a result of turbidity currents (Kuenen and Migliorini, 1950), for instance from a proglacial deltaic slope to the bottomsets in front (Fig. 45).

74

General characteristics of glacigenic sedimentation

I

i b.

I: Fig. 42. Inclined laminated (indicated with subscript 'S) in a channel within glaciofluvial deposits.

Fig. 43.Ripple-drift cross-lamination (subscript 'R).Photograph: A. Hahszczak.

Terminology and use of symbols

75

Fig. 44. Irregular wave ripples (subscript 'W) in the marginal deposits of a glacial lake.

Fig. 45. Normal, i.e. upward, grading (indicated with subscript ' G ) in proglacial bottomsets.

76

General characteristics of glacigenic sedimentation

Grading may also be reversed (Sallenger, 1979; Broster and Hicock, 1985). A varved succession (Kempe and Degens, 1979; Schluchter, 1979a,b; Schove, 1979; Sturm, 1979; Striimberg, 1983), commonly consisting of graded layers resulting from seasonal deposition (but aeolian varves are also known: Stokes, 1964) - alternating or not with turbidites - is denoted by the subscript V (Fig. 46). This code will be applied most commonly for varved bottomsets in glacigenic lakes.

Fig. 46. Typically varved (subscript 'V') glaciolacustrine deposits. Some dropstones are also visible.

Parallel lamination (Fig. 47) is to be designated by the subscript L. This quite common structure (McBride et al., 1975; Boyko-Diakonow, 1979; Mackiewicz, 1983; Mackiewicz et al., 1984) may have different origins, but distinguishing between them is considered beyond the scope of the present discussion. Laminated terminoglacial tunnel-mouth deposits formed under a high flow regime, subglacial channel deposits with a lamination due t o a low flow regime and proglacial lake-margin deposits that are laminated by swash and backwash thus only warrant their notation on the basis of a description of the structure and not of interpretation of their genesis. A special code is also considered useful t o indicate the presence of deformation structures within a layer. Such deformation structures

Terminology and use of symbols

77

Fig. 47. Parallel lamination (subscript 'L'),formed during transport of sand grains under upper flow-regime conditions.

(Anketell et al., 1970; Van Loon and Wiggers, 1975, 1976; Prescott and Lisowski, 1977; Boulton and Jones, 1979; Parriaux, 1979; Doe and Dott, 1980; Funder and Petersen, 1980; Krtiger and Humlum, 1980; Schwan et al., 1980b; Boulton, 1981; Mills, 1983; Van Loon et al., 1984, 1985; Van Loon and Brodzikowski, 1987) are quite common in water-saturated sediments, especially if there is a high silt content or a relatively large amount of organic material. Various types may occur as a result of plastic deformation but liquefaction is also common. The code applied for all these structures is the subscript D (Fig. 48). Apparent absence of sedimentary structures is also worth mention. The subscript M could be applied for such massive units (Fig. 49). There may be, e.g., englacial melt-out tills that could be described by this code. A specific unit may of course be characterised by a number of different sedimentary structures. All pertinent codes might be used in such a case; the order of the codes should indicate the relative importance of the various structures. Codes for bedding characteristics - The nature of the contacts between successive layers may be useful for the interpretation of the depositional history. It is therefore considered appropriate to add a specific code (a

78

General characteristics of glacigenic sedimentation

Fig. 48. Plastic deformation and liquefaction (sedimentary deformation structures are indicated with subscript 'D') in the foresets of a proglacial delta.

Fig. 49. Apparently structureless ( = massive; subscript ' M ) of unknown glaciofluvial origin.

Terminology and use of symbols

79

lower-case subscript) in some cases in order to indicate the nature of the lower boundary of the layer. The lower boundary may be erosive, designated by subscript e, indicating that the layer involved was deposited by a process related t o an erosive force (there are two contacts a t the same place if the erosive process had nothing to do with the layer involved) (Fig. 50). The contact may also be influenced by tectonic activity (glaciotectonic push, regional endogenic forces). In this case it is useful t o indicate the non-sedimentary nature of the contact by the subscript t (Fig. 51). A rather sharp contact without any sign of a sedimentary break is denoted by subscript s. This may be the case, for instance, if a terminoglacial mass-flow deposit is laid down on top of other sediments (Fig. 52). Gradual contacts are more common, indicating that the sedimentary processes did not change abruptly. Such contacts, denoted by the subscript g, may be present in e.g. coversands where slight changes in wind intensity or direction influenced the sedimentary succession (Fig. 53). Deformed contacts due to early diagenetic processes like load casting are quite common, especially so in water-saturated sediments with

Fig. 50. Deformed sediments (centre), being a remnant of a layer that had deeply incised the clay underneath. The light-coloured layer was then eroded itself, being preserved only in the erosion depressions made before. Such erosive contacts (the contact here is partly erosive in a duplicate way) are designated with subscript’e’.

80

General characteristics of glacigenic sedimentation

Fig. 51. Succession with several tectonic contacts (subscript 't') due to shearing as a result of glaciotectonism.

W

I m I :,-

. .

.

.

Fig. 52. Sharp contact (subscript 's') between a unit of silts and fine sands, and a sand layer of probably turbiditic origin.

Terminology and use of symbols

81

i

Fig. 53. Gradual grain-size transition (subscript'g') in coversand.Photo: J. Cegia.

(temporary) high sedimentation rates and alternating grain sizes. Such deformed contacts will be denoted by subscript d (Fig. 54). Relevance of coding

An outcrop in glacigenic sediments may consist entirely of sandy material. It is superfluous t o code each layer with the superscript S in such a case. Codes should be used only where appropriate and relevant. This implies that codes should be used in cases where they are necessary (or a t least helpful) for the interpretation of the sediments or where they may serve t o distin uish between various lithological units. The code I-A-3-CgFcCefor a layer can easily be understood by readers as referring t o a layer in supraglacial deltaic foresets, consisting of sand with a relatively large amount of fines (a considerable part of the particles consisting of small concretions), with current ripples and an erosive base. It is questionable, however, whether such detailed information should always be provided, even though field work implies that the investigator does make all these observations. It does not seem practical to provide generally applicable guidelines touching the details of coding. A short description may improve readability and be equally useful. Each researcher must decide how and in how

82

General characteristics of glacigenic sedimentation

Fig. 54. A diapir, representing an extreme form of deformed contacts (subscript '$1.

much detail coding should be used. The framework sketched above should therefore be considered only as a tool to facilitate communication among scientists. Use of incomplete coding

In practice, lack of data may make it impossible to establish the specific type of deposit within a particular facies. In such a case one might still use all codes that are considered correct and relevant. If one is not sure, for instance, whether a specific laminated deposit from a supraglacial deltaic facies should be interpreted as a supraglacial stream deposit or a supraglacial deltaic foreset, the pertinent deposit might be referred t o as I-AL, thus deleting the code for the specific facies and type of deposit. An erosive, massive diamict of unknown nature in the subglacial melting-ice facies might be referred t o as I-C-lMeD .