Sedimentary Geology, 49 (1986) 73-108 73

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE ORIGIN OF HORIZONTAL ALTERNATING BEDDING IN WEICHSELIAN AEOLIAN S A N D S IN NORTHWESTERN EUROPE

J. SCHWAN

Institute of Earth Sciences, Free University, P.O. Box 7161. 10(t7 MC Arnsterdarn (The Netherlands)

(Received August 15, 1985; revised and accepted February 4, 1986)

A BSTRACT

Schwan, J., 1986. The origin of horizontal alternating bedding in Weichselian aeolian sands in northwest-

ern Europe. Sediment. Geol., 49: 73-108.

Periglacial aeolian sands of Weichselian age are widespread in the lowlands of western and central

Europe. One facies of this sediment type, corresponding to the coversand sensu stricto, is characterised

by a wavy parallel bedding and a generally horizontal position of the strata. Since this layering results

from an alternation of finer- and coarser-grained beds, "'horizontal alternating bedding" is the ap-

propriate designation for it.

By a detailed analysis of representative lacquer peels eight characteristic stratification types could be

distinguished. These units are the smallest fundamental building blocks constituting the sediment under

consideration.

An idealised annual cycle of sedimentation is proposed. In summer, light and variable winds carry silt

in suspension to the site of deposition whilst the sandy saltation load is left behind in a throughput zone

farther upwind. Upon settling, the fines adhere to the surface which is wet or damp due to the thawing of

the seasonally frozen ground. In winter, wind strength is generally greater so that deposition of sand

prevails in the area whereas the fine-grained suspension load settles downwind. Deposition by snow-free

winds results in the accretion of patches of well-sorted sand. Snow flurries, on the other hand, produce

niveo-aeolian strata of generally more poorly sorted sand. In the cycle it is implied that: (1) The aeolian

sedimentation basin must have been large and uniform enough to permit areal separation of sand and

loess-like material; and (2) a seasonal shift in the centres of sand and silt deposition was inherent to the

atmospheric pressure system of the formative period.

The exposed part of the northern and eastern North Sea floor as it existed around the Middle

Weichselian glacial climax assumedly was the principal source area for the deposits.

The relationships between the periglacial aeolian sands with horizontal alternating bedding, loess and

their integrades as described from the Belgian lowlands and Niedersachsen can be interpreted in the

context of the proposed depositional cycle.

INTRODUCTION

Periglacial aeolian sands of Weichselian age occur in the lowlands of western and central Europe in a wide belt with approximately W - E orientation. The western

part of this zone is adjacent to the even larger area of loess deposition (Koster,

0037-0738/86/$03.50 c~ 1986 Elsevier Science Publishers B.V.

74

1982a). Though most reports on these sediments refer to the Weichselian, it is known that, at least in The Netherlands, similar materials were deposited during several earlier Glacials of the Pleistocene Epoch (Ruegg, 1983).

In exposures, the aeolian sands are regularly found to be associated with features indicative of a periglacial environment. These are:

- -Loess layers. In Belgium, The Netherlands and West Germany, aeolian beds of sandy texture coexist with, or laterally change into, loess.

--Waterlaid beds. These "fluvio-periglacial" sediments are either coarse sands infilling valleys or fine-grained material deposited in stagnant pools. Both melting snow and thawing ground ice presumably acted as water sources (Zagwijn and Van Staalduinen, 1975).

--Gelifluction deposits. Thin unsorted sand-gravel intercalations may occur in the aeolian sands where these occupy gently sloping valley sides. They are attributed to periglacial slope-processes (e.g. De Gans, 1981).

- -Deflat ion lags. Aeolian lag concentrates normally comprise single-grain strings of ventifacts or wind-polished particles. Mostly their lateral extent is insignificant, but the Beuningen Pebble Layer is one example of a desert pavement that has been recognised in several European countries.

--Cryogenic structures. Included in this group are seasonal frost cracks, ice-wedge casts, involutions and frost-mound remnants (e.g., Maarleveld, 1976; Vandenberghe, 1983). Whereas the frost cracks were formed contemporaneously with the wind- blown sands, the other features formed under permafrost conditions between periods of aeolian deposition.

It has been known for a long time that basically two varieties of Weichselian periglacial aeolian sand exist. Apart from details and intergrades to be considered later, one type shows a dune topography with related internal structure (facies 1) whereas the other is characterised by a very gently undulating or level surface expression and essentially horizontal bedding. This latter type is, in the strict sense of the word, a coversand.

On the basis of its internal structure, texture and, to a certain extent also its stratigraphic position, at least two subtypes of aeolian coversand can be dis- tinguished: (a) facies 2 with a prevalence of even, horizontal or slightly inclined parallel lamination, low-angle cross-lamination and occasional strings of granules or small pebbles; and (b) facies 3 with horizontal alternating bedding and irregular waviness of the layers as its characteristic features.

The principal facies of Weichselian periglacial aeolian sand in western Europe are summarised in Table 1. It should be understood that particularly the granulometric and chronostratigraphic data contained in Table 1 are only approximations. Also the distinction between the three facies is not hard and fast. Transitional types are found regularly and it may not always be possible to assign an exposed succession to either class with reasonable confidence.

Facies 3 is reported to occur in The Netherlands and adjacent parts of Germany,

TA

BL

E 1

Prin

cipa

l fa

cies

of

Wei

chse

lian

per

igla

cial

aeo

lian

san

d in

Wes

tern

Eur

ope.

Bas

ed o

n K

oste

r (1

982b

),

Pyri

tz (

1972

) an

d R

uegg

(19

83)

FACIES

CHARACTERISTIC SEDIMENTARY

STRUCTURES

DONE-FORESET CROSS-BEDDING

EVEN HORIZONTAL OR SLIGHTLY

INCLINED PARALLEL LAMINATION,

LOW-ANGLE CROSS-LAMINATION

AND OCCASIONAL STRINGS OF

GRANULES OR SMALL PEBBLES

HORIZONTAL ALTERNATING BEDDING

AND IRREGULAR WAVINESS OF

LAYERS

MOOAL RANGE OF

GRAIN-SIZE DISTRIBUI ION

IN urn

150 - 300 +

10

5

- 2

10

BIt,IOOAL WITH PEAKS

I05-150 AND EITHER

50-75 OR 16-63

SHAPE OF SEDIMENT

BODY

DU

NE

S

(At-

OL

[AN

D

UN

E

SA

ND

S)

SAND SHEETS

(AEOLIAN COVER

SANDS)

MORPHOLOGY

RIVER DUNES AND

INLAND DUNE FIELDS

GENTLY UNDULATING

MANTLE BLANKETING

PRE-EXISTENT

TOPOGRAPHY

DOMINANT PERIOD OF

DEPOSITION

WEICHSELIAN LATE

GLACIAL

WEICHSELIAN UPPER

PLENIGLACIAL

76

Belgium and Denmark (Kolstrup, 1983). Of all the three facies discussed above, this

sediment type has least in common with the classical windborne deposits described

in literature, and insight into its origin has remained limited. In this paper an attempt is made to unify the various hypotheses on its genesis into one coherent

concept.

TERMINOLOGY

Beds, in the present description, are depositional units distinguishable by a specific structure a n d / o r texture. They supposedly were laid down under more or less constant physical conditions (Otto, 1938). Their boundaries are defined either by units of different identity which over- and underly them or by bounding surfaces

resulting from erosion or nondeposition. According to Reineck and Singh (1980) beds have no limiting thickness, whereas

other sources (e.g. Collinson and Thompson, 1982) have suggested that units thinner

than 1 cm should be termed "laminae". This latter approach would create termino- logic inconsistency when individual depositional units thinner than I cm are

internally laminated. Since this case applies to a considerable part of the units

discussed in this text, the 1 cm-criterion is unsuitable for our purpose.

Laminae are the finest primary depositional features macroscopically observable within a bed.

A set, as the word is used here, is a succession of beds, either identical or different, which are believed to be genetically related.

A laver or stratum, in our usage, is a general term applying to any unit which is visibly separated from adjacent units. Thus, a lamina, a bed, a deflation lag or a pedogenic horizon may all be referred to as a layer or stratum.

Finally, the term texture is used throughout this paper in the exclusive meaning of grain-size composition.

DESCRIPTION OF THE SEDIMENT TYPE

In the Twente area in The Netherlands and in the adjacent Emsland in Germany the horizontal alternating bedding facies of Weichselian aeolian sand is generally well developed (e.g. Van der Hammen, 1951). Accordingly observation data col-

lected there form the basis of the facies description following below. Supplementary information is derived from Van der Hammen (1951), Van der Hammen and Wijmstra (1971) and Ruegg (1983). Textural classification follows the U.S. Depart-

ment of Agriculture (1975).

Sedimenta~ structures

The facies is primarily characterised by wavy parallel bedding and a generally horizontal position of the beds. This stratification results from an alternation of

finer- and coarser-grained layers. Normally, the finer-grained beds have a texture of

77

loamy (very) fine sand or even finer and a thickness of 5-10 mm. The coarser beds

have a (fine) sandy texture and thicknesses varying from 10 to 20 mm or more. As a

rule, the coarser-grained component of the alternating bedding is thicker than the

fine-grained one. The degree of development of the alternating bedding is variable. In its most

obvious form there is a distinct contrast between successive beds. To a variable

extent, however, the average thickness of the coarser layers may decrease so that ultimately a more or less homogeneous though laminated texture of loamy very fine sand will prevail. In some exposures these two stratification types-- textural alterna-

tion or general fineness throughout--occur in close association both laterally and vertically.

Wherever textural alternation is poorly developed or absent an overall horizontal, predominantly fine-textured bedding is still observable but the lamination is discon-

tinuous, crinkly and generally faint.

Deformations in the sediment are of various kinds: lamina waviness, frost cracks,

jointing, water-escape structures, load casts and flame structures. Here and there the

dominant stratification type is associated with shallow cut-and-fill structures and

thin beds with current-ripple lamination. In the study area the thickness of the

facies rarely seems to exceed 2 m but it is known that much greater thicknesses are found where it fills valleys.

Grain-size composition

All granulometric data given below refer to the fine earth, i.e. the fraction < 2000/~m.

In Dutch literature a bimodal grain-size distribution with peaks in the classes 50-75 and 105 150 Arm is regarded as characteristic of the facies (Van der Hammen, 1951).

Distributions with a maximum in the 16-63/~m range occur mainly in the south of the Netherlands, which is attributable to the admixture of loess (Di~cker and

Maarleveld, 1957; Ruegg, 1981). On the whole, the facies with horizontal alternating bedding is finer grained and more poorly sorted than the two other types mentioned in Table 1.

Unfortunately, practically all available data are from bulk samples so that very little information is available on the textural difference between the fine- and

coarse-grained beds which together make up the alternating bedding of the facies. In

Table 2, granulometric characteristics of an approximately 4 mm thick fine-textured bed are compared with those of a bulk sampled 10 cm layer which included this bed. The contrast between the bulk sample representing a mixture of many bed pairs and the single-bed sample is quite obvious. In other cases however it may be much less distinct. Without recourse to single-bed sampling, the textural alternation of the facies may be demonstrated by fixed-interval bulk-sampling in a vertical succession.

78

T A B L E 2

Textural characterist ics of a bulk-sampled layer and a single bed inc luded in it

FGATURL S INGLE BED BULK-SAMPLED LAYER

THICKNESS OF SAMPLED STRATUt t 4 ]00 I N mm

I ~IEAN GRAIN SIZE 57 90 It ; ,m i

MODAL GP~AIN SIZE q4 - 53 AND 53 - 63 AND Ct~SSES IN ;~rn 105 - 125 105 - 125

TEXTJRAL CLASS VERY FINE SANDY FINE SAND LOAM

O c m

2 1 0 c m

7 0 1 5 0 I I

M E A N G R A I N S I Z E I N ;am

Fig. I. Textural profi le based on 10 cm interval bulk-sampling; N = 20.

79

This is shown in the textural profile of Fig. 1 originating from an exposure with

well-developed alternating bedding.

Stratigraphic position

The rule that a specific sedimentary facies cannot be assigned to one or more formally defined geologic time periods unless it shows a very limited distribution in space and time also applies to the facies discussed here and thus its stratigraphic position can only be approximate. A few generalisations, however, can be made on the basis of data available from The Netherlands and neighbouring countries, though this information is not uniform in its degree of detail (Van der Hammen, 1951: D~cker and Maarleveld, 1957; Kolstrup, 1980, 1983; Ruegg, 1981, 1983; Vandenberghe and Gullentops, 1977; Vandenberghe, 1981; Van der Hammen and Wijmstra, 1971):

(1) Both aeolian low dunes (facies 1) and evenly laminated coversand (mainly facies 2) were formed during the cold phases of the Weichselian Late Glacial.

(2) Horizontally bedded aeolian sand was the prevailing sediment of the Weich- selian Upper Pleniglacial or Weichselian Pleniglacial as a whole.

As stated above, in The Netherlands the same sediment type was also deposited during pre-Weichselian cold periods of the Pleistocene. To just what extent these horizontally bedded sands really correspond to facies 3 is less clear and for at least two reasons this is not surprising since: (1) intergrades between facies 2 and 3 are quite common, as already mentioned; perhaps facies 11 and 3 should be considered as end members on a sliding scale rather than as discrete entities; and (2) the Weichselian Upper Pleniglacial was characterized by variable environmental condi- tions. Considerable climatic fluctuations occurred which, according to Kolstrup (1980, 1983), included periods of continuous permafrost, fluvial activity and severe deflation.

The uncertainty of the relationship between facies types and time stratigraphy is furthermore accentuated by the observation of Van der Hammen and Wijmstra (1971) that sediment similar to facies 3 sometimes occurs in stratigraphic intervals known to be of Weichselian Late Glacial age. Then, apparently local conditions conducive to its formation had dominated over the regional tendency of the time.

ENVIRONMENTAL FACTORS

Below conditions and processes are discussed which may have been instrumental in the deposition of the facies with horizontal alternating bedding. Six different factors will be considered.

Mode of transport and deposition

This concerns primarily aeolian depositional processes which produce horizontal (or slightly inclined) parallel bedding along with a more or less marked textural contrast between successive laminae.

80

(1) Otto (1938) has pointed out that a layer of differently graded laminae could be built up by a sequence of wind events with varying strengths. As noted by Hunter (1977) this mechanism refers to the thinnest types of stratification being formed by small erratic fluctuations in transporting power. Clearly, random varia- tions in wind speed are totally inadequate to account for the large scale--both in time and space--on which horizontal alternating bedding occurs in the sediment considered here. Moreover, Otto's concept refers to a different stratification type. It simply is the most general and obvious way to explain why parallel lamination

should occur at all i n any clastic sediment. (2) De Ploey (1977, 1980) has studied the vertical distribution of both transport

rate and mean grain size during episodes of sand flow on an inland dune. He found that close to the ground surface a thin but heavily laden flow layer occurs in which coarse sands and granules are intimately mixed with grains of (very) fine sands and silt size. It is only at higher levels that the flow becomes both less concentrated and better sorted. Thus a dense basal flow close to the ground surface coexists with an optimum saltation fraction at greater height during periods of strong deflation.

The bimodality of the basal flow is attributed to damping of turbulence. As soon as it passes over elements .of sufficient roughness, turbulent eddying will set in and then the floating fines are carried off to greater heights where they normally occur. De Ploey suggests that this process may bring about an areal separation of a well-sorted optimum saltation fraction and a bimodal sand resulting from basal flow with damped turbulence. By inference, a vertical stacking of the two types might also take place and would produce aeolian sand with textural alternation.

From this it is obvious that the dense basal flow layer, owing to its inherent

instability, must be a transient phenomenon of only short duration. Consequently it is hard to see how the process could have contributed to the formation of the aeolian facies with horizontal alternating bedding in any systematic way.

(3) Bagnold (1941) has described extensive sand accumulations with almost featureless surface expression occurring in the North African desert. Internally these sand sheets are characterised by layers of horizontally bedded sand separated by single-grain strings of pebbles. Since, in its origin, the pebbles play a critical role, and also because its textural make up is so much different, this sediment type has little in common with the facies under discussion beyond a similarity in overall body shape (cf. Ruegg, 1983).

Reineck and Singh (1980) pointed out that other equally large sand sheets exist in which no pebbles are found. Again horizontal bedding is the dominant stratification

type but alternation of finer- and coarser-grained beds is not described. Rather than being akin to the facies with horizontal alternating bedding, the internal structure of the pebble-free sand sheets may have the same origin as either the plane-bed lamination of Hunter (1977) or the low-angle aeolian deposits discussed by Fry- berger et al. (1979).

(4) At any one particular site of deposition, the settling of fines from suspension

may be either preceded or followed by the accretion of saltating and creeping grains

of coarser grade. Provided the accretion is in the form of thin, smooth-surfaced sand

sheets, this simple alternation of depositional regimes might be an acceptable basis for the interpretation of the horizontal alternating bedding (e.g. Krumbein, 1937). To put this scheme into operation it must be assumed that processes with inbuilt periodic change were at work over large areas during sufficient lengths of time, as

suggested by Van der Hammen (1951). (5) Niveo-aeolian sediments are windborne beds with interstratification of clean

snow, dirty snow and sand or other mineral material. They are discussed by many

students of the periglacial environment (e.g. Samuelsson, 1926: Van der Hammem

1951; Edelman and Maarleveld, 1958; Rochette and Cailleux, 1971: Cailleux, 1972, 1973, 1974; Jahn, 1972; Pissart et al., 1977; Washburn, 1979).

According to Cailleux (1972, 1973, 1974) sediments of this type are either perennial or annual, depending on the climate. In the latter case, the snowy strata

will melt out completely during the summer. He has described modern annual niveo-aeolian deposits in arctic Canada. The sediments bodies are in the shape of nearly fiat and discontinuous patches of limited extent and thicknesses varying from 1 t o 3 m .

The thickness of both the compacted snow layers and intervening sandy strata is in the cm or dm scale. The snow is contaminated with sand grains to various

degrees. Bedding is generally horizontal. Upon melting of the snow, the originally

flat surfaces of the interstratified patches obtain an irregular microrelief consisting

of little cones and roundish mounds with heights between 10 and 30 cm. In these

structures sand grains are often found to be aggregated into pellets of 3-8 mm diameter. Jahn (1972) studied the deflation of snow patches covering ploughed

fields in the Polish Sudetic Mountains. Snow was transported in drifting motion and heaped up near obstacles downwind. Along with a gradual uncovering of the ground

surface by deflation the successive snow layers become richer in soil particles with

the topmost strata being virtually free of snow. Depending on the survival time of the snow cover, one or more of such cycles are formed during a winter season. Jahn

found that, as compared to ordinary wind transport, the niveo-aeolian process has a lower capacity for size-sorting. This is due to the fact that in a snow flurry suspended and saltating particles are intercepted by snow flakes, temporarily aggregated and collectively deposited.

It is obvious that the structure and texture of fossil niveo-aeolian deposits will be determined to a large extent by the manner in which melting of the snow layers takes place. In the present discussion only annual melting will be considered.

In theory, disturbance of the original structural will be minimal when removal of snow is either by direct evaporation or slow melting on a permeable substratum

with horizontal topography. Under less stable conditions, however, the stratification formed by sand-snow interlayering will be distorted or altogether destroyed upon melting of the snow. Concurrently, the annual snow melting may produce tern-

82

porary surface run-off or shallow sheet-flooding leading to widespread reworking of the loose substratum. As a result, thin layers of contrasting texture would be both spatially separated and vertically stacked in areas subject to "niveo-fluvial" activity. This process has been invoked to account for the genesis of coversand with horizontal alternating bedding. Though beds of undisputable fluviatile origin some- times occur in the facies, it is unlikely that meltwater flushing could have signifi- cantly contributed to its formation.

One other deformational process worthy of consideration in the present context is liquefaction of niveo-aeolian sediment beds. A high rate of snow melting in spring combined with a still frozen substratum and a thick overburden of mineral material would be conducive to this process. Once soaked and liquefied, the bed looses its strength so that the gentlest slope or unevenness may bring about quick behaviour. As a result the original stratification will be partially or entirely obliterated with concomitant mixing of texturally different layers.

(6) Deposition by adhesion occurs when dry, wind-blown sand grains adhere to a damp or wet sandy surface or when they are trapped in shallow pools of still water (e.g. Hunter, 1973, 1980).

From experimental work, Kocurek and Fielder (1982) identified six factors which control the type of adhesion structures to be formed: topography and moisture content of the receiving surface, speed and directional variability of the wind, sediment supply rate and impact angle of saltating grains.

Ruegg (1975, 1981, 1983) thought that adhesion deposition could have played a role in the formation of periglacial aeolian sands with horizontal alternating bedding. According to him, the silty beds result from a fine-grained suspension load sticking to a damp surface, whereas pure sand beds are the product of tractional deposition on a dry surface. This alternation of sedimentary processes suggests a frequent change in both soil moisture content and mean wind strength.

Source areas

The source area of the periglacial aeolian sands in The Netherlands has been the subject of a long-standing debate (Vink, 1949; Crommelin, 1964, 1965; Maarleveld, 1968; Veenstra and Winkelmolen, 1971; Vandenberghe and Krook, 1981). Whilst dry and barren river beds as well as other local sources must have contributed, there is wide agreement that during the Weichselian Pleniglacial long distance transport from the North Sea basin prevailed. In the subsequent Late Glacial, conditions were different and interacted to promote transport from nearer sources by mainly westerly winds (e.g. Maarleveld, 1960). Recession of the ice sheet margin, reinunda- tion of the North Sea basin, changes in the atmospheric pressure system and a denser vegetation cover, all cooperated to modify the depositional regime. Virtually all the windborne sands formed at this later time are thought to have been derived from both pre-existing aeolian sediments and temporarily dry water courses. Thus a

83

distant northwestern source was gradually replaced by sources closer to the deposi-

tional sites (Edelman and Maarleveld, 1958). During the last glaciation, both deflation and aeolian deposition occurred in the

North Sea basin. Throughout the whole of the Weichselian cold time its southern part remained a non-glaciated land surface where only continental sediments were laid down (Jelgersma et al., 1979). Thus, in the Dutch North Sea sector, large expanses of coversand form part of a periglacial aeolian sand belt extending through northern Germany, "The Netherlands, Belgium and eastern England (Oele and

SchiSttenhelm, 1979). Further to the north in the Scottish, Norwegian and German sectors, windborne

sands appear to be absent or rare and here the setting is dominated by the former

41° 2 ° 01o 2 ° 4 o 6 o 9 ° 10 ° t ~ ~ ~ i~ ii !i~iiii~ii~iii!i!iii!ii~i~ii~iii~iii~il

62° i .62 °

Lake or Jnqand sea

Drainage valley

Ice margin

J Tunnel valley

4o 2 ° 0 o 2 o 4¢ 6 ° 8o 10 o

Fig. 2. Maximal extent of the Weichselian ice sheet. From Jansen et al. (1979).

84

activity of the Weichselian ice sheet whose approximate position is shown in Fig. 2. In the northern part of the basin glacial beds of considerable thickness were deposited by the British and Scandinavian ice caps which had their greatest extent from 20,000 to 15,000 years B.P. In the German sector, supply of sediment was provided by both the sandurs along the ice margin and the rivers Elbe, Weser and Eros. In this region the Scandinavian ice did not advance beyond the Elbe river valley (Behre et al., 1979).

Unlike the southern section of the North Sea basin its deeper northern and eastern reaches did not emerge during all of the Weichselian time. Yet there is ample evidence for a considerable lowering of the sea level, so that large parts of the extensive basin became dry during the Middle Weichselian cold climax. The barren or poorly vegetated surfaces must have been important sources of aeolian accumula-

tion downwind.

Wind patterns

Data on the prevailing wind directions during the Weichselian Pleniglacial are scanty. This is to be expected since in western Europe aeolian sediments of this time generally have a level or very gently undulating surface topography and, moreover, to a large extent they are covered by younger deposits.

As explained in the preceding section, the significance of northerly winds is implicit in the work of Veenstra and Winkelmolen (1971). Similarly, Edelman and Maarleveld (1958) inferred the dominance of northwesterly winds in Upper Pleni- glacial times from wind-shadow patterns in hilly country in The Netherlands. Ten Cate (1969) deduced a N to NNE wind from the dip direction of inclined strata in low linear sand dunes occupying the valley sides of small water courses. Gullentops et al. (1981) have described linear coversand and loess ridges of Brabantian (= Weichselian Upper Pleniglacial) age in the lowlands of northeast Belgium. Both types are thought to have been formed by a NE wind. They suggested that these landforms could be due to a local surface wind that not necessarily coincided with the general flow pattern of the time.

The dominance of a northerly wind can also be deduced from Fig. 3 if it is supposed that along-track size-sorting is a major cause of the coversand-to-loess transition. Again, on the basis of textural gradient, deposition by a northwesterly wind may be assumed for the sandy loess of Niedersachsen (Vierhuff, 1967).

Lamb (1977) discussed the atmospheric circulation in western Europe during the Weichselian glacial climax. According to his paleoclimatic reconstruction, much northerly windflow occurred all the year round over the sea area between Greenland and northern and western Europe. In that region a generally easterly surface wind regime prevailed in winter with lighter and more variable winds in summer. In winter the atmospheric circulation was largely controlled by the presence of a strong

85

N J

0

A/CP r ( 0 50 km ++J

~ J Coocse grained fluviatile deposits of Rhine and Meuse

~ PePiglacial aeolian sands partly covered by Holocene beds

[ • T r a n s i t i o n zone (.Sandloam")

~ L o e s s QPeQ

Fig. 3. West-east orientation of the boundary between coversand and loess in the Belgian lowland and adjacent Netherlands. From Zagwijn and Paepe (1968).

anticyclone centered somewhere over the ice cap as it existed during the glacial maximum.

Rutten (1954), in discussing the genetic relationship between coversand and loess, proposed an alternation of the following events: (1) westerly storms with bad weather, deflation of the dry North Sea basin and deposition of sand; and (2) anticyclonic flow with sunny wheather or katabatic wind, deflation of the

86

Weichselian ice sheet margin and deposition of loess. In this manner, both areal separation of sand and loess as well as successions of coarser and finer-grained strata could be produced. Vierhuff (1967), who studied the sand-loess interstratifi- cation in Niedersachsen arrived at a similar conclusion.

Though objection may be raised against the assumption of two texturally contrasting zones within the Weichselian North Sea basin, Rutten's concept is interesting in that it emphasises the alternation--seasonally or recurrent ly--of two different weather types, each having its own wind characteristics.

Climate

In all discussions on the climate of the Weichselian Upper Pleniglacial in The Netherlands, reference is made to the Beuningen Complex. This important marker horizon of large areal extent represents (1) a period of continuous permafrost associated with the glacial maximum, (2) a subsequent time of niveofluvial or glaciofluvial sedimentation, and (3) a final phase of large-scale deflation due to intense drought and strong wind. The Beuningen time was both preceded and followed by accumulation of aeolian sands with alternating bedding. During these episodes the climate, though periglacial, was much less extreme with respect to both winter temperature and precipitation. From the pollen content of slightly organic intercalations and also from the absence of syngenetic permafrost indicators (i.e. large ice-wedge casts and involutions), Kolstrup (1980) inferred a mean January air temperature of - 8 ° C and a mean July air temperature of 9 or 10°C for the pre-Beuningen phase of coversand deposition in The Netherlands. During the later post-Beuningen period of aeolian sedimentation the temperature regime was even slightly milder.

As pointed out by Williams (1975) evaluation of paleoprecipitation is difficult and to a large extent inevitably conjectural. He estimates that active growth of ice wedges is possible only when the snow cover has a thickness of less than 25 cm. According to him this figure corresponds to a winter precipitation of 10 cm (rain equivalent) and an annual one of approximately 25 cm. On this basis, annual precipitation during the periods of aeolian deposition could have been at least 25 cm but proof for this assumption is not available. In exposures of Weichselian Upper Pleniglacial coversands two features are suggestive of seasonal freezing and thawing. The crude, irregular wavy bedding is most likely due to frost thrusting, i.e. to the mainly lateral displacement of soil during freezing. Similarly, the frost cracks, with uniform widths of several centimeters, indicate thermal contraction at subfreezing temperatures (French, 1976; Washburn, 1979). On the other hand, features regarded as diagnostic of permafrost (e.g. levels of ice-wedge casts or periglacial involutions) are either absent or superimposed. In the latter case they developed only after the deposition of the coversand.

87

Vegetation cover

According to Bagnold (1941), in subhumid grass steppes undulating bodies of sand may develop in which dunes with the associated high-angle cross-bedding are absent. Laminations frequently interrupted by plant roots and patchwise accumula- tions of sediment between vegetation stands might be characteristic features of this

regime (Roep, 1968). Similarly, Pissart et al. (1977) found that a vegetation cover

helps in producing even parallel lamination. Bioturbation or other sedimentologic

evidence of plantgrowth (see Fig. 4) occurs rarely in the aeolian sands with horizontal alternating bedding. Yet it is believed by Kolstrup (1983) that " . . . parts

of the landscape were, at least periodically, covered by vegetation during the deposition of the sands". This she infers from the micropaleontologic record of the Weichselian Upper Pleniglacial, a period during which deposition of facies 3 occured on a large scale.

Topography of the depositional surface

Two points are relevant concerning the morphology of the terrain prior to and during the accumulation of the aeolian facies with alternating bedding.

Fig. 4. Shrub coppice mound (nebkha) on microscale. This rare feature presumably formed as a sand shadow behind the stem of a plant. Wind was from left to right.

~8

(1) Wherever a strong relief pre-existed, the periglacial aeolian deposits have mantled and greatly reduced it. To this effect the coversands owe their very name. A case of positive feed-back might be involved in that decreasing height differences of the terrain and the general horizontality of the mantling strata mutually enhanced each other.

(2) Vandenberghe (1981) and Vandenberghe and Krook (1981) noted that struct- ural changes in the Weichselian Upper Pleniglacial aeolian sands may be related to the local topography of the substratum. They found that whilst horizontal alternat- ing bedding in optima forma prevailed at low-lying sites, it was gradually replaced by incipient dune features near (now buried) watersheds higher-up. This is in agreement with the general variability of the facies as already mentioned, though its dependence on local topography may not be equally apparent everywhere.

OBSERVATION DATA

In order to get insight into the structural make up of the facies with horizontal alternating bedding, twelve lacquer peels from the Twente area in The Netherlands and the adjacent Emsland in Germany were analysed in detail.

S T R A T I F I C A T I O N T Y P E S

Poor ly sor ted sand w i t h coarse 1 I~°~:./%'~ ~or i nd i s t i nc t l a m i n a t i o n o r w i t h o u t

~ o ~ ~ I s t r u c t u r e a l toge ther .

We l l - so r ted sand w i t h coarse o r 2 E~'~ lindistinct l a m i n a t i o n or w i t h o u t

I s t r u c t u r e a l t oge the r .

~ 7 ~ W e l l - s o r t e d ( f ine) sand w i t h hor izonta l ~ p a r o l l e l l a m i n a t i o n o r l o w - a n g l e cross-

3 ~ l a m J n a t i o n . l n d i v i d u a l laminae are we l l -de f ined . v ~ . ~ ~ L =Thin set of s t ra t i f i ca t i on t y p e 3.

Si l t to f ine sand w i t h coarse o r indis- t i n c t l a m i n a t i o n o r w i t h o u t s t ruc - tur 'e a l t o g e t h e r

Si l t tO f ine sand w i t h w a v y o r even 5 pa ra l l e l bedd ing . W e l l - d e f i n e d bedding

planes are bounding surfaces of beds compr is ing m a n y l a m i n a e Ind iv idua l l a m i n a e are ind is t inc t .

Silt to f ine sand w i t h a iscont inuous 6 w a v y o r even , rough ly para l le l l a m i n a t i o n

Dis t inct bedding planes o r l a m i n a r surfaces are lacking.

F 7 7 7 7 ~ Sand f ine send o r si l t w i th c r o s s - l a m i n a t i o n . 7 ~ A R C " C r o s s - l a m i n a t i o n by adhes ion r ipp le c l imb,

} / ~ J l C F = C r o s s - l a m i n a t i o n by cu r ren t f l ow .

8 ~ cSuatr-d&'-nfil I ss~nudc?urr~,'st " with

~ Original structure disturbed by p~antroots or hgndhng of peel.

Fig. 5. Legend to Figs. 6-9.

VARIOUS STRUCTURES

- - ~ Frost crack.

~ Fuzzy (deformed) lammat#on,

F lame s t ruc tu res on cm scare.

i ~ ~ ~ ~ ~ ~ Poor ly so r t ed sand w i t h I pa r t i c les up to g r a n u l e size.

~ w~.__ Wind scour,

V

C o n v o l u t e l a m i n a t i o n on cm-sca le .

Eros ive con tac t .

C o a r s e n i n g - u p w a r d sequence.

89

STRATIFICA-

TION TYPES

VARIOUS

STRUC- TURES

Fig. 6. Lacquer peel I: coversand facies with horizontal alternating bedding. Standard type. Length of

profile = 125 cm.

90

STRATI FI C A- T I ('~ NI T Y IDI~ ~

VARIOUS

STRUC -

Fig. 7, Lacquer peel 1I: coversand facies with horizontal alternating bedding. Standard type. Length of

profile = 125 cm.

OUS

FURES

91

Fig. 8. Lacquer peel III: coversand facies with horizontal alternating bedding. Type with dominance of

fine-grained adhesion laminations. Length of profile = 105 cm.

92

ST RATI FI C ATI ON

TYPES

VARIOUS

STRUCTURES

. ~ . . . . ~ .

" . . . . ::-L.

ARC

r I

~ - ~ A R C

--CF

ARC

I C F

i

I

ARC

l ~ l l lI / l / l l l l i Fig. 9. Lacquer peel IV: coversand facies with horizontal alternating bedding• Type with dominance of climbing-adhesion-ripple structures and current-flow features• Length of profile = 100 cm.

93

Allowing for a certain degree of generalisation eight different stratification types

could be distinguished (see Fig. 5). In the present sense these types are uniform

units with generally well-defined bounding surfaces. They may correspond to one single bed or to a set of virtually identical beds. Their textural and structural

characteristics are thought to be attributable to a specific mode of deposition. Whereas most units seem to be tabular on the scale of a lacquer peel (see Figs. 6-9), in exposures of sufficiently large size their body shape appears to be lenticular with the vertical dimension measured in cm and the horizontal one in m. The bounding

planes of the units are horizontal and either even or wavy with the latter type being more common. As already stated in the section on climate, the wavy distortion is presumed to be due to frost thrusting.

Stratification types

Stratification type 1." Poorly sorted sand with coarse or indistinct lamination or

without structure altogether

As explained in the preceding section niveo-aeolian sedimentation tends to

produce sand deposits of poorer size-sorting than ordinary wind transport. This is

due to the interception by drifting snow flakes of both suspended fines and coarser saltating particles which are laid down collectively at the site of deposition. Moreover, upon ablation of the snow the sand-snow interlayering may collapse or

liquidise so that, particularly in the latter case, full diamictisation will ensue. At the same time these events will, to varying extents, disturb or entirely obliterate the

11cm

® ..... ~--Poorly sorted coarse sona

Poorly sorted sand

FWi~II/~rntde_~ i it Ond

®

F 3cm

L

<--- Poorly sorted sand • ~ . o . Q . o . o •

Fine sand-si l t

Fig. 10. Loading phenomena in beds of niveo-aeolian origin.

94

horizontal bedding originally present in the interstratified snow. Therefore, stratifi- cation type 1 is considered to be a fossil niveo-aeolian deposit. Though, in general,

beds of the discussed stratification type have well-defined bounding surfaces, in

some cases their transition towards an underlying stratum may be indistinct and gradational. The units in question are totally structureless and remind one of a heavy cloud with irregular and blurred outline which has sunk into its substratum (see Fig. 10A). This feature is interpreted as a load structure, i.e. it is thought to result from collapse of a bed which foundered into a liquefied substrate. Liquefac-

tion of the substrate could have been caused by soaking upon melting of inter- calated snow, and subsequent loading by rapid deposition of the overlying bed. At

least part of the small flame structures found in the peels (see Fig. 10B) must be due

to the same process. Occasionally, the units of stratification type 1 contain particles up to granule size.

Layers of this kind not only have a coarser than normal texture but they are also

very poorly sorted, so that it is not possible to interpret them as deflation lags. Rather, these strata are thought to result from storms of unusual strength which

temporarily supplied the coarser grades. Again, it may have been the snow which prevented the size-sorting and areal separation of grains which would have occurred

with ordinary wind transport.

Stratification type 2: Well-sorted sand with coarse or indistinct lamination or

without structure altogether

With regard to its texture, this unit is better sorted and finer grained than its predecessor, though not to the same degree as type 3 to be discussed below. In type

and variability of structure, it is very similar to stratification type 1. For this latter reason, type 2 is interpreted as a fossil niveo-aeolian sediment. Presumably it formed from deflation of nearby sand patches upwind. Interaction of three condi-

tions could account for the well-sorted texture of the resulting deposit: (1) absence of excessive wind speed; (2) a local source, which itself was graded and deposited by

snow-free wind; and (3) sand incorporated in drifting snow moving at a low level

above the ground surface, so that interception of suspended grains was excluded or

considerably reduced.

Stratification type 3: Well-sorted (fine) sand with horizontal parallel lamination or

low-angle cross-lamination

Individual laminae are well defined and have a mean thickness of 1 mm or less. The uniformity and distinctness of the lamination in this unit clearly contrasts with that of the two types already described.

For the interpretation of stratification type 3 the following observations are relevant: (1) features associated with current flow are lacking altogether; (2) in all units the basal laminae seem to be strictly parallel to the depositional surface

95

formed by the underlying stratum of different origin; (3) ripplemarks, even of the flattest type, were never found at the top of any unit; and (4) inversely graded laminae are absent, though admittedly their recognition would be difficult due to the extreme thinness of the lamination.

From these data it is concluded that type 3 is an aeolian accretion deposit. Its internal structure might be interpreted as either planebed lamination or subcritically climbing translatent stratification (Hunter, 1977). Though unlike in origin, the two categories are hard to tell apart when the angle of ripple climb is very small in the latter type. This is the more so since in that case, the ripples on the depositional surface become very flat (e.g. L = 60 mm and H = 2 or 3 mm). Ripples of this subdued type, though easily recognised on a modern depositional surface, have a considerable chance of losing their identity when they are buried by younger beds of different origin.

At first sight, the above features (2) to (4) are strongly suggestive of planebed lamination, but it cannot be excluded that at least part of type 3 results from subcritical ripple climbing.

Planebed lamination, as defined by Hunter (1977), is due to tractional deposition at wind velocities that are too high for ripple existence. The role of grain-size distribution in the genesis of a smooth, ripple-free depositional surface has been expounded by Bagnold (1941).

Since the type-3 units are exclusively in the form of thin sheets or very flat lenses, it is appropriate to ask why the accretion process proposed above did not result in the formation of dunes. It is suggested that seasonal variation in environmental conditions should provide the answer. First of all, during the season of high wind speeds, there was little supply of sand due to the fact that in winter all or most of the ground surface would be in a frozen state. In the source area this would have seriously hampered the entrainment of sand grains by wind. Consequently, at the receiving site down-wind, the sand supply rate fell below the threshold value required for the start of dune building. Secondly, the incidence of winds sufficiently

strong to produce sand drifting and accretion in the depositional area was presuma- bly restricted to the winter season. This implies that, in the long run, the supply of sand was discontinuous with time and was interrupted or considerably diminished for several months each year. In the third place, at least part of the sand deposited in winter would freeze and thus become immobile.

Freezing of the ground surface would fail to occur only when winter tempera- tures were very low and the air correspondingly dry. Extreme frigidity, however, would strongly reduce precipitation which is thought to be a necessary condition for the genesis of aeolian facies 3 (see also Discussion).

The effect of periglacial conditions on aeolian dune formation in the U.S.A. is discussed by Ahlbrandt et al. (1983). They emphasise modification rather than suppression of the dune building process.

96

Stratification type 4." Silt to fine sand with coarse or indistinct lamination or without

structure altogether

Because of their small grain size, layers of this type must have formed by settling from suspension during periods of low wind speed. The silty material may have fallen on a dry or a damp surface or perhaps it came to rest in shallow pools of stagnant water. In general, however, the structures in type 4 are too irregular and ambiguous to permit meaningful interpretation. Consequently it is not possible to be specific about the depositional environment of this unit beyond stating that it fell out from suspension. Since silty beds (units 4, 5 and 6) constitute an essential component of the facies with horizontal alternating bedding, it is most unlikely that they would result from accidental slackening of wind speed. Here, it is suggested that these characteristic strata are primarily, though not exclusively, due to a seasonal decrease in mean wind strength. Otherwise it would be hard to imagine how they were deposited over large areas during considerable lengths of time, alternating as they do with layers of coarser grainsize.

Stratification type 5: Silt to f ine sand with wavy or even parallel bedding

Well-defined bedding planes are the bounding surfaces of beds comprising many laminae. Individual laminae are indistinct, thin and crinkly in detail. In vertical peel sections, this type shows a typical bumpy microrelief on a millimeter scale. Cross- lamination of climbing adhesion-ripple origin is here and there found intercalated in beds of unit 5.

This type presumably formed by settling from suspension onto a moist surface of silty or fine sandy windborne material. The necessary moisture content of the slowly aggrading depositional surface was provided by capillary rise from a seasonally thawing substratum. Because of the fine texture, the upward movement of soil moisture could readily keep pace with the prevailing rate of deposition until in full summer it was exceeded by evaporation. In this way, a sustained accumulation of sediment over large areas can be accounted for. From the experiments conducted by Kocurek and Fielder (1982), it can be inferred that under these conditions adhesion laminations (quasi-planar adhesion stratification according to Hunter, 1980) will be the predominant stratification type. Fig. 3 of Kocurek and Fielder (1982) shows that plane-bedded adhesion laminations can be produced from vertical grainfall onto a wet or damp surface. Since deposition is assumed to be by fallout from a gentle wind, this case is most likely to apply. The bedding planes separating beds of silty laminae may result from short spells of stronger wind depositing very thin layers of coarser texture. The authors mentioned above also pointed out that adhesion plane beds could form over a much wider depositional area than that of adhesion ripples. As far as the coversands with alternating bedding are concerned, this statement agrees with the observed facts.

97

Stratification type 6." Silt to fine sand with discontinuous, waL~v or e~en, roughly parallel lamination. Distinct bedding planes or laminar surfaces are lacking

Like the preceding unit, vertical peel sections of this type exhibit a characteristic knobby microtopography and the two are believed to have a virtually identical origin. The poorer structural development in unit 6 may result from greater textural

uniformity a n d / o r less variation in the generally low wind speed during the time of

deposition.

Stratification type 7: Sand, fine sand or silt with cross-lamination Based on structural details and content two genetically different classes of

cross-laminated units can be distinguished: one type formed by current flow and

another one produced by adhesion-ripple climb.

Cross-laminated beds resulting from current flow are easily identified as they

always occur in association with other structures formed by the same process. The second type are climbing-adhesion structures whose genesis and characteristic morphology are discussed by Hunter (1973) and Kocurek and Fielder (1982). Cross-laminated units of this type may either be stacked into sets separated by bedding-plane disconformities or they occur as thin single beds intercalated in strata

of unit 5. The formation of climbing-adhesion-ripple structures requires both a relatively

high moisture content of the depositional surface as well as saltating sand. They are

a fairly rare phenomenon in the coversands with horizontal bedding. From this it

may be inferred that at the time of their formation, predominance of a sand-driving

wind and widespread dampness of the receiving ground tended to be seasonally

separated.

Stratification type 8: Sand, fine sand or silt with cut- and -fill structures The amazing fineness of the scours and associated features suggests shallow,

short-lived current flow most probably caused by runoff of melting snow. This type

is not very common in the facies under discussion and it is largely, though not exclusively, restricted to low-lying sites.

The lacquer peels

Lacquer peels I (see Fig. 6) and II (see Fig. 7) may be regarded as standard types of the periglacial aeolian sand with horizontal alternating bedding such as it occurs in the Twente Emsland region. In both peels there is a fairly regular alternation of coarser- and finer-grained beds and the waviness of their bounding surfaces is evident.

At the base of peel I, two sharp erosive contacts suggest melt water runoff. Half-way across the same peel an unusually coarse layer of stratification type 1

98

(deposits of niveo-aeolian origin) is present. As already mentioned it is attributed to snowstorm activity. Nearer the top of peel I a single bed with climbing-adhesion- structure is faintly discernible.

Type-3 units (aeolian accretion deposits with planebed lamination) are well represented in peel II. Clearly contrasting with them in the same profile are the basically structureless niveo-aeolian layers of stratification types 1 and 2. Four levels of flame structures in peel II demonstrate that the waviness, at least in part, must be due to loading, with frost thrusting presumably being its principal cause. This latter process involves horizontal compressive stress and may have triggered the internal motion in the soaked beds that resulted in the formation of flame structures.

In lacquer peel III (Fig. 8) textural alternation is much less regular than that of the profiles discussed above, and there is a clear dominance of fine-grained layers. Since stratification types 5 and 6 are well-represented here, settling from suspension of windborne silt or fine sand onto a damp surface must have been the major process by which the profile was built up.

The bumpy microtopography characteristic for vertical peel sections of units 5 and 6 is particularly distinct in the lower half of the peel, though the coarse blobs in the basal stratum may be artifacts of faulty peel preparation.

In one type-5 bed, incipient convolute laminations suggest a temporary hydro- plastic state of the sediment.

In the middle of peel III, the generally fine-grained succession is interrupted by a type-3 unit with sandy texture. The fineness and uniformity of its laminations is conspicuous and contrasts with that of the other stratification types. A frost crack cuts through most of the peel. Though well developed in general its level of departure is poorly defined.

Lacquer peel IV (Fig. 9) represents an unusual variety of the coversand facies with horizontal alternating bedding. Here, interstratification results from frequent vertical change in structure with textural variation being very subordinate. Water has been an essential agent in the deposition of this profile as is evidenced by the dominance of both climbing-adhesion-ripple structures (type-7 beds) and current- flow features (type-8 beds). The peel originates from the lowest part of an exposed paleodepression infilled by coversand, and this environment may account for its aquatic imprint. The stacked units with climbing-adhesion-ripple structure are not necessarily associated with regional summer thawing of the seasonally frozen ground. It may well be that in early spring or during sunny winterdays snow meltwater accumulated at low sites and there--before it could freeze--trapped saltating sand which happened to move over it. The rapid succession of meltwater influxes and subsequent (or simultaneous) aeolian deposition by climbing adhesion-ripples would imply an extraordinary high rate of sedimentation. This inference is supported by the steady alternation throughout the whole of the profile of type-7 and type-8 strata.

Many fine subvertical joints are present in the upper part of peel IV. Similar

99

linear elements can be seen in its lower half but these are suspected to be artifacts of peel making.

Rather than departing from distinct levels, the joints developed in an en-echelon pattern. They frequently intersect with the bounding surfaces of beds of contrasting texture without their rectilinear shape being affected in the least by these discon- tinuities. From these observations it is concluded that the joints cannot be contem- poraneous with the host sediment and that they developed when the material was in a frozen state (M.E. "de Smet, pers. commun., 1985). Joint patterns of this type are by no means exceptional in periglacial sediments in the Twente-Emsland area nor are they restricted to aeolian beds. It is here suggested that they were formed by frost thrusting though their supposedly postdepositional origin remains somewhat puzzling.

In conclusion, it must be emphasised that all the analysed successions were definitely nonsequential. Apart from an occasional microsequence, gradual vertical changes in sediment properties and, by inference, in rrgime have never been found. Rather, the broadly repetitive stacking of units with contrasting features and origin is a primary characteristic of the investigated sediment type.

TENTATIVE MODEL

Assumptions

The present hypothesis on the origin of horizontal alternating bedding in perigla- cial aeolian sands is based on the following assumptions:

(1) Inherent periodicity of the depositional process operating on a sufficiently large time and space scale. This requirement is imposed by the very sediment type, its areal distribution and the length of the time span concerned. Likewise, it appears to be seasonality which should account for the absence of dune formation in the deposit under consideration.

(2) A regional sedimentation system involving a source area, a throughput zone and a receiving site. In a process as dynamic and changeable as sedimentation, these three regions will not be stable either in place or in physical conditions. In the source area occurs a net loss of material. The proglacial margin of the Weichselian icesheet and the non-glaciated part of the dry North Sea floor will at one time or another have acted as source areas. Perhaps the land surface of Britain in as much as it was icefree should also be included. A more or less continuous supply of source material was provided by processes such as glaciofluvial or glaciolacustrine sedimen- tation, subglacial deposition followed by retreat of the ice front, and frost shattering of hard rock.

In the throughput zone a state of dynamic equilibrium characterises the long-term sedimentation budget. This zone should be located somewhere between the source area and the receiving site along the track of the prevailing winds.

100

At the receiving site a time-averaged net accumulation of sediment takes place. (3) Depending on the environmental conditions, several distinct modes of aeolian

deposition can coexist or alternate. Windborne particles may come to rest and accumulate by processes as different as accretion, settling from suspension, ad- hesion to a damp surface or niveo-aeolian deposition.

(4) During the deposition of the aeolian facies with horizontal alternating bedding, both sand and silt were available in a source area of considerable extent. Consequently, over the large distances involved, along-track aeolian sorting was the principal agent to account for both areal separation and vertical stacking of sand and fines. This is suggested by the loess-coversand distribution pattern in Europe. According to Bowler (1978), Chinese geologists have adopted a similar view with respect to the long-range textural gradients in the loess deposits of their country.

Though distance from source must have been of paramount importance, other factors may have contributed in shaping the details of the pattern. These include changes in altitude or topography of the terrain, density of the vegetation cover and soil moisture content of the depositional surface (Mi~cher, 1973).

(5) A state of seasonally frozen ground and a modest quantity of snow precipitation are prerequisites of the model. It should be imagined that extreme aridity created a cold desert with predominance of deflation as happened during the Beuningen time (Kolstrup, 1980, 1983).

(4) As already stated by Rutten (1954), coversand with horizontal alternating bedding and loess are close associates both in origin and material composition. Any attempt to explain either of them separately is deemed to be sedimentologically defective. So far, intergrades between these two sediment types such as "sand loam"

( ~ ) (S °u r ce a r e a I Th roughpU tzone I R e c e i v i n g s i t e I

j S u m m e r s e d i m e n t a t i o n wi th prevalence of o o o ~ ~ " , " , ' ) ~ - . " • " • ; J . ~ " " • " ~ / I gh t & v a r i a b l e w ings and t h a w i n g o

- - Winter sed imentQt ion wi th generalby ~ t ronger

/ ~ / / ~ / / / / / / / ~ / ~ 7 7 / / / / ~ / / / / / / ~ ' / ~ / / / / " / ' / / / / / / / / / / / / / ~ / ' ' / / / ' ' / ' ~ depositional surface.

A Def la t ion lag ntal Sand - LoeSs

In ters t rat i f icG- tion (left)

Sond~oa m & Sandy LOESS

Submergence by NOah S@Q* burial by younger sediment or reworking

E Loess

( r i g h t )

S e d i m e n t s of d e p o s i t i o n a l aeo l ian c l i m a x

PROCESSES :

Deflation

[ _~>~] Accretion of sand,

Niveo-aeolion deposition With mean [ - ~ g~ in - s i ze depending on distance f rom ~u rce

Sett l ing f rom sust~ension of f ines on damp surface (adhes ion deposi t ion) ,

$~ t t l i ng f r o m suspension of f ines on dry surface.

Fig. 11. Schematic representation of seasonal aeolian deposition in the periglacial environment. Vertical dimension of diagrams 1 and 2 is approximately 1 cm.

in Belgium (Van Damme and De Leenheer, 1972) and “sandy loess” (Duphorn et

al., 1973) or “sand-1oess interstratification” (Vierhuff, 1967) in NW Germany have

received insufficient attention. They are all facies of the same large-scale sedimen-

tary system (see Fig. 11).

Idealised annual cycle

Summer sedimentation

The periglacial summer scene involves both a prevalence of light and variable

winds (Williams, 1975; Lamb, 1977) and the thawing of the depositional surface.

These two conditions combined are thought to be conducive to the aeolian sedimen-

tation of mainly silt-sized material.

The winds directed towards the depositional site probably resulted from weak

pressure gradients and consequently surface wind speed would have dropped below

the critical threshold value for sand driving somewhere upwind of the depositional

site. Accordingly, temporary storage of sand might have occurred in a throughput

zone far north or northwest of the coversand belt. At the receiving site itself settling

of fines from suspension is likely to be the single, or dominant, process of

deposition in summer.

Preservation of the thin silty layers so formed is warranted by the water-logged or

moist condition which, during early summer, will have prevailed in the seasonally

frozen depositional sites apart from high elevations. In fine-grained sediment the

capillary rise of water is very effective and. once fixed, a stratum of this texture is

not readily displaced (Bagnold, 1941).

In exposures and lacquer peels of the coversand facies with horizontal alternating

bedding, summer sedimentation is represented by the stratification types 4. 5 and 6

and by the silty or fine sandy variety of stratification type 7.

Winter sedimentation

Following Lamb (1977) during the winters of the Middle Weichselian glacial

maximum, a general easterly surface wind has blown over NW Europe along with

much northerly wind flow all the year round. In addition, katabatic winds moved by

gravity rather than by pressure gradients may have completed the winter pattern. In

Lamb’s discussion of the paleoclimate it is implied that in winter, the winds were

generally stronger than during the summer season. This suggests that the coarser

sand-sized component of the facies with horizontal alternating bedding was de-

posited mainly during the winter period. It may well be that infrequent and

short-lived wind events were dominant in the formation of sandy winter layers.

Various features in the stratification types 1, 2 and 3 (see section on observation

data) indicate that winter sedimentation was controlled by winds of sufficient

strength to carry sand grains in saltating and creeping motion to their site of

deposition. In general, the supply of sand to the receiving site must have continued

102

at quite a low rate since deflation in the source area was impeded by a frozen state

of the ground surface. Moreover, part of the depositing wind events was in the nature of snow storms.

The activity of fairly high wind speeds may be concluded from both the relatively coarse texture of stratification type 1 and the aeolian planebed lamination observa- ble in type 3. The interference of snow is suggested by the niveo-aeolian origin attributed to stratification types 1 and 2.

The easterly winds, though perhaps prevalent in occurrence, were not necessarily the most effective in deposition Rather, it is thought that the northerly wind, also present in winter, was the principal agent in the sedimentary processes. It flowed over the sea area between Greenland and the Atlantic seaboard of Europe so that it was able to pick up moisture and bring snow to sites downwind. Furthermore, it had to pass over the North Sea basin which is supposed to be the principal source area of the Weichselian Upper Pleniglacial aeolian sands with horizontal alternating bedding (see section on source area). In the third place, it is conjectured that the aircurrent from the north, in contrast to the anticyclonic easterly flow, was capable of developing surface depressions (polar lows?) with concomitant high wind speeds and snowy precipitation. This would be in support of infrequent depositional events of short duration as suggested above.

In this manner, sand, mixtures of sand and silt trapped in dirty snow, and pure snow were all laid down along the fringes of the European loess area whilst the finest dust came to rest much further beyond in southeastern and southern direc- tions.

It follows from the present discussion that the cyclic depositional model (see Fig. 11) is based on two essential requirements:

(1) An aeolian sedimentation basin of sufficient size to permit areal separation of sand and loess-like material. In a large enough wind sedimentation system these two grades are deposited far apart since the saltation carpet moves at a lower speed than the suspension cloud, and also because the transporting wind will gradually decel- erate due to surface roughness. This process of along-track size-sorting can fully materialise only when a more or less uniform landscape with no abrupt changes in altitude extends over a considerable distance in the direction of the transporting wind.

(2) A seasonal shift in the centres of sand and loess deposition, since otherwise a vertical alternation of the two components occurring over vast areas would be impossible. This requirement, which involves a periodic change in mean wind strength, is thought to be a characteristic property of the climate prevailing during the period of deposition.

Discussion

The random element in the depositional process

Environmental parameters such as air temperature, precipitation, incidence of

103

soil freezing, soil moisture content, wind strength, wind direction and sediment transport rate fluctuate in a complex manner. Therefore these variables must be

treated as if they possess a random element. In the depositional model presented above, two different sets of these variables are assumed to operate in succession, each one within fairly narrow limits of frequency and magnitude. This again reduces

the chance that the anticipated event (the deposition of a bed pair) really will occur. Several causes can be thought of that would prevent the annual cycle from running its envisaged course. In any case, a storm of unusual strength, combined with a

non-frozen and dry state of the soil, would blow away- - i n perhaps no more than a

few hours - - the strata formed during a number of years. A drastic change in the sensitive sedimentary cycle would ensue from a drop in

mean annual precipitation. This would gradually diminish the soil moisture content

at the depositional surface so that in summer time the adhesion of fines had a

poorer change to take place. Likewise, the probability for niveo-aeolian deposition

to occur during the winter season, would lessen. Ultimately, an extended period of increased aridity might terminate the proposed cycle.

Older Coversand II is the designation of a Middle Weichselian stratigraphic unit

in The Netherlands having a mean thickness of 2 m and consisting predominantly of aeolian sand with horizontal alternating bedding. According to Kolstrup (1980)

the unit has formed in a time span of between 1000 and 1600 years. If a mean thickness of 20 mm is assumed for one bed pair, the average return period of a

depositional cycle ranges between 10 and 16 years.

Winter conditions in the source area

A frozen surface soil during the winter season was assumed for the depositional site and there is little reason to believe that a different condition should have

obtained for the source area. This raises the question of how in winter time the

entrainment by wind of sand or silt grains would be possible from a surface

supposedly subject to seasonal freezing. Here it is suggested that locally and

temporarily in the Middle Weichselian, dry North Sea basin conditions must have existed which permitted deflation during the winter season. The following argu-

ments are thought to support this assumption: (1) According to Lamb (1977), a general easterly surface wind regime occurred

over NW Europe during the winter season of the Middle Weichselian glacial

maximum. Though most of the available evidence indicates that air flow from the north and northwest was responsible for the deposition of sand and snow, it may well be that the easterly winds were important in another respect. As they were presumably much drier than their northerly counterparts, they might have caused the direct evaporation of ice from the frozen Surfaces affected by them. In this way, mineral grains may have been liberated from their ice-cemented matrix so that the wind could carry them away.

(2) From the work of Jansen et al. (1979) and others, it is known that the North

104

Sea floor, far from being a monotonous plain, has a varied topography with locally considerable height differences (see Fig. 2). It is conceivable that areas of greater

elevation, especially those consisting of permeable sands, were unable to retain much soil moisture. As a consequence, very little ice would form in the surface layer of these sites during the freezing season.

(3) As already explained, the incidence of ground freezing is seasonal in a

stochastic sense only, i.e. it may fail to occur during part or all of any one particular

winter. Naturally, the chance for this to happen would be slender in a periglacial climate of the very coldest type. That case however must be ruled out since extreme

frigidity would preclude or strongly reduce precipitation which is thought to be indispensable for the working of the depositional model.

(4) The expulsion of soil particles from developing ice is mentioned by Wash-

burn (1979). Though no details of this process appear to be known, it could mean that in a freezing soil mineral grains are worked up to the surface so that a wind of sufficient strength might get hold of them.

From the above it may be tentatively inferred that during winter the deflation and entrainment of sand grains in the source area, though occurring at a reduced rate, did not cease altogether.

Facies variation in coversand

In Table 1 two facies (2 and 3) of aeolian coversand are defined. In any exposure

where both facies are present, facies 2 always overlies facies 3. Whereas facies 3 is characterised by an alternation of coarser and finer beds, the overlying facies 2 has a

more homogeneous texture throughout. It cannot be excluded that this difference

has to do with a gradual evolution in the atmospheric pressure system. As already

mentioned the formation of facies 3 has been attributed primarily to a seasonal

contrast in mean wind strength. Therefore, by the same token, a more equable wind

pattern similar to the present one may have prevailed during the deposition of the younger facies 2.

CONCLUSIONS

(1) The periglacial aeolian sands with horizontal alternating bedding have been produced by an annual cycle of deposition.

In summer, light and variable winds carried silt in suspension to the site of deposition whilst the sandy saltation load was left behind in a throughput zone farther upwind. Upon settling, the fines adhered to the surface, which was damp or wet due to the thawing of seasonally frozen ground. In winter, wind strength was

generally greater so that during this season deposition of sand prevailed in the area under consideration whereas the fine-grained suspension load settled downwind. Northerly winds are thought to have been more effective in deposition than their anticyclonic easterly counterparts, which were also present during the winter season.

105

Deposition by snowless winds resulted in the accretion of patches of well-sorted sand. Snow flurries, on the other hand, produced niveo-aeolian strata of generally

poorer-sorted sand. (2) The above cycle is an idealisation in that the conditions required must have

recurred at irregular intervals. This results from the fact that all processes in nature constantly fluctuate both in frequency and magnitude. Thus the productive opti- mum was, to a large extent, dependent on chance for its incidence.

(3) Climate was the primary control of the depositional cycle since both thawing

of the seasonally frozen ground and snow precipitation were necessary conditions. Similarly, the periodic change in mean surface wind velocity must have been an inherent property of the ancient climate. A vertical alternation of coarser- and finer-grained strata occurring systematically over large areas might be adequately accounted for by a seasonal shift in the centres of sand and loess deposition.

(4) The dry parts of the northern and eastern North Sea floor as they existed shortly before, during and immediately after the Middle Weichselian glacial maxi- mum were the principal source areas for the depositional process. Ample quantities of material of the required grain-size range were provided by the ice sheet. Only in a sedimentary basin of this large size, could the process of along-track size-sorting bring about an areal separation of sand and loess-like material.

(5) Coversand with horizontal alternating bedding, loess and their intergrades are genetically related facies. For at least part of these three categories it is assumed that the differences between them result from modification of the seasonal effect as

control led--essential ly--by distance from the source. (6) Suppression of dune building in the depositional aeolian process considered

here is primarily attributed to seasonal separation of high-wind-speed events and greatest sand-supply potential in the source area. Moreover, at least part of the sand would freeze and consequently become immobile soon after its deposition.

A C K N O W L E D G E M E N T S

Thanks are due to the following persons: Dr. W. Roeleveld and Dr. J. Terwindt for critical reading of the manuscript; Mr. C. Kasse, Dr. J. Vandenberghe and Dr. H.F. Vugts for useful suggestions and discussions; Mrs. G.B. Snijder for typing of

the manuscript; Mr. H.A. Sion for preparation of the drawings and Mr. C. van der Bliek for photographic work.

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