the structure and genesis of weichselian to early hologene aeolian sand sheets in western europe

Sedimentary Geology, 55 (1988) 197-232 197

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

T H E S T R U C T U R E A N D G E N E S I S O F W E I C H S E L I A N T O E A R L Y

H O L O C E N E A E O L I A N S A N D S H E E T S I N W E S T E R N E U R O P E

J. SCHWAN

Institute of Earth Sciences, Free University, P.O. Box 7161, 1007 MC Amsterdam (The Netherlands)

(Received May 19, 1987; revised and accepted September 24, 1987)

ABSTRACT

Schwan, J., 1988. The structure and genesis of Weichselian to Early Holocene aeolian sand sheets in

western Europe. Sediment. Geol., 55: 197-232.

Weichselian to Early Holocene aeolian sands are widespread in the lowlands of western Europe. To a

large extent these deposits occur in the form of sheet-like coversands with slipfaced dunes being much

rarer. Whereas the latter type of landform corresponds to a single facies with dune-foreset cross-bedding

( = aeolian facies 1), two structurally different facies are distinguishable in the sand sheets. This paper

concentrates on the two sand-sheet facies which are referred to as aeolian facies 2 and aeolian facies 3.

Data on these two types are based on (1) a survey of existing literature, and (2) a detailed analysis of

lacquer peels and larger-scale exposures in England, The Nether lands and the Federal Republic of

Germany.

Aeolian facies 2 is defined by the spatial position of its beds, which may be either horizontal, inclined

or oppositely dipping in the case of structures formed by fluctuating winds. Horizontal bedding is by far

the most common type. Inclined bedding is related either to small, isolated dome dunes or to

scoop-shaped deflation surfaces. The structures resulting from fluctuating winds are rare and do not

represent an inherently large measure of directional variability of the wind regime. The internal structure

of the beds is dominated by aeolian planebed lamination with or without concordantly infilled

wind-scours. Degradation of this stratification type because of interference by coarse particles is

discussed.

Aeolian facies 3 is uniquely typified by the alternation of coarser- and finer-grained horizontal thin

beds that are either wavy or even in shape. Depositional models of facies 3 are given at both the local and

regional scales. In both cases the coarser-grained layers result from tractional deposition of saltating and

creeping grains whilst the finer-grained strata were laid down by settling from suspension. Periodic

changes in surface wind speed, the presence of a damp depositional surface and the availability of both

sand and silt in the source area are necessary conditions for the working of the models. The large-scale

model is associated with specific environmental conditions of the Weichselian Upper Pleniglacial. The

local-scale model accounts for the fact that facies 3 is also found in units of Weichselian Late Glacial or

Early Holocene age. The regional-scale model involves stepwise tractional transport over long distances

so that grains of distant provenance were mixed with material from sources nearer to the receiving site.

This process was an important control on the mineralogical composit ion of the resultant deposits. The

prevalence of aeolian planebed lamination in both facies 2 and the coarser-grained layers of facies 3 is

attributed to the interaction of three factors, viz. the rarity of topographic barriers, the sparseness of

vegetation cover and a high ratio between wind energy and sand availability during the processes of

transport and deposition.

In the lowlands of Europe, sand sheets are gradually replaced in an easterly direction by coeval wind

dunes. The possible causes of this phenomenon are considered.

0037-0738/88/$03.50 © 1988 Elsevier Science Publishers B.V.

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INTRODUCTION

In the lowlands of western and central Europe, aeolian sand deposits formed over large areas under periglacial climatic conditions during the Weichselian. Similar sediment was also laid down during the preceding ice ages of the Pleistocene (Ruegg, 1983) but information on this latter class is scanty.

In internal structure and surface expression the Weichselian windborne sands show considerable variation. A generalised description of their three principal facies is given in Table 1. With respect to this scheme the following points are relevant:

(1) The facies distinction is an idealisation in the sense that intermediate types showing characteristics of two standard facies are quite common.

(2) The facies presented in the table are aeolian sand deposits in the strict sense of the word. Sand-loess intergrades (Vierhuff, 1967) and periglacial aeolian sands reworked to some considerable degree by current flow or slope processes (Ruegg, 1983; Vandenberghe, 1985) are not included.

(3) The relationship between facies types and time stratigraphy as given in the table is tentative. Yet, from the work of Van der Hammen (1951), Diacker and Maarleveld (1957), Van der Hammen and Wijmstra (1971), Vandenberghe and Gullentops (1977), Koster (1978), Kolstrup (1980), Ruegg (1981), Vandenberghe (1981), Kolstrup (1983), Ruegg (1983) and Haest (1985) it can be inferred that: (a) In any exposure where both facies 3 and 2 are present, the latter always overlies

TABLE1

Pfincipalfaciesof Weichselian pef i~aoalaeol iansandin western Europe based on Koster(1982) and Schwan(1986)

I Shape of sediment body

Dunes

Morphology Characteristic sedimentary structures

I Facies

Dune-foreset cross-bedding (slip-face development)

Horizontal bedding, inclined bedding and fluctuating wind structures. Within beds prevalence of aeolian planebed lamination and occasionally X- lamination. Concordantly infilled wind scours

Alternation of coarser- and finer-grained, horizontal thin beds. Shape of layers is either wavy or even

(aeolian dune sands)

Sand sheets (aeolian coversands)

River dunes and inland dune fields

Gently undulating mantle blanketing pre-existing topography

Dominant period of deposition

Cold stadiats of Weichselian Late

\

\ \ i

"\

Weichsetian Upper I Pleniglacial i

199

facies 3. (b) Aeolian coversands with horizontal layering and a flat or gently undulating surface expression were the prevailing type of blown-sand deposit during the Weichselian (Upper) Pleniglacial. To an unknown extent strata of this age show an alternation of coarser- and finer-grained thin beds and, therefore, correspond to facies 3. (c) During the cold stadials of the Weichselian Late Glacial both dunes (facies 1) and coversands with sheet-topography were formed. In the second category, the characteristics of facies 2 tend to predominate. Locally, however, the facies 3 type alternation of coarser- and finer-grained horizontal beds is also present in coversands of Late Glacial age.

(4) The fact that Table 1 exclusively refers to the younger Weichselian does not imply that aeolian activity would be restricted to that time period alone. In many cases, deposition of sand sheets and dunes which began in the Weichselian Late Glacial extended for a varying period of time into the Early Holocene (Koster, 1982). Furthermore, the formation or modification of inland dunes during the younger Holocene, even up to the present day, has been reported from several places in the European lowlands (e.g., Dylikowa, 1969; Pyritz, 1972; Koster, 1978).

(5) The concept of facies 1 refers to Weichselian dunes which, irrespective of shape, have relative heights of over 5 m. Another criterion for this facies is that the dunes, rather than forming isolated features within aeolian sand sheets, should occur areawise as more or less extensive dunefields. Moreover, sediment bodies of this type are expected to have steep leesides because of slipface-de,~elopment though this would not be the case with dome dunes (McKee, 1979). As it is the objective of the present paper to explain the origin of the periglacial aeolian sand sheets in western Europe, attention will be focussed on facies 2 and 3 which are the principal constituents of these sediment bodies. However, because of the general fluidity of the facies-subdivision, facies 1 cannot be ignored altogether in this text. In a broad west-east trending zone from the Low Countries through the plains of northern Germany into central Poland and beyond, facies 1 gradually gains importance both in areal extent and in relative height of the dunes (Poser, 1951; Dylikowa, 1969 and Koster, 1978).

REGIONAL DATA

This section involves four countries in western Europe where periglacial blown sands occur mainly in the form of sheet-like sediment bodies. It is based on a survey of existing literature so that the information is inevitably heterogeneous in approach.

England

In England, extensive aeolian sand sheets of Late Devensian age (see Table 2 for Late Glacial stratigraphy) are found in Lancashire, Yorkshire and Lincolnshire. In East Anglia, a part of the same sediment type is attributed to the Early Devensian.

200

TABLE 2

Chronostrat igraphy of the Weichselian Late Glacial in Britain and The Netherlands

~4C yrs B.P. British Isles The Netherlands (Van

t (Lowe and Walker, 1984) Staalduinen et al., 1979)

I Flandrian Holocene

- - 10,000

Late Dryas Stadial

- - 11,000

- - 12,000

- - 13,000

- - 14,000

Loch Lomond

Stadiat

- - L a t e G l a c i a l

Interstadial

/

- - Late Devensian

ice-sheet)

Allerod Interstadial

Early Dryas Stadial

Boiling s.1. Interstadial

Weichselian Upper

Pleniglacial

r (succession of tundra

polar desert --* tundra

environments)

. 3

4 T

Included in these coversands are minor occurrences of dunes with relative heights of up to 6 m (Catt, 1977). According to Perfin et al. (1974) as quoted in Straw and Clayton (1979) a widespread aeohan sedimentation of sand and silt occurred in eastern England during the Late Devensian. The gently undulating sheets of coversand present in the region are one of the results of this large-scale event. Buckland (1982), Matthews (1970) and others have demonstrated that their deposition took place mostly during the Loch Lomond Stadial. It is very likely that this process continued for a time into the Early Flandrian until a sufficient vegetation cover had developed. Renewed sand drifting occurred in historical times as a result of forest clearance by man. The corresponding deposits differ from the coversands since they form dunes that are lacking in most of the older blown sands. The coversands are up to 6 m thick and from data obtained in the Vale of York by Matthews (1970), it may be assumed that northwesterly winds prevailed during their emplacement.

From the work of Buckland (1982) and Straw (1963), the characteristics of coversand in north Lincolnshire may be summarised as follows: (1) predominance of (sub)horizontal parallel bedding in well-sorted sand; (2) presence of beds formed by shallow current flow and intercalations of slightly organic sand, often with moss

201

remains; (3) occurrence of mesolithic artifacts and pebble-sized gelifluction layers where the blown sand is banked against steep cuesta scarps; (4) indications for deposition in a cold climate at the close of the Loch Lomond Stadial based on insect evidence and radiocarbon data, and (5) a substratum of frost-shattered limestone or shale intersected by sand-filled ice-wedge casts. In many places this material is overlain by a basal peat. Both in this latter stratum and in the superjacent coversands, cryogenic deformations are entirely absent.

On the basis of these observations Buckland (1982) proposed an annual cycle of sedimentation. He suggested that aeolian deposition by westerly storms in autumn was followed by deep snowfall in early winter and partial reworking of the sand by spring thaws. Faunal evidence for cold conditions can be reconciled with the absence of cryogenic deformations by assuming that the snow cover was thick enough to insulate the sands from frost action. The lack of frost structures in the coversand of north Lincolnshire sharply contrasts with the findings of Douglas (1982) in Leicestershire where the same sediment type is distorted almost beyond recognition by large periglacial involutions. However, the relevance of these data is limited since the synchroneity of the Leicestershire coversands with those in north Lincolnshire could not be unequivocally established.

The Netherlands

In the Netherlands, sandy aeolian surface deposits are to a large part in the form of sheets with a flat or gently undulating topography. Included in this category are small dune forms of varying and often poorly defined shape having relative heights of 5 m at most and maximum slopes of 6 °. Insofar these low dune forms have a recognisable surface expression at all, they are mostly longitudinal-linear or parabolic in plan, as is the case in the coversand areas of Belgium and Niedersachsen (Ten Cate, 1969; Koster, 1982).

In The Netherlands, traditionally two types of coversand are distinguished, viz. Younger coversand and Older coversand. The first category ranges in age from Weichselian Late Glacial to Early Holocene and was derived from local, nearby sand sources such as river beds and alluvial fans. Over large parts of the Dutch landscape it forms a surface deposit with generally subdued topography. The second type, the Older coversand, was laid down during the Weichselian (Upper) Plenigla- cial. Its provenance is more complex than that of its younger counterpart. Whilst a particle supply mainly from the local subsoil has been advocated by several authors (e.g., Crommelin, 1964; Maarleveld, 1968) on sedimentary-petrologic grounds, others have attributed a significant role to long-distance grain transport with the dry part of the North Sea floor being an important source area (e.g., Vink, 1949; Veenstra and Winkelmolen, 1971; Schwan, 1986). Further on in this paper it will be argued that these two viewpoints are not necessarily incompatible.

Apart from the Subatlantic dunebelt along the North Sea, wind-built accumulation forms truly deserving of the designation "dunes" are less common. They are

202

found mostly in the vicinity of river floodplains or otherwise they occur as isolated areas of inland dunes. The majority of the river dunes have a Late Dryas to Early Holocene age (Koster, 1982). Inland dunes, on the other hand, formed mainly during various phases of the Holocene as a result of deforestation (Koster. 1978),

Maarleveld (1960) demonstrated that an important change in the direction of the sand-transporting winds occurred during the Allerod Interstadial. This he inferred from the orientations of low dune forms, which suggest the prevalence of northwesterly to westerly winds during the Early Dryas Stadial and westerly to southwesterly winds during the Late Dryas Stadial. An identical trend has been observed in coeval blown sands in central Belgium (Vandenberghe, 1983a) and the lowland of central Poland (Dylikowa, 19691.

Quaternary aeolian deposits in The Netherlands are discussed in greater detail b\,' De Jong (1967), Koster (1982) and Ruegg (1983).

Belgium

In the Grote Nete floodplain in central Belgium, three generations of Weichselian aeolian sand deposits were studied by Gullentops et al. (1981) and Vandenberghc (1983a). The oldest class is closely similar to our facies 3. The younger types were deposited during the cold stadials of the Weichselian Late Glacial. Apart from age, they differ from their older counterpart in sand-transport direction, provenance and surface topography. The two younger units derive from nearby sources and they are present in the form of distinct linear ridges with relative heights of up to 5 m and slopes of 1-2%. The ridges run parallel to the Grote Nete and its tributaries. They reach their maximum development and even include low parabolic dunes along valley sections that are parallel to the prevalent palaeo-wind direction. From this, Vandenberghe (1983a) infers that the effectiveness of riverdune accumulation is co-determined by the angle between valley trend and direction of the sand-transporting wind.

The genesis of aeolian sands with horizontal alternating bedding is discussed by Haest (1985). In his study area in northern Belgium, these deposits accumulated both along valley sides and further away from watercourses, on interfluves. The deposition resulted mainly from northwesterly winds and took place during the Weichselian Upper Pleniglacial. A network of minor streams, including their smallest tributaries, was the primary source of sand. Because of springtime flooding following melting of the snow, the valleys remained barren of vegetation and could supply sand in considerable quantities, In late summer and autumn the wind-transported sand accreted on valley-flank sites where it was checked by vegetation. Haest (1985) believes that saltation of sandgrains should just be possible over surfaces bearing only a treeless tundra vegetation, and further suggests that snow cover may have had an additional roughness-reducing effect.

The presence of thin silty beds, an essential characteristic of the sediment under discussion, is attributed to fall out from suspension, Unlike the accretion of the

203

sandy strata, this process operated on a regional scale, with source and receiving site possibly being far apart. The above mentioned author emphasises that the alternation of coarser- and finer-grained thin beds lacks in younger blown sands also present in his study area. Since the latter type results from local reworking of the former, the fines were blown out and no longer replaced by large-scale settling from suspension.

Federal Republic of Germany

Pyritz (1972), who studied aeolian sands in the lowland part of Niedersachsen, distinguished dunes and sand sheets. Dunes, in his definition, are sand accumula- tions with distinct local relief having a relative height of 1.5 m or more and a height: width (or length)-ratio of 1:5. Sand sheets are another type of aeolian sand deposits that does not meet the definition just mentioned. The sand sheets, though presumably having a Weichselian Late Glacial origin, were modified in historical times by the agricultural activity of man. Their areal distribution is less obviously associated with river floodplains than that of dunes.

The dunes, for the greatest part, are found along the rivers Eros, Weser/Aller , Elbe and their tributaries. The vast majority were formed in the last 300 years or so as a result of human interference. The few river dunes of supposedly Weichselian Late Glacial age are interesting in that they have a recognisable parabolic or linear shape in contrast to the generally diffuse pattern displayed by their recent counterparts. Moreover, from west to east in the investigated area the Weichselian Late Glacial dunes show a tendency to become higher and steeper on their leesides.

FACIES DESCRIPTION

As already mentioned in the introductory paragraph, aeolian facies 2 and 3 (see Table 1) are the principal constituents of western European sand sheets. Accord- ingly, this description will concentrate on these two types only. The pertinent information is based on the analysis of lacquer peels and larger-scale exposures in sandpits. Observation data of this kind were compiled in England (north Lincoln- shire), The Netherlands (Twente and West Brabant) and the Federal Republic of Germany (Emsland and Grafschaft Bentheim).

Aeolian facies 2

The spatial position of the aeolian beds within the sediment body is the diagnostic characteristic of facies 2. Accordingly, its beds may be either horizontal, inclined or with opposed dips. The first case is by far the most common, and the third case refers to the flanks of fluctuating wind structures which are a rather rare feature. Postdepositional deformations of the blown sand form a fourth, separate class of structures which will be considered here.

204

Individual beds of facies 2 may show a variety of internal structures except for textural alternation as this feature defines the domain of facies 3.

Horizontal bedding Below, three examples of horizontal bedding in facies 2 are discussed in detaiN.

Profile I. The lacquer peel (Figs. 1 and 2) is from the uppermost part of the Pleistocene infill of the Dinkel valley in Twente, The Netherlands. In the succession of thin beds in zone B of the peel, two types of very fine intrabed lamination are present, viz. slightly wavy parallel lamination and cross-lamination. Furthermore, in the better part of the thin beds in zone B, a tendency towards normal grading is observable.

In the cross-laminated beds the laminae consistently dip to the west and, as visible in photograph B of Fig. 2, their dip angle is very small. One thin bed in zone B (see Fig. 2, photograph A) has an intrabed structure that is strongly reminiscent of complete rippleform lamination (Hunter, 1977). As thin units which are structurally transitional between this particular layer and the cross-laminated beds are also present in zone B, a common or at least a closely similar origin may be assumed for both types. Since complete rippleform lamination results from ripple climb, the same process must be held responsible for the formation of the cross-laminated beds. This implies that the "cross-laminae" in fact represent very thin climbing translatent strata in the sense of Hunter (1977). Their persistent westward dip tallies with the generally assumed prevalence of westerly winds during the main period of facies-2 deposition (e.g. Maarleveld, 1960).

In many layers of zone B, climbing translatent stratification or parallel lamination co-exists with normal grading within one and the same thin bed. It is suggested that each unit showing this structural duality, might correspond to a single depositional event with gradually decreasing wind speed.

The succession in zone B of thin beds with either climbing translatent stratification or parallel lamination is tentatively attributed to the depositional activity of a wind with fluctuating but generally high velocity. Furthermore, the very small dip angle prevailing in the thin beds with climbing translatent stratification is believed to suggest both a low rate of sand accumulation and instability of bedform, i.e. a situation where the climbing of flat ripples readily gives way to plane bedding upon a slight increase in wind speed. On the basis of these considerations, the parallel lamination in zone B is interpreted as aeolian planebed lamination. With respect to the above interpretation it is noted that:

(1) Aeolian planebed lamination is in principle characterised by straight and continuous laminae. In the periglacial coversands, however, this pattern may be disturbed to a variable degree by cryogenic activity. The waviness of the parallel lamination visible in Fig. 2C is attributed to that process.

205

"%. 270*

- A: Sedimentary structures ! r a t e d by bioturbation

=" E

°i!

• ~

E D: Cryoturbated subsoil =ded at its top by deflation

(Beuningen Pebble Layer)

L E G E N D

~ Coarse sandy or medium sandy bed

~ F i n e sandy or very fine sandy bed

~ Graded bed

~ Climbing translatent stratification i i

~ l c o r n p l e t e rippleform lamination

~ Wavy parallel lamination

o Particles of size-class 1-2 turn

~ Wind scour

~ Frost (?) crack

~ Ancient root channel infilled with sand

COS = Coarse sand

MS = Medium sand

FS = Fine sand

VFS = Very fine sand

Fig. 1. Profile I: facies 2 type horizontal bedding. Location: Twente, The Netherlands. Length of

profile = 160 cm.

206

E E A

B

t E E C

Fig. 2. Details of Profile I (see Fig. t~. (A) Complete rippleform lamination; (B) climbing translatent stratification; (C) wavy parallel lamination,

(2) Low-angle wind-ripple deposits are frequently laid down in the form of subcritically climbing translatent stratification (Hunter, 1977; Kocurek and Dott, 1981). There are two criteria by which this stratification type can be distinguished from aeolian planebed lamination. In the first place the low-angle wind-ripple type is characterised by inverse grain-size grading that seems to lack in aeolian planebed lamination (Hunter, 1977). Secondly, low-angle wind-ripple deposits show a lesser degree of internal parallelism than planebedded units. In the former type, sets are separated by low-angle truncation surfaces. Whereas laminae within a set tend to be parallel, they are at an angular discordance with laminae within other sets (Kocurek, 1986). In aeolian planebed lamination, on the other hand, laminae wilt remain parallel to each other on either side of a deflation level.

Profile 1I. In the lacquer peel (Fig. 3) which is from north Lincolnshire, four zones can be distinguished. Slightly wavy, parallel lamination and small, concordantly infilled wind scours are the dominant features of zone A. This combination is the principal characteristic of aeolian sand sheets in the area mentioned above.

The width of the generally shallow scours ranges from a few centimetres to a few metres. Within the parallel-laminated beds they occur either as a single feature or, much more rarely, groupwise as in zone C of the profile under discussion. Almost invariably their infilling is concordant, i.e. parallel to the concave-up bottom of the scour (Fig. 4).

The parallel lamination in zone A is interpreted as aeolian planebed lamination resulting from the prevalence of strong wind with the frequent occurrence of

207

ZONE A:Slightly wavy, parallel lamination & small concordantly- infilled wint~ scours

ZONE B : Partial disturbance of lamination

ZONE C: Scour-fill cross-bedding

ZONE D: Alternation of coarser- & finer-grained sandy layers

• Pedogenic lamellae ~ Loamy sand layer Load cast - - ~ - ] Structureless or poorly- [ ~ Indurated sand

laminated sand

[ ~ % ~ Strongly distorted lamination

Fig. 3. Profile II: facies 2 type horizontal bedding in zones A and B, scour-fill cross-bedding in zone C

and facies 3 type alternation of coarser- and finer-grained thin beds in zone D. Location: north

Lincolnshire, England. Length of profile = 162 cm.

208

Fig. 4. Concordantly-infilled wind scour in facies 2 type aeolian sand. I,ocation: north l.mcolnshirc. England. Measuring tape has centimetre scale.

scour-and-fill structures being due to the same condition (e.g. McKee, 1979). As stated earlier, Buckland (1982) has suggested that structures formed by shallo~. current flow are a characteristic feature of the north Lincolnshire blown sands.

Though it is likely that a part of the cut-and-fill structures are caused by tile

short-lived activity of shallow overland flow, it is presumed here that the above- mentioned author has overrated their importance.

In zone B, the lamination is much less regular than that of the overlying unit. It is

proposed that, originally, zone B had the same orderly planebed lamination as it.~ superjacent counterpart. Partial disturbance of this structure is tentatively attributed to a temporary water-saturated state and subsequent unequal loading of zonc B. This is suggested both by the small loadcasts at the base of zone A and thc indurated, iron-cemented layer in the lower part of zone B.

Zone C shows a type of cross-bedding which is supposed to have been produced by repeated aeolian scouring and infilling. Accordingly, the stratification of zone ( has been referred to as scour-fill cross-bedding. Locally, this type of structure is associated with wind-tipple-laminated beds (Fig. 5). Because of this, it is possible that the trough cross-bedding in zone C results from migration of wind ripples with sinuous crests rather than from repeated scour-and-fill. Since, however, wind ripples are normally straight crested (Bagnold, 1941; Collinson and Thompson, 1982) this alternative is not very likely. As shown in Table 3, the sand of zone C is much coarser and less well sorted than that of the overlying units A and B. This is probably due to lag concentrates that were left behind in the scour bot toms at the close of a deflation phase.

209

2 5 5 °

" - . 2 5 5 °

1 . 4 r 'n - - ~ ,

j J

~ - 1 . 4 m

Fig. 5. Aeolian planebed lamination (section B, top), wind-ripple foreset-lamination (sections A and B), scour-fill cross-bedding (sections A and B) and small fluctuating wind structure (section A, bottom). Location: north Lincolnshire, England.

The possibility that trough cross-bedding of aeolian origin was caused by the alternation of scouring and subsequent infilling is mentioned by Limarino and Spalletti (1986) who studied Permian windborne deposits in Argentina.

A Trough-Cross-Stratified Translatent Facies in Permian aeolian sandstone in Arizona (U.S.A.) is discussed by Blakey and Middleton (1983). The trough-shaped sets have 1-10 m dimensions and are thus much bigger than those reported here. Mostly, the laminae that fill and generally conform to the trough shape are climbing translatent strata in the sense of Hunter (1977). Blakey and Middleton (1983) tentatively suggest that troughs of the quoted size formed as depressions associated with blow-out dunes. Whilst the scours might have been carved by bimodal winds, the climbing translatent strata that fill them were deposited unidirectionally.

210

TABLE 3

Granulometric data on profile II (Fig. 3)

Zone Mean grain-size Modal grain-size S.D. in Textural class 3

in ~m 1 in/zm phi-units ~

A + B 190 229 0.89 fine sand

C 252 273 0.90 sand

D, coarser-

grained beds 182 273 1.38 fine sand

D, finer-

grained beds 121 273 1.83 loamy sand

2 Determined on fraction < 2000 #m.

2 S.D. = standard deviation of mean grain size.

3 Textural classification following U.S. Department of Agriculture (1975).

Scour-fill cross-bedding on an even larger scale has been described from the Palaeozoic Casper Formation (U.S.A.) by McKee (1979) who refers to it as "festoon cross-lamination'. Since it is by far the most dominant stratification type in an enormous volume of sediment, its origin is considered to be enigmatic.

The alternation of coarser- and finer-grained sandy layers in zone D of profile II will be considered in the paragraph on aeolian facies 3.

Profile 1H. The profile (Fig. 6) is from windborne sand of Late Dryas age from the Grafschaft Bentheim (F.R.G.). This deposit overlies ice-pushed sand and graveJ beds and has a leeside position with respect to the prevailing westerly winds (Fig. 7) The relevance of profile tII lies in the contrast between zones A and C.

In zone A, even parallel lamination in moderately sorted to well-sorted sand is the predominant stratification type. Also present in this unit is a wind scour infitled with very coarse sand and granules.

Zone C is characterised primarily by a succession of beds with variable mean grain size, degree of sorting and internal structure (Fig. 6, right-hand peel). In general, the transition from one type of layer to the other is gradational and poorly defined. Likewise, intrabed structures are weakly developed apart from a few strata with marked parallel lamination and some bedding planes picked out by ittuviation of ferri-oxyhydrate. Wind-ripple foreset-lamination is observable in one thin bed of very limited lateral extent.

Along with the general indistinctness of the structure, the overall coarseness of texture is a conspicuous feature of zone C. It contains 8.3 wt.% of the fraction greater than 0.841 mm and included in this class are particles of up to 9.3 mm diameter. In unit A, on the other hand, the same size-class is represented by 0.5 wt.% only with fragments greater than 2 mm being extremely rare (0.1 wt.%).

211

Dominant stratification

type

o L

o

© c N ~

=.{ ~ ~

g

< ~ .-= . . ~

Dominant stratification

type

>

a, E

m

e ~ ~o

o ._~ I

c

< . . . ~ 7

~ 0 S O L

- - - - ~ Wind- ripple foreset- lamination

~ l n t e r c a l a t i o n w i t h ( v e r y )

coarse sandy texture

- - - - ~ W i n d scour infilled with very coarse sand

~ Wlnd scour infilled with medium sand

~ Folding due to secondary growth of tree roots

~ Ancient root channels infilled with sand

~ T Scattered charcoal fragments

Fig. 6. Profile Ill: facies 2 type horizontal bedding. Upper half of profile to left and lower half to right.

For location see Fig. 7. Length of profile = 245 cm.

212

AD- ', ',V,.. ..,\....,. BI -ooo., ,,., x \ \ ~-- 7 - T - ' j - : ~

i / : ~ \ \ \ - . ' / : - - - . X ' , ' ' q " - - - ° t L c. e ..

" " "~ . .s 'v ~- ~ ' ~ ' o " % . t ' N i ::-" \'i \', ,

: '," \ ~ -- .<t 4 , o - 'z ,-,- I

' " ; ' I]~ ' ~ " o ~11~" r - r - r - . . . . 4

i - ~ ( ~ - L ' , , , t~E - ,, ',~' I . " . . ' . . . . . . ~ I

l: :& I l O O m I , \ \ \ \ ( : Sect ion A B . . . . . . . . . . . . . . . :~ '~:--SectJon BC---5 ,I

Fig. 7. Ice-pushed sand and gravel bex.ls covered with aeolian sand. Location: Orafschaft Benthcim. F.R.G. (A) Surface topography with heights in metres above mean sealevel; (B) cross-section

As with the preceding profiles I and II, the even parallel lamination in zone A is believed to correspond to aeolian planebed lamination. The underdeveloped state of the same stratification type in zone C is attributed mainly to the interference of particles in the very coarse sand to small pebble range. Because of the proximity and spatial orientation of the ice-pushed sand and gravel beds (Fig. 7), considerable quantities of these fragments must have been driven downhill in sliding or rolling motion under the impact of saltating sand grains.

From experimental work by Logie (1981), it is known that roughness elements covering a bed of blown sand, have an ambivalent role with respect to deflation threshold. Low cover densities reduce this parameter and, therefore, enhance erosion. Dense covers, on the other hand, increase it and consequently protect the sand from deflation. Logie (1981) also demonstrated that the critical value of cover density, named inversion point by her, depends on the diameter of the roughness elements. This relationship is approximately linear, i.e. the value of the inversion point varies proportionally with the mean size of the roughness elements. Thus, the larger the roughness elements, the greater must be their minimal cover density in order to shelter the sand surfacc.

It is suggested here that influxes of coarse-grained surface creep, set into motion during spells of strong wind. are rapidly dispersed on their way downhill. This would happen to the degree that their cover density dropped below the critical value, so that deflation of previously formed sandy layers ensued. In this way, the scarcity of planebed lamination in zone C could be accounted for. According to Logie (1981), at high wind speed a n d / o r low cover density, the roughness elements are undercut by deflation and thereby dig themselves into the sand. This should explain why the granules and small pebbles, rather than forming single-grain strings

213

of particles, mostly occur scattered throughout the sandy beds of unit C. The specific makeup of that zone must have resulted from the interaction of several different processes such as leeside expansion of flow and tractional deposition of both saltating sand grains and surface-creep material. It also involved often repeated deflation, which counteracted the full-scale development of planebed lamination. At a later stage of the depositional process, however, the supply of coarse particles gradually fell back when the source area became buried under aeolian sediment to an ever greater extent. Simultaneously, and for the same reason, the slope of the receiving site would diminish. It is presumed that these two events ultimately brought about a more uniform regime of deposition. This is reflected in both the structure and texture of zone A, which are much less variable than that of its older counterpart. That the supply of surface-creep material did not stop altogether during the younger sedimentation phase, is evidenced by the coarse scour-fill found in unit A.

Schwan (1986) has suggested that the combination of poor sorting and poor lamination in periglacial windborne strata could be a result of aeolian sand transport during snow flurries. However, both the gradual upward change in profile III as well as the proximity of a coarse-grained sediment source are clearly in favour of the explanation given above rather than one based on niveo-aeolian deposition.

Inclined bedding An inclined bed is an aeolian layer whose attitude visibly differs from horizontal.

Strata of this type acquired their sloping position during deposition and are not merely a product of sedimentation on a non-horizontal substrate. Dune-foreset cross-beds resulting from slip-face development are excluded since these, by definition, are the principal characteristic of facies 1 (see Table 1). As a rule, the dip angle of the inclined beds of facies 2 does not exceed 25 °. An example of the bedding type under discussion, from the Twente area of The Netherlands, is given in Fig. 8. Its detailed structure is shown in profiles A and B which are based on photographs of lacquer peels. The occurrence of adhesion structures in the lower part of profile B, and to a lesser degree also in that of profile A, suggests accumulation of windborne sand on a damp or wet surface (Kocurek and Fielder, 1982). This condition ceased to exist in a later stage of the depositional process since aeolian planebed lamination is the dominant stratification type higher up in both profiles.

Changes in the direction of the sand-transporting wind can be inferred from the ripple structures in profile B, provided, of course, that these features have been correctly interpreted. As will be explained later, the events recorded in the ripple structures are purely in the nature of short-lived deviations from the prevailing wind direction. Consequently, they should not be regarded as evidence for any large measure of directional variability being inherent in the ancient wind regime.

As can be seen in Fig. 8, profile A consists of four bedsets which are separated by angular unconformities. Leaving aside the lowest stratum which has a non-aeolian

214

Prof i le A Prof i le B

Paleo- wind directions inferable

from ripple structures /

L e g e n d :

Paraliel lamination

Structureless or poorly

laminated sandy bed

bed

statent

foreset -

,ination

.=sion - ripple

ruination

horizon with

n string of

rticles

~en Pebble Layer),

tic deformations

1cared top level

rtructurH

f biotu rbatlon $

e bounding surfac~

'~. 'i4 5 "

" J ~ ~ - ' ~ Y ~'----.~"--I ~ ' ~ L ~ " '.'-':'-:- ~1 < 3.1 m >

i -i ,ec.d be.,0g 0, .... F = q in aeolian land Deflation level Cryoturbeted subsoil

Fig. 8. Facies 2 type inclined bedding. Location: Twente, The Netherlands. Length of profile A = 75 cm.

Length of profile B = 90 cm.

215

Fig. 9. Facies 2 type inclined bedding. Upslope divergence of bedding planes in aeolian sand suggests truncation by present landsurface. For location see Fig. 7. Length of section = 17 m.

origin, the three wind-deposited units show a distinct upward increase in dip angle and a similar trend is found in profile B. On the basis of both its macrostructure

and its orientation with respect to the prevailing wind, the exposure under discussion is tentatively interpreted as the leeside of a small dome dune. According to McKee (1979) this type of dune is related to strong winds which retard or altogether

suppress vertical accumulation. This condition has also been invoked to account for planebed lamination, the commonest characteristic of horizontally bedded units of

facies 2. Thus, it is not surprising to find horizontal bedding associated with small dome dunes within the sand sheets of facies 2.

A somewhat different type of structure, possibly representing a low dome dune, is shown in Fig. 9. The aeolian beds cover a palaeo-surface that gently dips to the east and consequently occupies a leeside position with respect to the predominant

wind direction. The bedding planes of the windborne unit show a slight upslope

divergence suggesting truncation by the present landsurface. In contrast to the previous example, abrupt changes in dip angle of the bedding planes appear to be

absent here. It is therefore presumed that, prior to deflation of its upper part, the small dome dune consisted of gently curved and laterally continuous bedding planes. The whole structure may have been in the form of dome-shaped, parallel

layers draping an initial h u m p - - a l m o s t like storm-wave hummocks on a continental shelf.

Inclined bedding of yet another kind has been described from Weichselian Late Glacial low dunes in the Emsland, F.R.G. (Schwan, 1987). Here, the base, and in places also the top, of the dipping strata is sharply defined by planar, undulating or scoop-shaped deflation surfaces. As a result, the dip direction of the inclined beds is

216

not uniform and in part even opposed. In a strictly descriptive sense, the sediment body as a whole may be referred to as a low-angle aeolian deposit. It is believed. however, that the genetic implication of this term as formulated by Fryberger et al. (1979) would not be applicable to the case under discussion.

According to Nowaczyk (1976) Weichselian Late Glacial aeolian sands in central west Poland occur in the form of both dunes and coversands. The latter type is

characterised by a gently undulating surface topography with relative heights not

exceeding 3 m. The sediment was derived from nearby sources and has a thickness which ranges from 1 to 3 m. Below a structureless top layer, distinct parallel

lamination is the prevailing stratification type in the coversands. The parallel lamination is either horizontal or dips at angle of 15 ° at most. Coversands of the latter type are spatially associated with dunes with which they share a common direction of stratal dip. On the basis of these observations, Nowaczyk (1976)

suggests that the sand sheets with inclined bedding represent the basal remnants of large dunes that migrated in a downwind direction when they were in their mobile phase. A similar view is also found in discussions on aeolian sand sheets b~ Fryberger et al. (1984), Kocurek (1986), and Limarino and Spalletti (1986).

Although the conclusion of Nowaczyk seems sound with respect to his own stud~

area, it cannot apply to the sand sheets in western Europe. As different from Poland, the sand sheets in western Europe are not associated with extensive

contemporaneous dunefields, and this fact alone renders a dune-related origin of

these sediments most unlikely. Moreover, none of the sets of inclined beds encoun-

tered in sand sheets in England, The Netherlands and adjacent Germany resemble the dune-apron or dune-plinth deposits described by Kocurek (1986). This author explains that under conditions of low net preservation dune-apron deposits ( = low- angle toesets of slipfaces) and dune-plinth deposits ( = non-slipfaced bases of dunes with low-angle wind-ripple lamination) may be the sole record of former dune

existence.

Fluctuating wind structures In this type of structure, the tapering terminations of oppositely dipping layers

overlap each other in the manner shown in Fig. 10. The example is from a site in

north Lincolnshire where at a depth of 5 m the coversand overlies a peat layer radiocarbon dated at 10, 210 ± 60 B.P. (GrN-14045). On this basis a Loch Lomond Stadiai or Early Holocene age is attributed to the superjacent aeolian deposits which contain the structure under discussion.

In vertical sections the fluctuating wind structures normally occur in groups of two or three though, more rarely, also as single features. As a rule, they are buried under a horizontally bedded layer of windborne sand. Consequently, on the surface of the sand sheets, the fluctuating wind structures either remain altogether un- noticed or they stand out slightly as subdued mounds with oval outline. The feature under discussion is a rather rare phenomenon within the predominantly horizontally

217

/

J f ~

Obliteration of sedimentary structures by soil formation

Parallel lamination interspersed with s c o u r - & - f i l l s t ructures & few beds with wind- ripple foreset- lamination

Oppositely- dipping strata of fluctuating wind structure

Parallel lamination interspersed with scour- &-f i l l structures & few beds with wind- ripple foreset -lamination

Fig. 10. Facies 2 type fluctuating wind structure. Location: north Lincolnshire.

bedded sediment bodies of facies 2, and its dimensions, both horizontal and vertical, do not seem to exceed 5 m or so. As already suggested by the name used here, their origin is ascribed to directional fluctuations in the sand transporting wind.

Hunter and Rubin (1983) have pointed out that, depending on the shape and orientation of an initial unevenness, relatively small fluctuations in flow direction may cause significant shifts in the loci of maximum deposition. Moreover, deposition and erosion can take place simultaneously on different parts of uneven surfaces that are strongly curved. These effects are most pronounced on downcurrent convex salients or downcurrent pointed spurs of the original obstacle. In the fluctuating-wind structure of Fig. 10, the repeated alternation of deposition and deflation is clearly marked by the lateral pinchouts of the oppositely dipping layers. Hunter and Rubin (1983) refer to this pattern as "interleaved erosion surfaces" and they ascribe its origin to moderate fluctuations in flow direction over a downcurrent convex salient. The occurrence of fluctuating wind structures cannot be grounds for assuming that bimodality or a considerable degree of directional variability were inherent in the

218

wind regime which laid down facies 2. In the first place, given the appropriate bedform, relatively small fluctuations in flow direction already suffice to produce the structures under discussion (see above). Secondly, their quantitative insignifi- cance suggests that directional changes in the sand transporting wind were infre--

quent unless scarcity of suitable uneven surfaces would have been the limiting

factor.

Deformation feature,~ Cryogenic deformations (frost cracks, ice-wedge casts, sand wedges, cryoturbatic

involutions) in Weichselian aeolian sand sheets in The Netherlands and the Io~- lands of Belgium are discussed by e.g. Kolstrup (1980), Maarleveld (1976), Ruegg

(1983) and Vandenberghe (1983b, 1985). However, it is not always clear from these papers to which one of our facies 2 or 3 the descriptions refer. Moreover, at least a

part of the cryogenic features, in particular the ice-wedge casts and involutions, ma~ be epigenetic. This latter type, therefore, is not directly related to the environmental

conditions under which the aeolian host sediment itself was laid down. Frost cracks and frost fissures have been reported from windborne sands of Late Dryas age in

the northern and central part of The Netherlands (Van der Tak-Schneider, 1968).

Whereas seasonally frozen ground is a sufficient condition for the formation of frosE cracks, frost fissures, on the other hand, require a state of (dis)continuous permafrost to come into being (Maarleveld, 1976: Vandenberghe, 1983b). The work ol Van der Tak-Schneider (1968) is relevant in the present context since, in all likelihood, it refers to the facies 2 type of blown sands. Besides. her findings contrast with those of Buckland (1982) who emphasised the absence of cryogenic

features in approximately contemporaneous sand sheets in north Lincolnshire.

A eolian facies 3

In contrast to the structural variation found in the preceding type, aeolian facies 3 is uniquely characterised by alternation of horizontal coarser- and finer-grained

thin beds and an irregular wavy shape of the layers. As will be explained later, the latter feature is not present in all the aeolian deposits with alternating bedding. Normally, the fine-grained beds have a texture of loamy (very) fine sand and a thickness of up to 10 ram. The coarser beds. on the other hand. have a (fine) sand~ texture and thicknesses varying from 1 to 2 cm.

Schwan (1986) has proposed an idealised annual cycle of sedimentation for the facies 3 type of coversands.

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, the wind strength is generally greater so that deposition of sand now prevails in the area of relevance

219

whereas the fine-grained suspension load settles downwind of it. Deposition by snowless winds results in the accretion of thin, flat sheets of well-sorted sand. Snow flurries on the other hand produce niveo-aeolian strata of generally poorer-sorted sand.

Schwan (1986) further suggests that: (1) Climate was the primary control for 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 accounted for by a seasonal shift in the centres of sand and loess deposition.

(2) The dry part of the North Sea floor such as existed during the Weichselian Upper Pleniglacial, together with its extension into the present land area of NW Europe, formed the principal source regions for the depositional process. Ample quantities of material of the required grain-size range were provided by the ice-sheet margin that surrounded the emerged sea floor. Only in a sedimentary basin of this large size, having the proper orientation with respect to the prevailing northerly or northwesterly winds of the time, could a process of along-track size-sorting bring about an areal separation of facies 3 type sand and loess.

The above model of deposition needs amplification in two respects. In the first place, the process of stepwise tractional transport must be considered and secondly, it is necessary to distinguish between a local-scale and a regional-scale model of deposition of facies 3.

Stepwise tractional transport As already mentioned, the bed pairs of facies 3 result from two different

depositional processes. Whereas the finer-grained strata were laid down by settling from suspension, the formation of the coarser units is attributed to the accretion of saltating and creeping grains. Moreover, source area and receiving site were located far apart and, because of this, the tractional transport of sand grains must have proceeded discontinuously in discrete steps. This implies that the sand grains were moved over a certain distance during episodes of strong wind and then, upon slackening of the wind speed, came to rest on the ground surface. There, they had to await a next wind event of sufficient strength to entrain them anew. Thus, the particles originating from the emerged North Sea floor should have been in frequent contact with the substrate during their journey from source to receiving site. As a result, the grains of remote provenance would gradually mix with particles from sources nearer to the depositional area. Furthermore, it is likely that the farther down the wind track the more important the admixture of proximal material would become.

A more continuous mode of transport is envisaged for the finer component of facies 3, which presumably fell out from suspension. It can be envisaged that, near

220

the ice-sheet margin, the grams involved in this process were lifted high into the air. remained in suspension over long distances, and simply bypassed the extensive throughput zone of the aeolian sedimentation basin. As a result, the fines of distant origin would not have suffered any serious measure of contamination when the~

ultimately settled. The transport mechanisms discussed above may account for some of the com-

plexities reported in sediment-petrologic studies on the provenance of coversands in Belgium and The Netherlands (e.g., De Ploey, 1961; Crommelin, 1964: Maarleveld,

1968; Vandenberghe and Krook, 1981; Haest, 1985). Interpretation of the data invariably focuses on the question of whether the blown sands were derived from the local substrate that underlies them or from more distant sources such as the North Sea floor. With respect to this, it is pertinent to realise that the saltating and

creeping sand grains may have moved over long distances in a stepwise fashion and thereby became enriched in proximal components.

Less controversial than the provenance of the coversands is that of the loess.

According to Miacher (1986), who has summarised recent mineralogic work on this subject, it is generally accepted that the emerged North Sea floor should be the source area of the loess in the Low Countries. This would agree with our suggestion

that the suspended grains had remained aloft over long distances before they began

to settle.

Local-scale model of deposition As already mentioned in the Introduction. the facies 3 type alternation of

coarser- and finer-grained thin beds, though a characteristic of Weichselian (Upper~ Plenigiacial coversands, is occasionally also found in aeolian units of Weichselian

Late Glacial age. Examples of the latter type are zone D of profile II (Fig. 3) and zone B of profile IV (Fig. 11).

Profile II (Fig. 3) is from north Lincolnshire, England and has a late Loch Lomond Stadial age (Buckland, 1982). Zone D. in the basal part of the profile, shows an alternation of coarser- and finer-grained sandy layers that have an irregular wavy shape and a thickness varying from 2 to 4 cm, An irregularly distributed and laterally discontinuous lamination is observable in both types of strata. In the coarser-grained units, the laminae consist of silty material whilst those found in the finer-grained beds have a texture of medium sand. Thus, a confused pattern of textural alternation is present at both the within-zone and the within-bed levels.

The textural characteristics of the coarser- and finer-grained beds in zone D arc summarised in Table 3. The two units clearly differ in mean grain size but not in mode and, moreover, they are both more poorly sorted than the beds of the overlying zones A - C . These data are consistent with the structural makeup of zone D as just described.

221

/

K--

co

/

I \

- - ~ - - ~ Adhesion lamination

- - - ~ Climbing -adhesion- ripple structures

- ~ ] Aeolian planebed lamination

~ Thin layer with silty texture

[ ~ Alternation of coarser-& finer- grained thin beds or laminae

Structureless or poorly-laminated sand

~ eflation lag of grains in 1- 3 mm size- range

Fig. 11. Profile IV: facies 3 type alternation of coarser- and finer-grained thin beds mainly in zone B and

adhesion structures mainly in zone C. Location: West Brabant, The Netherlands. Length of profile = 125

c m .

222

Profile IV (Fig. 11) is from a Late Dryas Stadial coversand deposit in West Brabant in The Netherlands. The chronostratigraphic position of the profile is inferred from the fact that it is underlain by a radiocarbon dated peat bed of Aller~l age (GrN-12743:11.240 + 50 B.P.). In zone B. the alternation of coarser- and finer-grained thin beds or laminae is the predominant feature. The texturally contrasting layers are referred to as either " thin beds" or " laminae" depending on whether they are internally laminated or not. The alternating stratification in zone B occurs in association with well-sorted strata characterised by aeolian planebed lamination, In fact, there is a gradual transition from this latter type towards the coarser-grained thin beds (or laminae) in the units with alternating stratification. This implies that deposition of the coarser-grained strata was quantitatively of greater importance than that of the finer-grained ones.

In contrast with the preceding example, the layers of zone B are undistorted and even in shape. Post-depositional loading in a temporarily water-saturated state, and frost thrusting in seasonally frozen ground are two different processes that could explain the irregular waviness frequently found in facies 3 type coversands. Ap- parently, neither of them has been at work in the unit under discussion.

I 1 . S A N D - T R A N S P O R T I N G W I N D E V E N T

i

/ / ~ . / ~ s p e n s i o n Cloud

..... ~ ~G,ou.o..,,,, ~ . ...................

2. CESSATION OF WIND EVENT ]

T h i n s a n d p a t c h f o r m e d

Settling of fines from suspension E

. I [

b y t r a c t i o n a l O e p ~ i t i o n " - . - . i

j " . - . ' . . - .

Fig. 12. Local formation of aeolian facies with alternation of coarser- and finer-grained thin beds. Coarser component was drifted to receiving site from nearby deflation hollow. Finer component is transported from afar and settles from suspension upon cessation of wind event. Because of damp condition of receiving site, the fines adhere to previously deposited sand layer. Away from low-lying place, groundsurface is too dry to protect fine particles against entrainment by next wind event.

TABLE 4

Two models of deposition of aeolian facies 3

223

Regional-scale model Local-scale model

Entrainment of

coarser and finer com-

ponents from source

area

Throughput of

particles

Deposition of

coarser and finer com-

ponents at receiving

site

Formative wind

pattern

Moisture condition of

ground surface in

depositional area

In principle from one single

source of large extent,

i.e. from dry part of North

Sea Basin

Stepwise and over long

distance

Separated in time by

several months

Seasonal contrast in mean

wind strength

General wetness during

spring and early summer

due to thawing of season-

ally frozen ground

From spatially separated

"point-sources", i.e. from

small deflation hollows

with chance distribution

Cont inuous and over rela-

tively short distance

Almost simultaneous

Alternation of sand

transporting wind events

and calms

Prolonged wetness restricted

to topographic lows with

shallow groundwater level

The prevalence of adhesion structures in zone C suggests that, during its formative period, the depositional surface was damp most of the time.

An alternative for facies 3 deposition on a regional scale (Schwan, 1986) is

suggested in Fig. 12. The conditions necessary for the working of this local-scale sedimentation model are the occurrence of (1) randomly distributed deflation hollows acting as sources of both sand and silt; (2) low places with a shallow

groundwater level where adhesion to the damp substrate protects the fine-grained aeolian beds from deflation by strong winds; and (3) alternations of sand-transporting winds and calms that are inherent in most wind regimes. In Table 4 the two

models of deposition of facies 3 are compared. Features that can be accounted for on the basis of the local-scale model are the

following: (1) The occurrence of facies 3 type units in coversands of Weichselian Late

Glacial age. A general reduction in scale of the aeolian sedimentation processes due to climate-induced changes in environmental conditions is discussed by Schwan (1987). The dry part of the North Sea basin, which was the main source area for the wind-borne facies with alternating bedding, would shrink due to deglacial rise of the sea level. As a result, during subsequent aeolian sedimentation, supply from the emerged sea floor diminished and was gradually replaced by provenance from local

224

sources much nearer to the receiving site. Concurrently, the prevailing direction of the sand-transporting winds shifted from north or northwest to southwest. Further- more, the marked seasonal contrast in mean wind strength envisaged for the regional-scale model, gave way to a more uniform distribution of wind speed over the year when the younger coversands were laid down. These events took place during the time span comprising Weichselian Upper Pleniglacial and Weichselian Late Glacial.

(2) The generally greater thickness, within facies 3 units, of the coarser-grained layers with respect to their finer-grained counterparts. It is presumed that the tractional deposition of saltating and creeping sand grains is a more effective process than the settling of fines from suspension. The latter type of event takes place during spells of greatly reduced wind speed. As long as this condition lasts, the silt grains fall out from a suspension cloud which will have a much lower particle concentration than a saltation carpet. Thus, it may well be that both mean duration and sediment supply rate of a fallout event are small in comparison to those of an accretion event.

(3) The gradual lateral change from facies 3 to facies 2 or, conversely, that is regularly found in aeolian sand sheets in western Europe. In these cases, the position of the facies with alternating bedding is usually determined by a waterlogged substrate from which the necessary moisture is supplied by capillary rise.

(4) The association of facies 3 with an underlying unit dominated by adhesion structures. This feature is exemplified in profile IV (Fig. 11).

When wind-blown sands accumulate on a groundwater-fed substrate, the soil moisture content will diminish with increasing thickness of the sediment body. Within limits, this variable controls the nature of the stratification types that develop successively in the drying-up sequence. For the same reason, facies 2 always overlies facies 3 in successions where both types are present (cf. Ruegg, 1983).

ORIGIN OF AEOLIAN PLANEBED L AM INAT ION

Aeolian planebed lamination with or without concordantly-infiUed wind scours is by far the commonest intrabed structure in both facies 2 and the coarser-grained layers of facies 3. This stratification type results from tractional deposition at wind velocities that were too high for ripple existence (Hunter, 1977). Here, its predominance in the periglacial aeolian sand sheets of western Europe is attributed to the interaction of the following factors:

(1) Topography of the sedimentary basin. Large-scale topographic barriers that could have checked the free flow of the sand-transporting westerly winds are relatively rare in the lowland of NW Europe. In eastern England east of the Pennines, the Mesozoic cuesta scarps may have formed minor obstacles once the British icesheet had retreated to the north in Weichselian Late Glacial times. During that period and before, the southern North Sea sector had remained a non-glaciated.

225

gently undulating land-surface that connected southern England with the Low Countries. On the present continent of NW Europe, the NW-SE trending rivers Rhine, Ems, Weser, Aller and Elbe were important traps for blown sand as is evidenced by the tracts of Weichselian Late Glacial dunes that accompany them. Since the dune strings may occur on either bank of these rivers, their formation cannot be due solely to local deflation from emerged riverbeds and, consequently, an interceptive effect must be assumed. In the extensive areas that lie between these rivers, however, the surface topography would, in general, have been too subdued to offer serious resistance to the airflow passing over it.

(2) Sparseness of vegetation cover. Under periglacial climatic conditions, vegetation must have been very sparse and, therefore, did not significantly contribute to surface roughness. A somewhat greater density and height of the vegetation stand may be one reason why the building of cold-climate dunes was, by and large, restricted to the banks of major rivers. Similarly, the presence of low parabolic dunes on top of the sand sheets presumably represents a last-moment feature caused by climatic amelioration and the enhanced plant growth associated with it.

(3) High ratio between wind energy and sand availability. Because of seasonally frozen ground surface, the availability of sand in winter was much less than it might have been in the absence of soil moisture. The period of low potential sand supply would be further prolonged by a phase of wetness caused by springtime thaw of the ground ice. Insofar as winter was also the preferred season of strong, sand-transporting winds, a high ratio between wind energy and sand availability should have prevailed during the depositional process. This condition must have existed in a gradually decreasing measure throughout both the Weichselian Upper Pleniglacial and the cold stadials of the Weichselian Late Glacial.

It is envisaged that a combination of the factors discussed above created a state of high mobility, i.e. a situation in which the passing of sand grains greatly exceeded their trapping. The throughput of sand rather than its vertical aggradation would be the dominant process so that nucleation and subsequent slipface development were suppressed. Net deposition at a slow rate took place in the form of thin, flat sand patches (Dylikowa, 1969; Pyritz, 1972; McKee, 1979; Steele, 1983; Schwan, 1986).

SAND SHEETS v. DUNES IN THE PERIGLACIAL ENVIRONMENT

As already mentioned in the Introduction, sheet-like landforms are in an easterly direction gradually replaced by slipfaced dunes in the Weichselian to Early Holo- cene aeolian sandbelt found in the lowlands of northern Europe. In the central Polish lowland, vast fields of dunes occur along all major rivers. These dunes, which have been transported from ice-marginal valleys and sandurs, are to a large extent contemporaneous with the Weichselian Late Glacial sand sheets of western Europe (Dylikowa, 1969). Because of both this synchroneity and the prevalence of westerly winds in Weichselian Late Glacial times, the northern European lowland area,

226

which has a distinct W - E orientation, may be thought of as an aeolian sedimentation basin with downwind evolution of bedforms. The areal distribution of aeolian landforms and sand thicknesses are features that have been studied by man~. authors (e.g., McKee and Moiola, 1975; Fryberger et al., 1979, 1984: Lancaster, 1983; Mainguet and Chemin, 1983). For the zonation in the northern European aeolian landscape, a twofold explanation is tentatively proposed here. It involves the combined effects of climatic gradient and topographic corridor.

Cfimatic gradient

Dylik (1969) and Lea and Waythomas (1986) have suggested that, in a cold-climate environment, dunes would represent drier conditions than coversands. In this assumption, it is implied that (1) the formation of dunes requires a higher a n d / o r more constant sand supply rate from that of sand sheets, and (2) under arid periglacial conditions, seasonal freezing of the ground surface would fail to occur, so that, in principle, the av',filability of sand remained stable throughout the year. On this basis, it cannot be excluded that a greater dryness of the climate in the central European lowland during the cold stadials of the Weichselian Late Glacial played a role in the phenomenon under discussion. Poser (1951) also mentions a higher degree of continentality in the Late Pleistocene climate of Poland as a possible cause of the gradual sand sheet-to-dune transition in north Germany.

The measure of humidity of an ancient cold climate can be evaluated qualita- tively from the presence of snow-induced structures in windblown sands (e.g., Ahibrandt and Andrews, 1978: and several others quoted in Schwan, 1986). The absence of these features, however, does not necessarily signify lack of precipitation since snowy intercalations in aeolian deposits may melt and disappear without leaving any trace at all (E.A. Koster, pers. commun., 1986: Schwan, 19861.

Topographic corridor

In Weichselian Late Glacial times, the subsiding European Basin with W-F. orientation was bounded to the north by the continental ice sheet and to the south by the glaciated Variscan uplands (Woldstedt and Duphorn, 1974; Koster, 1978). When, during the close of the last Ice Age, westerly air-flow acquired overriding importance this setup was in the nature of a topographic corridor with respect to the prevailing winds. Across the European continent, this corridor had a variable width. Whilst its western par t - - reaching from the Low Countries to SW Denmark- -was very wide, it narrowed in an easterly direction to as far as the Oder-Neisse valley whence it gradually began to broaden again. Thus, the Oder-Neisse area presumably acted as a choke point in the one time corridor (cf. Fryberger et al., 1984). As a result, the westerly airflow would expand and loose speed upon leaving the funnel-shaped exit of the choke-point zone. That condition alone might have

227

sufficed to bring about a change in regime from primarily transportational to depositional.

Discussion

The origin of warm-climate aeolian sand sheets in the U.S.A. is considered by Kocurek and Nielson (1986). Their examples of both modern and ancient sand sheets all occur on the trailing edge, leading edge or lateral fringes of dune fields. Moreover, low-angle wind-ripple lamination is a prominent structural characteristic of these sediment bodies.

In a critical evaluation of their observation data the authors pose the question of how far aeolian sand sheets can exist in their own right rather than being erg-marginal, transient features primarily associated with dune-field degradation. Here it is proposed that an affirmative answer to this question should be given with respect to the periglacial aeolian sandbelt of the northern European lowland. As noted before, the structural makeup of the Weichselian sand sheets in northwestern Europe is wholly unsuggestive of a trailing-edge position and features unambiguously inter- pretable as dune-base relics are virtually absent in them.

CONCLUSIONS

(1) Weichselian to Early Holocene aeolian sand sheets with subdued surface topography occur over large areas in eastern England, the eastern and southern Netherlands, the lowland of Belgium and the northwestern part of the Federal Republic of Germany.

(2) Within the sand sheets, two structurally different facies of wind-deposited sand can be distinguished. Here they have been referred to as aeolian facies 2 and aeolian facies 3.

(3) The spatial position of the beds within the sediment body is the diagnostic characteristic of aeolian facies 2. Accordingly, its beds may be either horizontal~ inclined or oppositely dipping.

Horizontal bedding is by far the most common feature. Inclined bedding is related either to small, isolated dome dunes or to scoop-shaped deflation surfaces. Oppositely dipping beds on a decimetre to metre scale are found in structures resulting from fluctuating winds. The last mentioned feature is of rather rare occurrence and, presumably, does not represent an inherently large measure of directional variability in the palaeo-wind regime.

Individual beds of facies 2 may exhibit a variety of internal structures except for textural alternation, as this characteristic defines the domain of aeolian facies 3. Aeolian planebed lamination with or without concordantly-infilled wind scours is the stratification type most frequently seen in beds of facies 2. It may be either fully developed or degraded to a changeable degree by (1) unequal loading of the

228

waterlogged sediment during a transient phase of high groundwater level: (2) secondary growth of tree roots; (3) interference of coarse particles with the depositional process; and (4) distortion by frost thrusting.

(4) Aeolian facies 3 is uniquely typified by the alternation of coarser- and finer-grained horizontal thin beds that are either wavy or even in shape. Whereas the coarser-grained layers result from tractional deposition of saltating and creeping grains, the finer-grained strata were laid down by settling from suspension. Two models for the deposition of facies 3 are proposed, viz. a large-scale model and a local-scale model. Periodic changes in surface wind speed, the presence of a damp depositional surface and availability of both sand and silt in the source area are necessary conditions for the working of both models.

The large-scale model is thought to be related to the specific environmental conditions of the Weichselian Upper Pleniglacial and has been discussed in detail by Schwan (1986). The local-scale model, on the other hand, may operate in almost every wind-dominated milieu where, locally and temporarily, the afore mentioned requirements happen to be fulfilled. It accounts for the fact that facies 3 is also found in units of Weichselian Late Glacial or Early Holocene age.

(5) The regional-scale model of deposition of facies 3 involves stepwise tractional transport over long distances so that grains of remote provenance were mixed with material from sources nearer to the receiving site. This process was an important control on the mineralogic composition of the resultant deposits.

(6) The prevalence of aeolian planebed lamination in both facies 2 and the coarser-grained layers of facies 3 is attributed to the interaction of three factors, viz. rarity of topographic barriers, sparseness of vegetation cover and a high ratio between wind energy and sand availability during transport and deposition. These conditions caused a state of high sediment mobility whereby the transport of sand grains greatly exceeded their trapping. Throughput of sand rather than its vertical aggradation would be the dominant process, so that nucleation and slipface development were suppressed. As a result, the slow net deposition took place in the form of thin and flat sand patches.

(7) From literature sources it is known that, from west to east in the aeolian sandbelt of the European lowland, the sand sheets are gradually replaced by partly contemporaneous, slipfaced dunes. Two possible causes of this feature are (1) a greater aridity of the climate in the central European lowlands during the cold stadials of the Weichselian Late Glacial, and (2) the presence of a topographic corridor that was delimitated by the margin of the Weichselian Late Glacial ice sheet to the north and the glaciated Variscan uplands to the south. In the vicinity of the Oder-Neisse floodplain, a narrow passage in this corridor must have had a decelerating effect on the airstream that emerged from it, resulting in a change of the wind regime from primarily transportational to depositional.

229

ACKNOWLEDGEMENTS

T h a n k s are d u e to the f o l l o w i n g pe r sons : Dr . W. R o e l e v e l d a n d Dr . J. T e r w i n d t

for cr i t ica l r e a d i n g of the m a n u s c r i p t ; Mr . C. H e m k e r fo r d e v e l o p m e n t of s o f t w a r e

on gra in-s ize da ta ; Dr . J. G r i ede , Mr . C. Kasse , Dr . Th . Leve l t , Mr . Th . R o e p a n d

Dr . J. S c h o u t e for f i e l d w o r k ass i s tance a n d use fu l c o m m e n t s ; Dr . W . G . M o o k of t he

I s o t o p e Phys ics L a b o r a t o r y , G r o n i n g e n , for r a d i o c a r b o n d a t i n g o f t w o sample s ; the

m a n a g e r s o f Br i t i sh I n d u s t r i a l S a n d Ltd. , H u r d i s s Ltd . , a n d C l u g s t o n Ltd . for

p e r m i s s i o n to w o r k on the p r e m i s e s of the i r c o m p a n i e s ; Mrs . C. S c h w a n for t y p i n g

of the m a n u s c r i p t ; Mr . H. S ion a n d Mrs . S. K a r s for p r e p a r a t i o n o f the d r a w i n g s

a n d Mr. C. van de r Bl iek for p h o t o g r a p h i c work .

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