Comparative micromorphology of two late Pleistocene paleosols (in the Paris Basin)

CATENA

Vol. 9,227-251

Braunschweig 1982

COMPARATIVE MICROMORPHOLOGY OF TWO LATE PLEISTOCENE PALEOSOLS (IN THE PARIS BASIN)

N. Fedoroff, Grignon P. Goldberg, Jerusalem

SUMMARY The micromorphology of two late Pleistocene paleosols in the Paris Basin, St. Pierre-lesElbeufand Bois du Moulin were studied in order to characterize their pedogenic and sedimentary processes and to reconstruct and interpret the pedosedimentary history and climate of each profile. The micromorphological observations of greatest interpretive value were the types of clayey and silty textural accumulations (argillans and siltans), pedality and porosity; of lesser value were papules, bioactivity and concretions. During the last interglacial period on both profiles developed a sol brun lessiv6, comparable to those developed during the Holocene. With the onset of cold conditions of the last Glaciation, the two differentiate pedologically and sedimentologically. At Elbeuf, in the Seine Valley, we observe colluviation of silts and the development of grey forest soils with strong textural degradation under cold and moderately dry local conditions. At Moulin, on the plateau the grey forest phase is very weak and probably brief and is followed by a marked phase of churning and eluviation of silt; conditions were wetter than in the valley, such that the upland position protected the profile from colluvial truncation. The advantages ofmicromorphology in interpreting pedo-sedimentary sequences is discussed. RESUME Etude micromorphologique de deux paleoprofils developp6s sur limons eoliens ayant 6volu6 d'abord sous climat interglaciaire (Eemien), puis sous climats froids (Wechselien inferieur). L'un est en position de vall6e (vall6e de la Seine a Saint-Pierre-les-Elbeuf), l'autre est en position de plateau (Thimerais). Au cours de l'interglaciaire, il s'est developp6 darts les deux limons un sol de type brun lessiv6; mais le debut de cette p6riode (uniquement dans le sol du Thlmerals) est caractense a la difference du debut de 1 Holocene par une pedogenese d~gradation texturale encore marquee. Des le debut de la p6riode froide, les deux sols se differencient nettement. Le sol de vall6e a evolu6 en sol gris forestier accompagne d'une forte d6gradation texturale; ce type d'6volution s'est poursuiviiusqu'a l'enfouissement. Dans le sol du plateau, la phase de sol gris forestier est ~ peine marquee; il lui succ~de une phase cryogenique et d 61uviation grossi6re. I1est possible que 1enfouissement de ce sol soit post6rieur a celui de Saint-Pierre-les-Elbeuf. •

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1. INTRODUCTION The loesses and paleosols in the Paris basin have served as a focus for Quaternary research for more than a century (MICHEL 1969, BOURDIER & LAUTRIDOU 1974). It would be fair to say that in general the earlier works (e.g. LADRIERE 1890, BORDES 1953,

228

Fig. 1:

FEDOROFF & GOLDBERG

Location of exposures. E

Saint-Pierre-les-Elbeuf; B - Bois du Moulin.

BOURDIER 1969) tended to concentrate on building the stratigraphic framework of the deposits; soils, while clearly recognized and identified were used primarily for stratigraphic purposes although some paleoclimatic overtones were dear. Pedologists, on the other hand, working with surface soils, have looked at loessic soils from a different viewpoint. Using micromorphology, they have been primarily interested in soil genesis, neglecting historical and paleoclimatic considerations (JAMAGNE 1973), although more recent works (EIMBERCK-ROUX 1976, CAILLER 1977) have paid attention to temporal and climatic related problems. In recent years, LAUTRIDOU (1977) and DEWOLF (1978) have been less extreme in their outlook and have attempted to bridge the gap between these two schools of thought. Rather, they view a profile as an evolving pedo-sedimentary complex that reflects local and regional paleoenvironmental changes through time. Their vision, however more panchromatic, is still macroscopic in scale. Thus, it is our belief that in order to fully understand the history ofa pedo-sedimentary complex its micromorphology must be studied. In this paper we have used micromorphology to study two late Pleistocene buried soils developed on loessic material from the western part of the Paris basin. The first is located in the well-known exposure ofSaint-Pierre-les-Elbeuf(LAUTRIDOU et al. 1974) which is representative of a valley setting in the sequanien province (LAUTRIDOU & SOMME 1974). The second, in contrast, Bois du Moulin (Thimerais) was selected because of its plateau location and its stronger soil development (fig. 1). We had three goals in mind. We wished first to characterize the types ofpedogenesis in each profile and secondly to reconstruct the pedo-sedimentary history for each. Finally, we attempted to interpret each recognized pedo-sedimentary phase in terms of past climate.

MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS 2.

229

METHODS

Elbeuf thin sections (13x8 cm) were prepared in Grignon according to the method of GUILLORE (1980) and those from Bois du Moulin in Wagengingen following JONGERIUS & HEINTZBERGER (1963). Organic matter was determined on Elbeuf thin sections by a sodium hypochlorite treatment (BABEL 1964). We have used principally the terminology of BREWER (1964) and to a lesser extent definitions of FEDOROFF (1977) when we would not find an adequate BREWER term. Micromorphological interpretations were based on the methodology outlined by FEDOROFF (1973).

3.

3.1.

MACRO- AND MICROMORPHOLOGICAL DESCRIPTIONS OF PALEOSOLS

SAINT-PIERRE-LES-ELBEUF

The famous site of the Saint-Pierre-les-Elbeuf (hereafter as "Elbeuf') is in a former brickyard located in the Seine valley, 25 km S-E of Rouen (fig. 2) where loessic material rests on 30 m terrace of the Seine. We have chosen exposure 4 (fig. 2) because it is more easily accessible than the main section (exposire 1 and 2) and because it displays a relatively great thickness of Eemian and Wechselian deposits (LAUTRIDOU 1974). This exposure peviously described by LAUTRIDOU (1974) is as the following: Alb Large mottles, 10 YR 5/6, yellowish brown and 10 YR 7/8, yellow. 400-460 cm Massive structure with a tendancy to crumble. Crotovinas are probably present at the top. Calcium carbonate accumulations. IIArA2b Top, 10 YR 5/3, brown, bottom, 10 YR 4/4, dark yellowish brown. 460-480 cm Numerous washed zones in form of weakly defined horizontal lenses. Massive structure with a tendency to crumble. Top part has crotovinas and few pseudomycelia. Diffuse boundary. IIA2b 10 YR 5/6, yellowish brown. Less washed zones than in IIArA2. Scattered 480-520 cm gravels at the base. Massive structure. Abrupt boundary. IIIB2tb 7,5 YR 5/6, strong brown. Very coarse polyhedric structure. Common 520-600 cm clay coatings, 5 YR 3/4, dark reddish brown, on surface of peds and in small channels. Thick dark clay in-fillings in large vesicles (O 1 cm). The upper part of the exposure has not been studied (see description in LAUTRIDOU 1974).

3.2.

BOIS DU MOULIN

Bois du Moulin (hereafter, "Moulin) was a small brickyard located in eastern Thimerais, 18 km west of the Eure Valley between Chartres and Dreux (fig. 1). The geological and geomorphological setting have been previously discussed by FEDOROFF (1967), who described a small exposure. This, however, proved to be less detailed and less accurate than a subsequent description based on a larger section prepared for the 1969 INQUA congress (fig. 3). The paleosol described below is covered by a Late Pleistocene loess (DEWOLF 1978) on which subsequently developed a typical Holocene sol brun lessiv& From top to bottom (fig. 3):

230

FEDOROFF & GOLDBERG

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a - Location. Existing exposures in the quarry of Saint-Pierre-les-Elbeuf; E4 = studied exposure. b Analytical data of exposure 4. c - Schematic drawing of exposure 4. 1 to 4: studied horizons; location of samples for thin sections. d - Interpretation of exposure, a) development of a sot brun lessiv8 (At-A3-Bt2-Bt3); b) truncation of the sol brun lessiv8 (remnants of Bt3); T = truncation; c) deposition of a colluvium; S = sedimentation; d) development of a grey forest soil (A't-N2-Bt'2); e) deposition of a silty sediment; f) development of a weak grey forest soil (A"vA"2-Bt"2).

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MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS

231

5 % (Iron)

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Transition with the overlying loess 185-205 c m

10 YR 5/8, yellowish brown. Spots (O 1 mm) of washed silt with sharp boundaries. Silt loam. Massive structure. Regular and gradual transition.

A2b

10 YR 6/4, light yellowish brown. Numerous spots of washed silt. Silt loam. Massive structure. Clear transition. 10 YR 6/4, light yellowish brown. Numerous spots of washed silt. Black concretions, some with yellowish nuclei, distributed regularly throughout and forming about 40% of the horizon; average diameter is ca. 1 cm and decreases to 3 m m at the base; can be cut with a knife. Silt loam. Massive structure. Clear, irregular transition.

205-230 cm A2irb 230-255 cm

232

FEDOROFF & GOLDBERG

A and Bb A fraction 255-270 cm A andBb B fraction 255-270 cm B and Ab A fraction 270-285 cm B2tb 285-320 cm

B3tb 320-365 cm

Ctxb 365-400 cm

10 YR 6/4, light yellowish brown. Silt loam between the aggregates of Bb or with spots of washed silt (O 1 ram). Very clear and irregular transition with B fraction. 7,5 YR 5/6, strong brown. Numerous vertically oriented aggregates, with fringes, 7,5 YR 5/8, strong brown 2 cm thick. Some blackish coatings. Clay loam. Fine, distinct polyhedric structure. Some clay coatings integrated into the soil mass. Transition between B and A fractions distinct. 10 YR 6/4, light yellowish brown. A fraction penetrates the B fraction in the form of tongues; towards the base of the horizon the A fraction is present as silt coatings on aggregates. Transition of the B and Ab to the B2t gradual. Exterior of aggregates, 7,5 YR 5/8, strong brown; interior of aggregates, 7,5 YR 6/8, reddish brown. Spots of washed silt (O 1 cm). Clay. Very fine, distinct polyhedric structure. Clay coatings integrated into the soil mass. Smooth, distinct transition. 7,5 YR 5/8, strong brown. Spots of washed silt (O 1 cm), 20% at the top and 2% at the base. Very small fragments of flint. Clay loam. Medium, distinct polyhedric structure. Clay coatings integrated into the soil mass. Distinct transition. 7,5 YR 7/4, pink. Very small fragments of flint. Silt loam. Massive structure with horizontal layering and coarse prismatic substructure. Thick clay coatings, 7.5 YR 5/5 strong brown on aggregates and in intrapedal voids. Distinct transition.

4. DISCUSSION In this section we discuss for each section or profile the micromorphological data in terms of the inferred pedogenic and sedimentary processes and their palaeoclimatic interpretations. As will be seen, the occurrence of a given micromorphological feature in space does not necessarily denote contemporaneity with that horizon or level.

4.1.

SAINT-PIERRE-LES-ELBEUF

Based on field observations, LAUTRIDOU (1974) interpreted this exposure as the following: second, less developed steppic soil with strong burrowing; AI: II A1-A2: steppic soil developed on sediments of level 2 (supported by pollen of compositae liguliflora); II A2: colluvial reworking of A horizons of Eemian soil; III B2t: truncated Bt horizon of the Eemian soil. This interpretation can be expanded considerably by micromorphological observations.

Pedality

-do

Large square peds Large sub(2)<2 cm) horizontal joints loosely packed and narrow vertical Interconnected vughs and vesicles

Subrectangular peds (1 X2 cm)

llA2b top (Elb 3) (Photo 2)

llA:b base (Elb 2)

llIB2tb (Elb 1) (Photo 1)

Interpedal system of vertical and subvertical joints, Within peds vughs and vesicles, commonly circular to elongate

Relatively porous with circular, irregular, interconnected vughs and vesicles

Thin subhorizontal wavy joints, isolated and interconnected vughs and vesicles

-do-

I1ArA2b (Elb 4) (Photo 2)

do-

-do

Large elliptical void (O 6 cm) and smaller circular to elongate vughs and vesicles

Porosity

Aibase (Etb. 5)

-A~b Massive middle and top (Elb. 6 and 7)

Horizon

do-

-do

-dobut muscovite less abundant

-do

-do

Dominantly quartzic silt with some muscovite

Skeleton

dobut more abundant

Some more abundant mineral plasma silasepic, locally ske[sepic

Rare mineral plasma

-dobut more abundant with small plant fragments

do-

Rare organomineral, Silasepic

Plasma

(lOlo)

Ferruginous Features Brownish black concretions (O 200-300 ,am)

(2) (2-3%), (3) (1-2°/0), (5) (1-2°/0) Yellow brown argillans (7) comparable to type (6)

(2) (25-30%), (3) clayey matrans (5) (3-5%) comparable to type (4) slightly dusty argillans (6) Papules (2-3%)

(1), (2) (15-20%) (3) (1%) and laminated rusty argillans (4) (1%) Papules (1%)

(1), (2) (15-200/0)

5%

but less abundant

do-

Striotubules (10%) -do-

-do

(20%)

Abundant and large slriotubules

(40%)

-dobut more abundant

-dobut less abundant (25%), some in form of striotubules

Biological Features Burrows with loosely packed grains (50%)

-dobut some more abundant, 1%

-do-

(1) and siltans -do(2) 15o/0 Red brown laminar argillans (3) papules

Textural Features Diffuse zones of coarse silt (1) (15%)

Tab. 1: MICROMORPHOLOGICAL DESCRIPTIONS OF HORIZONS FROM SA1NT-PIERRE-LES-ELBEUF

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234 4.1.1.

FEDOROFF & GOLDBERG Pedogenesis

a) Yellow brown argillans (7, see Table 1) (hyaline, locally laminated, dusty or with embedded grains of quartz silt) developed on the loessic material in horizon B2t coat interpedal vertical voids and intrapedal vughs and vesicles (sometimes the latter are completely filled in) (Photo 1). Such an argillic fabric is very similar to that found in the lower part (B3t horizon) of a Holocene sol brun lessiv6 developed on the Weichselian "loess recent" (JAMAGNE 1978). Missing therefore are the overlying soil horizons which have been eroded and colluvially reworked into the sediments of horizon II A1-A2b (see section on sedimentation). b) Red brown clayey matrans (5) (flecked noncontinuous birefrigence, diffuse extinction) and laminated dusty argiUans (4) are closely related to one another. Matrans are present mostly in horizon B2t and to a lesser extent in horizon II A2b where they become better sorted. In horizon II AFA2b, they are present but very rare. The matrans postdate the yellow argillans with medium unconformity (moderately cross juxtaposed) which we interpret as a discontinuity in pedogenesis; in fact we think that the discontinuity is contemporaneous with the destruction of the top of the sol brun lessiv6 and colluviation. Furthermore, these clay accumulations are dark red brown and are therefore not part of the sol brun lessiv6 (characterized by yellow to yellowish brown argillans) nor of a sol fersiailitique iessiv6 (characterized by red bright argillans). Rather, they are found presently in degraded chernozons (grey forest soils) (KOWALINSKI 1969) and in argiudolls (observations of ECHEVARIA in Uruguay). The distribution of these clay accumulations is characteristic of a complete profile and it differs from that of a sol brun lessiv6 by its large spherical voids that are completely filled in. Also since these accumulations are fairly abundant, we can conclude that the corresponding soil was rather well developed. Their internal morphology, poor sorting and interbedding with the coarse material must be related to a seasonally contrasted water regime. In fact, such morphology has been observed in present day soils under continental climates KOWALINSKI (1969), TARGULAIN (1974) and FEDOROFF-observations in Finland and Quebec). We think therefore that these accumulations belong to a former degraded chernozem (grey forest soil) which in the field is expressed by the first humic horizon) (fig. 2). c) Red brown laminar argillans (3) differ from the matranic type by being better sorted and laminated. Their distribution is basically similar but they are found higher up in the exposure (up into the base of horizon Alb) and they clearly postdate with weak unconformity (weak cross juxtaposition) the matrans. Their pedological interpretation is similar to that of the matrans. However, there is only a slight disconformity between the two argillans and moreover the laminar argillans possess a more homogeneous internal fabric which signifies a less seasonally contrasted water regime. Finally from their uppermost occurrence in the base of horizon Batb, the laminar argillans would reasonably belong to the second humic horizon. d) Siltans and zones of washed silt occur in horizon B2tb and become more abundant in II A1-A2b where they reach a maximum; in horizon B2tb, they are less abundant, less diffuse and are partially interbedded with the matrans and laminated dusty argillans. These two types of silt features often merge with each other and they are generally not clearly juxtaposed. Therefore we believe these features to be a result of the same process which commenced roughly at the same time. Also judging by the position and the degree of expression of the silt and clay features within the exposure, we think they are related especially to the first

MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS

235

humic horizon. Such features can presently be observed from a wide range of Holocene soils (much wider range than grey forest soils) in boreal and continental climates where they are usually in association with laminated argillans. e) Ferro-manganlferous concretions and impregnations are very scarce in all levels. Most of them seem to be formed in place since the non-impregnated and impregnated material are the same and most of them show diffuse boundaries. They have no special significance; they are widespread in Holocene sol brun lessiv~ and in grey forest soils where they have a similar form and even a higher abundance. f) Weathering. It is difficult to make any definite statement about the degree ofweathering since good indicators like biotite are absent. Poorer substitutes, such as heavy minerals and muscovite do not show any real changes in abundance and in degree of weathering; this is also supported by the clay mineralogy (LAUTRIDOU, unpublished data). Strong weathering does not appear to have been present although in absence of sensitive mineral indicators we cannot rule out the possibility of milder weathering episodes. g) Organic matter in humic horizons is found in small amounts, (less than 1%), is well humified and is intimately mixed with the mineral plasma; the latter is a characteristic typical of present day mollic epipedons. Although in present grey forest soils, the amount of organic matter is somewhat higher (1-2%), it is difficult to compare their organic content since Elbeuf is much older and a great amount of aging has taken place. h) Bioactivity. Its effects are manifest in three types of features. All have a tubulic form, but they differ in size and fabric. Those of the first type, isotubulic/striotubulic type, are generally small (O 1 cm) and are delimited by a poorly oriented clay-silt. They are best expressed in horizons Ill A2b and II A1-A2b; in the base of level 4, they occur relatively rarely compared to the others (photo 2). The second type is an aggrotubule of medium size (O 2-3 cm), present only in horizon II A2b. The third one is larger (O 3-5 cm); it corresponds to the crotovinas of the field description. They are filled with a looser material that is slightly less organic than the surrounding one; they occur in horizon II AvA2b and top of horizon Alb.

4.1.2.

Sedimentation

In this exposure, three sedimentary units had been recognized in the field (LAUTRIDOU 1974): Horizon III A2t: Pre-Eemian eolian sediment truncated by a diffuse gravel layer; Horizons II A2b and II A1-A2b: colluvial silt of early Weichselian age; Horizon Alb: silty sediment also of early Weichselian age. From the micromorphological evidence alone it is difficult to make any conclusive statements about the environment of deposition of these sediments. This is due to the fact that the original sedimentary fabrics in all levels have been completely obliterated by pedogenesis. Evidence of truncation of the horizon B2tb comes from field observations (the diffuse gravel layer and clear contact between horizons II B2tb and II Azb) and from micromorphology (no higher order fabrics from the Eemian soil are found in horizon II A2b). Colluvial origin of horizon II A2b and II A1-A2b can be appreciated by the presence of papules from the sol brun lessiv~; -

236

FEDOROFF & GOLDBERG

-

the dispersion of the papules in the whole mass of the sediment; their rounding and the absence of remains of high order fabrics from the upper horizons of the sol brun lessiv6 suggest that these horizons have been reduced to elemental grains. Wind transport cannot be excluded, however; - the poor sorting of papules indicates that even if they were wind transported, the particles were moved only short distances; the size distribution and composition of heavy minerals, silt and sand fractions are the same in horizons III B2t, II A2b and II AI-A2b indicating that the colluvium was not enriched (or very little) by external eolian material. In horizon Alb thre is only a trace of papules which can be interpreted as representing a slight influence of colluviation. However the decrease in abundance of quartz sand and the slight change in composition of the heavy minerals (LAUTRIDOU 1974) suggest also a more distant source and eolian deposition. -

4.1.3.

Pedosedimentary

history and paleoenvironments

It should be evident from the above discussions that within this Elbeuf exposure we have telescoping of different and also repeated pedogenetic processes on an aggrading parental material. Below we attempt to resolve this exposure into its various pedosedimentary components and to interpret their paleoenvironmental significance (fig. 2d). a) Development of a sol brun lessiv4 similar to Holocene ones from which we infer a temperate climate comparable to today's. b) Truncation of the sol brun lessiv6 down to its B3t horizon. c) Deposition ofcolluviumderived principally from the top of the sol brun lessiv6 (horizons II A2b and II A~-A2b). We speculate that this colluviation represents a transition to colder conditions in which arboreal vegetation was reduced (cf. pollen analysis) and landscape subject to erosion. d) Development of a grey forest soil with a mollic epipedon (horizon II Al-A2b); eluviation has taken place in the mollic and in lower horizons (silt accumulations are the signs of the eluviation) and evidence of illuviation if found from base of mollic epipedon down to the truncated B3t of the sol brun lessiv6. Such soils do not exist from the Holocene in Western Europe. Instead, they are found nowadays under continental climates in transition between derno-podzolic soils and chernozeros. From the literature, it is clear that in these soils clay accumulations do occur (BRONGER 1978), but silt accumulations have not been described. Our experience is limited to the grey forest soils of southern Poland (FEDOROFF, personal observations; see also KOWALINSKI 1969). In these soils, silt accumulations were obvious, a feature also observed by Soviet colleagues (PARFENOVA & TARGULIAN, personal communication) from similar soils in Russia. Speculating on the environment, we believe that: - the climate was colder than today's, but not very cold since we have not observed the slightest evidence of frost action. - the strong eluviation is due to the melting of a thick snow cover, probably thicker than that in regions with Holocene grey forest soils; the last hypothesis is based on the greater development of silt accumulations in St-Pierre-les-Elbeuf. e) Deposition of a new silty sediment (horizon'Aab).

MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS

237

f) Development of a second weaker grey forest soil in which the mollic epipedon (horizon Alb) extends downward to the top of the first one and the eluvial-illuvial profile penetrates down to the B t horizon of the sol brun lessiv~. The overall conditions ofpedogenesis are comparable to those of the first grey forest soil although we think that the climate was drier. This is based on the lower amount of silt accumulations in the second soil and on the greater abundance of crotovinas, assuming the latter are more characteristic of steppic conditions.

4.2.

BOISDUMOULIN

FEDOROFF (1967) previously studied this paleosol and concluded that it was essentially a polyphased soil. The first phase is represented by a weakly rubefied sol lessiv~ tbllowed by a degradational phase in which the iron-clay complex is destroyed and concretions formed. At the time of observation, more subtle features such as silt accumulations were overlooked due to, among other things, the poor quality and small size of the thin sections.

4.2.1. 4.2.1.1.

Pedogenesis Textural accumulations

a) Brown dusty silt-argillans 1( see Table 2) (alternation of silty clay beds and dusty clayey laminae very diffuse extinction) are the first accumulations to result after the eolian silt had been stabilized. In the A and B horizons, they are found only in smaller intrapedal vughs and vesicles whereas in the B2tb, B3tb and Ctxb horizons they become more abundant and are present in bigger voids (Fig. 7). They cannot be related to a sol brun lessiv~ because of their coarseness, but rather are indicative of more aggressive eluviation, intermediate to that found in a sol brun lessiv~ and a derno-podzolic soil (TARGULIAN 1974). b) Yellow laminated argillans (2) are found in most cases on the dusty brown silt-argillans, their distribution being similar to that of the latter; the two are very weakly cross-juxtaposed. Their morphology is similar to ones found in Holocene sol brun lessiv~ developed on loess recent and we infer that they represent a similar type and degree ofpedogenesis. c) Yellow to yellowish brown clayey matrans (3) are juxtaposed to laminated argillans, showing a clear boundary. Their morphology (except color which is yellow to yellowish brown instead of red brown) and to a lesser extent their distribution are similar to those of the clayey matrans (2) from Elbeuf. Consequently we relate them to a very weakly developed phase corresponding to a grey forest soil. d) Silt accumulations, two sub-types were recognized; interpedal coarse silt and siltans. The interpedal coarse silt is well sorted and is found in a branching pattern between peds and concretions in horizons A2b and A2irb; washed silt is also an important component of the peds in the A2b and A2irb horizons. The siltans are present in major voids (fig. 5) and cap rounded peds and concretions in A2b and A2irb horizons. Both commonly contain silt size papules of argillans. The relationship of these silt accumulations with clay deposits is not clear for nowhere could we observe the transition between the silt and clay accumulations. In the B3tb horizon, the siltans were emplaced in horizontal joints that were clearly formed after the clay was

-do but muscotive is more rare

Massive

Ctxb (Photo 10l

Some horizontal joints, lndependant or elliptical vughs and elliptical crescentic vesicles

do

do but joints are more regular

-do but more muscovite

Rectangular peds (1-3 cm)

lmerpedal system of horizontal vughs merging into irregular horizontal joints. Few vesicles

-do

B3tb

Heterogeneous with whitish subvertical mottles within a rusty brown mass. Subangular and rounded peds (O 5-20 mm)

A and Bb and B and Ah (Photos 4 and 8)

do but more developed horizontal vughs

Dominantly quartzic silt with some nluscovite

--do

-do

A2irb (Photo 3 and Fig. 6)

Packing voids with horizontal elongated vughs

Skeleton

lnterpedal system of irregular horizontal and vertical large joints, horizontal ones expand to vughs

Rounded and subangular peds (('l 1-I, 5 cn]t

Axb (Photo 3 and Fig. 5)

Porosity

B2tb Subrectangular (Photos 5 and to rectangular 9 and Fig, 7) peds ( I X 2 cm)

Pedaln~

--do Clayey features present but less integrated into the s-matrix sdtans (t-2%) (5l

(I) is present but more abundant (-2j (1-2%), (2) and (3) are more abundant. half of them are integrated to the s-matrix (5%) (4) (1%) (5) (t5-20%)

Brown rusty iaminated sill argillans ( 1) ( I%); yellow laminated argillans (2) (2%); yellow to yellowish brown clayey matrans (31 (2)3%). Aquic silt argillans (4) (1%): siltans (5) (15-20%)

do but sittans are better laminated (30%). lnterpedal loose silt is less abundant ( 10%1

Papules, I% Poorly laminated sillans (20%) (5) lnterpeda[ coarsc loose silt (15%) (6)

Textural Features

do Only types ( l ) and but heterogeneous (2) are present. No integration into s-matrix

less abundant

More abundant skelsepic and lattisepic

More abundant lattisepic and masepic in rusty nlass

do-

Only present in peds, silascpic

Plasma

none

(_'ryogen IC Features

none

none

-do In the s-matrix But more abundant (2-3%)

Concretions (1%)

do but less abundant (1-2%)

Impregnation'; Rounded peds in neosesquans and some vughs concretions (10%)

Brownish black none heterogeneous concretions (10-15%)

none

t-'errugirlous Features

MICROMORPHOLOGICAL DESCRIPTIONS OF HORIZONS FROM BOIS DU MOULIN

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MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS

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Photo 1: Fabric of IIIB2t of Saint-Pierre-lesElbeuf. Small vughs coated with hyaline argillans and a large vugh filled with a clayey matran and a bedded argillan.

Photo 2: Fabric of IIA1-A2 of Saint-Pierre-lesElbeuf. Burrows with spongy material, one in form of a striotubule.

Photo 3: Fabric of A2b and A2irb horizons of Bois du Moulin. See details of fabric of A2b horizon in fig. 5, the fabric of a rounded concretion in fig. 6 and the impregnation of a plant fragment in photo 7.

Photo 4: Fabric of B and Ab horizon ofBois du Moulin. Interpedal system of horizontal vughs merging into irregular joints.

Photo 5: Fabric of B2tb horizon of Bois du Moulin. Interpedal system of irregular horizontal and vertical large joints. To vertical ones are associated laminated silt-argillans, laminated argillans and clayey matrans; in horizontal ones when they expand to vughs, thick siltans are present.

Photo 6: Fabric of Ctxb horizon of Bois du Moulin. Vughs are coated with silt-argillans and yellow argillans, see details on photo 10.

Fig. 5: Fabric of A2b horizon (Bois du Moulin). I - moderately eluviated s-matrix; II - strongly eluviated s-matrix; 1II interpedal coarse loose silt and laminated siltans; 1- argillic papules; V - voids.

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Photo 7: Fragment of a plant residue impregnated by iron in Airb horizon ofBois du Moulin.

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~hoto 8: Rounded peds coated with aquic silt-argillans in a large 1ugh in A and Bb horizon of Bois du Moulin.

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Photo 9: Textural accumulations in B2tb horizon of Bois du Moulin. In contact with s-matrix, dusty laminated silt-argillans, crossjuxtaposed on them laminated argillans and clayey matrans; note strong fragmentation.

Photo 10: Textural accumulations in Ctxb horizon of Bois du Moulin. In contact with s-matrix, thick dusty laminated silt-argillans, cross juxtaposed on them thin laminated argillans.

244

FEDOROFF & GOLDBERG

deposited (photo 5). If we reasonably assume that both types of silt features are contemporaneous, we have the problem of finding the clay that surely would have moved during this period of coarse translocation. We have two alternatives: either the clay was destroyed, which seems unlikely because of the cold environment, or that illuviation was so strong that clay was completely washed from the profile. We cannot exclude, however, the possibility that the uppermost level of clay features in lower B3tb and Ctxb horizons are contemporaneous with this coarse translocation. Thus if correct, washing of the ground mass in the A2b and A2irb horizons was a long process which increased in intensity through time producing first silty-clay deposits (matrans) and then silt features. These silt features are comparable to those described by TARGULI.~,J (1974) in a demo-podzolic soil north of Moscow. But in central Russia the transitkm between silt features present mostly in A> and A and B horizons and clay features situated in B horizons is clear. We conclude from this that in Bois du Moulin coarse translocation was more vigorous than it is nowadays in central Russia. e) Aquic silt-argillans (4) occur typically as thin coatings (40/~m) in voids ofAand Bb and B2tb horizons (photo 8). These coatings are found in vughs produced by incomplete packing of rounded peds. It represents the last stage ofilluviation before burial by loess recent.

4.2.1.2.

Ferro-manganiferous concretion and cutans

In the A2irb and A and Bb horizons, we have distinguished two major types of concretions. The first one is characterized by a moderately diffuse impregnation of a silty ground mass that is predominantly eluviated; the impregnation locally is more concentrated in the form of sesquans and neosesquans (fig. 6). With depth this type becomes more abundant and reaches a maximum in the A and Bb horizon. On the basis of their irregular morphology and diffuse boundaries, it is clear that these concretions have formed in situ. The inclusions of washed silt within the ground mass show that they continued to form well into the derno-podzolic phase. However, it is clear that they ceased to form during the period of most intense coarse translocation since they are commonly capped with siltans. The second type is found in the A2irb, Aand Bb, Btb and Ctxb horizons and is divisible into three subtypes that grade into each other. However all three are somewhat similar in that they are rounded, commonly fragmented in B3tb and Ctxb horizons, have sharp boundaries and are strongly impregnated with iron and manganese; most of them are opaque (fragment of flint in fig. 6). The first subtype contains predominantly carbonized wood fragments in the nucleus. They are best expressed in th A2irb horizon, but are found in lower horizons as well (photo 7). These charcoal fragments have been previously recognized by VAN VLIET-LANOE (1976) from the upper horizons of'degraded" sol brun lessiv6. In Bois du Moulin, they occur in a similar position. The second subtype is the most abundant and consists of strongly impregnated quartz silt with some quartz sand. It also is most bundant in the A2b horizon but similarly occurs down through the Ctxb horizon but is quite rare in the B3tb horizon. The third subtype is the rarest and is composed ofloessic material with an argillic fabric resembling that ofa B 3t horizon of sol brun lessiv6. While this type is most prominent in the A2irb horizon, traces of much smaller size concretions occur in the Ctxb horizon as well.

MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS

245

The concretions found in Ctxb and B3tb horizons have been inherited from other soils since they are usually fragmented, well rounded and have sharp boundaries (see below for more discussions on their origin). The rounded shapes and sharp boundaries of concretions in A2irb and Aand B horizons would point to a colluvial origin as in the B3tb and Ctxb horizons, but absence of fragmentation could suggest only an extensive churning. Coexistence of rounded shapes and clear concretions formed in situ indicate that their tbrmation was spread out over a long period of time and signify a deepening of the concretion process through time. Because the most rounded concretions in the A2irb and Aand Bb horizons impregnate an argillic fabric, we deduce that the process of concretion formation started probably at the end of the sol brun lessiv~ phase. Finally it should be pointed out that these types of concretions tend to occur in a number of Pleistocene paleosols regardless of locality and age, always in the same position in the sequence of evolution (BILLARD & FEDOROFF 1977).

4.2.2.

Pedogenic history and paleoenvironments

It is clear from the micromorphological observations that the pedogenic history of the Bois du Moulin profile is more complex than is apprent in the field. Its history differs from the Elbeuf case, however, in that it shows a succession of continuous pedogenic events taking place on a stable non-aggrading profile (fig. 4). Therefore, the effects ofmorphogenesis are not evident. a) Deposition of silty parental material. This material is probably colluvial in origin, at least in its lower part. Its age is not known. We assume, however, on the basis of the hierarchy between the silty ground mass and the clay accumulations that it was deposited during a cold period previous to the Interglacial during which the sol brun lessiv~ was formed. b) Formation ofa fragipan. This results from a post-depositional effect which is largely pene-contemporaneous with the silt deposition. c) Sol brun lessiv&demopodzolic soil phase. This phase corresponds probably to a climate still rather cold and moist with a winter snow cover; this explains the derno-podzolic characteristics of the dusty silt-argillans. It represents the beginning of an Interglacial period and is not known from the Paris basin during the transition between the last glacial period and the Holocene, d) Sol brun lessiv~ phase. This soil is similar to Holocene ones (except a redder colour, 5 YR instead of 7,5 YR) and on this basis we infer a temperate climate comparable to today's. It clearly represents an interglacial period. e) Grey forest soil phase. This weakly developed phase indicates a shift to a period of renewed cold. f) Frost action and coarse translocation phase. The absence of transition between the clayey matrans and the siltans shows that climatic conditions degraded rapidly. Association of the cryogenic porosity pattern with the siltans indicate a very cold and moist climate, something similar to the climate of central Finnish Lapland, but probably still colder. g) Hydromorphicphase. In the Moulin profile, this phase is very faint. It corresponds to probably a regional climate wetter than before although the pedological conditions remain to be clarified. h) Burial without any truncation, this paleosol was buried by the complexe du loess recent (DEWOLF 1978).

Fig. 6: Fabric of a rounded concretion of A2irb horizon (Bois du Moulin). I Medium eluviated smatrix; II - strongly eluviated s-matrix; III - interpedal coarse loose silt; a) weak ferruginous impregnation; b) medium ferruginous impregnation; c) strong ferruginous impregnation; d) very strong ferruginous impregnation of a fragment of flint; 1 - argillic papules and argillans fossilized by iron impregnations; 6 laminated siltans (with finer beds, in black); V - voids; Q - quartz grains.

Fig. 7: Fabric of very top of B2tb horizon (Bois du Moulin). RD - s-matrix; I - partially eluviated smatrix and silans (residual silt accumulations); 3 - clayey matrans (3 in text) and aquic silt-argillans (4 in text); 6 yellow laminated argillans (2 in text); 7 brown dusty laminated silt-argillans (1 in text); V voids; Q quartz grains.

248

-

-

FEDOROFF & GOLDBERG In terms of age, as all above phases form a continuous sequence, we consider that: The sol brnn lessiv~ corresponds to the last Interglacial (Eemian) (it is comparable to Eemian soils observed in the Paris Basin); the grey forest soil phase represents the deterioration of climatic conditions at the end of the last Interglacial; the frost and coarse illuviation corresponds to the first cold stade of the last Glaciation; prior to the sol brnn lessiv~, we noted the previous cold period by deposition of the silty material and fragipan formation.

4.2.1.3.

Porosity and pedality

The porosity and pedality of this profile are the result of a temporal succession of two pedogenically related phenomena. The isolated vughs and vesicles that occupy intrapedal positions, specially in the B~tb and Ctxb horizons are those inherited from the three first illuviation phases and are presumed to be the result of bioactivity. Although originally present in the upper horizons (A2b, A2irb, Aand Bb), they have been obliterated by cryoturbation (see below). Related to these vughs and vesicles is a system of vertical joints in the A and Bb, B2tb, B3tb and upper Ctx horizons. These joints also contain the three first types of clay accumulation, but not siltans and probably relate to shrinking and swelling during the three first phases (photo 5). Superimposed upon this porosity pattern are a set of well developed horizontal joints and rectangular peds best expressed in the B2tb, B3tb and less in Ctxb horizons (photo 5). Higher in the profile (top of A and Bb horizons), these joints give way to shorter, more lenticular voids (photo 4). At the top (A2b and A2ir horizons), the latter are much reduced or absent and instead we observe thin crescentic voids delimiting small circular and well rounded peds that are usually capped with fine siltans (photo 8). These concentric voids are characteristically filled with interpedal coarse silt, but never in clear association with aquic silt argillans. Significant is that the superimposed porosity pattern contains only siltans and coarse silt but no clay accumulations indicating that the coarse translocation phase is syngenetic and epigenetic with this porosity pattern. Such porosity pattern has not been observed in demo-podzolic soils (TARGULIAN 1974). Horizontaljointing in deep horizons appears under colder climate, as for example, FEDOROFF has observed it in central Finnish Lapland. Horizontaljointing associated with rounded peds in upper horizons is found under even colder climates; BUNTING (personal communication) has noted it in the area of the Great Slave Lake (Canada) and BUNTING & FEDOROFF (1973) have described it in the Candian Arctic Archipelago. In Bois du Moulin, this pedological phase corresponds no doubt to a cold climate, even very cold, but the presence of a permafrost remains hypothetical since ice-wedges have not been observed. Association of siltans and coarse silt with a porosity pattern, such as in Bois du Moulin has not been observed in high Canadian arctic deserts (BUNTING & FEDOROFF 1973) but such an association is well developed in Lapland for example. Thus we think that during this phase the climate was cold, but also moist in Bois du Moulin.

MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS 4.2.1.4.

249

Weathering

As at Saint-Pierre-les- Elbeuf mineraloical indications of weathering are not striking. Muscovite tends to be most common and more weathered in the B2tb and B3tb horizons and relatively poor in the Ctxb. The heavy minerals do not vary significantly in type or abundance within the profile. The more resistant tend to predominate whereas the less stable (epidote and hornblende) occur in very minor amounts. Similarly microscopic examination could not improve upon field observations in trying to elucidate the process of rubefaction.

4.2.1.5.

Silty ground mass

As at Elbeuf, it is not a simple task to determine unequivocally the source and mode of deposition of the material. The original depositional fabric of the parental silty material has been protected from pedogenesis only in the B3tb and Ctxb horizons. But, from the above discussion of the concretions it is clear that they, at last in these horizons, are inherited from other soils and probably from the base of eluvial horizons; their size and specific gravity would rule out wind transport, except perhaps by saltation. Based on these two facts, we assume that the parental material of this paleosol has a colluvial origin; most of it probably originated from the truncation of'degraded" lessiv~ soils somewhat similar to this profile. In the upper part of this profile, however longer distance transport might have been possible because of the virtual absence of concretions and coarse sand. The fragipan (Ctxb horizon) results from a close packing of silt and clay particles which we think is a post-depositional effect of sediment in a thixotropic state. The presence of a permafrost could not be excluded. This fragipan had to have developed before the first phase ofilluviation (intermediate between a sol brun lessiv~ and a derno-podzolic soil) because the very first clay accumulations have not at all been disturbed.

5. COMPARISON BETWEEN SAINT-PIERRE-LES-ELBEUF MOULIN AND CONCLUSIONS

AND BOIS DU

To our knowledge, these are the only two exposures from the Paris Basin that have been investigated micromorphologically. Nevertheless in terms of the macromorphological characteristics, they are each typical for this type of setting: St.-Pierre-les-Elbeuf is similar to the Seine and Eure valleys (Mantes and Chaudon, BORDES 1954 and DEWOLF 1978) whereas Bois du Moulin is not unlike other plateaux exposures of Thimerais and FauxPerche. It is interesting to compare those parts of both exposures that are stratigraphically equivalent in order to appreciate regional versus local contrasts. In both exposures the sol brun lessiv6 phase is quite comparable, although later events are different. At St-Pierre-les-Elbeufthe soil is truncated and overlain by an early Weichselian silty colluvium on which is developed a mollic epipedon of a well developed grey forest soil. At Bois du Moulin, on the other hand, the lower Weichselian is marked by a distinct but continuous deterioration of the climate culminating in extensive freeze-thaw activity in a cold, moist climate; at the beginning of this stade, signs of only a weakly developed grey forest soil

250

FEDOROFF & G O L D B E R G

phase are present. Although we cannot correlate the Weichselian phases, it is clear that at St-Pierre-lesElbeuf, the early Weichselian is less complete, more eroded and noticeably drier than at Bois du Moulin. We ascribe this to a local manifestation of its valley setting although the specific reasons are not dear. The preservation of a continuous sequence at Bois du Moulin seems to relate to its plateau location, less subjected to colluvial truncation. As we have repeatedly stressed above, micromorphology of soils and especially paleosols reveals information not observable in the field or accessible by other techniques. For example, under the microscope, it is possible to observe in fine detail a variety of textural features (including their composition, sorting, colour, internal fabric) that can be related to different porosity patterns. All these elements can be used to construct a detailed history of the profile. Thus at Bois du Moulin, it was possible to observe vertical joints (shrink-swell) and vughs (bioactivity) associated with argillans of a typical sol brun lessiv6; later these were disrupted by horizontal jointing and silt accumulation was associated with a much colder climate. A final point that needs emphasis is that the position of a micromorphological feature within a profile or section is commonly not contemporaneous with that"unit". The siltans in the "B2tb" at Moulin, for example, clearly do not relate to the formation of that interglacial horizon but correspond to one of the last events in the degradational phase prior to burial by the late Pleistocene loess. Similarly, the three types ofargiUans in the"Eemian" soil of Elbeuf do belong to the three soil forming events present at that profile; these are separated by an unknown but distinct period of time. Thus enlightened with a relatively complete history of a buried paleosol we can use this information to improve our understanding of surface soils. For example in the Thimerais and Faux-Perche, the Bois du Moulin is commonly on the surface where most of textural accumulations and frost porosity patterns have been obliterated by a well developed hydromorphic Holocene phase.

ACKNOWLEDGEMENTS

Part of this research was made possible by a study grant to Paul GOLDBERG from the French Government, Ministry of Foreign Affairs. In addition, we would like to thank Professor SOMME (Universit6 de Lille) for his moral support in the field and to J.P. LAUTRIDOU for demonstrating and discussing the exposure at Saint-Pierre-les-Elbeuf. Finally this work could not have been accomplished without the fine thin sections prepared by P. GUILLORE and G. HEINTZBERGER. Comments by G. KUKLA are greatly appreciated.

REFERENCES

BABEL, U. (1964): Chemische Reaktionen an BodendiJnnschliffen. Leitz-Mitteilungen f'tir Wlssenschaft und Technik 1, 3, 12-14. BILLARD, A. & FEDOROFF, N. (1977): Interglacial-Periglacial pedogenetic cycle in northern Italy and France. Abstracts Xth INQUA Congress, Birmingham, 35. BORDES, F. (1954): Les limons quatemaires du bassin de la Seine. Arch. Inst. Paleont. humaine 26, 472 p. BOURDIER, F. (1969): Etude compar6 des d6pots quaternaires des bassins de la Seine etde la Somme. Bull. Inf. G6ol. Bass. Paris 21, 169-231. BOURDIER, F. & LAURIDOU, J.P. (1974): Les grands traits morphologiques et structuraux des r6gions de la Somme et de la Basse-Seine. Bull. A.F.E,Q. 40-41, 109-115.

MICROMORPHOLOGY: PLEISTOCENE PALEOSOLS

251

BRONGER, A. (1978): Climaticsequenes of steppe soils from Eastern Europe and the USAwith emphassis on the genesis of the "argillic" horizon. CATENA 5, 33-51. BREWER, R. (1964): Fabric and mineral analysis of soils. Wiley, New York, 470 p. BUNTING, B. & FEDOROFF, N. (1973): Micromorphological aspects of soil development in the Canadian High Arctic. In: G.E. Rutherford (ed.), Soil microscopy. The Limestone Press, Kingston Ontario, 350-365. CAILLIER, M. (1977): Etude chronos6quentielle des sols sur terrasses alluviales de la Moselle. Gen~se et 6volution des sols lessiv+s glossiques. Th6se P6dologie. Universit6 de Nancy I, 87 p. DEWOLF, Y. (1978): Contribution a l'6tude des marges occidentales du Bassin de Paris. Etude g6omorphologique. Th~se Universit~ de Paris VII, to be published in M~moires et documents C.N.1L S. Paris. EIMBERECK-ROUX, M. (1976): Les sols lessiv6s glossiques ~tpseudogly de l'Argonne m6ridionale. Th~se P6dologie. Universit6 de Dijon, 107 p. FEDOROFF, N. (1967): Un exemple d'application de la micromorphologique/~ l'6tude des pal6osols. Bull. A.F.E.Q. 12, 193-209. FEDOROFF, N. (1973): Application des techniques nouvelles de la p6dologie a l'6tude des paleosols. Le Quaternaire. Geodynamique, Stratigraphie et Environnement. Comit6 national francais de I'INQUA. 200-201. FEDOROFF, N. (1979): Organisation du sol/~ l'echelle microscopique. In: M. Bonneau & B. Sohchier (ed)., P6dologie (vol. 2), Masson, Paris, 251-265. GUILLIORE, P. (1980): Methode m+canique de fabrication en serie des lames minces. Departement des sols. Ins. Nat. Agro. P-G, 22 p. JAMAGNE, M. (1973): Contribution/l l'6tude p6dogenotique des formations loessiques du Nord et la France. Th6se Fac. Sci. agronomiques Gembloux (Belgique), 445 p. JONGERIUS, A. & HE1NTZBERGER, G. (1963): The preparation of mammoth-sized thin sections. Soil survey papers 1. STIBOKA, Wageningen, 37 p. KOWALINSKI, S. (1969): Soils of Southwestern Poland. Excursion guide book. IIIrd Inter. Working Meeting on Soil Micromorphology, Wroclaw, Poland, 130 p. LADRIERE, J. (1890): Etude stratigraphique des terrains quaternaires du Nord de la France, Ann. Soc. g6ol. Nord 18, 93-149. LAUTRIDOU, J.P. (1974): La sequence loessique sequanienne du WiJrm a Saint-Pierre-les-Elbeuf. Bull. A.F.E.Q. 40-41,237-241. LAUTRIDOU, J.P. (1977): Loess et sables de la Basse-Seine: lithostratigraphie et chronostratigraphie. Bull. Soc. g6ol. Normandie 64, 81-91. LAUTRIDOU, J.P., MASSON, M., PAEPE, 1L, PUISSEGUR" J.J. & VERRON, G. (1974a): Loess, nappes alluviales et tuf de Saint-Pierre-les-Elbeuf, pros de Rouen; les terrasses de la Seine de Muids ~ Caudebac. Bull. A.F.E.Q. 40-41, 193-201. LAUTRIDOU, J.P. & SOMME, J. (1974b): Les loess et les provinces climato-s6dimentaires du Pleistoc6ne sup6rieur dans le Nord-Ouest de la France: essai de correlation entre le Nord et la Normandie. Bull. A.F.E.Q. 40-41,237-241. MICHEL, J.P. (1969): Historique et evolution des recherches sur les loess de la r~gion parisienne. Mem. hors serie, Soc. g+ol. France 3-5, 33-36. SOIL SURVEY STAFF (1975): Soil taxonomy. U.S. Dept. of Agriculture. Soil conservation service, Agriculture handbook 436, 754 p. TARGULIAN, V.O., BIRINA, A.G., KULIKOV, A.K., SOKOLOV& T.A. & TSELISHCHEVA, L.K. (1974): Arrangement, composition and genesis of sod-pale-podzolicsoil derived from mantle loams. Xth Inter. Congress of Soil Sci. Moscow, 34 p. VAN VLIET-LANOE, B. (1976a): Traces de segregation de glace en lentilles associ6es aux sols et ph6nom~nes p6riglaciaires fossiles. Biul. Peryglacjalny26. VAN VLIET-LANOE, B. (1976b): Correlation entre la presence de charbon de bois au sommet de pal6osols et les d6gradations climatiques. P6dologie, XXVI-1, 97.

Anschrift der Autoren: N. Fedoroff, Department des Sols, I.N.A. P-G 78850 Grignon, France P. Goldberg, Institute of Archaeology, Hebrew University Jerusalem. Israel