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.Catena 35 1999 99121

Facies and microfacies of slope deposits

Pascal Bertran ), Jean-Pierre Texier

Institut du Quaternarie, UMR 9933 du CNRS, Uniersite de Bordeaux 1, Aenue des Facultes, F-33405 Talence, Bordeaux, France

Received 5 June 1997; accepted 15 May 1998

Abstract

This paper reviews the facies and microfacies of the main types of slope deposits. Lamination

and sorting, when preserved, are good evidence for overland flow. Features due to deformation .folds, boudins, coatings and tails due to the rotation of clasts are associated with the early stages

of deformation in earth slides. Other mass-movements such as debris flows, rock avalanches, earth

flows, and to a certain extent, dry grain flows may be characterised by similar microscopic facies,typically a poorly sorted, porphyric material. Porosity gives evidence for both liquefaction debris. .flows and frost-induced mass-movement solifluction . However, this criterion is thought to be

irrelevant for fossil deposits owing to the rapidity and scale of post-depositional changes wetting.and drying, freezing and thawing, bioturbation . At present, only a detailed analysis of both the

macro- and micromorphological characteristics of the deposits allows an accurate identification of

past slope dynamics. q1999 Elsevier Science B.V. All rights reserved.

Keywords: Micromorphology; Slope deposits; Rockfall talus; Grain flow; Rock avalanche; Debris flow;

Periglacial solifluction; Earth slide; Earth flow

1. Introduction

Recently, work has focused on the transport processes and sedimentology of slope

deposits from various active depositional environments to improve the genetic interpre-tation of fossil deposits. Whereas the facies are fairly well documented for example, see

.Van Steijn et al., 1995 , the investigations on associated microfacies are sparse and .concern mainly solifluction review in Huijzer, 1993 . The aim of this paper is to present

a comparative review of the sedimentological characteristics that typify the main slope

mechanisms with a special reference to micromorphology. The results obtained so far

)

Corresponding author. Fax: q33-5-56-84-84-51

0341-8162r99r$20.00 q 1999 Elsevier Science B.V. All rights reserved. .P I I : S 0 3 4 1 - 8 1 6 2 9 8 0 0 0 9 6 - 4


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( )P. Bertran, J.-P. TexierrCatena 35 1999 99121100

Fig. 1. Classification of slope processes.

are variable and probably not always representative. Therefore, this paper should be

considered a preliminary report. .Results have been obtained from 1 modern slope deposits from the French Alps, the

.Bolivian Andes, southwestern France and Morocco, 2 Pleistocene deposits from

southwestern France and Portugal. . .The classification used here for transport processes Fig. 1 is based on Varnes 1978

.and Pierson and Costa 1987 , reflecting the style and velocity of movement, and the

grain-size and rheological properties of the moving material. Modifications have been

made to accommodate certain criteria useful for facies analysis.

Large thin sections were prepared from blocks of sediments vacuum-impregnated by

.a synthetic resin according to the method of Guillore 1980 . The terminology is adapted .from Bullock et al. 1985 .

2. Results

2.1. Low sediment concentration, interstitial fluidsair: rockfall deposits

In this case, rock clasts fall from a cliff as individual particles or highly dispersed

assemblages and come to the halt after rolling, bouncing andror sliding onto the talus.On the steep upper segment of the talus, the debris then experiences rapid creep due to

the impact of falling stones, washing, temperature variations and trampling by animals .Rapp, 1962; Gardner, 1979; Francou, 1991 .

Unmodified rockfall talus is characterised by a coarse clast-supported to openwork .structure and the lack of, or a very crude, stratification Francou and Hetu, 1989 . This

kind of deposit usually constitutes only part of screes, and is interlayered with

mass-movement deposits such as debris and grain flows. The clast fabric is random or

displays a weak preferred orientation parallel to the slope, particularly near the apex of

.the talus Caine, 1967; McSaveney, 1971; Bertran et al., 1997 .Very few data on microfacies are currently available, mainly because of the coarse

texture and the lack of cohesion of the material. We have been able to obtain thin

sections from an active talus developed in limestones in southwestern France because of


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( )P. Bertran, J.-P. TexierrCatena 35 1999 99121 101

rapid cementation. Under a microscope the scree appears as a loose accumulation ofpoorly sorted clasts with a monic to chitonic crf related distribution clean clasts or

. .clasts coated by clay- to sand-sized particles Fig. 2 . We do not see any textural

features despite the strong porosity, probably because of the high rate of talus accretion.

2.2. Low to intermediate sediment concentration, interstitial fluidswater or waterqfine

particles: oerland flow deposits

The thickness of water flowing on slopes is generally some millimetres and the

conditions of particle transport are significantly different from those in fluvial environ-ments. This results in poor separation of bedload and suspended particles Moss and

. .Walker, 1978 . Additional processes may interact with the flow: i raindrop impacts . splash favour the detachment of particles and disturb the flow Moeyersons and De

. .Ploey, 1976 ; ii sliding, fall or liquefaction of the walls of rills and gullies are .responsible for large sporadic sediment inputs De Ploey, 1971, 1974; Govers, 1987 ;

.iii vegetation induces a forced sedimentation of particles and flow concentration .Faury, 1990 . As a consequence, the sediment load may quickly change within a single

flow event.

Overland flow deposits are usually moderately sorted, with marked spatial grain-size .variation. The clast fabric ranges from random to weakly oriented Bertran et al., 1997 .

Deposits typically show laminated lenses interstratified with massive deposits. The

lenses with obvious lamination and evidence of sorting represent areas where vertical or

lateral accretion due to dilute flows is rapid. Such deposits may be extensive, as for

.Fig. 2. Loosely packed limestone clasts in a rockfall talus Lousteau, France . PPL.


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example downslope of large gullies. Massive deposits represent areas of hyperconcen- .trated flow accumulation Lowe, 1982; Wells and Harvey, 1987 or areas where

accretion is slow and post-depositional perturbations such as splash, bioturbation, and

freezing and thawing are strong. According to the morphology of the watershed, the

grain-size and the mechanical properties of the eroded material, hyperconcentrated flows

may be dominant and build thick faintly bedded cones.Compaction and pedogenic modification tend to erase lamination and bedding within

fine-grained deposits. Gravel lenses and gravel sheets corresponding respectively to

former rills and residual pavements, or coarse-grained fans are often the only features

which survive. Agricultural practices also destroy the sedimentary features and create .massive homogeneous deposits Allen, 1992 .

In thin sections the laminated materials appear as juxtaposed mineral grains and .rounded to subangular soil aggregates Fig. 3 . The relative abundance of soil aggregates

.Fig. 3. Laminated overland flow deposits in a small colluvial cone Vaise, France . Clay and silt particles

occur as sand-sized aggregates juxtaposed to quartz grains as well as secondary infillings in packing voids.

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depends mainly on the structural stability of the parent material. Overland flow deposits

derived from the erosion of clayey or humic soils may be composed predominantly of . .aggregates Alberts et al., 1980 Fig. 4 , formed from faecal pellets of the mesofauna in . humic soils Mucher et al., 1972 . The crfrelated distribution ranges from monic clean

. . sand or enaulic sand and soil aggregates to chitonicgefuric fine fraction forming

.coatings and bridges between grains . The latter comes from clay and silt infillingsduring decreasing flow stages or post-depositional modifications. Typically, such beds

alternate with massive, poorly sorted layers with a chitonic to single-spaced porphyric .i.e., matrix-support crf related distribution. They represent former structural crusts . Valentin and Bresson, 1992 , overland flowqsplash deposits Mucher and De Ploey,

. .1977 with frequent scattered flakes of silt laminae Fig. 5 , or hyperconcentrated flow .deposits Fig. 6 .

Anthropogenic colluvium typically displays a biological porosity together with .fragments of reworked older soils, charcoal and ferruginised organic debris manure

.Fig. 4. Soil aggregates in overland flow deposits derived from the erosion of an argillic horizon Morocco .

PPL.


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.Fig. 5. Poorly sorted layer interstratified with laminated deposits from a colluvial cone Vaise, France .

Disrupted silty flakes are scattered within the whole layer, which may reflect overland flowqsplash action.

PPL.

.MacPhail, 1992 . Sandy silt intercalations due to internal slaking and dusty clay .coatings are also characteristic of these deposits Jongerius, 1970; MacPhail, 1992 .

2.3. High sediment concentration, interstitial fluidsair: grain flows and rock aalanches

2.3.1. Grain flows

Grain flows correspond to dry cohesionless flows of debris on very steep talus slopes .Rapp, 1962; Whitehouse and McSaveney, 1983 . They are characterised by weakly

.developed lateral levees, fine-grained channel-bottom deposits sieved material and anelongated frontal lobe. Accumulation of grain flows typically produces stratified de-

posits with inverse grading, strong imbrication of clasts and slope-parallel preferred .orientation Hetu et al., 1995; Bertran et al., 1997 . Stratification and preferred orienta-

tion tend to disappear in the distal part of the talus, which is built by the stacking of

coarse lobes and by clasts falling in isolation. The dispersive pressures and the sieving


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Fig. 6. Massive sandy layer with an abundant fine matrix deposited by an hyperconcentrated flow over a .colluvial cone Vaise, France . PPL.

effect are responsible for the inverse grading and the fine-grained basal sole layer, which

may be some millimetres to several centimetres thick.

Grain flow deposits lack cohesion and are unsuitable for the manufacture of thin

sections. The microfacies in Pleistocene sole layers slightly indurated by secondarycarbonates are characterised by sand-sized grains scattered within a homogeneous fine

. matrix open porphyric crf related distribution with a crystallitic b-fabric randomly. .distributed small crystals of calcite Fig. 7 . Secondary infillings of silt form thick

.microlaminated coatings at the top of the sole layer Fig. 8 . A number of clast-sup-

ported fine-grained lenses which have been sampled are composed only of microlami-

nated accumulations of sand and silt. They are probably not directly related to the

emplacement of grain flows but correspond to banded accumulations formed by water

percolating through highly permeable deposits.

2.3.2. Rock aalanches 3 3.Rock avalanches are flows of very large volumes of rock material )10 m which

.disintegrate during the process of sliding or collapse Hsu, 1975; Melosh, 1987 .


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Fig. 7. Massive poorly sorted layer corresponding to the basal sole of a grain flow on a steep talus Belesten,.France . PPL.

Deposits are fan- or tongue-shaped depending on the underlying topography Eisbacher,.1979 . Most accumulation occurs in the distal part of the avalanche, which frequently

.displays a hummocky surface Goguel and Pachoud, 1972; Siebert, 1984 and a steepfront. This process may be of major importance for the morphogenesis of regions with

.high seismic activity or in paraglacial areas Gardner, 1979; Whalley et al., 1983 .

The main diagnostic criteria are the presence of megablocks, a very angular shape of

the clasts, often with a jigsaw fit, an inverse grading of the clasts, a basal layer with an

abundant cataclastic fine matrix and often distinct sliding surfaces. Thrusting planes mayappear in the frontal area of the avalanche Yarnold, 1993; Topping, 1993; Blair and

.McPherson, 1994; Bertran, 1996 . The texture varies according to the lithologies

involved so that rock avalanche deposits range from clast-supported, highly porous

debris to matrix-supported, slurry-like diamicton.Few data on microfacies of rock avalanches are currently available. Thin sections

.from a recent rock avalanche derived from a cliff fall in limestones and marls show: i . . .in situ fractured clasts jigsaw breccia not visible in the field Fig. 9 , ii a cataclastic


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.Fig. 8. Microstructure of the basal sole of a grain flow deposit Belesten, France . Dots correspond to the basal

silty sand layer and black areas correspond to postdepositional silt accumulations.

sandy silt matrix showing limited mixing so that there are juxtaposed areas of different . .colour and grain-size Fig. 10 , iii abundant plant fragments derived from the forest

. .which previously covered the talus, and iv a high fissural porosity Bertran, 1996 . The

microfacies of the basal sheared layer is significantly different and is characterised by a

more homogeneous matrix and the lack of a jigsaw fit of the clasts. The b-fabric is

crystallitic. Thick secondary silt accumulations are visible in some voids and are

attributed to the strong run-off activity which followed the cliff collapse.

.Fig. 9. In situ fractured clast in a rock avalanche Claix, France . PPL.


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.Fig. 10. Angular clasts scattered in a silty matrix from a rock avalanche Claix, France . Note the juxtaposition

of areas with distinct colour and grain size and the small organic fragments. PPL.

2.4. High sediment concentration, interstitial fluidswater or waterqfine particles:

debris flows, solifluction, earth slides and flows

2.4.1. Liquefaction of the debris: debris flows

Debris flows correspond to flows of liquefied sediments. They are initiated by the

removal of loose debris accumulated in gullies or by the transformation of landslides .into a slurry following a rainstorm Johnson and Rodine, 1984; Blijenberg, 1993 .

The accumulation of lateral levees together with gullying in the upper segment of thetalus lead to the formation of crudely stratified deposits characterised by coarse

openwork lenses channel lags, external part of levees, pavements due to post-deposi-. tional run-off activity in a diamicton Bertran and Texier, 1994; Van Steijn et al.,

.1995 . Further downslope, stacking of terminal lobes builds diamictic deposits with acrude horizontal bedding and frequent buried paleosols and laminated overland-flow

deposits. These facies characterize cones formed by channelised debris flows in large .gullies Hubert and Filipov, 1989 . The clast fabric shows weak to moderate preferred


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orientation, the highest strength being found in the lateral levees Nieuwenhuijzen and.Van Steijn, 1990; Bertran et al., 1997 .

Some data on microfacies in debris flows have already been published by Van . . .Vliet-Lanoe 1985 , Bertran 1993 and Bertran and Texier 1994 . These only concern

.cohesive debris flows Lowe, 1982 with significant amounts of silt and clay. Recent

deposits show a homogeneous fine-grained matrix, a porphyric crf relative distribution .and an undifferentiated or crystallitic b-fabric Fig. 11 . Plant debris may be abundant.

Voids are typically poorly interconnected irregular to rounded vesicles which reflect .liquefaction Fig. 12 . This porosity may rapidly disappear because of pedogenetic

activity due to soil fauna and flora, drying and wetting and freezing and thawing. When

debris flows are derived from sliding of clayey soils, remains of the soil may be .preserved as small rounded clay lumps or as mineral grains coated by clay Fig. 13 .

.According to Van Vliet-Lanoe 1985 , the debris flows due to liquefaction of slopedeposits in periglacial environments display silt cappings disrupted and scattered within

the matrix of the flow. The microfacies are close to those observed in the dense basalsole of grain flows as well as those from the lower layer of the rock avalanches.

2.4.2. No or partial liquefaction of the debris, no distinct sliding plane: periglacial

solifluctionSolifluction is a slow downslope displacement of sediments velocity from 1 to 10 cm

. .a year . It involves the combined effects of two elementary processes: 1 frost-creep .due to alternation of frost heaving growth of ice lenses and resettlement of the soil

. .melting of ice and 2 gelifluction caused by high porewater pressures occurring

during thaw consolidation and resulting in loss of soil strength and subsequent flow. The

Fig. 11. Poorly sorted debris with an abundant fine matrix in a debris flow derived from schists Vallon.Laugier, France . PPL.


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.Fig. 12. Typical vesicular porosity in a muddy debris flow Coulaures, France . PPL.

respective role of the two processes varies according to granulometry Harris et al.,.1993, 1995 and the amount of available water determined by the occurrence or absence

.of a water table at depth Benedict, 1970; Matsuoka, 1996 . Movement is distributed

throughout the whole soil mass and there is no distinct sliding plane.

.Fig. 13. Silty clay coating around a clast in a debris flow originating from an earth slide Aubagne, France .

PPL.


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Solifluction landforms comprise tongue-shaped lobes, terraces and sheets. The related .deposits display two main lithofacies: i a massive or crudely stratified diamicton,

stratification being sometimes marked by buried organic soils nonsorted turf-banked

lobes, sheets or terraces: Benedict, 1970; Matthews et al., 1986; Repelewska and Pekala,. .1993 , ii stratified deposits composed by alternating diamictic and openwork beds

.stone-banked lobes and sheets: Francou, 1990; Bertran et al., 1995 . Among all themacroscopic diagnostic criteria proposed by the authors, the most reliable are a

well-developed preferred clast orientation parallel to the slope Brochu, 1978; Nelson,.1985; Bertran et al., 1997 and the occurrence of a platy structure together with

deformation features like folds or downslope overturning of strata Benedict, 1970; Van.Vliet-Lanoe and Valadas, 1983; Van Steijn et al., 1995 .

The microfacies of solifluction deposits have been the subject of a number of studiesHarris and Ellis, 1980; Harris, 1981, 1987; Van Vliet-Lanoe, 1982, 1985, 1987; Bertran

. .et al., 1995 . Platy structures due to the growth of ice lenses are widespread Fig. 14 .

They are replaced by granular structures in the upper part of clay-rich soils. The melting

.Fig. 14. Platy structure due to ice lensing in the upslope part of a solifluction lobe La Cumbre, Bolivia .

Depth of the samples5 cm. PPL.


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of ground ice and summer rainfalls in areas where the vegetation cover is lacking orsparse may lead to oversaturation and to the collapse of the soil aggregates Harris,

. 1983; Van Vliet-Lanoe, 1985 . Subsequently, a surficial vesicular layer is formed Fig..15 . Features due to eluviation become very important and within a solifluction lobe, a

.downslope gradient of eluviation usually develops Bertran et al., 1995 . The distal part

.exhibits well-developed silt cappings on gravels and a washed sandy matrix Fig. 16 .Fine particles are translocated and accumulate at depth, particularly near textural

.discontinuities Van Vliet-Lanoe, 1985; Bertran, 1993; Bertran et al., 1995 . When ice .lensing interferes with accumulation, silts are incorporated in the whole matrix Fig. 17 .

In areas where freezing is shallow, for example in tropical mountains, the translocated

clay and silt form microlaminated coatings on voids. The b-fabric is undifferentiated or

crystallitic. Sometimes it may be weakly striated due to oriented clay domains around .aggregates Van Vliet-Lanoe, 1985 .

Fig. 15. Vesicular structure due to the partial collapse of cryogenic aggregates in a solifluction lobe depth of. . samples5 cm La Cumbre, Bolivia . Because of the lack of liquid precipitation in the area Bolivia, 5200

.m , this microstructure does not reflect crusting due to the impact of raindrops but likely indicates an .oversaturation of the soil due to water release following the melting of ice lenses gelifluction . PPL.


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.Fig. 16. Silt coatings on gravels in the frontal zone of a solifluction lobe same lobe as Figs. 14 and 15 .

Because of a stronger eluviation and a subsequent lower frost susceptibility of the soil than in the upslope zone .of the lobe, velocity is about 3 times weaker frost-creep .

.Van Vliet-Lanoe 1982, 1985, 1987 has proposed a classification of microstructures

in relation to the type of movement. Frost-creep should be characterized by a platy .structure and a well-sorted squeletan initial frost-creep or by assymetric aggregates

.with cappings of fines which thicken downslope frost-creep sensu stricto . Gelifluc-

tion should generate rounded aggregates or mineral grains coated by microlaminated

silts owing to the rotation they undergo during motion. Several objections can be made

to this proposal. .1 Frost-creep and gelifluction are intimately associated in natural solifluction

landforms. Their relative importance depends on several factors and particularly on

granulometry. Laboratory experiments show that gelifluction is only responsible for a

.minor part of movement in sandy sediments Harris, 1993; Harris et al., 1995 .Nevertheless, silt coatings around grains are typical of the sandy textured, slowly

moving frontal zone of solifluction lobes. They are also often mentioned in Pleistocene .soliflucted granitic sands Van Vliet-Lanoe and Valadas, 1983; Bertran, 1989 . The

adoption of Van Vliet-Lanoes classification would mean that the microstructure ob-served in sands would imply gelifluction, which is in direct contradiction to experimen-

tal data and field measurements. .2 The rate of particle rotation during flow depends on its shape and the amount of

.displacement Ildefonse et al., 1992; Jesek et al., 1996 rather than on the speed of

movement. Accordingly, the rounded or slightly elongated grains in the samples studied .were embedded with silt whereas flattened grains for example schistose clasts were

only capped on their upper surface. Both features may be associated within a single thin

section. Therefore, coatings around grains would be specifically related to rounded or


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.Fig. 17. Microstructures in a stone-banked solifluction lobe La Mortice, France .

subrounded clasts and reflect strong eluviation together along with displacement during

solifluction sensu lato.

Solifluction structures are commonly well preserved in relict deposits, but frequently

differ from those observed in present-day active environments. This is due to the

superimposition of successive frost structures during the progressive accumulation of

sediments. For example, vesicular structures are rare in relict deposits and the rounded

cryogenic aggregates either are integrated into lenticular aggregates or disappear owingto compaction. Likewise, it is generally difficult to identify simple cryopedogenic

profiles characterized by an increase in thickness of the platy structure and a fining of

textural pedofeatures with depth.


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.Fig. 18. Brecciated microstructure from an earth slide in varved silts Trieves, France . PPL.`

2.4.3. No or partial liquefaction of the debris, distinct sliding planes: earth slides and

earth flows

Earth slides and flows are mass displacements where movement concentrates along

basal and internal shear planes. The sliding mass breaks into a few blocks or disinte-

.Fig. 19. Ductile deformation of silt and sand laminae in an earth slide Trieves, France . Note the tails`indicating the rotation of the coarse grain. PPL.


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grates more completely and flows slowly. Earth slides are triggered by increase of shear

stress due to factors such as suppression of basal support or overloading, or by decrease

in soil strength brought about by high porewater pressures, weathering, and slow creep.

In periglacial environments, sliding replaces classic solifluction in clayey materials .Harris, 1987; Hutchinson, 1991 . Slides are also promoted by thawing of ice-rich

permafrost skinflows, active layer glides: Lewkowicz, 1992; Harris and Lewkowicz,.1993a,b .

The main field features which typify such movements in sections are the occurrence

of branching slickensided slip planes, brecciated structure composed of sedimentaryclasts in a homogeneized matrix Hutchinson, 1970; Brunsden, 1984; Harris and

.Lewkowicz, 1993a,b or deformation of the former bedding in the form of folds,

stretching or boudinage. When disintegration of the mass is weak, the main diagnostic .criterion is often the upslope tilting of the bedding. Harris and Lewkowicz 1993a,b

have also described injections and flame-like structures due to liquefaction at the bottom

of active layer glides on permafrost.

.Fig. 20. Scattered silt and clay lumps in a homogeneous matrix from an earth flow Trieves, France . This`sample comes from a secondary flow developed on the top of the earth slide shown in Figs. 15 and 16. PPL.


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.Available microscopic data about earth slides are scarce. Blondeau 1976 reports

laboratory experiments on the microstructure of soil samples under a shear stress. Slow

creep gives rise first to decrease in size of the voids, then to their complete closure. If

the strain leads to failure, the intergranular bonds are broken, the clay domains orientate

and microcracks appear. The strength of clay orientation increases with time from the

application of the strain. In the case of a heavy load leading to rapid failure, the strengthof clay orientation is weak and microcracks form quickly.

Thin sections in clay-rich earth slides reveal a heterogeneous material characterizedby sedimentary clasts scattered in a dense matrix open porphyric crf related

.distribution , with great variability of the deformation features at 1 cm scale: areas with . .angular to sub-rounded sedimentary clasts rigid deformation Fig. 18 are juxtaposed to

.areas with boudinaged and stretched clay lumps ductile deformation . The b-fabric

ranges from undifferentiated or crystallitic to striated in sheared bands. Fissures related

to post-depositional wetting and drying are commonly observed.

Brecciated microstructure is characteristic of earth slides where deformation is weak.Initial stages of deformation are also marked by features formed due to the rotation of

.rigid inclusions such as tails around gravels Fig. 19 . Silty accumulations related to

drainage through fissures during sliding may also be deformed. In earth flows by

contrast, the homogenization of the material is more important and only few sedimentary .clasts survive within the matrix Figs. 20 and 21 .

.Skempton et al. 1991 describe similar microfacies in clayey heads derived from

weathered argillites in England. The smooth and undulating sliding planes exhibit a

striated b-fabric. In the remaining earth slide mass, argillite clasts and small patches of

.oriented clay are randomly distributed throughout speckled b-fabric . .In active-layer glides of the Canadian Arctic, Harris and Lewkowicz 1993b

observed stretchings, boudins and folds in bedded deposits. Deformation concentrated

mainly in thin undulating sand layers in which grains are oriented parallel to the shear

plane. The upper part of the active layer shows a prismatic to a polyedric structure due

to cryodesiccation.

Fig. 21. Schematic microstructures in earth slides, earth flows and debris flows. Black: porosity, grey:

undisturbed clay, dashes: oriented clay domains in the remoulded matrix.


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3. Discussion and conclusion

The foregoing descriptions clearly indicate that different slope processes can generate

similar microfacies. For example, a massive poorly sorted material with an open

porphyric crf related distribution and a crystallitic b-fabric may typify debris flow

deposits, rock avalanches and earth flows, as well as the basal matrix-rich layer of grain

flow deposits. It is therefore necessary to use several criteria such as macro- and

microscopic structures and granulometry for reliable diagnosis of the studied sediments.Features due to deformation folds, boudins, coatings and tails due to the rotation of

. .clasts are associated with the early stages of deformation earth slides . However, they

probably also characterise other kinds of materials as for example tills which have been

deposited under a similar strain pattern.

Porosity is a very unstable feature and thus can rarely be considered as a reliable

criterion. Cryogenic porosity may be an exception. This forms in fine-textured frost-sus-

ceptible sediments and is caused by compaction and cryodesiccation of soil aggregates .during growth of ice lenses Van Vliet-Lanoe, 1985; Harris, 1987 . Nevertheless,

cryogenic microstructures may develop rather quickly and their use as an indicator of

solifluction remains questionable. For example, a platy microstructure similar to that

found in solifluction deposits was observed within the levees and the lobe front of a stillrecognizable debris flow in the French Alps, after ca. 40 years of frost action.

Post-depositional processes can deeply alter original microstructures and thus severely

complicate analysis and dynamic interpretation. Most of the time, sedimentation is

discontinuous and numerous processes such as biologic and anthropogenic activity,

freezing and thawing, run-off and surficial creep can modify the deposits in varyingdegrees between successive sedimentary events. When modifications are important,

macroscopic scale evidence seems to provide the most useful evidence for interpretation

of slope deposits. Indeed, sedimentary structures such as the morphology and spatial

distribution of gravel lenses or the sorting of pebbles are often preserved and can be

used most effectively for this purpose.

We believe that there is a need for much more information about the associated facies

and microfacies of contemporary slope deposits. Likewise, there is a need for a

programme of laboratory experiments designed to better understand the significance and

the formative conditions of the microstructures.

References

Alberts, E.E., Moldenhauer, W.C., Foster, G.R., 1980. Soil aggregates and primary particles transported in rill

and interrill flow. Soil Science Society of America Journal 44, 590595.

Allen, M.J., 1992. Products of erosion and the prehistoric land-use of the Wessex chalk. In: Bell, M., .Boardman, J. Eds. , Past and Present Soil Erosion, Archaeological and Geographical Perspectives. Oxbow

Books, Oxford, Monograph, 22, pp. 3752.

Benedict, J.B., 1970. Downslope soil movement in a Colorado alpine region. Arctic and Alpine Research 2,

165226.

Bertran, P., 1989. Evolution de la couverture superficielle depuis le dernier interglaciaire. Etude micromor-

phologique de quelques coupes types du sud de la France. These Universite de Bordeaux I, 210 pp.`


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Bertran, P., 1993. Deformation-induced microstructures in soils affected by mass movements. Earth Surface

Processes and Landforms 18, 645660.Bertran, P., 1996. Sedimentologie dune avalanche rocheuse survenue en fevrier 1995 a Claix Alpes `

. .francaises . Quaternaire 7 23 , 7583.Bertran, P., Texier, J.P., 1994. Structures sedimentaires dans un cone de flots de debris Vars, Alpes francaises

.meridionales . Permafrost and Periglacial Processes 5, 155170.Bertran, P., Francou, B., Texier, J.P., 1995. Stratified slope deposits: the stone-banked sheets and lobes model.

.In: Slaymaker, O. Ed. , Steepland Geomorphology. Wiley, Chichester, pp. 147 169.

Bertran, P., Hetu, B., Texier, J.P., Van Steijn, H., 1997. Fabric of subaerial slope deposits. Sedimentology 44,116.

Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on

morphology, hydraulic processes, sedimentary processes, and facies assemblages. Journal of Sedimentary .Research A 64 3 , 450489.

Blijenberg, H.N., 1993. Modelling of debris flows. Temporal occurrence and forecasting of landslides in the .European Community. In: Casale, R., Fantechi, R., Flageollet, J.C. Eds. , Final Report, European

Commission, Science and Research Development, pp. 161198.

Blondeau, F., 1976. Rapport general. Stabilite des talus. Versants naturels. Bulletin de Liaison des Labora- toires des Ponts et Chaussees, no. special II, pp. 6576.

Brochu, M., 1978. Disposition des fragments rocheux dans les depots de solifluxion, dans les eboulis de gravite et dans les depots fluviatiles: mesures dans lEst de lArctique nord-americain et comparaison avec dautres regions du globe. Biuletyn Peryglacjalny 27, 3551.

.Brunsden, D., 1984. Mudslides. In: Brunsden, D., Prior, D.B. Eds. , Slope Instability. Wiley, Chichester, pp.

365418.

Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U., 1985. Handbook for Soil Thin

Section Description. Waine Research Publications, Wolverhampton, 152 pp.

Caine, N., 1967. The texture of talus in Tasmania. Journal of Sedimentary Petrology 37, 796803.

De Ploey, J., 1971. Liquefaction and rainwash erosion. Zeitschrift fur Geomorphologie 4, 491496.De Ploey, J., 1974. Mechanical properties of hillslopes and their relation to gullying in Central semi-arid

Tunisia. Zeitschrift fur Geomorphologie N.F. 21, 177190, Suppl.-Bd. .Eisbacher, G.H., 1979. Cliff collapse and rock avalanches sturzstroms in the Mackenzie Mountains,

northwestern Canada. Canadian Geotechnical Journal 16, 309334.

Faury, O., 1990. Lerosion anthropique actuelle sur les hautes chaumes des Monts du Forez: mesures de suivi.Bulletin Rhodanien de Geomorphologie 2526, 4565.

Francou, B., 1990. Stratification mechanisms in slope deposits in high subequatorial mountains. Permafrost

and Periglacial Processes 1, 249263.

Francou, B., 1991. Pentes, granulometrie et mobilite des materiaux le long dun talus deboulis en milieu alpin. Permafrost and Periglacial Processes 2, 175186.

Francou, B., Hetu, B., 1989. Eboulis et autres formations de pente heterometriques. Contribution a une `terminologie periglaciaire. Notes et Comptes-rendus du groupe de travail Regionalisation du Periglaciaire,

XIV, pp. 11144.Gardner, J.S., 1979. The movement of material on debris slopes in the Canadian Rocky Mountains. Zeitschrift

.fur Geomorphologie N.F. 23 1 , 4557.Goguel, J., Pachoud, A., 1972. Geologie et dynamique de lecroulement du Mont Granier dans le massif de la

Chartreuse, en novembre 1248. Bulletin du Bureau des Recherches Geologiques et Minieres, 2e serie, III 1, ` 2938.

Govers, G., 1987. Spatial and temporal capacity of thin flows in relation to rill erosion. Catena 12, 3549.

Guillore, P., 1980. Methode de fabrication mecanique et en serie des lames minces. Institut National dAgronomie, Paris-Grignon, 22 pp.

Harris, C., 1981. Microstructures in solifluction sediments from South Wales and North Norway. Biuletyn

Peryglacjalny 28, 221226.Harris, C., 1983. Vesicles in thin sections of periglacial soils from North and South Norway. Proceedings of

the 4th International Conference on Permafrost, Fairbanks, AK, pp. 445449. .Harris, C., 1987. Solifluction and related deposits in England and Wales. In: Boardman, J. Ed. , Periglacial

Processes and Landforms in Britain. Cambridge Univ. Press, Cambridge, pp. 209223.


22/23


Harris, 1993.

Harris, C., Ellis, S., 1980. Micromorphology of soils in soliflucted materials, Okstindan, Northern Norway.

Geoderma 23, 1129.

Harris, C., Lewkowicz, A.G., 1993a. Form and internal structure of active-layer detachment slides, Forsheim

Peninsula, Ellesmere Island, Northwest Territories, Canada. Canadian Journal of Earth Sciences 30,

17081714.

Harris, C., Lewkowicz, A.G., 1993. Micromorphological investigations of active-layer detachment slides,Ellesmere Island, Canadian Arctic. Proceedings of the 6th International Conference on Permafrost, Beijing,

China. S. China Univ. of Technology Press, pp. 232237.

Harris, C., Gallop, M., Coutard, J.P., 1993. Physical modelling of gelifluction and frost-creep: some results of

a large-scale laboratory experiment. Earth Surface Processes and Landforms 18, 383398.

Harris, C., Davies, M.C.R., Coutard, J.P., 1995. Laboratory simulation of periglacial solifluction: significance

of porewater pressures, moisture contents and undrained shear strengths during soil thawing. Permafrost

and Periglacial Processes 6, 293311.

Hetu, B., Van Steijn, H., Bertran, P., 1995. Le role des coulees de pierres seches dans la genese dun certain ` `type deboulis stratifies. Permafrost and Periglacial Processes 6, 173194.

.Hsu, K.J., 1975. Catastrophic debris streams sturzstroms generated by rockfalls. Geological Society ofAmerica Bulletin 86, 129140.

Hubert, J.F., Filipov, A.J., 1989. Debris-flow deposits in alluvial fans on the west flank of the White

Mountains, Owens Valley, California, USA. Sedimentary Geology 61, 177205.

Huijzer, A.S., 1993. Cryogenic microfabrics and macrostructures: interrelations, processes, and paleoenviron-

mental significance. Thesis, University of Amsterdam, 245 pp.

Hutchinson, J.N., 1970. A coastal mudflow on the London clay cliffs at Beltinge, North Kent. Geotechnique20, 412438.

Hutchinson, J.N., 1991. Periglacial and slope processes. In: Forster, A., Culshaw, M.G., Cripps, J.C., Little, .J.A., Moon, C.F. Eds. , Quaternary Engineering Geology. Geological Society Engineering Geology

Special Publication no. 7, pp. 283331.

Ildefonse, B., Launeau, P., Bouchez, J.L., Fernandez, A., 1992. Effects of mechanical interactions on the

development of shaped preferred orientations: a two-dimentional experimental approach. Journal of

Structural Geology 14, 7383.

Jesek, J., Schulmann, K., Segeth, K., 1996. Fabric evolution of rigid inclusions during mixed coaxial and

simple shear flows. Tectonophysics 257, 203221. .Johnson, A.M., Rodine, J.D., 1984. Debris flows. In: Brunsden, D., Prior, D.B. Eds. , Slope Instability.

Wiley, Chichester, pp. 257361.

Jongerius, A., 1970. Some morphological aspects of regrouping phenomena in dutch soils. Geoderma 4,

311331.

Lewkowicz, A., 1992. Factors influencing the distribution and initiation of active-layer detachment slides on .Ellesmere Island, Arctic Canada. In: Dixon, J.C., Abrahams, A.D. Eds. , Periglacial Geomorphology.

Wiley, Chichester, pp. 223250.

Lowe, D.R., 1982. Sediment gravity-flows: II. Depositional models with special reference to the deposits of .high-density turbidity currents. Journal of Sedimentary Petrology 52 1 , 279297.

MacPhail, R.I., 1992. Soil micromorphological evidence of ancient soil erosion. In: Bell, M., Boardman, J. .Eds. , Past and Present Soil Erosion, Archaeological and Geographical Perspectives. Oxbow Books,

Oxford, Monograph 22, pp. 197215.

Matsuoka, N., 1996. Soil moisture variability in relation to diurnal frost heaving on Japanese high mountain

slopes. Permafrost and Periglacial Processes 7, 139151.

Matthews, J.A., Harris, C., Ballantyne, C.K., 1986. Studies on a gelifluction lobe, Jotunheimen, Norway: 14

C

chronology, stratigraphy, sedimentology and palaeoenvironment. Geografiska Annaler 68A, 345360. .McSaveney, E.R., 1971. The surficial fabric of rockfall talus. In: Coates, D.R. Ed. , Quantitative Geomor-

phology: Some Aspects and Application. S.U.N.Y., Annual Geomorphology Symposium, pp. 181198. .Melosh, H.J., 1987. The mechanics of large rock avalanches. In: Costa, J.E., Wieczorek, G.F. Eds. , Debris

FlowsrAvalanches: Process, Recognition and Mitigation. Geological Society of America, reviews in

Engineering Geology, 7, pp. 4150.


23/23


Moeyersons, J., De Ploey, J., 1976. Quantitative data on splash erosion, simulated on unvegetated slopes.

Zeitschrift fur Geomorphologie N.F. 25, 120131, Suppl.-Bd.Moss, A.J., Walker, P.H., 1978. Particle transport by continental water flows in relation to erosion, deposition,

soils and human activities. Sedimentary Geology 20, 81139.

Mucher, H.J., De Ploey, J., 1977. Experimental and micromorphological investigation of erosion andredeposition of loess by water. Earth Surface Processes 2, 117124.

Mucher, H.J., Carballas, T., Guitian Ojea, F., Jungerius, P.D., Kroonenberg, S.B., Vilar, M.C., 1972.Micromorphological analysis of effects of alternating phases of landscape stability and instability on two

soil profiles in Galicia, N.W. Spain. Geoderma 8, 241266.

Nelson, F.E., 1985. A preliminary investigation of solifluction macrofabrics. Catena 12, 2333.

Nieuwenhuijzen, M.E., Van Steijn, H., 1990. Alpine debris flows and their sedimentary properties. A case

study from the French Alps. Permafrost and Periglacial Processes 1, 111128.

Pierson, T.C., Costa, J.E., 1987. A rheologic classification of subaerial sedimentwater flows. In: Costa, J.E., .Wieczorek, G.F. Eds. , Debris FlowsrAvalanches: Process, Recognition and Mitigation. Geological

Society of America, Reviews in Engineering Geology, 7, pp. 112.

Rapp, A., 1962. Karkevagge. Some recordings of mass-movements in the northern scandinavian mountains.Biuletyn Peryglacjalny 11, 287309.

Repelewska, J., Pekala, K., 1993. The influence of local solifluction rates, Spitzbergen, Svalbard. In: Frenzel, .B. Ed. , Solifluction and Climatic Variation in the Holocene. European Palaeoclimate and Man 6.

European Science Foundation, G. Fisher, Stuttgart, pp. 251266.

Siebert, L., 1984. Large volcanic debris avalanches: characteristics of source areas, deposits and associated

eruptions. Journal of Volcanology and Geothermal Research 22, 163197.

Skempton, A.W., Norbury, D., Petley, D.J., Spink, T.W., 1991. Solifluction shears at Carsington, Derbyshire. .In: Forster, A., Culshaw, M.G., Cripps, J.C., Little, J.A., Moon, C.F. Eds. , Quaternary Engineering

Geology. Geological Society Engineering Geology Special Publication no. 7, pp. 381387.

Topping, D.J., 1993. Palaeogeographic reconstruction of the Death Valley extended region: evidence from

Miocene large rock-avalanche deposits in the Amargosa Chaos basin, California. Geological Society of

America Bulletin 105, 11901213.

Valentin, C., Bresson, L.M., 1992. Morphology, genesis and classification of surface crusts in loamy and

sandy soils. Geoderma 55, 225245.

Van Steijn, H., Bertran, P., Francou, B., Hetu, B., Texier, J.P., 1995. Review of models for geneticalinterpretation of stratified slope deposits. Permafrost and Periglacial Processes 6, 125146.

Van Vliet-Lanoe, B., 1982. Structures et microstructures associees a la formation de glace de segregation: ` leurs consequences. Proceedings of the 4th International Conference on Permafrost, Fairbanks, AK, pp.116122.

.Van Vliet-Lanoe, B., 1985. Frost effects in soils. In: Boardman, J. Ed. , Soils and Quaternary LandscapeEvolution. London, pp. 117158.

Van Vliet-Lanoe, B., 1987. Cryoreptation, gelifluxion et coulees boueuses: une dynamique continue en .relation avec le drainage et la stabilite de lagregation cryogenique. In: Pecsi, M., French, H. Eds. , Loess

and Periglacial Phenomena. Akademial Kiado, Budapest, pp. 203226.

Van Vliet-Lanoe, B., Valadas, B., 1983. A propos des formations deplacees des versants cristallins des massifs anciens. Bulletin de lAssociation Francaise pour lEtude du Quaternaire 4, 153160.

Varnes, D.J., 1978. Slope movement. Types and processes. Landslides. Analysis and Control, National

Academy of Science, Washington. Special Report, 176, pp. 1133.

Wells, S.G., Harvey, A.M., 1987. Sedimentologic and geomorphic variations in storm-generated alluvial fans,

Howgills Fells, north-west England. Geological Society of America Bulletin 98, 182198.

Whalley, W.B., Douglas, G.R., Jonsson, A., 1983. The magnitude and frequency of large rockslides in Iceland .in the Postglacial. Geografiska Annaler 65A 1 2 , 99110.

Whitehouse, I.E., McSaveney, M.J., 1983. Diachronous talus surfaces in the southern Alps, New Zealand and

their implications to talus accumulation. Arctic and Alpine Research 15, 5364.Yarnold, J.C., 1993. Rock avalanche characteristics in dry climates and the effects of flow into lakes: insight

from mid-Tertiary sedimentary breccias near Artillery Peak, Arizona. Geological Society of America

Bulletin 105, 345360.

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