Models for the genetic and environmental ... - Wiley Online Library

Permafrost and Periglacial Processes, Vol. 6: 125-146 (1995)

Models for the Genetic and Environmental Interpretation of Stratified Slope Deposits: Review Henk van Steijn, Pascal Bertran, Bernard Francou, Bernard H&u4 and Jean-Pierre Texier’ ‘The Netherlands Centre for Geo-ecological Research, Faculty of Geographical Sciences, Utrecht University, PO Box 80.115, 3508 TC Utrecht, The Netherlands. * Institut du Quaternaire, UMR 0033 du CNRS, UniversitC de Bordeaux 1, avenue des Facultis, 33405 Talence, France. 30RSTOM, Calle EE. UU. 1487, CP 9214 La Paz, Bolivia. Module de GCographie, UniversitC du QuCbec B Rimouski, 300, AllCe des Ursulines, Rimouski, QuCbec, G5L 3A1, Canada

ABSTRACT The principal aspects addressed are the relationships between transport processes and depositional environments, and the sedimentary properties of the deposits. The following ‘process-product’ models are discussed: (a) stone-banked solifluction sheets or lobes, (b) dry grain flows and frost-coated clast flows, (c) debris flows, and (d) niveo-aeolian transport within the context of talus development. Problems of convergence, the possible role of other processes, and the climatic significance of stratified slope deposits are briefly discussed.

RBSUMB Cet article traite des modkles concernant la genCse et l’interprbtation environnementale des dCp6ts de versant stratifiks. Les relations des mdcanismes de transport avec, d’une part, les milieux de stdimentation et, d’autre part, les CaractCristiquesstdimentologiques, sont dCcrites. Toutes les donnCes prCsentCes ici proviennent d’btudes rCalisCes en milieux actifs. Les modbles gknttiques et skdimentologiques trait& sont les suivants: (a) nappe-coulkes et lobes de solifluxion h front pierreux, (b) coulCes de pierres skches et coulCes de pierres glades, (c) flots de dCbris, et (d) transport nivCo-tolien sur un talus. A la fin de cet article, sont tvoquks les problkmes de convergence, le r61e possible de mCcanismes stdimentaires non intCgrCs dans les modCles, ainsi que le problCme de la signification climatique des dCp6ts stratifits. Ce bilan global peut donc servir de base introductive A I’interprCtation de cas concrets. KEY WORDS: Talus; stratified slope deposits; periglacial processes; palaeoclimates

(Guillien, 1951), and (b) sediments within which only crude stratification can be detected. A wide Stratified slope deposits (sometimes termed strati- range of facies is present, reflecting a diversity of fied screes) have been reported from within a environments and processes. The interpretation of stratified slope deposits range of geomorphological contexts. ‘Stratification’, as used here, includes all transitions be- with regard to transport processes and depotween (a) clearly and regularly stratified deposits, sitional environments has improved during the such as the gr2zes litkes of Charente, France last decade with the publication of new genetic INTRODUCTION

CCC 1045-6740/95/020125-22

0 1995 by John Wiley & Sons, Ltd.

Received 23 March 1995 Accepted 28 April 1995

I26 H. van Steijn, P. Bertran, B . Francou, B . H&u and J.-P. Texier

models. These are based on observations of processes active on talus within a number of environments The aim of the paper is to assemble the characteristics of the various models currently available for the genetic interpretation of stratified slope deposits. The discussion must include: (a) stone-banked solifluction sheets or lobes, as found in the high Andes of Bolivia and Peru (Francou, 1988, 1989, 1990; Bertran et al., 1993) and in alpine solifluction lobes (Bertran et al., 1993); (b) dry grain flows together with frost-coated clast flows (coulkes de pierres glackes), as occurring in northern GaspCsie, Canada (Hktu et al., 1994) and in the Pyrenees; (c) debris flows, as occurring in regions where cliff-talus slope systems are found, such as the southern French Alps (Van Steijn, 1988; Nieuwenhuijzen and Van Steijn, 1990; Bertran and Texier, 1994); and (d) niveo-aeolian transport combined with supranival clast sliding, as observed in northern Gaspesie (HCtu, 1991). A review of the functioning of each model (specification of processes, environmental conditions and constraints) and the diagnostic sedimentary properties of the deposits produced allows recognition of the processes involved and discussion of their (pa1aeo)environmental significance. STONE-BANKED SOLIFLUCTION SHEETS AND LOBES

their main components, is illustrated in Figure 1. The processes are those of solifluction: that is, (1) frost sorting by which coarse material reaches the surface, (2) surface creep of coarser material due to needle-ice formation, and (3) displacement of matrix-rich material by frost creep and gelifluction. Velocities observed at Andean sites are 3-10cm a-l for displacement of surface material by frost creep and up to 1m a-l (and sometimes more) by needle-ice development. At depths >5 cm, movement becomes negligible. Frontal velocities measured in Peru and Bolivia varied between 2-5cm a-' and 20-30cm a-*. In the Alps, surface displacements of about 2-5 cm a-l are measured; these decrease exponentially with depth and disappear below 30cm. Other processes may interfere with those mentioned. For example, surface runoff may be active in the Alps during summer. This favours pavement formation by washing, and locally rills develop where fine material is at the surface. Small fans may occur at the lower end of such rills. In the Andes, where the diurnal temperature range is large, there is a high frequency of surficial freeze-thaw cycles. Under such conditions, precipitation almost always reaches the ground as snow or hail. Runoff therefore relates to the melting of this frequently occurring but thin cover of either snow or hail. However, the quantities of water and velocities are insufficient to cause dissection of the sheets. Occasionally rills may develop but these are quickly obliterated by further frost activity (cf. Schumm, 1964). Rainstorms with intensities near 20mm h-' have return periods of about 10 years and may trigger debris flows.

The development of stratification by the movement of stone-banked solifluction sheets and lobes has been studied in the high-altitude (alpine) periglacial regions of low and temperate latitudes (Francou, 1988, 1989, 1990; Bertran et al., 1993). In the Andes of Peru and Bolivia, these sheets Development of stratification patterns and related and lobes are abundant at altitudes near the 0°C sedimentary properties isotherm (c. 4500-5400m ASL). The length of the lobes is of the order of metres. Sheets are some- During movement, the stony front is progressively what larger, having dimensions of tens of metres. buried by the matrix-rich core. The movement is For both, the length/width ratio is low compared like that of a caterpillar belt. Meanwhile, the with that for dry grain flows and debris flows. The front is continually being re-established by frost sheets and lobes occur on gentle (4" or less) to sorting of the coarse clasts and eluviation of fines; steep (35") slopes and develop mainly in moraines both affect the matrix-rich body of the sheet. The and talus accumulations. O n the latter their de- different frontal velocities observed on the velopment is widespread, being only restricted Andean sheets probably reflect the seasonal alterwhere rockfall activity becomes important (i.e. nation of movement types: (1) slow movement near cliff foots). The lobes found in the French during the dry season, dominated by frost creep, Alps are limited in dimensions, being 2-3m which causes the thick stony front (Figure 2); and wide, and occur in groups. The composition of (2) rapid movement (gelifluction) restricted to the the features, showing the spatial relationship of humid season, by which sheets of fine material

Models of Stratified Slope Deposits: Review 127

B Figure 1 Schematic section through (A) alpine stone-banked lobes and (B) Andean sheets. Matrix-rich layers are in white. In (A), notice the two-directional grading of the coarse, openwork layers caused by their composite origin (as explained in the text). Matrix-rich layers are Icnticular in frontal scctions. In (B), the surficial pavement is restricted to stone stripes. The matrixrich layers are continuous and their lowcr boundary typically displays folds in longitudinal sections and a wavy pattern in frontal sections.

overrun the front which is gradually buried. It follows that the deposition of an openwork, stony level and the formation of a matrix-rich layer are simultaneous processes, operating during the advance of a sheet or lobe. On the slope as a whole, several ‘trains’ of sheets or lobes may be moving simultaneously, leading to the development of a stratified slope deposit. This mechanism, thought valid for Andean sites, is complicated when transposed to the French Alps. There the result is a more irregular pattern of stratification. Eluviation of the lobes is intense because of both snowmelt (restricted to a relatively short period) and summer rain. Particles washed out from the lobes accumulate downslope. These silty sediments are sensitive to the development of mud boils (ostiofes)related to the annual freeze-thaw cycle. Sometimes the mud boils are the starting point for the development of new stone-banked lobes or of small debris flows. In sections, the solifluction sheets or lobes show the following characteristics: (1) a surficial pavement of coarse clasts (the surface expression of Figure 2 Front of a stone-banked sheet. Note digitated form of the front, passing upslope (on the surface of the sheet) into a series of stone stripes. Chacaltaya, Bolivian Andes, altitude 5200m ASL. Thickness of the front is about 30cm.

128 H . van Steijn, P. Bertran, B. Francou, B. Hetu and J.-P. Texier

Figure 3 Frontal section of a stone-banked sheet. A matrix-rich layer with coarse stripes is seen at the surface. Beneath this layer a unit of coarsening-downward. openwork gravel is visible, deposited over anothcr laycr of fine material. La Cumbre, Bolivian Andes, altitude 5000m ASL.

which may be a series of stone stripes); (2) a layer of heterogeneous, matrix-rich, clast-supported or matrix-supported material (the latter especially in the proximal parts of flows); and (3) a lower level of openwork clasts (Figure 3). In the Andes this stratification is regular and continuous, but alpine deposits show lenticular patterns due to the limited lateral extent of the lobes. The coarse fractions generally show welldeveloped grading. Inverse (coarsening-upward) grading corresponds to old pavements, i.e. reflecting a sieve effect within the clasts transported on top of the flow. Normal grading is caused by the burial of stony fronts; here, the finest clasts (in contact with the matrix-rich core of the flow) are the first to be buried when the core of the flow progresses downslope over the front (Figure 4). Two-directional grading is the most frequent case. It is caused by the superposition of a ‘pavement’ and a layer deposited by the burial of a front. The character of the contacts between openwork layers and those rich in matrix depends on local drainage conditions. Where drainage is good, the upper limit of the matrix-rich layer is sharply defined and tabular. The uppermost 2-5 cm of these layers show an increased silt content caused by illuviation. The lower boundary of these matrix-rich layers is relatively vague owing to the downward migration of the fine fraction.

On the lower parts of the slopes, where drainage is poor, the lower boundary is more clearly developed, and no increased silt accumulation is found. On top of the matrix-rich layers, elongated patches of fine sediments derived from small mud boils are sometimes found. Locally, other facies are also found. For example, (1) more or less clearly laminated (but often frost disturbed) silt and sand may occur as small fans deposited by surface runoff; (2) thick layers of openwork clasts may be caused by eluviation of inactive sheets, and (3) matrix-rich layers showing vertical grading in either sense may be caused by subsequent illuviation. Environmental and climatic significance Stone-banked solifluction sheets and lobes require an absence of vegetation. According to studies by Herz and Andreas (1966), Benedict (1970) and Van Vliet-Lanoe (1987), stony flows occurring at high latitudes do not develop stratification. On the basis of the observations made in the Andes and the Alps it is hypothesized that two conditions must be satisfied to allow stratification to develop. First, disturbance by cryoturbation of frost-susceptible layers must be absent or weak. Second, a high number of shal-


Figure 4 Front of an alpine solifluction lobe, showing a well-developed fining-upward grading of the coarse material. The smallest clasts are at the contact with a level of matrix-rich material. The tube visible in the centre of the photograph was expelled almost completely by frost heave since its installation in the matrix-rich layer three years before. La Mortice, Southern French Alps, altitude 3100 m ASL.

low (< 20cm) freeze-thaw cycles (with or without permafrost) must exist. The latter condition occurs where diurnal cycles exist and where snow cover is sporadic. The distribution of stratified slope deposits can be explained by this model. Their frequency of occurrence is highest within a belt between approximately 40"N and 40 "S, especially where there is aridity, as in parts of the Andes, the western Kunlun Shan, the Atlas Mountains, or the Canary Islands. Within more humid alpine environments they occur only at certain aspectdetermined sites, for instance on slopes where frost penetration is shallow owing to a relatively thick snow cover. Finally, lithological conditions are important. Stone-banked solifluction sheets and lobes develop preferentially on rocks that produce fine material by gelifraction, such as schists, volcanic and plutonic rocks, and certain types of limestones. FLOWS OF COHESIONLESS DEBRIS

Perez, 1985; HCtu et al., 1994). Two types are distinguished: (1) dry grain flows (see the definitions given by Bagnold, 1954 and Lowe, 1976); (2) frost-coated clast flows,reported from GaspCsie, QuCbec (HCtu and Vandelac, 1989; HCtu et al., 1994). Both belong to the family of 'grain flows' (Wasson, 1979; Ishii, 1988). Lowe (1976, p. 188) proposed the following definition for this group: 'the term grain flow is restricted to sediment gravity flow in which a dispersion of cohesionless grains is maintained against gravity by grain dispersive pressure and in which the fluid interstitial to the grains is the same as the ambient fluid above the flow.' Dispersive pressure, as introduced here, is the effect of multiple particle collisions after the first movement is triggered. Its magnitude is directly related to grain area: larger clasts are exposed to higher pressures (Bagnold, 1954). This appears to have important consequences with regard to grain-size distribution within the flowing mass. Several natural and artificial flows of this kind have been observed, which permits description of the transport process as well as of the facies of the resulting deposits (HCtu et al., 1994).

Flows of cohesionless (openwork) clasts are defined as small and relatively rapid slides (0.56 m s-l) mobilizing openwork clast accumulations Dry grain flows at the surface of scree slopes over a thickness of about 5-20cm (Rapp, 1960; Gardner, 1979; On talus, dry grain flows are frequently triggered

130 H. van Steijn, P. Bertran, B . Francou, B. HCtu and J.-P. Texier

B

Figure 5 Schcmatic longitudinal section of dry grain flow, showing different parts and materials with examples of fabrics. GF: dry grain flows. Upper fabric diagram: lobe, observed in frontal zone of a small artificial flow at Belesten (French Pyrenees). Note strong clast imbrication. Lowcr fabric diagram: slip planc artificial cone in quarry near Remollon, French Alps.

by the impact of rock fragments falling from cliffs above the talus. Increased loading and oversteepening by continuing debris supply at the top of the talus may also lead to their initiation. The flows are normally 0.4-1.0m in width, covering distances of some 3 to 10m, at velocities of about 0.5-2m s-’ depending on the grain size of the substratum. Length/width ratios are relatively high compared with solifluction sheets, but lower than those of the highly mobile frost-coated clast flows. Because of the high friction values involved in this sliding of dry material, dry grain flow development is only found on relatively steep slopes. During flow, a threefold segregation of grain sizes occurs (Figure 5 ) . Particle collisions and the resulting dispersive forces cause a dilatant behaviour of the moving mass. This in turn favours the downward migration (by gravity) of the finer elements, which therefore concentrate near the base of the flow (kinematic sieving: Allen, 1972; Carnie1 and Scheidegger, 1974), while the coarse debris becomes concentrated at the surface. Within the flow, the velocity of the particles increases from the bottom to the surface. Thus sand and silt, concentrated at the base of the flow, are deposited first at upslope positions,

while the coarse material continues downslope. During movement the coarse material itself is sorted, with the coarsest clasts being concentrated at the surface. There, they quickly travel to the front of the flow, which develops into a stony lobe. Smaller grains (fine gravel, coarse sand) form the tail of the flow. During the last seconds of the movement, the tail of finer material covers the upslope part of the already immobilized lobe of coarse clasts. The internal organization of a dry grain flow and its deposits is schematically shown in Figure 5 . Thus, an originally heterogeneous material (including matrix and grades of coarse clasts) is split into two distinct units after only a small distance of transport: (a) a matrixrich bed with finer clasts, and (b) a coarser bed of completely openwork clasts with inverse grading. After transit, dry grain flows leave a shallow, flat-bottomed depression the surface of which is covered by fine, infiltrated grains (Figure 6). In a few cases the depression is flanked by small, poorly developed lateral levees. The fine-grained surface serves as a slip plane for subsequent flows (ishii, 1988). If such flows reach an already immobilized lobe, loading of the latter together with the


Figure 6 Small artificial dry grain flow, showing flat-bottomed channel covered with fines concentrated at the base ot the How by kinematic sieving. Talus near Mont-Saint-Pierre (GaspCsie).

impact of the arriving flow may cause reactivation. The two masses then move downslope together. Eventually, stabilization of the track occurs owing to the gradual deposition of coarsegrained terminal lobes (Figure 7). To date few scree deposits consisting of material from dry grain flows have been analysed in detail (Ishii, 1988; Bertran et al., 1995). Important differences exist within the facies of dry grain flow deposits (Figure 8). O n the upslope parts of talus on which dry grain flows are active, sections show an alternation of thin beds (thickness 1-3cm) consisting of fine material concentrated at the base of the flows (the slip plane) and of beds of coarse, openwork material, characterized by well-developed inverse grading and strong clast imbrication. On average, 50-80% of the clasts show an upslope imbrication, while 1040% even dip in an upslope direction. These coarse beds correspond to terminal lobes. Their thickness (between 0.2 and 1.2m) is directly proportional to the grain size of the material. Imbrication of the clasts is due to the high pressures that exist within the terminal lobe when its movement stops (Figure 5). The upper boundary of a fine bed is sharp and fairly uniform owing to the abrasive passage of flows, while its base is less clearly defined, partly because of post-depositional percolation of water which locally removes

material. The layers of fine material develop under conditions of high shear stress. This is reflected in the long-axis orientation (parallel to the slip plane) of the few coarser clasts found within the fine beds (Figure 5 ) . Imbrication is much weaker: 6 0 % of the clasts are imbricated upslope; 34" for dry grain flows). Second, frost-coated clast flows do not leave silty to sandy slip planes because the fines do not migrate to the base of the flow during motion. The ice coatings stay humid as an effect of frictional heat produced during movement. Thus, fines stick around the clasts which are covered with silt for some time after the event. In the Gaspesie the silt was later washed away by summer rain. Third, within dry grain flows, the dilation coefficient is directly related to flow velocity, giving values 3 to 12 times higher for frost-coated clast flows than for dry grain flows (Hetu et al., 1994). Fourth, imbrication within lobes of frost-coated clast flows is stronger than in those of dry grain flows.

Environmental and climatic significance

The conditions favourable for the triggering of grain flows can be summarized as follows. First, debris production on the rockwalls above the talus is needed for a constant supply of material to talus. Second, a concentrated debris supply is needed on the proximal part of the talus, leading to local oversteepening or overloading. Both conditions are favourable for the triggering of dry grain flows. This implies the production of small, flaky and elongated clasts, while the amount of fines (sand or silt) stays small. If the quantity of fines is large, debris flows and transport by overland flow will dominate. Third, the influence of snow should be negligible, to prevent reworking of grain-flow deposits by snow creep or snow avalanches. Otherwise these processes may dominate over the grain-flow process (Hetu and Vandelac, 1989; Luckman, 1988; Gardner, 1979). The absence of snow does not necessarily mean an absence of precipitation as snow, for it may be caused by deflation cleaning the exposed talus, as in Gaspesie (Httu and Vandelac, 1989). There are a number of other possible contributing factors. For example, strong winds (blizzards or strong winds near glaciers) may play a role in the transport system on rockwalls and talus, especially by triggering grain flows (Httu, 1992). It

Models of Stratified Slope Deposits: Review

GRAIN-SIZE

A) SCHEMATIC MAP OF A FCCF

135

FABRIC

CHANNEL

r Channel

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Mean. 20.2 rnrn

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,

1-Terminal lobe

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80

,

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B) CROSS-SECTIONS Channel and Levees

Mean vector

Mean:35.7mrn 50 40

0

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40

60

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Figure 12 Properties of a frost-coated clast flow (using values from the Mont-Saint-Pierre, Gasptsie study area for grain-size distributions and fabric).

was found that the largest grain flows were often associated with periods of violent winds blowing in gusts around the cliffs (Wasson, 1979; HCtu et al., 1994). Second, vegetation has to be absent from the starting zone of dry grain flows, as observed on many active talus slopes in the Alps and the Pyrenees. In GaspCsie, however, frostcoated clast flows traversed zones of scattered shrubs before penetrating into the dense forest at the foot of the slope (HCtu, 1990; HCtu et al., 1994). Finally, local factors may be important in certain cases. High-intensity phenomena like earthquakes or the impact of coarse hail grains may trigger grain flows on talus near to its sta-

bility limit. Instances are mentioned by Cailleux (1967), Church et al. (1979), Wasson (1979) and HCtu et al. (1994). Another local factor is the removal of material at the talus foot leading to an increase of slope steepness, which triggers dry grain flows under any climate (Bones, 1973). Dry grain flows are a ubiquitous process observed in warm arid regions and the temperate and arctic zones (Bones, 1973). As such, relict deposits do not possess any palaeoclimatic significance. It is clear nevertheless that the periglacial climates in mid-latitudes during the cold phases of the Pleistocene were particularly favourable for their development. Cold conditions are also re-

136 H. van Steijn, P. Bertran, B. Francou, B. HCtu and .I.-P. Texier

LONGITUDINAL SECTION crn 0 -

TERMINAL LOBE

20

Bed 13

0

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Bed dip : 24'

60 Bed 7

80

7 29" 7 29"

0

50 q

t , N = 50

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100

120 Bed 6

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Figurc 13 Longitudinal section through frost-coatcd clast Row deposits (bed 11 cxceptcd: rockfall deposit). Talus slope near Mont-Saint-Pierre, Gaspisie. Beds of finc gravel are channel-bottom deposits (example: bed 6). Beds of coarse, openwork clasts showing moderate imbrication belong to levees (bed 7), and to frontal lobes if imbrication is strong (bed 13). The circular diagrams show elast long-axis dip for bcds 6, 7 and 13.

quired for frost-coated clast flows (implying the contribution of ice). However, the only known talus with active frost-coated clast flows have been found under a cold, temperate climate (MAAT +3"C) with a strong maritime influence, where freeze-thaw cycles are abundant. Within a palaeoclimatic context, regions that had marginal periglacial climates (i.e. with a high number of freeze-thaw cycles but no deep annual frost) were probably most prone to the development of frost-coated clast flows. It is concluded that the (pa1aeo)environmentaI significance of grain flows is not simple. They are part of complex systems, in which a wide spectrum of factors are involved.

DEBRIS FLOWS Debris flows are rapid mass movements of poorly sorted granular solids, water and air (Varnes, 1978). They involve a wide range of materials from completely cohesionless to more or less cohesive. Different rheological models are used to explain the movement of such mixtures. Within the scope of the present review, only small-scale hill-slope debris flows are relevant. These are characterized by flow-track lengths of the order of hundreds of metres. Composition may influence flow behaviour, although slope morphology and water content are also important. Three geomorphological zones can be dis-


(3) A terminal part where the levees form a frontal lobe. This part is often complicated by the presence of several lobes or levee systems arranged beside or on top of each other.

Figure 14 Debris-Row systems in the French Alps, showing source area (cliffs and chutes), transit zone (lower cliff zone and upper part of the talus), and depositional zone. Levees and lobes arc also well developed. Longest track on the talus measures about 150m.

tinguished o n the cliff-talus slope system which is the base for this kind of debris flow (Figure 14): (1) A source area consisting of rockwalls intersected by steep chutes or ravines. Coarse material accumulates (by single-particle rockfall or dry grain flow) on the floors of these chutes, while finer material occurs on the bordering slopes. Downslope, the chute cuts into the talus apex. As a whole, erosion is the dominant process in this part of the system. (2) A talus zone where ridges of heterogeneous sediment (levees) are found on both sides of the track. The lower part of these tracks may be sinuous, especially when talus slope angle is low. Transport and accumulation coexist, but in the steepest parts of this middle zone erosion may still exceed accumulation.

Debris-flow tracks are narrow, elongated features. A central depression can generally be distinguished between the levees. Deposition is not constant along an individual track. Normally the higher and steeper parts of the system are characterized by erosion processes. Although debris flows may be triggered by rapid melting of snow and (or) ground ice (Harris and Gustafson, 1988) they normally occur as a reaction to ‘high-intensity’ rainstorms (Caine, 1980). The minimum intensity required may be low under permafrost conditions or where deep seasonal frost exists (cf. Larsson, 1982). Movement of debris flows can be described either by a visco-plastic rheological model (Johnson, 1970; Johnson and Rodine, 1984) or by a flow model in which dispersive pressures caused by interparticle collisions maintain the mobility of the material (Takahashi, 1978, 1980). A typical debris-flow event consists of a sequence of surges (waves of highly concentrated material) which last a few seconds, separated from each other by phases of longer duration in which flow is quieter and concentration of sediment is lower. Estimated mean velocities for the relatively small debris flows discussed here are about 1.5-4.0m s f l (Van Steijn et af., 1988). The visco-plastic model implies that movement will only occur after a strength threshold within the debris has been surpassed. This yield strength is responsible for the velocity profile of the debris flow. Most characteristic is the presence of a central zone within the moving mass where velocity is constant with depth (the ‘rigid plug’ as defined by Johnson, 1970). Velocity decreases below this plug, to become zero at the base of the flow. According to this model, coarse material, often concentrated near the front of the flow, is pushed to the sides of the flow in a snow-plough fashion. The Takahashi model yields a different velocity profile, characterized by the effects of particle collisions (dispersive pressure) and (possibly) turbulence. As in the case of dry grain flows, coarse particles move towards zones of low shear force, i.e. the front and the outer parts of the flow. It is thought that the visco-plastic model is suitable for highly concentrated flows with a relatively high clay content (the ‘cohesive debris flow’ of

138 H. van Steijn, P. Bertran, B. Francou, B. HCtu and J.-P. Texier

event are lateral levees, channel deposits between the levees, and terminal lobes. In the French Alps, debris-flow deposits occur when the gradient falls below 30”. Gradual changes, found within the deposits along the track of debris flows, concern changes in levee height and mean cobble size (Van Steijn el af., 1988; Nieuwenhuijzen and Van Steijn, 1990; Bertran and Texier, 1994) or the general stratification pattern (from predominantly openwork units in proximal position to mainly matrix-rich elements at the distal side) as observed in small fans described by Bertran and Texier (1994). The spatial relationships between the different morpho-sedimentary units of a debris-flow deposit are schematically shown in Figure 15. Levees, channel deposits, and terminal lobes consist of heterogeneous, matrix-rich material. This is frequently clast-supported. Outer parts of levees are often concentrations of coarse, openDevelopment of stratification patterns and related work clasts in which preferred orientation is normally developed. Within the channel deposits sedimentary properties and the inner parts of the levees, long-axis orienDeposits formed during an individual debris-flow tation is parallel to flow direction. Along small

Lowe’s 1976, 1979 classification). Low slope angles are also more favourable for this ‘plug flow’. The Takahashi model is associated with high mobility, owing to the more diluted character of the debris, and a lower clay content, and the flows are transitory compared with Lowe’s ‘grain flows’ (Lowe, 1976, 1979). The ‘turbulent’ behaviour under such conditions is best developed on steep slopes (Nieuwenhuijzen and Van Steijn, 1990; Bertran and Texier, 1994). As stressed by Meunier (1991), most flows will possess elements of both types of behaviour during motion. The different flow conditions specified by the models may be reflected in the sediment characteristics (Van Steijn and Coutard, 1989). A common aspect of both is the process of ‘kinematic sieving’, leading to the concentration of fine material near the base of the flow.

lateral levee channel

levee

/

---

.

/

/

/

I

/

\

\

frontal lobe

a

laminated silts diarnict

openwork lens paleosol Figure 15 Systematic variations of sediment properties within debris-flow deposits along the track: example of deposits in Vallon Laugier, French Alps (see text). Sections within the proximal part display well-developed openwork lenses, 0.3-1 m wide, and of various forms: triangular, concave-upward, or convex-upward. These forms might represent outer lcvcc parts, channels, and lag dcposits on Icvces, respectively. They occur between matrix-rich material from the central part of a flow, as indicdtcd in thc figurc. By contrast, cross-sections in the distal part mostly rcvcalcd lcnticular diamictic units. 0.2-0.4m thick and 2-4m wide (frontal lobes) interstratified with palcosols. In this cxamplc, openwork lenses arc rarc in the frontal part, probably due to reduced velocity or increased viscosity in these lower parts, in combination with a less effective post-depositional washing out.


debris-flow tracks, or when channel deposits are very thin, preferred orientation may be completely lacking (Bertran and Texier, 1994). In most cases fabrics are moderately to strongly developed. The outer parts of levees often show a clastorientation trend oblique to flow direction, sometimes even tending upslope. This outer zone reveals weakly developed fabrics. Concentrations of coarse clasts without matrix may also be found on channel bottoms, in which case fabrics are also very weak (Bertran and Texier, 1994). The clasts in the external parts of the terminal lobes show orientations which follow the outline of the lobe. There, fabric strength is comparable with that of the channel material. Imbrication is observed where obstruction leads to a decrease of velocity - generally near the end of the track. Imbrication is found not only in terminal lobes, but in levees as well. Longitudinal sections of debris flows show parallel beds in which heterogeneous, matrix-rich material is dominant. Individual units may be elongated, mainly caused by the more or less sinuous outline of the debris-flow track and the accidental direction of the section. Within both matrix-rich and openwork units, inverse grading is often found. Central parts of levees and channel deposits show weak to absent clast imbrication. In contrast, clasts in terminal lobes may be strongly imbricated. This reflects the 'braking' effect imposed on the lobe when it stops.

Environmental and climatic significance Debris flows occur under a range of environmental conditions (Costa, 1984; Innes, 1983; Johnson and Rodine, 1984). They are most active in high-relief areas where a high production of both fine and coarse clasts exists, and where concentrated surface runoff is frequent. Lithological variability and a general lack of vegetation also promote debris-flow activity. Climate is the most important factor, because precipitation regimes play a dominant role in debris-flow triggering (cf. Caine, 1980). Virtually all events of the scale discussed here were caused by high-intensity rainstorms occurring in summer or autumn (Van Steijn, 1991; Blijenberg, 1993). Lower precipitation amounts in northern Scandinavia and Spitsbergen trigger comparable flows (Rapp and Nyberg, 1981; Larsson, 1982). In part this difference is due to the role of permafrost, especially in Spitsbergen.

It is clear from the foregoing discussion that periglacial conditions are not required for debrisflow formation. However, certain aspects of the periglacial environment favour their formation: gelifraction for debris production, and permafrost and lack of vegetation for favourable hydrological conditions and for transport.

NIVEO-AEOLIAN ACTIVITY RELATED TO TALUS DEVELOPMENT The systematic participation of niveo-aeolian activity together with supranival sliding of clasts and debris-flow activity justifies discussion of the resulting stratification pattern as a separate model. Talus slopes studied for niveo-aeolian activity are situated along the southern shore of the Saint Lawrence estuary. They generally possess a north-facing aspect. Their lengths, measured along the slope, are between 100 and 180m, and their basal widths are 15-65m. They are dominated by subvertical (>70") rockwalls, 60 to 130m high. Bedrock consists of thin-bedded Ordovician argilites which on weathering produce large amounts of fine material (sand, silt, but also thin flakes belonging to the gravel fraction). These flakes show very high flatness indices (median values between 9 and 12). The talus cones show a concave profile (38-42" near the cliff base, declining to