Post-glacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

DISCUSSION

Post-glacial colluvium in western Norway: depositionalprocesses, facies and palaeoclimatic record

PASCAL BERTRAN* and VINCENT JOMELLI *Institut du Quaternaire, UMR 5808 CNRS, BaÃtiment de GeÂologie, Avenue des FaculteÂs,33405 Talence, France (E-mail: [email protected]) Laboratoire de GeÂographie Physique, UMR 8591 CNRS, 1 place Aristide Briand,92195 Meudon, France

INTRODUCTION

In a recent paper, Blikra & Nemec (1998) presentedan interesting study of the facies and palaeocli-matic signi®cance of post-glacial colluvial depos-its from western Norway. Of particular interestare the data on snow¯ows because, until now,little was known about their deposits in spite oftheir frequent occurrence in mountainous areas.However, this discussion of Blikra & Nemec'sstudy raises some important issues concerningboth facies models and palaeoclimatic recon-structions using colluvium. Several points areworthy of discussion and, more generally, the useof colluvial deposits for palaeoclimate assessmentis questioned.

SUBAERIAL (DRY) GRAINFLOWSAND DEBRISFLOWS

Grain¯ows are treated by Blikra & Nemec (1998)as a variety of debris¯ow, and their deposits arenot distinguished from those of cohesionlessdebris¯ows. For subaerial deposits, a fundamen-tal distinction is made by geomorphologistsbetween cohesionless debris¯ows, in which theinterstitial ¯uid is water, and grain¯ows in whichthe ¯uid is air. Although both are granular ¯ows,their depositional context, triggering factors andfacies are different (Van Steijn et al., 1995).Debris¯ows are generated by the liquefaction ofpoorly consolidated debris, mainly by incorpora-tion of water in an earthslide or when run-offexceeds the hydraulic conductivity of loosedebris accumulated in a gully (Johnson & Rodine,1984; Blijenberg, 1993), and give rise to crudelystrati®ed deposits on slopes that may be verygentle. In contrast, grain¯ows are triggered onrockfall taluses when the slope is oversteepened

because of debris accumulation at the apex of thetalus or because of basal erosion, for instance bya river or glacier (Bones, 1973; Whitehouse &McSaveney, 1983; HeÂtu et al., 1995). Grain¯owsproduce steep (usually >33°) strati®ed depositswith well-developed size grading (HeÂtu et al.,1995; Van Steijn et al., 1995). Distinction betweenthe two processes is important for the recon-struction of past sedimentary and climatic envi-ronments. Grain¯ows are favoured by an intenseproduction of debris by rock walls, mainly inperiglacial environments (but not always, e.g.PeÂrez, 19981 ), whereas debris¯ows may occur inconnection with a wide range of climatic events(see below).

COARSE GRAVEL INFILTRATEDWITH LAMINATED SAND

Coarse debris that is ®lled with laminated sand isinterpreted by Blikra & Nemec (1998, ®g. 26A) assnow¯ow deposits, in which the ®ne-grained loadhas been redistributed by snowmelt. However,similar facies have not been observed in the snowavalanche deposits investigated in the French Alpsby Jomelli (1997). In these deposits, the keycharacteristics of snow¯ows are (Fig. 1): (1) asuper®cial coarse-grained openwork layer left bythe last snow avalanches that has been washed bysnow melt and summer rains; and (2) a lowermassive layer, totally or partially ®lled with anunsorted, ®ne-grained matrix. This clearly sug-gests that in situ snow melt (i.e. without channel-ization of subsequent run-off) is never able todeliver water discharges high enough to sort theparticles.

However, a facies similar to that shown inBlikra & Nemec (1998, ®g. 26A) has beenobserved in other contexts, corresponding to

Sedimentology (2000) 47, 1053±1068

Ó 2000 International Association of Sedimentologists 1053

rockfall taluses in which a couloir at the talusapex or a karstic pipe gave rise to occasionalhigh water discharges. The facies assemblage inthese talus deposits typically includes: (1) layersmade of rockfall blocks with an openworktexture or ®lled with laminated sands (Fig. 2),in which the sorting of sands results from ¯oodsthat occur as phenomena independent of theaccumulation of the coarse debris; and (2)diamictic layers that correspond to debris¯owsdeposited when the discharge exceeds thehydraulic conductivity of the debris accumu-lated at the downslope end of the couloir (orkarst) and cause a local increase in the intersti-tial water pressure.

The main part of the clear water that cascadesonto the talus percolates through the scree, exceptduring the ¯ood peak when super®cial water ¯owoccurs, and reappears in the distal part of the

talus with a high ®ne-grained load (concentra-tions of »40 g L)1 have been measured during theevent shown in ®g. 3). These observations suggestthat the facies shown in Blikra & Nemec (1998,®g. 26A) is not typical of snow avalanche depos-its, but represents coarse debris accumulationfrom mainly rockfall and possibly debris¯ows, asin the screes investigated in the French Alps,®lled with sand by abundant run-off from anindependent source of water.

SIMILARITY BETWEEN DENSESNOWFLOWS AND DEBRISFLOWS

The similarities between dense snow avalanchesand debris¯ows are probably not as important assuggested by Blikra & Nemec (1998), because ofthe very speci®c mechanical properties of snow,such as compaction, deformation by melting andrecrystallization of ice crystals and the hardeningof snow by bonding of crystals. Rapid changesduring ¯ow make modelling of rheologicalbehaviour dif®cult and, therefore, it cannot beassumed that the mechanisms of sorting of debrisand fabric development that occur in debris¯owsare also present in snow avalanches. For example,Blikra & Nemec (1998) assume that fresh snow-¯ow deposits may have a strong fabric of clasts,which is subsequently disturbed by snow melt-ing. Our measurements show that this is not thecase (Fig. 4), although the reason for this is notcompletely understood. The fabric vector magni-tudes calculated according to the method ofCurray (1956) in fresh deposits are always low,and the hypothesis of a random orientation ofclasts can be accepted at a 5% signi®cance level.Snowmelt promotes signi®cant movement of thedebris, particularly when the initial snow thick-ness is great. Pits form, because snow melts fasterin the dark debris-rich areas, whereby many rockfragments that were initially dispersed in thesnow¯ow deposit collapse into these pits. How-ever, snowmelt does not result in signi®cantchanges in the bulk fabric characteristics (Jomelli,1999).

A high preferential orientation of a-axes hasbeen observed only locally in ridges of debrisformed in the distal part of thick snow avalanchesthat spread over a snow-free substrate (Fig. 5). Inthe run-out zone of an avalanche, the snowstrength and density increase as a result ofcompression, and the ¯ow changes into a slide.Debris dragged along the ground is injected intothe thrusts and the longitudinal shear planes that

Fig. 1. Stratigraphy of an avalanche debris cone (Bri-ancËonnais, French Alps) showing an upper openworklayer overlying a massive partially in®lled material.The outcrop section is 0á80 m high.

1054 P. Bertran and V. Jomelli/L. H. Blikra and W. Nemec

Ó 2000 International Association of Sedimentologists, Sedimentology, 47, 1053±1068

separate snow¯ow tongues. After melting, thisgives rise to the ridges described, but unex-plained, by Blikra & Nemec (1998). Injection is

probably favoured by local snow fusion resultingfrom frictional heat release along the shearplanes.

Fig. 2. Laminated sand ®lling interstices between blocks in a rockfall talus near Bourg-d'Oisans (French Alps). Thelamination and sorting of the sand matrix is caused by high water discharges channelled by a gully to the talus apexduring summer rainstorms. Knife (18 cm) for scale.

Fig. 3. View of a rockfall/debris¯ow talus near Bourg d'Oisans (French Alps) a few minutes after a rainstorm.Super®cial water¯ow is restricted to a small gully in the upper part of the talus. Water percolates through the debrisand reappears in the distal part of the talus with a high ®ne-grained load. Car in the upper left corner for scale.

Facies and palaeoclimatic signi®cance of post-glacial colluvial deposits 1055


IMPORTANCE OF DEBRIS CREEP

We agree with Blikra & Nemec (1998) that debriscreep and soli¯uction, i.e. periglacial, frost-in-duced creep, play an important role in theformation of colluvial accumulations. In Europe,a number of Pleistocene slope deposits have beenattributed solely to soli¯uction (Harris, 1987;Bertran et al., 1994). Both soli¯uction and debris-¯ows generate predominantly diamictic depositsand roughly similar facies, i.e. a clast-supported,poorly sorted and crudely strati®ed gravel. How-ever, Van Steijn et al. (1995) have shown thatcreep and soli¯uction may be con®dently diag-nosed by three characteristics: (1) folds or down-slope bending of strata testi®es to a long-lastingdeformation by creep or soli¯uction; (2) a lamel-lar structure (platy aggregates 0á3±10 mm thick) ofthe ®ne-grained layers re¯ects frost activity andice lensing (Van Vliet-LanoeÈ, 1985), which is themechanism that provokes soli¯uction; and (3) inmost cases, the clast fabric strength is higher insoli¯uction deposits than in other types of mass-¯ow deposits in spite of some overlap of values(Bertran et al., 1997); exact comparisons requirea-axis orientation to be measured and the fabricparameters to be computed using the methods ofCurray (1956) and Woodcock (1977).

In the matrix-poor, predominantly openworksnow-avalanche deposits, classic soli¯uction can-not occur. However, a slow downslope creep ofcohesionless debris as a result of the impact ofblocks tumbling on the talus, snow avalanches,temperature variations and reorganization ofdebris caused by winnowing of ®ne particles(Church et al., 1979; Gardner, 1979) is likely to beresponsible for a large part of the ®nal sedimen-tary structure of the deposits.

DEBRISFLOWS AND CLIMATE

Blikra & Nemec (1998) consider that the phases ofdebris¯ow emplacement correspond to periodswith frequent and intense rainstorms. However,this should be validated by a precise comparisonwith other, independent climatic data, includingdetailed discussion of the available 14C dates.There is probably no simple relationship betweendebris¯ows and climate. For example, Ballandras(1997) in the French Alps and Jonasson (1993) inSweden have shown that debris¯ows were abun-dant during the Little Ice Age (fourteenth tonineteenth centuries) because of an increase insummer rainstorm frequency. However, Meyeret al. (1995) have demonstrated that debris¯ow

Fig. 4. Clast fabric of snow¯owdeposits before (1) and after(2) snowmelt, plotted inWoodcock's (1977) diagram. Thetriangle (3) corresponds to a ridge ofdebris injected along a shear plane.The clast fabrics produced by otherslope processes are indicated forcomparison, after Bertran et al.(1997). Isotropic fabric is towardsthe diagram origin, planar fabrictowards the r2 axis and preferentialorientation towards the r1 axis.



accumulation on colluvial cones in the Yellow-stone National Park, USA, originates mainly fromnatural forest ®res that occurred during the hotand dry phases of the Holocene, particularlyduring the mediaeval climatic optimum. Zimmer-mann & Haeberli (1992) have shown that glacialretreat, together with permafrost melting, exposeslarge quantities of poorly consolidated sedimentto erosion. In mid-latitude mountains, periods ofglacier recession are therefore associated withabundant debris¯ows, and such a phenomenonmay correspond to a milder climate or to a periodwith lower precipitation. The peak of debris¯owoccurrence at the end of the Lower Dryas(»13 000 BP) in western Norway recognized byBlikra & Nemec (1998) probably corresponds tosuch a situation.

THE MEANING OF A HIGHERSOLIFLUCTION RATE

An increase in soli¯uction rate is considered byBlikra & Nemec (1998) as evidence of a milderclimate with intense rainfall. However, a numberof studies in mid-latitude mountains have shownthat high soli¯uction rates are correlated with lowsummer temperatures (Todisco, 1999) or withsnowy winters (Gamper, 1983; Coutard & Ozouf,1996), which suggests that increased soli¯uctionactivity indicates a climatic deterioration.

CLIMATIC SIGNIFICANCE OFCOLLUVIUM

More generally, as pointed out by Van Steijn et al.(1995), several factors may render the potential ofcolluvium for palaeoenvironmental and palaeo-climatic reconstructions much lower than thatsuggested by Blikra & Nemec (1998). The mainfactors are:

1 Many processes that are relatively infrequenthave a major geomorphological impact on atalus, because they transport large quantities ofmaterial. Between these high-magnitude events,other sedimentary and soil-forming processesmay operate and lead to the degradation ofthe original sedimentary characteristics of thedeposits. In a number of cases, reliable inter-pretation of individual beds remains impossible.

2 Many slope processes are common in a widerange of continental environments, dependingon the source area lithology. This is particularlytrue with respect to run-off, debris¯ows, grain-¯ows and earthslides.

3 A wide range of intimately associated triggeringfactors are often re¯ected in slope sedimen-tation, including climate, human activities,intrinsic evolution and maturation of the slope,as well as tectonics. Making allowance for allthese factors renders the study of the ancientsedimentary record extremely dif®cult, evenfor historical times, for which abundant docu-mentation is available. For instance, slopesedimentation increased signi®cantly duringthe fourteenth to nineteenth centuries in manylow-altitude European areas. However, the rel-ative role of climate (Little Ice Age) and humanenvironmental pressure in investigating thissedimentation still remains debatable (Innes,1983; Dearing et al., 1990; Jorda & Provansal,1996). A comparison between sedimentary and

Fig. 5. Longitudinal ridge of debris injected along asubvertical shear plane separating two digitations of ahumid snow avalanche near Saint-Baudille-en-Pipet(French Alps). The snowmelt allows the ridge to emergeat the surface. The sur®cial veneer of debris shows anisotropic fabric, whereas the debris in the core of theridge exhibits strong preferential downslope orienta-tion. The area of the photograph is about 20 m long.



biological (pollen, charcoal and molluscs)records may help to distinguish the role ofclimate from that of other factors.

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couleÂes de pierres seÁches dans la geneÁse d'un certain

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des deÂpoÃts d'avalanches. GeÂomorphologie, 1, 1±9.Jonasson, C. (1993) Holocene debris ¯ow activity in northern

Sweden. In: Soli¯uction and Climatic Variation in theHolocene (Ed. B. Frenzel), pp. 179±196. European Science

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Manuscript received 4 October 1999;revision accepted 1 March 2000.

REPLY

LARS H. BLIKRA* and W. NEMEC *Geological Survey of Norway, N-7491 Trondheim, Norway (E-mail: [email protected]) Geological Institute, University of Bergen, N-5007 Bergen, Norway (E-mail: [email protected])

Bertran & Jomelli have called into question: (1)the classi®cation of sediment-gravity ¯ows used

in our study; (2) the feasibility of a tractionaldeposition of sand in connection with the melting



of snow¯ow lobes; (3) the relevance of theanalogies drawn between the mechanics of densesnow¯ows and debris¯ows; (4) our assessment ofthe importance of debris creep, or soli¯uction;and (5) the climatic signi®cance of debris¯ows,soli¯uction rate and colluvium in general. Wenote that the ®rst half of Bertran & Jomelli'sdiscussion has been written from the narrowperspective of sediment-gravity ¯ows on terres-trial hillslopes, and is based on an assumptionthat the colluvial processes and deposits inwestern Norway should necessarily be the sameas those, for example, in the French Alps. Bertran& Jomelli's further comments invoke a range ofexotic òther' factors, as if their relevance toour study area were a perfectly viable possibility,and proceed with reservations that are at oddswith the contents of our paper (Blikra & Nemec,1998).

However, colluvial sedimentology is a relat-ively new research frontier, previously exploredchie¯y by geomorphologists and physical geogra-phers, in which controversies and misunder-standings are almost inevitable. We thus takethe present opportunity to elaborate on some ofthe issues raised by Bertran & Jomelli, hopingto resolve the apparent incongruities in terminol-ogy, physical concepts and ®eld observations.

GRAINFLOWS AND DEBRISFLOWS

Bertran & Jomelli object to our consideration ofgrain¯ows as a variety of debris¯ow and suggestthat an ìmportant' and `fundamental distinction'should be made between a cohesionless debris-¯ow and a grain¯ow, which they de®ne ascontaining interstitial water and air, respectively,and which they attribute to different triggeringfactors. Debris¯ows are said to be `generated byliquefaction of poorly consolidated debris, mainlyby incorporation of water in an earthslide orwhen runoff exceeds the hydraulic conductivityof loose debris accumulated in a gully', whereasgrain¯ows àre triggered on rockfall taluses whenthe slope is oversteepened because of debrisaccumulation at the apex ¼ or because of basalerosion'. Bertran & Jomelli thus de®ne mass¯owcategories according to the presence or absence ofwater and the ¯ow-triggering conditions, ratherthan the ¯ow mechanism itself. We do not sharethis approach, and probably few sedimentologistsdo. Suf®ce it to note that grain¯ows are notnecessarily dry, that the same type of mass¯owcan be generated in different ways and that the

same triggering factor, such as liquefaction, mayresult in very different mass¯ows, dependingupon sediment type and slope conditions. Nota-bly, the only sediment-gravity ¯ows that canpossibly be categorized as grain¯ows in our ®eldstudy are the matrix-depleted tails of some of theavalanching watery debris¯ows (Blikra & Nemec,1998, p. 924), which are neither dry nor caused byrockfall talus collapses.

In modern sedimentology, the classi®cation ofsediment-gravity ¯ows has been based onvarious aspects of ¯ow mechanics, or rheologicalbehaviour, rather than the sediment source and¯ow-triggering factors (see Carter, 1975; Middle-ton & Hampton, 1976; Lowe, 1979, 1982; Nemec &Steel, 1984; Postma, 1986; Pierson & Costa, 1987;Mulder & Cochonat, 1996). Since the benchmarkpaper by Lowe (1982), the term debris¯ow hasgenerally been used as a synonym for a rheolog-ical plastic ¯ow. Grain¯ows are obviously plastic¯ows, for they are characterized by a ®nite shearstrength and come to rest by en masse `freezing'.Lowe (1982, ®g. 1) originally distinguished twoprincipal categories of debris¯ow: a cohesivedebris¯ow (or mud¯ow) and a grain¯ow. Theformer involves chie¯y cohesive strength, and thelatter is governed by frictional strength, which arethe two shear-strength components speci®ed bythe classic Coulomb criterion for rheologicalplastics and widely used in the engineeringclassi®cation of `soils', or natural clastic mater-ials. Notably, grain¯ow was considered to be avariety of debris¯ow in earlier publications (e.g.Johnson, 1970, pp. 461±463; Takahashi, 1978,1981), and the mechanism of particle collisionsand dispersive pressure has been an integralpart of general debris¯ow models (e.g. Iverson,1984; Johnson & Rodine, 1984; Takahashi, 1991,1993).

However, not every non-cohesive debris¯owmust necessarily be a grain¯ow, i.e. characterizedby a pervasive shear, high shear-strain rates,intense particle collisions and pronounced dila-tion. The momentum transfer in a granular ¯owmay occur by the particles persistently rubbingagainst one another without signi®cant collisions(see the `frictional regime' of Drake, 1990), or belimited to a thin basal layer of colliding particles(Campbell, 1989). Therefore, Nemec & Steel(1984) have slightly modi®ed Lowe's categoriesby classifying debris¯ows into cohesive andcohesionless (or frictional), with mud¯ow andgrain¯ow as end-members of a possible ¯owspectrum. In short, a grain¯ow is considered tobe a speci®c variety of cohesionless debris¯ow,



and there is nothing in the existing classi®cationor its use by Blikra & Nemec (1998) that mightpossibly prevent a researcher from distinguishinggrain¯ow processes and deposits in the ®eld (e.g.see Nemec & Kazancõ, 1999).

Bertran & Jomelli's de®nitions of mass¯owtypes are simplistic and unsuitable for wideruse in sedimentological research. The triggeringmode and implications of a particular type ofdebris¯ow may be different in different deposi-tional systems, and we do not share the opinionthat the mode of debris¯ow generation should bepart of the mass¯ow de®nition. This latter sug-gestion would mean that almost every localdepositional system, or slope-failure event, mightrequire a different ad hoc de®nition of a debris-¯ow, or a grain¯ow.

The presence or absence of water is important,but there is no reason why a grain¯ow should, byde®nition, be air dry. After all, the originalexperiments from which the term grain¯owderives (Bagnold, 1954, 1955, 1956) involvedsolid grains suspended in a liquid. Grain¯owdeposits have been reported from a whole rangeof widely disparate sedimentary systems, inclu-ding subaerial colluvial slopes (HeÂtu et al., 1995;Van Steijn et al., 1995; Nemec & Kazancõ, 1999)and aeolian dunes (Hunter, 1977; Kocurek, 1996),but also the slipfaces of subaqueous bedforms(Hunter, 1985; Hunter & Kocurek, 1986), beach-faces (Sallenger, 1979; Swirydczuk et al., 1980),steep underwater deltaic slopes (Postma, 1984;Postma & Roep, 1985; Nemec, 1990; Massari,1996; Sohn et al., 1997; Nemec et al., 1999), aswell as slopes, canyons and fans in the deep-searealm (Lowe, 1976, 1982; Mullins & Van Buren,19791 ; Nemec et al., 1980; Clifton, 1981, 1984;Lash, 1984; Dingler & Anima, 1989; Surlyk& Ineson, 1992; Schuppers & Martinius, 1994).Even an aeolian grain¯ow may not necessarily bedry (Hunter et al., 1983), and there are also casesin which a subaerial grain¯ow crosses the air±water interface when descending the steep slopeof a fan delta (Larsen & Steel, 1978; Nemec et al.,1984; Blikra & Nemec, 1998) or the slipface of an`aeolian delta' (Beadle, 1981, ®g. 11.5). Theliterature abounds in studies showing that allkinds of debris¯ow can be triggered by liquefac-tion, grain¯ows included (e.g. see the `gravitywinnowing' described by Postma, 1984; Nemec& Steel, 1984; Nemec et al., 1984), and it wouldbe quite misleading to assume that eachdebris¯ow type must necessarily be generateddifferently, even if this may be true in somecases.

COARSE GRAVEL INFILTRATED WITHLAMINATED SAND

Bertran & Jomelli question our observation thatthe large interstitial spaces in a bouldery gravelderived by snow avalanches tend to be in®ltratedwith a laminated sand resulting from relatedmeltwater run-off, which we have illustrated witha photograph of such a deposit (Fig. 26A). Thearguments raised against this observation are that:(1) no similar facies was noted by Jomelli (1997)to be associated with snow-avalanche deposits inthe French Alps; (2) an apparently similar facieswas found in a rockfall talus near Bourg d'Oisansin the Alps and attributed to the run-off ofrainstorm water percolating through the scree;and (3) the melting of a snow¯ow body `is neverable to deliver water discharges high enough tosort the [sand-sized] particles'. Consequently,Bertran & Jomelli assert that the deposit shownin our Fig. 26A must be of another origin, withthe implication that our criteria for the recogni-tion of snow¯ow deposits may be mistaken. Tosay the least, it is rather risky to attempt this kindof remote reinterpretation of the processes anddeposits in one region on the basis of observa-tions from another region, geographically andclimatically very different.

First, it is quite possible that the spectrum offeatures that characterize snow¯ow deposits inthese two different regions is not exactly thesame, depending on the local slope characteris-tics, the recurrence, prevalent type and sedimentload of the snow¯ows, and the snow-meltingconditions. It should be noted that the in®ltrationby waterlain sand is just one of the numerouscharacteristics of snow¯ow deposits discussed inour paper, and we have emphasized that a set ofcriteria should preferably be used for the recog-nition of depositional processes. The presence ofa waterlain in®ll alone cannot prove or disprovethe mode of gravel emplacement, which pertainsalso to Bertran & Jomelli's ®g. 2. As pointed out inour paper, it is rather the spatial characteristics ofthe gravel texture that help to discriminaterockfall and snow¯ow deposits.

Secondly, the deposit in Bertran & Jomelli's®g. 2 may well be a rockfall talus in®ltrated withlaminated sand, but we question what signi®-cance this example from Bourg d'Oisans has forthe slope conditions in western Norway?Nowhere in our paper is it written that a rockfalldeposit cannot be in®ltrated with waterlain sand,and we have reported on the local occurrence ofsuch cases (p. 915 and Fig. 8). We did not ®nd



this phenomenon to be particularly characteristicof the rockfall-dominated fans in our study area,because the vertical cliffs are negligible catch-ments, but this does not mean that the in®ltrationprocess cannot be common elsewhere. For exam-ple, the whole middle part of the QuaternaryAnatolian colluvium described by Nemec &Kazancõ (1999) shows this process to have beencommonplace, although limited to water¯owgullies in colluvium B and nearly absent in theunderlying colluvium A and the uppermostcolluvium D. In western Norway, most of themountain slopes with large recesses, cirques andmajor ravines are prone to snow avalanches, but asimilar mountain slope in a region with lessabundant snowfall may well be prone to rockfalls,while also yielding copious water run-off. Thesedifferences are important, for they may indicateregional climatic changes or inter-regional cli-matic differences. Instead of being preoccupiedwith facies similarities and precluding differen-ces, one should focus rather on the differencesand contemplate their possible causes.

Thirdly, western Norway and the French Alpshave greatly different climatic conditions (cf.Blikra & Nemec, 1998, p. 912). As a result of theoceanic climate controlled by the Gulf Stream,western Norway is notorious for its high annualprecipitation, around 2000 mm at sea level andincreasing to nearly 4000 mm at an altitude of1200 m. Most of the precipitation occurs in thelate autumn, winter and early spring. The wintersare relatively mild, with the air temperature oftenrising above 0 °C and phases of rapid snowaccumulation being interrupted by heavy rain.The rainfall intensity can reach 30 mm h±1 andcombines with a snowmelt intensity of around5 mm h±1 (Sandersen et al., 1996). A relativelythick snowpack, such as a snow¯ow lobe restingon a colluvial slope, tends to melt much fasterand yields more water. The water percolatesdown and ¯ows through a network of rapidlyexpanding and coalescing meltout conduits alongthe base of the snowpack. The ground may still befrozen, additional water is derived by run-offfrom the upper slope and the subsnowpackcon®nement of the water¯ow renders its rateslocally quite high. The meltwater current is strongenough to carry the sand and ®ne pebbles thathave melted out from the snow¯ow body and thesubstrate, as is testi®ed by a semi-continuouslobate sheet of strati®ed sand, with scatteredmeltout cobbles and boulders, that remains on theslope surface after a snow lobe has melted fully.Where the coarse gravel is more abundant, its

interstices appear to have been ®lled with stra-ti®ed sand. It is worth mentioning also that thecolluvial products of snowpack melting reportedby Guillien (1951, 1964) from the Pyrenees andNemec & Kazancõ (1999) from the Taurus Moun-tains consist of abundant strati®ed sand, withscattered gravel, which contradicts Bertran &Jomelli's assertion that tractional transport insuch conditions is infeasible.

Last, but not least, we wish to emphasize thatthe sedimentological criteria reviewed in ourpaper are based on a large number of carefullyrecognized sites of snow¯ow deposition, wherethe occurrence of rockfall deposits can be pre-cluded on the basis of slope morphology, runoutdistances and lack of gravel-size segregation. Therockfall-dominated fans and aprons in westernNorway are associated with vertical bedrock cliffscharacterized by negligible catchment areas and avery low capacity for snow accumulation. Snowavalanches are rare on these colluvial slopes, andthere are no stream¯ow gullies or other evidenceof signi®cant run-off. Therefore, rockfall depositsin this region are seldom in®ltrated with water-lain sand. Although rockfall debris may besporadically emplaced on other fans and becomein®ltrated, these are isolated occurrences in a fan-head area. Snow¯ow-dominated fans, by compar-ison, have much larger catchments and lowersurface inclinations, generally around 20°, butoften as low as 5±15° where long-runout ava-lanches prevail (Blikra & Nemec, 1998, ®gs 3 and4D; see also Rapp, 1959, 1960). The water run-offon these fans is considerable, but limited togullies and other sur®cial depressions. Theseare fully ®lled with snowdrift in the winter toearly spring time, whereby the snow¯owsdescending the mountain slope spread indepen-dently and their deposits commonly appear to beelevated, or perched, relative to the zones ofregular run-off. In short, the modern deposits ofsnow avalanches are relatively easy to recognize,and we have been careful to select sites where nostrong run-off other than the local meltwater ¯owcould occur.

SIMILARITY BETWEEN DENSESNOWFLOWS AND DEBRISFLOWS

Contrary to Bertran & Jomelli's supposition, aknowledge of debris¯ow mechanics is extremelyuseful to an understanding of dense snow¯ows,not least because the former do not melt and havebeen more thoroughly studied, and we are obvi-



ously not the ®rst to draw physical analogiesbetween the two mass¯ow types (see literaturereview in Blikra & Nemec, 1998, pp. 925±932). Theanalogies include such important aspects as theplasticoviscous behaviour, the mechanism ofshear and downslope spreading, the mode ofdebris transport, the formation of leveÂes, trans-verse ridges (compressional listric thrusts) andlongitudinal shear zones, as well as the develop-ment of clast fabric. It is true that the mechanicalproperties of snow are speci®c and highly varied(see our review, pp. 927±928), but so also are theproperties of sediment±water mixtures (see pp.917±919). Rapid rheological changes can occur inboth snow¯ow and debris¯ow avalanches (see pp.921±925), but are constrained by the avalanche'sshort duration (typically 10±20 s). It is true thatthe modelling of natural phenomena is dif®cult,depending on what speci®c aspect is to be mod-elled and for what purpose, but this does not meanthat the models for snow¯ows are less advancedthan those for debris¯ows. From a geological pointof view, the crucial question is whether thevarious mechanisms of debris emplacement canbe recognized from the sedimentary deposits, andour study has shown how this can be done on thebasis of sedimentological criteria.

Nowhere in our paper is it written that weexpect fresh snow¯ow deposits to show a strongfabric of the component rock debris. On thecontrary, we have pointed to the different modesof debris transport by different snow¯ows andindicated that the melting of the snow matrixand the settling of clasts are likely to add chaosto the fabric, which is consistent with Bertran &Jomelli's expectation. The debris carried pas-sively in the rigid plug of a dense cohesivesnow¯ow is likely to have a disorderly fabric,and is unlikely to become any less chaotic afterthe melting, unless the clasts are reoriented bysubsequent snow¯ows. But, if the dense snow-¯ow is not fully rigid and subject to transientlaminar shear, the large and more tabular clastsmay tend to assume a planar orientation. Tur-bulent powder snow¯ows roll large clasts alongthe base, but the blade-shaped clasts willprobably slide, and some of the debris depositedfrom the ¯ow head may be reoriented by the¯ow body and tail. In short, the chances forthe development of a strong fabric are very low.The patchy gravel sheets/lobes deposited byturbulent snow¯ows commonly show clustersof imbricate ¯ow-transverse clasts among hap-hazardly oriented ones, whereas the marginsshow clast alignment parallel to the lobe edge.

Gravel deposited from dense snow¯ows rarelyshows any particular organization, except on avery local scale. Our general ®eld observationwas that `the clast fabric of snow¯ow depositsvaries on a local scale, and is disorderly whenmeasured more systematically' (p. 935), whichis consistent with Bertran & Jomelli's own data.

The fabric data in Bertran & Jomelli's Fig. 4would be more useful if they included informa-tion on the type of snow¯ow and the size, shapeand original position (above base height) of theclasts. However, the fact that the fabric wasmeasured in `fresh snow¯ow deposits (beforesnow melt)' implies observations on clasts ¯oat-ing in dense snow¯ows, as the bedload debris ofturbulent powder snow¯ows is normally buriedunder several metres of snow and inaccessible. Ifso, it is not surprising that the original fabric isdisorderly, slightly planar and generally similarto that reported from debris¯ow deposits; and it iseven less surprising that a disorderly premeltingfabric appears to be no less chaotic after melting,`although the reason for this is not completelyunderstood' by Bertran & Jomelli. Notably, thepost-melting data as a whole indicate slightlyhigher fabric isotropy (disorder), even though astatistical test cannot distinguish between alesser and a greater chaos. Contrary to Bertran &Jomelli's opinion, their data are thus quiteconsistent with both our ®eld observations andthe suggested analogy between dense snow¯owsand debris¯ows.

It is unclear to what `debris ridges' Bertran &Jomelli are referring in the ®nal part of theircomment, but if the remark pertains to the`longitudinal debris ribs' described in our paper(p. 935 and Fig. 27A), then it is not true that theorigin of these depositional features was left`unexplained' by us. The explanation offered byBertran & Jomelli invokes a dense and rigidsnow¯ow, subject to discrete internal thrusting,whereas the gravel in our Fig. 27A is a deposit ofone or more turbulent powder snow¯ows.

Our ®eld observations indicate that the linearaccumulations of coarse clasts that have beenexpelled and/or shouldered aside by turbulentsnow¯ows are characterized by a fairly consistentfabric, but this does not preclude other causes oflocal orderly fabric, such as that suggested byBertran & Jomelli for dense snow¯ows. We haveobserved longitudinal shear zones (p. 930 andFig. 22D), but these involved detritus originallypresent within, and subsequently melting outfrom, the snow¯ow body itself. The same charac-teristic pertains to the systems of closely spaced



listric thrusts, manifested as sur®cial transverseridges at the snow¯ow surface (Fig. 23A), wherewe could not see any injection of substrate debris.We also could not see any post-melting transverseridges of debris that might be attributed to theinjection phenomenon described by Bertran &Jomelli. It is possible that this phenomenondepends on the moisture level of the snow¯owand the type and thermal state (wet vs. frozen) ofthe substrate. The snow¯ows in western Norwaytypically come to rest on colluvial slopes coveredwith a snowpack and often frozen, with sporadicexceptions.

IMPORTANCE OF DEBRIS CREEP

We agree with Bertran & Jomelli that debris creep,soli¯uctional or not, is an interesting and oftenimportant process. Although we could not seemuch of its direct in¯uence on the colluvialslopes in our study area, we have discussed thisprocess (pp. 938±941) for the sake of complete-ness and to draw the attention of sedimentolo-gists who may study colluvial deposits. Weperceive Bertran & Jomelli's comment as a con-tribution to our review.

Within the altitudinal range of the colluvialaccumulation zone in our study area, soli¯uc-tional creep is recognizable on slopes of 20±30°,which practically means the bedrock slopescovered with glacial till, and we have pointed tothe likely importance of creep in causing gravita-tional failures of the latter. Soli¯uction occurs ongentler slopes in high-altitude areas, but this factis irrelevant to our study. The rockfall-dominatedcolluvial slopes are steep, but show little evi-dence of creep. Some reorganization of sur®cialdebris may occur as a result of freeze±thaw cyclesand under the impact of falling blocks, assuggested by Bertran & Jomelli, but these subtlechanges are unrecognizable and of minor impor-tance, whereas the role of both snow avalanchesand water¯ow is minimal on these slopes. Thecolluvial slopes dominated by debris¯ows andsnow¯ows are probably not steep enough to showany substantial creep, whereas their steeperapical parts are subject to frequent reworking byavalanches and ¯owing water. The well-pre-served primary lamination in the waterlain sandyinterbeds and in®ll of coarse gravel clearlycontradicts Bertran & Jomelli's supposition thatsoli¯uction is `responsible for a large part of the®nal sedimentary structure of the deposits'. It is amistaken notion also that debris¯ow deposits are

`crudely strati®ed' and that soli¯uction alone canpossibly produce colluvial fans and aprons.

The fact that we have not found demonstrableevidence of creep on the colluvial slopes in ourstudy area does not mean, of course, that thisprocess is not well recognizable elsewhere inNorway (Matthews et al., 1986; Nesje et al.,1989), at high altitudes, and possibly much moreimportant in other regions. Creep processes arealso thought to have been more important in ourstudy area during the past stages of permafrostconditions, as in the Younger Dryas time.

DEBRISFLOWS AND CLIMATE

We agree with Bertran & Jomelli that, from aglobal perspective, there is probably no simplerelationship between colluvial debris¯ows andclimatic conditions. Debris¯ows are common-place and can be caused by a range of factors.However, our study pertains to a particularregion, with particular geoclimatic and geomor-phic settings, where a regional-scale impact offactors similar to the forest ®res in YellowstonePark or the timber harvesting in British Columbiacan readily be precluded. Palynological studies ofthe Holocene in Norway give no indication ofwidespread forest ®res, and the country's landarea of »324 000 km2 had a population of only723 618 as recently as AD 1769 (®rst of®cialcount). In the last century, Norway has seen itspopulation more than double and reach 4á3million but, with 40% of the population stillconcentrated around the Oslo fjord, the countryremains the sparsest populated part of the Euro-pean continent and one of the least deforestedand anthropogenically disturbed areas in theworld. Although local hillslope deforestation orincidents of forest ®re cannot be precluded in astudy area of more than 15 000 km2, we wish toemphasize that neither of these factors canpossibly trigger debris¯ows by itself, withoutthe involvement of a substantial amount of water,and that our data are not from a single hillslope,but from two dozen widely scattered localities.

Furthermore, the Fennoscandian craton is tec-tonically stable and lacks powerful earthquakes.The strongest earthquakes recorded south of ourstudy area had a magnitude of 4á0±4á5, and thosein offshore western Norway were rated as 4á9±5á3 (Bungum et al., 1991); none of them is knownto have triggered any debris¯ows. In short,Norway can be regarded as a kind of naturallaboratory as far as the topic of the present



discussion is concerned, and it is widely recog-nized that the principal factor triggering debris-¯ows on the mountain slopes in this region isepisodes of heavy rainfall combined with snowmelting (Rapp, 1963; StroÈmquist, 1976; Sander-sen et al., 1996; Matthews et al., 1997; Rappet al., 1997). If other triggering factors came intoplay earlier in the Holocene, they were mostprobably climatic too, although some exception-ally strong earthquakes cannot be precluded.

The increase in debris¯ow processes during theLittle Ice Age is by no means an established fact,particularly in Scandinavia. The claim that Jon-asson's (1993) data from northern Sweden sup-port this notion is a misjudgement, as the data arevery poorly constrained chronostratigraphically,and the deposits in question are products ofunspeci®ed `rapid mass-wasting activity', whichthe author has chosen to call `debris ¯ows'. Atbest, Jonasson's (1991, 1993) data can indicate anincrease in debris¯ow processes in the twentiethcentury, which may re¯ect the impact of rain-storms and snowmelt on mountain slopes whosevegetation cover had been depleted by the pre-ceding Little Ice Age.

The historical base of observational data isimportant, but should be considered with muchcaution. The historical accounts of natural cata-strophic events, even as recently as the earlytwentieth century, commonly failed to discrimi-nate between such processes as debris¯ows,rockfalls, snow avalanches or even ¯ash ¯oods.Most of these events have later been lumpedunder a geomorphologist's universal label `land-slide' and are now being invoked under variousspeci®c labels by various authors. Bertran &Jomelli's reference to Jonasson's (1993) data is acase in point, where it is essential to know whatprocess we are dealing with, because differentprocesses may have different causes.

The meteorological conditions promotingdebris¯ows on the mountain slopes in Norwayhave been studied in detail by Sandersen et al.(1996), who examined some 30 historical eventsand concluded that the triggering factor wastypically an episode of heavy rainfall of approxi-mately 1-day duration and intensity between 5and 30 mm h±1, accompanied by a 1- to 4-hrainstorm or combined with rapid snowmelt.

The ®rst recognizable peak of post-glacialdebris¯ow activity in our study area, around12á5 ka BP (see Fig. 39 in Blikra & Nemec, 1998),was caused by gravitational failures of the thick,metastable till mantle left on steep mountainslopes by the retreating ice-sheet. The high inci-

dence of debris¯ows in the Younger Dryas (11±10 ka BP), with some sedimentological indicationof snow-bearing ¯ows, was thus not related to aglacial retreat, as suggested by Bertran & Jomelli,but was probably caused by the sparse vegetationcover, permafrost conditions and abundant sum-mer snowmelt. Notably, the areas that wereactually covered by the ice-sheet in the YoungerDryas did not show any recognizable pulse ofdebris¯ow processes directly after the deglacia-tion, and a similar conclusion was reached byJonasson (1991) in the northern Scandinavianmountains. The Holocene debris¯ow deposits inour study area were rather poorly constrained byradiocarbon dates at the time of publication, butthis regional database is presently being improvedwith a large number of new dates from bothsubaerial and lacustrine sedimentary successions.

THE MEANING OF HIGHERSOLIFLUCTION RATE

It is indeed widely accepted that soli¯uctionprocesses occur in cold climatic conditions, orrepresent climatic deterioration, but the exactfactors that control or in¯uence the rate ofsoli¯uction are by no means clear (see review byMatthews et al., 1993). For example, an increasein soli¯uction rate has been attributed variouslyto a decrease in summer temperatures, anincrease in winter precipitation or a decrease inthe winter temperatures and precipitation (seereferences in Bertran & Jomelli; also Matthews &Berrisford, 1993). To say that a higher rate ofsoli¯uction `indicates a climatic deterioration' isan oversimpli®cation that obscures the problemand may be quite misleading.

The late Holocene soli¯uction processes inNorway correspond to generally colder climaticconditions. However, the available regional data(Matthews et al., 1986; Nesje et al., 1989) indicatethat the episodes of higher soli¯uction rate are notcorrelative with the phases of glacier advance orpronounced snow-avalanche activity, whereas theLate Holocene colluvial record indicates alternat-ing phases of snow¯ow processes and debris¯owsaccompanied by water run-off (Blikra & Nemec,1998, p. 950). This evidence suggests that anincrease in soli¯uction rate may thus not be dueto the phase of strong `deterioration' leading toglacial advance, but rather to an intervening phaseof slightly milder climatic conditions.

In short, the problem is by no means as simpleas Bertran & Jomelli propose, and a wider range of



detailed studies in different climatic zones isneeded to evaluate the climatic meaning ofchanges in soli¯uction rate.

CLIMATIC SIGNIFICANCEOF COLLUVIUM

Bertran & Jomelli's reservations notwithstanding,colluvial sedimentary systems have a remarkablyhigh potential to record the responses of moun-tain slopes to climate and climatic changes,especially in terms of extreme weather condi-tions. Colluvial fans and aprons are `closed-end'systems, characterized by a considerable episodicinput and negligible output. Unlike alluvial fansor ¯uvial plains, they are not subject to sweeping¯oods, seldom form catchments to streams, andtheir deposits suffer relatively little contempora-neous erosion or reworking. Bertran & Jomelli's®nal comments have clearly missed the point.

It is true that the deposition of debris byavalanches often combines with more subtlesecondary processes (e.g. winnowing, illuviation,local creep, wind action and pedogenesis), butthe aim of a sedimentological study is to recog-nize the nature and meaning of a particularcombination of processes, rather than discard itin advance as ìmpossible' to study. It is true alsothat some of the processes involved in colluvialsedimentation are known to occur in otherdepositional systems (see the ®rst section of thisreply), but it is the mountain-slope source anddepositional setting, the avalanching mode ofepisodic sedimentation and the unique combina-tion of processes, or facies, that render colluvialsystems speci®c and recognizable. Every depos-itional system is controlled by a range of factors,often inter-related, and colluvial systems are noexception. In fact, they are among the leastintricate. It should be noted that the principle of`complex control' underlies virtually all geologi-cal phenomena, that sedimentary systems farmore complicated than colluvial have beenstudied successfully (see Reading, 1996), and thatthe recognition of the controlling factors is one ofthe primary aims of geological research. In afrontier area such as colluvial sedimentology, oneshould preferably focus on relatively simple,laboratory-like ®eld cases, and our paper haspresented such a case study. Bertran & Jomelli'sspeculation on `possible' complications and`global' variations points to little more than theneed for wider research, emphasized in ourpaper.

CONCLUDING REMARKS

As we have demonstrated, Bertran & Jomelli'scomments in no way detract from the contents ofour paper. Our study of the colluvium in westernNorway had three principal aims: (1) to de®ne themain depositional processes and establish sedi-mentological criteria for their recognition; (2) toidentify the products of these processes in a largenumber of local stratigraphic sections; and (3) tocompare the sections, on a chronostratigraphicbasis of abundant radiocarbon dates, in order torecognize correlative regional changes in deposi-tional processes. We have attributed these facieschanges to regional climatic changes and veri®edour interpretation by comparing the colluvialrecord with other types of regional palaeoclimaticproxy data, such as the record of glacier ¯uctu-ations (Nesje & Kvamme, 1991; Matthews &KarleÂn, 1992; Dahl & Nesje, 1994, 1996) andextensive pollen and tree-line data (Bennet, 1984;Selsing & Wishman, 1984; Birks, 1988, 1990;Kullman, 1988, 1989, 1993; Solem, 1989;GunnarsdoÂttir, 1996; Blikra & Selvik, 1998). Wereferred in particular to this latter parallel publi-cation and regional review, and we fully agreewith Bertran & Jomelli's advice that à comparison[with other data types] may help to distinguishthe role of climate from that of other factors'.

We emphasized in our paper (pp. 951±952) thatìt is crucial to compare different types of climaticproxy record, because their correspondence anddiscrepancies may be meaningful'; and that `com-parative studies should show how the signal ofQuaternary climatic changes has been recorded incolluvial successions in other regions, in a widerrange of climatic zones'. We have also empha-sized that the `colluvial systems are highly depend-ent upon local slope conditions, and thepalaeoclimatic record is thus most legible whenfacies data from a larger number of localities arecompared in chronostratigraphic terms'. We wishto re-emphasize these points in the present reply.

One should avoid taking our study for morethan it is and was meant to be: a regional casestudy in colluvial sedimentology, with a prelim-inary attempt to use colluvial facies successionsas a proxy record of climatic conditions and theirchanges. A wider range of similar studies fromother regions and climatic zones are clearlyneeded (e.g. see Nemec & Kazancõ, 1999), and itis the comparative approach focused on bothfacies similarities and differences that is crucialto an understanding of colluvial systems. Wehope that the present discussion has clari®ed



some of the controversial issues and will stim-ulate further sedimentological research on collu-vial deposits. One of the important challengeswill be to assess the response of mountain slopesto climate change and future climatic conditionsin particular regions. An increase in extrememeteorological events, such as rainstorms orheavy snowfall, may have a serious impact onmountain slopes, which has to be taken intoconsideration in the development and planningof human habitats and activities in many parts ofthe world.

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Manuscript received 24 November 1999;revision accepted 1 March 2000.



Post-glacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

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