Geomorphology 83 (2007) 1–13www.elsevier.com/locate/geomorph

Ultramafic rock weathering and slope erosion processesin a South West Pacific tropical environment

Anicet Beauvais a,b,⁎, Jean-Claude Parisot b, Cécile Savin c

a IRD, UMR161-CEREGE, BP-A5, 98848 Nouméa, New Caledoniab IRD, UMR161-CEREGE, Europôle de l'Arbois-BP80, 13545 Aix-en-Provence, France

c GEOPHYSICAL, 19 rue Jenner 98800 Nouméa, New Calédonia

Received 25 October 2005; received in revised form 14 June 2006; accepted 15 June 2006Available online 4 August 2006

Abstract

Weathering and erosion processes are investigated using electrical resistivity tomography (ERT) imaging and the quantificationof geomorphic patterns at the edges of a lateritic plateau overlying ultramafic rocks in the north western region of the main island ofNew Caledonia (Southwest Pacific). The obtained ERT images document the structure and long-term evolution of the regolith,while source area parameters such as area (A) local slope above channel head (tanθ) and longitudinal river profiles allow thecharacterization of contrasting geomorphic patterns around the plateau. The geo-electrical profiles show a succession of hard rockprotrusions and weathering troughs, whose depth varies greatly. The area–slope relationship allows the distinction betweensaprolite- and ferricrete-mantled source areas. The former could result from a regolith erosion process by shallow landslides; thelatter from a secondary ferruginization process of reworked lateritic debris. The deepest troughs underlie saprolite-mantled sourceareas above channel heads, which are characterized by a low permeability saprolite, relatively high slope gradient, and lower area/slope ratios. Such source areas generate fairly high runoff, sustaining rivers and creeks with relatively high erosion power. Theferricrete-mantled source areas are characterized by higher permeability and area/slope ratios, leading to lower runoff and lesserosion but further chemical rock weathering. The ferricrete of those source areas acts as a protective hardcover against mechanicalerosion of the underlying regolith. This ferricrete reworks, at least partly, allochtonous lateritic materials inherited from a previousdisaggregated ferricrete that suggests past erosion processes driven by hydro-climatic condition changes.© 2006 Elsevier B.V. All rights reserved.

Keywords: Slope process; Electrical resistivity tomography; Weathering; Erosion; Ultramafic rocks; New Caledonia

1. Introduction

In tropical areas deep weathering mantles develop atthe expense of rocks, and they are very sensitive toerosion processes if relief is steep. For instance, shallowregolith landslides and/or debris flows can occur when awater-saturated regolith stands between rather steep

⁎ Corresponding author. Tel.: +33 687 26 07 59; fax: +33 687 26 07 69.E-mail address: Anicet.Beauvais@noumea.ird.nc (A. Beauvais).

0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2006.06.016

ground surface and bedrock topographies. Such hill-slope processes may control the way the channelnetwork develops and thus the erosion dynamics aschannel heads sap the regolith profiles at the edges of arelic lateritic plateau. We investigate the links betweenweathering and erosion processes involved in theshaping of a lateritic landscape by applying bothelectrical resistivity tomography (ERT) to image theregolith structure and a geomorphologic analysis of theproperties of source areas above channel heads.

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Slope process features frequently occur at the peripheryof the scarped ultramaficmassifs ofNewCaledonia,whichappear thus as suitable geomorphic objects to analyzehillslope erosion processes. Slope hazard fingerprints suchas shallow landslides, which are conditioned by a cyclonicclimate, characterize these hillslope processes. Ultramaficrocks, mainly peridotites belonging to an obductedophiolite of 34 Ma (Paris et al., 1979; Cluzel et al.,2001), are major geological features of New Caledonia asthey represent one third of the main Island area. Theserocks were deeply weathered under lateritic conditionssince the ophiolite over thrusting. The combined effects ofweathering and erosion processes and epeirogenic move-ments have shaped the peridotite massifs and have contri-buted to step relic planation surfaces (Chevillotte, 2005;Chevillotte et al., in press). Most of those massifs storenickel ore, which developed by supergene enrichmentprocesses. The geomorphic patterns of ultramafic rockmassifs may also reflect quite intense erosion processes,resulting in “lavaka” and/or shallow landslide erosionfeatures above channel heads. The links between weather-ing and erosion processes are roughly understood, butremain to be characterized and quantified, particularly inthe domain delimited between the drainage divide or theslope change and the channel heads, which is verysensitive to epeirogenic and/or eustatic movements.

The ERT technique used to investigate the weatheringpatterns has previously proved very useful to explorethick lateritic weathering mantles bearing stepped land-surfaces capped with ferricrete over granitic bedrock inWest Africa (Beauvais et al., 1999), providing a fullknowledge of the weathering mantle, its 2-D layerorganization over a thickness of several tens of meters,from the fresh rock to the ground surface. In particular,the image of the bedrock profile at the interface with thesaprolite horizons can contribute to appraise mineral ormetal ore resources as well as spatial variations inbedrock weathering (Palacky and Kadekaru, 1979;Beauvais et al., 2003). ERT data coupled to miningcore logs provide a rather accurate 2-D image of theweathering mantle, its thickness and variability, whilethe peripheral erosion patterns are quantified bymeasuring the source area parameters such as slopegradient and area above the channel heads (Montgomeryand Dietrich, 1989). A geomorphologic mapping of theconcave portion of source areas surrounding the areaunder ERT investigation, including measurements ofground surface elevation, bedrock elevation, slopegradient, and watershed area, may document the runoffpattern and thus the potential erosion around the edges ofa lateritic plateau in New Caledonia. The conjugatedgeophysical and geomorphologic approach is employed

to characterize the regolith structure, and thus theinfluence of the weathering patterns on the erosionprocesses around that plateau.

2. Field setting

The study area is centered on the Tiebaghi massif,which is one of the six ultramafic rock massifs of NewCaledonia's west coast (Fig. 1). This is the site of a formerchromite mine, and is now mined for nickel. The localbedrock consists of peridotite, the main lithological faciesbeing harzburgite with 40% olivine and 60% pyroxene,dunitewith 90%olivine and 10%pyroxene, and lherzolite(Moutte, 1979). The harzburgite represents 90% of theperidotites. Most of these rocks are affected by serpenti-nous weathering at their base. They are overall deeplyweathered under the effects of a subtropical climate,characterized by (i) a humid and warm season fromDecember to April, which is also the period of hurricanesand/or cyclones, (ii) a humid and cool season from July toSeptember, and (iii) two transition or intermediaryseasons, from October to November and from May toJune. The mean annual rainfall is 1600 mm at Tiebaghi,the mean annual temperature is 23 °C and the meanrelative air humidity is 76% (Giraud and Rignot, 1971).The vegetation is a “maquis type” bush with endemicspecies among which Myrtaceae, Cunoniaceae, Dillinia-ceae, Epacridaceae, Proteaceae and Casuarinaceae are themost widespread (Jaffré, 1980).

The Tiebaghi lateritic plateau is 20 km long and 8 kmwide (Figs. 1 and 2a). From the highest elevation point at600 m located close to source area No. 13 (Fig. 1), theplateau surface exhibits a 3.5 to 4% slope gradient to theNorthwest and the Southeast, respectively, and an 8% slopegradient to the West. Natural cross-sections at theboundaries of the plateau, mine quarries and drillingboreholes through the lateritic plateau and the underlyingregolith allow the distinction of four main weatheringlayers over the bedrock: a coarse saprolite with elements ofpoorlyweathered bedrock, a yellow fine saprolite, a red softlaterite layer including nodular ferruginous elements and 3to 10 m ferricrete at the top (Fig. 3). Such a weatheringmantle capped by a ferricrete uniformly underlies theplateau, ofwhich the boundaries stand as peripheral erosionfeatures corresponding to source areas above channel heads(Fig. 2a). The water table is quasi permanent under theplateau ferricrete. Groundwater often springs along thewestern slope of the plateau just below the ferricrete layerand/or lower at the interface between the coarse saproliteand the bedrock. It was also observed that water dischargeat the bottom of the regolith is higher on the westernhillslope than on the eastern one (Join et al., 2005) as a

Fig. 1. Simplified geomorphological map, with electrical resistivity tomography (ERT) profiles (white lines), plateau regolith cross-sections (black lines)and source areas (dark grey patches) at the edge of the ferricrete-mantled Tiebaghi plateau (grey surfaces). Light grey = incised hillslope mantled by softregolith; dashed line = boundaries between ultramafic rocks and alluvial formations; white star = 600 m elevation point).

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result of the westward bedrock slope and larger contribut-ing drainage area, the divide being close to the east side ofthe plateau. Many sinkhole structures also affect thislateritic plateau where the ferricrete is fragmented andcollapsed (Fig. 2b). Some creeks are alignedwith a series ofsuch sinkholes on the plateau surface according tostructural directions 130–140N (Fig. 2b). The sources ofthe creeks often occur at resurgences of sinkhole drainageon the flanks of very steep hillslopes as “lavaka” erosionstructures marked by a ferricrete scarp of 3–10 m. Manyerratic ferricrete blocks of meter size are scattered on theslopes down to the scarped edge (Fig. 3a), indicatingcollapse mechanisms of the regolith-mantled hillslope(Latham, 1986). The plateau hillslope is thus mantled bysaprolite as a result of erosion of the upper regolith layers(ferricrete and red soft laterite) and retreat of the plateaumargin. Two types of source area were investigated at theplateau-hillslope boundary, namely saprolite- and ferri-crete-mantled source area (Figs. 2 and 3). The former

results from the partial erosion of the regolith profile thatprovides natural but incomplete lithological cross-sections(Fig. 3a). The latter has not been incised, and the lateriticweathering profile is entirely preserved from the weather-ing front to the ferricrete (Fig. 3b). This ferricrete can bepolygenic as it contains allochtonous lateritic elements.

3. Methods and techniques

3.1. Geophysical survey

The electrical resistivity tomography (ERT) tech-nique was employed using the ABEM Lund ImagingSystem with a multi-electrode Wenner configuration,i.e., an array of 64 steel electrodes, to obtain a 2-D imageof the general geo-electrical structure of the regolith.This method usefully documents the ranges of resistiv-ity, ρ, of each layer or horizon composing the thicklateritic weathering mantles (Ritz et al., 1999; Beauvais

Fig. 2. Aerial views of the Tiebaghi plateau and its margins. (a) Overview. (b) Detailed view of the north part. Black triangle = upslope limit ofconcave source area; white triangle = channel head; white circle = sinkhole.

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et al., 1999). The apparent resistivity was measuredalong a 640 m straight line with an electrode spacing of10 m using a computer-controlled multichannel resis-tivity meter (Griffiths et al., 1990). A 10-m electrodespacing is the best compromise to investigate theregolith structure down to 80 to 100 m depth and toimage the interface between the fresh bedrock and thesaprolite with relatively good accuracy. The apparentresistivity data were inverted to obtain true resistivitytomography sections of the underlying mantle structures(Edwards, 1977; Daily and Ramirez, 1992; Beard et al.,1996; Loke and Barker, 1996; Storz et al., 2000). On thebasis of a non-linear least-squares optimization method,the interpretation software RES2-DINV was effectively

used to convert the measured resistivities to provide aninverted model of 2-D colour pseudo sections of theinvestigated structures (Griffiths and Barker, 1993;Loke, 2003). The topographic variations were alsoincorporated in the inversion model that finally providesmore realistic 2-D images of the terrains. As theresulting resistivity tomography of poorly stratifiedterrains does not really account for gradual change ofresistivity between the different regolith layers, boreholeinformation is required to thoroughly constrain thegeological interpretation of the 2-D electrical pseudosections (Beauvais et al., 1999, 2003). For instance,mining borehole logs were used to delineate moreprecisely the fresh bedrock elevation profile and thus to

Fig. 4. Descriptions of source area. (a) Plan view. Grey = source area.Dashed line = measurement line of length (L). (b) Cross-section. sc =slope change; ch = channel head; zsc = regolith thickness at slopechange; zch = regolith thickness at channel head; hsc = regoliththickness excluding the ferricrete layer; θ=slope angle; β=bedrockslope angle.

Fig. 3. Schematic cross-sections of source areas. (a) saprolite-mantledsource area. (b) ferricrete-mantled source area. Black and greytriangles stand for slope change and channel head of source area,respectively.

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estimate the regolith thickness along the geo-electricalsections.

3.2. Geomorphologic analysis

A geomorphologic analysis was carried out usingdifferent scalemaps. Fig. 1was drawn first using 1:50,000topographic maps with 20 m contour lines, on which thelocation of the analyzed source area is represented as wellas the lateritic plateau bearing a ferricrete and the incisedhillslope surface mantled by saprolite. Second, a 1:10,000topographic map of the study area, with 10 m contourlines and depicted channel network, has allowed thedelineation of the concave portion of source areas andestimation of their topographic properties. The sourcearea is defined as the drainage area upslope of the channelhead (Shreve, 1969) of which the only concave part wasinvestigated (Figs. 2 and 4a). Most of the western and allthe eastern source area are saprolite-mantled, the ferricrete

being broken down and eroded, while some westernsource areas are carved into ferricrete in downslope zonesof the lateritic plateau surface (Figs. 1 and 3). Third,1:5000 regolith isopach and bedrock isohypsemaps of theplateau were used to analyze the spatial relationshipsbetween the ERT-depicted regolith structure and theground surface topography including the edges of theplateau (Fig. 1). The 1:10,000 map-based geomorpholo-gic analysis also allows the examination of relationshipsbetween the regolith structure and the topographicproperties of each delineated source area (Fig. 4a) suchas local slope (tanθ), drainage area (A), source area length(L), and the elevation of the source area at slope change(sc), and that at the channel head (ch) (Fig. 4b). Themethod to depict the source area above channel heads andto measure A, L and tanθ on topographic maps waspreviously described byMontgomery andDietrich (1989)as shown in Fig. 4a. Source areas are thus delineated asunchanneled concave surfaces, the longitudinal limitsbeing perpendicular to the contour lines. Area, length andslope of the delineated source areas were measured on the1:10,000 topographic maps (Table 1). Available descrip-tions of boreholes located at the edges of the plateau,topographic data as ground surface elevation, data fromisopach and/or isohypse maps and ERT geo-electricalimages were used together for measuring the bedrock

Table 1Properties of source areas

San°

ΔE(m)

CHE(m)

L(m)

tanθ A(m2)

zsc(m)

hsc(m)

zch(m)

tanβ

a1 55 445 207 0.28 8000 32 23 27 0.252 48 482 139 0.37 6300 37 34 24 0.273 65 465 229 0.30 12500 37 32 25 0.244 80 420 220 0.39 7000 32 31 28 0.375 57 493 170 0.36 8000 40 35 24 0.256 40 510 186 0.22 6700 60 58 40 0.116b 38 512 126 0.32 4900 45 41 30 0.197 60 490 209 0.30 7000 30 27 20 0.258 52 498 159 0.35 6600 40 34 10 0.159 50 520 149 0.36 4400 45 41 25 0.2110 50 520 103 0.56 2700 40 35 22 0.3611 50 520 112 0.50 4300 45 40 22 0.2712 62 518 219 0.30 8100 45 39 22 0.1913 72 523 241 0.31 10500 48 41 22 0.2014 45 525 186 0.25 8300 44 37 22 0.1315 30 520 104 0.30 4200 40 35 22 0.1216 55 495 198 0.29 6300 40 35 24 0.20

b19 52 478 131 0.43 4300 37 33 25 0.3320 35 465 135 0.27 5400 32 27 25 0.2121 35 465 139 0.26 6600 32 27 25 0.2124 20 480 78 0.27 2100 32 27 25 0.1625 30 490 129 0.24 3300 36 28 24 0.1526 18 492 72 0.26 2200 34 29 24 0.1127 28 452 143 0.20 4700 29 23 26 0.1829 30 370 133 0.23 6600 18 11 31 0.3330 22 378 78 0.29 2000 18 14 30 0.4634 52 398 207 0.26 8000 20 11 20 0.2635 47 403 205 0.23 6300 25 24 45 0.3432 44 456 186 0.24 9800 30 26 40 0.3037 40 360 122 0.35 4100 15 12 31 0.4939 22 488 93 0.24 3200 35 29 1540 43 437 118 0.39 4200 15 13 10 0.3541 55 415 212 0.27 8800 28 23 28 0.2742 42 438 104 0.44 2500 29 27 27 0.4243 37 433 116 0.34 3400 28 25 27 0.33

c22 38 462 238 0.16 11000 32 24 26 0.1323 25 475 172 0.15 5200 32 23 25 0.1028 22 448 221 0.10 8700 28 23 26 0.0931 20 440 151 0.13 6800 35 29 1533 27 463 197 0.14 5600 30 21 25 0.1136 30 390 153 0.20 8600 30 26 40 0.2738 40 410 263 0.15 11800 30 23 29 0.15

(a) saprolite-mantled eastern source areas; (b) saprolite-mantledwestern source areas; (c) ferricrete-mantled source areas. Sa n°=source area number (Fig. 1); ΔE=elevation difference betweenchannel head and uppermost source area; CHE = channel headelevation; L=source area length; tanθ=source area slope abovechannel head; A=source area; zsc= total regolith thickness at slopechange; hsc= regolith thickness excluding the ferricrete at slopechange; zch= regolith thickness at channel head; tanβ=bedrockslope.

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elevation. Estimates were then derived for the regoliththickness including the ferricrete at the upslope (zsc), andchannel head (zch), and also the regolith thickness ex-cluding the ferricrete at slope change (hsc) for some sourceareas (Fig. 4b and Table 1).

4. Results and discussion

4.1. Geo-electrical structure of the lateritic weatheringmantle

Three representative ERT profiles were implementedparallel and close to the plateau edges, two on the easternside and one on the western side (Fig. 1), to investigatethe structure of the lateritic weathering mantle above thenearest source area. Three contrasted geo-electricallayers were depicted from the ERT profiles, and asuperficial resistive layer is characterized by resistivitiesranging from 800 to 5000 Ωm. This resistive layeroverlies a high conductivity layer with resistivitiesranging from 10 to 120 Ωm, and at depth a resistivelayer with a resistivity range of 120 to 5000 Ωm occurs(Fig. 5). Based onmining borehole logs, the two resistivelayers correspond to the upper ferruginous layers(consisting of hard ferricrete, a ferruginous nodularlayer and/or soft ferricrete) and to the unweatheredultramafic bedrock, while the intermediate less resistivelayer can be attributed to the coarse and fine saprolitelayers. In situ electrical resistivity measurements andborehole logs allow the delineation of the limits betweenthe three geo-electrical layers (Fig. 5). Very undulatedboundaries are depicted; the interface between thebedrock and the saprolite clearly reveals quite regularlyspaced weathering troughs and unweathered bedrockprotrusions previously reported by Savin et al. (2003).

The first ERT profile oriented 140°N exhibitsinteresting erosion and weathering differentiation pat-terns (Fig. 5a). A bedrock protrusion located at the slopechange between −175 and −65 m separates two partsalong the profile. The eastern half corresponds to therather flat summit area of the plateau. The ferricrete isrelatively thick, while the saprolite is only half as thick asthat of the western half, which also shows a 3–4° slopegradient westwards. The west extremity of the geo-electrical profile ends very closely to an elongatedsinkhole in which the ferricrete is disagreggated. Theelevation difference between the eastern and westernparts is roughly 30 m, which is also the difference insaprolite thickness (Fig. 5a).

The second ERT profile is oriented N–S subparallelto the eastern plateau edge. An obvious bedrockprotrusion, between −115 and 75 m along the transect

Fig. 5. Geo-electrical structure of the regolith mantle along measurement sections a, b and c. Dashed white line = interface between bedrock andsaprolite; black line = bottom of ferricrete; upper dashed black line in (a) = topographic elevation of the eastern part; lower dashed black lines in (a)and (c) = average lower bounds of saprolite. See Fig. 1 for section locations.

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(Fig. 5b), corresponds to a quite large bedrocktopographic convex landform separating two symmet-rical saprolitic troughs. The undulated bedrock topog-raphy may suggest thickness variations of the coarsesaprolite layer that could reflect contrasted weatheringrates of the peridotite rock. The ferricrete thickness isquite constant from north to south.

The third ERT profile is oriented N–S parallel to thenorthwestern plateau edge and centered at the head of amajor creek (Fig. 5c). The bedrock topography exhibitsregular undulations marked by successive symmetricalweathering troughs and rather small bedrock protrusionswith summits standing at the same elevation. Theferricrete thickness is quite constant, and the transitionbetween the fine saprolite and the ferricrete is sharp.

The ERT investigation and the existing boreholelogs have allowed the construction of regolith isopachand bedrock isohypse maps, from which two cross-sections perpendicular to the isopach-strikes weredepicted from the eastern to the western hillside ofthe plateau (Figs. 1 and 6). The ground surface andbedrock slopes are rather parallel indicating a long-termequilibrium between weathering and erosion processesat the scale of the plateau summit. The regolithstructure consisting of weathering troughs and bedrockprotrusions is however obvious under this part of theplateau. Differences in regolith thickness and slopegradient are also observed for the eastern and westernmargins of the plateau, i.e., in the source area domain(Fig. 6). Regolith thickness effectively increases at the

Fig. 6. Cross-sections through the regolith cover (grey) of the plateau surface and its margins. S = sinkhole; black and grey triangles = slope changeand channel head of source area #8, #35, and #36; dashed line = base of ferricrete. See Fig. 1 for section locations.

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channel head of some western source areas while theopposite trend is observed for the eastern ones (Table1a, b). In contrast, the gradient of the eastern sourcearea appears steeper than the western gradient, whilethe bedrock slope of the eastern source area is gentlerthan that of the western source area (Fig. 6). Theseobservations have prompted a systematic geomorphicanalysis of the source areas all around the plateau inorder to depict their general erosion and weatheringpatterns.

4.2. Morphometric and regolith patterns of the sourcearea

Measurements of morphometric parameters such asthe area A (m2), length L (m) and slope gradient tanθ (mm−1) of 41 source areas located immediately upslope ofchannel heads around the Tiebaghi plateau are collated inTable 1. Field observations and ERT images show thatweathering troughs underlie most source areas, whichare generally flanked by bedrock protrusions. Measure-ments of bedrock elevation at the slope change and the

channel head using borehole logs as well as isopach andisohypse maps have allowed the depiction of robustpower law and linear relations (Fig. 7). The best fitequations allow a sound estimation of missing measure-ments of regolith thicknesses at the slope change and thechannel head, zsc and zch, leading us to appraise thebedrock slope gradient, tanβ, for each source area (Fig.4b and Table 1). All the eastern source areas exhibit atopographic slope gradient greater than the bedrockslope and thus larger regolith thickness at the slopechange than at the channel head, while a bedrock slopesomewhat greater than the topographic slope charac-terizes some of the western source areas with thickerregolith at the channel head than at the slope change (Fig.8 and Table 1). Thicker profiles at the channel head couldbe a result of (i) in situ rock chemical weathering leadingto deep saprolite trough or (ii) the accumulation oflateritic materials mechanically transferred fromupslope, e.g., by shallow landslides, of which thesignatures can be tested by the examination of slope-area relationship. The two mechanisms can also beinvolved together.

Fig. 7. Scatter plots of (a) bedrock elevation vs. ground surfaceelevation at slope change, and (b) bedrock elevation at channel headvs. channel head elevation. Black dots = saprolite-mantled easternsource areas; grey dots = saprolite-mantled western source areas; whitedots = ferricrete-mantled source areas.

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4.3. Slope–area relationship

Fig. 9a shows the distribution of source area slopes,tanθ, as a function of the corresponding area, A. Threegroups are distinguished according to location, whether

Fig. 8. Scatter plot of source area slope (tanθ) vs. bedrock slope (tanβ).Black dots = saprolite-mantled eastern source areas; grey dots =saprolite-mantled western source areas; white dots = ferricrete-mantledsource areas.

eastern or western, and dominant materials mantling thebed of each source area (Montgomery and Foufoula-Georgiou, 1993). All the eastern source areas have aslope gradient of more than 0.2 m m−1. The westernsource areas are distributed around this slope gradientthreshold of 0.2 m m−1 that allows the separation ofsaprolite-mantled and ferricrete-mantled source areas.For a constant drainage area, the eastern source areas areon average steeper than the western source area (Fig. 9aand Table 1). The eastern plateau edge is effectivelysteeper than the western edge (Fig. 6). For an equal localslope, the source area surface of most of the saprolite-mantled eastern creeks is also larger than that of thewestern creeks (Fig. 9a and Table 1a,b). The ferricrete-mantled western source areas are as large as the onesmantled by saprolite.

The slope-area plot (Fig. 9a) may characterizeshallow landslide and/or debris flow process-controlledsource areas (Montgomery and Foufoula-Georgiou,1993), particularly those mantled by saprolite. Someof those source areas located on the western side are also

Fig. 9. Scatter plots of (a) tanθ vs. source area A, and (b) ln[A/tanθ] vs.tanθ. Dashed line shows tanθ=0.2. Black dots = saprolite-mantledeastern source areas; grey dots = saprolite-mantled western sourceareas; white dots = ferricrete-mantled source areas.

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characterized by a bedrock slope gradient larger than thetopographic slope (Fig. 8), and can have a rather thickferricrete, e.g., 4–9 m (zsc–hsc in Table 1b). The easternsource areas have thicker profiles at the slope changethan at the channel head, and also have morehomogeneous ferricrete and saprolite thicknesses atthe slope change (Table 1a) reflecting constant in situbedrock weathering and ferricrete development on theeastern part of the plateau. In contrast, the westernsource areas have heterogeneous ferricrete and saprolitethicknesses (Table 1b–c) suggesting more complexbedrock weathering and ferricrete development. It alsoappears that the less steep ferricrete-mantled source areahave also a relatively low bedrock slope, tanβ smallerthan 0.2 (Fig. 8 and Table 1c). The ferricrete cover of 5to 9 m thickness at the slope change of those sourceareas (Table 1c) can protect also the underlying softregolith from erosion.

Thick ferricrete mantling some western source areasposes the question of the autochthonous or allochtonousorigin of the lateritic materials composing that ferricrete.Autochthonous ferricrete development would implyquite long-term steady-state climatic conditions favor-able to chemical weathering rather than to mechanicalerosion, while an allochtonous origin of lateriticmaterials composing the ferricrete would suggestsalternative chemical and erosion processes driven byhydro-climatic changes.

Assuming that the drainage area of source area, A, is agood surrogate of the contributing area per unit contourlength with respect to channel head width (Montgomeryand Dietrich, 1989), ln[A/tanθ] was plotted against tanθ(Fig. 9b). The simple least square linear regression yieldedln[A / tanθ]=8–3.4 log(tanθ) (r2=0.6, n=41). Fig. 9bshows that less steep western ferricrete-mantled sourceareas are characterized by the highest ln[A/tanθ].Furthermore, the permeability of fractured ferricrete andof the underlying soft nodular lateritic layer is approxi-mately equal to 3–5×10−5 m s−1, while that of saproliticmaterials is 1–5×10−7 m s−1 (Join et al., 2005). Thesepermeability data and the variation of ln[A/tanθ] as afunction of tanθ (Fig. 9b) imply that for a constant rainfallrate the saprolite-mantled source areas could generatehigher runoff than the ferricrete-mantled source areas(Beven and Kirkby, 1979), and thus will undergorelatively rapid erosion compared to weathering.

4.4. Regolith erosion vs. bedrock weathering

4.4.1. Slope erosion processesThe shape of the slope vs. area plots reflects specific

erosion processes depending on climatic variables, such

as rainfall, soil, lithology, vegetation cover, and complexinteractions between all these parameters (Montgomeryand Dietrich, 1992). At Tiebaghi we can assumehomogeneous lithology, vegetation cover and rainfallpatterns on both sides of the plateau. The geomorphic andsoil properties of the investigated source areas suggesthowever a contrasting hydrological response to a constantrainfall input. The differences observed in materialpermeability suggest that the steep and small saprolite-mantled source areas can actually generate more runoffthan the larger and less steep western ferricrete-mantledsource areas (Fig. 9b). Relatively high runoff impliesmore mechanical erosion of regolith in saprolite-mantledsource areas, while lower runoff induces less regolitherosion in ferricrete-mantled source areas. The relativelythick ferricrete cover also protects the underlyingsaprolite, which could be further produced by bedrockchemical weathering with respect to higher water fluxesand better drainage conditions in ferricrete-mantled thanin saprolite-mantled source areas. This proposition shouldbe tested however by future hydro-geochemical analysesof ground and creek waters all around the Tiebaghiplateau.

4.4.2. Implications of ERT geo-electrical dataThe three ERT-geo-electrical profiles on the plateau

exhibit a succession of bedrock protrusions and weath-ering troughs, the thickness of which is highly variable(Fig. 5). The geo-electrical structure of the weatheringmantle and the ground surface topographic patterndisplayed by the first ERT section (Fig. 5a) suggest athree-stage development of the lateritic weatheringmantleaccording to long-term hydro-climatic variations. Firstlyboth saprolite and ferricrete layers are developed in situby dissolution–oxidation processes of the bedrock.Secondly, following cessation of theweathering processesan erosion event has contributed to the partial dismantlingof the ferricrete. Thirdly, bedrock chemical weatheringand ferruginization processes were reactivated. Thewestern downslope part of the ferricrete cover is partlyformed, as observed in the field, of reworked debris andrelics of the previous autochthonous ferricrete, the latterbeing better preserved on the highest part of the plateauclose to the eastern side. The thickest ferricrete layer isactually located in the western and northwestern part ofthe plateau, containing in its upper part allochtonouslateritic elements of various sizes. It can also directly capin an unconformable manner the peridotite bedrock at thenorthwestern edge of the actual plateau. These observa-tions suggest a past erosion event sufficiently intense tohave washed away a part of the previous regolith(stripping of saprolite), followed by a weathering period

11A. Beauvais et al. / Geomorphology 83 (2007) 1–13

favorable to the neoformation of a ferricrete made up ofallochtonous lateritic materials directly on the denudedbedrock surface. A similar process was described inWestern Africa on hillslopes (lateritic “glacis”) where aferricrete/granite bedrock unconformity was observed(Beauvais et al., 2003). A part of the lateritic materials ofthe ferricrete-mantled source areas could originate from insitu oxidation processes of saprolitic materials (Nahon,1986; Tardy, 1997; Beauvais, 1999), but also, at leastpartly, from lateral transfers of lateritic materials from theupper eastern part of the plateau.

4.4.3. Implications of longitudinal river profilesLongitudinal river profiles representative of eastern

and western Tiebaghi plateau hillslopes also documentthe contrasting erosion patterns of the massif and itsedges (Fig. 10). The eastern river profile is generallylinear with a slope of 0.32mm−1 above the second orderstream confluence, below which it is smoothly concavewith a slope of 0.23 m m−1, and the overall slope is0.25 m m−1. The maximum slope of 0.45 m m−1 occursalong the second order streams where they are incisedinto bedrock (Fig. 10a). A global slope of 0.2 m m−1

characterizes the western river profile. The linearupstream profile with a slope of 0.21 mm−1 is connected

Fig. 10. Longitudinal river profiles through (a) channel head (CH) #8 flowin2nd, 3rd and 4th order channels are shown. Dashed line is inferred ferricrete

to a steepmiddle profile of 0.45mm−1 located just abovethe confluence with a third-order channel that reflects thecomplete incision of the regolith down to bedrock.Downstream the profile becomes fairly concave with a0.2 m m−1 in mean slope (Fig. 10b).

The shape of longitudinal river profiles documentsthe lack of maturity of the channel networks, which arestill far from equilibrium with the incision dynamics.The west creek incises a lateritic glacis surface over a 1km length upslope of the third order bifurcation (Fig.10). This segment is effectively consolidated by aferricrete. The eastern hillslope is steeper and moredeeply incised into the saprolite (Fig. 10a), as indicatedby all saprolite mantled source areas. Therefore, theeastern hillslope has undergone net erosion that hascompletely disaggregated the ferricrete, while thesmoother profile of the western hillslope is locallycontrolled by the neoformation of a ferricrete consisting,at least in part, of allochtonous lateritic materials assuggested by the occurrence of a ferricrete-mantledsource area. These observations agree well with the factthe plateau and bedrock topographies slope down to thewest (Fig. 6). This may also constrain westwardhydrodynamic paths resulting in higher water dischargeon the western hillslope than on the eastern one. High

g to east, and (b) channel head #38 flowing to west. Confluences withbase.

12 A. Beauvais et al. / Geomorphology 83 (2007) 1–13

water fluxes and good drainage conditions can lead tobedrock chemical weathering rather than mechanicalregolith erosion, and this could be the particular case ofthe western ferricrete-mantled source areas.

5. Conclusion

Regolith evolution and geomorphic processes werestudied by an approach coupling geo-electrical investi-gations of the regolith mantle underlying a lateriticplateau with source area data from around that plateau.The interpretation of the geo-electrical images isoptimized using borehole logs that document bothgeneral structure and bedrock protrusions and weather-ing troughs. The spatial relations between the groundsurface topography and the geometry of the underlyingregolith structure including the bedrock topographysuggest alternating chemical weathering and mechanicalerosion periods driven by long-term hydro-climaticvariations. The slope–area relationship discriminatessaprolite-mantled and ferricrete-mantled source area allaround the plateau edges. Field observations withconjugated ERT images and investigations of sourceareas including longitudinal river profiles togetherreveal the downward reduction of the regolith and theallochtonous origin, at least partly, of the lateriticmaterials forming the ferricrete of the western ferri-crete-mantled source areas. Deep and wide weatheringtroughs sustain the saprolite-mantled source areas,which can result from shallow landslides, and cangenerate relatively high runoff and erosion. The lesssteep ferricrete mantled source areas are more propitiousfor saprolite production by rock weathering than forregolith erosion processes, the ferricrete cover protect-ing the underlying saprolite from mechanical erosion.

Acknowledgements

This is an IRD UMR161-CEREGE contribution. Themining company “Société le Nickel”, SLN, is thankedfor allowing us the access to the Tiebaghi site and forproviding us with borehole logs and aerial photogra-phies. Bernard Robineau is thanked for field assistanceand reading the manuscript. Catherine Costis, JohnButscher and Nicaise Daffond are also thanked for fieldassistance during geophysical survey. Derek Motters-head and an anonymous referee are also acknowledgedfor their useful critiques and remarks, and TakashiOguchi for handling the manuscript. The late NicolasPerrier is greatly acknowledged for English editing of anearly draft. This paper is dedicated to his cheerfulmemory.

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