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Development of the AMS method for unconsolidated sediments. Application to tsunami deposits

Développement de la méthode AMS pour les sédiments non consolidés. Application aux dépôts de tsunami
Patrick Wassmer et Christopher Gomez
p. 279-290

Résumés

L’AMS (Anisotropie de la Susceptibilité Magnétique) est une méthode géophysique qui utilise les propriétés électromagnétiques des échantillons pour offrir une vision simplifiée de leur fabrique. Généralement utilisée sur des roches résistantes ou des sédiments cohésifs, la méthode n’a jamais été appliquée sur les sédiments non consolidés. Nous présentons ici l’utilisation de la méthode AMS sur des formations meubles, plus particulièrement dans le cadre de l’étude de dépôts de tsunami. Nous avons travaillé sur les dépôts de la côte du nord de Sumatra (Indonésie), frappée le 26 décembre 2004 par un tsunami dévastateur qui remodela l’environnement côtier et la distribution des sédiments sur la frange littorale. Nous avons effectué l’analyse de 6 échantillons, qui apparaissent d’une constitution comparable : 1) une première couche déposée par un flux orienté vers le SSW ; 2) une phase de décantation ; 3) une couche déposée lors du lavage (orienté vers le nord) ; 4) deux autres « uprushes » ont déposé deux unités orientées vers le SSW et le SSE. Cette recherche prouve que l’AMS permet de déduire l’orientation et l’énergie des flux. Cette méthode offre ainsi des perspectives stimulantes tant en géomorphologie marine que fluviale, même sur des formations superficielles non consolidées.

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Notes de la rédaction

Article soumis le 4 août 2010, accepté le 6 mars 2011

Texte intégral

Introduction

1According to D.A. Clark et al. (1992), AMS is a fundamental data for geophysics and earth sciences. AMS stands for Anisotropy of Magnetic Susceptibility. It is a fast, non-destructive and low cost method for investigating rock fabrics. The AMS method describes the measure of the orientation of the magnetic ellipsoid generated during the exposure of a sample to a series of magnetic field (H). Traditionally in experiments, an AC electrical current induces H. For each measure, the induced magnetic moment (M) is proportional to H, with a relation described by a scalar k (the volume magnetic susceptibility) such as M = k.H. The volume magnetic susceptibility k depends on the enhancement of magnetic minerals that are present in a sample, and on the orientation of the sample. The anisotropy can be visualized in 3D thanks to a triaxial AMS ellipsoid. Three axis characterize the ellipsoid: Kmax, Kint and Kmin, with the following relation Kmax< Kint<Kmin, that are respectively the susceptibility maximum, intermediate and minimum axis (fig. 1). The volumetric fabric of the sample controls the latter axis, and the AMS is a tool for visualizing the orientation of these different axes. According to N. Hamilton and A.I. Rees (1970), the values of magnetic parameters are related to the primary sedimentary fabrics of natural sediments providing that (i) the angle between the plane of maximum-intermediate susceptibilities with the horizontal plane is less than 20°, and (ii) the shape factor q ranges between 0.06 and 0.7. Although the AMS technique relies on the use of an external magnetic field for its measurement, the methodology and concepts behind this technique are more akin to a traditional petrofabric approach than they are to paleomagnetic works.

2Up to present, AMS has been a common technique for consolidated and hard rocks (Argenton et al., 1975; Ellwood, 1975) and more especially volcanic rocks (e.g., Palmer and McDonald, 1999; Canon-Tapia and Pinkerton, 2000). AMS measurements have started in the early 1960s with studies of granitic rocks (e.g., Balsey and Buddington, 1960). The AMS method has been essentially used to determine the magma flow dynamics and the emplacement mechanisms, through the establishment of a relation between AMS patterns and tectonic structures. The AMS method has also numerous applications for marine and lacustrine deposits (e.g., Granar, 1958; Park et al., 2000) and for aeolian deposits (e.g., Bradak, 2009), and for tectonic applications (e.g., Borradaile and Henry 1997; Zak et al., 2009). Eventually, the method has been extended to rocks heated in laboratory, for which the heating process can enhance in some case the original AMS signal (Henry et al., 2003). Despite these technical evolutions, the studied material was mostly hard-rock or cohesive sediments. We based the present research on the hypothesis (Rees, 1965) that the AMS method could be extended to unconsolidated sediments, proving that we have an appropriate sampling technique. This contribution aims at presenting the results of the AMS method applied to unconsolidated sediments: tsunami deposits, and it highlights the opportunities that it brings to geomorphology.

Fig. 1 – Schematic representation of the electromagnetic ellipsoid H induced by a sample, with the three principal tensors Kmax, Kmin and Kint.
Fig. 1 – Représentation schématique de l’ellipsoïde électromagnétique H induit par un échantillon, avec les trois principaux tenseurs Kmax, Kmin et Kint.

Fig. 1 – Schematic representation of the electromagnetic ellipsoid H induced by a sample, with the three principal tensors Kmax, Kmin and Kint.Fig. 1 – Représentation schématique de l’ellipsoïde électromagnétique H induit par un échantillon, avec les trois principaux tenseurs Kmax, Kmin et Kint.

Application of the AMS in geomorphology

Interpretation of the AMS cyclographic representation

3Scientists conventionally display AMS data on a circular plot, which represents the projection plan of the lower hemisphere of AMS tensor (Kmax, Kint, Kmin). Therefore, these tensors are represented as a single point when the tensors meet the boundary of the lower hemisphere. They correspond to a cyclographic trace when the plan defined by Kmax and Kint meet the lower hemisphere. The angle of this plan is modified each time a measure is done (fig. 2, see 1, 2 and 3). The synthetic diagram of these series of measures (fig. 2, lower part) provides us with the tilting of the Kmax and Kmin tensors, Kint providing complementary information on Kmax. The position of Kmax provides the declination in degree of the sample, i.e. the orientation of the sample. The vertical inclination of the grain is inversely proportional to the distance between the position of Kmax and the centre of the circular diagram. The same concepts apply to Kint. Finally, Kint gives complementary information on Kmax and Kmin, as a confirmation of the data collected by Kmax and Kint. Kmin also provides information on the grain orientation when the tensors Kmax or Kint are vertical, and then do not provide information on their orientations, since their cyclographic trace appears at the centre of the diagram.

Preamble to the application to unconsolidated sediments

4The preamble of this research is based on the existence of a preferred orientation for most of grains in a sediment body. This orientation is due to the forces that have acted during sediments deposition. Therefore, the AMS, governed by the volume’s distribution should help retrieving this information. G.B. Jeffreys (1922) has conducted the first theoretical and experimental researches on grain fabrics and on their relations to the sedimentation mechanisms, to the flow power and its direction. For well-sorted sands (which is the case for our example in Sumatra), the planar lamination of the deposits comes from the superposition of layers of similar grain size. AMS studies benefit the analysis of rock fabrics, grain orientations in rocks, magmatic flow paths orientations, etc. (e.g., Rochette et al., 1992), because grain alignments result in anisotropy of magnetic susceptibility. Therefore it is a good proxy of sediment fabric, which allows the reconstruction of flow directions during settling phase. Studies of the magnetic fabric or experimentally deposited sediments indicate that the magnetic grains are mainly oriented parallel or close to the bedding plane, with their longer axes parallel to the flow direction, and with an imbrication, when existing, dipping upstream (Hamilton and Rees, 1970; Taira 1989). The reader will note that this relation is only true for sand grains.

Fig. 2 – Explanation of AMS cyclographics for lower hemisphere projections.
Fig. 2 – Visualisation schématique des graphiques AMS (projections en hémisphère inférieur).

Fig. 2 – Explanation of AMS cyclographics for lower hemisphere projections. Fig. 2 – Visualisation schématique des graphiques AMS (projections en hémisphère inférieur).

A: Spherical representation of an observed sample. a: observation plane; b: lower hemisphere. 1: Kmax; 2: Kmed; 3: Kmin.
B: Complete graphic representation of AMS results. 1:
Kmax; 2: Kmin; 3: Kint; 4: Kmean max; 5: Kmean min. Kmax values are weakly scattered showing a NW-SE direction. The position of the Kmean max value slightly below the equator can be interpreted as a plunge of the ellipsoid and, by then, as a flow direction indicator.
A : Représentation sphérique d’un échantillon. a : plan d’observation ; b : hémisphère inférieur. 1 : Kmax ; 2 : Kmed ; 3 : Kmin.
B : Représentation graphique de résultats AMS. 1 : Kmax ; 2 : Kmin ; 3 : Kint ; 4 : Kmean max ; 5 : Kmean min. Les valeurs de Kmax ont tendance à montrer une direction NW-SE. La position de la valeur Kmean max, légèrement sous l’équateur, peut être interprétée comme une déformation négative de l’ellipsoïde et, de ce fait, indiquer l’orientation du flux.

Application to tsunami deposits

Tsunami deposits: state of the art

5Tsunamis are events that can have catastrophic imprints on human settlements and the environment as well. The recognition and the analysis of tsunami deposits is of wide interest, because it provides an insight on paleotsunamis and potential hazards (Goff et al., 2001; Felton and Crook, 2003; Scheffers, 2004; Paris et al., 2007). Tsunami can displace large amount of sediments, impacting hundreds to thousands of kilometers of shorelines. Although the phenomena can last for a couple of minutes only, the environmental and sedimentary changes can span in geological times (e.g., Scheffers, 2004; Bondevik et al., 2005; Williams et al., 2005; Goff et al., 2006). Tsunami deposits have also been recently named tsunamiites (Dawson and Shi, 2000; Dawson and Stewart, 2007; Shiki et al., 2008). The term tsunamiites was most probably created in 1988 by the Chinese Gong Yiming. It was firstly used in a publication by T. Yamazaki et al. (1989) to describe a Miocene tsunami deposit of boulders, and it has become widely accepted with the last book on tsunami deposits named ‘tsunamiites, features and implications’ for instance (Shiki et al., 2008). Tsunamiites can comprise material mainly extending from large boulders (e.g., Kelletat et al., 2004; Scheffers, 2004; Goff et al., 2006; Paris et al., 2009) to sand deposits (Paris et al., 2007).

6D. Sugawara et al. (2008) have proposed a distinction in tsunamiites, between deposits that have sedimented in subaerial and other that have settled in subaqueous environments, the latter being divided into sea floor and enclosed environment deposits. Submarine deposits seem difficult to identify. The erosion of fine sediments at the surface of the sea-bottom, changes in diatom assemblages (Okamura et al., 2004), and the intercalations of coarser sediment particles within muddy layers (Fujiwara et al., 2000; Takashimizu and Masuda, 2000) are submarine sediments proxies of tsunamis. Underwater, the suspension of particles dominates the transport process; therefore free falling of sedimentary particles from suspension may create an upward-fining sequence in submarine tsunamiites (e.g., Tada et al., 2002). However, submarine evidences of tsunami deposits remain scarce, because of data accessibility (Paris et al., 2010b). Observations of tsunamiites in basin and/or in lacustrine environments have identified distinct sand layers in silty to clayey sediments. The units below the sand deposits are typically partially eroded, because of the traction flow effects induced by the tsunami (Bondevik et al., 1997). Such deposits have been observed on the Scilly Isles, seemingly because of the 1755 Lisbon earthquake and associated tsunami (Foster et al., 1993), in Japan (e.g., Hirose et al., 2002; Nanayama et al., 2002).

7On dry land, tsunamiites can be divided between uprush features and backwash units, although the distinction is not always simple. The most characteristic features of uprush units are sand-sheets that cover the sub-horizontal areas (Atwater, 1987; Dawson et al., 1988; Long et al., 1989, 1990; Minoura et al., 1996, 2000; Paris et al., 2007; Wassmer et al., 2007; Paris et al., 2009). Vertical grain-size variations within the sand layer can also form graded bedding, with often coarse sand at the base and fine sand at the top (Benson et al., 1997; Dawson et al., 1988, 1991; Gelfenbaum and Jaffe, 2003; Wassmer et al., 2007). The layers tend to diminish in depth with the distance to the sea, forming wedges (e.g., Goff et al., 2004). High-energy tsunamis, such as the one that struck the Indian Ocean in 2004, can also erode coral reefs and depose coral fragments inland. Although these deposits are mostly limited to the first hundred meters to the shoreline, they can be numerous and their thickness can reach several meters (Scheffers, 2004; Paris et al., 2009). Backwash units have been recognized in inhabited areas from the presence of human artefacts in the deposits (Barthomeuf et al., 2010). The backwash often erodes the substratum and the newly deposited tsunamiites before depositing a poorly sorted sandy mud (Sato et al., 1995) or fine grain sands. Bent plants (Nishimura and Miyagi, 1995), cobbles (Nanayama et al., 2000) indicate the direction of the backwash of a tsunami, which is not always the opposite direction of the run-up.

8However, in the absence of such proxies - for paleotsunamis, or tsunami with a low backwash energy for instance -, the AMS method on unconsolidated sediments can inform us on the flow direction and the flow energy. Scientists have carried out identification of tsunami and paleotsunami deposits all around the world, in Europe (e.g., Dawson et al., 1988), in Northern America (e.g., Clague and Bobrowsky, 1994), in Southern America (e.g., Cisternas et al., 2005), in Central Pacific (e.g., Moore et al., 1994), in Oceania (e.g., Goff et al., 2001; Dominey-Howes, 2007), in East Asia (e.g., Minoura and Nakaya, 1991), and in South-east Asia (Wassmer et al., 2007, 2010), using geomorphological, sedimentological, stratigraphical, archaeological, palynological and paleontological evidences. Despite these numerous researches, which have peaked after the December 2004 tsunami, methods and tools haven’t evolved much. C. Gomez et al. (2010) have applied geophysical investigation – GPR (Ground Penetrating Radar) – on sand deposits, but this method presents important technical limitations, and it doesn’t help with the distinction between a tsunami deposits and another sand-sheet, or between uprush and back-wash units. C. Chagué-Goff (2010) has re-explored the potentiality of chemical signatures for distinguishing tsunami deposits, but the decay of chemical signatures vary with time and environments, so that the method is hardly applicable. B. Mamo et al. (2009) and R. Paris et al. (2010) have respectively re-explored foraminifera assemblages and nanoliths, and they have used them as proxies to identify seawater flooding. These methods are also constrained by the presence of a proxy and it is not possible to reconstruct with precision the different stages of the shuttle movement from the sediment analysis. In this context, the AMS method applied to unconsolidated deposits come anew in this research field (Wassmer et al., 2010), and it offers the possibility to determine uprush from back-wash sandy tsunamiites. The present contribution aims at correcting and improving the 2010 vanguard paper, by bringing a more precise insight on the method, by voiding the statistical incorrectness, which brought mischievous results in the previous paper (because of the averaging process).

North Sumatra, the sampling area

9The Indian Ocean tsunami of 26 December 2004 devastated about 180 km km of the Aceh coast (Northwest Sumatra), mainly between Banda Aceh and Meulaboh (fig. 3). The tsunami eroded the coast back (e.g., Wassmer et al., 2007; Paris et al., 2009) up to 500 m and it removed a large number of beach landforms and sand dunes. The tsunami was generated by the largest earthquakes ever recorded (M9.3 on Richter open-scale), which was produced by the largest known earthquake rupture (Lay et al., 2005). The main-shock rupture started 2 mn before 8:00 AM, displacing up to 30 km3 of seawater (Bilham, 2005). This tsunami has materialized on shores by 15-35 m m high waves (Lavigne et al., 2009), with a decrease towards the South. The sampling area is located on the sub-horizontal coastal plain of the Kajhu Perumnas areas, east of Banda Aceh. The topography is very flat and limited by an east-west trending linear shoreline (fig. 3). The whole zone of the sea front was constituted by a draught board of shallow fish or shrimp breeding basins. These artificial depressions with an average depth of 1 m m had a clayey bottom and were bordered by sandy dykes. Beyond 1,5 km km landwards, very shallow coastal lagoons were present in some places. During the December 26, 2004 event, the first tsunami wave, which flooded the littoral zone, eroded the sea front dunes. The following waves easily crossed and leveled the dune field before spreading on the coastal plain, which was then under the direct influence of the surging water (Wassmer et al., 2007, 2010). The second wave has been the most important and it has reached 13 m-high in the study area (Lavigne et al., 2009). In the study area, all these depressions (breeding basins, shallow lagoons) were flooded by the arrival of the successive tsunami waves without significant backwash, except after the first wave, before the end of the flooding, according to eyewitnesses (Lavigne et al., 2009). This particular behavior, probably due to the flatness of the coastal plain, differs drastically from the normal swash cycle of each wave (uprush/flooding-slack/backwash) and must be taken in account to interpret the sedimentary signature.

Fig. 3 – Location map of the study area: Kajhu Perumnas, located to the East of Banda Aceh City.
Fig. 3 – Carte de localisation de la zone étudiée: Kajhu Perumnas, situé à l’est de Banda Aceh.

Fig. 3 – Location map of the study area: Kajhu Perumnas, located to the East of Banda Aceh City. Fig. 3 – Carte de localisation de la zone étudiée: Kajhu Perumnas, situé à l’est de Banda Aceh.

A: Location and topographic settings of the coastal plains.
B: Aerial photograph, acquired before the tsunami event.
C: Aerial photograph, acquired few days after the tsunami event. 1: flooded coastal plain; 2: non-flooded coastal plain; 3: uplands; 4: tsunami uprush; 5: tsunami backwash; 6: tsunami flow depth; 7: sample.
A : Localisation et présentation du contexte topographique de la plaine côtière.
B : Photographie aérienne, prise avant le tsunami.
C : Photographie aérienne, prise quelques jours après le tsunami. 1 : plaine côtière inondée par le tsunami ; 2 : plaine côtière non inondée par le tsunami ; 3 : zones en position élevée ; 4 : vague de flot du tsunami ; 5 : vague de retour du tsunami ; 6 : hauteur d’eau atteinte lors du tsunami ; 7 : échantillon prélevé.

Methodology for the samples of North Sumatra

10We have collected the samples with 20 mm wide cubic plastic non-magnetic boxes along vertical sections of the pits dug out in the tsunami deposits. The boxes were carefully pushed manually into the moist sediment along a horizontal direction, normal to the vertical sections of the deposits. In order to avoid air compression in the box during sampling, which could have induced modifications in the grains-structure, a small hole was previously drilled at the bottom of each box to allow air escape. Before sample removal from the sediment, each box was oriented (fig. 4). The sampling boxes were sealed after removing from the sections to avoid desiccation and, by then, disturbance of the sedimentary fabric. Samples were collected in each layer of the deposits according to visual facies changes. Thin layers were not collected because of sharp changes in sediment textures within the size of the sampling boxes. Sampling of some sections was not continuous or not complete (like top sequences of Section F for instance). For this contribution, in a first time, we have analyzed 6 samples from a pit located in a former breeding basin along the pre-tsunami coastline in Kahju area (fig. 3). We collected the 6 samples at 35, 19, 14, 10, 6 and 4.5 cm cm from the roof of the deposit. In a second time, at the laboratory of Lille University (CNRS-UMR 8157 ‘Geosystèmes’), we have processed the samples with a Kappabridges® KLY-2 (fig. 5). It consists of a pick-up unit control connected with the DPU1 data processing unit and a computer to save the data. We have analyzed each sample in 15 different directions in order to reconstruct the electromagnetic ellipsoid and to determine the magnitude and direction of the different tensors (Kmax, Kint, Kmin).

Fig. 4 – Sample boxes with orientation marks in tsunamiites.
Fig. 4 – Boîte à échantillon, avec marques d’orientation sur les dépôts de tsunami.

Fig. 4 – Sample boxes with orientation marks in tsunamiites.Fig. 4 – Boîte à échantillon, avec marques d’orientation sur les dépôts de tsunami.

Fig. 5 – The Kappabridges® KLY-2, in which samples are processed.
Fig. 5 – The Kappabridges® KLY-2, dans laquelle les échantillons sont traités.

Fig. 5 – The Kappabridges® KLY-2, in which samples are processed.Fig. 5 – The Kappabridges® KLY-2, dans laquelle les échantillons sont traités.

Results: distinction of tsunami deposits from different waves

11The AMS analysis of the 6 samples has provided us with the vertical tilting data of the Kmax, ranging from 8° to 20°, with an average of 10° and a standard deviation of 8.3°, which proves the heterogeneity of the dataset. Indeed, 4 data displays a Kmax tilting angle comprised between 14° and 20°, whereas two data describe sub-horizontal patterns with angles of 0° and 1° (fig. 6). These angles are the direct expression of the deposition angles of sand grains, therefore they are essential to understand the conditions of deposition. The data has shown two preferential orientations, with 2 samples oriented towards the north (samples 3 and 4), and 4 samples oriented between SSE and SSW. The AMS analysis provided some additional parameters that are useful for better understanding the characteristics of sediment emplacement processes. L is the lineation value and it reflects the importance of the traction during emplacement. F, the foliation value reflects the importance of the decantation processes. Fs is the alignment parameter. Linked to the shearing exerted by the flowing water on the bottom of the water column, it parallels the evolution of L and F (tab.tab. 1).

Tab. 1 – Characteristics of the AMS tensors from the tsunamiites sampled in North Sumatra.
Tab. 1 – Caractéristiques des tenseurs AMS appliqués aux tsunamites de la partie septentrionale de l’île de Sumatra.

Tab. 1 – Characteristics of the AMS tensors from the tsunamiites sampled in North Sumatra.Tab. 1 – Caractéristiques des tenseurs AMS appliqués aux tsunamites de la partie septentrionale de l’île de Sumatra.

12Chronologically, we can infer from the obtained dataset, that the first and the lowest unit in Sections B, constituted of reworked sand from the littoral dunes field could represent the sedimentary record of the first tsunami wave surge (Wassmer et al., 2010). This material corresponding to 42% of the sediment volume in this area has been deposited along a SW orientation, with a strong tilting of 20° (sample 1). The second sedimentary unit constituted by a fining upwards sequence corresponds to the second wave that reached 15 m m in height in Kajhu (22.5% of the sedimentary signature). At the base, the material is coarse and contains numerous bioclasts and small rip-up clayey clasts. Due to this coarse material, sampling was not possible in this layer. The deposit evolves upwards to fine sands (samples 2 and 3). Base and top of this unit seem to have been deposited in two opposite directions but the tilting was only 1° and 0° (samples 2 and 3), so that the grain deposition was horizontal. This has been confirmed by the Kmin, which is located at the center of the cyclogram (fig. 6), so that the Kmin tensor is sub-vertical. Unit 3 corresponds to 4.5 cm cm of coarse sand layer displaying a 14° northward tilting (sample 4). The sedimentary contribution of this unit reaches 11.25% of the total amount of the material brought by the tsunami. Unit 4 (sample 5) displays a 17° tilting to the SW had a contribution of 5%. Finally, unit 5, the closest to the roof of the deposit (sample 6) displays an 8° tilting, oriented to the South.

Fig. 6 – Synthetic cyclograph of the 6 samples, lower hemisphere projection of Kmax and Kmin tensors.
Fig. 6 – Représentation synthétique des résultats pour les 6 échantillons, projection en hémisphère inférieure pour les tenseurs Kmax et Kmin.

Fig. 6 – Synthetic cyclograph of the 6 samples, lower hemisphere projection of Kmax and Kmin tensors. Fig. 6 – Représentation synthétique des résultats pour les 6 échantillons, projection en hémisphère inférieure pour les tenseurs Kmax et Kmin.

13According to the position of the sample pit to the sea (fig. 3), the cyclogram has described:

14(i) A first uprush (sample 1) oriented to the SW that emplaced a thick unit of coarse sand (tab.tab. 2). The weak foliation factor (F) attests for a traction-dominated settling.

15(ii) A second uprush (samples 2 and 3). It corresponds to a fining upwards sequence. The base was not sampled but the coarse material and the presence of numerous rip-up clasts pleads for a strong energy at first. Samples have been collected in the fine material emplaced by the “tail” of the wave (tab.tab. 2). The very low energy did not allow a clear tilting to occur. Base (sample 2) and top (sample 3) are respectively deposited in a SSW and a SE orientation. This variation is probably due to the interference between a direct and a reflected wave. Lineation factor is weak while foliation increases correspond to a deposition process dominated by decantation (for the upper part of the deposit).

16(iii) A backwash (sample 4) oriented to the NNE that emplaced a 4.5 cm cm thick layer of sand. The tilting (14°) and the increasing of the lineation factor reflect traction-dominated emplacement.

17(iv) Two uprushes (samples 5 and 6) oriented to the SW and S respectively. This variation of the incidence angle is probably due to the arrival of a reflected/refracted wave. The mean grain size is the same for the two waves and the smallest of the whole deposit but tilting is one of the stronger of the whole section with 17° for sample 2 which is characterized by the maximum value of the alignment factor that confirms strong bottom currents during emplacement. The second uprush that corresponds to sample 1 shows a tilting of 8°. The sedimentation process is dominated by settling as attested by a high value of the foliation factor.

18This interpretation is inferred from the waves orientations, the mean grain size, but also from the declination angles of the grain deposited. The highest is the energy or the velocity of the waves that brought the sand, the higher the tilting is. Therefore, uprush waves usually present the highest declination angles, although the dataset has indicated backwash oriented units with a 14° tilting.

Tab. 2 – Grain size of the sand samples.
Tab. 2 – Granulométrie des échantillons sableux.

Tab. 2 – Grain size of the sand samples. Tab. 2 – Granulométrie des échantillons sableux.

Discussion

Outlines of the contribution

19Application of AMS method to the 2004 tsunami sandy signature has been of interest since we have a thorough knowledge of the whole tsunami wave train on this area of Kajhu from field and eyewitness evidence. The method that we have presented in this contribution has produced results that have been compared with eye witnesses reports of tsunami waves orientations, with, tilting of building structures and trees, and with orientations of scorch marks (Wassmer et al., 2010), as described by F. Lavigne et al. (2009). Data provided by AMS confirms the results that we have previously described on wave dynamics and it also provides further details on the processes involved in sediment emplacement. It allows the analysis of original parameters that could not be investigated by traditional methods. Thanks to the AMS method, we have worked on each individual deposit interval identified in the field from sedimentary evidences, i.e. fining upwards sequences with sharp limits on base and top that corresponds to the sediment mass brought by a single wave. We have evidenced the variation of the orientation of the water flow during emplacement of each individual sequence and to differentiate the sediments deposited by uprush from those deposited by backwash. The evidence of a tilting to the North, related to a backwash process, for sample 4 is surprising here because eyewitness testimonies have reported that no backwash has occurred in this area (except after the first wave) and that the water pushed landwards during the entire event (Lavigne et al., 2009). This data attests that the backflow begun before the last waves reached this zone located near the former coastline. Eyewitnesses were provided by people that took refuge in the remaining houses far from the seashore where all the buildings were wiped out by the tsunami. This new insight implies a spatial restriction of the information given by eyewitnesses to the distal zone only, i.e. far from the previous seashore.

Limits of the method

20This work has proven that the AMS method is a reliable proxy of the deposition conditions of sandy deposits. However, this method has intrinsic difficulties, which we need to address by protocol explorations. Five weak points are exposed:

21- Even if this method is not difficult to implement, it requires extreme care during the sampling process. As it is impossible to sample in non-cohesive dry fine material, sampling protocol requires moist sediments. Their cohesion prevents fabric disturbance during sampling process. The very thin boundary layer disturbed by the sampling box (≤1 mm) make this AMS well adapted to fine sediments.

22- The size of sampling box constitutes one of the main limits to this method. When the layer deposited by a sedimentary process is thinner than the size of the box, i.e. 20 mm., sampling is impossible. In Banda Aceh, due to this limit, it was impossible to sample the uppermost sequences of some sections in our study area because their sicknesses were below the box size and therefore we were not able to analyze these layers with the described technique. Future investigation techniques may allow overcoming this limit.

23- Coarse material, i.e. bioclasts or rip-up clasts contained in fine sediments, can disturb the sand fabric during the sampling when the box is pushed in the sediment. Due to this limit, on the investigated pit, the base of the second fining upwards sedimentary interval corresponding to the second uprush sequence identified cannot be sampled.

24- Another limitation of the method concerns poorly sorted sediments with an important fraction of coarse volcaniclastic particles. The individual magnetic susceptibility of the coarse volcanic grains can bias the bulk volume susceptibility of the whole sample itself.

25- The measured tilting (Kmax) of a sample reflects the sediment fabric and allows us to retrieve the flow direction during deposition. The absence or weakness of tilting is hard to interpret in term of flow direction. A weak tilting measure is related to a decantation dominating process during emplacement but might be a consequence of the difficulty to maintain the box perfectly horizontal during the sampling and therefore might induce a bias in the data interpretation. For instance, sample 2 corresponding to the lower part of the second wave tail deposit, i.e. second fining upwards sequence; the measured tilting is 1°. This weak value can be attributed to a very slow bottom current but at the same time could result from the box horizontality incertitude during sampling. We faced this problem during our study in Banda Aceh where we partially explained the reduction of the grain tilting by potential back and forth water movements (seiche effect) inside the breeding ponds (Wassmer et al., 2010).

26Hence, the method opens up new opportunities for understanding hydrodynamic conditions and for reconstitutions of flow directions during deposition processes (uprush or backwash). Although, we developed this contribution from unconsolidated deposits, AMS can bring new insights to old and indurated tsunami deposits using a driller for the sampling.

Conclusions

27The efficiency of the AMS method has not to be proven. It has been used for a long time by geologists on hard rock, i.e. granitic rocks to reconstruct orientation of the progression of magmatic rocks or volcanic rocks to retrieve lava flow direction. On lacustrine or marine sediments, AMS was applied to research the depositional conditions (traction or decantation). Applied to unconsolidated fine material, this method can be helpful for researchers to investigate various fields of geomorphology. In fluvial geomorphology, when the topography is not a good indicator (Mississippi for instance), AMS can help retrieving flow orientations from former channels or terraces over floodplains. It could come as a complementary approach to the study of dunes and bars structures (e.g., Reesink and Bridge, 2007, 2009), in order to link these deposits with flow velocity and hydrological power. Openings in volcanic geomorphology are also promising. AMS can bring new insights on deposition processes of block-and-ash flows for instance, which are still difficult to assess, despite improvements in subsurface geophysical investigations (Gomez et al., 2008, 2009). AMS could also provide information on the differentiations of complex layering in lahar deposits, that have been proven more complex than expected from outcrops analysis (Gomez and Lavigne, 2010). The AMS method has proven its compatibility with unconsolidated sediments, as long as the sediment is wet, and as long as the sampling protocol is strictly followed. This method brings a new way of working on tsunamiites, with the possibility to retrieve the flow orientation and energy, not only for the whole deposit but for each single wave constituting the tsunami wave train, as long as corresponding deposits are >2 cm cm thick. Within each sedimentary interval that corresponds to the sediment contribution of a single wave, it is possible to evidence flow orientation variations from base to top. The AMS for unconsolidated sediments developed by P. Wassmer et al. (2010) offers a new dataset for tsunami deposits. Also, we are presently extending AMS applications to riverine sand deposits, and to volcanic geomorphology, in order to notably improve the knowledge of deposition conditions of lahars and block-and-ash flows.

This research has been partially funded by the French Government in the aftermath of the boxing day tsunami in 2004, by the DIPT. The program was lead by Franck Lavigne and co-directed by Raphael Paris. Preliminary interpretation of the AMS were also fostered by J.-L. Schneider and laboratory analysis would not have been possible without the help of Olivier Averbusch. The authors are also in debt to Hervé Regnauld, Franck Lavigne and an anonymous reviewer who helped improving this manuscript.

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Bibliographie

Argenton H., Bobier C., Polvêche J. (1975) – La mesure de l’anisotropie de susceptibilité magnétique dans les flyschs : application à la recherche des directions des paléocourants. Sedimentary Geology 14, 149-167.

Atwater B.F., (1987) – Evidence of great Holocene earthquakes along the outer coast of Washington State. Science 236, 371-375.

Balsey J.R., Buddington A.F. (1960) – Magnetic susceptibility anisotropy and fabric of some adirondack granites and orthogneisses. American Journal of Sciences 258A, 6-20.

Barthomeuf B., Paris R., Grancher D., Wassmer P., Vautier F. (2010) – Accumulation de blocs par le tsunami du 26 décembre 2004 à Lhok Nga, Ouest Banda Aceh. In Lavigne F., Paris R. (Eds.) Le tsunami du 26 décembre 2004 en Indonésie. Presses de la Sorbonne, Paris, in press.

Benson B.E., Grimm K.A., Clague J.J. (1997) – Tsunami deposits beneath tidal marshes on northwestern Vancouver Island, British Columbia. Quaternary Research 48, 192-204.

Bilham R. (2005) – A flying start, then a slow slip. Science 308, 1126-1127.

Bondevik S., Svendsen J.I., Mangerud J. (1997) – Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway. Sedimentology 44, 1115-1131.

Bondevik S., Mangerud J., Dawson S., Dawson A., Lohne O. (2005) – Evidence of three North Sea tsunamis at the Shetland Islands between 8000 and 1500 years ago. Quaternary Science Reviews 24, 1757-1775.

Borradaile G.J., Henry B. (1997) – Tectonic applications of magnetic susceptibility and its anisotropy. Earth-Science Reviews 42, 49-93.

Bradak B. (2009) – Application of anisotropy of magnetic susceptibility (AMS) for the determination of paleo-wind directions and paleo-environment during the accumulation period of Bag Tephra, Hungary. Quaternary International 198, 77-84.

Canon-Tapia E., Pinkerton H. (2000) – The anisotropy of magnetic susceptibility of lava flows: an experimental approach. Journal of Volcanology and Geothermal Research 98, 219-233.

Chagué-Goff C. (2010) – Chemical signatures of paleotsunami: A forgotten proxy? Marine Geology 271, 67-71.

Cisternas M., Atwater B.F., Torrejon F., Sawai Y., Machuca G., Lagos M., Eipert A., Youlton C., Salgado I., Kamataki T., Shishikura M., Rajendran C.P., Malik J.K., Rizal Y., Husni M. (2005) – Predecessors of the giant 1960 Chile earthquake. Nature 437, 404-407.

Clague J.J., Bobrowsky P.T. (1994) – Evidence for a large earthquake and tsunami 100-400 years ago on western Vancouver Island, British Columbia. Quaternary Research 41, 176-184.

Clark D.A., French D.H., Lackie M.A., Schmidt P.W. (1992) – Magnetic petrology: application of integrated rock magnetic and petrological techniques to geological interpretation of magnetic surveys. Exploratory Geophysics 23, 65-68.

Dawson A.G., Shi S. (2000) – Tsunami deposits. Pure and Applied Geophysics 157, 875-897.

Dawson A.G., Stewart I. (2007) – Tsunami deposits in the geological record. Sedimentary Geology 200, 166-183.

Dawson A.G., Long D., Smith D.E. (1988) – The Storegga slides: Evidence from eastern Scotland for a possible tsunami. Marine Geology 82, 271-276.

Dawson A.G., Foster I.D.L., Shi S., Smith D.E., Long D. (1991) – The identification of tsunami deposits in coastal sediment sequences. Science of Tsunami Hazards 9, 73-82.

Dominey-Howes D. (2007) – Geological and historical records of tsunami in Australia. Marine Geology 239, 99-123.

Ellwood B.B. (1975) – Analysis of emplacement mode in basalt from deep-sea drilling project holes 319A and 321 using anisotropy of magnetic susceptibility. Journal of Geophysical Research 80, 4805-4808.

Felton E.A., Crook K.A.W. (2003) – Evaluating the impact of huge waves on rocky shorelines: an essay review of the book ‘Tsunami – The Underrated Hazard’. Marine Geology 197, 1-12.

Foster I.D.L., Dawson A.G., Dawson S., Lees J.A., Mansfield L. (1993) – Tsunami sedimentation sequences in the Scilly Isles, south-west England. Science of Tsunami Hazards 11, 35-45.

Fujiwara O., Masuda F., Sakai T., Irizuki T., Fuse K. (2000) – Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan. Sedimentary Geology 135, 219-230.

Gelfenbaum G., Jaffe B. (2003) – Erosion and sedimentation from the 17 July, 1998 Papia New Guinea tsunami. Pure and Applied Geophysics 160, 1969-1999.

Goff J.R., Chagué-Goff C., Nichol S.L. (2001) – paleotsunami deposits: a New Zealand perspective. Sedimentary Geology 143, 1-6.

Goff J.R., McFadgen B.G., Chague-Goff C. (2004) – Sedimentary differences between the 2002 Easter storm and the 15th century Okoropunga tsunami, southeastern North Island, New Zealand. Marine Geology 204, 235-250.

Goff J.R., Dudley W.C., deMaintenon M.J., Cain G., Coney J.P. (2006) – The largest local tsunami in 20th century Hawaii. Marine Geology 226, 65-79.

Gomez C., Lavigne F. (2010) – Transverse architecture of lahar terraces, inferred from radargrams: preliminary results from Semeru Volcano, Indonesia. Earth Surface Processes and Landforms 35, 1016-1022.

Gomez C., Lavigne F., Lespinasse N., Hadmoko D.S., Wassmer P. (2008) – Longitudinal structure of pyroclastic-flow deposits, revealed by GPR survey, at Merapi Volcano, Java, Indonesia. Journal of Volcanology and Gethermal Research 176, 439-447.

Gomez C., Lavigne F., Hadmoko D.S., Lespinasse N., Wassmer P. (2009) – Block-and-ash flow deposition: A conceptual model from a GPR survey on pyroclastic-flow deposits at Merapi Volcano, Indonesia. Geomorphology 110, 118-127.

Gomez C., Lavigne F., Lespinasse N., Saintenoy A., Leparoux D. (2010) – Mise en évidence de dunes tronquées par le tsunami du 26 décembre 2004 et de dépôts sableux corrélatifs par radar géologique. In Lavigne F., Paris R. (Eds.) Le tsunami du 26 décembre 2004 en Indonésie. Presses de la Sorbonne, in press.

Granar L. (1958) – Magnetic measurements on Swedish varved sediment. Arkiv foer geofysik 3, 1-40.

Hamilton N., Rees A.I. (1970) – The use of magnetic fabric in paleocurrent estimation. In Runcorn S.K. (Ed.) Paleogeophysics. Academic Press, London, 445-464.

Henry B., Jordanova D., Jordanova N., Souque C., Robion P. (2003) – Anisotropy of magnetic susceptibility of heated rocks. Tectonophysics 366, 241-258.

Hirose K., Goto T., Mitamura M., Okahashi H., Yoshikawa S. (2002) – Event deposits and environment changes detected in marsh deposits at Aisa, Toba City. Chikyu Monthly 280, 692-697 (in Japanese).

Jeffreys G.B. (1922) – The motion of ellipsoidal particles immersed in a viscous fluid. Proceedings of the Royal Society of London, Series A 120, 161-179.

Kelletat D., Scheffers A., Scheffers S. (2004) – Holocene tsunami deposits on the Bahaman island of Long Island and Eleuthera. Zeitschrift für Geomorphologie 48, 519-540.

Lavigne F., Paris R., Grancher D., Wassmer P., Brunstein D., Vautier F., Leone F., Flohic, F., De Coster, B., Gunawan, T., Gomez C., Setiawan A., Cahyadi R., Fachrizal (2009) – Reconstruction of Tsunami Inland Propagation on December 26, 2004 in Banda Aceh, Indonesia, through Field Investigations. Pure and Applied Geophysics 165, 1-23.

Lay T., Kanamori H., Ammon C.J., Nettles M., Ward S.N., Aster R.C., Beck S.L., Bilek S.L., Brudzinski M.R., Butler R., Deshon H.R., Ekström G., Satake K., Sipkin S. (2005) – The Great Sumatra-Andaman earthquake of 26 December 2004. Science 308, 1127-1133.

Long D., Smith D.E., Dawson A.G. (1989) – A Holocene tsunami deposit in eastern Scotland. Journal of Quaternary Sciences 4, 61-66.

Long D., Dawson A.G., Smith D.E. (1990) – Tsunami risk in northwestern Europe: A Holocene example. Terra Nova 1, 532-537.

Mamo B., Strotz L., Dominey-Howes D. (2009) – Tsunami sediments and their foraminiferal assemblages. Earth Science Reviews 96, 263-278.

Minoura K., Nakaya S. (1991) – Traces of tsunami preserved in inter-tidal lacustrine and marsh deposits: some examples from northeast Japan. Journal of Geology 99, 265-287.

Minoura K., Gusiakov V.G., Kurbatov A., Kateuti S., Svendsen J.I., Bondevik S., Ota T. (1996) – Tsunami sedimentation associated with the 1923 Kamchatka earthquake. Sedimentary Geology 106, 145-154.

Minoura K., Imamura F., Kuran U., Nakamura T., Papadopoulos G.A., Takahashi T., Yalciner A.L. (2000) – Discovery of Minoan tsunami deposits. Geology 28, 59-62.

Moore J.G., Bryan W.B., Ludwig K.R. (1994) – Chaotic deposition by a giant wave, Molokai, Hawaii. Geological Society of America Bulletin 106, 962-967.

Nanayama F., Shigeno K., Satake K., Shimokawa K., Koitabashi S., Miyasaka S., Ishii M. (2000) – Sedimentary differences between the 1993 Hokkaido-nansei-oki tsunami and the 1959 Miyakojima typhoon at Taisei, southwestern Hokkaido, northern Japan. Sedimentary Geology 135, 255-264.

Nanayama H., Kaga A., Kinoshita H., Yokoyama Y., Satake K., Nakata T., Sugiyama Y., Tsukuda E. (2002) – Traces of the Nankai earthquake tsunami discovered at Tomogashima, Kidan Strait. Kaiyo Monthly 28, 123-131 (in Japanese).

Nishimura Y., Miyagi N. (1995) – Tsunami deposits from the 1993 southwest Hokkaido earthquake and the 1640 Komagatake eruption, northern Japan. Pure and Applied Geophysics 144, 719-733.

Okamura Y., Satake K., Katayama H., Noda A., Sagayama T., Suga K., Uchida Y. (2004) – Effect of the earthquake and tsunami on the sea bottom. In Hirata N. (Eds.) Report of the Emergency Survey and Study on the 2003 Tokachi-Oki Earthquake. Tokyo University Earthquake Research Institute, 7, Tokyo (in Japanese).

Palmer H.C., McDonald W.D. (1999) – Anisotropy of magnetic susceptibility in relation to source vents of ignimbrites: empirical observations. Tectonophysics 307, 207-218.

Paris R., Lavigne F., Wassmer P., Sartohadi J. (2007) – Coastal sedimentation associated with the December 26, 2004 tsunami in Lhok Nga, west Banda Aceh (Sumtra, Indonesia). Marine Geology 238, 93-106.

Paris R., Wassmer P., Sartohadi J., Lavigne F., Barthomeuf B., Desgages E., Grancher D., Baumert P., Vautier F., Brunstein D., Gomez C. (2009) – Tsunamis as geomorphic crises: lessons from the December 26, 2004 tsunami in Lhok Nga, West Banda Aceh (Sumatra, Indonesia). Geomorphology 104, 59-72.

Paris R., Cachao M., Fournier J., Voldoire O. (2010a) – Nannoliths abundance and distribution in tsunami deposits : example from the December 26, 2004 tsunami in Lhok Nga (northwest Sumatra, Indonesia). Géomorphologie: relief, processus, environnement 1, 109-118.

Paris R., Fournier J., Poizot E., Etienne S., Morin J., Lavigne F., Wassmer P. (2010b) – Boulder and fine sediment transport and deposition by the 2004 tsunami in Lhok Nga (western Banda Aceh, Sumatra, Indonesia): A coupled offshore-onshore model. Marine Geology 268, 43-54.

Park C.K., Doh S.J., Suk D.Z., Kim K.H. (2000) – Sedimentary fabric on deep-sea sediments from KODOS area in the eastern Pacific. Marine Geology 171, 115-126.

Rees A.I. (1965) – The use of anisotropy of magnetic susceptibility in the estimation of sedimentary fabric. Sedimentology 4, 257-271.

Reesink A.J.H., Bridge J.S. (2007) – Influence of superimposed bedforms and flow unsteadiness on formation of cross strata in dunes and unit bars. Sedimentary Geology 202, 281-296.

Reesink A.J.H., Bridge J.S. (2009) – Influence of bedform superimposition and flow unsteadiness on the formation of cross strata in dunes and unit bars – Part 2, further experiments. Sedimentary Geology 222, 274-300.

Rochette P., Jackson M.J., Aubourg C. (1992) – Rock magnetism and the interpretation of anisotropy of magnetic susceptibility. Revue of Geophysics 30, 209-226.

Sato H. Shimamoto T., Tsutsumi A., Kawamoto E. (1995) – Onshore tsunami deposits caused by the 1993 Southwest Hokkaido and 1983 Japan Sea earthquakes. In Satake S., Imamura F. (Eds.) Tsunamis: 1992-1994. Their Generation, Dynamics and Hazard. Pageoph Topical Volumes, Birkhauser Verlag, 144, 3/4, 693-717.

Scheffers A. (2004) – Tsunami imprints on the Leeward Netherlands Antilles (Aruba, Curaçao, Bonaire) and their relation to other coastal problems. Quaternary International 120, 163-172.

Shiki T., Tsuji Y., Yamazaki T., Minoura K. (2008)Tsunamiites – Features and Implications. Elsevier, Amsterdam, 411 p.

Sugawara D., Minoura K., Imamura F. (2008) – Tsunamis and tsunami sedimentology. In Shiki T., Tsuji Y., Yamazaki T., Minoura K. (Eds.) Tsunamiites – Features and Implications, 9-49.

Tada R., Nakano Y., Iturralde-Vincent M.A., Yamamoto S., Kamata T., Tajika E., Toyoda K., Kiyokawa S., Delgado D.G., Oji T., Goto K., Takayama H. (2002) – Complex tsunami waves suggested by the Cretaceous-Tertiary boundary deposit at the Moncada section, western Cuba. Geological Society of America Special Paper 356, 109-123.

Taira A. (1989) – Magnetic fabrics and depositional processes. In Taira A., Masuda F. (Eds.) Sedimentary Facies in the Active Plante Margin. Terra Sciences Publishing, Tokyo, 43-77.

Takashimizu T., Masuda F. (2000) – Depositional facies and sedimentary successions of earthquake-induced tsunami deposits in Upper Pleistocene incised valley fills, Central Japan. Sedimentary Geology 135, 231-239.

Wassmer P., Baumert P., Lavigne F., Paris R., Sartohadi J. (2007) – Sedimentary facies and transfer associated with the December 26, 2004 tsunami on the north eastern littoral of Banda Aceh (Sumatra, Indonesia) / Faciès et transferts sédimentaires associés au tsunami du 26 décembre 2004 sur le littoral au nord-est de Banda Aceh (Sumatra, Indonésie). Géomorphologie: relief, processus, environnement 4, 335-346.

Wassmer P., Schneider J.L., Fonfrège A., Lavigne F., Paris R., Gomez C. (2010) – Use of Anisotropy of Magnetic Susceptibility (AMS) in the study of tsunami deposits: application to the 2004 deposits on the eastern coast of Banda Aceh, North Sumatra, Indonesia. Marine Geology 275, 255-272.

Williams H.F.L., Hutchinson I., Nelson A.R. (2005) – Multiple sources for late-Holocene tsunamis at Discovery Bay, Washington State, USA. The Holocene 15, 60-73.

Yamazaki T., Yamaoka M., Shiki T. (1989) – Miocene offshore tractive current-worked conglomerates – Tsubutegaura, Chita Peninsula, Central Japan. In Taira A., Matsuda M. (Eds.) Sedimentary Facies and the Active Plate Margin. Terra Sciences Publication, Tokyo, 483-494.

Zak J., Verner K., Klominsky J., Chlupacova M. (2009) – “Granite tectonics” revisited: insights from comparison of K-felspar shape fabric, anisotropy of magnetic susceptibility (AMS), and brittle fractures in the Jizera granite, Bohemian massif. International Journal of Earth-Sciences 98, 949-967.

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Annexe

Version abrégée en français

L’AMS (Anisotropie de la Susceptibilité Magnétique) est une méthode non destructive à faible coût, qui offre une vision simplifiée de la fabrique des sédiments. L’AMS décrit l’orientation de l’ellipsoïde du champ électromagnétique induit (M) d’un échantillon lors de son exposition à un champ électromagnétique externe (H). La relation qui lie M et H est : M=k.H pour laquelle k est un scalaire qui varie en fonction de l’organisation cristalline de l’échantillon et de son orientation physique en 3D. Grâce à ces propriétés, il est possible de définir la fabrique du matériau étudié. L’étude de la fabrique se fait alors par la mesure des angles des trois tenseurs de caractérisation 3D des volumes par rapport à un plan horizontal. Ces trois tenseurs sont Kmax, Kint et Kmin. Classiquement, la lecture de ces trois tenseurs se fait à partir d’un graphique cyclographique, i.e. un graphique circulaire orienté sur lequel sont projetées orthogonalement les intersections entre les tenseurs Kmax, Kint et Kmin avec un demi-hémisphère dont le plan de coupe est le plan de représentation cyclographique.

L’AMS est une méthode qui s’est répandue en sciences de la Terre dès la décennie 1960, tout d’abord avec les travaux sur l’orientation de la progression des roches magmatiques (Balsey et Buddington, 1960), puis plus récemment avec les recherches sur des dépôts consolidés marins, lacustres (Park et al., 2000) et éoliens (Bradak, 2009). Toutefois, les recherches en AMS ne se sont jamais tournées vers les matériaux non cohésifs, malgré l’hypothèse émise par A.I. Rees (1965) sur la possibilité d’étendre la méthode AMS à ces derniers. L’objectif de cette publication est donc de présenter une méthode développée pour des matériaux non-cohésifs ainsi qu’un exemple d’application aux dépôts du tsunami du 24 décembre 2004 à Sumatra (Indonésie) dans la province d’Aceh.

Le tsunami du 24 décembre 2004, déclenché au large de Sumatra Nord en Indonésie, a eu d’énormes impacts sur les sociétés humaines vivant sur le pourtour de l’Océan Indien (300 000 morts) et sur les environnements côtiers, notamment en matière de transferts sédimentaires. Les caractéristiques de ce tsunami ont été décrites en détail (e.g., Lavigne et al., 2009), tout comme les impacts sédimentaires sur la côte (e.g., Paris et al., 2007 ; Wassmer et al., 2007 ; Paris et al., 2009). De nouvelles réflexions méthodologiques ont vu le jour avec les travaux de C. Chagué-Goff (2010), qui a exploré les signatures chimiques des dépôts de tsunamis, de B. Mamo et al. (2009) et de R. Paris et al. (2010 a et b), respectivement sur les assemblages de foraminifères et les nanolithes, et de C. Gomez et al. (2010) sur la signature GPR des dépôts de tsunami. Toutefois, toutes ces méthodes demeurent impossibles à utiliser sur l’ensemble des dépôts de tsunami et la question cruciale de l’origine des matériaux (dépôts d’ « uprush » ou dépôts de « backwash ») déposés reste entière. Pour cette raison, la méthode AMS que développent P. Wassmer et al. (2010) est essentielle car elle permet de déterminer avec exactitude le mode de dépôt ainsi que l’orientation des vagues qui ont mis en place les dépôts sableux. Afin de démontrer la pertinence de cette méthode, nous proposons dans cette publication de nous appuyer sur un exemple pris sur la côte nord de Sumatra à Kajhu Perumnas, à partir de 6 échantillons prélevés sur une coupe stratigraphique intégrant la série sédimentaire du dépôt de tsunami.

Les résultats montrent que le dépôt de tsunami a été mis en place par au moins trois vagues d’ « uprush », une vague de « backwash » et une période durant laquelle la sédimentation s’est faite par décantation. Chronologiquement, une première vague d’ « uprush » orientée vers le SSW a déposé une couche de sable (échantillon 1), avant d’être suivie par une phase de décantation (échantillons 2 et 3), puis une vague de « backwash » orientée vers le nord (échantillon 4). Enfin deux autres vagues d’ « uprush » ont apporté les deux derniers dépôts étudiés d’orientation SSE et SSW (échantillons 5 et 6). Au-dessus de ces niveaux, les dernières unités déposées par le tsunami ont été trop fines pour être échantillonnées. La méthode AMS permet donc une reconstitution fidèle non seulement de l’orientation des vagues qui ont mis en place les dépôts mais aussi des conditions de dépôt. Cette méthode peut être étendue à tous les dépôts sableux.

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Table des illustrations

Titre Fig. 1 – Schematic representation of the electromagnetic ellipsoid H induced by a sample, with the three principal tensors Kmax, Kmin and Kint.Fig. 1 – Représentation schématique de l’ellipsoïde électromagnétique H induit par un échantillon, avec les trois principaux tenseurs Kmax, Kmin et Kint.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-1.png
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Titre Fig. 2 – Explanation of AMS cyclographics for lower hemisphere projections. Fig. 2 – Visualisation schématique des graphiques AMS (projections en hémisphère inférieur).
Légende A: Spherical representation of an observed sample. a: observation plane; b: lower hemisphere. 1: Kmax; 2: Kmed; 3: Kmin. B: Complete graphic representation of AMS results. 1: Kmax; 2: Kmin; 3: Kint; 4: Kmean max; 5: Kmean min. Kmax values are weakly scattered showing a NW-SE direction. The position of the Kmean max value slightly below the equator can be interpreted as a plunge of the ellipsoid and, by then, as a flow direction indicator.A : Représentation sphérique d’un échantillon. a : plan d’observation ; b : hémisphère inférieur. 1 : Kmax ; 2 : Kmed ; 3 : Kmin. B : Représentation graphique de résultats AMS. 1 : Kmax ; 2 : Kmin ; 3 : Kint ; 4 : Kmean max ; 5 : Kmean min. Les valeurs de Kmax ont tendance à montrer une direction NW-SE. La position de la valeur Kmean max, légèrement sous l’équateur, peut être interprétée comme une déformation négative de l’ellipsoïde et, de ce fait, indiquer l’orientation du flux.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-2.png
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Titre Fig. 3 – Location map of the study area: Kajhu Perumnas, located to the East of Banda Aceh City. Fig. 3 – Carte de localisation de la zone étudiée: Kajhu Perumnas, situé à l’est de Banda Aceh.
Légende A: Location and topographic settings of the coastal plains. B: Aerial photograph, acquired before the tsunami event. C: Aerial photograph, acquired few days after the tsunami event. 1: flooded coastal plain; 2: non-flooded coastal plain; 3: uplands; 4: tsunami uprush; 5: tsunami backwash; 6: tsunami flow depth; 7: sample.A : Localisation et présentation du contexte topographique de la plaine côtière. B : Photographie aérienne, prise avant le tsunami. C : Photographie aérienne, prise quelques jours après le tsunami. 1 : plaine côtière inondée par le tsunami ; 2 : plaine côtière non inondée par le tsunami ; 3 : zones en position élevée ; 4 : vague de flot du tsunami ; 5 : vague de retour du tsunami ; 6 : hauteur d’eau atteinte lors du tsunami ; 7 : échantillon prélevé.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-3.png
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Titre Fig. 4 – Sample boxes with orientation marks in tsunamiites.Fig. 4 – Boîte à échantillon, avec marques d’orientation sur les dépôts de tsunami.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-4.png
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Titre Fig. 5 – The Kappabridges® KLY-2, in which samples are processed.Fig. 5 – The Kappabridges® KLY-2, dans laquelle les échantillons sont traités.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-5.png
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Titre Tab. 1 – Characteristics of the AMS tensors from the tsunamiites sampled in North Sumatra.Tab. 1 – Caractéristiques des tenseurs AMS appliqués aux tsunamites de la partie septentrionale de l’île de Sumatra.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-6.png
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Titre Fig. 6 – Synthetic cyclograph of the 6 samples, lower hemisphere projection of Kmax and Kmin tensors. Fig. 6 – Représentation synthétique des résultats pour les 6 échantillons, projection en hémisphère inférieure pour les tenseurs Kmax et Kmin.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-7.png
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Titre Tab. 2 – Grain size of the sand samples. Tab. 2 – Granulométrie des échantillons sableux.
URL http://geomorphologie.revues.org/docannexe/image/9491/img-8.png
Fichier image/png, 43k
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Patrick Wassmer et Christopher Gomez, « Development of the AMS method for unconsolidated sediments. Application to tsunami deposits », Géomorphologie : relief, processus, environnement, vol. 17 - n° 3 | 2011, 279-290.

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Patrick Wassmer et Christopher Gomez, « Development of the AMS method for unconsolidated sediments. Application to tsunami deposits », Géomorphologie : relief, processus, environnement [En ligne], vol. 17 - n° 3 | 2011, mis en ligne le 15 septembre 2013, consulté le 19 août 2017. URL : http://geomorphologie.revues.org/9491 ; DOI : 10.4000/geomorphologie.9491

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Auteurs

Patrick Wassmer

Laboratoire de Géographie Physique de Meudon (CNRS UMR 8591) - Université Panthéon-Sorbonne (Paris 1) - 1, place Aristide Briand - 92195 Meudon Cedex - France. Université de Strasbourg - Faculté de Géographie et d’Aménagement - 3, rue de l’Argonne - 67000 Strasbourg - France

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Christopher Gomez

University of Canterbury - College of Sciences - Department of Geography - Private Bag 4800 - Christchurch 8140 - New Zealand (christopher.gomez@canterbury.ac.nz)

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