# Discriminating between tsunamis and tropical cyclones in the sedimentary record using X-ray tomography

Maude Biguenet, Éric Chaumillon, Pierre Sabatier, Raphael Paris, Pierre Vacher, Nathalie Feuillet

► To cite this version:

Maude Biguenet, Éric Chaumillon, Pierre Sabatier, Raphael Paris, Pierre Vacher, et al. Discriminating between tsunamis and tropical cyclones in the sedimentary record using X-ray tomography. Marine Geology, 2022, 450, pp.106864. 10.1016/j.margeo.2022.106864. hal-04254917

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# Discriminating between tsunamis and tropical cyclones in the sedimentary record using X-ray tomography

M. Biguenet<sup>ab</sup>, E. Chaumillon<sup>a</sup>, P. Sabatier<sup>b</sup>, R. Paris<sup>c</sup>, P. Vacher<sup>d</sup>, N. Feuillet<sup>e</sup>.

<sup>a</sup> Université de la Rochelle, UMR 7266 LIENSs, 17042 La Rochelle cedex, France.

<sup>b</sup> Université Savoie Mont Blanc, CNRS, UMR 5204 EDYTEM, 73376 Le Bourget du Lac cedex, France.

<sup>c</sup> Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France.

<sup>d</sup> Laboratoire SYMME, domaine Universitaire BP 80439, 74944 Annecy le Vieux Cedex, France.

<sup>e</sup> Institut de Physique du Globe de Paris, Université de Paris, CNRS, 75005 Paris, France.

## 1 Abstract

Scrub Island lagoon (Anguilla, Caribbean) provides the opportunity to display both tropical cyclones,

near-field and far-field tsunami deposits within the same sediment core. Here, we use X-ray

tomography to highlight the sedimentary fabric and link it to processes that occurred during these

events. Overall, the tsunami sedimentary fabric remains highly homogenous, and display fabric

trends that can be associated with variations in sediment concentration and /or flow strength. The

near-field tsunami deposit displays fabric characteristics of intense and highly-concentrated flow,

whereas the far-field tsunami fabric is less defined which may illustrate a less intense event.

Contrastingly, the sediment fabric of the tropical cyclone (hurricane) deposit is more variable with a

succession of different dipping-angles and orientations of the grains that can be interpreted as the

record of successive waves, likely infragravity waves. Observed differences in sedimentary fabrics

between tsunami and tropical cyclone-related deposits are an encouragement to generalize

sediment fabric parameters as criteria to distinguish them in coastal sedimentary archives.

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15 **Keywords:** tsunamis, tropical cyclones, Lesser Antilles, lagoons, X-ray tomography
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# 1. Introduction

Geological records constitute key-archives to reconstruct long-term series of marine flooding to extend our knowledge well beyond the observational and historical records. They allow defining return periods of tsunami induced by large earthquake (e.g. Minoura et al., 2001; Nanayama et al., 2003; Monecke et al., 2008) or tropical cyclone/storm activity related to climate variability (e.g. Liu and Fearn, 1993, 2000; Donnelly and Woodruff, 2007; Sabatier et al., 2012; Goslin and Clemmensen, 2017; Wallace et al., 2021). Therefore, it is crucial to differentiate tropical cyclones and tsunamis-related deposits in sedimentary archives in order to better reconstruct return time periods of those dramatic events in densely populated coastal areas. Unfortunately, there are still no unequivocal methods for differentiating between tsunami- and tropical cyclone- generated deposits. Moreover, criteria to distinguish those deposits seem to be site dependent and there are few locations where both types of deposits were clearly identified (e.g. Nanayama et al., 2000; Goff et al., 2004; Tuttle et al., 2004; Kortekaas and Dawson, 2007; Biguenet et al., 2021). The most widely recognized sedimentary signatures for tsunami deposits are sequences of normal (fining-up) or inversely (coarsening-up) graded beds, nongraded (massive) or multiple graded sandsheets, internal mud laminae, intraclasts (rip-up clasts), truncated flame structures, or backflow induced structures and erosional lower contact (e.g. Nanayama et al., 2000; Goff et al., 2004; Tuttle et al., 2004; Kortekaas and Dawson, 2007; Biguenet et al., 2021). Storm deposits tend to have many parallel, cross laminations and climbing ripples (e.g., Morton et al., 2007; Baumann, 2017), with a highly variable grain-size distribution and a sharp, non-erosional lower contact (e.g., Goff et al., 2004; Soria et al., 2017; 2018). Here, we use X-ray tomography on previously identified tropical cyclone (hurricane) and tsunami deposits recorded in a coastal lagoon located in Scrub Island (Anguilla, Biguenet et al., 2021) to present new criteria based on the sedimentary fabric. X-ray tomography enables characterization of the three-dimensional structure and texture of rocks and sediments (Cnudde and Boone, 2013). Thus, an increasing number of sedimentological studies use X-ray tomography to observe internal
sedimentary structures (e.g. Renter, 1989; Orsi et al., 1994; Flisch and Becker, 2003). Among these studies, the use of X-ray tomography to analyze tsunamis and or tropical cyclones deposits is relatively new (e.g. Cuven et al., 2013; May et al., 2016; Falvard and Paris, 2017; Paris et al., 2020). We propose in this study new diagnostic parameters based on X-ray tomography using a method developed by Falvard and Paris (2017) and Paris et al. (2020) to identify tropical cyclone, near-field and far field tsunamis deposits in coastal archives.

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## 2. Study area and available data

Scrub Island is located in the northern part of the Lesser Antilles Arc, southeast of the Anegada Passage (Fig. 1AB). This island is 1.5 km wide from north to south and about 3.5 km long from east to west (Fig. 1C). It consists of low-relief limestones (highest point about 25 m above mean sea-level, a.s.l.). In this study, we focus on a 400 m-long and 130 m-wide very shallow (maximum water depth of 80 cm) closed lagoon, located along the western coast of the island (Fig. 1D). This saltwater lagoon is separated from the ocean by a 2 to 5 m-high and about 50 m-wide sandy barrier, which is covered by small bushes at the top. A 110 cm-long sediment core (SCR1-18-01) was collected in March 2018 in this lagoon using an Uwitec gravity corer with hammer. Twenty-five sandy layers resulting from high-energy-marine floods were identified after sedimentological, geochemical, microfaunal and chronological analyses. Two of these layers are interpreted as historical tsunami deposits and the 23 remaining sandy layers were interpreted as tropical cyclone (hurricane) deposits, with the three most recent layers being correlated with historical tropical cyclones (Biguenet et al., 2021).

In order to compare tsunami and tropical cyclones sedimentary signature with X-ray tomography, we selected two tsunami deposits and one tropical cyclone deposit in the same core SCR1-18-01. Those deposits are all composed of fine-to-coarse bioclastic sand (mean grain-size of samples between 122 and 765 µm) interstratified with the silty organic-rich lagoonal sediment-fill. A short description of those deposits is given below (for further details, please refer to Biguenet et al., 2021). The most recent tsunami deposit is dated between 1652 and 1810 cal. CE. It is interpreted to result from the
Lisbon transoceanic tsunami that occurred in 1755 CE and also observed and identified in the sedimentary record of a large part of the Lesser Antilles (Engel et al., 2016; NGDC/WDS, 2021; Paris et al., 2021). This sediment record consists of a 2 cm-thick very poorly to poorly sorted sandy layer, with a rip-up clast and topped by a 1.9 cm-thick mud layer with a large amount of wood and plant remains (33.9 to 30 cm-depth in core). The older tsunami deposit is dated between 1364 and 1469 cal. CE and is interpreted to record the Pre-Columbian event also identified in St-Thomas (US Virgin Islands, Fig 1A; Fuentes et al., 2017) and in Anegada (British Virgin Islands, Fig 1A; Atwater et al., 2017). This sediment record is a 16.2 cm thick massive poorly to well-sorted sandy bed without any major grain-size variations (63 to 46.8 cm-depth in core). Two internal mud laminae were found at its base. The selected tropical cyclone deposit is interpreted as recording the Category 3 Hurricane Donna in 1960 CE (NOAA, 2020, Historical Hurricane Tracks). This sediment record corresponds to a 4.75 cm-thick, poorly sorted, medium to coarse sandy deposit (5 to 0.25 cm-depth in core). We choose to focus on the Hurricane Donna deposit because it is the thickest and it was more accurately dated than the other tropical cyclone deposits found in SCR1-18-01 core.

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## 3. Material and methods

The three event deposits were scanned at the SYMME laboratory (Annecy, France) using an X-ray tomography system (Easytom XL 150 from RX SOLUTION company) with a 50 µm resolution (voxel size). After reconstruction of the original projections, the deposits are imaged as stacks of horizontal cross-sections along the core (16 bits Tiff image files). The tsunami deposit dated between 1652 and 1810 cal. CE. that we interpret to be formed by the Lisbon tsunami (Biguenet et al., 2021) (299 images from base to top) and Donna deposit (757 images from base to top) were scanned at 150 kV and 70 µa, whereas the Pre-Columbian tsunami deposit (2360 images from base to top) was scanned at 150 Kv and 200 µa. These parameters were adapted for each deposit to obtain the best image quality.

The reconstruction of the sedimentary fabric in 3D and associated numerical values was made following the method of Falvard and Paris (2017), later updated in Paris et al. (2020). The sand grains
were semi-automatically separated using the morphological segmentation tool from MorpholibJ
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collection (Legland et al., 2016) included in the Fiji distribution of ImageJ (Schindelin et al., 2012). All
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sand grains were then counted and characterised in terms of size, shape, orientation, and dipping-
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angle, using the Blob3D program (Ketcham, 2005). Sand grains with a measured maximum axis
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length shorter than 250 µm were excluded to remove artifacts and avoid potential over-
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segmentation of larger grains. Furthermore, sand grains with an aspect ratio (longest axis / smallest
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axis of the ellipsoid) higher than 3 were also excluded to avoid overly elongated particles resulting
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from artifacts. Resolution and image quality allowed sedimentary fabric to be characterized (spatial
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arrangement of the grains) at 5 mm intervals down core. In every 5-mm subunit, the sedimentary
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fabric was characterized for three different grain-size fractions (medium sand 250-500 µm; coarse
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sand 500-1000 µm; very coarse sand > 1000 µm). Projections of the orientations (azimuth) and
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dipping-angle (plunge) of the longest grain axes (sedimentary fabric data) were plotted on
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stereographic lower hemisphere equal-area projection, using StereoNet. The eigenvalues (the
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normalized vector magnitudes) S1 (longest axis), S2 (intermediate axis) and S3 (shortest axis) of the
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ellipsoid fabric were also calculated for the three grain fractions in every subunit of 5 mm. This
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ellipsoid corresponds to the preferential orientations of the particles extracted from the sample and
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they were also represented in equilateral ternary diagrams (Benn, 1994). These diagrams describe
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sedimentary fabrics based on fabric isotropy (I = S3/S1) and elongation (E = 1-(S2/S1)) of each
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sample. The three poles of the diagram correspond to an isotropic fabric (S1 = S2 = S3; I = 1 and E =
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0), a planar girdle-shaped fabric (S1 = S2 > S3; I = E = 0, oblate ellipsoid), and a cluster shaped fabric
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(S1 > S2 > S3; I = 0, E = 1, a prolate ellipsoid). Finally, the detailed grain-size parameters of each
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deposit previously determined in Biguenet et al. (2021), were described here to enable a precise
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comparison with the sedimentary fabric. The grain-size parameters result from the grain-size
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analyses that were carried out at the EDYTEM laboratory using a Beckman Coulter LS 13320 XR
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particle size analyser using sonication, with a range size between 0.010 µm and 3000 µm. The
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sampling step was approximatively 5 mm through core SCR1-18-01 (n = 126) respecting facies
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boundaries. The mean grain size curve and the grain size contour plot were produced using MATLAB software (Biguenet et al., 2021).

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## 4. Results

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The use of high-resolution X-ray tomography allowed us to identify more sand layers than by eye and thus revise the boundaries of the selected tsunami and tropical cyclone-related layers to obtain more precise deposits delimitations. The sandy part of the deposit interpreted as being formed by the Lisbon Tsunami has been restricted to extend from 33.25 to 31.75 cm (Fig. 2). Thin sand layers alternating with mud at the base of the Pre-Columbian deposit have been interpreted as recording older events. Thus, the Pre-Columbian deposit is restricted to extend from 60.5 to 47.25 cm (Fig. 2). The sand layer (5-3.5 cm, Fig.2) separated from the base of Hurricane Donna deposit by a mud layer has been interpreted as recording and older tropical cyclone event. Thus, Hurricane Donna deposit is restricted to extend from 3.5 to 0.25 cm (Fig. 2). X-ray tomography is hence a promising method to precisely define facies boundaries at a high resolution.

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The study of sedimentary fabric from the three deposits has led us to use the same criteria distinguishing sedimentary fabrics as Paris et al. (2020), to facilitate the comparisons. Thus, all the studied stereograms were listed in the four categories defined by Paris et al. (2020) (Fig. 2).

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- Fabric A: moderate-dipping-angle (15 to 35°) to high-dipping-angle (>35°) fabric with one well-defined orientation, typically flow-parallel.

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- Fabric B: bimodal low-dipping-angle (<15°) fabric (two orientations with opposite dipping-angles), either flow parallel or flow oblique.

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- Fabric C: low-to-moderate dipping-angle fabric with at least two different orientations.

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- Fabric D: dispersed fabric.

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### 4.1 Pre-Columbian Tsunami
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Fourteen grain-size samples were collected in this deposit from 60.25 to 47.75 cm, a 1-cm interval (Fig. 2). The top of the deposit (56.75 to 47.75 cm) is a unimodal coarse sand with mode at 718 µm. The sand is well sorted and displays symmetrical skewness from 56.75 to 51.75 cm, and is moderately well sorted and finely skewed from 51.75 to 47.75 cm. The base of the deposit (60.25 to 56.75 cm) displays unimodal to trimodal, poorly sorted, mostly very fine skewed, fine to coarse sand. The major modes are at 718 µm (coarse sand), 122 µm (fine sand), 596 µm (coarse sand), 134 µm (fine sand), and 53 µm (coarse silt). The 53 µm and 596 µm modes are only present in the sample at 60.25 cm, the base of the deposit.

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As seen on the stereographic projections (Fig. 2), the Pre-Columbian tsunami deposit displays a bimodal low—dipping-angle (mean ellipsoid plunge <23° and often <10°) fabric (Fabric B) with two well-defined orientations of opposite dipping-angles through almost the entire deposit and in the three sand fractions (Fig. 2). However, the base of this deposit (60.5 to 59 cm) displays a progressive transition from a poorly- to well-defined fabric, transitional between a dispersed fabric (FD) and bimodal fabric (FB) with a slight variation of orientation (50° rotation), especially in the medium sand (MS) and coarse sand (CS) fractions (Fig. 2).

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The mean ellipsoid plunge varies between 0.2° and 22.4° (Fig. 3). The mean ellipsoid plunge is 8.7° for the MS fraction, 5.8° for the CS fraction and 4.3° for the VCS fraction. Thus, the MS fraction displays the highest plunges. The ellipsoid degree of elongation of this tsunami deposit is typically larger than 0.15 (Fig. 3 & 4), with a broad range of values from 0.07 to 0.53. Compared to the other deposits, the Pre-Columbian tsunami displays a relatively higher degree of elongation. The mean ellipsoid elongation is 0.17 (low elongation) for the MS fraction, 0.25 (low to moderate elongation) for the CS fraction and 0.38 (moderate elongation) for the VCS fraction. Thus, the degree of elongation is highest for the VCS fraction.

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The ellipsoid degree of isotropy for this deposit ranges between 0.24 and 0.78 (Fig. 4). The mean ellipsoid isotropy is 0.74 (medium to high isotropy) for the medium sand fraction (MS), 0.55 (medium isotropy) for the coarse sand fraction (CS) and 0.36 (moderate isotropy) for the very coarse sand fraction.
172 fraction (VCS; Fig. 4). Thus, the degree of isotropy is highest for the MS fraction and lowest for the
173 VCS fraction.

174 For each grain-size fraction, there is no major variation of isotropy and elongation degree, and mean
175 plunge from the base to the top of the deposit (Standard deviation inferior to 0.1 for both isotropy
176 and elongation degree, and inferior to 6° for the mean plunge).

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178 **4.2 Lisbon Tsunami**

179 Three grain-size samples were analysed for the deposit interpreted as being formed by the Lisbon
180 Tsunami (33.05 to 31.9 cm; Fig. 2). The grain-size sample at 33.05 cm is a trimodal, very poorly sorted
181 symmetrical fine sand. The sample has a mode at 718 µm (coarse sand), a second at 122 µm (fine
182 sand), and a third at 53 µm (coarse silt). The grain-size sample at 32.55 cm shows bimodal, poorly
183 sorted, very fine skewed medium sand. The sample has a mode at 718 µm (coarse sand) and a
184 second at 111 µm (fine sand). The grain-size sample at 31.9 cm is a trimodal, poorly sorted, very fine
185 skewed medium sand. The sample has a mode at 718 µm (coarse sand), a second at 111 µm (fine
186 sand) and a third mode at 48 µm (coarse silt).

187 In this deposit, the MS and CS fractions have dispersed fabrics (FD), with no identifiable orientation
188 (FD, Fig. 2). However, the VCS fraction displays a less-dispersed fabric, with a base-to-top evolution
189 from a transitional FB-FC fabric (low-to-moderate dipping-angle fabric with at least two different
190 orientations) to poorly-defined fabrics (FC to FD).

191 The Lisbon tsunami deposit is characterized by low ellipsoid degree of elongation (Fig. 3), with a
192 range of values between 0.01 and 0.26. The mean ellipsoid elongation is 0.05 for the MS fraction,
193 0.07 for the CS fraction and 0.18 for the VCS fraction. Thus, the degree of elongation is slightly higher
194 for the VCS fraction.

195 The mean ellipsoid plunge is low and varies between 1.1° and 19° (Fig. 3). The mean ellipsoid plunge
196 is 6.0° for the MS fraction, 6.3° for the CS fraction and 11.0° for the VCS fraction. Unlike the Pre-
197 Colombian tsunami deposit, the VCS fraction displays here the highest plunge values (Fig. 3).
For each fraction, there is no major variation of isotropy and elongation degree, and mean plunge from the base to the top of the deposit (Standard deviation inferior to 0.14 for both isotropy and elongation degree, and inferior to 9° for the mean plunge).

The ellipsoid degree of isotropy for the Lisbon deposit varies from 0.33 to 0.64 (Fig. 4). The mean ellipsoid isotropy is 0.60 (medium isotropy) for the MS fraction, 0.55 (medium isotropy) for the CS fraction and 0.48 (moderate to medium isotropy) for the VCS fraction (Fig. 4). Thus, the degree of isotropy is slightly lower for the VCS fraction.

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## 4.3 Hurricane Donna

Three grain-size samples were analysed for the Hurricane Donna deposit (2.8 to 1.05 cm; Fig. 2). They are all poorly sorted, very fine skewed medium to coarse sand. The distribution is bimodal at 2.8 and 2.1 cm with a mode at 718 µm (coarse sand) and a second at respectively 101 and 111 µm (fine sand), whereas the sample at 1.05 cm displays one unique mode at 788 µm (coarse sand). Thus, there is a coarsening of the grain-size from bottom to top of the deposit.

Hurricane Donna deposit displays vertical variations both in terms of fabric orientation and plunge, as well as variations between the different grain-size fractions analysed. Small variations of the dipping-angle of fabric are visible from base to top of the deposit (Fig. 2). The base of the deposit (3.5 to 2 cm) has a bimodal low dipping-angle (<15°) fabric with a moderately well-defined orientation for all the sand fractions, corresponding to FB (Fig. 2). The upper part of the deposit (2 to 0.25 cm) shows significant differences between the different sand fractions. The VCS still define a FB-type fabric, whereas, finer fractions (MS and CS) have higher dipping-angles (>15° and up to 62.4°) with one dominant orientation, this defining a FB to FA-type fabric.

The ellipsoid elongations calculated for the Hurricane Donna deposit vary between 0.08 and 0.39 (Fig. 3); a range that is slightly less dispersed than for the Pre-Colombian tsunami deposit (Fig. 3). The mean ellipsoid elongation is 0.16 (low elongation) for the MS fraction, 0.17 (low to moderate
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elongation) for the CS fraction and 0.29 (moderate elongation) for the VCS fraction. Thus, the degree of elongation is highest for the VCS fraction.

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Donna's deposit has a larger range of ellipsoid plunges (0.5° to 62.4°) compared to the two other deposits (Fig. 3). The mean ellipsoid plunge is 35.0° for the MS fraction, 11.3° for the CS fraction and 5.7° for the VCS fraction. Highest plunge values are found in the MS fraction.

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The ellipsoid degree of isotropy for this deposit ranges between 0.29 and 0.83 (Fig. 4). The mean ellipsoid isotropy is 0.76 (medium to high isotropy) for the MS fraction, 0.66 (medium isotropy) for the CS fraction and 0.41 (moderate isotropy) for the VCS fraction (Fig. 4). These values are almost identical to those in the Pre-Colombian tsunami deposit. The degree of isotropy is also highest for the MS fraction and lowest for the VCS fraction in this deposit.

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For each fraction, there is no major variation of isotropy and elongation degree from the base to the top of the deposit (Standard deviation inferior to 0.11 for both isotropy and elongation degree).

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However, the mean plunge values display a large broad of values and varies from base to top of the deposit (between 1.9° and 62.4°), especially in the MS fraction, where the standard deviation reaches 24.1°.

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# 5. Discussion

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## 5.1 Grain-size influence on the sedimentary fabric interpretation

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The separation of the grains from the matrix and their counting is dependent of the image resolution (50 µm here) because the larger grains are then better defined. The largest grain-size fraction (VCS fraction, >1000 µm) had a higher degree of anisotropy of the ellipsoid of grain orientation, and it is thus more suitable for identifying the primary fabric. Indeed, a more anisotropic ellipsoid of grain orientation indicates that the grains are preferentially aligned in a given orientation (Fig. 4). The smallest grain-size fraction (MS fraction, 250-500 µm) displays the lowest degree of anisotropy of the ellipsoid of grain-orientation, indicating a lesser defined grain orientation (Fig. 4). Thus, the stereographic projection of the smallest grain-size fraction must be interpreted with caution.

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However, the differences in the anisotropy degree between the grain-size fractions may also be the result of sedimentary processes such as interactions of coarse sand with finer-grained fractions, and the characteristics of the flow itself (Paris et al., 2020). For instance, the primary fabric is better preserved if fine-grained particles (silts and clays) are mixed with sands, as it was the case for the high plunges of the smallest grain-size (MS fraction, 250-500 µm) at the top of Hurricane Donna deposit (Fig. 3). Thus, these high-plunges could represent a secondary orientation because of hammering during coring or bioturbation for instance, but most likely a primary fabric resulting from sedimentary processes.

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## 5.2 Near-field vs far-field tsunami

The Pre-Columbian tsunami deposit is a massive sandy bed and it is thicker than all other high-energy event deposits recorded over the last 1600 years in Scrub Island. The absence of visible grading and laminations (except for the two mud laminae at the base) in this massive coarse sand unit could suggest a rapid and powerful event, maybe the strongest that hit the lagoon over this period. Thus, Morton et al. (2007) suggested that extremely rapid deposition occurs when flow decelerates between the uprush and backwash phases of inundation, often resulting in single-layer, homogeneous and structureless tsunami deposits, where inverse grading is rare. Alternatively, a deposit can also be massive if the source consists of very well-sorted sediments, or if it results from sediment transport convergence (deposition over time as more sediment comes into an area than leaves it) (Jaffe et al., 2012). However, the erosional base of the Pre-Columbian Tsunami deposit is typical of a rapid and powerful event, which remains coherent with the massive nature of the deposit. This type of tsunami deposit was already reported by Switzer et al. (2012). They found a mixture of upper flow regime planar beds and homogenous massive sand in the powerful 2004 Indian Ocean tsunami deposit. There are no historical observations for the Pre-Colombian tsunami, and its source is still debated. Because the sediment records associated with this event were found (for now) in the northern part of the Antilles, it is assumed that its source may be close to the Puerto
Rico Trench. Cordrie et al. (2022), showed that only a near-field tsunami, originated either from an Mw 8.5-8.7 mega-thrust earthquake on the subduction interface North to the Anegada Island or from a Mw 8.0 outer-rise earthquake along the trench, was able to reach the location of the deposits observed in Saint-Thomas, Anegada, Anguilla and create ~6-10 m height waves on Scrub Island coast leading to the flood of the lagoon.

In this study, the Pre-Columbian deposit sedimentary fabric remains highly homogenous (FB-type fabric) through all its thickness and grain-size fractions. FB is either flow-parallel or flow-oblique depending on the deposition rate, sediment concentration, and bed shear stress (Paris et al., 2020). Flow-parallel fabric is common for sands transported at high sediment concentration and rapidly deposited (e.g. Rees, 1968, 1983; Hiscott and Middleton, 1980; Gupta et al., 1987; Baas et al., 2007).

Such fabric can be explained by the reduction of the distance between colliding grains, thus creating transfers of angular momentum (Rees, 1983). The massive texture of the Pre-Columbian sand unit suggests a high depositional rate, and it is then assumed that the FB orientation observed here is aligned to the main flow during marine flooding. Given the north-south orientation of the barrier facing the Caribbean Sea to the west of the lagoon (Fig 1D), it is very likely that the flow propagated from the West to East in a cross-shore direction relative to the sediment barrier. Consequently, the main recorded flow/current orientation (FB orientation) during marine flooding should be approximately West-Est. The grain-size parameters are coherent with this observation. Thus, the top of deposit (56.75 to 47.75 cm) remains homogenous: unimodal well sorted sand (718 µm) with some little vertical variations of sorting and skewness. This sediment may then illustrate a source of mature sediment such as the sandy barrier and/or beach with a mode at 718 µm. Finally, the flow-parallel fabric maintains a low-dipping-angle because the sediment is quite well-sorted, as demonstrated by Paris et al. (2020).

The less-defined orientation at the base of the deposit, which is more or less sub-parallel to FB orientation illustrates a flow-oblique fabric. Although the base displays a different fabric compared to the rest of the deposit, there is no significant differences in terms of mean plunge, isotropy and
elongation of the ellipsoids. Thus, the presence of flow-oblique fabric may be explained by others
parameters. Unlike the rest of the deposit, the base (60.25 to 56.75 cm) is poorly sorted with vertical
variations in term of skewness and grain sizes (modes at 718 µm, 122 µm, 596 µm, 53 µm), which
tend to show several sediment sources. The modes at 53 µm and 596 µm are only present in sample
at the base of the deposit. It is thus likely that these sample illustrates a mix of different sources of
sediment. The mode at 53 µm (coarse silt) reveals for example silty lagoonal sediment that was
incorporated with the sand during the erosion part of the deposition. Moreover, the oblique
orientation may be the result of the bed shear stress that occurred at the bottom of the flow during
the event.

Paris et al. (2020) proposed that vertical variations of the fabric orientation could thus reflect
changes in sediment concentration and bed shear stress. The transition from a flow-oblique fabric to
a flow-parallel fabric could also illustrate an acceleration of the flow and an increase of the sediment
concentration.

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The deposit interpreted as being formed by the Lisbon Tsunami (Biguenet et al., 2021) is composed
of a 1.5 cm-thick coarse-sandy layer with rip-up clast, topped by a 1.9 cm-thick silty layer with a large
amount of plant remains and wood originated from the sandy barrier or the interior of the island
(Biguenet et al., 2021; Fig. 5). Historical archives and sedimentological records (Atwater et al., 2017;
Fuentes et al., 2017; Paris et al., 2021) suggested that the Lisbon Transoceanic Tsunami reached the
Lesser Antilles, and numerical modelling (Roger et al., 2011) showed that it induced moderate
tsunami waves in the Lesser Antilles region. The tsunami has been observed at 18 locations across
the north-eastern Caribbean from Bridgetown (Barbados) to Santiago de Cuba (Cuba)
(NGDC/WDS,2021), with a maximum wave height of 6.4 m at Saba Island (Fuentes et al., 2017) and a
4.5 m-high run-up observed in Saint-Martin, close to Scrub Island (Affleck, 1755).

In the studied samples, the MS and CS fractions show a dispersed fabric (FD), whereas the VCS
fraction a- base-to-top evolution from a transitional FC fabric to a poorly defined fabric (FC to FD).
327 Falvard and Paris (2017) explained that collision between clasts during a tsunami may produce
328 isotropic ellipsoids in the resulting deposits. In some cases, a flow with a very high concentration of
329 sediment may produce dispersed fabric (Rees, 1983; Paris et al., 2020). The Lisbon tsunami deposit
330 displays no significant differences in terms of mean plunge, isotropy and elongation of the ellipsoids
331 through all the deposit. The entire sandy part of the deposit is composed of multimodal very poorly
332 to poorly sorted sand without any major grain-size variation. The mode at 718 µm, already major in
333 the Pre-Columbian deposit, is also part of all the grain-size samples of this deposit. This may again
334 indicate a contribution of the sandy barrier and/or beach. However, the sorting and multimodal
335 distribution of the Lisbon deposit grain-size samples illustrate also other sources of sediment. The
336 basal sample displays the same grain-size modes (122 µm and 53 µm) as the Pre-Columbian deposit,
337 indicating an incorporation of lagoonal sediment during the erosion phase of the deposition. The two
338 others grain-size samples (middle and top of the deposit) display almost identical grain-size
339 parameters. The only difference is the presence of a mode at 48 µm (coarse silt) in the top deposit.
340 This is most likely because of the presence of rip-up clasts and/or the 1.9 cm-thick silty layer
341 originated from the barrier or the interior of the island that is topped over the sandy part of the
342 deposit. Thus, the grain-size parameters are relatively homogeneous and cannot explain the VCS
343 fraction fabrics. The poorly-defined fabric at the base of the deposit, turning to less-defined,
344 dispersed fabric in the VCS fraction may illustrate a variation in flow regime during the event. The
345 scenario of inundation could be similar to a simulation produced for the same event on the
346 Martinique's island shore (Anse Meunier lagoon, Paris et al., 2021). The water level rises
347 progressively and it can last several hours until the barrier breaks locally, causing a washover that
348 floods the lagoon. The deposits of the Lisbon tsunami in Scrub Island record a similar washover
349 causing dispersed fabric. This hypothesis is in accordance with far-field tsunami inundation.

350

351 **5.3 Tropical cyclone deposit**
The sediment record of Hurricane Donna is a 3.25 cm thick poorly-sorted very fine skewed laminated medium to coarse sandy deposit. Donna was a Category 3 tropical cyclone (hurricane) that hit Scrub Island in September 1960. The tropical cyclone passed 21 km South-West with respect to Scrub Island. Winds were at 110 kt and pressure at 952 mb (NOAA, 2020, Historical Hurricane Tracks).

There is no significant difference in the isotropy and elongation of the ellipsoids throughout the deposit of Hurricane Donna (with values in the same range than those of the Pre-Colombian deposit), but there are important variations of the fabric from base to top, and also between the different grain size fractions. The lower part (3.5-2 cm depth) of the deposit is characterized by a dominant moderately well-defined FB fabric, with variations of grain orientations. Here, FB fabric is less defined than the one developed in the Pre-Colombian tsunami deposit. The grain-size mode at 718 µm probably illustrates again the arrival of barrier and/or beach sand. The base does not show silty lagoonal sediment which may indicate a less erosive event than the two tsunami deposits. Thus, the base of the deposit tends to show a less intense and/or concentrated initial flow than for the tsunami deposits. The absence of an erosional surface at the base of storm deposits was also observed in washover deposits related to the 1991 Halloween storm in the northeastern U.S. coastline by Switzer and Jones (2008) and in Typhoon Haiyan overwash sediments from Leyte Gulf coastlines by Soria et al. (2018). This was related to a lower inundation velocity compared to tsunami events. This is consistent with a less intense and/or concentrated initial flow during storms compared to tsunamis.

The upper part (2-0.25 cm depth) is clearly different, with FB to FA fabric in CS and FB fabric in VCS (Fig. 2). There is then a transition to FB/FA moderate-to-high dipping-angle fabric, typical of a rapidly deposited highly concentrated flow. Very high-dipping-angle fabric seen in the MS fraction should be considered with caution, since it mostly illustrates the fact that MS acts as a matrix between the coarser grains (CS and VCS). This observation is restricted to the tropical cyclone deposit, as both the Lisbon and Pre-Colombian tsunami deposits have very little vertical variations of the mean plunge.

The primary fabric is here preserved thanks to a finely-skewed grain size distribution (i.e. the presence of a fine fraction, here the medium sand, intercalated between the coarser fractions). The
lamination (upper plane beds) observed in the deposit is consistent with the deposition under a near-supercritical flow regime. Fabric in upper plane beds is usually flow-parallel, which is consistent with the orientation of the fabric throughout the deposit. Moreover, the coarsening at the top of the deposit (788 µm mode at 1.05 cm) tends to show an intensification of the flow. It could also record a more energetic flow entering the lagoon related to the lowering or opening of the sandy barrier. Inverse grading may also be the result of dispersive pressures due to grains collision, that causes larger grains to move toward the zone of lower pressure at the top of the sediment layer (Bagnold, 1954). Inverse grading may also result from kinetic sieving, which implies downward migration of smaller grains and upward moving of larger grains during deposition (Middleton, 1970). Leatherman and Williams (1983) concluded that both these mechanisms may occur during the flow and are then part of the primary fabric of the deposit. Overall, the lamination (upper plane beds), and the coarsening of the deposit at the top of Hurricane Donna deposit is consistent with other storms deposits such as the Haiyan overwash deposit (Soria et al., 2017), which were interpreted as indicating upper flow regime conditions of a unidirectional, turbulent, high-velocity flow. Moreover, the small variations of dipping-angle of fabric through the deposit are reminiscent of sediment impulses related to flow variations creating laminations. Plane or laminated bedding in coastal deposits is generally attributed to repeated inundation by storm waves (Switzer et al., 2012; Tuttle and al., 2004). Here, the fabric orientation and dipping-angle variations tend to show successive waves arrival, which is typical of infragravity waves (Baumann et al., 2017). Infragravity waves correspond to surface ocean gravity waves with periods in the range 20–200s and can increase rapidly to over 1 m in nearshore water depths (Baumann et al., 2017). Thus, infragravity waves are known to dominate the hydro-sedimentary dynamics of dissipative beaches, particularly under storm conditions (Russell, 1993). These waves have significant contribution to wave runup and overwash formed during storms (Bertin et al., 2018) and may have brought the sand deposit of Hurricane Donna in Scrub Island lagoon.
404

**6. Conclusion**

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This work is based on an exceptional 500 year-long sediment record located in a small coastal lagoon (Anguilla, Caribbean) including three contrasted high-energy events: a near-field tsunami, a far-field tsunami and a tropical cyclone. Thus, this sediment record gave us the opportunity to explore differentiation between tsunami and tropical cyclone deposits, using X-ray tomography. We found that X-ray tomography is a promising tool to distinguish tsunami from tropical cyclone sandy deposits because it is able to document important sediment fabric variations in those three deposits. Each tsunami deposit displays mostly similar fabric parameters from base to top, but their fabric are contrasted. The near-field Pre-Colombian tsunami deposit displays mostly bimodal low-dipping-angle flow-aligned fabrics from base to top. The homogenous low-dipping-angle fabric of the Pre-Colombian tsunami deposit is both related to its well sorting and probably to an extremely rapid and intense character of tsunami waves generated by the near-field earthquake (Cordrie et al., 2022).

The thick and coarse Pre-Colombian tsunami deposit is obviously related to massive erosion of the sandy barrier after this near-field event. The more dispersed fabric of the Lisbon tsunami deposit is both related to its bimodal grain size distribution and to lower energy far-field tsunami waves. The far-field Lisbon tsunami is characterized by a more or less dispersed fabric from base to top.

Contrastingly, the tropical cyclone deposit displays vertical variations in medium and coarse sand fabric, with two orientations and opposite low dipping-angle fabric at base and one dominant orientation with moderate to high dipping-angle at top. The more homogenous fabric of the two tsunami deposits from base to top is interpreted as the result of rapid sedimentation induced by the arrival of few inundation waves in the lagoon over a period of minutes to hours, which is typical during coastal tsunamis (e.g., Tuttle et al., 2004). The vertical variations in orientation and dipping-angle of the tropical cyclone deposit are interpreted as resulting from a larger number of waves that occurred during a period of hours to days. These waves can be attributed to infragravity waves, frequently observed during storms. Our observations are promising but still too scarce to be generalized as general criteria to distinguish tsunami and tropical cyclone deposits. Nevertheless,
430 they are consistent and complementary with the results of Falvard and Paris (2017) and Paris et al.
431 (2020). Similar studies of comparable deposits should be conducted in order to obtain reproducible
432 observations and generalize sediment fabric parameters as criteria to distinguish tsunami and
433 tropical cyclone deposits. It appears that these parameters are subtle, leading to the idea that
434 potentially many past high-energy events have been misidentified.

435

436 **Data availability**

437 All data generated or analyzed during this study are included in this article or available from the
438 corresponding author on reasonable request.

439

440 **Acknowledgments**

441 This work is part of the CARQUAKES project (ANR-17-CE03-0006). The objective of the CARQUAKES
442 project is to improve the catalog of large earthquakes and tsunamis in the Lesser Antilles and
443 characterize the related hazards by applying an innovative and novel multidisciplinary approach
444 combining several state-of-the-art methods of offshore and onshore paleoseismology and tsunami
445 modelling. We thank Simon Falvard for helping us in images processing and sediment fabric analyses.

446 Thanks are due to the reviewers and the editor Edward Anthony for their constructive comments
447 that helped improve the manuscript.

448

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620 **Figure captions**

621 Figure 1. (A) The Antilles Arc with main faults and islands (PR: Puerto Rico, VI: British and US Virgin
622 Islands, S: Saba, StE: Sint Eustatius, StK: Saint Kitts, N: Nevis, B: Barbuda, Ant: Antigua, Mo:
623 Montserrat, G: Guadeloupe, D: Dominica, Ma: Martinique, modified from Feuillet et al., 2002, and
624 references therein). The bathymetry and topographic DEM (digital elevation model) are derived from
625 the SHOM (Service hydrographique et océanographique de la Marine) website (SHOM, 2018). The
626 small yellow square corresponds to the Anguilla-Saint-Barthélemy platform shown in panel B. (B) The
627 Anguilla-Saint-Barthélemy platform with its islands. (C) Scrub Island with topographic data obtained
628 by LIDAR (Pinet, 2011) and the bathymetry from SHOM
629 (data.shom.fr/donnees/legend/RASTER_MARINE_3857_WMTS). (D) Satellite image of the
630 westernmost coastal lagoon from Scrub Island and its sandy barrier (Google Earth Pro V 7.3.4.8248.
November 4, 2020. Scrub Island, Anguilla. 18°17′04,68″N, 62°57′11,82″W, Eye alt 561 meters. Maxar Technologies 2021 [September 10, 2021]). The white dot corresponds to the location of SCR1-18-01 sedimentary core.

Figure 2. Data for core SCR1-18-01. From left to right: tsunami and tropical cyclone events (in grey), sedimentary log, grain-size contour plot, X-ray photography and their associated projections of the orientations (azimuth) and dipping-angle (plunge) of the longest grain axes for Hurricane Donna, Lisbon Tsunami and Pre-Colombian Tsunami. The grey rectangles in the X-ray photography of the Pre-Columbian Tsunami indicate area with no data because some sedimentary levels were sampled before this study (Biguenet et al., 2021). The FA to FD stereographic representations are relative to the facies observed in the studied deposits.

Figure 3. Elongation of the ellipsoids compared to the plunges for all grain-size fractions in the three studied events. Examples of ellipsoid shapes depending on their elongation value are displayed below the horizontal axis (for a plunge of 0°). Examples of ellipsoid dipping-angle depending on their plunge value are displayed left to the vertical axis (for a degree of elongation of 0.3).

Figure 4. Anisotropy of the ellipsoids of grain orientation for different types of fabric (Benn, 1994), subdivided by grain-size fraction for every studied deposit. The shape of the ellipsoid is represented at each end of the central plot: isotropic (top), oblate (low left) and prolate (low right).

Figure 5. Three-dimensions visualisation of the Lisbon tsunami deposit. (A) Entire deposit; (B) Observation of the sediment fabric in the centre of the deposit and of its erosive base.