Supplementary material

We applied the method established by Glicken (1996) and Bernard (2008) to calculate the grain-size class weight percentages from the three grain-size analysis methods (image analysis, sieving and laser diffraction).

The volume percentages of the identified clasts were converted to weight percentages (wt%) using the following relationships:

Wc= 100 ρc Vcρc Vct+ ρf (100−Vct ) Eq. S1

Wf = 100 – Wc Eq. S2

where Wc is the wt% of size class coarser than -5 ϕ, Wf is the wt% of size class finer than -5 ϕ, Vc is the volume percent (vol%) of size class coarser than -5 ϕ, Vct is the total vol% of all size classes coarser than -5 ϕ, ρc is the density (g/cm³) of material coarser than -5 ϕ, and ρf is the density of material finer than -5 ϕ. In these calculations, we used mean ρc and ρf values of 1.94 and 1.61 g/cm3, respectively. These values correspond to the mean densities measured by Glicken (1996) for clasts of andesite and dacite lithology, and for Mount St. Helens debris

flow deposit’s matrix, respectively.

The w% of each grain size class (Wi) of samples composed of particles with diameters < -5 ϕ used for sieving and laser diffraction were calculated using the following relationship:

Wi = Wf (ws /wi) Eq. S3

where ws and wi represent the total weight of the samples and the weight of each grain size class, respectively.

Table S1 Latitude and longitude (WGS84) coordinates (ddd.ddddd°) and altitudes (m a.s.l.) of sampling sites

Latitude Longitude AltitudeRG-2 0.01048 -78.15238 2758RG-7 0.01076 -78.15355 2785RG-19 0.05402 -78.16558 2768RG-21 0.00886 -78.17136 2696AY-24 0.07204 -78.13433 2859AY-29 0.07107 -78.13333 2861AY-30 0.01784 -78.08976 3248AY-32 0.01756 -78.08893 3230RG-34 -0.00998 -78.16043 2786RG-36 -0.00651 -78.18877 2789

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Fig. S1 a) Digital image of a 1 m x 1 m window of the debris flow deposit matrix facies (site RG-19); b) Image reframed and corrected with Photoshop© to erase the effects induced by perspective and lens distortion; c) Image processed with Illustrator©: each element >-5 ϕ was drawn and recorded by size class (pink: refusal at -5 ϕ; yellow: refusal at -6 ϕ; blue: refusal at -7 ϕ), d) Final processed image; the number of pixels corresponding to elements >5 ϕ were counted using Illustrator©. The grid spacing is 5 cm.

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Table S2 Grain size analysis in phi (ϕ) units of the debris flow deposits. Results are expressed in wt%.

φ -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 - ∞ wt%RG-2 1.7 4.5 3.3 11.1 6.5 5.3 6.0 5.7 7.9 7.1 9.4 3.5 4.6 3.7 3.1 1.9 14.5RG-7 3.9 5.7 1.8 5.9 7.0 9.2 9.2 5.2 7.4 5.6 5.6 4.2 4.1 3.9 3.2 2.0 16.1RG-19 0.0 4.4 2.5 14.1 6.8 7.8 9.5 4.9 8.3 6.6 6.6 3.2 3.6 3.8 3.1 2.0 12.8RG-21 2.8 3.8 5.2 5.1 8.8 7.7 9.2 5.1 7.7 6.3 5.3 5.8 4.8 3.8 2.9 1.9 13.8AY-24 3.5 6.1 3.7 3.5 4.2 5.1 7.7 5.4 8.8 7.8 10.0 5.7 6.8 4.4 3.7 2.4 11.1AY-29 2.7 6.6 5.0 7.5 5.8 5.3 8.3 4.7 7.3 6.6 8.9 4.9 6.4 3.9 3.5 2.3 10.3AY-30 9.0 8.2 3.9 8.0 8.0 6.7 7.8 4.6 7.2 6.5 6.9 3.4 3.0 2.8 2.7 1.6 9.7AY-32 7.4 3.4 4.7 8.7 8.7 5.2 8.3 3.8 6.4 6.0 7.4 4.4 4.5 3.7 3.5 2.2 10.8RG-34 20.4 7.5 8.3 10.5 7.3 9.2 6.4 6.9 2.5 2.9 4.0 3.2 1.9 9.0RG-36 8.3 2.9 1.6 2.1 8.1 7.1 11.0 5.6 7.9 6.4 7.8 4.5 5.3 4.6 3.4 2.1 11.3

Fig. S2 Photographs of dung beetle (Coprinisphaera) fossil brood balls found in cangahua a) above (RG-36) and b) below (AY-24) the debris flow deposit.

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Fig. S3 Drawings of clasts (black) in the 1-m² windows at various sites through the debris flow deposit. The texture corresponds invariably to a matrix facies. The dashed contour at site RG7 delineate an elongated clast (see Fig. 3e). The grey shape delineated by a dashed contour at site RG 21 corresponds to a clast with jigsaw cracks (see Fig. 3d).

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Fig. S4 X-ray diffractograms of the clay-sized fraction for samples a) RG-36, b) AY-29 and c) AY-32. The arrows indicate the main diffraction peaks corresponding to smectite (montmorillonite) (S.), halloysite (H.), plagioclase feldspar (F.), cristobalite (C.), kaolinite (K.), jarosite/natrojarosite (J.), alunite (A.), greigite (G.).

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Fig. S5 Sequences of oriented X-ray diffractograms of the clay-sized fraction for samples a) RG-36 and b) AY-29. The materials were subjected to K saturation (K) followed by heating at 105 °C, 300 °C or 550 °C, and Mg saturation (Mg) followed by ethylene glycol solvation (Mg Eg). Diffraction peaks are labelled by d-spacings (Å). See Fig. S4 for the legend. Quartz, illite and smectite all contribute to the 3.34-Å peak observed at 550°C after K saturation, whereas only quartz and illite contribute to this peak at 20 °C.

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Fig. S6 Sequences of oriented X-ray diffractograms of the clay-sized fraction for samples a) RG-34 and b) AY-32. The materials were air dried and treated with formamide (Form). Formamide treatment allows identification of kaolinite (K.) and halloysite (H.).

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