# Facies analysis provides new insights into event bed deposition in a hadal trench environment

Mishelle Muthre, Jean-noël Proust, Charlotte Pizer, Ken Ikehara, Jyh-jaan

Steven Huang, Hajime Naruse, Michael Strasser

► To cite this version:

Mishelle Muthre, Jean-noël Proust, Charlotte Pizer, Ken Ikehara, Jyh-jaan Steven Huang, et al. Facies analysis provides new insights into event bed deposition in a hadal trench environment. Sedimentology, 2026, 73 (2), pp.441-477. 10.1111/sed.70069. insu-05512992

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Sedimentology (2026) 73, 441–477

doi: 10.1111/sed.70069

# Facies analysis provides new insights into event bed deposition in a hadal trench environment

MISHELLE MUTHRE* ID, JEAN-NOËL PROUST† ID, CHARLOTTE PIZER* ID, KEN IKEHARA‡ ID, JYH-JAAN STEVEN HUANG§ ID, HAJIME NARUSE¶ ID and MICHAEL STRASSER* ID

*Department of Geology, University of Innsbruck, Innrain 52f, Innsbruck 6020, Austria
(E-mail: mishelle.muthre@uibk.ac.at)

†Géosciences-Rennes, CNRS, Université Rennes, Campus de Beaulieu, Rennes 35042, France

‡National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Ibaraki, Japan

§Institute of Oceanography, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan

¶Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Kyoto 606-8502, Japan

Associate Editor – Gregor Eberli

## ABSTRACT

Hadal trenches preserve exceptional sedimentary archives of past geological events, yet their depositional processes remain poorly constrained. The Japan Trench captures complex earthquake-triggered event beds that record repeated sediment-gravity flows that deliver terrigenous and biogenic material into the trench, informing palaeoseismic reconstructions. Classical event stratigraphy treats such event beds as products of brief, isochronous processes. This study aims to build on that concept to document hydrodynamic conditions and the relative depositional timing and duration within single event beds. Using four well-documented historical earthquake-triggered beds recovered during IODP Expedition 386, high-resolution analyses of sedimentary structures were conducted—integrating physical, chemical and biogenic characteristics—to develop a systematic, facies-based framework for identifying event-internal characteristics. Six event-internal facies (F1–F6), bounded by hemipelagic facies (F0), form a fining-upward sequence that records the shift from high-energy, non-cohesive, bedload-dominated flows to low-energy, cohesive, suspension-dominated deposition. A 32-µm grain-size threshold separates coarse-grained (F1–F3; >32 µm) from fine-grained (F4–F6; <32 µm) facies. Event-internal facies organise into pulses and pulse groups that stack hierarchically into three patterns: single-pulsed, multi-pulsed and amalgamated. Amalgamated patterns comprise two or more single- or multi-pulsed successions separated by breaks in the fining-upward trend. Breaks marked by opportunistic trace fossils or by F6 indicate pauses during deposition (quasi-synchronous flows), whereas their absence suggests synchronous flows. Most event beds are amalgamated and comprise flows emplaced at different times and sourced from different directions, as reflected by variable composition, facies and palaeo-flow indicators. Basin physiography strongly influences facies development and thickness: basin highs record erosion–deposition stages, whereas depocentres pond turbid water masses in prolonged suspension, producing thick fine-grained tops. Transitional facies (F4) forms where new flows interact with a still-settling
suspension cloud, providing a basis to interpret complex mainshock– aftershock sequences. By providing new insights into hydrodynamic condi- tions and relative depositional timing and duration, this facies-based frame- work advances event stratigraphy and improves global understanding of deep-marine sedimentary dynamics.

**Keywords** Event stratigraphy, Turbidite stacking pattern, Hydrodynamic flow conditions, Earthquake-triggered event bed, Relative depositional tim- ing, Japan Trench.

INTRODUCTION

Hadal trenches are the deepest submarine envi-
ronments on Earth, occurring at >6000 meters
below sea level (mbsl). Some of these unique
environments, such as the Japan Trench, have
recently gained attention in sedimentological
and biogeochemical studies for their role in cap-
turing ‘event beds’ that record sediment gravity
flows triggered by shaking in past earthquakes
which, in turn, lead to episodically enhanced
carbon turnover in ultra-deep water (Ikehara
et al., 2016, 2017; Bao et al., 2018; Kioka
et al., 2019; McHugh et al., 2020; Schwester-
mann et al., 2020, 2021; Kanamatsu et al., 2022,
2023; Chu et al., 2023, 2025; Strasser et al.,
2024). Sediment cores capturing such archives
offer a valuable opportunity to reconstruct long,
continuous records of key geological processes,
advancing understanding of the dynamics of
sediment remobilization and deposition and,
ultimately, constraining earthquake recurrence.

Sedimentary records have classically been
interpreted using ‘event stratigraphy’ to recon-
struct past geological processes by distinguishing
beds produced by brief, chronostratigraphic
events from the intervening background sedimen-
tation (Seilacher, 1982; Einsele, 2000). This con-
cept uses frameworks underpinned by widely
adopted facies sequences and process models
(Bouma, 1962; Mutti & Ricci Lucchi, 1978;
Piper, 1978; Stow & Shanmugam, 1980;
Lowe, 1982; Stow & Piper, 1984; Pickering
et al., 1986). For event beds associated with sedi-
ment gravity flows (e.g. turbidity currents), some
of these models often consider grain size distribu-
tion and trends, sedimentary structures, bed
thickness and composition to define a facies
linked to different depositional stages of the
flows (e.g. Middleton, 1993; Talling et al., 2012).
Within the palaeoseismology community, the
concept of event stratigraphy has been adapted to
denote a common triggering mechanism (i.e. an

earthquake sequence) that initiates multiple, spa-
tially distributed and temporally offset slope fail-
ures and sediment-gravity flows; the resulting
deposits stack in trench and lacustrine basins as
multi-source composite deposits (e.g. Nakajima &
Kanai, 2000; Goldfinger et al., 2012; Moernaut
et al., 2014; Ikehara et al., 2021). In this usage,
determination of ‘event beds’ slightly differs from
the classical concept: the isochronous event is
the trigger, not the associated deposition. Accord-
ingly, criteria have been proposed to diagnose a
coseismic imprint in event beds, including the
following: (i) a multi-pulsed or amalgamated pat-
tern at the turbidite base resulting from
quasi-synchronous flow accumulation of
co-seismic sediment remobilisation (Nakajima &
Kanai, 2000; Noda et al., 2008; McHugh
et al., 2016; Kioka et al., 2019; Ikehara
et al., 2021); (ii) over-thickened, homogeneous
mud deposits resulting from decantation of a
large volume of suspended particles and the fol-
lowing oscillatory movements of the whole water
column in semi-enclosed or enclosed basins
(Polonia et al., 2013); (iii) soft sediment deforma-
tion structures related to liquefaction during epi-
sodes of ground acceleration (Goldfinger
et al., 2003; Molenaar et al., 2022); (iv)
multi-directional turbidites in single ‘event beds’
as an indication of simultaneous triggering of
sediment gravity flows (Van Daele et al., 2017);
(v) recurring trace-fossil successions and conse-
quent changes in substrate consistency, oxygena-
tion and organic matter delivery and
remineralisation (Uchman & Wetzel, 2011; Hovi-
koski et al., 2025).

While these descriptions are useful for inter-
preting earthquake-triggered event beds in the
geographic context in which they were devel-
oped, applying such site-specific or more classi-
cal frameworks (Seilacher, 1982; Einsele, 2000)
to other settings present several challenges. Fun-
damentally, this stems from a lack of consensus
in the definition of commonly used facies
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Japan Trench overview and studied trench basins. (A) Regional bathymetry off the eastern coast of Honshu showing IODP Expedition 386 core sites in central Japan Trench (red circles; Strasser *et al.*, 2023), the track of along-strike topographic profile shown in E (yellow line; Kioka *et al.*, 2019) and slope-to-trench flow-accumulation pathways (grey lines; Kioka *et al.*, 2019). (B–D) High-resolution bathymetry of Basins C2, C1 and S3, showing core sites (red circles) at basin depocentres (M0083, M0091) and basin highs (M0089, M0090). (E) Along-strike topographic profile of the trench indicating basin locations and core sites (Kioka *et al.*, 2019). (F) Sub-bottom profile across Basin C2 showing acoustically transparent event beds that thicken in the basin depocentre and thin towards the basin high. MSCL bulk density logs from IODP Exp. 386 cores are plotted alongside (Strasser *et al.*, 2024). The four most recent ‘thick event beds’ (Event Beds 1–4) are linked to the CE 2011, CE 1454, CE 869 and ~2.3 ka earthquakes (Kioka *et al.*, 2019; Strasser *et al.*, 2024).</figcaption></figure>

sequence terminology (e.g. ‘event beds’, ‘event layers’, ‘compound turbidite’, ‘composite flow’, ‘multi-pulsed turbidite’, ‘amalgamated turbidite’, ‘stacked turbidite’, ‘flow deposit’, ‘mass transport deposit’, ‘seismoturbidite’, ‘graded bedding’, and ‘homogenite’). This inconsistency leads to ambiguity in interpreting event bed deposition in each context. Many diagnostic criteria for event interpretation were developed from outcrop-scale analogues in sediment-rich environments such as canyon-fed deep-sea fan systems or in delta-fed submarine ramp systems within foreland basins (Bouma, 1962; Mutti & Ricci Lucchi, 1978) and passive margin basins (Piper, 1978; Stow & Shanmugam, 1980; Lowe, 1982; Stow & Piper, 1984). Moreover, the lack of exhumed trench analogues that preserve primary structures without later overprinting deformation by subduction or accretion (Vannucchi *et al.*, 2006) further limits the direct applicability of those criteria to modern settings. Because hadal trenches remain among the least

explored aquatic environments (Jamieson *et al.*, 2010; Kioka & Strasser, 2022; Hovikoski *et al.*, 2025), no sedimentary framework yet exists that integrates coherent facies-based interpretation with a process-oriented approach sensitive to trench physiography and its control on facies development. To address these limitations, a tailored framework was proposed to identify facies sequences and stacking patterns shaped by the unique depositional dynamics of tectonically active hadal trenches. This framework is essential to define an event bed in these environments and to fully tap into the potential of hadal trenches as records of past geological events.

This study aims to develop a systematic, facies-based framework for identifying event-internal characteristics, building on the concept of event stratigraphy to enable the documentation of hydrodynamic conditions and relative depositional timing and duration within a single event bed, using the Japan Trench as a case study. The trench axis,
located at >7000 mbsl, is a sediment-depleted
environment shaped by topographic complexities
that form semi-confined basins and highs (Fig. 1).
The Japan Trench is also one of the world's most
active subduction zones with the highest seismic
productivity (Bilek & Lay, 2018). Thus, seismic
shaking in large earthquakes is the primary trigger
of turbidity currents induced by large-scale surfi-
cial sediment remobilisation, supplying episodic
sediment delivery to the trench axis (Ikehara
et al., 2016, 2021; Kioka et al., 2019; Schwester-
mann et al., 2020). The resulting event beds pre-
served in the Japan Trench hold an exceptional
archive for palaeoseismological studies and their
connections to deep-ocean biogeochemical pro-
cesses (Bao et al., 2018; Kioka et al., 2019; Chu
et al., 2023, 2025; Strasser et al., 2024).

Recent results from the International Ocean Dis-
covery Program (IODP) Expedition 386 outline a
preliminary event stratigraphy from ~40-m sedi-
ment cores along the trench axis, targeting both
depocentre sites—where event beds are thick—
and topographic high sites—which preserve lon-
ger records with thinner, harder-to-identify event
beds (Strasser et al., 2023, 2024). These findings
highlight the need for a comprehensive strategy to
consistently define facies across both physiogra-
phies in order to distinguish individual events. A
high-resolution analysis of sedimentary structures
—integrating physical, chemical and biogenic
characteristics—was conducted on an example
dataset comprising four recent event beds and
their intervening deposits from the
southern-central Japan Trench, which were previ-
ously correlated with historically documented
earthquakes (Ikehara et al., 2016; Kioka
et al., 2019; McHugh et al., 2020; Schwestermann
et al., 2020, 2021; Kanamatsu et al., 2022, 2023;
Strasser et al., 2024). By examining the sedimen-
tary characteristics of these previously identified
event beds, a detailed interpretation of facies and
their stacking patterns was built, thereby develop-
ing a process-based understanding of event bed
deposition in a hadal trench environment.

BACKGROUND

Geological and oceanic setting in the Japan
trench

The Japan Trench is located east of Honshu
(36°N–40.4°N; Fig. 1), where the Pacific Plate sub-
ducts beneath the Okhotsk Plate at a rate of ~7.3–
8.6 cm/yr (DeMets et al., 2010; Loveless &

Meade, 2010). The margin is considered an
end-member case of tectonic erosion, characterised
by long-term landward retreat developing forearc
subsidence and normal faulting (Von Huene &
Culotta, 1989; Von Huene & Lallemand, 1990). Tec-
tonic erosion strongly influences the forearc slope
morphology, which features a gently sloping upper
terrace (1°–2°), narrow fault-bounded mid-slope
terraces at 4000–6000 mbsl, and a steeper lower
slope (average–5°) (Kawamura et al., 2012; Arai
et al., 2014). The resulting slope morphology con-
trols sediment-gravity flow pathways from the
upper slope into the trench axis (Kioka
et al., 2019), which is bounded by the Erimo and
Daiicimi-Kashima subducting seamounts (Fig. 1A).
Between them, the trench axis is commonly parti-
tioned into the northern, central, and southern sec-
tions located at ~7600, ~7500 and ~8000 mbsl,
respectively (Fig. 1A).

The slightly oblique subduction and bending of
the Pacific plate as it approaches the trench forms
N-S to NNW-SSE trending horst-and-graben struc-
tures, creating a series of semi-enclosed basins at
the trench axis with varying along-strike connec-
tivity (Fig. 1E). Trench basins at the central Japan
Trench (37.5°N–39°N, Fig. 1C) are relatively small
(with basin areas defined as connected areas of flat
seafloor <4 km²) and have no direct connection to
canyons at either end of the trench axis (Kioka
et al., 2019). As a result, sediment accumulation in
the trench basins is driven mainly by episodic
slope-sourced turbidity currents (Kioka
et al., 2019; Ikehara et al., 2020, 2021; Schwester-
mann et al., 2021), yielding high bulk sedimenta-
tion rates up to 4.4 m/kyr (Ikehara et al., 2016).

Hemipelagic sediments primarily consist of dia-
tomaceous silty clay to clay (Saino et al., 1998; Ike-
hara et al., 2013, 2016), and accumulate at lower
rates of <2.1 mm/yr (Strasser et al., 2024). With
water depths >5000 m, the trench environment
lies below the carbonate compensation depth
(CCD, Berger et al., 1976; Usami et al., 2018). Pri-
mary productivity—and thus the diatomaceous
flux forming hemipelagic sediments—is controlled
by the Sanriku Confluence of the warm Kuroshio
and Tsugaru currents with the cold Oyashio
(Yasuda et al., 1996; Saino et al., 1998; Ikehara
et al., 2013, 2016).

Event beds in the Japan Trench

In the aftermath of the 2011 Mw 9.0 Tohoku-Oki
earthquake, high-resolution sub-bottom profiler
(SBP) surveys were conducted to investigate
earthquake-triggered event bed deposition at the
trench axis. The results show a series of thick, acoustically transparent units with lateral pinch-out geometries enabling links between earthquake shaking and sediment accumulation in the hadal trench basins (Kioka et al., 2019; Strasser et al., 2024; Pizer et al., 2025). Several studies of sediment cores confirm that these units imaged by sub-bottom profilers (SBP) are event beds, each comprising a coarse-grained layer overlain by a thicker fine-grained layer (Kioka et al., 2019; Ikehara et al., 2021; Strasser et al., 2024). The sediments derive from the upper few centimetres of young, unconsolidated, organic carbon (OC)-rich layers on the upper and lower slopes, which are remobilised during long-lasting, low-frequency shaking to generate turbidity currents that transport sediments downslope from the upper and lower slopes (McHugh et al., 2016; Kioka et al., 2019; Moleelaar et al., 2019; Schwestermann et al., 2020). Based on this, the three uppermost event beds in the trench have been linked to shaking in the CE 2011 Tohoku-Oki, CE 1454 Kyōtoku and CE 869 Jogan historical earthquakes (Ikehara et al., 2016, 2017; Bao et al., 2018; Kioka et al., 2019; McHugh et al., 2020; Schwestermann et al., 2020, 2021), with a fourth attributed to a seismic event at ~2.3 ka (Kanamatsu et al., 2022, 2023). However, the distinction of these historical event beds remains based mostly on their outstanding thickness and overall structure, rather than a robust, process-based interpretation of facies sequences. Accordingly, four event beds interpreted from IODP Exp. 386 sediment cores in the southern-central Japan Trench (Strasser et al., 2024) are used as examples to develop a facies-based framework for interpreting event beds in hadal trenches. Since age/earthquake implications are irrelevant for this study, these deposits are hereafter referred to as Event Beds 1–4 (youngest to oldest).

## Terminology and classification of sedimentary characteristics

Accurate interpretation of event beds requires precise terminology to delineate depositional characteristics, stratigraphic sequences and sedimentary facies. While previous studies have sometimes used varying terms, this study has developed standardised definitions to ensure clarity and consistency throughout this study (Table S1).

Sedimentary structures are defined as the physical characteristics within sediments,

classified as (i) primary structures, formed during or immediately after deposition on a forming seafloor and (ii) secondary structures, developed after deposition on a stable seafloor. For sedimentary structures, terminology follows the classical literature (Walker, 1965; Jopling & Walker, 1968; Allen, 1973; Baas, 1994, 1999), and for trace fossils, terminology is compiled from Wetzel & Uchman (1998, 2012), Uchman & Wetzel (2011) and Hovikoski et al. (2025), all summarised in Table S1. For instance, standard metrics, such as plan-view morphology, cross-sectional geometry and angle of climb, are used to classify the bedform migration patterns of ripple cross-laminations (Allen, 1973; Baas, 1994). To complement descriptions of sedimentary structures, lithological terminology follows the grain-size classification of Wentworth (1992) and the textural classification of Shepard (1952).

A facies is defined as a group of sediment that shares specific physical, chemical and biogenic characteristics, reflecting specific hydrodynamic conditions during event bed deposition. Facies are distinguished from observed data—principally granulometry and sedimentary structures—with composition (XRF-based) used as supporting evidence for distinguishing fine-grained facies. Facies within an event bed are termed event-internal facies. Arranged vertically, these facies form a fining-upward sequence that may record deposition from initial bedload to suspended load, before returning to background sedimentation. Facies of background sediments, termed here as hemipelagic, bound the event-internal facies above and below. Specific event-internal facies that co-occur form a facies association. A discrete part of the event-internal facies is considered a single pulse. Successions of these pulses define stacking patterns (multipulsed and amalgamated).

## METHODOLOGY

Summary of site/core locations, dataset acquisition, processing workflow and analytical approaches used to characterise facies and their stacking patterns.

### Site locations

To establish a facies framework for hadal trench sediments, Event Beds 1–4 and their intervening deposits were examined in sediment cores from
the Japan Trench collected during IODP Exp. 386 (Strasser *et al.*, 2023). Using the same nomenclature as in Strasser *et al.* (2023, 2024), data from 18 holes were analysed across four sites, which are located in three basins S3 (site M0091, Fig. 1D), C1 (site M0090, Fig. 1C) and C2 (sites M0089 and M0083, Fig. 1B,E) near the central zone of the Japan Trench (Fig. 1A). Basins S3 and C1 are ~100 km apart, while basins C1 and C2 are ~45 km apart (Fig. 1A, E). These sites were selected as they exhibit morphological irregularities that enable us to assess trench physiography-controlled sediment transport and deposition, with two sites located at basin highs at the edge of basins C1 and C2 (sites M0090 and M0089; hereafter referred to as 'basin-high sites', Fig. 1F), and two sites in the depocentre of basins S3 and C2 (sites M0091 and M0083; hereafter 'basin-depocentre sites', Fig. 1F).

Basin C2, containing sites M0089 on the topographic high (38° 43.202′ N, 144° 7.538′ E, at 7595 mbsl, Fig. 1B, E) at the southern edge of the basin and M0083 in the depocentre (38° 45.413′ N, 144° 7.755′ E, at 7635 mbsl, Fig. 1B, F), is a flat NNE-SSW-elongated basin (~14 km long). Its floor slightly shallows towards the S-SW and N-NE, reaching depths of ~7595 m and ~7605 m, respectively. The asymmetric basin features a steeper northwestern side (25–30% slope gradient) and a gentler southeastern side (15–20% slope gradient). Sedimentation at the basin-high site M0089 has been documented to be stratigraphically condensed compared to the expanded depocentre site M0083 (Fig. 1E, Strasser *et al.*, 2024), while the depocentre displays the largest flow accumulation (source area of 1.2 × 1010 m²) with an up to 160 m trench-fill (Kioka *et al.*, 2019; Pizer *et al.*, 2025).

Basin C1, where site M0090 is located on a topographic high (38° 17.834′ N, 144° 3.549′ E, at 7445 mbsl, Fig. 1C, E), is an N-S-elongated basin (~3.5 km long). Its floor gradually deepens towards the north, where the trench axis veers to the NE-SW. The basin has a comparably small source area (~0.41 × 10¹⁰ m²; Kioka *et al.*, 2019) and exhibits a pronounced asymmetry, with steep western margins (~20% slope gradient) contrasted by gentler eastern slopes (6–10%), and it spans ~1 km in width near site M0090.

Basin S3, where site M0091 is located in the depocentre (37° 24.747′ N, 143° 43.729′ E, at 7815 mbsl, Fig. 1D, E). It is a flat NE-SW-elongated basin (~10 km long), oriented

obliquely to the trench axis but parallel to the sharp topographic boundary at the northern basin high. Much of the basin floor exhibits irregular morphology, especially to the southwest, becoming elevated towards the northeast. The asymmetric basin varies from ~7800 to 7825 mbsl near site M0091, featuring a steeper western side (~20% slope gradient) and a gentler eastern side (3% slope gradient). Flow accumulation in S3 is the highest among the central trench basins (source area of ~2.2 × 10¹⁰ m²; basin size 8.2 km²; Kioka *et al.*, 2019).

## Analytical methods

A multi-proxy analysis integrating quantitative and semi-qualitative approaches was applied to capture observable and measurable sediment characteristics, developing a facies-based framework from sediment cores.

### Shipboard data

To develop detailed facies and stacking-pattern analyses for interpreting event beds in hadal trench environments, high-resolution shipboard datasets from IODP Expedition 386 (Strasser *et al.*, 2023) were integrated with new scanning-and sample-based data acquired for the 18 holes. The shipboard dataset includes (i) photo spectroscopy and line-scan images of split-core sections (1 m long, 12 cm wide), with colour reflectance at 2-cm intervals and RGB values at 100-µm intervals, using histogram equalisation to enhance colour-based facies contrast, (ii) sagittal slice images of x-ray computed tomography (CT) data (3D-radiodensity ~600 µm-voxel resolution), (iii) visual core descriptions of lithology defined by informed grain size, composition and sediment structure estimates, and (iv) Multi-Sensor-Core-Logging (MSCL)-derived physical property data including magnetic susceptibility (MS), gamma-ray attenuation bulk density (D) and natural gamma radiation (NGR) (see details on data acquisition methods in Strasser *et al.*, 2023).

### Scanning-based data

In addition to shipboard CT images, higher-resolution two-dimensional x-ray radiographs (100 µm-pixel resolution) were acquired from selected U-channels (1 m long, 6 cm wide) at sites M0083D (from 0 to ~10 meters below seafloor, msbf), M0090C and M0089C (from 0 to ~2 mbsf each) using the SCOPIX device at the EPOC Laboratory, University of Bordeaux, following Migeon *et al.* (1998). X-ray CT and
radiograph datasets were combined to perform 2D and 3D analyses. The high-resolution x-ray radiographs enabled the observation of sedimentary structures in the fine-grained sediments that are not readily distinguished on CT images. These images were used to identify and measure the dimensions and occurrence of primary and secondary structures, including trace fossils. Ichnological analysis is crucial to distinguish opportunistic versus equilibrium-fauna colonisation and thus unstable versus stable seafloor conditions. Recorded characteristics included incipient ichnofossils, ichnogenera and cross-cutting relationships, following Hovikoski et al. (2025).

Relative palaeo-flow orientation was inferred via 3D CT analysis of consistently oriented core sections. Reliable vector indicators were derived from observations of asymmetric sedimentary structures (Van Daele et al., 2017; Peakall et al., 2024 and references therein), including sole marks (e.g. gutter casts), foreset-dipping ripple cross-lamination and overturned syn-folds. Measurements were compiled into rose diagrams with 5° bins to summarise directional trends and dominant flow orientations. Although the cores were not azimuth-oriented, relative interpretations were constrained by integrating sedimentological observations with physiographic and stratigraphic relationships—including slope-to-trench flow accumulation pathways (Fig. 1A) and basinward thickening, lateral pinch-out directions and fining trends of acoustically transparent event beds (core and sub-bottom profile data, Fig. 1E).

High-resolution x-ray fluorescence (XRF) core scanning was acquired at 2 mm intervals on continuous U-channels through full cores using ITRAX-XRF Core Scanner at the Department of Geology, University of Innsbruck (UIBK) and at the Geological Survey of Japan (AIST) (Huang et al., 2025). XRF scanning is semi-quantitative, providing relative element intensities; interpretations rely on down-core compositional trends. Because measured intensities can be influenced by moisture content and matrix composition, elemental counts were calibrated using a centred-log ratio transformation to correct for matrix effects and down-core variations in physical properties (Weltje et al., 2015). XRF analysis is useful for capturing the distinct chemical signature of each event bed and helps distinguish them from the background hemipelagic sediment. Therefore, seven elements (Ca, Sr, Fe, Ti, Mn, Si and Cl) were selected to characterise

variations in sediment composition, following the XRF-based chemical proxy framework outlined by Rothwell & Croudace (2015). Element variations can indicate sediment provenance of event beds by reflecting shifts between terrigenous inputs (Fe, Ti) and biogenic remains (Ca, Sr) below CCD. They can also indicate changes in primary productivity (Si) and authigenic processes, such as redox zonation with surface oxic sediments (Mn, Fe) under clear-water and stable seafloor conditions, and the balance between dissolution and allochthonous input, or even post-depositional precipitation (Ca, Si).

## Sample-based data

Sample-based data generation included laser-diffraction and dynamic image analyses of granulometric measurements obtained through two strategies: systematic and site-specific sampling. For systematic sampling, data were collected at 1–2 cm intervals. In Event Bed 1, measurements were made on two cores (M0090C and M0089C); in Event Bed 2, on one core (M0083D); and in Event Bed 3, on six cores (M0091B, M0091D, M0090B, M0090D, M0083B and M0083D). Measurements for Event Beds 1 and 2 were conducted with a laser-diffraction analyser (Malvern Mastersizer 3000) at the Department of Geology (UIBK), while for Event Bed 3, a dynamic image analyser (Camsizer X2) at the Disaster Prevention Research Institute (Kyoto University) was used. In addition, site-specific sampling was performed at selected coarse and fine-grained fractions of event beds and within hemipelagic sedimentation using the Malvern Mastersizer 3000 at UIBK. Assessment of samples from the same depth, measured using both the Malvern Mastersizer 3000 and the Camsizer X2, shows that the results are comparable (see Fig. S5).

Granulometric data from each core were quantified using five statistical parameters: mean, mode, d10 (10th percentile; finest fraction), d50 (median, can exceed the mode in coarse-grained distributions), d90 and d99 (90th–99th, coarsest fraction). To better constrain event-internal facies, pulses and pulse groups, four textural parameters were calculated: fines or fines content—the percentage (%) of particles <32 µm relative to the total sediment (Gladstone et al., 2018)—sorting, skewness, kurtosis, using calculated percentiles from granulometric data (Folk & Ward, 1957). Event-internal facies were characterised using binary cross-plots with granulometric and textural parameters. In particular,
<figure><img src="image_4.png" /></figure>
**Fig. 2.** Example of multi-proxy data used to build a facies-based framework for Event Bed 2 (CE 1454 Kyōtoku earthquake): (A) Basin-high site (M0089) showing the complete event bed. (B–D) Basin-depocentre site (M0083) showing (B) the upper section, (C) the lower section and (D) the complete event bed, given the exceptional thickness at this site (see full dataset in Fig. S2). Imaging includes sagittal-slice x-ray computed tomography (CT), altered-colour line-scan images and x-ray radiographs. High-resolution downcore data include bulk density (D; blue), magnetic susceptibility (MS; red) (Strasser *et al.*, 2023) and elements from x-ray fluorescence (XRF) core scanning expressed as centred-log ratio (clr; Huang *et al.*, 2025). New datasets comprise lithologic sketches and grain-size estimates from visual core descriptions, sedimentary-structure symbols, granulometric measurements (with d10, d50, d90 in µm) and relative palaeo-flow direction inferred from CT axial slices (black arrow). The trench-axis topographic profile indicates core sites. The vertical axes in panels (A–C) show section depth (cm) and in (D) show core depth (cumulative section depth; m). Symbols for primary (P*) and secondary (S*) structures and trace fossils (TF*) are defined in Fig. 3.

sorting versus fines content plots reveal a fining-
upward trend and fine-dispersion conditions
associated with the development of parallel,
cross and convolute lamination. These plots also
identify grain-size thresholds that support the
organisation of facies sequences into different
pulses and help explain the differences in
sediment-transport and deposition modes. Sort-
ing versus skewness cross-plots are used to
assess how grain-size distribution shape (disper-
sion and asymmetry) differentiates facies. Distrib-
bution can help to separate pulses and different
stacking patterns within an event bed. Lastly,
d99 versus d50 cross-plots show the potential
differences between the hemipelagite and
sequence-end facies, highlighting a larger pro-
portion of the coarsest grains (d99).

RESULTS

This study presents collated datasets and addi-
tional analyses of four example event beds
(Events 1–4) and their intervening deposits
(examples in Figs 2, 3; and full compilations in
Figs S1–S4). Firstly, primary and secondary sed-
imentary structures with trace fossils were
described, including their textural and composi-
tional characteristics and stratigraphic occur-
rence. Secondly, six event-internal facies were
defined from the vertical arrangement of sedi-
mentary structures; together they form a fining-
upward sequence. Facies are grouped into
coarse-grained (F1–F3, >32 µm) and fine-grained
(F5–F6, <32 µm) facies, with a fine-grained ‘tran-
sitional’ facies (F4) between them, all bounded
by hemipelagic background sediment facies (F0).
Thirdly, three organisational stacking patterns at
the pulse and pulse-group scales were outlined,
linking facies sequences to stacking patterns
produced by multiple hydrodynamic flows.

Sedimentary structures

Sedimentary structures were described and clas-
sified using the defined terminology (**Terminol-
ogy and classification of sedimentary
characteristics** section; Table S1) and exempli-
fied in Figs 2–4. Occurrences were indexed by
grain-size domain: (coarse-grained, >32 µm ver-
sus fine-grained sediments, <32 µm; Figs 5–8)
and by setting (basin highs versus depocentres;
Table S2).

Primary structures

*Structureless mud.* Structureless silty clay or clayey silt (5–10 cm thick) occurs mostly at the tops of event beds across both basins’ physiographies (Fig. 4). Here, burrows infilled with dark-coloured sediments (RGB-bluish in line-scan images) coincide with high CT density and a slight upward increase in bulk density (e.g. 89–93 cm depth in M0083D-1H-8 W, Fig. 2C). Trace fossils include slender helicoidal traces (*Gyrolithes*) and vertical to horizontal, deep-penetrating, thread-like and ramifying burrow systems (e.g. *Trichichnus*, *Pilichnus*). At depocentre sites (Figs 2B, 3B), multiple structureless intervals occur as (i) ungraded silty clay overly-ing normal grading or fine parallel- or cross-laminated intervals (e.g. 14–16 cm depth in M0083D-1H-3D, Fig. 2B) or (ii) packages alter-nating discordantly with deformed intervals (e.g. 27–42 cm depth in M0083D-1H-8D, Fig. 2C).

*Normal and inverse grading.* Normally and inversely graded silty clay–clayey silt (Figs 4, 5A) increases in occurrence towards event bed tops, especially in depocentre sites. Both trends contain similar fines contents (50–60%, >32 µm; Fig. 5A) and typically form thicker fine-grained intervals (10–50 cm) than the underlying coarse-grained sediments (3–10 cm). At depocentre
<figure><img src="image_5.png" /></figure>
**Fig. 3.** Example of multi-proxy data used to build a facies-based framework for Event Bed 3 (CE 869 Jógan earthquake): (A) Basin-high site (M0089) showing the complete event bed. (B–D) Basin-depocentre site (M0083) showing (B) the upper section, (C) the lower section and (D) the complete event bed, given the exceptional thickness at this site (see full dataset in Fig. S2). Imaging includes sagittal-slice x-ray computed tomography (CT), altered-colour line-scan images, and X-ray radiographs. High-resolution downcore data include bulk density (D; blue), magnetic susceptibility (MS; red) (Strasser *et al.*, 2023) and elements from x-ray fluorescence (XRF) core scanning expressed as centred-log ratio (clr; Huang *et al.*, 2025). New datasets comprise lithologic sketches and grain-size estimates from visual core descriptions, sedimentary structure symbols, granulometric measurements (with d10, d50, d90 in µm) and relative palaeo-flow direction inferred from CT axial slices (black arrow). The trench-axis topographic profile indicates core sites. The vertical axes in panels A–C show section depth (cm), and in D show core depth (cumulative section depth; m). Symbols for primary (P*) and secondary (S*) structures and trace fossils (TF*) are defined below panel D.

sites (Figs 2B, 3B), normally graded silty clay commonly occurs, typically overlying mm-scale parallel or cross laminations (e.g. 9–14 cm depth in M0083D-1H-3 W, Fig. 2B), whereas inversely graded silty clay to clayey silt usually underlies mm-scale laminations (e.g. 11–14 and 38–46 cm depth in M0083D-1H-8 W, Fig. 2C). Abundant burrows with horizontally to sub-vertical axes and concentric/eccentric lining enveloping a central tube (e.g. *Artichnus* in 89–93 cm depth in M0083D-1H-8 W, Fig. 2C) are present in fine-grained sediments. By contrast, a moderately sorted, normally graded medium sand–coarse silt with <20% fines (Figs 4, 5A) is observed only in the coarse-grained sediments of Event Beds 2–3 at the basin high C1 (site M0090) and lacks identifiable trace fossils (e.g. 12–14 cm depth in M0090B-1H-2 W, Fig. 3A). The line-scan RGB values indicate that graded intervals lighten upward.

**Parallel lamination.** Thick parallel laminations of sand to coarse silt (laminae 0.5–1 cm thick), moderately sorted with <20% fines, occur in the coarse-grained fraction of event beds, with higher fines at the basin high than at depocentre sites (Fig. 5B). Closely spaced thin interlamina-tions (laminae 0.1–0.5 mm thick) of fine sand–coarse silt alternating with silt–clay and 25–45% fines (e.g. 15–20 cm depth in M0090-1H-2 W, Fig. 3A) are also common in the coarse-grained fraction of event beds at depocentre sites. Towards the event bed tops, very thin parallel laminations form 10–50 cm packages (laminae <0.2 mm thick) that become gradually widely spaced and indistinct and may alternate with graded or structure-less silty–clay intervals (e.g. 35–40 cm depth in M0090C-1P-1 W, Fig. S1). These indistinct laminations contain 50–60%

fines and are slightly better sorted than the sand–silt interlaminations (Fig. 5B). At depocentre sites, bioturbated parallel laminations within deformed intervals are common in the fine-grained fraction of event beds (e.g. 50 cm depth in M0083D-1H-8 W, Fig. 2C), with occasional escape burrows deforming adjacent laminations (e.g. *Teichichnus* burrow at 39–48 cm depth in M0083D-1H-3 W, Fig. 2B). Below some erosive bases of event beds, localised ‘rhythmic’ thin–thick clayey silt parallel laminations are observed (e.g. 84–89 cm depth in M0083D-1H-8 W, Fig. 2C; 39–45 cm depth in M0090B-1H-2 W, Fig. 3A, S7). Here, trace fossils are dominated by straight and ramifying burrow systems (*Gyrolithes, Pilichnus, Trichichnus*).

**Cross lamination.** All cross-bedforms are mm–dm-scale ripple cross laminations (height versus length thresholds in Fig. 6A), mostly as trough-cross laminations. Following Allen (1973), four types are distinguished by angle of climb (α) relative to the stoss slope (Fig. 4): non-climbing ripples (α = 0°); and climbing ripples (α > 0°) Type-A, (α < stoss), Type-B (α < stoss) and Type-S (α > stoss). The coarsest ripples: Non-climbing ripples (medium–fine sand) and Type-A ripples (fine sand–coarse silt) are moderated sorted (<25% fines) in both physiographies (Fig. 6B). Type-B (fine sand–coarse silt) and Type-S (fine silt–clayey silt) are more poorly sorted at depocentre sites (>50% fines) than at basin-high sites (20–40% fines) (Fig. 6B). Sinuous and linguoid crests—typically atop Type-B and Type-S ripples—may develop mm–cm-scale Type-A convolute lamination (e.g. at 21 cm depth in M0090B-1H-2 W, Figs 3A, 4). Climbing ripples can also occur in heterolithic patterns—flaser, wavy and lenticular laminations—(Fig. 6C).
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Characteristics of graded and parallel-laminated sedimentary structures: (A) fines (% >32 µm) versus sorting for normal- and inverse-grading; and (B) fines (% >32 µm) versus mean grain size for parallel lamination at basin-high and basin-depocentre sites.</figcaption></figure>

Flaser (medium sand–coarse silt) has the lowest fines (~10%); wavy (fine sand–fine silt) ranges 10–40% fines; lenticular (fine silt–clay) is fines-rich (40–50%). Ripples at depocentre sites tend to appear in heterolithic patterns. Tabular ripple cross-lamination (Fig. 4)—dm-scale non-tangential foresets forming straight crests up to 15 cm thick (e.g. 22–28 cm depth in M0089B-1H-5 W, Fig. S4)—is rare and observed only in Event Bed 4. At depocentre sites, mm-scale clayey-silt ripple laminations occur as isolated or single sets (Figs 4, 7A), typically as Type-A and Type-B ripples (e.g. 22–25 cm depth in M0083D-1H-1 W, Fig. 2C). Convergent and down-lapping laminae locally suggest silty clay-floccule ripples (Fig. 4).

**Contacts and sole marks.** Sedimentary contacts can be erosive, gradational, deformed or bioturbated (Figs 4, 7B). Erosive contacts (particularly scoured) form sole marks (Fig. 4), which are mainly found at the basal contacts below sandy

beds, including sandy ripples, thick parallel laminations and/or normally graded sand (e.g. axial slice of gutter casts at 27 and 31 cm depth in M0090B-1H-2 W, Fig. 3A). Most sole marks are linear features, ranging from gutters (relief <0.1 cm) to tool- and scour-generated marks typically (ridge relief >0.5 cm relief; Fig. 4). These marks indicate relative flow direction (Peakall *et al.*, 2024; and references therein) and reflect multidirectional behaviour of turbidity currents (Van Daele *et al.*, 2017) at the basal contacts, commonly in basin-high sites (e.g. rose diagram at site M0090B; Fig. 3A).

Deformed contacts, such as flame and load structures (Figs 4, 7B), commonly occur at grain-size interfaces, and are often overturned in the flow direction (Fig. 4). The greater the grain-size contrast, the larger the resulting structures. For example, multiple intervals at the base of Event Bed 3 show mm–cm-scale flame structures (1.5–2 cm high) at the boundary between normally graded coarse sand and coarse silt and
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>
**Fig. 6.** Characteristics of cross-laminated sedimentary structures: (A) Ripple dimensions (height versus length) with thresholds for mm-scale (≤4 cm length, ≤ 0.7 cm height), cm-scale (≤ 10.5 cm length, ≤ 3 cm height) and dm-scale (>10.5 cm length, >3 cm height); (B) fines (% >32 µm) versus sorting for ripple cross-lamination at basin-high and basin-depocentre sites; and (C) fines (% >32 µm) versus mean grain size for heterolithic cross-lamination and soft-sediment deformation structures (SSDS).

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Occurrences of contacts and sole marks by sedimentary-structure type.</figcaption></figure>

clayey silt (e.g. axial slice at 14 cm depth in M0090B-1H-2 W, Fig. 3A), and load casts (0.5 cm deep) at the boundary between fine sand and silty clay interlaminations (e.g. axial slice at 18 cm depth in M0090B-1H-2 W, Fig. 3A). The basal contact of an event bed shows no deformation and is typically sharp and/or scoured (e.g. 38 cm depth in M0090B-1H-2 W, Fig. 3A). Bioturbated contacts (Fig. 7) can occur below silt laminae within a clay matrix (e.g. 9 and 11 cm depth in M0083D-1H-3 W, Fig. 2B) and below sandy cross- and parallel-laminations, forming downward scape structures, with trace fossils such as *Rhizocorallium*, *Teichichnus*, *Artichnus* (e.g. 46 cm depth in M0083D-1H-3 W, Fig. 2B).

*Soft-sediment deformation structures.* In addition to load structures described in contacts and sole marks, detached pseudonodules forming ball-and-pillow structures descend up to 20 cm into the underlying clayey silt-silty clay matrix (e.g. 36–46 cm depth in M0083D-1H-15 W, Fig. 3C), which is common at depocentre sites. Soft-sediment deformation structures (SSDS; Fig. 7) also include Type-A and Type-B convolute laminations and fluid-escape structures. Type-A convolute laminations are mm-scale syn-folds (0.5–1 cm high and 0.8–1.2 cm wavelengths) that develop above Type-B and Type-S ripple crests (e.g. 21 cm depth in M0090B-1H-2 W, Fig. 3A), particularly

above coarse silt ripples forming wavy or lenticular laminations that contain 25–45% fines (Fig. 6C). Type-B convolute laminations can occur as (i) cm-dm-scale continuous overturned folds (1–3 cm high with 2–10 cm wavelengths; Fig. S6) that form coarsening-upward intervals up to 20 cm thick (e.g. 68–77 cm depth in M0083D-1H-8 W, Fig. 2C) and (ii) cm-scale, discontinuous overturned folds (0.5–2 cm high with 1–3 cm wavelengths; Fig. S6) that show upward-thickening laminae while remaining overall fining-upward (45–52 cm depth in M0083D-1H-15 W, Fig. 3C). Type-B convolute laminations are composed of silty clay to (clayey) coarse silt with 55–65% fines (Fig. 6B) and occur just below the coarse- to fine-grained interface of event beds at depocentre sites of Basins S3 (e.g. 12.20–12.50 m depth in M0091B-1H-13 W, Fig. S3) and C2 (e.g. 13.60–13.80 m depth in M0083D-1H-13 W, Fig. S3). Towards the top of the event bed, convolute laminations may also be present at interfaces between clayey silt structures (e.g. mm-scale cross or parallel laminations) and structureless clay, but they are absent in basin-high sites (e.g. 53–60 cm depth in M0083D-1H-2 W, Fig. 2B).

## Secondary structures

*Sediment slumping and faulting.* Folded and contorted laminations (Figs 2B–D and 3B–D) are present in the fine-grained overthickened part of
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Fines (% >32 µm) versus mean grain size for all measured sedimentary structures, with reference mean thresholds at 32 µm (fine- versus coarse-grained facies) and 63 µm (sand- versus coarse silt-dominated facies).</figcaption></figure>

event beds in depocentre sites of Basins S3 (e.g. 9.40–10.10 m depth in M0091B-1H-10/11 W, Fig. S3) and C2 (e.g. 2.5–3.2 and 4.5–5.4 m depth in M0083D-1H-4 W and -6 W, Fig. 2D). These laminations contain deformed sedimentary structures with trace fossils that alternate discordantly with non-deformed intervals within the same fine-grained fraction (e.g. 30–40 cm depth in M0083-1H-8 W, Fig. 2C). Fractures (3–10 cm long) and microfractures (up to 1 cm long) are observed in the coarse-grained fraction (e.g. 14–17 cm, 25–27 cm, 35–38 cm depth in M0090B-1H-1 W, Fig. 2A) and the fine-grained fraction (e.g. 7–12 cm depth in M0090B-1H-1 W, Fig. 2A) of Event Bed 2.

*Burrowing and mottling.* Small burrow motting (~0.5–1 mm long) recorded by *Phycosiphon* and/or *Nereites* traces (Fig. 4) is present in structureless silty clay in the fine-grained fraction of event beds (e.g. 85–88 cm depth in M0083D-1H-8 W, Fig. 2C). These traces are cross-cut by deeply penetrating, straight and

ramifying burrow systems, including *Zoophycos*, *Gyrolithes*, *Pilichnus* and *Trichichnus* (e.g. 89–93 cm depth in M0083D-1H-8 W, Fig. 2C). Burrow infills may contain iron monosulphides that form as greigite or pyrite (Hovikoski *et al.*, 2025), resulting in high CT density and an increase in bulk density (e.g. 89–92 cm depth in M0083D-1H-8 W, Fig. 2C). Trace fossils such as *Rhizocorallium*, *Teichichnus* and *Artichnus* may occur in sandy ripples at the base of event beds (e.g. 46 cm depth in M0083D-1H-3 W, Fig. 2B).

## Sedimentary facies

Based on analyses of sedimentary structures (Figs 4–7; Tables S1–S2) in the four event beds (Figs S1–S4) across various basin physiographies, granulometric parameters for specific structures were plotted using binary cross-plots. The fines (%) versus mean grain-size cross-plot (Fig. 8) reveals an inverse relationship: a small increase in mean grain-size leads to a large decrease in fines. This approach allowed the
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Idealised facies sequence showing an overall fining-upward trend of five event-internal facies (F1–F5) bounded above and below by hemipelagic background facies (F0). Coarse-grained facies (F1–F3) pass upward into fine-grained facies (F4–F5); facies F4 (Type-B convolute) marks the coarse- to fine-grained transition. Pulses represent individual (often incomplete) parts of this sequence. Note that this complete sequence is conceptual and rarely observed in natural examples (see text in *Variability in the facies sequence section*).</figcaption></figure>

identification of a fining-upward sequence with a distinct grain-size threshold at 32 µm, distinguishing coarse-grained (>32 µm; coarse silt to fine sand) from fine-grained (<32 µm; clay to coarse silt) structures. These grain-size criteria underpin an idealised facies sequence that fines upward from coarse-grained (F1–F3) to fine-grained (F4–F6) event-internal facies, bounded above and below by hemipelagic background facies F0 (Figs 9–10). F0 was also characterised by granulometric-textural parameters, XRF-based chemical signatures and ichnological fabric (Fig. 11). A schematic representation of the full idealised F0–F6 facies sequence is illustrated in Fig. 9; this complete sequence is conceptual and rarely observed in natural examples (see *Variability in the facies sequence section*). Detailed descriptions of each facies (and sub-facies, where further distinction is needed) are described below and in Fig. 10.

### Coarse-grained event-internal facies (F1–F3)

The coarse-grained facies comprise three facies (F1–F3) and seven sub-facies (F1A–B, F2A–D,

F3), arranged in a 5–15 cm fining-upward sequence with scoured or sharp base contacts (Figs 9, 10). Parallel lamination (F1) includes (F1A) normally graded sand (2–5 cm thick) or (F1B) thick, parallel-laminated fine sand to coarse silt (0.5–1 cm laminae). This grades into cross-laminated (F2) cm–dm-scale sets with convergent foresets, laminae pinch-outs and downlapping contacts, expressed as (F2A) non-climbing medium–fine sand ripples, (F2B) Type-A climbing ripples (fine sand–coarse silt), (F2C) Type-B climbing ripples (coarse–fine silt) and (F2D) Type-S climbing ripples (coarse–fine silt). Type-A convolute lamination forming mm–cm-scale syn-folds (~1 cm high) commonly overlies Type-B/Type-S ripple crests. The sequence then fines upward into (F3) thin parallel interlamina-tions (0.1–0.5 mm laminae) of fine sand–coarse silt alternating with silt–clay.

### Fine-grained event-internal facies (F4–F6)

The fine-grained facies comprise three facies (F4–F6) and five sub-facies (F4, F5A–F5B and F6A–F6B) that form recurrent successions
<figure><img src="image_13.png" /><figcaption>Fig. 10. Sedimentary facies F0 to F6. For each facies, a brief description highlights key characteristic, with a representative example line-scan, sagittal CT and X-ray radiograph images alongside an interpretative line drawing and grain-size distribution. Also shown is a comparison with analogous facies in prior facies sequences (*S1: Bouma, 1962; *S2: Stow & Shanmugam, 1980; *S3: Hovikoski et al., 2025).</figcaption></figure>

<caption>**Fig. 10.** Sedimentary facies F0 to F6. For each facies, a brief description highlights key characteristic, with a representative example line-scan, sagittal CT and X-ray radiograph images alongside an interpretative line drawing and grain-size distribution. Also shown is a comparison with analogous facies in prior facies sequences (*S1: Bouma, 1962; *S2: Stow & Shanmugam, 1980; *S3: Hovikoski et al., 2025).</caption>
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Discrimination of hemipelagic facies (F0) from the uppermost event-internal facies (F6B) using multi-proxy data: (A) grain-size plots (d99 versus d50, mean versus mode, sorting versus skewness) with dashed lines outlining facies clusters; and (B) clr-XRF elemental plots (clr-Fe versus clr-Ca; clr-Si versus clr-Mn) showing facies trends indicated by solid black lines.</figcaption></figure>

marked by alternating coarsening- and fining-upward trends (Figs 9, 10). The group begins with a unique transitional facies (F4) characterised by a cm-dm-scale Type-B convolute lamination in silty clay to coarse silt, showing either coarsening-upward or upward-thickening laminae at the coarse-to-fine-grained interface. The overlying facies consists of mm-scale laminations (F5) that occur as (F5A) ripple cross-lamination or (F5B) very thin, parallel clayey-silt lamination. Particularly, below F5 sub-mm erosive surfaces, inversely graded silty clay-clayey silt transitions upward to thin, continuous, parallel fine-silt laminae (F5B). The succession then fines upward into (F6A) indistinct, wispy laminations in normally graded clayey silt-silty clay, and into ungraded, structureless silty clay (F6B). Locally, detached pseudonodules form ball-and-pillow structures that penetrate up to ~20 cm deep into the underlying silty-clay matrix (F4–F6). Some F5–F6 intervals may appear slumped, with most burrows deformed;

however, undeformed burrows that cross-cut the slump are also present.

### Hemipelagic facies (F0)

Event beds are typically bounded by bioturbated, structureless, ungraded silty clay or clayey silt (F0), consistent with hemipelagic background sedimentation over a stable seafloor (Figs 9, 10). Hemipelagic facies (F0) is typically homogeneous and closely resembles the uppermost event-internal facies (F6B), so distinguishing them requires granulometric, textural, XRF-based chemical and ichnological analysis. Compared to F6B, F0 shows broader grain-size dispersion (Fig. 11A), as indicated by d99 (110–175 µm), d50 (9–17 µm), mode (10–30 µm) and mean (10–18 µm). This reflects coarser/larger biogenic silica fraction (diatoms, sponge spicules, silicoflagellates, and radiolarians; Strasser *et al.*, 2023), whereas F6B is mainly radiolarian-rich (Fig. S8). F0 also exhibits higher asymmetry and poorer sorting,
with skewness (−0.15–0.1) and sorting (3–4). These parameters show a dispersion region 3–3.5 times wider in F0 than in F6B (Fig. 11A).

XRF-based chemical signatures also distinguish F0 from F6B (F11B). F0 typically shows higher clr-Fe (0.05–0.20) and clr-Mn (−0.10–0.50) than F6B (clr-Fe −0.20–0.10; clr-Mn −0.80–0.00). F6B exhibits higher clr-Ca (−0.15–0.15) and clr-Sr than F0 (clr-Ca −0.25–0.05). clr-Si is variable in both facies but is slightly higher in F6B (−0.20–0.22). In clr-Fe versus clr-Ca plot, F0 defines a negative linear trend (β ≈ −0.51), whereas F6B shows a positive trend (β ≈ +0.51). In clr-Si versus clr-Mn plot, both facies show negative co-variation, steeper for F0 (β ≈ −0.34) than for F6B (β ≈ −0.13).

The hemipelagite facies (F0) typically appears as a thin ‘capping’ (3–5 cm thick) layer overlying bioturbated F6A or F6B. Common trace fossils include slender helicoidal traces (*Gyrolithes*) and vertical to horizontal, thread-like and ramifying burrow systems (e.g. *Trichichnus*, *Pilichnus*). Their burrows can extend into underlying sediments, forming mm- to dm-scale intervals of bioturbation and often cross-cutting the F6B trace-fossil fabric or descending into F6A, reaching depths of ~20 cm. Hemipelagic preservation of F0 facies may be compromised if an erosive contact occurs above.

## *Variability in the facies sequence*

A complete facies sequence is rarely found in the dataset (Figs 9, 10). Two forms of event-internal facies variability were recognised: by dominant lamination type, and by incomplete facies sequences (Fig. 12).

1. Dominant lamination type and grain size reflect preferred facies association within a facies sequence. The parallel-laminated association (Fig. 12A) is characterised by near-parallel lamination, normally graded silt-clay, with fine-grained facies predominating, mostly F3, F5B and F6 and rare F2 and rare cross-laminations (F5A). The cross-laminated association (Fig. 12B) is dominated by ripple cross-lamination and convolute lamination and by coarser facies such as F2, F4, F5 and rare F6 (Figs 9, 10). Mixed parallel-cross-lamination associations comprise both facies’ types (F2–F6) but also include the coarsest facies F1, yielding the most complete sequences (Figs. 9, 10, 12C).

2. Most event beds are thin (5–10 cm thick), truncated or vertically incomplete (yet laterally

continuous), especially at basin-high sites C1 and C2 (M0090 and M0089), and preserve either the uppermost facies (F5 to F6, base-cut-out) or the lowermost facies (F1–F3, top-cut-out) of a facies sequence (see pulse schematics in Fig. 13). Extreme base-cut-out sequences consist of silty clay or clayey silt without any coarse silt lamination (e.g. 15–20 cm depth in M0090B-1H-2 W, Fig. 14A), whereas top cut-out sequences grade from sand-dominated structures (F1, F2A–B) to silt-dominated structures (F2C–D, F3) (e.g. 32–40 cm depth in M0090B-1H-2 W, Fig. 14A). In few cases, only the middle facies are either preserved (F2A–D) or removed (F1A–B and F5–F6, middle-cut-out), forming parallel- and/or cross-laminated facies associations (e.g. 27–32 cm depth in M0090B-1H-2 W, Fig. 14A).

## *Stacking event-internal facies*

Event beds comprise multiple stacked sequences ('pulses') of event-internal facies arranged in an overall fining-upward trend. Documenting the occurrence (Fig. 12) and stacking patterns of these pulses (Figs 13, 14) can reveal how deposition evolved during event-bed emplacement (see further discussion in the section Advanced process-based understanding of event beds in hadal trenches). See the idealised stacking pattern in Fig. 13 and a core example in Fig. 14.

## *Pulses*

A pulse is a single, typically incomplete (top-, middle- or base-cut-out) facies sequence composed of coarse-grained (F1–F3) and/or fine-grained (F4–F6) event-internal facies. Boundaries between pulses dominated by coarse-grained facies (mostly top cut-out, Fig. 13B; e.g. 5–14 cm depth in M0090B-1H-2 W, Fig. 14A) can be distinguished by abrupt grain size breaks with sharp/scoured contacts (e.g. a shift from 40 to 90 µm at 31 cm depth in M0090B-1H-2W, Figs 3A and 14A), or occasionally inverse/normal grading alternating with thin parallel laminations (e.g. 30–50 cm depth in M0091D-1H-14 W, Fig. S3). Within each pulse, grain size trends vary with some XRF-based elements (e.g. with clr-Ca in Event Bed 3, Fig. 3A, or with clr-Mn in Event Bed 1, Fig. S1). Additionally, each coarse-grained facies pulse usually exhibits either parallel-laminated (e.g. dominant in Event Beds 1 and 4; Fig. 12A) or cross-laminated facies association (e.g. dominant in Event Beds 2 and 3; Fig. 12B), as well as a consistent palaeo-flow direction evidenced by linear sole marks, down-
A Parallel-laminated facies association

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

B Cross-laminated facies association

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

C Mixed parallel- and cross-laminated facies association

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

**Fig. 12.** Examples of event beds showing the variability in facies sequence: (A) parallel-laminated facies association; (B) cross-laminated facies association; and (C) combined parallel- and cross-laminated facies association. Each panel includes their respective dataset, facies interpretation with colour code (Fig. 10 for more details), lithology sketches and pulses colours partitioning as coarse-grained (F1–F3) and fine-grained facies (F4–F6).
<figure><img src="image_21.png" /><figcaption>B Multi-pulsed pattern</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>C Amalgamated pattern</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Fig. 13.** Idealised stacking patterns of event beds: (A) single-pulsed; (B) multi-pulsed; (C) amalgamated. Each panel shows the interpretation of facies, pulses and pulse groups; pulse groups are delimited by breaks in the fining-upward trend (blue italics).</figcaption></figure>

lapping laminae on ripple foresets or overturned syn-folds of convolute lamination (e.g. rose diagrams and axial slice in Fig. 3A).

Boundaries between pulses dominated by fine-grained facies (mostly base cut-out, Fig. 13B) are less distinct, as they exhibit gradational transitions of F5B (Figs 9, 10) via upward-increasing very thin parallel laminations alternating with normal or inverse grading (e.g. 85–88 cm depth in M0083D-1H-10 W, Fig. 14C). However, unlike in the coarse-grained pulses, the (secondary) slumped intervals in fine-grained pulses often exhibit discordant contacts between deformed and non-deformed intervals.

### Pulse stacking patterns

Successive pulses stack to build the architecture of event beds and occur in three organisational patterns (Figs 13, 14) that may provide relative depositional timing and reflect physiographic controls on the stacking and distribution of coarse- and fine-grained facies.

1. **'Single-pulsed' (Fig. 13A)**—a single fining-upward facies sequence, with neither internal erosion nor facies repetition. When bounded above and below by hemipelagic sediments (F0) and/or by bioturbated sediments with equilibrium-trace

fossils, the interval constitutes an event bed formed by a single pulse (e.g. 78–86 cm depth in M0089B-1H-2 W, Fig. 14B).

2. **'Multi-pulsed' (Fig. 13B)**—several pulses of coarse-grained facies (F1–F2) are followed by pulses that progressively increase in fine-grained facies (F4–F6), forming a continuous interval of a coarse-to-fine-grained facies succession with an overall fining upward trend. Here, lower pulses tend to be top-cut-out and upper pulses base-cut-out (e.g. 14–39 cm depth in M0090B-1H-2 W, Fig. 14A).

3. **'Amalgamated' (Fig. 13C)**—two or more multi-pulsed (or single-pulsed) successions form a composite stack within a single event bed, with boundaries between successions defined by abrupt breaks in the fining-upward trend, marked by erosive contacts (e.g. sharp contact at 7 cm depth in M0089B-1H-3 W, Fig. 14B) or deformed contacts (e.g. flame structure at 14 cm depth in M0090B-1H-2 W, Fig. 14A), or gradational transitions into F4 (e.g. 8–26 cm depth in M0083D-1H-15 W, Fig. 14C) or F5B (85–95 cm depth in M0083D-1H-10 W, Fig. 14C). Here, lower intervals contain pulses with higher proportions of coarse-grained than fine-grained facies; and upper intervals, vice versa (Fig. 13C).
A Event Bed 3 in basin high C1 (site M0090)

<figure><img src="image_24.png" /></figure>

B Event Bed 3 in basin high C1 (site M0089)

<figure><img src="image_25.png" /></figure>

C Event Bed 3 in basin high C2 (site M0083)

<figure><img src="image_26.png" /></figure>

**Fig. 14.** Facies-based framework for interpreting Event Bed 3 and adjacent deposits across three core sites: (A) C1 basin high (site M0090); (B) C2 basin high (site M0089); (C) upper and lower part of >5 m-tick Event Bed 3 in C2 basin depocentre (site M0083). Each panel shows their respective dataset and the interpretation of facies, pulses and pulse groups, with stacking patterns indicated by breaks in the fining-upward trend (blue italics). Event Bed 3 is highlighted with a light-blue line.
## Pulse groups

Successive pulses of event-internal facies often exhibit similar grain-size distributions and textures, resulting in similar stacking patterns (Fig. 13), with occasionally comparable XRF-based chemical signatures. These similarities are documented and used to define 'pulse groups', which could allow pulses to be distinguished from the same flow versus different flows. Group boundaries are best identified by a break in the fining-upward trend in a multi-pulsed (or single-pulsed) succession. Key markers include abrupt contacts (erosive or deformed), gradational transitions (F4, F5B) and facies F6B (sequence end-member). Event Bed 3 illustrates how pulse stacking builds the deposit and how physiography modulates coarse- versus fine-grained facies content (Fig. 14). At the C1 basin high, three pulse groups (A, B, C) bounded by abrupt breaks are dominated by coarse-grained facies, still showing an upward increase in fine-grained facies: ~10% in A, ~40% in B, 100% in C (Fig. 14A). At the C2 basin high, three pulse groups show a similar trend: ~40% in A, ~60% in B, 100% in C (Fig. 14B). At the C2 depocentre, multiple pulse groups stack into a 4.5-m-thick event bed; pulses are thick and dominated by fine-grained facies: ~50% in A, >90% in B–D and 100% in subsequent groups (Figs 14C, S3).

## DISCUSSION

High-resolution analyses of Japan Trench Event Beds 1–4 (see Figs S1–S4) clarify the relationships between facies sequences, flow pulses and basin physiography, improving recognition of earthquake-triggered sedimentary events and flow reconstruction in the hadal trench environment.

## Linking between facies and depositional conditions

### Grain size-based distinction of sedimentary structures for interpreting hydrodynamic flow behaviour

Sedimentary structures and key grain-size parameters (e.g. fines versus mean, Fig. 8) convey how the different event-internal facies and their associations relate to flow dynamics (Figs 4–7; Tables S1–S2). The dataset reveals two grain-size thresholds at 32 µm and 63 µm (Fig. 8). The 32 µm threshold differentiates the fining-upward, coarse-grained facies (F1–F2; >32

µm) from the fine-grained facies (F4–F6), and the intervening transitional Type-B convolute lamination (F4). This facies boundary also suggests a 32 µm threshold marking the transition from a non-cohesive to cohesive flow behaviour (Fig. 15). Similar thresholds are reported from facies studies in other environments (e.g. Pickering et al., 1986; Shanmugam, 2000).

In the hadal trench, coarse-grained facies (F1–F3; Figs 9, 10, 15) consist of hydraulically sorted, non-cohesive grains that settle rapidly as traction or saltation bedload or short-travel suspension (Allen, 1984). An additional 63 µm threshold separates sand-dominated (F1, F2A–B; Figs 8, 15) from coarse silt-dominated facies (F2C–D, F3; Figs 8, 15). Where fines are <20%, sand-dominated facies show high erosion potential expressed in abundant sole marks at their base (Fig. 7). By contrast, coarse silt-dominated facies exhibit upward-increasing fines (~20–50%), more gradational and deformed basal contacts (Fig. 7), and thus reduced erosion potential because fines may act as ‘binding’ agents increasing cohesion (Baas et al., 2016; Baker et al., 2017). The fine-grained facies (F5–F6) reflect suspended, cohesive, long-range transport and suspended-load fallout of flocculated aggregates (Schieber et al., 2007). Locally, interlaminations of coarse- and fine-grained laminae (e.g. heterolithic facies F2C–D and F3) point to fluctuating flow conditions due to variable sediment supply (Yawar & Schieber, 2017).

Hydrodynamic conditions are inferred from specific sedimentary structures (i.e. parallel-, cross- and convolute lamination, Fig. 15). Parallel-laminated facies F1B (<20% fines) and F3 (~45% fines) (Figs 9, 10) are characteristic deposits of low-density turbidity currents (e.g. Shanmugam, 2000); F1B is analogous to Bouma Tb division and F3 to Bouma Td and Stow and Piper T3 divisions (see facies comparison in Fig. 10). Both F1B and F3 may form in the near-bed layer of hydraulically smooth turbulent flow, but under different regimes and sediment concentrations (Fig. 15). Experiments also associate parallel laminations with low-amplitude cross-bed forms (e.g. Bridge & Best, 1988). F1B develops under high shear and high sand-coarse-silt concentrations in the upper-flow regime (often supercritical), where sheet-like bedload transport and shear-thinning enhance grains’ mobility in the near-bed part of the flow, producing mm-scale thick, normally graded, and well- to moderately sorted laminae (Allen, 1984; Postma & Cartigny, 2014). As flow wanes,
<figure><img src="image_27.png" /><figcaption>**Fig. 15.** Hydrodynamic controls on facies development. Facies are shown as 3D cubes: front face shows a sagittal CT slice; lateral face shows sedimentary structures with grain-size estimates. Vertical axes show mean grain size, and fines content (% >32 µm). A 32-µm threshold (dashed) separates coarse-grained facies (F1–F3) and fine-grained facies (F4–F6); a 63 µm threshold (dashed) separates sand- and silt-dominated facies. A 32 µm transition zone (grey) marks the occurrence of Type-A and Type-B convolute lamination and thin parallel lamination. Increasing fines correspond to a shift from bedload to suspended-load transport and higher cohesion. Horizontal axes indicate relative flow velocity and preferred facies associations (parallel- versus cross-laminated).</figcaption></figure>

suspended-load concentration increases, forming a fines-rich near-bed layer that damps turbulence and reduces near-bed grains' mobility. A bedward-directed outer-flow perturbation (e.g. surges, bores in turbidite currents) then increases flow resistance (or frictional stress), triggering cohesive strengthening and deposition of fine silt–clay poorly sorted laminae. Repetition yields mm-scale interlaminations of sand–coarse silt with silty clay that define F3 (Walker, 1965; Allen, 1984; Al-Mufti & Arnott, 2024). Consistent with the F3 interpretation, mm-scale lamina contacts show shallow load casts where dense sand–coarse silt settles onto ductile fine silt–clay, marking loading transitions across F3 interlaminations (e.g. 15–20 cm in M0090-1H-2 W, Fig. 3A) and reflecting transient perturbations in the near-bed shear stress in the lower-flow regime (subcritical).

Ripple cross-laminations (F2A–D, Fig. 15) documented here vary in grain size (e.g. mean, fines content) and geometry, reflecting bedform development under non-uniform or unsteady flow (Allen, 1970). Accordingly, the angle of climb increases where the sediment deposition

rate exceeds the bedload transport rate, either along the flow direction (shallow to deep, Fig. 15) or through time as velocity wanes (low to high, Fig. 15). F2A–D facies are analogous to Stow and Piper T0, T1 and T2 divisions (see facies comparison in Fig. 10). Grain size is a key control on bedform development: grains >63 µm favour non-climbing and Type-A ripples (<20% fines), whereas <63 µm favour Type-B or Type-S ripples (20–50% fines). Type-A ripples (>63 µm, F2B) form under limited sediment availability, where high bedload transport rates (traction–saltation) cause trough erosion and crest deposition, inhibiting significant ripple growth and incipient convolute lamination—in contrast to the T0-to-T1 transition of Stow and Piper divisions—(Allen, 1970, 1973; Hunter, 1977). By contrast, Type-B and Type-S ripples (F2B–C) develop under bedload transport and rapid suspended-load fallout, where sediment deposition rates match (Type-B) or exceed (Type-S) the angle of climb, producing downstream migration and preventing net erosion (Jopling & Walker, 1968; Allen, 1973; Hunter, 1977) and favouring stoss-side deposition that can nucleate
Type-A convolute lamination on ripple crests (Kuenen, 1953; Gladstone *et al.*, 2018).

Particularly, fines content can produce recurrent coarse-silt to clay admixtures that generate heterolithic cross-laminations (Fig. 15) during the rapid suspended-load fallout as the flow wanes (Baas *et al.*, 2016). Fines increase upward from flaser (>10% fines; F2A–F2B) to wavy (10–40% fines; F2B–F2C) and ultimately to lenticular laminations (40–50% fines; F2C–F2D). Fine silt that remains suspended can flocculate with clay, settling into ripple troughs and forming coarse-silt–silty clay interlamina- tions during waning flow (Yawar & Schieber, 2017). Larger troughs (e.g. lenticular lamination) more effectively trap fines and iso- late ripples. Critical velocities for clay floccula- tion (~25 cm/s in flume experiments; Schieber *et al.*, 2007) distinguish clay-floccule-rich rip- ples (~50% fines) from coarse silt ripples. Facies F2D, characterised by lenticular lamina- tion (Figs 9, 10), marks this transitional zone within the fine-to-coarse silt range (~20–40 µm). It exhibits the highest fines content (40–50%), occurring near the ~32 µm grain-size threshold that separates non-cohesive and cohesive sedi- ment behaviour (Fig. 15). Heterolithic patterns are commonly attributed to unsteady flows (e.g. waxing–waning); they have also been linked to downslope flow transformation (e.g. Baker & Baas, 2020; Łapcik, 2023). In the central Japan Trench, no clear evidence of such transforma- tion is observed; if present, heterolithic inter- vals may appear limited to turbulent–transitional regimes without a hybrid- event-bed phase.

Two types of convolute laminations were also recognised (Figs 4, 10): Type-A (F2C–D) and Type-B (F4), which are distinguished by their location, grain size and vertical trend. Occurring on either side of the ~32 µm threshold (Fig. 8), their coexistence with other sedimentary struc- tures illustrates the driving mechanisms of their formation (Fig. 15). Previous studies have pro- posed various origins for convolute laminations (e.g. Owen & Moretti, 2011; Gladstone *et al.*, 2018; Molenaar *et al.*, 2022); but in this study, their granulometric, textural and struc- tural characteristics are linked to specific insta- bility processes. For example, Stow & Piper (1984) identified divisions T1 and T5 con- taining convolute laminations (see facies com- parison in Fig. 10), with T1 reflecting incipient ripple lamination or loading processes, while the processes behind the wispy T5 remain less

clearly defined. Type-A convolute laminations likely form when perturbations nucleate on lin- guoid ripples (F2C–D) with 25–45% fines, driven by buoyancy forces related to Rayleigh– Taylor instability (McClelland *et al.*, 2011; Glad- stone *et al.*, 2018), contrasting with the transi- tional Type-B convoluted facies (F4), which occur independently of ripple features and have higher fines content (55–65%). Because the Rayleigh–Taylor instability only forms at wave- lengths smaller than or comparable to the under- lying ripple wavelength (~10 cm threshold, Gladstone *et al.*, 2018), it cannot account for the formation of cm–dm scale Type-B convolute lamination (F4). Instead, Type-B convolute lami- nations record Kelvin–Helmholtz instability, when a new flow overriding a fine-rich density- stratified layer creates strong velocity shear at the interface, driving wave growth and roll-up/overturning to produce convolute lami- nations up to 20 cm high (Kuenen, 1953; Dra- zin, 2002; Gladstone *et al.*, 2018).

*Linking facies sequence to event emplacement*
Empirical and experimental studies indicate that turbulence intensity is the primary control on flow velocity and grain interactions that ulti- mately determine sedimentary structures and facies transitions (Middleton, 1993; Baas, 2004; Halsey *et al.*, 2017). At first order, deposition from turbidity currents occurs when turbulence can no longer support the near-bed bedload or suspension load, either due to waning flow or high sediment concentration that damps turbu- lence (Halsey *et al.*, 2017). In turbidity currents, F1A records thick, structureless sand that occurs when near-bed sediment suspension becomes so loaded that fine-scale eddies are suppressed, leading to ‘en masse’ settling (Baas, 2004). As sediment concentration drops down and turbu- lence recovers, grain-by-grain settling resumes, allowing laminated structures to form. F1B develops when an initially smooth turbulent flow and high bed shear generate parallel lami- nation in the upper-flow regime by sustaining sheet-like bedload transport and low-amplitude bed waves, thus preventing the formation of cross-lamination. Once the flow becomes sub- critical, lower-regime turbulence produces cm– dm-scale non-climbing ripples (F2A) and/or Type-A climbing ripples (F2B). With fluctuating turbulence, the rapid suspended-load fallout forms Type-B (F2C) and Type-S (F2D) climbing ripples at moderate and high rates, respectively. Variations in sediment supply produce
heterolithic lamination, with increased fallout yielding wavy-lenticular lamination (F2C–D). Under low-flow, steadier-flow regime, parallel interlaminations (F3) form as coarse silt and silt-clay floccules move as traction bedload, overprinting the culmination of the coarse-grained facies (F1–F3).

Two facies associations: parallel-laminated (Fig. 12A) and cross-laminated (Fig. 12B) reflect specific sediment transport conditions (Fig. 15). The parallel-laminated association (F3, F5B, F6; Fig. 12A) develops when turbidity currents behave as near-laminar, unidirectional flows, either because high fines concentrations dampen turbulence (Halsey et al., 2017) or a low-flow regime produces weak velocity shear and limits turbulence generation (Middleton, 1993). Under these conditions, episodic shifts between sheet-like near-bed bedload and suspended-load fall-out build F3 interlaminations; consequently, thick F1B laminae are absent (see high-flow-regime facies in Fig. 15). In contrast, the cross-laminated facies association (F2, F4, F5, F6, Fig. 12B) reflects turbidity currents that behave as non-uniform or unsteady flow (Allen, 1970) and are supplied with sufficient sediment to sustain ripple downstream migration and vertical growth. Transport is dominated by bedload (traction and saltation), where grains are intermittently lifted off the bed and rapidly returned by gravity and grain-grain collisions (Middleton, 1993).

In these facies associations, suspended-load concentration increases as flow wanes in all basin physiographies. Particularly in depocentres, the suspended load forms turbid water masses that develop thick sediment-suspension layers ('clouds'), pond immediately after the initial passage of a flow and persist for up to a few years following major events (e.g. earthquakes, slope failures). These thick suspension clouds can interact with outer-flow perturbations that are damped by weak velocity shear, yet can still generate sub-mm cross- and parallel-laminations (F5), alternating with pure-settling facies (F6). The recurring F5–F6 facies are diagnostic criteria for constraining suspension-load settling and its relative timing. For example, following the 2004 off-Kii Peninsula earthquakes, an ~2.5 m-thick near-bottom turbid mass was measured in Nankai Trough slope basins (via a submerged heat-flow probe), coincident with acoustically transparent layers >2 m thick on sub-bottom profiler images in

semi-enclosed basins (Ashi et al., 2014). In particular, F5 sub-mm traction structures likely result from a combination of shear-driven currents produced by (i) long-lasting nepheloid layers, (ii) seiche-like oscillations within semi-enclosed mini-basins, and/or (ii) newly formed turbidity currents disturbing the still-settling sediment cloud at the sediment-water interface.

Post-2011 Tohoku-Oki trench-axis seafloor monitoring with stationary cameras at ~7000 mbls captured a gradual increase in seafloor light over the 4 months following the main-shock, consistent with persistent nepheloid layers likely sustained by ongoing aftershocks (Oguri et al., 2013). Seiche-like oscillations are likewise implicated in the formation of over-thickened fine-grained fractions of event beds, based on magnetic-fabric analyses (Yakupoğlu et al., 2022; Ikehara et al., 2023), underscoring the role of semi-enclosed basins in ponding turbid water masses. Furthermore, the recurrence of subtle mm-scale sedimentary structures in the fine-grained facies suggests episodic sediment remobilisation by low-energy turbidity currents periodically entering semi-enclosed trench basins (e.g. depocentre sites in Basins S3 and C2, Figs 2D, 3D), disturbing the still-settling suspension cloud at the unstable sediment-water interface. These findings shed new light on the evolution of seafloor conditions during and after the event-bed emplacement, enhancing interpretations of post-earthquake processes (e.g. aftershocks) in the hadal trench.

The hemipelagic facies (F0) is the baseline for understanding background sedimentation and seafloor stability in the hadal trench environments. Crucially, F0 sediments bound event beds above and below, marking non-synchronous deposition between successive event beds. Although F0 and uppermost event-internal facies F6B can appear similar (both structureless and bioturbated), they differ systematically in three diagnostic criteria:

1. **Granulometric-textural distribution** (Fig. 11A): F0 shows a broader dispersion than F6B, with higher d99, greater asymmetry, poorer sorting (Fig. 11A), and coarser biogenic silica grains (e.g. diatoms, sponge spicules). This is consistent with dilute background fallout under minimal hydrodynamic selection. By contrast, F6B is more hydrodynamically sorted in the suspended load within waning flows, exhibiting a
narrower dispersion region (3–3.5 times smaller than F0).

2 Geochemical signatures (Fig. 11B): F0 shows higher clr-Fe and clr-Mn, indicating redox sensitivity under stable seafloor conditions, and clr-Si reflecting primary biogenic silica, particularly below the CCD. By contrast, F6B exhibits higher clr-Si, clr-Ca and clr-Sr, consistent with allochthonous input, with clr-Si possibly indicating clay-rich aluminosilicate input (Luo et al., 2025), while below the CCD, the Ca–Sr enrichment chiefly records dissolution and import of biogenic carbonate from upslope or outer shelf remobilisations rather than primary productivity (Schwestermann et al., 2020).

3 Ichnology: F0 is dominated by deeply penetrating, straight, and ramifying equilibrium-trace fossils (e.g. *Gyrolithes*, *Trichichnus* and *Pilichnus*) indicating sustained colonisation by microbe-farming and chemosymbiotic invertebrates (Hovikoski et al., 2025). By contrast, F5 and F6 are characterised by opportunistic-trace fossils (e.g. *Phycosiphon*, *Nereites*). Distinguishing F0 from F6B enhances stratigraphic correlation, improves age models and strengthens palaeoenvironmental reconstructions.

Advanced process-based understanding of
event beds in hadal trenches

*Physiographic influence: Basin high versus
depocentre sites*

The proposed facies sequence (F1–F6; Figs 9,
10) differs from established frameworks in prior
literature and other settings (see facies compari-
son in Fig. 10). The classic Bouma (1962)
sequence is not directly applicable to the Japan
Trench’s predominantly fine-grained event
beds, dominated by low-density flows (Ikehara
et al., 2020). Although intervals of alternating
clayey silt–silty clay (F5–F6) resemble Bouma
D–E divisions, the facies sequence provides finer
discrimination of mm–cm-scale sedimentary
structures, particularly in thick event beds at
basin depocentres. The facies sequence of Stow
& Shanmugam (1980) or Stow & Piper (1984) is
yet comparable to the fine-grained facies (Stow
and Piper T3–T8 divisions) but is less

descriptive of the coarse-grained facies, with the
coarsest facies F1 absent (Bouma's A and B divi-
sions). So, while these differences can reflect
variations in sediment source, supply rate and
climatic or oceanic conditions (Mutti & Nor-
mark, 1987), they may also reflect the unique
physiography of hadal trench basins of steep
slope gradients and semi-confined basin geome-
try at 7500–8000 mbls.

The facies framework has been adapted to
both unconfined (basin high) and semi-confined
(basin depocentre) characteristics of hadal
trench physiography. Here, coarse-grained facies
(F1–F3) occur in both basins with broadly com-
parable thicknesses, whereas fine-grained facies
(F4–F6) are systematically thicker and more per-
sistent in depocentres. This pattern implies ini-
tial deposition of coarse facies by waning
turbidity currents, followed by prolonged sus-
pension settling of large volumes of remobilised
fine-grained sediments that are preferentially
ponded in depocentres. The interpretation is
consistent with observations of acoustically
transparent, laterally pinching-out bodies in
sub-bottom profiler profiles (Kioka et al., 2019;
Fig. 1E) and with maps of thick, largely homoge-
neous event beds distinguished by physical and
isotope-geochemical properties (McHugh
et al., 2020; Strasser et al., 2024). Both criteria
have been inferred as unique sedimentary
expressions of earthquake-triggered event bed
characteristics in the Japan Trench.

An important result of detailed facies analysis
is documenting the overthickened nature of fine-
grained facies in the basin depocentre (e.g.
Event Beds 2–3 in Basin S3 and C2; Figs 2D,
3D), which exhibit structural complexities that
remain unaddressed in existing facies literature.
Here-presented detailed observation provides
valuable insights into the emplacement and evo-
lution of the still-settling sediment and its
impact on hydrodynamic conditions in the
trench basin. Particularly, Type-B convolute
lamination (F4) is interpreted as a transition
facies formed when a new flow (e.g. turbidity-
current pulse or seiche-modulated shear) arrives
at a depocentre and encounters a still-settling
suspension cloud left by prior activity. This

**Fig. 16.** Physiographic controls and depositional timing and duration of event beds. (A) Basin-high event bed deposited by synchronous flows. (B) Basin-high event bed deposited by quasi-synchronous flows. (C) Multiple event beds deposited by non-synchronous flows. (D) Depocentre event bed deposited by quasi-synchronous flows.
A Basin-high sites

Synchronous flows

<figure><img src="image_28.png" /></figure>

D Basin depocenter site

Synchronous and quasi-synchronous flows

<figure><img src="image_29.png" /></figure>

B Basin-high sites

Quasi-synchronous flows

<figure><img src="image_30.png" /></figure>

C Basin-high and depocenter sites

Non-synchronous flows

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>
encounter creates strong velocity shear across a
density-stratified interface, generating Kelvin-
Helmholtz instability (often amplified by
bedward-directed perturbations during flow
waning-waxing). Even with damped near-bed
turbulence in a clay-rich suspended load, shear
at the interface can generate instability waves
whose roll-up and breakdown produce
soft-sediment deformation (Fig. 14C). The grada-
tional F4 transition at depocentres laterally cor-
relates to a sharp contact at basin-high sites (e.g.
see A–B group boundary in Fig. 14A, B), consis-
tent with basin highs being above the direct
influence of the ponded, suspension cloud.

Recognising how facies record depositional
variations across different physiographic settings
is therefore valuable for advancing a process-
based understanding of the dynamics, duration
and ultimate facies expression of event emplace-
ment in hadal trench environments. This
approach has implications for both interpreting
complex sequences of event-internal deposition
and subsequent sediment remobilisation linked
to the megathrust earthquake sequences that
exhibit long aftershock sequences, as well as
building on the concept of event stratigraphy
(Seilacher, 1982; Einsele, 2000) to enable the
documentation of relative depositional timing
and duration within a single event bed (see
more discussion in Stacking patterns for inter-
preting event stratigraphy section).

As this facies-based framework is developed
to encompass both unconfined and semi-
confined depositional environments, the frame-
work may be applicable to interpreting event
beds in other hadal trench settings. However,
applicability may be physiography-dependent:
the Japan Trench's rough seafloor reflects
horst-and-graben structures formed by flexural
bending of the Pacific plate. These faults create
semi-enclosed trench-fill and graben-fill basins
with variable along-strike connectivity, resulting
in steep slope-to-basin gradients that may create
unique hydrodynamic conditions—including
increased turbulence, flow expansion and dilu-
tion and enhanced seafloor erosion (Mutti &
Normark, 1987; Middleton, 1993)—that may be
less common elsewhere. For example, at sites
M0089 (basin high) and M0083 (depocentre)
located only ~1 km apart in the same trench
basin, fine-grained facies of a single event bed
differ by up to ~6 m in thickness. This work
therefore highlights how highly compartmenta-
lised physiography concentrates sediment

suspension clouds into depocentres, driving lat-
eral facies variability and deposit thickness.

Stacking patterns for interpreting event
stratigraphy

In the literature, a suite of terms is commonly
used to describe the complex grading patterns in
event beds (e.g. Van Daele et al., 2017). For
example, a 'multi-pulsed turbidite' is defined as
a deposit formed by multiple successions of
inverse-to-normal grading, which are interpreted
in the classical concept of event stratigraphy
(Seilacher, 1982; Einsele, 2000) as recording
deposition under episodes of waxing and wan-
ing in a single isochronous flow (Kneller &
McCaffrey, 2003; Ho et al., 2018). However,
observations from this study adapt the concept
of event stratigraphy to interpret earthquake
sequences in which a series of multiple pulses
from several spatially distributed and temporally
offset flows stack together to form a single event
bed. Most event beds in the central-southern
Japan Trench comprise multiple pulses, and the
critical distinction between depositional scenar-
ios lies in the organisational stacking pattern of
coarse-to-fine-grained facies pulses (Figs 13, 14).
Here, the key distinguisher is the delay time (or
pauses) between successive pulses inferred from
facies analysis (i.e. whether pulses form a con-
tinuous multi-pulsed succession or are separated
by breaks in the fining-upward trend into an
amalgamated succession).

Using this facies-based framework, relative
pause duration can be constrained by two key
criteria (Fig. 16):

1 Occurrence of the uppermost fine-grained facies (F6) between two successive pulses (e.g. 7–15 cm depth in M0089B-1H-2 W, Fig. 14B) indicates an interval of continuous and quiescent settling from suspension cloud on an unstable seafloor, implying time. Relative pause duration can be constrained by bioturbation: unbioturbated F6 indicates a ‘short’ pause and bioturbated F6 indicates a ‘long’ pause (Fig. 16B, D), both within quasi-synchronous flow arrivals, whereas the absence of F6 (coarse-grained facies only) indicates synchronous flow arrivals (Fig. 16A). For example, an amalgamated event bed (Figs 14A, 16A) with multiple multi-pulsed successions dominated by coarse-grained facies (F1–F3) implies insufficient time for F6 to settle, as successive synchronous, bedload-bearing flows arrive rapidly (e.g.
15–40 cm depth in M0090B-1H-2 W, Fig. 14A). By contrast, F6 occurring between pulses dominated by coarse-grained facies (Figs 14B, 16B) records quiescent pauses that allow suspended load to settle, as quasi-synchronous flows arrive after a delay (e.g. 51–57 cm in M0083D-1H-2 W; Fig. 14B).

2 Occurrence of undeformed *Phycosiphon* and *Nereites* that cross-cut *Artichnus*, *Rhizocorallium*, *Teichichnus* and *Palaeophycus*-like burrows within F5 indicates a period of colonisation of opportunistic-trace fossils immediately after deposition, implying a ‘short’ pause (Fig. 16D), whereas equilibrium-trace fossils imply a sustained pause on a stable seafloor and re-establishment of background conditions, regardless of hemipelagic occurrence (Fig. 16C).

For context, in the Japan Trench, synchronous deposition can occur when a single episode of ground shaking triggers flows in multiple upslope catchments that, despite different travel times, reach a basin depocentre within minutes to hours (Ho et al., 2018; Howarth et al., 2021). By contrast, quasi-synchronous deposition can develop during a prolonged aftershock sequence that triggers smaller, localised flows over months to decades (Omori, 1894; Utsu et al., 1995). In the latter scenario, which is common for megathrust earthquakes, there is often enough time between successive flows for suspended load to settle and facies F5–F6 to form, but not enough time for full seafloor stabilisation and development of facies F0. This helps to explain why quasi-synchronous event beds associated with megathrust earthquake sequences can become very thick, particularly in the upper fine-grained fraction in the Japan Trench (Ikehara et al., 2016; Strasser et al., 2023).

Documenting pulses and pulse groups indicates that most of event beds are amalgamated and comprise flows emplaced at different times and arriving from different directions, as reflected by variable composition, facies, and palaeo-flow indicators. Within these groups, along-strike variability in thickness and in facies of individual pulses provides a practical tool to explore sediment sources, flow direction and sediment-routing directions. Accordingly, systematic mapping of pulse occurrence, thickness gradients and palaeo-flow indicators can yield testable hypotheses for sediment provenance and pathway evolution across the Japan Trench.

As practical examples, Event Bed 2 and 3 illustrate the use of this approach to develop hypotheses on flow directions and sediment source areas. Pulses and pulse groups of Event Bed 2 (CE 1456 Kyōtoku Event, Figs 2, S2) suggest a north-to-south source hypothesis. Coarse-grained pulses range in thickness from ~10 cm with 1–3 pulses in depocentre sites of Basins S3 (south) and C2 (north), to ~30 cm thick with 6 pulses in Basin C1 (central). The fine-grained top contains more pulse groups at depocentre sites of Basin C2 (up to 6 m thick) than in Basin S3 (up to 50 cm thick), which is located ~100 km farther south. For Event Bed 3 (CE 869 – Jogan Event, Figs 3, 14C), coarse-grained pulses range from ~5 to 15 cm in Basin C2 (north) to 35 cm in Basins C1 (central) and S3 (south). It also contains more pulses in Basins C1 (5 pulses) and S3 (6 pulses) than in Basin C2 (2 pulses). Pulse groups dominated by fine-grained facies are similar in number and thickness at both depocentre sites, Basins C2 (~4 m thick) and S3 (~4.5 m thick). These observations suggest that the main sediment source area for Event Bed 2 in the southern-central Japan Trench may lie farther north than that of Event Bed 3, which appears to have been farther south. This inference is consistent with recent interpretation of source area of the respective event beds as derived from XRF cluster-based chemo-stratigraphy analyses (Huang et al., 2025). Yet, a conclusive palaeoseismological interpretation of these event beds remains speculative at this stage and requires further investigation in future studies. Nevertheless, the new ability to robustly distinguish relative spatio-temporal trends within event beds in hadal trenches improves conceptual understanding of hydrodynamic conditions, depositional timing and duration and sediment provenance (Fig. 17).

## SUMMARY

This study highlights the valuable role of facies analysis for interpreting event beds, building on the concept of event stratigraphy to document hydrodynamic conditions and the relative depositional timing and duration within a single event bed in a hadal trench environment. By examining the four most recent event beds and their intervening deposits in the southern-central Japan Trench sediment cores, a tailored
<figure><img src="image_33.png" /><figcaption>**Fig. 17.** Conceptual model of earthquake-triggered event-bed deposition. Text annotations in black regular indicate hydrodynamic conditions inferred from facies analysis; in blue italics denote depositional timing and duration; in bold highlight key depositional stages.</figcaption></figure>

facies-based framework that leverages 2D and 3D multi-proxy observations was developed (e.g. grain size analysis, high-resolution images, XRF-based chemical composition) to detail the full spectrum of sedimentary structures in the different facies. An idealised facies sequence that fines upward from coarse-grained (F1–F3) to fine-grained (F4–F6) event-internal facies was proposed, including the unique transitional F4, and bounded by hemipelagic background sediments (F0). Event beds are hierarchically

organised into pulses and pulse groups which can form single-pulsed, multi-pulsed and amalgamated stacking patterns.

This study provides evidence that specific sedimentary structures and facies sequences can be directly linked to hydrodynamic flow conditions. Key thresholds (32 µm and 63 µm) in grain size delineate cohesive versus non-cohesive depositional behaviour, while parallel- and cross-laminations provide insights into flow regime, balance between erosion and deposition.
The distinction between Type-A and Type-B
convolute laminations (F2D and F4) clarifies the
respective roles of Rayleigh-Taylor- and Kelvin-
Helmholtz-type instabilities, respectively. Simul-
taneously, each facies is shown to reflect the
development of deposition under flow condi-
tions, whether parallel-lamination versus cross-
lamination in coarse-grained facies (F1–F3), or
recurrent subtle structures in fine-grained facies
that record settling of the suspension cloud in
basin depocentres (F5–F6).

This refined perspective advances understand-
ing of event bed deposition in hadal trenches by
integrating hydrodynamic conditions, relative
depositional timing and duration across basin
highs and depocentres, while highlighting the
key role of semi-enclosed basins in ponding tur-
bid water masses and producing thick fine-
grained tops. In particular, it clarifies how
unique transitional (F4) and the rest of the fine-
grained facies (F5–F6) develop in response to
the interaction with the still-settling suspension
cloud, thereby damping new flow arrivals
within depocentres.

Within an event bed, event-internal facies
stack hierarchically as pulses and pulse groups
to produce three fining-upward patterns: single-
pulsed, multi-pulsed and amalgamated. An
amalgamated pattern comprises two or more
multi-pulsed (or single-pulse) successions, sepa-
rated by breaks in the fining-upward trend.
Breaks marked by opportunistic-trace fossils or
by the uppermost facies F6 indicate a time delay
and thus quasi-synchronous arrivals; their
absence (coarse-grained facies only) is consistent
with synchronous flows. Documenting pulses
and pulse groups shows that most event beds
are amalgamated, recording flows emplaced at
different times and from different directions, as
reflected by variable composition, facies and
palaeo-flow indicators.

Overall, the systematic, facies-based frame-
work offers a robust, process-oriented approach
for identifying and interpreting earthquake-
triggered event beds in the Japan Trench and
holds promise for similar applications in other
hadal trenches worldwide. Grounding event
stratigraphic interpretations in the relative depo-
sitional timing and duration advances the classi-
cal concept of event stratigraphy (Seilacher,
1982; Einsele, 2000). This enables more consis-
tent reconstructions of depositional stages
improving global understanding of deep-marine
and hadal sedimentary dynamics.

ACKNOWLEDGEMENTS

This study used samples and data provided by
the International Ocean Discovery Program
(IODP). We gratefully acknowledge the IODP
Expedition 386 Lithostratigraphy Team for ship-
board visual core description. We thank Z. Cai
(KyotoU), H. Braun (UIBK) and A. Pflügler
(UIBK) for grain-size analyses conducted with a
Malvern Mastersizer 3000 (UIBK) and a Camsi-
zer X2 (KyotoU). We also thank the lab man-
agers A. Amano (AIST) and H. Braun (UIBK), as
well as the student assistants J.-T. Lin, Y.-H.
Wu, Y.-S. Shao (NTU) and M. Niederstätter
(UIBK), for performing Itrax XRF measurements.
We thank the Associate Editor Gregor Eberli and
the reviewers for thoughtful comments that sub-
stantially improved this manuscript. This
research was funded by the Austrian Science
Fund (FWF, grant https://doi.org/10.55776/
P36809 to M.M., C.P. and M.S.), the Ministry of
Science and Technology of Taiwan (grants 111-
2116-M-002-013-MY3 and 113-2116-533 M-002-
009 to J.-J. H.), the National Taiwan University
Core Consortia Project (112L894206 to J.-J.
H.) and the Japan Society for the Promotion of
Science (KAKENHI 23K22586 to K.I.). For open-
access purposes, the authors has applied a CC
BY public copyright license to any Author-
Accepted Manuscript arising from this submis-
sion. Open Access funding provided by Univer-
sitat Innsbruck/KEMÄ.

DATA AVAILABILITY STATEMENT

Standardised IODP Expedition 386 data used in
this study are accessible at https://iodp.pangaea.de/front_content.php?idcat=653; new data generated in this study will be made available on
http://iodp.pangea.de.

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Manuscript received 7 June 2025; revision accepted 1 December 2025

## Supporting Information

Additional information may be found in the online version of this article:

**Fig. S1.** Interpretation of Event Bed 1 (blue area) at basin-high sites M0090 (Basin C1) and M0089 (Basin C2), and at basin-depocentre sites M0091 (Basin S3) and M0083 (Basin C2). Image datasets include sagittal slice medical x-ray computed tomography (CT), line-scan with altered colour and x-ray radiographs (see Methods). High-resolution downcore data include density (blue), magnetic susceptibility (MS; red) (Strasser *et al.*, 2023) and selected elements (centred-log ratio) from x-ray fluorescence (XRF) core scanning (Huang *et al.*, 2025).

**Fig. S2.** Interpretation of Event Bed 2 (blue area) at basin-high sites M0090 (Basin C1) and M0089 (Basin C2), and at basin-depocentre sites M0091 (Basin S3) and M0083 (Basin C2). Image datasets include sagittal slice medical x-ray computed tomography (CT), line-scan with altered colour and x-ray radiographs (see Methods). High-resolution downcore data include density (blue), magnetic susceptibility (MS; red) (Strasser *et al.*, 2023) and selected elements (centred-log ratio) from x-ray fluorescence (XRF) core scanning (Huang *et al.*, 2025).

**Fig. S3.** Interpretation of Event Bed 3 (blue area) at basin-high sites M0090 (Basin C1) and M0089 (Basin C2), and at basin-depocentre sites M0091 (Basin S3) and M0083 (Basin C2). Image datasets include sagittal

slice medical x-ray computed tomography (CT), line-scan with altered colour and x-ray radiographs (see Methods). High-resolution downcore data include density (blue), magnetic susceptibility (MS; red) (Strasser *et al.*, 2023) and selected elements (centred-log ratio) from x-ray fluorescence (XRF) core scanning (Huang *et al.*, 2025).

**Fig. S4.** Interpretation of Event Bed 4 (blue area) at basin-high sites M0090 (Basin C1) and M0089 (Basin C2), and at basin-depocentre sites M0091 (Basin S3) and M0083 (Basin C2). Image datasets include sagittal slice medical x-ray computed tomography (CT), line-scan with altered colour and x-ray radiographs (see Methods). High-resolution downcore data include density (blue), magnetic susceptibility (MS; red) (Strasser *et al.*, 2023) and selected elements (centred-log ratio) from x-ray fluorescence (XRF) core scanning (Huang *et al.*, 2025).

**Fig. S5.** Grain-size distribution of sample M0090B-1H-2-W, 5–6 cm, measured using two methods: Malvern Mastersizer 3000 (blue) and Camsizer X2 (pink). Both methods show excellent alignment, confirming consistency in grain-size characterisation across measurement techniques.

**Fig. S6.** Examples of Type-B convolute lamination in depocentre sites Basin S3 (M0091) and Basin C2 (M0083) for Event Beds 2 and 3, along with interpretations of the associated syn-folds formed within each event bed.

**Fig. S7.** Examples of pre-event beds occurring beneath Event Beds 2 and 3 across different basin physiographies. The erosive surface is indicated by the black arrow.

**Fig. S8.** Components of two samples: hemipelagic sediment (F0) and the ungraded, structureless event-internal facies (F6B). F0 shows more heterogeneous, terrigenous components, whereas F6B is more homogeneous, well sorted and radiolarian-rich.

**Table S1.** Definitions of sedimentary structures: primary, secondary and trace fossils.

**Table S2.** Occurrence of sedimentary structures in event beds across different basin physiographies.