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

Hadal trenches uniquely 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, influencing both paleoseismic reconstruction and deep-sea carbon cycling on multi-millennial timescales. Classical event stratigraphy treats these event beds as the product of a brief, isochronous process. This study aims to build on this concept to enable the documentation of relative depositional timing within a single event bed. Within four well documented historical earthquake-triggered beds recovered during IODP Expedition 386 and linked to megathrust earthquakes (CE 2011 Tohoku-Oki, CE 1454 Kyotoku, CE 869 Jogan, and ~2.3 ka), we develop a systematic, facies-based framework to identify event-internal characteristics. This integrated approach resolves sedimentary structures—physical, chemical and biogenic characteristics—by combining visual core description with high-resolution 2D and 3D imaging (X-ray computed tomography and X-ray radiography), grain-size measurements, and geochemical datasets (X-ray fluorescence). 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. This shift is marked by a 32 µm grain-size threshold that separates coarse-grained (F1–F3; >32 µm) from fine-grained (F4–F6; <32 µm) facies. Grain-size breaks at this threshold mark the transition from non-cohesive to cohesive deposition, and from bed-load to suspended-load dominated regimes. Event-internal facies organize into pulses and pulse groups that stack hierarchically into three patterns: single-pulsed, multi-pulsed and amalgamated. The amalgamated pattern comprises 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 paleo-flow indicators. Basin physiography strongly influences facies development and deposit thickness: basin highs record erosion–deposition stages, whereas depocenters favor ponding turbid water masses in prolonged suspension, producing thick fine-grained tops. Here, transitional facies (F4) forms where new flows interact with the still-settling suspension clouds, allowing interpretation of complex mainshock–aftershock sequences; this process is enhanced in depocenters but absent at basin highs. By providing new insights into hydrodynamic conditions, relative depositional timing and duration of event bed deposition in hadal trenches, the resulting facies-based framework advances the event stratigraphy concept and improves global understanding of deep-marine and hadal sedimentary dynamics and supports the use of hadal event beds as robust natural archives of mainshock–aftershock and source-to-sink processes.