Confinement‐controlled architecture of stacked resedimented volcaniclastic beds in the Miocene Eoil Basin, SE Korea

Abstract Volcano‐sedimentary basins record complex interactions among volcaniclastic sediment input, accommodation creation and pre‐depositional relief, yet outcrop‐scale constraints on how topographic confinement controls resedimented volcaniclastic architecture remain limited. In the Miocene syn‐rift Eoil Basin, SE Korea, metre‐thick volcaniclastic beds interbedded with lacustrine‐to‐swamp deposits are exposed along a ca 100 m roadcut. Seven measured Sections were correlated to document lithofacies architecture and its relationship to evolving syndepositional topography. Units II–VI comprise stacked subunits produced by successive debris flow pulses and related flow transformations, expressed by recurrent transitions from massive, poorly sorted conglomerate (GSm) to normally graded conglomerate to sandstone (GSn) and/or crudely stratified gravelly sandstone to sandstone (Ss). These successions record composite sediment gravity flows in which concentrated debris flow divisions passed into more dilute hyperconcentrated‐flow and tractionally modified trailing divisions during runout. Unit III provides the strongest evidence for a probable multi‐surge event, based on lateral splitting and amalgamation of compositionally similar subunits within common boundaries and the absence of persistent repose deposits. In Units II and IV–VI, the available evidence does not uniquely distinguish intra‐event surges from separate events occurring in close succession; these units are therefore described as deposits of successive flow pulses. Unit II records deposition within a localised topographic low, Unit III shows pronounced cross‐channel facies partitioning and Units IV–VI record compensational stacking across progressively lower‐relief surfaces. Pumice‐rich Units II, IV and V reflect selective remobilisation with limited incorporation of intrabasinal material, whereas mixed pumice–scoria–lithic Units III and VI record either local channelised entrainment or upstream integration of multiple sediment stores. These results demonstrate that topographic confinement and sediment‐routing pathways jointly control facies architecture and compositional variability in volcano‐sedimentary basin fills.