The Suwolbong pyroclastic sequence in the western part of Cheju Island, Korea, comprises partly preserved rim beds of a Quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline. The sequence consists of breccia, lapillistone, lapilli tuff and tuff. Eighteen sedimentary facies are established and organized into six lateral facies sequences (LFS) and seven vertical facies sequences (VFS). The LFS 1, 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar‐bedded (LFS 1), undulatory‐bedded (LFS 4) or climbing dune‐bedded (LFS 5) (lapilli) tuff units. They are representative of relatively ‘dry’ base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting. The LFS 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent. It results from relatively ‘wet’ base surge in which sorting is poor due to the cohesion of damp ash. The LFS 3 comprises well‐sorted lapilli tuff and stratified tuff further downcurrent, suggestive of deposition from combined fall and surge of relatively ‘dry’ hydroclastic eruption. All seven vertical facies sequences generally comprise two facies units of coarse‐grained fines‐depleted lapilli tuff and an overlying fine‐grained tuff. These sequences are suggestive of deposition from base surge that consists of a turbulent head and a low‐concentration tail. Depositional processes in the Suwolbong tuff ring were dominated by a relatively ‘dry’ base surge. The base surge comprises turbulent and high‐concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport. As the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air, it develops large‐scale turbulent eddies and is segregated into coarse‐grained bedload and overlying fine‐grained suspension forming thinly stratified units. Further downcurrent, the base surge may be either cooled and deflated or pushed up into the air, depending on its temperature. The Suwolbong tuff ring comprises an overall wet‐to‐dry cycle with several dry‐to‐wet cycles in it, suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise.

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The Songaksan mount in the southwestern part of Cheju Island, Korea, is a Taalian tuff ring produced by phreatomagmatic explosions at an aquifer. A detailed analysis of proximal‐to‐distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies; one lateral facies sequence (LFS) and three vertical facies sequences (VFS). The VFS 1 and 2 are representative of facies relationships in horizontal near‐vent deposits. The VFS 1 comprises scour‐fill bedded tuff, inversely graded tuff, massive tuff and laminated tuff from base to top. The VFS 2 is a variant of the VFS 1, replaced by an inversely graded lapilli tuff unit at the base. The sequences suggest traction carpet, suspension and minor traction sedimentation from a high‐concentration near‐vent base surge. The LFS 1 and the VFS 3 are distilled from outward‐dipping flank deposits. Both sequences begin with disorganized lapilli tuff, followed successively by stratified (lapilli) tuff, dune‐bedded (lapilli) tuff, very thinbedded tuff and accretionary lapilli. They are suggestive of waning base surge which decreases in particle concentration, suspended‐load fall‐out rate and flow regimes with an increase in traction and sorting. These facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space. A near‐vent base surge is turbulent, uniformly mixed and highly concentrated and produces scour‐fill bedded tuff. As capacity decreases, the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part (gravity transformation), depositing inversely graded, massive and normally graded (lapilli) tuff. Ensuing loss of sediment load and mixing of ambient air result in flow dilution (surface transformation). Stratified and dune‐bedded units are produced by tractional processes of turbulent and low‐concentration surge. Further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension.

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Debris flow and hyperconcentrated flow in a submarine channel • Y. K. Sohn et al.