DEPOSITIONAL MECHANICS AND SEQUENCES OF BASE SURGES, SONGAKSAN TUFF RING, CHEJU ISLAND, KOREA

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. Copyright © 1990, Wiley Blackwell. All rights reserved