Tephra-filled volcanic neck (diatreme) of a mafic tuff ring at Maegok, Miocene Eoil Basin, SE Korea

Abstract

The Eoil Basin is one of the Miocene sedimentary basins in SE Korea, containing abundant volcanic deposits in addition to sedimentary deposits. Origin of a mafic volcaniclastic deposit at Maegok in the southeastern basin margin has remained enigmatic although it was interpreted as peperite by some workers. New features of the deposit were revealed by recent re-exposure of the outcrop, including 1) the occurrence of the deposit as an isolated vertical cylinder, 2) an abundance of hydroclastically fragmented sideromelane shards within the matrix, 3) tilting, faulting, and ductile deformation of the surrounding sedimentary strata, indicating synvolcanic vent-ward collapse of the strata, 4) dike-like structures composed of fine-grained tephra, some of which intruded into the surrounding sedimentary deposits, and 5) some clasts arranged in a jigsaw fashion, indicating in situ fragmentation by shock waves. These features suggest that the deposit represents a tuff pipe or the diatreme (tephra-filled volcanic neck) of a phreatomagmatic volcano. Plastically deformed mafic clasts, formerly regarded as evidence for peperitic interaction between magma and wet sediment, show neither the features of simultaneous intrusion and mingling of magma with wet sediments nor the features of in situ fragmentation, which are essential characteristics of peperite. An abundance of sideromelane ash and the lack of juvenile lapilli in the deposit suggest that the diatreme belongs to a phreatomagmatic volcano that was produced by contact-surface steam explosivity, and that the volcano had probably the morphology of a tuff ring. The Maegok exposure is inferred to belong to the deeper part of the diatreme in spite of its occurrence at a shallow level, about 100 m below the pre-eruption surface. The conduit of the tuff ring is therefore interpreted to have had a shallow (a few hundred meters deep) and flared-up geometry.