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.

Shallow magma diversions during explosive diatreme-forming eruptions

“…The rheological behavior of the diatreme’s infill will therefore vary depending on the degree of rock induration at the time of intrusion, which will be strongly dependent on (1) the time interval between excavation and new magma intrusion events, (2) sediment properties affecting porosity (i.e., sorting, size, shape) 62 , and (3) the depth of the diatreme fill. As such, diatreme infills exhibit a variety of structures, such as brittle faults and intrusions (dike, sills) 40 , non-brittle folds 63 , and peperite 60 , which reflect elastic to elastoplastic to fluid-like viscous behaviors.…”

Influence of the substrate on maar–diatreme volcanoes — An example of a mixed setting from the Pali Aike volcanic field, Argentina

“…In many cases, the nature of the diatremes is not known, but there are examples where diatremes appear much narrower than overlying craters, and possibly not as deep as those found in hard substrates, creating "champagne glass" shapes for maardiatreme-systems in cross-section (e.g. : Ellendale, Australia: Smith and Lorenz, 1989; possibly Maegok, Korea: Kwon and Sohn, 2008; Fig. 13d).…”

“…Depending on the thickness of the soft units and hydrologic conditions, the phreatomagmatic explosion sites may or may not penetrate into the basement units. Where the unconsolidated deposits are sufficiently thick, downward propagation of the diatreme may be limited by lowpermeability sediments at depth (Kwon and Sohn, 2008;Lorenz, 2008). In the case of thinner soft units, hydrologic conditions in the basement may dictate if the maar acquires a bowl shape (no or low penetration in the basement: essentially a soft substrate-only maar) or becomes deeper (true mixed substrate).…”

Formation and Evolution of the Miocene Ipcheon Subbasin in Yangbuk-myeon, Gyeongju, SE Korea

“…반면, 어일분지 의 팔조리응회암은 5~10 mm 내외의 암회색 암편과 10~30 mm의 부석편이 흔히 관찰되나 흑운모편은 전 혀 관찰되지 않는다 (Fig. 10F) (Son et al, 2000aKwon and Sohn, 2008;Kwon et al, 2011). 와읍분지의 전기 마이오 세 분지충전물은 일부 고지의 현무암을 제외하면 대 …”