1:45 PM - 3:15 PM
[SVC36-P05] Submarine phreatomagmatic explosion and slope failure inferred from volcaniclastic rocks of the Middle Miocene Kakuda volcanic rocks
Keywords:ancient submarine volcano, phreatomagmatic explosion, boulderly submarine debris flow deposits, gravelly turbidite flow deposits
Submarine volcaniclastic deposits are originated from explosive eruptions, flank failure and debris avalanche, and post-eruptive sedimentation. In considering the history of volcanoes, it is important to clarify the transport and deposition processes of volcaniclastics and the eruptive events that originated the volcaniclastic material.
Neogene Kakudayama volcanic rocks, distributed in the Kakudayama area in the central part of Niigata Prefecture, is composed of volcaniclastic rocks I, two pyroxene andesiteI, aphyric andesite, basaltic andesite, volcaniclastic rock II, hornblende two pyroxene andesite, hornblende two pyroxene dacite, two pyroxene andesite II, volcaniclastic rock III, and two pyroxene dacite II from the lower to the upper. The volcaniclastic rock II contains large blocks of the aphyric dacite and the two pyroxene dacite, although they are not found as a lava layer in the study area. The two pyroxene andesite I, aphyric andesite, and basaltic andesite are all composed of hyaloclastite, which outflowed and emplaced on the seafloor. The hornblende two pyroxene andesite, hornblende two pyroxene dacite, and two pyroxene andesiteII are composed of massive lava and hyaloclastite, and the hornblende-bearing two pyroxene andesite and two pyroxene daciteII are composed of massive lava.
We found that the volcaniclastic rocks II was originated from the flank failure and submarine debris flow and the continued phreatomagmatic eruption of hornblende two pyroxene andesite magma under seawater. The lower beds consists of boulderly submarine debris flow deposits, and the middle to upper beds are gravelly turbidite flow deposits.
The boulderly submarine debris flow deposits are composed of stratified layers of volcanic breccia. The individual layers are2.8 to 5.8 m in thickness. Each layer has an inverse-graded structure, and often contains lava blocks with diameters of 20 cm or more in the upper part, and rarely contains large lava blocks of about 2~3 m size. Blocks up to 10 m long and 2 m thick are also found in the deposits. These lava blocks are aphyric, and are dacite in composition. The blocks are partly reddish in color, and are thought to have emplaced as a terrestrial lava flow. Small amount of rounded gravels are contained in the deposits, which indicates beach environment, and suggest slope failure of the volcanic island.
The gravelly turbidite flow deposits are composed of stratified beds of volcanic breccia, tuff breccia, and lapilli stone, and show upward thinning and upward fining structure. Some of the beds show inverse- and normal-grading structures, but most of them are ungraded, with plate-like lava blocks imbricated almost parallel to the bedding plane. The lowest bed is 2.5 to 3.6 m thick, and rich in large polymict gravels about 10 cm in diameter. It also contains rare subrounded gravels.
The upper beds consist of lenses of fine-grained thin layers with different grain sizes. They are rich in vesicular, intrinsic rock fragments associated with occasional dense rock fragments. The other gravels are dacite, basaltic andesite, andesite, tuff, and siltstone gravels. The amount of vesiculated fragments is around 45%, and are glassy hornblende-two pyroxene andesite with various vesicularity, up to 60~70%. Such intrinsic rock fragments are found throughout this gravelly turbidite flow deposits, and in small amounts in the boulderly submarine debris flow deposits, suggesting the submarine phreatomagmatic eruption.
The harmonic boundary between the lower boulderly submarine debris flow deposits and overlying gravelly turbidite flow deposits suggest that the phreatomagmatic explosion was triggered by the slope failure that generated a low mud content debris flow.
Neogene Kakudayama volcanic rocks, distributed in the Kakudayama area in the central part of Niigata Prefecture, is composed of volcaniclastic rocks I, two pyroxene andesiteI, aphyric andesite, basaltic andesite, volcaniclastic rock II, hornblende two pyroxene andesite, hornblende two pyroxene dacite, two pyroxene andesite II, volcaniclastic rock III, and two pyroxene dacite II from the lower to the upper. The volcaniclastic rock II contains large blocks of the aphyric dacite and the two pyroxene dacite, although they are not found as a lava layer in the study area. The two pyroxene andesite I, aphyric andesite, and basaltic andesite are all composed of hyaloclastite, which outflowed and emplaced on the seafloor. The hornblende two pyroxene andesite, hornblende two pyroxene dacite, and two pyroxene andesiteII are composed of massive lava and hyaloclastite, and the hornblende-bearing two pyroxene andesite and two pyroxene daciteII are composed of massive lava.
We found that the volcaniclastic rocks II was originated from the flank failure and submarine debris flow and the continued phreatomagmatic eruption of hornblende two pyroxene andesite magma under seawater. The lower beds consists of boulderly submarine debris flow deposits, and the middle to upper beds are gravelly turbidite flow deposits.
The boulderly submarine debris flow deposits are composed of stratified layers of volcanic breccia. The individual layers are2.8 to 5.8 m in thickness. Each layer has an inverse-graded structure, and often contains lava blocks with diameters of 20 cm or more in the upper part, and rarely contains large lava blocks of about 2~3 m size. Blocks up to 10 m long and 2 m thick are also found in the deposits. These lava blocks are aphyric, and are dacite in composition. The blocks are partly reddish in color, and are thought to have emplaced as a terrestrial lava flow. Small amount of rounded gravels are contained in the deposits, which indicates beach environment, and suggest slope failure of the volcanic island.
The gravelly turbidite flow deposits are composed of stratified beds of volcanic breccia, tuff breccia, and lapilli stone, and show upward thinning and upward fining structure. Some of the beds show inverse- and normal-grading structures, but most of them are ungraded, with plate-like lava blocks imbricated almost parallel to the bedding plane. The lowest bed is 2.5 to 3.6 m thick, and rich in large polymict gravels about 10 cm in diameter. It also contains rare subrounded gravels.
The upper beds consist of lenses of fine-grained thin layers with different grain sizes. They are rich in vesicular, intrinsic rock fragments associated with occasional dense rock fragments. The other gravels are dacite, basaltic andesite, andesite, tuff, and siltstone gravels. The amount of vesiculated fragments is around 45%, and are glassy hornblende-two pyroxene andesite with various vesicularity, up to 60~70%. Such intrinsic rock fragments are found throughout this gravelly turbidite flow deposits, and in small amounts in the boulderly submarine debris flow deposits, suggesting the submarine phreatomagmatic eruption.
The harmonic boundary between the lower boulderly submarine debris flow deposits and overlying gravelly turbidite flow deposits suggest that the phreatomagmatic explosion was triggered by the slope failure that generated a low mud content debris flow.