韓半島南部陽南および浦項盆地の古環境変遷 Paleoenvironmental change of the Tertiary Yangnam and Pohang basins of the southern Korean Peninsula

韓半島南部の陽南および浦項盆地は最大の第三紀堆積盆地である。日本海南東縁にあたるこれらの堆積盆地における, 環境の変化のような地質事件は, 日本海の起源の問題と密接な関係を持っている。前期〜中期始新世には, 激しいデイサイト質火山活動がこの2つの盆地において起こり, 陸上で行われた活動は大量の溶結凝灰岩・火山角礫岩をもたらした。上部始新統〜漸新統は発見されず, これら地域は静穏で, かつ侵食地域であったと考えられる。前期中新世には, 激しい火山活動が陽南盆地に起こったが, 浦項盆地は静穏な陸域であった。陽南盆地の火山活動はデイサイトと安山岩・玄武岩のバイモーダルな活動を示し, 陸域でかつ大半は水中で行われた。当時陽南盆地には広大な湖が存在していたことがわかる。中期中新世には浦項盆地の全域にわたって海進が起こり, 陽南盆地ではいくつかの陥没盆地が発生した。中期中新世の海進は, Hataiarca, Vicaryaのような暖海生の軟体動物要素をもたらした。

The Yangnam and Pohang basins are the largest Tertiary basins in the southern Korean Peninsula. The geologic events, such as paleoenvironmental change in chronologic order, of these basins are closely related to the origin of the Japan Sea, for these basins bound the southwestern border of the Japan Sea. In the Early-Middle Eocene time, dacitic volcanic activity occurred heavily all over the two basins. The dacitic volcanic activity occurred on land, and a large amount of welded tuffs and volcanic breccias were erupted. The rocks of the Late Eocene through the Oligocene are not found yet, and the area of the Yangnam and Pohang basins seems to have been in a state of quiet and weathering condition. In the Early Miocene time, heavy volcanic activity occurred only in the Yangnam Basin, and the Pohang Basin was in a state of quiet upland condition. The volcanic activity in the Yangnam Basin was bimodal in composition, that is, dacitic and andesitic-basaltic. The dacitic volcanic activity preceded the andesitic-basaltic activity and occurred on land. The andesitic-basaltic volcanic activity occurred mainly under water, and this suggests that the Yangnam Basin underwent fresh water invasion and existed as a large lake during the Early Miocene time. In the Middle Miocene time, marine water transgressedin to the whole of the Pohang Basin and several collapsed basins in the Yang-nam Basin. The Middle Miocene marine transgression brought warm water molluscan elements, such as Hatai area, Vicarya, into the basins.