沖縄県与那国島の八重山層群,久部良岳層のシーケンス層序  [in Japanese] Sequence stratigraphy of the Kuburatagi Formation of the Yaeyama Group, Yonaguni-jima Island, Okinawa, southwest Japan  [in Japanese]

沖縄県与那国島に分布する八重山層群 (前期中期中新世) の入部良岳層をシーケンス層序学的に解析した。その結果, 層厚およそ100 mの入部良岳層のなかに6つの堆積シーケンスが識別された。個々のシーケンスは, 相対的海水準上昇期の薄い沖浜堆積物と, それに続く高海水準期に沖浜から外浜へと浅海化する過程で形成された厚い地層から構成される。シーケンス境界は, シャープな海進面 (ラビーンメント面) である。久部良岳層のシーケンスは, 上位のものほど浅海化する傾向があり, 前進型の累重様式をとっている。入部良岳層の個々のシーケンスに見られる相対的海氷準上昇期の沖浜堆積物は, ほとんど生物撹拌作用を被っていないことに特徴づけられる。これは, 急激な海水準上昇に伴う外浜侵食により, 沿岸域から大量の砕屑物がタービダイトとして頻繁に供給されたため, 底生生物の活動が著しく阻害され, 生物撹拌作用が進行しなかった結果と考えられる。

Sequence stratigraphic analysis was examined on the Kuburatagi Formation of the Yaeyama Group (early Middle Miocene) distributed in the Yonagunijima Island, Okinawa, southwest Japan. Six depositional sequences can be recognized in the Kuburatagi Formation (SA to SF in ascending order). A single depositional sequence is from 50 m to 10 m thick and consists of cuplets of finding-and coarsing-upward facies successions. Fining-and coasing-upward facies are interpreted to represent transgressive (TST) and highstand (HST) systems tract, respectively. Sequence boundaries are characterized by sharp/flat or erosional surfaces and coincide with ravinment surfaces formed by shoreface erosion in response to relative rises in sea level. Sedimentary structures formed at the time of deposition are well preserved at both mud and sand beds in the sediments of TST in each depositional sequence. A part from the presence of small amount of trace fossils, these sediments are characterized by the sparsity of biogenic structures. Such a characteristic lithology can be interpreted to be the product of frequent influx of a large amounts of muddy sediments, which may be caused formed shoreface erosion related to rapid elevation of sea-level. The sediments characterized by the sparsity of biogenic structures change into the overlying intensely bioturbated sandy mudstone or muddy sandstone. This change in lithology may be ascribed to the rapid increase of biogenic activity associated with decresing of accumulation rate caused by the reduce of velocity of sea-level rising.