砂岩組成とシーケンス層序学  [in Japanese] Sandstone composition in sequence stratigraphy  [in Japanese]

堆積シーケンスの発達にともなう砂岩組成の変化を, 前弧海盆で形成された地層を例に検討した.海進期に形成された地層は, 石英粒子の相対的含有量が少なく, 長石粒子や火山岩片などの相対的含有量の多い砂岩組成で特徴づけられる.これに対し, 高海水準期に形成された地層は, 石英粒子の相対的含有量の多い砂岩組成を特徴とする.また, 低海水準期に形成された地層は, 組成領域の最も大きいばらつきを示す.このような堆積シーケンスの形成にともなう砂岩組成の変化は, 海進と海退にともなう陸源砕屑物の海域への運搬プロセスと相対的供給量の変化, ならびに火成弧の発達と開析に対応するものと考えられる.従来, 砂岩組成の時間的変化の解析には岩相層序学に基づく検討が行なわれてきた.しかし, 今後は, 堆積シーケンスの形成にともなう砂岩組成の変動を詳しく解析し, シーケンス層序学的な枠組みに基づいた砂岩組成の時間的変化の解析が不可欠と考えられる.

Sequence-stratigraphic variation in framework composition of sands and sandstones was investigated based mainly on sedimentary infills of forearc basins in the Japanese Islands. In general, basin-fill successions are characterized by duration of one to a few million years and represent third-order depositional sequences. Such a third-order depositional sequence consists mainly of transgressive (transgressive sequence set) and regressive (highstand sequence set) successions. Transgressive deposits are characterized by more feldspathic and volcanogenic framework composition, indicating uplift and renewed volcanism in provenance terranes and reduction in active supply of terrigenous clastic sediments to a marine environment. Regressive deposits are characterized by more quartzose petrofacies and indicate active supply of terrigenous clastic sediments from incised provenance terrances, associated with progradation of coastal depositional systems that are affected by abrasion and destruction of unstable lithic fragments by nearshore dynamic processes. Component systems tracts of high-frequency depositional sequences that constitute a third-order basin-fill succession also show compositional variations similar to those of a third-order depositional sequence. In particular, lowstand systems tract deposits show more variable petrofacies than highstand and trangressive systems tratcs deposits. This variable petrofacies may be a result of erosion of preceding highsgtand systems tract deposits and the direct fluvial supply of terrigenous clastic sediments that were not subjected to abrasion and destruction of unstable lithic fragments by nearshore dynamic processes. Therefore, variation in framework composition of sands and sandstones is interpreted to be controlled not only by variety of geologic components of a provenance terrane but also by variations among systems tracts of a depositional sequence. In conclusion, temporal variation in framework composition of sands and sandstones is clearly detected much more by a sequence-stratigraphic framework than by a traditional lithostratigraphic framework.