シーケンス層序学的地層観  [in Japanese] A sequence stratigraphic view of sedimentary successions  [in Japanese]

シーケンス層序学は, 地層形成を堆積プロセスと外的要因に基づいて成因論的に解析する方法である。従来の岩相層序学は, mappableという厚さや広がりの制限を持つ, ある一定の岩相で特徴づけられる岩相単位を認定し対比することで特徴づけられる。これに対し, シーケンス層序学は, 様々なスケールで地層境界面の特徴を認定し対比することで特徴づけられる。地層境界面は外的要因の変動に対応した一連の堆積プロセスの状態変化を示すものであり, 地層中に広い意味での時間面を刻んでいる。シーケンス層序学では, 様々な時空スケールで地層の形成過程が解析されるため, 形成期間の異なった堆積シーケンスの複合累重様式が認定される。これにより, 外的要因ならびに内的要因に支配された堆積プロセスの時空変化が階層的に解析される。

Sequence stratigraphic analysis is a genetic classification and correlation of stratigraphic successions within a chronostratigraphic framework. Identification and correlation of major bounding surfaces, whose development corresponds to temporal variation in dynamic condition in response to a relative sea-level cycle, are critical of sequence straitgraphic analysis. Comparing to the lithostratigrahic principles, sequence stratigraphy provides us the following new and/or renewed concepts on stratigraphic successions: (1) Bounding surfaces are represented by erosional, by passing and nondepositional surfaces. Identification of these surfaces predicts transportation and deposition of clastic sediments in other peculiar environments. (2) Facies organization and architecture of sedimentary successions are controlled by the interplay between sediment supply and relative sea-level changes. (3>This interplay controls variations in timing and nature of transgressive-to-regressive cycles. This develops unique combination of depositional processes and systems at transgressive and regressive stages, resulting in asymmetrical stacking of lithofacies associations in a transgressive-to-regressive cycle. (4) Sequence stratigraphic classification is scale-independent and multi-fold stacking patterns of depositional sequences with different cyclicity have been identified and analyzed. This results in hierarchical discrimination of spatial and temporal variations in sedimentary processes in response not only to allogenic but also to autogenic causal mechanisms.