更新統の海浜堆積物に対するMINI-SEQUENCE STRATIGRAPHY  [in Japanese] Mini-sequence stratigraphy for the Pleistocene beach deposits  [in Japanese]

海浜堆積物は砂鉄質の平行葉理で特徴づけられる。それらの平行葉理は海浜の地表面で形成されたものであるから, それぞれの葉理は同時間面を示すことになる。更新統の海浜堆積物の露頭断面で葉理構造を詳細に調べた。その断面はシークェンス層序学で用いる音響断面と同じ意味をもつので, その解析法を露頭規模での海浜堆積相断面に適用して, 海浜堆積物の発達過程を明かにした。その結果, 前浜や後浜の堆積物はそれぞれシステマティックな累重様式をしていることがわかった。この葉理構造を利用する方法は堆積物の発達過程を解析する新しい方法として提唱できる。

Sequence stratigraphic method was applied to a succession in an outcrop of the Pleistocene beach deposits (upper part of the Kioroshi Formation, Shimosa Group). The beach deposits are characterized by heavy mineral concentrated, parallel-laminated and well-sorted sands. Each lamina of modern beach deposits was formed on a particular beach surface. The laminae, therefore, provide the same information as the synchronous reflection lines in a seismic section. This allows a seismic-stratigrahic approach to be applied to an outcrop of lateral terminations of laminae within beach deposits. In the lower part of foreshore deposits, lamina sets developed with onlap pattern on an erosional surface which formed by a storm event. The lamina sets in the upper part show the backstep structure and the top shows an erosional truncation surface. Their developments were controlled by "beach stage" and longer sea-level changes. Backshore deposits cover the foreshore lamina sets with downlap surface formed the overwash process.