三陸沿岸, 種市層(Santonian)におけるヘリングボーン斜交層理の保存 Preservation of herringbone cross-stratification in a transgressive sequence of the Santonian Taneichi Formation, northeast Japan

種市層は三陸北部沿岸に細長く分布する浅海性堆積層を主とする上部白亜系である。宿戸(シュクノヘ)付近では, 種市層は海進作用による典型的な上方細粒化の連続露頭を示している。またこの露頭の下部では, 見事なヘリングボーン斜交層理(HBCS)が見られる。筆者は海進作用を示す一連の堆積層において, HBCSが保存され得た堆積環境を考察した。その結果, 該当するHBCSは潮汐卓越型の潮流口で形成・保存されたとの結論を得た。

The Santonian Taneichi Formation is well exposed along bluffs of the Sanriku Coast, northeast Japan. Facies of a typical tidal channel deposit, herringbone cross-stratification (HBCS) exists in a transgressive sequence that includes a ravinement surface and shows an overall fining-upward succession. Because of shoreface wave action, foreshore and uppermost shoreface sediments are generally scarcely incorporated into the transgressive stratigraphic record. However, the studied HBCS bed was preferentially preserved because the bed was formed in a deeply scoured-and-filled tide-dominated channel and was not thoroughly bevelled by ravinement during transgression.