中部更新統金剛地層の堆積環境とシーケンス層序学的解析 Depositional Environments and Sequence Stratigraphic Analysis of the Middle Pleistocene Kongochi Formation
Sequence stratigraphic analysis of the Middle Pleistocene Kongochi Formation (Kazusa Group) and the bottom part of the overlying Jizodo Formation (Shimosa Group) distributed in the Boso Peninsula, Japan was made, based on the recognition of the facies stacking pattern and stratigraphic discontinuity. The Kongochi Formation consists of three shallowing-upward facies successions. The lower, middle and upper successions are made up of, respectively, outer shelf to upper shoreface, inner shelf to lower shoreface, and inner shelf to salt marsh sediments. The lower succession is interpreted to be an upper segment of a depositional sequence (maximum thickness 270 m), of which lower one is the underlying outer shelf deposits of the upper part of the Kasamori Formation, and to represent highstand systems tract. The others constitute two depositional sequences whose maximum thickness are 43 m and more than 23 m, in ascending order. As the upper part of the Kazusa, and Shimosa Groups were said to be formed under the strong influence of 5th to 6th order glacio-eustatic sea-level changes, the overall upward-shallowing and thinning pattern among the sequences may reflect decreasing of accomodation spaces possibly induced by longer-term (i.e. 4th or 5th order) sea-level falling.