八潮GS-YS-2コアに見られる更新統下総層群の堆積サイクルとテフロクロノロジー

Abstract

Sedimentary facies and tephrochronology of the Pleistocene Shimosa Group in the GS-YS-2 core recovered from Yashio, Saitama Prefecture, central Japan are examined. Our detailed examination reveals that the depth range of 37.02-94.40 m which corresponds to the Shimosa Group is divided into 10 lithofacies units, A to J. Of them, Units G and J are composed mainly of humic mud interpreted as marsh facies. Units A, D, E, and H are characterized by bioturbated sandy mud and/or muddy sands indicating bay facies, and Units B, F, and I consist of well-sorted sands interpreted as shoreface to beach facies. We recognize four depositional cycles corresponding to the formations in the standard division of the Shimosa Group. Each of them comprises the marsh, bay, and shoreface to beach facies in ascending order. Consequently, the examined interval in the core section is divided into four formations; they are Formation I (Units A and B), Formation II (Units C, D, E, and F), Formation III (Units G, H, and I), and Formation IV (Unit J). The core section intercalates some tephra layers. The lowermost tephra layer in the core section is A1Pm (TE-5a) of the Omachi APm series, known as a marker tephra indicating MIS 11. It is intercalated in Formation II. A tephra layer which is similar to BT-72 considered to have falled at MIS 10, is recognized in the lowermost part of Formation III. Therefore, Formations I to IV are correlative with the Kasamori Formation of the Kazusa Group, the Jizodo Formation, the Yabu Formation, and the Kamiizumi Formation of the Shimosa Group, respectively. The correlation between the GS-YS-2 and the other cores in the central Kanto Plain makes it clear that each formation of the Shimosa Group becomes thicker and represents deeper distribution northeastward from the GS-YS-2 drill site. These characteristics are distinctive particularly in the lower formations. These indicate that the area northeast of the GS-YS-2 drill site was continuously subsiding during the deposition of the Shimosa Group.