부산대학교 도서관

Seismic reflection survey was conducted at the Higashi-Ibaraki Terrace, the eastern foot of the Tsukuba Mountain Block, in order to clarify the shallow geological structures and the shape of the pre-Neogene basement. As a result, the followings are found. In the eastern part of the surveyed area, there are two major horizontal formation boundaries at about 50m and about 150m in depth. The former corresponds to the boundary between the middle-upper Pleistocene series (the Shimosa group) and the Pliocene-lower Pleistocene series (the Kazusa group), and the latter to that between the Kazusa group and the Miocene series. The Miocene series are unconformably overlain by the Kazusa group. The Shimosa and the Kazusa groups are nearly horizontal, but the Miocene series shows about 15-20° dip to the east. In the western part, there are also two major horizontal formation boundaries at the nearly same depths as in the eastern part. The former corresponds to the boundary between the Shimosa group and the unidentified formation, and the latter to that between this unidentified formation and the pre-Neogene basement, stretching from the Tsukuba Mountain Block. This basement is relatively flat in the western part, but it begins to dip eastward at the angle of about 15-20°, and is overlain by the Miocene series. The unidentified formation may be the heteropic facies of the Kazusa group in the eastern part, although no firm evidence is now available. The relation between the Miocene series and the pre. Neogene basement in the surveyed area is the same as that in the Toriashi, Torinoko and Yamizo Mountain Blocks, where both the Miocene series and the pre-Neogene basement outcrop at higher levels.