抄録
The Tertiary system distributed in the eastern and the northeastern margin of the Kanto mountains, such as in the Chichibu basin, the Iwadono hill, the Hiki hill, and the Itsukaichi basin, is difficult to correlate stratigraphically because of their separate distribution, and therefore, the geological development of the Kanto region since the Tertiary has not been clarified enough. We recognized that the detailed geological mapping of each areas is neccessary to solve this problem, and started our collaborative research group in 1976. We have surveyed four years the Neogene system in the Itsukaichi basin in the eastesn margin of the Kanto mountains. As a result, we obtained the following facts against to the previous reports: 1. The Neogene Itsukaichi-machi group is divided into the Akikawa and the Ajiro formation, and the former is subdivided into the seven members on the basis of rock facies, that is, the Sajikami conglomerate member, the Kosho mudstone member, the Habu tuff member, the Ina sandstone member, and the Yokosawa sandstone and mudstone member. The Ajiro formation overlies the Akikawa formation with slight clino-unconformity. 2. The basin where the Neogene system was deposited is a kind of the collapse basin, which is estimated to be thrown down at about 300 m. 3. The formation of the basin is fundamentally similar to that of green tuff regions although the volcanism is scarecely approved in the basin. To make clear such a difference, which may reflect the difference of the state deep in the earth's crust, is the problem for future discussion. 4. The Neogene system is developed taking the form of the so-called "sedimentary imbricate structure", and the migration of the basin shows the direction nearly parallel with the structural trend of the basement terrain. Consequently, it is necessary to examine the historic change of the sedimentary basin from the Poleozoic to the Mesozoic. 5. The Neogene system has the closed fold with two pairs of the anticline and syncline, which might be ascribed essentially to the block movement of the basement. 6. The Neogene system is cut by the three systems of faults with different strikes to be divided into some blocks. As the conclusion of this report, we show the model of the development of geological structure of the Itsukaichi basin.
