房総半島南端地域の新生代堆積盆地と褶曲構造の形成過程  [in Japanese] The formative process of the Neogene sedimentary basin and folded structures in the southern part of the Boso Peninsula  [in Japanese]

房総半島南端地域には, 新生界が東西方向の褶曲構造に支配されて分布している。本地域の新生界は, 下位から西岬層, 千倉層および豊房層に区分される。これらの地層は, おもに砂岩泥岩互層から構成されている。千倉層と豊房層および西岬層と豊房層は, それぞれ不整合関係で接するが, 各層内には不整合は認められない。本地域における層序学的解析から, 現在の背斜構造付近を境として層相・層厚が変化し, 地層の堆積過程と褶曲の形成が密接な関係を有していることが示唆された。本地域南部に位置する作名背斜と神余向斜の解析では, 千倉層の堆積期に作名背斜が萌芽し, 豊房層堆積時の再活動により現在の構造が形成されたこと, 豊房層堆積時の変形は作名背斜の北翼形成から南翼形成へと段階的に進行し, これは豊房層の堆積盆地の移動に伴っていること, さらに神余向斜は下部豊房層堆積盆地の堆積中心部に形成されていることが明らかとなった。

Many E-W trending folds composed of Neogene formations are distributed in the southern part of the Boso Peninsula. The formation of this area are divided into the Nishizaki, Chikura and Toyofusa Formations in ascendeng order. These formations are mainly composed of alternating beds of sandstone and siltstone. The Toyofusa Formation has an unconformable relationship with the underlying Nishizaki and Chikura Formations. Each formation was deposited without any remarkable unconformity. The results of the stratigraphical analysis indicate that the thickness and facies of these formations change near the axis of the present anticlines, and these changes show the folding initiated during their deposition. The analysis of the Sakuna anticline and the Kanamari syncline situated the southern part of this area reveal the fact that the Sakuna anticline was formed during the stage of deposition of the Chikura Formation and the Toyofusa Formation. In the depositional stage of the Toyofusa Formation, the deformation of the Sakuna anticline was shifted from the part of south limb to the part of north limb. These movements were accompanied by the northward shifting of the depositional centers of these fomations. Kanamari syncline was formed in the center part of the lower Toyofusa sedimentary basin.