秩父盆地における中新世の堆積盆地末期の造構過程(その1)

層序と構造

抄録

In the Chichibu basin, which is located in the northeastern part of the Kanto mountainland, the Neogene Tertiary deposits are widely distributed underlaying Paleozoic to Mesozoic basement rocks of the Chichibu belt. According to investigated geological data of this area, the total thickness of the deposits attains to about 5000m. And the deposits have the so-called imbricated structure (Ijiri & Fujita, 1949) showing a east-southeastward migration of the depositional field. Since 1985, the writers have surveyed geologically the uppermost part of the Neogene Tertiary deposits distributed in the Chichibu basin. As a result, they established the detailed geological succession and structure based on effective key beds. This paper (1st report) is mentioned about geological succession and structure because of making clear the tectonic process of the latest period of the Chichibu sedimentary basin. The conclusions of this paper are as follows: (1) The thick pile of strata developping in the uppermost part of the Neogene System in the Chichibu basin, was separated from the so-called Chichibu-machi Group. The writers redefined it and named newly as the Yokoze-machi Group. (2) The newly defined Chichibu-machi Group is divided into the Yaoi and the Kawahigashi Formations in the ascending order. On the other hand, the Yokoze-machi Group is divided into the Sekinoiriya, the Nakago, the Karugome and the Negoya Formations respectively, in the ascending order. (3) In each formation of the Yokoze-machi Group, the lithological facies laterally changes strongly. There are many strata with a lot of breccia within the Yokoze-machi Group, and these breccias are developed to eastern part near the basement. (4) The Nakago Formation was deposited in the depression maked by uplift-collapse. The collapsed basin is considered to have been occured at first time of sedimentation of the Sekinoiriya Formation. (5) The Yokoze-machi Group has a synclinal structure (the Une syncline), of which the axis shows WNW-ESE trend. This axis show echeron. The Une syncline is considered to have been created in the sedimentary process of the each strata of the Yokoze-machi Group.