The geology of the area in and around the Seiwa Prefectural Forest Park, Chiba Prefecture, central Japan, with special reference to the stratigraphy and structure of the Kiyosumi and Anno formations, Neogene Awa Group

摘要

The Neogene Amatsu, Kiyosumi and Anno formations of the upper part of the Awa Group are well exposed in ascending order in the Seiwa Prefectural Forest Park, the central part of Boso Peninsula, central Japan. The Amatsu Formation is mainly formed of deep-water mudstones, and the Kiyosumi and Anno formations are mainly made up of turbidite successions. In this region, the detailed geologic and stratigraphic works based on the trace of many tuff marker beds have not been done owing to the existence of the Mishima and Toyofusa lakes. We conducted the geologic survey using the motor boats in the lakes and clarified the detailed stratigraphy and structure of the formations, especially the Kiyosumi and Anno formations, distributed in this area by making a detailed geologic map drawn with the distribution of many tuff marker beds.The Kiyosumi and Anno formations are gently folded along a couple of anticline and syncline axes trending WNW-ESE with dominant of younger N-S directional faults, and some older E-W directional low-angled reversal faults at the southern part of the syncline in the Kiyosumi Formation. The new facts on the lithology and stratigraphy of these formations are follows: a deep basal erosion and channel deposits with pebbly sandstone are developed across the syncline around Toyofusa Lake; maximum erosion of the channel reach to the horizon of Am78 tuff marker in the upper Amatsu Formation; depositional centers of the Kiyosumi and Anno formations are moved northward during the deposition of these formations. This area is very important for the investigation of the depositional process of the Anno Formation as opposed to other areas due to the existence of the basal erosion and the channel deposits, about 1 km wide in E-W direction and a few kilometer long in N-S direction.