[H-QR24_1PO1] Stratigraphy and tectonics of the alluvial plain and related area

[HQR24-P02] Geological overview of the Mobara District: Quadrangle Series, 1:50000, GSJ/AIST

*Futoshi NANAYAMA¹, Hiroomi NAKAZATO², Shinzou OOI³, Rei NAKASHIMA¹ (1.Geological Survey of Japan, AIST, 2.NARO, 3.GSI)

Keywords:Chiba Prefecture, Mobara District, Quadrangle Series, 1:50000, GSJ/AIST, Geology, Geomorphology, overview

Between 2010 and 2014, geological and geomorphological research for the Quadrangle Series, 1:50000 of the Mobara district was performed by Geological Survey of Japan, AIST. In this poster presentation, we presents the proto version of geological map due to have opinions from everyone.The Mobara district is located in the northeastern part of Boso Peninsula of the Kanto region. The district partly includes the Pacific Ocean in the eastern district. The land area is geomorphological divided into hills, diluvia uplands, river terraces, alluvial lowlands and Kujyukuri strand plane. The hills and uplands occupy the southwestern to western and northwestern part of the district, respectively. The hills constitute parts of the Kazusa Hills and the uplands are parts of the Shimosa Uplands. River terraces and alluvial lowlands are distributed along Ichinomiya River, Isumi River and Murata River. The Murata River runs through the northwestern district flows into the Tokyo Bay. On the other hand, the Ichinomiya River runs through the central district eastward into the Pacific Ocean. Also the Isumi River runs through the southern district eastward into the Pacific Ocean.In the Mobara district, there are mainly two stratigraphic units, the lower to middle Pleistocene Kazusa Group and the middle to upper Pleistocene Shimosa Group, trending northeast to southwest and gently dipping northwest. Furthermore Upper Pleistocene terrace deposits with Kanto Loam, Holocene terrace deposits and alluvial deposits arc mostly distributed along the Ichinomi, Isumi and Murata Rivers.The Kazusa Group is divided into seven formations, Otadai, Umegase, Kokumoto, Kakinokidai, Chonan, Kasamori and Kongochi Formations. These were conformably deposited upward the continuous change of the sedimentary environments from the lower bathyal through upper bathyal to inner shelf.