Abstract
This excursion has been carried out in every autumn since 2008, visiting the southern Kanto gas field
（“Minami-Kanto gas field”） located in the Boso Peninsula. Since the ?rst time, we have visited the natural gas seepage spot, the production plant of natural gas and iodine, and outcrops of the Pleistocene Kazusa Group which produces natural gas. From the last excursion, we added a new spot known as “Chiba Section”, a part of the Kokumoto Formation of the Kazusa Group, which records the geomagnetic boundary of Matuyama-Brunhes geomagnetic boundary about 0.77 Ma and was officially approved as a Global boundary Stratotype Section and Point （GSSP） of the middle Pleistocene subseries just on 17 January 2020. This excursion was carried out on 1 November 2019. At the first stop, we visited Godo Shigen Co., Ltd., a major iodine-producing company occupying 7 % of the world iodine share, observed the production plants of natural gas and iodine. Then, we visited the natural gas seepage site and observed the active bubbling phenomenon of methane gas at the surface of the Mizusawa River. After lunch, we visited outcrops around a public cultural and sport facilities, observed sedimentary structures of turbidite sandstones and related deposits of the Otadai Formation of the Kazusa Group. After that, we visited the Otaki Natural-gas Memorial Museum which show the development of natural gas in Otaki Town, where natural gas dissolved in water was firstly discovered and developed. At last, we visited and observed only the nearby area of the Chiba Section, because we could not approach to the just point of the section, an outcrop along the Yoro River, due to the destruction of the stairway lastly reaching the floor of the river by torrential rain on 25 October 2019.
