IMPACT STRUCTURE IN JAPAN

Takamatsu MKT crater is buried circular crater located in Busshozan-Takamatsu, Kagawa Prefecture, Japan, which is defined by ca.4-km diameter geophysical signature with negative gravity anomaly and by ca. 8-km semi-circular structure visible on topographic images of Late Cretaceous (ca.90 Ma) Rhyoke granitic rocks. Purpose of the present paper is to make a new report on the borehole sample collection. The Takamatsu MKT borehole project was carried out in the parking lot of the Hotel Kansui, which is in the northern part of the structure and inside of the Takamatsu crater rim. From these drilled samples, below surface soils, we found conglomerate and clay of Mitoyo group (230-m thickness), brownish and black brecciated rocks, and (finally) granitic rock from 1,135-m depth. In these drilled samples, there are no volcanic rocks below 230 m, we found basement granitic rock below 1,135 m. After formation of this impact crater (which is proved by shocked quartz grains), melted breccias of brownish and black mixed layers were deposited up to ca. 660-m depth These contain Fe-Ni-bearing grains and spherules. These drilled samples show typical sedimentary layer process following crater formation. Original crater size of ca.8 km with 1,135 m depth was cut by intrusion of a small andesitic body, thus reducing the gravity anomaly size to ca.4km in size.