Stratigraphy and sedimentary environments of the Sorachi and Yezo Groups in the Yubari-Ashibetsu area, Hokkaido, Japan

Abstract

The Upper Mesozoic Sorachi and Yezo Groups are widely distributed in the Yubari-Ashibetsu region, located in the axial zone of central Hokkaido, the northernmost island of Japan. The Sorachi Group in this area consists mainly of volcanic rocks and volcaniclastic turbiditic rocks, and is divided into the Gokurakudaira, Ashibetsugawa, Nakatengu and Shirikishimanaigawa Formations in ascending order. The Gokurakudaira Formation constitutes mostly picrites, accompanied by intercalation of dacitic-andesitic volcaniclastic rocks and arkosic terrigenous turbiditic sandstones derived from the East Asian Continent. The Ashibetsugawa Formation is composed of medium-bedded red mudstone with several andesitic volcanic sandstone beds. The other two formations are characterized by andesitic volcaniclastic rocks. The rocks of the Nakatengu Formation are thick breccia beds and very thin-bedded mudstones, while those of the Shirikishimanaigawa Formation are thick- to medium- bedded sandstones, mudstones and tuff. The Yezo Group conformably overlying the Sorachi Group comprises alternating beds of terrigenous turbiditic sandstone and hemipelagic mudstone, presumably resulted from the erosion of the uplifted Jurassic accretionary prisms (Oshima Belt) located in southwestern Hokkaido. The two stratigraphic marker units of olistostrome unit containing "Orbitolina Limestone" blocks (the Upper Aptian Kirigishiyama Olisto- strome Member) and felsic tuffaceous unit (the Upper Albian Maruyama Formation), are recognized in the Yezo Group. Based on the biostrati-graphic study using radiolarians and planktonic foraminifers, the Sorachi Group and the lower part of the Yezo Group range in age from Tithonian to early Albian. Many previous works have been considered that the Sorachi Group consisted of basaltic rocks and cherts. However, we found intercalation of thick sedimentary sequence composed of arkosic sandstones within the basaltic rocks in the investigated area. This suggest that the basaltic eruptions, in part, occurred in an adjacent area close to the Asian continent. Moreover, some volcaniclastic rocks of the Sorachi Group also contain many ooids including angular andesite fragments. The occurrence of these rocks also indicates shallow marine environment on the volcanic mound.