open access

https://hdl.handle.net/2115/17248

Re-evaluation of the intra-Yezo disturbance and the Cretaceous tectonics in Hokkaido, Japan

日本語

felsic volcanism

1999-10-15

日本地質学会

Journal of the Geological Society of Japan

105(10)

711-728

Many previous works on the Central Hokkaido support a tectonic model that exposure of Kamuikotan metamorphic belt occurred in the middle Cretaceous, and that the Yezo sedimentary basin was divided into the western and eastern terranes by the movement. The recognition of this event named as "the intra-Yezo disturbance" is based mainly on the exsitence of unconformity where basal conglomerates of "the Middle Yezo Group" (the middle part of Yezo Group) overlie alternating beds and limestones of "the Lower Yezo Group" (the lower part of Yezo Group).On the basis of lithology, sedimentary facies and microfossil biostratigraphy in both basal conglo-merate beds and the underlying strata, conglomerates and breccias distributed in the Soeushinai and Oyubari-Ashibetsu areas of the western terrane of the Yezo Group are recognized as debris flow deposits associated with felsic tuff beds which are correlated with the late Albian Maruyama Formation, the middle part of the Yezo Group. In the western Kanayama, southern Kanayama, Soshubetsu, Hidaka and Idonnappu areas of the eastern terrane, basal conglomerates reported by the previous workers are olistostromes or redeposited conglo-merates which belong to the lower part of the Yezo Group and are assigned to the late Aptian. Hence, coglomerate beds in previous works can be classified into the Aptian olistostromes and the late Albian debris flow deposits associated with tuff beds. These two deposits are distributed widely in both eastern and western terranes of the Yezo Group. These observations suggest that conglomerates assignable to "the intra-Yezo disturbance" are not typical basal conglomerates, and the uplifted model of Kamikotan ridge needs to be reviced. Instead, the extensive distributions of olistostromes and tuff beds in the Central Hokkaido indicate two mid-Cretaceous tectonic events which are huge submarine sliding in the late Aptian, and the following large-scale felsic volcanism in the late Albian.

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