The Intra-Yezo Disturbance and Sedimentation of the Middle Yezo Group

To make efficient use of isopach maps and lithofacies maps that are indispensable in oil exploration, it is required that such maps were prepared on the basis of stratigraphic units distinguished by significant time plane.The Cretaceous sediments which are widely distributed in the axial zone of Hokkaido are divided, in descending order, into the Hakobuchi group, the Upper Yezo group, the Middle Yezo group, the Middle Yezo group, the Lower Yezo group, and the Sorachi group. However, since this stratigraphic division is based largely on lithology, the boundaries of the respective groups may sometimes intersect the time plane obliquely. If such is the case, then the maps that were prepared from the stratigrephic division would be meaningless.Therefore, the present writer examined the propriety of the past methods of defining the base of the Middle Yezo group. His aim was to discern the base of the Middle Yezo group as a time plane and clarify the scope and significance of the Intra-Yezo disturbance.The writer has reached the following conclusion : The Intrayezo disturbance took place in a limited period of time during late Albian. It formed the uplifted zone that stretches north-south centering on the Ashibetsu mountains, and also upraised the west marginal area of the Hidaka mountains. This disturbance probably represents a crustal movement in the middle period of the history of development of a sedimentary basin.