Geometrical Structural Analysis of Cretaceous System in South Central Hokkaido

Abstract

The writer presents here the result of his geometrical structural analysis of Cretaceous System which crops out in south central part of Hokkaido. Cretaceous geology is megascopically compiled on the basis of photo-geologic interpretation (Fig. 1).Of two commonly used stereographic methods (Wulff's net, lower hemisphere projection), π-diagram (S-pole diagram) is adopted for finding the direction of fold axis. Measurements of 1598 bedding planes observed in the surveys conducted by his many colleagues before 1961, have been used as data for construction. Some typical projections are figured as several kinds of stereogram (Fig. 2).Structural homogeneity decided from regular orientation (acute distribution of S-poles on great circle on the net) of π-diagram, isdiscernible in the intrasub-area. Orientation of axial surfaces and fold axes are shown (Fig. 3 and 4), from which fold forms are inferred. Taking into consideration the subareal S-poles counting densities (Fig. 5), structurally homogeneous domains have been studid (Fig. 6).