Geospans and Quantitative Approach to Tectonic Analysis in the Upper Cretaceous System, South Central Hokkaido

Abstract

This paper deals with the structural analysis of the Upper Cretaceous System in SouthCentral Hokkaido, on Lie basis of grasping quantitatively the change in velocity of downwarping during the entire period.The writer attempts to measure quantitatively the movement of subsidence on the assumption that the rate of subsidence is equivalent to the velocity of downwarping, because the subsidence during sedimentation is misty accepted as common or/and true evidences in the ralation of upheaval of source areas and sinking of sedimentary basins.Zonal succession, composed of the species of genus Inoceramus, is represented as widest common divisor by six zones, namely; shikotanensis•orientalis•japonicus•uwajimensis•hobetsensis and concentricus nipponicus, in descending order. Each zone is extended over the whole areas under discussion without any breaks throughout the time. These zones roughly correspond to the international stages. (Fig. 1.)On the otherhand "Holmes's symposium" has presented the Time Scale in the worldwide grade, in which the Cretaceous dating from 136 million years to 65 million years subdivided into eleven stages of apploximately equal duration.Recently the laboratory research of University of Alberta and Geological Survey of Canada have supplied some noticeable results concerning the dating of the Cretaceous System mainly in South Western Alberta. However, assumption of equal duration of each stage in Upper Crtaceous Epoch is not entirely ascertained even by so many materials on the western part of the country.Upper Cretaceous Zoning (space) and corresponding Duration (time) present the factors to resolve the movement on the subjects quantitatively. Velocities of downwarping in six stages are quite interesting and distinguishable especially in the middle stage ranging from Turonian to Santonion, instead of clear coincidence of upper and lower stages in velocity gradient. (Fig. 5). Figures (in Fig. 4) and subsequent plotts (in Fig. 5A) may be changed according to true determined ages in future, however, characteristic differences and its gradients in velocities among stages remain unaltered.Upper Cretaceous sedimentary basin might be divided into four subbasins, representing different tendency of subsidence which controlls the sedimentation during the Epoch. (Fig. 7)