1980 Volume 30 Issue 162 Pages 229-240
The purpose of this study is to deliniate geological background of coalification and to interprete characteristic nature of coal rank variation in the Ishikari coalfield from a view point of orgnic metamorphism in determining and representing rank of coalification by vitrinite reflectance (R 0). In order to elucidate the coalification variation covering the entire coalfield, numerous vitrinite samples were collected both from coal seams of the major coal-bearing formations and from phytoclasts of non-coal-bearing formations of selected sampling routes.
The measured R 0 values are provided to analyze both stratigraphic coalification pattern within the sucsession and regional variation pattern within the major coal-bearing formations. The stratigraphic coalification patterns in the selected sampling sucsessions reveal rather low reflectance gradient with some remarkable retardation near the bottom horizon of the Noborikawa coal-bearing formation. Regional rank variation pattern of the major coal-bearing formations is characteristically concordant with the variation of overlying thickness, and is cut and shifted by major faults. In comparing these rank variation patterns with sedimentary and structural data and with R 0-gradient of the deep oil exploring bore holes that penetrate the same horizons, the geological processes of coalification in the Ishikari coalfield are summarized as follows. The coalification process underwent through the diagenetical alteration during sedimentation of the Ishikari and overlying groups which accumulated in a paleogeothermal condition of low heat flow, and the consequent coalification pattern was modified by differential sinking and upheaving movements of the basin which acted as a precursor of the Hidaka orogeny. The present pattern was constructed during the latest stage of the orogeny resulting in physical deformation of the original pattern by faulting and folding movements under a condition of falling temperature in coalification reaction due to the total uplift of the basin.
The vitrinite reflectance gradient in the Cenozoic groups of the present high heat flow region in the continent side (inner zone) of the Japanese Islands is relatively high compared with that of the Ishikari coalfield. This contrasting coalification condition is attributed to the contrasting crustal thermal structure of both sides of the island arc in the Cenozoic Era, which suggests a paired organic metamorphism in the tectonic development as the pair of the higher grade of metamorphism formed by the older orogenic movements in the island arc. The feature of the coalification of the Ishikari coalfield can be designated as a typical example of the lower temperature and/or higher pressure organic metamorphism in the Pacific side of the Japanese Islands during the Cenozoic development of the arc.
