Geological information obtained from coal and coalified dispersed organic matter in the Tertiary system of Japanese islands

Abstract

The geological structure of the Tertiary system of the Japanese islands is rather complicated because the islands were subjected to the Alpine orogeny at the northeastern margin of the Asian continent which faces the active Pacific and Philippine Sea plates that moved toward the continent. The major coal-bearing formations are Tertiary in age and were affected by tectonic movements throughout their geologic history that began with sedimentation, diagenetic alteration, and ended in final geotectonic deformation. Because geological conditions were unstable, coal and oil fields are smaller in size and tend to induce short-lived mining operations and difficult or fruitless exploration. Though the geological background is thus unfavorable for economic development of low-cost hydrocarbon resources, coal and coalified dispersed organic matter (MOD) in the sediments offer abundant valuable information to solve geological problems on the Cenozoic island-arc system within limited areas of the islands. The Japanese islands are composed of several arcs: Ryuku, Honshu, Kurile, and Hidaka-Saghalien from southwest to northeast. The Honshu arc is subdivided into two parts northeastern and southwestern by the Fossa Magna (great fissure). The major coal and oil fields are restricted in distribution; i.e., the former are distributed in the northeastern Pacific side and contrasting continent side of the western edge of southwestern Honshu. The major oil fields are restricted mostly to the continental side of northeastern Honshu. Regional variation of coal-seam thickness in the major coal fields suggests a change in the sedimentary conditions of the basins (Aihara, 1980a,b,c). Variation of maceral composition provides more detailed information about differences of sedimentary conditions in peat swamps of the coal basins. The characteristic maceral composition of less fusinite and micrinite in the Japanese Tertiary coal suggests that the peat swamp was kept wet and buried quickly, thus preventing desiccation (Takahashi and Aihara, 1989). Diagenetic and thermal alteration of the Tertiary System can be analyzed by the degree of coalification (rank) of coal and coalified MOD in the sediments. vitrinite reflectance (R0) was adopted as the rank parameter. The variation of R0 regionally, with stratigraphic position, and at various depth in boreholes was studied. The result has revealed regional variation of the paleogeothermal gradient which can be related to the present terrestrial heat flow conditions of the islands (Aihara, 1980a,b,c; Takahashi and Aihara, 1989). Paleogeothermal conditions in characteristic geotectonic provinces of the coalfields is recognized; i.e., lower heat flow conditions in the northeastern Pacific side where there are thicker sediments, and higher heat flow conditions in the southwestern continental side where there are thinner sediments. Geological structure also contrasts between the two provinces; i.e., coalfields in the former are folder and faulted, whereas those in the latter are gently dipping and simple half-grabens. These characteristic geotectonic conditions provided contrasting temperature-pressure conditions during coalification (or organic metamorphism) of each province; i.e., lower temperature and/or higher pressure conditions predominated in the northeastern Pacific side, and higher temperature and/or lower pressure conditions predominated in the southwestern continental side behind the active volcanic front. The delicate difference in temperature-pressure conditions of coalification reactions affected the chemical composition in vitrinite of the two provinces. Minute examination of ultimate analytical data of vitrinite from major coalfields (Sugimura et al., 1966) reveals that the vitrinite in the northeastern Pacific has slightly higher H/C atomic ratios than that on southwestern continental side. The hydrogenous active fractions generated during coalfication tended to be more preserved in the chemical structure of the coal under lower temperature and/or higher pressure conditions than under higher temperature and/or lower pressure conditions that caused their release (Aihara, 1980a,b,c). Gas chromatographic analysis of released gas from fresh samples collected from coal-mine faces has detected aliphatic fractions only from coalfields in the northeastern Pacific side. Coaly laminae and coalified MOD in the non-coal-bearing sediments in the Pacific side of southwestern Honshu, which are contemporaneous with the coal-bearing sediments on the continental side, can be utilized to analyze the paleogeothermal history and tectonic deformation of the extremely complicated accreted body of the Shimanto Terrain. Regional and stratigraphic variation of R0 of MOD from the Kyushu Shimanto Terrain has been studied, and thermal effects from the newly born, warm Philippine Sea plate was detected. Measurement of the direction of optical anisotropy ofo R0 (bireflectance, Rbi = Rmax - Rmin) of coaly laminae in regions of high R0 value (> 1.0) reveals the direction of stress and strain during the low-temperature metamorphism. Geological information obtained from coal and coalified MOD of the Japanese Tertiary system in the field and the laboratory was successfully utilized to analyze the sedimentological, diagenetic, paleogeothermal, and tectonic problems. it is evident that the same methods can also be adapted to older clastic sediments if they contain MOD. © 1990.