Organic geochemistry and mineralogy of mudstones affected by contact metamorphism

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The content of organic carbon (Co), the composition of visual kerogen, vitrinite reflectance (Ro), statistical Thermal Alteration Index (stTAI) and X-ray diffraction in two series of Miocene mudtones near the dykes at Owani-machi, Minami-Tsugaru, Aomori, were measured to investigate the thermal effect of contact metamorphism on organic maturation and minerals in sedimentary rocks. The organic maturation by contact metamorphism progressed with decreasing distance from the dykes with regard to the content, composition and maturity of organic matter in sedimentary rocks. In the samples within 1.5 m from the dyke at Sabaishi, the contents of Co and amorphous kerogen increase with decreasing distance from the dyke. This phenomenon may be explained by the idea that liquid constituent generated from the kerogen under high temperature and high pressure was recombined the residual kerogen to form the coke-like material. It is recognized in a diagram of Ro against stTAI that the pathway of organic maturation caused by contact metamorphism is different from that by diagenesis. This difference between these two pathways attributes to the large gap of heating time and temperature scales, and of sensitivity of Ro and stTAI to heating time and temperature. Without suffering effect of contact metamorphism, organic maturation indicates equivalent between two series, while silica mineral zoning indicates that the Sabaishi series has more highly progressed diagenesis than the Moriyama series. Kuroishi and Mitsumenai faults made the Sabaishi area 330 m deeper than the Moriyama area just after the movements of the faults, Pliocene epoch. It is concluded that organic maturation was fixed after the fault movements, and that silica mineral zoning has maintained from the time before the fault movements.