青森県五所川原市東部に分布する中新統に含まれるケロジェンと珪酸鉱物の続成変化

抄録

The diagenesis of kerogen as well as silica mineral was investigated in Miocene sediments distributed in the east of Goshogawara City, Aomori Prefecture. The Miocene sediments in this area are composed of the Nagane, Umanokamiyama, Gempachimori and Fudotaki Formations in ascending order and have a dome-shaped structure with a maximum thickness more than 1500 m. It is assumed from the elemental analyses that kerogens in these sediments may be derived from a mixture of pollen, spore, plankton, bacteria and terrestrial higher plant, and that the terrestrial higher plant contributes much to kerogens in more deeply underlying formations, the Nagane and Umanokamiyama Formations. This assumption is concordant with the continuous transgression in Miocene age at this area. The spin-lattice relaxation time (T₁) of kerogen in ¹H-NMR increases exponentially from higher to lower stratigraphical horizon in the sediments. Silica mineral in the Fudotaki Formation is amorphous silica (opal A), that in the Gempachimori and Umanokamiyama Formations is cristobalite (opal CT), and that in the Nagane Formation is quartz. So it is obviously observed in these sediments that amorphous silica was transformed to quartz through cristobalite by diagenesis. The d (101) spacing of cristobalite progressively decreases toward low horizon in the Gempachimori and Umanokamiyama Formations. Therefore it is concluded that both kerogen and silica mineral show typical diagenetic changes in the Miocene sediments. Logarism of ¹H-NMR T ₁ value of kerogen reversely correlates to d (101) spacing of cristobalite (coefficient of correlation=-0.89) except for a few samples which show extremely low ¹H-NMR T₁ value of kerogen with small d (101) spacing of cristobalite.