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Diagenesis and distribution of sterenes in Late Miocene to Pliocene marine siliceous rocks from Horonobe (Hokkaido, Japan)

Amo, Miki*; Suzuki, Noriyuki*; Shinoda, Toshio*; Ratnayake, N. P.*; Takahashi, Kazuharu

Late Miocene to Pliocene marine siliceous rocks in Horonobe, Hokkaido, Japan are rich in sterenes and steradienes. Diagenetic degradation and transformation of sterenes and steradienes in immature Horonobe marine siliceous rocks of Wakkanai and Koetoi Formations were studied using the borehole core samples from HDB-3 and HDB-4 drilled by Japan Atomic Energy Agency. The paleo-temperatures of marine siliceous rocks were estimated to be in the range from 30 to 60 $$^{circ}$$C considering the silica mineral transformation from Opal-A to Opal-CT during diagenesis and the present day geothermal gradient in Horonobe region. Concentrations of steradienes and sterenes decrease quickly in the temperature range from 45 to 60 $$^{circ}$$C. The significant increase of sterane concentration in the same temperature range shows that a part of steranes is derived from sterenes and steradienes. The sterane/sterene ratios of C$$_{27}$$, C$$_{28}$$, and C$$_{29}$$ compounds change similarly with increasing maturity. The sterane/sterene ratio of Horonobe marine siliceous rocks can be a useful indicator of paleo-temperature. Either sterenes or steranes, the relative abundance of C$$_{27}$$, C$$_{28}$$, and C$$_{29}$$ compounds is kept nearly similar all through the Horonobe boreholes, showing that their relative abundance is still applicable to the characterization of organic matter in immature sediments.

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Category:Geochemistry & Geophysics

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