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

Late Miocene to Pliocene marine siliceous rocks from Horonobe (Hokkaido, Japan) are rich in sterenes and steradienes. Diagenetic degradation and transformation of these components in sediments from the Wakkanai and Koetoi Formations were studied using core samples from boreholes HDB-3 and HDB-4 drilled by the Japan Atomic Energy Agency. The palaeotemperatures of the rocks were estimated to be in the range 30–60 °C on the basis of the transformation of Opal A to Opal CT and the present day geothermal gradient in the Horonobe region. Concentrations of steradienes and sterenes decrease rapidly in the range 45–60 °C. The significant increase in sterane concentration in the same temperature range shows that part of the steranes is derived from sterenes and steradienes. The sterane/sterene ratios of the C27, C28, and C29 components change similarly with increasing maturity, indicating that the ratio can be a useful indicator of palaeotemperature in the early diagenetic stage. For sterenes and steranes, the relative abundance of the C27, C28, and C29 components is quite similar through both boreholes, showing that it is not significantly affected by the diagenetic transformation of sterenes.