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Geochemical studies of the Miocene Onnagawa Formation of northern Japan and the Monterey Formation of California are reported.Geochemical parameters indicative of organic source are similar between the Onnagawa and Monterey Formations, suggesting marine plankton is the major source of organic matter.The degree of preservation at depositional site is a major factor influencing quality and quantity of organic matter.The total organic carbon (TOC) considerably varies among samples collected at millimeter-to centimeter-scale stratigraphic intervals, and TOC shows a distinct positive correlation with detritus content. This close relationship between TOC and detritus content suggests that the organic matter could be largely adsorbed to the surface of detritus particles which might have formed aggregates with a greater resistance to bacterial degradation.The detritus contents varied in centimeter-to decimeter-scale and control organic richness. Biomarker data indicate the Monterey Formation was deposited under more reducing condition than the Onnagawa Formation.This intensive anoxic condition could contribute to the higher contents of organic carbon and sulfur in the Monterey Formation compared to the Onnagawa Formation.

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