2021 Volume 86 Issue 5 Pages 384-405
Fine-grained clastic rocks of the Miocene Nanatani Stage（17.0-12.3Ma）in the northern Niigata and central Yamagata areas, northern Japan, record the transitional phase from the syn-rift to post-rift regimes related to backarc spreading in the Sea of Japan. They are characterized in places by biosiliceous lithology, often with a relatively high content of sedimentary organic matter. Observations of rock facies along with the results of the total organic carbon, RockEval, and X-ray fluorescence analyses of outcrop sections revealed that the organic matter content was primarily controlled by the enhanced primary production of photosynthetic algae in the northwestern Pacific since the Middle Miocene. This global-scale process was modified locally by regional tectonics in terms of the rate of clastic input and subsea topographic relief, which possibly served as a barrier to ocean circulation. This explains the tens of kilometerscale lateral variations in the source rock potential, as well as the time lags of the organic matter accumulation between the basins within the coastal region of the Sea of Japan.
