Siderite nodules formed in the oxygen-poor marine environments were obtained from the Middle and Upper Yezo Groups (Upper Cretaceous), Hokkaido, Japan. The siderite nodule is resulted from follwoing reaction:
Fe⁺⁺+2HCO₃^-=FeCO₃+CO₂+H₂O.
The observed siderite nodules occurred from dark gray, massive mudstone. The depositional environment of the mudstone is interpreted as quiet offshore facies. Compared with that of sediments deposited under higher oxygenated environments in the studies area, the dark gray mudstone is characterized by low diversity of benthic fauna, occurrence of siderite nodule and higher sulfur/carbon contents. The siderite nodules without pyrite suggest the sulfate-reduction by anaerobic bacterial decomposition of organic matter would not have been taken place. In addition to the paleoecological evidences, these facts also indicate post-oxic, nonsulfidic environments.
The nodules contain autochthonous low-oxygen tolerant inoceramids. For example, Inoceramus amakusensis is one of the low-oxygen tolerant inoceramids that would live on the soft muddy bottom with stagnant bottom water.
The fluctuation of post-oxic nonsulfidic and slightly sulfidic environments resulted in the two different mode of occurrences of inoceramids; concretion overgrowth around inoceramids during post-oxic nonsulfidic environments and concretion-free inoceramids in the mudstone during slightly sulfidic environment.