Chemistry of Late Early Triassic Siliceous Claystone (‘Toishi‐type’ Shale) from the Oritate Area, Sambosan Belt, Kyushu, Southwest Japan

Abstract

Abstract. Inorganic chemical compositions are determined for late Early Triassic siliceous claystone (‘Toishi‐type’ shale) and associated chert from Oritate area, Sambosan Belt, Kyushu, southwest Japan. The siliceous claystone is similar in chemical composition to Post‐Archean Australian Shale (PAAS), but is depleted in Ca, Sr, Mn, and Fe and slightly enriched in Cu, Zn, P, and rare earth elements (REEs). The siliceous claystone and associated chert have flat REE patterns with positive Eu anomalies and no distinct Ce anomaly. The siliceous claystone and associated chert have largely constant Ti/Al, Th/Al, and Nb/Al ratios compared to the variable ratios found in siliceous shale and sandstone deposited close to land in a trench setting. This suggests that the claystone and chert were deposited in a deep‐sea pelagic environment and were derived mainly from the suspended fraction, including eolian dust and material transported from distant lands, rather than from turbidity currents that occurred close to land. The low Ca contents of the analyzed rocks indicate deposition below the calcium‐carbonate compensation depth. The depletion of Mn and Fe, and no distinct Ce anomaly in the Oritate siliceous claystone are also evident, being similar to those in samples from the Sasayama and Kinkazan sections in the Mino‐Tanba Belt, where oceanic anoxia developed during the Late Permian to earliest Triassic. This might suggest that oceanic anoxia prevailed through to the late Early Triassic.