栃木県塩原層群(更新統)の層序と珪藻質葉埋泥岩のタフォノミー <Bull. Natn. Sci. Mus., Tokyo>Stratigraphy and Taphonomic Features of Diatomaceous Shale of the Pleistocene Shiobara Group, in Tochigi, Japan
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The Shiobara Group, the Pleistocene lacustrine deposits, is distributed at the northern slope of the Quaternary Takahara Volcano in Tochigi Prefecture. It is subdivided into two formations, the Kamishiobara and Miyajima formations redefined here. The Kamishiobara Formation consists of gravel and sand beds of talus, debris apron, alluvial fan/fan-delta and rock fall facies, and corresponds to marginal facies of the paleo-Shiobara lake. The Miyajima Formation is composed of laminated diatomaceous mudstone, and represents a lacustrine profundal facies in the central part of lake. The southern coast was bordered by lava flow on gentle slope of the Takahara Volcano. These features imply a caldera lake. The laminated mudstone of the Miyajima Formation yields numberous well-preserved fossils such as leaves, insecta and fishes, which are flattened but retain details of soft tissue. Diatomaceous opal-A in the original laminated soft mudstone (opal-A zone) changes diagenetically into opal-CT in"hard shale" (opal-CT zone). Preservational states of plant megafossil are closely related to the diagenesis of opal. The primary organic structure is preserved in opal-A zone. However, a few fossil occurs in opal-A zone. The plant fossils from opal-CT zone are abundant and morphologically well-preserved, while organic materials are altered. Extraordinary well-preserved fossils occur from the opal-CT zone.