キーワード：生痕化石，堆積環境，中新統，秩父盆地層群

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Trace Fossils From the Miocene in the Chichibu Basin, Saitama Prefecture, Central Japan, and Some Discussion on Depositional Environments

Kiichi OBATA
Saitama Museum of Natural History. Nagatoro, Chichibu-gun, Saitama 369-1305

Abstract
　　It have been researched that a lower half of the Miocene Chichibu Basin Group exposing along the river Akahira-gawa, from Hisanaga, Yoshida-machi to Nagura, Ogano-machi, Saitama Pref., central Japan.
　　As a result of it, several ichnofossils were found from the various horizons of the former. These ichnofossils and lithofacies of their host rocks are described in this paper.
　　Ichnoassemblage of these horizons are different from each other, reflecting sedimentary environment of each horizon, namely, the one of the part of the Shirasu Formation with Psylonichnus isp. shows littoral to sublittoral zone; the one of the Nenokami Formation with Schaubcylindrichnus freyi shows shoreface to offshore; the one of the upper part of the Ogano-machi Formation with Helminthopsis isp. and Planolites isp. shows submarine fan of the bathyal zone; and the one in the lower half of the lower part of the Chichibu-machi Formation are divided into alternatively a lot of Rosselia socialis and Schaubcylindrichnus freyi, supposed to be a sediment of the upper shoreface to the outer shelf.
　　From the mode of occurrence of ichnofossil in the former last formation, it is considered that the horizon with R. socialis was the product of relatively rapid sedimentation, on the contrary the one with S. freyi reflected slow sedimentation.