要　旨　埼玉県秩父盆地東部,大野原周辺の荒川右岸および横瀬川両岸に露出する秩父盆地層群(中新統)を調査し,岩相によりA〜Gの7部層に区分した.そして, 5層準から12種の生痕化石を発見した.これらの生痕化石を記載し,それらの古環境を推定した.
　A部層とC部層には生痕化石が認められず, B部層上半部の成層細粒砂岩やE部層の砂質泥岩からは,Schaubcylindrichnus freyiだけが発見された.いっぽう, D部層, F部層の泥岩砂岩互層やG部層の礫岩砂岩泥岩互層からは,多様な生痕種が産出した.
　B部層下半部の現状細粒砂岩にふくまれるRosselia socialisの密集層は海進期の堆積物をしめし, F部層の 泥岩砂岩互層にふくまれるArenicolites isp.やSkolithos linearisは,急速な埋積を示す日和見生痕種である.
　また,調査地の地層は全体として上方粗粒化が認められる.
　これらのことから,今回調査した地層は海進期から高海水準期にかけて,沿岸域に急速に堆積したものと推定される.

キーワード:生痕化石,日和見生痕種,堆積環境,中新統,秩父盆地層群

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The Trace Fossils from the Miocene Chichibu Bonchi Group in the District of Onohara, Chichibu City, Saitama Pref., Central Japan, and Discussion on the Paleoenvironment of Them

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

Abstract
　　The Miocene Chichibu Bonchi Group has been surveyed by the writer along the right bank of the River Ara- kawa and either bank of the River Yokoze-gawa in the district of Onohara, in the eastern part of Chichibu basin in Saitama Pref., central Japan. As a result of the survey, the group was divided into 7 members of A to G by their lithological facies, and the following 12 ichnospecies of trace fossils were found from different 5 horizons of them.
　　Arenicolites isp., Beaconites capronus, Chondrites isp, Paleophycus tubuaris, Psilonichnus aff. upsilon, Rosselia socialis, Schaubcylindrichnus freyi, Skolithos linearis, Skolithos cf. linearis, Teredolites isp., Thalassinoides suevicus, Thallassinoides? isp.
　　The above ichnofossils were described, and the paleoenvironment of them was discussed.
　　The members of A and C yields no trace fossil. Only one of Schaubcylindrichnus freyi occurred in the layered fine sandstone of the upper part of the member B and in the sandy mudstone of the member E. On the contrary, various ichnofossils were found from the the alternated beds of mudstone and sandstone of the members of D and F, and from the alternated beds of conglomerate, sandstone and mudstone of the member G.
　　The layer of dense occurrence of Rosselia socialis in the massive fine sandstone of the lower part of the member B is presumed to be the product of the transgressive stage. And Arenicolites isp. and Skolithos linearis found from the alternated beds of mudstone and sandstone of the member F are opportunist trace fossils, showing the rapid sedimentation.
　　Their lithological facies become coarser upward in succession as a whole.
　　Considering the facts above, it is presumed that sediments treated in this paper might be deposited rapidly in the sea near coast, during the term from the transgressive stage to the one of high sea-level.