埼玉県立自然史博物館研究報告 第15号 (1997)
入間市牛沢町の仏子層(下部更新統)産の
貝化石群集および生痕化石と古環境
小 幡 喜 一
埼玉県立自然史博物館． 369-1305埼玉県秩父郡長瀞町長瀞1417-1
要 旨 関東平野西縁丘陵の入間市牛沢町に露出する下部更新統上総層群の仏子層を調査し，貝化石の産出層準を検討した．そして，およそ1，000個の貝化石を採集し，巻貝3種，二枚貝20種を確認した．
この貝化石群集について， HDM特性曲線， VDM特性曲線および底質の特性曲線を描いた．これらによって本層準は，湾奥部の砂泥質干潟の堆積物であり，周囲には砂底の浅海が広がり，一部に岩礁も存在したと推定される．また，気候は現在よりもわずかに暖かで，湾は東に開いていたと考えられる．また，貝化石層の直上より生痕化石Psilonichnus isp.が産出した．これも潮間帯付近の堆積環境を示唆する．
キーワード: 関東平野西縁丘陵，仏子層，下部更新統，貝化石，生痕化石，古環境．
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図版4の説明
1 Cerithideopsis djadjarensis (K.Martin) 外形雌型
2 Batillalia cf. zonalis (Bruguiere) 同上
3 Neptunea (Barbitonia) sp. 外形雄型
4 Scapharca cf. subcrenata (Lischke) 外形雌型
5 Tegillarca granosa (Linnaeus) 外形雄型
6 Septlfer (Mytilisepta) cf. keenae Nomura 同上
7 Solamen spectabilis (A. Adams) 同上
8 Anomia sp. 同上
9 Crassostrea gigas (Thunberg) 同上
10 Ostea circumpicta Pilsbry 同上
11 Lucinoma sp. 外形雌型
12 Crcladicama sp. 外形雄型
13 Diplodonta gouldi Yokoyama 同上
14 Merisca (Pistris) subtruncata (Hanley) 同上
15 Fabulina cf. peitahoensis (Grabau & King) 同上
16 Macoma (Macoma) incongrua (Martens) 同上
17 Trlapezium (Neotrapezium) liratum (Reeve) 同上
18 Corbicula (Corbicula) cf. japonica Prime 同上
19 Dosinia (Phacosoma) japonicus (Reeve) 同上
20 Ruditapes philippinarum (Adams & Reeve) 同上
21 Paphia? sp. 同上
22 Cyclina sinensis (Gmelin) 同上
23 Thracia? sp. 同上
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Molluscan Fossil Assemblage and the Trace Fossil, Paleoenvironments of the Early Pleistocene
Bushi Formation at Ushizawa-cho, Iruma City in Saitama Prefecture, Japan
OBATA, Kiichi
Saitama Museum of Natural History, Nagatoro, Chichibu-gun, Saitama 369-1305
Abstract
The writer studied the fossil occurrence on the stratigraphical succession of the Early Pleistocene
Bushi Formation exposed along the drainage of the River Iruma-gawa in the district of Ushizawa-cho, Iruma City in Saitama Prefecture.
He has collected about one thousand specimens of themolluscan fossilsfrom the former formation. Hecould discreminate 3 species of gastropoda and 20 species of bivalvia in them. Considering from the HDM and VDM curves made from the molluscan assemblage, he assumed the palaeoenvironment of them as follows.
These fauna hadbeen living on the sandy mud beach at ebb tide along the interrior part of abay openning to the east, where the shallow sea with sand bottom extended, and the shore reefs might probably existed insome laces of the Sea. The climate of that time was deemed to be slightly warmer than that of the Recent.
He detected Psilonichnus isp., which has been considered to indicate littoral zone.