中国地方下部石炭系の微化石層序学的研究

Microbiostratigraphical Studies on the Foraminiferal Faunas of the Lower Carboniferous Formations of the Chûgoku Region, Southwest Japan

沖村 雄二

廣島大學地學研究報告

Geological report of the Hiroshima University

In the present paper, the Carboniferous of the Chûgoku region, Southwest Japan, especially in the Atetsu, Taishaku and Akiyoshi limestone areas, are microbiostratigraphically studied regarding to the foraminiferal zones of them. These limestones are lithologically and biostratigraphically subdivided into two parts of the lower and the upper. The lower consists mainly of well bedded limestone with schalstein and chert, and contains a lot of such "smaller Foraminifera" as endothyrids, palaeotextularids, archaediscids, tuberitinids, tetrataxids, etc. which take 60-90% of all foraminiferal specimens recognized in thin sections. The upper part consists of the unstratified limestone overlying disconformably the lower, and is characterized with the primitive fusulina-ceans as Millerella spp., Eostafella ikensis, Ozawainella sp., Pseudoendothyra spp., and Pseudostaffella antiqua.
Thus the lower facies of smaller Foraminifera and the upper one of primitive fusulinids are dis-criminated, and it is plain enough that the former indicates the faunal characters of the Viséan and the latter that of the Bashkirian to Moscovian in age.
On the basis of the faunal characters and the most rich species of Foraminifera, the strata of the lower facies are subdivided into three zones and also those of the upper can be separable into four zones in descending order as follows:
7. Fusulinella imamurai-F. biconica (Fusulinella-Fusulina) zone
6. Profusulinella toriyamai - P. beppensis zone
5. Pseudostafella antiqua zone
4. Eostafella sp. A - Millerella sp. A zone
Disconformity
3. Mediocris sp. A - M. mediocris zone
2. Endostafella delicata zone
1. Endothyra sp. A zone
The correlation among these zones of the Atetsu, Taishaku and Akiyoshi districts and their geologic ages are tentatively shown on the Table 2.
The zonal species of the lowest zone, Endothyra sp. A is similar to E. cara SCHLYKOVA from the Oka substage of the eastern Jugoslavia.
The foraminiferal fauna of the Endostafella delicata zone is composed of such endothyroids as Endothyra omphalota samarica RAUSER-CHERNOUSSOVA, E. similis RAUSER-CHERNOUSSOVA, E. pauciseptata RAUSER-CHERNOUSSOVA, E. masanae (OKIMURA), Endostafella delicata ROSOVSKAYA, Mediocris breviscula (GANELINA), Palaeotextularia vulgaris (REITLINGER), P. consobrina LIPINA and some others, and it is referable to the Middle Viséan in age.
The characteristic faunal elements of the Foraminifera listed below are common to both of the lower subzone of the Mediocris mediocris zone of Akiyoshi and the Onimaru formation, the latter of which is the type of the upper Lower Carboniferous of Japan; Endothyranopsis hirosei OKIMURA, E. compressa RAUSER and REITLINGER, E. spp., Mediocris mediocris (VISSARIONOVA), Endostafella parva (MÖLLER), Palaeotextularia spp., and primitive Climacammina. They are the important species of the foraminifetal fauna from the lower Serpukhova substage (Upper Viséan) of USSR.
On the other hand, the foraminifcral assemblage from the lower part of the Nagaiwa formation listed below closely resembles that from the Eostafella sp. A - Millerella sp. A - zone and the Pseudostafella antiqua zone of the Chûgoku region; Eostafella ikensis VISSARIONOVA, Globivalvulina spp., Millerella sp., Planoendothyra spp., Ozawainella cf. aurora SCHLYKOVA, Climacammina antiqua (BRADY) and a few others.
Consequently the upper part of the Mediocris mediocris zone may be referable to the hiatus between the Onimaruan and the Nagaiwan in age, and also to the uppermost Lower Carboniferous, Namurian Protova horizon of USSR.
The writer would like to propose the new stage name of Ohkuboan for the upper part of the Mediocris mediocris zone in the Chûgoku region.

日本語

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676