秋吉石灰岩層群 Profusulinella beppensis 帯以下の有孔虫化石帯

Foraminiferal Zones Underlying the Profusulinella beppensis Zone of the Akiyoshi Limestone Group

OKIMURA Yuji

廣島大學地學研究報告

Geological report of the Hiroshima University

The Carboniferous formations widely distribute on the southern slope of Minamiyama and Daiyama of Akiyoshi Plateau, Southwest Japan, where the type localities of the Millerella sp. a Zone of Toriyama, and the Endothyra Zone of Murata were recognized. They show a general trend of NEE strike and following 5 sedimentary facies can be recognized probably in ascending order.
1) Sandstone and chert facies
Fault?
2) Schalstein facies with chert and limestone lenses
3) Oolitic limestone facies with intercalated thin chert
4) Algal limestone facies with small limestone pebbles in its lower part
5) Massive limestone facies (chiefly fine calcarenite)
The strata of the lower three facies indicate the south dip while the dip and strike of the upper two ones are uncertain. Throughout the faunal arrangements of 120 localities on 6 routes in this area, it was rocognized that the characteristic variation of foraminiferal faunas exceedingly matches that of rock facies. And following five fossil zones can be distinguished by each peculiarity of their foraminiferal faunas, probably in ascneding order.
a) Endothyra sp. A zone
b) Pseudoendothyra spiroides zone
c) Millerella sp. A zone
d) Pseudostaffella sp. A zone
e) Profusulinella beppensis zone
The above-stated faunal zones can be established by the comparable thin section study of the foraminiferal faunas mainly composed of Endothyroids, Palaeotextula rids, Tuberitinids, Archaediscids and primitive Fusulinids. It is interesting to have confirmed the Endothyra sp. A zone, Pseudoendo-
thyra spiroides zone both without Millerella, and the Pseudostaffella sp. A zone with similar foraminiferal fauna to that of the Millerella zone without any form of Profusulinella. But the Pseudostaffella sp. A zone may be included in the Millerella zone or Profusulinella zone by a detail study in future.
The above-stated five fossil zones in this area can 'safely be correlated to the five foraminiferal zones of the Mitsudo group of the Atetsu limestone plateau, Okayama Prefecture, as follows;
　　　Akiyoshi　area 　　　 Atetsu area
a. Endothyra sp. A zone -------------------- Plectogyra communis zone
b. Pseudoendothyra spiroides zone ----- Endothyra spiroides zone
c. Millerella sp. A zone---------------------- Endothyra symmetrica zone
d. Pseudostaffella sp. A zone-------------- Atetsuella meandera zone
e. Profusulinella beppensis zone---------- Profusulinella zone
Foraminiferal fauna of the Pseudostaffella sp. A zone is comparable to that of the Atetsuella mean-dera zone except Profusulinella and Atetsuella. The above-mentioned five fossil zones of this area probably range from the upper Lower to the lower Middle Carboniferous in age.
Palaeontological study of endothyroid Foraminifera has been complicated in confusion according to the two different original definition of genus "Endothyra", but recently it has made great strides and Endothyridae has been divided into 5 subfamilies, 19 genera. As a result of the study progressed, zone fossil names of the Atetsu limestone are unavoidably retouched as follows; Plecto-gyra communis = Endothyra sp. A, Plectogyra primaeval = Endothyra sp., and Endothyra = Pseudoendothyra.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676