帝釈台とその周辺の古生層，とくに石灰岩層の堆積相について

The Upper Paleozoic Formations in and around Taishaku-dai, Chugoku Massif, Southwest Japan; with Special Reference to the Sedimentary Facies of Limestones

HASE Akira

OKIMURA Yuji

YOKOYAMA Tsuruo

廣島大學地學研究報告

Geological report of the Hiroshima University

It has well been known that in the Inner Zone of Southwest Japan there are several isolated large masses of the Carboniferous-Permian limestone, surrounded by contem-poraneous thick series of sandstone, shale and chert. An example is in and around Taishaku-dai, a part of the Kibi plateau, Central Chugoku.
Two distinctive facies, the central and the marginal, can be recognized in the limestones and the overlying clastic rocks of Taishaku-dai. The stratigraphy is as follows in ascending order.
Central facies
Dangyokei formation (Endothyra zone), 150 m. thick: Characterized by the predominance of basic volcanic and pyroclastic rocks, with some intercalation of limestone.
Eimyoji formation (Millerella, Profusulinella, Fusulinella and Fusulina zones), 150m. thick: Composed exclusively of massive limestone.
Uyamano formation (Pseudoschwagerina, Parafusulina, Neoschwagerina and Yabeina zones), 400～500 m. thick: Massive limestone. An unconformity is at the base, and a hiatus, of smaller scale is also inserted between the Parafusulina and Neoschwagerina zones.
Oshigatani formation (Yabeina zone), 100～300 m. thick: Massive black shale, containing lenses of limestone. An ill-sorted, heterogeneous conglomerate occurs locally.
Marginal facies
Lower Idaniyama formation (Endothyra zone), 200m. thick: Chert with intercalation of limestone and basic volcanic rocks.
Upper Idaniyama formation (Millerella, Profusulinella and Fusulinella zones), 200～250 m. thick: Chert and limestone repeated in various thickness, often showing a graded bedding as in the Lower Idaniyama.
Zenbutsujidani formation (Pseudoschwagerina and Parafusulina zones) and Arita formation (Neoschwagerina and Yabeina zones), 400～500 m. thick: Massive limestone with a small amount of chert. An unconformity is at the base of each formation.
Maedani formation (Yabeina zone), 1000 m.± thick: Sandstone and shale in graded alternation, succeeded by thick sandstone with a conglomerate at the top. Small lenses of limestone are included in places, and layers of basic volcanics and acid tuffite are also intercalated.
Notabiyama formation: Chert and the succeeding siliceous shale, with intercalation of acid tuffite.
The microscopic examination of limestones from more than 600 localities shows that the two facics have their respective suites of rock-types concerning the sedimentary petrographic features. The limestones of the central facies are mostly sparry; oo-sparrudite and -sparite are dominant in Carboniferous and biosparrudite (especially fusuline biosparruditc) in Permian. An autochthonous biolithite is also characteristic of this facies, though the occurrence is rather limited. Most of the Carboniferous limestones of the marginal facies are, on the contrary, micritic; bio-micrudite and -micrite rich in fragments of crinoid, bryozoa etc. are common. In the Permian marginal facies develops a calclithite, which consists of pebbles of contemporaneous limestones as well as those of older ones, cemented by biomicritic matrix.
Judging from the distribution of macro- and micro-facies mentioned above, it may be concluded that the central facies represents sediments on reef proper (probably like a table reef) and the marginal facies those on fore-reef slope to near-reef basin. They are combined to form an organic reef complex on the submarine volcanic mound. The facies-development and the sedimentary history arc diagrammatically shown on Plate V.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676