T. Nakazawa, Carboniferous reef succession of the Panthalassan Open-ocean setting: example from Omi Limestone, Central Japan, FACIES, 44, 2001, pp. 183-210
The Carboniferous-Permian (Visean-Midian) Omi Limestone in the Akiyoshi Ter
rane, central Japan is a large carbonate unit developed on a seamount in th
e Panthalassa Ocean. As the seamount subsided during Carboniferous and Perm
ian time, the carbonate deposition at the top of a seamount was almost cont
inuous. Terrigenous siliciclastic sediments are absent, because the seamoun
t was situated in an open-ocean setting. The lower part of this seamount-ty
pe limestone records a nearly continuous Carboniferous reef succession.
Sedimentary facies in the Carboniferous part of the Omi Limestone are gener
ally highly diverse, but their diversity varies in each age. The Upper Carb
oniferous part consists of highly diversified facies including fore reef, r
eef front, reef crest, sand shoal, and lagoon facies, while a simple facies
assemblage, composed only of fore reef, reef front, and sand shoal facies,
occurs in the Lower Carboniferous.
The Carboniferous reef succession consists of four phases characterized, in
ascending order, by the coral-bryozoan-crinoid community, problematic skel
etal organism-microencruster community, chaetetid-microencruster community,
and calcareous algal community. The first phase, comprising the coral-bryo
zoan-crinoid community, occurs in the Endothyra spp. Zone to the Eostaffell
a kanmerai Zone (Visean to Serpukhovian). This community acted only as sedi
ment-bafflers and/or contributors. The second phase, represented by the pro
blematic skeletal organism-microencruster community, is developed in the Mi
llerella sp. Zone to the Akiyoshiella ozawai Zone (Bashkirian to lowermost
Moscovian), and the third phase, comprising the chaetetid-microencruster co
mmunity, occurs in the overlying Fusulinella biconica Zone (Lower Moscovian
). These two communities are characterized by highly diversified reef-build
ing organisms that had the ability to build rigid frameworks. Calcareous al
gae and incertae sedis such as Hikorocodium, solenoporaceans and phylloid a
lgae characterize the fourth phase, which occurs in the Beedeina sp. Zone (
Upper Moscovian). The changes of the reef communities were successive for a
long period of more than 40 m.y., and each community was distributed in va
rious environments. In addition, the continuous subsidence of the isolated
seamount resulted in environmental stability. These properties indicate tha
t this succession represents the biotic evolution of reef-building organism
s.
The problematic skeletal organism-microencruster community and chaetetid-mi
croencruster community of the Late Carboniferous formed wave-resistant and
rigid frameworks along with abundant submarine cements. The growth of these
reef frameworks resulted in the formation of highly diversified sedimentar
y facies comparable to those of a modern reef complex. Such reefs are also
recognized in the seamount-type Akiyoshi Limestone, but rare on Carbonifero
us Pangean shelves. Therefore, the formation of these types of reefs appear
to be characteristic of open-ocean seamount settings, which differed from
epicontinental shelf settings in having no siliciclastic input, being expos
ed to relatively strong open-ocean waves and swells, and probably more envi
ronmental stability resulting from the relatively continuous subsidence of
the seamount.