Carboniferous reef succession of the Panthalassan Open-ocean setting: example from Omi Limestone, Central Japan

Authors
Citation
T. Nakazawa, Carboniferous reef succession of the Panthalassan Open-ocean setting: example from Omi Limestone, Central Japan, FACIES, 44, 2001, pp. 183-210
Citations number
53
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
FACIES
ISSN journal
0172-9179 → ACNP
Volume
44
Year of publication
2001
Pages
183 - 210
Database
ISI
SICI code
0172-9179(2001)44:<183:CRSOTP>2.0.ZU;2-H
Abstract
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.