Tsutomu Nakazawa

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

SummaryThe Carboniferous-Permian (Visean-Midian) Omi Limestone in the Akiyoshi Terrane, central Japan is a large carbonate unit developed on a seamount in the Panthalassa Ocean. As the seamount subsided during Carboniferous and Permian time, the carbonate deposition at the top of a seamount was almost continous. Terrigenous siliciclastic sediments are absent, because the seamount was situated in an open-ocean setting. The lower part of this seamount-type limestone records a nearly continuous Carboniferous reef succession.Sedimentary facies in the Carboniferous part of the Omi Limestone are generally highly diverse, but their diversity varies in each age. The Upper Carboniferous part consists of highly diversified facies including fore reef, reef 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 coralbryozoan-crinoid community, problematic skeletal organism-microencruster community, chaetetid-microencruster community, and calcareous algal community. The first phase, comprising the coral-bryozoan-crinoid community, occurs in theEndothyra spp. Zone to theEostaffella kanmerai Zone (Visean to Serpukhovian). This community acted only as sediment-bafflers and/or contributors. The second phase, represented by the problematic skeletal organism-microencruster community, is developed in theMillerella sp. Zone to theAkiyoshiella ozawai Zone (Bashkirian to lowermost Moscovian), and the third phase, comprising the chaetetid-microencruster community, occurs in the overlyingFusulinella biconica Zone (Lower Moscovian). These two communities are characterized by highly diversified reef-building organisms that had the ability to build rigid frameworks. Calcareous algae and incertae sedis such asHikorocodium, solenoporaceans and phylloid algae characterize the fourth phase, which occurs in theBeedeina sp. Zone (Upper Moscovian). The changes of the reef communities were sucessive for a long period of more than 40 m.y., and each community was distributed in various environments. In addition, the continuous subsidence of the isolated seamount resulted in environmental stability. These properties indicate that this succession represents the biotic evolution of reef-building organisms.The problematic skeletal organism-microencruster community and chaetetid-microencruster 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 sedimentary facies comparable to those of a modern reef complex. Such reefs are also recognized in the seamount-type Akiyoshi Limestone, but rare on Carboniferous 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 exposed to relatively strong openocean waves and swells, and probably more environmental stability resulting from the relatively continuous subsidence of the seamount.

Bacterial and Archaeal 16S rRNA Genes in Late Pleistocene to Holocene Muddy Sediments from the Kanto Plain of Japan

Microbial communities in ancient marine sediments composed of clay and silt obtained from the terrestrial subsurface were phylogenetically analyzed based on their 16S rRNA gene sequences. Chloroflexi and Miscellaneous Crenarchaeotic Group were predominant in bacterial and archaeal clone libraries, respectively. Of 44 operational taxonomic units (OTUs) that had close relatives in the database, 30 were close to sequences obtained from marine environments. Some sequences belonged to the candidate groups JS1, ANME-I, and Marine Benthic Group-C, which are typically found in marine sediments. Low chloride concentrations in the sediments suggest that these marine-affiliated sequences may not reflect currently active microbial communities. Our results indicate the existence of long-term preserved DNA or descendants of ancient oceanic microbial components in subsurface muddy sediments in a temperate region, which may reflect indigenous population of paleoenvironments.

Pennsylvanian–Early Permian cyclothemic succession on the Yangtze Carbonate Platform, South China

Abstract Pennsylvanian (Late Carboniferous)–Early Permian cyclothems are documented for the first time from the shallow-marine carbonate succession on the Yangtze Carbonate Platform, South China. The Zongdi section in southern Guizhou Province comprises bioclast-rich grey limestones with dark-coloured intervals yielding macro- and microfabrics related to emergent events. We recognized 26 depositional sequences (consisting essentially of basal peritidal sediments, middle shallow subtidal sediments and upper peritidal sediments affected by meteoric diagenesis and subaerial exposure) separated by 25 lowstand events. These are interpreted as having been formed by alternating marine transgression and regression resulting from variations in the Gondwanan ice sheets. They are compared with the Midcontinent (Kansas-type) cyclothems in North America. However, the Zongdi cyclothems are relatively thin and consist entirely of very shallow-marine sediments formed above wave base, whereas the Midcontinent cyclothems reflect deposition far below wave base. The Zongdi depositional environment was an upper shelf area, on a platform within the epeiric sea on the South China Craton, where sedimentation occurred in a relatively small (shallow) accommodation space during the highest stands of sea level. They were formed by major (large-scale) transgressions that submerged high-shelf areas, and most of them correspond to the major cyclothems of the North American Midcontinent Basin. The fusuline biostratigraphical framework reveals an uneven stratigraphical pattern of the Zongdi cyclothems, reflecting subtle fluctuations of tectonic subsidence on the epeiric Yangtze Carbonate Platform. Less erosion and karstification on the tops of cyclothems in the Zongdi section, together with the presence of partly calcareous palaeosols, indicates that a dry climate prevailed on the South China Block during the Pennsylvanian–Early Permian when the Zongdi cyclothems were formed.

Middle Permian sponge–microencruster reefal facies in the mid-Panthalassan Akiyoshi atoll carbonates: observations on a limestone slab

A Murgabian (Middle Permian) reef-core facies in the Akiyoshi Limestone, southwest Japan, which is of mid-Panthalassan atoll origin, is here described from a limestone slab. The reef-core facies is composed mainly of boundstone with submarine cement and matrix. The reef-building community is characterized by a high-diversity biota, including sponges (sphinctozoans, inozoans, and chaetetids), bryozoans (fistuliporids and cryptostomates), crinoids and various microencrusters (Tubiphytes, Archaeolithoporella, reticular and laminar microbialites, encrusting foraminifers and problematic laminar red algae). Among them, sphinctozoan sponges and microencrusters such as Tubiphytes and microbialites are the most abundant. This community is similar to a time-equivalent lagoonal mound (patch-reef) biota on the Akiyoshi atoll in terms of the dominant sponges, Tubiphytes and microbialites, but differs in the additional occurrence of bryozoans, Archaeolithoporella and encrusting foraminifers, which probably preferred higher-energy conditions in the reef-core environment. Among temporally changing reef-building communities on the Carboniferous–Permian Akiyoshi atolls, the sponge-dominated reefal community described here flourished on mid-Panthalassan atolls in a relatively stable warm-water environment during a Middle Permian post-deglacial period.

Temporal changes in biotic and abiotic composition of shallow-water carbonates on submerged seamounts in the northwestern Pacific Ocean and their controlling factors

ABSTRACT The lithology of Cretaceous to Pleistocene shallow-water carbonates, which were collected from 29 sites on 24 submerged seamounts in the northwestern Pacific Ocean using the Deep-sea Boring Machine System, are described. The shallow-water carbonate deposits examined in the present study can be roughly divided into three types based on their composition: Cretaceous, Eocene (to lowest Oligocene?), and Oligocene to Pleistocene. The Cretaceous type is characterized by an abundance of molluscs (including rudists), smaller foraminifers, microencrusters, non-skeletal grains (e.g., peloids, cortoids, and intraclasts), and microbial sediments. Most components have been micritized and possess thick micrite envelopes. The Eocene type is characterized by the dominance of larger foraminifers, Halimeda spp., nongeniculate and geniculate coralline algae, bryozoans, and dasycladacean algae. Scleractinian corals are very minor components. The Oligocene to Pleistocene type is similar in composition to the Eocene type, but it differs from the latter by the abundant occurrence of scleractinian corals and nongeniculate coralline algae. Corals, nongeniculate coralline algae, and Halimeda spp., which precipitate carbonates within closed to semi-closed spaces in and around their bodies (intra-tissue), are major components of the Eocene and Oligocene to Pleistocene types. In contrast, the Cretaceous-type sediments contain relatively more carbonates of extra-tissue origin (i.e. carbonates deposited in relatively open spaces around the bodies of organisms, such as rudists, as well as microbialite and ooids) than the Eocene and Oligocene to Pleistocene types. The changes in the major constituents of the carbonate factory depend on local environments, such as nutrient availability, as well as a global factor: seawater chemistry in the surface waters. Temporal variations in the abundance of the shallow-water carbonates on the examined seamounts suggest that carbonate accumulation was not necessarily controlled by climatic conditions; instead, it was related to the volcanism and tectonics that served as the foundations for reef/carbonate-platform formation.

Simple assessment of shallow velocity structures with small-scale microtremor arrays: interval-averaged S-wave velocities

This article describes a method for processing microtremor records from a small-scale seismic array that allows interval-averaged S-wave velocities to be estimated for 10-m depth ranges down to a depth of 30 m. The method was applied to microtremor data obtained in the town of Mashiki, Kumamoto Prefecture, Japan, and the analysis results were evaluated through a comparison with available PS logs and sections obtained by surface-wave methods. It turned out that the interval-averaged S-wave velocity estimates may be subject to errors of up to 20–30% in absolute values, but it was shown that the method can help evaluate relative spatial variations in those S-wave velocities. In view of the simplicity of analysis, the analyser-independent nature of the results and the limitations of analysis accuracy, the interval-averaged S-wave velocity estimation method presented here could be used as an effective tool for the preliminary analysis of microtremor data from small-scale seismic arrays. This article describes a simple method for estimating interval-averaged S-wave velocities for 10-m depth ranges down to a depth of 30 m. Possibilities and limitations of this method are examined by using the analysis results of the microtremor data obtained in the town of Mashiki, Kumamoto Prefecture, Japan.