Natsuko Adachi

Earliest Triassic Microbialite Micro- to Megastructures in the Huaying Area of Sichuan Province, South China: Implications for the Nature of Oceanic Conditions after the End-Permian Extinction

Abstract The end-Permian extinction reflects one of the greatest biotic crises in earth history. Evidence of the event and its aftermath is well preserved in Permian/Triassic transitional strata of Sichuan Province in South China, originally deposited in the western part of the Yangtze epicontinental sea. Lowest Triassic strata (the Hindeodus parvus conodont Zone) lie with apparent conformity on uppermost Permian skeletal packstone and wackestone, and are overlain by deeper subtidal thinly bedded argillaceous lime mudstone. These beds are characterized by thrombolitic mesostructures that exhibit macrostructures of highly variable columnar shapes, showing a distinctive stratigraphic succession in bed form from planar to domed and subspherical forms. Spheroidal and ellipsoidal micritic bodies (30 µm in average diameter) are preserved in clusters and are interpreted as calcified coccoidal microbes. The earliest Triassic thrombolitic columns and masses are considered to have been constructed by vertical and lateral accretion of mesoclots of microbial origin. Even after the end-Permian extinction, microbial formation of carbonates apparently predominated in Sichuan in localized subtidal to intertidal environments. Microbialites that formed immediately after the end-Permian extinction represent not only disaster-related forms in stressed environments, but also space- and time-specific, environmentally induced carbonates that may be related in part to the causal mechanism of extinction and delay of biotic recovery. The microbialite sequences in Sichuan Province, South China provide a glimpse of the pre-existing “background” and following “foreground” microbial world, and hence a window that was open temporarily in the post-extinction interval.

Geobiological Aspects of the Earliest Triassic Microbialites Along the Southern Periphery of the Tropical Yangtze Platform: Initiation and Cessation of a Microbial Regime

Abstract Microbialite masses within the lowest Triassic strata along the southern periphery of the tropical Yangtze Platform were produced in a post-Permian microbial regime. These microbialites (the Hindeodus parvus Zone), which are represented by thrombolites, occur only where terrigenous sediment influx was rare, even in shallow-marine settings. The lower parts of the thrombolites lie upon a distinctly unconformable Permian-Triassic boundary and exhibit a thin-bedded to thick-bedded planar structure. In contrast, the upper parts of the thrombolites contain domed macrostructures that interact in complex ways with skeletal grainstones and packstones. Irregular frameworks of thrombolite bodies differ in degree of lateral and vertical accumulation and in the amalgamation of mesoclots of microbial origin; they exhibit marked variations in texture. A transgressive episode occurred in the earliest Triassic following the mass extinctions, and this included the initiation of microbial regimes that usually formed planar thrombolite masses in lower-energy, deep subtidal environments. The varied textures and structures of thrombolites during deposition may reflect a combination of sea-level fluctuations, physicochemical ocean conditions, microbial activity, skeletal-sediment influx, and other factors. These earliest Triassic, uniquely microbial regimes collapsed in stepwise fashion and were succeeded by the Isarcicella staeschei and I. isarcica zones, which contain a predominance of mudstones, suggesting a marked sea-level transgression. Space-specific and time-specific, the earliest Triassic microbialites record short-term, high-resolution paleoenvironmental fluctuations immediately after the end-Permian extinctions.

Early Ordovician shift in reef construction from microbial to metazoan reefs

Abstract The Ordovician is a period when novel reef ecosystems appeared along with new reef constructors and skeletal-dominated reefs. The Lower Ordovician (late Tremadocian) Fenhsiang Formation of the Three Gorges area in South China contains the oldest known bryozoan reefs (lithistid sponge–bryozoan and bryozoan–pelmatozoan reefs) alongside lithistid sponge–microbial reefs. The latter are characterized by the dominance of microbialites that encrusted and bound the frame-building sponges and inter-sponge sediments. In contrast, the lithistid sponge–bryozoan and bryozoan–pelmatozoan reefs are generally characterized by bryozoans that encrusted the frame-building sponges or pelmatozoans and grew to fill the inter-frameworks. These sponges and pelmatozoans did not construct the rigid frameworks unaided; their association with bryozoans enabled the development of small skeletal-dominated reefs with rigid frameworks. Skeletal-dominated reefs, for which frame-constructing and encrusting roles are conspicuous, were largely unknown before the Early Ordovician. The appearance of skeletal organisms (specifically colonial, encrusting bryozoans) enabled the development of skeletal-dominated reefs, which were pioneers in the rise of Middle–Late Ordovician reefs. The Early Ordovician establishment of skeletal-dominated reefs at the earliest stages of the Great Ordovician Biodiversification Event would have created novel niches and biological interactions that further promoted the evolution of reef-building and -dwelling organisms, as well as ensuing reef ecosystems.

Lower Triassic stromatolites in Luodian County, Guizhou Province, South China: evidence for the protracted devastation of the marine environments

Stromatolites are one of the oldest and most intriguing organosedimentary deposits. In contrast to stromatolites of the Precambrian to Early Ordovician, Phanerozoic equivalents occurred episodically under specific conditions. A group of previously undescribed stromatolites in composition occur in the Lower Triassic (Olenekian) at the Dajiang section in the Luodian region of Guizhou Province, South China. We described the textures of these stromatolites with the aim of determining the genetic mechanisms and revealing the nature of interactions between micro-organisms and marine environments. Mesoscopic features show that the stromatolites consist of several sets of stacked slices, and that they are embedded in alternating beds of fine and coarse microsphere packstones that include aggregates of microspheres, forming grapestones and lumps. Microscopically, the stromatolites consist of spar- and dolomite-infilled microspheres (average diameter, 100 μm), micrites, peloids, small-sized pyrite framboids (average diameter, 5.8 μm) and fenestrae. Micrite-dominant intercalations accentuate laminated textures at a mesoscopic level and are laterally continuous with micrite-rich parts in surrounding interstromatolites, indicating the simultaneous, widespread deposition of these layers. The microspheres and associated micrites were the products of in situ microbial activity, probably sulphate-reducing or anoxygenic phototrophic bacteria, which led to the formation of these unusual stromatolites. Even during a protracted period of harsh marine conditions, the micrite-rich carpets were deposited intermittently on the stromatolites and their surroundings under severely anoxic/sulphidic conditions. The presence of Early Triassic stromatolites and their subtle but important vertical variations in texture provide a record of temporal changes in marine conditions during geobiologically critical intervals.

Erratum to: Early Ordovician reefs in South China (Chenjiahe section, Hubei Province): deciphering the early evolution of skeletal-dominated reefs

The Lower Ordovician (late Tremadocian–early Floian) Fenhsiang and the overlying Hunghuayuan Formations at the Chenjiahe section in the Three Gorges area of Hubei Province, South China, include four types of reef: microbe-dominated (lithistid sponge–stromatolite and lithistid sponge–calcimicrobial) reefs, and skeletal-dominated (lithistid sponge–bryozoan and bryozoan–pelmatozoan) reefs. The microbe-dominated reefs are characterized by the dominance of microbial sediments that encrusted and bound the surfaces of sponges to reinforce the reef frameworks. In contrast, the skeletal-dominated reefs are distinguished by bryozoans that encrusted frame-building sponges and pelmatozoans, and that grew downward to fill the open spaces available within the frameworks. A series of these reefs shows a temporal succession in reef type, with a decline in the lithistid sponge–stromatolite reefs and an increase in the lithistid sponges and receptaculitids within the lithistid sponge–calcimicrobial reefs in the Hunghuayuan Formation; the lithistid sponge–bryozoan reefs are common in both the Fenhsiang and Hunghuayuan Formations. These features of the Chenjiahe reefs are in marked contrast to other coeval reefs on the Yangtze Platform and elsewhere. Skeletal-dominated reefs first developed in the Three Gorges and adjacent areas, located on the central part of the platform. Likewise, lithistid sponges and receptaculitids first developed in the Three Gorges area and then expanded their range. In contrast, stromatolites declined over time, but remained abundant on a marginal part of the platform. The spatial–temporal distributions of these reefs on the Yangtze Platform reflect the initiation of the Great Ordovician Biodiversification Event and its consequences, although influenced by local environmental conditions. The Three Gorges area was a center for the development of skeletal-dominated reefs, which were established earlier here than elsewhere in the world. These reef types and their spatial–temporal successions provide invaluable clues to the earliest evolution of skeletal-dominated reefs and their ensuing development during the Middle–Late Ordovician.

Late Early Cambrian archaeocyath reefs in Hubei Province, South China: modes of construction during their period of demise

The Lower Cambrian (lower Toyonian) Tianheban Formation of Hubei Province, South China, includes small archaeocyath–calcimicrobial reefs that formed by combinations of archaeocyaths (e.g., Archaeocyathus yichangensis) and calcimicrobes (including Epiphyton, Renalcis, and Girvanella). The archaeocyaths provided substrates onto which the calcimicrobes became attached. In particular, Girvanella encrusts directly upon the surfaces of archaeocyaths, and bush-shaped aggregations of Epiphyton, growing downward, spread over the Girvanella. The remaining spaces among these reef framework-builders are infilled by chambered forms of Epiphyton and/or Renalcis. These calcimicrobes made a strong contribution to reinforcement of the reef frameworks. The archaeocyath-bearing reefs in South China first appeared during the Atdabanian and are last seen in the early Toyonian Tianheban Formation in Hubei Province. Subsequent reefs are purely microbial reefs without archaeocyaths. The Tianheban reefs are therefore the last representatives of archaeocyath-bearing reefs in South China. These reefs, reported herein, record the transition from archaeocyath–calcimicrobial reefs to purely microbial reefs during the Toyonian. Further studies involving integrated geobiological and geochemical analyses are needed to identify the factors that led to the demise of archaeocyath-bearing reefs and that subsequently hindered the development of skeletal-dominated reefs for 40 million years.

Early Ordovician stromatoporoid Pulchrilamina spinosa from South China : Geobiological significance and implications for the early development of skeletal-dominated reefs

Abstract. The thin, laminar to low domical, encrusting stromatoporoid Pulchrilamina spinosa is reported from the Tremadocian—Floian in Hubei and Guizhou provinces, South China. The Chinese Pulchrilamina appeared earlier (late Tremadocian—early Floian) than North American equivalents (early Floian), which possess large domical forms and are the main framework-builders. Pulchrilamina appeared much earlier than the observed diversification of other stromatoporoids. These skeletal reef-builders thus provide excellent clues for understanding the initial evolution of the stromatoporoids and the subsequent development of the skeletal-dominated (especially stromatoporoid-dominated) reefs that reached their first acme in the late Middle—Late Ordovician.

Lower Ordovician Stromatolites from the Anhui Province of South China: Construction and Geobiological Significance

The Cambrian through Early Ordovician was a lengthy interval when microbialites once again dominated after their decline from a peak in the Middle Proterozoic (e.g., Riding 2006a). These microbialites declined again in the Middle–Late Ordovician, although they remained locally common (e.g., Webby 2002; Shapiro 2004; Riding 2006a).

MICROBIALITE DEVELOPMENT DURING THE PROTRACTED INHIBITION OF SKELETAL-DOMINATED REEFS IN THE ZHANGXIA FORMATION (CAMBRIAN SERIES 3) IN SHANDONG PROVINCE, NORTH CHINA

Abstract: Coralomorph-lithistid demosponge-microbial reefs are developed in deep subtidal settings of the lower part of the Zhangxia Formation (Crepicephalina Zone: Drumian of Cambrian Series 3) in the Jiulongshan section of the Laiwu area of Shandong Province, North China. These reefs are several tens of centimeters thick and formed upon stabilized substrates of microbial origin. The coralomorph-bearing reefs are clearly distinguishable from demosponge-calcimicrobial reefs and Epiphyton bioherms by the development of dendritic thrombolites and the immediately overlying columnar-layered stromatolites. The microbial degradation of sponges facilitated the precipitation of micritic, peloidal, and clotted carbonates, leading to the formation of coralomorph-bearing thrombolitic cores. In contrast, Epiphyton preferentially grew upward, and the accumulation of Epiphyton clumps formed the dendritic thrombolites and the overlying columnar frameworks of the stromatolites. Marked changes in the microbialite texture are considered to reflect some kind of ecological succession, possibly changing environmental conditions. Therefore, the reefs originated from both heterotrophic and photoautotrophic microbial activities. Concave-upward infillings and lateral bridges are common in the interthrombolite and intercolumnar spaces, suggesting episodic sedimentation on the lateral outgrowths of biofilms and highlighting the columnar-layered structures. The coralomorph-lithistid demosponge-microbial reefs are characteristic of the pioneering, stabilizing, and climax stages of a microbial succession. They are age-specific features of microbial-dominated reefs from deep subtidal settings in the Drumian Stage, when skeletal-dominated reefs were inhibited globally, long before the regime shift in reef construction around the Early Ordovician.