Hiroyoshi Sano

Erosional truncation of uppermost Permian shallow-marine carbonates and implications for Permian-Triassic boundary events

On shallow-marine carbonate buildups in south China, Turkey, and Japan, uppermost Permian skeletal limestones are truncated by an erosional surface that exhibits as much as 10 cm of topography, including overhanging relief. Sedimentary facies, microfabrics, carbon isotopes, and cements together suggest that erosion occurred in a submarine setting. Moreover, biostratigraphic data from south China demonstrate that the surface postdates the uppermost Permian sequence boundary at the global stratotype section and truncates strata within the youngest known Permian conodont zone. The occurrences of similar truncation surfaces at the mass-extinction horizon on carbonate platforms across the global tropics, each overlain by microbial buildups, and their association with a large negative excursion in δ 13 C further suggest a causal link between erosion of shallow-marine carbonates and mass extinction. Previously proposed to account for marine extinctions, the hypothesis of rapid carbon release from sedimentary reservoirs or the deep ocean can also explain the petrographic observations. Rapid, unbuffered carbon release would cause submarine carbonate dissolution, accounting for erosion of uppermost Permian skeletal carbonates, and would be followed by a pulse of high carbonate saturation, explaining the precipitation of microbial limestones containing upwardgrowing carbonate crystal fans. Models for other carbon-release events suggest that at least 5 × 10 18 g of carbon, released in <100 k.y., would be required. Of previously hypothesized Permian-Triassic boundary scenarios, thermogenic methane production from heating of coals during Siberian Traps emplacement best accounts for petrographic characteristics and depositional environment of the truncation surface and overlying microbial limestone, as well as an associated carbon isotope excursion and physiologically selective extinction in the marine realm.

Lowermost Triassic (Griesbachian) microbial bindstone-cementstone facies, southwest Japan

SummaryOn the basis of the lithostratigraphy and microscopic characters, the paper describes the facies interpretation of the upper Upper Permian (Changhsingian) and Lower Triassic (Griesbachian to Spathian) carbonates of southwest Japan, with a focus upon the lowermost Triassic (Griesbachian) microbial bindstone-cementstone. We emphasize the significant sediment-binding and stabilizing agencies of microbes chiefly of cyanobacteria along with the syndepositional cementation for the carbonate deposition on a Panthalassan buildup in a period of the Scythian reef gap. Cyanobacteria flourished as postmass extinction disaster forms in the beginning of the Triassic. The Griesbachian microbial bindstone-cementstone we describe comprises the oldest known Triassic microbial facies.Examined were the Changhsingian Mitai Formation and the Triassic Kamura Formation (Griesbachian to Norian) in southwest Japan. These units consist entirely of carbonates and are reconstructed as relict of a shallowmarine buildup upon a seamount in the Panthalassa.The Changhsingian Mitai carbonates (ca. 35 m thick) consist mainly of grainstone and packstone with a small amount of lime-mudstone. The topmost part is intensely dolomitized. The carbonate succession is characterized by an upward-decrease in number and taxonomic diversity of shallow-marine skeletal debris and an increase up-section in an amount of peloidal particles. The lower Mitai rocks are interpreted to have accumulated as skeletal sand in an oxygenated subtidal environment and the upper Mitai carbonates are considered to have been formed in a quiet intertidal environment where peloidal particles predominantly accumulated. The facies interpretation suggests the late Changhsingian regression, which led to an increase of an inhospitable condition for shallow-marine benthic communities and to an intensive dolomitization.The Kamura Formation (ca. 38 m thick) disconformably rests upon the Mitai Formation with a drastic lithologic change. The Lower Triassic rocks we focused reach 15.5 m thick and comprise the Griesbachian and Dienerian to Spathian sections.The lower part (ca. 5.5 m) of the Griesbachian section consists of dark gray carbonaceous limestone composed of thinly layered triplets of a gastropod-bearing peloidal grainstone layer, a spar-cemented frame of clotted peloids, and a thin-laminated and occasionally stromatolitic cover of cryptomicrobial micrite in ascending order. The upper two members of a triplet often form a bindstone-cementstone layer characterized by a low-relief domed structure, or a broad hump. The upper part (ca. 2 m thick) of the Griesbachian section is composed of oncolitic limestone that contains laminae packed with gastropods. The Dienerian to Spathian section (ca. 8 m thick) consists of coquinites comprising an explosive flourish and accumulation of pectinacean bivalves.We interpret the Griesbachian rocks to have accumulated in a stagnant, ecologically rigorous tidal flat, where microbes, of possible cyanobacteria, flourished. The flourish of gastropods reflects an intermittent inundation by spring tide into the peritidal environment. The deposition of gastropods was followed by a dominant cyanobacterial activity that formed a microbial bindstone-cementstone layer along with the syndepositional cementation in an intertidal zone. The cyanobacterial activity contributed to the formation of gently undulated, sediment-binding and stabilizing mats. The oncolitic limestone in the upper part of the Griesbachian section also suggests the cyanobacterial, or algal activity. The Griesbachian microbial-controlled sedimentation was followed by the mass accumulation of bivalves that most possibly reflects a rapid transgression in Dienerian time.All the results permit us to conclude that possible cyanobacteria were the significant rock-forming organisms as post-mass extinction disaster forms on a panthalassan buildup in the beginning of the Scythian reef gap. The Griesbachian carbonates here described are similar in having the important microbial control on the sedimentation to the Lower Triassic stromatolitic and thrombolitic carbonates previously known in the Tethyan platform.

Changes in productivity and redox conditions in the Panthalassic Ocean during the latest Permian

The Gujo-Hachiman section in central Japan provides a rare window into environmental conditions within the Panthalassic Ocean, which encompassed more than half the Earth’s surface at 251 Ma. The section is characterized by a sharp transition from green-gray organicpoor cherts to black siliceous shales in the uppermost Permian. Normalization to the clay fraction demonstrates that apparent increases in the concentrations of organic matter and trace metals above this transition were due primarily to the loss of a diluent biogenic (radiolarian) silica fl ux and only secondarily to a small shift toward more reducing bottom waters. In the black shale, pyrite abundance increases by a factor of ~30× and is dominated by framboidal grains of probable syngenetic origin. These observations suggest that the expansion of lowoxygen conditions within the Panthalassic Ocean was focused within the oxygen-minimum zone rather than at the seafl oor. Such a pattern implies that (1) changes in nutrient fl uxes and primary productivity rates, rather than stagnation of oceanic circulation, were a key factor infl uencing oceanic redox conditions around the Permian-Triassic boundary, and (2) large regions of the Panthalassic Ocean underwent only limited redox changes, providing potential refugia for marine taxa that survived into the Triassic.

Paleogeographic reconstruction of accreted oceanic rocks, Akiyoshi, southwest Japan

The depositional setting of an accreted oceanic rock succession of limestone, chert, and greenstone is reconstructed by using the stratigraphy, age, and lithologic features in the Akiyoshi terrane, southwest Japan. Stratigraphic and radiolarian paleontological results identify four coeval oceanic rock units, ranging in age from late early Carboniferous to late middle Permian. Sediments on the top, the upper flank, the lower flank, and the foot of the seamount and on the surrounding ocean floor are represented by reef limestone, detrital limestone with spicular chert, spicular chert with displaced limestone, and radiolaria-bearing chert, respectively; all units rest on greenstone. Radiolaria-bearing chert is interpreted as a more distal facies than spicular chert. These rocks are interpreted as sediments deposited on and around a basaltic seamount formed in an open-ocean realm that had no input of terrigenous materials.

Microbial controls on Panthalassan Carboniferous-Permian oceanic buildups, Japan

SummaryThis contribution emphasizes the significant rock-building role of microbial encrusting organisms includingTubiphytes, filamentous cyanobacteria (mainlyGirvanella andOrtonella), andArchaeolithoporella in the Carboniferous to Permian buildups that formed on seamounts in the Panthalassa ocean. The description concentrates on the paleontological characters of these microbes and the petrographic properties of the microbial bindstones and related cryptomicrobial fabrics. Comparisons of major rock-building biotas in the Carboniferous-Permian Panthalassan buildups and Pangean reefs are briefly discussed.The Lower Carboniferous (Visean) to upper Middle Permian (Murgabian) limestone units and the upper Lower to upper Middle Permian (Artinskian to Murgabian) limestone units in southwest Japan were examined. All these limestone units are underlain by oceanic island-type basalts and are totally free from terrigenous materials. The limestones are lying in disrupted accretionary terranes of Japan and are regarded as relicts of oceanic buildups upon seamounts in the Panthalassan open-ocean realm.Thin-section studies revealed that the sediment-binding and stabilizing of filamentous cyanobacteria,Tubiphytes, andArchaeolithoporella contributed significantly to the formation of several varieties of microbial bindstone and related cryptomicrobial fabrics. Sessile foraminifers,Girvanella and related filamentous cyanobacteria formed bindstones and cryptomicrobial micritic laminae/mats in which skeletal debris and peloidal grains have been trapped. Minute shrubs ofOrtonella and related filamentous cyanobacteria acted as small-scale sediment-bafflers as well as sediment-binders.Tubiphytes bindstone was formed by the low-laminar and dense insitu growth ofT. obscurus, often encrusted by filamentous cyanobacteria.T. sosioensis formed complicated bodies that are completely encrusted byArchaeolithoporella and cemented by syndepositional carbonate cement.These microbial bindstones and related cryptomicrobial fabrics are present in almost all the stratigraphic levels, especially in Upper Carboniferous and Permian carbonate sections. In contrast, boundstones, characterized by large-sized, wave-resistant frameworks and built by rugose corals, chaetetids, and bryozoans, only occurring in laterally discrete patches, dominate in an interval ranging from the Upper Visean to the Lower Moscovian. The microbialites and filamentous cyanobacteria-related cryptomicrobial fabrics, though less conspicuous and much smaller-sized, dominanted on the Panthalassan oceanic buildups during Carboniferous to Permian time when most large-sized frame-builders were globally impoverished.The Visean to early Moscovian abundance of the coralchaetetid-bryozoan association in the examined Panthalassan buildups is of special importance for the discussion on the recovery of the reef ecosystem after the end-Devonian crisis. The Panthalassan buildups differ from their time-equivalent Pangean reefs in the ability to construct wave-resistant reefs. These wave-resistant coral-chaetetid-bryozoan reefs were replaced by the post-Moscovian microbial carbonates, which characterize the Upper Carboniferous and Permian development of the carbonate buildups of the Akiyoshi and Mino teranes. A modification of the ‘Akoyoshi reef model’ is proposed.

Is mid-late Paleozoic ocean-water chemistry coupled with epeiric seawater isotope records?

Isotopes of epeiric sea carbonates are used to construct seawater records for modeling global changes in Paleozoic ocean chemistry, climate, and for intercontinental correlation. We present for the first time geochemical results of Paleozoic brachiopods (biogenic low-Mg calcite, bLMC) from open-ocean Permian–Carboniferous seamounts of Japan situated in the tropical mid-Panthalassic Ocean. Strontium isotope values of bLMC from the Panthalassic and Paleotethys Oceans are coupled with those of coeval specimens from epeiric seas of North America, Europe, and Russia ( p = 0.393), but not with those of epeiric sea whole rocks (matrix aragonite/calcite, mAC; p = 0.029) and conodonts (biogenic apatite, bA; p = 0.031). Oxygen isotope values of bLMC from the Panthalassic and Paleotethys exhibit mixed results with studies of counterparts from epeiric seas ( p = 0.596) reflecting overprinting of local environmental conditions on global trends. Carbon isotope values of bLMC and mAC from the Panthalassic and Paleotethys Oceans are generally dissimilar to those of coeval material from epeiric seas of North America, Europe, and Russia ( p = 0.001 and 0.002, respectively). Factors such as water mass stratification, evaporation, dilution, depth, temperature, carbon burial and/or oxidation variations, and syndepositional diagenesis within the local environment probably influenced the chemistry of the fauna and accumulating sediments. This decoupling of carbon and oxygen isotope values from the open ocean with those from epeiric seas makes questionable the use of isotope results from epeiric seas for international correlation, constructing global seawater records, determining fluxes in the global carbon cycle, and for modeling climate changes and subsequently atmospheric carbon dioxide levels.

Tectonostratigraphy of Mino terrane: Jurassic accretionary complex of southwest Japan

Abstract The Mino terrane, a disrupted accretionary terrane in central Japan, consists of four fault-bounded mappable units of Mesozoic and Paleozoic unmetamorphosed sedimentary rocks. The four units are described as (1) Permian greenstone-carbonate-chert unit, (2) Lower Triassic to lowest Cretaceous siliceous pelagite-distal turbidite unit, (3) Middle Jurassic proximal turbidite unit, and (4) upper Lower Jurassic to lowest Cretaceous olistostrome-slump unit. The first unit is interpreted as a sedimentary cover which was formed on and around a seamount in an open-ocean setting. The second unit records the accumulation of deep-water radiolarian-rich sediments in a pelagic realm and a tapering wedge of trench turbidites. The third unit contains submarine fan sediments deposited in a possible trench-slope basin. The forth unit, labeled as sedimentary melange containing blocks derived from accretionary prisms, is considered to be submarine deposits which accumulated in a lower trench-slope basin to trench floor setting. An originally southward-vergent imbricate stacking of structural wedges of these four units characterizes the highly complicated structure of the Mino terrane rocks. Collision and subsequent offscraping accretion of Permian to Middle Jurassic oceanic rocks and Lower Jurassic to lowest Cretaceous clastic rocks provide the most satisfactory explanation for juxtaposition of the structural wedges of the four units which were originally laid down in largely different depositional sites. These tectonic events most probably took place during Early Jurassic to Early Cretaceous time along an active continental margin of East Asia.

Panthalassan Seamount-Associated Permian-Triassic Boundary Siliceous Rocks, Mino Terrane, Central Japan

Abstract. We describe the lithology and age of an intact section (NF 1212R) and two reference sections of Panthalassan seamount-associated Permian-Triassic boundary (PTB) siliceous rocks. The sections occupy the upper part of the Hashikadani Formation of the Mino terrane in the Mt. Funabuseyama area, central Japan. Section NF 1212R comprises a lower unit of gray chert (ca. 1.7 m thick), a middle unit of dark gray to black chert (ca. 0.8 m) with a pyrite-rich layer at the top (ca. 0.1 m), and an upper unit of black claystone with thin, intermittent beds of black to dark gray chert (ca. 1.2 m), in ascending order. The chert of the lower and middle units is rich in radiolarian remains with minor siliceous sponge spicules. The black chert of the middle unit is carbonaceous and includes tiny pyrite grains. The black claystone consists of microcrystalline quartz and clay minerals rich in carbonaceous matter. The chert of the upper unit is also carbonaceous and rich in radiolarian remains. The lower and middle units are correlated with the Neoalbaillella optima Zone (Changhsingian). The basal part of the upper unit is referable to the Hindeodus parvus Zone (basal Griesbachian), and the major part of the upper unit is possibly correlated with the middle to upper Dienerian. We position the PTB at the sharp lithologic boundary between the upper Upper Permian chert and lower Lower Triassic black claystone. The examined PTB siliceous rocks are stratigraphically attributed to the upper part of the Hashikadani Formation, reconstructed as an oceanic rock unit characterized by Lower Permian to Lower Triassic siliceous rocks that accumulated upon the lower flank of a mid-oceanic seamount in a pelagic realm of the Panthalassa Ocean. Our results present the worlds first record of deep-marine PTB siliceous rocks associated with a Panthalassan seamount.

Tubiphytes-archaeolithoporella-girvanella reefal facies in Permian buildup, Mino terrane, central Japan

Abstract The Lower to Middle Permian Okumino buildup of the Mino terrane, central Japan, formed a carbonate cap on a seamount which was sitting in an open-ocean realm. Microscopic examination reveals considerable amounts of Tubiphytes, Archaeolithoporella , and Girvanella in these rocks. These low laminar encrusting organisms together with cystopore bryozoa and syndepositional radial-fibrous cements formed bindstones. The bindstones are interpreted as having formed wave-resistant algal reefal mounds on the marginal terrace of the Okumino buildup which also has the lagoonal flat, sand bar or shoal, and foreslope facies. The Okumino buildup is closer in its biotic association of major encrusting organisms to the Trogkofel buildup in southern Alps than to the Capitan Reef Complex in New Mexico and Texas. The similarity implies that Tubiphytes and Archaeolithoporella were the most predominant and significant rock-forming encrusting organisms in Early to early Middle Permian times.

Permian hydrothermal deposits in the Mino Terrane, central Japan: Implications for hydrothermal plumes in an ancient ocean basin

Abstract Geochemical features of metalliferous deposits, cherts and basalts from the Early Permian Hashikadani Formation in the Mino Terrane, central Japan were studied. The metalliferous deposits occur as thin Fe-rich layers about 5 cm thick and Mn-rich crusts up to 30 cm thick in cherts overlying basalt. The Fe-rich layers contain large amounts of magnetite and are intercalated in the massive chert unit just above the basal basalt. The Mn-rich crusts occur in the bedded chert unit, which always overlies massive cherts, and their dominant Mn-mineral is braunite. These metalliferous deposits show highly fractionated MnO/Fe 2 O 3 ∗ (total iron as Fe 2 O 3 ) values and are depleted in such elements as Ni, Co, Cu and Zn; these features indicate their hydrothermal origin. The values of Fe 2 O 3 /FeO and MnO/Fe 2 O 3 ∗ in samples abruptly increase above the lithologic boundary between the massive chert and the bedded chert units. This stratigraphic change of lithofacies and chemical characteristics can be attributed to the precipitation of Fe-oxides prior to Mn-oxides from a hydrothermal solution that was gradually diluted with seawater and became oxic. The formation process of the Permian metalliferous deposits from central Japan can be inferred by analogy with modern hydrothermal deposits recently discovered along active spreading centres.