Rie S. Hori

Offscraping accretion of Jurassic chert-clastic complexes in the Mino-Tamba belt, central Japan

Abstract Detailed structural and biostratigraphical analysis of the Jurassic Inuyama Sequence, a coherent chert-clastic complex in the Mino-Tamba Belt, central Japan, clarifies the evolution of accretionary processes at shallow structural levels. The Inuyama Sequence is characterized by a series of stacked thrust sheets. Each sheet consists of an Early Triassic to Middle Jurassic oceanic plate stratigraphy composed of four lithologic units which are, in ascending order: siliceous claystone; ribbon chert; siliceous mudstone; and clastic rocks. The structural features of the Inuyama Sequence demonstrate a four-stage progressive deformation. (1) A decollement was initiated within the siliceous mudstone when this sequence was just seaward of the deformation front. Clastic dikes and sills formed at the horizon just above the decollement at this time. (2) The stratigraphic section above the decollement was imbricated by in-sequence thrusting in the frontal part of the wedge. This initial stage of thrusting and imbrication was followed by (3) the formation of duplex structures with fault-related folds within the lower stratigraphic section as the decollement stepped down-section to the lowest siliceous claystone interval. Finally, (4) these thrust packages were overprinted by secondary prism thickening in the form of out-of-sequence thrust faulting.

The Toarcian radiolarian event in bedded cherts from southwestern Japan

Abstract A drastic faunal turnover of radiolarians in the early Toarcian, termed the Toarcian Radiolarian Event (TRE), recognized in bedded chert sequences of southwestern Japan, is marked by the first co-appearance of the genus Praeparvicingula and Mesosaturnalis hexagonus and by high diversity among multicyrtids. The event resulted in the replacement of latest Triassic through earliest Jurassic taxa by Late Jurassic to Early Cretaceous taxa. At TRE levels in bedded cherts in southwestern Japan, the size of radiolarians decreased drastically, reworked conodont fossils appeared, and anoxic sediments (black cherts with FeS2 nodules) were deposited. The lithologic and faunal changes suggest that global marine environmental changes, including the occurrence of anoxia, caused the TRE.

Marine osmium isotope record across the Triassic-Jurassic boundary from a Pacific pelagic site

The Triassic-Jurassic (T-J) boundary ca. 200 Ma represents one of the major mass extinction events of the Phanerozoic; however, the cause of this event remains controversial because of a paucity of geological evidence. In this study we present an isotopic record of osmium extracted from a bedded chert succession across the T-J boundary in the Kurusu section of Japan, deposited within a Paleo-Pacific (Panthalassa) deep basin. The data show a gradual decrease in seawater 187 Os/ 188 Os values during the Rhaetian, followed by a sharp increase in the latest Rhaetian, and a subsequent stable phase across the T-J boundary. The decreasing trend of 187 Os/ 188 Os values during the Rhaetian indicates a gradual increase in the relative supply rate of unradiogenic Os from the mantle associated with emplacement of the Central Atlantic Magmatic Province. The subsequent shift toward radiogenic values reflects an increased supply of radiogenic Os due to enhanced continental weathering. This interval marks more negative isotopic values of organic carbon, the onset of radiolarian faunal turnover, and conodont extinctions, indicating that the rapid increase in continental weathering rate was closely linked to the perturbation of the carbon cycle and the T-J biotic crisis.

Triassic Radiolaria from Kaka Point Structural Belt, Otago, New Zealand

Abstract Triassic Radiolaria are documented from phosphatic nodules obtained from the Karoro Formation and Potiki Siltstone from Kaka Point Structural Belt, Otago, New Zealand. There are abundant Glomeropyle and Spumellaria, and rare Nassellaria and Entactinaria. By comparison with radiolarian faunas from Japan, Thailand, European Tethys, and other Triassic strata in New Zealand, the Karoro Formation is correlated with upper Lower Triassic (Olenekian), older than the Potiki Siltstone. We believe that the fauna is strongly affected by non‐Tethyan radiolarians based on the abundance of Glomeropyle, rare occurrence of Tethyan taxa, and the presence of many undescribed forms. Three new species are described herein: Glomeropyle bispinosa, G. campbelli, and Poulpus (?) caveaformis.

Triassic radiolarians from the ocean‐floor sequence of the Waipapa Terrane at Arrow Rocks, Northland, New Zealand

Abstract Triassic radiolarians are reported from two horizons in Waipapa Terrane at Arrow Rocks, Whangaroa area, Northland. This relatively undisturbed succession represents an ocean‐floor sequence, consisting (in ascending order) of basalt with limestone layers, bedded chert, black shale, and red, maroon and green siliceous mudstone. The age of the lower part of the section is Middle‐Late Permian. The radiolarian assemblages reported here indicate Early or Middle Triassic and Middle Triassic (Anisian) ages for maroon siliceous mudstones in the upper part of the section. Between strata of known Permian and Triassic age there is a thin potential Permian/Triassic boundary interval consisting of alternating black shale and grey chert. Radiolarian paleogeography indicates that the Arrow Rocks sequence experienced long distance plate tectonic displacement from a position of relatively low latitude in the Middle Permian and to a high latitude in mid‐Triassic time.

Rock magnetic record of the Triassic-Jurassic transition in pelagic bedded chert of the Inuyama section, Japan

The end-Triassic mass extinction event is regarded as one of the fi ve largest extinction events of the Phanerozoic. The emerging consensus points to volcanic activity at the Central Atlantic Magmatic Province (CAMP) as the ultimate cause of the extinction, yet the underlying mecha- nisms and the nature of global environmental changes that accompanied the biotic turnover remain elusive. We present a rock magnetic study of the extinction interval found within a continuous chert sequence that provides an uninterrupted record of pelagic sedimentation in the Panthalassa Ocean. The variations in the relative abundances and characteristics of authi- genic magnetic phases indicate that the Triassic-Jurassic transition progressed in two stages. The initial stage, characterized by a disappearance of the previously ubiquitous magnetofos- sils, started a few tens of thousands of years to 100 k.y. prior to the formal Triassic-Jurassic boundary as identifi ed by the diagnostic radiolarian species. The second stage, defi ned by signifi cant changes in optical and magnetic properties of hematite pigment, lasted a few tens of thousands of years. The stepwise change in magnetic properties is suggestive of the protracted environmental deterioration, likely prompted by the early episodes of the CAMP volcanism, which was followed by a sudden ocean acidifi cation event, perhaps triggered by a catastrophic release of gas hydrates.

Pseudopodial features and feeding behavior of living nassellarians Eucyrtidium hexagonatum Haeckel, Pterocorys zancleus (Müler) and Dictyocodon prometheus Haeckel

ABSTRACT Light microscopic studies, using both video imaging and normal camera, were made on living nassellarian radiolarians Eucyrtidium hexagonatum Haeckel, Pterocorys zancleus (Müler) and Dictyocodon prometheus Haeckel from the East China Sea off Okinawa Island, Japan. Continuous and careful observations revealed pseudopodial activity related mostly to feeding behavior. The results are presented in detail with the definition of 11 descriptive terms for nassellarian pseudopodia. Among them, terminal projections (TP) extending as straight projections from the aperture of the shell body and forming a conical structure (terminal cone: TC) and axial projections (XP) prolonged outward from the center of the aperture and proximally surrounded by TP are used for feeding. Although there are some minor differences, feeding activities of the three species can be subdivided into the following three cyclic phases; Phase 1 characterized by gradual extension of TP and XP from the aperture, Phase 2 marked by an abrupt withdrawal of TP and XP toward the aperture to engulf captured prey, and Phase 3 during which entangled TP used for capturing prey are completely retracted into the aperture. These lines of evidence suggest that the shell aperture plays the same role of a feeding apparatus as does a mouth. Interestingly, XP of E. hexagonatum sometimes shows abnormal branching, fusion and separation, which is different from the spumellarian axoflagellum.

A new Early Jurassic radiolarian fauna from the Murihiku Supergroup of the Otago coast, New Zealand

Abstract Lower Jurassic Radiolaria are documented from Aratauran strata of the Murihiku Supergroup at the northern end of Sandy Bay, south of Nugget Point, Otago, New Zealand. The fossils were extracted from small carbonate nodules in fine grey sandstone in the middle part of the Aratauran sequence. The following genera were identified: Eptingium(?), Gigi, Hsuum, Orbiculiforma, Pantanellium, Paronaella(?), Poulpus(?), Saitoum, Spongostaurus(?), and Thetis. Comparison with radiolarian faunas of Europe, southwest Japan, and North America suggests that the study fauna is of Pliensbachian age, probably Early Pliensbachian, indicating the possibility that the Aratauran local stage, previously equated approximately with Hettangian—Sine‐murian internationally, extends into the Early Pliensbachian.

Pseudopodial silica absorption hypothesis (PSA hypothesis): a new function of pseudopodia in living radiolarian polycystine cells

The secretion process of the siliceous skeleton in polycystine radiolarians has drawn a great deal of interest during the last century; however, little is known about the actual physiological process of silica deposition. Recently, the PDMPO (2-(4-pyridyl)-5-[(4-(2-dimethylaminoethylaminocarbamoyl) methoxy)-phenyl] oxazole) method for staining silica deposition sites in polycystines was developed. In the present study we examined over 30 polycystine cells with PDMPO and found that both the skeletons and pseudopodia of three species (Lithelius sp., Rhizosphaera trigonacantha and Arachnosphaera hexasphaera) were stained and emitted green fluorescent light. Staining of the skeleton was probably the result of skeletal thickening growth, whereas staining of the pseudopodia may indicate that siliceous matter is assimilated within pseudopodia. We refer to this hypothesis as the ‘pseudopodial silica absorption hypothesis’ (PSA hypothesis). If this hypothesis is correct, PSA is an intermittent process, and the absorbed silica within pseudopodia is quickly transferred to the cytokalymma where it is deposited on the skeleton. To date, the PSA process has been observed in only the three species cited above; therefore we are unable to evaluate whether the PSA process is unique to these species or a common process that occurs in all polycystines; further investigation is necessary.

Late Triassic Phaeodarian Radiolaria from the Northern Chichibu Belt, Shikoku, Japan

Abstract. A new genus (Triassiphaeodina gen. nov.) and two new species (Medusetta japonica sp. nov. and Triassiphaeodina niyodoensis sp. nov.) of Late Triassic (Rhaetian) phaeodarian Radiolaria are described from a phosphatic nodule found in mélange rocks of the Northern Chichibu Belt, Shikoku, Japan. The Rhaetian age of the nodule and of the phaeodarian new taxa is based on co-occurring Polycystina Radiolaria, including Bipedis acrostylus Bragin, Livarella densiporata Kozur and Mostler, Fontinella primitiva Carter, and Ferresium sp. A of Carter (1993). This finding shows that phaeodarian Radiolaria were already represented in Late Triassic oceans, with morphologies similar to those known in the Late Cretaceous and Cenozoic, from which they have previously been reported. The new taxa described herein represent the oldest known phaeodarian fossils.