Atsushi Takemura

Neogene deformation of the back-arc shelf of Southwest Japan and its impact on the palaeoenvironments of the Japan Sea

Abstract The evolutionary process of the back-arc basins of Southwest Japan is presented on the basis of geological and geophysical data-sets. Surface geology and borehole information demonstrate rapid subsidence throughout the back-arc margin in the Early Miocene as a result of continental rifting. Uplift in the Middle Miocene and extensive truncation in the Late Miocene on the back-arc margin tend to become more intense eastward and westward, respectively. Palaeomagnetic studies show that the Middle Miocene uplift is linked to collision of the Izu-Bonin Arc with the eastern part of Southwest Japan and confined deformation of the island arc. Offshore seismic data illustrate that the Late Miocene unconformity event was caused by intra-arc folding under N-S compression. Coeval resumption of subduction of the Philippine Sea Plate is the probable cause of this episode. Seismic profiles also indicate that the folds converge into the Tsushima Strait. The crustal contraction brought about closure of the Tsushima Strait at the western end of Southwest Japan, blocking the warm Tsushima current, and so impacting strongly on the Neogene palaeoenvironments of the Japan Sea.

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.

Eocene and Oligocene radiolarian biostratigraphy from the Southern Ocean: correlation of ODP Legs 114 (Atlantic Ocean) and 120 (Indian Ocean)

Abstract Eocene-Oligocene radiolarians from Ocean Drilling Program Sites 699, 702, and 703, Leg 114 of the Subantarctic Atlantic were examined in order to extend the tripartite zonation for the recovered cores based on results of similar analysis of Leg 120 submarine sediments from the Indian Ocean. Correlation of the two oceans is made by examining 23 biohorizons and the three zones, Eucyrtidium spinosum, Axoprunum irregularis, and Lychnocanoma conica, in ascending stratigraphic order. One new species, Eucyrtidium nishimurae, is described.

A probable shark dorsal fin spine fragment from the Early Triassic of the Arrow Rocks sequence, Whangaroa, northern New Zealand

The ornament on a small external cast in pink chert shows considerable similarity with that of various Middle Palaeozoic and Triassic fish genera. It comes from the Permian–Triassic Oruatemanu Formation of Arrow Rocks, Whangaroa area, eastern Northland. Conodont faunas from a few metres above and below the sample allow correlation with the Neospathodus pakistanensis zone of the Early Triassic, which is assigned to the late Dienerian (late Induan), with adjacent conodont zone faunas in their correct stratigraphic association. The cast is assumed to be that of a small fragment of fin spine, most likely from the junction area of the crown and root on the right-hand side of a dorsal fin spine, possibly anterior, of a marine ctenacanthoid shark, a basal shark order not previously recorded from New Zealand.

Mesozoic phosphatic and calcareous nodules containing well-preserved radiolarian fauna from the North Island, New Zealand

ニュージーランドには日本とよく似たチャート・砕屑岩を含む中・古生代の地質体が広く分布するが, 放散虫化石による生層序学的研究はようやく端緒に着いたばかりある. 我々は1995, 96年の2年にわたり, 国際学術研究 (代表: 酒井豊三郎)による, ニュージーランドの中生界を中心とした含放散虫岩の調査を行った. その結果のうち, ワイパパ, ムリヒク両テレーンの, 中生代南半球の極めて保存のよい放散虫化ヨを産するノジュールの産状を紹介する.