Yoshihito Kamata

Triassic radiolarian faunas from the Mae Sariang area, northern Thailand and their paleogeographic significane

Abstract Early to Late Triassic (Spathian to Carnian) radiolarians were obtained from the bedded chert sequence of the Mae Sariang Group distributed in northern Thailand. Based on the similarity of radiolarian fauna and petrographical characteristics, it is inferred that the fine-grained siliceous and calcareous sediments of the Mae Sariang Group are equivalent to those belonging to the eastern marginal facies of the Sibumasu Block. Moreover, the occurrence of an early (?) Carnian radiolarian assemblage from bedded chert shows that the closure of the Paleotethys Ocean between the Sibumasu and Indochina Blocks in northern Thailand occurred after the early Carnian.

Middle Triassic radiolarians from West Timor, Indonesia

Moderately well-preserved Middle Triassic radiolarians were recovered from bedded limestone exposed at about 3 km west of Kefamenanu, West Timor, Indonesia. This limestone probably from the Aitutu Formation is considered to be an allochthonous block and is embedded in the Neogene Bobonaro Complex. The radiolarian fauna in this limestone is characterized by abundant radiolarians of typical Tethyan forms and is identical to that of the early Fassanian (early Ladinian) of European Tethys and other related faunas reported from the Philippines, Russian Far East, and Japan. The Aitutu Formation is thought to be deposited in an ocean environment dominated by a warm-water current system originating from the low latitude Tethyan realm. Fifty-nine species belonging to 34 genera, including five unidentified genera, are systematically treated, among them, five new species; Parentactinia suparkai, Pseudostylosphaera timorensis, Cryptostephanidium? megaspinosum, Tetrarchiplagia compacta, and Planospinocyrtis kefaensis.

Late Jurassic radiolarians from pebbles of Lower Cretaceous conglomerates of the Hayang Group, southeastern Korea

Permian to Late Jurassic radiolarians were obtained from granules and pebbles in conglomerates of the Donghwachi and Gisadong formations of the Lower Cretaceous Hayang Group, southeastern Korea. Radiolarian faunas and the petrologic character of the pebbles resemble those of Jurassic accretionary complexes of East Asia. Considering lines of evidence such as the lithologic characters of pebbles, affinity of the radiolarian faunas, reconstructed paleocurrent direction in the Hayang Group, and the paleogeographic position of southwestern Japan, it may be inferred that these pebbles were derived from the accretionary complexes mainly of southwestern Japan and the Far East. New evidence for the occurrence of Late Jurassic radiolarians in the Gisadong Formation suggests that radiolarian-bearing pebbles would be not only chert but also siliceous shale or shale of the Jurassic accretionary complexes, and gives an exhumation age for the latest accreted units of the complexes.

Middle Jurassic radiolarian fauna from Rotti Island, Indonesia

Abstract A well-preserved Middle Jurassic radiolarian fauna was discriminated from calcareous shale of the Wai Luli Formation on Rotti Island, Indonesia. This fauna is characterized by the presence of Tricolocapsa plicarum, Tricolocapsa ? fusiformis, Stichocapsa japonica, S. convexa, Cyrtocapsa mastoidea, Protunuma turbo, Transhsuum maxwelli, Eucyrtidiellum sp., Archaeodictyomitra sp. A. and others. These radiolarians represent the T. plicarum Assemblage reported from Bajocian and early Bathonian (Middle Jurassic) sequences in Japan and southeastern Europe. Based on accumulated micropaleontological evidence in Timor Island contributed by the present authors, Rotti Island was probably positioned within a warm water current system originating in the low latitude Tethyan realm through the Middle Jurassic. Fifteen species belonging to seven genera are systematically investigated. Among them, Tricolocapsa multispinosa and Tricolocapsa matsuokai are described as new.

An Early Jurassic ammonite from a limestone conglomerate in the Kuzu Complex of the Ashio Belt

Abstract A specimen referable to an Early Jurassic ammonite, Cleviceras cf. chrysanthemum (Yokoyama, 1904), was discovered from a limestone conglomerate layer of the Kuzu Complex in the Ashio Belt, central Japan. Cleviceras chrysanthemum, a species comparable to the present specimen, is a diagnostic ammonite of early Toarcian age in the Circum-Pacific. The limestone conglomerate layer in question is a part of a sequence composed of, from top to bottom, siliceous shale, limestone conglomerate itself, calcareous sandstone, and again siliceous shale. The siliceous shale above and below the ammonite-bearing conglomerate bed carries radiolarian fossils which indicate the Unuma echinatus Assemblage Zone or Tricolocapsa plicarum Zone of Middle Jurassic age. The whole sequence is judged to be normally superposed by frequently observable normal graded bedding. Thus, the age indicated by the fossil ammonite for the limestone conglomerate is in contradiction with the age of the overlying and underlying beds. These facts suggest that the ammonitebearing pebble of Early Jurassic age is a fragment transported from elsewhere and redeposited in its present position during a Middle Jurassic time. The description of the ammonite is given.