Takeshi Kozai

The welding of Shan-Thai

The Shan-Thai Terrane is viewed as a remnant of paleo-Tethys in South East Asia. The more internal “Thai” elements of Shan-Thai, bordering with Indochina, are of Cathaysian type, while the more central part of the terrane is of transitional “Sibumasu” character: The external “Shan” elements of Shan-Thai that left Gondwana last have a clear cold-water imprint. Petrological and paleontological evidence corroborates the end Triassic—earliest Jurassic Late Indosinian orogeny, as the main Paleotethyan tectonic closure event. Its main axis consists of the Mae-Sariang Zone, which can be followed over Mae Sot to Kanchanaburi and Chanthaburi, from where it extends into southern Thailand and Peninsular Malaysia. Cenozoic Himalayan escape tectonics, alternating strike-slip movements and rotation severely compressed Shan-Thai, opened the Gulf of Thailand, disrupted the original alignment of the Mae Sariang zone and Gondwana-Tethys divide, and shaped the present tectonic configuration of SE Asia.

New micropalaeontological evidence for a Late Triassic Shan-Thai orogeny

The Shan-Thai block is viewed as a remnant of Paleotethys in South East Asia. The general consensus about its origin is that it happened through the rifting from Gondwanaland and final amalgamation to Eurasia, sealed by fluvial-shallow marine Jurassic deposits. As the main Paleotethyan closure, the Nan-Uttaradit/Nan-Chantaburi and Bentong-Raub sutures were proposed by other workers. However, a suture further west, flanked by the Gondwana-Tethys divide (GTD), is advocated here as the main Paleotethyan closure. The Middle and Late Triassic radiolarian faunas were extracted from a chert-sequence in the Mae-Sot and Umphang areas of NW Thailand. The radiolarian faunas indicate early Ladinian, early-middle Norian, and Norian-Rhaetian, individually. The Triassic chert-sequence is overlain by the ‘Jurassic base-conglomerate’. The limestone and chert clasts in the conglomerate yield Early-Late Triassic conodonts and Middle-Late Triassic radiolarians, respectively. Chert clasts in the conglomerate yield among others Norian-Rhaetian radiolarians that are correlative with those from TM3, while Early-Late Triassic conodonts are found in limestone clasts. The silici-pelagic origin of the clasts suggests the Mae Sariang Zone that amalgamated the parts of the Shan-Thai block. This first finding of Late Triassic radiolarians from chert-sequence, next to the Middle Triassic and older radiolarian faunas, adds another element to the reconstruction of the sequence now comprised in the Mae Sariang Zone, west of the Nan-Uttaradit Suture. The occurrence of Triassic limestone, as that of the Chaiburi Formation in the Mae Sariang Zone, or the Kodiang Limestone in the “Western Zone”, may elucidate the question about the provenance of the Triassic conodont-bearing limestone clasts in the Jurassic base-conglomerate that seals the Mae-Sariang Zone. The newly dated Triassic sequence is further sealed by the continental-shelf deposits of the Toarcian-early Bajocian Hua Fai Group.

Gravel bearing radiolaria as tracers for erosional events: a review of the status of recent research in SW Japan and Korea

Abstract The authors introduce a case study of research on the provenance of radiolaria-bearing gravels and clasts of the monomictic chert-pebble conglomerates in the Lower Cretaceous Monobegawa Group, a paralic molasse in the Outer Zone of Southwest Japan. Reconstruction of oceanic plate stratigraphy from gravels and clasts in monomictic chert-pebble conglomerates based on their lithology and radiolarian ages is possible and effective for the provenance research in the case of the Monobegawa Group. A review of provenance studies of radiolaria-bearing gravels in Mesozoic orogenic sediments in SW Japan and Korea is included. The paper concludes with the discussion of the erosional events of the Jurassic and pre-Jurassic accretionary complexes in SW Japan with reference to the Late Mesozoic geological history in the eastern margin of Asian continent.

Early Cretaceous Terrestrial Weathering in Northern China: Relationship between Paleoclimate Change and the Phased Evolution of the Jehol Biota

The Jehol Biota from lower Cretaceous deposits in northern China provides an important record of terrestrial fauna and flora, including feathered dinosaurs and one of the earliest angiosperms. This biota underwent three development phases, with a relatively limited biodiversity in the early phase that rapidly diversified in the middle phase. This study analyzes the conditions of terrestrial paleoweathering during the lower Cretaceous as inferred from the geochemistry of mudstones, with the aim of assessing the role of paleoclimate change as a background factor that led to the phased evolution of the Jehol Biota. The analysis focuses on the Dabeigou and Dadianzi formations of Hebei Province, northern China, which record the early and middle phases of the Jehol Biota, respectively. These fluviolacustrine sequences can be lithologically divided into lower and upper units. Geochemical weathering indices (e.g., W, ΣREE [total amount of rare earth elements], and ΔW) show a significant increase from the lower to the upper unit, indicating enhanced weathering of the hinterland. Based on a comparison with the W values of recent soils that developed under various climates, the obtained increase in W can be interpreted as indicating temporal increases in temperature and humidity. Therefore, the increase in hinterland weathering from the lower to the upper unit was possibly induced by a shift in the paleoclimate to a more temperate and humid state. The timing of this change in paleoclimate closely coincides with a shift in the Jehol Biota to an evolved phase. Consequently, this preliminary result indicates that paleoclimate change in terrestrial regions of northern China might have contributed to the development of the Jehol Biota.

Faunal affinity of Toarcian-Aalenian (Early Jurassic) bivalves from Mae Sot and Umphang (Tak Province), Northwestern Thailand

In the Mae Sot and Umphang areas (NW Thailand), Jurassic strata seal the Permian and Triassic substratum of the Shan-Thai terrane with a brecciated conglomerate. Pliensbachian to Early Bajocian shallow marine strata are intercalated within the partly terrestrial Jurassic sequence. Thirty-five Toarcian-Aalenian bivalve species from Mae Sot and Umphang were identified. Among these,Eomiodon chumphonensis Hayami is known only from Thailand, whereasParvamussium donaiense (Mansuy),Myophoriella saurini Hayami,Thracia loducensis Hayami,Pholadomya (Bucardiomya) fontainei Hayami,Thracia loducensis Hayami andModiolus sestiniae Hayami were originally described in Vietnam.M. sestiniae also occurs in Iran. The greater part of the fauna, however, shows varying degrees of affinity with Myanmar (6 species), Cutch (10 species), Japan (13 species) and Europe (9 species), Levant (2 species). In view of these, it appears that these highly endemic Toarcian-Aalenian bivalves from Thailand characterize the Southeastern Asian Province of Tethys.

Radiolarians, bivalves and the J/K boundary in the Birafu Formation, southern Kurosegawa Belt, Central Shikoku, SW Japan

The Oxfordian-Berriasian Birafu Formation in the southern Kurosegawa Belt (Permian accretion terrane), Central Shikoku, SW Japan, yields micro- and mega-faunas that contribute to the biostratigraphy across the Jurassic-Cretaceous boundary. The type section consists of turbiditic sandstone and mudstone with an upward-fining tendency in the lower members (A1–A3). The middle members (B1–B2) are coarse sandstone and mudstone with muddy limestone intercalations, whereas the upper member C is characterized by a fine-grained turbiditic succession. Radiolarians define the assemblage zones (AZ):Kilinora spiralis AZ (Oxfordian) in member A2,Loopus primitivus AZ (Tithonian) in the upper part of member A3, andPseudodictyomitra carpatica AZ (Berriasian—lower Valanginian) in members B2 and C. These give at least an Oxfordian—Berriasian total time-range to the Birafu Formation. The mixed marine and brackish bivalve assemblage of members B1 and B2 (lower part) comprisesGrammatodon takiensis Tamura,Pterotrigonia toyamai (Yehara) andCtenoides tosanus Kimura, the range of which is Late Jurassic to Earliest Cretaceous, andAguilerella nagatoensis (Ohta),Miltha japonica Tashiro andIsocyprina japonica Tashiro and Kozai, having an Early Cretaceous range. The concurrent range of Jurassic to Cretaceous bivalves and exclusively Cretaceous species is significant for the clarification of bivalve evolution across the Jurassic—Cretaceous boundary. The Berriasian appearance of Cretaceous marine and non-marine bivalves takes place while Late Jurassic marine bivalves still survived. We consider Member B1 as Berriasian with the J/K boundary situated at its base.

The “Homeland” of the Torinosu-Type Limestone in Relation to Jurassic Accretionary Tectonics in SW Japan

Upper Jurassic–Lower Cretaceous fore-arc basin deposits cover the oceanic plate sequences (OPSs) of the Permian and Jurassic accretionary complexes (ACs) in the Outer Zone of SW Japan, facing the Pacific. In this study, special attention is given to the Torinosu-type reef limestone blocks in the fore-arc basin deposits, in search for a continental shelf facies from which to derive them, in relation to the subduction–accretion tectonic evolution of the Asian margin in the Japan segment. Finding such a margin facies may resolve questions about the palaeogeographical relationships and identity of the Kurosegawa Permian accretionary terranes and South Chichibu Jurassic accretionary terranes. The in situ Torinosu-type reef limestones in the Toyonishi Group (Akiyoshi Terrane in the Inner Zone) as well as those in the Birafu Formation of the Sakashu Group (Kurosegawa Terrane, Outer Zone) of SW Japan, suggest a “homeland” for the Torinosu-type limestone blocks, which were transported as olistoliths by submarine mass-wasting events from the continental shelf thrust nappe into the fore-arc basins during subduction–accretion.