Tetsuo Matsuda

Well‐documented travel history of Mesozoic pelagic chert in Japan: From remote ocean to subduction zone

The Mino-Tanba belt in southwest Japan, a segment of the Cordilleran-type orogenic chain of Jurassic east Asia, is composed mainly of a Middle-Upper Jurassic subduction-accretion complex in which Triassic and Lower Jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with Middle-Upper Jurassic clastic rocks. High-resolution microfossil (conodont and radiolaria) research has identified very low average sedimentation rates of about 0.5 g/cm²/1000 yr in the chert units, similar to those of modern pelagic sediments accumulated in open ocean environments. Judging from the low average sedimentation rate, high purity of biogenic silica, long duration of continuous deposition (>50 m.y.), and wide along-strike extent (>1000 km), the bedded radiolarian cherts in the Mino-Tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments; accordingly, the alleged origin in smaller marginal basins is untenable. Upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench. The tectonic interlayering of these cherts and coarse-grained terrigenous elastics is a secondary feature that was added through duplexing-underplating in the subduction zone. On the basis of the primary stratigraphy and field occurrence of Triassic bedded chert in the Mino-Tanba belt, newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a Cordilleran-type bedded chert from its birth at a mid-oceanic ridge to its demise at a subduction zone.

Kurosegawa zone and its bearing on the development of the Japanese Islands

Abstract The Kurosegawa zone in southwest Japan is a 600 km long serpentinite melange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone: 1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma, 2. (2) Siluro-Devonian formations, 3. (3) Upper Carboniferous to Jurassic formations, 4. (4) Upper Jurassic to Lower Cretaceous formations, 5. (5) serpentinite and 6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar). The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units. Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.

Detailed Stratigraphy across the Permo-Triassic Boundary at Chaotian in Northern Sichuan, China

To reveal environmental changes across the Permo-Triassic boundary (PTB), detailed lithostratigraphy was analyzed of the PTB interval at Chaotian in northern Sichuan, China. The study interval is composed mainly of shelf carbonates that are lithologically divided into 7 units, i.e., Units A to G in ascending order. Units A-E (8.5 m) correspond to the uppermost Permian, and Units F-G (3.5 m) to the lowermost Triassic. On the basis of a field study and microscopic observations of more than 200 thin sections, the main extinction horizon is recognized at the Unit DIE boundary. Various fossils, such as ammonoid, brachiopod, bivalve, radiolaria, and conodont occur abundantly in Units A-D, while they are absent (or nearly absent) in Units E-G. Radiolarians in particular, show a clear contrasting mode of occurrence : abundant in Unit D to its top, while barren in Unit E and above. The Unit D/E boundary is thus identified as the event PTB horizon with major extinction. The PTB extinction terminated various Late Permian organisms, particularly radiolarians, which are representative planktons in open ocean, suggesting a sharp decline in total marine productivity. On the other hand, the Griesbachian (Early Triassic) index conodont Hindeodus parvus first appears at the base of Unit F, thus the Unit E/F boundary is recognized as the biostratigraphically-defined PTB horizon. The marl of Unit E represents the interval of strong environmental stress that appeared around the PTB. The population of the latest Permian radiolarians decreased remarkably across the Unit B/C boundary, while their mean shell size stayed constant, and even increased rapidly within Unit D. As large-shelled radiolarians often predominate in the cool waters of modern oceans, the radiolarian size increase in Unit D may indicate that the depositional site may have been invaded by a cooler water mass at the end of Permian period.