Thermal maturity of the Upper Triassic to Lower Jurassic Nampo Group, a sediment‐fill of the Chungnam Basin located in the central western part of South Korea, was assessed by illite crystallinity measurement and sandstone microtexture analysis. The Nampo Group consists of a fluvio‐lacustrine deposit bearing meta‐anthracitic coals and was over‐thrusted by the basement rocks. Sandstones are characterized by down sequence increasing illite crystallinity, from anchizone to epizone, which is strongly suggestive of burial heating. Deep‐burial diagenesis and deformation are evidenced by well‐developed pressure solution textures, whose intensity tends to increase down sequence, and by ductile deformation in the lowermost strata. On the basis of the result of illite crystallinity measurement, the maximum paleo‐temperature and total burial depth of the Nampo Group are estimated to be ca 340°C and 10 km, respectively; these conditions are in good agreement with the observed ductile deformation features. The absence of strata younger than the Nampo Group in and around the Chungnam Basin suggests that deep burial of the Nampo Group was caused by tectonic crustal loading. The tectonic overload was because of basement over‐thrusting that occurred during the Jurassic Daebo orogeny, which is closely related to the orthogonal subduction of the Izanagi Plate beneath the East Asian continent. Subsequent hydrothermal alteration disturbed the thermal maturity pattern, resulting in anomalously high illite crystallinity and meta‐anthracitization.
Provenance and tectonic history of the Jurassic accretionary complex, Mino terrane, located in the Inner Zone of south-west Japan, were studied using sandstone framework composition and mudrock geochemistry. Modal analysis of sandstones shows that the tectonic setting of the source area for the studied Mino terrane clastic rocks was uplifted basement, largely dominated by highgrade metamorphic terrain composed of quartz and feldspar, especially plagioclase. The textural and mineralogical immaturity, extent of alkali and alkaline earth element leaching, low chemical index of alteration values and depleted rare earth element (REE) contents suggest rapid uplift and erosion within the source terrain and a relatively weak weathering intensity. Factor analysis revealed that grain-size effects governed compositional heterogeneity in the studied sediments. Provenance of the sediments is interpreted as being plagioclase-enriched felsic basement rock, such as granodiorite, within a continental margin and evolved arc tectonic setting rather than active volcanic arc. Lack of a contribution from active volcanic arcs may have resulted from the cessation of volcanism during the reorganization of the subducting plate system and/or erosion of arc volcanics and exposure of basement. Considering the previous studies on palaeogeography and palaeocurrent reconstruction, the north-eastern part of the Yeongnam massif in the Korean Peninsula is interpreted as the most probable source area for the studied turbidites. The results of mixing calculation for Mino terrane sediments suggest that Precambrian leucocratic granite and the basement rock of the Cretaceous Gyeongsang Basin shed large amounts of sediments to the Mino trench, whilst Precambrian granitic gneiss and the Triassic pluton supplied lesser amounts. The results of this study reveal that, although active subduction-accretion processes were occurring, the Mino trench was bordered by continental basement rocks. This knowledge contributes to enhanced understanding of the Jurassic palaeogeography of the east Asia continental margin.
The Dumugol Formation (Lower Ordovician) in the southern part of the Baegunsan syncline, South Korea, contains mixed siliciclastic and carbonate ramp deposits. The ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone‐mudstone couplets, bioclastic grainstones to packstones, flat‐pebble conglomerates, a skeletal lag layer and laminated calcisiltites. All tempestites are characterized by an erosive to sharp base, poor grading and a transitional upper boundary. The difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality. For example, laminated calcisiltites have developed on the shallow carbonate ramp, flat‐pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps, and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting. The stratigraphic succession of the Dumugol Formation represents an initial transgression followed by a regression. The vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp, to a basin, shallowing to a deep carbonate ramp, and to a shallow carbonate ramp. Storm effects are mostly well preserved in shallow to deep ramp deposits.
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