• The Journal of the Petrological Society of Korea
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• v.14 no.4 s.42
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• pp.195-211
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• 2005

The Kusandong Tuff, known as a representative key bed in the Cretaceous Gyeongsang Basin, is a crystal-rich tuff of pyroclastic flow and surge origin. It is 1-4 m thick and laterally extends for more than 200 km intercalated in the upper part of the Hayang Croup. Observations and analyses of the feldspar crystals in the tuff, using polarizing microscopes, EPMA, and BSE images, reveal that the plagioclase crystals in the tuff were completely albitized (>$97\%$ Ab) whereas those in the southernmost localities where the tuff is rich in fine ash matrix are unaltered or partly albitized. K-feldspars are partly albitized at all localities, irrespective of the matrix content of the tuff, Perthitic textures, chessboard twinning, albitization along micro-fractures and cleavages, and the relationship between matrix content and the degree of albitization suggest that feldspars in the Kusandong Tuff were albitized by Na-rich fluid after burial. Albitization is interpreted to start preferentially along micro-fractures and cleavages and be hampered in matrix-rich tuffs with a low permeability. Original composition of the plagioclases in the Kusandong Tuff is also interpreted to have ranged between oligoclase and andesine ($Ab_{62.5}-Ab_{83.3}$) before the albitization.

• Economic and Environmental Geology
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• v.27 no.4
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• pp.411-420
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• 1994

Euiseong sub-basin, one of three sub-basins in Kyungsang basin, consists of various sedimentary and igneous rocks of Cretaceous age. Kusandong tuff and Yucheon volcanic rocks from the sub-basin were collected for the anisotropy of magnetic susceptibility (AMS) study. Maximum directions of the AMS for Kusandong tuff and Yucheon volcanic rocks are used to detect possible source areas. Although the dispersion of the maximum directions of the AMS, mainly due to low susceptibility and/or low percent anisotropy of individual specimens, is rather large, it is possible to reveal several source areas for the volcanic rocks. Areas near the Keumseongsan and Hwasan, calderas in the study area, are identified as source areas for Yucheon volcanic rocks, while the western part of Sunamsan, another collapsed caldera in Euiseong sub-basin, is inferred to be the source area for Kusandong tuff. However, it is not possible to determine detailed source areas for groups of Yucheon volcanic rocks of different lithologies, because of poor degree of convergence of the maximum directions of the AMS results from the volcanic rocks. It is also concluded that several episodic volcanic activities centered at Keumseongsan and Hwasan calderas were responsible for the formation of Yucheon volcanic rocks in Euseong area.

• The Journal of the Petrological Society of Korea
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• v.22 no.3
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• pp.235-249
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• 2013

The Gusandong Tuff (Kusandong Tuff), known as a very significant key bed in the Cretaceous Gyeongsang Basin, is divided into (1) Northern Gusandong Tuff (NKT), (2) Southern Gusandong Tuff (SKT), and (3) Sinsudo Tuff, which were derived from different vents. In order to suggest their more accurate eruption times and to contribute to establishing stratigraphy of the basin, SHRIMP U-Pb zircon ages were determined from the three tuffs. As a result, the virtually same ages of $103.0{\pm}1.2$ Ma and $104.1{\pm}1.3$ Ma were obtained from NKT and SKT, respectively, which mean that they simultaneously erupted during 103~104 Ma. The zircon ages obtained from the Sinsudo Tuff are however divided into two groups i.e. $103.4{\pm}2.1$ and $95.79{\pm}0.98$ Ma. Based on distinctive morphology and cathodoluminescence image of the younger zircons, the younger age, $95.79{\pm}0.98$ Ma, is much more reasonable as the eruption time of the Sinsudo Tuff.

• Economic and Environmental Geology
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• v.32 no.4
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• pp.353-363
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• 1999

The Cretaceous and the Upper Jurassic strata of the Korean Peninsula, entirely of continental facies, form a sedimentary mega-unit subdivided into three unconformity-bounded units. The lower, Upper Jurassic-early Lower Cretaceous unit (Jasong Synthem) occurs profusely in North Korea and is characterized by volcanic rocks of intermediate to acidic, calc-alkaline to alkaline compositions; but strata of this unit is very rare in South Korea. The middle, Hauterivian-Lower Albian unit occurs commonly in the Korean Peninsula, but some alkalinesubalkaline basalt and andesite occur only in South Korea. A recently obtained U-Pb isochron age about 113.6 Ma (Chang et at, 1998) from the zircon grains of the Kusandong Tuff in the uppermost part of the Haman Formation has thrown much light on the age of this unit. The stratotype of this Hauterivian-L. Albian unit is the Sindong and Hayang Groups of the Kyongsang Basin, where the unit is about twice thick and has more conglomerates than in sedimentary basins in North Korea. The unit shows various sedimentary cycles in different basins showing that the cyclicity is controlled by local crustal motion. The upper, Upper Albian-Upper Cretaceous unit is abundant in South Korea with prolific volcanic rocks which are intermediate to acidic and notably calc-alkaline. In North Korea, however, this unit occurs in only one locality without volcanic rocks and is not voluminous. The distribution of these three unconformity-bounded units shows a stepwise younging toward the Pacific Ocean: the lower unit occurs mainly in N Korea, the middle unit occurs in both N and S Korea, and the upper unit occurs mainly in the southern part of S Korea. The Cretaceous sedimentary basins of S Korea were genetically controlled by paralleling sinistral strike-slip faults parallel to the Pacific margin.