• Journal of the Korean earth science society
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• v.33 no.4
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• pp.329-336
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• 2012

In September 2009, a perfectly preserved fossil of a dinosaur egg nest was discovered in the Cretaceous formations of the Aphaedo area in Shinan, Jeollanam-do, South Korea. In order to estimate the age of dinosaur eggshells and the depositional age of the Cretaceous sediments in Aphaedo area, a whole-rock K-Ar dating was carried out on volcanic pebbles showing a sedimentary structure contemporaneous with the Aphaedo strata, acidic tuffs overlaying the strata conformably, and acidic dike rocks intrude to both of them. Volcanic rocks observed in the strata are 3-20 cm in diameter as pebbles found in lenticular conglomerate and pebble bearing mudstone strata. K-Ar whole-rock dating was performed on six different volcanic pebbles which show a sedimentary structure contemporaneous with the dinosaur egg nest contained in the strata, and all samples show Late Cretaceous ages: Cenomanian ($97.6{\pm}1.9$Ma), Coniacian ($87.6{\pm}1.7$ Ma), Santonian ($84.5{\pm}1.7$Ma) or Campanian ($82.5{\pm}1.6$, $77.3{\pm}1.5$, $75.7{\pm}1.5$ Ma). The K-Ar whole-rock age of acidic tuffs overlaying the Cretaceous formation conformably was estimated to be Campanian ($79.2{\pm}1.6$ or $77.3{\pm}1.5$Ma), when the dating was carried out under the same conditions. The acidic dike intruding both Cretaceous formation and acidic tuff showed a K-Ar whole-rock age of $70.9{\pm}1.4$Ma (Campanian). Therefore, the depositional age of the Cretaceous formation in the Aphaedo area and the time when dinosaurs lived in the study area are considered to be 77-83 Ma. Such results indicate that the ages of dinosaur eggshells from Aphaedo area can be correlated with the ages of the Seonso Formation (81Ma) with dinosaur egg nest fossils and the Uhangri Formation (79-81Ma) with dinosaur, pterosaur and web-footed bird tracks.

• Economic and Environmental Geology
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• v.30 no.6
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• pp.613-624
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• 1997

A study of anisotropy of magnetic susceptibility (AMS) was conducted on the Ordovician-Eocene strata in the Taebaek area. The study area is a northeastern part of the Okchon belt, sometimes called as Paegunsan Synclinal Area. A total of 600 independently oriented samples were collected from 60 sites covering the whole area. With a few exception of late Cretaceous-Eocene volcanic rocks, all the sampled strata are nonmetamorphosed sedimentary rocks, mainly sandstones. Among the 60 sites, 5 sites showed flow lineation lying on the bedding plane, 11 sites showed load foliation parallel to the bedding plane, and 21 sites showed tectonic foliation unrelated to the bedding plane. The tectonic foliations are defined by $k_1-k_2$ ($k_{max}-k_{int}$) anisotropy plane, and are considered as a result of tectonic forces acted perpendicularly to the foliation plane in the geologic past. Regardless of sample-site locations, tectonic force directions defined by $k_3$ ($k_{min}$) axis perpendicular to the tectonic foliation are consistent among the strata of the same geologic age. In the course of geologic time, however, the tectonic force directions showed a clockwise rotation: approximately E-W in the Ordovician sites, NW-SE in the Permian sites, N-S in the Triassic sites, and lastly NE-SW in the late Cretaceous-Eocene sites. The pre-Permian directions showed better clustering in the in-situ (geographic) coordinates, while the younger directions become better clustered after the bedding-tilt correction. It is interpreted that the major tectonic structures of the Taebaek area were controlled by the above-mentioned tectonic forces: The Paegunsan Syncline and the Hambaeksan Fault must have been generated by the NW-SE force of late Permian-early Triassic time. It was then reactivated in the reverse (dextral) sense by the N-S force of Triassic time. The Osipchon Fault in the eastern part of the study area was either generated or reactivated by the NE-SW force of late Cretaceous-Eocene time. The Permo-Triassic NW-SE force should be an expression of the Songnim Disturbance in the Korean peninsula, which is in turn related with the SCB/NCB collision in China.

• Economic and Environmental Geology
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• v.45 no.5
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• pp.565-578
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• 2012

Sedimentary facies of the Middle to Upper Cambrian Sesong Formation, Taebacksan Basin, are analyzed using detailed field mapping and stratigraphic section measuring. As a result, five sedimentary facies are recognized in the formation, which include lime nodule bearing shale facies, anastomosing wackestone-packstone facies, well-laminated siltstone facies, fine to medium sandstone facies and lime pebble conglomerate facies. Together with sedimentary facies analysis, study on vertical facies variation indicates that the Sesong Formation was deposited in an outer to inner shelf during relative sea-level fall. Especially, shallow marine aspects of the upper part of the Sesong Formation including 10-m-thick, fine to medium-grained sandstones appear to be very similar with the shallow marine strata accumulated during the Steptoean Stage (Dunderbergia) in Laurentia. These lithofacies comparisons of coeval strata between two continents suggest that sedimentation in the Sesong Formation reflects the influence of global sea-level fall occurred during the late Middle Cambrian to early Late Cambrian. As well, a stratigraphic discontinuity surface that may have sequence stratigraphic significance is recognized within the shallow marine sandstone beds of the uppermost Sesong Formation. This stratigraphic discontinuity surface may correspond to the Sauk II-III sequence boundary in Laurentia. Therefore, results delineated in this study will use a new stratigraphic paradigm for regional correlation of the Middle to Late Cambrian strata (e.g., the Sesong Formation) in the Taebacksan Basin, and will provide very useful information on intercontinental stratigraphic correlation in the future.

• Tunnel and Underground Space
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• v.26 no.5
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• pp.363-374
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• 2016

All five independent elastic constants of a transversely isotropic rock sometimes need to be determined from a single specimen. Saint-Venant approximation has been widely used for a long time in the analysis of single specimen test. This paper has proven how this empirical equation can be mathematically transformed into a form of the apparent Young's modulus based on theory of elasticity. The transformed equation is a monotonous function on anisotropic angle and can be useful in the analysis of the in-situ stress measurement in an anisotropic rock mass. The estimations of data in literatures have shown that the measured values of $G^2$ are uniform on anisotropic angles and smaller than that of Saint-Venant's case. This decrement may be caused by sliding of the interface of strata and the decrement rate is inferred to relate well with the combination of bonding condition of strata and strength of rock material. Accumulation of these kinds of studies in the future enables to define the decrement and to determine elastic constants of a transversely isotropic rock from a single specimen from modifying Saint-Venant approximation.

• Geomechanics and Engineering
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• v.12 no.4
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• pp.657-673
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• 2017

Preventing water seepage and inrush into mines where close multiple-seam longwall mining is practiced is a challenging issue in the coal-rich Ordos region, China. To better protect surface (or ground) water and safely extract coal from seams beneath an aquifer, it is necessary to determine the height of the mining-induced fractured zone in the overburden strata. In situ investigations were carried out in panels 20107 (seam No. $2-2^{upper}$) and 20307 (seam No. $2-2^{middle}$) in the Gaojialiang colliery, Shendong Coalfield, China. Longwall mining-induced strata movement and overburden failure were monitored in boreholes using digital panoramic imaging and a deep hole multi-position extensometer. Our results indicate that after mining of the 20107 working face, the overburden of the failure zone can be divided into seven rock groups. The first group lies above the immediate roof (12.9 m above the top of the coal seam), and falls into the gob after the mining. The strata of the second group to the fifth group form the fractured zone (12.9-102.04 m above the coal seam) and the continuous deformation zone extends from the fifth group to the ground surface. After mining Panel 20307, a gap forms between the fifth rock group and the continuous deformation zone, widening rapidly. Then, the lower portion of the continuous deformation zone cracks and collapses into the fractured zone, extending the height of the failure zone to 87.1 m. Based on field data, a statistical formula for predicting the maximum height of overburden failure induced by close multiple seam mining is presented.

• The Journal of the Petrological Society of Korea
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• v.21 no.2
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• pp.89-112
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• 2012

The Korean Peninsula consists of three Precambrian blocks: Nangrim, Gyeonggi and Yeongnam massifs. Here we revisited previous stratigraphic relationships, largely based on new geochronologic data, and investigated the crustal evolution history of the Precambrian massifs. The Precambrian strata have been usually divided into lower crystalline basements and upper supracrustal rocks. The former has been considered as Archean or Paleoproterozoic in age, whereas the latter as Paleoproterozoic or later. However, both are revealed as the Paleoproterozoic (2.3-1.8 Ga) strata as a whole, and Archean strata are very limited in the Korean Peninsula. These make the previous stratigraphic system wrong and require reconsideration. The oldest age of the basement rocks can be dated as old as Paleoarchean, suggested by the occurrence of ~3.6 Ga inherited zircon. However, most of crust-forming materials were extracted from mantle around ~2.7 Ga, and produced major portions of crust materials at ~2.5 Ga, which make each massif a discrete continental mass. After that, all the massifs belonged to continental margin orogen during the Paleoproterozoic time, and experienced repeated intracrustal differentiation. After the final cratonization occurring at ~1.9-1.8 Ga, they were stabilized as continental platforms. The Nangrim and Gyeonggi massif included local sedimentary deposition as well as igneous activity during Meso-to Neoproterozoic, but the Yeongnam massif remained stable before the development of Paleozoic basin.

• The Journal of the Petrological Society of Korea
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• v.26 no.3
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• pp.167-181
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• 2017

Cheongryangsan area ($49.51km^2$) has been designated as the Provincial Park in 1982, because it has magnificent aspect and seasonally spectacular landscapes. Especially, Cheongryangsa sitey ($4.09km^2$) has been designated as Noted Scenery No. 23 in 2007, because it has the same topographical landscape as rock cliffs, rock peaks and caves. The most spectacular landscapes are exhibited in the Cheongryangsan Conglomerate and Osipbong Basalt. There are twelve rock peaks on the ridges of the two strata, and many rock cliffs in the several valleys of strata, in which a few caves are formed by differential weathering and erosion. The valleys, in which flow Cheongryang, Bukgok and Cheonae streams, are classified as fault valleys along WNW-ESE faults. The rock cliffs were generated from vertical joints parallel to WNW-ESE faults in the two strata, and the caves were formed by differential weathering and erosion along bedding of sandstones and shales intercalated in the conglomerates. The rock peaks are landscapes formed by differential erosion along crossed vertical joints in the ridges. The vertical joints are developed subparallel to two WNW-ESE faults and a NNE-WWS fault. Therefore the topographical features are caused by existence of the faults and Lithologic difference in the Cheongryangsan Conglomerate and Osipbong Basalt, and by differential weathering and erosion along them.

• The Journal of the Petrological Society of Korea
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• v.23 no.3
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• pp.197-208
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• 2014

Zircon separated from a biotite schist of the Guryong Group in Odesan area, eastern part of the Gyeonggi Massif in Korea were analysed for SHRIMP U-Pb ages. CL images display composite core-rim structures of the zircon, indicating an in-situ overgrowth of zircon through a high-grade metamorphism. The metamorphic zircon rims give a weighted mean age of $247{\pm}6Ma$. While the detrital zircon cores have zoning patterns and Th/U ratios indicative of a magmatic origin. Among 53 analyses from the cores, 46 data yield near concordant ages which are concentrated at $378{\pm}10Ma$ (n=9), $420{\pm}4Ma$ (n=6) and $1845{\pm}9Ma$ (n=18) with sporadic Neoproterozoic ($687{\pm}9Ma$) to late Archean ($2519{\pm}20Ma$) ages. The age data constraint sedimentation age of protolith of the Guryong Group, so far unknown, as late Paleozoic. The Guryong Group of this study is the first late Paleozoic strata reported from eastern Gyeonggi Massif, and its maximum depositional age (ca 378 Ma) is identical with those of the late Paleozoic strata in the southwestern Ogcheon Belt. The Triassic metamorphic age and abundant middle Paleozoic provenance (361~425 Ma) of the Guryong Group are similar with those reported from the Triassic collisional belt in central China. Thus this study indicates that the Odesan area would be an possible eastward extension of the Triassic collisional belt in central China.

• Journal of the Korean GEO-environmental Society
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• v.24 no.10
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• pp.51-58
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• 2023

The dynamic behavior of the group piles supporting the superstructure in an earthquake is influenced by different complex dynamic mechanisms by the inertia force of the superstructure and the kinematic force of the ground. In an earthquake, The dynamic p-y curve is used to analyze the dynamic behavior of the pile foundation in consideration of the interaction of the ground, pile foundation, and superstructure due to the inertia force and the kinematic force. Most of the research has been conducted in order to confirm the dynamic p-y curve of the pile foundation by applying to the pile foundation installed on the single layered ground consisting of sand and clay, but the research for the multiple layered ground is insufficient. In this study, 1g shaking table tests were conducted to analyze the effect of the strata ratio of the top and bottom ground of the two layered sandy ground which has different relative densities on the dynamic behavior of group piles supporting the superstructure. The result shows that the maximum acceleration in the ground, the pile cap, and the superstructure increases as the strata ratio increases, and the location of the maximum bending moment of the pile foundation is changed. In addition, it was confirmed that the slope of the dynamic p-y curve of the pile foundation increased and decreased according to the strata ratio.

• Economic and Environmental Geology
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• v.56 no.6
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• pp.831-846
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• 2023

The Jucheon-Pyeongchang area in the northwestern Taebaeksan Zone of the Okcheon fold-thrust belt preserved several thrust faults placing the Precambrian basement granite gneisses of the Gyeonggi Massif on top of the Early Paleozoic Joseon Supergroup and the age-unknown Bangrim Group. Especially, the thrust faults in the study area show the closed-loop patterns on the map view, showing older allochthonous strata surrounded by younger autochthonous or para-autochthonous strata. These basement-involved thrusts including Klippes will provide important information on the hinterland portion of the fold-thrust belt. For defining Klippe geometry in the thrust fault terrains of the Jucheon-Pyeongchang area by older on younger relationship, the stratigraphic position of the age-unknown Bangrim Group should be determined. The Middle Cambrian maximum depositional age by the detrital zircon SHRIMP U-Pb method from this study, together with field relations and previous research results suggest that the Bangrim Group overlies the Precambrian basement rocks by nonconformity and underlies the Cambrian Yangdeok Group (Jangsan and Myobong formations). The structural geometric interpretation of the Pyeongchang area based on newly defined stratigraphy indicates that the Wungyori and Barngrim thrusts are the same folded thrust, and can be interpreted as a Klippe, having Precambrian hanging wall granite gneisses surrounded by younger Cambrian strata of the Joseon Supergroup and the Bangrim Group. Further detailed structural studies on the Jucheon-Pyeongchang area can give crucial insights into the basement-involved deformation during the structural evolution of the Okcheon Belt.