The Geounri shear zone in the Paleozoic Taebaeksan Basin of Korea: Tectonic implications

Lee, S.Y.,Min, K.,Ree, J.H.,Han, R.,Jung, H. Pergamon Press 2012 Journal of structural geology Vol.42 No.-

The Songrim and Daebo orogenies represent two major Phanerozoic tectonic events that are well-preserved across much of the present-day Korean Peninsula. The Songrim orogeny corresponds to the Late Permian-Triassic collision of the North and South China cratons whereas the Jurassic Daebo tectonic event represents a thin-skinned contractional deformation in a continental arc setting. It is well-established that the Songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the Korean Peninsula (e.g., the Pyeongnam Basin and the Imjingang belt) with only a minor impact on the geology of southern Korea (e.g., the Okcheon and Taebaeksan basins). It was the Daebo tectonic event, however, which generated most of the deformational structures observed within the Okcheon and Taebaeksan basins. The Deokpori thrust in the Taebaeksan Basin is a significant fault structure that formed during the Daebo tectonic event; no other regional structures related to the Songrim orogeny have been found in the Taebaeksan Basin. In the vicinity of the Deokpori thrust, we have identified in this study a previously undocumented reverse-slip shear zone, which is named the Geounri shear zone. Microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at ~400 <SUP>o</SUP>C, in contrast with the nearby Deokpori thrust, which formed in a brittle deformation regime. A geochronological analysis of muscovite isolated from phyllonite samples of the Geounri shear zone, yielded two <SUP>40</SUP>Ar/<SUP>39</SUP>Ar age spectra with a combined weighted mean age of 209 +/- 5 (2σ) Ma. Thermal modeling of the Ar data, combined with structural interpretations, suggests that this muscovite <SUP>40</SUP>Ar/<SUP>39</SUP>Ar age represents a robust lower age limit for the timing of the Geounri shearing event, therefore linking formation of the shear zone with deformational events caused by the Songrim orogeny. These results imply that structural features caused by 'Songrim deformation' were developed at higher temperatures than structures caused by 'Daebo deformation' as found within the Taebaeksan Basin.

Recent advances of trilobite research in Korea: Taxonomy, biostratigraphy, paleogeography, and ontogeny and phylogeny

Duck K. Choi,Tae-Yoon S. Park 한국지질과학협의회 2017 Geosciences Journal Vol.21 No.6

Trilobites are among the most diverse and abundant fossil groups in Korea and occur in the Cambrian–Ordovician Joseon Supergroup of the Taebaeksan Basin. The Cambrian–Ordovician trilobites of the Joseon Supergroup have been intensively studied during the last quarter century, with emphasis on taxonomic revision, refining biostratigraphic zonation, paleogeographic implications, and ontogeny and phylogeny. A total of 243 species have hitherto been known to occur in the Joseon Supergroup of the Taebaeksan Basin: 118 and 110 species were reported in the Taebaek and Yeongwol groups, respectively, whilst 15 species were described from the Mungyeong Group. Contrasting trilobite faunal contents of the Taebaek and Yeongwol/Mungyeong groups resulted in two separate biostratigraphic schemes for the Cambrian–Ordovician of the Taebaeksan Basin: 22 biozones or fossiliferous horizons were recognized in the Taebaek Group; 19 zones were established in the Yeongwol Group; and four biozones were known from the Mungyeong Group. These trilobite biozones of the Taebaeksan Basin can be correlated well with those of North China, South China, and Australia. The distribution of some of the Cambrian and Ordovician endemic trilobites and the detrital zircon spectra of the pertinent areas suggest that the Sino-Korean Craton was located at the margin of east Gondwana and was separated from the South China Craton by an oceanic basin, and that the Taebaeksan Basin was a part of an epeiric sea in east Gondwana. Focus of the ontogenetic research of Korean trilobites was initially on the Cambrian trilobites from the Machari Formation of the Yeongwol Group, and then, was shifted to the silicified sclerites dissolved out of the carbonates of the Guzhangian to Tremadocian strata of the Taebaek Group, which have played a crucial role in resolving the trilobite phylogeny. Still, much of the carbonate strata of the Taebaek Group containing silicified trilobites has remained unstudied. The future ontogenetic research on the trilobites from the middle Cambrian (Cambrian Series 3) and the Furongian strata of the Taebaek Group would significantly contribute to elucidating the muddled trilobite phylogeny.

Permo-Triassic changes in bulk crustal shortening direction during deformation and metamorphism of the Taebaeksan Basin, South Korea using foliation intersection/inflection axes: Implications for tectonic movement at the eastern margin of Eurasia during

Kim, H.S.,Ree, J.H. Elsevier Scientific Publishing Co 2013 Tectonophysics Vol.587 No.-

The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving complicated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural and metamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., the Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among the terranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shortening directions in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basin with other Phanerozoic mobile belts in eastern Asia based on a combined analysis of foliation intersection/inflection axes (FIA) trends and metamorphic P-T and T-t (time) paths. The orientations and relative timing of FIA preserved as inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups of inclusion trails in the Pyeongan Supergroup at the northeastern margin of the Taebaeksan Basin. These microstructures indicate the development of early NNW-NNE-trending structures and fabrics, followed by later E-W-trending ones. These observations suggest a change in the orientation of bulk crustal shortening from E-W to N-S during the Songrim orogeny. Based on the similar microstructures and temperature-time paths of the three Paleozoic terranes, we interpret that the E-W bulk crustal shortening influenced the eastern part of the Korean Peninsula during the early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terrane and the eastern margin of Eurasia, whereas the N-S bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny, related to collision between the South and North China blocks.

Sequence Stratigraphy of the Yeongweol Group (Cambrian-Ordovician), Taebaeksan Basin, Korea: Paleogeographic Implications

Y.K. Kwon(권이균) 대한자원환경지질학회 2012 자원환경지질 Vol.45 No.3

전기 고생대 태백산분지 영월층군은 탄산염-규산쇄설성 퇴적암 복합체로서 하부로부터 삼방산층, 마차리층, 와곡층, 문 곡층, 영흥층으로 이루어져있다. 영월층군에 대한 순차층서학적 분석에 따르면 중기 캠브리아기에 일어난 범람에 의해 최하부의 규산쇄설성 사질 퇴적암이 우세한 삼방산층이 퇴적되었다. 이어지는 후중기 캠브리아기 ~ 전후기 캠브리아기 에 지속적으로 발생한 빠른 해수면 상승으로 마차리층 하부에는 셰일, 입자암, 각력암층을 협재한 사면 혹은 심부 램프 시퀀스가 형성되었다. 후기 캠브리아기 동안 지속된 해수면 상승은 실질적인 퇴적가능공간을 창출하였고, 조하대 환경에 탄산염 퇴적물 공장이 만들어졌으며, 탄산염 대지에는 마차리층을 구성하는 탄산염암이 우세한 조하대 시퀀스가 형성되 었다. 마차리층 상부의 와곡층은 후후기 캠브리아기의 완만한 해수면 상승국면에서 만들어진 탄산염 램프 시퀀스로 해 석되며, 퇴적 당시에는 리본 탄산염암과 탄산염 역암을 포함하는 이회암으로 구성되었던 것으로 보인다. 와곡층은 퇴적 직후에 일차적으로 캠브리아기와 오르도비스기 사이의 해수면 하강국면에서 불안전 백운암화 과정을 거치고, 후에 심부 매몰 속성환경에서 광범위한 백운암화 작용을 받은 것으로 해석된다. 전기 오르도비스기에도 세계적인 해수면 상승과 해 침은 지속되었으며, 영월층군의 조하대 램프 퇴적환경은 그대로 유지되어 탄산염 역암층을 협재하는 석회이암과 이회암 이 교호하는 전형적인 램프 시퀀스인 문곡층이 형성되었다. 문곡층은 중기 오르도비스기에 퇴적된 것으로 알려진 영흥 층에 덮여 있다. 영흥층은 주로 윤회층리를 보이는 조석대지 탄산염암으로 이루어져 있으며, 문곡층의 최상부에서 조하 대 퇴적환경이 영흥층의 조석대지 퇴적환경으로 변화한다. 세계적 1차 규모 순차 경계면인 소크(Sauk)와 티피카누 (Tippecanoe) 시퀀스의 경계는 영흥층 중부에서 관찰되는 최소퇴적가능공간 부근에서 인지된다. 중기 오르도비스기 초기 의 세계적 해수면 하강과 이어지는 해수면의 급격한 상승은 영흥층의 전반적인 상향 천해화 윤회층의 전진퇴적체를 형 성하였다. 영월층군이 퇴적된 영월 탄산염 대지의 상대적 해수면 변동곡선을 복원해 보면 같은 태백산 분지의 태백층군 이 퇴적된 태백 탄산염 대지의 해수면 변동 곡선과 유사함을 확인할 수 있다. 이것은 두 개의 탄산염 대지가 유사한 조 구조적 운동 역사를 갖는다는 것을 의미하며, 이러한 유사성은 영월층군이 형성된 영월 탄산염 대지가 비록 태백층군이 퇴적된 태백 탄산염 대지와 상이한 퇴적시스템을 갖기는 하지만 상대적으로 가까운 지역에 속해 있었음을 암시한다. 퇴 적층서 분석결과에 따르면 영월 탄산염 대지는 태백 탄산염 대지에 비해 상대적으로 열린 천해 환경이었을 것으로 추측 된다. 고생대 후기와 중생대 전기에 걸쳐 발생한 북중국지괴와 남중국지괴의 충돌 시기에 영월 탄산염 대지와 태백 탄 산염 대지가 복잡한 이동과정을 거쳐 현재의 태백산 분지에 모이게 된 것으로 해석된다. The Yeongweol Group is a Lower Paleozoic mixed carbonate-siliciclastic sequence in the Taebaeksan Basin of Korea, and consists of five lithologic formations: Sambangsan, Machari, Wagok, Mungok, and Yeongheung in ascending order. Sequence stratigraphic interpretation of the group indicates that initial flooding in the Yeongweol area of the Taebaeksan Basin resulted in basal siliciclastic-dominated sequences of the Sambangsan Formation during the Middle Cambrian. The accelerated sea-level rise in the late Middle to early Late Cambrian generated a mixed carbonate-siliciclastic slope or deep ramp sequence of shale, grainstone and breccia intercalations, representing the lower part of the Machari Formation. The continued rise of sea level in the Late Cambrian made substantial accommodation space and activated subtidal carbonate factory, forming carbonate-dominated subtidal platform sequence in the middle and upper parts of the Machari Formation. The overlying Wagok Formation might originally be a ramp carbonate sequence of subtidal ribbon carbonates and marls with conglomerates, deposited during the normal rise of relative sea level in the late Late Cambrian. The formation was affected by unstable dolomitization shortly after the deposition during the relative sea-level fall in the latest Cambrian or earliest Ordovician. Subsequently, it was extensively dolomitized under the deep burial diagenetic condition. During the Early Ordovician (Tremadocian), global transgression (viz. Sauk) was continued, and subtidal ramp deposition was sustained in the Yeongweol platform, forming the Mungok Formation. The formation is overlain by the peritidal carbonates of the Yeongheung Formation, and is stacked by cyclic sedimentation during the Early to Middle Ordovician (Arenigian to Caradocian). The lithologic change from subtidal ramp to peritidal facies is preserved at the uppermost part of the Mungok Formation. The transition between Sauk and Tippecanoe sequences is recognized within the middle part of the Yeongheung Formation as a minimum accommodation zone. The global eustatic fall in the earliest Middle Ordovician and the ensuing rise of relative sea level during the Darrwillian to Caradocian produced broadly-prograding peritidal carbonates of shallowing-upward cyclic successions within the Yeongheung Formation. The reconstructed relative sea-level curve of the Yeongweol platform is very similar to that of the Taebaek platform. This reveals that the Yeongweol platform experienced same tectonic movements with the Taebaek platform, and consequently that both platform sequences might be located in a body or somewhere separately in the margin of the North China platform. The significant differences in lithologic and stratigraphic successions imply that the Yeongweol platform was much far from the Taebaek platform and not associated with the Taebaek platform as a single depositional system. The Yeongweol platform was probably located in relatively open shallow marine environments, whereas the Taebaek platform was a part of the restricted embayments. During the late Paleozoic to early Mesozoic amalgamations of the Korean massifs, the Yeongweol platform was probably pushed against the Taebaek platform by the complex movement, forming fragmented platform sequences of the Taebaeksan Basin.

Kayseraspis (Trilobita) from the Mungok Formation in Yeongwol area and its significance for the Lower Ordovician biostratigraphy and chronostratigraphy of the Taebaeksan Basin, Korea

이승배 한국지질과학협의회 2020 Geosciences Journal Vol.24 No.4

This study reports the occurrence of a trilobite Kayseraspis from the uppermost part of the Mungok Formation, Yeongwol Group, Korea. This genus has been known to occur in the upper part of the Dumugol Formation of the Taebaek Group and in the lower part of the Yeongheung Formation of the Yeongwol Group, and to indicate the lowermost Floian in the Taebaeksan Basin. However, the occurrence of Kayseraspis from the Mungok Formation and the comparison between the biostratigraphy of trilobites and graptolites suggest that the Kayseraspis-bearing faunas in Korea can be assigned to the upper Tremadocian. Accordingly, it is plausible that the base of the Floian may be positioned in the overlying strata, the Yeongheung Formation of the Yeongwol Group and the Makgol Formation of the Taebaek Group. In order to consolidate the Lower Ordovician chronostratigraphy of the Taebaeksan Basin, additional integrative studies on trilobites, graptolites, and conodonts are required.

A new lithostratigraphic scheme of the Ordovician Jeongseon Formation of the Yongtan Group in the Taebaeksan Basin and sequence stratigraphic analysis with discussion on basin geometry

권유진,권이균 한국지질과학협의회 2024 Geosciences Journal Vol.28 No.3

Lithostratigraphy of the Yongtan Group is newly established based on detailed sedimentologic and stratigraphic studies on recently exposed, less deformed, relatively continuous outcrops and comprehensive review of previous studies, mainly focusing on the Jeongseon Formation which occupies most of the group spatially and stratigraphically. The Yongtan Group is divided into the Jeongseon, Haengmae, and Hoedongri formations in ascending order, and all the units are assigned to the Ordovician. The Jeongseon Formation is divided into three members based on lithology. Eleven sedimentary facies are recognized in the Jeongseon Formation and organized into five facies associations (FAs), FA1 (tidal flat), FA2 (inner-ramp), FA3 (shoal), FA4 (mid-ramp), and FA5 (ramp-slope to outer-ramp). The formation is interpreted to have been deposited on a homoclinal ramp. The Yongtan Group is divided into two supersequences separated by type 1 sequence boundary and the lower and middle members of the Jeongseon Formation belong to Supersequence I and the Upper Member to Supersequence II which includes the Haengmae and Hoedongri formations. The relative sea-level curve inferred from sequence stratigraphic analysis suggests that the formation evolved through two depositional stages in accordance with second-order sea-level changes separated by the boundary between the Sauk and Tippecanoe sequence, and the curve is greatly similar to that of the Taebaek and Yeongwol groups. The southeastward thickening of FA3 and northwestward prevalence of shale-rich facies suggest that topographic highs and lows were present in the Taebaeksan Basin and a slope might have developed along the carbonate platform during the early stage of deposition. The correlation and lateral thickness variation of the facies associations additionally suggest that the basin was deeper toward the northwestern part of the Jeongseon area. The deepening is likely attributed to the weakening of the productivity of the carbonate factory during the early Ordovician.

Temperature and Timing of the Mylonitization of the Leucocratic Granite in the Northeastern Flank of the Taebaeksan Basin

김형수 한국지구과학회 2012 韓國地球科學會誌 Vol.33 No.5

The Mesozoic leucocratic granite in the northeastern margin of the Taebaeksan Basin was transformed to protomylonite and mylonite. Mylonitic foliations generally strike to NWWNW and dip to NE with the development of a sinistral strike-slip (top-to-the-northwest) shear sense. Grain-size reduction of feldspar in the mylonitized leucocratic granite occurred due to fracturing, myrmekite formation and neocrystallization of albitic plagioclase along the shear fractures of K-feldspar porphyroclasts. As the deformation proceeded, compositional layering consisting of feldspar-, quartz- and/or muscovite-rich layers developed in the mylonite. In the feldspar-rich layer, fine-grained albitic plagioclase and interstitial K-feldspar were deformed dominantly by granular flow. On the other hand, quartz-rich layers containing core-mantle and quartz ribbons structures were deformed by dislocation creep. Based on calculations from conventional two-feldspar and ternary feldspar geothermometers, mylonitization temperatures of the leucocratic granite range from 360 to 450oC. It thus indicates that the mylonitization has occurred under greenschist-facies conditions. Based on the geochemical features and previous chronological data, the leucocratic granite was emplaced during the Middle Jurassic at volcanic arc setting associated with crustal thickening. And then the mylonitization of the granite occurred during the late Middle to Late Jurassic (150-165 Ma). Therefore, the mylonitization of the Jurassic granitoids in the Taebaeksan Basin was closely related to the development of the Honam shear zone.

Revision of the conodont zonation of the uppermost Hwajeol Formation (Furongian), Taebaeksan Basin, Korea

이병수,박영숙 한국지질과학협의회 2015 Geosciences Journal Vol.19 No.4

The uppermost interval of the Hwajeol Formation, Taebaeksan Basin, has been assigned to the Fryxellodontus inornatus- Monocostodus sevierensis-Semiacontiodus lavadamensis Zone. The main species in the zone are Fryxellodontus inornatus Miller, Hirsutodontus hirsutus Miller, Monocostodus sevierensis (Miller), Semiacontiodus lavadamensis (Miller), S. nogamii Miller, Utahconus utahensis (Miller), and Cordylodus proavus Müller. These species were re-examined to evaluate the biostratigraphy of the Bangteogol and Maesangol sections. Data were added from eleven sections around the Taebaeksan Basin, which were studied previously by the first author. Conodont occurrences above the Cambrooistodus minutus Zone are characterized by a lack of vertical and lateral continuity in sections. Conodont recovery is relatively poor but is sufficient for biostratigraphic assignment. This trend of conodont occurrence seems to be related to sedimentary process, including debris slides and eustatic sea-level fluctuation. Nevertheless, detailed study of the characteristic species of the zone leads to the conclusion that retention of the zone is reasonable. However, it is renamed the M. sevierensis-S. lavadamensis Zone because Fryellodontus inornatus occurs stratigraphically lower than the other two nominate species. The M. sevierensis-S. lavadamensis Zone is correlated with equivalent zones of western USA, Canada, North China, and Australia.

Deformation-induced right-side-up pseudo-stratigraphy of the early Paleozoic Joseon Supergroup in the southeastern Danyang area, South Korea

엄태훈,이진한,김형수 한국지질과학협의회 2024 Geosciences Journal Vol.28 No.3

The formations in the early Paleozoic Joseon Supergroup (mainly carbonates with subordinate siliciclastics) within the Taebaeksan Basin in the Danyang area, South Korea, show an apparent right-side-up homoclinal stratigraphy without repetition or omission of any formation, and it was therefore thought that the NW-dipping formation boundaries are primary depositional contacts. Our detailed examination revealed, however, that the formation boundaries are reverse-slip shear zones parallel to the second-generation foliation (S2) crenulating transposed S0//S1 foliation and that WNW-dipping bedding planes (S0) are only locally preserved. The most penetrative regional planar structure in the area is the NNW-dipping first-generation foliation (S1) that is defined by compositional layering and is parallel to the axial planes of isoclinal F1 folds. Isoclinal to close F1 folds occur on both the mesoscopic and macroscopic scale, whereas tight to close F2 folds occur locally only on the mesoscopic scale. D1 deformation presumably involved a NNW-SSE horizontal contraction and resulted in the buckle folding and transposition of S0. D2 deformation involved a NW-SE contraction, probably at a high angle to the S0//S1 transposed layers, and produced passive shear folding of S0//S1 with S2 crenulation cleavages as discrete shear surfaces. Shearing deformation was strongly localized along some S2 foliation planes, resulting in reverse shear zones that represent the current ‘pseudo’-formation boundaries. Based on zircons U-Pb ages, we suggest that the Okdong Fault was initiated from an unconformable boundary between the Precambrian basement gneiss and overlying sediments of the Paleozoic Taebaeksan Basin, probably during the Middle Jurassic.

Middle Furongian (late Cambrian) polymerid trilobites from the upper part of the Sesong Formation, Taebaeksan Basin, Korea

박태윤,Jang Won Sohn,최덕근 한국지질과학협의회 2012 Geosciences Journal Vol.16 No.4

The upper part of the Sesong Formation of the Taebaek Group, Taebaeksan Basin, Korea, is known to contain the Kaolishania Zone which can be equated to the Kaolishania Zone of North China. Silicified trilobite sclerites were recovered from six horizons of the upper part of the Sesong Formation at the Sagundari section, which include a total of thirteen polymerid species: i.e., Shirakiella elongata Kobayashi, 1935, Shirakiella sp. 1, Taishania? sp. 1, Acanthometopus sp. 1, Pagodia sp. cf. P. spina Qian, 1994, Kaolishania granulosa Kobayashi, 1933, Gumunsoia triangularis gen. et sp. nov., Gumunsoia sp. 1, kaolishaniid genus and species indeterminate 1, Elaphraella? taebaeksanensis Park and Choi, 2012, Elaphraella microforma Lu and Qian, 1983, Elaphraella nodus (Qian, 1994), and Lingyuanaspis sp. The occurrence of Kaolishania suggests that the studied interval may be part of the Kaolishania Zone. However, Kaolishania occurs only from the lowermost horizon, and the full stratigraphic range of Kaolishania is unknown at present. Accordingly, the studied interval is provisionally called the Kaolishania fauna. The uppermost interval of this Kaolishania fauna produces Acanthometopus sp. 1, and thus can be correlated with the Acanthometopus Zone in Northeast China, which is underlain by the Kaolishania Zone