Duck K. Choi

Tectonic and sedimentary evolution of the Korean peninsula: a review and new view

Abstract This review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts (Late Proterozoic–Neogene) in the Korean peninsula. The Korean peninsula is part of the Amuria Plate and represents an important link between continental blocks of North and South China and the island arcs of Japan. The basement rocks, exposed in the Kyonggi and Yongnam massifs, consist of 2.7 to 1.1 Ga high-grade gneiss and schist. These massifs are separated by the Okchon Fold Belt which comprises metasedimentary rocks and bimodal meta-volcanic rocks. The stratigraphy of the Okchon Group is unclear at present. The Okchon Basin was probably initiated as an intraplate rift prior to the Late Proterozoic. The Hwanggangri Formation (clast-bearing phyllite) most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin. The stratigraphic relationship between the Okchon Group and the Choson Supergroup (Cambro-Ordovician) of the Taebaeksan Basin is poorly constrained. The Choson Supergroup unconformably overlies the Yongnam Massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments, reflecting numerous sea-level fluctuations. The sequence is disconformably overlain by siliciclastic sequence of Pyongan Supergroup (Carboniferous–?Triassic) which formed most likely in shallow marine, deltaic, and fluvial environments. The Imjingang Belt is an east-trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics (Devonian–Carboniferous), underlain unconformably by Proterozoic basement rocks. Late Proterozoic amphibolites of oceanic affinity were metamorphosed at about 8–13 kbar and 630–740°C during the late Permian to the early Triassic. The south-vergent contraction and top-down-to-the-north normal faulting are suggestive of a suture belt between the North China Block (Sino-Korea Craton) and the South China Block (Yangtze Craton), an extension of Sulu Belt across the Yellow Sea. Entire peninsula experienced strong deformation and metamorphism during the suturing event, namely Songrim orogeny. During this orogenic event, the Kyonggi Massif (and the Okchon Basin) accreted to the Yongnam Massif (and Taebaeksan Basin) along the South Korean Tectonic Line running northeast–southwest. A series of northward-trending thrust formed along the boundary zone to the east (Kaktong and Kongsuwon thrusts and others). Piggyback basins locally developed along the thrust faults, forming the Taedong Group. The crustal deformation resumed in the early to late Jurassic (Daebo event) under contractional setting. Dextral ductile shearing associated with thrusting and folding continued in the mid-southern part of the peninsula. It was due to orthogonal (northwestward) subduction of the Izanagi Plate under the Asian continent. The Yongnam Massif experienced continuous dextral offset along the Honam Shear Zone. In the early Cretaceous, the Izanagi Plate began to subduct northward and caused formation of strike–slip basins in retroarc setting, i.e., Kyongsang Basin in the southeastern part and a number of small-scale basins in the mid-southwestern part of the peninsula. Small-scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions. Extensive intrusion of granitoids occurred from Triassic to Early Tertiary with a gap between 160 and 100 Ma, representing continental magmatic arc. In the Tertiary, the southeastern margin of the Korean peninsula experienced back-arc opening. Pull-apart basins formed in the Miocene, bounded by the Yangsan and Hupo faults. The Yonil Group, sedimentary fill of the Pohang Basin, comprises more than 1-km-thick siliciclastic sequence which represents deposition in fan-delta systems on the hanging wall of the Yangsan fault. Thick (more than 10 km thick) sediments in the Ulleung Basin margin were deformed in the late Miocene due to the northward movement of Kyushu Block. Quarternary volcanic events in Cheju Island represent intraplate hot spots.

Micro Electrochemical Machining of 3D Micro Structure Using Dilute Sulfuric Acid

Abstract Micro electrochemical machining (ECM) using ultra short pulses with tens of nanosecond duration is presented. 0.1 M sulfuric acid was used as electrolyte and 3D micro structures were machined on stainless steel. To prevent taper, a disk-type electrode was introduced. Using the disk-type electrode, taper could be eliminated. To improve productivity, multiple electrodes were applied and multiple structures were machined simultaneously. Since the wear of tool electrode is negligible in ECM, micro wire can be used as tool electrode. Using a platinum wire electrode with 10 urn diameter, various 3D features were machined on stainless steel plate.

The Yongwol* Group (Cambrian-Ordovician) redefined: a proposal for the stratigraphic nomenclature of the Choson Supergroup

The Yongwol Group is newly defined to comprise exclusively the Cambrian-Ordovician strata distributed in Yongwol area and replaces the Yongwol-type Choson Supergroup. It consists of the Sambangsan, Machari, Wagok, Mungok and Yonghung formations in ascending order. The Yongwol Group is bounded to the east by the Kaktong thrust, to the north by the Sangri thrust, and to the northwest by the Chonggok-Yonbongjong thrust. On the other hand, the southwestern extent of the group is obscure, as the Tanyang-Chechon area is represented by metasedimentary sequence of complex stratigraphy and contrasting lithologic successions. Based on the trilobite and conodont fossils, the Yongwol Group ranges in age from the middle Middle Cambrian to the early Late Ordovician. Other Cambrian-Ordovician sequences in the Taebaeksan region are accordingly renamed or redefined: i.e., Taebaek, Yongtan, Pyongchang, and Mungyong groups which replace the Tuwibong-type, Chongson-type Pyongchang-type, and Mungyong-type Choson supergroups, respectively. The Taebaek Group is further subdivided into the Chiktong and Sangdong subgroups.

Trilobites and zircons link north China with the eastern Himalaya during the Cambrian

A paucity of integrative data can lead to disparate pre-Pangean paleogeographic reconstructions, such as those for the Neoproterozoic–Cambrian paleogeography of the blocks of modern-day China. Reconstructions for the north China block, in particular, have relied on sparse paleomagnetic and biogeographic data and, as a result, have yielded discordant paleogeographic models. Here we present new detrital zircon grain age distributions from siliciclastic rocks, coupled with species-level polymerid trilobite biogeography, that suggest close ties between north China and the northeastern Indian margin during the Cambrian. In combination, these data require north China to have been in paleogeographic continuity with northern India as a part of core Gondwanaland, contrasting with the traditional view that north China was an isolated outboard terrane. The shared record of Cambrian–Ordovician tectonism in both northern India and north China likely represents the same event, which affected this region of Gondwanaland.

Real-Time Tool Breakage Monitoring for NC Milling Process

Abstract This paper presents real-time tool breakage detection in NC milling process by monitoring cutting force. Cutting force was indirectly measured by the feed drive AC motor current. Modeling of the feed drive system and calibration with a tool dynamometer show that the sensitivity of the feed drive motor current is sufficient to characterize tool breakage. AR 1 model with tool-runout is used to determine tool breakage. The algorithm was implemented on a Digital Signal Processing (DSP) board for real-time tool breakage monitoring. Experimental works showed an excellent real-time monitoring capability of the proposed tool breakage detection system.

Taebaek Group (Cambrian-Ordovician) in the Seokgaejae section, Taebaeksan Basin: a refined lower Paleozoic stratigraphy in Korea

The Taebaek Group (Cambrian-Ordovician) in the Taebaeksan Basin comprises mixed carbonate-siliciclastic sequence and is exposed well in the Seokgaejae section located in the central-eastern part of the Korean Peninsula. The group in the Seokgaejae section consists of in ascending order the Myeonsan, Myobong, Daegi, Sesong, Hwajeol, Dongjeom, Dumugol, Makgol, Jigunsan and Duwibong formations. This study describes in detail the well-exposed outcrop sections of the Taebaek Group in the Seokgaejae Pass in order to refine the lithostratigraphy of the lower Paleozoic strata in the Taebaeksan Basin, Korea. The refined lithostratigraphy delineates clearly the lithologic boundary between the Myobong and Daegi formations, the Hwajeol and Dongjeom formations, the Dumugol and Makgol formations, and the Makgol and Jigunsan formations. The preliminary information on trilobite faunal assemblages suggests that the Cambrian-Ordovician boundary can be placed within the lowermost part of the Dongjeom Formation.

The Cambrian-Ordovician stratigraphy of the Taebaeksan Basin, Korea: a review

The lower Paleozoic sedimentary rocks in Korea, Joseon Supergroup, are mainly exposed in the Taebaeksan Basin. The Joseon Supergroup is a siliciclastic-carbonate succession that ranges from late Early Cambrian to Middle Ordovician in age. It can be subdivided into the Taebaek, Yeongwol, Yongtan, Pyeongchang, and Mungyeong groups, based on lithologic characteristics and geographic distribution. The stratigraphy of the Taebaek and Yeongwol groups is relatively well established due to prolific occurrence of trilobites and conodonts, whereas the latter three groups are poorly understood. The Taebaek Group comprises in ascending order the Jangsan/Myeonsan, Myobong, Daegi, Sesong, Hwajeol, Dongjeom, Dumugol, Makgol, Jigunsan, and Duwibong formations. The Cambrian-Ordovician boundary in the Taebaek Group can be drawn within the lowermost part of the Dongjeom Formation. The Yeongwol Group consists of the Sambangsan, Machari, Wagok, Mungok, and Yeongheung formations in ascending order. The Cambrian-Ordovician boundary in the Yeongwol Group has been placed at the base of the Mungok Formation. The trilobite faunal assemblages of the Taebaek and Yeongwol groups display a profound contrast in faunal contents, which resulted in two separate biostratigraphic schemes. A total of 19 biozones are recognized in the Taebaek Group comprises in ascending order theRedlichia, Elrathia, Mapania, Bailiella, Megagraulos, Solenoparia, Olenoides, Stephanocare, Drepanura, Prochuangia, Chuangia, Kaolishania, Dietyites, Eoorthis, Pseudokainella, Asaphellus, Protopliomerops, Kayseraspis, andDolerobasilicus zones. However, most of these biozones have not been well defined. On the other hand, the biostratigraphy of the Yeongwol Group is well established: from oldest to youngest, theMetagraulos sampoensis, Megagraulos semicircularis, Tonkinella, Lejopyge armata, Glyptagnostus stolidotus, G reticulatus, Proceratopyge tenuis, Hancrania brevilimbata, Eugonocare longifrons, Eochuangia hana, Agnostotes orientalis, Pseudoyuepingia asaphoides, Fatocephalus hunjiangensis, Yosimuraspis vulgaris, Kainella euryrachis, Shumaridia pellizzarii, andKayseraspis zones. Little attention has hitherto been paid to the Cambrian-Ordovician chronostratigraphy of the Taebaeksan Basin. The Taebaek area includes the Iyeonnaeian and Homyeongian series for the Cambrian and the Mungogian and Yemisanian series for the Ordovician. Stages for the Cambrian-Ordovician of the Yeongwol area are the Eodungolian, Deokuan, Bundeokchian, Gonggirian, Garamian, and maepoan stages in ascending order. The refined biostratigraphy and chronostratigraphy provide an enhanced and more reliable correlation with coeval units elsewhere.

Ordovician trilobite faunas and depositional history of the Taebaeksan Basin, Korea: implications for palaeogeography

The Taebaeksan Basin occupies the central-eastern part of the Korean peninsula and was a low-relief shallow marine carbonate shelf on which the Cambro-Ordovician Choson Supergroup was deposited. In the Taebaeksan Basin trilobites are among the most dominant fossil groups in the Lower Ordovician, but they become less important in Middle Ordovician faunal assemblages. The Early Ordovician trilobite faunas of the Taebaeksan Basin are characterised by the common occurrence of pandemic genera such as Jujuyaspis, Leiostegium, Asaphellus, Protopliomerops, Hystricurus, Apatokephalus, Shumardia, Asaphopsoides, and Kayseraspis. Biogeographically significant trilobite taxa include Yosimuraspis, Dikelokephalina, Koraipsis, and Chosenia. These Ordovician trilobite faunas, which thrived in shallow marine environments, show a remarkable similarity with faunas from North China, implying that the Taebaeksan Basin was connected through contiguous shallow waters to North China. These Sino-Korean faunas exhibit a close biogeographic connection with Australian faunas, with which they share some endemic genera, whereas they are more distantly related to the faunas of South China, South America, and North America. Based on these palaeobiogeographical features, it is suggested that in the early Palaeozoic much of the present Korean peninsula including the Taebaeksan Basin belonged to the Sino-Korean block, while part of the peninsula was derived from the Yangtze block.

Origin of Limestone Conglomerates in the Choson Supergroup (Cambro-Ordovician), mid-east Korea

Abstract The Choson Supergroup (Cambro–Ordovician) of mid-east Korea consists mainly of shallow marine carbonates and contains a variety of limestone conglomerates. These conglomerates are largely comprised of oligomictic, rounded lime–mudstone clasts of various size and shape (equant, oval, discoidal, tabular, and irregular) and dolomitic shale matrices. Most clasts are characterized by jigsaw-fit (mosaic), disorganized, or edgewise fabric and autoclastic lithology. Each conglomerate layer is commonly interbedded with limestone–dolomitic shale couplets and occasionally underlain by fractured limestone layer, capped by calcareous shale. Limestone conglomerates in the Choson Supergroup consist of: (1) disorganized polymictic conglomerate (Cd), (2) horizontally stratified conglomerate (Cs), (3) mosaic conglomerate (Cm), and (4) disorganized/edgewise oligomictic conglomerate (Cd/e). These conglomerates are either depositional (Cd and Cs) or diagenetic (Cm and Cd/e) in origin. Diagenetic conglomerates formed under the conditions of: (1) alternating deposition and preservation of carbonate-rich and carbonate-poor layers, and (2) early differential cementation of carbonate-rich layers. In situ fragmentation may result from brittle fracturing and ductile deformation due to compaction and dewatering, enhanced by dissolution processes. Fluidization is promoted by the overlying impermeable calcareous shale beds and has probably reinforced in situ fragmentation of the underlying limestone beds. In situ fragmentation and rounding process is important, whereby earlier semiconsolidated portions of limestone are incorporated into the still fluid portion, which can be termed ‘autoconglomeration’ process. A synthesis of 145 beds of limestone conglomerates in the Choson Supergroup suggests that less than about 7% of the conglomerate layers (10/145) are depositional, whereas the rest, more than 93% (135/145), are identified as diagenetic conglomerates. The common occurrence of diagenetic conglomerates warrants further study on limestone conglomerates elsewhere in the world.

Trilobite faunal successions across the Cambrian–Ordovician boundary intervals in Korea and their correlation with China and Australia

Abstract The Cambrian–Ordovician boundary in Korea lies within the Choson Supergroup exposed in the Taebaeksan Basin and has been recognized in the Taebaek and Yongwol groups. The Taebaek and Yongwol groups show slightly different trilobite faunal successions across the putative Cambrian–Ordovician boundary intervals. In the Taebaek Group, saukiid-dominated, Missisquoia/Onychopyge, and kainellid-dominated faunas occur successively in the basal part of the Tongjom Formation. While the remaining part of the Tongjom Formation is poorly fossiliferous, the succeeding Tumugol Formation yields fairly diverse trilobite faunal assemblages of late Tremadocian age. This implies that the Cambrian–Ordovician boundary in the Taebaek Group should be placed somewhere within the interval yielding the kainellid-dominated fauna. In the Yongwol Group, an uppermost Cambrian Fatocephalus fauna is more or less contemporaneous with the saukiid-dominated fauna of the Taebaek Group, while three Tremadocian trilobite faunas, i.e. Yosimuraspis, Kainella, and Shumardia zones in ascending order, are well represented in the Mungok Formation. The occurrence of Jujuyaspis in the Yosimuraspis Zone provides a firm basis for locating the Cambrian–Ordovician boundary at the base of the Mungok Formation. These Cambrian–Ordovician trilobite faunas of Korea are in general well correlated with those of China and Australia.