Sung Kwun Chough

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.

Sediment distribution, dispersal and budget in the Yellow Sea

Abstract Information on the surface sediment distribution in the Yellow Sea is newly compiled based on all available data produced by Chinese and Korean workers and is used to refine the dispersal patterns and budget of sediments derived from the adjoining landmass. The central Yellow Sea is largely covered with fine-grained sediments originating from the Huanghe River (2.2 × 10 8 t/yr, about 20% of the total discharge). Thick sediment accumulation (exceeding 5 mm/yr) occurs in the Gulf of Bohai and around the Shandong Peninsula (about two thirds of the total Huanghe River discharge). The old Huanghe Delta suffers from reworking by strong tides and waves and forms a mud patch southwest of Cheju Island (accumulation rate, 2 × 10 7 t/yr). More than half of the sediments derived from the Changjiang River are deposited along the coast of the East China Sea; the remainder accumulate on the inner shelf off the river mouth. In the southeastern Yellow Sea, the Keum River dominates sediment discharge forming a Holocene mud belt along the southwestern coast of Korea (accumulation rate, 5 × 10 6 t/yr). Turbid water plumes occur nearshore throughout the year, and their suspended sediments are transported along the tip of the peninsula to the western coastal embayments in the South Sea. The Nakdong River (discharge of 1 × 10 7 t/yr) is responsible for the eastern nearshore mud of the South Sea. Relict sands occur in the northern part of the Yellow Sea, in the northern East China Sea and in the Korea Strait.

Zoned facies of mass-flow deposits in the Ulleung (Tsushima) Basin, East Sea (Sea of Japan)

Abstract High-resolution (3.5 kHz) reflection profiles in the Ulleung (Tsushima) Basin made it possible to classify the uppermost (ca. 50 m thick) sedimentary deposits into rock fall, slide, slump, debris-flow deposit (debrite) and turbidite in terms of their acoustic properties. These deposits generally occur in a distinctive zonal facies: slides and slumps on the slope; debrites on the base-of-slope; and turbidites on the basin floor. The rock-fall deposits occur along the bases of the Korea Plateau and Ulleung and Dok Islands. The zonal distribution of mass-flow deposits suggests that they most likely are derived from a line source at the shelfbreak. Shelf sediments prograde toward the shelfbreak. On the slope, sediment masses move downslope mainly, accompanied by progressive slumping upslope. Individual slides and slumps on the slope probably develop into debris flows on the base-of-slope and successively into turbidity currents on the basin floor. Mass-flow processes on the Ulleung Basin slope played an important role in transporting a large volume of terrigenous sediments into the deep basin.

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.

High-resolution acoustic characteristics of epicontinental sea deposits, central-eastern Yellow Sea

Abstract Large amounts (45390 line km) of closely spaced (2.2–4.4 km) high-resolution subbottom profiles (Chirp, 2–7 kHz) made it possible to identify detailed echo types and their distribution in an epicontinental shelf environment. On the basis of seafloor morphology, surface bedforms and subbottom acoustic characters, 10 echo types were identified in the uppermost sedimentary sequence of the central-eastern Yellow Sea. Flat seafloor with sharp bottom echoes (echo types 1-1, 1-2 and 1-3a; transgressive sediment sheets or relict sands) is widespread in the offshore area and underlain to the west by an acoustically transparent wedge (echo type 1-3b; highstand muds). Mounded seafloor with either smooth surface or superposed bedforms (echo types 2-1, 2-2 and 2-3; tidal ridges) and flat seafloor with regularly spaced, wavy bedforms (echo type 1-4; large-scale dunes) are dominant in the eastern nearshore area. Large-scale mounds with continuous, inclined internal reflectors (echo type 2-4; giant mud bank) occur in the southeastern nearshore area. Various-scale eroded seafloor (echo types 3-1 and 3-2; channels) and flat seafloor with regularly spaced, wavy bedforms (echo type 1-4; large-scale dunes) are present in the northern part of the sea. The distribution pattern of echo types in the central–eastern Yellow Sea reflects depositional processes and sediment dispersal systems during the Holocene transgression and highstand period: (1) development of tidal ridges and large-scale dunes in response to strong tidal currents and waves in the eastern nearshore area; (2) construction of transgressive to highstand mud bank (Huksan mud belt) by deposition of muds derived from the Keum river in the southeastern nearshore area; (3) active erosion due to intensified currents in the northern part; and (4) highstand mud deposition derived from the Huanghe river on the transgressive sediment sheets in the offshore area.

Depositional processes of late Quaternary sediments in the Yellow Sea: a review

This review focuses on the depositional processes of late Quaternary sediments in the eastern Yellow Sea, an epicontinental sea with a flat and broad seafloor (less than 100 m in water depth) and extensive tidal flats along the southeastern coast. The Yellow Sea was subaerially exposed during the last glacial period when sea level was about 120 m below the present level. During erosional retreat of shorefaces and river mouths, sedimentation was largely controlled by high-amplitude rise in sea level, forming transgressive sheets (echo type 1–3a) and sediment ridges (echo types 2-1 and 2–2) with extensive development of ravinement surfaces. The distribution of surface sediments reflects an interplay of sediment input from the surrounding landmass and the hydrodynamic regime in response to sea-level rise. Muddy sediments in the central part represent the Huanghe-River source and form a highstand sheet (echo type 1–3b). Large birdfoot-like sand bodies off the Jiangsu coast also represent highstand deposits when sea level reached the present position at about 6 ka. In the southeastern part of the Yellow Sea, sediments are dominated by sand ridges (echo types 2-1, 2–2 and 2–3), largely shaped by tidal currents. The southwestern corner of the Korean Peninsula is dominated by a thick deposit of mud, the Heuksan mud belt. The muds largely originate from the Geum River, whose distribution is controlled by strong southward coastal currents. In the coastal regions of the southeastern Yellow Sea, sedimentation is controlled by a combined effect of waves and tides with distinctive season-alities in sedimentary facies owing to the monsoonal climate: tidedominated mud deposition in summer and wave-dominated sand deposition/erosion in winter: Winter storms play a role in sedimentation on intertidal flats. Quantitative monitoring of sediment transport suggests that the textural variation results from the overwhelming role of winter waves superimposed on tidal currents in pulling sands and resuspending muds. Due to low sedimentation rate, the tidal flats formed retrogradational, coarsening-upward pattern during the Holocene sea-level rise.

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.

Chirp (2–7-kHz) echo characters of the South Korea Plateau, East Sea: styles of mass movement and sediment gravity flow

Abstract The South Korea Plateau is a complex of ridges, seamount chains, troughs and a basin. Detailed analysis of 12 019 line-km Chirp (2–7-kHz) subbottom profiles from the South Korea Plateau and the eastern continental slope of Korea reveals a distinctive zonal distribution of echo types depending systematically on seafloor morphology. Pelagites/hemipelagites (type I-2) and basement highs (type III-1) prevail on the ridge summits, seamount chains and the upper to middle part of the eastern continental slope of Korea, and are commonly bounded downslope by creeps (type III-3) and slides/slumps (type IV-1) that occur extensively over the entire slope areas of the ridges, seamount chains and eastern continental slope. The mass-movement deposits change downslope to debrites and turbidites (types II, III-2, IV-2 and IV-3) in the troughs and Onnuri Basin, suggesting successive downslope evolution from slide and slump to debris flow and turbidity current. The voluminous creeps, slides and slumps over the entire slope areas of the plateau and eastern continental slope, deeper than 300 m in water depth, were most likely generated by frequent seismic shakings.

Chirp (2–7 kHz) echo characters in the Ulleung Basin

High-resolution (Chirp, 2–7 kHz) reflection profiles in the Ulleung Basin (35–37°N) made it possible to classify the uppermost (ca. 50–70 m thick) sedimentary sequence into eleven discrete echo types. They are either (1) distinct (Echo Types IA, IB, IC, ID, IE, and IF), (2) indistinct (Echo Types IIA, IIB, and IIC), (3) hyperbolic (Echo Type III), or (4) combined (Echo Type IV).Echo Type IA is characterized by a sharp, continuous bottom echo with no subbottom reflectors (interpreted as deposits of sand and gravel). Echo Type IB shows a sharp bottom echo with discrete, continuous subbottom reflectors and either flat or undulatory surface topography (most likely turbidites). Echo Type IC is characterized by a distinct, smooth bottom echo (inclined) with parallel subbottom reflectors (interpreted as deposits of bottom currents or hemipelagic settiing). Echo Type ID is represented by a sharp bottom echo with diffuse, discontinuous, subparallel subbottom reflectors (probably deposits of sand and gravel). Echo Type IE is characterized by a sharp bottom reflector on the channelized subsurface topography with either inclined, channelized, or transparent internal reflectors (interpreted as buried fluvial channels). Echo Type IF shows a distinct bottom reflector with prolonged subbottom echoes and irregular surface topography (basement highs). Echo Type IIA is represented by a semi-prolonged bottom echo with intermittent subbottom reflectors and smooth or undulatory surface topography (most likely turbidites). Echo Type IIB shows a prolonged bottom echo with no subbottom reflectors (interpreted as turbidites). Echo Type IIC is characterized by laterally wedged, transparent subbottom echoes with variable bottom echoes (interpreted as debrites). Echo Type III shows regular, over-lapping hyperbolae with slightly varying vertex elevations (interpreted as debrites). Echo Type IV is represented by irregular blocky, lumpy, or hyperbolic masses, with various amount of internal deformation, bounded upslope by scars (slide/slump deposits and mass-failure scars).The shelfal areas are dominated by distinct echoes: the southern shelf by Echo Types IA, ID, and IE; the eastern shelf by Echo Type ID; the western shelf by Echo Type IF. The upper-to-middle slope region is generally characterized by Echo Type IV; the western slope by Echo Type IC. The lower slope is commonly occupied by Echo Types III and IIC (southern margin) and Echo Type IIC (western and eastern margins). The basin floor is generally dominated by Echo Types IIB, IIA, and IB. Echo characters show a zonal distribution: Echo Type IV (slump and scars) on the upper slope, Echo Types IIC and III (debrites) on the middle to lower slope and the base-of-slope, and Echo Types IIB, IIA, and IB (turbidites) on the basin floor. This distribution generally suggests that sediments were supplied from a linear source with large-scale flow transformations of sediment gravity flows.

Sequence aggradation and systems tracts partitioning in the mid-eastern Yellow Sea: roles of glacio-eustasy, subsidence and tidal dynamics

Abstract In the mid-eastern Yellow Sea, late Pleistocene–Holocene glacio-eustatic sea-level fluctuations and a local subsidence regime have formed an aggradational set of high-resolution sequences (sequences I–V in descending order). Extensive tidal scour-and-fill processes during transgressions played a significant role in marked partitioning of non-marine to paralic clast-rich lowstand systems tract (LST) and tidal fine-grained transgressive–highstand systems tract (TST–HST) within each sequence. The lateral extent and thickness of sequences and component systems tracts reflect differences in magnitude and duration between eustatic sea-level cycles. Sequence V is characterized by extensive LST, indicating sea-level fall to the shelf-margin area. Sequences IV and III display a limited extent of LST, reflecting small-scale sea-level falls during the late Pleistocene. Sequence II displays seaward restriction of TST–HST owing to a small-scale sea-level rise prior to the last glacial maximum (LGM). Sequence I has formed since the LGM and is characterized by discontinuous TST–HST over the entire shelf area, reflecting rapid retreat of sediment sources during the post-glacial sea-level rise. Along with the predominant controls of local subsidence, glacio-eustasy and transgressive tidal dynamics on the overall stratal architecture, minor variations in architectural characteristics were added by other local factors such as changing sediment flux, antecedent topography and basin physiography.