Daekyo Cheong

Tephrostratigraphy in the Ulleung Basin, East Sea: Late Pleistocene to Holocene

Three sediment cores, obtained from the Ulleung Basin, reveal five tephra layers that are easily distinguished from the other core intervals by coarse grain size, density grading, and sediment color. The tephra layers are well correlated among the cores and show a systematic lateral decrease in thickness and grain size away from the source of the explosive volcanic eruption. Three marker tephra layers were derived from Ulleung Island and consist predominantly of white vesiculate pumices of phonolitic or trachytic alkali magma. The disorganized Ulleung-I tephra layer was due to the last explosive eruption in Ulleung Island. It consists of only white vesiculate pumices, disturbed by strong bioturbation. The Ulleung-II tephra (ca. 9300 yr B.P.), the most explosive one among the Ulleung Island-derived tephras, shows two units: a lower density-graded unit and an upper pumice-scattered unit. The Ulleung-II tephra layer shows little bioturbation and is characterized by pumice-scattered texture. An unnamed tephra layer shows a distinct density grading by dark fine-grained crystals and white vesiculate pumices. This marker tephra originated from a phonolitic alkali magma of unknown source. The Aira-Tanzawa ash (ca. 22000 yr B.P.), derived from the Aira Caldera in southern Japan, consists commonly of bubblewalled glass shards from rhyolitic subalkali magma.During the Holocene period, sediments were accumulated at a rate of 17.5–24.7 cm/kyr, based on the Ulleung-II tephra, which increased toward the center of the basin. During the last glacial maximum (LGM) period which is represented by the interval between the Ulleung-II tephra layer and the Aira-Tanzawa ash, a very thick turbidite mud was deposited in the unchannelized Ulleung Basin plain with higher accumulation rate (mean 26.1 cm/kyr) than that during the Holocene period. The very thick turbidite mud of LGM period occurs only in the unchannelized Ulleung Basin plain, which is not correlated with other layers in the channelized basin plain.

Effects of in-phase and out-of-phase sediment supply responses to tectonic movement on the sequence development in the late Tertiary Southern Ulleung Basin, East (Japan) Sea

Stratigraphic inverse modeling using the SEDPAK stratigraphic simulator established the size of the physical parameters that together controlled the development of the stratal patterns in the late Tertiary Ulleung Basin, East (Japan) Sea. The modeling results provided a quantitative geohistory of the basin. This included the dimension of variations in rates of tectonic subsidence (or uplift) and sediment supply. Input variables were based on the ages and geometries of strata determined from well and seismic interpretations and the given base-level control of the Haq et al. eustatic curve. The simulation results indicate that changes in the rate of sediment supply clearly correlate with local back-arc tectonic events that occurred both with (out-of-phase) and without (in-phase) a time lag associated with the local tectonic movements. The initial package of the basin stratigraphy was generated by an in-phase response of sediment supply to back-arc spreading in the Early and Middle Miocene, during which a high rate of sediment supply caused a series of downlap surfaces onto which strata prograding northward. In contrast, the basin experienced an out-of-phase response to the sediment supply during the second stage of basin growth, a back-arc closure characterized by rapid uplift, thrusts, and folds during the Late Miocene. In the latter case, the time lag in sediment supply response, caused by sediment trapping behind the thrust zone, produced transgressive surfaces and halted basin growth, and then gradually forced the shelf front to migrate basinward as the sediment supply slowly increased in response to the further tectonic uplift.

Delta Formation in the Nakdong River, Korea, during the Holocene as Inferred from the Diatom Assemblage

ABSTRACT Cho, A.; Cheong, D.; Kim, J.C.; Shin, S.; Park, Y.-H., and Katsuki, K., 2017. Delta formation in the Nakdong River, Korea, during the Holocene as inferred from the diatom assemblage. The causes and succession of the Nakdong River delta formation related to sea-level change and precipitation in the early–middle Holocene were discussed based on diatom analysis. On the basis of ecology, stratigraphic changes of diatom flora were divided into five divisions, labeled zones I–V. According to the diatom assemblage, the first marine transgression occurred at about 10.5 ka at this research site. At that time, marine species gradually increased, and freshwater species had their highest values. This site was an estuary influenced by inputs of fresh and marine water until 9.8 ka (zone I). Zone II is dominated by bay and offshore species, meaning that sea level rose continuously after about 9.8 ka (zone II) when the Nakdong River mouth region was an inner bay. Then, the environment of this site changed to an outer bay after 8 ka (zone III) because of sea-level rise, which is supported by increasing numbers of oceanic species. At the end of this zone, at 6–7 ka, sea level highstand occurred. A prodelta began forming just after the highstand under slow sea-level rise (zone IV), and the delta front arrived at its present location at 5 ka by voluminous sediment input from the Nakdong River related to high precipitation during the Holocene Climate Optimum (zone V). After 5 ka, the delta front remained around its present site because of stable or decreasing sea level. Compared with other Asia deltas (Yangtze and Mekong), the initiation of the Nakdong River delta was delayed by about 1000–2000 years because of differences in relative sea-level changes.

Structural and provenance analyses of a newly defined major fault: The Homyeong Fault as a northern boundary fault of the Cretaceous Nagdong Trough, South Korea

The Homyeong Fault on the Yecheon Shear Zone is newly defined and described in the northwestern part of North Gyeongsang Province, South Korea. In the study area, the basement rocks consist, of the Precambrian rocks and Jurassic granitoid, and are dextrally deformed by the Jurassic Yecheon Shear Zone to the north of the Cretaceous Gyeongsang Supergroup in a ductile manner. These basement rocks are covered with unconformity by the sindong Group, the lowest part of the Gyeongsang Supergroup. The Homyeong Fault runs on the Yecheon Shear Zone, and is marked by cataclasite that contains mylonite fragments originating from the mylonitic rocks of the Yecheon Shear Zone and that shows sinistral sense of shear with normal-fault component dipping basinward, which indicates reactivation of the Yecheon Shear Zone. The conglomerates of the northern part of the Sindong Group usually contain the gravels of granite, quartzite, blackish quartzite, mylonite, ultramylonite, and mudstone. Petrological similarity between these gravels and the basement rocks suggests that most of the gravels were very likely derived from the Yecheon Shear Zone and its vicinity. Considering the Korean Cretaceous basins mostly formed by sinistral reactivation of the pre-existing shear zones, it can be judged that the reactivation of the Yecheon Shear Zone caused the basement rocks nearby the Homyeong Fault, or the Yecheon Shear Zone and its vicinity, to be deposited as the gravels to the Nagdong Trough where the Sindong Group was filled on the south of the fault. This gives new proposals that the Homyeong Fault had played a role as a northern boundary fault of the Nagdong Trough and also that the inversion tectonics from Jurassic dextral to Cretaceous sinistral motions, observed in the eastern margin of Asia, had an effect on the formation of the Nagdong Trough.

Recent variations in sediment organic carbon content in Lake Soyang (Korea)

Four gravity cores were taken from Lake Soyang, which was created by the construction of an artificial dam in 1973. The variation of organic carbon content of the bottom sediments since the construction was interpreted using a variety of sedimentological and geochemical measurements. Based on the textural properties of sediments and the total organic carbon (TOC) content, it is clear that sediment organic carbon has been deposited very consistently throughout the postconstruction period, except for local sudden inputs caused by summer flooding events near fish farms or farmland areas. The autochthonous input resulting from the enhanced primary production led by phytoplanktons each year is likely to become a major problem at downstream sites within Lake Soyang. The vertical variation of δ13C values of sediment organic matter also reflects the fact that the level of autochthonous supply of organic carbon becomes high downstream.

Reconstruction of burial history and analysis of the hydrocarbon potential using sedimentary modeling the middle Bengal Fan, Myanmar

The burial history and sedimentation rate of the middle part of the Bengal Fan, which is one of the world’s largest submarine fan, was reproduced using the sedimentary basin modeling software, SEDPAK. Modeling of the sedimentation rates, subsidence rates, and burial history of the Bengal Fan suggests three stages of basin evolution: Stage I (4.5–1.81 Ma), Stage II (1.81–0.79 Ma), and Stage III (0.79–0 Ma). Stage I was characterized by low sedimentation and subsidence rates, whereas the highest sedimentation and rapid subsidence occurred during Stage II. During Stage III, high sedimentation and the slowest rate of subsidence occurred. Biogenic gas is the typical hydrocarbon found in the study area. Based on the general features of the petroleum system, the generation and migration of the biogenic gas is interpreted to have occurred immediately after the deposition of Unit 2. The modeling results also support the generation and preservation of biogenic gas rather than thermogenic gas. Thermal maturity analysis established that the TTI (time-temperature index) and the thermogenic hydrocarbon potential in the study area are low because of relatively low temperatures and a short burial.

Graphical simulation of clastic margin progradation: Ulleung Basin, offshore Korea

The evolution of the southern part of the Ulleung Basin, East Sea (Sea of Japan) is analysed using a two-dimensional computer-based graphical simulation (SEDPAK) on a multichannel seismic profile. Iterative editing of the input data file includes various factors: initial basin configuration, local tectonic behavior, eustatic sea level, and the amounts and direction of clastic sedimentation. The sedimentary succession on the seismic profile can be divided into seven sequences bounded by sequence boundaries. The simulation relatively well reproduces evolving geometry shown on the interpreted seismic profile and burial history of sediments through time. The over 3000-m-thick sedimentary body is composed of sandstone and shale which have been accumulated in deltaic setting and as slumping or turbidites in a shelf-slope setting since the middle Miocene (16.5 Ma). The SEDPAK simulation shows all sequences except for the upper part of Sequence 5 which displays progradational clinoforms and onlapping transgressive units. It well reflects that the simulated sequences were formed during the mature stage of the back-arc basin development since the early Miocene. Scouring by incised valleys occurred extensively after the deposition of Sequence 5 on the margin, most obviously during the lowstand of sea level. Scouring may be related to the late Miocene thrusting and wrenching caused probably by collision of the Bonin Arc with the Amurian Plate. Simulation also shows the evolutionary development of the sedimentary sequences, which can track the burial history of individual layers throughout the run. When the 16.5 Ma surface is assumed to be the initial basin surface (the top of the pre-16.5 Ma sequence), the burial path of the 17.5 Ma to 16.5 Ma sequence reveals that it was at a depth of 3000 m at 12.5 Ma; 3200 to 3900 m at 6.3 Ma; at 3500 to 4200 m at 3.8 Ma; and at 4000 to 4700 m today. In addition, one can follow the burial history at a particular location using the time-depth-elevation plot for a specific column of the output within the simulated section. These features aid in the prediction of sedimentary facies distribution and geothermal history of the sequences.

Geochronology and sedimentary environment at the Udu-dong archeological site, Chuncheon, South Korea

The Udu-dong archeological site in Chuncheon, South Korea, dates back to the Proto–Three Kingdoms Period (approximately 100 BC to AD 350). Many artifacts, including some earthenware, have been excavated in these ancient dwelling sites. We applied three geochronological dating methods (radiocarbon, optically stimulated luminescence (OSL), and archeomagnetic dating) to the archeological remains of this large-scale human settlement and reconstructed the history of depositional processes prior to human settlement. The timing of the ancient community’s settlement was investigated by radiocarbon dating of the charcoal fragments collected from old furnaces. Archeomagnetic dating allowed us to constrain the time period of the settlement’s abandonment by dating the last use of fire. The timing and development of fluvial deposits underlying the settlement site were reconstructed by OSL dating combined with sedimentary facies analysis. Our results show that, following the deposition of coarse sediments starting 10,000 years ago, the region formed a stable floodplain environment starting around 3000 years ago; people began to form clustered settlements approximately 50 years later. For the subsequent 150 years or so, the area was heavily used as a settlement site, with people evenly distributed across it, before eventual abandonment of the site around AD 200–250. Because the sedimentary deposits do not show any significant facies change during this period, we conclude that any catastrophic events were not the main reason for settlement abandonment. This study suggests that combining scientific and archeological analyses have significant benefits for studies of such archeological sites. Therefore, continuous collection of such data can provide important information for the excavation and protection of prehistoric or historic sites.

On the validity of archeomagnetic dating method in Korea: a case study

An archeomagnetic study has been performed for six archeological relics of the Proto-Three Kingdoms period excavated in Jungdo Island, South Korea. Among a total of 90 samples from hearths remains of six dwelling sites, the characteristic remanent magnetization components were isolated from 70 samples using the progressive alternating field demagnetizations and considered to record faithfully a thermoremanent magnetization at the timing of the last cooling of the hearths. Two different approaches were made to determine the archeomagnetic ages: One is the conventional method using the relocated paleosecular variation (PSV) curve obtained from southwestern Japan. The other method adopts a Matlab archaeo_dating tool using a Bayesian statistics with global geomagnetic field prediction model (ARCH3K.1). Both archeomagnetic ages using two individual dating methods generally overlapped with the radiocarbon age ranges for each sites. However, no systematic relationship between archeomagnetic ages and radiocarbon ages was observed, implying that neither the relocated Japanese PSV data nor the global model accurately represent the Korean secular variation during the archeological period of this study. At present, archeomagnetic dating in Korea should be carefully applied in conjunction with other dating methods on the consideration of archeological context.