Young Kwan Sohn

Debris Flow and Hyperconcentrated Flood‐Flow Deposits in an Alluvial Fan, Northwestern Part of the Cretaceous Yongdong Basin, Central Korea

The alluvial‐fan deposits in the Cretaceous Yongdong Basin, Korea, consist of conglomerates with a muddy or sandy matrix and sandstones with thick or thin laminations. The conglomerates and sandstones occur commonly in couplets, constituting apparently single sedimentation units. Facies transitions in the deposits can be summed up in a tripartite facies sequence: (1) a clast‐supported conglomerate with a muddy or sandy matrix and parallel clast alignment (facies A and B), (2) a matrix‐supported coarse‐tail normally graded conglomerate with random clast orientation (facies C) and thickly stratified pebbly sandstone (facies D), and (3) laminated sandstone (facies E). The clast‐supported conglomerate is interpreted as deposits of debris flows dominated by frictional grain interactions. Development of pervasive parallel clast alignment, lacking large floating clasts and inverse grading, suggests deposition via incremental aggradation rather than en masse freezing. The matrix‐supported conglomerate and thickly stratified sandstone are interpreted as deposits of dense inertia layers or traction carpets developed beneath a high‐concentration bipartite flow. The laminated sandstone indicates traction sedimentation associated with dilute flows. The facies sequence therefore suggests deposition from a composite sediment flow that comprises a preceding debris flow, a trailing watery flow, and an intermediate flow between. The intermediate flow is regarded as a hyperconcentrated flow on the basis of its bipartite nature because a hyperconcentrated suspension has a meager yield strength and is prone to be density stratified. The measured section comprises three depositional sequences, decameters thick and separated by thick mudstone beds, which could be interpreted in terms of fan evolution (progradation and retreat) under an influence of changing sediment supply from a drainage basin. Close association of sediment type with constituent facies in the three sequences suggests that composite sediment‐flow deposits are favorably developed by sand‐matrix debris flows drained from large and less rugged catchments.

Hydrovolcanic processes forming basaltic tuff rings and cones on Cheju Island, Korea

Tuff rings and tuff cones are small volcanoes produced by explosive magma-water interactions and have been regarded as resulting from relatively dry and wet eruptions, respectively, which are related to low and high mixing ratios of water to magma. However, comparative work on four Pleistocene basaltic tuff rings and cones on Cheju Island, Korea, shows that there are dry and wet types in both tuff rings and tuff cones, and their variations are not satisfactorily explained by the prevailing model. Instead, it is inferred that the morphological variations are directly caused by depositional processes (pyroclastic surge–dominated in tuff rings and fallout-dominated in tuff cones), irrespective of water-magma mixing ratios. The depositional processes are interpreted to be in turn controlled by a number of fundamental controls, which include depositional settings, type, level, and lithology of aquifers, strength of country rocks, ground-water behavior, and properties and behavior of magma. These controls determine the explosion depth, conduit geometry, mode of magma-water interaction, magnitude of explosion, eruption-column behavior, and subsequent depositional processes. The Suwolbong and Songaksan tuff rings, which formed almost entirely on land above fragile and permeable sediments and granites with some aquiclude beds, were produced by contact-surface steam explosivity at depth because of the fragility of country rocks, insufficient and inhibited supply of shallow-level external water into the vents, and interaction of nonvesiculated magma with interstitial water. These conditions led to generation of buoyancy-dominated eruption columns and pyroclastic surges, resulting in tuff rings. On the other hand, the Ilchulbong and Udo tuff cones formed in shallow seas above extremely permeable but rigid basalt lavas. The explosions occurred at shallow depths mainly by bulk-interaction steam explosivity because of the rigidity of country rocks, sustained supply of shallow-level external water into the vents, and interaction of vesiculated magma with free water. This process resulted in the generation of dense, inertia-dominated jets and the formation of tuff cones mainly by fallout processes. It is thought that the morphological and sedimentological variations of these volcanoes are more successfully explained by the fundamental controls rather than solely by the water-magma ratio. It is suggested that the water-magma ratio can explain the evolution of a single volcano or a group of volcanoes under otherwise identical conditions, but cannot explain the variability of tuff rings and cones in different hydrogeologic settings because the nature of hydroeruptions is governed by a number of fundamental controls.

Characteristics and Depositional Processes of Large-scale Gravelly Gilbert-Type Foresets in the Miocene Doumsan Fan Delta, Pohang Basin, Se Korea

ABSTRACT The Doumsan fan delta in the Miocene Pohang Basin (SE Korea) includes large-scale gravelly Gilbert-type foresets that are more than 150 m high and dip at about 20°. The foresets consist of six sedimentary facies: medium- to thick-bedded sandy gravel deposits with variable grading patterns and bed geometries (Facies A), thin- to medium-bedded, commonly inversely graded sandy gravel deposits (Facies B), sheet-like layers, a few grains thick, of pebble gravel (gravel sheets; Facies C), lensoidal layers, a few grains thick, of cobble to boulder gravel (gravel lenses; Facies D), thin-bedded sand with outsized clasts (Facies E), and very thick-bedded (> 10 m thick) disorganized gravel deposits in the toeset area (Facies F). These facies are indicative of deposition from cohesionless ebris flows (Facies A and B), debris falls (Facies C and D), turbulent flows (Facies E), and very thick debris flows that are related to large-scale foreset failure (Facies F). These facies are in close association with one another, except for Facies F, and are either vertically superposed or laterally juxtaposed within single sedimentation units. This suggests that these facies originated from a series of evolving sediment gravity flows. A cohesionless debris flow generated at the topset-foreset boundary or on the middle of the foreset slope segregated its sediments into a pebble-rich lower division and a sandy upper division with sparse cobble-to-boulder clasts by preferential upward drift of large clasts and surface transformation of fine-grained material. The pebbly lower division was emplaced by frictional freezing, forming Facies A beds. On the other hand, the sediments in the upper division continued to move as a sandy turbulent flow and bouldery debris fall, resulting in Facies E and D deposits downslope. The cohesionless debris flow occasionally transformed into a series of thinner flows by development of roll waves along the top of the flow, producing Facies B layers on the margin of a Facies A bed. Otherwise the cohesionless debris flow transformed into grain-assemblage debris fall and single-grain debris fall consecutively by removal of interstitial sand via downward percolation and stripping into ambient water. The debris falls produced gravel sheets (Facies C) and gravel lenses (Facies D). During this flow transformation, cobble- to boulder-size clasts and sandy material were selectively transported farther downslope, resulting in prominent textural bimodality of the prodelta deposits, which comprise isolated large clasts and lensoidal deposits of cobble-to-boulder gravel set in sandy background material.

Stratigraphy, petrochemistry and Quaternary depositional record of the Songaksan tuff ring, Jeju Island, Korea

Abstract The Songaksan tuff ring (STR) is one of several recent hydrovolcanic centers on Jeju Island, Korea, which provides an excellent example of proximal-to-distal facies changes in wet pyroclastic surge deposits. A multidisciplinary study has been carried out on the STR and adjacent lithostratigraphic units to constrain absolute age, geochemical characteristics, and Quaternary depositional history. A number of rock units were identified inside the crater of the STR, including Scoria deposit I, trachybasalt lava, Scoria deposit II, and a late-stage basaltic tuff, indicative of a rather complex sequence of magmatic and phreatomagmatic eruptions after the construction of the tuff ring. Petrochemical analysis shows that the STR was generated from different magma batches that fractionated from a homogenous magma chamber, and the early erupted magma was more evolved and volatile-rich. Reworking of the STR commenced shortly after the hydromagmatic eruption in a high-energy nearshore environment, resulting in deposition of the Hamori Formation. The formation is composed of planar-stratified and low- to high-angle cross-stratified tuffaceous (pebbly) sandstones and occurs up to an altitude of about 4 m above present sea level. 14C dating of molluscan shells beneath the formation indicates that it began to be deposited after about 4000 yr BP. Detailed sedimentary logging reveals that the formation consists of several stratal packages bounded by laterally persistent and distinct lithologic boundaries, probably formed by millennial-scale sea-level fluctuations. Occurrence of another hydrovolcanic sequence (the Sinyangri Formation) on the opposite side of Jeju Island, having similar sedimentary characteristics and ages, suggests that the sea-level fluctuations as seen in the Hamori Formation have affected a wide area of Jeju Island, probably related to the high-frequency sea-level oscillations during the post-6 ka BP regression period in the East Asian region. It can be concluded that the formation of the STR was possible because of the Holocene transgression, which made the present coastal areas water-saturated and adequate for hydrovolcanic eruptions. The STR in turn contributed to record high-frequency sea-level fluctuations during the Holocene via acting as a local and short-lived but affluent source of loose sediment.

Early-stage volcanism and sedimentation of Jeju Island revealed by the Sagye borehole, SW Jeju Island, Korea

Jeju Island comprises extensive lava flows and hundreds of monogenetic volcanic cones with rare and thin sedimentary deposits. However, a number of boreholes reveal that the lavas are only 50 to 120 m thick along the coastal regions and are underlain by a 100 m-thick volcaniclastic sedimentary formation, which is correlative with the Seoguipo Formation. Detailed examination of the subsurface Seoguipo Formation, as recovered from the Sagye borehole in the southwestern part of the island, reveals that the formation consists of primary hydrovolcanic facies (massive or inclined stratified lapilli tuff) emplaced by Surtseyan fallouts and debris flows in subaerial or subaqueous settings and reworked hydrovolcanic facies (stratified, massive, or normally graded pebble conglomerate and sandstone) and nonvolcanic facies (homogenous or laminated mudstone) that were deposited in subaerial to submarine (nearshore to offshore) settings. The predominance of volcaniclastic deposits in the formation demonstrates that the early stage of Jeju volcanism was characterized by repetitive hydrovolcanic activity probably because of an abundance of external water for hydroexplosion. The Seoguipo Formation is envisaged to consist of multiple, superposed hydromagmatic volcanoes with intervening, marine or nonmarine sedimentary sequences. This finding warrants redefinition of the Seoguipo Formation as a sedimentary succession, above the U Formation, composed of fossil-barren and and fossiliferous, subaerial and subaqueous, volcaniclastic and nonvolcanic sedimentary deposits beneath the plateauforming lavas.

Coarse-grained debris-flow deposits in the Miocene fan deltas, SE Korea: a scaling analysis

Abstract The viscoplastic and inertial grain-flow models have been widely used as tools for description and interpretation of ancient debris-flow deposits, providing a basis to estimate yield strength, viscosity, cohesion, and internal friction angle. Recent studies suggest, however, that a debris flow is an intimate mixture of solid and fluid, in which a number of momentum-transfer mechanisms operate. It is therefore necessary to evaluate the relative importance of different momentum-transport processes to properly describe a debris flow. Scaling analysis may be useful to this end, using the Bagnold number (the ratio of inertial grain stress to viscous shear stress), Savage number (the ratio of inertial grain stress to shear stress borne by sustained grain contacts), friction number (the ratio of frictional shear stress to viscous shear stress), and Darcy number (the ratio of grain–fluid interaction stress to inertial grain stress). Scaling analyses of Miocene gravelly debris-flow deposits in Korea suggest that different types of debris flows can be better distinguished by the analyses, providing some implications for debris-flow processes.

Geology of Tok Island, Korea: eruptive and depositional processes of a shoaling to emergent island volcano

Detailed mapping of Tok Island, located in the middle of the East Sea (Sea of Japan), along with lithofacies analysis and K-Ar age determinations reveal that the island is of early to late Pliocene age and comprises eight rock units: Trachyte I, Unit P-I, Unit P-II, Trachyandesite (2.7±0.1 Ma), Unit P-III, Trachyte II (2.7±0.1 Ma), Trachyte III (2.5±0.1 Ma) and dikes in ascending stratigraphic order. Trachyte I is a mixture of coherent trachytic lavas and breccias that are interpreted to be subaqueous lavas and related hyaloclastites. Unit P-I comprises massive and inversely graded basaltic breccias which resulted from subaerial gain flows and subaqueous debris flows. A basalt clast from the unit, derived from below Trachyte I, has an age of 4.6±0.4 Ma. Unit P-II is composed of graded and stratified lapilli tuffs with the characteristics of proximal pyroclastic surge deposits. The Trachyandesite is a massive subaerial lava ponded in a volcano-tectonic depression, probably a summit crater. A pyroclastic sequence containing flattened scoria clasts (Unit P-III) and a small volume subaerial lava (Trachyte II) occur above the Trachyandesite, suggesting resumption of pyroclastic activity and lava effusion. Afterwards, shallow intrusion of magma occurred, producing Trachyte III and trachyte dikes.The eight rock units provide an example of the changing eruptive and depositional processes and resultant succession of lithofacies as a seamount builds up above sea level to form an island volcano: Trachyte I represents a wholly subaqueous and effusive stage; Units P-I and P-II represent Surtseyan and Taalian eruptive phases during an explosive transitional (subaqueous to emergent) stage; and the other rock units represent later subaerial effusive and explosive stages. Reconstruction of volcano morphology suggests that the island is a remnant of the south-western crater rim of a volcano the vent of which lies several hundred meters to the north-east.

Geometry and kinematics of the Ocheon Fault System along the boundary between the Miocene Pohang and Janggi basins, SE Korea, and its tectonic implications

Detailed geological mapping and observations of various structural elements were made in order to determine the geometry and kinematics of the Ocheon Fault System (OFS) along the boundary between the Early Miocene Janggi and the Middle Miocene Pohang basins, SE Korea, and to reveal its roles on the basin evolutions. The OFS is a NE-trending relayed fault system composed of a number of NE or NNE-trending normal-slip and sinistral-normal oblique-slip faults, and has a scissor fault geometry decreasing in vertical offset southwestward. The constituent faults created independent grabens or half-grabens on the hanging-walls for the deposition of the Early or Middle Miocene strata. The OFS was initially the northwestern border fault of the Janggi Basin which acted as normal faults by the WNW-ESE tensional stress associated with the NNW-directed dextral simple shear caused by the East Sea opening. Afterwards, it experienced clockwise rotation with change of slip sense from normal-slip to sinistral-normal oblique-slip in response to the progressive dextral simple shear. At about 17 Ma, the shear stress propagating westward was released rapidly by the dextral strike-slip faulting of the NNW-trending Yeonil Tectonic Line (YTL) and the normal faulting of the NNE-trending western border faults of the Pohang Basin. At that time, the depocenter suddenly migrated northward and the depositional environment also changed rapidly from terrestrial to marine due to dramatic subsidence of the Pohang Basin. The Pohang Basin is interpreted to be a pull-apart basin extended at releasing bend/overstep between two PDZs (Principal Displacement Zones), i.e., the YTL and probably the East Korea Fault. The OFS was also reactivated as the eastern border faults of the Pohang Basin. In contrast to the western border faults, the OFS was rotated clockwise and could not be linked with the YTL because of its scissor fault geometry. Our results suggest that the NNW-trending regional dextral shear stress persisted for a considerable period of time in SE Korea during the East Sea opening, supporting the pull-apart opening of the East Sea rather than the fan-shaped opening. Most of the previous studies advocating the pull-apart opening emphasize the role of the NNE-trending strikeslip faults, like the Yangsan fault and OFS, as PDZs. In contrast, this study suggests that the NNE-trending faults in SE Korea acted as major normal faults at releasing bends or stepovers in the NNW-trending dextral fault system during the East Sea opening.

Shallow-marine records of pyroclastic surges and fallouts over water in Jeju Island, Korea, and their stratigraphic implications

Explosive volcanism results in a wide range of volcaniclastic deposits in many of Earth9s subaerial and subaqueous environments. In this paper, we introduce a unique, shallow-marine volcaniclastic deposit from Jeju Island, Korea, for which the materials were transported to the water surface by pyroclastic clouds and then settled from the surface as they were entrained in the water. The deposition occurred under alternating currents and still waters, which is most plausibly attributed to tidal processes. Mud flasers or drapes intercalated in the deposit, which indicate periods of slack water during tidal cycles, suggest that the deposit accumulated in a very short period of a fortnight or a month, about a million times faster than the adjacent sedimentary strata. Because of the unusually high sedimentation rate, the volcaniclastic deposit could record the “usual” fair-weather processes in the depositional site at a resolution that is almost never provided by ordinary sedimentary deposits. This finding highlights the biases in Earth9s stratigraphic records and teaches us that volcanic deposits, commonly regarded as the products of catastrophic events, can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot.

Lithofacies and architecture of a basinwide tuff unit in the Miocene Eoil Basin, SE Korea: Modes of pyroclastic sedimentation, changes in eruption style, and implications for basin configuration

The Miocene Eoil Basin, SE Korea, is a small half-graben basin that was rifted by both extensional and dextral strike-slip deformations during backarc opening of the East Sea (Sea of Japan). The basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits. The Paljori Tuff is a 2–20-m-thick, basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation. The tuff consists of two pyroclastic units (units II and III), which are underlain and overlain by two resedimented volcaniclastic units (units I and IV). Unit I at the base is composed mainly of fine-grained resedimented tephra; unit II is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin; unit III is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin-margin relief; and unit IV at the top is composed of coarse-grained resedimented tephra. The general coarsening-up of these units, the lack of nonvolcaniclastic sedimentary interbeds, and the offset stacking of the units toward the northeast suggest an episodic, short-duration eruption from the southwest that waxed in mass-eruption rate and changed eruption style from phreatomagmatic to magmatic. These units show basin-scale variations in geometry, including: (1) thick accumulations of unit I in the northeastern and southwestern extremities of the basin; (2) ponding of unit II in the southwestern part of the basin; (3) truncation of unit III in the same area; and (4) thickening of unit IV toward the northeastern basin margin. These provide important clues to the distribution of and temporal changes in accommodation space and, hence, the configuration and structural evolution of the Eoil Basin. It is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin; the southwest-northeast–trending axes of the basin plunge gently toward the northeast, and the major intrabasinal faults of the basin were produced before the eruption of the Paljori Tuff. The basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax. The Paljori Tuff shows that important clues to, and high-resolution records of, the large-scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit.