• Economic and Environmental Geology
• /
• v.21 no.2
• /
• pp.117-129
• /
• 1988

The purpose of this study is to determine the stratigraphy of the volcanic rocks in the Chilpo-Weolpo area, the north of Pohang basin, based on field survey and lithological properties of the rocks. The volcanic pile(Chilpo tuff) overlies the Cretaceous sedimentary formation and is unconformably overlain by the Miocene Yeonil Group. The Chilpo tuff comprises a thick sequence(>200m) of pyroclastic flow deposits. Five members are distinguished, each representing separate flow units, comprising none(or weakly) to densely welded rhyolite tuff. The Chilpo tuff consists of, in ascending order, greenish weakly welded tuff, volcanic conglomerate, alternation of tuff breccias and fine tuffs, greenish none to densely welded tuff and red-brownish densely to weakly welded vitric tuff. This study revealed that the volcanic rocks in this area were formed by 4 volcanic stages. On the basis of K-Ar age($44.7{\pm}1.1\;Ma$) and lithologic data, geological age of the Chilpo tuff may be Eocene.

• Economic and Environmental Geology
• /
• v.18 no.2
• /
• pp.125-134
• /
• 1985

The Chisulryoung Volcanic Formation comprises a thick sequence of pyroclastic flow deposits. Six members are distinguished, each representing separate flow units, comprising weakly to intensely welded acidic tuffs. A stock of welded acidic tuff, 1 km in diameter, intrudes hornblende granodiorite and sediments of Late Cretaceous age and the lower part of the Chisulryoung Volcanic Tuff Formation and may represent the vent through which the upper flows of the Chisulryoug Volcanic Formation were erupted.

• The Journal of the Petrological Society of Korea
• /
• v.5 no.1
• /
• pp.108-120
• /
• 1996

This study reports petrography and geochemical characteristics of the Cretaceous volcanic rocks that are distributed in the vicinity of the Cheonsungsan area, Yangsan-Gun, Gyeongsangnam-Do. The Cretaceous volcanic rocks composed of andesitic rocks, Wonhyosan tuff, Cheonsungsan tuff in ascending order. Sedimentary rock is the basement in the study area cofered with volcanic rocks. These volcanic rocks are Wonhyosan tuff and Cheonsungsan tuff that represented the early phase of the Bulgugsa igneous activity. Wonhyosan tuff are classified into dacite tuff and dacite welded tuff based on the rock texture and their mineral composition. They are covered with Cheonsungsan tuff. Dacite tuff composed of lithic lapilli ash-flow tuff and vitric ash-flow tuff. Most dacite welded tuff are lapilli ash-flow tuff. Cheonsungsan tuff overlying the Wonhyosan tuff consists of rhyolite tuff and rhyolite welded tuff. Rhyolite tuff are lithic crystal ash-flow tuff and crystal vitric ash-flow tuff with somewhat accidental fragments of andesitic and sedimentary rocks. Rhyolite welded tuff is distinguishe from rhyolite tuff by is typical eelded fabrics and its rock color. According to petrochemical data, the volcanic rocks in study area belong to high-K orogenic suties. On the discriminant diagrams such as La/Yb versus Th/Yb, these rocks falls into the discriminant fields for the normal continental margin arc.

• Economic and Environmental Geology
• /
• v.55 no.3
• /
• pp.295-307
• /
• 2022

We studied the Cretaceous Jeonggaksan Formation to determine depositional processes of pyroclastic density currents entering into the lacustrine environments. This formation is composed largely of sandstone-mudstone couplets and (tuffaceous) normally graded sandstones deposited in lacustrine environments, interbedded with two pyroclastic beds: welded massive lapilli tuff and normally graded lapilli tuff. The welded massive lapilli tuff (10 m thick) is composed of poorly sorted, structureless lapilli supported by a welded ash matrix. The normally graded lapilli tuff (4 m thick) is characterized by moderately to well sorted natures and multiple normally graded divisions in the lower part of the bed with internal boundaries. The contrasting depositional features between these lapilli tuff are suggestive of different physical characteristics and depositional processes of pyroclastic density currents in the lake. Overall poorly sorted and massive natures of the thick, welded massive lapilli tuff are interpreted to have been formed by rapid settling of pyroclastic sediments from highly concentrated and sustained pyroclastic density currents. In this case, the pyroclastic density currents were able to displace lake water from shoreline and the pyrolclastic density currents preserved their own heat except for frontal parts of the currents. As a result, welded textures can be formed despite entrance of pyroclastic density currents into the lake. The internal boundaries of the normally graded lapilli tuff reflect unsteady natures of the pyroclastic density currents at the time of the deposition and the pyroclastic density currents can not provide sufficient pressure to displace lake water. As a consequence, the pyroclastic density currents transformed into water-saturated turbidity currents, forming relatively well sorted, normally graded lapilli tuff.

• Economic and Environmental Geology
• /
• v.21 no.4
• /
• pp.335-348
• /
• 1988

The author aimed to describe the volcano-stratigraphy and petrology of the volcanic mass in the Koheung peninsula, South Cheolla province. The volcanic mass is composed of the volcanics and intrusives of late Cretaceous which extruded the Pre-cambrian metamorphic(Jirisan gneiss complex) and the early Cretaceous sedimentary(Duwon Formation) basement. The volcanic pile consists of, in ascending order, Bibongsan andesite, Koheung tuff and breccia, and Palyeongsan welded tuff, and are intruded by ring intrusives( intrusive breccia, andesite porphyry, intrusive rhyolite and fine-grained quartz-diorite) and central pluton(diorite, quartz monzodiorite, biotite granite and micrographic granite). Bibongsan andesite mainly consists of andesite tuff and lava. Koheung tuff consists of alternation of fine tuff, coarse tuff and lapilli tuff, and Palyeongsan welded tuff which overlies Koheung tuff, comprises K-feldspar and quartz phenocrysts, elongated brown fiamme, lithic fragments in matrix of devitrified brown glass shards, and mainly consists of rhyodacite to rhyolite vitric ash-flow tuff. The results of petrochemical studies of the igneous rocks suggest that the rocks were a serial differentiational products of fractional crystallization of calc-alkaline magma series. This study reveals that the volcanic mass in this area is inferred to the remnant of the resurgent cauldron, measuring 30 by 25 km in diameter. The cauldron block was lowered at least 1,000 m by ring fault displacement.

• Economic and Environmental Geology
• /
• v.28 no.4
• /
• pp.415-423
• /
• 1995

A welded tuff with a near-vertical parataxitic fabric crops out as an elliptical shape($500{\times}350m$) in horizontal section, the Geumseongsan volcanic field. It intrudes the Cretaceous sedimentary rocks of the upper Hayang Group, surge tuff and rhyolite of the Geumseongsan volcanic complex. Generally it displays an ubiquitous, steeply inward-dipping welding foliation, subparallel to the margins of the intrusion, and a subvertically inward-inclined lineation defined by extremely stretched fiammes on the welding foliation plane. These fabrics suggest its overall form may be of an inverted cone-shaped plug representing a flared vent that served as a feeder for extrusive welded ash-flow tuff sheets.

• The Journal of the Petrological Society of Korea
• /
• v.23 no.4
• /
• pp.325-336
• /
• 2014

Even though Mesozoic Mudeungsan tuff, located within Neungju Basin, has been named several rock names, it should be named as Mudeungsan tuff due to several evidences, such as fiamme, welded texture and rock fragments in the Mudeungsan tuff. Volcanic eruption boundary between the Cheonwangbong and Anyangsan areas is not clear, but petrochemical and mineral chemical evidences with different ages indicate clear petrological boundary between Cheonwangbong and Anyangsan. The Mudeungsan tuffs from Cheonwangbong and Anyangsan is welded crystal tuff with dacitic composition and were generated from cogenetic calc-alkaline magma in the volcanic arc environment. Geochemical events indicate that magma beneath Cheonwangbong was seems to have been evolved from the magma beneath Anyangsan due to fractional crystallization dominated by plagioclase.

• Economic and Environmental Geology
• /
• v.25 no.2
• /
• pp.101-114
• /
• 1992

Occurrences of many kaolin-pyrophyllite deposits in the Youngnam area is related to the Late Cretaceous volcanic rocks, which are widely distributed through southern part from Tongnae-Yangsan to Miryang-Wolsung. The mode of occurrence and genesis of the kaolin-pyrophyllite deposits related to the volcanism was studied. This area is covered by andesitic rocks, rhyolite and rhyolitic welded tuff in ascending order. Lower most andesitic part is almost fresh. The altered rocks in the rhyolitic welded tuff can be classified into the following zones: silicified, pyrophyllite-kaolin, and argillic zone from the center part of ore deposit. The clay deposits occur as irregular massive, layer and funnel type about 5~20 m in width and is accompanied by thin diaspore bed outside of ore shoot. The clays chiefly consist of kaolinite, sericite, pyrophyllite, a little amount of diaspore, alunite, dumortierite, corundum and pyrite. The process of kaolinization-pyrophyllitization has a close relation to a local acidic hydrothermal solution originated from granitic rocks. Acidic hydothermal alteration occurrs mainly in the rhyolitic welded tuff. Initial solution containing $H_2S$ and others was oxidized near the surface and formed hydrothermal sulfuric acid solution.

• Korean Journal of Mineralogy and Petrology
• /
• v.34 no.4
• /
• pp.293-311
• /
• 2021

The volcanic rocks that make up Yeongdo island, an administrative district located on the southern coast of Busan, are composed of andesitic and rhyolitic rocks. Andesitic rock is mainly composed of volcanic breccia has a phenorysts of plagioclase and contains rock fragments. The rhyolitic rock is composed of volcanic angular rock at the base of Mt. Bongnae, and welded tuff forms the main mass of Mt. Bongnae. The fiamme structure can be easily observed with the naked eye, and the higher the altitude, the weaker the welded structure develops and the less the amount of rock fragments and crystals constituting the welded tuff. It is indicated that the magma that formed this study area is related to the tectonic environment of the continental margin related to subduction, and that it erupted after undergoing fractional cystallization at the same time with some contaminant in the continental crust. As a result of analyzing the main elements by altitude, it is believed to be the result of mixing at least 4 times or more of magma batches.

• The Journal of the Petrological Society of Korea
• /
• v.18 no.3
• /
• pp.195-209
• /
• 2009

Juwangsan area($107.4km^2$) has been designated as the twelfth National Park in 1976, because it has magnificent aspect and seasonally spectacular landscapes. Juwang valley($9,177.5m^2$) has been designated as Noted Scenery No. 11 in 2003, because it has the same topographical landscape as rock domes, rock cliffs, caves, waterfalls and plunge pools. The most spectacular landscapes are exhibited in the densely welded zone of the Juwangsan Tuff. The rock cliffs generated from vertical joints in the densely welded zone, in which there are the same many rock cliffs as Geupsudae, Haksodae, Sinseondae, Mangwoldae, Giam and Byeongpungbawi, three caves such as Juwanggul, Mujanggul and Yeonhwagul in the cliff. The cliffs and caves are landscapes that were generated from vertical joints in the densely welded zone, and the rock domes and waterfalls are landscapes of erosional vertical joint planes in the zone. The vertical joints are columnar joints formed during cooling of the Juwangsan Tuff. Therefore the topographical features cause the cooling columnar joints that are vertically intersected in the densely welded zone of the Juwangsan Tuff.