• Journal of the Korean earth science society
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• v.30 no.1
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• pp.10-18
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• 2009

Two fruit fossils of Albizia miokalkora were collected from the Miocene Duho Formation of Yeonil Group in the northern Pohang Beach and Changpodong, Gyeongsangbug-do, Korea. The legume is flat and long and has 7 rounded seeds. Although the legume fossils are preserved as impression, they show their whole shape well. It is considered that the fossil Albizia might have flourished in a warm temperate climate in East Asia. This discovery is the second record of Albizia from the Neogene of Korea.

• Economic and Environmental Geology
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• v.25 no.3
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• pp.351-358
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• 1992

The gravity measurement has been conducted at 327 station with an interval of 25 m along the survey lines of 1.6 km and 1.7 km traversing Hyungsan river and of 2.35 km and 2.42 km running N-S direction near Heunghae-eup in Pohang basin. Bouguer gravity anomalies were obtained, and geologic structure along four survey lines were interpreted by applying Fourier series and Talwani methods for two demensional body. A fault is in existence along the Hyungsan river, and northern block of it is displaced down by 150 m to 200 m relative to southern one. The thicknesses of Yeonil Group vary from 250 m to 550 m and from 150 m to 300 m in the northern and southern blocks of the fault, respectively. Another fault is in existence running E-W direction near Heunghae-eup, and its southern block is displaced down by about 250 m relative to its northern block. The thicknesses of Yeonil Group vary from 200 m to 400 m and from 500 m to 700 m in the southern and northern blocks of the fault, respectively. Above two faults are normal faults and make a graben structure, which results the age of rocks in the central region between the faults is younger than those of outside regions. This result coincides with that of paleontological study.

• Journal of the Mineralogical Society of Korea
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• v.11 no.1
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• pp.1-12
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• 1998

In Pohang area, basaluminite accompanying a little amounts of hydrobasalumnite, super-genetically occurs as whitish cryptocrystalline (2-4 $\mu\textrm{m}$) clay-like aggregates in the vicinity of altered carbonate concretions embedded within mudstones of the Tertiary Yeonil Group. A hydrobasaluminite changed readily into a basaluminite at room temperature in air, and, in turn, into a metabasaluminite when heating to 150$^{\circ}$~30$0^{\circ}C$. For the basaluminite, a monoclinic unit-cellparameters (a=14.845$\AA$, b=10.006$\AA$, c=11.082$\AA$, $\beta$=122.15$^{\circ}$) were calculated by X-ray powder diffraction data. Its basal reflections (001 and 002) are XRD analyses strongly indicate that the aluminum sulphate mineral has a layer structure and, at least, three types of water, i.e., (1) interlayer water (9.0 wt %), (2) crystal water (8.0 wt %), and (3) structural water (19.0 wt %). may present in its lattice. Based on TG-DTG data combined with EDS and IR analyses, a new chemical formula of Al5SO4(OH)134H2O was given to the basaluminite. Field occurrence and stable isotope data ($\delta$18O, $\delta$D, $\delta$34S) for the basaluminite seem to reflect that it was formed by the leached meteoric solution from surrounding mudstones during or after uplifting. An interaction of the acid solution with carbonate concretion and the resultant local neutralization of the fluid rich in Al3+ and SO42- are major controls on the basaluminite formation.

• Economic and Environmental Geology
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• v.25 no.3
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• pp.337-349
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• 1992

The Tertiary basins in Korea have widely been studied by numerous researchers producing individual results in sedimentology, paleontology, stratigraphy, volcanic petrology and structural geology, but interdisciplinary studies, inter-basin analysis and basin-forming process have not been carried out yet. Major work of this study is to elucidate evidences obtained from different parts of a basin as well as different Tertiary basins (Pohang, Changgi, Eoil, Haseo and Ulsan basins) in order to build up the correlation between the basins, and an overall picture of the basin architecture and evolution in Korea. According to the paleontologic evidences the geologic age of the Pohang marine basin is dated to be late Lower Miocence to Middle Miocene, whereas other non-marine basins are older as being either Early Miocene or Oligocene(Lee, 1975, 1978: Bong, 1984: Chun, 1982: Choi et al., 1984: Yun et al., 1990: Yoon, 1982). However, detailed ages of the Tertiary sediments, and their correlations in a basin and between basins are still controversial, since the basins are separated from each other, sedimentary sequence is disturbed and intruded by voncanic rocks, and non-marine sediments are not fossiliferous to be correlated. Therefore, in this work radiometric, magnetostratigraphic, and biostratigraphic data was integrated for the refinement of chronostratigraphy and synopsis of stratigraphy of Tertiary basins of Korea. A total of 21 samples including 10 basaltic, 2 porphyritic, and 9 andesitic rocks from 4 basins were collected for the K-Ar dating of whole rock method. The obtained age can be grouped as follows: $14.8{\pm}0.4{\sim}15.2{\pm}0.4Ma$, $19.9{\pm}0.5{\sim}22.1{\pm}0.7Ma$, $18.0{\pm}1.1{\sim}20.4+0.5Ma$, and $14.6{\pm}0.7{\sim}21.1{\pm}0.5Ma$. Stratigraphically they mostly fall into the range of Lower Miocene to Mid Miocene. The oldest volcanic rock recorded is a basalt (911213-6) with the age of $22.05{\pm}0.67Ma$ near Sangjeong-ri in the Changgi (or Janggi) basin and presumed to be formed in the Early Miocene, when Changgi Conglomerate began to deposit. The youngest one (911214-9) is a basalt of $14.64{\pm}0.66Ma$ in the Haseo basin. This means the intrusive and extrusive rocks are not a product of sudden voncanic activity of short duration as previously accepted but of successive processes lasting relatively long period of 8 or 9 Ma. The radiometric age of the volcanic rocks is not randomly distributed but varies systematically with basins and localities. It becomes generlly younger to the south, namely from the Changgi basin to the Haseo basin. The rocks in the Changgi basin are dated to be from $19.92{\pm}0.47$ to $22.05{\pm}0.67Ma$. With exception of only one locality in the Geumgwangdong they all formed before 20 Ma B.P. The Eoil basalt by Tateiwa in the Eoil basin are dated to be from $20.44{\pm}0.47$ to $18.35{\pm}0.62Ma$ and they are younger than those in the Changgi basin by 2~4 Ma. Specifically, basaltic rocks in the sedimentary and voncanic sequences of the Eoil basin can be well compared to the sequence of associated sedimentary rocks. Generally they become younger to the stratigraphically upper part. Among the basin, the Haseo basin is characterized by the youngest volcanic rocks. The basalt (911214-7) which crops out in Jeongja-ri, Gangdong-myon, Ulsan-gun is $16.22{\pm}0.75Ma$ and the other one (911214-9) in coastal area, Jujon-dong, Ulsan is $14.64{\pm}0.66Ma$ old. The radiometric data are positively collaborated with the results of paleomagnetic study, pull-apart basin model and East Sea spreading theory. Especially, the successively changing age of Eoil basalts are in accordance with successively changing degree of rotation. In detail, following results are discussed. Firstly, the porphyritic rocks previously known as Cretaceous basement (911213-2, 911214-1) show the age of $43.73{\pm}1.05$ 및 $49.58{\pm}1.13Ma$(Eocene) confirms the results of Jin et al. (1988). This means sequential volcanic activity from Cretaceous up to Lower Tertiary. Secondly, intrusive andesitic rocks in the Pohang basin, which are dated to be $21.8{\pm}2.8Ma$ (Jin et al., 1988) are found out to be 15 Ma old in coincindence with the age of host strata of 16.5 Ma. Thirdly, The Quaternary basalt (911213-5 and 911213-6) of Tateiwa(1924) is not homogeneous regarding formation age and petrological characteristics. The basalt in the Changgi basin show the age of $19.92{\pm}0.47$ and $22.05{\pm}0.67$ (Miocene). The basalt (911213-8) in Sangjond-ri, which intruded Nultaeri Trachytic Tuff is dated to be $20.55{\pm}0.50Ma$, which means Changgi Group is older than this age. The Yeonil Basalt, which Tateiwa described as Quaternary one shows different age ranging from Lower Miocene to Upper Miocene(cf. Jin et al., 1988: sample no. 93-33: $10.20{\pm}0.30Ma$). Therefore, the Yeonil Quarterary basalt should be revised and divided into different geologic epochs. Fourthly, Yeonil basalt of Tateiwa (1926) in the Eoil basin is correlated to the Yeonil basalt in the Changgi basin. Yoon (1989) intergrated both basalts as Eoil basaltic andesitic volcanic rocks or Eoil basalt (Yoon et al., 1991), and placed uppermost unit of the Changgi Group. As mentioned above the so-called Quarternary basalt in the Eoil basin are not extruded or intruaed simultaneously, but differentiatedly (14 Ma~25 Ma) so that they can not be classified as one unit. Fifthly, the Yongdong-ri formation of the Pomgogri Group is intruded by the Eoil basalt (911214-3) of 18.35~0.62 Ma age. Therefore, the deposition of the Pomgogri Group is completed before this age. Referring petrological characteristics, occurences, paleomagnetic data, and relationship to other Eoil basalts, it is most provable that this basalt is younger than two others. That means the Pomgogri Group is underlain by the Changgi Group. Sixthly, mineral composition of the basalts and andesitic rocks from the 4 basins show different ground mass and phenocryst. In volcanic rocks in the Pohang basin, phenocrysts are pyroxene and a small amount of biotite. Those of the Changgi basin is predominant by Labradorite, in the Eoil by bytownite-anorthite and a small amount pyroxene.

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

• The Journal of the Petrological Society of Korea
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• v.24 no.4
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• pp.323-335
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• 2015

Although the Daljeon Basalt in the Pohang Basin is important for interpreting the basin evolution, its relative chronology, stratigraphic position, and isotopic age still remain controversial. In order to clear up the controversies, this study carried out detailed field investigation to determine its distribution and occurrence together with reanalysis of its previous geochemical data and $^{40}Ar/^{39}Ar$ age dating. Based upon the field investigation, the basalt occurring in the central part of the Pohang basin is composed of three main bodies and a dozen of minor dikes and sills that intruded into the Yeonil Group. Their mineral assemblages consist of phenocrysts such as olivine and clinopyroxene and fine groundmasses of clinopyroxene, plagioclase, olivine, and opaque oxide, impling the porphyritic texture of alkaline basalt. All their geochemical data also show the similar geochemical characteristics in TAS, Zr-Ti, and REE/trace elements distribution diagrams. The samples are plotted on alkalic field in the total alkali-versus-silica diagram and show similar patterns to enrichment oceanic basalt or within plate basalt in trace elements. In addition, $^{40}Ar/^{39}Ar$ isochron age of 13.82Ma is obtained. These results indicate that the Daljeon Basalt is an alkaline intrusive rock belonging to the middle Miocene Yeonil Group.

• Journal of the Mineralogical Society of Korea
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• v.19 no.4 s.50
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• pp.311-324
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• 2006

In the Pohang-Gampo area, several diatomite beds occurred in mostly thinner than 1 m are embedded in the Pohang Formation of marine environment and the pyroclastic Eoil Formation. The diatomite from the Eoil Formation is constituting the high-grade ore altered slightly by diagenesis. In contrast, the diatomite intercalated within the upper horizon of the Yeonil Group is comparatively low-grade and highly altered in places. During diagenesis, an increasing of crystallinity of opal, i.e., the original mineral component of diatom, results in ultimately the mineral transition to quartz with accompanying a drastic change in morphology and texture of the altered diatomite. The diagenetic alteration appears to have undergone by way of the chemical diagenesis, which is largely controlled by degree of fluid contact, rather than burial diagenesis. For the diatomite from the Pohang-Gampo area, careful SEM observations, XRD, chemical analyses, and determination of specific surface area were done to identify the fossil species, mineral and chemical composition, and other physical properties in the view of assesment of grade and quality. The domestic diatomite ores are evaluated to be not good in grade and quality, compared to those of famous foreign localities. However, some diatomite deposits of marin,: origin from the Pohang Formation is constituting a peculiar clay-rich type, i.e., moler applicable to the special usage such as a manufacturing of lightweight brick. Because such a diatomite is frequently intercalated relatively as a thicker bed in the upper part of the Yeonil Group, a systematic and careful investigation should be done for the exploitation and development of an economic diatomite deposit of the moler type.

• The Journal of the Petrological Society of Korea
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• v.11 no.3_4
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• pp.182-199
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• 2002

We attempted to show the evolution of the magma and the geochemical characteristics of dikes and dike swarms by using the petrographic and geochemical data from 287 dikes, SE Korea. The dikes can be divided into mafic, intermediate, and felsic dikes in the field. And each of them is subdivided into three groups, two groups, and two groups, respectively. The group (I) among the mafic dikes most pervasively occurs and are distributed in both sides of the Yeonil Tectonic Line (YIL), which petrographic and geochemical characteristics are the same. These facts thus, strongly support the results of the previous studies which showed that they were intruded contemporaneously and that YTL was a main tectonic line which restricted the crustal clockwise rotation during the Early Miocene. The geochemical characteristics are discriminated according to the seven groups divided petrographically. The mafic, intermediate and felsic dikes belong to basalt and basaltic andesite, andesite and facile, and rhyolite, respectively, and the magmas mostly belong to calc-alkaline series. The geochemical data indicate that there were the fractional crystallizations of olivine, clinopyroxene, and plagioclase in the mafic dikes. And the content of characteristic elements and tectonic discrimination diagrams show that the dikes were formed from the magma related to the subduction of plate and that the tectonic setting was related to orogenic volcanic arc.

• Journal of the Mineralogical Society of Korea
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• v.13 no.1
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• pp.1-14
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• 2000

알루나이트는 포항지역의 제3기 연일층군의 이암 층내의 탄산염 결핵체 주변에서 할로이사이트와 함께 극미립 변질물 (1-2 $\mu\textrm{m}$)로서 산출된다. 알루나이트는 정육면체와 유사한 능면체 결정형을 이루고 침상 내지 단주상의 할로이사이트와 밀접한 공생관계를 이룬다. X-선회절 분석에 의해서 이 알루나이트는 a=6.9897(1) $\AA$, c=17.2327(4)$\AA$, V=728.75(3) $\AA$3의 격자상수값을 갖는 것으로 밝혀졌다. X-선형광된 이 알루나이트의 화학식은 (K0.94N0.06)(Al2.55Fe3+0.45)(SO4)2(OH)6 으로서, 나트로알루나이트 단성분을 6-7 mole%정도 함유하는 것으로 분석되었다. 또한 시차열분석 (TG-DTG-DTA)을 통해서 알루나이트의 승화성 성분들 (H2O와 SO3)의 존재와 함유 정도를 조사하였고, 고온X-선회절분석을 병행하여 이 광물의 OH기의 이탈 반응 (52$0^{\circ}C$)과 고온상으로의 전이 반응 ($600^{\circ}C$ 및 $700^{\circ}C$)을 감정 하였다. K/Ar 법으로 측정된 알루나이트의 생성 연대 ($0.342\pm$0.008 Ma)와 안정동위원소들의 분석 결과 ($\delta$18Oso4=-1.7, $\delta$DSMOW=-31, $\delta$34S=-10.8)는 이 알루미늄 황산염 광물이 연일충군의 융기 이후에 야기된 민물의 유입에 의한 표성기원의 변질작용의 결과로 생성되었음을 지시한다. 알루나이트+할로이사이트 공생군의 침전은 이암 내에서 조성된 강산성 (pH=2-3)의 알루미늄 황산염 용액이 탄산염 결핵체를 만나 반응하여 pH가 국지적으로 증가되어 (pH=4) 과포화되는 과정에 의해서 야기되었다. 컴퓨터를 이용한 Al3+의 포화지수에 관한 화학적 평형 모델링 실험 결과, 알루미늄 황산염 용액으로부터의 알루나이트와 할로이사이트의 침전은 pH=4 및 \ulcornerSO42-=10-4M 조건에서 K+과 Si(OH)4의 농도가 10-4M 이상 유지되어야 가능한 것으로 밝혀졌다.

• The Journal of the Petrological Society of Korea
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• v.16 no.2 s.48
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• pp.73-81
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• 2007

A basaltic tuff formation (Upper Basaltic Tuff of the Janggi Group) occurs in close association with basalt (Yeonil Basalt) at the Tertiary Janggi basin. The purpose of this paper is to describe the occurrence of the basaltic tuff and associated basalt and to determine their mode of formation. The basaltic rocks of the study area show few distinct lithofacies, all of which are originated from the interaction of basaltic magma with external water. The four lithofacies include (1) sideromelane shard hyaloclastite, (2) pillow breccia, (3) entablature-jointed basalt, and (4) in-situ breccia. The sideromelane shard hyaloclastite constitutes most of the Upper Basaltic Tuff and has a gradual contact with the pillow breccia. The pillow breccia consists of a poorly sorted mixture of isolated and broken pillows, and small basalt globules and fragments engulfed in a volcanic matrix of sideromelane shard hyaloclastite. The entablature-jointed basalt occurs as a small body within the hyaloclastite. It is characterized by irregularly-curved joints known as entablature. The in-situ breccia occurs as a marginal facies of entablature-jointed basalt, and its width varies from 10 to 30m. The result of this study indicates that the basaltic tuff and associated basalts of the study area were produced by the volcanic activity of same period and the basaltic tuff was formed by subaqueous eruption of basaltic lava followed by nonexplosive quench fragmentation.