• Korean Journal of Heritage: History & Science
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• v.45 no.1
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• pp.218-231
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• 2012

A total of 126 fossil scaphopods (121 specimens from the Hagjeon Formation and 5 specimens from the Duho Formation) were found from the Cenozoic strata, Hagjeon and Duho Formations, Pohang Basin, Korea. Five species belonging two genera (Fissidentalium yokoyamai, F. sp. A, B, and Rhabdus sp. A, B) were classified: the most dominant species is Fissidentalium yokoyamai. The species of Fissidentalium yokoyamai is characterized by curved shell (accuration=3.90%) and very closely spaced longitudinal ribs on shell surfaces, while the species of Rhabdus is a nearly straight shell characterized by concentric growth lines without longitudinal ribs. Identification of two genera is somewhat easy due to such morphological differences but classification at generic level is hard because diagnostic features (e.g., cross section and apical structure) are lost in the most specimens. Consequently, except for Fissidentalium yokoyamai, the rest were classified temporarily as F. sp. A, B, and Rhabdus sp. A, B. Two types of preservation state were recognized: one is three-dimensionally preserved specimen (3D specimen) and the other is compressed specimen. Internal parts of the 3D specimen is filled with clastic sediments identical to the surrounding sediments of the shells, which is not observed in the compressed specimens. It is, thus, concluded that the 3D fossils were originally empty but internal cavity were immediately filled with the sediments, which may have protect from the compaction due to pressure during deposition of the gravelly to coarse sandstone of the Hagjeon Formation.

• Economic and Environmental Geology
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• v.23 no.2
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• pp.201-213
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• 1990

Mineralogy, chemistry, physical property, and fossil abundance have been studied for the samples collected from three weathering profiles, two from the Duho Formation and one from the Hagjeon Formation in the Tertiary sediments in the Pohang area. The mineralogy of the samples from the Duho Formation shows somewhat different from that of the Hagjeon Formation. Kaolinite is more abundant and shows higher crystallinity in samples from the Duho Formation than those in the Hagjeon Formation, but clay mineral assemblage in each weathering profile remain fairly constant with depth. This difference in mineral distribution seems to be inherited from original source materials. It indicates that little or not severe leaching has been taking place in these three weathering profiles. Weathering indicies indicate different degrees of susceptibility to chemical weathering in these two formations. The Duho Formation has a higher degree of susceptibility to weathering than that of the Hagjeon Formation which is mainly due to differences in clay mineral assemblages in both formations. A noticeble colour difference between oxidized and unoxidized zones in each profile can be easily recognized which is definetely due to different decomposition rate organic carbon by various oxidation state from surface to bottom of the profile. Weathering process have also intensively influenced microfossil preservation about up to 7-10 m in depth in the Duho Formation. Consequently, characteristics observed at weathering surface should be used as a subsidiary tool in setting geologic boundary or establishing formation. Great care must be taken to choose sampling site for microfossil study.

• Economic and Environmental Geology
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• v.26 no.4
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• pp.507-518
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• 1993

Paleomagnetic investigations have been carried out on the Tertiary sedimentary formations (Yeonil Group) in Pohang basin, southeastern Korea. A total of 215 samples were collected from 26 sites. Progressive thermal demagnetization indicates that many samples have unstable magnetization and do not reveal a characteristic direction. However, some samples from the lower and upper Duho Formation show a characteristic direction $D/I=7.8^{\circ}/48.3^{\circ}$ (${\alpha}_{95}=3.7^{\circ}$, k=174.1). Stepwise thermal demagnetization data show that some samples from the Hagjeon and middle Duho Formations reveal great-circle distributions moving from the present to a reversed direction of geomagnetic field. The mean of intersection points of the great-circles is nearly antipodal to the characteristic normal direction of the lower and upper Duho Formation. We infer that the Hagjeon Formation was formed during the reversed polarity chron C5B (16.2~14.7 Ma) and the Duho Formation 14.7~11.6 Ma based on our results and previous paleontologic and age dating data. Paleomagnetic direction for the Middle Miocene of Korea, analysed from the combined results of stable endpoints and great circles, is $D/I=8.7^{\circ}/53.9^{\circ}$ (${\alpha}_{95}=4.2^{\circ}$, k=74.8), and the corresponding paleopole is Lat./Long.=$82.7^{\circ}/230.2^{\circ}$ (${\delta}p=2.8^{\circ}$, ${\delta}m=5.9^{\circ}$). On the basis of this, we interpret that the opening of the East sea (Japan sea) or the synchronous clockwise rotation of the Southwest Japan exerted no structural influence on the Yeonil Group in the Middle Miocene.

• Economic and Environmental Geology
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• v.42 no.4
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• pp.367-376
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• 2009

Forty specimens of fossil ophiuroids were collected from two formations of the Cenozoic marine deposits, Duho Formation and Hagjeon Formation, Pohang Basin, Korea. A few specimens were three-dimensionally preserved: most of them were remained flattened and articulated. Although a gross morphology is well preserved in some specimens, the details such as disk shape and plates which is the most important diagnostic features were not observed. Most of the arms are disarticulated, and thus arm vertebra, arm spines and/or arm plates are separately preserved. Only an oral side is recognized in an attached specimens because crossing arms into disk are clearly visible. It is, thus almost impossible to identify Korean ophiuroid fossils as a species level or even a genus level. The fossils were classified into 3 groups by gross morphology of disk and arm, and architecture of vertebra. The most abundant fossils (32 specimens), were found only in the Hagjeon Formation. The majority of specimens are partially disarticulated, having only proximal and median portions of their arms preserved. Arm plates are disarticulated from arm vertebra: most of them show structure of arm vertebra. It has long and well-preserved arm spines, and large tentacle pore. Some specimens (4 specimens) from the Duho Formation is characterized by short and conical arm spines, and well-developed arm plates. Lateral arm plates are small in compared to dorsal and ventral arm plates. The others (4 specimens) is poor in preservation state showing circular oral disk and relatively short sinuous arms. No arm plates are either identified.