Myogok Formation

Myogok Formation
Stratigraphic range: Berriasian
~138.6 Ma
Outcrop of the Myogok Formation
TypeGeological formation
UnderliesUllyeonsan Formation
OverliesWonnam Formation
ThicknessUnknown
Lithology
PrimaryShale, mudstone and sandstone
Location
Coordinates36°30′N 128°35′E / 36.50°N 128.58°E / 36.50; 128.58
RegionNorth Gyeongsang Province
Country South Korea
Type section
Named forMyogok, Bonghwa
Named byLee and Lee, 1963
Myogok Formation is located in South Korea
Myogok Formation
Myogok Formation (South Korea)

The Myogok Formation(Myogog Formation in older literatures)(Korean: 묘곡층; Hanja: 卯谷層) is a Lower Cretaceous geologic formation in South Korea. Myogok Formation is chronologically intermediate between Upper Triassic-Middle Jurassic Daedong Supergroup and the Lower Cretaceous Hauterivian-Upper Cretaceous Gyeongsang Supergroup.

Geology

The Myogok Formation is locally distributed at the Yeongyang Subbasin of the Gyeongsang Basin, and is contiguous to the boundary of the Yeongnam Massif. The lithology of the Myogok Formation is mainly consists dark gray to black shale and mudstone, and subordinately of light to dark grey siltstone and sandstone. In the shale deposited in the lower part of the formation, two or three thin coal beds are present.

Regarding the interpretation of the paleoenvironment of the Myogok Formation, multiple suggestions are present. Chun et al. (1991) suggested the Myogok Formation was deposited at the point-bar of the meandering river system, whereas Kong et al. (2020) suggested paleoienviornment of the Myogok Formation was lacustrine setting. This is caused by poor outcrop continuity and severe deformation of the formation, which makes it hard to draw multiple correlatable columnar sections.

Age

Myogok Formation was initially suggested as Upper Jurassic based on plant and mollusc fossils. While it is definite that the Myogok Formation was deposited later than the Daedong Supergroup based on plant fossils, segregation with the Gyeongsang Supergroup was uncertain based on plant fossils, due to most of taxa except Podozamites sp. are overlapping with the Nagdong Formation. Chang and Yang (1970) suggested that the depositional age of the Myogok Formation might be similar to the lower part of the Gyeongsang supergroup, but taxa presented in the research are not suitable for comparison due to wide range from Late Jurassic to Early Cretaceous. Whereas, bivalve fossils took an important role to show chronological gap was present with the Nagdong Formation, by showing distinct species not found in entire Gyeongsang Supergroup.

Since 1990s, researches based on palynofossils, molluscan fossils and the detrital zircon U-PB ages show the depositional age of the Myogok Formation dates back to the Early Cretaceous, especially Berriasian(138.6±2.1Ma).

Fossil Records

Molluscs

Molluscan fauna of the Myogok Formation could be related to Japanese assemblages, including fossil records from the Tatsukawa Formation, Izuki Formation, Okurodani Formation and the Kuwajima Formation, based on similarities of the Korean Viviparus cf. ongoensis and the Japanese V. ongoensis. But precise comparisons between Korean and Japanese fossils are difficult since the former is much better preserved than the latter.

Molluscs of the Myogok Formation
Genus Species Location Stratigraphic position Material Notes Images
Koreania K. cheongi
Nagdongia N. leei
Cuneopsis C. kihongi
Viviparus V. cf. ongoensis

Plants

Plants of the Myogok Formation
Genus Species Location Stratigraphic position Material Notes Images
Onychiopsis O. elongata
Cladophlebis C. tenue
C. falcata
C. argutula
C. haibumensis
C. sp. cf. C. undulata
C. raciborskii
C. acutipennis
C. sp.
Ruffordia R. sp.
Coniopteris C. sp.
Nilssonia N. sp.
Equisetites E. sp.
Neocalamites N. sp.
Dictyozamites D. sp.
Otozamites O. sp.
Zamites Z. sp.
Schizolepis S. sp.
Podozamites P. distans

References

  1. 1 2 Lee, Dai Sung; Lee, Ha Young (December 30, 1963). "Explanatory text of the geological map of Yean sheet (SHEET 6924-II SCALE 1:50000)". Geological Survey of Korea: 1–6. doi:10.22747/data.20211221.4669.
  2. 1 2 3 Lee, Seung-Eun; Kwak, Yujung; Park, Seung-Ik (28 April 2025). "A Review on the Geology and Basin Evolution History of the Myogok Formation". Economic and Environmental Geology. 58 (2): 151–161. doi:10.9719/EEG.2025.58.2.151.
  3. 1 2 3 4 5 Cheong, Chang Hi; Lee, Ha Young (31 December 1966). "The Myogog Formation and Its Tectonic Significance". Journal of the Geological Society of Korea. 2 (2): 21–35. doi:10.14770/jgsk.1966.2.2.21.
  4. 1 2 Chun, H.Y.; Bong, P.Y.; Choi, S.J.; Kim, B.C.; Kim, Y.B.; Kwon, Y.I.; Lee, H.I.; Lee, M.S.; Um, S.H. (December 1991). Stratigraphic and Paleontologic Study of Myogog Formation (PDF). Korea Institute of Energy and Resources. pp. 1–77.
  5. ↑ Kong, EunHye; Lim, Seonmi; Cheong, Daekyo (30 June 2020). "Stratigraphy and sedimentary facies of the Myogok formation and the lower sequences in the Yeongyang Subbasin of the Cretaceous Gyeongsang Basin". Journal of the Geological Society of Korea. 56 (3): 327–354. doi:10.14770/jgsk.2020.56.3.327.
  6. 1 2 Yang, Seong-Young (1976). "On the Non-Marine Molluscan Fauna from the Upper Mesozoic Myogog Formation Korea". Transactions and proceedings of the Paleontological Society of Japan. New series. 1976 (102): 317–333. doi:10.14825/prpsj1951.1976.102_317.
  7. ↑ Chang, Ki Hong; Yang, Sung Yung (30 June 1970). "STRATIGRAPHIC POSITION OF GYEONGJEONGDONG AND MYOGOG FORMATIONS". Journal of the Geological Society of Korea. 6 (2): 129–133. doi:10.14770/jgsk.1970.6.2.129.
  8. 1 2 3 Yang, Seong-Young (31 March 1984). "Paleontological Study on the Molluscan Fauna from Myogog Formation, Korea. (pt. 2)". Journal of the Geological Society of Korea. 20 (1): 15–27. doi:10.14770/jgsk.1984.20.1.15.
  9. ↑ Yi, Myung-Suk; Chun, Hee-Young (30 June 1993). "Palynomorphs from the Myogog Formation, Korea, and Their Stratigraphic Significance". Journal of the Geological Society of Korea. 29 (3): 246–257. doi:10.14770/jgsk.1993.29.3.246.
  10. 1 2 Kozai, Takeshi; Ishida, Keisuke; Hirsch, Francis; Park, Sun-Ok; Chang, Ki-Hong (1 February 2005). "Early Cretaceous non-marine mollusc faunas of Japan and Korea". Cretaceous Research. 26 (1): 97–112. doi:10.1016/j.cretres.2004.11.003.
  11. ↑ Lee, Tae-Ho; Park, Kye-Hun; Yi, Keewook; Geng, Jian-zhen; Li, Huai-kun (June 2015). "SHRIMP U-Pb ages and Hf isotopic composition of the detrital zircons from the Myogok Formation, SE Korea: development of terrestrial basin and igneous activity during the early Cretaceous". Geosciences Journal. 19 (2): 189–203. doi:10.1007/s12303-014-0042-6.