# Geology of South Korea

The geology of South Korea includes rocks dating to the Archean and two large massifs of metamorphic rock as the crystalline basement, overlain by thick sedimentary sequences, younger metamorphic rocks and volcanic deposits. Despite the country's small size, its geology is diverse, containing rocks formed during the Precambrian to Cenozoic eras.

## Geologic history, stratigraphy, and tectonics

The K'yŏnggi Massif and Yongnam Massif lie north and south of the Okch'on-T'aebaeksan Zone respectively. They are both polymetamorphic gneiss and schist complexes from the Precambrian and underlie the entire Republic of Korea. Units range in age from the Archean to the Proterozoic, with metamorphic facies from greenschist to amphibolite grade. Some geologists have attempted to correlate the rocks with North China-North Korea Paraplatform and Yangtze Paraplatform rocks although these categorizations are uncertain.

### Precambrian (–539 million years ago)

The rate which Precambrian rocks occupy in Korean Peninsula is 40%. The Korea Peninsula formed mostly after the Paleoproterozoic (2,500 to 1,600 million years ago), and Archean rocks appear in some regions.

### Paleozoic (539–251 million years ago)

Rock formations formed during the Paleozoic are composed of the Joseon Supergroup (lower) and Pyeongan Supergroup (upper).

#### Joseon Supergroup

From the early Cambrian to the Ordovician in the early Paleozoic, the Joseon Supergroup was formed, and it was distributed in the Pyeongnam Basin and Okcheon Fold Belt. The early Paleozoic Joseon Supergroup in Korea has been divided into the Taebaek, Yeongwol, Pyeongchang, Yongtan and Mungyeong groups, depending on the sequence of lithology. Taebaek group that located in Taebaek, Samcheok is composed of the Jangsan, Myobong, Daegi, Sesong, Hwajeol, Dongjeom, Dumugol (Dumu-dong), Makgol (Makgol), Jigunsan, Duwibong Formations. The Yeongwol group is composed of the Sambangsan, Machari, Wagok, Mungok, Yeongheung Formations. The geological table of the Joseon Supergroup is as follows.
| Geologic time scale | Taebaek Area | Taebaek Area | Taebaek Area | Taebaek Area | Yeongwol Area | Yeongwol Area | Yeongwol Area | Pyeongnam Area |
| Geologic time scale | division | Formation Name | Thickness(m) | Distribution region | Formation Name | Rock | Thickness(m) | Pyeongnam Area |
| Ordovician 443.8–485.4 Mya | upper great limestone group | Duwibong limestone | 50 | | Yeongheung | shale limestone | - | Sangseo-ri Mandal Singok |
| Ordovician 443.8–485.4 Mya | upper great limestone group | Jigunsan shale | 50~100 | | Yeongheung | shale limestone | - | Sangseo-ri Mandal Singok |
| Ordovician 443.8–485.4 Mya | upper great limestone group | Makgol limestone | 250~400 | Taebaek | Yeongheung | shale limestone | - | Sangseo-ri Mandal Singok |
| Ordovician 443.8–485.4 Mya | upper great limestone group | Dumugol shale (Odu) | 150~200 | Baegun Mt. Syncline Zone | Mungok | limestone Dolomite | 120~200 | Sangseo-ri Mandal Singok |
| Ordovician 443.8–485.4 Mya | upper great limestone group | Dongjeom Quartzite (Od) | 50 | Baegun Mt. Syncline Zone | Mungok | limestone Dolomite | 120~200 | Sangseo-ri Mandal Singok |
| Cambrian 485.4–538.8 Mya | lower great limestone formation | Hwajeol (CEw) Sesong | 200~260 | Baegun Mt. Syncline Zone East Danyang | Wagok | limestone Dolomite | 200~500 | Gopung |
| Cambrian 485.4–538.8 Mya | lower great limestone formation | Daegi(Pungchone limestone) (CEp) | 150~300 | Baegun Mt. Syncline Zone Southeast Jeongseon | Machari (Om) | shale limestone Dolomite | 420 | Mujin |
| Cambrian 485.4–538.8 Mya | Yangdeok Formation (C) | Myobong Slate (CEm) | 80~250 | | Sambangsan (cs) | Sandstone shale | - | heuggyo(C) |
| Cambrian 485.4–538.8 Mya | Yangdeok Formation (C) | Jangsan Quartzite (CEj) | 150~200 | Baegun Mt. Syncline Zone Danyang | | | | Junghwa(C) |

#### Pyeongan Supergroup

From the Carboniferous to Triassic, the Pyeongan Supergroup was formed, and it was distributed in the Pyeongnam Basin and Okcheon Folded Zone.
The geology and stratigraphy of the Pyeongan Supergroup are as follows.
| Geologic time scale | Geologic time scale | Geologic time scale | Area | Area | Area | Area | Area | Area | Area | Area |
| period | Epoch | Age | Samcheok | Gangneung | Jeongseon-Pyeongchang | Yeongwol | Danyang | Boeun | Pyeongnam Basin | Duman Basin |
| Mesozoic Triassic 201.3–252.17 Mya | Mid 237–247.2 Mya | Anisian | Donggo (Formation) | | Bakjisan | | Donggo | | taejawon | |
| Mesozoic Triassic 201.3–252.17 Mya | Early 247.2–252.17 Mya | Induan | Donggo (Formation) | | Sangwonsan | | Donggo | | taejawon | |
| Paleozoic Permian 252.17–298.9 Mya | Lopingian 252.17–259.8 Mya | | | | | | | | Gobangsan | Songsang |
| Paleozoic Permian 252.17–298.9 Mya | Guadalupian 259.8–272.3 Mya | Capitanian | Gohan | Unbyeolri | Okgapsan | | Gohan | | Gobangsan | Songsang |
| Paleozoic Permian 252.17–298.9 Mya | Guadalupian 259.8–272.3 Mya | Wordian | Dosagok | Mangdeoksan | Okgapsan | | Dosagok | | Gobangsan | Songsang |
| Paleozoic Permian 252.17–298.9 Mya | Guadalupian 259.8–272.3 Mya | Roadian | Dosagok | Mangdeoksan | Okgapsan | | Dosagok | | Gobangsan | Songsang |
| Paleozoic Permian 252.17–298.9 Mya | Cisuralian 272.3–298.9 Mya | Cisuralian | Hambaeksan | Hambaeksan | Okgapsan | | Hambaeksan | Hambaeksan | Gobangsan | Gyeryongsan |
| Paleozoic Permian 252.17–298.9 Mya | Cisuralian 272.3–298.9 Mya | Artinskian | Jangseong | Jangseong | Jangseong | Mitan | Jangseong | Jangseong | Sadong | Gyeryongsan |
| Paleozoic Permian 252.17–298.9 Mya | Cisuralian 272.3–298.9 Mya | Sakmarian | | | Bamchi | Bamchi | | Bamchi | Sadong | Amgi |
| Paleozoic Permian 252.17–298.9 Mya | Cisuralian 272.3–298.9 Mya | Asselian | | | Bamchi | Bamchi | | Bamchi | ibseog | Amgi |
| Paleozoic Carboniferous 252.17–298.9 Mya | Pennsylvanian 298.9–323.2 Mya | Moscovian | Geumcheon | Geumcheon | Geumcheon | Pangyo | Geumcheon | | hongjeom | |
| Paleozoic Carboniferous 252.17–298.9 Mya | Pennsylvanian 298.9–323.2 Mya | Moscovian | Manhang | Manhang | Manhang | Yobong | Manhang | Manhang | hongjeom | |
| Paleozoic Carboniferous 252.17–298.9 Mya | Pennsylvanian 298.9–323.2 Mya | Bashkirian | Manhang | | Manhang | Yobong | Manhang | | | |

#### Okchon (folded) Zone

The Okch'ŏn Zone likely formed in mid-Cambrian times with faulting, interpreted from olistolith limestone breccia.
In contrast to the Chŏsun Supergroup, the Okch'ŏn Supergroup crops out in the central Okch'on-T'aebaeksan Zone with thick sequences of metasedimentary and metavolcanic rocks. Some geologists have interpreted the supergroup as a series of nappe formations that took shape in a Cambrian intracratonic basin. Above the volcanic and sedimentary sequence in the middle part of the supergroup are jumbled rocks formed from submarine debris flows during rifting and contain granite, gneiss, quartzite, limestone, mudstone and basic volcanic rock fragments.
At first, the Taebo orogeny was credited with the folding, thrusting and metamorphism but Cluzel, Jolivet and Cadet in 1991 named the Okch'on orogeny to explain Silurian and Devonian deformation.
South Korea has no Silurian or Devonian sedimentary rocks, but sedimentation began again on a sinking paralic platform inland from the proto-Japan as it formed beginning in the Carboniferous. The P'yŏngan Supergroup outcrops northeast of the Okch'on Zone, subdivided into the Hongjom Formation gray mudstone, limestone and mudstone, Sadong Formation sandstone, mudstone and coal seams, Kobangsan Formation coarse terrestrial sandstone and mudstone, and Nogam Formation green sandstone and mudstone.

### Mesozoic (251–66 million years ago)

During the Triassic at the beginning of the Mesozoic, as sedimentation continued in the P'yŏngan Supergroup, the Sŏngnim tectonic event affected the Okch'on-T'aebaeksan Zone, although it only caused slight faulting and warping of the major supergroup strata. Geologists have inferred that the event was related to deformation further west in Indonesia.
The event generated dextral strike-slip faulting in intermontane troughs in the Kyonggi Massif, in which the terrestrial sediments of the Taedong Supergroup accumulated. The rocks in the basin include two sequences of conglomerate grading to sandstone, mudstone, and coal beds. The Taedong Supergroup has extensive fossils, particularly crustaceans.

#### Taebo Orogeny

The Taebo orogeny in the Jurassic is broadly similar to the Yenshanian tectonism in China, although its effects are believed to have been less dramatic. The event ended with the batholith intrusions of the Taebo granites which outcrop over 30 percent of the country.

#### Gyeongsang Basin

The Gyeongsang Basin is a basin that was formed during the Cretaceous. It occupies most of the Yeongnam region and part of the Honam region.
The Gyeongsang Basin is composed of the Gyeongsang Supergroup, which consists of Sindong, Hayang, Yucheon group, and Bulguksa Granite. The Sindong Group is the lowest stratigraphic sequence in the Cretaceous Gyeongsang Basin and comprises three stratigraphic units: the Nakdong, Hasandong, and Jinju Formation in ascending order. Hayang group is mid group of Gyeongsang supergroup. Yucheon group is the top group of Gyeongsang supergroup.
Kyŏngsang rocks are intruded by microlite, diorite and granodiorite from the late Cretaceous.

### Cenozoic (66 million years ago–present)

Volcanic activity continued into the Cenozoic ending 50 million years ago. No Paleogene sedimentary rocks have been found onshore. Yangbuk Group conglomerates and alluvial fan sediments gathered in small fault-bounded basins in the Miocene. During the last 2.5 million years of the Quaternary, Jeju and other offshore islands formed from volcanism.