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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.^[1] Despite the country's small size, its geology is diverse, containing rocks formed during the Precambrian to Cenozoic eras.^[2]

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%.^[3] It is confirmed that Korea peninsula is formed mostly after Paleoproterozoic (2,500 to 1,600 million years ago) and Archean rocks appear in portion region.^[4]

Paleozoic (539–251 million years ago)

The rock formation which is formed during Paleozoic is composed of 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 distributed in the Pyeongnam Basin and Okcheon Fold Belt. The early Paleozoic Joseon Supergroup in Korea has been divided into Taebaek, Yeongwol, Pyeongchang, Yongtan and Mungyeong groups, depending on the sequence of lithology.^[5] Taebaek group that located in Taebaek, Samcheok is composed of Jangsan, Myobong, Daegi, Sesong, Hwajeol, Dongjeom, Dumugol (Dumu-dong), Makgol (Makgol), Jigunsan, Duwibong Formation. Yeongwol group is composed of Sambangsan, Machari, Wagok, Mungok, Yeongheung Formation.^[6] The geological table of the Joseon Supergroup is as follows.^[7]^[8]^[9]

Geologic time scale	Taebaek Area				Yeongwol Area			Pyeongnam Area
Geologic time scale	division	Formation Name	Thickness(m)	Distribution region^[10]	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
		Jigunsan shale	50~100
		Makgol limestone	250~400	Taebaek
		Dumugol shale (Odu)	150~200	Baegun Mt. Syncline Zone	Mungok	limestone Dolomite	120~200
		Dongjeom Quartzite (Od)	50	Baegun Mt. Syncline Zone	Mungok	limestone Dolomite	120~200
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
	lower great limestone formation	Daegi(Pungchone limestone) (CEp)	150~300	Baegun Mt. Syncline Zone Southeast Jeongseon	Machari (Om)	shale limestone Dolomite	420	Mujin
	Yangdeok Formation (C)	Myobong Slate (CEm)	80~250		Sambangsan (cs)	Sandstone shale	-	heuggyo(C)
	Yangdeok Formation (C)	Jangsan Quartzite (CEj)	150~200	Baegun Mt. Syncline Zone Danyang				Junghwa(C)

Pyeongan Supergroup

From Carboniferous to Triassic, The Pyeongan Supergroup was formed, and it distributed in the Pyeongnam Basin and Okcheon Folded Zone.^[13]^[14]

The geological table and stratigraphy of the Pyeongan Supergroup are as follows.^[15]

Geologic time scale			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
	Guadalupian 259.8–272.3 Mya	Capitanian	Gohan	Unbyeolri	Okgapsan		Gohan
		Wordian	Dosagok	Mangdeoksan			Dosagok
		Roadian	Dosagok	Mangdeoksan			Dosagok
	Cisuralian 272.3–298.9 Mya	Cisuralian	Hambaeksan	Hambaeksan			Hambaeksan	Hambaeksan		Gyeryongsan
		Artinskian	Jangseong	Jangseong	Jangseong	Mitan	Jangseong	Jangseong	Sadong	Gyeryongsan
		Sakmarian			Bamchi	Bamchi		Bamchi	Sadong	Amgi
		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
		Moscovian	Manhang	Manhang	Manhang	Yobong	Manhang	Manhang	hongjeom
		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. By 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. ^[16]

Mesozoic (251-66 million years ago)

In the Triassic at 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 the basin that is formed at Cretaceous.^[18] it occupies most of the Yeongnam region and part of Honam region.

The Gyeongsang Basin is composed of 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.^[19] Hayang group is mid group of Gyeongsang supergroup.^[20] Yucheon group is the top group of Gyeongsang supergroup.^[21]

Kyŏngsang rocks are intruded by microlite, diorite and granodiorite from the late Cretaceous. ^[22]

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.