Thegeology of South Korea includes rocks dating to theArchean and two large massifs ofmetamorphic rock as thecrystalline 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 thePrecambrian toCenozoic eras.^[2]

The K'yŏnggi Massif andYongnam Massif lie north and south of the Okch'on-T'aebaeksan Zone respectively. They are both polymetamorphicgneiss andschist complexes from thePrecambrian and underlie the entire Republic of Korea. Units range in age from theArchean to theProterozoic, withmetamorphic facies fromgreenschist toamphibolite grade. Some geologists have attempted to correlate the rocks with North China-North Korea Paraplatform and Yangtze Paraplatform rocks although these categorizations are uncertain.

The rate which Precambrian rocks occupy in Korean Peninsula is 40%.^[3] The Korea Peninsula formed mostly after thePaleoproterozoic (2,500 to 1,600 million years ago), and Archean rocks appear in some regions.^[4]

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

From the early Cambrian to the Ordovician in the early Paleozoic, the Joseon Supergroup was formed, and it was distributed in thePyeongnam Basin and Okcheon Fold Belt. The early Paleozoic Joseon Supergroup in Korea has been divided into theTaebaek,Yeongwol,Pyeongchang, Yongtan andMungyeong groups, depending on the sequence of lithology.^[5] Taebaek group that located inTaebaek,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.^[6] The geological table of the Joseon Supergroup is as follows.^[7][8][9]

From theCarboniferous toTriassic, the Pyeongan Supergroup was formed, and it was distributed in thePyeongnam Basin and Okcheon Folded Zone.^[13][14]

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

The Okch'ŏn Zone likely formed in mid-Cambrian times with faulting, interpreted fromolistolith limestonebreccia.

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 ofnappe 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, theTaebo orogeny was credited with the folding, thrusting and metamorphism but Cluzel, Jolivet and Cadet in 1991 named theOkch'on orogeny to explainSilurian andDevonian 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 theCarboniferous. TheP'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]

During theTriassic at the beginning of theMesozoic, 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 inintermontane 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.

The Taeboorogeny in theJurassic 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.

The Gyeongsang Basin is a basin that was formed during theCretaceous.^[18] It occupies most of theYeongnam region and part of theHonam region.

The Gyeongsang Basin is composed of the Gyeongsang Supergroup, which consists of Sindong, Hayang, Yucheon group, and BulguksaGranite. The Sindong Group is the lowest stratigraphic sequence in the Cretaceous Gyeongsang Basin and comprises three stratigraphic units: the Nakdong,Hasandong, andJinju 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 bymicrolite,diorite andgranodiorite from the late Cretaceous.^[22]

Volcanic activity continued into theCenozoic ending 50 million years ago. NoPaleogene sedimentary rocks have been found onshore. Yangbuk Group conglomerates and alluvial fan sediments gathered in small fault-bounded basins in theMiocene. During the last 2.5 million years of theQuaternary,Jeju and other offshore islands formed from volcanism.

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