Geology of South Korea

Most Paleozoic rock outcrops are found only in the Okch'on-T'aebaeksan Zone. The Chŏsun Supergroup in the northeast of the zone is cut by a northeast rending fault zone called the Tangyang-Chongsŏn Thrust Zone. It splits the Tuwibong facies marine siliclastic and carbonate rocks from the two kilometer thick Yŏngwol Cambrian and Ordovician fossil-bearing sandstone, mudstone and limestone. The Tuwibong rocks are the same age and reach up to one kilometer thick, beginning with a basal quartzite layer on top of the Yongnam Massif and ascending through limestone, dolomite and mudstone units.

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. [2]

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.

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. The Kyŏngsang Supergroup encompasses more terrestrial rocks, also formed in small basins. The Shindong Group is the oldest: a coarse alluvial fan two kilometers thick, overlain by the five kilometer thick alluvial, lacustrine and fluvial sediments with interspersed basalt and andesite of the Hayang Group. The Yuch'on Group is the top unit, with volcanic rocks and interbedded sediments. In the southern Kyŏngsang Supergroup low shields formed from cooling andesite flows, followed by explosive rhyolite volcanism. The entire supergroup formed in the Cretaceous prior to Japan's southward migration from Asia.

Kyŏngsang rocks are intruded by microlite, diorite and granodiorite from the late Cretaceous. [3]

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, Cheju and other offshore islands formed from volcanism.