Geology

[punuri]

[Introduction]

	The first geological map of Korea, with a scale of 1:1,000,000, was published in 1928. In 1974, geological maps with a scale of 1:250,000 covering the whole of the Republic were published. The Geological Survey of Korea started to publish geological quadrangles with a scale of 1:50,000 from 1961, and by 1996, 83% of the landmass was covered. The Geological Society of Korea was established in 1964, the Korea Institute of Mining Geology in 1968, and the Paleotological Society of Korea in 1984. The Geological Survey of Korea was renamed the Korea Institute of Geology, Mining and Materials (KIGAM) in 1991.

      [Geological Overview]

	Physiographically, Korea is a mountainous peninsula extending south-southeast from the northeastern part of the China mainland. The north-northwest, south-southeast trend forms the Taebaeksanmaek range, which is close to the east coast. The east coast is of an uplifted topography, showing a relatively straight shoreline, whereas the west coast shows the features of a submerging shoreline. The mountains are not high, rarely exceeding 1,200 meters, but they are found almost everywhere. As a consequence, the terrain is rugged and steep. Only near the west and southwest coasts are there extensive flat alluvial or deluvial plains and more subdued rolling hilly lands. Being a mountainous peninsula, Korea is of a diverse geologic make-up. It is composed largely of Precambrian rocks, such as granite gneisses and other metamorphic rocks. Two separate blocks of Paleozoic Strata are found in South and North Korea. The one in the South covers the Taebaeksanmaek range, and the one in the North is near Pyeongyang. Mesozoic Strata are found in the southeastern part of the peninsula and Cenozoic Strata are limited to some small areas scattered around the peninsula. Jurassic and Cretaceous granites intrude through the older rocks in a northeastward-southwestward direction in some places, but show no specific trend in others. Unlike nearby Japan, Korea is a stable landmass with no active volcanoes and rare earthquake shocks, although the islands of Ulleungdo and Jejudo are of volcanic origin. Mt. Baekdusan in the North is capped with a caldera lake, and Mt. Hallasan on Jejudo island has a small crater lake. For more details about Korea's geological make-up, please refer to the appendix II located at the end of this chapter.

      [Earthquakes]

	Nearly 1,800 earthquakes are recorded in various historical documents of past dynasties from A.D. 2 to 1907 and more than 200 have been scientifically recorded since 1905 on the Korean Peninsula. Thus, the total number of recorded earthquakes in Korea is a little more than 2,000 of which only 48 were destructive - far fewer than those recorded in Japan but more than in Manchuria. Frequency and intensity of earthquakes in these areas are directly related to their proximity to the Circum-Pacific Earthquake Belt. Japan is located on the belt, while Korea and Manchuria are located some distance from it. Thus, seismicity in Korea is much stronger than in Manchuria, but much weaker than in Japan. In Korea, earthquakes occur mainly on faults or tectonic plates, which, viewed on the surface, are river courses. However, during last century, earthquakes had occurred frequently along mountain ranges such as Mt. Jirisan. South Korea is a comparatively stronger seismic area than North Korea, and the west half of the Korean Peninsula has shown stronger seismicity than the eastern half. The downstream basins of each of the main rivers flowing into the Yellow Sea show a much denser distribution of earthquake epicenters than the middle and upper stream basins. However, both the Geumgang and Nakdonggang rivers show a higher seismicity in both the lower and middle stream basins. The southwestern edge of the Gyeonggi Massif, located in the central part of the Korean Peninsula, or the northern margin of South Korea, has shown much stronger seismicity than the inland and northeastern coastal areas. Almost the entire area of the Okcheon Zone has shown higher seismicity. The northeastern part of the Yeongdong-Gwangju, however, has shown a relatively higher seismicity than the southwestern region. The southwestern region of the Taebaeksan Block of the Yeongnam Massif has shown a higher seismicity than the northeastern region, which is located in the eastern coastal area. The northeastern edge of the Jirisan Block of the Yeongnam Massif, a southwestern continuity of the Taebaeksan block, and the central zone of the southern half along which Mt. Jirisan runs, have shown relatively higher seismicity than other areas. The entire area of the Gyeongsang Basin, occupying the southeastern edge of the Korean Peninsula, has shown high seismicity, and particulary both the eastern and western edges have historically shown an intensive seismicity with the southwestern edge of the Gyeonggi Massif. The Chugaryeong Graben running through the central zone of the Gyeonggi Massif in the north-northeast to south-southwest directions, has shown a higher seismicity in both past and present times.

      [Tectonic and Geological Provinces]

	The Korean Peninsula lies within the Korea-China Heterogen. Generally speaking, the Precambrian basement of the peninsula is tectonically related to that of Manchuria and China. The Pyeongbuk-Gaema Massif forms the southern part of Liao-Gaema Massif of southern Manchuria, and the Gyeonggi and Mt. Sobaeksan massifs of the peninsula can be correlated with the Shandong and Fujian Massifs of China. The Paleozoic sediments, lying on the Precambrian Massifs in the Pyeongan and Okcheon basins, can be correlated with those in the Yellow River and Yangtze basins. On the other hand, the Mesozoic rocks of the Gyeongsang Basin in the southeastern part of the peninsula can be said to extend toward the Kwanmon Basin in the southwestern tip of Japan across the Korea Strait. The Pohang Tertiary sedimentary rocks, distributed mainly in the southeastern corner of the land, lie on Mesozoic rocks. Precambrian Geology The Precambrian geology of Korea is yet to be fully explored. The 1:250,000 scale geological maps published in 1974 offer a more detailed Precambrian stratigraphy of South Korea. In general, the Gyeonggi and Yeongnam Massifs belong to a low-pressure facies series, and the Okcheon Metamorphic Belt to an intermediate pressure series. Yeoncheon System This system, belonging to the Gyeonggi Massif, is distributed around Seoul extending to Yeoncheon-gun in a northeasterly direction. The system i s divided into upper and lower parts. The lower part is composed of biotite-quartz-feldspar schist, marble, lime-silicate, quartzite, and graphite schist, and the upper part is of mica-quartz-feldspar schist, mica schist, quartzite, augen gneiss, and garnet-bearing granitic gneiss. Precambrian Granite Gneiss This gneiss is also known as grey granite gneiss or Goguryeo granite. Outcrops of it cover one-third of the surface of Korea, and are especially preval‎ent in Pyeongan-do and Hamgyeongnam-do provinces. Intrusions of Goguryeo granite are also found in the Yeoncheon System in the Gyeonggi Massif. Sangwon System This system was first observed in the northern part of Korea, Sangwon, Pyeongannam-do province, and Hwanghae-do province in central Korea, the main area of its distribution. Collenia fossils have been found in the limestone interbedded in the system. The presence of Collenia suggests that the system belongs to the Proterozoic. Paleozoic Erathem Paleozoic sediments in Korea are represented by the lower Paleozoic Joseon Supergroup and the upper Paleozoic Pyeongan Supergroup. The Joseon System comprises the Cambrian to Middle Ordovician sedimentary rocks, the Pyeongan System, the Middle Carboniferous to Permian, and the Unidentified Triassic sedimentary rocks. Lower Silurian limestone was discovered from conodont fossils in 1980. The Upper Ordovician, Devonian and Lower Carboniferous systems are not known to exist. Cambro-Ordovician Strata The Cambro-Ordovician Joseon Supergroup is widely distributed in the limestone plateau in northwest Korea. There are many other localities with scattered patches of the Joseon Supergroup. The Joseon Supergroup has been divided into two parts according to lithology, the Samcheok Group and the Sangdong Group. These groups are conformable to each other. The Samcheok Group overlies the Precambrian rocks unconformably. Samcheok Group The Samcheok Group is divided into the Jangsan, Myobong, Pungcheon and Hwajeol formations in Gangwon-do province. The Jangsan Formation consists mainly of white quartzite that is pebbly throughout, but it starts with thin basal conglomerates. The overlying Myobong Formation, which is predominated by greenish grey slates, follows the Jangsan with alternating zones of quartzite and slate at its base. The average thickness of each of these two formations is about 200 meters. Sangdong Group The Sangdong Group, distributed in Gangwon-do province, which is located in the east central part of Korea, is a sequence of thick limestone-rich sediments, as is the case with the strata in northern Korea. The strata have been divided into four formations. The lowest stratigraphic unit of the Sangdong Group is the Dongjeom Quartzite which also overlies the Hwajeol Formation. The Dumugol Limestone is of a lesser worm-eaten appearance. The Maggol and Duwibong limestones consist of gray limestones. A bed of fossiliferous shale, known as the Jigunsan Shale, is intercalated between the two limestones. Trilobite, cephalopod, graptolite and other fossils are found in the limestones of the Sangdong Group. Recently, a number of conodonts have been identified in these strata. Middle Paleozoic Strata Middle Paleozoic strata from the Upper Ordovician to the Lower Carboniferous are not known to exist. However, the existence of the Lower Silurian Hoedong-ri Formation has been established by the finding of Silurian conodont fauna. Some formations in the Okcheon Supergroup are thought to be middle Paleozoic in age by some geologists, although no conclusive evidence has been found to substantiate this claim. Carboniferous-Triassic Strata Carboniferous-Triassic Strata of the Pyeongan Supergroup, distributed exclusively overlying the Middle Ordovician disconformably, are found roughly in four locations in North Korea, as well as three areas in Gangwon-do and Chungcheongbuk-do provinces in South Korea. Gomok Group The main rocks of this group are slightly metamorphosed green, red, gray or mottled sandstones and shales with some light-colored limestone beds. Most shales are characterized by dark fine ottrelite. The limestone beds are fossiliferous with primitive fusulinids and other foraminifers, corals, brachiopods and others. The presence of fusulinids indicates the age of the group to be Moscovian of Middle Carboniferous. The Gomok Group is about 220 meters thick on the average. Cheoram Group The Cheoram Group is characterized by somewhat metamorphosed gray to dark-gray sandstones, shales, coaly shales, coal beds, and dark grey limestone beds. Three or more coal beds are intercalated in the upper part of the series, and are an important source of anthracite. Upper Moscovian fusulinids are found in the Samcheok and Danyang coalfields, while Sakilometersarian fusulinids are found in the Yeongwol coalfield. The thickness of the Cheoram Group averages 150 meters. Hwangji Group The group locally overlies the Cheo lam Group conformably, and is composed mainly of white quartzites intercalating some black shales. The age of the group is Middle to Late Permian, possibly even extending to Triassic. The thickness of the Dosagok and Gohan Formations is about 880 meters. The upper part of the group (Donggo Formation) is composed mainly of green arkose sandstones with some conglomerate beds. The age of the Donggo Formation has not been clarified by fossils, though it has been thought to be Triassic because of its thickness. This formation is 400 - 2,000 meters thick. Mesozoic Erathem The lower Mesozoic is represented by the Donggo Formation of the upper part of the Pyeongan Supergroup. The rest of the Mesozoic sediments in Korea are represented by the middle Mesozoic Daedong System and the upper Mesozoic Gyeongsang System. The Daedong System represents the Jurassic, and the Gyeongsang the Cretaceous. In the Jurassic Period, a deformation known as Daebo Orogeny took place. This mountain forming process was the most intensive on the Korean Peninsula, which caused all earlier formations to fold, thrust and fault drastically. Daedong System The Daedong System is not widely distributed in Korea. Long, narrow, and patchy, it is scattered with or without distributional relationship with the Pyeongan Supergroup, though showing deformations as does the latter. It is subdivided into the Lower Nampo, Upper Nampo and Bansong groups in South Korea, which consist mainly of conglomerates, milky white pebble-bearing arkosic quartzose sandstones, black sandy shales, coaly shales, and coals. The Daedong System near Pyeongyang in North Korea is subdivided into the Seonyeon and Yugyeong series, which consist of conglomerates, sandstones, and shales ranging up to 1,300 meters in thickness. Plutonism in the Jurassic Period Most of the granite bodies, except the Cretaceous ones, intruded during the Jurassic Period. In South Korea, granite batholiths show a characteristic distribution trend north northeast-south southwest (the so-called Sinian). Most of these granite bodies are biotite granites, and some are hornblende granites. Gyeongsang System The Gyeongsang Supergroup is distributed across a wide area within Gyeongsang-do province in the southeastern part of Korea. Animal fossils indicate that sediments were deposited in shallow water environments. They are of fresh to brackish water genera. No marine fossils have been found. The Gyeongsang Basin was formed subsequent to the Daebo Orogeny and can be divided into three minor basins: the Yeongyang, Uiseong, and Miryang basins. The Gyeongsang Supergroup is composed of the Sindong and Hayang groups and the Bulguksa Intrusives. Plutonism in Cretaceous Period The biotite granite intruded in the Gyeongsang Supergroup is called Bulguksa Granite. The radiometris ages of intrusions are dated to be Late Cretaceous to Early Tertiary. A remarkable characteristic of the Plutonism of this period is that the granites are not of an orogenic production. Sindong Group This group is distributed in the western part of Gyeongsang-do province. It consists mainly of shales, sandstones, conglomerates, and one or two thin coal seams. The formations of the group have gentle monotonous eastward dip in contrast to the marked deformation of the Jurassic and Pre-Jurassic rocks. Ripple marks and sun cracks are found in many localities of the group, indicating that the group was formed in shallow-water environment. Animal fossils indicative of the Early Cretaceous age are preval‎ent. They include Viviparus, Hydrovia, Bulimus, Itometamia, Brotiopsis, Anisus, Trigonioides, Plicatounio, Nakamuranaia, Schistodesmus, and Estherites. Plant fossils include: Group 1, Cladophlebis browniana, Onychiopsis mantelli, Ruffordia goepperti, and Nilssonia schaumbur gensis; Group 2, Cladophlebis lobifolia, Coniopteris hymenophylloides, Dictyozamites falcatus, Ptilophyllum pecten, and Nilssonia compta. Group 1 indicates Early Cretaceous and Group 2 Middle Jurassic. Because the Sangdong Group overlies granite gneiss unconformably, its relationship to the Daedong System cannot be determined. Hayang Group This group, widely distributed throughout Gyeongsang-do province, consists of conglomerates, sandstones, shales, and volcanic rocks such as andesites, basalts, rhyolites, and tuffs, especially in the upper part of the group. The thick basal conglomerates form the boundary dividing the groups from the underlying Sindong Group. The Jeokgak-ri Formation in South Korea and the Daebo Series near Pyeongyang in North Korea are correlated to the Hayang Group. The former overlies the Pyeongyang Supergroup, and the latter Daedong System unconformably. The Hayang Group has more ripple marks and sun cracks than the Sindong Group, but the latter is more fossiliferous. Plant fossils found in this group indicating the whole range of the Cretaceous period are Filicales, Cycadales, Coniferales, and Angiospermae. Bones, eggs and tracks of dinosaurs have also been found in the Hayang sediments. Cenozoic Geology Neogene Tertiary Strata are distributed across small areas along the eastern coast of the Korean Peninsula. They are, from north to south, the Gilju-Myeongcheon Basin in North Korea, and the Bukpyeong, Yeonghae, Pohang and Ulsan basins in South Korea. The rocks are poorly lithified sandstones, shales, conglomerates, lavas, and sills. An alternation of land and marine deposits is characteristic of the strata. Bongsan Series This series is distributed throughout Hwanghae-do and Pyeongannam-do provinces of North Korea, and consists of an alternation of shales and sandstones, conglomerates, and coal beds. Fossils indicating the late Eocene include Colodon, Caenolophus, Desmatotherium and Portianotherium (animal fossils), and Populus, Platanus and Vipurunus (plant fossils). The series is about 350 meters thick. Yongdong Series This series is distributed in Hamgyeongbuk-do province, North Korea, and consists of sandstones, shales, interbedded coal beds, and alkali-basalts in the lower part of the series. Plant fossils indicating Middle to Late Oligocene include Pinus, Glyptostrobus, Sequoia and Juglans. The series is 80 meters thick. Yangbuk Group This group is distributed in Gyeongsangnam-do province, South Korea, and consists of conglomerates, volcanic rocks, sandstones, shales, and coal beds alternating with tuffs. Plant fossils indicating the early to middle Miocene include Sequoia, Salix, Carpinus, Alnus, Populus, Betula, Fagus, Fagophyllum, Castanea, Colylus, Zenthoxyl-on, Planea, Ficus, Uities, Acer and Juglans. The series is approximately 1,400 meters thick. Yeonil Group This is distributed throughout Yeonil, South Korea, and consists of conglomerates (200 meters thick) and thick shales (400 meters thick). Fossils indicating the Miocene include Turborotaria, Globigerina and Globigerinoides, which are all foraminifers; Caridium, Solen, Lucina, and Potamides, which are all marine mollusks, from the sandstones; Leda, Cardium, Dosinia, Ostrea and Pecten which are from the shales; and Salix, Quercus, Cinnamomum, Sapindus and Aagus, which are all plant fossils, from the upper part of the shales. Seogwipo Formation This formation is distributed in Jejudo island off southwestern Korea. The formation consists of sandstones and mudstones. Fossils indicating the late Pliocene include marine mollusks and foraminifers.