Marine Geology of Korean Seas

Marine Geology of Korean Seas 2nd Edition

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Marine Geology ofKorean Seas 2nd Edition

S.K. CHOUGH Department of Oceanography, College of Natural Sciences, Seoul National University, Seoul 151-742, Korea

HJ. LEE Marine Geology Laboratory, Korea Ocean Research and Development Institute, Ansan, P.O. Box 29, Seoul 425-600, Korea

S.H. YOON Department of Oceanography, Cheju National University, Cheju 690-756, Korea

2000

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Contents Preface

xi

Acknowledgements CHAPTER 1

Introduction

CHAPTER 2.1 2.2 2.3

2 Korean Peninsula Introduction Kyonggi and Yongnam Massifs Okchon Fold Belt 2.3.1 Okchon Group 2.3, LI Hwanggangri Formation 2.3.2 Deformation and Orogenic Setting 2.4 Taebaeksan Basin 2.4.1 Choson Supergroup 2.4.1.1 Taebaek Group 2.4.1.2 Yongwol Group 2.4.2 Pyongan Supergroup 2.5 Imjingang Belt 2.6 Orogenic Events 2.7 Cretaceous Non-Marine Basins 2.7.1 Kyongsang Basin 2.7.2 Other Basins 2.8 Pohang Basin 2.9 Jangki Group 2.10 Quaternary Volcanism in Cheju Island 2.11 Sedimentation and Tectonic History

CHAPTER 3.1 3.2 3.3 3.4 3.5 3.6 3.7

3 Yellow Sea Physiography Geologic Setting Northern Yellow Sea Basin Southern Yellow Sea Basin Basin Evolution Shallow Structure Surface Sediments 3.7.1 Distribution 3.7.2 Mineral and Geochemical Composition

xiii 1 7 7 9 12 12 14 16 17 17 17 21 24 26 26 28 30 32 34 40 43 43 47 47 47 49 52 55 56 62 62 65

Contents

3.8

3.9

3.10

3.11

3.12

3.13

Dispersal of Fine Sediment in the Western Part 3.8.1 GulfofBohai 3.8.2 Central Part 3.8.3 Old Huanghe Delta 3.8.4 Changjiang Estuary Dispersal of Fine Sediment in the Southeastern Part 3.9.1 Clay Mineral Distribution 3.9.2 Distribution of Trace Elements 3.9.3 Dispersal of Fine Sediment Mass Physical Properties 3.10.1 Sediment Texture and Structures 3.10.2 Water Content 3.10.3 Shear Strength 3.10.4 CaCOs and Organic Matter 3.10.5 Atterberg Limits Tidal Flats 3.11.1 Surface Sediments 3.11.2 Benthic Biota 3.11.3 Sedimentary Structures of Holocene Sediments 3.11.4 Pre-Holocene Oxidized Mud 3.11.5 Holocene Lithostratigraphy 3.11.5.1 Unit 1 3.11.5.2 Unit II 3.11.5.3 Unit III 3.11.6 Holocene Sea-Level Curve Reclamation Effect on Sedimentation: Daeho Area 3.12.1 Geologic Setting 3.12.2 Tidal Flat Morphology and Sediments 3.12.3 Nearshore Suspended Matter 3.12.4 Seasonal Sedimentary Processes 3.12.5 Suspended Sediment Budget Transgressive Holocene Sequence Stratigraphy 3.13.1 Northern Part 3.13.1.1 High-Resolution Seismic Stratigraphy 3.13.1.2 Lithofacies 3.13.1.3 Interpretations 3.13.2 Southern Part 3.13.2.1 High-Resolution Seismic Stratigraphy 3.13.2.2 Lithofacies 3.13.2.3 Interpretations

VI

68 68 69 69 71 71 72 72 75 78 79 82 82 82 85 87 89 92 92 94 100 100 100 103 103 107 107 110 Ill 114 116 117 119 120 125 128 131 134 135 141

Contents CHAPTER 4.1 4.2 4.3

4.4

4.5

4 South Sea and East China Sea Geologic Setting Sedimentary Basins Coastal Embayments 4.3.1 Gamagyang Bay 4.3.1.1 Physiography 4.3.1.2 Acoustic Stratigraphy 4.3.1.3 Deposition of Fine Sediment 4.3.1.4 Late Quaternary History Surface Sediments 4.4.1 Distribution 4.4.2 Mass Physical Properties 4.4.2.1 Water Content 4.4.2.2 CaCOj and Organic Matter 4.4.2.3 Shear Strength and Atterberg Limits 4.4.3 Recent Depositional Processes Late Quaternary Transgressive Deposits

CHAPTERS East Sea 5.1 Physiography 5.2 Crustal Structure 5.3 Magnetic and Gravity AnomaHes 5.4 Heat Flow 5.5 Age and Type of Crust 5.6 Stratigraphy 5.6.1 Seismic Stratigraphy 5.6.2 Lithostratigraphy 5.7 Tectonic Evolution 5.7.1 Tectonic Origin 5.7.2 Opening Mode 5.7.3 Tectonic History 5.8 Surface Sediments 5.8.1 General Statement 5.8.2 Distribution 5.8.3 Geochemical Composition 5.9 Late Quaternary Sediments 5.9.1 Lithology 5.9.2 Holocene-Pleistocene Boundary 5.10 Late Quaternary Paleoceanography CHAPTER 6

Eastern Continental Margin Vll

145 145 145 149 150 150 152 155 156 158 158 160 160 163 164 165 167 173 173 175 176 177 178 181 181 184 186 186 187 189 191 191 191 193 ...193 193 195 196 199

Contents

6.1 6.2

6.3

6.4

6.5

6.6 6.7

Physiography Geologic Structures 6.2.1 UlleungFauh 6.2.2 Hupo Fault 6.2.3 Yangsan Fault 6.2.4 Dolgorae Thrust Belt 6.2.5 Small-Scale Faults and Folds Seismic Stratigraphy 6.3.1 Eastern Margin 6.3.1.1 Acoustic Basement 6.3.1.2 Sedimentary Unit 1 6.3.1.3 Sedimentary Unit II 6.3.1.4 Sedimentary Unit III 6.3.2 Southeastern Margin 6.3.2.1 Succession 1 6.3.2.2 Succession II 6.3.2.3 Succession III Sedimentary Basins 6.4.1 Pohang-Yongduk Basin 6.4.2 Mukho Basin 6.4.3 Hupo Basin Evolution History 6.5.1 Eastern Margin 6.5.2 Southeastern Margin Surface Sediments Late Quaternary Sediments 6.7.1 Mass Physical Properties 6.7.1.1 Water Content 6.7.1.2 Shear Strength 6.7.1.3 CaCOs and Organic Matter 6.7.1.4 Atterberg Limits 6.7.2 Sedimentary Facies 6.7.3 High-Resolution Echo Characters 6.7.3.1 Shelf Region 6.7.3.2 Slope Region 6.7.4 Slope Failure Features 6.7.5 Slope Stability 6.7.6 Depositional Processes

CHAPTER 7 Ulleung Basin 7.1 Physiography

199 201 201 205 205 206 207 208 208 208 210 210 211 211 212 214 216 217 217 218 219 220 220 221 222 224 224 224 225 225 225 226 227 227 228 233 235 237 239 239

vni

Contents 12 7.3 7.4

7.5 7.6

7.7

Crustal Structure Gravity and Magnetic Anomalies Seismic Stratigraphy 7.4.1 Acoustic Basement 7.4.2 Sedimentary Sequence Tectonic Evolution Late Quaternary Sediments 7.6.1 Distribution and Echo Characters 7.6.2 Chronostratigraphy 7.6.3 Turbidite Facies 7.6.3.1 General Statement 7.6.3.2 Sedimentary Facies 7.6.3.3 Provenance 7.6.4 Hemipelagic Facies Late Quaternary Sedimentation

References Subject Index

241 243 243 243 245 252 254 254 256 258 258 258 260 262 264 269 307

IX

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Preface (2nd Edition) •^•^A.---:;vmimm»%-»!if.t&mmmmfmsmm^mimii^mmm-m'miiVf^'^:m>^mn!fi-M?'f-^

Marine Geology of Korean Seas was first published in 1983. Since that time tremendous progress has been made in the geological understanding of the Korean Seas with the advances in sophisticated exploration technique and reinforcement of research personnel, specifically in the areas of marine geophysics, sedimentology, geochemistry, and paleoceanography. Over the past two decades, the number of research scientists in marine geology has been doubled (or tripled) in most academic institutions (15 universities), the Korea Ocean Research and Development Institute (KORDI), and the Korea Institute of Geology, Mining and Materials (KIGAM). In the Yellow Sea, continuous efforts have been made to explore hydrocarbon in the concession blocks. Although regional basin analysis in the eastern part of the Yellow Sea (Concession Blocks I-III) was instigated in 1970 by Gulf Oil Limited, additional data were acquired and reinterpreted in 1987 by Marathon Oil Company in cooperation with the Korea National Oil Corporation (KNOC). Twenty holes have since been drilled throughout the Yellow Sea basins. On the other hand, both shallow subsurface mapping using high-frequency profiling and deep cores (up to 60 m deep) into the Holocene/Pleistocene boundary have been made by the KIGAM to reveal late Quaternary depositional processes and sequence stratigraphy in this unique epicontinental sea. Studies have also been active by the KORDI and the academic institutions for environmental changes in the eastern part of the sea, estuaries, and tidal flats, delving into aspects of sediment transport and deposition, physical oceanography (tides, waves, and coastal currents), geochemistry, and air-sea interactions. The sea south of the Korean Peninsula, South Sea, is characterized by numerous islands that have been submerged during the last transgression. Shallow subsurface mapping using high-resolution seismic profiling has revealed that the sea is characterized by complex incised valley systems and transgressive deposits during the rise in sea level. This is an area for further detailed studies of high-resolution sequence stratigraphy. A number of offshore exploratory wells have also been drilled, revealing hydrocarbon potential. In the East Sea (Sea of Japan), studies have focused on the Ulleung Basin and its surrounding margins, using single- and multi-channel seismic profiling, magnetic and gravity data, closely spaced (5.5-km interval) highfrequency profiling (Chirp), and multibeam mapping. More than ten exploratory wells have been drilled in the southern margin of the basin (Block VI) where commercial development of gas is being sought. The XI

Marine Geology of Korean Seas

coverage of Chirp and Seabeam profiling by the National Oceanographic Research Institute (NORI) provides an unprecedented data base. Deep piston coring in the basin and analyses of sedimentary facies, microorganisms, and oxygen and carbon isotope data help reveal paleoceanographic and environmental changes in the sea. Aspects of water circulation and the formation of deep water masses in the deep basins have also been described by physical and chemical oceanographers. The subsurface geology of the Korean Seas is intimately related to that on the adjacent land; especially, the tectonic evolution of the Mesozoic and Cenozoic sedimentary basins is contiguous to that on land. For this reason, an expansion has been made in this edition to relate details of basin evolution on land to those under the sea. At this stage, it is timely to summarize the hitherto-revealed knowledge on the geology of the Korean Seas for a second time. In this edition, we have incorporated the new results and interpretations that help formulate geological hypotheses and corollary on the evolution of the Korean Seas in relation to the adjacent continents. We have followed the basic framework of the first edition, but amply expanded the volume to include recent developments in every realm of marine geology in the past 16 years. Because of the lack of our knowledge on the northern part of the peninsula (north of 38th parallel; DMZ), this book focuses on the sea floor off the Republic of Korea. Geographic names follow the current-use Romanization proposed by the Government of Korea.

xn

Acknowledgements We would like to thank the following publications and copyright holders for their cooperation: Korea Institute of Geology, Mining and Materials, Korea Ocean Research and Development Institute, Korea National Oil Corporation, National Oceanographic Research Institute, Geosciences Journal, Geological Society of America, Inc. (Geological Society of America Bulletin and Geology), International Association of Sedimentologists (Sedimentology), SEPM (Journal of Sedimentary Petrology, Journal of Sedimentary Research), Blackwell Science (The Island Arc), SpringerVerlag (Geo-Marine Letters), Taylor & Francis (Marine Geotechnology), American Geophysical Union (Tectonics), Elsevier Science Ltd. (Continental Shelf Research), and Elsevier B.V. (Marine Geology, Sedimentary Geology, Tectonophysics, and Earth Science Reviews). We are indebted to many colleagues in Korea and abroad for invaluable dialogue and support, especially the members of the Marine Geology and Geophysics Divisions, the Korea Ocean Research and Development Institute (Drs. B.K. Park, S.J. Han, S.K. Chang, B.C. Suk, G.H. Hong, K.S. Jeong, M.Y. Choe, C.H. Park, and S.M. Lee), the colleagues of the Korea Institute of Geology, Mining and Materials (Drs. Y.H. Kwak, K.S. Park, J.H. Chang, K.R Park, G.H. Min, C.W. Lee, I.G. Hwang, J.H. Jin, and W.H. Ryang), and the colleagues of the Korea National Oil Corporation (Drs. J.H. Han, S.J. Park, B.G. Choi, and M.S. Kim) who generously permitted us the use of unpublished data and helped in acquiring data and preparing illustrations. Discussions with colleagues, Drs. Y.A. Park, S.J. Kim, J.H. Kim, C.-E. Baag, D.K. Choi, M. Cho, and M.S. Lee (Seoul National University), K.M. Yu and S.-T. Kwon (Yonsei University), J.-H. Ree (Korea University), D.J. Lee (Andong National University), S.S, Chun (Chonnam National University), K.S. Woo (Kangwon National University), S.C. Park (Chungnam National University), K.C. Na, C.W. Rhee, and J.S. Kim (Chungbuk National University), YK. Sohn (Gyeongsang National University), B.G. Jo (Chunbuk National University), M.C. Suh (Kongju National University), H.W. Shon (Paichai University), G.H. Lee (Kunsan National University), J. Ko (Seoul), and W.R. Fitches (Llandudno,U.K.), were useful in clarifying points in their field of interest. Drs. M.R. Gipp and M.W. Milner (Toronto, Ontario) and D. Barber (Boulder, Colorado) made helpful suggestions on the manuscript. We are indebted to the Korea Science and Engineering Foundation, the Research Institute of Basic Sciences (Seoul National University), and the Korea Research Foundation for their continuous support through grants. We xin

Marine Geolo^ of Korean Seas

thank Drs. Femke Wallien for successful publication of this work. We are grateful to graduate students of the Sedimentology Laboratory (SedLab), Seoul National University (H.R. Jo, S.B. Kim, S.H. Lee, JJ. Bahk, J.W. Kim, Y.H. Kim, Y.K. Kwon, and H.K. Ha) for continuous discussions and editorial help for the preparation of this book. We thank Ms. J. Cho for editorial assistance and the Instructional Media Center (IMC), Seoul National University for the preparation of figures.

XIV

CHAPTER 1

Introduction The Korean Seas (Fig. LI) are geologically unique. The Yellow Sea (or West Sea) is a shallow (less than about 100 m), postglacially submerged epicontinental sea bounded on the east by a long stretch of ria-type coast. The western part of the East Sea (Sea of Japan) is characterized by a narrow shelf with a straight coastline. The Yellow Sea floor is rather flat and progressively deepens toward the southeast to form the Okinawa Trough in the northern East China Sea. The East Sea deepens abruptly seaward, forming a number of deep basins between ridges and surrounding margins that are related to the opening of a back-arc basin associated with subduction of the Pacific Plate. The South Sea, bounding the southern coast of the Korean Peninsula, is also shallow and flat, similar to the Yellow Sea, but characterized mostly by rocky embayments. Regional studies on the geological structure of the Yellow Sea were made in a joint survey (Emery et al., 1969; C.S. Kim et al., 1969) supported by the Committee for Co-Ordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas (CCOP) (Fig. 1.2). An airborne magnetic survey was also conducted in the Yellow and South seas and the southern part of the East Sea (Bosum et al., 1971) (Fig. 1.2). Regional basin-scale studies on the concession blocks (Blocks I-V; Fig. 1.3) were made by the Marathon Oil Company (1987) and the Korea National Oil Corporation (KNOC) (PEDCO, 1997) based on gravity, magnetic, seismic, and drilling data. These studies in the Yellow Sea showed the existence of two large-scale Mesozoic-Cenozoic non-marine basins (North and South Yellow Sea basins) bounded by basement highs (massifs) (Fig. 1.3). In the Cretaceous, these basins were contiguous to those on land in a retroarc basinal setting. Attempts have been made by the Korea Institute of Geology, Mining and Materials (KIGAM) mapping projects since the early seventies to obtain data on the geological structure of the shallow portions of the Yellow Sea (Chough, 1983a). These were followed by deep drilling of Quaternary deposits in the southeastern Yellow Sea revealing depositional history and sequence stratigraphy of the regressive/transgressive systems (KIGAM, 1996; Jin and Chough, 1998). The surface sediment distribution in the entire Yellow Sea has been compiled and interpreted in terms of physical processes (H.J. Lee and Chough, 1989). Recently, closely spaced, high-resolution seismic data have been obtained in the entire Yellow Sea by the National

2

Marine Geology of Korean Seas

120°

125°

135°

Fig. 1.1. Bathymetry of the Korean Seas: the Yellow Sea, northern part of the East China Sea, and the East Sea (Sea of Japan). Contours in meters. Modified after Mammerickx et al. (1976) by permission of the Geological Society of America, Inc. Oceanographic Research Institute (NORI) using the Chirp system. In the meantime, drilling activities have also been greatly increased in the South Sea and the northern East China Sea, aiming at the deformed Tertiary strata whose economic hydrocarbon potential was strongly predicted by earlier studies. On the continental margin of the East Sea, the KIGAM conducted a cooperative seismic survey with the Federal Institute of Geoscience and Mineral Resources of Germany (Schltiter and Chun, 1974) to reveal possible

Introduction -T-7—[—3 \ Geol. Survey of Japan'-.

Seismic ^ . ^ . Q^ survey ^ \ \ V

^Vema 28 * \ (1971) \

134°

Fig. 1.2. Tracklines and blocks of major geological and geophysical surveys (prior to 1983) in the seas around the Korean Peninsula. KIER = Korea Institute of Energy and Resources. seaward extensions of coal seams in the northern part (south of 38th parallel) and hydrocarbon potential off Pohang (Fig. 1.2). A high-resolution seismic study was also made in the latter area by Huntec (1968) under the auspices of the United Nations Economic Commission for Asia and the Far East (ECAFE) and the United Nations Development Programme (UNDP). In the past decade, the KNOC has acquired multichannel seismic profiles in the Ulleung Basin and drilled more than ten exploratory wells in the southwestern margin of the Ulleung Basin (Block VI-1) (Figs. 1.3 and 1.4). The KNOC is currently developing a gas field in the Block VI-1 north of the thrust front (undeformed region). The data revealed thus far indicate that the Ulleung Basin consists of transitional crust overlain successively by a thick sedimentary sequence of layered and transparent sediments largely modified by volcanism (Chough and Lee, 1992). The Ulleung Basin was most likely formed in the early Miocene by southward extension of the Kyushu Block and subsequently began to close in the late Miocene, forming a thrust front

Marine Geology of Korean Seas

123°

126°

129°

Fig. 1.3. Major sedimentary basins and concession blocks (I-VI) for hydrocarbon exploration in the Korean Seas. Also note the Korea-Japan Joint Development Zone (JDZ) (1-6). Dots represent well locations. (Chough and Barg, 1987; Yoon and Chough, 1995; Chough et al., 1997a). It is now beyond doubt that the East Sea was formed by oblique back-arc extension and rotation in the late Oligocene-early Miocene (Otofuji and Matsuda, 1983; Lallemand and Jolivet, 1985; Jolivet and Tamaki, 1992; Yoon and Chough, 1995). Using ocean bottom seismometers, efforts have also been made to detect seismicity and the nature of the underlying crust (C.H. Park et al., 1996; G.H. Lee et al., 1999; H.J. Kim et al., 1999).

Introduction

-^ ^

I® I®

NORI(2-7kHz) KORDI (sparker) KNOC (multichannel) • KORDI (multichannel) I KNOC (multichannel) I i KORDI (2-7 kHz)

129° E 130° 131° 132° 133° Fig. 1.4. Tracklines of major geological and geophysical surveys (1983-1999) in the East Sea. Contours in meters. NORI = National Oceanographic Research Institute, KORDI = Korea Ocean Research and Development Institute, KNOC = Korea National Oil Corporation.

Recently, the Ulleung Basin was extensively surveyed by the NORI using the Chirp and Seabeam profiling systems on a 5.5-km grid (Fig. 1.4). These unprecedented data provide details of seafloor morphology and sedimentary facies of the topmost (up to 50 m thick) sedimentary layer, revealing depositional processes of slide/slump, debris flows, and turbidity currents in the margins and basin plain (Chough et al., 1997b). Analyses of a number of long piston cores in the basin plain and deep-sea channels have also revealed

Marine Geology of Korean Seas

details of the sedimentary processes and paleoceanographic changes as well as volcanic eruptions in the surrounding margins (Bahk et al., 2000). In the postglacial period, fine-grained sediments have been transported from offshore as well as along the shore and trapped in the coastal embayments and tidal flats, creating a unique environment of geologic interest. These fine-grained sediments typically contain high levels of organic carbon. Geological processes related to the postglacial sea-level changes proved to be important along the coasts of the peninsula, both for the Quaternary history and for the problems related to the human land-use activity. Efforts have been made to investigate the characteristics and sedimentary processes of extensive tidal flats of the eastern Yellow Sea which have been reclaimed one after another in recent years. These tidal flats show a coarsening-upward trend during the Holocene transgression due to the lack of sediment supply, different from other tidal flats in the world (Y.H. Kim et al., 1999). Many geological findings that emerged from the studies outlined above may be summarized as problems related to the geologic structure and tectonics, stratigraphy and sequence stratigraphy, sedimentary facies and processes, and origin and development of the seas around the Korean Peninsula. They include the epicontinental Yellow Sea and the shelf of the northern East China Sea, the East Sea with a narrow continental shelf and deep basins, and the South Sea with numerous rocky embayments. The geology of the Korean Peninsula is briefly described in the next section before its surrounding seas are discussed.

CHAPTER 2

Korean Peninsula 2.1

Introduction

The Korean Peninsula has inherited a rugged terrain with scenic mountain chains and intervening valleys, generally running NE-SW. These mountain chains represent the denuded remnants of deformed basement rocks and sedimentary successions as well as granitic intrusions and volcanics, concealing a long history of basin formation and crustal deformation. The Korean Peninsula represents an important tectonic link between eastern China and the Japanese Islands, The peninsula consists of Precambrian tectonic blocks (Nangrim, Kyonggi, and Yongnam massifs) that are bounded by fold belts (Fig. 2.1). The Nangrim and Kyonggi massifs are separated by the Imjingang Belt (Cho et al., 1995; Ree et al., 1996). The Kyonggi and Yongnam massifs are separated by the Okchon Fold Belt (Figs. 2.1 and 2.2). In the Taebaeksan Basin (Cambro-Ordovician), carbonates were deposited in shallow platforms unconformably overlain by shallow marine and fluvial sequences (Carboniferous-Triassic). The Cretaceous Kyongsang Basin, formed in the southeastern part of the peninsula, comprises gently eastwarddipping fluvial and lacustrine successions (Chang, 1975a; H.I. Choi, 1985; Rhee et al., 1998). A Tertiary sequence was deposited in the Pohang Basin (Chough et al., 1990), a strike-slip basin formed in association with back-arc opening of the East Sea (Yoon and Chough, 1995). Hypotheses on the tectonic history of the Korean Peninsula have focused on the location of the suture zone between the North China Block (SinoKorea Craton) and the South China Block (Yangtze Craton), with the location being either the Imjingang Belt (Liou et al., 1994; Cho et al., 1995; Ree et al., 1996) or the Okchon Fold Belt and Honam Shear Zone (Yanai et al., 1985). The former hypothesis assumes that the Quinling-Dabie Belt in China extends into the Imjingang Belt across the Yellow Sea (Liu, 1993; Yin and Nie, 1993; Liou et al., 1994; Ree et al., 1996). Cluzel et al. (1990, 1991a) suggested that the Okchon Fold Belt initially formed in the early Paleozoic as an intra-plate failed rift within the South China Block and underwent polyphase orogenic events in the late Silurian-Devonian, late Permian-Triassic, and the middle Jurassic-early Cretaceous. Cluzel et al. (1991b) also suggested that the two different Cambro-Ordovician platforms (Yongwol and Taebaek groups) were fused together by a dextral ductile fault

8

Marine Geology of Korean Seas

KM Kyonggi Massif NM Nangrim Massif YM Yongnam Massif DSB IB OFB |SB 115°

120"

laS''

130"

135°

Dabieshan Belt imjingang Belt Okchon Fold Belt Sulu Belt 140°

Fig. 2.1. Outline of major sedimentary basins, orogenic belts, cratonic blocks (massifs), and other geologic features of the northeastern Asian margin. (NE-SW displacement of 200 km) between the North China Block and the South China Block. Recent reliable data suggest that the Okchon Basin was initiated in the late Proterozoic in an intra-plate setting (Kwon and Lan, 1991) and underwent a series of compressional deformation events in the Mesozoic (H. Kim et al., 1995; Koh and Kim, 1995; Min et al., 1995; Oh et al., 1995; J.H. Kim, 1996). According to stratigraphic analyses of Cambro-Ordovician sequences in the Taebaeksan Basin (Yongwol region) using trilobites (D.K. Choi and Lee, 1995; J.G. Lee, 1995; D.K. Choi et al., 1996), the shallow-water carbonate sequences were successively thrusted eastward. Faunal assemblages in these sequences are similar to those of the North China Block, implying that these basins were part of an inter-connected sea in a continental margin. A recent review of the tectonic evolution in the Korean Peninsula (Chough et al..

Korean Peninsula

2000) suggests that the Imjingang Belt represents the collision zone between the North China Block and the South China Block in the early Triassic (Songrim Orogeny). The collision was followed by dextral displacement (ca, 200 km) along the Okchon Fold Belt and Honam Shear Zone in the Jurassic (Daebo Orogeny), which constitutes the major tectonic framework of the Korean Peninsula. In the early Cretaceous, oblique subduction of the Izanagi Plate under the Asian continent caused opening of strike-slip basins in a retroarc setting. In the Miocene, strike-slip basins were formed in the East Sea in a back-arc setting.

2.2

Kyonggi and Yongnam Massifs

The basement rocks of the Korean Peninsula (Fig. 2.2) consist of highgrade gneisses and schists (late Archean to early Proterozoic) that are unconformably overlain by supracrustal sequences (schists, quartzites, marbles, calcsiHcates, and amphibolites) (D.S. Lee, 1987). These rocks experienced amphibolite-granulite facies metamorphism. In large part, the Kyonggi Massif is composed of banded biotite gneiss, porphyroblastic garnet-bearing gneiss, migmatitic gneiss, and granitic gneiss and biotite schist, crystalline limestone, and quartzite of metasedimentary origin (Na, 1987). Mineralogically, these rocks are composed largely of microcline, plagioclase, biotite, and hornblende with accessory garnet, zircon, epidote, andalusite, sillimanite, and cordierite of amphibolite facies (Na, 1980). Various types of schist, such as biotite schist, biotite-sillimanite schist, graphitic schist, quartz-muscovite schist, as well as amphibolite and marble are also interspersed within the gneiss. According to isotopic age determinations on whole rock (Rb-Sr) and gneiss (U-Pb), the massif ranges in age from about 2.7 to 1.1 Ga (Hurley et al., 1973; Na and Lee, 1973; Gaudette and Hurley, 1973; Choo et al, 1982). The Yongnam Massif comprises granitic gneiss, porphyroblastic gneiss, augen gneiss, migmatitic gneiss, banded gneiss, and anorthosite with minor schist and amphibolite (Na, 1987). Three metamorphic episodes are recognized in the massif: (1) amphibolite-granulite facies, (2) amphibolite facies, (3) epidote-amphibolite to greenschist facies (S.M. Lee et al., 1981). Rb-Sr and Pb-Pb whole-rock ages for felsic orthogneiss range from 2.2 to 1.6 Ga (D.H. Kim et al., 1978; Y.J. Kim and Lee, 1983; Choo and Kim, 1986; Choo, 1987; M.S. Lee, 1988; K.H. Park et al., 1993; S.-T. Kwon et al., 1995). U-Pb zircon ages (2.1-1.9 Ga) for the granitic gneisses are indicative of felsic igneous activity during the late Paleoproterozoic to early Mesoproterozoic time. Metamorphic ages are poorly known, although D.Y. Kim et al. (1998) obtained a Sm-Nd garnet age of 1,820±11 Ma for a

10

Marine Geology of Korean Seas

GEOLOGICAL MAP OF KOREA Modified after the Map (I:l,000,CXX)) by the Korea Institute of Energy and Resources

k km 50 _i

I .

100 I

L E G E N D SEDIMENTARY ROCKS TERT,ARr[T3v^N«.^t,3,

39*

^ I"-- v " - ! YUCHON OR. P;-v^V^ NCUWGJO OR. CRETACEOUS [ V . O ; ] JINAN OR. SINLA SER. HAYANO OR. NAKTONG SER.

IC 7r._ • L ^ r -\ NAMPO OR. NOQAM OR. OOBANdSAN a SAOONO-FMS.

37T'

I 1 LIMESTONE OR.

METAMORPHIC ROCKS

ri AOE UNKNOWN

OKCHON OR.

p ^ ' / : ) YULLl OR. | ~ ~ ~ [ WONNAM OR. SANGV^ON OR. YONCHON

mm

OR.

MACHEONRYEONG

lONEOUS ROCKS

35*

QUATERNARY g i g ] 5 « ^ J ^ TERTIARY P 7 ^ HYPABYSSAL TERTIARY^ A j VOLCANIC »

r L * » * J MASANITE C R E T A C E O U S ^ 3y^y^3^

(^

•'"''*^®'^ ^ ^ * \ ° * ^ ^ ORANITE



TRIASSIC [* ^*\

S O U T H

125"

127**

SEA

129"

GRANITE

r L * ^ * J INTERMEDIATE a «/>c ..«.>-wyii«|fc- * 3 BASIC PLUTON AOE UNKNOWN • ^ „ L |N^'