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
<2>
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^'
Korean Peninsula
6E0CHR0N0L0GICAL SCALE (Ma)
QUATERNARY //////////////A
VOLCANISM
YONIL GROUP YANG8UG GROUP
TTTli BULGUGSA GRANITE ^•"•^1 SERIES
DAEBO GRANITE SERIES SONG RIM GRANITE SERIES
iLi
if So > go o ggo
SIS O K U S ^ UJ <
SQ
< ZQC
«o F"TT^ GRANITE l*-^-^l PEGMATITES pTT:« HONGJESA |-^--^-»( GRANITES
2 I^Vv^l Volcanic Rocks t ^ ^
Non-morlne Sediments
Y//^
Marine Sediments
.^-^.^^r^^ Unconformity tonguior) Oisconformity
Fig. 2.2. (a) Simplified geologic map of the Korean Peninsula (modified after KIER, 1981a); (b) major stratigraphic units (modified after Reedman and Um, 1975).
11
12
Marine Geology of Korean Seas
chamockite.
2.3
Okchon Fold Belt
The Okchon Fold Belt is a NE-SW-trending fold-and-thrust belt, separating the Kyonggi Massif to the NW and the Yongnam Massif to the SE (Figs. 2.1 and 2.3). The belt includes the Okchon Basin to the SW and the Taebaeksan Basin to the NE. The Okchon Basin consists of ageunknown non-fossiliferous, low- to medium-grade metasedimentary and metavolcanic rocks, whereas the Taebaeksan Basin consists of fossiliferous, weakly metamorphosed sedimentary rocks of the Paleozoic to early Mesozoic. 2.3.1
Okchon Group
The Okchon Group comprises phyllite, quartzite, clast-bearing phyllite, and limestone and is represented by more than 14 formations, generally trending NE-SW (Fig. 2.3) (M.S. Lee and Park, 1965; O.J. Kim, 1970; Son, 1970; D.S. Lee et al., 1972; Reedman et al., 1973; C.H. Lee et aL, 1980; C.H. Lee, 1987). The Kyemyongsan Formation in the western margin consists of sandy phyllite. It is similar in lithology to the Unkyori Formation (Koesan district) (low-grade psammitic rocks) in which argillaceous and crystalline limestone beds are intercalated. Dark brown phyllite (shale with tripartite foliations) intercalated with black slate (Sochangri Fm.) largely occurs in the northeastern part of the belt. It is similar in lithology to the Changri Formation in the southern part. The western part of the belt is dominated by quartzite interbedded with biotite schist (Taehyangsan and Midongsan fms.). The Munjuri Formation consists of greenish to dark grey phyllite and associated black slate, dark-grey schist, quartzose sandstone, and quartzite. The limestone beds (Kumgang and Hansu fms.) are laterally persistent along the boundary between the Hwanggangri and Myongori (Changri) formations. The Hwanggangri and Puknori formations consist of clast-bearing phyllite. The matrix contains sericite, quartz, amphibole, chlorite, and clinozoisite and represents chlorite to amphibolite facies in metamorphic grade (O.J. Kim et al., 1977). These rocks are present in the central part of the fold belt (Fig. 2.3). The definitive age and stratigraphy of the Okchon Group sequence are still speculative, because the only two fossil occurrences in the entire group suffer from uncertainties in that they have not been reproduced even in the same locations: a single piece of Archaeocyatha in the Hyangsanri Formation (dolomite) is Cambrian (D.S. Lee et al., 1972), whereas a few
Korean Peninsula
13
Fig. 2.3. Simplified geologic map of the Okchon Basin. After Chough and Bahk (1992).
14
Marine Geology of Korean Seas
pieces of conodont and trilobite in a limestone pebble (Hwanggangri Fm) are Ordovician (J.H. Lee et al., 1989). On the other hand, isotopic ages provide some constraints on the stratigraphy of the Okchon Group. Ages from K-Ar on mica and Rb-Sr on biotite are mostly Jurassic (Cliff et al., 1985; J.H. Kim, 1987; Min et al., 1995). Some K-Ar and Ar-Ar amphibole ages vary from the Triassic (230 Ma) to the late Proterozoic (M.S. Lee, 1988; Min et al., 1995). A CHIME monazite age suggests that the metamorphism occurred in the late Permian-early Triassic, conforming to the Songrim orogenic event. A protolith age on amphibolite suggests 677 ± 91 Ma (Sm-Nd on hornblende-whole rock-plagioclase) (Kwon and Lan, 1991). Within the Hwanggangri Formation, a number of primary depositional features such as inverse (to normal) grading, debris plug, load casts, and pseudonodules are in line with the stratigraphic order proposed by M.S. Lee and Park (1965), i.e., from base to top, Sochangri, Puknori, Myongori, and Hwanggangri formations (Fig. 2.3). These results suggest that the Munjuri Formation was thrusted over the Hwanggangri Formation (Fig. 2.4) (Cluzel et al., 1991a; H. Kim et al., 1995). In the northeastern part of the Okchon Belt (Ponghwajae area), the thrust runs along the boundary between the quartzite (and phyllite) of the Okchon Basin and the limestone sequence of the Taebaeksan Basin (cf Khim et al., 1996; Chough et al., 2000). 2.3.1.1
Hwanggangri
Formation
The Hwanggangri Formation contains abundant floating clasts (3-30% by volume) which consist of quartzite, granite, shale, limestone, and dolomite (Chough and Bahk, 1992). Quartzite clasts are generally subequant, subrounded to well rounded and range in size from granule to boulder (up to 2 m in long axis). Shale clasts are pebble- to boulder-size chips and are of the same composition as the matrix of the clast-bearing phyllite. They display a wide range of deformed shapes ranging from thin chips to long ribbons or irregular fragments. Limestone clasts are also abundant, containing both lime-mudstone chips and limestone pebbles and boulders. The lime-mudstone chips are largely deformed. The matrices range from 70 to 97% by volume and are generally poorly sorted and either siliciclastic or calcareous. Although the pebbly phyllites of the Hwanggangri Formation have been interpreted as tillites of glacial environments (Reedman et al., 1973; Reedman and Um, 1975; M.S. Lee et al., 1998), the presence of contemporaneous limestone and shale clasts negates the possibility of glacial origin (e.g.. Chough and Bahk, 1984/85; Yoon et al., 1991). The existence of contemporaneous lime and shale clasts suggests that the deposit was formed by submarine slump/slide and associated debris flows (Chough, 1981;
LLI
Korean Peninsula
\5
16
Marine Geology of Korean Seas
Chough and Bahk, 1992) (Fig. 2.5). The presence of primary depositional features such as inverse (to normal) grading and debris plugs also supports the latter interpretation. 2.3.2
Deformation and Orogenic Setting
Cluzel et al. (1990, 1991a) suggested that the Okchon Fold Belt experienced three orogenic events: (1) the Caledonian (or Okchon) Orogeny (Silurian-Devonian) causing Dj and D2 deformation events, (2) the Indosinian (or Songrim) Orogeny (late Permian-Triassic) producing D3 deformation event, and (3) the Daebo Orogeny (middle Jurassic-early Cretaceous) resuhing in D4 deformation (Fig. 2.4). Cluzel et al. (1991a) argue that the main framework of the regional structures in the Okchon Belt developed during the Caledonian Orogeny. The existence of the Caledonian
Older Sequence
Siliciclastic Sediments
Sea Level A
Fig. 2.5. Conceptual diagrams showing depositional model of the Hwanggangri Formation, (a) During sea-level lowstand, coarse-grained terrigenous sediments were deposited on the basin margin, initiating debris flows, (b) When sea level was high, carbonate debris flows were generated from slumping of semi-consolidated sediments on the slope. Ky = Kyemyongsan Fm., Qz = Quartzite, So = Sochangri or Changri Fm., Hj = Hwajonri Fm., Ls = Hansu or Kumkang limestone bed, Hw = Hwanggangri Fm.
Korean Peninsula
17
Orogeny in the Okchon Belt, however, remains uncertain and needs more rehable radiometric age constraints (J.H. Kim, 1996). The metasedimentary sequence was certainly affected by the Songrim and Daebo orogenies (Chough et al., 2000). Although earlier studies have interpreted the metavolcanic rocks (especially along the boundary between the Okchon Basin and the Taebaeksan Basin) as part of an ophiolite suite (O.J. Kim and Kim, 1974, 1976; D.S. Lee, 1980; H.Y. Lee et al., 1980) or island-arc volcanics (B.K. Park and So, 1972), recent petrological and geochemical analyses indicate that these rocks represent an intracontinental rift setting (Kwon and Lan, 1991; Cluzel, 1992; Kwon and Lee, 1992).
2.4
Taebaeksan Basin
The Taebaeksan Basin lies along the northern margin of the Okchon Fold Belt and is filled by the Choson Supergroup (Cambro-Ordovician) and the unconformably overlying Pyongan Supergroup (Carboniferous-Triassic) (Fig. 2.6). The former comprises largely carbonates with minor siliciclastics, whereas the latter consists of siliciclastics with minor carbonates. 2.4.1
Choson Supergroup
The Choson Supergroup overlies the Precambrian granitic gneiss and metasedimentary rocks (Yulli Group) and consists mainly of limestone beds overlain unconformably by the Pyongan Supergroup (Cheong, 1973). The latter consists primarily of non-marine and marine elastics including important coal measures. A Silurian sequence occurs on top of the Choson Supergroup (H.Y. Lee, 1980). The Choson Supergroup can be divided into five lithologic units based on type locality: Taebaek, Yongwol, Yongtan, Pyongchang, and Mungyong groups (D.K. Choi, 1998) (Fig. 2.6). These sequences yield diverse and abundant invertebrate fossils. 2.4.1.1
Taebaek
Group
The Taebaek Group is present in the southern part of the Paekunsan Syncline and comprises 10 lithologic units (Kobayashi, 1966; Cheong, 1969; D.K. Choi et al., 1996; D.K. Choi, 1998) (Table 2.1; Fig. 2.7). The Changsan Formation is the basal unit of the Taebaek Group and consists of 50-200 m thick, milky white, light brown quartzites with trough and herringbone cross-beds and conglomerate beds of well-rounded clasts (granitic gneiss, slate, and quartzite). The clasts are generally 5-50 cm in diameter. No fossils are reported. It was formed probably in barrier beach
18
Marine Geology of Korean Seas
127030'
128°00'
128*^30'
129°00'
Fig. 2.6. Simplified geologic map of the Taebaeksan Basin. After D.K. Choi (1998). environments (Yun, 1978). The Myobong Formation (100-250 m thick) conformably overlies the Changsan Formation, and comprises dark grey to greenish grey slate, phyllite, and shale with interlayered sandstone and limestone beds. Kobayahsi (1966) recognized four biozones: Redlichia, Elrathia, Mapania, and Bailiella zones in ascending order. It was probably deposited in shallow marine environments (B.K. Park et al., 1994). The overlying Taegi (Pungchon) Formation (200-300 m thick) consists of milky white, light grey to dark grey massive limestone with argillaceous lime mudstone, intraformational carbonate breccia, oolitic limestone, and dolomitic limestone. Kobayashi (1966) established three trilobite zones: Megagraulos, Solenoparia, and Olenoides zones in ascending order (Middle Cambrian). It was probably deposited in carbonate shelf environments (Yun, 1978), arid hypersaline environments (J.Y. Kim and Park, 1981), or peritidal flat and oolitic shoal-reef slope environments (B.K. Park and Han, 1986, 1987).
Korean Peninsula
19
Table 2.1. Stratigraphic nomenclature of the lower Paleozoic Choson Supergroup in Taebaek-Samchok area. GICTR = Geological Investigation Corps of Taebaeksan Region, GLS = Great Limestone Series, YS = Yangdok Series. Geok)gic s^e
Kobayashi (1953, 1966)
GICTR (1962)
D.K. Choi (1998)
Cheong (1969)
Ashgfll
Old.
Caradoc
Tuwibong Fm
lianvim
Ougunsan Fm
Aieti^
Tuwibong Fm Makkol Fm Sangdong Group
Makkol Fm Tumugol Fm
Tremadoc
GLS
Tongjom Fm
Sangdong Subgroup
Makkol Fm
T^miu^l Fm
Tumugol Fm
GLS
Tumugol Fm
Tongjom Fm
Hwajol Fm Late
Hwajol Fm
Hwajol Fm
Tongjom Fm Taebaek Group
Hwajol Fm
Sesong Fm
Camb.
YS Early
Sesong Fm
Taegi Fm
Middle
Myobong Fm Changsan Fm
Tuwibong Fm Oiigunsan Fm Makkol Fm
Pungdion Fm
YS
Samdbiok Group
Taegi Fm
Chiktong Subgroup
Taegi Fm
Myobong Fm
Myobong Fm
Myobong Fm
Changsan Fm
Changsan Fm
Changsan Fm
The conformably overlying Sesong Formation (10-30 m thick) consists of dark grey-red slate intercalated v^ith thin-bedded fine-grained sandstone and light-grey limestone beds. Intraformational limestone conglomerate beds are present. Two trilobite zones {Stephanocare and Drepanurd) indicate early Late Cambrian age (Kobayashi, 1966). It v^as probably deposited in a deepwater environment (deep-water fan) (B.K. Park et al., 1985). The Hwajol Formation (200-260 m thick) conformably overlies the Taegi Formation and consists of ribbon rocks (alternation of limestone, marl, and limestone conglomerate). Locally quartzite beds are interbedded in the upper part. Abundant fossils of trilobites, brachiopods, gastropods, hyolithids, and conodonts are indicative of Late Cambrian age (Kobayashi, 1966; H.Y. Lee, 1975). Recent studies on conodonts indicate that the upper part of the formation was deposited in the Late Cambrian-Early Ordovician. The alternating lithofacies of ribbon rocks are generally interpreted as deposition in subtidal to peritidal environments (Woo and Park, 1989). The Tongjom Formation (10-50 m thick) conformably overlies the Hwajol Formation and consists of grey to pinkish white, well-rounded quartzose sandstone and calcareous sandstone. Fossils rarely occur, except for a trilobite species {Pseudokainella iwayai) which is indicative of early Ordovician age. It was probably deposited in (sub)tidal environments (Y.S. Choi, 1990). The overlying Tumugol Formation (150-270 m thick) consists of greenish grey marl to calcareous shale and ribbon rock (shale/limestone) with limestone conglomerates. Abundant fossils occur: trilobites, brachiopods, pelecypods, gastropods, cephalopods, and conodonts (Kobayshi, 1966; H.Y, Lee, 1975; D.K. Choi and Lee, 1988; D.K. Choi and Kim, 1989; Y.S. Choi,
20
Marine Geology of Korean Seas 129^00'
129^05'
LEGEND
OL
Tongjom Fm.
I C a r ^ gX^e^r^oup
1 Hi
Hwa,ol
|Tw 1 ,^^'^"9
1 Pc Pungchon Fm.
| C g 1 ^n^^nsan
1 Mb Myobong Fm.
| M g | K^^^-'
1 Cs
[Tm|^S"^9^'
IpC
|Gp| Porphyry
^
Changsan Fm. Precambrian Basement Rocks
Fig. 2.7. Simplified geologic map of Taebaek area. Modified after Son and Kim (1962) and KIGAM (1979).
1990; K.H. Kim et al., 1991). These fossils are suggestive of Tremadoc (Lower Ordovician). The associated lithofacies are indicative of a shallowmarine carbonate platform and ramp (Y.I. Lee and Choi, 1987; Y.L Lee and Kim, 1992; J.C. Kim and Lee, 1996). The limestone conglomerates are mostly of pseudoconglomerates (autoconglomerates) that originated from autoconglomeration of lime-mud during compaction (Kwon, 2000). The conformably overlying Makkol Formation (250-400 m thick) comprises lime mudstone-dolomitic limestone, limestone conglomerate, skeletal and non-skeletal grainstone, quartz siltstone, and thin-bedded mudstone (Paik, 1985, 1986, 1987). Abundant fossils occur, i.e., brachiopods, pelecypods, gastropods, trilobites, and conodonts (Kobayashi, 1966; H.Y. Lee, 1976; Paik, 1985) which are of middle Ordovician (Llanvim) age (D.K. Choi, 1998). Stromatolites, ripple marks, desiccation cracks, rhythmic beds, bird's eye structures, and bioturbated features are all suggestive of tidal flat (sub- and peri-tidal) environments (Paik, 1985; Woo, 1999). Some intercalated intraformational conglomerate beds show jigsawfit or mosaic structures which are suggestive of autoconglomeration processes (Kwon, 2000). The Chigunsan Formation (30-60 m thick) conformably overlies the Makkol Formation and consists of dark grey calcareous shale. It contains
Korean Peninsula
21
abundant microfossils (brachiopods, trilobites, gastropods, cephalopods, and conodonts) indicative of the Llanvim. It was deposited most likely in relatively deep subtidal environments (Y.I. Lee, 1988; Hyeong, 1991). The uppermost Tuwibong Formation (about 50 m thick) conformably overlies the Chigunsan Formation and is unconformably overlain by the Pyongan Supergroup (Carboniferous-Triassic). The Tuwibong Formation consists of light grey massive limestone with intercalated calcareous shale. Fossils include brachiopods, pelecypods, gastropods, hyolithid, cephalopods, trilobites, riberioid, sponges, bryozoans, crinoid stems, and conodonts (Kobayashi, 1966; H.Y. Lee, 1977). The Tuwibong Formation was formed in shallow marine (subtidal-intertidal) (Han, 1987), open marine platform (Y.I. Lee, 1988), or intertidal environments (Hyeong, 1991; Hyeong and Lee, 1992). 2.4.1.2
Yongwol
Group
The Yongwol Group comprises limestone, dolomitic limestone, and dolomite-interbedded shale beds with abundant invertebrate fossils. The entire sequence can be divided into five formations (Yosimura, 1940; Kobayashi, 1966; J.G. Lee, 1995; D.K. Choi et al., 1996; D.K. Choi, 1998): (1) Sambangsan Formation (sandstone and siltstone) of middle Cambrian age, (2) Machari Formation (thin-bedded limestone and black shale), (3) Wagok Formation (massive dolomitic limestone), (4) Mungok Formation (limestone/dolomite), and (5) Yonghung Formation (dolomitic limestone with shale) in conformable ascending order (Table 2.2). Recent work on Table 2.2. Stratigraphic nomenclature of the lower Paleozoic Chosen Supergroup in Yongwol area. Geologic age
Yosimura (1940) Kobayashi (1966)
GICTR (1%2) Sambangsan Fm
Post-Ordovidan Yonghung Fm
O.J. Kim et al. (1973)
D.K. Choi (1998)
Sambangsan Fm (Hongjom Fm)
AshgiU Catadoc Ordovidan
Llanvim Aienig Tremadoc Late
Yonghung Fm Mungok Fm Wagok Fm Machari Fm
Cambrian
Middle Early
Sambangsan Fm
Samtaesan Fm Hungwolri Fm Machari Fm
Yonghung Fm Samtaesan Fm (Mungok Fm) Hungwobi Fm (Wagok Fm)
Yonghung Fm
Yongwol Group
Mungok Fm Wagok Fm
Madiari Fm Sambangsan Fm Taegi Fm
Machari Fm Sambangsan Fm
,
22
Marine Geology of Korean Seas
trilobites (J.G. Lee, 1995; D.K. Choi et al., 1996) made it clear that the entire sequence consists of thrust sheets with a successive eastward stacking pattern (Fig. 2.8). The middle Cambrian Sambangsan Formation (400-700 m thick) consists predominantly of light grey, green or brown, massive or thick-bedded sandstone (upper part) and red or green shale and sandy shale (lower part). Abundant trilobites of Middle Cambrian age occur in light brown, thickbedded, medium- to coarse-grained, micaceous sandstone beds in the upper part of the formation. The overlying Machari Formation (up to 200 m thick) is characterized by rhythmic alternation of thin-bedded, dark grey to light grey limestone and laminated black shale with banded structures. The lower part comprises finely laminated grey or brown shale, thick-bedded bioclastic grainstone to packstone, and lime breccia and contains trilobites (Olenoides, Tonkinella, and Peronopsis). The middle part is dominated by dark grey to black laminated shale and yields abundant trilobites of Late Cambrian age. The upper part consists of alternating layers of light grey dolomitic limestone and laminated black shale. It occurs mainly in the thrust sheets west of the Machari Thrust Fault. These lithofacies suggest that the Machari Formation was formed in relatively deep-water environments. The formation also contains abundant layers of intraformational breccia and conglomerates that resulted from soft-sediment deformation during compaction. The overlying Wagok Formation consists mainly of a thick (200-250 m thick) sequence of light grey to grey, massive dolostone. The uppermost part of the formation is characterized by dark grey oolitic grainstone to packstone and is overlain by the basal member of Mungok Formation (J.G. Lee, 1995). Kobayashi (1960) reported an occurrence of poorly preserved brachiopods and trilobites from the middle part of the formation. Based on these faunal assemblages, the Wagok Formation was dated at late Cambrian (Kobayashi, 1966). The overlying Mungok Formation (ca. 200 m thick) (Tremadoc) consists of limestone with minor shale (Paik et al., 1991; Y.S. Choi et al., 1993; Chung et al., 1993). The basal member (ca. 50 m thick) comprises ribbon rock and grainstone to packstone with intraformational conglomerates as well as chert layers, peloidal or bioclastic grainstone to packstone, and bioturbated ribbon rock. The ribbon rock is often cross-laminated and normally graded. The lower member (30-35 m thick) is recognized by a monotonous sequence of light grey to grey, massive to crudely bedded dolostone. The middle member (35-60 m thick) consists of ribbon rocks and intraformational conglomerates as well as grainstone and packstone. The upper member (50-60 m thick) comprises ribbon rock, grainstone to
Korean Peninsula
23
Fault •*
Thrust fault Road
128»22'30"
128'27'30"
Fig. 2.8. Geologic map of Yongwol area, showing successive Cambro-Ordovician thrust sheets. C = Chonggok Fault, P = Pyongchang Fault, Y = Yonbong jong Fault, M = Machari Fault, S = Sangri Fault, K = Kaktong Fault. After D.K. Choi (1998).
24
Marine Geology of Korean Seas
packstone, pseudoconglomerates (Kwon, 2000), and marlstone to shale and contains abundant trilobites, brachiopods, ostracods, and pelmatozoan stems (D.K. Choi, 1998). The Mungok Formation was most likely formed in tidal (supratidal to subtidal) environments on a carbonate platform with patches of oolitic shoals (Paik et al., 1991; Chung et al., 1993) or in a shallow ramp to basinal environment (Y.S. Choi et al., 1993). The overlying Yonghung Formation consists mainly of massive to thickbedded, dark grey, fine- to medium-crystalline dolostones in the lower part and grey to bluish grey limestones in the upper part (H.Y. Lee, 1987; Y.S. Choi et al., 1993). In the western part of the Yongwol area, it is intercalated with thin beds of mud-flake conglomerate. Primary sedimentary structures including mud-cracks, ripple marks, and trace fossils are well preserved. Based on lithofacies characteristics, Yoo (1991) and Yoo and Lee (1998) suggested that it was formed in shallow carbonate-ramp environments. S.J. Choi and Woo (1993) recognized eight lithofacies and four transgressive cycles and suggested deposition in sabkha-type tidal flat environments. Based on trace fossils in the lower part of the formation, Jeong and Choi (1993) suggested that the lower Yonghung Formation was deposited in shallow shelf or sheltered lagoon. Based on the occurrence of Bailiella and some poorly preserved cephalopods, Kobayashi (1966) correlated the formation with the Chigunsan and Tuwibong formations and the Wolungian to Toufangian Series (Middle Ordovician) of North China. The conodont assemblage suggests that the formation is of the Arenig to Caradoc (H.Y. Lee, 1979, 1980; S.J. Lee, 1990). 2.4.2
Pyongan Supergroup
The Pyongan Supergroup (Carboniferous-Triassic) disconformably overlies the Choson Supergroup (Cheong, 1969). It is partly distributed in the marginal belt of the Okchon and Taebaeksan basins (notably Samchok, Yongwol, Kangnung, and Mungyong areas), and contains economically important coal measures. The sequence began with a limestone-sandstone succession followed by deltaic and fluvial sequences. In the Taebaek area, the Pyongan Supergroup (ca. 1,700 m thick) is present along the axis of the Paekunsan Syncline (Fig. 2.7) and comprises the Manhang, Kumchon, Changsong, Hambaeksan, Tosagok, Kohan, and Tonggo formations in ascending order (Cheong, 1969). For these units petrographic studies have been made for the provenance (Y.I. Lee, 1990; Y.I. Lee and Lim, 1995; Yu et al., 1997; Y.I. Lee and Sheen, 1998; Ko et al., 1999). The Manhang Formation consists of reddish and greenish gray sandstone, mudstone, light gray limestone, and pebbly conglomerate. Cheong (1973) recognized three fusulinid zones of Moscovian age. The Kumchon Formation comprises dark gray sandstone, shale.
Korean Peninsula
25
limestone, and coal (S.I. Park, 1989). The Changsong Formation (Artinskian) consists of dark gray sandstone and shale. The Hambaeksan Formation consists of thick arenitic sandstone. The Tosagok Formation consists of pebble-bearing sandstone and red shale. The Kohan Formation comprises gray to greenish sandstone and siltstone. The uppermost Tonggo Formation comprises alternating red and green sandstone and siltstone. In the Yongwol area, the Pyongan Supergroup is distributed in the eastern part of the Machari thrust. Cheong (1969) classified the Upper Paleozoic rocks into Yobong, Pangyo, Bamchi, and Mitan formations in ascending order: the Yobong and Pangyo formations are equivalent to the Hongjom Formation, whereas the Bamchi and Mitan formations to the Sadong Formation. Yosimura (1940) reported the occurrence of brachiopods and crinoids from the Sadong Formation. The Pyongan Supergroup yields diverse fusulinids (Cheong, 1973; Cheong and Park, 1977; Cheong et al., 1983; C.Z. Lee, 1984; C.Z. Lee et al., 1988; C.Z. Lee and Kim, 1993) and conodonts (C.Z. Lee et al., 1988; S.I. Park, 1993). The Yobong Formation rests unconformably on the Ordovician Yonghung Formation. It is characterized by reddish and greenish yellow shale or mudstone, sandstone, conglomerate, and light grey limestone (100-150 m thick). C.Z. Lee (1992) recognized two fusulinid zones in the formation, the lower Eostaffella-Pseudostafella Zone and the upper Profusulinella Zone. He correlated the lower one with the Bashkirian, and the upper with the lower Moscovian, the Manhang Formation of the Samchok coalfield, and the Penchi Series of China. J.D. Lee (1985) recognized two conodont biozones in the formation in the Machari-Bamchi area, namely, the lower Idiognathoides Zone and the upper Idiognathodus Zone. J.D. Lee (1985) confirmed that the Hongjom Formation was formed in the lower Morrowan (lower Bashikirian) to middle Desmoinesian (middle Moscovian). The Pangyo Formation consists of alternations of grey sandstone, shale, and nodular chert-bearing limestone which attains thicknesses of up to 150 m (C.Z. Lee, 1992). The basal part of the formation is composed of a 2-mthick grey sandstone bed (Cheong, 1969). C.Z. Lee (1985, 1992) recognized two fusulinid zones, namely, the lower Beedeina Zone and the upper Neostaffella-Fusulina Zone. The lower one was correlated with the Neostaffella sphaeroidea Subzone of the Samchok coalfield, whereas the upper with the Kumchon Formation, the Moscovian of the Russia, and the Fusulinella provecta and Fusulina cylindrica subzones of North China (C.Z. Lee, 1992). The Bamchi Formation (80 m thick) rests paraconformably on the Pangyo Formation and grades conformably to the Mitan Formation (C.Z. Lee, 1984). It consists mainly of alternating dark grey limestone, shale, and sandstone beds. The limestone beds produce abundant fossils including fusulinids.
26
Marine Geology ofKorean Seas
foraminifers, brachiopods, corals, and conodonts (S.I. Park, 1993). The Mitan Formation (Permian) (200 m thick) consists mainly of black shale and grey sandstone. Three coal seams are intercalated within the upper part of the formation.
2.5
Imjingang Belt
The Imjingang Belt is an E-W-trending fold and thrust belt, characterized by the occurrence of a Devonian-Carboniferous sequence (>200 m thick) and is underlain unconformably by Proterozoic and lower Paleozoic basement rocks (Ree et al., 1996). The Imjin Group consists of originally siliciclastic and carbonate sequences with volcaniclastics. The Yonchon complex underlies the Imjin Group sequence and is divided into Jingok and Samgot units (Ree et al., 1996). The Jingok unit consists of pelitic schist, phyllite, and quartzite, whereas the Samgot unit consists of calc-silicate rocks and amphibolite. The latter is in contact to the S with deformed granitoids of unknown age (Ree et al., 1996). Structural, petrological, and geochronological data (Ree et al., 1996) suggest that the northern Jingok unit consists of Barrovian-type metapelites and the southern Samgot unit comprises calc-silicate and amphibolitic rocks. Reverse-sense shearing is dominant in the Jingok unit, whereas late normalsense shearing is pervasive in the Samgot unit and the deformed granitoid to the S, corresponding to extensional deformation associated with uplift following compression (Ree et al., 1996). Pressure-temperature conditions of the amphibolites (Late Proterozoic) were about 8-13 kbar and 630-740°C, respectively, and occurred in the Permian-Triassic.
2.6
Orogenic Events
A series of polycyclic deformation occurred in the Korean Peninsula, accompanied by granite intrusion that disturbed the pre-Mesozoic sequences. A synthesis on tectonic evolution (Chough et al., 2000) suggests that orogenic events in the Korean Peninsula were largely due to collision of the North and South China blocks and subduction of the Pacific Plate (Fig. 2.9). The first orogeny occurred in the early Triassic (Songrim Orogeny) in the middle part of the peninsula (J.H. Kim, 1996; Ree et al., 1996). It caused thrust deformation of the intra-cratonic basins, the Imjingang Belt (Ree et al., 1996) and the Okchon Fold Belt (Cluzel et al., 1991b; H. Kim et al., 1995; Min et al., 1995). Formed in the post-deformational inland basins (piggyback basins), the Taedong Group (late Triassic-early Jurassic) consists of non-marine siliciclastics (Thomas et al., 1976; Chun et al., 1990;
Korean Peninsula
27
(a) Early Triassic
(b) Jurassic
Fig. 2.9. Orogenic events in NE Asia, (a) Late Permian-early Triassic (Songrim Orogeny), (b) Jurassic (Daebo Orogeny). NCB = North China Block, SCB = South China Block, IB = Imjingang Belt, KM = Kyonggi Massif, YM = Yongnam Massif, TB = Taebaeksan Basin, OB = Okchon Belt, HSZ = Honam Shear Zone, PP = Pacific Plate. After Ree et al. (2000) by permission of the American Geophysical Union.
28
Marine Geology of Korean Seas
Yu, 1983; Yu and Lee, 1992; Yu et al., 1992). The second orogeny (Daebo Orogeny) occurred in the middle Jurassic causing strike-slip deformation in the middle-southern part of the peninsula (Honam Shear Zone) with extensive granite intrusion (Figs. 2.10 and 2.11) (Turek and Kim, 1995; C.-B. Kim and Turek, 1996; J.H. Kim, 1996; Kwon and Ree, 1997). The metamorphic grade increased from greenschist to amphibolite facies toward the northeastern and southwestern margins of the Okchon Fold Belt (Reedman et al., 1973; Cluzel et al., 1991a). Mineral assemblages in pelitic rocks include almandine, staurolite, and kyanite, suggestive of an intermediate-pressure metamorphic environment (Min et al., 1995; Oh et al., 1995). These tectonic movements resulted in the formation of thrust faults and both dextral and sinistral ductile wrench zones (Yanai et al., 1985; Chang and Han, 1989; Cluzel et al., 1991a; Otoh and Yanai, 1996). The dextral ductile shear zone, Honam Shear Zone (Yanai et al., 1985), consists of two sets of nearly vertical ductile shear zone trending N and NE extending for about 400 km (ca. 80 km in width) (Chang and Hwang, 1984; Chang, 1985; Chang and Han, 1989; Cluzel et al., 1991a, b; J.H. Kim and Kee, 1994; Otoh and Yanai, 1996). It dissects the Yongnam Massif, Jurassic granite, and gneissose granite (Otoh and Yanai, 1996; Kwon and Ree, 1997) and occupies a pre-Miocene paleogeographic position similar to the Akiyoshi, Maizuru-Tamba, and Hida belts in the inner zone of SW Japan (Yanai et al., 1985; Otoh and Yanai, 1996). The mylonitic rocks in the shear zone are characterized by vertical foliation with a dextral sense of shear. The dextral motion in the Honam Shear Zone continued until the middle-late Jurassic (Otoh and Yanai, 1996). High-angle extensional and divergent faults prevailed, parallel to the major fault zone. Small-scale NW-SEtrending sinistral, NNE- and NS-trending normal and en echelon faults and folds commonly occur with reverse and thrust faults.
2.7
Cretaceous Non-Marine Basins
The Cretaceous non-marine sedimentary sequences and volcanics in the Korean Peninsula largely occur in the southeastern part (Kyongsang Basin) with subordinate exposures in the southwestern part (Haenam, Neungju, Jinan, Kyokpo, Yongdong, Kongju, and Eumsung basins) generally trending NE-SW (Fig. 2.12). These basins are interpreted as transtensional basins formed on the overriding continental plate (Asian Plate) in a retroarc setting (Chun and Chough, 1992; S.B. Kim et al., 1997). The Kyongsang Basin most likely represents part of a large-scale strike-slip basin which is contiguous to the southwestern part of Japan (Fig. 2.12).
Korean Peninsula
fa
NM
29
TB
^
r ^^2^r YMy NCB
f!0 OB/
/
...
300 km
r^^
1
/
SCB
Pre-Triassic
[b
NM /
V^
PB \ ,
NCB ^ ^
#K YM 7 HSZ
1 ^ ^
SCB
/
Fig. 2.10. Mesozoic plate tectonic setting for crustal deformation in the Korean Peninsula, (a) Pre-Triassic, (b) Jurassic. Note that the Yongnam Massif (and Taebaeksan Basin) was fused together with the Kyonggi Massif (and Okchon Basin) by dextral fault movement in the Jurassic. NM = Nangrim Massif, PB = Pyongnam Basin, IB = Imjingang Belt, SB = Sulu Belt, OB = Okchon Belt, TB = Taebaeksan Basin, YM = Yongnam Massif, KB = Kyongsang Basin, NCB = North China Block, SCB = South China Block, FP = Farallon Plate, DSB = Dabieshan Belt, HSZ = Honam Shear Zone. After Chough et al. (2000) by permission of the Elsevier Science B.V.
30
Marine Geology of Korean Seas
^
Fig. 2.11. Distribution of granitoid rocks in the southern part of the Korean Peninsula. IB = Imjingang Belt, KM = Kyonggi Massif, OB = Okchon Belt, YM = Yongnam Massif, KB = Kyongsang Basin. After Chough et al. (2000) by permission of the Elsevier Science B.V. 2.7.1
Kyongsang Basin
The Kyongsang Supergroup sequence (ca. 9,000 m thick) can be divided into 3 major lithostratigraphic units on the basis of the presence (dominance) or absence of volcanics: the lower Sindong Group of siliciclastic sediments, the middle Hayang Group of non-volcanic sediments v^ith subordinate volcanics, and the upper Yuchon Group of predominant volcanic sediments (Chang, 1975a, b). Studies on fossils including molluscs, vascular plants,
Korean Peninsula
31
Cretaceous Basins
Fig. 2.12. Tectonic setting of the Korean Peninsula and the adjacent Japanese islands in the Cretaceous. Note oblique subduction of the Izanagi Plate and formation of obliqueslip basins and the associated volcanic activity. MTL = Median Tectonic Line.
calcareous algae and crustaceans (Tateiv^a, 1929; Suzuki, 1940; Ota, 1960 Yang, 1978, 1992), estheria (Myeong, 1980), charophyta (Seo, 1985; S J Choi, 1987), palynomorphs (D.K. Choi, 1985; J.B. Park, 1986; D.K. Choi and Park, 1987), and dinosaur and bird tracks (B.K. Kim, 1969; Yang, 1982
32
Marine Geology of Korean Seas
Urn et al., 1989; Lockley et al., 1991, 1992; Lim and Yang, 1992) have revealed that the basin was initiated in the Early Cretaceous (Hauterivian to Barremian) and continued into the Aptian to Albian. Conchostracan fossils from the upper part of the Sindong Group suggest that the basin was formed in shallow, temporary reservoirs/swamps which experienced seasonal drying and repeated wetting events (Myeong, 1980). Charophyta fossils from the Hayang Group are indicative of shallow lacustrine depositional environments (Yoo, 1970; Seo, 1985). Bird and dinosaur tracks also suggest that shallow lakes persisted for a significant period (Lockley et al., 1992). Based on the scarcity of fern spores and the abundance of Corollina and Ephedripites, D.K. Choi (1985) inferred that an arid and warm climate prevailed in the Kyongsang Basin during the Early Cretaceous. Studies of desiccation cracks indicate a seasonal climate with wetting and drying cycles during the Early Cretaceous (Paik and Lee, 1998). The Kyongsang Supergroup can be grouped into three facies associations, representing alluvial fan, floodplain, and lacustrine environments (Chang, 1977, 1988; H.L Choi, 1979, 1981, 1985, 1986; K.C. Lee, 1985; Chang et al., 1990; Son, 1990). The occurrence of rhizoliths, ooids, and stromatolites suggests that the basin contained a saline lake under evaporative conditions, indicating semiarid to arid climate (Woo et al., 1991; K.C. Lee, 1992; Paik and Chun, 1993). Detailed sedimentological work in the Pyonghae Basin (northeastern extension of the Kyongsang Basin) reveals that the succession represents a terminal fan system (Rhee, 1991; Rhee and Chough, 1993). In the northwestern comer of the Kyongsang Basin, the sequence comprises alluvial deposits of conglomerate, gravelly sandstone, sandstone, and mudstone which can be grouped into four allostratigraphic units based on stratigraphic discontinuities (Fig. 2.13) (Rhee et al., 1998). The discontinuities trend NW-SE and are marked by distinct facies transitions, abrupt emplacement of conglomerates, and persistent mudstone key beds. The allostratigraphic units are characterized by channel-fill architecture, clast composition of the conglomerates, and the sandstone/mudstone ratio. The four units successively shifted eastward, stratigraphically upsection and each unit developed nearly parallel to the direction of overall sediment dispersal, forming alluvial fans in the northern margin and fluvial channel networks toward the basin center to the southeast (Fig. 2.13). 2.7.2
Other Basins
In the southwestern Korean Peninsula, a series of strike-slip faults, the Kongju and Kwangju Fault systems, occur trending NE-SW. They have the characteristic normal and en echelon patterns associated with high-angle
Korean Pen insula
33
34
Marine Geology of Korean Seas
transtensional faults. These major faults were activated by sinistral strikeslip movements in the late Jurassic to the Cretaceous, forming small-scale rhomboidal basins (Chun and Chough, 1992, 1995). These elongated basins were filled with alluvial to lacustrine sediments during the Early to Late Cretaceous, controlled by left-lateral strike-slip faults. The rhomb-shaped Eumsung Basin (-7 km x 33 km), located within the Kongju Fault system, is bounded by two left-stepping sinistral master faults which are in contact with foliated cataclastite, microbreccia, and mylonite in the basin margin (Precambrian gneiss and Jurassic granite) (Fig. 2.14; Y.S. Choi, 1996). An analysis of mylonitic foliation and minor fault orientations suggests that the major faults have attitudes of N13°E/82°W in the eastern part and N38°E/75°E in the western part (Cheong, 1987) and indicates strike-slip or high-angle normal faults bordering the basin margin. The Eumsung Basin contains a thick (>8 km thick) sequence of volcanics, conglomerate, conglomerate/purple mudstone, grey sandstone/green-grey mudstone, purple mudstone, and green-grey mudstone deposited in alluvial fan, fluvial, and lacustrine environments (Fig. 2.15) (Ryang and Chough, 1997). Another Cretaceous Basin, Jinan Basin, is filled with siliciclastic and volcaniclastic sedimentary sequences formed in a strike-slip fault system (Fig. 2.16) (S.H. Lee and Chough, 1999). Bouldery conglomeratic successions are present along the eastern margin, successively overlapping toward the N and NE. These successions consist of axial (longitudinal) fill and transverse (lateral) fill. The northeastward overlapping geometry of the lateral-fill sequence is indicative of sinistral fault movement. The consistent oblique progradation (i.e., northeastward) of the lateral-fill sequence to the margin suggests a northeastward basin-floor tilting, induced by differential subsidence along the fault-bounded margin. An increase in both depositional slope and water depth from the lower to the upper part of lateral-fill sequence refiects an enhanced dip-slip fault movement with time. The marginal conglomeratic successions in the eastern part of Jinan Basin formed under conditions of a northeastward increase in vertical fault movement accompanied by sinistral strike-slip faulting.
2.8
Pohang Basin
The Pohang Basin is bounded on the west by the Yangsan Fault which runs N-NE for 170 km from the southeastern tip of the Korean Peninsula to the East Sea (Fig. 2.17). The Yangsan Fault experienced about 35 km of post-Eocene dextral strike-slip movement (Chang et al., 1990). In the earlymiddle Miocene, however, the western margin of Pohang Basin was under
Korean Peninsula
35
lay^ao*
LEGEND
Fig. 2.14. Geologic map and cross section (A-A') of Eumsung Basin. For location see Fig. 2.12. After Ryang and Chough (1997) by permission of the SEPM.
36
Marine Geology ofKorean Seas
DEPOSITION OF BERJAE SEQUENCE NORTHWARO-SI^WED ALLUVIAL SYSTEM Repetitive conglonienrte sheet/channd fiHs: laHuvialfan
EMPLACEMENT OF GREEN SILTSTONE WITH CONGLOMERATE Transgression of lake ievd or Formation of small-scale marginal lake Intensified fault displacemettf
1 DEPOSITION OF DOOTASAN SEQUENCE NORTHEASTWARD-SKEWED ALLUVIAL SYSTEM Debris-flow depostts in the basin margbv Debris flow dominated aHuvial ran Channel fWs within purple sMstone: Channel systems ki aHuvial plain
PLAN VIEW
Fig. 2.15. Depositional model of the southeastern part of Eumsung Basin. After Ryang and Chough (1997) by permission of the SEPM.
an extensional regime (Chough et al., 1993). Recent paleostress analysis in the northern part of the Yangsan Fault revealed several episodes of strikeand dip-slip movements in the early to middle Miocene (Chae and Chang, 1994). The Yonil Group, a sedimentary fill of the Pohang Basin, comprises a more than 1-km-thick siliciclastic sequence. Analysis of sedimentary facies and depositional architecture reveals that the Yonil Group represents six fandelta systems (Fig. 2.17). The Gohyun, Duksung, and Doumsan fan deltas are of Gilbert type; the Maesan system is of steep-slope type; the Malgol and Yugye systems are of scree-apron type (Chough et al., 1993). Overlapping
Korean Pen insula
37
FAi mm FAIIIJi FAVf^ ^ —
Paleoflows Syndepositional normal fault Fault
Fig. 2.16. (a) Geologic map of the northeastern part of Jinan Basin (for location see Fig. 2.12). FA I = alluvial fan, FA II = small-scale Gilbert-type delta, FA III = large-scale steep delta slope, FA IV = base of the delta slope and prodelta, FA V = lacustrine plain deposits, (b) Depositional model of the northeastern Jinan Basin, showing progressive changes in sedimentary facies and stratal patterns. (1) Deposition of the marginal fill (FA I, II, and V) and the longitudinal fill (FA III, IV, and V). Relatively low magnitude of subsidence with sinistral strike-slip displacements along the southeastern bounding fault. Slightly oblique progradation due to the northward basin-floor tilting. (2) Emplacement of lacustrine plain deposit (FA V) associated with intensified basin subsidence. (3) Deposition of the upper marginal fill (FA III and IV). Relatively high magnitude of basin subsidence accompanied with sinistral strike-slip displacements along the southeastern bounding fault. Slightly oblique progradation due to the northward basin-floor tilting. After S.H. Lee and Chough (1999) by permission of the Elsevier Science B.V.
38
Marine Geology of Korean Seas
3. Deposition of Upper Marginal Fill -""
"""••-
h\
Longitudinal Fill
2. Emplacement of Lacustrine Plain Deposit Longitudinal
N ^
Lower Marginal
1. Deposition of Lower Marginal Fill
Longitudinal
Lower Marginal
Fig. 2.16. Continued.
Korean Peninsula
39
Fig. 2.17. Simplified geologic map of the western part of Pohang Basin (Miocene) and outline of fan-delta systems. After Chough and Hwang (1997) by permission of the SEPM. patterns and architecture of these fan deltas indicate that they were formed in shallow marine water, followed by rapid subsidence and deposition in a deep-marine setting (Hwang et al., 1995; Chough and Hwang, 1997). Largescale drainage systems were developed at the junction between the strikeslip zone and the oblique transfer faults, acting as major sources for coarsegrained sediments. The high sediment supply rate and rapid subsidence of the hanging wall resulted in the progradation of large-scale Gilbert-type fan deltas with a radial distribution pattern. Along the steeply inclined footwall scarps, the drainage systems were relatively small. Here, the low rate of sediment supply formed scree-apron-type and steep-faced slope-type fan deltas.
40
Marine Geology of Korean Seas
The evolution of the Pohang Basin can be divided into four stages (Hwang et al., 1995). During the first stage (Early Miocene), alluvial fans prograded into shallow-marine setting (Figs. 2.18 and 2.19). In the Doumsan and Duksung systems, a high rate of sediment supply and rapid rise of relative sea level resulted in the transformation of shoal-water-type to Gilbert-type foresets. In the Malgol and Maesan systems, rapid sea-level rise accompanied with a lower sediment supply rate caused transformation of alluvial fans to scree-apron-type and steep-faced slope-type fan deltas, respectively. In the second stage, subsidence of the hanging wall formed large-scale truncation surfaces in the Doumsan and Duksung fan deltas. Coarse-grained sediments were continuously supplied in the Doumsan system, whereas only fine-grained sediments were deposited in the Duksung system. In the Malgol system, sediment supply was almost terminated in this stage. In the third stage (Middle Miocene), sediment supply rate of the Doumsan fan delta decreased abruptly and the system was overlain by finegrained Gilbert-type deposits. The Duksung fan delta was overlain by the Maesan and Gohyun systems. The Maesan system prograded further basinward. During the last stage (Late Miocene), sediment supply of the Doumsan and Maesan systems was also terminated and the entire basin was draped by fine-grained biogenic materials (mainly diatoms).
2.9
Jangki Group
The Jangki Group (Tertiary) occurs along the southeastern coast of Korea (Tateiwa, 1924). It is distributed in the Kumkwangdong, Jangki, and Eoil areas and consists of pyroclastic and epiclastic deposits of Early to Middle Miocene age which rest on rhyodacitic ignimbrites of Eocene age (B.K. Kim et al., 1975; J.Y. Kim, 1982; Jin et al., 1988; H.K. Lee et al., 1992). These deposits are associated with contemporaneous basaltic flows and dykes (Yoon, 1986; Chwae et al, 1988). The Jangki Group near Jangki area was subdivided by Tateiwa (1924) into Jangki Conglomerate, Nultaeri Trachyte Tuff, Lower Coal-bearing Formation, Lower Basaltic Tuff, Upper Coalbearing Formation, Keumori Andesitic Tuff, and Upper Basaltic Tuff in ascending order. The Eocene volcanic basement includes pheno-rhyodacitic ignimbrite containing abundant pumice fragments and crystal grains of quartz and feldspar in an eutaxitic matrix (J.Y. Kim, 1982). Clasts derived
Fig. 2.18. Depositional model for fan-delta systems in the Miocene Pohang Basin: (a) Stage 1 (Early Miocene), (b) Stage 2 (Middle Miocene), (c) Stage 3 (Middle-Late Miocene), (d) Stage 4 (Late Miocene). After Hwang et al. (1995) by permission of the Elsevier Science B.V.
Korean Peninsula Malgol
Q
C
Malgol
Malgol
Doumsan
Doumsan
Doumsan
Maesan
Maesan
Maesan
Duksung Gohyun Yugye
Duksung Gohyun Yugye
Duksung Gohyun Yugye
41
42
Marine Geology of Korean Seas
C
•; MS-2
_i ;;D-4
A
is MS-1
D-2 Truncation surface
(D
O O
1 M-2 \
Lii
1 M-1 1
Malgol
u '
1" 1 Doumsan
1
Maesan
/ \ DS-3
CD
O /
CO
DS-2 \
^Mruncation surfac^w
DS-1
1
Duksung
Fig. 2.19. Inferred rate of relative sediment supply from the alluvial feeder systems in the Pohang Basin. The width of each pinnacle represents relative growth rate. After Hwang et al. (1995) by permission of the Elsevier Science B.V. from this basement are abundant in epiclastic gravelstone and gravelly sandstone of the Jangki Group. The Nultairi Trachyte Tuff and the Lov^er Basaltic Tuff are mainly composed of vitric grains (vitric tuff) (J.Y. Kim, 1982). The eruption centers of these pyroclastic deposits are presently unrecognized near Jangki town. Plagioclase composition in the sandstones of the Lower Coal-bearing Formation varies in the range of AnlO-60, indicating diverse provenance (Noh and Boles, 1989). The Jangki Group in the Jangki area comprises non-marine pyroclastic and epiclastic deposits (Bahk and Chough, 1996). A detailed fades analysis reveals that it consists of 16 volcaniclastic and sedimentary facies based on a four-tier system of grain size, types, vitric components, and primary sedimentary structures. These facies can be organized into syn- and intereruption deposits. The former mainly comprises deposits of volcanic debris flows and hyperconcentrated flood flows which were initiated from turbulent flood surges, whereas the latter consists of channel and interchannel deposits which show abrupt compositional transition to and
Korean Peninsula
43
from those of the syneruption deposits. During the syneruption periods, volcaniclastic aprons of volcanic debris-flow and hyperconcentrated-floodflow deposits prograded onto the axial fluvial systems which were preserved as encased small-scale channel-fill units. During the intereruption periods, the axial fluvial system was re-established on dissected volcanic aprons and produced thick successions of braided stream and interchannel deposits.
2.10
Quaternary Volcanism in Cheju Island
Cheju Island is a shield volcano composed mainly of basaltic lava flows and minor pyroclastic rocks (Won, 1976; M.W. Lee, 1982). Major and trace element studies of the volcanic rocks suggest that the island was produced by alkaline basaltic magma (M.W. Lee, 1982). Eruptions have occurred mainly during the middle Pleistocene and have continued into historic time (D.H. Kim et al., 1989). The island contains about 360 scoria cones and 10 tuff rings and cones. The former are largely concentrated in inland areas, whereas the latter are distributed along the coastal regions. Sedimentological studies on the tuff rings and cones (Fig. 2.20; Sohn and Chough, 1989, 1992, 1993; Chough and Sohn, 1990; Sohn, 1996) suggest the existence of both dry and wet pyroclastic surges. The Suwolbong and Songaksan tuff rings were produced by relatively powerful explosions at deeper levels and were emplaced mainly by dry or wet pyroclastic surges. Shallow-level ground water was not readily supplied to the deeper explosion sites because of the presence of some aquiclude beds. These conditions resulted in the generation of eruption columns of ash and led to the formation of tuff rings. On the other hand, the Ilchulbong and Udo tuff cones were produced by relatively weak explosions at shallow depths and emplaced mainly by fallout of dry to wet tephra (mostly lapilli) and a number of remobilization processes.
2.11
Sedimentation and Tectonic History
Sedimentation in the Korean Peninsula began as early as late Proterozoic in the Okchon Basin. Shallow-water carbonates and siliciclastics were deposited in this failed-rift basin. In the long span of Cambrian to Ordovician, carbonate deposition was prevalent in the Taebaeksan Basin in shallow (subtidal, tidal, and peritidal) platform of calcite sea. A Silurian sequence conformably occurs locally. In the Imjingang Basin, siliciclastic and carbonate sediments (Devonian) were formed. In the Carboniferous to Permian, non-marine and shallow-marine siliciclastic sediments (Pyongan Supergroup) were deposited in the Taebaeksan Basin, unconformably overlying the Ordovician carbonates.
44
Marine Geology ofKorean Seas
b. Songaksantuffnng
Reworked deposit Lava [vovj Scoria cone WM Tuff ring/cone c.
Ilchulbong tuff cone
Fig. 2.20. Geological outline and tuff rings and cones in Cheju Island. After Sohn (1996) by permission of the Geological Society of America, Inc.
In the early Triassic, collision of the North and South China blocks occurred along the Imjingang Belt (Fig. 2.9), a probable extension of the Sulu Belt in Shandong Peninsula and Dabieshan Belt in China. The collisional deformation also occurred in the Okchon and Taebaeksan basins with extensive thrust faults and folds. Both siliciclastic and carbonate sequences of the late Proterozoic to the Permian were influenced by compressional deformation events during this time (Songrim Orogeny). The Jurassic compressional deformation was due to northwestward subduction of the Pacific (Farallon) Plate beneath the Asian continent (Maruyama and Seno, 1989; Maruyama et al., 1997). The deformation (Daebo Orogeny) was dominated by thrust movements during which the Yongnam Massif (with Taebaeksan Basin) was fused together with the Kyonggi Massif (with Okchon Basin) by a dextral fault (ca. 200 km) along the Okchon Fold Belt and Honam Shear Zone (Fig. 2.10). It was associated with intrusion of granites and extrusion of basic-rhyolitic magmas in the entire fold belt, affected by low-angle subduction of the Farallon Plate under the Asian continent. In the thrust depressions, non-marine sequences
Korean Peninsula
45
(Taedong Group) were locally deposited, forming piggyback basins. In the late Jurassic-early Cretaceous, the Izanagi Plate began to subduct northward, oblique to the Asian continent (Engebretson et al., 1985; Otsuki, 1992). It caused oblique-slip deformation of the crust and formation of strike-slip basins (Fig. 2.12). A number of large- and small-scale basins were formed during this time, notably the Kyongsang Basin in Korea and Kanmon Basin in Japan. In the middle part of the southern Korean Peninsula, the Kongju and Kwangju Fault systems were the locus of strike-slip deformation, forming a series of retroarc basins. The scale, orientation, shear sense, and offset of the Tan-Lu Fault also suggest that the transcurrent movement resulted from continental shearing due to oblique motion of the Izanagi Plate relative to the Asian continent (Klimetz, 1983; Zhang et al., 1984; Xu and Zhu, 1994). Granitoid rocks and acidic volcanics were widespread in these arc and retroarc settings (Maruyama and Seno, 1989; Chough et al., 2000). The Cretaceous basins in the Korean Peninsula were terminated by the Paleogene, whereas the Yellow Sea and Bohai Bay basins were further expanded, forming thick non-marine deltaic and lacustrine deposits (K.S. Park et al., 1997). In the Neogene, the Pohang Basin was formed in association with the pull-apart opening of the Ulleung Basin in the East Sea (Yoon and Chough, 1995). The large-scale Ulleung Fault was formed extending for about 500 km from the Korea Plateau to the Korea Strait. Thick (>10 km thick) sediments were accumulated in the Ulleung Basin margin and later deformed (Late Miocene) due to the partial closure of the sea caused by northward movement of the Kyushu Block. This movement was felt in all parts of the Korean Seas in the late Miocene and brought stabilization for marine transgression.
This . Page Intentionally Left Blank
CHAPTER 3
Yellow Sea 3.1
Physiography
The Yellow Sea (often called West Sea by Koreans) is an epicontinental sea with about 500,000 km^ in area, and is arbitrarily bordered to the northern East China Sea by a line connecting Cheju Island and south of the Changjiang (Yangtze) River mouth (Fig. 3.1). The shallow area south of the peninsula between Cheju Island and Tsushima Island has been named the South Sea by Koreans. The Yellow Sea is characterized by a flat, broad, and featureless seafloor with average water depth of about 55 m (maximum less than about 100 m. Fig. 3.1). The western part of the seafloor is bordered by the deltas of both the Huanghe and Changjiang rivers and the isobaths are parallel approximately to the coastline. The eastern Yellow Sea is fringed by numerous islands and a long stretch of tidal flat along the coast. Tidal sand ridges are ubiquitous in the eastern Yellow Sea in water depth less than about 70 m, trending slightly oblique to the coastline. The seafloor deepens progressively toward the axis that lies in about the eastern two-thirds of the sea. The seafloor of the shelf deepens progressively southeastward to form the northern extension of the Okinawa Trough. The seafloor around Cheju Island exceeds 100 m in water depth.
3.2
Geologic Setting
Geological studies on the Yellow Sea were instigated in the sixties with a general reconnaissance (aeromagnetic) survey by the Geological and Mineral Institute of Korea in cooperation with the Economic Commission for Asia and the Far East (ECAFE) (Emery et al., 1969; C.S. Kim, 1976). These studies have revealed that the Yellow Sea comprises a number of deep-seated sedimentary basins with isolated ridges (Chough, 1983). Ideas on the geologic evolution of the Yellow Sea and East China Sea have heavily depended on single-channel seismic and aeromagnetic data, whereas the abundant multichannel seismic and well data by both domestic and foreign exploration activities have not been integrated. The existing models on the evolution of eastern China and Yellow Sea are based largely on Chinese efforts in concert with American workers (Watson et al., 1987; Tao,
48
Marine Geology of Korean Seas Fig. 3.1. Bathymetric chart of the Yellow Sea. Contours in meters.
120'
122°
128°
1992). Although these studies revealed, to some degree, tectonic settings in northeast Asia during the Mesozoic and Cenozoic, the nature of the basement, basin evolution, and sedimentary processes have yet to be unraveled. The Yellow Sea is underlain by two large-scale Mesozoic and Cenozoic basins, generally trending E-W (Fig. 1.3). These basins are commonly contiguous to those onland, especially in China and western part of the Korean Peninsula. The acoustic basement, which forms part of the SinoKorean Massif, lies at about 3-5 km below the seafloor. The Sulu Belt in China probably extends across the Yellow Sea into the Imjingang Belt of Korea, a narrow suture zone of high-grade metamorphic event in the early Triassic (Ree et al., 1996) (Fig. 2.1). Information on the Paleozoic sedimentary sequence and the underlying basement (late Proterozoic) rocks in the Yellow Sea stems from a number of deep exploratory wells in the western part of the South Yellow Sea Basin off Subei Basin onland (eastern coast of China). The late Proterozoic sequence
Yellow Sea
49
(Sinian) was formed in a shallow carbonate shelf environment, followed by Cambrian-Silurian paralic-shallow marine carbonates. In the Devonian, nonmarine sequence was dominated and overlain by sequences of paralic, restricted carbonate platform. In wells WX 5-ST-l, WX 13-3-1, and CH 121-1, Carboniferous, Permian, and Triassic carbonates and shelf sequences were identified. The Mesozoic and Cenozoic sedimentary basins of northeast Asia are generally bounded by and parallel to the strike of the Tan-Lu Fault in eastern China and the subduction zones of the northwestern Pacific (Fig. 2.1). The Tan-Lu Fault with a sinistral displacement of about 540 km is a major transcurrent fault activated on the continental side of the northwestern circum-Pacific margin in the Mesozoic (Xu et al., 1987; Xu and Zhu, 1994). According to Meng and Zhang (1999), the Tan-Lu Fault system was due to the collision between the North and South China blocks whose large sinistral displacement was related to the oblique northward subduction of the Kula Plate under the Asian continent. On the west of the Tan-Lu Fault, Songliao, Bohai, Hefei and Jianghan basins occur, whereas on the east, Korea Bay, Subei, and Yellow Sea basins are present (Fig. 2.1). The latter are contiguous to those in the Korean Peninsula trending NE-SW (Chun and Chough, 1992).
3.3
Northern Yellow Sea Basin
The northern part of the Yellow Sea Basin was formed on the South China (Yangtze) Block bounded on the north by Qianliyan Massif and on the south by the Central Massif (Fig. 1.3). It is about 40,000 km^ in area and comprises over 7,000-m-thick sedimentary sequence, generally thickening westward. The basin consists of four depressions bounded by left-lateral strike-slip faults (NE-SW) and conjugate faults (NW-SE). The small-scale Kunsan Basin occurs in the eastern part of the basin, which is bounded on the south and east by growth faults which trend NEE-SWW. Exploratory wells delineate Cretaceous volcanic layers, purple conglomerate, sandstone, and mudstone overlain by dark-grey and purple sandstone and mudstone beds (Fig. 3.2). The basement is characterized by simple continuous reflection patterns with moderate amplitude and strong reflectors with high amplitude and chaotic reflections (Fig. 3.3). A seismic section across the northern depression delineates a thick sedimentary sequence, up to 2.6 s (twt) (Fig. 3.3). The sequence is generally offset by high-angle transpressional faults with an antithetic component. The acoustic basement shows high-amplitude reflection with moderate continuity. It is interpreted as the top of pre-Cretaceous strata. Anticlinal folds occur
50
Marine Geology of Korean Seas
TWO-WAY TRAVEL TIME IN SECONDS
Yellow Sea
51
52
Marine Geology of Korean Seas
with displacement up to 1.0 s (twt) along the listric faults (Fig. 3.4). The preMesozoic strata were pulled apart and faulted, forming half-graben during the Late Cretaceous, followed by subsidence in the Paleogene. In Block II, NE-SW-trending right-lateral strike-slip and NW-SE trending conjugate fault systems are dominant (Fig. 3.4). The acoustic basement is characterized by listric faults (K.S. Park et al., 1997). The sedimentary sequences underwent a transpressional fault regime and generally thicken southwestward (Fig. 3.5). Exploratory wells IIH-IXA and IIC-IX reveal brownish shale and interbedded sandstone in the lowermost portion (Late Cretaceous) (Bong et al., 1986; PEDCO, 1997) (Fig. 3.2). In well Inga-1, basaltic rocks were recovered at 3,400 m below sea level. Whole-rock K-Ar dates on these rocks (110 Ma) indicate subsidence of 5456 m/m.y. since the Eocene (Jin and Lee, 1991). In the Oligocene, NE-SWtrending major wrench and conjugate faults were dominant (Fig. 3.4). The Eocene to Oligocene sequence comprises conglomerate, sandstone, siltstone, brownish shale and evaporites, and generally fines upwards. The Oligocene to Miocene sequence was probably deposited in fluvial or shallow lacustrine environments (C.S. Kim et al., 1986, 1987; K.S. Park et al., 1997). Alluvial fan deposits occur in the faulted margins and show a coarsening-upward trend. During the Late Oligocene to Middle Miocene, compression was dominant. Block II contains 3 extensional pull-apart basins that are similar in stratigraphy to the oil-producing basins of the eastern China. Drill wells (IIC-IX and IIH-IXA) have revealed that a large part of the Oligocene section is missing. The Oligocene fluvial and alluvial reservoirs are interbedded with and sealed by organic-rich lacustrine shales and evaporites (PEDCO, 1997).
3.4
Southern Yellow Sea Basin
The Southern Yellow Sea Basin is bounded on the north by the Central Uplift (Massif) and on the south by Wunansha Uplift (Fig. 1.3). On the west, it is transitional to the Subei Basin on land. The basin is about 13,000 km^ in area and comprises over 3,000-m-thick sedimentary sequence. The basin contains a thick sedimentary sequence overlain by a regional unconformity (Liu, 1983). The sequence is folded and shows a tapered termination in the south where the faults dip northward. Along the northern margin these reflections are bounded by a fault dipping in the opposite direction (southward). The overlying reflectors also delineate a thick sedimentary sequence which generally thins toward the apex of the fold axis and thickens in the depression. The sequences reveal roll-over termination against the
«
•
-
•
-
»
Yellow Sea
fa
o
.S o
00
c^ o
o O ^ o
Si I o ctj CD
O
II
•S 8 ^
C/3 r-^
(L)
O C
^ .2
UH
^
53
o U
(U ON C ON
c/3 ( N
1>
54
Marine Geology of Korean Seas
Transfer fault Inferred transfer fault Basenr^ent involved listric fault
N
A
123X)0'
123^30'
124*30*
Fig. 3.5. Time structure map of acoustic basement in Block II. Contour interval, 500 ms. After K.S. Park et al. (1997) courtesy of the Korea National Oil Corporation. fault. The acoustic basement lies in depth from 1.5 to 3 s (twt), characterized by a continuous reflection doublet. The subbasins were formed on a previously folded and thrusted terrain of the Upper Paleozoic and Triassic shallowmarine carbonates, filled with lacustrine sediments (Liu, 1983). The sequence in the western part of the basin consists of conglomerate, sandstone, and mudstone, formed in fan-deltaic and lacustrine environments. The Oligocene section comprises siltstone and mudstone deposited in fluviodeltaic to lacustrine environments. Normal faulting was prevalent during the deposition and ceased at the end of the Oligocene. In the late Miocene, marine transgression occurred and deposited predominant sandstone and
Yellow Sea
55
subordinate siltstone and mudstone interbeds. These rocks are partly carbonaceous and pyritic. The PUocene sequence is made up of sandstone, siltstone, and mudstone, deposited in the lower shoreface. Possible traps occur in four types of structural patterns: negative flower structure, buried hill and drape, antithetic and synthetic forms, and overthrusts. There are three potential source rock units. The Sanduo Formation (Late Oligocene) contains organic mudstone deposited in a restricted lagoonal environment, whereas the Funning Formation is made up of brownish mudstone and organic-rich, dark grey calcareous mudstone. The Taizhou Formation represents an earliest sag-fill in the northern part. Reservoir rocks include fluvial and fan-deltaic sand bodies and deep-sea turbidites, as well as karstified carbonates.
3.5
Basin Evolution
In the Mesozoic, sedimentary basins in the Eurasian margin were largely under a transtensional regime due to oblique convergence of the subduction zone (Tang, 1982; Otsuki, 1985; Watson et al, 1987). These basins were intimately associated with the Tan-Lu Fault and the Honam Shear Zone (Figs, 2.9 and 2.10). The Tan-Lu Fault was initiated as early as late Proterozoic (Fletcher et al., 1995), contemporaneous with the collision of the North and South China blocks (Meng and Zhang, 1999). The Tan-Lu Fault was reactivated with sinistral transcurrent movements in the Triassic and continued in the late Jurassic-Cretaceous (Xu et al, 1987; Li, 1994; Xu and Zhu, 1994), resulting from continental shearing at the suture zone (Dabie Belt). In the early Cretaceous (about 135 Ma), the rate of subduction of the Izanagi Plate rapidly increased, causing the maximum offset of the Tan-Lu Fault and the formations of strike-slip basihs in the Korean Peninsula and adjacent seas (Chun and Chough, 1992; S.B. Kim et a l , 1997). The Cretaceous basins in the Korean Peninsula terminated in the late Cretaceous, whereas the Yellow Sea and Bohai Bay basins were further expanded. Due to the continuous transtension, NE-SW-trending wrench faults and NW-SE and N~S faults were reactivated in the shear zone, resulting in the development of en echelon pull-apart basins trending N E SW. The ponded occurrence of the Cretaceous strata occupying only local depressions within the South Yellow Sea Basin suggests that the Cretaceous strike-slip basins transformed and coalesced into a large-scale extensional basin in the Tertiary. Meso-Cenozoic basins in eastern China and southwestern Japan were subject to a temporal cessation of basin subsidence and extension in the late Cretaceous. Cretaceous sequences of the Songliao and Bohai Bay basins in
56
Marine Geology of Korean Seas
China were deformed and unconformably overlain by the Tertiary deposits. The Cretaceous Kanmon and Himenoura groups in Japan are both unconformably overlain by Paleogene strata that show different structural trends from those of the Cretaceous strata. In the Korean Peninsula, no Paleogene sequence was formed. These interruptions of basin subsidence might have been produced only by changes in fault regime from transtension to transpression along the strike-slip fault system. It is interesting to note that the obliquity of motion of the proto-Pacific Plate (Kula Plate) relative to the Asian continent changed to near normal at about 85-75 Ma (Engebretson et al., 1985). This event is thought to have been responsible for the cessation of strike-slip movement on Mesozoic faults in Japan (Otsuki, 1992). The Tan-Lu Fault also stopped transcurrent movement in the late Cretaceous, changing to extensional motion (Xu and Zhu, 1994). This event could have affected the whole eastern margin of Asia, thereby terminating the opening of the transtensional basins. In the Tertiary, these basins were largely influenced by the collision of the Indian Plate with the Asian continent (Tapponier and Molnar, 1976; Tapponier et al., 1986). These basins have undergone tectonic inversion in the Neogene (Lambiase and Bosworth, 1995).
3.6
Shallow Structure
The shallow geological structure of the west coast offshore of the Korean Peninsula east of 125°E has been revealed through a series of seismic (both air-gun and Uniboom) and magnetic surveys by the KIGAM teams (Koo et al., 1971; Yang et al., 1971; C.S. Kim et al., 1972; Koo, 1972; J.H. Lee et al., 1972; Frazier et al., 1976; S.W. Kim et al., 1980a, b; C M . Kim et al., 1981, 1987; S.W. Kim and Min, 1981; C.S. Kim et al., 1982), whose results have been published as a map series. A 3.5 kHz subbottom profiler was used to obtain high-resolution nearbottom profiles. The acoustic basement appears to be covered with a veneer of unconsolidated or semiconsolidated Quartemary sediments less than about 30 m thick nearshore (locally up to 60 m) and slightly thicker seaward (Fig. 3.6). On the Huksan Platform, sediments are thin and often basement rocks are exposed on the seafloor. This occurs near islands where strong tidal currents preclude the deposition of sediments. The acoustic basement consists of the seaward extension of onland rocks such as Precambrian gneiss and schist, Jurassic granite, metasedimentary rocks, and others (Cho and Choi, 1970; N.Y. Park et al., 1972) that can be correlated positively with the weak magnetic intensity. Locally, some extrusive rocks and schists with magnetite bodies signal strong magnetic intensity (Yang et al., 1971; C M .
Yellow Sea
57
35°30'
125°30'
126°30'
Fig. 3.6. Isopach map of sediment thickness above the acoustic basement (Paleogene) in the southeastern Yellow Sea (see inset). Hatched area for gas-charged sediments. Contours in meters. After KIER (1981b). Kim and Lee, 1974a; C M . Kim et al., 1981). The sediments can be divided into several layers (or sequences) by a number of mid-reflectors (Fig. 3.7). The reflectors are named a, (3, and y, etc. from the top. Each probably consists of hardground corresponding to a sealevel lowstand during the Quaternary period. The first reflector, reflector a, resulted most likely from the Wisconsinan sea-level low^stand and occurs at various depths below the seafloor or in some areas is exposed on the surface (Figs. 3.7 and 3.8a). The sequence above reflector a, designated sequence A (transgressive sequence), consists of unconsolidated Holocene sediments deposited during the postglacial transgression, and ranges in thickness from a few^ meters to about 40 m (average thickness, 10 m) (Fig. 3.9a). Sequence B, the sediment layer between the reflector a and the reflector p, also is distributed uniformly in most of the nearshore area of the southeastern Yellow Sea with a thickness of up to 50 m (Fig. 3.9b) (S.W. Kim et al., 1980a; C.S. Kim et al., 1982). Sequence B reveals complicated structure, that is, beds are often inclined, incised in many areas and refilled
58
Marine Geology of Korean Seas
Fig. 3.7. Uniboom profile showing the mid-reflectors a and p near Hatae Island (34°20'N, 125°30'E). Also shown are the unconsolidated transgressive sequence A overlying a, sequence B (regressive sequence) below it, and the sequence C above the acoustic basement (AB). Vertical scale in two-way travel time in milliseconds. For location of the profile see Fig. 3.9a. Profile courtesy of the Korea Institute of Geology, Mining and Materials.
with sediments (Figs. 3.8a and 3.9b). Sequence B represents most likely the sediments deposited during the Wisconsinan glacial regression (regression sequence) that have undergone erosion. The underlying beds are truncated v^here the reflectors are exposed (Figs. 3.7 and 3.8a). Sequence C, the sediment layer betw^een the reflector p and the acoustic basement, occurs locally and is exposed on the seafloor adjacent to the islands (Fig. 3.9c) where it is scoured by strong tidal currents. Reflector a is diachronous in that it is the datum plane on which postglacial deposition has occurred. ^^C dating in the Gamagyang Bay (Kang and Chough, 1982) suggests that it formed prior to about 4.5 to 5.0 ka. Highresolution seismic profiles also reveal numerous paleochannels that are approximately perpendicular or subparallel to the shoreline. These channels are most likely erosional channels that were activated during the lower stand of sea level in the last glacial period. Postglacial sediments are nondepositional or erosional where strong currents meet a topographic high or an island. Here, sediments are often stripped off forming moats, probably by strong tidal currents. The activities of tidal currents are also indicated by the sand dunes and large-scale tidal ridges on the shelf (Figs. 3.10 and 3.11) (CM. Kim and Lee, 1974b; S.W. Kim et al., 1980a; C M . Kim et al., 1981; C S . Kim et al., 1982; Klein et al., 1982). Tidal currents measured in the area off Anmyon
Yellow Sea
ms^s^^iiSi
59
&^mmm^^^mmmm^m^mm
I km
Fig. 3.8. (a) 3.5 kHz seismic reflection profile (ORE) east of Hatae Island (34°22'N, 125°30'E) showing inclined strata in sequence A (transgressive sequence) and underlying mid-reflector a, which is exposed on the sea floor toward the east. Sequence B and the underlying mid-reflector P are also shown, (b) Transgressive sequence A showing a coastal onlap (arrow) toward the Korean Peninsula. Vertical scale in two-way travel time in milliseconds. For location of the profiles see Fig. 3.9a. Profiles courtesy of the Korea Institute of Geology, Mining and Materials. Island are up to 1 m/s flov^ing mainly northeast- and southw^estw^ard (Fig. 3.11). Sand ridges are about 1-3 m in height and spaced at about 90-155 m trending dominantly in the current directions. In the South Sea north of Cheju Island, a thin sediment layer (<30 m) occurs above the acoustic basement of probable Neogene semiconsolidated sedimentary sequence. The surficial sediment is slightly thicker in the southern part. The lack of sediment here is due to strong bottom or tidal current activity, through v^innowing and removal of the fine-grained material. This part of the sea is also characterized by coarse-grained sediment that contains abundant shell fragments resulting in a CaC03 content of up to 40% (S.W. Kim et al., 1980b).
60
Marine Geology of Korean Seas
3.10a
SEQUENCE A (meters) _J
No A or B Sequence
Fig. 3.9. (a) Isopach map of sequence A. (b) Isopach map of sequences A and B. (c) Surface distribution of sequences A, B, C, and acoustic basement (AB) in the southeastern Yellow Sea northeast of Sohuksan Island. Profile courtesy of the Korea Institute of Geology, Mining and Materials.
Yellow Sea
t
61
DISTRIBUTION Of A.BondC
fe" Fig. 3.9. Continued.
Fig. 3.10. Side-scan sonar record over the sand dune field northeast of Hatae Island (34°30'N, 125°35'E). For location of the profile see Fig. 3.9a. Actual dimension of the asymmetrical dunes: length, about 3-3.5 m; height, 0.22-0.36 m; slope angle, about 2530°. Currents from NE to SW. Courtesy of the Korea Institute of Geology, Mining and Materials.
62
Marine Geology of Korean Seas K
LEGEND Ship Track Tidal Current (Kt) Measured Inferred Cun^ents from Sand Ridge Sand Ridge .... ^^
^
-
1.75
*—
ANMYON IS.
/ A .75
^ /
A
^
i «.
I
>v
-
^ ,1.75^ .
^ ":...•
f
i \
• "
i±. / 1.75 0
•^"i
10
I
I
I
OCHONG IS.
1.75/ / 125°30'
126°30'
Fig. 3.11. Tidal sand ridges offshore Anmyon Island. After CM. Kim and Lee (1974b) courtesy of the Korea Institute of Geology, Mining and Materials.
Some parts of the sediment sequence in the eastern Yellow Sea appear to be gas-charged (CO2 and CH4) resuhing in attenuation of the acoustic energy. This feature is rather common along the southwestern coastal area (Fig. 3.9a,b). On the high-resolution seismic profiles, some of the acoustically turbid layers show oblique and discontinuous stratification, suggesting that the acoustic turbidity could also result from the concentration of shells or other materials of different acoustic impedance.
3.7 3.7.1
Surface Sediments Distribution
The epicontinental Yellow Sea is tectonically a stable, postglacially submerged depocenter of elastics derived from the adjoining landmass of China and Korea. The Huanghe and Changjiang rivers, first ranked among the world's rivers with respect to sediment and water discharge, respectively, together empty enormous amounts of fine-grained sediments to the western Yellow Sea (Table 3.1). On the other hand, limited amounts of fine-grained sediments debouch into the eastern side of the sea via some of the smaller,
Yellow Sea
63
Table 3.1. River characteristics associated with Yellow and South seas. After HJ. Lee (1991). River
Draining area (lO' km")
Water discharge (kmVyr)
Sediment discharge References (10* t/yr)
Huanghe
77
49
1080
Changjiang
194
900
478
Han Keum Nakdong
26 9.9 24
19 7 15
— 5.6 10
Somjin
5
3
Milliman and Meade (1983) Milliman and Meade (1983) Chough and Kim(1981) Chough and Kim(1981) Chough (1983); Yu et al. (1985) Ministry of Construction of Korea (1978)
steep Korean rivers, especially the Han and Keum rivers. These recent sediments cover the extensive, otherv^ise transgressive sand sheet in a progradational pattern, w^hich is further complicated by frequent changes of both the Huanghe and Changjiang river courses even in historic time. The western part of the Yellow^ Sea and the Gulf of Bohai are almost entirely covered with the Huanghe-derived mud (silty and clayey mud) (Fig. 3.12). Sandy sediments (sand, muddy sand, and sandy mud) largely occur south of the Shandong Peninsula. Large birdfoot-like sand bodies are also present north of the Changjiang River mouth having formed when the old Changjiang River entered the Yellow Sea prior to its southward shift in course. These sands are remnants of tidal current winnowing (Li, 1979; Zhang et al., 1985; Wang and Aubrey, 1987). Between these sandy deposits, silty mud of the old Huanghe River delta, deposited prior to 1855, prevails along the coast of Jiangsu Province (Fig. 3.12). This has been partly winnowed by waves and tidal currents and acted as a source of a mud patch southwest of Cheju Island (Nittrouer et al., 1984; Butenko et al., 1985; DeMaster et al, 1985; Milliman et al., 1985b). Mud also blankets the nearshore area south of the Changjiang River mouth (Fig. 3.12). In the eastern part of the Yellow Sea, a prominent sand deposit extends north of about 36°N to nearshore off Aprok River, comprising well-sorted sand of 2-3 (j) in mean grain size (the details offshore the north Korean coast are unknown). H.J. Lee et al. (1988) suggest that the deposit represents a transgressive basal layer formed during the Holocene sea-level rise. The coastal area, however, receives small amounts of muddy sediment from the Korean rivers, which deposit muddy sand and sandy mud (Chough, 1983). A distinctive mud belt of nearly equal amounts of silt and clay embraces the southwestern coast of the Korean Peninsula (20-70 m in water depth).
64
Marine Geology of Korean Seas
Fig. 3.12. Map showing surface sediment distribution in the Yellow Sea and adjacent area. Sediment classification according to grain size using Folk's (1954) scheme. After H.J. Lee and Chough (1989) by permission of the Elsevier Science B.V.
extending to the north of Cheju Island (Fig. 3.12). This mud deposit, proved Holocene in age, has been designated "Unit A" from interpretations of highresolution seismic profiles (Jin and Chough, 1998); it w^as alternatively named "Sequence A" (Chough, 1983) or "Fluksan deposif (Werner et al., 1984). The mud deposit ranges in thickness from a few^ meters to 60 m. The sand deposit off the mouth of Changjiang River contains a series of tidal ridges, suggesting Holocene transgressive sand sheets (Yang and Sun, 1988). These sands are mixed w^ith recent muds, forming sandy mud or muddy sand in the southern Yellow Sea and northern East China Sea (Fig. 3.12).
Yellow Sea 3.7.2
65
Mineral and Geochemical Composition
The bulk of sediments in the Yellow Sea consists of detrital fractions with minor amounts of insoluble grains of biogenic and authigenic origin. The calcium carbonate content is about 10% and increases southeastward. In the east, the light mineral fractions of sand size are subrounded to angular and consist mainly of quartz (43-95%), K-feldspar ( 3 ^ % ) , plagioclase (3-25%)), and rock fragments (1-10%) (Fig. 3.13) (N.J. Kim et al., 1970; Seo et al., 1971; S.W. Kim et al., 1972; H.I. Choi and Hahn, 1975; C O . Kim et al., 1975; S.W. Kim et al., 1977; Koo et al., 1980). This mineralogy would be equivalent to arkose or lithic arkose (when compacted). The total amounts of heavy minerals in the sand deposit offshore vary widely between 0.8 and 17.6%) of the 63-125 |um size fractions and between 0.7 and 22.5% of the 125-250 |Lim size fractions (H.J. Lee et al., 1988). Green hornblende comprises up to 80% of the heavy mineral assemblage in both size fractions, whereas garnet (0-20%o), zircon (0-10%)) and epidote (zoisite and clinozoisite, 10-20%o) are in subordinate amounts. Relatively unstable minerals such as pyroxene also occur (up to 8%). Opaque and altered minerals exceed 10%o. Other minerals are less common or present in trace amounts. Foraminiferal content in the sediments ranges from about 5%o in the central Yellow Sea to about 40%) in the shelf area south of Cheju Island (Niino and Emery, 1961; J.J. Kim, 1970; C O . Kim et al., 1975). In the eastern Yellow Sea they are abundant in the muddy sediments south of about 35°30'N (B.K. Kim et al., 1970; J.J. Kim, 1970; Chang and Kim, 1976). A total of 256 faunal species (87 genera) of both planktonic and benthonic foraminifera are found in the Yellow Sea. Planktonic foraminifera include both cold-water (and temperate) species such as Globigerina bulloides, G. falconensis, G. pachyderma, G. quinqueloba and warm-water species such as Globigerinoides ruber, G. sacculifer, Globoquadrina dutertrei, and Pulleniatina obliquiloculata. The latter, according to B.K. Kim et al. (1970), are due to incursion of the warm Kuroshio Current into the southeastern Yellow Sea. Dominant benthonic foraminifera in the sea includes Pararotalia nipponica, Ammonia beccarii, and Hanzawaia nipponica which are also found in muddy sediments with less than about 40%) sand. Also common are shell fragments, diatoms, ostracodes, echinoid spines, fish teeth, and other organic remains. Organic carbon content in the bulk sediment is less than about 0.3%o ( 0 . 1 0.9%)) and is usually found in the fine-grained sediments. In the nearshore embayments and estuarine environments, it is relatively high, ranging up to 3.0% in Kyonggi Bay (S.W. Kim et al., 1979; Koo et al., 1980) and off the Yongsan Estuary (Chang et al., 1978; S.W. Kim and Chang, 1979).
66
Marine Geology of Korean Seas
no
eo
40
0 Plogioclose
Orthocias6 4 South Sea»
4 lb A: Offshore,
•
Yel low (West) S M
•D©A:Nearshore
Fig. 3.13. Classification of sediment composition according to 3 light minerals (quartz, orthoclase, and plagioclase) in sediments from the Yellow and South seas, (a) Coastal region, (b) Off- and nearshore. After Koo et al. (1980) courtesy of the Korean Ocean Research and Development Institute.
Yellow Sea
67
Nitrogen content in the bulk sediments is approximately between 0.005% and 0.01% (C.G. Kim et al., 1975). Analysis of inorganic bottom sediments in the eastern Yellow Sea shows that Si02 (71-86%) is dominant, followed by AI2O3 (6-16%), K2O (3.23.9%), Fe203 (1.2-3.8%), Na20 (1.2-2.5%), CaO (0.5-6.7%), and MgO (0.1-2.3%)). In the coastal mudflats and nearshore area of the eastern Yellow and South seas, it also includes dominantly Si02 (46-88%)) and AI2O3 (2.817.5%) followed by Fe203 (0.3-6.0%), CaO (0.6-6.7%), Na20 (1.6-3.8%), K2O (1.2-3.9%), MgO (0.2-3.0%), P2O5 (0.01-0.3%), Mn02 (0.02-0.2%), Ti02 (0.05-0.7%) and S (0.005-0.7%) (Koo et al., 1980). Trace elements in these sediments include Cu, Ni, Co, Cr, B, V, Ba, Sr, Pb, Zn, In, Zr, Th, U, and Au. Fig. 3.14 compares the relative amounts of total iron (expressed as Fe203) plus magnesia (MgO) with those of Na20 and K2O found in the Yellow and South sea sediments. Iron and magnesia are found in ferromagnesian minerals such as biotite and hornblende, as well as illite, chlorite, and montmorillonite. K2O and Na20 are present in alkali feldspars, muscovite, and illite. The sediments fall in the middle part of the ternary diagram, characteristic of arkose. Fe2p3+MgO
YELLOW SEA SOUTH SEA EAST SEA
NGSO
Fig. 3.14. Chemical composition (Fe203 plus MgO vs. alkalies [Na20 and K2O]) in sand and mud from the Yellow, South, and East seas.
68
3.8 3.8.1
Marine Geology of Korean Seas
Dispersal of Fine Sediment in the Western Part GulfofBohai
Although the Huanghe River entered the western Yellow Sea south of the Shandong Peninsula between 1128 and 1855 AD, it has debouched into the Gulf of Bohai during most of the historic times. The Huanghe River forms no bird-foot-like delta due to the frequent changes of outlet location (Ren and Shi, 1986). Suspended sands and coarse silts rapidly settle out near the river mouth, forming spits of silty sand. The remainder of the Huanghe River sediment is carried by southward littoral drift (Fig. 3.15) with a high concentration of suspended matter (over 100 mg/1 in surface water), visible as a turbidity front on Landsat images (Ren and Shi, 1986). The estimated
ISO'^E
Fig. 3.15. Schematic dispersal patterns (arrows) of recent sediments in the Yellow Sea. BC = Bohai Current, JCC = Jiangsu Coastal Current, CRFW = Changjiang River Freshwater, TWC = Taiwan Warm Current, SKCC = South Korean Coastal Current. After H.J. Lee and Chough (1989) by permission of the Elsevier Science B.V.
Yellow Sea
69
amount of Huanghe-derived suspended sediments exported to the Yellow Sea is variable, ranging from 15 to over 30% (Alexander et al., 1991a; Pang and Si, 1980). The suspended particulates retain high CaC03 and CaO contents, derived from the Loess Plateau in China (dominantly carbonates, gypsum, and halite) (Hu et al., 1982; Li et al., 1984). Along with large amounts of montmorillonite (more than 10% of the clay minerals. Table 3.2), these components distinguish the Huanghe-derived sediments from those of other sources (Milliman et al, 1985b). 3.8.2
Central Part
Suspended sediments from the Huanghe River are carried southward by coastal currents (Bohai Current) and bypass the Shandong Peninsula (Fig. 3.15). Off the eastern and southern coasts of Shandong Peninsula, these turbid water plumes (5 mg/1 in summer) (Milliman et al., 1986) enter the Yellow Sea, depositing significant amounts of suspended particulates. This process has built a more than 4-m-thick Holocene muddy sediment unit with a high CaC03 content (Ren and Shi, 1986; Khim, 1988). Most of the remaining suspended sediments are transported further south into the central part of the Yellow Sea (Qin and Li, 1983; Milliman et al., 1986) rather than along the Chinese coast (Ren and Shi, 1986) (Fig. 3.15). This is also evident from the occurrence of palimpsest, transgressive sandy sediment south of the Shandong Peninsula (Fig. 3.12) where near-bottom concentrations of suspended sediment are less than 1 mg/1 in summer (Milliman et al., 1986). According to the preliminary results of ^^^Pb dating of surface sediments (Khim, 1988; Alexander et al., 1991a), accumulation rates on a scale of 100 yr range between 4 and 6 mm/yr east of the Shandong Peninsula and linearly decrease southward to a value as low as 1 mm/yr northwest of Cheju Island. In the eastern Yellow Sea, the Huanghe River sediment appears to have little effect (Chough, 1983; Y.A. Park et al., 1986; H.J. Lee et al., 1987). The Yellow Sea Warm Current and diluted Changjiang River freshwater (Zheng and Klemas, 1982; Lie, 1986) flow northward near Cheju Island and most probably restrict eastward transport of sediments from the Huanghe River. 3.8.3
Old Huanghe Delta
The old Huanghe Delta is made up of great volumes of sediment in the coastal area of the northern Jiangsu Province (Fig. 3.12) when the Huanghe River was diverted into the Yellow Sea during the period between 1128 and 1855 AD. Since then, the delta has undergone active reworking due to strong tides and waves, such that the coastal retreat rate reaches 200 m/yr (Schubel et al., 1984). The wintertime concentration of suspended material is
70
Marine Geology of Korean Seas
Table 3.2. CaC03, CaO and clay mineral contents. After H.J. Lee (1991). Clay minerals (%) Area Loess Plateau (China) Huanghe River
CaCOj CaO (%) (%) 13.7 11.6 7-9
Bottom sediment off Huanghe River moutt\ Bohai Sea 5-10 Bottom sediment East of Shandong Peninsula 10 Old Huanghe Delta Central Yellow Sea Changjiang River Bottom sediment. Changjiang River mouth East China Sea Keum River Bottom sediment off Keum River mouth
7
Yongsan River Aprok River, 50 km from The mouth South Sea
9.63
References I 60-68 59.00 60.60 67
M
C
9-19 23.23 10,06 13
11-12 9.29 9.60 12
58-62 8-12 67-76 8-14
K 10-11 8.48 19.20 8
Liu (1966) Ren and Shi (1986) Xu(1983) He (1985) Ren and Shi (1986)
11-13 15-20 He (1985) 9-14 8-11 Qin and Li (1983) Khhn(1988)
63.7 20.50 8.30 59.03 23.97 8.10
7.70 8.89
65-72 5-17
6-12
6-15
Qin and Li (1983) Xieetal.(1983) Xu(1983) Khim(1988)
65 4 49-74 0-15
25 6 14-49 0-15
Xu(1983) ShenetaL(1983) Xieetal.(1983) Ren and Shi (1986) Wang and Eisma (1988) Chen (1978) Aokietal.(1983)
63.70 72-79 55-79 60-81 63.90 59
19.30 12-16 13-26 9-20 16.80 30
17.00 8-14 8-20 8-19 19-20 10
J.H. Choi (1981) Khim(1988) Chough (1985) Y.A. Park etal. (1986) D.C.Kim (1980) Ren and Shi (1986)
21 23 21
BKF^aniHan(1965) B.K.Paik etal (1976) Y.A. Park etal. (1984)
67.97 70-77 3.5-4.2 72.3 75-79 58
5.52 2-8 6.4 2-4 6
trace trace trace <2 trace 1
12.65 13.86 12-16 5-11 10.7 10.8 19-21 • 23 13
25 4 50 27 5 45 57 trace 21 I - illite, M - montmorillonite, C - chlorite, K - kaolinite •CandK.
extremely high (exceeding 500 mg/1) (Milliman et al., 1986). In summer, tidal currents dominate the bottom velocity component and resuspend silty sediment from the delta during all tidal cycles (Milliman et al., 1984; Larsen et al., 1985; Sternberg et al., 1985). These sediments are rich in CaC03 and montmorillonite (Table 3.2). The resuspended sediments are advected by the Jiangsu Coastal Current eastward and southward (Fig. 3.15), forming a large
Yellow Sea
71
mud patch with a high calcite content about 150 km southwest of Cheju Island (Butenko et al., 1985; Milliman et al., 1985a) (Fig. 3.12), This offshore clayey mud is extensively bioturbated and shows a low to moderate accumulation rate (maximum, 3 mm/yr) (Nittrouer et al., 1984; DeMaster et al., 1985), 3.8.4
Chang] iang Estuary
The Changjiang River is the largest in China in terms of water discharge, although its sediment discharge is much lower than that of the Huanghe River (Table 3.1). Suspended sediment in the Changjiang River is mostly of fine silt size, the peak sediment load of which occurs during the flood season (July-September). The clay mineralogy of the Changjiang sediment is characterized by an intermediate amount of montmorillonite (about 5%) between the extremes of the Huanghe and the Korean rivers (Table 3.2). The sediment behavior in the Changjiang Estuary is controlled by both tidal action (mesotidal) and river runoff (Milliman et al., 1984). Although low river runoff and neap tides allow the estuary to act as a temporary sink for riverine sediment, most sediment eventually escapes the estuary (Milliman et al., 1985b). DeMaster et al. (1985) estimated that more than half of the sediment escaped from the Changjiang Estuary is deposited temporarily on the inner shelf north of 30°N (accumulation rate, 4.4 cm/month). Much of this mud is resuspended during subsequent winter storms and transported further south (Fig. 3.15) (McKee et al., 1983; DeMaster et al., 1985; Milliman et al., 1985b). A turbid nearshore region south of the Changjiang Estuary typically appears on satellite imagery with a sharp front between the coastal water and the Taiwan Warm Current (Beardsley et al,, 1985). The well-developed mudflat on the coastal plains of the East China Sea, with only limited sediment supply from nearby rivers, further underscores the significance of sediment from the Changjiang River (Wang and Eisma, 1988). Hydrographic data indicate that a surface plume of low salinity (less than 26%o) derived from the Changjiang River flows to the northeast towards Cheju Island during summer (Beardsley et al., 1985; Lie, 1986), although current and suspended sediment measurements indicate that only small quantities of the Changjiang sediments are transported eastward or northeastward (Sternberg et al., 1985).
3.9
Dispersal of Fine Sediment in the Southeastern Part
Fine-grained sediments delivered by the Keum River predominate in the southeastern part of the Yellow Sea. Suspended sediments of the Keum
72
Marine Geology of Korean Seas
River are generally about 7-8 ^ in mean size, compared with 4-6 (j) of the bottom sediments. The coastal area near the Keum Estuary is macrotidal, with a spring tidal range of up to 6 m; surface ebb currents are slightly stronger than flood currents (1.0-1.6 m/s) (S.C. Kim, 1982). Dispersal patterns of the suspended sediments were investigated by D.C. Kim (1980), J.H. Choi (1981), Chough and Kim (1981), Chough (1983), and M.S. Choi et al. (1995) by studying less-than-2 |im fractions of clay minerals and some trace elements in the seafloor and suspended sediments. 3.9.1
Clay Mineral Distribution
The clay minerals in this region are represented by an assemblage of kaolinite-chlorite-illite (Table 3.2). Montmorillonite generally occurs in only trace amounts. High concentrations of kaolinite (>20%) occur nearshore, especially near the mouth of Keum River, and extend about 120 km both northwest- and southwestward (Fig. 3.16a). The northwest extension follows the shore closely and is deflected southwestward off the Taean Peninsula. In the southwest extension, the zone of high kaolinite is confined to depths less than 20 m. Further south, the amount of kaolinite decreases gradually away from the Keum and Yongsan estuaries. The chlorite distribution resembles that of kaolinite, but is less distinct (Fig. 3.16b). It is generally in high concentrations near the shore and at the mouths of both rivers where it amounts to more than 20%. lUite comprises more than 75% of the clays at the expense of kaolinite and chlorite (Fig. 3.16c). It increases seaward, away from the influence of the Keum and Yongsan rivers, and is concentrated in the offshore area south of 35°N. 3.9.2
Distribution of Trace Elements
Of the six trace elements analyzed, four (Zn, Cu, Ni, and Fe) show the trend of enhanced concentrations near the Keum and Yongsan estuaries and the adjoining nearshore area. The other two (Pb and Co) are rather scattered, and bear no relationship to the distributional pattern of fine-grained sediments (Chough, 1983, 1985). The distribution of Zn (Fig. 3.17a) shows some similarities with those of kaolinite and chlorite: increased values (over 150 ppm) offshore the Keum and Yongsan estuaries and in the nearshore area within about 50 km from the shore. The Zn values at the mouth of the Keum River range from about 60 to 170 ppm (W.Y. Kim and Park, 1978). Koo et al. (1980) also reported similar values in the suspended particulate matter that ranges between 15 and 50 mg/1, with an average of 25.5 mg/1 (K.W. Lee et al., 1979). The distribution of Zn is probably associated with the influx of material from the
Yellow Sea
o
O
OX)
00 ON
I e
>-» c/3 ^
o
.2 ^4H
o O
CO
S O
d
o ^^ o .ti
73
to fc=
74
Marine Geology of Korean Seas
126°
127°
126°
127°
Fig. 3.17. Concentration (ppm) of trace elements in the bulk sediments from the southeastern Yellow Sea. (a) Zinc (Zn). (b) Copper (Cu). After Chough (1983) by permission of the CCOP Technical Bulletin.
Precambrian gneiss and schist in the drainage basins of the Keum and Yongsan rivers. The Zn concentration is generally high in the muddy sediments (Figs. 3.12 and 3.17a), suggesting that Zn is mainly associated with clay minerals and their colloidal material. The Zn concentration in the southeastern Yellow Sea is slightly higher than the world average of about 50-100 ppm in soils (Aubert and Pinta, 1977). The distribution of Cu concentrations (Fig. 3.17b) may also reflect sediment yield from the acidic rocks in the drainage areas of the Keum and Yongsan rivers. The distribution pattern follows that of Zn, being high near the Keum and Yongsan estuaries. In suspended matter at the mouth of Keum River, K.W. Lee et al. (1979) found an average of more than 20 mg/1 of Cu and ascribed the higher concentration (compared to other estuaries in Korea) to the pollution in the upstream of the river. The general decrease offshore to the west of the Keum Estuary is correlated with the sand content of the
Yellow Sea
75
sediments and appears to indicate that Cu is mainly associated with clay minerals and other colloidal suspensions. The nearshore enrichment of Fe (Chough, 1983) may also be explained as the result of Fe-rich clastic sediment influx derived from metamorphic rocks. The Fe concentration at the mouth of Keum River is generally higher than about 25,000 ppm in the bottom sediments and up to 3,600 |ug/l in suspended matter (W.Y. Kim and Park, 1978). The iron concentration in the southeastern Yellow Sea is also associated with muddy sediment and may be present as finely divided oxides and hydroxides. Concentrations of Ni are rather low, due probably to the low concentration in acidic rocks and leached podzolic sediments in the temperate region. However, Ni tends to be more abundant in the nearshore area, with an increasing trend southwestward. The distributional pattern of both Pb and Co deviates from the above. Pb is concentrated in the upper and lower thirds of the southeastern Yellow Sea whereas the Co concentration is rather patchy. Although the Co content in acidic or metamorphic rocks is generally low, 5-30 ppm (Aubert and Pinta, 1977), the lower content of Co in the southeastern Yellow Sea may be due to its dispersion. The Pb values in the suspended matter in the Keum Estuary and the adjoining southern nearshore range from 5 to more than 20 mg/1, owing to the pollution upstream (K.W. Lee et al., 1979). 3.9.3
Dispersal of Fine Sediment
The isotherm patterns compiled by Hahn et al. (1978a, b) seem to agree with the observed distributional pattern of the fine-grained sediments. The horizontal temperature distributions at fixed-depth intervals observed during the period between 1961 and 1975 reveal that in winter, typically in February (Fig. 3.18a), the low temperature (3-6°C) isotherms indicate the existence of clockwise nearshore currents in the eastern Yellow Sea. In contrast, the summertime isotherms, especially at 30 m and 50 m depths, extend from the south, suggestive of a counterclockwise current gyre (Fig. 3.18b). A series of in situ current measurements in the area slightly north of 35°N made during the period between July 1979 and June 1980 (Nam and Seung, 1980) also indicate a southwestward residual current of about 4 cm/s in winter; this current pattern is essentially reversed in summer. In spring and autumn the temperature structure is transient due to the unstable weather conditions. Chough and Kim (1981) suggested that the NW-extending belt of fine-grained sediments rich in kaolinite and chlorite results from the transport of fine fractions out of the Keum River by the counterclockwise nearshore currents during summer, whereas the clockwise circulation during winter is responsible for the southwestern mud belt (Figs. 3.18a,b and 3.19).
76
Marine Geology of Korean Seas
' • • ^ • '
"7
1
E 8
-Ma
-o c cd
^ 0/J =3 O
VH
^ U
(U
<« < c 3
»—J ^-—V,
X)
b
s
cd
UH
X) (L)
,^—>, 03 03
^
C>0
O
J^>
(U
e
>
C/D
03 (U
O C/5
^-•->J3
s
^
C
C/0
P-1 o
s
;-i
>.
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<1>
g
C/3 C/3
03 o
S
03
;Zr
S^ 03 X ) OO 00 00 0^
^
e
r*^ "^-^ W) [^
Yellow Sea J
Fig. 3.19. Possible transport routes of fine-grained sediments in the southeastern Yellow Sea. Modified after Chough and Kim (1981) by permission of the SEPM.
. ^
/ ^
^
TF^
1 •«= summer f ^ winter
<=>
*
i
'
11
-^
X
r
*!
&^°j 0
1
30 km
, 126^
D°0 0 127°
The influence of the Kuroshio Current in transporting illite-rich sediments into the southeastern Yellow Sea is considered minimal. Wells and Huh (1984) and Wells (1988) report an autumn onset of a turbid water plume pushed southward by the northerly winter monsoon in the nearshore area south of the Keum River. Throughout the well-mixed waters (20-80 m deep), concentrations of suspended sediments are extremely high with a maximum of over 250 g/1. Such a high turbidity is caused by resuspension of bottom sediments from the nearshore and tidal flats in this area. Numerous fine laminae made exclusively of well-sorted silt grains (4-6 (j)) found in the mud-belt deposit also attest to the repeated resuspension processes during winter (H.J. Lee et al., 1987). The winter-time high turbidity of the coastal water lasts through the subsequent early spring (Y.A. Park et al., 1986), and moderately turbid waters still remain herein during the summer as seen on Landsat images (Yoo, 1986). Some of the materials derived from the Keum River are thought to be transported to the
78
Marine Geology of Korean Seas
coastal embayments in the South Sea (Kang and Chough, 1982; Wells and Huh, 1984; Wells, 1988) (Figs. 3.15 and 3.19).
3.10
Mass Physical Properties
A number of sediment cores (up to 3.5 m long) have been retrieved from the various depositional units in the southeastern Yellow Sea (Fig. 3.20). The units include part of the central Huanghe-derived mud, the Keumderived mud belt and nearshore mud off the southwestern coast of Korea, and the transgressive sand sheet off Taean Peninsula (Fig. 3.20). Mass physical properties of the sediment cores are distinctly different from one area to another, largely reflecting the lithology of surface sediments. Downcore variations of the properties are expected according to the changes
Fig. 3.20. Map showing core locations (dots) from the southeastern Yellow Sea. Surface sediment distribution, according to the Shepard's (1954) scheme, based on analysis of more than 300 surface samples.
Yellow Sea
79
in sediment texture and structures. No measurements of Atterberg limits and shear strength with a vane apparatus were made for the transgressive sand deposit, due to the predominance of sand. 3.10.1
S ediment Texture and Structures
Sediments sampled near the estuary of the Keum River (cores 606 and 1003) (Fig. 3. 20) are dark olive grey (5Y3/2) sandy silt or clayey silt with large amounts of silt (50-70%). No primary sedimentary structures are found. A series of cores in the mud belt (cores 608-1 through 1703, all less than 70 m water depth) consist generally of olive grey (5Y4/2) clayey silt (more than 50% silt). In water depths of less than 40 m, cores 608-1, 1302, and 1402 show that silty layers alternate with clay-rich layers (0.5-1 cm thick), except for the extensively bioturbated interval (Fig. 3.21). Clay layers contain numerous thin laminae (less than 0.5 cm thick), whereas the silty layers are very well sorted (mean and mode, both 4-6 ([)). In cores 1303 and 1402, however, these alternating layers are mostly masked by extensive bioturbation. In cores 611, 1604, and 1703, collected at water depths of 4 0 70 m, clay content increases to 40-60%) with occasional discrete silty laminae (<0.5 cm thick) (Fig. 3.21). In this area, bioturbation generally dominates physical sedimentary structures. Sediments in the central depression (cores 1108 through 1815) (Fig. 3.20) are extensively bioturbated with distinctive burrows (Fig. 3.21). In the topmost 20 to 30 cm of cores 1113, 1111, 1213, and 1415, sediments generally consist of greyish brown (2.5Y5/2) clayey muds (50-70%) clay), below which dark grey sandy (or clayey) silts prevail (60-70%) silt). The latter are exposed on the seafloor as in nearby cores 1108, 1511, and 1815, suggesting that relatively thick silty muds are regionally overlain by thin clayey muds in the central Yellow Sea. Cores from the transgressive sand sheet (Fig. 3.22) show that sediments are olive grey (5Y4/1 or 5Y4/2), well-sorted, fme-to-very-fme sands in the uppermost part (-10 cm), but rapidly change to muddy sand or sandy mud within 50 cm subbottom depth as mud contents increase markedly downcore (Fig. 3.23). Shell fragments and gravels are embedded in the uppermost sandy sediments, whereas mud chips of various size occur scattered in the lower muddy section. Although bioturbation is dominant, alternations of sand and mud layers and laminae are preserved at some intervals (normally less than 5 cm thick) in the lower sandy muds (cores YC-11, YC-28, YC-29 and YC-54) (Fig. 3.22). Peats are relatively abundant in both matrix and the mud chips in the lower section.
80
C/1
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Marine Geology of Korean Seas
tn
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Yellow Sea
1
1
I
I
1
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1
r
-58
Fig. 3.22. Map showing core locations (dots) from transgressive sand sheet in the eastern Yellow Sea off the Taean Peninsula, Korea.
r
•
YG-62
yc~67
YC-54
\YC-1 r
"
YC-35 YC-29
TC- -32
YC-16
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81
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126'00'
YC-2 0
20 4 0
YC-9 60 80 100(%)
0
YC-11
20 4 0 60
80 100(X)
0
20
YC-12
4 0 60 80 100(9()
0
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I
20 4 0 60 80 100(X) "r—I—I—TT:
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1 B' 100(CRO
m YC-15 0
20 40
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20 40 60 -I
YC-27
YC-25
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1
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^m
0
20 40
'
60
80 100(X)
' ' ';^
0
20 40
60 80 100(90
y:rrr:hz
lOOH (OR)
Fig. 3.23. Sediment texture of cores from the transgressive sand sheet off Taean Peninsula. Note that the topmost sand changes to muddy sand or sandy mud within the 50 cm subbottom depth in most cores. For core location see Fig. 3. 22. After H.J. Lee (1991).
82
Marine Geology of Korean Seas
3.10.2
Water Content
In the sandy silty sediments of core 606, water content increases gradually from 35 to 50% downcore (Fig. 3.24). Silty sediments of core 1003 contain low water content of about 30% to the depth of 50 cm, below which it increases up to 60%). This increase is associated with an abrupt increase in clay content. Water contents of the sediments from the mud belt (cores 1303, 1402, and 611) show a large depth variation, ranging from 40 to 110%) (Fig. 3.24). This large variation with depth is attributable to the existence of the fine-sediment layers and laminae with various degrees of bioturbation. Sediments of core 608-1 show a downcore decrease in water content (7040%), but sandy sediments of core 608-2 have a consistently very low water content (<30%) (Fig. 3.24). In the central mud, cores 1108 and 1415 contain higher than 60%o water content (higher than 80%) in cores 1111 and 1213) in the uppermost 30 cm interval (Fig. 3.24). Fine-grained material and extensive bioturbation probably cause the high water content in this interval. Below 30 cm and in other cores (1511 and 1810), water content is relatively constant (<50%) (Fig. 3.24). Sandy silts of core 1815 are much lower in water content (2540%). Water content in the transgressive sand is extremely low (about 20%)), but increases slightly downcore to 30-40% with mud content. Although core YC-29 contains relatively high amounts of water (50-90%o), the prevailing low water content throughout most cores indicates that the transgressive deposit is in a considerably compacted state. 3.10.3
Shear Strength
Shear strength values in cores from the mud belt show a large depth variation, ranging from 3 to 13 kPa (Fig. 3.25). Off the Keum River, cores 606 and 1003 display a downcore decrease in shear strength (12-5 kPa) (Fig. 3.25). In the central mud, cores 1108, 1111, 1113, 1213, and 1415 exhibit a gradual downcore increase (up to 8 kPa) (Fig. 3.25). In silt-dominant sediments (cores 1511 and 1815), shear strength increases up to 10 kPa. Core 1511 displays an abrupt change in most mass physical properties at about 100 cm depth where shear strength is high (22 kPa) (Fig. 3.25), which seems associated with increasing silt, CaC03, and organic contents. 3.10.4
CaC03 and Organic Matter
Calcium carbonate contents of cores 1402 and 1703 are relatively high (average, 7.0%), but organic contents vary with depth, ranging from 1.0 to
Yellow Sea 0402 0
0502
606
20 40 60 80 100(X) 0 20 40 60 80 100(X) 0 20 40 60 80 100(»Q
83
1003 0 20 40 60 80 100(X) T
1
1
Fig. 3.24. Water content of cores from the southeastern Yellow Sea. Plastic (x) and liquid (o) limits are also shown. Note a good correlation between water content and sediment texture; water content tends to increase with increasing clay content. For core location see Fig. 3. 20. After H.J. Lee (1987).
84
Marine Geology of Korean Seas 1213
1511
1810
1815
0 20 40 60 80 100M -| ! 1—r
0 20 40 60 80 K)OM
0 20 40 60 80 100M
0 20 40 60 80 100(X) T-v^l 1 1 1
— 100-
<
< i
- {
200H
300(CM)
1113
1415
1108
1111
0
0 20 40 60 80 100M
0 20 40 60 80 100M "IP" T 1
0 20 40 60 80 100(90
20 40 60 80 loom ^1 1^—I—r-^
«\
I / I
T
"T
1—I
y lOOH
I
y
200H
300H
(no
Fig. 3.24. Continued.
i V
Tj—y
Yellow Sea 1604
611 0
85
1703
20 40 60 80 100(X) 0 20 40 60 80 100(X) 0 20 40 60 80 100{%} 1 "T—r -r
T
/ lOOH
/
/
/
200 H
1 ./
300 H (OR)
\
Fig. 3.24. Continued. 7.0% (average, 4.1%). Sediments off the Keum River show relatively constant CaCOs and organic contents (4.5-6.5%o, 2.0^.0%), respectively). In the central mud, high organic content (more than 5%)) is found in sediments of cores 1113, 1213, and 1415. In others, it is less than 4%) and nearly uniform downcore. Carbonate content is generally more than 10%) in the central mud. CaC03 content in the transgressive sand and sandy mud is markedly uniform in the range of 3 to 5%, whereas organic matter content varies considerably with sediment texture. Sands contain only 1-2%) organic matter, but sandy muds possess more than 3%. Occasionally mud chips or muddy matrix involving peats cause the organic matter contents to be abnormally high (20-40%) (cores YC-29, YC-32, YC-37, YC-54, and YC-58). Variable amounts of CaC03 and organic matter appear to control values of specific gravity (G) of the sediments, ranging from 2.52 to 2.75. 3.10.5
Atterberg Limits
On the plasticity chart, most cores (except core 1703) plot on the division of CL, the field of inorganic clayey-silty or silty-clayey sediments with lowto-medium plasticity (Fig. 3.26). Atterberg limits of core 1703 are in the division of CH (inorganic clays of high plasticity). Some silty and sandy sediments off the Keum River (cores 606 and 1003) are very low in
86
o
T-
<0
o o
o
M O
o o
0«—2
2
-I
-i
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1
r
r
A. / N ^ "T—
Marine Geology of Korean Seas
1
^
AvV
"A r\
:
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O
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/
^
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ou
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Yellow Sea
87
LL(dry wt. %)
Fig. 3.26. Atterberg limits of sediments from the southeastern Yellow Sea in the plasticity chart. Most sediments fall on the division of CL, inorganic clays or silty clays of low to medium plasticity. Sediments of core 1703 and part of core 1604 are, however, included in the division of CH, inorganic clays of high plasticity. After H.J.Lee et al. (1987) by permission of the Taylor & Francis. Atterberg limits and are in the division of ML, inorganic silts and fine sand with a lov^ plasticity (Fig. 3.26). Sediments in the mud belt (cores 608-1 through 1604) fall in the division CL. Muddy sediments in the central mud (cores 1113 through 1815) are also in the division CL.
3.11
Tidal Flats
Along the ria-type west coast of Korea tidal flats drape almost all embayed areas and reentrants except for rare patches of sandy beach (Fig. 3.27). With as much as 10 m of extreme macrotidal range, the extensive tidal flats are recognized worldwide as rivaling those of the North Sea. Although the Korean tidal flats display no distinct, wave-related features such as
Marine Geology of Korean Seas
Ji^TH
36°N
100 k m ^ ^
C
•A^ X
125"E
o 126^
125°E
130"E
Fig. 3.27. Map showing distribution of tidal flats on the west coast and part of the south coast of Korea. Inset shows physiographic location of the region. longshore bars and barrier islands, in contrast with those of the North Sea, single, small cheniers occasionally develop on the innermost part of the Korean tidal flats. These cheniers reflect wave dominance over the tidal process at least during the winter monsoon season (November-March). The Korean tidal flats usually lack a substantial landward area of salt marsh, and their tidal channel systems are more or less simple and persistent. Comprehensive studies of the tidal flats have been accomplished through vibracoring, can coring, and surface observations in several localities, distributed fairly regularly along the west coast, including Inchon, Taean Peninsula, Keum River, and Gomso tidal flats, among others (Fig. 3.27).
Yellow Sea 3.11.1
89
Surface S ediments
The surface sediments on most of the intertidal flats in the west coast of Korea show a seaward-coarsening trend, generally from (sandy) mud to silty sand to sand. In Inchon tidal flats, there are three distinctive zones in a seaward direction (Frey et al., 1989): intensely bioturbated muddy inner flat; wavy bedded, sandy silt mid flat; and ripple-laminated, sandy outer flat. The inner flat typically undergoes bioturbation due largely to crabs, rarely including faint remnants of wavy beds, flaser bedding, and convolution. The muds comprise watery clay on top but become rather compacted below 1015 cm. The mid flat is dominated by wavy beds, although ripples and parallel laminations may prevail locally. Here the degree of bioturbation and water content is still high. The surface of the sandy outer flat is hardened and extensively molded by ripples. Bioturbation is also severe. This textural and sedimentary pattern of the Inchon tidal flats can also be seen in other tidal flats, with only minor modifications. On the Taean Peninsula and Keum River tidal flats, sediments also generally coarsen seaward, although these flats are coarser-grained as a whole than the Inchon flat (KORDI, 1989). In these places, the amounts of complete shells and shell fragments rapidly increase seaward. Bioturbation is extensive over the entire surface but ripples and parallel laminations become prevalent on the outer sand flat. The most extensively studied tidal flat of Gomso Bay also conforms to the seaward-coarsening textural trend (Fig. 3.28). According to the sand content, the tidal flat can be divided into mud flat, mixed flat, and sand flat environments. The sand flat near the low-water line consists of sand (>75%) and subordinate silt and clay with a mean grain size ranging from 0 to 4 (j) (Fig. 3.29). Ripple cross-laminae and burrows are common with occasional shell layers composed of imbricated broken bivalves. Reactivation surfaces caused by reversing tidal currents are also common. Bedforms of varying scale are well-developed throughout the sand flat. By contrast, the mud flat occurs near the high-water line, and is predominated by medium to fine silt with a mean grain size of 5 to 8 (|) (Fig. 3.29). The mud flat sediments are intensely bioturbated by benthic fauna that includes crabs, polychaetes, and molluscs. In the near-surface of the mud flat, thin parallel laminae are occasionally preserved and are similar to typical tidal bedding such as flaser, wavy, and lenticular beddings. Patches of salt marsh occur colonized by small halophytes (e.g., Sueda Japonica). The mixed flat between the sand and mud flats consists of very fine sand and silt with a mean size in the range of 4 to 5 (j) (Fig. 3.29). In the higher mixed fiat, small-scale ripple cross-laminae, parallel laminae, scattered shell fragments, and burrows occur. In the lower mixed fiat, small-scale ripple cross-laminae are preserved less frequently and occur with parallel laminated or deformed mud layers (5-10
90
Marine Geology of Korean Seas
,9eose
G
m
OX) G 03 T3
LP
C/3 ^^—1
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e
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td
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i
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b OJ W)
,n
cd 03 (U
c: C/3 o o
^
Yellow Sea
100 T
JMMdflat
80
average
Mixed flat
average
Sand flat 100 t
Fig. 3.29. Grain size-frequency curve of surficial sediments on the mud, mixed, and sand flat. After Y.H. Kim et al. (1999) by permission of the SEPM.
91
92
Marine Geology of Korean Seas
cm thick). A chenier lies on the boundary between the mixed and mud flats near the bay mouth (H.J. Lee et al., 1994) (Fig. 3.28). 3.11.2
Benthic Biota
Frey et al. (1987) investigated in detail the benthic animal distributions on the Inchon intertidal flats. They found that many marine species lived there, and that these animals were distributed in a tripartite zonation pattern depending largely on the sediment type, as follows: (1) Brachyuran zone on the sandy mud inner flat characterized by the crabs Ilyoplax pingi and Macrophthalmas japonicus, (2) Molluscan zone on the mid-flat of sandy clayey silt dominated by the bivalves Solen stricus and the gastropod Umbonium thomasi, and (3) Holothurian zone on the outer flat of sandy silt to silty sand characterized by the holothurian Protankyra bidentata. Such a three-fold, sediment-animal coupling also has been noted on other tidal flats in Garorim Bay and near Panwol in the northern west coast (KORDI, 1981a, b; Shin, 1984). This tripartite zonation however seems nonuniversal over the entire tidal flats of the west coast of Korea. Because of differences in morphologies and geologic settings, each tidal flat appears to retain its own intrinsic distributional pattern and dominant species. In the vicinity of Inchon, Shin et al. (1989) studied a broad region ranging from intertidal to shallow subtidal flats on both sides of a large tidal channel running through Kyonggi Bay. They divided the intertidal flats into two biological zones (zones I-l and I2): in most tidal-flat areas (zone-I-1) the bivalve Mactra veneriformis and the gastropod Hinia festiva were the characteristic species, whereas near the low-water line (zone 1-2) the polychaetes Aricidea jeffreysii and Haploscoloplos elongatus were dominant. In the nearby subtidal flats, they have also recognized two benthic faunal assemblages, represented by the amphipod Harpiniopsis on the flank of the channel and by the polychaetes Mediomastus and Sternaspis scutata in the deep channel axis. Shin et al. (1989) regarded the difference in submerging period as the chief control on the animal distribution. On the tidal flats at the estuarine mouths of the Mangyung and Dongjin rivers, located immediately south of the Keum River, the dominant species are remarkably similar to those of the Inchon counterparts. However, the lower flat corresponding to the Incheon Holothuroidean zone becomes increasingly diverse with additional bivalves, particularly Mactra veneriformis and Mactra chinesis. 3.11.3
Sedimentary Structures of Holocene Sediments
The systematic analysis of sedimentary facies and structures has been
Yellow Sea
93
performed for the tidal-flat deposits less than 5 m thick in Gomso Bay, based on many vibracores and can cores collected from two cross-lines normal to the shore (Y.H. Kim, 1997) (Fig. 3.28). A total of 10 sedimentary facies were identified using a two-tier classification scheme of mean grain size and primary sedimentary structures. Class S: Sand (>75% sand): According to internal structures, the sand facies can be further divided into 4 subfacies. The massive sand facies (Sm) consists largely of massive medium to coarse sand with abundant shell fragments. Sand content is often more than 90%. Each unit, generally less than 0.1 m thick, is bounded by a sharp erosional base or loading structures. Shell fragments are usually disorganized or occasionally lie convex upward. Facies Sm is less bioturbated than any other facies, and frequently grades upward into parallel- to cross-laminated sand or muddy sand (Facies Sp, Sx, SMp, and SMx). These features together strongly suggest a storm-deposit origin for Facies Sm (see Kumar and Sanders, 1976; Allen, 1982). The bioturbated sand (Facies Sb) is characteristically devoid of primary structures owing to severe bioturbation. Small burrows (<2 cm in diameter and 1-5 cm long) and biogenic escape structures are occasionally observed, including skolithos, arenicolites, cylindrichnus, and pelecypodichnus. Discontinuous, remnant laminae or strata are rarely present. Intense bioturbation suggests relatively slow accumulations on the sand flat under fair-weather conditions (see Frey et al., 1989). The parallel laminated sand (Facies Sp) is composed of several-millimeter-thick, parallel-laminated, fine to medium sand layers or alternations of sand and muddy sand layers. Individual layers are usually horizontal or sometimes slightly inclined, and occasionally exhibit thinning- or thickening-upward trend. The parallellaminated nature of Facies Sp indicates high-current activities, which are frequently encountered on sand bars and shoals, channel slopes, and point bars. The cross-laminated sand facies (Sx) largely comprises ripple crosslaminated fine to medium sand layers with subordinate layers of muddy sand, sometimes showing herringbone structures. Thickness of the cross-laminae set ranges from 1 to 2 cm. This facies is slightly bioturbated and rich in scattered shell fragments. The centimeter-scale thickness of cross-laminae set suggests that it was produced by migration of small ripples. Class SM: Sand/mud (25-75% sand): This facies contains a large quantity of silt/mud fractions (up to 75%) but its sand content is still 25%), or higher. The class SM is subdivided into 4 subfacies. The bioturbated sand/mud facies (SMb) comprises structureless, poorly sorted muddy sand and sandy mud, rarely with rock and shell fragments, peats, and mud lumps. Burrows and biogenic escape structures are abundant. This facies appears to be typical of the mixed-flat deposit that is subjected to strong bioturbation. Facies SMp, parallel laminated sand/mud, consists of alternating thin
94
Marine Geology ofKorean Seas
laminae of muddy sand and sandy mud. The thickness of each lamina is on the order of millimeters to centimeters. The laminae are rarely undulatory and are laterally either continuous or discontinuous. These sand/mud facies (SMx) comprise ripple cross-laminated muddy sand with occasional herringbone cross-laminae. The thickness of cross-laminae sets ranges between 1 and 2 cm. Like the sand-flat analogue (Facies Sx), this facies reflects migration of small ripples on the mixed flat. The deformed sand/mud layers occur with inclusions of thin sand lens. This facies is closely related to soft-sediment deformations (liquefaction or fluidization), caused by rapid sedimentation of water-rich sand/mud layers, particularly near tidal channels and creeks (see Reineck and Singh, 1980; Allen, 1982). Tensional faulting and slumping on the channel bank may be another cause of the mechanical disturbance. Class M: Mud (<25% sand): This class contains more than 75% silt and mud, and can be fiirther divided into 2 subfacies. The bioturbated mud facies (Mb) is rich in such diverse burrows as skolithos, cylindrichnus, thalassinoides, teichichnus, and dilocraterion. Sediments are thoroughly bioturbated by benthonic organisms including crabs, molluscs, and polychaetes. Most burrows, particularly those across the facies boundary, are filled with materials from the overlying deposit. This facies represents the depositional mode typical of the mud flat, i.e., suspension settling and intense syn- and post-depositional bioturbation. Facies Mp, parallel laminated mud, is composed of alternating coarse (sandy silt to silt) and fine (silty clay and clay) laminae. Individual laminae are generally horizontal and slightly undulatory, and rarely exceed 1 cm in thickness. The alternating laminae probably result from variations in currents during tidal cycles (see Reineck and Singh, 1980). 3.11.4
Pre-Holocene Oxidized Mud
In Gomso Bay, semiconsolidated, oxidized mud of brown color lies below the Holocene tidal-flat mud with a sharp erosional boundary in the uppermost tidal flat (cores SW-5, SW-8, and HJ-3) (Fig. 3.30). Although the grain size is almost identical for the two mud deposits, radiocarbon ages for
Fig. 3.30. Stratigraphic cross section of the Gomso tidal flat along the line SW (for location see Fig. 3. 28) based on detailed description of vibracores, showing retrogradational distribution of units O, I, II, and III. Facies Sm = massive sand, Sb = bioturbated sand, Sp = parallel laminated sand, Sx = cross-laminated sand, SMb = bioturbated sand/mud, SMp = parallel laminated sand/mud, SMx = cross-laminated sand/mud, SMd = deformed sand/mud, Mb = bioturbated mud, Mp = parallel laminated mud. After Y.H. Kim et al. (1999) by permission of the SEPM.
Yellow Sea
95
96
Marine Geology of Korean Seas
the organic samples from immediately below and above the erosional boundary are 13,840 and 7,000 yrs B.P., respectively, indicating a considerable hiatus between them. The oxidized mud typically contains no microfossils diagnostic of marine environments. This type of oxidized mud also has been documented elsewhere (Frey et al., 1989; Y.A. Park et al., 1997), testifying that it is a widespread depositional unit on the western coast of Korea. The sedimentological results from Gomso Bay suggest that the mud was accumulated and oxidized in non-marine environments during the lowered sea level, and was then reworked to varying extents by the Holocene transgression, followed by deposition of tidal-flat muds (H.J. Lee etal., 1994). The oxidized deposit ranges in thickness from 60 to 180 cm down to the surface of weathered rock fragments and soil, and contains neither sand nor gravel. The sedimentary structures include parallel laminated silt/mud couplets, spreiten burrows, and vertical plant-root remains. The deposit can be subdivided into the upper massive mud layer and the lower parallel laminated silty mud layer, the former experienced more severe soil-forming processes than the latter. In core SW-8, the thickness of individual silty laminae in the parallel laminated mud tends to decrease upward from about 10 mm to less than 2 mm, and this thinning-upward tendency repeats cyclically at approximately 5-8 cm intervals (Fig. 3.31). Microscopic examinations of silt/mud couplets demonstrate that the laminae are distinguished only by changes in mud content without any sharp contacts. This means that the parallel laminated mud might represent seasonal fluctuations in energy level and that the mud was deposited by suspension settling rather than current traction, most probably from low-density sheet flows spilling over levees or banks of fluvial channels. The rhythmic thickness variations in silt laminae therefore suggest more or less cyclic shifts of channel courses during the time of deposition. The burrows in the oxidized mud are limited in diversity, and are characteristically endogenic and of spreiten type (Fig. 3.32). The main rootremains occurring in the middle and upper portions of the oxidized mud are of cylindrical shape and about 1 cm in diameter, and extend vertically downward up to 15 cm with numerous roothairs (Fig. 3.33). Plants with such vertically long roots are interpreted to favor relatively elevated areas with a lowered ground-water level, such as fluvial-channel levees or interfluves (Klappa, 1980; Retallack, 1983). Perhaps consistently meager sediment supply or even intermittent non-depositional conditions might have allowed the growth of plants to be maintained at least during the late depositional stage of the oxidized mud. Under such extensive subaerial exposure, the deposit must have been intensely affected by nearly syndepositional soil-forming processes.
Yellow Sea
97
"- -^tm
Fig. 3.31. Typical X-radiographs of yellowish brown oxidized mud. (a) Intensely bioturbated sediment, core SW-8, 150-180 cm. (b) Moderately to highly bioturbated sediment with thin interval of parallel lamination (lower one third) and vertical plant root, core SW-8, 215-245 cm. (c) Parallel laminated sediments with spreiten burrows, core HJ-3, 240-270 cm. Bar scale = 1 cm. For core location see Fig. 3.28. Geochemical analyses indicate that iron oxides (Fe203) and aluminum oxides (AI2O3) are consistently richer by 2-3% in oxidized mud than in the Holocene tidal muds (Table 3.3). In addition, authigenic sulfate minerals such as barite are present most probably derived from bacterial degradation of plants. Even though the overall clay mineral assemblage of the oxidized mud and Holocene tidal mud is quite similar to each other, the kaolinite content of the former show^s a slight but definite increase compared to that of the latter (Table 3.4). However, it is still unknow^n w^hether the increase in kaolinite is caused by w^eathering products from illite or by the difference in sources of the tw^o muds.
98
Marine Geology of Korean Seas Fig. 3.32. Typical Xradiographs of biogenic structures from the yellowish brown oxidized mud. (a) Sediment-filled vertical burrow, core SW-8, 130-160cm. (b) Crab hole, core HJ-3, 350-362 cm. (c) Protrusive spreiten-type burrow, core HJ-3, 250-267 cm. (d) Completely cemented burrows, core SW-5, 100-130 cm. Bar scale = 1 cm. For core location see Fig. 3.28.
Fig. 3.33. Typical X-radiographs of fossil roots from the yellowish brown oxidized mud. (a) Partially preserved plant root, core SW-8, 230-260cm. (b) Root mould (white vertical lines), core SW-5, 120-150 cm. (c) Partially cemented roots (top and base), core SW-8, 185-215 cm. (d) Completely cemented vertical root, core SW-8, 220-250 cm. Bar scale = 1 cm. For core location see Fig. 3.28.
Yellow Sea
99
Table 3.3. Chemical composition of late Pleistocene and Holocene sediments in Gomso tidal flat (*oxidized deposit). After KORDI (1994). Sample
Major Elements (weight percent)
Depth
Trace Elements (ppm)
AI2Q3 FesOa MgO CaO Na^O K^O TiO^ P2O5 MnO Ba Co Cr Sr Ni
HJ-3,190 cm 12.29 3.91 1.28 0.77 2.43 2.81 0.57 0.08 0.04 571 9 61 164 21 HJ-3,290cm* 14.06 4.61 1.06 0.46 1.87 2.84 0.66 0.08 0.03 626 9 67 137 23 HJ-3,300 cm* 15.04 4.62 1.20 0.43 2.13 2.89 0.70 0.06 0.03 915 8 79 137 28 HJ-3,320 cm* 14.19 6.74 1.14 0.43 2.08 2.81 0.65 0.08 0.03 558 12 8 133 25 SW-5,10 cm 12.51 3.48 1.22 0.92 2.54 3.01 0.60 0.09 0.07 562 7 58 174 21 SW-5,150 cm* 15.35 5.24 1.45 0.45 2.10 2.97 0.73 0.08 0.05 577 9 82 127 34 SW-8,200 cm* 15.15 5.84 1.28 0.38 2.05 2.84 0.77 0.08 0.05 572 16 83 119 33 SW-8,240 cm* 16.02 5.42 1,39 0.40 2,08 2.92 0.76 0.07 0.04 610 13 93 122 33 BC-A, 440 cm* 10.97 25.45 0.88 0.39 1.21 2.06 0.44 0.47 0.31 1366 54 198 174 45 Table 3.4. Clay mineral composition of late Pleistocene and Holocene sediments in Gomso tidal flat (*oxidized deposit). After KORDI (1994). Sample Depth
Smectite
Illite
Kaolinite
Chlorite
HJ-3
20 cm
2.02
66.44
18.49
13.05
80 cm
7.94
60.08
16.30
15.67
120 cm
18.68
56.73
12.50
12.09 11.81
SW-5
190 cm
15.46
60.51
12.22
230 cm
12.53
56.75
20.95
9.77
250 cm
26.88
52.19
10.75
10.18
260 cm*
8.68
54,58
24.22
12.51
290 cm*
5.90
58.79
24.08
11.23
320 cm*
17.98
59.62
15.42
6.98
10 cm
4.84
67.78
15.53
11.85
40 cm
12.64
55.11
14.85
17.40
70 cm
11.26
58.96
14.22
15.56
90 cm
11.20
59.19
15.72
13.89
120 cm*
14.57
62.14
15.83
7.46
150 cm*
11.31
62.20
18.54
7.95
100
Marine Geology of Korean Seas
3.11.5
Holocene Lithostratigraphy
Based on the sedimentological results of vibracores from Gomso Bay, the Holocene Gomso tidal-flat deposits can be divided into three lithostratigraphic units (Units I, II, and III) (Fig. 3.30). Unit I defines the base of the Holocene succession and grades upward into Units II and III. In dip section. Units II and III thin out landward, that is, wedge-shaped (Fig. 3.30). Oxidized mud (Unit O) is found to be unconformably overlain by Unit I, particularly in the landward cores. As a whole, the units are stacked to be a retrograding, coarsening-upward succession, a complete reverse of the prograding, fining-upward succession of the tidal-flat deposits in the North Sea and the Bay of Fundy. The retrograding, coarsening-upward, lithological trend appears to represent tidal-flat successions in the west coast of Korea that is characterized by low sedimentation rates (e.g., 0.2-2.0 mm/yr in Gomso Bay) (KORDI, 1991, 1994). 3.11.5.1
Unit I
The boundary between Units I and II marks a conformable facies change and deepens seaward (Fig. 3.30). This unit is characterized by intensely bioturbated dark grey (5Y4/1) silt with a mean grain size of 5 to 7 (j). It consists primarily of bioturbated mud (Facies Mb) and parallel laminated mud (Facies Mp) (Figs. 3.30 and 3.34). Although the mud content is usually greater than 75%, the mean grain size of Unit I is slightly coarser than the underlying oxidized mud. Many burrows are filled with coarse sand and granule, probably derived from the overlying units or the adjacent exposed rocks by biogenic processes. Faint, thin, parallel and undulatory laminae occasionally occur as alternations of silt- and clay-rich layers. Mottled, dark yellowish brown mud aggregates of granule grade are occasionally present in the lower part of this unit. The high mud content and intense bioturbation in Unit I suggest that it was accumulated on a mud flat near the high water line. C dating of unbroken foraminifera and shells suggests a mean accumulation rate of about 0.4-0.6 m/2,000 yrs (equivalent to 0.2-0.3 mm/yr). The presence of mud aggregates (mottled mud) in the lower part of Unit I indicates that the muddy sediments of the supratidal or higher mud flats underwent intermittently repeated inundation and exposure (see Dalrymple et al., 1990; Alexander et al., 1991b). 3.11.5.2
Unitll
Unit II conformably overlies Unit I and is conformably overlain by Unit III (Fig. 3.30). Unit II is characterized by moderately bioturbated or
Yellow Sea
Unit II
Unit III
101
I
Fig. 3.34. Typical X-radiographs of sedimentary structures from the Holocene sequence in vibracores. (a) Weathered soil, core HJ-11, 120-150 cm. (b) Bioturbated mud containing faint parallel laminae (upper part) with peat in black central part, core HJ-3, 318-348 cm. (c) Intensely bioturbated mud with remnant laminae, core SW-9, 437^67 cm. (d) Mud aggregates (scattered millimeter-scale dots) in the lower part, core HJ-27, 469-499 cm. (e) Cross-laminated and bioturbated silty sand (upper half) and shell concentrations (near base), core HJ-23, 30-60 cm. (f) Alternation of cross-laminated sand and deformed silty sand, core HJ-3 9, 335-365 cm. (g) Slightly bioturbated, parallel laminated sand with a thin layer of shells at base, core HJ-43, 157-187 cm. A cross on top marks the trace of a hand vane, (h) Slightly inclined and parallel laminated sand, core SW-27, 30-60 cm. Bar scale = 2 cm. For core location see Fig. 3.28.
102
Marine Geology of Korean Seas Fig. 3.34. Continued.
i^^^r;^-
\o\\
Unit
mrt
Unit I
^
laminated, dark grey (5Y4/1) sandy silt or silty sand with a mean grain size of 4-5 (j). It consists primarily of bioturbated sand and mud (Facies SMb) and parallel to cross-laminated sand/mud (Facies SMp and SMx) (Figs. 3.30 and 3.34). Unit II also contains massive (shelly) sand (Facies Sm), parallel to cross-laminated sand (Facies Sp and Sx), and deformed sand/mud layers (Facies SMd) (Figs. 3.30 and 3.34). In the vicinity of subaerially exposed rocks, the unit contains angular granule-sized fragments of Cretaceous volcanic and Jurassic igneous rocks. The remnant laminae (millimeter to centimeter thick) are commonly undulatory, parallel, or cross-laminated. Small burrows (several centimeters long) are abundant. Shelly sand layers with large oyster shells (up to several centimeters long) are present
Yellow Sea
103
occasionally. Mechanically deformed structures, including water-escape structures, are well developed where silty layers occur between sandy layers (Fig. 3.34). Unit II is considered to represent mixed-flat sedimentation, based largely on a sand content of 25-75% that is quite similar to that of modem-day mixed-flat areas. ^^C dating of fresh shells suggests a mean accumulation rate of 1.0-1.3 m/2,000 yrs (equivalent to 0.5-0.6 mm/yr), significantly higher than that of Unit I. Shelly sand layers in Unit II represent parts of tidal channel or storm deposits. 3.1L5.3
Unitlll
Unit III conformably overlies Unit II, and its upper boundary is defined by the modem seabed (Fig. 3.30). This unit is characterized by greenish to olive grey (5Y5/2) very fine to fine sand with a mean grain size of 3-4 (|). It is slightly to moderately bioturbated with relatively well-preserved laminae. Shell fragments are abundant and burrows are occasionally preserved (Fig. 3.34). Unit III consists of massive (shelly) sand (Facies Sm), bioturbated sand (Facies Sb), and parallel to cross-laminated sand (Facies Sp and Sx); it also contains subordinate bioturbated sand/mud (Facies SMb) and parallel laminated sand/mud (Facies SMp) (Fig. 3.34). The high sand content (>75%) and the slight to moderate bioturbation of Unit III suggest that it was deposited in a sand-flat environment. An unbroken shell {Crassostrea gigas) at the lower boundary of Unit III (3 m subbottom in core HJ-43) was ^^C-dated at 1,762 ± 80 yrs B.P.; this date is regarded as the initiation time of sand deposition near the bay mouth. The C date, combined with the 3 m of overlying sediment, implies that the mean accumulation rate of the unit III was about 2.7-3.6 m/1,800 yrs (equivalent to 1.5-2.0 mm/yr). 3.11.6
Holocene Sea-Level Curve
The Holocene sea-level history of the westem Yellow Sea has largely been derived from coastal stratigraphic work on the Chinese coast (Pirazzoli, 1991). A number of the Chinese coastal sea-level curves, based on elevations of peat layers and ^"^C dating, show a rapid rise in sea level from -40 m present mean sea level (MSL) at the beginning of the Holocene to about -10 m MSL around 7 ka. This was followed by a gradual rise to the present sea level, with minor oscillations of several meters (Yang and Xie, 1984a, b; Feng and Wang, 1986; Pirazzoli, 1991). Few reconstmctions of Holocene sea level have, however, been made for the westem coast of the Korean Peninsula partly because of the scarcity of well-preserved peats. A
104
Marine Geology of Korean Seas
sea-level curve constructed by Bloom and Park (1985) using ^^C data from the Korean west coast displays a gradual rise from -8 m mean high w^ater line (MHWL) around 8.5 ka to -2 m MHWL around 5 ka and a gradual rise thereafter. J.-M. Kim and Kennett (1998) have reported that the Holocene marine transgression started in the central Yellow Sea at about 11.3 ka and approached a present-day lower tidal flat of Korea at about 7.5 ka, based on radiocarbon dating on benthic foraminifera. A Holocene sea-level curve can be constructed for Gomso Bay based on nine radiocarbon dates obtained from plant remains, peats, and shells (Fig. 3.35; Table 3.5). Paleo-mean sea level (paleo-MSL) was determined using the assumption that tidal range and frequency, wave regime, and bathymetric slopes in Gomso Bay have changed little during the mid to late Holocene. It was also assumed that the distribution of subenvironments within the tidal flat has been, and still is, controlled primarily by water depth. Paleo-MSL at the time when a dated sample was deposited in a specific subenvironment was calibrated using the respective present-day water depth ranges for the mud-, mixed-, or sand-flat subenvironments (relative to MSL). Further details are published elsewhere (Y.H. Kim et al., 1999). All paleo-depth calculations for the '^C-dated samples in the Gomso Bay tidal flats are plotted in Fig. 3.35, which includes supplementary data obtained from mid
8 ka
Fig. 3.35. Holocene sea-level curve based on radiocarbon dates from the Gomso deposits (open circle), modified from J.H. Chang et al. (1996). After Y.H. Kim et al. (1999) by permission of the SEPM.
Yellow Sea
105
Table 3.5. '"^C ages for sea-level index samples from Gomso Bay and western coast of Korea (*relative position to present MSL). After Y.H. Kim et al. (1999). Core No. Depth
Material dated
Elevation of sample site (m)*
Depositional unit
"Cage (ka)
Estimated paleo-MSL
HJ-03,082 cm
Crassostrea
0.07
Unit I
1925±58
-1.9311
HJ-23,114 cm
Ostreadae unid. sp.
-2.08
Unitn
1515±86
-2.0811
HJ-31,130cm
Crassostrea gigas
-2.37
Unitn
1128±58
-2.3711
HJ-35,163 cm
Meretrix
-2.75
Unitm
1152±51
-1.2510.5
HJ-43,060cm
Dosiniajaponica
-2.27
Unitm
229±81
-0.7710.5
HJ-43,180 cm
Crassostrea gigas
-3.47
Unitm
414±81
-1.9710.5
HJ-43,295 cm
Crassostrea gigas
-4.62
Unitm
1762±80
-3.120.5
SW-07,579cm
Plant remains
-3.49
Unit I
7032±98
-5.4911 -2.1811
SW-09,168 cm
Crassostrea gigas
-0.18
Unit I
1619±81
Pyongtaek-1
Peat
1.00
4157±67
-3.3010.5
Pyongtaek-1
Peat
0.30
54701100
-4.0010.5
Daechon-1
Peat
-3.63
72701270
-7.2310.5
Daechon-2
Peat
-2.96
• • • •
69101450
-6.5610.5
Holocene peats at Pyongtaek and Daechon on the northern west coast. Fig. 3.28 shows that paleo-MSL on the west coast of Korea had a relatively rapid rise (at a rate of approximately 1.3 mm/yr) from 17 m MSL to - 5 m MSL between 7 and 5.5 ka. Between 5.5 ka and present, paleo MSL rose at a slower rate of 0.9 mm/yr on average. Holocene sea-level change can also be reconstructed from a litho- and chronostratigraphic analysis of the Gomso tidal-flat deposits by using the ^^C-derived mean accumulation rates of 0.2-0.3 mm/yr for Unit I, 0.5-0.6 mm/yr for Unit II, and 1.5-2.0 mm/yr for Unit III (Y.H. Kim et al., 1999). For example. Fig. 3.36 illustrates two isochrons that represent the paleoseabed at 1.1 and 1.8 ka, respectively. Based on the same assumptions as previously applied, the seabed-sloping line connecting the Unit Il-Unit III contact in cores is the lithofacies locus for points of a constant paleo-water depth equal to that of the present-day lithofacies boundary between the surficial sediments of the mixed flat (Unit II) and sand flat (Unit III), i.e., each point on the line formed about 1 m below MSL. The interception point of the seaward-dipping Unit Il-Unit III boundary with the 1.8 ka isochron is 3 m deeper than the present-day boundary. This suggests that paleo-MSL at 1.8 ka was 3 m lower than present MSL (Fig. 3.36). The gradual updip and landward migration of the Unit Il-Unit III boundary through time suggests that sea level has risen continuously since 1.8 ka without any significant episodes of sea-level fall or stillstand.
106
Marine Geology of Korean Seas
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Yellow Sea
3.12
107
Reclamation Effect on Sedimentation: Daeho Area
Much of the tidal flat in the west coast of Korea has been reclaimed by constructing seawalls and dykes, particularly since the early 1980s. Despite the short period of time elapsed, marked changes in depositional processes are evident in most of the reclaimed areas. Such human-induced changes have been well documented by a comprehensive study on the Daeho tidal flat/nearshore of the Taean Peninsula (H.J. Lee, 1994) (Fig. 3.37). 3.12.1
Geologic S etting
The Daeho area was once a large, funnel-shaped embayment (formerly
GRAB SAMPLE STATION WATER SAMPLE STATION
TRANSECT LINE
Fig. 3.37. Map of the Daeho area showing a variety of stations for grab and water sampling, anchor survey, and sedimentation rate monitoring. The present Daeho tidal flat in front of the Daeho seawalls I and II is small and narrow, a remaining outermost part of the formerly extensive Sosan Bay tidal flats. Most of the latter was reclaimed for farming by constructing the seawalls. Contours in meters. After H.J.Lee et al.(1999) by permission of the Elsevier Science B.V.
108
Marine Geology of Korean Seas
called Sosan Bay), characterized by well-developed tidal flats (Fig. 3.37). The bay was 7 km wide at the bay mouth and 10 km across. Like many other major bays on the west coast of Korea, Sosan Bay had not received terrigenous sediments directly from the rivers. The tidal flats within the bay were graded seaward from mud flat, mixed flat to sand flat, a tripartite zonation typical of the Korean tidal flats. Semi-halophyte plants {Sweda and Salicornia sp.) were colonized on the supratidal flats (Y.A. Park and Lee, 1987). After construction of the seawall near the lower water line, the remaining, only the most seaward sand flat exists as the present-day Daeho tidal flat. A trough, remnant of the former main tidal channel, divides the
—
^
^
V
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^
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^
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jt p
y
.
n H P $1 $1
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Fig. 3.38. Results of finite differential tidal modeling for the Daeho area without (a) and with (b) seawall constraint. In front of the seawall II, tidal currents were intensified after construction of the seawalls, 1.2 to 1.5 times stronger than those prior to the construction. In contrast, the flow of tidal currents near the seawall I has become almost null. After H.J.Lee etal.( 1999) by permission of the Elsevier Science B.V.
Yellow Sea
109
Daeho tidal flat into the northeastern and southwestern parts (Fig. 3.37). Tidal modeling, which has compared the tidal current regimes before and after the construction of the seawalls, indicates drastic changes in tidal current vectors, particularly in front of the seawalls (Fig. 3.38). Textural analysis of sediments of the nearshore seabed as well as of the tidal flats was performed from more than 50 grab samples (Fig. 3.39). The tidal flat is dominantly sandy in the NW, but becomes increasingly muddy toward the SW (silty sand to muddy sand). The muddy texture suggests the sheltering effect of islands (Daeranjido and Soranjido) against wave attacks induced by northwesterly monsoon winds during winter. Between Daeranjido and Daedobido there is a large area of exposed relict gravels which extends to Seawall I along the course of the former main tidal channel (Fig. 3.39). The gravel deposit is covered with modern silty sand between Daeranjido and Songmun-gak and in the nearshore of Samgilpo. Lithostratigraphy from vibracore analyses shows that the Holocene
TTOTHT _'9. QQ CL
m^^^^
oC^Oo^OVuoOOr: .oooopoo/L-: uoodbooooooooooor.-. Op OCT.
op-.-j^r.-r.
op 0 o op op op op op (f^l ^ -
v " " J ^ O ' ^ DAEDOBIDO
•"• A o n ^ , ; ^ \
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GRAVEL GRAVELLY SAND GRAVELLY MUDDY SAND| SANDY MUD SILTY SAND CHART DATUM
0
1
126*25
Fig. 3.39. Surface sediment distribution based on textural analysis of 55 grab samples. Facies classification after Folk's (1980) scheme.
110
Marine Geology of Korean Seas
sequence of the Daeho tidal flat is coarsening-upward from basal muds to the uppermost sand (HJ. Lee, 1994). The thickness of the sand flat amounts to more than 2 m near the low water line. Below the Holocene sequence lie either pre-Holocene oxidized muds of fluvial origin or rock basements as seen in Gomso Bay (H.J. Lee et al., 1994). 3.12.2
Tidal Flat Morphology and Sediments
The relative elevation of the tidal flat is highest in the northeast, near the area of Line I and gradually lowers toward Line IV in the southwest (Fig. 3.40). Most of the tidal flat is monotonously flat-lying, less than 0.4° in slope, towards the subtidal zone, in which the slopes abruptly increase up to 27° (Fig. 3.40). On the originally sandy tidal flat occur two contrasting
DISTANCE FROM THE SEAWALL (x100m ) 1
£
2
3
4
5
6
7
8
-4L
CO
-2
IS O -3 a:
-4 L -2 LU
-V
-3 -4L
Line II LEGEND )K 7 Apr.'93
'v_
A
20 July '93
D 18 Oct.'93 0 15 Jan.'94
Line I
•
1
27 Apr.'94
-2 -3 -4
Line IV
^s^ \
Fig. 3.40. Vertical sections across Lines I to IV showing changes in relief with time. Sand shoals around low-water line in Lines I and II moved 20-30 m landward during 1993-1994. This migration of sand shoals occurred only in winter time by an interplay of wave and tidal currents. On the other hand, Lines III and IV on the muddy southwestern part of the tidal flats show almost no variations in relief. For location of lines see Fig. 3.37.
Yellow Sea
111
sedimentary components, reflecting quite different hydrodynamic conditions: mobile sand shoals and depositional mud blankets. A prominent sand shoal lies parallel to the shoreline near the low-water level of Line I, with a steep landward face (Fig. 3.40). During the period of 1993-1994, this sand shoal expanded in both shore-parallel and shore-normal directions and migrated 20-30 m landward in winter while it was largely degraded during the summer. This suggests a significance of winter-storm wave activities on the development of sand shoals, despite some protection of the offshore islands (Daeranjido and Soranjido). A small, incipient sand shoal is growing on the northern side of the former main tidal channel, near the innermost part of Line III (Fig. 3.37). On the outer portion of the southwestern tidal flats, the mud accumulation likely began far before the construction of seawalls. In contrast, mud blankets at the bottom of the former tidal channel appears to have formed after the construction. 3.12.3
Nearshore Suspended Matter
Suspended particulates within the water column were taken from a total of 27 stations, located for optimum coverage of the study area (Fig. 3.37). Water depths for the stations vary from 2 to 20 m. The summer and winter measurements show remarkable seasonal variations in concentration as well as in the spatial distribution of suspended matter. The average concentration of suspended sediments in summer increases from 10 mg/1 at the surface to more than 30 mg/1 near the bottom (Fig. 3.41). In the absence of apparent sediment input from adjacent rivers, this relatively constant vertical profile may reflect that the bottom sediments are continually resuspended by tidal currents. In winter the concentration of suspended sediments increases up to 100 mg/1 even at the surface (Fig. 3.42), due largely to enhanced resuspension processes by strong wave action. The vertical suspended-matter profiles exhibit fairly consistent values in both concentrations and grain size throughout the water column, with limited variations of 10-20 mg/1 and less than 5 |am, respectively (Fig. 3.42). This means that a marked mixing of water masses results from wave orbital motion superimposed on tidal currents during winter. The wintertime sediment concentrations are characteristically higher in the offshore rather than nearshore of the Daeranjido (Fig. 3.42), implying that the core of the suspended sediment plume is located offshore north or northwest of the study area. Summertime suspended particulates show two different types of size spectrum, although most of the largest floccules were apt to be destroyed while sampled and size-analyzed. Type I is characterized by high percentage (>25%) of more than 100 |am fractions, whereas type II contains virtually no
112
Marine Geology of Korean Seas Fig. 3.41. Distribution of total suspended matter (TSM) in the Daeho area during summer, at the sea surface (a) and near the bottom (b). Average concentrations of TSM increase from 10 mg/1 at the surface to more than 30 mg/1 near the bottom. After HJ.Leeetal.(1999)by permission of the Elsevier Science B.V.
12d'25'
Sampling stations
1 I
TSM (mg/1) BOTTOM LAYER AUG. 30-31.1995
such coarse fractions (Fig. 3.43). Throughout the water column, both vertically and laterally, each type maintains its general spectral shape with little variations (Fig. 3.43). The mean grain size ranges from 40 to 60 jam and from 22 to 25 |am for the types I and II, respectively. It is interesting that the mode (about 15 |Lim) for the less than 100 |am fractions of type I is almost coincident with that of the whole fractions of type II (Fig. 3.43). The
Yellow Sea 12^25"
113
Fig. 3.42. Distribution of total suspended matter (TSM) in the Daeho area during winter, at the sea surface (a) and near the bottom (b). Note that the concentration of TSM increases up to 100 mg/1 even at the surface. After H.J.Lee et al. (1999) by permission of the Elsevier Science B.V.
26
Sampling stations
TSM (mgyi) BOTTOM LAYER FEB. 23-24,1994
w^intertime size spectra also are very similar to the type II spectra, although the mode of the former (11 \xm) is slightly smaller than that of the latter (1416 |am). Such a consistency in size spectra throughout the year suggests that the suspended matter in the nearshore of the study area is in a steady-state condition after experiencing an exponential decrease in sedimentation rates (see Syvitski and Murray, 1981). In this context, the existence of the coarsest
114
Marine Geology of Korean Seas SUMMER (TYPE I)
s m O a:
SUMMER (TYPE II)
WINTER
BOTTOM 50
m r^^^^^^
CL
^^^^^^^\ 0
^^^.
^^N" PARTICLE SIZE (nm)
100
PARTICLE SIZE (Mm)
10
PARTICLE SIZE (jun)
Fig. 3.43. Size-frequency curves of total suspended matter (TSM) in the nearshore off the Daeho tidal flats. Summertime curves are divided into two types, I and II, based largely on the presence/absence of the more than 100 fim fraction. This coarsest fraction may represent some sustainable floccules associated with biogenic production. By contrast, wintertime curves show a remarkable similarity over the area and lack the coarsest fractions seen in the summertime counterparts. fractions of type I and the slightly larger mode of type II, relative to the wintertime size spectrum, are thought to be attributable to the production of biogenic remains and some sustainable floccules. These biogenic products however appear to be subject to some breakdown due to current shear while settling down from the sea surface, since the averaged mean and mode of the summer suspension tend to decrease toward the bottom. 3.12.4
Seasonal Sedimentary Processes
Sediment accumulation and erosion on the tidal flat was monitored by measuring thickness variations from the surface to a buried plate during the period of 1994-1995 (Table 3.6; Fig. 3.44). In the winter season of 1994, all measurements indicated erosion at a maximum rate of over 30 mm/month.
Yellow Sea
115
Table 3.6. Sedimentation rates along Lines I to IV on the Daeho tidal flat during 19931995. After H.J. Lee et al. (1999). Seasonal Sedlmentatioii Rate (mm/moath)
AD'
Aniiual
(mm)
(on^)
-77.1
-3.9
Site Winter S^pring Sumner Autumn Winter Sluing Sununer Jl'2J9y-7J9Ai (3m-5/M) (6/94-«/94) (9/94-11/94) (12/94-2/95) (3/95-5/95) (6/95-«/95) LINE I
-7.8
-2.6
4.8
-0.3
-6.9
-0.4
-0.7
LINEH
-15.5
-2.4
6.1
-8.7
-8.5
-1.5
1.6
-166.2
-8.3
LINfEUI
-22.6
-5.8
5.9
-W.3
-17.5
-1.1
0.4
-300.7
-15.0
LINE IV
-16.1
-0.4
133
-U.l
-9.4
2.5
-1.6
-247.0
-12.4
Average
-15.5
-2.8
7.5
-8.9
-10.6
-0.1
-197.8
-9.9
-0.1
* AD » cumulative sedimentation or erosion for tfae period (12/93-8/95) between the initial and last measur^nents.
40 Winter (-16.5)
Summer (7.5)
Spring (-2.8)
Autumn (-8.9)
Winter (-10.6)
Spring (-0.1)
Summer (-0.1)
+
!l
+
I 20 E & CO
c o
I
-f
04
t % •
I
c E CO
-40
-1
2/94
1
4/94
1
1——I
6/94 8/94
\
1
1—
10/94
12/94
2/95
4/95
6/95
8/95
Date (mon/yr)
Fig. 3.44. Seasonal variations in the surficial sedimentation rate on the Daeho tidal flats during 1993-1995. Sedimentation rate was monitored by measuring (to an accuracy of 5 mm) the vertical distance from surface to an acrylic plate buried about 30 cm below the surface. Note that despite scattered values, the sedimentation rate clearly oscillates between the maximized erosion in winter and the highest deposition in summer. Monthly average sedimentation rate is in the range of-15.5 to 7.5 mm/month, with an annual average of 9.9 cm/yr of erosion. For location of measured sites see Fig. 3. 37. After H.J. Lee et al. (1999) by permission of the Elsevier Science B.V.
116
Marine Geology of Korean Seas
As the seasons progressed, the tidal flat began to undergo mud accumulation that culminated with a maximum rate of over 20 mm/month in June to July. The transition from the depositional to the erosional phase occurred prior to September, followed by another almost identical sedimentary cycle which ensued through the summer of 1995. The size frequency curves for the surficial sediments well reflect this seasonal variation in depositional environments (Fig. 3.45). The grain size spectrum is governed exclusively by 2-3 (j) fractions during winter as a result of wave sorting but becomes increasingly poorly sorted as mud accumulates during the subsequent fairweather conditions (Fig. 3.45). As a consequence, the 2-year observation accounts for the annual erosion at 9.9 cm/yr over the Daeho tidal flat (Table 3.6). 3.12.5
Suspended Sediment Budget
The 12-hour collections of hydrodynamic data from the four anchored stations (Al to A4) all exhibit semi-diurnal tides with a tidal range of 8-10
3
4 5 Size in phi unit
Fig. 3.45. Average grain size-frequency curves illustrating drastic seasonal changes. Each curve represents an average of size data from all surface samples taken during a specific survey. Note that the sand dominance (mode, 2.5 (j)) in winter diminishes rapidly over time until summer when poorly-sorted, muddy sediments are deposited. During autumn, however, the coarse-grained fraction begins to increase again. After H.J. Lee et al. (1999) by permission of the Elsevier Science B.V.
Yellow Sea
117
m (Figs. 3.46a and 3.47a). Wintertime currents are generally asymmetrical, reaching the maximum speed of over 1 m/s at surface during ebb: the flood maximum is less than 0.8 m/s (Fig. 3.46b). The bottom currents also are slightly stronger and hence total suspended matter (TSM) concentrations are much higher throughout the water column during ebb than at flood (Fig. 3.46c). Although suspended matter consists largely of silt (65-75%), sand fractions rapidly increase up to 20% concurrently with peak currents, suggestive of resuspension of bottom sediments (Fig. 3.46e,f). The summertime data also indicate a definite but slight asymmetry of tidal currents: surface peak speeds are 70-80 and 50-60 m/s during ebb and flood, respectively (Fig. 3.47b). TSM concentrations accordingly show higher values during ebb although their magnitude is a factor of 3 lower than the wintertime counterpart (Fig. 3.47c). The evaluation of the net flux of suspended matter was made on the basis of the anchor-station measurements of currents and suspended matter concentrations (Fig. 3.48). All of the estimates except for the station A3 suggest westward or southwestward flux, regardless of season. Due to the higher degrees of asymmetry in both the currents and suspended matter concentrations for the winter season (Fig. 3.46b,c), the wintertime flux is significantly higher than that for the summer (Fig. 3.48). These findings indicate that a suspension plume is constantly flowing southward in the nearshore of the study area throughout the year and that it is greatly enhanced during winter (see J.Y. Choi and Park, 1998).
3.13
Transgressive Holocene Sequence Stratigraphy
The Yellow Sea retains a Holocene history of marine transgression over a tectonically stable terrain. The eustatic sea-level rise has dominated the building of transgressive successions in the eastern part of the Yellow Sea where terrigenous sediment supply has been scant. Although some of the previous works with fragmental seismic data inferred possible lithologies of the subsurface shelf deposit of the early Holocene (and pre-Holocene), systematic reconstructions of sequence-stratigraphic architecture for the entire Holocene now have been made for the eastern part of the Yellow Sea (H.J. Lee and Yoon, 1997; Jin and Chough, 1998). Off the west-central coast of Korea, a transect of high-resolution seismic profiling and sediment coring reveals relatively simple Holocene successions that consist of tidal/estuarine sandy mud, tidal sand ridges, deltaic mud and transgressive sands (H.J. Lee and Yoon, 1997). To the south, on the southwestern innershelf of Korea, extensive collection of high-resolution seismic profiles (airgun and sparker as well as 3.5 kHz) and drill cores was made by Korea Institute of Geology,
118
Marine Geology of Korean Seas WINTER(FEB. 17,1995) 1^
-
10-
?
8^
UJ
6-
20-
(a) TIDAL RANGE (7.8)
^
•--....„^^
1
1
1
1
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
10
11
12
13
TIME (HOURS)
Fig. 3.46. Time series ofhydrodynamic measurements from anchor station Al (for location see Fig. 3.37) during winter. Note ebb-dominant tidal regime. Maximum ebb currents exceed 90 cm/s compared to less than 80 cm/s at flood. Accordingly, bottom TSM is higher (>80 mg/1) in ebb than during flood. Sand grains are entrained during the peak currents, suggesting resuspension from the bottom. After H.J. Lee et al. (1999) by permission of the Elsevier Science B.V.
Yellow Sea
Fig. 3.47. Time series of hydrodynamic measurements from anchor station Al (for location see Fig. 3.37) during summer. Note an ebb-dominant tidal regime with stronger ebb currents, although with much less overall energy compared to the winter counterpart (Fig. 3.46). Bottom concentrations of TSM tend to increase with intensified currents, suggesting some resuspension. After H.J. Leeetal. (1999)by permission of the Elsevier Science B.V.
SUMMER(AUG.29,1995)
<=> A0\
2
CO
(C)
119
Surfao0(4-2O.8) ~ (1(3.5-34.5)
20
6
8
10
12
TIME (HOURS)
Mining and Materials. Several type sections across and long drill cores (3050 m long) from the Huksan mud belt demonstrate somewhat different successions owning largely to the presence of unusually thick mud deposits, These successions include incised channel fills, ridges of rew^orked sand, and partitioned mud. In the follow^ing section, the Holocene successions in the northern and southern parts are discussed in detail. 3.13.1
Northern Part
The northern part encompasses the eastern half of the northern Yellow Sea immediately south of the passageway connecting the Gulf of Bohai and the Yellow Sea (Fig. 3.12). Water depth ranges from 19 to 82 m between the nearshore area and the deep outer shelf Sand dominates over the inner-shelf seafloor, but mud content tends to increase offshore. Further offshore, clayey deposits form a wedge on the almost flat seafloor (>0.05°) in water 72-79 m deep. Thus, the sea-floor morphology grades from nearshore sand ridges through an inner-shelf sand sheet to the offshore mud delta.
120
Marine Geology of Korean Seas
12€r25'E
Fig. 3.48. Vector diagram showing net flux of TSM for one tidal cycle during summer and winter. Based on 12-hour hydrodynamic measurements at 4 anchor stations (refer to Figs. 3.46 and 3.47). Note that all calculations for both the winter and summer data indicate the existence of a suspension plume flowing southwestward, hence bypassing rather than entering the Daeho tidal flats. This flow is enhanced in winter. After H.J. Lee et al. (1999) by permission of the Elsevier Science B.V.
3.13.1.1
High-Resolution
Seismic
Stratigraphy
High-resolution (3.5 kHz and Chirp) seismic profiles illustrate 30 m of vertical section that consists of three units (I, II, and III in descending order) separated from one another by strong reflectors and irregular erosional surfaces (Fig. 3.49). Units I, II, and III may correspond to Units A, B, and C, respectively, as described in the southern part. Unit III is the lowermost detectable unit in the section and appears to consist of pre-Holocene deposits. It is acoustically stratified and tectonically deformed (Figs. 3.50-52). This unit occurs consistently as an acoustic base along the entire section, W\Xh a more or less flat, erosional upper boundary that is intensely incised by paleochannels of various scale. Unit II overlies Unit III and underlies part of Unit I (Figs. 3.50-52). It is acoustically transparent or partly diffused. Characteristically, the thickness of Unit II is relatively uniform, averaging about 3 m (Figs. 3.50 and 3.52)
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Marine Geology of Korean Seas
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although it reaches up to 8 m in a large paleochannel. This unit is exposed largely on the seafloor of the inner-shelf area. In some places, it contains topographic rehef in the form of subdued sand ridges or banks with internal reflectors dipping landward rather than seaward (Fig. 3.51). Unit II is interpreted to be transgressive deposits accumulated during rapid sea-level rise, with the implication that it was deposited during an erosional (starved) transgression with low sediment supply (see Swift et al., 1971; Demarest and Kraft, 1987; Trincardi et al., 1994). The uppermost unit in the seismic section (Unit I) displays two contrasting styles of deposition: outer-shelf deltaic mud and nearshore sand ridges (Figs. 3.49-52). The sand ridges reach up to 15 m in thickness with extremely well-defined internal reflectors, downlapping seaward onto Unit II or Unit III (Fig. 3.52). Offshore deltaic muds are acoustically transparent, and the lower boundary of Unit I is traceable as a relatively weak reflector, but becomes increasingly obscure toward the Korean Peninsula (Figs. 3.50 and 3.52). The mud is thickest in the westernmost part and thins steadily in the form of a wedge toward the east (Fig. 3.50). This mud deposit is part of the central mud derived from the Huanghe River (H.J. Lee and Chough, 1989; Alexander et al., 1991a), whereas the sand ridges appear to have originated from the coastal region of Korea largely by tidal currents (Klein et al., 1982; D.R. Choi et al., 1992; Jung et al, 1998). Therefore, Unit I most probably represents deposition during the slowly rising or highstand phase of sea level (i.e., a type of highstand systems tract). 3.13.1.2
Lithofacies
A series of sediment cores from select locales along the transect allow the seismic stratigraphy to be interpreted with substantial lithologic control. A total of 4 sedimentary facies constitute Unit I and Unit II: basal sandy mud (Unit II), surficial sheet sand (Unit II), ridge sand (Unit I), and clayey mud (Unit I). The basal sandy mud facies occurs in the lowermost part of most of the cores (Fig. 3.53). Its sediment texture is remarkably constant, consisting of 33% sand, 35% silt, and 32% clay on average (Table 3.7). The water content is low (about 30%)) (Table 3.7), and remnant laminae of silt-clay couplets are present in some places, despite intense bioturbation with abundant horizontal burrows throughout (Figs. 3.53 and 3.54). Although the contents of organic matter and calcium carbonate are variable downcore, the shear strength of the sandy mud shows a definite increase downcore (Fig. 3.55; Table 3.7). In the inner-shelf area, the surficial sheet sand facies overlies the basal sandy mud facies (Figs. 3.53 and 3.54). Most of the sheet sand is too thin (<50 cm) to be detectable on the seismic profiles. Sand (mostly 3 (j) fraction)
126
Marine Geology of Korean Seas
(UJ) Hld3a 3dOD
Yellow Sea
127
Table 3.7. Sedimentary characteristics and mean values of mass physical properties of the Holocene sediments in the northeastern Yellow Sea. After HJ. Lee and Yoon (1997). Sediment
Pkcc Depontiood Maxit Occupied Envinminents Thioiaien Sand (Si
Silt
Basalundymud
Regional
Surface sheet sand
Inner shelf
Clay
Mean r W (g/cm*)
W (%)
e
n (%)
SS ftPa)
OM (%)
CaOO, (%L-
Tidalflat aad/orestuaiy
3
33
35
32
6.5
1.82
29
1.0
50
2.4
3.3
2.4
Shorafitteto
1
92
4
4
3.2
1.95
23
0.8
44
—
1.8
1.8
15
96
2
2
22
2.00
20
0.7
40
•—
1.5
>1
2
>1
19
81
9J
1.45
52
2.9
74
1.1
7.1
2.1
inner shelf Ridgesand
Neanboce
Neanhore
Clayey mud
Outer sfadf
Outer shdf
Y " wet bulk dennty, W - water content,«" void ratio, n " povoeity. SS " ihev «raigd&, OM - tota^
is predominant with trace amounts of mud, and water content is low, less than 25% (Table 3.7). Within the sheet sand, no physical structures are discernible and shell fragments and some gravels are present. In core P5 a shell layer marks the sharp contact with the underlying sandy mud (Fig. 3.54), but the corresponding contact in core P6 appears to be less distinct and lacks a shell base (Fig. 3.53). The ridge sand facies from the crest of the sand ridges shows intense bioturbation with dominantly horizontal burrows throughout the core (cores P7 and PIO) (Figs. 3.53 and 3.54). The uppermost 1 m is composed of fine sand (mean size, 2-3 (j)), whereas in the lower section either mud content generally increases to 20% (P7), or sand grains coarsen slightly (mean, <2 (j), core PIO) (Fig. 3.55). Shell layers are present in some places and are thought to be of probable storm origin, on the basis of irregular scattering and basal concentration of shell fragments. Water content is variable or decreases slightly downcore with an average of 20% (Table 3.7). Total organic matter is 1-3%), and calcium carbonate is in trace amounts (Table 3.7). In contrast, cores from the edge of the ridge and a trough between ridges (cores P8, P9, and PI 1) usually show a thin cap of (shelly) fine sand underlain by the sandy mud deposit with remnants of silt-clay couplets (Fig. 3.53). Intense bioturbation with vertical and horizontal burrows characterizes the clayey mud facies in the deepest shelf (cores P2, P3, and P4) (Figs. 3.53 and 3.54). This facies also overlies the sandy mud facies (Unit II). Grain texture is constant through the core, with clay fractions constituting up to 80% (Table 3.7). Water content is highest in this facies, more than 50%) (Table 3.7). Shear strength generally increases downcore, and accordingly water content tends to decrease although with some variations (Fig. 3.55). The contents of organic matter and calcium carbonate are relatively variable through the core (Fig. 3.55; Table 3.7).
128
Marine Geology of Korean Seas P2
P5
P10
(125>145cm)
(40-60 cm)
(70-90 cm)
Fig. 3.54. X-radiographs of representative cores. Thoroughly bioturbated clayey mud (125-145 cm, core P2) shows abundant burrows, whereas the surficial sheet sands displaying a shell hash layer overlie the sandy mud (40-60 cm, core P5). Note the relatively sharp boundary between the latter two facies, indicated by triangle arrows. Also, note the existence of remnant laminae in the sandy mud facies. Tidal sand ridge (70-90 cm, core PIO) is composed of bioturbated sand with rare horizontal burrows. After H.J. Lee and Yoon (1997) by permission of the SEPM.
3.13.1.3
Interpretations
The association of two sedimentary facies, sheet sand and sandy mud, in the inner-shelf area is interpreted as a transgressive sequence built of marine sand and tidal/estuarine sandy mud. The silt-clay couplets that are common in the sandy mud facies (Fig. 3.53) may be wavy bedding or rhythmites reflecting flood-ebb or neap-spring cyclicity (Dalrymple et al., 1991). A
Yellow Sea Mz(phi) 0 2 4 6 8 10 0 — T — r [ iP 1 1 1 1 1 r t
0M(%) 0
2
4
6
8
SS (kPa)
W(%) 10
0
129
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] 300 • P2 (Clayey Mud + Basal Sandy Mud)
• P5 (Sand + Basal Sandy Mud)
+ Pi 0 (Ridge Sand)
Fig. 3.55. Core log of mass physical properties for different sedimentary facies in the northern part of the eastern Yellow Sea. The basal sandy mud facies has consistent ranges in values of mean grain size, organic-matter content, and water content at different places far apart from each other (below 1.5 m, core P2 and below 0.5 m, core P5; for core location see Fig. 3. 49). Profile of shear strength for the basal sandy mud shows a typical normal consolidation. After H.J. Lee and Yoon (1997) by permission of the SEPM. shell hash layer truncating the top of the sandy mud shown in core P5 is probably of storm origin or a transgressive lag. However, the marine sand sheet merges with tidal sand ridges in the coastal area, and is entirely replaced by marine clays in the outer shelf (Fig. 3.53). Because the contact between the sheet sand and sandy mud facies in the inner shelf (cores P5 and P6) marks a pronounced shift in depositional environments from tidal/estuarine to marine, it can be considered a ravinement surface, although its nature is erosional in only a few places. Such a ravinement surface can be correlated laterally over a wide area of the inner shelf, judging from many of the previous cores, which also show a sudden lithologic change from sand to sandy mud at shallow subsurface depths (Fig. 3.56) (H.J. Lee, 1991). In the outermost, deepest shelf, however, the watery clays directly overlie the sandy mud facies, reflecting part of a basinwide muddy prodelta wedge that has been derived from the Huanghe River. The sand ridges in the nearshore are considered to be active at present, with no mud blankets on either crests or troughs, as revealed by cores P7
130
Marine Geology of Korean Seas
,2
Mz (phi) 3 4_ 5
6
2
Mz (phi) 3 4 5
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20 W(%)
• Grain Size (IViz) o Water Content (W) Fig. 3.56. Core log of mean grain size and water content from the transgressive sand in the northern part of the eastern Yellow Sea. For core location see Fig. 3. 49. Note that mean grain size decreases markedly from about 3 (f) to less than 4 ^ within 50 cm subbottom depth in most cores as surface sand grades into tidal/estuarine sandy mud. Water content accordingly shows a consistent increase downcore. After H.J. Lee and Yoon (1997) by permission of the SEPM.
Yellow Sea 131 through P l l (Fig. 3.53). D.R. Choi et al. (1992) also suggested such contemporary activities of the sand ridges in adjacent areas, on the basis of extensive, closely spaced grids of seismic profiles. Here, the sand ridges are 25-35 m high and 13-22 km wide, and have low-angle cross-bedded internal structures generally dipping to the SW; the crestline accordingly is parallel or slightly oblique to the main axis of tidal ellipses, which trend NE-SW (Larsen et al., 1985). These results strongly indicate that the sand ridges in the nearshore region of the northeastern Yellow Sea are tidally driven and are moving SW. Such active tidal sand ridges have been reported elsewhere in the Yellow Sea, mostly confined to the nearshore area (Off, 1963; Klein et al., 1982; Liu et al., 1989), but some of the relict or inactive analogs remain on the outer shelf, even near the shelf edge (Yang and Sun, 1988; Yang, 1989). Fig. 3.57 summarizes the Holocene depositional history in the northeastern Yellow Sea in a conceptual diagram. In the early phase of Holocene transgression, tidal/estuarine sandy muds first accumulated along an initial shoreline around the flat axial zone of the Yellow Sea Basin. As sea level continued to rise rapidly, some of the initial tidal/estuarine deposits remained exposed on the sea bottom without further deposition. Meanwhile, marine sands began to accumulate after undergoing weak shoreface erosion above the tidal/estuarine deposits on the flank of the basin. Together, the sands and estuarine sandy muds constitute transgressive sediments with an average thickness of about 3 m (Fig. 3.57b). In general, the marine sand section is less than 1 m thick, and as a base the ravinement surface is rather indistinct, a phenomenon expected for a regionally tide-dominated setting of low wave energy and limited sediment supply. As sea-level rise has slowed and approached the highstand position in the mid to late Holocene, two contrasting highstand-type deposits, deltaic muds and well-developed tidal sand ridges, have accumulated on the deepest shelf and shallow nearshore area, respectively (Fig. 3.57c). The mud is derived from the Huanghe River and rests discordantly on the early Holocene tidal/estuarine deposits, whereas the sand ridges originate in the recent coastal sand of the Korean Peninsula and travel offshore over the transgressive sand sheet. 3.13.2
Southern Part
The southern part encompasses the southeastern part of the Yellow Sea, a region from inner shelf to the deepest shelf (> 100 m) off the southwestern and part of the southern coast of the Korean Peninsula (Fig. 3.58a). On the inner shelf, the seafloor is generally flat and featureless and deepens southeastward, forming a bank-like feature which is covered with mud (Fig. 3.58a). Further offshore, the seafloor is characterized by ridges and swales.
132
Marine Geology of Korean Seas
Fig. 3.57. Evolutionary model showing the stratigraphy of the Holocene shelf sequence in the northern part of the eastern Yellow Sea. (a) During the last sea-level lowstand, the Yellow Sea shelf was subaerially exposed with pervasive channel incision into preHolocene sequences, giving rise to an unconformable sequence boundary at the top of Unit III. (b) As eustatic sea level rose progressively in the early Holocene, nearshore depositional systems of shoreface and tidal flats/estuaries retrograded across the inundated shelf, emplacing Unit II as a transgressive systems tract, (c) Recently, slowed eustatic sea-level rise allowed nearshore depositional systems to prograde into the outer shelf region, forming Unit I of two contrasting deposits, tidal sand ridges and prodelta muds from Korean and Chinese coastal areas, respectively. After H.J. Lee and Yoon (1997) by permission of the SEPM.
Yellow Sea
133
trending N E - S W in the northern part, and N W - S E in the southern part (Fig. 3.58a). The ridges are tens of meters high, tens of kilometers long, and a few kilometers apart. Seafloor sediments among the ridges and swales comprise
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125°00'E
126°00'E
•33°00'N 127°00'E 125°00'E
126°00'E
127°00'E
Fig. 3.58. Geographic location, bathymetry (a) and surface sediment distribution (b) of the southem part of the eastern Yellow Sea. The seafloor is relatively shallow and monotonous and deepens southward. Note ridges and swales in the west of Cheju Island. The seafloor is relatively deep (>110 m) in the vicinity of Cheju Island, where it is adjacent to the shelfbreak in the East China Sea (inset). Sandy sediments (sandy mud and muddy sand) occur offshore, whereas mud is dominant in the nearshore area, trending N-S(E). Note location of drill cores (YSDP 102 and YSDP 103). After Jin and Chough (1998) by permission of the Elsevier Science B.V.
134
Marine Geology of Korean Seas
muddy sand and sandy mud (Fig. 3.58b). In the vicinity of Cheju Island, the seafloor is dominated by slightly gravelly sand/mud (Fig. 3.58b). 3.13.2,1
High-Resolution
Seismic
Stratigraphy
Three distinctive units occur here, designated A, B, and C in descending order. Unit A is dominant in the northern area (bank-like area) which corresponds to the mud deposit trending N-S (Fig. 3.59). Unit B comprises ridges and swales in the southern area which are covered with sand/mud (Figs. 3.58a,b and 3.59). Unit C is confined to the vicinity of Cheju Island where the seafloor is depressed near the shelfbreak in the East China Sea (Fig. 3.59). Unit C underlies gravelly sand/mud in the southeastern area (Figs. 3.58b and 3.59). Unit C, the lowermost unit, consists primarily of sediments encased within eroded topographic lows, i.e., incised valley fills (Figs. 3.60a,b and 3.61). The topographic lows are presumed to have formed mainly by fluvial incision during the last regression and the following transgression. Although Unit C is commonly exposed on the seafloor, it is completely overlain by a thick younger deposit in the southwestern part (Fig. 3.60b). In the northern area, the surface of acoustic basement, the inferred lowstand surface of erosion (LSE), has low relief with few incised valleys (Fig. 3.61a,c). Unit B includes ridge-and-swale topography in the southwestern part (Figs. 3.59, 3.60b and 3.61a). The unit is exposed on the seafloor offshore and is generally covered with younger sediment nearshore (Figs. 3.60b and 3.61a). Its lower surface is sharply erosional and is deep in the offshore and shallow in the nearshore (Fig. 3.61b). This boundary is interpreted to be a transgressive surface of erosion (TSE). The ridges are ubiquitous in the offshore area where the lower boundary of the TSE is underlain by older sediments. These are, however, sparse in the nearshore area and tend to overlie shallow acoustic basement (Fig. 3.61b,c). The sparse distribution of ridges in the nearshore area suggests a limited sand supply that was fed by extensive reworking of predeposited sediments (see Swift and Thome, 1991). Unit A consists largely of mud deposits (up to 60 m thick) that trend N-S (Fig. 3.59). It unconformably overlies Units B, C, or acoustic basement in the northern area, and terminates southward off the northwestern Cheju Island. Unit A shows either transparent acoustic character or subparallel reflectors (Fig. 3.61a,b,c). Based on a distinct mid-reflector with high amplitude and lateral continuity. Unit A is divided into two subunits: upper unit (Unit Al) and lower unit (Unit A2) (Fig. 3.61b,c). Unit A2 is occasionally exposed on the seafloor and forms a flat-topped bank which is fringed by steep slopes and terminates off the northwestern shore of Cheju
Yellow Sea 36°30'N • - ~ Seismic Tracklines Seismic Unit Boundaryl• - - Inferred Unit Boundary f
36°00'N H
•
Deep Drilling Site
V
Short Coring Site
35°00'N -t-rir^r
135
Fig. 3.59. Distribution of seismic units (A, B and C). Note that each unit is largely covered with either mud, sand, and gravelly sand/ mud (cf. Fig. 3.58b). Dotted lines are seismic survey tracklines. Location of seismic profiles in Figs. 3.60 and 3.61 is also shown. Triangles = piston core locations, circles = drill core sites (YSDP 102 and YSDP 103). After Jin and Chough (1998) by permission of the Elsevier Science B.V.
l ^ » - ^ J. 4. a , 3 JyC « ^ 9 0 P 0 f . !^ ^ -_ u. L _ .i i ^ -, =. 96P254"0* 8 P , I T |0P14 ^ 3 ^ ^ p ^ ^
33°00'N 125°00'E
126°00'E
127°00'E
Island (Fig. 3.61a). Inclined reflectors within Units A2 and Al generally dip to the south (KIGAM, 1996). Further south, Unit Al becomes a thin (<10 m) blanket which partly covers the flank of the shallow-water bank and overlies Unit B or C. 3,13.2.2
Lithofacies
A total of 12 short cores were recovered from the deep (>100 m) water
136
Marine Geology of Korean Seas
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Marine Geology of Korean Seas
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Fig. 3.62. Radiocarbon dates and simplified lithofacies of piston cores (for details see Table 3.8). After Jin and Chough (1998) by permission of the Elsevier Science B.V. where Unit C is largely exposed on the seafloor and thins frequently below the limit of seismic resolution (Figs. 3.59 and 3.60a). Two cores contain fine-grained sediments immediately below the lower boundary of Unit C (cores 96P02 and 96P25) (Figs. 3.59 and 3.62). Radiocarbon dates on foraminiferal tests in the mud (Table 3.8) are close to or older than the age of
Yellow Sea
139
Table 3.8. Radiocarbon dates on core samples from the southeastern Yellow Sea. For core locations see Fig. 3.59. After Jin and Chough (1998). Sample No.
Depth (cm)
Seismic Unit
Sediment Type
89P2-2
150
A2
Biotuibated mud
Foraminifera tests
6,573±85
AMS
89P2.2
250
A2
Bioturbated mud
Foraminifera tests
6,538 ±79
AMS
89P12
168-170
Al
Bioturbated mud
Foraminifera tests
1,880 ±66
AMS
89P12
264-284
C
Gravelly
MoUuscan shell
8,640±150
Scintillation
sand/mud
fragments MoUuscan shell
9,150±160
Scintillation
6,380±160
Scintillation
9,640±170
Scintillation
Material
Age in Ka
Method
90P01
30-125
C
Gravelly sand/mud
fragments
90P02
30-140
B
Biotuibated
MoUuscan shell
sand/mud
fragments
Gravelly
MoUuscan sheU
sand/mud
fragments
Laminated mud
Foraminifera tests
4,579±74
AMS
Foraminifera tests
24,130±130
AMS
Foraminifera tests
17,130±100
AMS
90P06
120-200
94S41
7-11
96P02
63-68
C
Al
Below C Biotuibated sand/mud
96P25
70-80
Below C Laminated mud
YSDP 102
2160-2165
A2
Biotuibated mud
Foraminifera tests
6,080±60
AMS
YSDP 102
4134-4154
A2
Biotuibated mud
Foraminifera tests
6,910±110
AMS
YSDP 102
4340-4349
A2
Biotuibated mud
Foraminifera tests
6,670±70
AMS
YSDP 102
4802-4812
A2
Bioturbated mud
Foraminifera tests
6,480±110
AMS
YSDP 102
5027-5037
A2
Bioturbated mud
Foraminifera tests
6,500±100
AMS
YSDP 102
5162-5172
A2
Biotuibated mud
Foraminifera tests
6,44Q±100
AMS
YSDP 103
1295-J315
Al
Laminated mud
Foraminifera tests
2,393±80
AMS AMS
YSDP 103
1830-1840
A2
Biotuibated mud
Foraminifera tests
8,311±68
YSDP 103
2700-2710
A2
Biotuibated mud
Foraminifera tests
11,780±120
AMS
YSDP 103
2940-2950
A2
Bioturbated mud
Foraminifera tests
13,430±140
AMS
YSDP 103
3157-3177
A2
Biotuibated mud
Foraminifera tests
13,830±170
AMS
the last glacial maximum (16-18 ka) (see Curray, 1961; Fairbanks, 1989), suggesting that the lower boundary of Unit C may be the regressive surface of erosion (RSE). Other cores show that the upper part of Unit C is dominated by decimeter-to-meter thick, poorly-sorted, shell-rich, gravelly sand/mud (cores 90P01 and 90P06) (Fig. 3.62). ^"^C ages from shells in Unit C are about 9 ka (cores 89P12 and 90P06) (Table 3.8; Fig. 3.62). These cores and their radiocarbon ages as well as seismic stratigraphy suggest that the incised valley fills (Unit C) were formed in the early stage of the
140
Marine Geology of Korean Seas
following transgression and are partially covered with younger transgressive deposits. Unit B (ridges) consists largely of graded gravelly sands and well-sorted massive sands (core 89P13) (Fig. 3.62), covered with decimeter-to-meter thick, intensively bioturbated sand/mud. In situ shell materials from the bioturbated sand/mud yield an age of about 6.3 ka (core 90P02) (Table 3.8). The acoustic and lithologic characteristics indicate that ridges and swales (Unit B) formed on the transgressive surface of erosion (TSE). Two deep drill holes were retrieved along the longitudinal axis of the mud deposits of Unit A: YSDP 103 in the north and YSDP 102 in the south (Figs. 3.59 and 3.61a,b). X-radiographs of the drilled sediment cores reveal that the mud deposits consist largely of four sedimentary facies; (1) laminated mud, (2) homogeneous (silty) clay, (3) thin-bedded homogeneous silt, and (4) bioturbated mud (Fig. 3.63). The laminated mud is generally several centimeters thick, and comprises rhythmically alternating laminae of silt and clay. Individual laminae range in thickness from less than 1 mm to 1 cm. Thick silt laminae tend to show bidirectional cross-lamination (Fig. 3.64a). The clay laminae are persistent laterally and show bimodal grain size distribution. The homogeneous (silty) clay is characterized by a sharp lower boundary and minor bioturbation. It is commonly intercalated with laminated muds. The thin-bedded homogeneous silt is represented by silt lamina thinner than several millimeters. The lamina tend to be persisitent laterally and intercalated with intensely bioturbated mud. The bioturbated mud comprises moderate to intensely bioturbated mud, centimeters to meters thick, which gradually alternates with thinner facies such as laminated mud, homogeneous silty clay, and homogeneous silt. The mud deposits in core YSDP 103 are dominated by laminated mud with minor amounts of bioturbated mud (Fig. 3.63a). A thin layer of shellrich gravelly sand/mud (centimeter scale in thickness) occurs at the probable boundary between subunits Al and A2 (Fig. 3.63a). A coarse-grained layer is also present on the erosional surface of the lower mud (subunit A2) that is exposed on the seafloor (core 89P2-2) (Fig. 3.62). The predominance of laminated mud in core YSDP 103 suggests a significant influence of tidal currents. In contrast, core YSDP 102 consists of bioturbated mud, which most likely represents lower-energy conditions. In core YSDP 103, age constraints are well within the expected range, i.e., the base of the cored interval is approximately 14 ka. Here, the long-term sedimentation rate of the mud is about 3 mm/yr. On the other hand, radiocarbon age dates of core YSDP 102 are much younger (6-7 ka) relative to the thickness (Fig. 3.63b). This inconsistency is most likely due to the dating of bulk foraminifera tests from YSDP 102.
Yellow Sea YSDP 102 C Z S G
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ioi 2393+80 yrs B.P.
8311 ± 6 8 yrs B.P.
20 H
,11,780+120 yrs B.P.
.6,080+60 yrs B.P.
141
Fig. 3.63. Radiocarbon dates from drill cores YSDP 102 and YSDP 103 (for location see Figs 3.58b and 3.59). Each drawing schematically shows dominant lithofacies type, i.e., laminated muds (Facies Ml) in YSDP 103 and bioturbated mud (Facies Mb) in YSDP 102. C = clay, Z = silt, S = sand, G = gravel. After Jin and Chough (1998) by permission of the Elsevier Science B.V.
13,430+140 yrs B.P. 13,830+170 30 yrs B.P.
6,670+ 70 ' yrs B.P. 6,480+110 'yrs B.P. 6.500+100 'yrs B.P. 6,440+100 /rs B.P.
60 m
3.13.2.3
Interpretations
Fig. 3.65 schematically summarizes the distribution of three unconformity-bounded sedimentary units in the southeastern Yellow Sea. During the last glacial period, the Yellow Sea experienced a high-magnitude (>100 m) sea-level fall. Most of its shelf area was exposed subaerially and underwent shelf-wide regressive incisions, extending to the shelfbreak. The distribution of Unit C near the shelfbreak was affected most significantly by the antecedent bottom topography of the transgressive surface. As the transgression began, sediment accommodation space was preferentially created in the incised valleys. Unit B comprises older sediments which were reworked during the last
142
Marine Geology of Korean Seas
Fig. 3.64. X-radiographs of mud deposits, (a) Laminated mud (Facies Ml). Note microscale cross laminations (triangle arrows) with reversed direction in silt laminae, (b) Laminated mud (Facies Ml). In the interval a, clay laminae show bimodal grain size [cf. bright laminae and darker laminae (triangle arrows)], whereas silt laminae are very thin and laterally discontinuous. In the interval b, pairing of thin silt and thick clay laminae is prominent, (c) Bioturbated mud (Facies Mb) with faint biogenic structure (triangle arrow), (d) Bioturbated mud (Facies Mb) intercalated with thin silt layers (Facies Zh; triangle arrows). Scale bar = 2 cm. After Jin and Chough (1998) by permission of the Elsevier Science B.V. transgression and remolded into sand ridges and swales. The lower boundary of Unit B corresponds to the transgressive surface of erosion (TSE) (Fig. 3.65). Its sharp contact with the lower Unit C suggests that the TSE was formed by wave and storm erosion on a retreating beach shoreface. The shallow acoustic basement with strong resistance to transgressive erosion would not have yielded sufficient amounts of sediments to form the transgressive sand ridges (Unit B). Unit A, confined in the nearshore area (Fig. 3.65), reflects supply of large amounts of fine-grained sediments during the late transgressive phase and highstand in sea level. The prograding strata in Unit A indicate that sediments were transported south(east)-ward. The approximate sedimentation rate of the mud deposit in core YSDP 103 suggests that the fine-grained sediments have been derived primarily from the Keum River, transported southward by coastal currents. A thick transgressive deposit
Yellow Sea
143
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Post transgressive upper mud (Unit A1) Late transgressive lower mud (Unit A2) Transgressive sand ridges (Unit B) Regressive to transgressive incised-valley fills (Unit C)
Fig. 3.65. A schematic diagram showing three unconformity-bounded sedimentary units of lowstand to transgressive incision fills (Unit C), transgressive sand ridges (Unit B), and transgressive to highstand mud deposits (Units Al and A2) in the southeastern Yellow Sea. The regressive surface of erosion (RSE) can be traced across the entire shelf area. The transgressive surface of erosion (TSE) is represented by an erosional contact between the lower incision fills (Unit C) and the upper sand ridges (Unit B). The sealevel inflection surface of erosion (SLISE) is represented by an erosional boundary between the transgressive lower mud (Unit A2) and the highstand upper mud (Unit Al). The SLISE is confined landward and is correlative to the maximum flooding surface (MFS) basinward that is represented by the surface of Units B and C. After Jin and Chough (1998) by permission of the Elsevier Science B.V. might be formed even in the high-frequency sea-level fluctuations because rising sea level probably increases rate of newly added accommodation space. The erosional boundary between the lower mud (Unit A2) and the upper mud (Unit Al) exists almost totally across the mud deposit, although it is relatively confined landward in comparison with other shelf-wide erosional surface such as regressive surface of erosion (RSE) and transgressive surface of erosion (TSE). The boundary probably represents an inflection of rising sea level during the last transgression. It may be analogous to the erosional surface formed during a deceleration of the rate of sea-level rise, tentatively designated as a sea-level inflection surface of erosion (SLISE) (Fig. 3.65).
144
Marine Geology of Korean Seas
The overlying mud (Unit Al) has prograded south(east)-ward during the highstand, while the lower mud (Unit A2) remains partly under an erosional regime governed by modem hydrodynamic conditions and sediment supply.
CHAPTER 4
South Sea and East China Sea 4.1
Geologic Setting
The South Sea is the name Koreans use to refer the sea south of the Korean Peninsula between Cheju and Tsushima islands, whereas the East China Sea is arbitrarily demarcated from the Yellow Sea by the YangtzeCheju line (Fig. 4.1). In these seas, thick sedimentary sequences are present in a number of interrelated basins (Domi, Cheju, and Socotra basins) (Figs. 4.2 and 4.3) that form northeast extensions of the East China Sea Basin (ECSB). These basins trend NE-SW, parallel to the Taiwan-Sinzi Fold Zone and Okinawa Trough, and are covered by Blocks IV, V, and VI-2 and the Korea-Japan Joint Development Zone (JDZ) (Fig. 1.3). More than 14 exploratory wells have been drilled in this region, revealing some oil and gas shows, but not of commercial values.
4.2
Sedimentary Basins
The Domi Basin consists of three small-scale subbasins and intervening arches bounded by normal faults trending NE-SW (S.W. Kim et al., 1987; Hirayama, 1991; K.S. Park et al., 1992). Although the basin was initiated in the late Cretaceous, sediments were largely deposited in the Neogene, filling NE-SW-trending elongate depressions. Two wells (Domi-1 and Sora-1) penetrated more than 3,000 m of Neogene sediments (Fig. 4.3) (C.S. Kim et al., 1986; PEDCO, 1994). Here, sediments were deposited in alluvial-fan to coastal-plain environments (PEDCO, 1994). In the shelf area further south, the predeformational sequence is concentrated in the northern part toward the epicontinental sea and is underlain by an angular unconformity that lies about 500-1,500 m deep. The postdeformational sequence also consists of Neogene sediments ranging in thickness from tens of meters off Cheju Island and China to more than 2,000 m southeastward away from the continent. Possible reservoir rocks are present at the subsurface depth of 1,300-1,700 m (Domi-1, porosity, 15-23%) and 1,700-2,400 m (Sora-1; porosity, 10-20%). In Cheju Basin (Block V and JDZ), a number of exploratory wells were drilled (Fig. 4.3). The acoustic basement consists of gneiss and Cretaceous granite (in well JDZ-V-1), forming graben and half-graben. The sedimentary
146
Marine Geology of Korean Seas
Fig. 4.1. Detailed bathymetric chart of the southern Yellow Sea and the northern East China Sea. Contours in meters. Modified after Hahn (1979) courtesy of the Korea Ocean Research and Development Institute. fill comprises sandstone, shale, volcanic debris, and peat deposited in alluvial-lacustrine environments (Y.I. Kw^on et al., 1995). According to Zhou et al. (1989), the basin has experienced rifting (Late CretaceousOligocene), dow^nw^arping (Miocene-Pliocene), and regional subsidence (Pleistocene-Recent). Based on paleontologic data, K-Ar age determination, and correlation w^ith tuff layers in exploratory wells, Y.I. Kw^on et al. (1995) divided the Tertiary sequence into 5 units: A (Pleistocene-Recent), B (Pliocene), C (Late Miocene), D (Early-Middle Miocene), and E (Oligocene). Unit A is generally uniform in thickness across the entire basin, whereas Unit B is largely confined in channel-fills and is bounded with Unit C by an angular unconformity, caused by strike-slip faults trending NE-SW. Units D and E comprise coarse-grained sediments and volcanics (green tuff), respectively. Sediments largely prograded southwestward, draining the uplifted continental crust to the northeast. In Socotra Basin (northwestern part of ECSB and Cheju Basin), a thin Miocene sequence unconformably overlies late Eocene-Oligocene units formed by compressive deformation (Bong and Lee, 1994). According to
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gravity and magnetic survey results, the Socotra Basin comprises JurassicCretaceous volcanics, forming graben and horst, overlain by non-marine elastics (Paleogene) (Hyun et al., 1980; W.Y. Lee et al., 1985; Zhou et al., 1989). In the ESCB, extension began in the early Tertiary (Paleocene), and continued with subduction of the proto-Pacific Plate beneath the Asian continent (Sharp, 1992). Syn-rift deposits consist of a non-marine sequence (alluvial, fluvial, and lacustrine deposits) in the northern part of the basin (Oh et al., 1991). Towards the end of the Eocene, a major reorganization of the proto-Pacific Plate occurred, leading to the formation of the Philippine Sea and Pacific plates. The resultant change in movement of the Pacific Plate led to an increase in extension along the ECSB transform zone. This phase of increased extension lasted until the mid-Oligocene (PEDCO, 1992). Deposition continued as alluvial-fluvial plains developed with lakes and swamps. Deeper lakes evolved in the Oligocene. Near the end of this period, regional uplift occurred, coincident with the initiation of sea-floor spreading in the East and South China seas. Between the mid-Oligocene and early Miocene, thermal subsidence led to an increased area of post-rift sedimentation. At the end of the Miocene, collision occurred between Taiwan and a subducting arc on the Philippine Sea Plate. This led to propagation of a transpressive force through the ECSB region, leading to inversion. Continued subduction of the Pacific Plate led to the formation of the Okinawa Trough, a back-arc basin. This subsidence caused deposition of shallow marine sediments in the ECSB.
4.3
Coastal Embayments
The southern coast of the Korean Peninsula is characterized by numerous postglacial embayments and nearshore islands, forming a ria-type coast like that along the western coast. Sedimentation is controlled largely by moderate tidal currents depositing fine-grained sediments. These sediments are either riverbome or transported from the offshore, such as in Jinhae Bay (B.K. Park et al., 1976). Only a few large rivers drain into the southern coast, such as the Somjin and Nakdong rivers. These rivers deliver a substantial volume of clastic sediments, forming estuarine environments. The Nakdong River discharges approximately 63 million tons of water and delivers about 10 million tons of sediment into the sea annually (Ministry of Construction of Korea, 1974). The major portion of discharge (about 71%) occurs during the summer floods. The Nakdong River drains an area of about 24,000 km^ which is composed largely of Cretaceous sedimentary rocks (Kyongsang Supergroup) and granites. Sand and silty sand prevail in the upstream and
150
Marine Geology of Korean Seas
mouth of the river, whereas finer-grained sediments are dominant seaward (W.H. Kim and Park, 1981). Several embayments with Hmited drainage systems have been studied in detail, such as Gamagyang Bay (Kang and Chough, 1982), Deugryang Bay (Chang et al., 1980), and Jinhae Bay (B.K. Park et al., 1976; Hahn and Kim, 1977). These studies provide a model of processes operating in the coastal embayments in the South Sea wherein sediment transport is dominated by tidal currents, with no significant sediment yield from the surrounding drainage area. 4.3.1 4.3.1.1
Gamagyang Bay Physiography
Gamagyang Bay (150 km^ in area) is bounded on the north by relatively high mountains and steep hills which rise more than 400 m above sea level (Fig. 4.4). The bay is connected to the South Sea by numerous tidal inlets between relatively low islands, whereas a tidal inlet is open to Kwangyang 27«»30'
127** 40*
Fig. 4.4. Index map of Gamagyang Bay. DB = Deugryang Bay, JB = Jinhae Bay, KB= Kwangyang Bay, KI = Koje Island, NI = Namhae Island. After Kang and Chough (1982)by permission of the Elsevier Science B.V.
South Sea and East China Sea
151
Bay in the northeast (Fig. 4.4). The lithology of the Gamagyang Bay region is characterized by late Cretaceous rhyolitic tuffs. The drainage basin of the bay is limited, involving only small streams and creeks through which sediment discharge is minimal. The bay is shallow (mean water depth, about 9 m) and characterized by a semidiurnal tide flowing largely N and S (Fig. 4.5), The maximum spring tidal range is up to 350 cm with a mean of 195 cm (National Hydrographic Office of Korea, 1973). In the southern inlets, the surface tidal currents reach a maximum velocity of about 1 m/s (National Hydrographic Office of Korea, 1973). The bathymetric chart (Fig. 4.6a) generally shows a monotonous and flat seafloor except for the moats and depressions near islands and points. Sediment waves occur near the mouth of the northeastern inlet. They are about 30 to 50 m in wavelength and about 30 to 50 cm in amplitude. The waves occur at a depth of 7 to 8 m below the seafloor. Mean grain size of the sediments comprising the waves is about 7.5 (j) and slightly coarser than the adjacent seafloor sediments. The latter are composed primarily of silty clay
127* 40^
Fig. 4.5. Tidal cun-ents in Gamagyang Bay. Arrow length indicates average velocity of a series of observations, (a) Flood current, (b) Ebb current.
152
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Fig. 4.6. Bathymetry (a) and grain-size distribution (b) of Gamagyang Bay. More than 70 surface sediment samples show a progressive northward decrease in grain size. Bathymetry in meters is corrected in mean sea level. After Kang and Chough (1982) by permission of the Elsevier Science B.V. (Fig. 4.6b). The depressions or moats near the islands and points are ubiquitous, elongated in the N-S direction of flood and ebb currents. They are usually 15 to 25 m deep and more than 500 m across. In cross section they are Ushaped, whereas elliptical in a longitudinal view. The low reflectivity on the bottom of each moat suggests that the moats are covered with fine-grained sediments. A gradual decrease in mean grain size toward the center is observed in some moats (Fig. 4.7). 4.3.1.2
Acoustic
Stratigraphy
The acoustic basement deepens progressively southward with paleovalleys (500 to 1,000 m wide and 5 to 10 m deep) running both northand northeastward. The thickness of the sediment sequence above the acoustic basement ranges from a few meters to more than 30 m (Fig. 4.8). Thick accumulation occurs in the central part of the bay. Fig. 4.9 shows the
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47 4648 44 43
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Fig. 4.7. Mean grain-size variation and grain-size spectra map of a series of surface samples taken from the moats in Gamagyang Bay. (a) West of Baekdo Island, (b) East of Samdo Island. For location see Fig. 4.4. After Kang and Chough (1982) by permission of the Elsevier Science B.V.
existence of a relatively strong mid-reflector w^ithin the sedimentary sequence. The mid-reflector, characterized by a relatively large contrast in acoustic impedance, occurs at the depth of about 15 to 20 m below the seafloor and at a progressively shallower depth toward the margin. It can be traced over the entire bay except for over local highs of the acoustic basement. According to the seismic characteristics, the sedimentary sequence in the moats or depressions may be classified as either depositional or depositional-erosional. The former is characterized by laterally continuous and concordant reflectors, whereas the latter exhibits a depositional phase modifled by erosional downcutting. The occurrence of both phases varies in time and space. In the vicinity of the Baekdo Island, both phases are present, suggesting the instability of the moat. By comparison, close to Samdo Island, the reflectors are continuous and exhibit only a depositional sequence (Fig. 4.9). Here, the mid-reflector also is laterally continuous, filling the incised acoustic basement on both sides of the moat, which seems to have been stable for some time. The depressions in the southern tidal inlets show exclusively the depositional-erosional sequence with a discontinuous midreflector. The moats of both depositional and depositional-erosional origin
154
Marine Geology of Korean Seas Fig. 4.8. The thickness of sediments above the acoustic basement in Gamagyang Bay. Assumed sound velocity is 1,600 m/s in the sediments. The dotted lines indicate the estimated thickness, whose layers are masked by the turbid layer apparently caused by gas bubbles in the sediments. After Kang and Chough (1982) by permission of the Elsevier Science B.V.
127M0'
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appear to be similar to those of the large-scale, deep-sea moats on the Line Islands archipelagic apron reported by Normark and Spiess (1976). The thick sediments contain gases characterized on the Uniboom records by a lack of acoustic penetration (Fig. 4.9). This acoustic response is similar to the acoustically turbid sediments in other shallov^-w^ater environments (Reeburgh, 1969; Schubel, 1974). In Gamagyang Bay, the gas-charged zone occurs mainly in the central bay comprising about 12 km^ in area. Schubel (1974) and others attributed the acoustically turbid character to the scattering and attenuation of acoustic energy by gas bubbles contained within the sediment interstices. D'Olier (1979) also ascribed the "bright spot" reflections (Fig. 4.9) to either the highly organic muds and gases or the thick accumulation of shells in the estuaries. The sediments in the gas-charged zone are also very fine, almost the same as the clayey silts of the adjacent seafloor. The gases probably consist of CH4, CO2, and H2S produced by the biochemical degradation of organic matter in the sediments. According to the ^"^C age dating of the shell fragments from two cores
South Sea and East China Sea
155
a 20
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Fig. 4.9. Uniboom profile across the eastern margin (west of Dolsan Island) of Gamagyang Bay showing mid-reflectors (arrows) at a depth of about 30 ms interval (for location see Fig. 4.4). Vertical scale: two-way travel time in milliseconds; 10 milliseconds correspond to approximately 8 m sediment thickness. The mid-reflectors represent probably the erosional surface prior to the Holocene transgression about 4.5ka. The turbid layer (bright spot) on both ends of (b) represents the gas-charged sediments through which acoustic energy attenuates. After Kang and Chough (1982) by permission of the Elsevier Science B.V. located on the northern and southern flanks of a depositional moat near Samdo Island, respectively, the sediments have accumulated at a rate of 134 cm/ky (1,645 ± 200 yrs B.P. at a depth of 220 cm from the seafloor). Extrapolated at a constant rate, it yields an age of about 4.5 ka for the midreflector found approximately 6 m beneath the seafloor. 4.3. L 3
Deposition
of Fine
Sediment
The progressive northward decrease in grain size (Fig. 4.6b) suggests that the influx of suspended sediments has occurred through the southern tidal inlets in w^hich finer sediments travel farther northv^ard and settle on the bottom due probably to settling and scouring lag effects. A lack of distinctive bed forms and of primary sedimentary structures may be caused by the fineness of sediments and bioturbation by bottom-dwelling organisms. The lenticular bedding or coarsely interlayered bedding found occasionally
156
Marine Geology of Korean Seas
in a number of cores is typical of the subtidal environment where mud deposition dominates. The beds are normally thin and seem to be formed by the occasional transport and deposition of sand and silt during sporadic heavy storms, each lasting a few days. The coarsely interlayered bedding may form when relatively large amounts of sand or silt are available, whereas the flat isolated lenticular bedding may represent a meager sand or silt supply. The mechanics of moat formation may be shown by the Bernoulli equation. On approaching an island or a point, the tidal currents are deflected, causing an increase in current speed. The pressure gradient, therefore, increases normal to the closer streamlines, precluding deposition. The net deposition of sediments near the island or point probably occurs only when the current speed reaches a minimal stage. The decrease of mean grain size toward the center of the moat may be due to the segregation of sediments by an eddy or secondary helical flow. The complexity of depositional-erosional features shown in some uniboom profiles suggests that the current direction has shifted at various stages of moat formation. 4.3.1.4
Late Quaternary
History
The existence of a mid-reflector as found in Gamagyang Bay has been documented in many embayments along the coast of the South Sea (Song and Cho, 1978; Chang et al., 1980). Hahn and Kim (1977) reported a subaerial sand and gravel layer formed prior to the recent marine transgression at a depth of 16.5 m below the seafloor (13 cm above the basement) in a core retrieved in the northwestern part of Jinhae Bay (Fig. 4.4). Here, sediments were accumulated at a rate of about 113 cm/ky (Hahn and Kim, 1977). This sand and gravel may cause the mid-reflector in Deugryang Bay (Chang et al., 1980). The mid-reflectors and the overlying sedimentary sequence in areas such as Gamagyang and nearby Deugryang bays show evidence of onlap over the acoustic basement (Fig. 4.10), suggestive of marine transgression. Further south, at the present water depth of about 40 to 60 m, the sediment sequence beneath the mid-reflector is thicker, and its top surface is eroded and truncated by numerous erosional channels (Fig. 4.11). These paleochannels were probably the extensions of rivers and streams formed during Wisconsinan time when sea level was lower, providing the mechanism for eroding the pre-mid-reflector sequence. In summary, the bays along the southern coast seem to have been eroded subaerially or to have lain above the depositional base level during the last glacial period. During the postglacial transgression, the bays were submerged, but little or no deposition occurred until the rate of sea-level rise decreased about 4.5 ka.
South Sea and East China Sea
157
158
Marine Geology of Korean Seas
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4.4 4.4.1
Surface Sediments Distribution
A number of surface sediment samples were retrieved from various areas of the nearshore and shelf of the South Sea, and their textural characteristics and distribution have been published in separate reports and monographs (Chang et al, 1978; Chough, 1983; S.W. Kim et al., 1986, 1987, 1988). These results have been mosaicked into a composite map of the surface sediment distribution for the South Sea (Chough et al., 1991a) (Fig. 4.12). Fine-grained sediments (silty clay or clayey silt) prevail along the coastal area and extend to water depths of about 60 m. The fine-grained materials off Kohung Peninsula and Koje Island are somewhat coarser, and comprise sandy mud and muddy sand. In the eastern part of the sea, mud deposits are
South Sea and East China Sea
159
Fig. 4.12. Surface sediment distribution in the eastern South Sea. Sediment classification according to Folk's (1954) scheme. Recent mud is prevalent along the entire nearshore area and in some embayments. Sand increases seaward. Note sandy mud rather than pure mud in the westernmost part. After Chough et al. (1991a) by permission of the Elsevier Science Ltd. distributed in a narrow belt landward of the 110 m isobath (Fig. 4.12). Toward the deeper part of the seafloor, the mud generally grades through a mud-sand mixture (sandy mud and muddy sand) to relict sand with abundant gravels and shell fragments. Sediment cores (1-3 m long) were taken from numerous sites encompassing the full spectrum of subenvironments (Fig. 4.13). Sediment texture is usually consistent throughout each core and follows that of the seafloor sediments. Cores of mud (clayey silt, silt-clay, and silty clay) show a tendency toward slightly decreasing in grain size seaward (Fig. 4,14). Muddy sediments are consistently bioturbated but relatively compacted with occasional large shell fragments. Between the mud and sand deposits, core J102 displays a fming-upward sequence with sand decreasing from 90% near
160
Marine Geology ofKorean Seas
Fig. 4.13. Map showing locations of geotechnical core samples (dots) from the eastern South Sea. Contours in meters. After Chough et al. (1991a) by permission of the Elsevier Science Ltd. 150 cm subbottom to less than 5% at the surface (Fig. 4.15). Core P-20 contains shelly sand with abundant gravels below^ the uppermost 20 cm of compacted sandy mud. This textural change represents the Holocene transgression as revealed on seismic profiles. 4.4.2 4.4.2.1
Mass Physical Properties Water Content
Water content in muddy sediments varies along the coast largely depending on sediment texture (Fig. 4.15; Table 4.1). With burial depth, most cores attain constant values in both w^ater content and grain texture (Figs. 4.15 and 4.16). Water content generally increases seaward (over 80%), correlating well with the seaward decrease in grain size (Fig. 4.14). Silty
South Sea and East China Sea
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25
30
CaC03l%) Fig. 4.14. (a) Relationships of silt content vs. water depth. Each point with core number represents silt contents averaged throughout the core (for core location see Fig. 4.13). Note a consistent decrease in silt contents toward deeper water. The boundary between nearshore and offshore areas is determined based on depth contour configuration (see Fig 4.13). (b) Relationships of organic matter vs. CaC03 contents. Toward the eastern part, both contents distinctly increase. Each point stands for averaged value of the core. After Chough et al. (1991a) by permission of the Elsevier Science Ltd. sediments off the Kohung Peninsula (K-series cores) are considerably lower in water content, ranging from 55 to 65%. U-series cores off Ulsan have higher and more variable water contents than others (more than 150%) (Fig. 4.15). This probably reflects a complicated depositional structure where laminated sections with abundant organic matter frequently occur in
162
Marine Geology of Korean Seas Core K - 6
^0
Texture W» PL(x) a L L ( o ) 20 4 0 60 80 I00(%)0 50 100 I50t%)
71777172^
0
SS 5 to 1—I—I—r
CoCOjCOaOMCO) l5(kPa) 0 10 20 30 40(%) -I
{
50
<: 100
|T-_-SILT-"- - " ^ H
^<
150
( hti-c
I
H 250 (cm)
Core P-ll
^m T
50
» f I
-\—^—I—I—r
I
\
100
•-"-"_~j:>fxLAY:
\
\ • ^
\rsss;^^
IT , /\
•(
'ff<
^
250 (em)
/
\ / \ / V-c
Core C - 7 0| 50
I
I
I
T"
I
:->:t L--V zz
f
I 00 I (cm)
Core J-102
h
/ ,-^*
Fig. 4.15. Geotechnical properties of selected sediment cores (for core location see Fig. 4.13). Core K-6 comprises silty sediments, whereas core C-7 is composed largely of clay. U-series cores consist of about equal amounts of silt and clay, showing the highest values for all geotechnical properties. Transgressive deposits (core J-102) display a close relation between grain texture and water content. W = water content, PL = plastic limit, LL = liquid limit, SS = shear strength, OM = organic matter. After Chough et al. (1991a) by permission of the Elsevier Science Ltd.
South Sea and East China Sea
163
Core N-4 Texture 0
I-
• {
1"
•/
CaCOj^C) aOM(O)
ss
W, P L ( x ) a L L l o )
10 5 T—r—I—
20 40 60 80 I00(%)0 50 100 I50(%) —1—1—1—1 T — r - n
l5(kPa)0 I
-^
»
/ •>
M
0 1
1
1
"11=1:
VTH^
-??
30
1
1
7
r^^
—r
-
7 —I
^ \ •. V
>N ^ 1
'
1
f
n~ \^^ \ 'S 1
—p—1—-I
1
<
K \
\ /s . \, /N. \ \\
------tV 350 (cm)
1
n- f'
.-^
^>>>i
40(%)
\ .^S. /
\ Core U-OI
20
1
I
Tt
"
10
r>
}
/
V
^---^
<'
• >
< \
\r J'
Fig. 4.15. Continued. otherwise bioturbated sediments. The fining-upward sequence of core J-102 illustrates an increasing-upward profile from 35 to 130% in water content (Fig. 4.15). 4.4.2,2
CaCO^ and Organic
Matter
Nearshore cores show a consistent increase eastward in CaC03 (3-20%) and total organic matter (3-10%, from combustion loss at 550''C) (Fig. 4.14). Off Ulsan, mud and sandy mud of U-series cores exhibit the highest values among muddy cores in both CaC03 and organic matter (20-25% and 7-12%, respectively) (Table 4.1). In shelly muddy sand and shelly sand collected deeper than 70 m, CaCOs content is extremely high (generally over 30%) but
164
Marine Geology of Korean Seas
Table 4.1. Geotechnical properties of surface (and subsurface) sediments in the South Sea. After Chough et al. (1991a). Sediment texture (%) (Sub)Surface Depositional sediment environments
Ccwre samples Sand Silt
Clay
Alterberg limits (%) Mz W* SS CaCOjOM —(<j>) (%)(kPa) (%) (%) Pb LL PI
Clayey silt
Nearshore (up to 30 m deep)
K-2,K-4,K-6, 5-10 50-80 20-40 7-8 50-110 4-8 3-15 4-10 10-20 40-60 20-40 P-13,P-23,C-5, N-1
Silt-clay
Offshore (30-100 m deep) and embayment
P-11, P-22, C-1, <5 N-2,N-3,N-5, U-01,U-02,U-05, U-U, U-12, U-15, U-17, U-19
Silty clay
Oflfehore C-4, C-6, C-7, between C-8,C-9,N-4 Yosuand Koje Is. (30-60 m deep)
Sandy mud
Coast and tidal inlets
C-2,C-3,N-8, N-9
20-50 20-40 20-40 5-6 40-80 3-7
Shelly sand and gravel
Offshore (deeper than 60 m)
U-03.U-08, U-14,J-302
>90
Transgressive Deposit P-20,N-6, muddy sand between N-7, N-11, modem mud N-12,J-102, and relict J-305, U-06, sand (50-120 U-07,U-13 mdeep)
40-50 40-50 7-9 80-160 4-10 10-30 8-12 10-30 50-90 40-70
<1 30-40 60-70 8-10 100-130 2-4
<5
<5
1-3 20-30
5-95 5-50 5-50 1-7 20-130
15-20 50-60 30-40
10-20 60-7040-60 10-40 0-3
10-30 0-1210-50*40-90 20-60
* For symbols,referto Fig. 4.15. ^ Only for the uppermost muddy sediment Mz = mean grain size PI = plasticity index
organic matter is much lower (1-3%). 4.4.2.3
Shear Strength and A iter berg L im its
Vane shear strength of the muddy sediments is relatively high at the surface and near-surface (3-5 kPa), then gradually increases with burial depth: 8-9 kPa at 2 m depth of K- and P-series cores, and 7-14 kPa at 3 m in U-series cores (Fig. 4.16). Sensitivity varies between 1 and 3, indicating low to medium sensitive clays according to the Skempton and Northey's (1952) scheme. Atterberg limits generally increase toward the east, in good accord with an eastward increase in organic matter and CaC03. K-series cores show
South Sea and East China Sea 165 P-core
K- core
C-core
SS 2
4
2
4
0
2
4
6
8
0
i
2
4
200
w 50
2
L
\ \
50 100 150 irn—I—I—I
7
0
50 100 150 1—I—ipr—I—I
0 50 I—I—r
100
150
r
'
0
4
6
8(kPa)
< )
^
250h (cm)' 0
0
I
(
[
V s
I50h
8
' '
\
\ \
6
• I
50
lOOh
U-core
N-core
\ 50
100
150
0
50
100
150 (%)
<
lOOh
150, 200
250] (cm)
Fig. 4.16. Averaged vertical profiles of shear strength (SS) and water content (W) for each core series (for core location see Fig. 4.13). Note that all the shear-strength profiles exhibit a downcore increasing trend, whereas water content remains constant with burial depth. After Chough et al. (1991a) by permission of the Elsevier Science Ltd.
low^er values of liquid limits and plasticity index compared to the P- and Cseries cores (Table 4.1); the former fall in the division of inorganic silty clays of lov^ plasticity (CL), w^hereas the latter are in inorganic clays of high plasticity (CH) in the plasticity chart (Fig. 4.17). The U- and N-series cores occupy the higher CH division, with the highest measured liquid limits (8090%) (Fig. 4.17). 4.4.3
Recent Depositional Processes
Grain size analyses reveal that silt fractions gradually decrease seaward at the expense of increasing clay fractions in the entire coastal area of the South Sea. This seaward-fming feature typically represents sediment supplies from the adjacent landmass rather than onshore input from the open sea. Based on preliminary studies of clay minerals, some workers suggested that illitic sediments have been derived from the East China Sea by the
166
Marine Geology of Korean Seas
I00(%} LIQUID LIMIT
Fig. 4.17. Plasticity chart of South Sea sediments. K-series cores fall in the division of inorganic clays of low to medium plasticity (CL). P- and C-series cores are plotted in the lower portion of the division of inorganic clays of high plasticity (CH). Easternmost cores (N- and U-series) are the highest in the CH division, although some of the N-series cores with varying amounts of sand are scattered. All plots show a good linearity above the A-line. For symbols of points see Fig. 4.14. After Chough et al. (1991a) by permission of the Elsevier Science Ltd.
Kuroshio and Tsushima currents (Aoki et al., 1974; B.K. Park et al., 1976; B.K. Park and Han, 1985). However, it seems unlikely that the w^arm and saline Tsushima Current could intrude across the strong nearshore "thermal front" into^he cold, less-saline coastal water (Huh, 1982; Zheng and Klemas, 1982). Paleontological studies of ostracoda for the surface sediments from the area between Yosu and Ulsan support this areal constraint in water mass distribution, showing that coastal-water species, typically Bicornucythere bisanensis, predominate nearshore, whereas warm-water species, represented by Bradleya nuda and Bradleya japonica, are restricted to offshore sandy sediments (S.W. Kim et al., 1987). Y.A. Park et a l (1987) have reported that springtime suspensate concentrations are higher nearshore than offshore between Yosu and Pusan and increase toward bottom, as a result of terrigenous input and resuspension.
South Sea and East China Sea
167
Landsat imagery clearly shows that suspended sediments flushed from the Nakdong River in the flooding season are advected northeastward along the shore (M.S. Kim et al., 1987). Preliminary hydrographic surveys (Chu, 1975) also reveal that a low-salinity plume extends from the mouth of Nakdong River, and is subsequently deflected to the east and north along the coast. These observations also suggest that the Nakdong River is the most probable source to the U-series muddy sediments. West of the Kohung Peninsula, different processes seem to supply relatively small amounts of suspended materials resulting in somewhat more consolidated sandy mud (part of the K- and P-series cores). On late-fall-season Landsat images, welldefined turbulent plumes envelop the southwestern and southern coast (Wells and Huh, 1984; Yoo, 1986; Wells, 1988), indicating that some suspended sediments from the Yellow Sea are supplied to the coastal embayments in the South Sea (Fig. 3.15). The clayey muds in the area between Kohung Peninsula and Koje Island may have been derived from the Somjin River. S.C. Park and Yoo (1988) have also suggested that the Somjin and Nakdong rivers are possible sources for the recent muddy coastal-innershelf sediments. The muds in the South Sea contain high quantities of clay particles (typically more than 50%) causing an abundance of interstitial water (up to 200%) as well as enhanced preservation of organic matter and calcium carbonates. Substantial values in vane shear strength (3-5 kPa) near the seafloor may reflect the state of apparent overconsolidation (see Almagor, 1967), a common trend in subaqueous muddy environments having low accumulation rates. The vertically uniform water content most probably results from a combined effect of poor permeability, extensive bioturbation, and low overburden. Off Ulsan, comparatively high quantities of organic matter and calcium carbonates in the U-series cores (Fig. 4.14) probably result from high productivity in the surface plume due to upwelling induced by an onshore intrusion of deep water from the East Sea (J.C. Lee, 1983; J.C. Lee and Na, 1985; Byun, 1987). Some cold-water species of ostracode have been reported to be enriched in the surface sediments herein compared to those further south (S.W. Kim et al., 1987). Such high contents of organic matter appear to be reflected in the plasticity chart, where U-series cores are the most plastic (Fig. 4.17).
4.5
Late Quaternary Transgressive Deposits
An extensive survey (more than 6,000 km) of high-resolution seismic reflection profiles (sparker, uniboom, and 3.5 kHz) was conducted in areas off Somjin River to Pohang by the KIGAM and the Agency of Defence
168
Marine Geology of Korean Seas
Development during the period between 1986 and 1995 (Min, 1994; Yoo, 1997). Along with seismic surveys, numerous piston and gravity core samples, 0 . 5 ^ m long, were taken to complement the seismic interpretations. Although both contain a shallow shelf (mostly less than 100 m deep) and a ria-type coast with abundant embayments, the South Sea is quite different in geologic setting from the Yellow Sea. The South Sea is a narrow (60-100 km wide), meso-tidal strait highlighted by a deep (>200 m) central trough, over which a well-established thermohaline current, Tsushima Current, flows to the East Sea. Riverine input into the South Sea is meager compared to that of the eastern Yellow Sea, and is delivered mostly via the two rivers, Somjin and Nakdong (Fig. 4.17). Seismic investigations reveal that in addition to the recent mud deposits along the coast, diverse relict sediments occur around the central trough. Within this physiographic and geologic framework, the seismic facies analyses suggest a sequence-stratigraphic development for the late Quaternary (S.C. Park and Yoo, 1988; Min, 1994; Yooetal., 1996; Yoo, 1997). During the lowered sea level, sediments accumulated as lowstand deposits in the vicinity of the central trough, thickening (up to 100 m) toward the basin axis (Fig. 4.18). The central trough lowstand unit forms a wedgeshaped, seaward-dipping clinoform (Fig. 4.19a,b). The sub-units display sigmoid-oblique or shingled internal reflections and are occasionally chaotic or hummocky. Both the upper and lower boundaries of the lowstand wedge are highly erosional. Radiocarbon ages of shell samples from the lower unit range from 40 ka to more than 50 ka (Yoo, 1997). Together with the intertidal to shallow-marine origin of the dated shells, these ages suggest a sea-level lowstand stage prior to the last glacial maximum. Similarly, ^'^C dating of shells from the upper units, though much younger (15-15.5 ka), also indicates the sea-level lowstand during the last glacial period. These sediments represent episodic progradation. Cored sediments from the shelf margin and the trough are composed of a sand-mud mixture, i.e., compacted mud or muddy sand. Although massive muddy sand with scattered shell fragments is the predominant sedimentary facies in most cores, layered muddy (or sandy) facies and gravel-rich muddy sand facies also occur. The Holocene sea-level rise has formed a transgressive deposit that consists of a variety of depositional systems including beach-shoreface complex, incised channel fill, and transgressive deposits (transgressive sand sheet and transgressive estuary/delta) (Min, 1994; Yoo, 1997). The beachshoreface complex occurs to a limited extent along the shelf margin in about 120-150 m of water, as a narrow fringing belt (>100 km long, 2-4 km wide, and 5-10 m high) parallel to the present isobaths (Fig. 4.18). In seismic profiles across the complex, it takes a lobe or bank shape with a steep-sloped,
South Sea and East China Sea
169
35**N
RMD ^
128^ I
^
TSR
BSC 1 ^ ]
ICF
34®N
I29OE
Fig. 4.18. Areal distribution pattern of a variety of transgressive deposits and recent mud in the eastern South Sea. RMD = recent mud deposits, TSR = transgressive sand ridge, TSD = transgressive deposits, BSC = beach-shoreface complex, ICF = incised channel fill, LSD = lowstand deposits. Modified after Yoo (1997). seaward side and contains hummocky or chaotic reflections with some inclined reflectors (Fig. 4.19a). The incised channel-fill system is a cut-andfill structure with an irregular erosional base (Fig. 4.20). Internal reflection patterns are dominated by well-stratified reflectors as well as chaotic or hummocky ones. This system can be well traced further seaward off the Somjin and Nakdong rivers (Fig. 4.18). Transgressive estuary/delta system
170
Marine Geology of Korean Seas W
h-'''''^^^-'Pleistocene sedimentary strata / \ v ' ' 7 ' y • • / \ . \ \ \ N \ N \ N \ \ N N \ \ \ \ N \ \ S N \ \ \ N '
•31cm-
I
LSD
W/"x"x"^"xV^^"x"x^x^x'^x'^. Pleistocene sedimentary strata x ^ ^ ' ^ / \ \ V V \ \ V V x W x V W y 1
\\\ Fig. 4.19. Sparker seismic profiles and their interpretations from the shelf and trough region of the eastern South Sea, illustrating two different lowstand deposits (beachshoreface complex and lowstand deposits). For location see Fig. 4.18. The lowstand deposits (LSD) overlying the sequence boundary (SB) are well defined by seaward progradational reflection pattern, whereas the beach-shoreface complex (BSC) overlying the transgressive surface (TS) shows a chaotic or hummocky reflection pattern with some steeply inclined, accretionary internal reflectors. AT = acoustically turbid zone. Modified after Yoo (1997).
South Sea and East China Sea
171
-1 km-
0)
> CO
^ 60 o
RMD
Cut-and-fill structure
f f f f f f f / / f ^ f / ^
/
f
f
/
y
y
^
f
f
f
f
f
f
f
f
f
/
f
^
f
S''S^XV%'SVX^VS'^\''N'Pleistocene sedimentary Strata,,'^-',, V N ' N ' X V X ' ' S W > > >., > ^ '^ > ^ ^ > ^ \ ^
-^.jb.
\ ^ ^ >>^ >
Fig. 4.20. High-resolution uniboom profile and interpretation (for location see Fig. 4.18), showing three different deposits, recent mud deposits (RMD), transgressive sands (TSS), and transgressive estuarine/delta deposits (TED). RS = ravinement surface, SB = sequence boundary. Modified after Yoo (1997). is well preserved on the inner shelf east of Koje Island and off Namhae Island (Fig. 4.20). Its internal reflectors usually show obliquely inclined foreset bedding and a prograding pattern. A transgressive sand sheet, thinner than a few meters, blankets the transgressive deposits and wide areas of the mid-shelf (Figs. 4.18 and 4.20). In some cases, the sand sheet evolves into a series of large-scale sand ridges with seaward inclined internal reflectors, e.g., off Koje and Namhae islands (Fig. 4.18). These form a ridge-and-swale topography. The erosional ravinement surface is present between the transgressive estuarine/delta and transgressive sand deposits (Fig. 4.20). The cored sediments consist largely of moderately sorted, medium to fine sand (up to 90%) with shell fragments and gravels. Sands commonly contain iron-
172
Marine Geology of Korean Seas
stained quartz grains. The overlying muddy deposits occur in the coast-to-innermost-shelf region (Fig. 4.18), and are characterized acoustically by a variable degree of stratification. Internal reflectors are continuous and well stratified near the river mouth but become progressively weak and discontinuous seaward (Fig. 4.20). In other parts of the inner shelf, however, semi-transparent acoustic characters are common. The mud is poorly sorted with a mean grain size of 7-9 ([). Sand content is less than 5%. Cored sediments are either homogeneous, mostly from bioturbation, or laminated with silt-clay couplets.
CHAPTER 5
East Sea 5.1
Physiography
The East Sea (Sea of Japan) is a semi-enclosed marginal sea or back-arc basin surrounded by the east Asian continent and the Japanese Islands (Fig. 5.1). The seafloor has been mapped earlier by Terada (1934), Zenkevitch (1959, 1961), U.S. Naval Oceanographic Office (1969), and Mogi (1979). Average water depth is about 1,350 m and the maximum depth is about 3,700 m in the northeastern part. The sea is connected with the Pacific Ocean through shallow straits, i.e., Korea and Tsushima straits (140 m deep), Tsugaru Strait (130 m deep). Soya Strait (55 m deep) , and Tartar Strait (12 m deep). There are three deep basins (Japan, Yamato, and Ulleung basins) separated by submarine topographic highs such as the Korea Plateau, Oki Bank, and the Yamato Ridge that rise to within about 500 m of the sea surface (Fig. 5.1). The Japan Basin in the northern part of the sea is 200-300 km wide and 700 km long trending NE-SW. Its basin floor ranges in water depth from 3,500 to 3,700 m and the deepest portion is located between Sikhote-Alin and the southwestern part of Hokkaido. The southeastern part of the sea is occupied by the Yamato Basin trending NE-SW. It is shallower than the Japan Basin with an average water depth of 2,500 to 2,700 m (maximum of 2,970 m). The floors of the Japan and Yamato basins are rather smooth and flat except for a few seamounts and hills that rise up to 2,000 m above the surrounding basin floor. The Ulleung Basin (see Chapter 7) in the southwestern part of the sea is a bowl-shaped depression, 2,000-2,300 m deep, that is separated from the Japan and Yamato basins by the Korea Plateau and Oki Bank. Its basin floor gently deepens northward and is connected to the Japan Basin through the Ulleung Interplain Gap (Chough, 1983) between the Korea Plateau and Oki Bank (Fig. 5.1). The continental margins of Sikhote-Alin and northernmost Korea north of 40°N are characterized by a relatively narrow shelf and steep slope (Fig. 5.1). The southwestern margin is contiguous to the Korea Plateau which consists of two ridges (South Korea Plateau and North Korea Plateau). The South Korea Plateau is characterized by ridges, seamounts, and troughs. The North Korea Plateau is bounded in the north by the Wonsan Trough. The two ridges are partly separated by the Japan Basin. Ridges and troughs are also
174
Marine Geology of Korean Seas
45^N!
Fig. 5.1. Major physiographic features of the East Sea. Dots represent location of deepsea drilling (DSDP and ODP) sites including Leg 31 (Sites 299-302), Leg 127 (Sites 794-797), and Leg 128 (Sites 794, 798, and 799). TDSC = Toyama Deep-Sea Channel. Bathymetry in meters. UIG = Ulleung Interplain Gap. predominant near the eastern margin of the Yamato Basin and are aligned subparallel to northeastern Honshu. The Yamato Ridge in the central part of the sea also comprises several topographic highs, separated from each other by transverse depressions. A NE-SW-trending longitudinal depression, the Kita-Yamato Trough, divides the ridge into a northwestern part (KitaYamato Bank) and a southeastern part (Yamato and Takuyo banks). A deepsea channel, Toyama Deep-Sea Channel, runs north in the NE Yamato Basin (Fig. 5.1). Various canyons and channels are present along the entire margins.
East Sea
5.2
175
Crustal Structure
The crustal structure in the deep basins of the East Sea (Fig. 5.2) has been constrained with seismic reflection and refraction data (Kovylin and Neprochnov, 1965; Murauchi, 1966; Ludwig et al., 1975; Hirata et al, 1987; C.H. Park et al. ,1996). The upper crust (layer 2), mainly composed of basaltic sills and lava flows possibly interlayered with sediment layers (Shipboard Scientific Party, 1990), occurs at a depth of 4 km below sea level in the Ulleung and Yamato basins and at about 6 km below sea level in the Japan Basin (Fig. 5.2). It ranges in thickness from 1.5 to 4.0 km and is characterized by P-wave velocity of 3.5 km/s (consolidated sediments or green tuff) or 5.8 km/s, the latter being the basement rocks exposed on the adjacent margins. The lower crust (assumed layer 3) in the sea is characterized by apparent velocity in the range between 6.2 and 8.2 km/s (Fig. 5.2). In the Japan Basin, the lower crust is about 4 to 5 km thick and occurs at depths between 7 and 12 km below sea level (Fig. 5.3); the velocity structure deduced by ocean bottom seismometer (OBS) experiments in part of the basin is of typical
ULLEUNG BASIN
On
Cemral Northern p^ _ KurasNmo et al. (1993) (1996)
JAPAN BASrN
YAMATO BASIN Southern Northern Hirata etal. Ludwig etal. Shinohara (1989) (1975) etai.(1992)
MuraucN (1972)
Hiiataetal. (1991)
1.5
.;4tt;
IITT :2.^3.o
h5
10 H
h 10
15 i
h 15
20-' km
20 km water I;:;::;:::;:! sediment
^ ^ ^ layer2 R88883 layers
••"•«">""• Moho
Fig. 5.2. Crustal structure of the East Sea. Numbers denote P-wave velocity in km/s. After C.H. Park etal. (1996).
176
Marine Geology of Korean Seas JAPAN BASiN E 149 0)344-5 351-2 I T I L
YAMATO RIDGE
YAMATO BASIN 5
OKI RfDGE-TROUGH 150
151
7N
YAMATO BASIN SHELF
p
6S
@
Q>
0 km ' 3.0
Fig. 5.3. Schematic geologic structure (layers 1, 2, 3, and Moho) across the East Sea interpreted from reflection and refraction profiles. After Ludwig et al. (1975) by permission of the Geological Society of America, Inc. oceanic crust (Hirata et al., 1991). In the Yamato and Ulleung basins, the lower crust ranges in apparent velocity from 6.2 to 7.8 km/s, and lies at about 5 to 9 km below sea level (Ludwig et al., 1975; Hirata et al., 1987; C.H. Park et al, 1996) (Figs. 5.2 and 5.3). The depth to the upper mantle in these basins is 16 to 18 km below sea level, which is nearly twice as deep as that below normal oceanic crust. Due to the presence of anomalously thick sediment layer, the nature of the crust in the Yamato and Ulleung basins is still controversial (see Hirata et al., 1987; Tamaki et al., 1992; H.J. Kim et al., 1999; G.H. Lee et al., 1999). On the other hand, Yoshii (1973) estimated the depth to the mantle beneath the Yamato Ridge to be about 23 km based on gravity data.
5.3
Magnetic and Gravity Anomalies
The magnetic data compiled by Yasui et al. (1967), Isezaki and Uyeda (1973), Isezaki (1975, 1986), and Tamaki and Kobayashi (1988) define
East Sea
111
weak linear magnetic anomalies trending N60°E in the Japan Basin. The amplitude of the anomalies is smaller than 300 nT p-p and the wavelength is about one third of that in the western Pacific. Tamaki and Kobayashi (1988) correlated the magnetic polarity pattern with the dated magnetic anomaly time scale between 20 Ma (C6N) and 25 Ma (C7N). Anomalies 5E (19 Ma) through 7A (27 Ma) with a N70°E trend were later suggested following the Ocean Drilling Program (ODP) Legs 127 and 128 (Shipboard Scientific Party, 1990). The detailed magnetic anomaly pattern in the Japan Basin indicates complex pseudofault patterns similar to those generated by propagating spreading ridges (Tamaki and Kobayashi, 1988). On the other hand, the Ulleung and Yamato basins have no pronounced linearity in magnetic anomaly patterns, and instead show chaotic, high-frequency, lowamplitude anomalies (Isezaki, 1986). This is suggestive of laterally variable volcanic terrain rather than oceanic basement emplaced by typical seafloor spreading. The free-air anomaly in the sea is generally positive, 10 to 20 mgal, except for small minima near the foot of continental margin or ridge (Stroev, 1971; Ludwig et al., 1975; Joshima, 1978). This anomaly pattern suggests that the seafloor is in isostatic equilibrium and there is presently no oceanic spreading. Large positive anomalies of up to +50 mgal were also observed near such topographic highs as the Oki and Koshiki banks and the Korea Plateau. Negative anomalies (-20 mgal) were observed in the deeper part of the sea. In the Ulleung Basin and its southern margin near Korea Strait, the free-air anomaly is as low as -30 mgal (Joshima, 1978; Suh et al., 1993; C.H. ParketaL, 1996).
5.4
Heat Flow
Heat flow is relatively high (2.2-2.5 HFU on average) in the East Sea, particularly toward the south, reaching 3 HFU in the Yamato and Ulleung basins (Fig. 5.4). It diminishes over the intervening ridges between the basins (Yasui and Uyeda, 1972; Watanabe et al., 1977). The heat flow gradually decreases eastward to the Japanese Islands but drops abruptly along the continental margins of Korea and Sikhote-Alin. The high values are interpreted as a residual effect of an earlier phase of back-arc spreading, because no shallow earthquakes occur in relation to the suspected spreading ridges. As mentioned before, the positive free-air gravity anomaly suggests that the seafloor is in isostatic equilibrium and spreading has ceased. According to Schlanger and Combs (1975), the high heat flow in the basinal area gives rise to kerogen-hydrocarbon transformation in young and shallow strata. However, Kobayashi and Nomura (1972) identified
178 120°
Marine Geology of Korean Seas 130°
140°
150°E 50°N
Fig. 5.4. Heat flow values in the East Sea and adjacent areas. After Watanabe et al. (1977) by permission of the American Geophysical Union. ferromagnetic iron sulfides in cores from the sea, which suggests a stagnant environment during the glacial periods. They suggested that the oxidation of sulfides as well as sulfuration of oxides accounts for the high heat flow in the deep basins of the sea.
5.5
Age and Type of Crust
Parts of the basement rocks crop out in the Yamato and Kita-Yamato banks, Korea Plateau, and other seamounts. Age determinations of dredged basement rocks from these topographic highs (Hoshino and Homma, 1966; Ueno et al., 1971; Lelikov and Bersenev, 1975; Gnibidenko, 1979) reveal that they are composed both of old (generally >60 Ma) igneous and
East Sea
179
metamorphic rocks (granite, diorite, rhyolite, andesite, dacite, schist, and gneiss) and much younger (<60 Ma) sedimentary (sandstone and shale) and volcanic rocks (basalt and tuff). Metamorphic rocks of amphibolite facies, and samples of granite-gneiss dredged from the Korea Plateau north of the UUeung Basin, give Rb-Sr ages ranging from 2,729 to 1,983 Ma (Lelikov and Bersenev, 1975). K-Ar ages of the same samples turn out to be younger (180-250 Ma) which, according to Lelikov (cited in Gnibidenko 1979), is due to thermal events during granite intrusion in the Cretaceous causing Ar loss and isotopic resetting. K-Ar ages of granite and granodiorite dredged from the Yamato Ridge yield ages of 197 Ma, whereas 20 Ma is the average age for basalts and andesites in the region. From the Oki Bank slope Yuasa et al. (1978) reported the occurrence of biotite granite and gneiss, tuff and tuff breccia, and volcaniclastic sandstone that contain characteristic reddish brown olivine and volcanic glass. The age of andesitic and basaltic rocks dredged from the seamounts near northwestern Honshu ranges from 7.7 to 4.2 Ma (late Miocene to late Pliocene). Pliocene (4.6-2.5 Ma) basalts and trachytes are also reported on Dok Island of Korea (Y.K. Kim et al., 1987). Basements of the Japan and Yamato basins were penetrated during ODP Legs 127 and 128 (Tamaki et al., 1990). Drilling in the northeastern margin of the Japan Basin (Site 795) recovered massive to brecciated, calc-alkaline basalt with sparse phenocrysts of plagioclase and clinopyroxene, and basaltic andesite lava flows. In the Yamato Basin (Sites 794 and 797), two types of igneous-sedimentary rock suites were recovered. The upper suite consists of aphyric, brecciated to massive basaltic lava flows interbedded with sandstone. The lower suite consists of basaltic and doleritic sills of tholeiitic composition with plagioclase phenocrysts ranging from abundant to absent, which were intruded into and chilled against sandstone and tuffaceous sediments. The recovered volcanic rocks were analyzed by whole-rock stepwise-heating experiments for "^^Ar-^^Ar radiometric age dating (Kaneoka et al., 1992). Samples from the northeastern margin of the Japan Basin are consistent with crystallization ages of 24 to 17 Ma. In the Yamato Basin, "^^Ar-^^Ar analysis shows ages ranging from 21 to 18 Ma. Tamaki (1988) mapped the distribution of crust types on the basis of seismic refraction/reflection studies, bottom sampling from topographic highs, morphology of the basement and seafloor, magnetic anomalies, and heat flow. Fig. 5.5 shows that the oceanic crust is essentially limited to the eastern part of the Japan Basin, whereas the Ulleung and Yamato basins are floored by thinned or extended continental crust. On the other hand, Hirata et al. (1987), H.J. Kim et al. (1994, 1999), and G.H. Lee et al. (1999) have suggested that the Yamato and Ulleung basins also consist of oceanic crust, thickened due to the activity of a hot mantle plume or a perturbation of the thermal structure in the upper mantle. In the southern part of the East Sea,
180
Marine Geology of Korean Seas
48°N
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^m Contiental crust {S3 Bitted continental cru$t 45° h
CZ] Ext«icted continental crust
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145°E
Fig. 5.5. Crust types of the East Sea. The discrimination of crust types is based on seismic reflection/refraction data, bottom sampHng data, geomagnetic data, basement depth, and topography. Contours in meters. Modified after Tamaki (1988).
fragmented continental crust forms topographic highs (e.g., Yamato Ridge, Korea Plateau, and Oki Bank), which contrast markedly with the deep basin plain in the northern part. The areas of rifted continental crust surrounding the continental crust blocks are characterized by the ubiquitous distribution of ridges and troughs bounded by normal faults. These faults are remnants of the listric faults formed during extension of the continental crust. The fault patterns suggest that the Ulleung Basin is underlain by extended continental crust interlayered with volcanic sills.
East Sea
5.6
181
Stratigraphy
Isopach maps of the East Sea compiled by Gnibidenko (1979), Ishiwada et al. (1984), Tamaki (1988), and Chough and Lee (1992) indicate that thick sediment sequences (1,500 to 3,000 m) overlie acoustic basement in the basinal areas. The sediment thickness is more than 2.0 s (twt) in the Japan Basin, up to 1.6 s (twt) in the Yamato Basin, and 2.8 s (twt) in the Ulleung Basin (Fig. 5.6). Extensive single-channel reflection profiles, together with stratigraphic data from the Deep Sea Drilhng Project (DSDP) and ODP cores, piston cores, and dredges, indicate that the stratigraphic sequences are consistent in most basinal areas, despite significant differences in overall sediment thickness (Karig et al., 1975; Ludwig et al., 1975; Honza, 1979; Tamaki et al., 1978). In the deep basins, the sequences consist of undeformed Quaternary sediments, up to 1 km thick, underlain by slightly deformed Pliocene and Miocene sediments. 5.6.1
Seismic Stratigraphy
The sedimentary sequences in the East Sea are characterized by an upper well-stratified and highly reflective unit (former opaque layer) and a lower transparent to moderately-reflective unit (Fig. 5.7). The upper unit consists mostly of late Pliocene to Holocene siliciclastic sands, silts, and/or clays, and is characterized by well-defined reflectors traced over tens of kilometers (Fig. 5.7). Generally, the upper unit is horizontally stratified except near the ridges and troughs and ranges in thickness from 100 to 1,000 m in the Japan and Ulleung basins (Gnibidenko, 1979). In the central part of the Ulleung Basin south of Ulleung Island, it is about 600 m thick (Tamaki et al., 1978). Here, the top of the unit is characterized by a smooth surface, but becomes irregular and tilted toward both the northwestern and southeastern margins. The tilting is gentle toward the southeast but rather abrupt toward the northwest. Piston cores collected by Honza et al. (1978) reveal that the uppermost part of this unit consists predominantly of thinly-laminated turbidite sequences alternating with minor amounts of hemipelagic sediments (Chough, 1982). The lower unit is composed dominantly of pelagic sediments, mainly diatomaceous oozes which ubiquitously drape over acoustic basement in most areas of the sea and are remarkably uniform in character (Karig et al., 1975; Tamaki et al., 1990). Drilling at DSDP Site 301 confirmed that acoustically transparent subunits in the upper portion of the lower unit are correlative with the middle to late Miocene diatomaceous deposits in a number of Neogene sequences along the coasts of Honshu and eastern Korea near Pohang (Ingle, 1975). The unsampled older and seismically more
182
Marine Geology of Korean Seas
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184
Marine Geology of Korean Seas
reflective and stratified subunit above acoustic basement is thought to include diagenetically altered biogenic siliceous and calcareous sediments similar to and correlative with the porcellanites and hard siliceous shales of the Miocene Onnagawa and Funakawa formations of Honshu (Ingle, 1975; Suzuki, 1979; Kuramoto et al., 1992). 5.6.2
Lithostratigraphy
During ODP Leg 127, a complete sequence from the lower Miocene to Quaternary was recovered at 4 drilling sites (797, 794, 795, and 796) in the deep basinal part of the East Sea (Fig. 5.1). Tamaki et al. (1990) described the entire sequence and identified 6 lithologic units (Fig. 5.8): (1) alternating basaltic sills and flows and volcaniclastic sandstones and siltstones (>300 m thick) (lower to middle Miocene), (2) middle Miocene calcareous and phosphatic claystone (100-200 m thick) and welded dacitic tuff (up to 20 m thick), (3) upper middle Miocene to lower upper Miocene chert and siliceous clay (80-150 m thick), (4) upper Miocene diatom clays (ca. 100 m thick), (5) uppermost Miocene to Pliocene diatom oozes (100-150 m thick), and (6) Quaternary clays (80-100 m thick). The chronostratigraphy of the sequence was established by Burckle et al. (1992) based on the analysis of siliceous microfossils (i.e., diatoms, radiolarians, silicoflagellates, and ebridians). The estimated sedimentation rates in the basinal area range from 5 to 77 m/m.y. (Burkle et al., 1992). The upper 5 hemipelagic units (Fig. 5.8) were described in more detail by Tada and lijima (1992). Unit 1 (Pliocene to Quaternary) is characterized by decimeter- to meter-scale alternation of laminated, dark-colored silty clay and bioturbated, light-colored silty clay with abundant intercalations of thin ash layers. The dark layers are richer in organic matter and pyrite than the light layers. Low and fluctuating carbon/sulfur (C/S) ratios and the sedimentary structures suggest that the dark-light cycles reflect the fluctuation of bottom-water oxygenation level between oxic and anoxic or even euxinic conditions. The sediments are poor in biogenic silica, and the terrigenous component is characterized by a high and slightly variable Ti/Al ratio and abundant quartz, plagioclase, illite, and kaolinite as well as chlorite contents, which indicate a significant contribution of eolian materials. Unit 2 (upper Miocene to Pliocene) consists of heavily bioturbated and mottled diatom ooze and diatomaceous clay and their diagenetic equivalents. The lithology is nearly homogeneous throughout the sediment column, except for sporadic intercalations of yellowish brown carbonate layers and volcaniclastic sand layers. The C/S ratio of the unit is comparable to that of normal marine oxic sediments. Unit 3 (upper Miocene to Pliocene) comprises bioturbated diatom clay.
East Sea SITE 797 (Yamalo Basin) ra WD = 2862 m 0
SITE 794 (Yamato Basin) WO = 2811m
SITE 795 (Japan Basin) WD = 3300 m
185
SITE 796 (Okushiri Riclge) WD « 2571 m
TD. 762 m
TD. 903 m
Fig. 5.8. Columnar sections of the ODP cores of Leg 127 (for drilling location see Fig. 5.1). Modified after Tamaki et al (1992).
faintly-laminated siliceous claystone, and silty claystone with thin chert layers and minor calcareous layers and nodules. The unit is characterized by alternations of faintly-laminated dark layers and moderately- to heavilybioturbated light layers. The mode of bioturbation together with the relatively low and variable C/S ratio suggests a fluctuating bottom-water oxygenation level between oxic and anoxic or euxinic conditions. Unit 4 (middle to upper Miocene) is characterized by alternations of dark chert and light siliceous shale and laminated to weakly bioturbated siliceous claystone. The C/S ratio of the sediments is generally within the range of normal marine sediments, resulting from reactive iron limitation of pyrite formation caused by the dilution effect of biogenic silica input. The amount of diagenetic silica is relatively high, generally between 20 and 60% by weight.
186
Marine Geology of Korean Seas
Unit 5 (middle Miocene) is composed of dark claystone with carbonate micronodules and stringers and horizontal burrows. The C/S ratio for the claystone is close to the normal marine ratio and its diagenetic silica content is generally low. Subaqueous tuff layers as thick as 20 m are abundantly intercalated in the unit. The composition of the claystone is characterized by a high Mg/Al ratio and high smectite content, which may reflect the contribution of pyroclastic materials.
5.7 5.7.1
Tectonic Evolution Tectonic Origin
The East Sea principally lies on the eastern margin of the Eurasian Plate (or Amurian microplate) bounded on the east and south by the Pacific and Philippine Sea plates, respectively (Fig. 5.9). The hypotheses on the tectonic origin of the East Sea generally focus on back-arc spreading by south- to southeastward migration of the Japanese Arc in the late Oligocene to middle Miocene. Uyeda and Miyashiro (1974) and Hilde et al. (1976) suggested that the East Sea was formed by extension as a result of collision and subduction of the hypothetical Kula-Pacific Ridge from the late Cretaceous to Oligocene. Uyeda (1979) among many others proposed that the sea was rifted by a tensional force, common in Mariana-type subduction, caused by the heat produced from small-scale convection in the asthenosphere wedge located above the subducting lithosphere. Earlier, Bersenev (1971) and Karig (1971) proposed a hypothesis similar to that above but instead suggested a discontinuous diapiric rise of mantle material from the subducting oceanic crust. Independent of the plate-boundary processes, Miyashiro (1986) suggested that back-arc spreading in the East Sea resulted from a mantle plume associated with a migrating hot region. Recent studies on the origin of the East Sea, however, commonly adopt the kinematic spreading tectonics (Dewey, 1980) suggesting extensional inter-plate deformation caused by relative motion between the Pacific and Eurasian plates (Lallemand and Jolivet, 1985; Jolivet, 1986; Kimura and Tamaki, 1986; Tamaki and Honza, 1991; Tamaki et al., 1992). In particular, Kimura and Tamaki (1986) attributed the opening of the East Sea to the India-Eurasia collision that caused widespread deformation of the Eurasia continent since the late Eocene. The stress of continental collision and deformation was transmitted through the movements of several microplates between the Himalaya and the Okhotsk Sea (Zonenshain and Savostin, 1981), resulting in NNE retreat of the Amurian Block from the Pacific and Philippine Sea plates, eventually causing continental rifting and the subsequent spreading in the East Sea region (Fig. 5.10).
East Sea
187
Fig. 5.9. Distribution of tectonic plates around the East Sea region. Plate boundaries are marked by deep-sea trenches.
5.7.2
Opening Mode
The hypotheses on the opening mode of the East Sea include (1) a fanshaped opening model implying differential rotation of the Japanese Islands (Otofuji et al., 1985; Celaya and McCabe, 1987; Faure and Lalevee, 1987) and (2) a pull-apart opening model involving two subparallel shear zones in the eastern and western margins of the sea (Lallemand and Jolivet, 1985; Jolivet et al., 1989). Paleomagnetic data from the Cretaceous to Miocene igneous and sedimentary rocks in the Japanese Islands suggest that SW Japan rapidly rotated between 45° and 56° clockwise during middle Miocene (15-12 Ma), while NE Japan underwent more progressive counterclockwise rotation by more than 45° in early to middle Miocene time (Otofuji et al., 1985, 1991; Otofuji and Matsuda, 1987; Tosha and Hamano, 1988). The model based on paleomagnetic data suggests that this differential rotation occurred about two pivot axes at either ends of the Japanese Islands and resulted in fan-shaped opening of the East Sea. On the other hand, Lallemand and Jolivet (1985) proposed a pull-apart opening model based on onland and offshore structural data. According to this model, the East Sea opening was initiated by southward pull-apart movement of the Japanese Islands detached from the Korean Peninsula and Sikhote-Alin. The pull-apart opening was guided by two regional-scale
188
Marine Geology of Korean Seas
Sredinny Range
Fig. 5.10. Middle Tertiary microplate movement in the east Asian continent. IndiaEurasia collision resulted in NE retreat of the Amurian Block, which in turn gave rise to back-arc opening of the East Sea and Kuril Basin. After Tamaki (1988). right-lateral strike-slip fault systems; the w^estem fault lay along the eastern margin of the Korean Peninsula and the eastern fault along the w^estem margin of NE Japan and Sakhalin. Later, the pull-apart opening model v^as modified and made more sophisticated, based on the results of ODP Legs 127 and 128 (Jolivet and Tamaki, 1992; Jolivet et al., 1995). In the modified model, the pull-apart opening is accommodated in part by differential rotation of the Japanese Islands, but the rotation is less than 30° and more progressive (Fig. 5.11). The eastern shear zone extends more than 2,000 km from central Japan to northern Sakhalin (Fig. 5.11). Yoon et al. (1997) suggested that the Ulleung and Tsushima faults (see section 6.2.1) acted as a western counterpart of the pull-apart fault system.
East Sea
189
Fig. 5.11. Schematic summary of the East Sea opening based on the results of ODP drilHng. Seafloor spreading and crustal extension were guided by two right-lateral strike-slip fault systems at the eastern and western margins of the sea, and in part by differential rotation of the Japanese Islands. The spreading center in the Japan Basin propagated westward to increase the area of the oceanic crust. After Jolivet and Tamaki (1992).
5.7.3
Tectonic History
According to the results of ODP Legs 127 and 128 (Ingle, 1992; Tamaki et al., 1992), the East Sea opened between 32 and 10 Ma accompanied by rapid subsidence and vigorous volcanism. Crustal thinning and initial subsidence took place in the late Oligocene (>23 Ma) at rates compatible with simple thermal decay of the crust (Ingle, 1992) (Fig. 5.12). The initial subsidence was accompanied by widespread deposition of non-marine sediments and volcaniclastic deposits (Tamaki et al., 1992). This juvenile phase was followed by two periods of accelerated subsidence in the early Miocene (23-19 Ma) and middle Miocene (15-12.5 Ma) at rates (>900 m/m.y.) far exceeding those expected for thermal subsidence (Ingle, 1992) (Fig. 5.12). This rapid subsidence phase is correlated with the initial appearance of bathyal marine sediments in the back-arc area (W.H. Kim, 1990; Sato et al., 1991; Tamaki et al, 1992), and with radiometric ages of basalts and dolerites sampled from the top of the basement (Kaneoka et al., 1992). The accelerated basin subsidence reflects a mechanical or fault-controlled opening phase of the sea accompanied by seafloor spreading in the Japan
190
Marine Geology of Korean Seas Extension Fast spreading and subsidence arc rotation
- Compression - • Accelerating deformation of arc and basin
Age (Ma)
Fig. 5.12. Summary of tectonic (residual) and total subsidence patterns in the East Sea back-arc area, derived from back-stripping analysis of five stratigraphic sections. The envelope of maximum and minimum limits of vertical motion represents extremes defined by individual subsidence curves. Average total and tectonic subsidence/uplift were calculated by averaging the slopes of individual curves for each 2.5 m.y. interval. After Ingle (1992). Basin and rapid crustal extension/thinning in the Ulleung and Yamato basins (Tamaki et al., 1992). The initial seafloor spreading occurred in the northeastern part of the sea, triggered by breakup of the lithosphere along the eastern strike-slip margin; whereupon the spreading center propagated southwestward into the area of continuing crustal extension and generated a basin floored by oceanic crust (Jolivet and Tamaki, 1992) (Fig. 5.11). During the opening of the Japan Basin, the NE Japanese Islands migrated southeastward more than 400 km (Jolivet and Tamaki, 1992). This motion was accommodated by dextral shear movement along the eastern margin of
East Sea
191
the East Sea, and in part by counterclockwise rotation. At the same time, crustal extension/thinning occurred in the Ulleung and Yamato basins as the SW Japanese Islands drifted southward with a minor clockwise rotation. This block movement resulted in dextral strike-slip movements along the Ulleung and Tsushima faults. The final phase of thermal subsidence occurred between about 12.5 and 10 Ma, just prior to the initial indications of uplift between about 10 and 7 Ma (late Miocene) (Ingle, 1992) (Fig. 5.12). This latter event apparently signaled a major change in regional tectonic regime from tensional to compressional. Rates of uplift began to accelerate at about 5 Ma with rates of Pliocene-Pleistocene uplift commonly exceeding 500 m/m.y. This compressional regime is dramatically illustrated by folding and thrust faulting in the southern margin of the Ulleung Basin (Chough and Barg, 1987) and by obduction along the eastern margin of the Japan Basin (i.e., Okushiri Ridge) (Tamaki, 1988).
5.8 5.8.1
Surface Sediments General Statement
The distribution of surface sediments in the East Sea was compiled by Skomyakova (1961), Kaseno (1972), and Repechka (1973) (Fig. 5.13). Sediments may be classified into gravel, sand, silt, silty clay, and clay. Pelagic components are composed mainly of diatoms (Hasegawa, 1970; Koizumi, 1970, 1978) and subordinate silicoflagellates (Shitanka et al., 1970). Foraminifera (and thus CaCOa) in the deep basins are rather rare (Asano, 1957; Kozak, 1974; Ichikura and Ujiie, 1976), also due to the shallow carbonate compensation depth (CCD) of about 2,000 m caused by deep circulation of highly oxygenated water (Niino et al., 1969). Terrigenous components of the hemipelagic sediments in the deep basins originate from the Asian continent and Japanese Islands, and are highly oxidized and brownish in color. The influence of the Tsushima Current has also been suggested in the transport of fine-grained materials from the southwest (Aoki etal., 1974). 5.8.2
Distribution
Sandy and gravelly sediments occur along the shallow portion of the sea, some of which were transported by ice to the northern part of the sea (Fig. 5.13) (Skomyakova, 1961; Niino and Emery, 1966). Sandy gravels on the Yamato Ridge are composed of rock fragments of volcanic origin including diabase, gabbro-diabase, porphyrite, andesite, and andesite-basalt. Gravels of
192
Marine Geology of Korean Seas
48°
440
128°
132°
136°
140°
Fig. 5.13. Grain size distribution of surface sediments in the East Sea. Modified after Skomyakova (1961).
basic and alkali extrusive rock and some amounts of granite and quartz porphyry also occur (Fig. 5.13) (Ueno et al., 1971). Sandy sediments are confined mainly to a depth less than 70 m on the shelf, but also occur extensively along the Korea Strait. These sediments are usually noncalcareous (0.15-3.04% CaC03). Calcareous shell sands are found in the Korea Strait and along the coast of Japan where carbonate content is more than 15%. Calcareous foraminiferal sands occur in abundance (up to 30%) on the Yamato Ridge and Korea Plateau, and are dominated by Globigerina glutinata, Globigerinoides rubescens, and Globigerinoides ruber (Kozak, 1974). Coarse silts are usually green and greyish green in color, and contain abundant shell fragments. They occur mainly on the shallov^ portions of the sea (i.e., shelf and the upper slope), and also near the islands along the coast
East Sea
193
of Japan (Fig. 5.13). Carbonate content in silty sediment ranges from 0.3 to 2.1%. Fine silt and clay (0.05-0.01 mm) are dominant in the lower slope and the deep basin floor, as well as in some coastal embayments. They are generally low in carbonate content (0.13-1.56%). In the northern part of the sea, the silty clay contains abundant plant fragments and siliceous algae. In contrast to the Yellow and South seas, montmorillonite occurs in abundance (up to 25%)) in the East Sea (Niino et al., 1969; Aoki and Oinuma, 1973; Aoki et al., 1974). This is attributed to the influx of weathering products of volcanic, igneous, and pyroclastic rocks and soil on the adjoining margin. Kaolinite is less abundant (up to 15%)) than illite (up to 50%)). Chlorite content is about 30%o. It is believed that the large amounts of illite originate from the Korea Strait (Han, 1979) and partly from eolian transport by the jet stream. The kaolinite content is about average for the north Pacific region. 5.8.3
Geochemical Composition
Coarse-grained and calcareous sediments on the shelves and banks contain minor amounts of organic carbon (Niino and Emery, 1966), whereas the fine-grained, slightly calcareous sediments on the slope contain relatively large amounts, up to 2% (Fig. 5.14) (Solov'ev, cited in Strakhov, 1962; Niino et al., 1969). In the basin floor, yellowish brown muds are less calcareous (<1%) CaCOa) and contain less than 1%) organic carbon. Chemical composition of the sediments, excluding biogenic amorphous silica and carbonate, was determined by Sakanoue et al. (1970) and Repechka (1973), and shows that silica content is generally high (up to 70%o) in sandy sediments, whereas it is less than 50%) in pelitic oozes. CaO (2.41.2%o) and total alkali elements are also low in the pelitic sediments, whereas AI2O3 (11.7-17.6%), MgO (1.2-2.6%), Fe203 (2.7-3.7%), P2O5 (0.080.34%)), and MnO (0.04-0.73%o) are generally abundant in the fine-grained sediments. In volcanogenic sediments, the amounts of silica, magnesia, total iron, MnO, CaO, AI2O3, and Ti02 are generally high compared with those in sandy sediments. The relative amounts of total iron plus magnesia compared with those of Na20 and K2O in the East Sea sediments are shown in Fig. 3.14. East Sea sediments fall largely in the high ferromagnesia portion of the plot, characteristic of a tectonically active setting.
5.9 5.9.1
Late Quaternary Sediments Lithology
Ichikura and Ujiie (1976) examined in detail 26 piston cores from the East
194
Marine Geology ofKorean Seas [
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Fig. 5.14. Distribution of organic carbon (%) in surface sediments of the East Sea. Data are compiled from Solov'ev (cited in Strakhov, 1962), Niino et al. (1969), and G.H. Lee (1983). Sea taken aboard RfV Vema and R/V Robert Conrad. In the three deep basins, homogeneous and laminated clays are dominant, whereas either diatomaceous clay or ooze prevails on the ridges. The topmost sediment ( 0 2 m) is usually homogeneous brown clay, whereas the lower portion of the cores is commonly laminated. On the Yamato Ridge, however, the clayey sediments are replaced by sandy clay composed of quartz, volcanic glass, glauconite, mica, and some mafic minerals. The sandy clay is usually barren of biogenic components. Homogeneous brown clay is rich in diatoms with subordinate amounts of radiolarians, volcanic glass, and minute quartz grains. Foraminiferal sands occur in the laminated units (turbidites), suggesting that they were transported from the shallower part of the sea by turbidity currents.
East Sea
195
Other sediment types found in the sea include sandy clay, sands, and glauconitic sands, the latter of which are found exclusively on the Yamato Ridge. Found exclusively in the deep basins, the sands contain angular to subangular quartz, igneous rock fragments, and plant fragments. Microfossils such as foraminifera, sponge spicules, and ostracods also occur in the sands. Sands containing abundant shell fragments are widely distributed in the southern margin of the Ulleung Basin. Volcanic ash layers are abundant in the sea and contain light olive grey (5Y6/1) volcanic glasses with minor quartz, pyrite, and other mafic minerals. Manganese micronodules indicative of oxidizing conditions were found in surface sediments in parts of the sea deeper than about 2,100 m. Tiny framboidal pyrite (3-25 mm in size) aggregates of octahedral form also were found in foraminiferal tests, radiolarian tests, diatom frustules, or in other tubular organisms in the pre-Holocene sediments. 5.9.2
Holocene-Pleistocene Boundary
In three deep basins of the East Sea, ^^C dates on the lithologic boundary between the upper homogeneous hemipelagic clay and the lower laminated turbidite mud yield ages of about 9 ka. Ujiie and Ichikura (1973), Ichikura and Ujiie (1976), and Arai et al. (1981) found that the coiling pattern of Neogloboquadrina pachyderma changes from left to right across this lithologic boundary. This is also accompanied by the extinction of Globigerina umblicata which Ujiie and Ichikura (1973) ascribed to a change from a reducing environment to an oxidizing one across the boundary. This interpretation was substantiated by an abrupt increase of benthonic foraminifera resulting from an increase of oxygenated bottom water in Holocene time. Manganese micronodules are associated with this environment, whereas framboidal pyrites occur below the boundary when the sea was more isolated from the open ocean and thus was a reducing environment. The depth to the Holocene-Pleistocene boundary differs not only regionally, but also depending on the identifying method applied. It ranges from less than 1 m below the seafloor in the Japan Basin (Koizumi, 1970) to about 2.8 m in the Yamato Basin. In the Ulleung Basin, it occurs at about 2 m (Chough, 1982). The accumulation rate of hemipelagic sediments above the boundary averages about 12 cm/ky, which is substantially higher than the rate (1.5 cm/ky) determined by the lo/Th method employed earlier by Miyake et al. (1968). The data on diatoms and foraminifera (Koizumi, 1978) agree with the former. Detailed works on volcanic ash in core sediments (Mizuno et al., 1972; Arai et al. 1981) indicate that there are several ash layers that are widespread
196
Marine Geology ofKorean Seas
and can be correlated with each other. They include Akahoya ash from Kikai Caldera, Kyushu (ca. 6.3 ka), Aira-Tn ash from Aira Caldera (ca. 21-22 ka), and Aso-4 ash from Aso Caldera (ca. 50 ka). In the central and southeastern part of the sea, the marker layers include Ulleung (ca. 9.3 ka) and Yamato ashes (ca. 25-35 ka) from Ulleung Island (Chough, 1982).
5.10
Late Quaternary Paleoceanography
The late Quaternary paleoceanographic conditions of the East Sea have been documented by multidisciplinary studies on lithology, CaCOa and organic carbon contents, oxygen and carbon isotope ratios, and microfossil assemblages (Ujiie and Ichikura, 1973; Kato, 1978; Koizumi, 1989; Aral et al., 1981; Oba et al., 1991). The studies of oxygen isotope ratios in benthic and planktonic foraminiferal tests (Aral et al., 1981; Oba, 1984) suggested that the water mass in the sea was relatively constant (salinity of 33-34%o) with relatively low temperatures (8-10°C) during the period between 85 ka and 23 ka, influenced only by minor amounts of sea water from the Yellow and East China seas. A decrease in salinity (about 28%o) accompanied by an abrupt decrease in 5^^0 values of planktonic foraminifera tests between 22 and 20 ka indicates an inflow of fresh water probably from the Huanghe River in China (Oba et al., 1991). Between 20 and 16 ka, a benthic foraminiferal fauna presently existing in the northwestern Pacific Ocean appeared for the first time in the East Sea, and the planktonic 6^^0 values became heavier (Oba et al., 1991). These data suggest an inflow of Pacific waters at this time. At about 11-8 ka, warm water (7-8°C) species of foraminifera appeared accompanied by the appearance of dextral-coiling Neogloboquadrina pachyderma (Ujiie and Ichikura, 1973; Kato, 1984). The latter was also observed by Kato (1978) in four cores from the Ulleung Basin in which the dextral form replaced the sinistral form at about 10 ka, corresponding to the climatic change from glacial to interglacial. Various lines of evidence presented in the foregoing sections suggest that the East Sea was dominated by cold surface water and anoxic bottom conditions throughout the late Pleistocene glacial period. The water in the sea was most likely stagnant during this time with a restricted exchange of surface waters with the Yellow and East China seas (Oba et al., 1991). The reducing bottom water environment probably permitted pyrite and sulfide minerals to form on the sea bottom. At the end of the last glacial maximum (20-16 ka), the cold Pacific water flowed into the East Sea through the Tsugaru Strait between Honshu and Hokkaido, reestablishing deep-water ventilation. The influx of the Pacific water mass indicates that the eustatic sea level during the last glacial maximum was above the sill depth (130 m
East Sea
197
below the present sea level) of the Tsugaru Strait (Oba et al., 1991). Between 11 and 8 ka, postglacial sea-level rise resulted in a subsequent inflow of a warm water mass (Tsushima Current, a branch of Kuroshio) through the Korea and Tsushima straits flowing northeastward along the Japanese side of the sea. Winter cooling in the north end of the sea caused the formation of the bottom water mass resulting in the high concentration of soluble oxygen in the deep basins (Niino et al., 1969). Due to respiration and oxidation, this bottom water is progressively enriched in CO2. As a result, the CCD is very shallow and the hemipelagic sediments are deficient in calcareous organic remains (Ichikura and Ujiie, 1976).
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CHAPTER 6
Eastern Continental Margin 6.1
Physiography
The eastern shoreline of the Korean Peninsula is largely straight and lacks embayments except Yongil Bay near Pohang (Fig. 6.1). The coastal region is characterized by a number of late Quaternary terraces that are classified into 4 to 6 groups according to the elevation, ranging from 3 to 130 m above sea level (S.W. Kim, 1973; Oh, 1977; D.Y Lee and Kim, 1991). Individual terraces are 2-40 m thick, and are either abrasional or depositional in origin. The depositional terraces generally consist of unconsolidated sands and well-rounded gravels. ^"^C ages on peat and charcoal fragments from two terraces suggest an uplift rate of 1.1-1.4 mm/yr. At this rate, the highest terrace (90-130 m above sea level) on the east coast could have formed since the last interglacial period. Difference in the elevations of these terraces may be due to differential rates of uplift or faulting during epeirogenic deformation. The eastern continental shelf to the north of 35°45'N is dominantly flat and narrow (<20 km wide) and abruptly drops off into the steep continental slope at water depth of 130 to 150 m. Between 36^20' and 37°20'N, the continental slope is characterized by a submarine ridge (Hupo Bank) and a trough (Hupo Trough), which are aligned parallel to the shoreline (Fig. 6.1). The Hupo Bank is about 100 km long and varies in width from less than 1 to 14 km. Its top is relatively flat and lies between 10 and 200 m below sea level. The Hupo Trough is about 230 m deep at the eastern boundary and shoals gradually landward. To the east of the Hupo Bank, the slope gradient gradually increases downslope to 8°. In the absence of significant terrigenous sediment input from fluvial systems, the slope lacks distinctive submarine canyons and channels. Instead, the slope has been dominantly shaped by large-scale slope failures and associated mass-flow deposits (Chough et al., 1991b; H.J. Lee et al., 1991). The failure scars generally occur at water depth between 300 and 1,500 m, and are usually accompanied by failed sediment masses downslope. The continental slope abruptly passes into the flat basin floor of the Ulleung Basin at water depths of 1,500 to 2,100 m. North of 37°20'N, the continental margin trends NNW-SSE and is bounded on the east by the Korea Plateau that is characterized by irregular
200
Marine Geology of Korean Seas
Fig. 6.1. Bathymetric map of the eastern continental margin of Korea. Contour interval is 100 m. Location of Dolgorae-1 well and seismic profiles (Figs. 6.3, 6.4, 6.7, and 6.8) is also shown. Bathymetric data are compiled from H.J. Lee et al. (1993), Yoon (1994), and KORDI (1993, 1995).
Eastern Continental Margin 201 topography including ridges, seamounts, and troughs. The slope gradient is generally less than 3° and increases northward; the northern steeper slope meets a trough at about 1,200 m depth which extends northward into the Japan Basin. Small transverse channels and gullies extend from the shelfbreak to depths of about 800 m (HJ. Lee et al., 1991). Mass-failure scars are distinct along the slope area deeper than about 500 m in water depth and slide/slump deposits extend further downslope, showing rugged morphology (Chough et al, 1991b). The southeastern margin (south of 35°45'N) of the Korean Peninsula is characterized by broad shelf and slope between southeast Korea and southwest Japan (Fig. 6.1). The shelf is typically smooth gently dipping northward, but is occasionally incised by broad channels or troughs trending NE-SW. The shelfbreak generally occurs at water depth between 300 and 400 m, where the shelf is transitional to a broad northward-dipping slope. The relatively steep upper to mid slope (300-1,400 m depth) exhibits a complex morphology of gullies and slump scars except for the westernmost part of the slope (Fig. 6.1). The gullies commonly start near the shelfbreak, and progressively widen downslope. On the gentle lower slope (> 1,000 m depth), slide/slump and debris-flow deposits are ubiquitous giving rise to a low-relief seafloor topography (H.J. Lee et al., 1993).
6.2 6.2.1
Geologic Structures Ulleung Fault
The Ulleung Fault is a deep-seated, basement fault which runs N - S to NNE-SSW along the western margin of the Ulleung Basin (Fig. 6.2). It forms a major boundary fault system of the East Sea that was activated as a strike-slip fault during the Miocene back-arc opening of the sea (Yoon et al., 1997; S.J. Park, 1998; see section 7.5). The major displacement zone of the fault occurs as a buried, eastward-dipping basement escarpment (Yoon, 1994; Yoon et al., 1997). This feature generally lies deeper than 1,500 m below sea level underlain by a thick sediment sequence in the lower continental slope and basin margin (Fig. 6.3). Although the escarpment locally shows a subdued relief marked by small-scale hyperbolic reflectors, it is generally characterized by a steep (>30°) and smooth topography. In the southwestern margin of the Ulleung Basin, the fault movement along the basement escarpment is evident from the sequence geometry (Yoon et al., 1997) (Fig. 6.4). On the hanging wall block, the upper part of the lower Miocene sequence forms a wedge-shaped sediment package, in which internal reflectors frequently merge or converge away from the fault plane. In particular, the lower strata of the wedge-shaped sediment
202
Marine Geology of Korean Seas 38°N STRIKE-SLIP FAULT NORMATL FAULT REVERSE FAULT
35°N
50 km 129°E
130°
13fE
Fig. 6.2. Major geological structures of the eastern margin of Korea. DTB = Dolgorae Thrust Belt, HF = Hupo Fault, PYB = Pohang-Yongduk Basin, TF = Tsushima Fault, UF = Ulleung Fault, YF = Yangsan Fault. Modified after Yoon (1994) and Yoon et al. (1997).
Eastern Continental Margin
203
Fig. 6.3. Airgun profile and interpretation across the eastern margin of Korea, showing geologic structures and stratigraphy. For location see Fig. 6.1. BE = basement escarpment (Ulleung Fault), DA = debris apron. After Yoon and Chough (1995) by permission of the Geological Society of America, Inc.
204
Marine Geology of Korean Seas NW
5.5 Ma
Fig. 6.4. Multichannel seismic profile and interpretation illustrating the Ulleung Fault (for location see Fig. 6.1). Shaded part highlights sediment wedge deposited during fault movement. After Yoon et al. (1997). Profile courtesy of the Korea National Oil Corporation. package are progressively tilted toward the fault plane producing a reversed dip relative to the basinward sequences. This sequence geometry is interpreted as a hanging-wall rollover reflecting more or less continuous extensional faulting along the basement escarpment. With multichannel seismic data, Yoon et al. (1997) and S.J. Park (1998) mapped the fault trace further south and connected it with the well-defined Tsushima Fault (Fig.
Eastern Continental Margin 205 6.2). This observation leads to an inference that the regional-scale fault extends along the entire western margin of the Ulleung Basin from the southwestern tip of the Korea Plateau to the south of the Tsushima Island. 6.2.2
Hupo Fault
The Hupo Fault is the most prominent deformation structure in the eastern continental margin. The fault consists of three segments which can be traced for more than 140 km along the upper slope (Fig. 6.2). The northern segment between 36°30' and 37°20'N runs along the western foot of the Hupo Bank and offsets both acoustic basement and overlying Miocene and lower Pliocene sedimentary sequences (Fig. 6.3). Vertical displacement of the fault reaches approximately 1,000 m in the center of the fault trace and decreases to less than 100 m near the northern termination of the fault. The main fault zone is often accompanied by narrow (3-4 km wide) contractional deformation zone in the upthrown block. This deformation zone is characterized by several anticlinal folds and eastward-dipping thrust faults which are oblique to subparallel to the main fault trace. South of 36''30'N, the Hupo Fault bifurcates into two curvilinear branches (Fig. 6.2). The eastern branch (SE segment) can be traced southward by narrow horsts along the shelfbreak, where it offsets the acoustic basement and the overlying Miocene sequences with only minor vertical displacement. On the other hand, the western branch (SW segment) of the Hupo Fault appears as a buried basement fault bounding the early Miocene PohangYongduk Basin (Fig. 6.2). This segment dips steeply (50-75°) westward with a normal sense of offset. Based on these contrasting features, Yoon and Chough (1995) proposed three episodes of the fault movement: extensional deformation in the early Miocene, and two events of contractional deformation in the late Miocene and early Pliocene. Furthermore, they suggested that the fault movements in the early and late Miocene were mainly caused by strike-slip shear deformation. The evidence includes a narrow and laterally-persistent curvilinear trace with a steep fault plane and an offset involving the basement, and en echelon faults/folds along one or both sides of the coeval master fault (Fig. 6.2). 6.2.3
Yangsan Fault
The Yangsan Fault runs N-S to NNE-SSW for a distance of 140-170 km along the southeastern coast of the Korean Peninsula (Fig. 6.2). The fault plane plunges eastward with a very steep dip gradient (>80°) and the eastern block is apparently down-dropped (Y.H. Kim and Lee, 1988). South of
206
Marine Geology of Korean Seas
Pohang, the main fault zone is up to about 1 km in width, and appears severely fractured by numerous subparallel faults (Chae and Chang, 1994). North of Pohang, the fault zone becomes gradually narrower (<100 m) and is partly discontinuous with horse-tail bifurcations and splay faults (Y. Kim et al., 1990; Chae and Chang, 1994). Here, the down-dropped eastern block sinks northward beneath the Pohang-Yongduk Basin (Fig. 6.2). An analysis of satellite images suggests that the fault extends to the south of Hupo where it enters the continental shelf (Kang, 1979). However, offshore seismic reflection profiles provide no evidence for the submarine extension of the Yangsan Fault (Yoon, 1994). Pre-faulting geometric reconstruction of the Cretaceous sedimentary sequence along both sides of the Yangsan Fault suggests that the fault experienced dextral strike-slip motion of 25-35 km (Chang et al., 1990). Evidence for dextral displacement is also seen in the NE-SW-striking en echelon normal faults on the eastern fault block (Fig. 6.2), which deformed non-marine deposits of the Yangbuk Group (early Miocene). These faults are interpreted to be consequences of NW-SE extension associated with NNE-SSW dextral strike-slip displacement of the Yangsan Fault. The dextral fault movement along the Yangsan Fault appears to have dissected the Eocene rhyolitic tuff extruded prior to 45 Ma (Shibata et al., 1979). This faulted tuff is, in turn, overlain by the unfaulted lower to middle Miocene Yonil Group (Chang et al., 1990; Hwang, 1993). Thus the field relationships indicate that the last movement of the Yangsan Fault took place during the middle Eocene to early Miocene time. This is coeval with the initial strikeslip motion along the Hupo Fault. 6.2.4
Dolgorae Thrust Belt
The Dolgorae Thrust Belt is the most prominent deformation structure in the southeastern margin of Korea (Fig. 6.2). This belt is a NE-SW-trending structural zone consisting of complex thrust faults and associated folds; it is generally 6-8 km wide and is laterally persistent for more than 60 km. North of Tsushima Island, the Dolgorae Thrust Belt is connected to the Tsushima Fault (Fig. 6.2) which was reactivated in the late Miocene. Individual thrust faults have slightly curving traces consistently striking NE-SW and dipping SE. The folds generally occur not only as separate anticlines or synclines but also as drag and force folds attached to the thrusted sheets. The western part of the Dolgorae Thrust Belt is characterized by a contractional horst block that is flanked on each side by a faulted, upturned ankle-bend monocline. This feature is interpreted to result from wrench faulting with a significant component of lateral displacement (S.J. Park, 1998). The horst block passes northeastward into a gentle monocline which is incorporated into the eastern
Eastern Continental Margin 207 part of the thrust belt. Based on the chronostratigraphy of the sediment sequences within the deformation structures, Yoon (1994) suggested that the thrust belt formed during two episodes of contractional deformation. The initial deformation, which took place at the end of the middle Miocene, produced folding at the Dolgorae-I well site (Fig. 6.1). The deformation subsequently propagated southwestward throughout the entire thrust belt until the early late Miocene. The second episode of the deformation took place at the end of the late Miocene, giving rise to small-scale local folds and high-angle reverse faults, as well as angular unconformities with limited distribution. 6.2.5
Small-Scale Faults and Folds
Small-scale normal faults typically occur as basement faults which are buried beneath sedimentary sequences. These faults are recognized in the shelf and the slope regions north of 36°30'N, and along the coastal area between 35^40' and 36°N. Strikes of the faults are variable from NE to NW and the dips are toward either E or W. The vertical fault displacements are up to a few hundreds meters. In the shelf and slope regions the normal faults usually occur as boundary faults of local grabens or half-grabens. Some NEstriking faults are arranged in a left-handed en echelon pattern subparallel to the overall trend of the Hupo Fault (Fig. 6.2). Yoon and Chough (1995) suggested that these en echelon faults resulted from the shear deformation along the Hupo Fault in the early Miocene. Within the Pohang-Yongduk Basin, the basin-fiU sequences appear to have been significantly shortened and uplifted. On the western and eastern margins of the basin, N- to NNE-trending thrust zones are developed subparallel to the Yangsan and Hupo faults; in particular, the eastern thrust zone lies on the southwestern segment of the Hupo Fault (Fig. 6.2). These thrust faults threw up the intervening basin-fiU sequences, within which a number of small-scale reverse faults and folds are traced for a short distance (<10 km), striking dominantly N or NNE (Fig. 6.2). North of 36°40'N, contractional deformation structures of various scales are also recognized within the sedimentary sequences in the upper to mid slope area which occur as basement-involved reverse faults, folds, and flexures (Fig. 6.2). The reverse faults have relatively steep (generally >30°) fault plane, and are usually accompanied by anticlinal folds or fault drags within the uplifted hanging wall blocks. The strikes of faults and fold axial planes are typically either NE or NW. The NE-striking reverse faults dominantly exist within middle and upper Miocene sequences, whereas the NW-trending faults and folds are found within the upper Miocene and lower Pliocene sequences (Yoon, 1994).
208
6.3
Marine Geology of Korean Seas
Seismic Stratigraphy
6.3.1
Eastern Margin
The eastern margin is covered with a sedimentary sequence ranging in thickness from a few tens of meters to 1,200 m depending on basement topography (Schluter and Chun, 1974). Yoon and Chough (1995) described the seismic stratigraphy including the acoustic basement and the overlying 3 sedimentary units. These sedimentary units are usually bounded by extensive erosional or nondepositional unconformities. 6.3,1.1
Acoustic
Basement
The acoustic basement is characterized by either a faintly stratified reflection or an opaque reflection (lack of internal reflectors). The former occurs in some parts of the continental shelf south of Hupo, whereas the latter is dominantly identified from elsewhere along the margin. The basement topography is characterized by a complex series of elongated ridges, troughs, and domes, except in the shelf region (Fig. 6.5a) where the basement surface is marked by a relatively flat and high-amplitude reflection doublet (Fig. 6.3). The ridges and troughs generally trend N-S bounded in some cases by normal faults. The high-relief topography of the basement delineates intra-shelf and slope-perched sedimentary basins of various dimensions (Fig. 6.5a). Beneath the lower slope, the acoustic basement is much steeper (>30°) than that of the upper-to-mid slope, and forms a buried escarpment having relatively subdued surface morphology with small-scale hyperbolic reflectors (Fig. 6.3). Yoon et al. (1997) traced the basement escarpment along the entire western margin of the Ulleung Basin and suggested that this could be part of the Ulleung Fault (Fig. 6.2) (see section 6.2.1). The acoustic basement is probably an extension of crystalline or consolidated rock complexes exposed along the eastern Korean Peninsula. North of 36°40'N, the coastal region largely comprises Precambrian gneiss and Paleozoic sedimentary rocks that were intruded by Jurassic to Cretaceous granites (Fig. 2.2a). This rock complex seems to extend to the shelf region and the northern part of the slope (north of 37°15'N), where it forms acoustically opaque or faintly-stratified basement with a relatively subdued topography. The basement with opaque reflection has a P-wave velocity of about 5.6 km/s (Schliiter and Chun, 1974). Along the slope south of 37°15'N, the basement consists of extrusive volcanic rocks characterized by acoustically opaque reflections and very irregular topography with knolls and domes (Fig. 6.5a). This volcanic
Eastern Continental Margin aracNi
209
Zt^dHx
3/lOO'h
3^30'
3€fl0O'l
123'3ff
12SP30'
37°30'N|
37lD0|
37^>
3^30'F
36r30>
3eroo'i
13(ft)0'E
3^00*'
129^30'
13(f00'E
Fig. 6.5. Time-structure map of acoustic basement (a) and isopach maps of Unit I (b), Unit II (c), and Unit III (d) in the eastern margin of Korea. Contours in seconds (twt). After Yoon and Chough (1995) by permission of the Geological Society of America, Inc.
210
Marine Geology of Korean Seas
basement is probably an extension of the NE-SW-trending volcanic belt exposed along the coastal region south of 36°30'N. The volcanic belt consists mainly of andesite, felsite, and basalt, extruded through the widespread Cretaceous sedimentary succession (KIER, 1981a; Hwang, 1993) (Fig. 2.2a). K-Ar ages of the volcanic rocks indicate that the volcanic belt north of Pohang formed during two episodes of volcanic activity: middle Eocene ( 5 0 ^ 4 Ma) and early Miocene (22-20 Ma) (Jin et al., 1989; H.K. Lee et al., 1992). 6.3.1.2
Sedimentary
Unit I
The lowermost unit (Unit I) consists of acoustically faintly- to wellstratified sedimentary sequence deformed by reverse faults and folds (Fig. 6.3). The unit largely occurs within basement lows and reaches to the coast only in the southern Yongduk Basin. More than 0.6 s (twt) of Unit I sequence accumulated in intra-shelf and slope-perched basins and beneath the lower continental slope, whereas the shelf north of Hupo and basement highs in the slope region are barren of Unit I sequence (Fig. 6.5b). This unit can be correlated with the marine deposit (Yonil Group) exposed on land near Pohang (Fig. 6.5b), based on spatial distribution and similarity of the internal sequence configuration (e.g., structural deformation) (Huntec Ltd., 1968). The Yonil Group occurs to the east of the Yangsan Fault, where it comprises E-dipping sedimentary sequences of conglomerate, sandstone, and mudstone, unconformably overlying the Eocene igneous rock complex. Sedimentologic studies reveal that the sequence formed in fan-delta systems and adjacent deeper marine environments (Choe and Chough, 1988; Chough et al., 1990; see section 2.8). Paleontologic studies of siliceous microfossils (H.Y. Lee, 1975; Koh, 1986; Ling et al., 1988), planktonic and benthic foraminifers (Yoo, 1969; B.K. Kim, 1970, 1988; B.K. Kim and Choi, 1977; W.H. Kim, 1990), and mollusks (Yoon, 1975) from the Yonil Group suggest that deposition occurred during the period between the early and late Miocene. 6.3.1.3
Sedimentary
Unit II
Unit II, the middle sedimentary unit, is found predominantly in the Hupo Basin and the slope region (Fig. 6.5c), and consists of acoustically faintly- to well-stratified sedimentary sequences that are deformed by variable-scale faults and folds; the unit is further divided into pre-deformational and syndeformational subunits (IIA and IIB) around the Hupo Fault (Fig. 6.3). In the shelf and upper slope, the lower boundary of Unit II is an erosional
Eastern Continental Margin 211 unconformity marked by a flat and high-amplitude reflector. The unconformable base of the unit off Pohang is correlated with a stratigraphic horizon within the upper Miocene sequence (about 5.5 Ma) of the UUeung Basin (Yoon and Chough, 1995). In the slope region, the unit is <250 m thick and conformably overlies Unit I. The boundary between Unit IIA and Unit IIB occurs as an onlap-bounding surface on the east of the Hupo Bank, and is approximately correlated with a stratigraphic horizon within the lower Pliocene sequence of the Ulleung Basin (Yoon, 1994). The Hupo Basin is bordered on the east by the Hupo Fault and is filled with a thick syndeformational sequence (Unit IIB) (Fig. 6.3). This unit is disproportionately thicker (up to 700 m) within the basin, four to seven times thicker than Unit IIB on the slope region. Individual reflectors are slightly inclined (1-4.5^) toward the Hupo Fault, and inclination of the reflectors increases toward the lower part of the unit. Beneath the present shelfbreak, Unit IIB forms prograding deltas which pass basinward into a sheet-form deposit (Fig. 6.3). 6.3.1.4
Sedimentary
Unit III
The uppermost sedimentary unit (Unit III) consists of undeformed sequences with continuous, parallel, and well-stratified reflections (i.e., high amplitude and high lateral continuity). The unit reaches up to 200 m in thickness in the slope region (Fig. 6.5d); reflections at the lower boundary onlap onto uplifted fault blocks and anticlines (Fig. 6.3). On the steep midto-lower slope, the upper part of the unit is commonly remolded by largescale slope failures characterized by acoustic transparency (Fig. 6.3). The lower boundary of Unit III off Pohang is correlated approximately with the upper part of the lower Pliocene sequence of the Ulleung Basin (Yoon and Chough, 1995). 6.3.2
S outheastem Margin
Multichannel seismic surveys and chronostratigraphic work on exploratory wells (H.Y. Lee, 1994) in the southeastern margin reveal that the present shelf and slope region is underlain by thick (>8 km in thickness) Tertiary sequences within a narrow, fan-shaped depression (Barg, 1986; K.S. Park, 1990; S.J. Park, 1998). The sequences are characterized by a recurrent progradational stacking pattern as a result of N to NE advance of the shelfslope system since the early Miocene (Fig. 6.6). The shelf-slope sequences comprise various parasequence sets (see Van Wagoner et al., 1988) that have distinctive stacking patterns. Chough et al. (1997a) divided the sequences into 3 successions according to parasequence set organization, and presented characteristic features of the stratal patterns as a combined effect of eustasy,
212
Marine Geology of Korean Seas ISCOO'E
130°30'
Z
Fig. 6.6. Paleoposition of shelfbreak in the southeastern margin of Korea. The shelf prograded northeastward until 12 Ma; thereafter progradation turned northward as the eastern part of the shelf merged with the uplifted thrust zone (Dolgorae Thrust Belt). Capital letters denote the stratigraphic horizons shown in Figs. 6.7 and 6.8. Modified after Chough et al. (1997a) by permission of the Springer-Verlag. sediment supply, and basement subsidence/uplift controlled by active backarc tectonism. 6.3.2.1
Succession
I
Succession I (A-C interval in Fig. 6.7) consists of a single sequence unit which in turn comprises 6 parasequence sets with 3 types of progradational patterns: sigmoid progradational, oblique progradational, and retrogradational (see Mitchum et al., 1977) (Fig. 6.7). The lower part of the succession (A-B interval) is characterized by 3 recurring sigmoid progradational parasequence sets bounded by downlap-bounding surfaces in the foresets. Above horizon B, shelf-margin retreat is recognized by an abrupt landward shift of topset and foreset beds without notable retrogradational stacking of the strata. This is covered by another sigmoid parasequence set which is in
Eastern Continental Margin
213
214
Marine Geology of Korean Seas
turn overlain by an oblique progradational parasequence set with an erosional upper boundary. The recurrent sigmoid progradational units in Succession I reflect a persistent increase in accommodation space and, therefore, continuous rise of relative sea level. During the deposition of Succession I (16-12 Ma), there were two events of 3rd-order eustatic sea-level fall at 15.5 and 13.8 Ma (Haq et al., 1988). Thus, the continuous rise of relative sea level during this period reflects a tectonic subsidence rate greater than the maximum rate of eustatic sea-level fall. In this case, the seafloor subsides more rapidly than eustatic sea level fall; consequently, there is no local relative sea-level fall. Instead, the local relative sea-level rise is slowed and then accelerates as eustatic sealevel fluctuates. If the accommodation space continuously increases, repeated sigmoid progradational parasequence sets can be introduced due to high rate of sediment deposition which always exceeds the rate of accommodation space increase during the sea-level fluctuations. The occurrence of retrogradational and oblique progradational parasequence sets in the upper part of Succession I indicate a progressive decrease in deposition rate and slowed subsidence, resulting in relative sea-level fall. 6.3.2.2
Succession
II
Succession II (C-G interval; 12-6.5 Ma) consists of four sequence units bounded by erosional truncation surfaces with channel or valley incisions. Close to the Dolgorae Thrust Belt, the succession is characterized by a downstepping of the sequence units (Fig. 6.8). Each sequence is divided into two types of parasequence sets depending on their internal stacking pattern: retrogradational to aggradational lower parasequence sets and tangential oblique progradational upper parasequence sets (Fig. 6.8). The lower parasequence sets have continuous and high-amplitude reflections; they invariably occur as very thin (<20 ms twt) deposits mainly covering the shelf area. In the upper oblique progradational parasequence sets, the foreset strata terminate against the lower surface of the clinoform (i.e., downlap), or pass laterally into the thinner bottomset. The clinoforms grade landward into thin sheet- or wedge-shaped deposits that are characterized by very poorly organized internal reflections and ubiquitous channel-incision features. In the Dolgorae-I well, the poorly-organized deposits consist of two foraminiferal zones which reflect shallowing from neritic (C-D interval) to coastal (D-G interval) settings (KIER, 1982). Northwestward away from the thrust belt, the lower parasequence sets change into thicker sigmoid progradational parasequence sets (Fig. 6.7). Boundaries of the sigmoid progradational sequences are marked by frequent channel erosions in the topset beds and downlap in the foreset beds. No retreat of the shelf-slope system is recognized either between or along the sequence boundaries.
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Eastern Continental Margin
(s) eujji laARi) AeM-o/vy.
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216
Marine Geology of Korean Seas
Succession II at the front of the Dolgorae Thrust Belt is characterized by a downstepping shelf-slope sequence that consists of a thin retrogradational lower parasequence set and an oblique progradational upper parasequence set (Fig. 6.8). The downstepping of the sequence with retrogradational parasequence set is indicative of overall forced regression intermittently perturbated by short-term rises of relative sea level. The absence of a sigmoid progradational parasequence set suggests very low rates of sediment accumulation during the deposition of Succession II. The lower sedimentation rate suggests that sediments derived from the thrust belt mostly bypassed the shelf-slope system and were delivered to deeper marine environments. The shelf-slope system is thought to have been received large amounts of sediments from the uplifted deep-sea deposits of the thrust belt, while sediments from the Korean Peninsula and SW Honshu were mainly trapped behind the thrust belt. The sediment bypassing can most likely be attributed to increased slope gradient due to the uplifting, and finer grain size in the source area. In the undeformed region to the northwest of the Dolgorae Thrust Belt, Succession II is characterized by a building-up of prograding sequences with erosional boundaries (Fig. 6.7). This reflects intervals of relative sea-level rise, which, in turn, suggests a tectonic subsidence. The absence of a retrogradational parasequence set in spite of the increased rate of relative sea-level rise indicates relatively high rates of sediment supply and deposition onto the shelf-slope system. The enhanced sediment deposition in the undeformed region reflects a vigorous sediment supply both from the newly-formed thrust belt to the southeast and from the Korean peninsula to the west or southwest. 6.3.2.3
Succession
III
Succession III above horizon G consists of 4 sequence units of which the boundaries are downlap- or rarely onlap-bounding surfaces that are either erosional (horizons I and J) or nonerosional (horizon H) (Fig. 6.8). Individual units are commonly divided into 4 parasequence sets with sigmoid progradational to aggradational, retrogradational, sigmoid progradational, and oblique progradational stacking patterns in ascending order (Fig. 6.8). Shelf margin retreat is recognized in the retrogradational parasequence sets. Stacking patterns of the sequences in Succession III indicate risedominated relative sea-level fluctuations like those in Succession II of the undeformed region. However, the 4-parasequence set organization of a sequence indicates the effect of reduced sediment supply on shelf-margin progradation. When relative sea level begins to rise after the maximum regression, sediment supply is larger than the increase in accommodation
Eastern Continental Margin 217 space because sea-level rise is still slow. This results in development of a sigmoid progradational parasequence set. As the relative sea-level rise becomes faster, sediment supply can no longer keep pace with the increase in accommodation space, thereby producing a retrogradational parasequence set. This is followed by another sigmoid progradational parasequence set as the relative sea-level rise slows toward the maximum transgression. During the relative sea-level fall, terrigenous sediments accumulated primarily on the slope and basinal areas, bypassing the shelf. This resulted in the development of an oblique progradational stacking pattern with either no or very thin shelf strata.
6.4 6.4.1
Sedimentary Basins Pohang-Yongduk Basin
The Pohang-Yongduk Basin is a parallelogram-shaped graben between the right-stepping Yangsan and Hupo faults (Fig. 6.9). The coeval dextral shear deformation along the two bounding master faults postulates a pullapart opening of the Pohang-Yongduk Basin because right-stepping dextral strike-slip faults generally produce a zone of tension and depression between two master faults (Yoon and Chough, 1995). The basin consists of a main basin and two marginal basins which are separated by basement highs (Fig. 6.9). The main basin is a deep and narrow (15-20 km wide) trough with a longitudinal dimension of about 60 km. The marginal basins comprise shallower depressions that extend NE-SW at the ends of two master faults. The onland Pohang Basin (see section 2.8) is the southwestern extension of the offshore Pohang-Yongduk Basin (Fig. 6.9). The basin is dominantly filled with a Miocene sequence and is unconformably overlain by a thin (about 30 m thick) veneer of Quaternary sediment (Yoon, 1994). The basin-filling sequence rarely exceeds 700 m in thickness except in the center of the main basin where it is up to 1,000 m thick (assuming an interval velocity of the unit 2,500 m/s, Huntec Ltd., 1968). The sequence in the southwestern part of the basin (i.e., Yonil Group in the Pohang Basin) is composed of conglomerate, sandstone, and shale deposited in prograding marine fan-delta systems (Chough et al., 1990; Hwang and Chough, 1990). The sedimentary sequence in the northern marginal basin reaches about 300 m in thickness, and the western part of the sequence outcrops in the coastal region (i.e., Yonghae Basin). The lower part of the sequence is suggested to have been deposited in non-marine environments, whereas the upper fossiliferous sequence is correlated with the upper Miocene sequence of the Yonil Group (B.K. Kim, 1970, 1977).
218
Marine Geology of Korean Seas
30 km
Fig. 6.9. Distribution of offshore sedimentary basins along the eastern margin of Korea. After Yoon and Chough (1993).
130^
6.4.2
Mukho Basin
The Mukho Basin (Fig. 6.9) is a slope-perched sedimentary basin that is bordered on the east by a basement high projecting northward (Yoon and Chough, 1993). The basin is 20-30 km wide and more than 35 km long, and filled with a sedimentary sequence more than 1,400 m thick in the northern part (Schliiter and Chun, 1974). The acoustic basement is characterized by subparallel, discontinuous and low-amplitude reflections and partly by
Eastern Continental Margin
219
opaque reflection without internal reflectors, which appear to be an extension of Precambrian gneiss and Paleozoic sedimentary rocks that occur in the coastal region. Rugged morphology of the basement is strongly indicative of block faulting that can be traced for a short distance; the basin formed as a basement depression between basement highs uplifted by complex block faulting (Yoon and Chough, 1993). The basin is filled with Miocene to lower Pliocene sequences which are characterized by moderately- to faintly-stratified reflections with variable amplitude and lateral continuity (Yoon, 1994). The sequences are highly disturbed by contractional deformation structures including variable-scale faults and folds. The deformation structures tend to diminish near the top of the lower Miocene sequence. The structural axis changes in strike from N-S in the southern part to NW-SE in the northern part. A major fault zone occurs along the eastern margin of the basin and is accompanied by anticlinal folds within the uplifted western fault block. The uppermost unit is characterized by an undeformed, post-deformation sequence that is less than 150 m in thickness and is acoustically well stratified showing subparallel, high-amplitude reflections. 6.4.3
Hupo Basin
The Hupo Basin is a slope-perched half-graben bounded on the east by the flat-topped Hupo Bank (Figs. 6.3 and 6.9). The basin was formed by uplifting of the Hupo Bank during the late Miocene contractional deformation along the Hupo Fault. The basin is 5-17 km wide and 95 km long, extending in N - S direction (Fig. 6.9). The Miocene sequence (Unit I in Fig. 6.3) is acoustically faintly stratified and about 0.25 s (twt) thick. This unit thins landward onlapping onto the acoustic basement. The upper boundary of the Miocene sequence is an E-dipping (3-5°) high amplitude reflector over which the Pliocene and Quaternary sequences onlap (Fig. 6.3). The landward shift of the onlap is indicative of progressive deepening of the basin probably due to sediment loading and compaction as well as fault movement. Individual reflectors are slightly inclined eastward and increase in dip toward the deeper part. The boundaries of the sequence units within the basin are generally marked by onlap-bounding surfaces. Along the western margin of the basin, at least 3 sets of prograding clinoforms are identified in the upper part; each individual set (13-30 km in width) has prograded 2-5 km eastward. In the eastern margin of the basin, entire sequence reaches up to 0.7 s (twt) in thickness and is faintly or crudely stratified. Debris aprons built out from the Hupo Bank taper toward the center of the basin and interfinger with sediments derived from the coastal region (Fig. 6.3).
220
Marine Geology of Korean Seas
6.5
Evolution History
6.5.1
Eastern Margin
The tectonic evolution of the eastern continental margin of Korea was reconstructed by Yoon (1994) based on sequence and structural analyses of deep-seismic reflection profiles. In the early Miocene, the Hupo and Yangsan faults activated concurrently, forming a right-stepping strike-slip fault system (Fig. 6.2). The dextral shear movement along this fault system resulted in a pull-apart opening of the Pohang-Yongduk Basin. This pullapart basin initially underwent rapid tectonic subsidence in association with the strike-slip faulting; it was subsequently filled with terrigenous sediments supplied through the Pohang fan-delta systems (see section 2.8). At the same time, the northern part of the eastern margin experienced block faulting, which resulted in the opening of the Mukho Basin on the downthrown fault block. At the end of the early Miocene, the strike-slip deformation in the eastern margin was practically terminated and consequently subsidence rate of the margin began to decrease. This led to a thick accumulation of marine sediments on the prograding slope as well as in the shelf- and slope-perched basins until the end of the middle Miocene. In the late Miocene, back-arc spreading in the East Sea ceased and the stress field was inverted from tension to compression (Chough and Barg, 1987; Ingle, 1992). The compressional stress regime resulted in an E-W shortening accompanied by thrusting and folding of the early to middle Miocene sequences, particularly in the Pohang-Yongduk Basin. Contemporaneous uplift of the eastern margin gave rise to a widespread erosion of the upper part of the Miocene sequence, producing an angular unconformity in the shelf region. During the end of the late Miocene, the second episode of contractional deformation occurred along the eastern margin. This was accompanied by a convergent strike-slip movement along the northern and newly-formed southeastern segments of the Hupo Fault, as well as by widespread reverse faulting and folding within the slope sequences. During the strike-slip reactivation of the Hupo Fault, the eastern block (i.e., Hupo Bank) was uplifted to form a half-graben, the Hupo Basin (Fig. 6.3). Sediments derived from the Korean Peninsula and the uplifted Hupo Bank were mainly trapped in the Hupo Basin where they formed a thick onlapping sequence, whereas a thinner sequence was deposited in the progressively deepening middle to lower slope due to obstruction of the Hupo Bank. The third episode of contractional deformation began in the middle of the early Pliocene, resulting in significant deformation of sedimentary sequences
Eastern Continental Margin 221 north of Yongduk. This regional deformation was associated with E-W or NE-SW compressional stress regime, which caused block faulting in the Mukho Basin and reactivation of the northern segment of the Hupo Fault. Since the late Pliocene, the eastern continental margin has attained its present tectonic and depositional settings. The margin has continuously subsided causing progressive onlapping aggradational sequences and smallscale normal faults within the sedimentary sequence. Sediments from the Korean Peninsula mostly bypassed the narrow shelf and were deposited in the Hupo Basin and the slope region where mass failures were frequently generated. 6.5.2
Southeastern Margin
Chough and Barg (1987) suggested that the tectonic subsidence in the southeastern margin of Korea was initiated in the late Oligocene (about 28 Ma) as a result of the back-arc opening in the East Sea region. A backstripping analysis of the Dolgorae-I well indicates that initial tectonic subsidence proceeded at a rate of about 300 m/m.y. (Chough and Barg, 1987) (Fig. 6.10), which is more than three times that estimated by McKenzie's (1978) model for passive continental margins. The rapid initial subsidence was associated with the strike-slip faulting along the UUeung and Tsushima faults. Sediments were deposited in a paralic environment during the initial stage of sedimentation, followed by a progressive northeastward deepening of the margin in the early Miocene. This deepening formed a deep marine (bathyal) facies composed mainly of turbidites. The tectonic subsidence of the southeastern margin continued into the middle Miocene, but the subsidence rate progressively decreased (Fig. 6.10). The slowed tectonic subsidence, coupled with active sediment supply from the Korean Peninsula, resulted in a northeastward progradation of the shelf-slope system (Fig. 6.6). At the end of the middle Miocene (about 12-10 Ma), compressional stress regime prevailed in the southeastern margin due to the back-arc closing (Chough and Barg, 1987). The initial effect of the contractional tectonism was uplifting and folding of the Miocene sequences around the Dolograe-I well site (Figs. 6.8 and 6.10). The contractional deformation had propagated southwestward, and the continued NW-SE compression produced the Dolgorae Thrust Belt comprising several subparallel thrust faults and folds. In the late Miocene, angular unconformities became more pronounced particularly on the uplifted deformation zones. Eroded sediments from the uplifted areas were redeposited in the topographic lows at the front of the thrust belt (Succession II in Fig. 6.8), The progradation of the margin turned to the north as the northeastward prograding shelf-slope system merged with
222
Marine Geology ofKorean Seas
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6.6
Surface Sediments
The distributional pattern of surface sediments on the southern half of the eastern margin of the Korean Peninsula follows, in general, bottom topography such that coarse-grained sediment, dominated by sand and sandmud mixture, occurs on the shelf (to a depth of about 500 m), whereas the offshore margin is covered with mud (Fig. 6.11). In the former, sandy gravel, muddy gravel, gravelly muddy sand, sand, and gravelly mud also occur
Eastern Continental Margin 223 Fig. 6.11. Distribution of surface sediments on the eastern margin of the Korean Peninsula, classified by grain size according to Folk's (1980) scheme. Modified after the Chinhae Machine Depot (1979).
locally. Exceptions are found on the shelf between Pohang and Ulsan and in the southern embayments where silt and mud prevail. Recent sediment distribution on the southeastern shelf of the Korean Peninsula appears to be influenced strongly by currents which winnow fine particles into deeper water. The existence of sand and gravel in the middle part of the shelf is suggestive of bottom currents flowing from north to south whose effect is diminished south of Pohang. The coarse-grained particles on the southeastern comer are shaped most likely by the Tsushima Current flowing N and NE. Sand and gravel in this area are probably relict sediments
224
Marine Geology ofKorean Seas
only partly covered by muddy and silty sediments. The lack of riverine sediment yield results in a thin sediment cover on the eastern shelf of the Korean Peninsula, often with a rocky seafloor exposed. The bulk of sediments is believed to be produced by coastal cliff erosion and other local sources (CM. Kim et al., 1971). The influence of the Kuroshio Current on the ecology of foraminifera has also been emphasized by B.K. Kim and Han (1971, 1972) in the southern Korea Strait w^here w^arm water species such as Globorotalia menardii and Pulleniatina obliquiloculata prevail. They also found cold water species such as sinistrally coiled Neogloboquadrina pachyderma in the southeastern shelf, signaling cold water infiltration along the coast from the northern East Sea.
6.7
Late Quaternary Sediments
6.7.1
Mass Physical Properties
In the eastern and southeastern margins of the Korean Peninsula, mass physical properties of the late Quaternary sediments were evaluated in a number of piston-core samples obtained mainly by the Korea Ocean Research and Development Institute. The results of mass physical analyses were presented by Chough and Lee (1987), H.J. Lee (1991), and H.J. Lee et al. (1991, 1993). 6.7.1.1
Water Content
Water content varies closely with sediment texture which displays a regional trend of fining with water depth (Table 6.1) (H.J. Lee et al., 1991, 1993). On the shelf, interstitial water is uniformly distributed downcore in the range of 25-75%. Clayey silt or silty clay, which is typical of the upper slope, has a moderate to high water content in the range of 50-200% which remains constant or slightly decreases downcore. The middle and lower Table 6.1. Geotechnical properties of topmost sediments on the eastem and southeastem continental margin of Korean Peninsula, (modified after HJ. Lee et al., 1991, 1993). Province
Texture sand
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(%)
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(»)
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Shelf 20-100 2-50 <5 1.5-6.5 25-75 5-15 5-15 2-5 2.65-2.70 0-40 0-65 (<300m) Upper slope 0-20 25-70 20-75 6.0-9.5 50-200 2-20 5-10 5-10 2.55-2.60 40-95 60-130 (300-700) Middle slope -0 15-50 45-90 8.0140-220 2-5 2-10 5-10 2.60-2.65 60-100 85-140 (700-1400) 10.0 Lower slope 0-5 20-40 55-80 8.0-9.5 140-210 1-5 0-5 5-15 2.55-2.60 55-85 85-135 (> 1400 m) W = wat«- ccmtent; SS = shear strei^^ OM = oiganic matter, G = grain specific gravity; Ip = plasticity index; W, = liquid limit. 'Averaged for the uppermost 1 m interval.
Eastern Continental Margin 225 slope cores attain much higher water contents, generally exceeding 180%. However, a noticeable reduction of water content, to less than 150%, occurs in the lower sections of many cores from the middle slope, accompanied by a considerable increase in silt (up to 50%). A distinct change in water content is also recognized between upper hemipelagic muds (ca. 200%) and lower turbidite muds (150%)). 6.7.1.2
Shear
Strength
The shear strength of the shelf cores is consistently high (>4 kPa) at the surface, and increases linearly downcore; sand-rich (>30%)) sediments in the top 50 cm of some cores have higher shear strengths of up to 12 kPa (Table 6.1) (H.J. Lee et al., 1991). In the upper slope, shear strength varies widely (2-10 kPa) at the surface, due to variable sediment texture and water content. Cores from the middle slope show a subdued, consistent strength profile which increases steadily downcore. Although cores on the lower slope differ in their ranges of shear strength, they generally display uniform values downcore. Comparison among cores collected from slope failure scars and from unaffected, stratified sediment areas characteristically shows differences in the vertical profiles of shear strength (H.J. Lee et al., 1991). For the failure scars, shear strength tends to decrease slightly with depth, whereas stratified sediment shows shear strength increasing downcore. 6.7.1.3
CaCOj and Organic
Matter
Most of shelf sands contain more than 10%) CaC03 (maximum up to 60%)) (Table 6.1), whereas the CaC03 contents in slope muds, generally less than 5%), gradually decrease downslope. In the lower slope below 1,500 m water depth, calcium carbonate abruptly decreases to less than 2%o. This suggests that the CCD or lysocline in the East Sea lies at a relatively shallow water depth, about 1,500 m (H.J. Lee et al, 1991). Organic content also reveals two discrete ranges related to sediment type: l-4%o for shelf sands and 8-14%) for slope mud. There is no marked trend in organic content with either burial depth or water depth. 6.7.1.4
Atterberg
Limits
Atterberg limits in the eastern margin of Korean Peninsula generally increase with water depth. Shelf sediments in the northern part of the margin have lowest plasticity, falling in the inorganic sandy clays category of low to medium plasticity (ML-CH). Slope sediments are widely distributed
226
Marine Geology ofKorean Seas
throughout the CH division (inorganic clays of high plasticity); lower-slope sediment tends to plot close to the A-line. In the southern part of the slope, the liquid limit values are about 20% higher than those from the northern slope, but the plasticity index is similar between the two areas. Plots of these data on the plasticity chart show that the slope sediments cluster just below the A-line in the range designated as MH (diatomaceous silty clays) or OH (organic clays of medium to high plasticity). 6.7.2
Sedimentary Facies
Sediment cores from the shelf characteristically show abundant massive sand layers that consist of well-sorted medium to fine sand, with abundant shell fragments (Fig. 6.12a) (H.J. Lee et al., 1991, 1993). The shell fragments are randomly oriented and tend to increase in abundance downcore. The shelf sediments are generally structureless with a weak to moderate degree of bioturbation. Chough et al. (1991a) described these types
Fig. 6.12. X-radiographs of representative cores from the eastem margin of Korea, (a) Massive sand with shell debris, 40-52 cm, core S-1. (b) Bioturbated mud, 40-52 cm, core S-12. (c) Thoroughly bioturbated mud with pyrite filament, 222-237 cm, core N-36, (d) Laminated mud (E-division of turbidite), 100-115 cm, core N-12. (e) Turbidite mud with silty-clay couplets, H = clay units, Z = silt units, L = laminated mud, 220-232 cm, core S-3. For core location see H.J. Lee et al. (1991, 1993).
Eastern Continental Margin 227 of sedimentary facies from the Korea Strait, and interpreted them as the rehct sand or the transgressive sand sheet formed during the early phase of the Holocene sea-level rise. The cores from the slope predominantly show a bioturbated mud facies throughout (Fig. 6.12b). The strongly bioturbated, olive (5Y3/2) to dark olive (5Y4/4) colored, silty clay or clayey silt suggests uniform hemipelagic sedimentation (H.J. Lee et al., 1991, 1993; Yoon et a l , 1996). In the slope deeper than about 1,000 m, the water-rich, muddy sediments have a mottled black color with a strong odour, due probably to H2S (H.J. Lee et al., 1991). Pyritized filaments occur scattered through the core and benthic animal burrows are common (Fig. 6.12c). In contrast to the upper slope, cores collected from the lower slope contain turbidite mud, particularly in the lower core section generally below 150 cm subbottom. The turbidite mud facies consists of homogeneous mud and/or thinly-laminated mud, with varying thickness from millimeters to several centimeters (Fig. 6.12d). In places, the turbidite mud occurs as a silt-clay couplet composed of a basal laminated silt unit and an overlying homogeneous clay unit (Fig. 6.12e). 6.7.3
High-Resolution Echo Characters
High-frequency (3.5 kHz) reflection surveys in the Ulleung Basin have served as a powerful tool for delineating the late Quaternary sedimentary processes (Chough et al., 1985a; H.J. Lee et al., 1991, 1993; Yoon et al., 1996). In 1996, the National Oceanographic Research Institute (NORI) has acquired about 8,330 line-km of high-frequency Chirp sonar profiles in the Ulleung Basin between 35°00' and 37°00'N, providing an unprecedented database for detailed analysis of late Quaternary sedimentary processes (Fig. 6.13). Based on these data, Chough et al. (1997b) presented a detailed map of echo characters (Table 6.2; Fig. 6.14). In addition, the NORI obtained high-frequency Chirp sonar profiles in the north of 37''N in 1997. 6.7.3,1
Shelf Region
The shelf area of the southeastern margin is generally dominated by 3 types of sharp bottom echoes: lA, ID, and IE (Fig. 6.14). Echo type lA is characterized by a sharp, continuous bottom echo with no subbottom reflectors, and reflects flat or slightly irregular seafloor topography (Table 6.2). This echo is interpreted to be characteristic of sand and gravel deposits formed by shallow marine processes. In echo type ID, the bottom echo is distinct and laterally continuous, whereas the subbottom echo is more or less diffuse and laterally discontinuous. This echo is interpreted as shallow marine sand and gravel deposits. Echo type IE is characterized by a sharp
228
Marine Geology of Korean Seas
129°E
Fig. 6.13. Tracklines of Chirp sonar survey by the National Oceanographic Research Institute (NORI). Location of other seismic profiles, cores, and a seafloor image (box) is shown. bottom reflector with inclined or channel-like subbottom reflectors showing a scour-and-fill geometry. This echo is interpreted to reflect buried fluvial channels and their sediment fills. 6.7.3.2
Slope
Region
The eastern continental slope records three types of distinct echo (IF, IC, and IV) (Fig. 6.14). Echo type IF, mainly from the Hupo Bank and its peripheral area, shows a distinct bottom echo with prolonged subbottom reflectors; it is generally associated with irregular surface topography (Table 6.2). This echo reflects a hard-rock basement covered by thin sediment layers. The upper- to mid-slope area is dominated by echo type IC, often extending to the lower slope (Fig. 6.14). This type is characterized by distinct, inclined bottom echoes with continuous and parallel subbottom reflectors (Table 6.2; Fig. 6.15a,b). It generally drapes an irregular
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Fig. 6.15. High-resolution (Chirp) seismic profiles showing typical echoes on the east Korean slope (for location of the profiles see Fig. 6.13). Characteristics and interpretation of echo types are described in Table 6.2. Horizontal scale bar = ca. 3 km, vertical scale bar = ca. 50 m. After Chough et al. (1997b). subsurface topography, which is suggestive of deposition by hemipelagic settling. Deposits of echo type IC are commonly truncated to form slopefailure scars at w^ater depths of 300-850 m (Fig. 6.15a). The scars are arcuate in shape with a relief of 30-50 m and marked by either sharp glide planes or irregular drapes of thin (<7-10 m), acoustically-transparent sediment layers. They are usually accompanied by a series of blocky, lumpy, or hyperbolic masses resting along the lower slope (echo type IV) (Figs. 6.14 and 6.15a). These masses are often separated by concave-up shear planes
Eastern Continental Margin 233 dipping downslope. Internal reflectors are slightly to highly deformed showing mingled or transparent appearance. In the southeastern continental slope area, echo type IV generally occurs on the steep upper part, and changes downslope into echo type III which is, in turn, substituted by echo type IIC on the base of slope (Fig. 6.14). Echo type III is dominantly recorded from the broad area of the mid to lower slope, extending basinward for several tens of kilometers. It is characterized by regularly-overlapping hyperbolic bottom echoes with slightly varying vertex elevations (<20 m) (Table 6.2; Fig. 6.15c). Piston cores obtained in this area commonly show a homogeneous mud facies with semiconsolidated mud clasts (KORDI, 1995; H.J. Lee et al, 1996). Echo type IIC occurs in the form of a positive lens or a flat-topped trough with slightly irregular surface (Table 6.2; Fig. 6.15d). This is characteristic of debris flow deposits. In the proximal area, several debris-flow units are stacked to form singled architecture, whereas, in the distal area, individual units are separated each other and scattered on the gentle base of slope. 6.7.4
Slope Failure Features
A number of failure masses occur on the upper to lower slope areas along the entire margin in water depths of 300-1,500 m (Fig. 6.16; Table 6.3). These masses include rockfall, slump, slide, grain- and debris-flow deposits, and turbidite with corresponding slide scarps and scars. Morphology, acoustic character, and areal distribution of the mass failure features on the slope were described by Jeong (1983), Chough et al. (1985a, 1991b), H.J. Lee et al. (1991), and Yoon et al. (1996). These mass-failure deposits are well defined by both rugged surface morphology and disorganized or remolded internal structures. On airgun profiles, these features are characterized by hummocky or blocky, hyperbolic surface reflectors, often accompanied by partially transparent, hyperbolic or mingled subsurface reflectors (Fig. 6.17). With relief on the order of 5-10 m, individual slide sheets reach up to 50 m in thickness and 10 km in length. At the base, discontinuous glide planes are often traceable. On some seismic profiles, the failed mass has a step-like appearance, strongly suggesting retrogressive sliding. In plan view, the scars upslope of the slide masses are crescentshaped, measuring up to 275 km^ in area and 70 m in depth (Chough et al., 1991b) (Table 6.3). At the base of slope, the slide masses rest on the stratified basin-floor turbidite sequence and are occasionally fringed with transparent debris-flow deposits. Debris-flow deposits in water depths of about 1,100-2,100 m form a transitional facies between the slide/slump facies on the slopes and turbidites on the basin plain. They are occasionally transparent, lens-shaped masses
234
Marine Geology of Korean Seas
Fig. 6.16. Distribution of slope-failure scars (shaded) and slump/slide masses (dotted). Contours in meters. Modified after Chough et al. (1991b) by permission of the Talyor & Francis.
Table 6.3. Summarized characteristics of slope failure scars in the eastern continental slope of Korea. Modified after Chough et al. (1991b). Slope gradient (degr.)
Water depth (m)
Scar depth (m)
Scar area
~ 1.0-10.0
300-1500
20-70
45-275
Removed sediment (slide/slump) volume bulk density mass (km3) (Mg/m^) (kg x 10'^) 2-8 1.4-1.6* 0.8-4.8
• Estimated with the GDP data for the upper 100 m of silty clay from the Japan and Yamato basins (Nobes et al., 1992), as well as measured values of surface sediment in this study.
Eastern Continental Margin 235
Fig. 6.17. Airgun profile (for location see Fig. 6.16) showing slope failure scar and dislocated sediment masses. Slump/slide mass (S), located immediately downslope of the failure scar (SS), is relatively transparent and grades at the base of the slope into a stratified sediment wedge (T) thickening basinward. Vertical scale is two-way travel time in seconds. After Chough et al. (1991b) by permission of the Taylor & Francis. and commonly overlie previously deposited slumps and older debris-flow^ deposits at the base of the slope (Fig. 6.15d). On the basin floor, turbidites form an extensive layered sequence (see section 7.6.3). 6.7.5
Slope Stability
A few attempts to evaluate slope stability have been made for the slopes of the eastern margin using an infinite slope stability analysis, a simple pseudo-static approach (H.J. Lee et al., 1991, 1993). The basic equations of the infinite slope stability relate the sediment resistance mobilized at failure to the dov^nslope force w^hich is a sum of body force (gravity due to overburden) and any external forces (dominantly storm waves and earthquake acceleration). Morgenstem (1967) has proposed a simplified.
236
Marine Geology of Korean Seas
working formula for undrained slumping under earthquake loading: CJiZ = 0.5 sin 2a + ky/y' cos^a
(1)
where C^ = undrained strength mobilized at failure, y' and y = buoyant and total unit weight of the sediment, respectively, a = slope angle, Z = sediment thickness, and k = horizontal earthquake acceleration. For slopes of gentle inclination (<10°), equation (1) reduces to: CJYZ = sin a + ky/y'.
(2)
With the measured values of the parameters for the eastern margin, the estimated earthquake acceleration necessary to trigger slope stability is extremely high (>20% of gravity) throughout the margin. However, most of the seismic profiles delineate internal bedding planes within the upper 30-60 m of the sedimentary deposit where slope failure has occurred (Fig. 6.17). These discontinuities may well preclude an assumption of homogeneity of the deposit which is commonly adopted in regional stability assessments of a given area because shear strength of the deposit tends to be much lower across the discontinuities. The evaluation of the earthquake-loading effects can be further elucidated by the method of H.J. Lee and Edwards (1986) modified from Morgenstem (1967) and Seed and Rahman (1978): K = {Y/y)[K Ad U S (OCR)" - sina]
(3)
where k^ = the critical pseudo-static horizontal earthquake acceleration (expressed as a percent of gravity) required to cause failure, A^ = correction factor for the difference between shear strengths under isotropic (laboratory) and anisotropic (field) confining pressure, A^ = correction factor for staticstrength degradation by cyclic loads, U = degree of consolidation (U = 1 for normal- and overconsolidation and U < 1 for underconsolidation), S = ratio of undrained shear strength to consolidation stress, and m = sediment parameter (typically equal to about 0.8). Incorporating triaxial measurements into the equation (3), H.J. Lee et al. (1993) assessed the stability of the southern slope of Ulleung Basin. The results show that the calculated values of k, are in the range of 0.08-0.13, 0.12-0.18, and 0.18-0.28 for the upper, middle, and lower slopes, respectively. This suggests that the surface sediments of the upper slope have the greatest relative susceptibility to slope failure by cyclic loading from earthquakes. However, H.J. Lee et al. (1993) conclude that the southern margin of Ulleung Basin appears to be presently
Eastern Continental Margin 237 stable because the Yangsan and Hupo faults, the nearest probable seismic center, are too distant to trigger virtual slope failures. The triaxial measurements additionally indicate that the slope sediments of the southern margin of Ulleung Basin are quite stable under static undrained (rapid) loading conditions (HJ. Lee et al, 1993). Under static drained (long-term) loading conditions, this area also is very stable because the maximum slope sustaining a fully drained gravitational loading is equal to (j)', the effective angle of internal friction (Edwards et al., 1980); the frictional angle is 18-23° for the southern slope of Ulleung Basin. Furthermore, the entire slope is evaluated to be more stable under undrained static conditions compared to drained conditions. Some possible exceptions, however, exist on localized walls of canyons and gullies: sediments on oversteep slopes far exceeding 18° might have failed or have a great potential for static drained failure. The slope-failure features found ubiquitously on the slope of the eastern margin and the Ulleung Basin (Fig. 6.16) are probably remnants of ancient slope failures, most likely having occurred during the Pleistocene lower stands of sea level when terrigenous materials were more actively transported to the slope. The most likely triggering mechanisms for the slope failures would have been earthquakes rather than storm waves, becasue the failed sediment and rear scarps are located in deep water (>500 m) to have any effect of storm waves. Earthquakes associated with regional contractional tectonic movements have most probably influenced the East Sea slope region. The Yangsan and Hupo faults have been active since the Neogene, accompanied by significant historically-recorded seismic events (M>3.5, K. Lee et al., 1984). In addition, a weak depositional bedding might have behaved as a major discontinuity which would significantly reduce the shear resistance of the sediment under cyclic loading by earthquakes. 6.7.6
Depositional Processes
The eastern continental margin is a sediment-starved, fine-grained shelfslope system incised by ubiquitous slope-failure scars and a few small-scale submarine canyons and gullies. In the absence of significant terrigenous input during the late Quaternary, the shelf and slope sediments were introduced mainly by persistent hemipelagic settling and subordinate episodic mass flows from the shelf to the base of slope (Fig. 6.18) (H.J. Lee et al., 1993, 1996; Yoon et al., 1996). During the last glacial maximum, fine-grained hemipelagic sediments on the upper to mid slope were frequently reworked by large-scale slope failures accompanied by slide/slump, debris flow, and turbidity current (H.J. Lee et al., 1993, 1996). The submarine slope failures are due to both
238
Marine Geology of Korean Seas Sea Level
0
P " " " ^ Turbidity Current : \ ^ Contour Current
O Hemipelagic Settling
T^SIope Failure ^^^lide/Slump
Debris
^^^^^^^^^^rp^:^^^
Turbidity C u r r e n t ^ ^ ^ ^ ^ C ; ^
Nephelold Flow (Layer)
^ ^ ^ i l i ^ t i i ' i i iSil IIIII""'! r " ^
\
Fig. 6.18. Schematic illustration of Quaternary depositional processes on the eastern margin of the Korean Peninsula. Sediments were deposited mainly by persistent hemipelagic settling and episodic mass flows induced by retrogressive slope failures. After YoonetaL (1996). seismicity associated with faulting and the steep gradient of the slope (Chough et al., 1991b). In particular, the cyclic loading by earthquakes was suggested as a major cause of slope failures under the condition of insignificant loading by sediment overburden (H.J. Lee et al., 1993). In the later stage of the slope failures, large amounts of muds released from mass flows were suspended forming sediment-laden nepheloid layers (Yoon et al., 1996). Turbid layers remobilized enough sediments to activate hemipelagic settling, or directly flowed further downslope and filled the deeper basin floor and topographic lows in the Korea Plateau forming thick homogeneous mud facies. In the late Pleistocene, head scarps of failure scars retrogressively failed, giving rise to thin mass flow deposits in the scarred regions and downslope areas (Yoon et al., 1996; Chough et al., 1997b). Sediment redistribution by mass failures practically terminated at the end of the Pleistocene and hemipelagic sedimentation has played an important role in shaping the sediment-starved slope during the Holocene (H.J. Lee et al., 1996).
CHAPTER 7
Ulleung Basin 7.1
Physiography
The Ulleung Basin occupies the southwestern part of the East Sea (Sea of Japan) (Fig. 5.1). In plan view, the basin is rhomboidal, bounded by continental slopes of the Korean Peninsula and the southwestern Japanese Islands on the west and south, respectively, and by submarine topographic highs of the Korea Plateau and the Oki Bank on the north and east, respectively (Fig. 5.1). The northern and western margins of the basin are relatively steep with gradients of up to 10° and are characterized by largescale slope-failure scars and associated downslope slide/slump and debrisflow deposits. At the base of the western margin slope, a subdued deep-sea fan (Uljin Deep-Sea Fan) is present with a meager feeder system upslope (Fig. 7.1). Along the eastern and southern margins, the basin is surrounded by rather gentle (<3°) slopes of Oki Bank, a submarine extension of Honshu shelf off Shimane Peninsula, and the slope of San-in district of southern Honshu. These slopes are also characterized by various scarp features in the mid to lower slope area that have resulted from slumps and slides. In addition, small-scale gullies occur ubiquitously on the steep upper to mid slope. The gullies generally start near the shelfbreak and extend to water depths of about 1,000 m (H.J. Lee et al., 1993). The basin floor lies at water depths of 2,000 to 2,500 m and gradually deepens to the north and northeast. It is fairly smooth and flat except for a few islands and seamounts of volcanic origin in the northeastern part. The basin extends to the northeast through the Ulleung Interplain Gap (UIG) between the Ulleung and Dok islands (Chough, 1983) (Fig. 7.1). The UIG is a narrow and long interbasin plain between the Ulleung and Japan basins (Fig. 5.1). It cascades onto the deeper Japan Basin near the southwestern end of Kita-Yamato Ridge. Along the axis of the UIG, a deep-sea channel (Ulleng Interplain Channel, UIC) (Chough, 1983), runs northeastward largely controlled by topographic features (KORDI, 1996, 1997) (Fig. 7.2). The UIC is intermittently joined by numerous submarine chutes and channels radiating from the slopes of the Korea Plateau, Ulleung Island, Ulleung Seamount, Dok Island, and Dok Seamount. In some cases, these chutes and channels form an interwoven
240
Marine Geology of Korean Seas
1290
130°
1310
1320E
Fig. 7.1. Bathymetric map of the Ulleung Basin. Contour interval is 100 m. NUE = North Ulleung Escarpment, Is. = Island, Smt = Seamount, UIG = Ulleung Interplain Gap. Courtesy of the National Oceanographic Research Institute.
network. They are either erosional or depositional in origin. The UIC is formed most likely by bottom currents flowing through the long axis of the UIG (Han et al, 1997; KORDI, 1998). The Korea Plateau to the north of the Ullueng Basin is characterized by a number of ridges, seamounts, and intervening troughs that are aligned either N - S or NE-SW (Fig. 7.1). In the western part of the Korea Plateau, the intervening basins and troughs generally trend N-S (e.g., Onnuri Basin, KORDI, 1995) with numerous seamounts of volcanic origin. To the north of Ulleung Island, a trough deepens northward along the North Ulleung
Ulleung Basin 241 Fig. 7.2. Seabeam image of the northeastern part of the Ulleung Basin showing submarine channel system within the Ulleung Interpalin Gap. For location see Fig. 6.13. Photograph courtesy of the Korea Ocean Research and Development Institute.
13f00'E
13f30'E
13?001
Escarpment and extends to the Japan Basin (Fig. 7.1). In the eastern part of the Korea Plateau, there is a chain of small, but prominent seamounts trending NE-SW (Fig. 7.1).
7,2
Crustal Structure
Models on crustal structures in the Ulleung Basin (Ludwig et al., 1975; C.H. Park et a l , 1996; H.J. Kim et al., 1999; G.H. Lee et al., 1999) commonly suggest a two-gradient velocity structure (layers 2 and 3) beneath the unconsolidated sedimentary sequence (layer 1). The models of Ludwig et al. (1975) and C.H. Park et al. (1996) suggest that layer 2 lies at depths between 4 and 8 km below sea level, where the P-wave velocity ranges from about 3.7 to 5.8 km/s and the density varies from 2.58 to 2.63 g/cm^ (Figs.
242
Marine Geology of Korean Seas
300 200-
gravity profiles
150 H
Bouguer
• • ••- calculated 1^
50
I
100
150
"T
200
250
300
Fig. 7.3. Crustal structure (a) in the Ulleung Basin on the basis of gravity modeling (b) and seismic velocity analysis. Numbers in rectangles denote P-wave velocities in km/s; bold numbers are density values in g/cm\ For location of the profile see Fig. 6.13. Modified after C.H. Park et al. (1996). 5.2 and 7.3). Based on seismic profiles, Chough and Lee (1992) postulated the upper part of layer 2 as volcanic flov^s and sills intercalated with sediment layers based on the seismic reflection configuration (see section 7.4.2). Alternatively, G.H. Lee et al. (1999) and H.J. Kim et al. (1999) interpreted this part of layer 2 as a sediment sequence (layer 1), and suggested that layer 2 occurs at a depth of about 6 km below sea level with a thickness ranging from 1.7 to 3.3 km. In their model, the P-wave velocity of layer 2 progressively increases with depth, from 4.8 to 6.3 km/s. A density model shows a basinward transition of average density in this layer from 2.6 g/cm^ in the southwestern margin to 2.84 glcw? in the basin center (C.H. Parketal., 1996).
Ulleung Basin 243 The lower crust (layer 3) is characterized by P-wave velocities of 6.4-7.1 km/s and average densities of 2.88-2.90 g/cm^ (C.H. Park et al., 1996; H.J. Kim et al., 1999; G.H. Lee et al., 1999) (Fig. 7.3). The upper boundary of layer 3 typically lies at a depth of 8 km below sea level. The thickness of this layer is about 8 km in the basinal area. This is thicker than normal oceanic layer 3 by 2-3 km. The Moho occurs at a depth of about 15 km below sea level in the basinal area, and abruptly deepens to 28 km below sea level at the basin margin (C.H. Park et al., 1996).
7.3
Gravity and Magnetic Anomalies
Gravity and magnetic anomaly patterns in the Ulleung Basin were analyzed by Suh et al. (1993, 1998), C.H. Park et al. (1996, 1997), and Han et al. (1997). The free-air gravity anomaly is generally positive, about 40-60 mgal near foot of the Korea Plateau and Oki Bank and about -20-+20 mgal in the central part of the basin. The Bouguer anomaly shows that the basin center is characterized by a NE-SW-trending zone of positive values, about 150 mgal. In the basin margin, this anomaly is generally less than 40 mgal (Figs. 7.3 and 7.4). Suh et al. (1998) suggest that the abrupt increase in both free-air and Bouguer anomalies toward the basin center reflects high-density crustal material beneath the basinal area. The magnetic anomalies in the Ulleung Basin are relative weak, generally less than -400 nT, because of the thick sedimentary sequence above the basement (Fig. 7.5). Although Suh et al. (1998) suggest symmetrical magnetic anomalies in some NW-SE profiles, distinct lineation patterns are not evident across the entire basin (Fig. 7.5).
7.4 7.4.1
Seismic Stratigraphy Acoustic Basement
The acoustic basement in the basinal area typically occurs at a depth of 5-6 s (twt) below sea level (Chough and Lee, 1992) (Figs. 7.6 and 7.7). The top of the acoustic basement is generally marked by a high-amplitude reflector with small-scale relief. Although no distinct graben or half-graben structures are observed, there are two basement lows in the southern and northeastern parts of the basin which are oriented NE-S W and are separated by a median high (Chough and Lee, 1992; G.H. Lee et al., 1999). Along the western margin of the basin, the basement is offset by the steep, N - S trending Ulleung Fault scarp (Figs. 6.2 and 6.4).
244
Marine Geology of Korean Seas 39'N
129'E
134'E
35'N
Fig. 7.4. Bouguer anomaly map of the Ulleung Basin. Contour interval is 20 mgal. Modified after Suh et al. (1998). In the southern and central parts of the basin, the acoustic basement is characterized by high-amplitude, discontinuous parallel reflectors with a low-relief surface (Fig. 7.7). Chough and Lee (1992) have interpreted that this seismic facies reflects volcanic flows and sills. In the northern part of the basin, the reflection from the acoustic basement is often obscure. In some cases, it instead shows reflection of the overlying sedimentary sequence (Unit III-2). The obscure basement zone trends NE-SW (Fig. 7.7). The northern margin of Ulleung Basin and Korea Plateau is underlain by a shallow acoustic basement at depths of 3-4 s {\vA) below sea level (Figs. 7.6 and 7.7). The acoustic basement has no internal reflections; instead the top of the basement is marked by irregular reflectors with high amplitude and high frequency. Chough and Lee (1992) interpreted that this re flection-free basement reflects volcanic ridges or eruption vents. They have also suggested that these volcanic features are progressively younger northward. According to the crustal cross-section of Ulleung Basin suggested by G.H.
Ulleung Basin 245 39°N
35°N
129'E
Fig. 7.5. Magnetic anomaly map of the Ulleung Basin. Contour interval is 50 nT. Modified after Suh et al. (1998). Lee et al. (1999) and H.J. Kim et al. (1999), the acoustic basement is comparable to the upper part of layer 2 with a P-wave velocity ranging from 4.8 to 6.3 km/s. 7.4.2
Sedimentary Sequence
Based on the reflection configuration of multichannel seismic profiles, K.E. Lee (1992) and Chough and Lee (1992) divided the sedimentary sequences of the Ulleung Basin into 4 seismic sequence units (Units III-2, III-l, II, and I in ascending order). The lowermost sequence unit (Unit III-2) ranges in thickness from 0.4 to 1.2 s (twt) (Fig. 7.8a) with an interval velocity of 3.6-4.8 km/s. The unit shows variable-amplitude, parallel to shingled reflections with poor lateral continuity in the northern part of the basin, and low-amplitude, parallel reflection in the southern part (Figs. 7.6 and 7.8a). Chough and Lee (1992) interpreted the former seismic facies as
246 CO ^
Marine Geology of Korean Seas CM m
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DISCONTINUOUS HIGH-AMPUTUDE PARALLEL REFLECTION (Ca. 4.7 k m / s e c , VOLCANIC FLOWS & SILLS)
Fig. 7.7. Time structure and reflection types of acoustic basement in the Ulleung Basin. Contour interval is 0.5 s (twt). Modified after Chough and Lee (1992) by permission of Blackwell Science. volcanic flov^s and sills intercalating w^ith sedimentary layers, and the latter as bedded clastic or volcaniclastic debris. The transition zone between the acoustic basement and the overlying low^ermost sedimentary sequence is also characterized by high-amplitude, discontinuous reflections, probably due to an abrupt contrast in acoustic impedance betw^een the volcanics and the sediments. Unit III-l, mainly comprising the middle Miocene sequence, is relatively uniform in thickness and ranges from about 0.4 to 0.6 s (twl;) in the central part of the basin (Fig. 7.8b). This unit is generally characterized by variableamplitude, parallel reflections v^ith poor lateral continuities. The seismic facies of the unit is indicative of marine shale of various origins (K.E. Lee, 1992). In the western margin of the basin, reflectors in the lower part of the unit onlap onto the basement high. This unit also includes mound
248
Marine Geology of Korean Seas
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249
250
Marine Geology of Korean Seas
configurations and inclined reflectors downlapping the lower boundary, which indicates significant influx of volcaniclastics from the basin margins. In the northeastern part of the basin, Unit III-l is characterized by two distinct reflection configurations: low-amplitude, discontinuous parallel reflections in the lower part, and high-amplitude, discontinuous parallel reflections in the upper part (Fig. 7.8b). The former is interpreted as massive sandstone/shale and volcaniclastics, and the latter as turbidite sequences (Chough and Lee, 1992). The middle seismic unit (Unit II) is characterized by basin-wide, lowamplitude (occasionally high-amplitude), parallel reflections. In the southern part of the basin. Unit II is more than 1.2 s (twt) thick, and progressively thins northward to less than about 0.5 s (twt) (Fig. 7.8c). A thick interval of distinct low-amplitude or quiet reflections with occasional high amplitude reflections is indicative of a thick section of marine shale interbedded with thin sandstone and siltstone beds, probably deposited by turbidity currents (Chough and Lee, 1992). In the southeastern part of the basin, the unit is characterized by chaotic reflections indicative of slide or slump deposits (Fig. 7.8c). In the western part of the basin, the upper unit boundary is frequently marked by erosional truncations and channels. This unit correlates approximately with the upper Miocene and lower Pliocene sequences of Dolgorae-I well. The uppermost unit (Unit I) consists of upper Pliocene and Quaternary sequences. This unit is characterized either by high-amplitude and highfrequency continuous parallel reflections, or by choppy reflections (Fig. 7.8d). In the central part of the basin, the unit is relatively uniform in thickness (0.6-0.8 s twt) (Fig. 7.8d). The continuous parallel reflectors are indicative of interbedded hemipelagic and terrigenous sediments, whereas the choppy configurations represent volcanic ash mixed with hemipelagic sediments (Chough and Lee, 1992). G.H. Lee and Suk (1998) further divided the upper Pliocene and Quaternary sequences into 5 subunits, within which they distinguished 5 seismic facies on the basis of reflection configuration and geometry. The lower two units (upper Pliocene) are typically dominated by structureless-tochaotic internal reflections without distinct external form; these are interpreted as mass-flow deposits (Fig. 7.9a). Thickness variations and seismic facies distribution of the upper Pliocene sequence units suggest that these sediments were derived mainly from the western and southern margins of the basin. In the remaining upper units (Pleistocene and Holocene), the occurrence of mass-flow deposits with structureless-to-chaotic reflections is limited near to the base-of-slope area (Fig. 7.9b), suggesting a marked decrease in the generation of mass movements. Instead, these units are dominated by well-stratified, continuous reflectors that are interpreted as
Ulleung Basin
37^N
37^N
130^ Fig. 7.9. Seismic facies maps of the upper Pliocene seismic unit (a) and the lower Pleistocene unit (b) in the Ulleung Basin. Contours in meters. After G.H. Lee and Suk (1998) by permission of the Elsevier Science B.V.
251
252
Marine Geology of Korean Seas
mixed deposits of turbidites and hemipelagites. Near the axis of the Ulleung Interplain Gap, the occasional influence of bottom currents is evoked by low-relief undulating mounds (Fig. 7.9b).
7.5
Tectonic Evolution
Based on stratigraphic and structural analyses, Yoon and Chough (1995) reconstructed the tectonic evolution of the Ulleung Basin: (1) a pull-apart opening stage in the late Oligocene to early Miocene, (2) a fan-shaped opening stage with a differential rotation of the Japanese Islands in the early to middle Miocene, and (3) a back-arc closing stage since the late Miocene. G.H. Lee et al. (1999) also suggested a N-S pull-apart opening accompanied by the emplacement of thick oceanic crust on the basis of crustal structure and seismic character of the acoustic basement. In the late Oligocene when initial back-arc opening of the East Sea began in the rigid continental block (Fig. 7.10a, inset), the Japanese Arc was relatively straight without its present bent shape. Seafloor spreading was initiated at about 28 Ma in the northeastern Japan Basin; at the same time crustal extension commenced in the Yamato and Ulleung basins (Tamaki et al., 1992) (see section 5.7.3). The initial crustal extension in the Ulleung Basin presumably took place along a narrow zone of intensive magmatism between the Korea Plateau and the SW Japanese Islands (Fig. 7.10a). The major opening of the East Sea occurred in the early Miocene between 23 and 18 Ma (Ingle, 1992). Opening was achieved by complex pull-apart block movement accompanied by differential rotations of the SW and NE Japanese Islands (Jolivet and Tamaki, 1992) (Fig. 7.10b, inset). During the major back-arc opening, the SW Japanese Islands detached from the east Asian continent, and migrated southward (250-300 km) (Fig. 7.10b). This block movement resulted in a right-lateral strike-slip deformation in the Ulleung Fault, a major shear zone in the western margin of the Ulleung Basin (see section 6.2.1), which in turn caused divergent strike-slip deformation along the Hupo and Yangsan faults as secondary shear zones in the rigid continental crust (Fig. 7.10b). As a consequence, the Pohang-Yongduk Basin between the Hupo and Yangsan faults opened rapidly as a pull-apart basin. The strike-slip faulting in the eastern margin of Korea appears to have nearly ceased at the end of the early Miocene (Yoon and Chough, 1995), but strike-slip deformation along the Japanese margin was suggested to have continued into the middle Miocene (Fig. 7.10c, inset) (Ingle, 1992; Jolivet and Tamaki, 1992). Based on these observations, Yoon and Chough (1995) proposed that the Ulleung Basin further expanded during the middle
Ulleung Basin
253
Fig. 7.10. Hypothetical evolutionary reconstruction of the Ulleung Basin, (a) Initial opening stage, (b) Pull-apart opening stage, (c) Fan-shaped opening stage, (d) Back-arc closing stage. Insets show the tectonic evolution of entire East Sea. Reconstructions illustrate paleogeography at the end of each evolution stages. Thick arrows represent relative motions of the continental blocks. DTB = Dolgorae Thrust Belt, HF = Hupo Fault, NKP = North Korea Plateau Block, OB = Oki Bank Block, SKP = South Korea Plateau Block, TF = Tsushima Fault, UF = Ulleung Fault, PYB = Pohang-Yongduk Basin, YF = Yangsan Fault. Darkly- and lightly-shaded areas denote basements of oceanic crust and thinned/extended continental crust, respectively. Modified after Yoon and Chough (1995) by permission of the Geological Society of America, Inc.
254
Marine Geology of Korean Seas
of the SW Japanese Islands (Fig. 7.10c). However, G.H. Lee et al. (1999) argued against the middle Miocene fan-shaped opening in the Ulleung Basin, emphasizing the unreasonably high rotation angle and the absence of middle Miocene basement. The Ulleung Basin continuously widened and deepened during the middle Miocene, as has been deduced from subsidence analysis of the southern margin of the basin (Chough and Barg, 1987) (Fig. 6.10). At the end of the middle Miocene (ca. 12 Ma), the tectonic regime in the East Sea region inverted from tensional to compressional. This tectonic inversion was a consequence of the collision of the Bonin Arc with central Japan (Matsuda, 1979), which caused the subduction hinge to retreat landward at the Ryukyu Trench (Chough and Barg, 1987). This led to crustal shortening in the southern and western margins of the Ulleung Basin, producing uplift, thrust faulting, and folding in the Pohang-Yongduk Basin and the Dolgorae Thrust Belt (Fig. 7.10d). After a short tectonically-quiescent period, at least two events of crustal shortening occurred at the end of the late Miocene and early Pliocene. These tectonic events were accompanied by convergent strike-slip movement along the northern and newly-formed southeastern segments of the Hupo Fault, as well as by widespread faulting and folding in the southern margin (Fig. 7.10d). During the strike-slip reactivation of the Hupo Fault, the eastern block (i.e., Hupo Bank) was progressively uplifted to form a half-graben, the Hupo Basin. Since the late Pliocene, the Ulleung Basin has attained its present tectonic setting.
7.6 7.6.1
Late Quaternary Sediments Distribution and Echo Characters
Closely spaced, high-resolution seismic profiles obtained in the Ulleung Basin reveal various types of late Quaternary deposits. Jeong (1983) and Chough et al. (1985a) recognized a contour-parallel zonal distribution of mass-flow deposits based on high-resolution (3.5 kHz) echo characters: rockfall, slump, and slide deposits on the slope, debris flow deposits at the base of the slope, and turbidites in the central part of the basin. Chough et al. (1997b) presented a more detailed echo character map based on Chirp sonar data (Figs. 6.13 and 6.14). Lee et al. (1999) identified nine debris lobes in the southern part of the basin and analyzed downslope variations in the acoustic character and geometry. Echo type IB shows a sharp bottom echo with discrete, continuous subparallel subbottom reflectors and either flat or undulatory seafloor topography (Table 6.2; Fig. 7.11a,b). It forms large-scale mounds (tens of
Ulleung Basin 255 a. Echo type IB
b. Echo type IB
'2150 B
c. Echo type IjA
d. Echo type I IB
Fig. 7.11. High-resolution (Chirp) seismic profiles showing typical echo types in the Ulleung Basin plain (for location of each profile, see Fig. 6.13). Characteristics and distribution of each echo type are in Table 6.2 and Fig. 6.14, respectively. Horizontal scale bar = 3 km, vertical scale bar = ca. 50 m. After Chough et al. (1997b). km in width) and sometimes fills pond-like depressions and onlaps onto the adjacent mounds. Chough et al. (1997b) interpreted echo type IB as thinbedded turbidites on the basis of the extensive, subparallel reflectors and onlap-fill geometry (Table 6.2). Echo type IB is recognized mainly in the central part of the basin (Fig. 6.14). Echo type IIA is characterized by a semi-prolonged bottom echo With several intermittent subbottom reflectors, and shows smooth or undulatory seafloor topography (Table 6.2; Fig. 7.11c). This echo type is observed mainly on the basin-floor and lower-slope regions. It is succeeded by echo types IIB and IIC upslope, and changes basinward into echo type IB (Fig. 6.14). Chough et al. (1997b) suggest that the semi-prolonged bottom echoes and the intermittent subbottom reflectors are indicative of turbidites, which probably contain large amounts of bedded silt/sand (Table 6.2). Echo type IIB is characterized by very prolonged, flat (or slightly irregular) bottom echoes with no discrete subbottom reflectors, but shows locally two or three subbottom reflectors which are very prolonged and
256
Marine Geology of Korean Seas
diffuse (Table 6.2; Fig. 7.1 Id). This echo type belongs to the part of the turbidite spectrum which contains coarser sediments than echo type IIA (Chough et al., 1997b). Echo type IIB mainly occurs on the rims of the southern and eastern parts of the basin floor, and is transitional downslope to echo types IIA and IB (Fig. 6.14). Echo type IIC is characterized by laterally wedged, acoustically transparent subbottom echoes; acoustic characters of bottom echoes are highly variable, ranging from weak echoes through seafloor-tangent hyperbola to very prolonged echoes (Table 6.2; Fig. 6.15d). This echo type occurs as either a solitary unit or a stacked form of several units. On contour-parallel profiles, this echo type shows either a positive lens or a flatfloored trough. Chough et al. (1997b) interpreted echo type IIC as debrisflow deposits (Table 6.2). This echo is mainly observed in the lower-slope region and is transitional downslope to echo type IIA or IIB (Fig. 6.14). 7.6.2
Chronostratigraphy
The abundant ash layers in the sediment cores provide a tool for the correlation of turbidite and hemipelagic layers. The composition of ash layers and the times of eruptions have been elaborated for the entire region by Arai et al, (1981), Chough (1983), and Chun et al. (1997). The Ulleung ash erupted from Ulleung Island about 9.3 ka (Arai et al., 1981) is composed primarily of colorless pumiceous glass and occurs in the top hemipelagic layer at about 180-190 cm core depth (Fig. 7.12). Phenocrysts in this ash include alkali feldspar and hornblende. Another prominent ash layer is the Aira-Tn ash erupted from the Aira Caldera in southern Kyushu about 21-22 ka. This vitric ash consists mainly of bubble-wall and pumiceous glass shards, and can be correlated through the entire basin at about 300 to 720 cm below the sea bottom (Fig. 7.12). A ^"^C date from the depth of 360-364 cm (core PI03) gives an age of 18,000 ± 640 yrs B.P. This is slightly younger than that postulated by the ash correlation. Other ash layers underlying the Aira-Tn ash include the probable Yamato ash (25-35 ka) from Ulleung Island and the Aso-4 ash from Aso Caldera (50 ka) (Arai et al., 1981). In core PI06, a number of ash layers were found below the Aso-4 ash. In addition to the pelagically settled ash particles, there are a few ash layers composed mainly of ash debris that were transported and redeposited by turbidity currents. The correlation of ash layers and lithologic changes in the cores yields an average sedimentation rate for the pre-Holocene turbidites of about 40 cm/kyr (Bahk and Chough, 1983; Chough and Bahk, 1984/85).
Ulleung Basin
257
25
4J
^.-..; r^Td
900-J
Fig. 7.12. Descriptions of sediment cores from the Ulleung Basin floor. For location of cores see Fig. 6.13.1^= cross-laminated sand, 1^= laminated silt, Ej = laminated mud, E2= graded mud, E3 = homogeneous mud, E4= convolute mud, Fi = bioturbated mud, F2 = bioturbated mud with pyrite filaments, G = indistinctly-laminated mud, H = mud-clast mud. The top ash layer (<^) is called the Ulleung ash (Oki ash) erupted from the Ulleung Island about 9.3 ka; the second layer is the Aira-Tn ash (t) erupted from the Aira Caldera in southern Kyushu about 21-22 ka; the third layer (-^) is either the Yamato ash (25-35 ka) or the Aso-4 ash from Aso Caldera (50 ka) (Arai et al., 1981). Modified after Chough et al. (1984) and Chough and Bahk (1984/85) by permission of the SEPM and the Springer-Verlag.
258
Marine Geology of Korean Seas
7.6.3 7.6.3.1
Turbidite Facies General Statement
Since about the late Pliocene, the accumulation of turbidites has predominated over pelagic sedimentation in the deep part of the Ulleung Basin. The evidence for mass-flow processes on the slopes of the Ulleung Basin corresponds to the existence of thick, layered sediment sequences in the basin plain (Tamaki et al., 1978) that are dominantly turbidites (Chough, 1982, 1984; G.H. Lee, 1983). Mass flows have been more common during glacial periods when sea level was lower, thereby allowing the transport of terrigenous sediments directly to the outer shelf and upper slope via adjoining rivers and streams. Thick turbidite sequences are dominated by thinly laminated and homogeneous muds and alternate with non-turbiditic muds (Figs. 7.12 and 7.13). The latter include indistinctly laminated mud probably deposited by bottom currents, hemipelagic bioturbated mud, and crudely laminated mud. The thinly laminated and homogeneous turbidite mud facies are associated with the minor occurrence of massive or graded gravel and sand layers, micro cross-laminated sands, and silts. The hemipelagic facies are confined mainly to the upper 2 m of the cores, whereas the turbidites are predominant below it (Fig. 7.12). Some fine-grained turbidite layers also consist of abundant biogenic sediments such as planktonic foraminifera. 7.6.3.2
Sedimentary
Facies
Sedimentary facies of turbidites in sediment cores from the Ulleung Basin were analyzed by Chough (1982, 1984), Chough et al. (1984), Chough and Bahk (1984/85), and Bahk et al. (2000). Coarse-grained turbidites of sand and gravel were found mainly in the Ulleung Interplain Gap. The layers are up to 30 cm thick and show distinct graded or massive sedimentation structures (TJ. They are bounded at the base by a scour surface, and are commonly overlain by parallel-laminated sand division. Gravel clasts up to 12 mm in diameter include large amounts of resedimented volcanic debris. Distinct or crude parallel-laminated sand unit (Tb) overlying the Tg-division is composed of terrigenous, volcanic, and foraminiferal sands. Crosslaminated or climbing-ripple cross-laminated sands and silts (T^) also occur in the coarse-grained turbidites, but rarely exceed 1 cm in thickness. In some cases, convoluted mud units are interlayered, consisting of slumped and sheared intervals about 20 cm thick. Fine-grained turbidite facies identified in the Ulleung Basin include laminated silt (or sandy silt), laminated mud, graded mud, and homogeneous mud (Fig. 7.12) (Chough et al., 1984; Chough and Bahk, 1984/85). The
Ulleung Basin 259 95PC-3 350-380 cm
. 95PC-3 D TTS-aOScm
95PC-6 C 286"-316cm
. 95PC-7 d 430-460 cm
BSM
LM\HM
LM\HM HM
Fig. 7.13. X-radiographs of various mud types in the Ulleung Basin. LM = laminated mud, CLM = crudely laminated mud, BM = bioturbated mud, HM = homogeneous mud, BSM = bioturbated silty mud. For core location see Fig. 6.13. Photographs courtesy of the Korea Ocean Research and Development Institute. laminated silt facies (T^) is thinly laminated at the base w^ith laminae becoming indistinct upw^ard. This facies is composed of sand and silt grains w^ith a mud matrix. The maximum grain size is usually about 125 \xm. This facies generally fines upw^ard, characterized by a decrease in both maximum and mean grain sizes. Parallel-laminated mud (Fig. 7.13), designated Ei following Piper's scheme (1978), comprises about 50% of the total occurrence in the Ulleung Basin cores (Chough et al., 1984). It show^s various colors of olive grey and grey olive (5Y5/2-5Y3/2). In most cases, the unit repeats itself, although it occasionally overlies the microripple or cross-laminated division. The
260
Marine Geology of Korean Seas
individual depositional units are ill-defined (Fig. 7.13) and rarely exceed 1 cm in thickness. Sand fractions in the laminated muds are composed mainly (ca. 80%) of foraminiferal grains of planktonic origin. These are concentrated near the base of each unit with discrete lenses of silt and linear particles oriented parallel to bedding plane. Carbonate and organic carbon contents in the laminated mud have average values of 24 and 1.2%, respectively. Graded mud (E2) usually occurs in association with the laminated mud in thin units (less than about 5 mm in thickness) (Fig. 7.12). Where the individual depositional units of each laminae are discernible, the grading is emphasized by a higher concentration of slightly coarser-grained foraminiferal and terrigenous grains near the base of each layer. The top of the parallel-laminated unit is generally overlain by homogeneous mud (E3) (Figs. 7.12 and 7.13). In X-radiographs, this division is devoid of parallel laminae and is thereby differentiated from the underlying laminated division. The thickness of each E3 unit ranges up to 4 cm and consists of terrigenous as well as biogenic ooze components. The homogeneous mud is characterized by the near-absence of grains coarser than 30 |am and is thus well sorted. The carbonate content in the homogeneous mud is 20%) on average, slightly lower than that in the laminated mud; the organic carbon content is about 1.3%) on average. 7.6.3.3
Provenance
The provenance of turbidites in the Ulleung Basin was deciphered in detail by Chough et al. (1981), Bahk (1982), and Bahk and Chough (1983). They determined the relative amounts of heavy minerals found on the margins of the basin and statistically compared these proportions with those in turbidite layers in cores from the basin. This was possible because the basin is bordered by the eastern shelf of Korea, the Korea Plateau, the northwestern shelf of SW Japan (San-in Coast) and the Oki Bank (Fig. 7.1), which are composed of rocks having different assemblages and therefore different detrital heavy mineral provinces. In the western margin of the Ulleung Basin (eastern continental shelf of Korea), the total amounts of heavy minerals in the 63-125 |Lim size fraction range from 0.4 to 5.2%. The heavy mineral assemblage is dominated by green hornblende and stable minerals of metamorphic origin such as garnet, epidote, sillimanite, staurolite, and actinolite-tremolite (Fig. 7.14a). Sillimanite and staurolite are restricted to the western margin of the basin and are practically absent elsewhere. Garnet and brownish tourmaline grains are also abundant in the western margin (13.9 and 2.5%o on average, respectively).
Ulleung Basin 261 In the eastern and southern margins of the basin (Oki Bank and San-in Coast of Honshu), the total amounts of heavy minerals in the 63-125 |Lim size fraction are between 1.6 and 8.3%. Green hornblende is predominant as in the western margin, but followed by unstable minerals such as orthopyroxene, clinopyroxene, and basaltic hornblende with minor amounts
Continental Shelf (Korea) I 63-125 /im D 125-250 /im
4o^
Oki Spur I 63-125 ftm D 125-250 ftm
30
I
JQ
^ > . l L l l , J1^ilrfli.rf1>n
EP CZ GH BH FH HB AT OP CP OV AD SI GT ST ZR TL AP
h^ lllilL
i
I — I •-. • 1 - . . EP CZ GH BH FH HB AT OP CP OV AD Sf GT ST ZR TL AP 520
545
PI03-2 I 63-125 Mm D 125-250 ^m
I
tJl
ui
EP CZ GH BH FH HB AT OP 5> OV AD SI
PI05-3 I 63-125 ^m
v..
6T ST ZR TL AP
\i
• • I I I
••
EP CZ GH BH FH HB AT OP CP OV AD SI GT ST ZR TL AP
I 63-125 fim D 125-250 ftm
30
%
JHu
W\W\d\
h
EP CZ GH BH FH HB AT OP CP OV AD SI
itlti
GT ST ZR TL AP
Fig. 7.14. Histograms showing average frequency of heavy minerals from the eastern continental shelf of Korea (a) and from the Oki Bank (b). A representative turbidite layer (PI03-2) (c) shows a similar assemblage to (a), whereas PI05-3 (d) corresponds to (b). The PI 04-6 (e) sample contains assemblages of both (a) and (b). For core location see Fig. 6.13. EP = epidote, CZ = clinozoisite + zoisite, GH = green homblende, BH = brown homblende, FH = ferrohastingsite, HB = basaltic homblende, AT = actinolite + tremolite, OP = orthopyroxene, CP = clinopyroxene, OV = olivine, AD = andalusite, SI = sillimanite, GT = garnet, ST = staurolite, ZR = zircon, TL = tourmaline, AP = apatite. After Bahk and Chough (1983) by permission of the SEPM.
262
Marine Geology of Korean Seas
of ferrohastingsite, epidote, brown hornblende, and olivine (Fig. 7.14b). Basaltic hornblende (up to 47%) and ferrohastingsite (up to 26%) are the most diagnostic minerals in this region. Turbidite layers in the Ulleung Basin have an average heavy mineral content of 2.4% (calculated from 26 samples) in the 63-125 \xm size fraction. The heavy mineral assemblages are dominated by green hornblende with various mixtures of stable and unstable minerals in different turbidite layers. Some layers contain either the assemblage most similar to those on the western margin (Fig. 7.14c) or to those on the other margin (Fig. 7.14d). Other layers contain an assemblage of minerals common to both margins (Fig. 7.14e). This has been demonstrated further by using Q-mode factor analysis Fig. 7.15 is the result of factor analysis for heavy mineral data in which samples from the east coast and continental shelf of Korea (represented by Factor I) are shown in good contrast to those from the San-in coast and Oki Bank of Japan (represented by Factor II). The turbidite layers can be divided into three groups. The first group includes P103-1, 2, 3, P104-1, 3, 4, 5, and PI05-6 and is close to the Factor I axis. This suggests that the first group was derived mainly from the stable craton of the east Korean continental margin. Most of these turbidite layers (such as P103-1, 2, 3; P104-1, 5; PI05-6) were deposited between major volcanic eruptions. PI04-4 and 5, deposited prior to the third tephra layer, also were derived from the eastern continental margin of Korea. The second group (P103-4, P105-3, 5, 7, and the turbidite layers of PI06) falls in or near the Factor II axis. This suggests a derivation from the San-in coast and Oki Bank. PI03-4 and PI05-7 were deposited nearly coincident with the eruption of Aira Caldera. Two representative layers (PI06-5 and 8) of the core from the Ulleung Interplain Gap consist of a different mineral assemblage from the others, suggesting that the turbidites of PI06 were derived from the adjoining volcanic islands and the Oki Bank. Other turbidite layers (third group), such as PI04-2 and PI05-4, belong to the region of either margins. These layers contain mineral assemblages common to both margins. 7.6.4
Hemipelagic Facies
The present seafloor of the Ulleung Basin is covered with hemipelagic sediments rich in diatoms and fine-grained terrigenous materials with minor amounts of silicoflagellates, spicules, and others. The sediments are bioturbated largely by benthic deposit feeders and contain abundant pyritized filaments of similar origin (Fig. 7.12) (Chough and Bahk, 1984/85; Chough et al., 1984). In the sediment column, materials retain evidence of near-bottom current activity or incorporation into dilute turbid suspensions,
Ulleung Basin 263
1.0
Legend • San-ln Coast and Okl Bank o East Coast and Continental Shelf (Korea)
/ 0.5 h
• y ^
/
\ \ I / •/ /
y.-^A 5-5 A 5-7 A 5-3
A Ulleung Basin
A 4-2
y
• 4-6 A 5-4
_A4-1
^4-5
^^--a.-4-?iVi /
-^-3^
o
°o
o
/ o
0
1
o 1
1
I
0.5
I
I
o oc^ I
L-
1.0 1
Fig. 7.15. Factor loadings for the heavy mineral data. Factor I, heavily weighted on garnet, actinolite-tremolite, sillimanite, staurolite, and zircon, represents the samples from the east coast and continental shelf of Korea. Factor II of orothopyroxene, basaltic hornblende, and ferrohastingsite is due to contributions from the Oki Bank and San-in coast. Modified after Bahk and Chough (1983) by permission of the SEPM. referred to as nepheloid layers (Chough et al., 1984). Other than the pyritized filaments, burrows rarely occur in the pelagic sediments and in the topmost sections of each turbidite layers. Chondrites and rind burrow^s are found commonly. In many sections, the sediments are completely mottled and disrupted by organisms. In some sediment cores from the Ulleung Basin plain, Bahk et al. (2000) described a crudely laminated mud facies (Fig. 7.13) w^hich is also hemipelagic in origin. This facies is represented by poorly sorted, dark olive grey (5Y3/2) mud (or sandy mud). Each facies unit commonly show^s two discrete modal peaks of well-preserved planktonic foraminifera that are randomly scattered throughout the unit. The mud fractions are composed of diatom frustules, terrigenous silt and clay, articulated foraminiferal tests, and pyrite framboids in decreasing order of abundance. Individual units exhibit
264
Marine Geology of Korean Seas
no systematic vertical trend in both clarity and thickness of lamination. Unit thickness ranges from 1 to 9 cm with an average of 3 cm. The poorly sorted diatomaceous matrix and the randomly scattered foraminiferal sand with discrete modal peaks as well as the absence of systematic vertical variation in texture and lamina thickness indicate that this facies was mainly formed by hemipelagic sedimentation. The preservation of the crude lamination also suggests that periodic variations in biogenic and detrital influx played an important role under poorly oxygenated bottom water conditions where bioturbating macrofauna was suppressed (Bahk et al., 2000). The indistinctly laminated muds (Fig. 7.13) are found in cores from the Ulleung Interplain Gap (Chough et al., 1984) where the existence of bottom currents is strongly suggested by scoured channels and moats observed in high-resolution seismic profiles (Chough et al., 1985a). These laminated muds are sparsely bioturbated and better sorted than the other mud facies. The laminae boundaries are generally poorly defined, but are sometimes marked by concentrations of siliceous shell fragments. This facies consists of light olive grey (5Y5/2, 5Y5/1) mud layers alternating with slightly darker olive grey (5Y3/2) mud layers. In the latter, sediments consist mostly of biogenic material rich in diatoms, are largely bioturbated, and contain some pyritized filaments.
7.7
Late Quaternary Sedimentation
Late Quaternary depositional processes in the Ulleung Basin plain have been affected by variations in slope stability of the basin margin. In the late Tertiary, active regional tectonism in the basin margins frequently gave rise to slope failures, which in turn resulted in the emplacement of thick massflow deposits on the basin plain (G.H. Lee and Suk, 1998). In the Pleistocene and Holocene, however, as regional deformation waned, the occurrence of slope failures has been controlled mainly by eustatic sea-level changes (H.J. Lee et al., 1996). During this time, the scale and frequency of slope failures decreased significantly. Calculations of sedimentation rates based on ^"^C dates of several ash layers indicate that the pre-Holocene turbidites on the basin plain accumulated at an average rate of about 40 cm/ky (Bahk and Chough, 1983; Chough and Bahk, 1984/85). The average thickness of individual turbidite layers is about 2 cm (H.J. Lee et al., 1996). This gives a minimum recurrence time of about 50 years, assuming that the individual layer was deposited in a relatively brief period (days or months). Mass-flux estimates between the basin plain and the western margin of the basin suggests that most sediments were merely displaced as slides or slumps to the lower slope. Only about 10% of the sediment mass was
Ulleung Basin 265
y
95PC4
95PC3
MB97PC19
(9.3 ka)
1-13 to I l k a
Fig. 7.16. Summary of sedimentary logs and correlation of cores (for core location see Fig. 6.13). Solid arrows indicate tephra layers with known eruption ages; open arrow indicates location of AMS^'C date (15,090 ± 200 yrs B.P.). After Bahk et al. (2000) by permission of the Elsevier Science B.V.
LEGEND
10
11
I I Bioturbated mud • i Crudely laminated mud I I Homogeneous mud • i Laminated mud • 1 1 Lamina^ sand and s i • I Tephra 49 ka
transported further downslope by turbidity currents and deposited on the basin plain (H.J. Lee et al., 1996). On the basis of the vertical distribution of mud facies together with a chronostratigraphic framework derived from correlative tephra layers with known eruption ages (Arai et al., 1981; Chun et al., 1997), Bahk et al. (2000) differentiated three lithologic units that reflect paleoenvironmental changes
266
Marine Geology of Korean Seas
-13 to 11 ka-Present
a
UIG
Korea Strait Tsushima Current
Sea-level rise Pelagic sedimentation
Stable slope
n \/
n \/
n \/
Well-oxygenated bottom water
^ J. :&i. ^ -^X- ^^X- w
Intensive bioturbation
2 3 — 1 3 to 11 ka Fresh water input
Sea-levei fall _sz_
ly Slope failures
^
i>
Turbidity currents
c
Density stratification
Hemipelagic sedimentation
Poorly oxygenated bottom water Preservation of hemipelagic laminae
49-23ka
Surface cun-ent (?) ^2,
-^ Sea-level fluctuation
Hemipelagic sedimentation
Fig. 7.17. Schematic illustration of paleoenvironmental changes in the Ulleung Basin during the late Quaternary. Cross-sections cut through the Korea Strait and the Ulleung Interplain Gap (UIG). After Bahk et al. (2000) by permission of the Elsevier Science B.V.
Ulleung Basin 267 during the late Quaternary (Figs. 7.16 and 7.17). Units III (49-23 ka) and II (23—12 ka) consist of repetitive sequences of fine-grained turbidites and hemipelagic muds. The fine-grained turbidites generally comprise lower laminated and upper homogeneous muds and were deposited by turbidity currents derived mainly from frequent slope failures during sea-level lowstand. The hemipelagic muds in Unit III are intensively bioturbated, whereas those in Unit II are crudely laminated. This lithologic change between Units III and II is attributed to a variation of the degree of bottomwater oxygenation: relatively well-oxygenated (Unit III) to poorly oxygenated (Unit II). The change of bottom-water oxygenation state may have resulted from increased water-column stratification caused by eustatic sea-level lowering to the sill depth during the last glacial maximum (Oba et al., 1991; Keigwin and Gorbarenko, 1992). Unit I (<~12 ka) consists mainly of clayey bioturbated muds which represent Holocene pelagic sedimentation at the sea-level highstand. H.J. Lee et al. (1996) suggested that the facies change from the turbidite to hemipelagic mud in the absence of significant terrigenous sediment input would reflect physical effects of glacio-marine eustatic sea-level change. Sea-level lowering during the glacial period may have generated excess pore pressure within the sediments by reducing the hydrostatic confining pressure, thereby facilitating the triggering of deep-water slope failures by earthquakes. Storm waves may also have affected the uppermost slope area to induce small-scale slope instabilities during the lowered sea-level stand. Slope destabilization by these glacio-eustatic factors appears to have enhanced large-scale slope failures during the late Pleistocene. In contrast, the topmost hemipelagic mud attests to a stable slope phase associated with higher eustatic sea level in the Holocene, consistent with the infinite slope stability analyses (Chough and Lee, 1987; H.J. Lee et al., 1991, 1993).
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References Alexander, C.R., DeMaster, DJ., Nittrouer, C.A., 1991a. Sediment accumulation in a modem epicontinental-shelf setting: the Yellow Sea. Mar. Geol. 98, 51-72. Alexander, C.R., Nittrouer, C.A., DeMaster, D.J., Park, Y.A., Park, S.C., 1991b. Macrotidal mudflats of the southwestern Korean coast: a model for interpretation of intertidal deposits. J. Sediment. Petrol. 61, 805-824. Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis. Elsevier, Amsterdam, 539 pp. Almagor, G., 1967. Interpretation of strength and consolidation data from some bottom cores off Tel-Aviv-Palmakhim, Israel. In: Richards, A.F. (Ed.), Marine Geotechnique. University Illinois Press, Urbana, pp. 131-153. Aoki, S., Oinuma, K., 1973. Clay minerals in the sediments of the continental shelf, off San-in, the Japan Sea, J. Assoc. Geol. Collaboration Japan 27, 35-39. Aoki, S., Oinuma, K., Sude, T., 1974. The distribution of clay minerals in the recent sediments of the Japan Sea. Deep-Sea Res. 21, 299-310. Aoki, S., Oinuma, K., Okuda, K., Matsuike, K., 1983. Clay mineral composition in surface sediments and the concentration of suspended matter of the East China Sea. In: Acta Oceanologica Sinica (Ed.), Sedimentation and Sedimentation Rate of the Continental Shelf, with Special Reference to the East China Sea. China Ocean Press, Beijing, pp. 473^82. Arai, F., Oba, T., Kitazato, H., Horibe, Y., Machida, H., 1981. Late Quaternary tephrochronology and paleo-oceanography of the sediments of the Japan Sea. Quat. Res. Japan 20, 209-230. Asano, K., 1957. The foraminifera from the adjacent seas of Japan, collected by the S.S. Soyo Maru 1922-1930. Scientific Report of Tohoku University, Series 2 (Geology), V. 28, pp. 1-52. Aubert, H., Pinta, M., 1977. Trace Elements in Soils. Elsevier, Amsterdam, 395 pp. Bahk, J.J., Chough, S.K., 1996. An interplay of syn- and intereruption depositional processes: the lower part of the Jangki Group (Miocene), SE Korea. Sedimentology 43,421^38. Bahk, J.J., Chough, S.K., Han, S.J., 2000. Origins and paleoceanographic significance of laminated muds from the Ulleung Basin, East Sea (Sea of Japan). Mar. Geol. (in press). Bahk, K.S., 1982. Provenance of Turbidites in the Ulleung Back-Arc Basin, East Sea. M.S. thesis. Seoul National University, Seoul, Korea, 77 pp. Bahk, K.S., Chough, S.K., 1983. Provenance of turbidites in the Ulleung (Tsushima) back-arc basin. East Sea (Sea of Japan). J. Sediment. Petrol. 53, 1331-1336. Barg, E., 1986. Cenozoic Geohistory of the Southwestern Margin of the Ulleung Basin, East Sea. M.S. thesis. Seoul National University, Seoul, Korea, 174 pp. Beardsley, R.C., Limebumer, R., Yu, H., Cannon, G.A., 1985. Discharge of the
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Subject Index * •v<»'*^-'««%*i4'.-A.«
Age dating, 9, 14, 58, 69, 96, 103-105, 138-140, 154-155, 168, 177, 179, 195,210,256 Aira-Tnash, 196,256 Akahoyaash, 196 Aluminum, 67, 97, 193 Anmyon Island, 58 Ash layers, 184, 195,256 Aso-4ash, 196,256 Atterberg limits Eastern Continental Margin, 225226 South Sea, 164-165 Yellow Sea, 85, 87
B Back-arc opening, 4, 7, 186, 252 Baekdo Island, 153 Bamchi Formation, 25 Basement escarpment, 201-204, 208 Benthic animals, tidal flats, 92-94 Bohai Basin, 49, 55 Bohai Current, 69 Bouguer anomaly, 243 Bonin Arc, 254
c Carbon East Sea, 193 South Sea, 163 Ulleung Basin, 260 Yellow Sea, 6, 65 Carbonate compensation depth (CCD), 191,197,225 Cenozoic, Korean Peninsula, 34-43 Central Massif, 49 Changjiang Estuary, 63-64, 71 Changjiang River, 47, 63-64, 71 Changri Formation, 12 Changsan Formation, 17-18 Cheju Basin, 145-146
Cheju Island, 47, 145 sedimentary basin near, 146-149 sediment layer near, 56-64, 131-140 Chigunsan Formation, 20-21, 24 Chlorite, 72, 193 Choson Supergroup, 17, 24-25 Taebaek Group, 17 Yongwol Group, 7, 17 Chronostratigraphy, Ulleung Basin, 256 Clay minerals. Yellow Sea, 72 Coastal embayments. South Sea, 149158 Cobalt, Yellow Sea, 75 Conodonts, 14, 19-21,24-25 Copper, Yellow Sea, 74-75 Crustal Structure East Sea, 175-176 Ulleung Basin, 241-243
Dabieshan Belt, 44 Daebo Orogeny, 16-17, 28, 44 Daeho tidal flat, 107-117 accumulation rate, 114-116 Holocene lithostratigraphy, 109-110 morphology and sediments, 110-111 nearshore suspended matter, 111114 seasonal sedimentary processes, 114-116 suspended sediment budget, 116-117 tidal currents, 117 Deugryang Bay, 150, 156 Dok Island, 179,239 Dok Seamount, 239, 240 Dolgorae Thrust Belt, 206-207, 254 Domi Basin, 145 Doumsan fan delta, 40 Duksung fan delta, 40
East China Sea, 1, 47, 64, 71, 134, 165,
308
Marine Geology of Korean Seas
196 geological setting, 145 sedimentary basins, 145-149 East China Sea Basin, 145, 149 East Sea, 1, 173 age and type of crust, 178-180 crustal structure, 175-176 geochemical composition, 193 heat flow, 177-178 Holocene-Pleistocene boundary, 195-196 late Quaternary sediments, 193-196 lithostratigraphy, 184-186 magnetic and gravity anomalies, 176-177 physiography, 173-174 Quaternary paleoceanography, 196197 seismic stratigraphy, 181-184 surface sediments, 191-193 tectonic evolution, 186-191 Eastern continental margin, 199 depositional processes, 237-238 geologic structures, 201-207 high-resolution echo characters, 227233 late Quaternary sediments, 224-238 physiography, 199-201 sediment mass physical properties, 224-226 sedimentary basins, 217-219 sedimentary facies, 226-227 seismic stratigraphy, 208-217 slope failures, 233-235 slope stability, 235-237 surface sediments, 222-224 tectonic evolution, 220-222 Eumsung Basin, 28, 34 Eurasian Plate, 186
Fan delta, 36, 39-40 Farallon Plate, 44 Fine-grained turbidites, Ulleung Basin, 258-260 Foraminifera Eastern Continental Margin, 224
East Sea, 192, 195-196 Yellow Sea, 65 Free-air anomaly, 177, 243 Fusulinids, 24-25
Gamagyang Bay, 58, 150-158 acoustic stratigraphy, 152-155 late Quaternary history, 152 physiography, 150 sediment deposition, 155-156 Geologic structures. Eastern Continental Margin, 201-207 Geotechnical properties Eastern Continental Margin, 224226 South Sea, 160-167 Yellow Sea, 78-87 Gomso Bay, 89-106 accumulation rate, 100, 103 Holocene lithostratigraphy, 100-103 Holocene sea-level curve, 103-106 pre-Holocene oxidized mud, 94-98 sedimentary structures of Holocene sediments, 92-94, 100-103 Gravity anomaly East Sea, 176-178 Ulleung Basin, 243 Gulf of Bohai, 68-69 Gumchon Formation, 25
H Haenam Basin, 28 Han River, 63 Hansu Formation, 12 Hayang Group, 30, 32 Heat flow. East Sea, 177-178 Heavy minerals, Ulleung Basin, 260262 Hefei Basin, 49 Hida Belt, 28 High-resolution seismic stratigraphy, 120-125, 134-135, 167-172 Himenoura Group, 56 Holocene sea-level change, 103-106 Honam Shear Zone, 7, 9, 28, 44, 55 Hongjom Formation, 25
Subject Index Huanghe Delta, 69 Huanghe River, 47, 62, 68-69, 125, 129, 131,196 Huksan deposit, 64 Huksan Platform, 56 Hupo Bank, 199, 205, 219, 220, 254 HupoBasin, 219, 254 Hupo Fault, 205, 252, 254 Hupo Trough, 199 Hwajol Formation, 19 Hwanggangri Formation, 14-16 Hydrocarbon potential, 2-3
I Elite, 72, 97, 193 Imjin Group, 26 Imjingang Basin, 43 Imjingang Belt, 7, 9, 26, 44, 48 Ilchulbong tuff cone, 43 Inchon tidal flat, 89, 92 Indian Plate, 56 Iron, Yellow Sea, 67, 75, 97 Izanagi Plate, 9, 45
Jangki Group, 40 Japan Basin, 173, 175, 177, 179, 181, 189, 192, 195 Jianghan Basin, 49 Jiangsu Coastal Current, 70 Jinan Basin, 28, 34 Jingok unit, 26 JinhaeBay, 150, 156 Juksan Seamount Chain, 240
K Kanmon Basin, Japan, 45, 56 Kaolinite, Yellow Sea, 72, 97 Keum Estuary, 72, 75, 79 Keum River, 75, 142 discharge, 63, mud belt, 78-79, 82, 87 suspended sediment, 63, 71-77 Kita-Yamato Bank, 174, 178 Kita-Yamato Trough, 174 Kohung Peninsula, 158, 163, 167
309
Kojelsland, 158, 167, 171 Kongju Basin, 28 Kongju Fault, 32, 34, 45 Korea Bay Basin, 49 Korea Plateau, 173-174, 177-180, 192, 199,239-241,244,252,260 Korea Strait, 173-174, 192, 197, 224 Korean Peninsula Cenozoic, 34-43 Mesozoic, 26-32 orogenic events, 26-28 Paleozoic, 17-26 Precambrian, 9-17 Quaternary volcanism, 43 sedimentation and tectonic history, 7-9, 43-45 KoshikiBank, 177 Kula-Pacific Ridge, 186 Kula Plate, 56 Kumgang Formation, 12 Kunsan Basin, 49 Kuroshio Current, 65, 77, 166, 224 Kwangju Fault, 32, 45 Kwangyang Bay, 151-151 Kyemyongsan Formation, 12 Kyonggi Massif, 7, 9, 12, 27, 44 Kyongsang Basin, 28, 30-32, 45 Kyongsang Supergroup, 30-32, 149 Kyokpo Basin, 28 Kyushu ash, East Sea, 196 Kyushu Block, 3, 45
Lead, Yellow Sea, 75 Lithostratigraphy East Sea, 184-186 GomsoBay, 100-103
M Machari Formation, 21-22 Machari Thrust Fault, 22 Magnesia East Sea, 176-178 Yellow Sea, 67 Magnetic anomaly East Sea, 176-178 Ulleung Basin, 243
310
Marine Geology of Korean Seas
Yellow Sea, 56 Makkol Formation, 20 Manhang Formation, 25 Mariana-type subduction, 186 Marine terraces, 199 Mass physical properties Eastern Continental Margin, 224226 South Sea, 160-167 Yellow Sea, 78-87 Mesozoic, Korean Peninsula, 26-32 Midongsan Formation, 12 Mitan Formation, 25-26 Moats, South Sea, 151-152, 155-156 Montmorillonite East Sea, 193 Yellow Sea, 69-71 Mukho Basin, 218-219 Munjuri Formation, 12, 14 Mungok Formation, 21-22, 24 Mungyong Group, 17 Myobong Formation, 18
N Nakdong River, 149, 167-168, 169 Nangrim Massif, 7 Neungju Basin, 28 Nickel, Yellow Sea, 75 Nitrogen, Yellow Sea, 67 Non-marine basins, 1, 28-34 North China Block, 7-8,49, 55 North Korea Plateau, 173, 183 North Ulleung Escarpment, 240-241
Okchon Basin, 8, 12, 17, 24, 44 Okchon Fold Belt, 7, 9, 12-17, 26, 28, 44 Okchon Group, 12-16 Hwanggangri Formation, 12, 14-16 Kyemyongsan Formation, 12 Munjuri Formation, 12, 14 Unkyori Formation, 12 Oki Bank, 173, 177, 179-180, 239, 260262 Okinawa Trough, 1, 145, 149 Onnuri Basin, 240
Organic matter Eastern Continental Margin, 225 South Sea, 163 Yellow Sea, 82-85
Pacific Plate, 149, 186 Paekunsan Syncline, 17, 24 Paleochannels, 120, 125, 156 Paleozoic, Korean Peninsula, 17-26 Pangyo Formation, 25 Pench Series, China, 25 Philippine Sea Plate, 149, 186 Physiography East Sea, 173-174 Eastern Continental Margin, 199201 Gamagyang Bay, 150-152 Ulleung Basin, 239-241 Yellow Sea, 47 Pohang Basin,7, 34-40, 45, 217 Pohang-Yongduk Basin, 206, 217, 220, 252 Potassium, Yellow Sea, 67 Precambrian, Korean Peninsula, 9-17 Proto-Pacific Plate, 56, 149 Pseudoconglomerates, 20, 24 Puknori Formation, 12 Pull-apart opening, 187, 252 Pyongan Supergroup, 17, 21,24-26, 43 Pyongchang Group, 17 Pyonghae Basin, 32
Qianliyan Massif, 49 Quaternary paleoceanography, East Sea, 196-197 Quaternary volcanism, 43 Quinling-Dabie Belt, 7
Ravinement surface, 129, 131, 171 Reclamation of tidal flat, 107-117 Relict deposit, 109, 131, 159, 168 Ryukyu Trench, 254
Subject Index
Sadong Formation, 25 Sambangsan Formation, 21-22 Samdo Island, 153, 155 Samgot unit, 26 Sanduo Formation, 55 Scoria cone, 43 Sea of Japan, see East Sea Sedimentation rate, Yellow Sea, 69, 71, 103, 114-116, 140 East Sea, 195, 256, 264 Sediment dispersal. Yellow Sea, 68-78 Sedimentary basins Eastern Continental Margin, 217219 Korean Peninsula, 12-43 South Sea and East China Sea, 145149 Yellow Sea, 49-55 Sediments, East Sea distribution, 191-193 geochemical composition, 193 Sediments, Eastern Continental Margin depositional processes, 237-238 echo characters, 227-233 mass physical properties, 224-226 sedimentary facies, 226-227 Sediments, South Sea depositional processes, 165-167 distribution, 158-160 mass physical properties, 160-165 Sediments, Ulleung Basin depositional processes, 264-267 distribution and echo character, 254256 hemipelagic facies, 262-264 turbidite facies, 258-262 Sediments, Yellow Sea accumulation rate, 69, 71, 103, 114116,140 clay mineral distribution, 72 dispersal, 68-78 distribution, 62-64 foraminiferal content, 65 mass physical properties, 78-87, 125,
311
127 mineral and geochemical composition, 65-67 trace element distribution, 67, 72-75 transgrassive sheet, 63-64, 69, 78-79, 82,85,125-129,131,142-143 Seismic stratigraphy East Sea, 181-184 Eastern Continental Margin, 208217 Gamagyang Bay, 152-155 Ulleung Basin, 243-252 Yellow Sea, 56-62, 120-125, 136137 Sequence stratigraphy. Yellow Sea northern part, 119-131 southern part, 131 -144 Sesong Formation, 19 Shear strength Eastern Continental Margin, 225 South Sea, 164-165, 167 YellowSea,82, 125, 127 Sikhote-Alin, 173-174, 177, 187 Sindong Group, 30, 32 Sino-Korean Precambrian Massif, 48 Slope failure, 199, 233-235, 264 Slope stability, 235-237, 264 Sochangri Formation, 12, 14 Socotra Basin, 145-146, 149 Sodium, Yellow Sea, 67 Somjin River, 149, 167-168, 169 Songaksan tuff ring, 43 Songliao Basin, 49, 55 Songrim Orogeny, 9, 14, 16-17, 26, 44 South China Block, 7-8, 49, 55 South Korea Plateau, 173, 183 South Sea, 1,47,59 coastal embayments, 149-158 geologic setting, 145 late Quaternary transgressive deposits, 167-172 moats, 151-152, 155-156 sediment mass physical properties, 160-167 sedimentary basins, 145-149 surface sediment, depositional processes, 155-156, 165-167
312
Marine Geology of Korean Seas
surface sediment distribution, 158160 suspended sediment, 155 Soya Strait, 173 Specific gravity, Yellow Sea, 85 Stromatolites, 20, 32 Subei Basin, 48-49, 52 Sulu Belt, 44, 48 Surface sediments East Sea, 191-193 Eastern Continental Margin, 222224 South Sea, 158-167 tidal flats, 89 Yellow Sea, 62-67 Suspended sediment. Yellow Sea, 6878, 111-114,116-117 Suwolbong tuff ring, 43
Taebaek Group, 7, 17-21, Taebaeksan Basin, 8, 12, 17-26, 43-44 Taedong Group, 26, 45 Taegi Formation, 18 Taehyangsan Formation, 12 Taiwan-Sinzi Folded Zone, 145 Taiwan Warm Current, 71 TakuyoBank, 174 Tan-Lu Fault, 45, 49, 55-56 Tartar Strait, 173 Tectonic history East Sea, 189-191 Eastern Continental Margin, 220222 Korean Peninsula, 26-28, 43-45 Ulleung Basin, 252-254 Yellow Sea, 55-56 Tidal flats, 87-88 Tidal sand ridges, Yellow Sea, 47, 5859,117,125,127, 129,131-134, 142 Tongjom Formation, 19 Toufangian Series, North China, 24 Toyama Deep-Sea Channel, 174 Trace elements. Yellow Sea, 72-75 Transgressive deposits South Sea, 167-172
Yellow Sea, 57, 63-64, 69, 78-79, 82, 85, 125-129, 131, 142-143 Trilobites, 8, 18-19,22,24 Tsugaru Strait, 173, 196 Tsushima Current, 166, 168, 191, 197, 223 Tsushima Fault, 188, 190, 196, 202, 204,206,221 Tsushima Strait, 173-174, 197 Tuff ring, 43 Tumugol Formation, 19-20 Turbidites, 256, 258>262, 264-267 Turbidity current, 265 Tuwibong Formation, 21, 24
u Udo tuff cone, 43 Uljin Deep-Sea Fan, 239 Ulleung ash, 196,256 Ulleung Basin, 3, 173,239 crustal structure, 175, 241-243 gravity and magnetic anomalies, 177, 243 hemipelagic facies, 262-264 heavy minerals, 260-262 high-resolution echo characters, 254256 late Quaternary chronostratigraphy, 256 late Quaternary sedimentation, 264267 physiography, 239-241 seismic stratigraphy, 243-252 submarine channels, 239-240 tectonic evolution, 252-254 turbidites, 258-262, 264-267 Ulleung Fault, 188, 191, 201-205, 252 Ulleung Interplain Channel, 239 Ulleung Interplain Gap, 173, 239, 258, 262, 264 Ulleung Island, 239, 241, 256 Ulleung Seamount, 239 Unkyori Formation, 12
Volcanic ash layers, see ash layers Volcanism, Cheju Island, 43
Subject Index
w Wagok Formation, 21-22 Water content Eastern Continental Margin, 224 South Sea, 160-163, 167 Yellow Sea, 82, 125, 127 West Sea, see Yellow Sea Wolungian Series, North China, 24 Wonsan Trough, 173, 183 Wunansha Uplift, 52
Yamato ash. East Sea, 196, 256 YamatoBank, 174, 178 Yamato Basin, 173-175, 177, 179, 181, 190-191, 195 Yamato Ridge, 173-174, 179-180, 191192, 194-195 Yangsan Fault, 205-206, 217, 220, 252 Yangtze River, 47 YellowSea, 1,196 acoustic basement, 48, 56, 134 basin evolution, 49-56 benthic animals in tidal flats, 92-94 CaC03 and organic matter in sediments, 65, 70, 82-85 clay mineral distribution, 72 geological setting, 47-49 high-resolution seismic stratigraphy, 56-62 Holocene sea-level curve, 103-106 Holocene sequence stratigraphy, 117-144 Northern Yellow Sea Basin, 49-52 organic carbon, 65 physiography, 47
313
sediment dispersal, 68-78 sediment mass physical properties, 78-87 sediment shear strength, 82, 125, 127 sediment texture and structures, 79, 92-94, 125-127, 140 sediment water content, 82, 125, 127 shallow structure, 56-62 Southern Yellow Sea Basin, 52-55 surface sediment composition, 65-67 surface sediment distribution, 62-64 surface sediments of tidal flats, 8992 tidal flats, 87-117 tidal sand ridges, 117, 125, 127, 129, 131-134, 142 trace element distribution, 72-75 transgressive deposts, 63-64, 69, 7879,82,85,125-129,131,142143 Yobong Formation, 25 Yonchon complex, 26 Yongdong Basin, 28 Yongduk Basin, 210 Yonghung Formation, 21, 24-25 Yongnam Massif, 7, 9, 12, 28, 44 Yongtan Group, 17 Yongwol Group, 7, 17 Yonil Group, 21-24, 36, 210 Yuchon Group, 30
Z Zinc, 72, 74
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