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Yangtze delta, eastern China: 2. Late Quaternary subsidence and deformation
Marine Geology, 112 (1993) 13-21 Elsevier Science Publishers B.V., Amsterdam
13
Letter Section
Yangtze delta, eastern China: 2. Late Quaternary subsidence and deformation Z h o n g y u a n Chen a and Daniel Jean Stanley b'l aDepartment of Geography, East China Normal University, Shanghai, 200062, P.R. China bDivision of Sedimentology, E-207 NMNH, Smithsonian Institution, Washington D.C. 20560, USA (Received January 25, 1993; revision accepted April 15, 1993)
ABSTRACT Chen, Z. and Stanley, D.J., 1993. Yangtze delta, eastern China: 2. Late Quaternary subsidence and deformation. Mar. Geol., 112: 13-21. Late Quaternary deposits, examined on high-resolution seismic profiles, provide evidence of Recent subsidence of the outer Yangtze delta in the East China Sea. Deformed late Pleistocene strata and mud diapirs, tilted bedding, and gas-deformed structures in Holocene sequences record remobilization of underconsolidated sediment within and adjacent to the Yangtze depocenter. Deformation types and their specific position relative to the depocenter record displacement of sediment as a result of Quaternary overburden and probable compaction and deep-seated tectonic motion. The configuration of depositional sequences demonstrates that subsidence in the outer Yangtze delta has continued from the late Pleistocene to the present.
Introduction
Recent subsidence of the lower Yangtze (Changjiang) delta and its contiguous submarine depocenter in the East China Sea is suggested by lithostratigraphic analyses of cores collected on land and seaward of the Yangtze estuary (Stanley and Chen, 1993, this issue). Preliminary observations indicate that the Holocene section is thickest, and that the underlying late Pleistocene surface is concave-up and deepest ( ~ 60 m) just seaward of Hengshan Island. Moreover, calculated long-term averaged rates of subsidence of the Yangtze depocenter are highest (~3.0 to 4.4mm/year) just seaward of the estuary. High-resolution seismic profiles made in the region just seaward of the Yangtze coast (Marine Geological Bureau, Shanghai, 1986) complement stratigraphic information from 11 offshore core 1Author to whom all correspondenceshould be addressed, 0025-3227/93/$06.00
sites (Fig. 1) and geological data obtained on land (Yan and Xu, 1987). Positioning of cores along and near seismic track lines facilitates reliable correlation between acoustic reflectors and cores, some of them radiocarbon dated (Huang et al., 1985; Qin and Zhao, 1987). Seismic records reveal that sediment sequences of late Pleistocene to Holocene age are locally deformed in the vicinity of the Yangtze depocenter. This paper defines the nature, age and origin of these deformed late Quaternary sections. Methodology This study is based primarily on high resolution seismic records obtained during two expeditions off the Yangtze delta coast. Both surveys, using research vessel Fungdou No. 2 in 1982 arid 1983, were made in the area within 122° to 124°E long., and 30 ° to 32°N lat. (Fig. 1). The total length of seismic track lines is about 3000 km (records avail-
© 1993 - - Elsevier Science Publishers B.V. All rights reserved.
14
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able at Marine Geological Bureau, Shanghai, 1986). Subbottom profiling was ma-d~ with an E G & G U N I B O O M seismic system, with an outgoing pulse of 300 to 500 J. Horizontal lines are plotted at 0.01 second intervals on acoustic records. A sediment velocity of --~ 1700 m/s is determined (Huang et al., 1985) on the basis of measured Holocene section thickness in core Y-52 recovered along one of the track lines (Fig. 1). Penetration depths range from 30 to 80 m, and resolution to 0.5 m is attained locally. Seismic profiling was made at a ship speed of approximately 6 knots during both surveys. Litho- and chronostratigraphic interpretations of seismic reflectors are made primarily on the basis of correlation with sediment borings collected close to track lines. We consulted stratigraphic logs of 59 cores recovered at sea and on land (Fig. 1), most with rotary drilling equipment. Of
these borings, 11 were collected in the offshore study area, and 48 cores were recovered on the lower Yangtze delta plain. The length of offshore cores ranges from ~ 3 to 100 m, and those on land range from ~ 2 0 to >300 m; the diameter of most borings is 110 mm. A total of 19 radiocarbon dates were obtained for sections in I1 cores, five of them collected offshore. Information pertaining to these borings is summarized in Stanley and Chen (1993, this issue, their table 1). Correlation between acoustic facies and cores
Detailed information on late Quaternary stratigraphy of the outer delta is determined from the 59 borings recovered on land and at sea (Sun, 1981; G o u et al., 1987; Qin and Zhao, 1987; Hua, 1988; Chen et al., 1991; Stanley and Chen, 1993, this issue). Pre-Quaternary stratigraphic and neotec-
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA. E CHINA
tonic information for the region is provided by several long cores positioned on the outer margin of the Yangtze delta. Particularly valuable in this respect is Y-21 ( ~ 3 4 4 m in length) described by Chen and Yang (1991). The following Holocene sediment facies ( ~ 4 to 47 m thick) are recognized downward from top of borings: lower delta plain and estuarine mud; delta-front sand and mud; and prodelta finegrained mud. These facies bury early Holocene to late Pleistocene transgressive silt and sand ( ~ 1 to 25 m) of shallow marine origin. The above facies are underlain by uppermost Pleistocene fluvial stiff mud and sand of variable thickness; for example, in core Y-21 Upper Pleistocene deposits form a 58 m section. Unconformities separate late Pleistocene from early Holocene transgressive sequences, and the transgressive unit from midHolocene prodelta deposits. Thicknesses and depths of late Pleistocene and Holocene sections in cores of this region are recorded in Stanley and Chen (1993, this issue, their table 1). Three sets of subbottom seismic reflectors have been defined as acoustic facies I, II, and III by Huang et al. (1985). These acoustic facies (Fig. 2) are correlated with lithofacies in five offshore cores (Y-49, Y-52, Y-54, Y-56 and Y-59) positioned adjacent to seismic profiles (Fig. 1). Facies I is characterized by numerous distinct reflectors, usually continuous and parallel to the seafloor (Fig. 2A and B). Acoustic facies I is equivalent to mid- to upper Holocene prodelta deposits mapped in cores. Acoustic facies II comprises less distinct reflectors, which are also parallel or subparallel to the seafloor. In some instances, this (or sections of) facies II is partially transparent (Fig. 2B). The lower portion of these reflectors is interpreted as the transgressive silt and sand unit of early Holocene age. The upper part of facies II correlates with the basal part of the prodelta mud section of mid-Holocene age. Acoustic facies III is distinguished by contorted reflectors underlying the more horizontal reflectors of facies I and II (Fig. 2A-C). Facies III correlates well with fluvial sands and muds of late Pleistocene age in cores. The sharp acoustic demarcation between facies III and above-lying horizontal
15
reflectors II is the unconformity (~12,000-10,000 years before present, yr B.P.) between late Pleistocene and Holocene sections observed in cores. This break, wide-spread across the study area, is interpreted as a truncation surface (cf. Qin and Zhao, 1987; Chen et al., 1991). The areal distribution of the three acoustic facies shows that late Quaternary sections exposed at the seafloor become progressively older (i.e. I, II, III) in a seaward direction east of the Yangtze coast (Fig. 3B). Coring indicates that facies II and III, on the middle shelf east of 123°E long., are covered by a thin (1 to 2m) layer of reworked sand. Moreover, Holocene delta-front sands, reworked by modern strong littoral and offshore currents, cover a large part of the inner shelf beyond the mouth of the Yangtze estuary (Xu et al., 1982). Subbottom penetration by the UNIBOOM system is poor in inner and mid-shelf areas where these two types of reworked sand layers are thick and extensive (cf. Academia Sinica, 1982). Deformation types recorded by seismic profiling Throughout much of the study area, acoustic facies III strata are deformed, in contrast to rather continuous and near-horizontal (gradients usually well under 1 : 1000) layers of facies I and II. Four distinct types of stratal deformation are recorded and interpreted. Disrupted late Pleistocene strata Late Pleistocene horizons forming facies III are intensely deformed as tight folds (Fig. 2A-D) and isoclinal bedding. These latter are, in some cases, related to channeling (Fig. 2A). Strata are as highly deformed at the top of the facies III section as they are at depth, indicating that disruption of strata continued throughout the late Quaternary. Gradients of tilted strata to 1 : 7 are measured. Seismic lines indicate that facies III sections become somewhat less deformed in a seaward direction, i.e. bedding subparallel to the seafloor is recorded east and southeast of 123°15'E long. (Yuan, 1986). Clear evidence of fault displacement is not observed and may be apparent with deeper penetrating seismic systems. Strata, whether in
16
folds or isoclines, are ubiquitously truncated by the sharp horizontal surface (unconformity) that separates acoustic facies II from III. Seismic records indicate that Holocene strata are only locally affected by underlying deformation (exampies in Fig. 2C-F). Processes that deformed facies III are particularly active in the region underlying the outer Yangtze margin and inner shelf. Marked trunca-
z CHEN A N D D.J. STANLEY
tion at top of this facies is the result of eustatic lowering of sea level and subaerial exposure of the inner shelf surface for a period lasting at least 15,000years, i.e. from about 25,000 to 10,000 yr B.P. (cf. Yang and Xie, 1984). During subaerial exposure (maximum low at about 20,000-18,000 yr B.P.; cf. M6rner, 1971; Fairbanks, 1989), erosion was induced by large fluvial channels, some of them well defined (Fig. 2B), to ,-~ 1.5 km wide
17
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA, E CHINA
Fig. 2. Examples of late Quaternary deformation noted on seismic profiles (positions shown in Fig. 1). (A and B) Channels (CH) in deformed facies III. (C and D) Mud diapirs (M.D.) penetrating facies II and I. (E) Tilted strata of facies II and III. (F) Gasdeformed strata (G.D.) in facies I and II. Acoustic facies /=mid- to upper Holocene; H=lower to mid-Holocene; Ill=late Pleistocene; U= unconformity between facies II and III. Distance between vertical lines=l km; heavy vertical bar= ~17m. Explanation in text.
a n d ,,~ 70 m deep. B o t h e r o s i o n a n d d e p o s i t i o n o f alluvial s a n d a n d m u d sequences a r e related to these channels t h a t flowed s e a w a r d t o w a r d the shelf b r e a k . Isoclinal stratification r e c o r d s c h a n n e l m i g r a t i o n (Fig. 2A).
M u d intrusion and diapirs D i a p i r s , f o r m e d by facies I I I units, p e n e t r a t e u p w a r d into (Fig. 2C), a n d c o m p l e t e l y across (Fig. 2D), H o l o c e n e s t r a t a o f facies I I a n d I.
18
Z. CHEN AND D.J. STANLEY
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Fig. 3. (A) Isopachous map of Holocene sediment thickness, including transgressive silt and sand, reveals configuration of major outer Yangtze delta depocenter (after Stanley and Chen, 1993, this issue). Positions of deformed strata types are noted. (B) Distribution at seafloor surface of three major acoustic facies L H and III (after Huang et al., 1985) and position of major Yangtze depocenter at and beyond estuary mouth. Inset, schematic NW-SE section showing relationship among deformed late Quaternary strata, major depocenter and subsidence.
Diapirs are concentrated a b o u t 30 to 40 k m northeast o f Z h o u s h a n archipelago, and a b o u t 80 k m southeast o f the Yangtze estuary m o u t h (Fig. 3). They are interpreted as m u d diapirs on the basis
o f their steep-sided geometry (cf. Shepard et al., 1968; Shepard, 1973). These n a r r o w ( 1 0 0 - 1 5 0 m wide at their u p p e r sections), steep-sided structures closely resemble m u d diapirs off the Mississippi
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA, E CHINA
19
South Pass (Morgan et al., 1968)and Columbia's Magdalena delta (Shepard et al., 1968). They differ from broad, more gentle-sided salt diapirs and domes on many continental margins (cf. Moore and Curray, 1963). Moreover, on the basis of deep cores, it is demonstrated that salt does not underlie the outer Yangtze delta plain and contiguous offshore sector (Chen and Yang, 1991). Some diapirs are completely buried by acoustic facies I and have only slightly disturbed overlying strata. For example, the diapir shown in Fig. 2C suggests that mud pierced across facies II at some time during the past 5000 years; time of initial upward motion is unknown, and only strata in the lower section of facies I is disturbed. Taking into account the 10.2 m thick section of facies II intruded by late Pleistocene mud, a minimal penetration rate of ~ 2.0 ram/year is calculated. Since it is likely that penetration occurred more recently than 5000 years ago, the rate is undoubtedly much higher. Another example of diapirism (Fig. 2D) reveals late Pleistocene mud that has completely penetrated facies II and I, and rises ~ 22 m above the seafloor. If diapirism began as early as 12,000 years ago, the long-term penetration rate has been about 3.4 mm/year. However, since intrusion displaced both late Pleistocene and Holocene strata along its margins, a much higher rate of vertical displacement is likely. Offset of Holocene sections may record damming effects of the diapir which obstructed sediment transport by currents on the seafloor. The marked depression along the structure's northwestern margin is probably a result of bottom current erosion (cf. Stanley et al., 1974).
also tilted in the same manner, thus recording displacement at least as old as late Pleistocene.
Upward-tiltedHolocenestrata Further evidence of deformation during the Holocene to the present is provided by several west-to-east seismic profiles which traverse acoustic facies II exposed at the seafloor (Fig. 3B). In this sector, east of the main Yangtze depocenter (Fig. 3A), strata are commonly tilted (cf. Academia Sinica, 1982; Yuan, 1986). The width of the tilted strata zone may exceed 10 km. Strata usually dip landward (Fig. 2E), with gradients of at least 1 : 60. In most areas, deposits of underlying facies III are
Gas-intruded layers Patches of disrupted Holocene sections are mapped southeast of the Yangtze estuary (Huang and Shen, 1986) over the major delta depocenter (Fig. 3A), where facies I and II strata are normally parallel to the seafloor. These columnar structures appear as discontinuous, non-stratified sediment; they are sharply bounded by acoustically layered Holocene deposits (Fig. 2F). They are about 500 to 800 m wide and extend vertically from acoustic facies III to the seafloor. Unlike diapirs, these columnar structures are not formed by mud intrusion but, rather, by in situ disruption of formerly layered deposits. An origin related to vertical flow of biogenic gas, released from underlying organicrich sediments, has been proposed (Huang and Shen, 1986). Interpretations and conclusions Four types of post-depositional deformation of late Pleistocene to Recent strata are related to geologically recent subsidence in the outer Yangtze delta. These features are specifically sited relative to the major depocenter which extends from the Yangtze estuary in the northwest to the thickened sediment lobe southeast of the coast (Fig. 3A). Deformed structures originated by remobilization of underconsolidated sediment within and adjacent to the depocenter (Fig. 3B, inset). Acoustic facies III of late Pleistocene age is particularly deformed beneath and immediately adjacent to the thickest sector of the late Quaternary lobe. Diapirism, closely associated with this lobe, likely resulted from additional weight of the rapidly deposited Holocene overburden (cf. Paine, 1965; Morgan et al., 1968). Columnar structures, believed to have formed by release of biogenic gas in facies I and II, are localized in the sector immediately overlying the thickest part of the depocenter. Seaward of the thickest part of the lobe are the less deformed strata of acoustic facies III (late Pleistocene) and tilted bedding of facies I and II (Holocene).
20
Z. CHEN AND D.J. STANLEY
In s u m m a r y , d e f o r m a t i o n o f late Q u a t e r n a r y
strata records displacement of unconsolidated sedim e n t as a result o f m a r k e d b u i l d u p o f Q u a t e r n a r y d e p o s i t s a n d their R e c e n t subsidence in the o u t e r Y a n g t z e delta (Fig. 3B). Subsidence is likely related tO c o m p a c t i o n ( Z h u et al., 1964) a n d d e e p - s e a t e d tectonic d i s p l a c e m e n t ( D u a n et al., 1989; S t a n l e y a n d Chen, 1993). O n - g o i n g d e f o r m a t i o n o f the seafloor has direct i m p a c t s e a w a r d o f the c o a s t on such activities as e m p l a c e m e n t o f drilling platforms, h y d r o c a r b o n pipelines a n d i n t e r n a t i o n a l c o m m u n i c a t i o n cables. C l o s u r e o f large d a m s a l o n g the Y a n g t z e River, such as those p l a n n e d at the T h r e e G o r g e s , w o u l d m a r k e d l y reduce sedim e n t i n p u t to the lower Yangtze delta. R e d u c t i o n o f s e d i m e n t i n p u t is likely to alter stability o f seafloor d e p o s i t s in the e s t u a r y n e a r a n d b e l o w S h a n g h a i a n d o n the d e p o c e n t e r i m m e d i a t e l y offshore. This p o t e n t i a l l y i m p o r t a n t i m p a c t w a r r a n t s f u r t h e r research,
Acknowledgements Messrs. W . Yang, B. Shen, Y. Z h a n , Q. W u a n d MS. H. H u a n g o f the M a r i n e G e o l o g i c a l Bureau,
Shanghai, provided essential assistance in gathering a n d a n a l y z i n g seismic a n d c o r e d a t a used in this study. Senior G e o l o g i c a l E n g i n e e r Q. Y a n g gave useful advice for the field investigation. C a p t a i n , officers a n d m e n o f the Fungdou No. 2 are t h a n k e d for their assistance at sea. Dr. A . G . W a r n e k i n d l y reviewed this p a p e r . F u n d i n g was p r o v i d e d b y S m i t h s o n i a n S c h o l a r l y Studies P r o g r a m g r a n t 1233S-204 (to D.J.S.), a n d a fellowship s t i p e n d f r o m the D e p a r t m e n t o f P a l e o b i o l o g y N M N H (to Z.C.).
References Academia Sinica, 1982. Geology of Yellow Sea and East China Sea. Mar. Geol. Div., Inst. Oceanol., Beijing, Science Press, 219pp. Chen, Z., Xu, S. and Yan, Q., 1991. Sedimentary facies of Holocene subaqueous Changjiang river delta. Oceanol. Limnol. Sinica, 22: 29-37 (in Chinese, with English summary). Chen, Z. and Yang, W., 1991. Quaternary paleogeography and paleoenvironment of Changjiang river estuarine region. Acta Geogr. Sinica, 46:436-447 (in Chinese, with English summary),
Duan, G., Gao, D., Bi, X. and Wang, Y., 1989. The wedgeshaped fracture system in Shanghai and the area adjacent to it. Shanghai Geol., 2:23-33 (in Chinese, with English summary). Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: Influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature, 342: 637-642. Gou, X., Xu, S., Wan, J. and Li, C., 1987. Stratigraphy and areal subdivision of Holocene deposits of Yangtze estuary region. In: Q. Yan and S. Xu (Editors), Recent Yangtze Delta Deposits. East China Normal Univ. Press, pp. 92-102 (in Chinese, with English summary). Hua, D., 1988. On characteristics of the microfauna of Holocene in the mouth bars of the Changjiang estuary and its significance. J. East China Normal Univ., 1:87-96 (in Chinese, with English summary). Huang, H. and Shen, B., 1986. Evaluation of engineering geology conditions in area outside Changjiang river mouth. Mar. Geol. & Quat. Geol., 6:25-34 (in Chinese, with English summary). Huang, H., Shen, B. and Wu, Q., 1985. Geological implications of geophysical data in Holocene Changjiang subaqueous delta. Mar. Geol. & Quat. Geol., 5:81-94 (in Chinese, with English summary). Marine Geological Bureau, Shanghai, 1986. Report of Geological Investigation of Holocene Subaqueous Yangtze Delta. Inter. Rep., 123 pp. (Unpubl.) Moore, D.G. and Curray, J.R., 1963. Structural framework on the continental terrace, northwest Gulf of Mexico. J. Geophys. Res., 68: 1725-1747. Morgan, J.P., Coleman, J.M. and Gagliano, S.M., 1968. Mudlumps: diapiric structures in Mississippi delta sediments. In: J. Braunstein and G.D. O'Brien (Editors), Diapirism and Diapirs. Am. Assoc. Pet. Geol. Mem., 8: 145-161. M6rner, N.A., 1971. Eustatic changes during the last 20,000 years and a method of separating the isostatic and eustatic factors in an uplifted area. Palaeogeogr., Palaeoclimatol., Palaeoecol., 9: 153-181. Paine, W.R., 1965. Recent peat diapirs in the Netherlands: A comparison with Gulf coast salt structures. In: J. Braunstein and G.D. O'Brien (Editors), Diapirism and Diapirs. Am. Assoc. Pet. Geol. Mem., 8: 271-274. Qin, Y. and Zhao, S., 1987. Sedimentary structure and environmental evolution of submerged delta of Changjiang river since Late Pleistocene. Acta Sediment. Sinica, 5:105-11 (in Chinese, with English summary). Shepard, F.P., 1973. Sea floor off Magdalena Delta and Santa Marta area, Colombia. Geol. Soc. Am. Bull., 84: 1955-1972. Shepard, F.P., Dill, R.F. and Heezen, B.C., 1968. Diapiric intrusions in foreset slope sediments off Magdalena Delta, Colombia. Am. Assoc. Pet. Geol., 52: 2197-2207. Stanley, D.J. and Chen, Z., 1993. Yangtze delta, eastern China: 1. Geometry and subsidence of Holocene depocenter. Mar. Geol., 112:1-1 I. Stanley, D.J., McCoy, F.W. and Diester-Haass, L., 1974. Balearic Abyssal Plain: An example of modern basin plain deformation by salt tectonism. Mar. Geol., 17: 183-200.
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA, E CHINA
Sun, S., 1981. Sedimentary characteristics of the Holocene Yangtze Delta. Acta Oceanol. Sinica, 3:15-30 (in Chinese, with English summary). Xu, S., Li, P. and Wang, J., 1982. A sedimentary model of the Chang Jiang (Yangtze) river Delta. Sinica Quat. Res., 6: 83-88 (in Chinese, with English summary). Yan, Q. and Xu, S., Editors, 1987. Recent Yangtze Delta Deposits. Shanghai, East China Normal Univ. Press, 438 pp. (in Chinese, with English summary). Yang, H. and Xie, Z., 1984. Sea-level changes along the east
21
coast of China over the last 20,000 years. Oceanol. Limnol. Sinica, 15:1-13 (in Chinese, with English summary). Yuan, Y., 1986. Shallow seismic sequences in the sea area of Yangtze fiver delta. Acta Sediment. Sinica, 4:65-72 (in Chinese, with English summary). Zhu, S., Yang, Y.B., Wang, Q.T. and Zhu, J.A., 1964. Neotectonic movement around the Changjiang river deltaic plain and its effect on the ground sinking. (Research on Yangtze Estuary, 1.) Inst. Estuarine Coastal Res., East China Normal Univ., pp. 252-269.
13
Letter Section
Yangtze delta, eastern China: 2. Late Quaternary subsidence and deformation Z h o n g y u a n Chen a and Daniel Jean Stanley b'l aDepartment of Geography, East China Normal University, Shanghai, 200062, P.R. China bDivision of Sedimentology, E-207 NMNH, Smithsonian Institution, Washington D.C. 20560, USA (Received January 25, 1993; revision accepted April 15, 1993)
ABSTRACT Chen, Z. and Stanley, D.J., 1993. Yangtze delta, eastern China: 2. Late Quaternary subsidence and deformation. Mar. Geol., 112: 13-21. Late Quaternary deposits, examined on high-resolution seismic profiles, provide evidence of Recent subsidence of the outer Yangtze delta in the East China Sea. Deformed late Pleistocene strata and mud diapirs, tilted bedding, and gas-deformed structures in Holocene sequences record remobilization of underconsolidated sediment within and adjacent to the Yangtze depocenter. Deformation types and their specific position relative to the depocenter record displacement of sediment as a result of Quaternary overburden and probable compaction and deep-seated tectonic motion. The configuration of depositional sequences demonstrates that subsidence in the outer Yangtze delta has continued from the late Pleistocene to the present.
Introduction
Recent subsidence of the lower Yangtze (Changjiang) delta and its contiguous submarine depocenter in the East China Sea is suggested by lithostratigraphic analyses of cores collected on land and seaward of the Yangtze estuary (Stanley and Chen, 1993, this issue). Preliminary observations indicate that the Holocene section is thickest, and that the underlying late Pleistocene surface is concave-up and deepest ( ~ 60 m) just seaward of Hengshan Island. Moreover, calculated long-term averaged rates of subsidence of the Yangtze depocenter are highest (~3.0 to 4.4mm/year) just seaward of the estuary. High-resolution seismic profiles made in the region just seaward of the Yangtze coast (Marine Geological Bureau, Shanghai, 1986) complement stratigraphic information from 11 offshore core 1Author to whom all correspondenceshould be addressed, 0025-3227/93/$06.00
sites (Fig. 1) and geological data obtained on land (Yan and Xu, 1987). Positioning of cores along and near seismic track lines facilitates reliable correlation between acoustic reflectors and cores, some of them radiocarbon dated (Huang et al., 1985; Qin and Zhao, 1987). Seismic records reveal that sediment sequences of late Pleistocene to Holocene age are locally deformed in the vicinity of the Yangtze depocenter. This paper defines the nature, age and origin of these deformed late Quaternary sections. Methodology This study is based primarily on high resolution seismic records obtained during two expeditions off the Yangtze delta coast. Both surveys, using research vessel Fungdou No. 2 in 1982 arid 1983, were made in the area within 122° to 124°E long., and 30 ° to 32°N lat. (Fig. 1). The total length of seismic track lines is about 3000 km (records avail-
© 1993 - - Elsevier Science Publishers B.V. All rights reserved.
14
Z. C H E N
20*E
121 *E
122"E
Good =ubbottom penetration Poor eubbottom penetration
...... ~
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Sea
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,
.............. ,
Zhouehan
Archipelago
12~°E
123"E
124°F
Fig. 1. Map showing position of seismic track lines an~t core sites on the outer Yangtze (Changjiang) delta, East China Sea. Arrows pointing to letter codes A - F refer to segments of seismic lines shown in Fig. 2.
able at Marine Geological Bureau, Shanghai, 1986). Subbottom profiling was ma-d~ with an E G & G U N I B O O M seismic system, with an outgoing pulse of 300 to 500 J. Horizontal lines are plotted at 0.01 second intervals on acoustic records. A sediment velocity of --~ 1700 m/s is determined (Huang et al., 1985) on the basis of measured Holocene section thickness in core Y-52 recovered along one of the track lines (Fig. 1). Penetration depths range from 30 to 80 m, and resolution to 0.5 m is attained locally. Seismic profiling was made at a ship speed of approximately 6 knots during both surveys. Litho- and chronostratigraphic interpretations of seismic reflectors are made primarily on the basis of correlation with sediment borings collected close to track lines. We consulted stratigraphic logs of 59 cores recovered at sea and on land (Fig. 1), most with rotary drilling equipment. Of
these borings, 11 were collected in the offshore study area, and 48 cores were recovered on the lower Yangtze delta plain. The length of offshore cores ranges from ~ 3 to 100 m, and those on land range from ~ 2 0 to >300 m; the diameter of most borings is 110 mm. A total of 19 radiocarbon dates were obtained for sections in I1 cores, five of them collected offshore. Information pertaining to these borings is summarized in Stanley and Chen (1993, this issue, their table 1). Correlation between acoustic facies and cores
Detailed information on late Quaternary stratigraphy of the outer delta is determined from the 59 borings recovered on land and at sea (Sun, 1981; G o u et al., 1987; Qin and Zhao, 1987; Hua, 1988; Chen et al., 1991; Stanley and Chen, 1993, this issue). Pre-Quaternary stratigraphic and neotec-
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA. E CHINA
tonic information for the region is provided by several long cores positioned on the outer margin of the Yangtze delta. Particularly valuable in this respect is Y-21 ( ~ 3 4 4 m in length) described by Chen and Yang (1991). The following Holocene sediment facies ( ~ 4 to 47 m thick) are recognized downward from top of borings: lower delta plain and estuarine mud; delta-front sand and mud; and prodelta finegrained mud. These facies bury early Holocene to late Pleistocene transgressive silt and sand ( ~ 1 to 25 m) of shallow marine origin. The above facies are underlain by uppermost Pleistocene fluvial stiff mud and sand of variable thickness; for example, in core Y-21 Upper Pleistocene deposits form a 58 m section. Unconformities separate late Pleistocene from early Holocene transgressive sequences, and the transgressive unit from midHolocene prodelta deposits. Thicknesses and depths of late Pleistocene and Holocene sections in cores of this region are recorded in Stanley and Chen (1993, this issue, their table 1). Three sets of subbottom seismic reflectors have been defined as acoustic facies I, II, and III by Huang et al. (1985). These acoustic facies (Fig. 2) are correlated with lithofacies in five offshore cores (Y-49, Y-52, Y-54, Y-56 and Y-59) positioned adjacent to seismic profiles (Fig. 1). Facies I is characterized by numerous distinct reflectors, usually continuous and parallel to the seafloor (Fig. 2A and B). Acoustic facies I is equivalent to mid- to upper Holocene prodelta deposits mapped in cores. Acoustic facies II comprises less distinct reflectors, which are also parallel or subparallel to the seafloor. In some instances, this (or sections of) facies II is partially transparent (Fig. 2B). The lower portion of these reflectors is interpreted as the transgressive silt and sand unit of early Holocene age. The upper part of facies II correlates with the basal part of the prodelta mud section of mid-Holocene age. Acoustic facies III is distinguished by contorted reflectors underlying the more horizontal reflectors of facies I and II (Fig. 2A-C). Facies III correlates well with fluvial sands and muds of late Pleistocene age in cores. The sharp acoustic demarcation between facies III and above-lying horizontal
15
reflectors II is the unconformity (~12,000-10,000 years before present, yr B.P.) between late Pleistocene and Holocene sections observed in cores. This break, wide-spread across the study area, is interpreted as a truncation surface (cf. Qin and Zhao, 1987; Chen et al., 1991). The areal distribution of the three acoustic facies shows that late Quaternary sections exposed at the seafloor become progressively older (i.e. I, II, III) in a seaward direction east of the Yangtze coast (Fig. 3B). Coring indicates that facies II and III, on the middle shelf east of 123°E long., are covered by a thin (1 to 2m) layer of reworked sand. Moreover, Holocene delta-front sands, reworked by modern strong littoral and offshore currents, cover a large part of the inner shelf beyond the mouth of the Yangtze estuary (Xu et al., 1982). Subbottom penetration by the UNIBOOM system is poor in inner and mid-shelf areas where these two types of reworked sand layers are thick and extensive (cf. Academia Sinica, 1982). Deformation types recorded by seismic profiling Throughout much of the study area, acoustic facies III strata are deformed, in contrast to rather continuous and near-horizontal (gradients usually well under 1 : 1000) layers of facies I and II. Four distinct types of stratal deformation are recorded and interpreted. Disrupted late Pleistocene strata Late Pleistocene horizons forming facies III are intensely deformed as tight folds (Fig. 2A-D) and isoclinal bedding. These latter are, in some cases, related to channeling (Fig. 2A). Strata are as highly deformed at the top of the facies III section as they are at depth, indicating that disruption of strata continued throughout the late Quaternary. Gradients of tilted strata to 1 : 7 are measured. Seismic lines indicate that facies III sections become somewhat less deformed in a seaward direction, i.e. bedding subparallel to the seafloor is recorded east and southeast of 123°15'E long. (Yuan, 1986). Clear evidence of fault displacement is not observed and may be apparent with deeper penetrating seismic systems. Strata, whether in
16
folds or isoclines, are ubiquitously truncated by the sharp horizontal surface (unconformity) that separates acoustic facies II from III. Seismic records indicate that Holocene strata are only locally affected by underlying deformation (exampies in Fig. 2C-F). Processes that deformed facies III are particularly active in the region underlying the outer Yangtze margin and inner shelf. Marked trunca-
z CHEN A N D D.J. STANLEY
tion at top of this facies is the result of eustatic lowering of sea level and subaerial exposure of the inner shelf surface for a period lasting at least 15,000years, i.e. from about 25,000 to 10,000 yr B.P. (cf. Yang and Xie, 1984). During subaerial exposure (maximum low at about 20,000-18,000 yr B.P.; cf. M6rner, 1971; Fairbanks, 1989), erosion was induced by large fluvial channels, some of them well defined (Fig. 2B), to ,-~ 1.5 km wide
17
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA, E CHINA
Fig. 2. Examples of late Quaternary deformation noted on seismic profiles (positions shown in Fig. 1). (A and B) Channels (CH) in deformed facies III. (C and D) Mud diapirs (M.D.) penetrating facies II and I. (E) Tilted strata of facies II and III. (F) Gasdeformed strata (G.D.) in facies I and II. Acoustic facies /=mid- to upper Holocene; H=lower to mid-Holocene; Ill=late Pleistocene; U= unconformity between facies II and III. Distance between vertical lines=l km; heavy vertical bar= ~17m. Explanation in text.
a n d ,,~ 70 m deep. B o t h e r o s i o n a n d d e p o s i t i o n o f alluvial s a n d a n d m u d sequences a r e related to these channels t h a t flowed s e a w a r d t o w a r d the shelf b r e a k . Isoclinal stratification r e c o r d s c h a n n e l m i g r a t i o n (Fig. 2A).
M u d intrusion and diapirs D i a p i r s , f o r m e d by facies I I I units, p e n e t r a t e u p w a r d into (Fig. 2C), a n d c o m p l e t e l y across (Fig. 2D), H o l o c e n e s t r a t a o f facies I I a n d I.
18
Z. CHEN AND D.J. STANLEY
121°E
122°E
I
123°E
HOLOCENE <20 m
~
silt and sand) mmml=l _ . .
,
,[
A
THnCKNESS
(including transgressive
%..
•
<2ore
• "~General area of gas-deformed strata . . . . . !
i
i
.o.~
,
diapirs~
! o,1
Z
.. . . . . . . . . . . . . . . . . 121 °E
122°E
o 123°E
124°E
Fig. 3. (A) Isopachous map of Holocene sediment thickness, including transgressive silt and sand, reveals configuration of major outer Yangtze delta depocenter (after Stanley and Chen, 1993, this issue). Positions of deformed strata types are noted. (B) Distribution at seafloor surface of three major acoustic facies L H and III (after Huang et al., 1985) and position of major Yangtze depocenter at and beyond estuary mouth. Inset, schematic NW-SE section showing relationship among deformed late Quaternary strata, major depocenter and subsidence.
Diapirs are concentrated a b o u t 30 to 40 k m northeast o f Z h o u s h a n archipelago, and a b o u t 80 k m southeast o f the Yangtze estuary m o u t h (Fig. 3). They are interpreted as m u d diapirs on the basis
o f their steep-sided geometry (cf. Shepard et al., 1968; Shepard, 1973). These n a r r o w ( 1 0 0 - 1 5 0 m wide at their u p p e r sections), steep-sided structures closely resemble m u d diapirs off the Mississippi
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA, E CHINA
19
South Pass (Morgan et al., 1968)and Columbia's Magdalena delta (Shepard et al., 1968). They differ from broad, more gentle-sided salt diapirs and domes on many continental margins (cf. Moore and Curray, 1963). Moreover, on the basis of deep cores, it is demonstrated that salt does not underlie the outer Yangtze delta plain and contiguous offshore sector (Chen and Yang, 1991). Some diapirs are completely buried by acoustic facies I and have only slightly disturbed overlying strata. For example, the diapir shown in Fig. 2C suggests that mud pierced across facies II at some time during the past 5000 years; time of initial upward motion is unknown, and only strata in the lower section of facies I is disturbed. Taking into account the 10.2 m thick section of facies II intruded by late Pleistocene mud, a minimal penetration rate of ~ 2.0 ram/year is calculated. Since it is likely that penetration occurred more recently than 5000 years ago, the rate is undoubtedly much higher. Another example of diapirism (Fig. 2D) reveals late Pleistocene mud that has completely penetrated facies II and I, and rises ~ 22 m above the seafloor. If diapirism began as early as 12,000 years ago, the long-term penetration rate has been about 3.4 mm/year. However, since intrusion displaced both late Pleistocene and Holocene strata along its margins, a much higher rate of vertical displacement is likely. Offset of Holocene sections may record damming effects of the diapir which obstructed sediment transport by currents on the seafloor. The marked depression along the structure's northwestern margin is probably a result of bottom current erosion (cf. Stanley et al., 1974).
also tilted in the same manner, thus recording displacement at least as old as late Pleistocene.
Upward-tiltedHolocenestrata Further evidence of deformation during the Holocene to the present is provided by several west-to-east seismic profiles which traverse acoustic facies II exposed at the seafloor (Fig. 3B). In this sector, east of the main Yangtze depocenter (Fig. 3A), strata are commonly tilted (cf. Academia Sinica, 1982; Yuan, 1986). The width of the tilted strata zone may exceed 10 km. Strata usually dip landward (Fig. 2E), with gradients of at least 1 : 60. In most areas, deposits of underlying facies III are
Gas-intruded layers Patches of disrupted Holocene sections are mapped southeast of the Yangtze estuary (Huang and Shen, 1986) over the major delta depocenter (Fig. 3A), where facies I and II strata are normally parallel to the seafloor. These columnar structures appear as discontinuous, non-stratified sediment; they are sharply bounded by acoustically layered Holocene deposits (Fig. 2F). They are about 500 to 800 m wide and extend vertically from acoustic facies III to the seafloor. Unlike diapirs, these columnar structures are not formed by mud intrusion but, rather, by in situ disruption of formerly layered deposits. An origin related to vertical flow of biogenic gas, released from underlying organicrich sediments, has been proposed (Huang and Shen, 1986). Interpretations and conclusions Four types of post-depositional deformation of late Pleistocene to Recent strata are related to geologically recent subsidence in the outer Yangtze delta. These features are specifically sited relative to the major depocenter which extends from the Yangtze estuary in the northwest to the thickened sediment lobe southeast of the coast (Fig. 3A). Deformed structures originated by remobilization of underconsolidated sediment within and adjacent to the depocenter (Fig. 3B, inset). Acoustic facies III of late Pleistocene age is particularly deformed beneath and immediately adjacent to the thickest sector of the late Quaternary lobe. Diapirism, closely associated with this lobe, likely resulted from additional weight of the rapidly deposited Holocene overburden (cf. Paine, 1965; Morgan et al., 1968). Columnar structures, believed to have formed by release of biogenic gas in facies I and II, are localized in the sector immediately overlying the thickest part of the depocenter. Seaward of the thickest part of the lobe are the less deformed strata of acoustic facies III (late Pleistocene) and tilted bedding of facies I and II (Holocene).
20
Z. CHEN AND D.J. STANLEY
In s u m m a r y , d e f o r m a t i o n o f late Q u a t e r n a r y
strata records displacement of unconsolidated sedim e n t as a result o f m a r k e d b u i l d u p o f Q u a t e r n a r y d e p o s i t s a n d their R e c e n t subsidence in the o u t e r Y a n g t z e delta (Fig. 3B). Subsidence is likely related tO c o m p a c t i o n ( Z h u et al., 1964) a n d d e e p - s e a t e d tectonic d i s p l a c e m e n t ( D u a n et al., 1989; S t a n l e y a n d Chen, 1993). O n - g o i n g d e f o r m a t i o n o f the seafloor has direct i m p a c t s e a w a r d o f the c o a s t on such activities as e m p l a c e m e n t o f drilling platforms, h y d r o c a r b o n pipelines a n d i n t e r n a t i o n a l c o m m u n i c a t i o n cables. C l o s u r e o f large d a m s a l o n g the Y a n g t z e River, such as those p l a n n e d at the T h r e e G o r g e s , w o u l d m a r k e d l y reduce sedim e n t i n p u t to the lower Yangtze delta. R e d u c t i o n o f s e d i m e n t i n p u t is likely to alter stability o f seafloor d e p o s i t s in the e s t u a r y n e a r a n d b e l o w S h a n g h a i a n d o n the d e p o c e n t e r i m m e d i a t e l y offshore. This p o t e n t i a l l y i m p o r t a n t i m p a c t w a r r a n t s f u r t h e r research,
Acknowledgements Messrs. W . Yang, B. Shen, Y. Z h a n , Q. W u a n d MS. H. H u a n g o f the M a r i n e G e o l o g i c a l Bureau,
Shanghai, provided essential assistance in gathering a n d a n a l y z i n g seismic a n d c o r e d a t a used in this study. Senior G e o l o g i c a l E n g i n e e r Q. Y a n g gave useful advice for the field investigation. C a p t a i n , officers a n d m e n o f the Fungdou No. 2 are t h a n k e d for their assistance at sea. Dr. A . G . W a r n e k i n d l y reviewed this p a p e r . F u n d i n g was p r o v i d e d b y S m i t h s o n i a n S c h o l a r l y Studies P r o g r a m g r a n t 1233S-204 (to D.J.S.), a n d a fellowship s t i p e n d f r o m the D e p a r t m e n t o f P a l e o b i o l o g y N M N H (to Z.C.).
References Academia Sinica, 1982. Geology of Yellow Sea and East China Sea. Mar. Geol. Div., Inst. Oceanol., Beijing, Science Press, 219pp. Chen, Z., Xu, S. and Yan, Q., 1991. Sedimentary facies of Holocene subaqueous Changjiang river delta. Oceanol. Limnol. Sinica, 22: 29-37 (in Chinese, with English summary). Chen, Z. and Yang, W., 1991. Quaternary paleogeography and paleoenvironment of Changjiang river estuarine region. Acta Geogr. Sinica, 46:436-447 (in Chinese, with English summary),
Duan, G., Gao, D., Bi, X. and Wang, Y., 1989. The wedgeshaped fracture system in Shanghai and the area adjacent to it. Shanghai Geol., 2:23-33 (in Chinese, with English summary). Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: Influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature, 342: 637-642. Gou, X., Xu, S., Wan, J. and Li, C., 1987. Stratigraphy and areal subdivision of Holocene deposits of Yangtze estuary region. In: Q. Yan and S. Xu (Editors), Recent Yangtze Delta Deposits. East China Normal Univ. Press, pp. 92-102 (in Chinese, with English summary). Hua, D., 1988. On characteristics of the microfauna of Holocene in the mouth bars of the Changjiang estuary and its significance. J. East China Normal Univ., 1:87-96 (in Chinese, with English summary). Huang, H. and Shen, B., 1986. Evaluation of engineering geology conditions in area outside Changjiang river mouth. Mar. Geol. & Quat. Geol., 6:25-34 (in Chinese, with English summary). Huang, H., Shen, B. and Wu, Q., 1985. Geological implications of geophysical data in Holocene Changjiang subaqueous delta. Mar. Geol. & Quat. Geol., 5:81-94 (in Chinese, with English summary). Marine Geological Bureau, Shanghai, 1986. Report of Geological Investigation of Holocene Subaqueous Yangtze Delta. Inter. Rep., 123 pp. (Unpubl.) Moore, D.G. and Curray, J.R., 1963. Structural framework on the continental terrace, northwest Gulf of Mexico. J. Geophys. Res., 68: 1725-1747. Morgan, J.P., Coleman, J.M. and Gagliano, S.M., 1968. Mudlumps: diapiric structures in Mississippi delta sediments. In: J. Braunstein and G.D. O'Brien (Editors), Diapirism and Diapirs. Am. Assoc. Pet. Geol. Mem., 8: 145-161. M6rner, N.A., 1971. Eustatic changes during the last 20,000 years and a method of separating the isostatic and eustatic factors in an uplifted area. Palaeogeogr., Palaeoclimatol., Palaeoecol., 9: 153-181. Paine, W.R., 1965. Recent peat diapirs in the Netherlands: A comparison with Gulf coast salt structures. In: J. Braunstein and G.D. O'Brien (Editors), Diapirism and Diapirs. Am. Assoc. Pet. Geol. Mem., 8: 271-274. Qin, Y. and Zhao, S., 1987. Sedimentary structure and environmental evolution of submerged delta of Changjiang river since Late Pleistocene. Acta Sediment. Sinica, 5:105-11 (in Chinese, with English summary). Shepard, F.P., 1973. Sea floor off Magdalena Delta and Santa Marta area, Colombia. Geol. Soc. Am. Bull., 84: 1955-1972. Shepard, F.P., Dill, R.F. and Heezen, B.C., 1968. Diapiric intrusions in foreset slope sediments off Magdalena Delta, Colombia. Am. Assoc. Pet. Geol., 52: 2197-2207. Stanley, D.J. and Chen, Z., 1993. Yangtze delta, eastern China: 1. Geometry and subsidence of Holocene depocenter. Mar. Geol., 112:1-1 I. Stanley, D.J., McCoy, F.W. and Diester-Haass, L., 1974. Balearic Abyssal Plain: An example of modern basin plain deformation by salt tectonism. Mar. Geol., 17: 183-200.
SUBSIDENCE AND DEFORMATION OF THE YANGTZE DELTA, E CHINA
Sun, S., 1981. Sedimentary characteristics of the Holocene Yangtze Delta. Acta Oceanol. Sinica, 3:15-30 (in Chinese, with English summary). Xu, S., Li, P. and Wang, J., 1982. A sedimentary model of the Chang Jiang (Yangtze) river Delta. Sinica Quat. Res., 6: 83-88 (in Chinese, with English summary). Yan, Q. and Xu, S., Editors, 1987. Recent Yangtze Delta Deposits. Shanghai, East China Normal Univ. Press, 438 pp. (in Chinese, with English summary). Yang, H. and Xie, Z., 1984. Sea-level changes along the east
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coast of China over the last 20,000 years. Oceanol. Limnol. Sinica, 15:1-13 (in Chinese, with English summary). Yuan, Y., 1986. Shallow seismic sequences in the sea area of Yangtze fiver delta. Acta Sediment. Sinica, 4:65-72 (in Chinese, with English summary). Zhu, S., Yang, Y.B., Wang, Q.T. and Zhu, J.A., 1964. Neotectonic movement around the Changjiang river deltaic plain and its effect on the ground sinking. (Research on Yangtze Estuary, 1.) Inst. Estuarine Coastal Res., East China Normal Univ., pp. 252-269.