Gao Zhenzhong, He Youbin, , Li Xiangdong, Duan Taizhong

1.School of Geosciences, Yangtze University, Jingzhou 434023, China

2.Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

3.Marathon Oil Company, Houston, Texas 77056, USA

1 Introduction*

Internal-waves are subaqueous waves that develop either between water layers of different densities, or within layers where vertical density gradients are present (Lafond, 1966).Internal-tide is an important type of internal waves whose period is equal to the semi-diurnal or diurnal tide (Rattray, 1960).The study of internal waves has a long history in oceanography which can be traced back to the study of the interfacial wave theory by Stocks in 1847(Munk, 1981).At present, a comparatively deep understanding of internal waves has been achieved on the following aspects: ① the generation, superposition, propagation and boundary layer condition of internal waves(Nakamura and Awaji, 2001; Tanakaet al., 2003; Hibiya 2004; Lemckertet al., 2004; Nash and Moum, 2005; Aguilar and Sutherland, 2006; Rainville and Pinkel, 2006); ②the breaking, reflection, diffraction and attenuation of internal waves when interacting with submarine topography(Legg, 2003; Small, 2003; Troy and Koseff, 2005; Mercieret al., 2008); ③ the swash-back wash flows generated by internal waves (Umeyama and Shintani, 2004, 2006); ④the influence from various submarine topography (Kunze,2002; Pietrzak and Labeur 2004; Martinet al., 2006); and⑤ long-period internal waves and short-period internal waves (Marcet al., 1992; Anohinet al., 2006; D’asaro andLien, 2007).In addition, numerical simulation and suspension of sediments related to internal waves have also been documented (Boguckiet al., 1997; Venayagamoorthy and Fringer, 2006).

The velocity of deep-water bidirectional currents generated by internal waves and internal tides is about 20-50 cm/s, in general, as shown in oceanographic investigations (Gaoet al., 1998).Research from submersibles indicates that these currents can transport sediment of up to fine-grained-sand size, and produce a large number of wave-ripples in water depths of up to several kilometers(Mullinset al., 1982).These investigations suggest that internal waves and internal tides are important deep-water processes that influence deep-water sedimentation and should be preserved in the stratigraphic record.Unfortunately, sedimentologists have not paid enough attention to these important geological agents, and their significance in sedimentology has been largely ignored.Some researchers have noted evidence of internal tides in deep-water deposits,e.g., the bidirectional cross-beds produced by deep-water tides in the pre-Devonian of New Zealand(Laird, 1972), and flaser, wavy and lenticular bedding in Quaternary-Cretaceous cores from the Ontong-Java Plateau at water depths of 2.2?3 km (Klein, 1975).However,these authors did not carry out their studies from the viewpoint of internal-wave and internal-tide deposition.Ancient internal-tide deposits were first recognized by Gao and Eriksson (1991)in Ordovician deep-water sediments of the central Appalachians.Since then, a number of Chinese scholars have made significant contributions to the study of internal-wave and internal-tide deposits in the stratigraphic record.The first detailed Chinese example of internal-wave and internal-tide deposits was identified in the Upper Ordovician Yankou Formation, Tonglu, Zhejiang Province (Gaoet al., 1997; Heet al., 1998; He and Gao, 1999).Over the past decade, 9 additional examples have been discovered and described (Fig.1), including the Middle-Upper Ordovician in the central Tarim Basin (Gaoet al., 1996, 2000; Heet al., 2003); the Upper Paleozoic and Mesozoic in the western Qinling Mountains (Jinet al.,2002; Wanget al., 2005); the Precambrian Anle and Xiushui Formations of northwestern Jiangxi Province (Guoet al., 2003, 2004); the Upper Ordovician in Linan, Zhejiang Province (Liet al., 2005a); the Precambrian Madiyi Formation, Taojiang, Hunan Province (Liet al., 2005b);the Lower Cambrian Balang Formation, Shimen, Hunan Province (Heet al., 2005); the Middle Ordovician Pingliang Formation, western Ordos Basin (Heet al., 2007); andthe Middle Ordovician Miboshan Formation (Dinget al.,2008)and Xiangshan Group (Liet al., 2009, 2010; Heet al., 2011), Ningxia Autonomous Region.

Fig.1 Locations of the study sites of internal-wave and internal-tide deposits in China.

In addition to the discovery and detailed case studies of internal-wave and internal-tide deposits, Chinese authors have also contributed in: ① summarizing the sedimentary characteristics, the typical vertical sedimentary successions and depositional models (Gaoet al., 1998, 2006;He and Gao, 1999; Heet al., 2004, 2008); ② interpreting some deep-sea large-scale sediment waves as having an internal-wave origin, and theorizing their mechanisms(Zhanget al., 1999, 2002; Wanget al., 2005; Heet al.,2007; Gao and He, 2009); and ③ demonstrating that the reservoir properties of internal-wave and internal-tide deposits provide new potential targets for petroleum exploration (Tonget al., 2006; Heet al., 2008).In the internal-wave and internal-tide deposits drilled in the Tazhong Low Salient, Tarim Basin, Xinjiang, there are good shows in 17 members of 5 wells.The accumulative length of core with oil shows, such as oil-bearing, oil-soaked, oil immersion, oil slick, fluorescence, is 64.3 m.It indicates that there is a practical possibility of formation of an oil pool(Gaoet al., 2000).

2 The main characteristics of internalwave and internal?tide deposits

2.1 Occurrence in relative deep-water (oceanic)environments

Internal tides and internal waves are usually well-developed in deep-water environments (such as more than 200-250 m deep), so internal-tide deposits are easily formed and preserved in deep-water environments.In shallow-water areas, internal waves also exist, but internal-wave deposits have a low preservation potential due to the action of more significant waves and tides.So the sedimentary environment of internal-wave deposits is different from that of shallow-water tidalites.

Internal-tide and internal-wave deposits, turbidity current deposits and contour-current deposits are all formed in deep-water environments, mostly in continental slope and rise environments.Because internal-tide and internal-wave deposits are usually the products of reworked fine-grained turbidity current deposits by internal tides and internal waves, their clastic compositions are similar.The grain-size of sandstone (grainstone)of internal-tide and internal-wave deposit origin is similar to that of fine-grained turbidites and sandy contourites.Distinguishing correctly internal-tide and internal-wave deposits, turbidites and contourites is also the key to recognizing internal-tide and internal-wave deposits.There are distinctions between them in the terms of sedimentary structures, relationships between the direction of directional sedimentary structures and palaeogeographical patterns, vertical successions, bioturbation, and so on.

2.2 Sedimentary structures and lithofacies types

Internal-wave and internal-tide deposits are generally composed of mud to fine-grained sand, and potentially a little medium- to coarse-grained sand at lower flow velocities (about 20-50 cm/s max.).Generally, internal-wave and internal-tide deposits in submarine canyons and other gullies are sand dominated, whereas deposits formed in flat and open, unchannelized continental slope environments range from sand to silt or mud-sized particles, where the mudstones are generally black or dark gray.

The most typical sedimentary structures of internalwave and internal-tide deposits are bidirectional crossbeds (Figs.2, 3)and unidirectional cross-beds with laminae dipping up the submarine canyon or regional slope(Gao and Eriksson 1991; Gaoet al., 1998; He and Gao,1999).Additional structures include compound bed-sets consisting of rhythmic thin layers of sandstone and mud-stone (Gaoet al., 1997, 2000; Heet al., 1998, 2003), and flaser, wavy and lenticular bedding (Gaoet al.1997; Jinet al., 2002; Guoet al., 2003, 2004).Recently, various types of wave-ripples generated by internal waves and internal tides were documented, which include fascicular lenses superposed with cross-laminations, complexly interweaving cross-lamination structures (Jinet al., 2002)and wave ripples (Jinet al., 2002; Guoet al., 2003, 2004), as well as undulatory lamination, bunchy cross-lamination and cross-laminated lenses.

Fig.2 Charcoal drawing (A)and rose diagrams of foreset azimuths (B)of bidirectional cross-bedding in internal-tide deposits (simplified from Gao and Eriksson, 1991).

Fig.3 Bidirectional cross-bedding in internal-tide deposits.A-Argillaceous siltstone with bidirectional cross-bedding, Xujiahuan Formation, Langzuizi area, Zhongwei County, Ningxia, coin diameter is approximately 2 cm; B-Calcareous siltstone with bidirectional cross-bedding, note that the dip direction of the laminae in set a is opposite to that in set b, Xujiahuan Formation, Langzuizi area,Zhongwei County, Ningxia, length of the scale is 5 cm; C-Calcareous sandstone interbedded with shale, bidirectional cross-bedding are developed in which sets a, b and c are intercalated with shale, upper part of Xujiahuan Formation, northern Mopanjing section, Zhongwei County, Ningxia, coin diameter is approximately 2 cm; D-Calcareous siltstone with bidirectional cross-bedding, b-internal erosion surfaces, i-polished surfaces of core, Well TZ10, O2+3, Tarim Basin, Xinjiang, the line at lower left is 1 cm in length; E-Calcareous siltstone with bidirectional cross-bedding, b-polished surfaces of core, Well TZ10, O2+3, Tarim Basin, Xinjiang, the line at lower left is 1 cm in length (A, B, C from He et al., 2011, D and E from Gao et al., 2000).

In this article, the lithofacies classification is mainly based on sedimentary structures.A summary of the sedimentary structures, lithofacies types and related hydrodynamic interpretation of internal-wave and internal-tide deposits is shown in Table 1.Three flow effects of internal waves and internal tides correspond to three typical sedimentary structures respectively: ① bidirectional cross-bedding generated by alternating bidirectional currents of internal-wave and internal-tide origin; ② unidirectional cross-bedding generated by unidirectional dominated currents which are formed by the superimposition of an internal tide and a long period internal wave,with laminae dipping up-channel (or slope)providing a diagnostic characteristic; ③ undulatory laminations andcross-laminated lenses generated by deep-water oscillatory flows which are formed by internal waves and internal tides interacting with submarine topography close to wave base.

2.3 Vertical sedimentary successions

Four basic sedimentary successions of internal-wave and internal-tide deposits are recognized, which include:① a coarsening-up and then fining-up succession (bidirectional graded succession), ② a fining-up succession(unidirectional graded succession), ③ a coarsening-up and then fining-up succession with couplets of sandstone and mudstone (bidirectional graded couplet succession),and ④ a mudstone-oolitic limestone-mudstone succession(Fig.4).

The fundamental feature the of coarsening-up and then fining-up succession is that the dominant grain size is fine sand, and the coarsest part is located in the middle, whereas the grain size decreases both upwards and downwards(Figs.4a, 4b).These features suggest a weak?strong?weak hydrodynamic condition (Gaoet al., 2000; Heet al., 2004)which is closely related to the period of internal wave and internal tide.In short, this is a vertical succession that closely reflects the period of internal waves and internal tides.

The feature of the fining-up succession is that the dominant grain size is fine sand, with the coarsest portion located in the lower succession where grain size is gradually fining upwards.The basal contact with the underlying mudstone is sharp, but the contact with overlyingmuddy deposits is gradational.Two subtypes of this succession can also be identified by variations in grain size and sedimentary structures (Figs.4c, 4d).The responsible mechanism of this succession is that only the deposits related to the waning period are preserved,i.e., the normally graded portion, due to the erosion of previously deposited fine-grained sediments by subsequent strong currents produced by rapidly increasing current speed during the waxing period (He and Gao, 1999; Gaoet al., 2000; Heet al.,2004).This is a vertical succession that is related to the denudation of internal waves and internal tides.

Table 1 Sedimentary structures and lithofacies types of internal-wave and internal-tide deposits and their hydrodynamic interpretation

Fig.4 Vertical successions of internal-tide and internal-wave deposits (modified from He et al., 2004).a-Coarsening-up and then fining-up succession consisting of cross-laminated sandstone; b-Coarsening-up and then fining-up succession consisting of medium-scale and small-scale cross-laminations; c-Fining-up succession consisting of cross-laminated sandstone; d-Fining-up succession consisting of medium-scale cross-laminations and bidirectional cross-laminated sandstone; e-Coarsening-up and then fining-up succession consisting of sandstone and mudstone couplets; f-Mudstone-oolitic limestone-mudstone succession.

The coarsening-up and then fining-up succession with couplets is usually developed in very gentle and open areas.In this environment, the velocities of bidirectional currents caused by internal waves and internal tides are less than in channelized environments, but the slack-water periods between current reversals are longer.Mud layers deposited from suspension during the “tidal stillstand” and sand layer deposited during the “flood tide” or “ebb tide”are interbedded and preserved.And because of the control exerted by the longer period, these frequently alternating beds also display symmetrical graded couplet successions(Fig.4e)(Gaoet al., 1998; He and Gao, 1999).This is a vertical succession that is controlled by double periods of internal waves and internal tides.

The mudstone-oolitic-limestone mudstone succession(Fig.4f)is mainly developed in the clastic-dominated portion of the Middle and Upper Ordovician in the central Tarim Basin, and consists of oolitic limestone or sandy oolitic limestone and mudstone.The contacts of the oolitic limestone with the underlying and overlying mudstone are mostly sharp.Sometimes the top contact is gradational(Gaoet al., 2000).This is a vertical succession that is possibly related to the denudation and paroxysm of internal waves and internal tides.

2.4 Depositional models

Three depositional models for internal-wave and internal-tide deposits were established: a model for internal-wave and internal-tide deposits in submarine channels;a model for internal-tide deposits in unchannelized continental-slope environment; and a depositional model for internal-tide deposits in oceanic plateau settings (Fig.5).

During a sea-level lowstand in channelized continentalslope environments, coarse-grained gravity flows commonly develop and the energy of internal waves and internal tides is too weak to rework the sand and gravelsize terrigenous sediments introduced by gravity flows.It is therefore difficult for recognizable internal-wave and internal-tide deposits to form.With a rise in sea level, the distance from sediment source areas to depositional areas gradually increases, coarse-grained clasts are stranded closer to source areas, and internal waves and internal tides become dominant in reworking fine-grained gravity-flow deposits.Internal-wave and internal-tide deposits formed in this environment are mainly bidirectional cross-laminated sandstone facies and unidirectional cross-bedded and cross-laminated sandstone (siltstone)facies.

In unchannelized continental-slope environments, internal-tide currents have lower flow velocities than in submarine channels.Under these conditions, the distinctive, thin interbeds of sandstone (or grainstone)and mudstone aredeveloped in response to alternating bed load and suspension load deposition.Broad plateaus at abyssal and bathyal depths are also advantageous locations for the development of internal-tide deposits.The topography of a plateau is generally flat and its resistance to flow is small.Thus,internal tidal currents can maintain critical velocities over a long distance, transport fine-grained sediments and form internal-tide deposits.

Fig.5 Depositional models of internal-tide and internal-wave deposits (modified from Gao et al., 1998).A-Depositional model for internal-wave and internal-tide deposits in submarine channels; A1-Sea-level lowstand, coarse-grained clastic gravity-flows dominate;A2-Sea-level highstand, internal-wave and internal-tide deposits become more important.B-Depositional model for internal-tide deposits in unchannelized continental-slope environments.C-Depositional model for internal-tide deposits on oceanic plateaus.1-Gravity flows; 2-Internal-tide currents; 3-Internal-wave and internal-tide currents; 4-Internal-wave and internal-tide deposits; 5-Sandstone;6-Mudstone/shale; 7-Limestone; 8-Underlying rocks.

We emphasize that the cases we discussed above are true in general conditions, but some exceptions also exist.For example, turbidites with regressive successions associated with internal-wave and internal-tide deposits were discovered in the western Qinling Mountains (Jinet al.,2002).In addition, internal-wave and internal-tide deposits developed in unchannelized continental-slope environments lacking flaser, wavy and lenticular bedding were discovered in Xiangshan Group, Ningxia, China (Liet al.,2009).Lack of flaser, wavy and lenticular bedding in this case may be due to the superimposition of short period internal-waves which do not allow the deposition of suspened mud during the weaker flow period.

3 Existing challenges

Up to now, detailed examples of internal-wave and internal-tide deposits are still very limited, so the primary objective in internal-wave and internal-tide deposits research should be to continue refining existing criteria for identification and recognition of deposits formed by internal waves and internal tides.

As our previous discussion suggests, the sedimentary characteristics of internal-wave and internal-tide deposits are mainly related to the following factors: ① flow effects generated by internal waves and internal tides (i.e., bidirectional cross-bedding and bidirectional currents; unidirectional cross-bedding and unidirectional-dominated currents; wave-ripple bedding and deep-water oscillatory flows); ② the periods of internal waves and internal tides(bidirectional graded vertical succession; rhythmic thin alternating layers of sandstone and mudstone; flaser, wavyand lenticular bedding); ③ erosion by internal-wave and internal-tide currents (i.e., unidirectional graded vertical succession).These three factors are closely related to not only the generation, superimposition and propagation of internal waves and internal tides, but also to the breaking,reflection, diffraction, attenuation and swash-backwash flow, which are all caused by the interaction between submarine topography and internal waves and internal tides.All these phenomena mentioned above have been relatively well studied in oceanographic physics.However,the findings from modern oceanographic physics have not been effectively applied to the study of internal-wave and internal-tide deposits.Similarly, mechanisms inferred from the stratigraphical record of internal-wave and internal-tide deposits have not been used to advance our understanding of the physical characteristics of internal waves and internal tides.This is a potential second objective in internal-wave and internal-tide deposit research.

The comprehensive research method of integrating modern sedimentation, ancient deposits and laboratory experimentation has a long history beginning with the pioneering studies of turbidity currents and turbidites.In contrast, the research of internal-wave and internal-tide deposits has not benefited from the application of flume experiments.In addition, a large gap between studies of modern internal-wave and internal-tide deposits and ancient examples from the rock record also exists,i.e., significant lack of study of ancient examples.Therefore, the third objective in internal-wave and internal-tide deposit research should be to establish comprehensive research methods and techniques.

Furthermore, due to the limited number of published studies, there are some other more specific issues pertaining to internal-wave and internal-tide deposits: ① lack of systematic research on controlling factors, formation conditions and tectonic setting of internal-wave and internal-tide deposits, especially concerning their relationship with palaeo-ocean environments and palaeoclimate; ②limited studies on seismic response of internal-wave and internal-tide deposits, particularly on the recognition of ancient internal-wave and internal-tide deposits in seismic sections where there are currently only a few case studies documented; ③ very limited studies regarding diagenesis and petroleum significance of internal-wave and internal-tide deposits.

4 Conclusions

The study of internal-wave and internal-tide deposits is a very young research topic in the field of deep-water sedimentology.Although numerous achievements have been documented in the nearly twenty years since the first discovery of the internal-wave and internal-tide deposits in the stratigraphic record, there are many opportunities to expand our current knowledge.Perhaps one of the most important tasks in internal-wave and internal-tide deposits research at present involves integrating the characteristics of internal-wave and internal-tide deposits with the dynamic theory of internal waves and internal tides consistently in the following three study areas: modern sedimentation, ancient deposits and flume experimentation.

Acknowledgements

This research was funded by the National Natural Science Foundation of China (No.40672071 and 41072086)and the Research Fund for the Doctoral Program of Higher Education in China (No.20104220110002).We express our sincerest gratitude to Prof.Feng Zengzhao (Editor-in-Chief ofJournal of Palaeogeography)and two reviewers for their constructive comments.

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Umeyama, M., Shintani, T., 2004.Visualization analysis of run up and mixing of internal waves on an upper slope.Journal of Waterway, Port Coast and Ocean Engineering, 130(2): 89-97.

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Wang Hongwei, He Youbin, Sun Yixian, 2005.Internal-wave and internal-tide deposits in the Upper Permian of Xiahe area of Gansu Province.Journal of Oil and Gas Technology, 27(4): 561-562 (in Chinese with English abstract).

Wang Qingchun, Bao Zhidong, He Ping, 2005.The generational mechanism of orientating sedimentary structure by internal-waves.Acta Sedimentologica Sinica, 23(2): 255-259 (in Chinese with English abstract).

Zhang Xingyang, Gao Zhenzhong, Yao Xuegen, 1999.Internal-wave deposits in the North-eastern Rockall Trough, North Atlantic Ocean-Reinterpretation of deep-water sediment waves formation.Acta Sedimentologica Sinica, 17(3): 80-83 (in Chinese with English abstract).

Zhang Xingyang, He Youbin, Luo Shunshe, Bie Biwen, 2002.Deep-water sediment waves formed by internal waves.Journal of Palaeogeography, 4(1): 83-88 (in Chinese with English abstract).