Elsevier

Marine and Petroleum Geology

Volume 43, May 2013, Pages 370-380
Marine and Petroleum Geology

Seismic characteristics and processes of the Plio-Quaternary unidirectionally migrating channels and contourites in the northern slope of the South China Sea

https://doi.org/10.1016/j.marpetgeo.2012.12.010Get rights and content

Abstract

The Plio-Quaternary unidirectionally migrating channels (UMCs) and contourites in the northern slope of the South China Sea were investigated in this paper using seismic data. The UMCs include thalweg deposits (TDs) and laterally migrating deposits (LMDs), which result from the interaction between gravity flow and contour current. The LMDs migrating directions are northeast (NE) and west-southwest (WSW) and also display weak seismic reflection and obvious multi-stages. By contrast, the TDs show high seismic reflection and lateral aggradation.

Giant elongated, confined, and slope sheeted drifts, as well as sediment waves are widespread in the northern slope of the South China Sea. Helicoidal contour currents can generate giant elongated and confined drifts associated with moats. By contrast, tabular contour currents can develop slope sheeted drifts with rare moats. NE-migrating channels, giant elongated drifts, and a number of sediment waves are exclusively formed at water depths of 200 m–1200 m. Drifts, sediment waves, and few WSW-migrating channels are developed at water depths of 1200 m–3000 m.

The intermediate water contour current circulates clockwise, moving from Xisha Islands to Dongsha Islands and extending into the eastern part of Dongsha Islands because of the bathymetry. Deep water contour current may circulate counterclockwise and be transported northeastward through Dongsha Islands to Xisha Islands, bifurcating in the Xisha Islands because of topographical prominences.

Highlights

► Distributions of unidirectionally migrating channels and contourites had been studied. ► Circulations of intermediate and deep water contour current had been improved. ► Process of migrating channels and drifts had been worked.

Introduction

Gravity flow channels and contour current deposits are commonly developed on slopes. The channels are U- or V-shaped and include thalweg, levee, and slump deposits. These deposits are controlled by sediment supply, morphology, slope gradient, tectonic movement, and so on (Antobreh and Krastel, 2006; Armitage et al., 2009; Bertoni and Cartwright, 2005; Cunningham et al., 2005; Jobe et al., 2011; Lastras et al., 2009; Mountjoy et al., 2009; Shepard, 1981; Shepard and Emery, 1973; Weaver et al., 2000). The erosion and deposition of turbidity current and debris flow may contribute to the formation of channels (Shepard, 1981; Weaver et al., 2000). An increasing number of migrating channels attributed to the interaction between gravity flow and contour current has also been reported in West Africa, Brazil, and so on (Biscara et al., 2010; Rasmussen, 1994, 2003; Séranne and Abeign, 1999; Viana et al., 1999; Zhu et al., 2010). Fortunately, a series of NE- and WSW-migrational channels have developed in the northern slope of the South China Sea (SCS) (He et al., 2012; Zhu et al., 2010). However, studies on the genesis of these channels are limited (He et al., 2012; Zhu et al., 2010).

Contourites can reach kilometers of thickness and cover huge area (Faugères et al., 1993; Rebesco and Camerlenghi, 2008). Previous works have concentrated on bedforms, flow directions, frameworks, variability, and so on (Faugères et al., 1993, 1999; Hollister and Heezen, 1972; Kenyon and Belderson, 1973; Masson et al., 2004; Rebesco and Camerlenghi, 2008; Rebesco and Stow, 2001; Wynn and Stow, 2002). Plio-Quaternary contourites are well developed in the northern slope of the SCS (Lüdmann et al., 2005; Shao et al., 2001, 2007; Wang et al., 2010; Zhu et al., 2010; Gong et al., 2012). Accordingly, the present study investigated slope areas of the SCS near Dongsha Islands. Focus was given on the significant issue of the distribution of contourites in the northern slope of the SCS. This study also aimed to 1) summarize the seismic features of unidirectionally migrating channels (UMCs) and contourites, 2) examine the distribution of UMCs and contourites, 3) analyze the processes of UMCs and drifts, and 4) improve contour current circulation models.

Section snippets

Geological setting

The SCS, located in the Asian continent and surrounded by Indochina, Kalimantan Island, the Philippine Islands, and Taiwan Island, is a semi-enclosed deep water basin. The SCS has an area of 3.5 × 106 km2 and is more than 5000 m deep, trending northeast. The Luzon strait (300 km wide and 2500 km deep) is the main channel that connects the SCS with the Pacific Ocean (Li, 2002; Zhu et al., 2010). The SCS is the largest marginal sea in the west Pacific Ocean and is located in the southeastern

Data and methods

This study was based on two-dimensional (2D) and three-dimensional (3D) seismic data provided by the China National Offshore Oil Corporation Research Institute. The 2D seismic profiles covered almost the entire study area with water depths of approximately 200 m–3000 m in the PRMB and QDNB, northern SCS, as shown in Figure 2. The study profiles consist of NW–SE and NE–SW seismic profiles. The space of NW–SE and NE–SW profiles are 400 and 250 m, respectively. Sampling rate is 2 ms and group

Unidirectionally migrating channels (UMCs)

Two kinds of UMCs are found in the northern slope of the SCS. The first kind is developed at water depths of approximately 200 m–1200 m with NE migration. The second one is distributed at water depths of >1200 m, with WSW migration.

NE-migrating channels are developed in southeastern Hainan Island, Baiyun, Liwan1, and Dongsha Islands at water depths of 200 m–1500 m. Most of them orientate N–S, oblique to the slope. These channels are U or V shaped, indicating obvious migrational characteristics,

Processes of UMCs

The UMCs had been reported in Gabon (West Africa), Campos Basin (Brazil), and northern slope of the SCS (Biscara et al., 2010; He et al., 2012; Mulder et al., 2008; Rasmussen, 1994, 2003; Séranne and Abeign, 1999; Viana et al., 1998, 1999; Zhu et al., 2010). These UMCs probably result from the interaction between gravity flow and contour current (He et al., 2012; Rasmussen et al., 2003; Zhu et al., 2010).

On one hand, the sediment supply is able to influence the channel location. UMCs are

Conclusions

In this paper, a series of Plio-Quaternary channels and contourites are studied based on seismic data in the northern slope of the SCS. A number of points are determined as follows:

  • (1)

    Plio-Quaternary NE- and WSW-migrating channels, as well as contourites are developed. Unidirectionally migrating channels result from the interaction between gravity flows and contour currents, where channels migrate in the direction of contour current flow. The contourites include giant elongated, confined, and

Acknowledgments

This work was supported by the National Basic Research Program of China (NO. 2009CB219407) and the National Natural Science Foundation of China (NO. 40972077). We gratefully acknowledge the CNOOC Research Institute for providing the seismic data. We would like to thank Thomas Hadlari, Michele Rebesco, Doctor Chenglin Gong, Pin Yan, Qiliang Sun and anonymous reviewers for their fruitful suggestions.

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