The morphologies and genesis of mega-pockmarks near the Xisha Uplift, South China Sea Qiliang Sun a, b, * , Shiguo Wu a , Martin Hovland c, d , Peng Luo e , Yintao Lu f , Tailai Qu g a Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China c Centre for Geobiology, University of Bergen, 5014 Bergen, Norway d Statoil ASA, Stavanger, Norway e CNOOC Energy Technology & Services Ltd., Supervision & Technology Co, Tianjin 300452, China f PetroChina Hangzhou Institute of Geology, Hangzhou 310023, China g Research Institute of Petroleum Exploration and Development, Beijing 100083, China article info Article history: Received 13 August 2010 Received in revised form 24 November 2010 Accepted 4 March 2011 Available online 11 March 2011 Keywords: Mega-pockmarks Bottom currents Fluid flow Crescentic pockmarks South China sea abstract Pockmarks are normally regarded to be manifestations of fluids escape through the seabed. Kilometer- wide depressions, here called mega-pockmarks occur as widespread features near the Xisha Uplift, northern South China Sea. Most of the pockmarks observed in this area are multi-kilometers and much larger than normal-pockmarks reported worldwide. The maximum diameter observed is 3210 m and the maximum depth observed is 165.2 m based on multibeam data and 2D seismic data. The pockmarks are circular, elliptical and crescentic in plan view. Seismic profiles show that the genesis of pockmarks is related to fluid (gas and/or pore water) escape. According to the fluids pathways, the pockmarks fall into four types: (1) gas chimney-related; (2) depositional boundary-related; (3) gas chimney and inclined structure (fault)-related; (4) inclined structure-related. Bottom currents are strong and complex in the study area. The multibeam data and seismic profiles indicate that they may play an important role on extension, maintenance and shaping of pockmarks. The research of the study area is in its initial stage, and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In the last few years, more and more attention has focused on the fluid escape in continental margins and slopes, because the escaped fluids (mainly methane gas) are as a greenhouse gas in global climate change, and also as an energy resource. It can also influence gravitational instabilities (Fernández-Puga et al., 2007). Pockmarks are the most common manifestations of fluid flow through the seafloor (King and MacLean,1970; Hovland and Judd,1988; Judd and Hovland, 2007; León et al., 2010). They have been found in a number of widely spread locations (Harrington, 1985; Berndt, 2005; Cartwright et al., 2007; Gay et al., 2007; Hovland et al., 2010). Pockmarks are suspected to be formed abruptly by local over- pressured pore water and gas erupting through the seafloor surface sediments. They are subsequently maintained by slow pore water and gas seepage (Hovland et al., 2010; Cathles et al., 2010). However, bottom currents may also contribute on the maintenance and shaping of the pockmarks (Hovland et al., 2002; Hammer et al., 2009). The range of sizes of pockmark is wide, covering more than 4 orders of magnitude. However, the majority of those observed fall within the 10e250 m diameter range and 1e25 m depth range (Pilcher and Argent, 2007). The maximum size of pockmarks, to our knowledge, is diameters in excess of 1500 m and up to 150 m deep, documented by Pilcher and Argent (2007). In this paper, we document numerous mega-pockmarks, of average diameter 1640 m and average depth of 96.7 m (45 pock- marks). The largest has a maximum diameter of 3210 m, and the deepest has a maximum depth of 165.2 m. The possible origin of these mega-pockmarks will be discussed here, based on the mul- tibeam data and 2D seismic data. 2. Geological setting The study area is located to the south of Qiongdongnan Basin and Huaguang Reef, and between the Guangle Uplift and Zhongjian Island, which belongs to the Xisha Uplift (Fig. 1). It is a new study * Corresponding author. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China. Tel./fax: þ86 0532 82898544. E-mail address: sunqiliang@qdio.ac.cn (Q. Sun). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2011.03.003 Marine and Petroleum Geology 28 (2011) 1146e1156