Marine Geology 361 (2015) 136–146 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo Provenance of Upper Miocene sediments in the Yinggehai and Qiongdongnan basins, northwestern South China Sea: Evidence from REE, heavy minerals and zircon U–Pb ages Licheng Cao a,b,TaoJianga,⁎, Zhenfeng Wang c, Yingzhao Zhang c, Hui Sun a a Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China b State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China c Zhanjiang Branch of China National Offshore Oil Corporation, Zhanjiang 524057, China article info abstract Article history: The Yinggehai and Qiongdongnan basins in the northwestern South China Sea preserve a large volume of Ceno- Received 31 August 2014 zoic sediments. However, their sources are still barely understood. In this paper, we combine rare earth element Received in revised form 18 January 2015 (REE) geochemistry, heavy mineral analysis and detrital zircon U–Pb geochronology for samples from potential Accepted 19 January 2015 source areas and basin boreholes to explore the Late Miocene source-to-sink system. Our reconstructed model Available online 20 January 2015 reveals that the Upper Miocene sediments were supplied from nonunique sources. Hainan Island mainly con- trolled the supply to the outer shelf, resulting in the relatively high REE concentration, zircon–tourmaline–rutile Keywords: – Yinggehai Basin (ZTR) index and the Yanshanian zircon U Pb ages. By contrast, sediments from the northern Yinggehai Basin Qiongdongnan Basin show a lower REE concentration and the assemblage of garnet, magnetite, epidote, amphibole and staurolite, sug- South China Sea gesting the influence of metamorphic rocks along the Red River Fault Zone. Despite the geological data shortage Upper Miocene in Central Vietnam, it may still be distinguished from the other provenances in the Central Depression of the Provenance Yinggehai Basin where is characterized by little metamorphic minerals and the relatively low ZTR index. It is noteworthy that sediments in the Central Depression of the Qiongdongnan Basin are distinct from those offshore Hainan Island in terms of each provenance proxy. We interpret that these sediments probably were transported from the southeastern Yinggehai Basin through the submarine canyon. The relative sea level fall during the Late Miocene and the structural inversion in the northern Yinggehai Basin could together facilitate this long-distance sediment transport. The local mafic volcanism is also found in the Qiongdongnan Basin, as indicated by the Eu enrichment and the mineral group of augite and olivine. © 2015 Elsevier B.V. All rights reserved. 1. Introduction area of 160 × 103 km2 and a discharge of 120 km3/yr respectively (Milliman and Farnsworth, 2011). Accordingly, it has been previously The intensive petroleum exploration in the northwestern South held that the Red River dominated the Neogene sediment supply to China Sea (SCS) has led to a number of oil and gas discoveries (e.g. the the basins (Clift and Sun, 2006). Recent detrital zircon U–Pb geochro- DF1 and YC13 wells; Hao et al., 2000; Xie et al., 2008b) in the Yinggehai nology on Upper Miocene–Quaternary sediments by Wang et al. and Qiongdongnan basins (the YGHB and the QDNB). Moreover, the (2014) also favored this viewpoint that the Yangtze Craton along the basins preserve an important sedimentary record of Cenozoic Tibetan upstream Red River was always the major source to the YGHB. But Plateau uplift, tectonic activity of adjacent plates and East Asian mon- this conclusion may be questionable at the whole basin scale, as sample soon evolution (e.g. Clift and Sun, 2006; van Hoang et al., 2010b). locations are just confined to the Dongfang area of the central YGHB. Hence, their sedimentary filling, structural evolution and hydrocarbon There is already growing evidence that the YGHB and the QDNB were accumulation have received great scientific attentions. As an important not solely sourced from the Red River. Another possible provenance is component of basin research, provenance analysis has been widely ap- Hainan Island, which can be exemplified by progradational slope plied in e.g. paleogeographic reconstruction and reservoir evaluation clinoforms along its margin (Xie et al., 2008a). U–Pb ages peaking at (e.g. Nie et al., 2012; Vincent et al., 2013). The Red River is one of the 90–110 Ma of detrital zircons from the joint area of the two basins also largest rivers along the northwestern SCS (Fig. 1), with a drainage suggest that Hainan Island was an important source throughout the Oli- gocene–Miocenetime(Yan et al., 2011). Meanwhile, a contribution from Central Vietnam is usually disregarded because of the scarcity of pub- ⁎ Corresponding author at: 388 Lumo Rd., China University of Geosciences, Wuhan 430074, China. lished research in this source area. Clift et al. (2004) speculated that Cen- E-mail address: [email protected] (T. Jiang). tral Vietnam mainly feeds the Mekong River drainage basin, thus http://dx.doi.org/10.1016/j.margeo.2015.01.007 0025-3227/© 2015 Elsevier B.V. All rights reserved. L. Cao et al. / Marine Geology 361 (2015) 136–146 137 Fig. 1. (a) Location of the Yinggehai and Qiongdongnan basins relative to SE Asia and the South China Sea. (b) Map displaying the topography and sample locations. Topography data are after Ryan et al. (2009). Major rivers in potential source areas are also shown: RR, Red River; NSR, Nam San River; CR, Ca River; SR, Son River; TBR, Thu Bon River; CHR, Changhua River; LSR, Lingshui River; WQR, Wanquan River. Crosses, circles and stars represent the analyzed samples of rare earth element (REE), heavy mineral and detrital zircon U–Pb age, respectively. The locations of LK, LS4 and LS33 are also marked for a comparison of REE data (Clift et al., 2008; Shao et al., 2010; Chen, 2012). delivering little sediment to the YGHB. However, recent offshore seismic Yingdong Slope, the Yingxi Slope, the Lingao Low-uplift and the Central profiling revealed eastward progradation from the Vietnamese margin to Depression (Fig. 2). The maximum thickness of the Cenozoic sedimen- thewesternQDNBsincetheMiocene(Yao et al., 2008; van Hoang et al., tary fill reaches up to approximately 17 km (Gong et al., 1997). The 2010b), suggesting a potential provenance from Central Vietnam. high geothermal gradient and strong overpressure facilitate a complex Large-scale submarine fans are widely distributed in the Upper petroleum system (Hao et al., 2000). Miocene strata of the YGHB and the QDNB (Zhang et al., 2013b). Their The QDNB is surrounded by Hainan Island, the Shenhu Uplift and the sedimentary provenance is still not well understood. Wang et al. Xisha Islands (or Paracel Islands) with its orientation perpendicular to (2011) interpreted them as a part of the Red River depositional system, the trend of the YGHB (Fig. 1). The tectonic framework of the QDNB whereas Sun et al. (2014) suggested that the coeval gravity flow system shows obvious differences between its western and eastern parts in the central YGHB was probably fed by the Ca River in Central (Zhang et al., 2013a), and can be subdivided into the Northern Depres- Vietnam. A provenance study is thus presented in this paper, based on sion, the Central Depression and the Southern Uplift (Fig. 2). analyses on river sands from Hainan Island and Upper Miocene The structure and stratigraphy of the YGHB and the QDNB have been sediments from the YGHB and the QDNB. A combination of rare earth el- studied for decades. Several mechanisms e.g. the strike-slip of the Red ement (REE) geochemistry, heavy mineral analysis and detrital zircon River Fault Zone (RRFZ), the Indochina block extrusion and the SCS sea- U–Pb geochronology is employed to constrain the provenance informa- floor opening are used to explain their complex evolution (e.g. Zhu et al., tion. Major controlling factors like regional tectonic activity and eustatic 2009; Wang et al., 2013b; Zhang et al., 2013a; Zhu and Lei, 2013). The sea level change are also briefly discussed in order to establish a basic basins feature the typical passive continental margin with a two- model for the Late Miocene source-to-sink system. phase evolution: a rift phase from the Paleocene to the Oligocene and a post-rift phase from the Miocene to the recent (Fig. 3; Chen et al., 2. Geological setting 1993). During the early-stage Neogene post-rifting, the basins underwent a thermal subsidence with attenuated fault activities. The 2.1. Geological background of the YGHB and the QDNB stress pattern of the northern YGHB changed from transtensional to transpressional after the Middle Miocene, and this resulted in the exten- The YGHB is situated between Hainan Island and Vietnam with a sive inversion in the Hanoi Basin and the northern part of the YGHB long axis oriented NNW–SSE (Fig. 1). The basin can be parted into the (Rangin et al., 1995; Clift and Sun, 2006). During the Late Miocene, the 138 L. Cao et al. / Marine Geology 361 (2015) 136–146 Fig. 2. Tectonic units of the Yinggehai and Qiongdongnan basins and the simplified geological map (modified from Commission for the Geological Map of the World, 1975)ofpotential source areas showing the lithological distribution along river courses. Only rivers that directly flow into the northwestern South China Sea are displayed. Major fault zones are modified from Lepvrier et al. (2008). relative sea level fall (Fig. 3) led to the retreat and partial drowning of southeastern YGHB and the western QDNB were mainly characterized carbonate platforms along the southeast Vietnamese margin and by progradational slope clinoforms favored by high rates of sediment around the Xisha Islands (Fyhn et al., 2009; Wu et al., 2014). The supply, whereas the eastern QDNB showed an aggradational pattern Fig.