The SCS is one of the largest passive continental margin seas in the Western Pacific Ocean (Ru and Pigott, 1986). ZRMB-QDNB are bounded by Taiwan to the east, Hainan Island and Hong Kong to the north, Yinggehai Basin to the west, Xisha Massif to the south (Fig. 1). They consist of the tectonic units of the Northern Uplift, Northern Sag, Central Uplift, Southern Sag, and Southern Uplift from north to south (Fig. 2). Both of the ZRMB and QDNB are the Cenozoic basins, which are developed on a Mesozoic igneous basement. They mainly experience three tectonic evolution stages of rifting, thermal subsidence, and accelerated thermal subsidence (Zhang et al., 2009; Zhang, 2010; Tian et al., 2015) (Table 1). Basin ﬁlls are in response to the three tectonic evolutionary stages. The depositional environment transforms from alluvial to lacustrine, onshore to neritic and shelf-slope to abyssal environments from the Paleocene to the present day (Xie et al., 2006; Tian et al., 2015; Liu et al., 2016).
Figure 1. Bathymetric map from the northern South China Sea (SCS). The yellow solid arrows are the inferred intermediate water circulation (modified from Zhu et al. (2010); Chen et al. (2014) and Sun et al. (2017)); the pink solid arrows represent the assumed deep water circulation pathways in the northern SCS (modiﬁed from Shao et al. (2007), Zhu et al., (2010) and Chen et al., (2014)); the jacinth solid arrows are pathways for the Northern Paciﬁc Deep Water Current into the SCS via the Bashi Channel and the Luzon Strait (modiﬁed from Gong et al. (2012) and Tian et al. (2015)). QDNB means Qiongdongnan Basin; ZRMB means Zhujiang River Mouth Basin.
Figure 2. Tectonic units in the ZRMB-QDNB and the locations of the middle Miocene channel and mound reflections.
Stratigraphy Lithological stratigraphy
Erathem era System period Series epoch Cenozoic Quaternary Pleistocene Ledong Fm. Wanshan Fm. 1.9 T20 Pliocene Yinggehai Fm. Aohai Fm. 5.5 T30 Neogene Miocene upper Huangliu Fm. Hanjiang Fm. 11.6 T40 middle Meishan Fm. Zhujiang Fm. 13.8 T41 lower Sanya Fm. Zhuhai Fm. 15.5 T50 Oligocene upper Lingshui Fm. Enping Fm. 23.3 T60 lower Yacheng Fm. Wenchang Fm. 29.3 T70 Paleogene Eocene Lingtou Fm. Shenhu Fm. 32.0 T80 pre-Cenozoic – – – 53.5 Tg Note: – represents no data.
Nine stratigraphic horizons/surfaces from Eocene to Pleistocene have been seismically identiﬁed (Figs 3 and 4) and were tied to well (YLx) in the study areas. They correspond to Tg (53.5 Ma), T80 (32.0 Ma), T70 (29.3 Ma), T60 (23.3 Ma), T50 (15.5 Ma), T41 (13.8 Ma), T40 (11.6 Ma), T30 (5.5 Ma), and T20 (1.9 Ma) (Xie et al., 2006; Chen et al., 2014). The mounds are within the T41–T40 seismic horizons (13.8–11.6 Ma), which correspond to the upper middle Miocene Meishan Formation (Table 1). The channels are distributed between the mounds at the bottom of the upper Miocene.
Figure 3. Characteristics of the mounds at the top of the middle Miocene in the Liwan Sag, ZRMB. The mounds are distributed on the slope and central sag. The bottom of channels between mounds exhibits relatively high amplitudes. The profile location is shown in Fig. 2. TWT: two-way-travel time.
Figure 4. The middle Miocene mounds are distributed in the slope of the Beijiao Sag, QDNB. The profile location is shown in Fig. 2.
Figure 5. The characteristics of Miocene channels and mounds in the 3D seismic survey in the Liwan Sag, ZRMB. a. The isochronous map of the remnant mounds (T40−T41); b. the mean-square-root (RMS) amplitude map of time window from the channel bottom T40 to 30 ms upward; c. a north-south 3D seismic profile perpendicular to the channels; d. a near east-west seismic profile along the channels.
Due to the influence of the East Asia monsoon and intrusion of Kuroshio current, ocean currents in the northern SCS are still controversial and complex (Xue et al., 2004) (Fig. 1). In the nearby Luzon Strait, the ocean circulation can be mainly divided into surface circulation (less than 350 m water depth), intermediate water circulation (350–1350 m), and deep water circulation (more than 1 350 m) (Chen and Wang, 1998). This water core division scheme is widely accepted for bottom currents interpretation, but it is still controversial, especially in the intermediate water depth of division. Newly acquired data indicate that the main scope of the intermediate water can easily exceed a water depth deeper than 1 500 m (Chen et al., 2013; 2016). In the northern QDNB, the unidirectionally migrating canyons, resulted from the interaction between gravity flows and bottom currents, and influenced by northeastward flow bottom currents, were widely developed in water depth from 450 m to 1 500 m. (He et al., 2013). Honeycomb shaped drifts, associated with bottom currents flowing across undulate seafloor, are reported and developed since the early late Miocene (11.6 Ma) in the southwestern QDNB (Sun et al., 2017). Elongated and mounded drifts, resulting from bottom currents and related to seamount, can be at least traced back to the early late Miocene (11.6 Ma) in the western ZRMB (Chen et al., 2014). The flow direction of the above-mentioned bottom currents in the ZRMB-QDNB moves from southwest to northeast. It is intimately associated with the intermediate water, which seems to indicate that the bottom currents in the study areas are also associated with the intermediate water. According to present ocean circulation, bottom current deposits are divided into mid-water bottom current deposits (300 m and 2 000 m) and deep-water bottom current deposits (more than 2 000 m) (Stow et al., 2002). Based on these divided schemes, bathyal deposits (current water depth: 600–2 000 m) may be subjected to the influence of the intermediate water in the study areas.
Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea
- Received Date: 2019-10-01
- Accepted Date: 2020-05-25
- Available Online: 2021-04-02
- Publish Date: 2021-02-25
Abstract: Numerous elongated mounds and channels were found at the top of the middle Miocene strata using 2D/3D seismic data in the Liwan Sag of Zhujiang River Mouth Basin (ZRMB) and the Beijiao Sag of Qiongdongnan Basin (QDNB). They occur at intervals and are rarely revealed by drilling wells in the deepwater areas. Origins of the mounds and channels are controversial and poorly understood. Based on an integrated analysis of the seismic attribute, palaeotectonics and palaeogeography, and drilling well encountering a mound, research results show that these mounds are dominantly distributed on the depression centres and/or slopes of the Liwan and Beijiao sags and developed in a bathyal sedimentary environment. In the Liwan and Beijiao sags, the mounds between channels (sub) parallel to one another are 1.0–1.5 km and 1.5–2.0 km wide, 150–300 m and 150–200 m high, and extend straightly from west to east for 5–15 km and 8–20 km, respectively. Mounds and channels in the Liwan Sag are parallel with the regional slope. Mounds and channels in the Beijiao Sag, however, are at a small angle to the regional slope. According to internal geometry, texture and external morphology of mounds, the mounds in Beijiao Sag are divided into weak amplitude parallel reflections (mound type I), blank or chaotic reflections (mound type II), and internal mounded reflections (mound type III). The mounds in Liwan Sag, however, have the sole type, i.e., mound type I. Mound type I originates from the incision of bottom currents and/or gravity flows. Mound type II results from gravity-driven sediments such as turbidite. Mound type III is a result of deposition and incision of bottom currents simultaneously. The channels with high amplitude between mounds in the Beijiao and Liwan sags are a result of gravity-flow sediments and it is suggested they are filled by sandstone. Whereas channels with low-mediate amplitudes are filled by bottom-current sediments only in the Beijiao Sag, where they are dominantly composed of mudstone. This study provides new insights into the origins of the mounds and channels worldwide.
|Citation:||Yufeng Li, Gongcheng Zhang, Renhai Pu, Hongjun Qu, Huailei Shen, Xueqin Zhao. Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea[J]. Acta Oceanologica Sinica, 2021, 40(2): 65-80. doi: 10.1007/s13131-021-1759-5|