Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea
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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.
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Key words:
- South China Sea /
- middle Miocene /
- channels and mounds /
- contour currents
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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.
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.
6. The characteristics of RMS amplitude maps for channels (a) and mounds (b), the time thickness map (c) and profile of mounds (d, e) in the Beijiao 3D seismic survey. The amplitude of upper Miocene channels is high to mediate-low from the southwestern to northeastern Qiongdongnan Basin, respectively; the mounds between channels are characterized by low amplitudes.
Figure 7. Time thickness map at the centre of the Beijiao Sag (T40–T41), showing the middle Miocene mounded reflections extending in a near east-west direction as parallel lines. See Fig. 6c for the location.
Figure 8. Horizontal distribution (a) and three types (b and c) of internal structures of mound reflections in the Beijiao Sag. Polygonal faults are developed within the mounds (Li et al., 2017a), to some extent, which disturb the discipline of the three types mounds and make seismic reflections within mounds unclear.
Figure 9. Isopach map of the ZRMB-QDNB (T50−T40), indicating that both Beijiao Sag and Liwan Sag with channels and mounds reflections are located in the tectonic depression with relatively thick sediments.
Figure 10. The comparative section of sedimentary facies for connecting wells of the middle and lower Miocene in the ZRMB. The section location is shown in Figs 2 and 11. GR: natural gamma ray, RILD: resistivity of deep investigate induction log, SP: spontaneous potential, AC: acoustic, RT: true formation resistivity, RD: deep investigate double lateral resistivity log.
Figure 11. Middle Miocene (T50−T40) sedimentary facies map of the ZRMB and QDNB, indicating that the channels and mounds are developed in the bathyal environment.
Figure 12. Well YLx logging and lithology profiles corresponding to the mound reflections in the Meishan Fm. in the Beijiao Sag of the Qiongdongnan Basin. Fm.: represents formation.
Figure 13. The wave impedance profile for the Liwan Sag. The average value for the wave impedance of mounds is less than 6.5×106 kg/(m2∙s). Location is shown in Fig. 2.
Figure 14. The wave impedance profile for the Beijiao Sag. The average value for the wave impedance of mounds is less than 6.0×106 kg/(m2∙s). Location is shown in Fig. 2.
Figure 15. The migration characteristics of bottom-current sediment waves. It shows crests and troughs of sediment waves have migrated southwestward since the early late Miocene, which suggests that bottom currents persistently move northeastwards in the early late Miocene. Green and blue solid arrows represent unidirectional migration of crests and troughs of sediment waves, respectively. See location in Fig. 2.
Figure 16. Seismic profile showing seismic contact characteristics among mounds, channels and polygonal faults. Blue and dark blue dots stand for truncations and onlaps, respectively. See locations in Fig. 6a.
Table 1. Absolute ages and basin evolution stages corresponding to the Miocene channels and mounds in the deep-water area in the ZRMB-QDNB (modified from Zhang et al. (2009) and Zhang (2010))
Stratigraphy Lithological stratigraphy
of QDNBLithological stratigraphy
of ZRMBAge
/MaSeismic
horizonErathem 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. 
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