Sedimentary characteristics and genetic mechanism of the giant ancient pockmarks in the Qiongdongnan Basin, northern South China Sea
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Abstract: In the late Miocene, giant ancient pockmarks, which are fairly rare globally, developed in the Qiongdongnan Basin. In this paper, to determine the sedimentary characteristics and genetic mechanism of these giant ancient pockmarks in the Yinggehai Formation of the Qiongdongnan Basin, based on high-resolution 3D seismic data and multiattribute fusion technologies, we analyzed the planar distribution and seismic facies of the ancient pockmarks and compared the characteristics of the ancient pockmarks with those of channels, craters, and hydrate pits. Moreover, we also discussed the implications of the fluid escape system and paleo-bottom current activity in the ancient pockmark development area and analyzed the influence of the ancient pockmarks on the paleoclimate in this region. Finally, an evolutionary model was proposed for the giant ancient pockmarks. This model shows that the giant ancient pockmarks in the southern Qiongdongnan Basin were affected by both deep fluid escape and lateral transformation of paleobottom currents. In addition, the giant ancient pockmarks contributed to the atmospheric CO2 concentration in the late Miocene and played a great role in the contemporary evaluation of deepwater petroleum exploration.
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Key words:
- giant ancient pockmark /
- bottom current /
- fluid escape /
- Yinggehai Formation /
- Qiongdongnan Basin
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Figure 1. Location of the study area (red rectangle) in the Qiongdongnan Basin (black dashed line boundary), northern South China Sea (insert) (modified from Lei and Ren (2016), Cao et al. (2015) and Jiang et al. (2013)). The elevation data were derived from Tozer et al. (2019). Moreover, the deep-water well data were derived from Lei and Ren (2016).
Figure 2. Simplified stratigraphic, age, and lithological column information characterizing the Qiongdongnan Basin (modified from Jiang et al. (2013) and Cheng et al. (2021)). The sedimentation rate series was derived from Zhao et al. (2018). The sea level curves were derived from Zhao et al. (2018), Li (2009) and Haq et al. (1987).
Figure 3. Sequential stratigraphy of the Qiongdongnan Basin, modified from Cheng et al. (2021) (the section location is shown in Fig. 1).
Figure 5. Time structure diagram of the T30 Interface. Bottom current direction modified from Li et al. (2018c).
Figure 7. Root mean square (RMS) amplitude map between Horizon T27 and 40 ms above Horizon T27 (a); RMS amplitude map between Horizon T40 and 40 ms above Horizon T40 (b); paleo-topographic map of Horizon T40 (c) (source direction from Xiong et al. (2021)); planar distributions of sedimentary systems between Horizons T27 and T20 (d) (source direction modified from Cheng et al. (2022)); characteristics of the Late Middle Miocene on the seismic profiles (e). See d for the location of e. MTDs: mass-transport deposits.
Figure 8. Typical section structures of meteorite craters (a, b) (modified from Keerthy et al. (2019)); Xiuyan meteorite craters (c) (modified from Wang et al. (2013)); typical channel section structures (d, e) (modified from Tian et al. (2017)); and high-curvature channel deposits (f) (modified from Mayall et al. (2006)).
Figure 9. The evolution model of the giant ancient pockmarks in the Qiongdongnan Basin. Overpressure built up, and deep fluids accumulated in the channel sands (a); the fluid in the channel sands escaped following accumulation, forming near-circular ancient pockmarks on the seafloor (b); affected by bottom-current erosion, these ancient pockmarks were transformed into different shapes (c); and the overlying effective capping prevented the ancient pockmarks from developing in multiple stages, causing the pockmarks to be buried in the stratum (d).
Figure 10. Slight leakage of deep-formation fluids, most of which reacted with seabed sediments to form carbonate precipitation, while a few reacted with seawater and failed to break through the hydrosphere (a); intense deep-formation fluid leakage, most of which broke through the hydrosphere and entered the atmosphere, while a small part reacted with the seawater and seafloor sediments (b) (redrawn after Katz et al. (1999) and Dickens (2003)).
Figure 11. Reconstructed records of atmospheric CO2 concentrations from different metrics ranging from 7.5 Ma to 4.0 Ma (redrawn after Wei and Tian (2022); dates are referenced from Sosdian et al. (2018) and Breecker and Retallack. (2014)).
Table 1. Summary of ancient pockmark characteristics in the study area
Number Long axis/km Short axis/km Depth/km Dip angle of the long axis/(°) Trend Profile shape Planar shape 1 4.95 2.21 0.34 7.87 near NE U oval 2 2.29 1.25 0.43 21.52 near NE U oval 3 4.54 2.44 0.40 10.10 near NE W oval 4 4.18 1.56 0.36 9.87 near EW U crescent 5 2.75 1.68 0.60 25.00 near EW U oval 6 4.67 2.01 0.33 8.10 near NE U oval 7 12.04 10.66 1.03 9.80 near EW V chain 8 14.63 10.22 1.17 9.16 near EW V chain 9 9.16 7.38 1.25 15.64 near EW W oval Note: The information provided for ancient Pockmarks 7, 8, and 9 comprises only the data collected within the research area, although these pockmarks extend past the study area. -
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