Two isolated Neogene carbonate platforms (Xisha and Guangle carbonate platforms) have developed in the rifted uplifts since the Early Miocene. A large-scale submarine canyon system, the Zhongjian Canyon (ZJC), has developed in the tectonic depression between the two platforms since the Middle Miocene. High-resolution bathymetry data and 2D and 3D seismic data reveal the existence of the ZJC on the present seafloor, as well as in Neogene intervals. It exhibits typical characteristics of deepwater canyons that cut the surrounding rocks and indicate strong erosional features. The ZJC resulted from northwest–southeast strike-slip fault activities during synrift and postrift stages, and it periodically grew during the development of carbonate platforms since the Middle Miocene. We identified four cycles of parallel to subparallel high amplitude and dim reflectors in seismic data, which we interpreted as alternating canyon fill, based on the interpretation of seismic facies. Thus, the sedimentary evolution of the ZJC can be divided into four typical stages, which were in the Middle Miocene, Late Miocene, Early Pliocene, and Pleistocene. Considering the tectonic background of the carbonate platforms, as well as the on-going igneous activities, the sediment filling the canyon could be derived from a mixture of carbonate clasts, igneous clasts, mud, and silt. The laminar high-amplitude reflectors and dim-reflector package represented a fining-upward sedimentary cycle. The coarse-grained sediment in canyon fillings could be turbidites, carbonate debrites, and even igneous clasts. In contrast, the fine-grained sediment is likely to be dominated by pelagic to hemipelagic mud, and silt. This case study describes a deepwater canyon under a carbonate-dominated sedimentary environment and has significant implications for improving our knowledge of periplatform slope depositional processes. Furthermore, the insight gained into periplatform slope depositional processes can be applied globally.
Pockmarks, as depression morphology related to fluid escape on the seafloor, are revealed by three-dimension (3D) seismic data on the northwestern South China Sea (SCS) margin. The pockmarks can be classified into two groups by their various shapes in plan-view, which are circular group and elongating group. These pockmarks in the study area could be defined as mega-pockmarks, as their maximum diameters can reach to 7.5 km. They commonly develop more than one crater, which are central crater and secondary crater. The seismic data illuminated their complicated internal architectures in the subsurface, as well as their evolution periods, such as initiation stage, mature stage and abandonment stage. According to the buried structures and their genesis mechanism, the mega-pockmarks could be classified into linear faults-associated pockmarks and volcano-associated pockmarks. The linear faults-associated pockmarks root on the top Middle Miocene, where the linear faults distribute. The linear faults on the top of fluid reservoir in Middle Miocene act as conduits for fluid seepage. The fluid seepage is driven by the break of balance between the hydrostatic and pore pressure. When the fluid seepage initiate, they will migrate along the linear faults, making the linear feature of pockmarks on the seafloor. Both thermogenic gas from deep intervals and biogenic gas in shallow intervals may be fluid source for the genesis of pockmarks. On the other hand, the volcanic activities control the genesis and evolution of volcano-associated pockmarks. The volcano-associated pockmarks root on the craters of volcanoes. The volcanoes underneath the pockmarks provide volcanic hydrothermal solutions, such as phreatomagmatic eruptions through the volcanic craters. The confined fluid seepages make the pockmarks on exhibiting more circular shape on the seafloor. Long-term, multi-episode fluid expulsions generate the complicated internal architecture that leads to multi-cratered mega-pockmarks on the northwestern margin of SCS.
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