Mixed Carbonate-Siliciclastic Deposits in a Channel Complex in the Northern South China Sea

New high-resolution 3D seismic data image a submarine channel complex in the northern slope of the South China Sea. The channel complex stretches hundreds of kilometers across the slope and flows into the deepsea from the siliciclastic shelf margin, linking neritic environment to the pelagic plain. The evolution of the channel complex developed two sedimentary stages, stage I (19.1–18.5 Ma) and stage II (18.5–17.5 Ma), separated by erosional surfaces. In the first stage, the complex was filled with pure siliciclastic sediments, forming thick-massive sandstone intercalated by thin layers of mudstone. During the stage II, the channel complex was deposited five carbonate-siliciclastic cycles. The unexpected channel-fill carbonate deposits present allochthonous characteristics, suggesting the siliciclastic channel was surprisingly used to transport carbonate sediment from the adjacent neritic carbonate platform. By analyzing the periodical carbonate sedimentary process in the siliciclastic channel complex, we infer that it was related to the in situ carbonate production of the neritic carbonate platform and was most likely to be controlled by the relative sea-level changes. Unlike line-source carbonate slope aprons or small-sized carbonate channels, the large-sized siliciclastic channel complex links directly neritic carbonate platform to deepwater basin and can transport large volumes of neritic carbonates to the pelagic environment in a short period. The new findings help to estimate the contributions of neritic siliciclastic shelf and carbonate platform to deepwater slope more accurately. This study suggests that channel systems are more complex than expected and have significant implications on the conceptual models describing the deepwater sedimentary theory.