Along-strike Quaternary morphological variation of the Baiyun Sag, South China Sea: The interplay between deltas, pre-existing morphology, and oceanographic processes

Continental margins usually display along-strike morphological variation, with diverse underlying controls around the world. Based on 3D seismic data from the Baiyun Sag, northern South China Sea, this work provides a new case study to understand the combined controls on along-strike morphological variation over short strike distances. The northern slope of the Baiyun sag was characterized by a gently dipping ramp at the beginning of the Quaternary, and the Quaternary along-strike morphological variation in this area is mainly displayed in the deltas, submarine canyons and contourite depositional system. The deltas were featured by lobate form and mainly distributed at the western area, with shoreline parallel and oblique to the ramp strike in the west and east, respectively. The scales of the deltas and the associated submarine canyons decreased towards the east. The eastern plastered contourite drifts have a greater areal extent than those in the west, and the former are terraced. The Quaternary slope evolution took place in three stages (stage 1–3), of which the time intervals are ca. 2.6 - ca. 1.6 Ma, ca. 1.6 - ca. 0.14 Ma and ca. 0.14–0 Ma, respectively. The slope was characterized by a gentle ramp, deltas and contourites from stage 1–3 respectively. The along-strike morphological variation of the northern slope of the Baiyun Sag was predominantly influenced by the interplays between the deltas, pre-existing morphology, and oceanographic regimes. The localization of the lobate-form deltas with respect to the pre-existing submarine canyons on the ramp exerted a great influence on the along-strike variation of these canyons within stage 2. The interplay between the delta morphology (i.e., delta position and delta-front gradients) and the oceanographic regimes (i.e. contour currents and internal waves) controlled the along-strike variation in contourite processes during the stage 3. This study provides a new insight into the combination of controls responsible for along-strike morphological variation of post-rift continental margins evolving from a ramp. The underlying depositional processes revealed by this study would be helpful for understanding the lateral variability of sequence architectures and the deep-water sands deposition, which are important basis for guiding future deep-water hydrocarbon exploration on such continental margins elsewhere.