Multiple sediment sources and topographic changes controlled the depositional architecture of a palaeoslope-parallel canyon in the Qiongdongnan Basin, South China Sea


Submarine canyon deposits have drawn attention due to their significance on source-to-sink analysis and hydrocarbon exploration. High-resolution 2-D and 3-D seismic and exploration well data recently collected in the Ledong-Lingshui segment of the Qiongdongnan Basin are used to investigate the depositional architecture of the palaeoslope-parallel Central Canyon, which is distinct from other slope-perpendicular canyons. This study indicates that the canyon developed along the thalweg of a multiple stepped palaeotopography with a slope-parallel descending trend eastwards. The location of the thalweg is controlled by regional tectonics and progradational slope clinoforms in the western Qiongdongnan basin. Geographic changes in an extending direction and slope gradient of the palaeotopography resulted in variations in the depth and width of the canyon. Analysis of the canyon infillings indicates multiple sediment sources including an axial sediment source from the Central Vietnam and the western Hainan Island and a canyon-side source from the northern slope of the Qiongdongnan basin. Provenance study shows that the former source supplied relatively coarse-grained turbidites and the later supplied fine-grained mass transport deposits (MTD). Most of such MTDs originated from the northern slope of the basin. Evolution of the Central Canyon can be classified into three stages. Stage 1 is characterised by significant incisions that are responsible for the formation of the canyon. Subsequently or contemporaneously, the sharp bend at the beginning of the middle segment of the canyon likely resulted in lateral erosion, which triggered large-scale and small-scale canyon margin failures in the middle and lower segments of the canyon, respectively. The subsequent early filling stage (Stage 2) refers to the deposition of turbidites supplied by the axial sediment source. However, the morphology of the stepped thalweg slope resulted in sediment bypass in the upper segment of the Central Canyon. During the late filling stage (Stage 3), MTDs supplied by the canyon-side sediment source were dominated, and interbedded with turbidite deposits. The deposition of the MTDs resulted in the sharp decreases in canyon accommodation space and the abrupt southeastwards stepping of the deepest part of the canyon. Moreover, complex interactions between debris-flows and turbidity-flows occurred during this stage. Such variations in architecture of the canyon were controlled by multiple sediment supplies and topographic changes. The proposed conceptual model of canyon infilling and the resulting stratigraphic architecture could be applied in other analogous canyons for hydrocarbon exploration.
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