--> Morphology, classification and controlling factors of the Late Quaternary submarine canyon system on the northern margin of the South China Sea

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Morphology, classification and controlling factors of the Late Quaternary submarine canyon system on the northern margin of the South China Sea

Abstract

Eighteen Late Quaternary slope-confined canyons, developed on the northern margin of South China Sea, were investigated using 3D seismic datasets, displaying morphological variations from west to east. According to the geomorphology and geological settings, this study divided these Late Quaternary submarine canyons into two types. Type I canyons are close to the present shelf-break (<10 km), which was mainly formed by the progradation of last shelf-edge deltas at 135 kry BP Type I canyons developed paleo-delta front failures at canyons heads and they are presently characterized by relatively smooth, concave-upwards thalwegs without distinguished terraces along longitudinal profiles, as well as sands-rich lobes at canyon mouths. Type I canyons could be further morphologically classified into Type IA and Type IB, Type IA canyons, directly linked to paleo-shelf-edge delta front, have steeper gradients with head-scarps at the upper segments, while Type IB canyons have gentle upper segments and develop smoother seafloor composed of muddy belts at canyons heads. However, the eastern Type II canyons are far from the shelf edge with distances larger than 19 km, but close to the tectonically active Dongsha Uplift. They are characterized by undulated bedforms at canyons heads and faults-associated terraced morphology along thalwegs, suggesting that their developments were associated with open slope failures. The last active time of these slope-confined canyons was ca. 135 kry BP and got into the quiescent period subsequently, thus their resultant modern morphologies are mainly controlled by paleo-geomorphology, sediment supply, tectonic activities and oceanographic hydrodynamic regimes in the past 135 kry. The origin of Type I canyons was likely to be closely related to the higher sediment supply provided by the contemporary shelf-edge deltas, while Type II canyons responded much more strongly to tectonic activities of the adjacent region. Internal waves played an important role in the maintaining of these canyons during the quiescent period. Our results imply that the slope-confined canyons of Type I, dominated by the shelf-edge deltas, might be much more efficient in transporting sands across the shelf-edge through the turbidity currents generated by the delta front failures, especially for Type IA canyons with direct linkage to the shelf-edge deltas front.