Deep-water Sedimentary Processes And Responses In The Pearl River Canyon,South China Sea

Deep-sea contains multiple complicated sedimentary processes,and it is necessary to systematically illuminate these different sedimentary processes and their interactions between each other due to the significance for revealing the formation of deep-water sedimentary morphologies and sandy sediments distribution.The Pearl River Canyon,South China Sea,is largely influenced by gravity flows,contour currents and internal waves(tides)together,which is a favorable region for studying integrated deep-water sedimentary processes and the associated interactions between them.Therefore,by utilizing 3D seismic data and gravity cores,as well as published in-situ measurement data on physical oceanography,this study dissected the integrated deep-water sedimentary processes and the corresponding responses in the Pearl River C.anyonThe results showed that the gravity-flows-dominated morphologies are mainly composed of slope migrating canyons,slope channels and submarine landslides.The western slope migrating canyons were largely influenced by shelf-edge deltas,and they were dominated by gravity flows triggered by delta-front failures,and their scales were thus relatively larger when compared to the eastern slope canyons,which are far from the shelf-edge delta and dominated by gravity flows generated by slope failures collapsed from slope canyon heads and flanks.The paleo-slope migrating canyons were gradually abandoned since Pliocene at the area dominated by the present slope channels during the Quaternary.These Quaternary slope channels were initially eroded by lowstand gravity currents generated by shelf-edge delta-front failures due to rapid progradation.It is noteworthy that these slope channels show turbidites formed when the shelf edge had a water depth greater than 130 m and was far from the paleo-shoreline(>44 km).These turbidites were most likely deposited by turbidity currents triggered by the persistent along-slope erosion of the South China Sea Branch of Kuroshio current(SCSBK).The largest submarine landslides are distributed at the canyon center of the middle segment of the Pearl River Canyon,and they could be divided into two major phases most likely occurred at 0.54 Ma and 0.19 Ma,respectively,and the older one is larger.The occurences and evolution of these two phases of landslides were largely controlled by sediment properties,morphological features of slope migrating canyons,sediment supply fed from shelf,as well as neotectonics.As for the contourite depositional system in the Pearl River Canyon,they are mainly composed of contourite drifts,contourite terrace and sediment waves,and there are sandy contourites at the canyon center of the upper segment of the Pearl River Canyon.The contourite sedimentary processes within the study area are mainly influenced by the SCSBK(<450 m water depth)and the Intermediate Water Current(IWC)(>450 m water depth).SCSBK is relatively stronger and thus contributes to the sedimentary features dominated by erosion and bypass at the region from shelf edge to upper slope.Meanwhile,the SCSBK preferentially eroded the root of the steep failure scars and resulted in the formation of the V-shaped furrows.As for the IWC,it is relatively stronger at the canyon center of the upper segment of the Pearl River Canyon,with the maximum velocity being up to 30-42 cm/s,which is strong enough to rework unconsolidated seafloor sediments and result in the formation of sandy contourites.The northern slope of the Pearl River Canyon is shown as critical-supercritical slope for the internal waves and internal tides in this region,thus the internal waves and tides are much more prone to break and generate bottom currents.The bottom currents have two major effects on the seafloor sediments:on one hand,they could re-suspend fine-grained sediments and result in the formation of nepheloid layers;on the other hand,the along-slope component of the bottom currents generated by breaking internal waves(tides)could erode the seafloor and form along-slope erosive furrows;the downslope component of the bottom currents and turbidity currents triggered by internal waves could transport coarse sediment from shelf-edge delta front into slope migrating canyons,and mostly deposit at the upper-middle reaches of the slope canyons,with typical features of relatively frequent interbeds of thin sands and mudsBased on the synthetic analyses of the integrated sedimentary processes dominated by gravity flows,contour currents and internal waves(tides)within the Pearl River Canyon,this study revealed that these different sedimentary processes were influenced between each other.Under the background of shelf-edge deltas,these different sedimentary processes and their interactions jointly resulted in the present morphological features and sediment distribution.Based on the systematical analyses of the deep-water sedimentary processes within the Pearl River Canyon,this study proposed that deep-water interactive sedimentary processes could be divided into three major types,including the triggering mechanism of the previous sedimentary processes on the subsequent processes,the overlap between the different contemporaneous sedimentary processes when they occurred at the same site,and rework of the subsequent processes on the previous sediments.Within the Pearl River Canyon,previous contour currents and internal waves(tides)could trigger subsequent gravity flows,respectively;synchronous overlap-interactions of gravity currents and contour currents,contour currents and internal waves(tides)could occur;contourite drift collapsed ana then being reworked by gravity flows,and internal waves could erode the top surface of contourite drifts,and the reworking processes of contour currents on previous gravity-flow deposites are also revealed.Under the control of these interactions between different sedimentary processes,the sand-rich sediments within the Pearl River Canyon are mainly distributed at three major areas,including 1)the delta-front area without contourite drifts covering upon,2)the canyon center at the upper segment of the Pearl River Canyon,and 3)the upper-middle reaches of western slope migrating canyons.The canyon center of the upper segment of the Pearl River Canyon has the largest coverage area of sand-rich sediments with relatively less mud content.This study points out that the controlling mechanisms of the integrated sedimentary processes on the formation and distribution of deep-water sedimentary morphologies and sand-rich reservoirs,which is meaningful for other related researches around the world.