Abstract:
The upper edge of the continental slope of the East China Sea is a transitional zone between two main geomorphological units: the continental shelf of the sea and Okinawa Trough. Research into its geomorphological characteristics and origination is meaningful for better understanding the interaction between these two units, and the sea level changes during the late period of Late Pleistocene. Multibeam sounding and subbottom profile surveys revealed sea bottom valleys at the upper edge of the continental slope in the northern East China Sea. The valleys extend in NNE-SSW direction in quasi-parallel to the continental shelf break with an average distance of about 7 km. The depth of the slope break is about 150 m on average. It is also generally parallel to normal faults in the continental slope. They can be subdivided into single valleys and locally double-valleys. The later ones consist of a main west valley and an auxiliary east valley with an interval about 2km. All the valleys have a higher west slope and a lower east one, 400~1600 m wide and 20~25 m deep in general. The single valleys have straight depth contours and gentle bottom topography, while the double-valleys have rather complicated topography, with minor gullies cutting down valley slopes and frequently uneven bottom and so on. Subbottom profiles across the west valley show the upper 10~15m strata having parallel reflection waves, forming an eastward-dipping structure, which is the paleo-Changjiang prodelta of the late period of Late Pleistocene. Cone penetration pest (CPT) measurements of 3m deep at 170 m depth west of the west valley are interpreted as surficial loose silty sands, and interbedded silt/clay with sands downward. Side-scan sonar images near the CPT site showed sea bottom sandwaves with amplitude about 0.5 m and wave length of 6~9 m. It is inferred that the location and layout of the valleys were affected by normal faults in the continental slope related to the third phase of the Okinawa Trough expansion. Taking into account the shell
14C dating of nearby borehole samples and sea level change curves, the valleys were formed mainly during 17.4~14.5 kaBP, which is the early stage of sea level rising after the last glacial maximum (LGM), when the area was in offshore-shallow sea environment and underwent tidal current scouring. Therefore, these valleys are fault-scouring valleys. The complicated topography of the double-valleys was related to submarine landslides of paleo-Changjiang predelta deposits. The measured 200 m depth contour lines of sea bottom are mostly within the west valley but partly to the east of the valley, resulting in the intersection of each other; and their similar stretching directions were considered to be resulted from the regional tectonic framework.