| 摘要: |
| GEOPULSE甚高分辨率地震地层记录的研究表明，广东省汕头市南部近海埋藏有古河曲,曲率半径为 6-7km；发育有主河道和主、支汊道。主河道宽 2— 3km，最深约 25m；汊道宽200-1500m，深4-5m。在弯道处，主槽强烈东凸，侧向侵蚀显著。古河曲的形成在距今14kaB.P.之后，随着晚第四纪末次海面上升，在距今约12.3kaB.P.，古河曲的远岸部分开始淤浅 ，距今约11kaB.P.时整个河曲被淤平而消失；尔后，古河曲的近岸部分被韩江现代水下三角洲所掩埋。古河曲为古练江的一部分：从现在的练江口附近向东流去，在企望湾口转向南，再顺时针拐弯流向海洋。 |
| 关键词: 埋藏古河曲 晚第四纪 地震地层学 南海 |
| DOI： |
| 分类号: |
| 基金项目:福建省自然科学基金资助项目 ,B9910021号 |
|
| A BURIED MEANDERING RIVER OF LATE QUATERNARY OFF SHANTOU CITY GUANGDONG PROVINCE |
|
LIU A-Cheng,Lü Wen-Ying,CAI Feng
|
| Abstract: |
| The survey area covers the areas between 23°00′–23°30′ N, 116°30′–117°00′ E. It is near the Qiwang Bay, Shantou City, Guangdong, China. This area faces the Taiwan Shoal in the east, which is the southern part of the Taiwan Strait. The area extends from the coast to approximately 25 km offshore (35 m below sea level). The sea bed dips gently southeastwards at from 10 minute in shallow area (< 20m) to 5 minute in the areas beyond. Gravity core samples show that the near shore seabed is covered with fine sediments, such as clayey silt and silty clay. Form 25–30 m contour line, the deposit changes to coarse-grained sands.
The Hanjiang River, the largest river in the study area, has a large number of tributaries and runs into the northern part of the area with an annual runoff of 296×108m3 and sediments load of 11×106 t. The Lianjiang River with a smaller discharge is located in the west of the area. Most Hanjiang sediments deposited in the river mouth with part of them washed away by tides.
From seismic profiles, four seismic sequences were identified in terms of seismic stratigraphy. The upper layer or Layer A, is characterized with high frequency, continuous, strong amplitude, parallel, and vertically uniform reflection waves. It is between 0–6 m in thickness, decreasing from the coast to the sea and wedged out at the area 16 km from the shoreline. Gravity cores S1 and S25 indicate that Layer A consists of mainly clayey silt which deposited in the late Holocene high sea level period around 6 ka B.P. It constitutes the forehead of modern Hanjiang subaqueous delta. Layer B has similar wave characteristics to Layer A, but with wavy surface with various dipping directions. Layer B, between 0–20 m thick, is comprised of channel deposits. In the near shore zone it was transgressed by Layer A but in the offshore zone it directly exposed without younger cover. The base of Layer B deeply incised into the underlying sediments. Layer C waves, observable mainly in the open sea with a thickness of 20m, are high frequency, medium amplitude, continuous to sub-continuous and generally parallel to the sea floor. The top of the layer mostly exposes on the sea floor and was locally cut and overlain by Layer B in the near shore zone. Sampling at S26 shows that Layer C consists of mainly coarse sands similar to the surface sediments in western Taiwan Shoal. Its 14C dated age is (14107–13927)±(349–219) a B.P. or geologically Late Pleistocene. Layer D, the bottommost one, is a continental formation that has the greatest variation in wave characteristics.
In this area, paleo-river channels and major and minor tributaries were identified from seismic profiles. The main channels were 2–3 km wide and 22 m deep, jointed by tributaries with various sizes. The base of the channel near shore is 44 m below the present sea level, which cut into Layer D and underlies Layer A. The main channel offshore is 2.5 km wide and 25 m deep, 55 m below present sea level, which cut the Layer C. The age of the river is later than 14ka B.P. Observations on two main channels showed that the past sea floor dipped to the south that is similar to the present sea floor. Strong asymmetry and clear curvature observed in this study indicated the existence of a meandering river system in this area. This system is a part of the Lianjiang River in the past.
Based on the limited study on sea level changes since the Late Pleistocene in this area, the authors concluded that the paleo-meandering-river existed between 14–11 ka B.P. or from Late Pleistocene to Early Holocene. Due to the sea level continues rising, at 6 ka B.P., the Hanjing River underwater delta came into being. Therefore, the previous meandering river system was buried. |
| Key words: Paleo-meandering-river, Late Quaternary, Seismic stratigraphy, South China Sea |
