末次盛冰期以来长江下关-栖霞山段下切深度及沉积旋回

The Downcutting Depth and Sedimentary Cycles of the Yangtze River′s Paleo-Channel in the Xiaguan-Qixia Mountain Reach since the Last Glacial Maximum

选择长江下关-栖霞山段作为研究河段,搜集了南京长江大桥、四桥附近的76个钻孔资料.根据地质钻孔资料,绘制南京长江大桥和长江四桥附近长江古河槽地质剖面.在拟建长江四桥附近的4个钻孔采集了沉积物样品,在-59.33m深度上河床相沉积物的年代为(14 682±110)a BP,据此判断位于该沉积层下部的上一期沉积旋回应形成于末次盛冰期;在约-42.7m深度上河床相沉积物的年代为(11 305±130)a BP,该层以上应是全新世以来的沉积.末次盛冰期长江南京下关-栖霞山段古河槽埋深在-90m以下,形成局部深切.古河槽中充填了3次从粗到细的沉积旋回:约-65m以下的深槽中为末次盛冰期时的沉积旋回,-65～-40m为晚冰期的沉积旋回,-40m以上为全新世以来的沉积.

The Xiaguan-Qixia Mountain reach（Nanjng） of the Yangtze River was selected as the research reach.Data of 76 borehole cores from the Nanjing Yangtze River Bridges No.1 and No.4 were collected for study in this paper.Depositional sequences of two cross-sections of paleo-channel were established by Mapinfo software.Samples were collected from four drilling cores near the bridge No.4,which is under operation now,and the age of the sedimentary samples was dated to be（14 682±110） a BP at the depth of-59.33 m.It is deduced that in the lower part of the core there was a sedimentary cycle in the Last Glacial Maximum.The age of the-42.7 m sediment was determined to be（11 305±130） a BP.It is deduced that a sedimentary cycle in the upper part of the core took place in the Holocene.Therefore,it is believed that a downcutting below the depth of-90 m in the reach partly formed in the period of Last Glacial Maximum（LGM）.The drilling shows that the deep downcutting channel,deeper in somewhere,was narrow with water flowing swiftly.It is also found that the sedimentation of the Yangtze River in the reach have three obvious rhythms from graver and coarse sand to fine sand since the LGM.The time range of paleo-cannel at the depth lower than-65 m in the reach was within the period of the LGM.The sediments of the deep downcutting channel consist of pebble gravel,gravel sand,coarse sand and medium sand from bottom to top.The time range of paleo-cannel at a depth of-65^-40 m was within the Late Glacial Epoch.The sediments consist of pebble gravel,gravel sand,medium sand and fine sand from bottom to top.The sedimentary cycle in the part above-40 m is within the Holocene.The sediments consist of coarse sand and medium sand,fine sand,powdery sand and sandy clay from bottom to top.