江苏南通近岸区晚第四纪沉积序列的沉积相特征与定年

近三十多年来长江三角洲地区晚第四纪3期下切河谷的形成与演化得到了学术界的关注，但以往对晚第四纪早期和中期下切河谷充填层序年龄的限制还很欠缺。为了探讨这些下切河谷及其沉积充填的形成时间以及下切河谷形成与全球海平面变化的关系，对江苏南通近岸区孔深79 m的全取芯钻孔（JC-1205）岩芯开展了综合测试（沉积物粒度、底栖有孔虫、光释光（OSL）测年和AMS 14C测年）和沉积相分析。结果表明，该孔所揭示的晚第四纪沉积序列可划分为4个沉积单元（从下至上为DU 4—DU 1），中—下部3个沉积单元（DU 4—DU 2）为3期直接接触的河流沉积，上部DU 1为近岸海洋沉积（从下至上包括潮道沉积、浅水潮下带—内陆架沉积和潮坪沉积）。晚第四纪海平面变化是控制钻孔位置附近区沉积序列形成的主要因素，晚第四纪早期和中期的下切河谷及其充填沉积（DU 4和DU 3）分别形成于MIS 6和MIS 4，而与晚期下切谷相关的河间地的洪泛平原沉积（DU 2）形成于MIS 2；MIS 4时期下切谷的发育使得其下伏的MIS 5海洋沉积被侵蚀殆尽，而MIS 3时期相对较高的地势导致了沉积间断的出现。DU 1形成于MIS 1晚期，其底部的潮道沉积在其发育过程中对其下伏沉积物（可能形成于MIS 2晚期—MIS 1中期）的侵蚀造成了DU 1和DU 2之间超过10 kyr的沉积缺失。本文的研究成果为长江三角洲地区晚第四纪下切河谷的形成演化提供了重要的时间约束。

Sedimentary facies characteristics and dating of the late Quaternary sedimentary sequence in the nearshore coastal area of Nantong,Jiangsu Province,China

The formation and evolution of three-stage incised valleys in the Yangtze River delta during the late Quaternary have attracted much research attention. However, age constraints on the infilling sequence for the early and middle stages of incised valley formation are lacking. In this study, a 79-m-long sediment core (JC-1205) obtained from the nearshore coastal area in Nantong (Jiangsu Province) was analyzed, including measurements of down-core changes in grain size and benthic foraminiferal assemblages, optically stimulated luminescence and AMS 14C dating, and sedimentary-facies characterization, to determine the ages of valley incision and infilling and the responses of these processes to global sea-level variation. The sedimentary succession in core JC-1205 can be divided into four depositional units (DU 4 to DU 1) from bottom to top. Units DU 4 to DU 2 in the lower to middle parts of the core are interpreted as fluvial deposits with distinct contacts between vertically adjacent units. The uppermost unit DU 1 is interpreted as having been deposited in a nearshore shallow-marine environment and consists of tidal-channel, shallow-subtidal, and tidal-flat deposits from bottom to top. Late Quaternary sea-level variations are considered to have been a key control on the late Quaternary sequence at and around the core site. The early- and middle-stage valley incisions and corresponding infilling sequences (DU 4 and DU 3) were formed during the lower sea-level periods of MIS 6 and MIS 4, respectively, and the flood-plain deposit (DU 2) in the interfluve area, which is genetically linked to the late-stage valley incision, was formed during MIS 2. Fluvial incision during MIS 4 is interpreted to have eroded the underlying marine sediments that were deposited during MIS 5, and the development of relatively high terrain during MIS 3 resulted in a hiatus between DU 2 and DU 3. There is an unconformity corresponding to a hiatus of >10 kyr between DU 2 and DU 1, which was deposited during late MIS 1. This hiatus is attributed to the occurrence of tidal-channel scour, as evidenced at the bottom of DU 1, which is interpreted to have eroded the underlying deposits that presumably formed from late MIS 2 to middle MIS 1. This study provides important temporal constraints on the formation and evolution of late Quaternary incised valleys in the Yangtze River delta.